621.31:535.215 on Manuscript Rights YERKIN SHABDAN One-Dimen

Al-Farabi Kazakh National University

UDC: 621.38 – 022.532: [621.31:535.215 On manuscript rights

YERKIN SHABDAN

One-dimensional (1D) nanostructured materials for solar energy conversion study

6D074000 – Nanomaterials and Nanotechnologies

Thesis is submitted in fulfillment of the requirements for the degree of Doctor of Philosophy (Ph.D.)

Scientific consultants:

Candidate of Phys. & Math. Sciences, Head of lab. K. K. Dykhanbayev Department of Physics and Technical, Al-Farabi Kazakh National University, Almaty, Kazakhstan

Prof. N. Nuraje, PhD Department of Chemical Engineering, Texas Tech University, Lubbock, USA

The Republic of Kazakhstan Almaty, 2018 CONTENTS

SYMBOLS AND ABBREVIATIONS …………………………………… 3 Acknowledgment ………………………………………………………….. 5 Introduction ……………………………………………………………….. 6

1 LITERATURE REVIEW …………………………………………. 13 1.1 Renewable energy ……………………………………………………. 13 1.1.1 Solar Energy …………………………………………………….. 15 1.1.2 Dye-sensitized solar cell …………………………………………. 18 1.1.3 Photocatalytic Water splitting …………………………………… 28 1.2 Nanostructured materials, One-dimensional (1D) nanostructured materials and methods……………………………………………………. 34 1.2.1 1D nanostructured materials for Dye-Sensitized Solar Cell ……... 38 1.2.2 1D nanostructured materials for Photocatalytic Water Splitting …. 39 1.2.3 Sol-gel and Electrospinning methods ……………………………. 41 Conclusions for section 1……………………………………………………. 44

2 EXPERIMENTAL, RESULTS AND DISCUSSION FOR DYE- SENSITIZED SOLAR CELL …………………………………………. 45 2.1 Experimental section for Dye-Sensitized Solar Cell …………………. 45 2.1.1 Fabrications of Titania based nanofibers …………………… 47 2.1.2 Assembly of device …………………………………………… 51 2.2 Dye-sensitized solar cell results and discussion …………………… 55 2.2.1 Characterizations of fibers …………………………………….. 55 2.2.2 Discussion of experimental results (analysis of device)………….. 63 Conclusions for section 2………………………………………………….. 70

3 EXPERIMENTAL, RESULTS AND DISCUSSION FOR PHOTOCATALYTIC WATER SPLITTING ……………………… 72 3.1 Experimental section for Photocatalytic Water splitting ……………… 72 3.1.1 Fabrications of Strontium Titanate nanofibers………………. 72 3.1.2 Experimental setup for hydrogen evolution ……………………… 75 3.2 Photocatalytic Water splitting results and discussion …………………. 77 Conclusions for section 3………………………………………………… 92

CONCLUSION ……………………………………………………………. 94 REFERENCES ……………………………………………………………. 95 APPENDIX A ……………………………………………………………. 108 APPENDIX B ……………………………………………………………. 110

SYMBOLS AND ABBREVIATIONS:

1D One-dimensional NSMs Nanostructured Materials UV Ultraviolet SEM Scanning electron microscopy HRTEM High-resolution transmission electron microscopy TGA Thermogravimetric analysis XRD X-ray diffraction EDS Energy dispersive spectroscopy XPS X-ray photon spectroscopy EIS Electrochemical impedance spectroscopy DSC Differential scanning calorimetry GC Gas chromatography PL Photoluminescence PV Photovoltaic PECs Photoelectrochemical cells DSSCs Dye-sensitized solar cells DC Direct current FF Fill factor IV Current-voltage IPCE Incident photon to current efficiency PCE Power conversion efficiency LHE Light harvesting efficiency TCO Transparent conducting oxide FTO Fluorine doped tin oxide (SnO2: F) ITO Indium tin oxide CNTs Carbon nanotubes MWNTs Multi walled carbon nanotubes SWNTs Single walled carbon nanotubes TiO2 Titanium dioxide SrTiO3 Strontium Titanate STO Strontium Titanate STO-NFs Strontium Titanate nanofibers (SrTiO3-nanofibers) STO-NPs SrTiO3-nanoparticles DSRT Strontium titanium metal alkoxide N719 Ruthenium-based dyes (C58H86N8O8RuS2) LUMO Lowest unoccupied molecular orbital HOMO Highest occupied molecular orbital CB Conduction band VB Valence band TIP Titanium (IV) isopropoxide DI Deionized water EC Ethyl cellulose

PVP Polyvinylpyrrolidone Eredox-1 Oxidation potential of the HTM EF(TiO2) Fermi level of TiO2 EF(CE) Fermi level of counter electrode EF(redox-1) Fermi level of the HTM PEC Photoelectrochemical NHE Normal Hydrogen Electrode wt% Weight Percent η Efficiency e− Electron τ Electron lifetime L Diffusion length VOC Open circuit voltage ISC Short circuit current Jsc Short-circuit current density λ Wavelength c Speed of Light (3.00 x108 m/s) e- Electron eV Electron Volt (Unit of Energy) h Planck’s Constant (6.63 x10-34 J.s.) hv Energy of Photon h+ Hole OER oxygen evolution reaction HER hydrogen evolution reaction

ACKNOWLEDGMENT

My thanks and appreciation go to Dr. Nurxat Nuraje who served as my advisor throughout the time it took me to complete this research and write this dissertation. I am very proud to work with him from beginning to completion of my Ph.D thesis research at Texas Tech University. I would like to acknowledge the support of Kazakh National University and thank my advisor, Prof. K. K. Dikhanbayev at Kazakh National University. Especially thank Professor Nurxat Nuraje, without his support and help, this thesis cannot be accomplished. I especially thank Dr. Nuraje for guiding and advising me from starting my research to accomplishment. Most of the research works in the thesis has been conducted in Prof. Nuraje lab. Some characterizations have been carried out in the Materials Characterization center of Texas Tech University. I also take this opportunity to thank my colleagues, Md. Moniruddin, Hanford Blake, Amir Ronasi and Parfait Coulibay. I especially want to thank my friend Md. Moniruddin for research discussions, including those for using electrospinning instruments and assistance of our experiments. I also thank Dr. Bo for characterizing nanomaterials using SEM. Special thanks to Professor Z. Zhanabayev and Dr. A.Temirbayev supported during the PhD study. Finally, I must thank those who are always supporting my life and the success of my PhD: my parents, relatives and my wife and children. Without their help and support, I couldn’t focus on my PhD study.

INTRODUCTION

The general characteristics of the study

From the general point of view, this work studied one-dimensional (1D) nanofiber obtained by electrospinning/sol-gel methods for solar energy conversion purposes. The first part was devoted to a fundamental study of electron transport mechanisms in one-dimensional core-shell nanostructures of titanium dioxide (TiO2) and carbon nanotubes with different (TiO2@CNT) properties fabricated by the electrospinning/sol-gel methods in dye-sensitized solar cell (DSSC). The second part of the thesis discussed crystallization growth of 1D SrTiO3 nanomaterials in polymeric nanotemplated system. The effect of calcination temperature and precursor concentration on the photocatalytic activity of the fabricated Strontium Titanate nanofibers (STO-NFs) were also assessed by hydrogen generation rate from water splitting under UV irradiation. Each purpose used the experimental study of 1D nanostructured assembled materials. We mainly investigated 1D nanostructures of SrTiO3, and also TiO2 with carbon nanotubes (CNT’s) which were multi walled carbon nanotube (MWCNT) and single walled carbon nanotube (SWCNT). These materials were synthesized by electrospinning/sol-gel methods, and then characterized by related instruments including Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photon spectroscopy (XPS), etc. The produced 1 D nanomaterials were further utilized to study for the solar energy conversion including solar cell and water splitting. The experimental results from the solar cell and water splitting were explained via mechanistic study and our theoretical explanation was further discussed based on our experimental evidences.

Relevance of the study At this moment, most of the energy comes from fossil fuels, however, it is clear that our current hydrocarbon based energy resources are limited – according to the scientific modelling or prediction it will last for 100 years [1]. Solar energy is considered one of the important renewable energy sources to fulfill the world’s growing energy demand since it has enormous magnitude – approximately 105 terawatts on earth surface [2]. At this moment, the utilization of solar energy can practically be divided in the following two ways: One is its direct conversion into electricity. In this case, the utilization of solar cells is a promising way to produce electricity from solar light. For instance, a dye-sensitized solar cell (DSSC) is one type of solar cells which use ruthenium dye to harness solar light and titania nanoparticle to transfer electron to external circuit to produce electricity. To utilize the solar energy, the photovoltaic study, which converts solar energy to electricity, has been very successful and made a tremendous progress. The storage of solar energy by converting solar energy into chemical energy such as hydrogen gas is another promising option since hydrogen gas has high energy density, which is also easily compatible with current energy producing facilities. 6

Therefore, water splitting research becomes very intensive with a high demand. The utilization of hydrogen as the primary energy carrier produced from the photocatalytic watersplitting is very important and is a good way to easily extend anywhere on Earth. DSSC has many advantages, for instance, low cost fabrications, decent photovoltaic energy conversion and ecofriendly assembly system [3/2]. It consists of mesoporous photoelectrodes with high surface area, and a large amount of absorbed dyes. These advantages of DSSC technology make people proud to produce large-scale production. With this in mind, many researches are focusing on improving DSSC energy conversion efficiency and extending lifetime of DSSC. Most recent records show that energy conversion efficiency for TiO2 based DSSC reaches more than 12%. One of the key components of DSSCs is metal oxide nanomaterials, which performs a dual function: (1) a scaffold which provides large surface areas to contain or fix the sensitizer molecules (dyes), and (2) a transport material to transfer photo injected electrons from dye molecules to the external circuit. Both dye absorption on the photo electrode and electron transport properties to some degree depend on the size, morphology and defect of the metal oxide scaffold. In the DSSC mechanism, electron transport is a very important step to enhance the solar cell performance. The quasi-Fermi level position of electrons in the semiconductor is determined by the metal oxide optical properties under the light radiation. The energy conversion efficiency of DSSCs consisting of TiO2 photo electrodes is usually considerably higher than that of the cells produced from ZnO photoelectrodes. Currently, nanostructured TiO2 is most widely studied in DSSCs. TiO2 is a wide-band gap metal oxide with a band gap energy of ~ 3.3 eV and interest for use in a variety of optoelectronic devices, especially in DSSC. Unlike ZnO, TiO2 nanocrystals have a large variety of different morphologies with sufficiently high specific surface area, which can be obtained by methods of synthesis from solutions at temperatures lower than 100 ºC. Furthermore, the electron mobility in TiO2 (anatase structure) has been investigated broadly. For this reason, there is considerable interest in improving the manufacturing technology and electron transport in DSSCs by using TiO2 nanostructures with two different CNTs as electrodes. Most researches on TiO2 -based DSSCs are concentrated on two main issues: (1) development of synthesis methods of one-dimensional TiO2 nanostructures, which have good conductive properties, and (2) findings of suitable dyes for TiO2. Thus, the study of transport and recombination of photo injected electrons in DSSCs based on TiO2 nanostructures has both fundamental and practical interest. These studies allow us to understand the mechanisms of transport and recombination of charge in low-dimensional nanostructures and to evaluate the effect of the semiconductor properties on the structure-sensitive and photovoltaic properties of solar cells. At the same time, they provide us the opportunity to improve photovoltaic performance of DSSCs. The other focus is water splitting in solar energy conversion, since the solar energy can be directly converted into the simple chemical energy form of hydrogen and also hydrogen possess high energy density. Hydrogen gas can be further utilized

7 to generate electricity with ecofriendly water by-product from fuel cell. Inspired by the first study conducted by Fujishima and Honda , who demonstrated photoassisted water splitting using titania as a photoanode material under UV irradiation, broad photoactive materials including inorganic and organic dyes have been explored. Among them, SrTiO3 has been extensively studied due to its relative high conduction band energy position which is beneficial for hydrogen production. Strontium titanate is stable under strong solar illumination and its band gap is easily engineered to the broad-band region. However, the synthesis of perovskite nanomaterials with controlled size, proper morphology, and highly crystalline structure is challenging by using conventional methods and it is even more challenging to prepare ternary metal oxides of different morphologies of 1D network system. Therefore, our goal was to develop a porous 1D strontium titanate nanofibers (STO-NFs) by electrospinning technique in combination with sol-gel method where polymer is used as nanotemplate. In addition, the growth behavior of STO-NFs including crystallite size and the diameter of the fiber was simultaneously investigated under different conditions such as different precursor concentration and calcination temperature. This study focused on an important understanding of crystallization growth of SrTiO3 in polymeric nanotemplated system. The effect of calcination temperature and precursor concentration on the photocatalytic activity of the fabricated STO-NFs was also discussed and confirmed by hydrogen production from water splitting under UV irradiation. 1-D nanomaterials such as nanowires and nanofibers are receiving broad attraction and showing potential applications in electronics, photonics, and other related fields since they provide faster electron transport relative to nanoparticles. Porous nanoparticles- based photo anodes are suffered from low electron transport due to diffusion trap limitations. Therefore, the photoelectrode of DSSC consisting of 1D nanomaterials are superior to the above mentioned photoanode since they can provide faster electron transport and higher diffusion length. An electrospinning method in combination with sol-gel and hydrothermal approaches provided a convenient approach to produce 1D nanomaterials. This technique was originally developed to produce polymeric nanofibers by applying electrical field, which is further adapted to fabricate nanocomposite fiber with inclusion of inorganic precursors. The produced nanocomposite fibers were heat-treated to further crystalize and produce one dimensional assembled nanocrystals. The developed synthesis technique of the nanostructured materials offers an unique capabilities to control their architectures and properties of the assembled materials since their surface areas and sizes can be controlled in the nanometer range. However, these controlled properties are crucial to the efficiency of either DSSCs or photocatalytic water splitting, including different morphologies of low dimensional nanostructures, such as nanoporous films, nanorods, nanoplates, tetrapods, core- shell, composite and different hierarchical nanostructures.

The purpose of research: Using electrospinning unique method and sol-gel methods to generate one- dimensional nanostructured materials for solar energy conversion study. Specifically two different applications include 1D core-shell nanostructured materials for DSSC, there its electron transport mechanics, and 1D nanostructured materials for water splitting, specifically, effects of material’s concentration, temperature and crystal size effects on hydrogen evaluation.

Tasks of the research: Study electron transport mechanism of one dimensional core-shell nanostructures in Dye-Sensitized Solar Cell produced by using sol-gel method and electrospinning techniques. 1. Investigate 1D nanostructured core-shell nanofibers of titanium dioxide with metallic MWCNTs (TiO2@MWCNTs) and semiconducting SWCNTs (TiO2@SWCNTS). 2. Characterize the fabricated two core-shell nanostructures via scanning electron microscopy, energy dispersive spectroscopy (EDS) and X-ray photon spectroscopy (XPS). These techniques have been applied to characterize morphology and confirm the presence of carbon nanotubes inside the core- shell nanostructures. 3. Assemble Dye-sensitized solar cells using the above nanostructured materials to compare their device efficiencies and electron transport mechanism by corresponding cell measurements and mechanistic study methods. 4. Find out diffusion lengths and lifetimes for the two different core-shell nanostructures by using electrochemical approaches including I/V curve and Bode plot. The second part of the research is to study crystal growth of 1D semiconducting nanomaterials and their effect on photocatalytic hydrogen evolution. 5. To fabricate one dimensional semiconducting strontium titanate nanomaterials, an one pot synthesis technique has been developed to produce one dimensional structure using single precursor (strontium titanium metal alkoxide) via both sol-gel and electrospinning techniques. 6. To find out the crystallization temperature, both thermal gravity analysis and differential scanning calorimetry are applied to identify the temperature and composition. 7. Confirm the crystal structure of the strontium titanate via XRD and EDS analysis. 8. Study the morphologies of the nanomaterials by SEM and TEM. 9. Measure hydrogen evolution amount via gas chromatography.

Objects of research: Study One-dimensional Strontium titanate nanostructured materials for Water Splitting and one dimensional core-shell titanium dioxide with MWCNT (TiO2@MWCNTs) and SWCNT (TiO2@SWCNTS) for Dye-Sensitized Solar Cells.

Subject of study To investigate one dimensional core-shell titanium dioxides with MWCNT (TiO2@MWCNTs) and SWCNT (TiO2@SWCNTS) and their electron transport mechanism in DSSCs; and also investigate One-dimensional Strontium titanate nanostructured materials for waters splitting, more specifically, study charge separation mechanism and their concentration and crystal size effects on hydrogen production.

The main provisions for the defense DSSC study: 1. TiO2@SWCNT, semiconductive type, electron lifetime τ = 10.8 ms (or diffusion length) larger than TiO2@MWCNT, metallic property, lifetime τ = 0.108 ms (important finding) was obtained. 2. As a result of our 1D core-shell nanofiber study, we found that the diameter of the core-shell nanofiber was between 50 nm and 100 nm. The core-shell nanostructures (TiO2@SWCNT and TiO2 @MWCNT) were produced by using combinations of two methods: sol-gel and electrospinning. Water splitting study: 3. As a result, 1D SrTiO3 nanostructures with three different nanoparticle sizes were fabricated by the method of electrospinning for the first time. 4. As a result of the experiment, it was established that the increase in the size of the crystal of the strontium titanium oxide increases the hydrogen evolution rate.

Scientific novelty First, in the DSSC study: 1. For the first time, core-shell nanostructured titanium with a different property of MWCN and SWCNT and their Fundamental physics of electron transport mechanisms were studied in photovoltaic (PV). 2. For the first time, investigate 1D core-shell nanostructured TiO2 with CNT for fundamental physics of electron transport mechanisms, clearly say, found SWCNT and MWCNT lifetime advantages for DSSC. 3. An unique two syringe electrospinning method was developed to obtain core-shell nanostructured nanofiber for DSSC study 4. Electron transport properties of 1D core-shell nanostructure was found to be higher compare to typical titanium dioxide anode materials in DSSC; 5. A high open-circuit voltage of the solar cell was achieved as a result of using the core-shell method. Second, in the photocatalytic water splitting: 6. A new structure of 1D-nanostructured materials was easily and simply formed by electrospinning method. 7. For the first time, it was found that the concentration of a precursor of strontium titanate had a significant effect on the efficiency of hydrogen evolution. 8. For the first time, high-quality formation of SrTiO3 crystals was obtained

10 as a result of a smooth increase the annealing tempperatures of nanocrystalline fibers, the optimal temperature ( 800 ˚C) found.

Scientific and practical significance of the work As far as the solar energy conversion process is concerned, two major steps are common for water splitting and photovoltaics: light harnessing, and charge separation or recombination. One exception is the catalytic reactions on the surface of photo-catalyst for photocatalytic hydrogen evolution. The charge separation problems of one dimensional nanostructures were studied in the following two cases. One is a direct conversion of solar energy to electricity via a dye-sensitized solar cell. The second one is a conversion of solar energy into hydrogen fuels via photocatalytic watersplitting. The study of influence of morphology of one-dimensional nanostructured material is very important in device engineering such as photovoltaic, electronics, sensors, tissue-engineering etc. Using electrospinning method to produce one- dimensional various shaped nanostructured materials can be low –cost, easy process able promising method. Fundamental of understanding electron lifetime is important in solar cell. We first time attempted to explain the property of SWCNT and MWCNT decorated with titania shell structure and their fundamental mechanism of electron transport and collection problem in these systems. These findings can be further used to make the solar cell more efficient. To study crystal growth of one dimensional semiconducting nanomaterials and their effect on photocatalytic hydrogen evolution, we specifically investigated the charge separation mechanism in one dimensional semiconducting strontium titanate nanomaterials since the band gap of the strontium titanate can be easily engineered into visible light region via nitrogen or metal doping. The parameters including precursor concentration, calcination temperatures, flow rate of pump, and distance between syringe and electrodes for producing 1 D nanostructured materials were investigated. An effective synthesis technique was developed for producing stable nanocomposite fibers. It was found that the size of the assembled nanoparticles in the 1D dimensional nanostructures can be tuned via precursor concentration and calcination temperatures. 1D nanofibers with the size range between 50-100 nm were fabricated, and characterized by analysis using different equipment such as scanning electron microscopy(SEM), energy dispersive spectroscopy (EDS),X-ray photon spectroscopy (XPS), Electrochemical impedance spectroscopy(EIS), thermal gravity analysis(TGA), differential scanning calorimetry(DSC), XRD, TEM, gas chromatography(GC) and electrospinning.

Personal contribution of the author The author directly involved in the experimental research part, obtained all the results of physical and chemical experiments, and performed some equipment analysis, and numerical calculations. The experimental design and discussion of the

11 results were carried out by applicant together with scientific advisers and other contributed PhD students.

Reliability of results and the scientific conclusions of the work are confirmed by the reproducibility of the experimental results, the correspondence of the experimental results, the consistency of the results obtained with the theoretical and experimental assumptions and conclusions obtained by other authors in studies close in content, and by the use of proven techniques.

Approbation testing of thesis 21 articles were published based on the materials of the thesis, including 7 articles published in journals recommended by the Committee for Control in Education, Science of the Ministry of Education and Science of the Republic of Kazakhstan. Three articles were published in journals with high impact factor (i.e. Applied Surface Science 419, 2017 (IF: 3.387), AIP Advances 7, 2017 (IF: 1.568), Journal of Experimental Nanoscience, 2017, Taylor & Francis (IF: 0.91) and 9 publications in international conferences bulletins, 2 book chapters including one domestic and one foreign were published. All of these publications were made during the PhD program.

Relation of the dissertation topic with the plans of scientific works This dissertational work was carried out in the framework of the scientific project “Development of scientific basics for producing an efficient solar cell with silicon nanowires as an enveloping layer and without front contacts”, funded by under the financing grant of project, № GF/3205 from the Science Committee of the Ministry of Education and Science of Republic of Kazakhstan.

Structure and volume of the dissertation. The dissertation consists of introduction, literature review, experimental details, results and characterization discussions, conclusion, the list of references. The thesis is presented in 110 text typed pages, includes 77 illustrated pictures, brought in 42 equations, 11 tables, includes the list of used references total 185.

1 LITERATURE REVIEW

As a literature review, this chapter discussed energy and environmental issues correlating with world population growth, and general concerns of solar energy. In the solar energy conversion, we introduced the basic principles of Dye-Sensitized Solar cell and Photocatalytic Water Splitting. Further information on various dimensions of nanostructures, especially one-dimensional interest in solar energy conversion study, is provided. Moreover, the electrospinning and sol-gel methods are explained for future studies.

1.1 Renewable energy Over the past few decades, our world's energy consumption has risen dramatically so that we are facing some serious problems in this century. The widespread use of fossil fuels has led to an unexpected issues in urban health, economic dependence, political instability and many war cases. Cause of the above issues is to some degree ascribed to the population outburst, and industrialization in developing countries. Due to the rapid growth of population in the last century, the present earth's energy resources couldn’t meet the need in our planet. The world population has been estimated to be 7.590 Billion by December 27th, 2017 and it is expected to raise even more rapidly according to the World Population Report of the United Nations Population Fund and Worldometers [3]. It will rise to 9 billion in 2040. The total world energy consumption is expected to increase 28%, which indicate the jump from 575 quadrillion British thermal units (Btu) in 2015 to 736 quadrillion Btu in 2040. Energy demand will be higher than current consumption [4]. At this moment, most of the energy we use in our routine life rely on fossil fuels such as coal, oil and natural gas, which consume more quickly than their replacement. However, it is clear that fossil fuel reserves are limited - this is only a matter of time. This means that someday in the future we could run out of these fuels. The present energy economy remains highly dependent on fossil fuels. Presently, 85% energy production comes from oil, natural gas and coal. However, the world demand for fossil fuels will be expected to soon surpass annual production, while scarcities of fossil fuels could lead to international economic and political crises and conflicts. In addition, fossil fuel combustion emits greenhouse gases to the environment, which cause local and global effects. In fact, the cause of the greenhouse effect is as the result of burning fossil fuels. The amount of carbon dioxide increases with burning of fossil fuels. When CO2 and other gasses release in the air, which is called greenhouse gasses, a decreased airflow with the subsequent isolation of the warm air occurs, which lead to the temperature increase, increase of sea level, and reduction of ice caps on the mountains. It is predicted that the global temperature will be between 0.6 and 4.0 degrees Celsius in the next century, according to the simulation models with parameters including world population increase, basic energy source, and economic development [5, 6]. In figure 1, based on the report in 2017, it is forecasted that by

2040, renewable energy accounts for two-thirds of the global investment in power plants, as many countries use the lowest-cost source as their main energy stay. By that time, the renewable energy reaches 40% in total power generation.

Figure 1 – Global energy demand by fuel type [5]

Energy and environmental problems are very anxious for the public and they will increase in the nearest future. Currently we are living in the world for high demand on cutting-edge technology and economy development, consequently we are in strong need of using extra energy to catch up the economy growth and the demand on energy over the last few years. Future economy development not only requires the ample energy sources, but also emphasizes the availability of green energy source which can be obtained without bringing negative impact on our global living environments. Therefore, development of future energy meets strict requirements. As can be seen from the above chart, energy storage will maintain leadership in the market (even if coal is forecasted to decline) but demand for renewable energy will increase substantially. So far, peoples consider to discuss two major energy issues related to fossil fuels: resource depletion (economic issues) and environmental issues. For the past several years, the production and use of renewable fuels have grown rapidly, resulting in cleaner, less polluting environment. The sun, wind and water are ideal sources of energy, up to our location. They are pollution-free, renewable and efficient. A natural resource is a renewable resource that can be replenished at a same or larger than its depletion rate, for instance, solar, wind, geothermal and biomass resources. Today, politically and economically, Renewable resources are a crucial point, which affects the environment movement.

The term “renewable energy” means energy produced by a wide resource spectrum including sunlight, wind, flowing water, geothermal heat, and biomass which are crops, agricultural, industrial and municipal waste. Above mentioned resources might be utilized to generate electricity for all industrial needs, fuels for vehicles, and heat for facilities and manufacturing lines. To be competitive with current energy source, alternative energy sources must be efficient, low-priced, eco-friendly, and sustainable. For instance, there are some energy resources like nuclear power energy. While it is considered as an alternative energy, it still couldnot be determined as sustainable due to the fact that the use of nuclear plants is correlated to a number of detrimental environmental issues plus public safety. More specifically it requires the management of the radioactive nuclei and radioactive waste. Among the entire applicable technologies generating renewable energy, photovoltaic energy is the most promising one for future energy needs. Earth receives 1.2×1017 W insolation or 3×1024 Joule of solar energy a year [4]. Solar cells with a 10% efficiency which covers only 0.13% of the Earth’s surface would fulfill our current necessity [2]. Moreover, photovoltaic cell has advantages including little requirement of preservation, easy installment, wireless control, and benefit of remote control and removal management. According to the Solar Generation 6 report prepared by the EPIA (European Photovoltaic Industry Association) and Greenpeace International, the photovoltaic will be 9% of the world energy supply in 2030 [7]. With the rising demand in energy use, solar energy has become a promising potential energy source to be exploited due to its ample source and free availability. At present, the utilization of solar energy can usually be divided in the following two ways: One is its direct conversion into electricity. In this case, the utilization of solar cells is a promising way to produce electricity from solar light. Furthermore, various types of solar cells have been developed so far, including Silicon Solar Cells, Polymer Solar Cells, Dye-Sensitized Solar Cells(DSSCs), Perovskite Solar Cells, Quantum based Solar Cells, etc. (Accordingly, the DSSCs research is mentioned in Chapter 2). Secondly, the storage of solar energy by converting solar light into chemical energy such as hydrogen gas is a promising option since hydrogen gas has high energy density and the availability of the solar source is seasonal and terrestrial. In addition, hydrogen gas infrastructure and transport system are also compatible with current energy facilities (Accordingly, the SrTiO3 based research is mentioned in Chapter 3). However, both cases include solar energy conversion processes.

1.1.1 Solar Energy There is no life on earth without solar energy. It is emitted in the form of black body radiation. The sun which supplies the Earth with huge amount of solar energy, is a compound reactor which has been burning more than 4 billion years. It can supply our planet with the energy within an hour which is equal to one year’s need on the earth. The energy that is naturally available from the solar source is quite enormous. The sun delivers 1.2 x 105 TW of power onto the Earth [8], which surpasses any other energy resource by capacity and availability. The sun is the

15 ultimate energy source for human beings. Solar energy is transmitted onto the Earth’s surface through a constant flow of electromagnetic radiation waves which propagates at the speed of light, c (3 × 108 m s-1), in vacuum. As a wave, it can be described by its oscillation frequency, ν, or equivalently by its wavelength, λ, in a vacuum. The relationship between frequency ν and wavelength λ is given by:

λ ν = c (1)

A light wave is also recognized as quantized energy chunks known as photons. The energy of a single photon is equal to the frequency multiplied by the Plank’s constant h. The higher the frequency, the more energy a single photon possesses [2]. Light possesses dual functions which are particle and wave characteristics. Equation (2) shows that energy of a photon (E) is inversely proportional to its wavelength, λ, according to,

E = h ν (2)

Where h is Planck’s constant (h = 6.62606876(52) × 10−34Js), ν the frequency and c the speed of light. The energy of photons can take the unit of electron volt (eV), in Equation (3) and (4), where e is the value of an eV.

퐶 E = h × ν = h × (3) 휆

ℎ×푐 1240 E (eV) = = (4) 푒× 휆 휆(푛푚)

An electron volt is considered to be the energy which is required for an electron to pass through 1 volt. The unit conversion between the electron volts and joules is shown as below: 1 eV = 1.6 × 10-19 J

Equation (4) shows that a photon energy is inversely proportional to wavelength (table 1).

Table 1 – The energy ranges of an UV, Visible and Infrared light and their corresponding wavelengths

Wavelength, λ (in nm) name Energy, E (in eV)

280-400 UV 4.43-3.10 400-800 Visible light 3.10-1.55 1240 infrared 1.0

Solar light’s availability on the earth's surface is contingent on atmospheric conditions, time of the day, distance between Earth and Sun, and earth self-rotation speed and tilt angle. While the solar spectrum are subject to lots of factors, it is important to build a standard solar spectrum and power density which can be used to make reasonable comparison for various photovoltaics. Solar energy is electromagnetic radiation. Solar radiation near the earth surface is in the range of λ between 290 – 2500 nm. Quantum (unit energy) of electromagnetic radiation - photon (equation (2)) - is an often a more convenient term in the mechanism of solar conversion. The solar spectrum includes the ultraviolet (UV, 3-5%), visible (42-43%) and infrared (IR, 52-55%) regions of electromagnetic spectrum, with a maximum peak at around 500 nm. Its spectrum follows a blackbody radiation at 5670 K which is affected by some molecules absorption in the atmosphere, such as O3, CO2, H2O, as shown in figure 2 [9]. This figure shows the initial spectrum of solar radiation at the top of the atmosphere (orange) and atmospheric (multicolor, black). Black spots are the light that is invisible to our eyes. The radiation energy decreases from left to right.

Figure 2 – The spectrum of solar radiation. (Adapted from Wikimedia [10])

The Earth receives around 174 Petawatts (PW) (10 to the order of 15 watts) of incoming solar radiation in the upper atmosphere. Among the total radiated energy in the solar system, only the Earth captures a very small portion. Some of the captured energy is ultimately converted into wind and ocean flows, and some will be converted and stored by plants. The total power of solar radiation is 384.6 yotta watts (3.846 × 1026 W). This huge amount of energy flow is uniformly radiated into all directions from the Sun. At the distance of 150 million kilometers away, the exposed area of the Earth receives approximately 1368 W m-2. As 30% of the 17 irradiated solar energy is reflected by the atmosphere, approximately 1000 W m-2 of solar energy is received everywhere on the surface of the Earth [6, 8, 9]. While this amount of total energy provided by the Sun is far more than sufficient, the challenge is about how to efficiently collect and use the solar energy.

1.1.2 Dye-Sensitized Solar Cell (DSSC) Dye-Sensitized Solar Cell (also known as Grätzel cells created by Gratzel and O’Regan in 1991) is belonging to 3rd generation solar cells. Its photoanode is made of metal oxide nanoparticles which provide high surface area and are able to adsorb huge amount of dye molecules. On contrary to other solar cells, it is very popular due to low production cost, decent solar energy conversion efficiency, and ambient assembly condition [11, 12]. These priorities enable the DSSC technology to be scalable production. Wide-ranging researches have been done to increase energy conversion efficiency and prolong its lifetime [12 - 17]. The efficiency of DSSC is recently reported to be greater than 12% [18, 19] as seen in figure 3. The significant collective efforts of the scientific community over the past 20 years have not only promoted the improvement of efficiency, but also brought several new ways to make the DSSC affordable, durable, and efficient. The improvements include significant progresses on inorganic oxide forms, sensitizers, co-adsorbents, co-sensitization, new counter-electrodes, and new redox electrolytes etc. DSSCs can be competitive with traditional Silicon based Solar Cells as they have advantages of low cost, physical flexibility and easy made. It can be also used indoor and outdoor applications like mobile charger, portable consumer electronics.

Figure 3 – Efficiency of certified DSSCs and other related PVs. (Adapted from the NREL Research Cell Efficiency Records [20]) 18

The basic structure and working principle of DSSCs The typical DSSC structure is described as in figure 4: it mainly consists of a wide band gap semiconductor nanomaterial (Metal Oxide semiconductor materials) which is layered on the top of glass which is covered with a transparent conducting oxide (TCO), a ruthenium-complex sensitizers (N719, N3, Black dye), an - - electrolytic solution containing an iodide/ triiodide (I /I3 ) redox couple and a counter electrode (Platinum, graphite -coated). Between two glasses a sealing gasket is used to avoid direct contact of both electrodes.

Figure 4 – The main components of typical DSSC structure

The DSSC converts solar energy directly into electric current. It is usually built as it is described in the following stages: First, a TCO anode is made by a glass sheet coated with a transparent conductive oxide layer; Second, on the top of TCO glass, a mesoporous layer of wide bandgap mesoporous oxide (TiO2) is constructed; Third, monolayer sensitizers are attached to the surface of TiO2 layer to harness solar light; Fourth, an electrolyte organic liquid solution including redox mediator are sandwiched between two electrodes, which can help to regenerate dyes. The sealing gasket keeps from the electrolyte leaking; Fifth, a glass sheet cathode coated by platinum thin layer acts as counter electrode to facilitate electron collection. Sixth, finally the DSSC both sides are clipped by the paper clips. 19

Here, we describe assembly of the DSSC device step by step. Figure 5 shows the basic working principle of DSSC and introduces the charge transfer mechanisms in the photocurrent generation. 1. Sunlight is first caught by the dye which is immobilized onto the surface of a wide bandgap semiconductor. Furthermore, photon of sunlight is caught by the dye (a ruthenium dye, a common dye in DSSC) on the surface of wide bandgap semiconductor which can be either TiO2 or ZnO.

Figure 5 – working principle of DSSCs (Energy level diagram and mechanism of photocurrent generation)

2. The electron is excited from the HOMO level to the LUMO-level when a dye absorbs a photon. On the other hand we can say that the photosensitizer immobilized on the surface of the semiconductor absorbs the sunlight and transform into an excited state (or changes from the ground state (S) to the excited state (S*))

TiO2|S + hν  TiO2|S* (5)

3. The electron is injected into the wide bandgap semiconductor, TiO2. This operation occurs between 100 fs - 100 ps, depending on practical conditions. An exact analysis of injection kinetics can be found elsewhere [21, 22]. Alternatively, we can say the excited electrons are injected into the semiconductor’s conduction band, appearing in the oxidation of the sensitizer (S+)

+ − TiO2|S*  TiO2|S + 푒퐶퐵 (6)

4. Oxidized dye regeneration will be performed in microseconds scale. The regeneration kinetic operation has been considered with Marcus theory, see 20 elsewhere [23]. On the other hand we can say the oxidized sensitizer (S+) is regenerated by accepting electrons from the iodide ion

- 3 - 1 − TiO2|S + I  + 퐼 + TiO2|S (7) 2 2 3

5. The dye returns to the ground state by both radiative and non-radiative processes. At the same time, the triiodide redox mediator moves to the counter electrode and accepts electron to reduce to iodide

− − - 퐼3 + 2푒퐶퐵(푃푡)  3I (8)

6. Photoinjected electrons in TiO2 are possible to return to recombine with the holes in the oxidizing species in the electrolyte. When the process occurs the dye at the excited state is decayed to the ground state

S*  S (9)

7. Photo-injected electrons in the TiO2 can be also recombined with the holes in the oxidized dye. Equations (10) shows the recombination of the injected electrons with the dye cations

+ − S + 푒퐶퐵  S (10)

8. Equation (11) shows that the injected electrons recombine with the triiodide redox mediator

− − - 퐼3 + 2푒퐶퐵  3I (11)

The above mentioned steps describe the electron/charge processes which occur in the DSSC during dye excitation and recombination steps Let us introduce in more detail each part of the DSSC structures. The Transparent Conducting Oxide: TCO - coated glasses are the basic substrates to formulate DSSC electrodes. This is due to their high transmission to the visible light range and low ohmic resistance, which are the basic requirement for such DSSCs. TCO is an n-type semiconductor and its bandgap is around 4 eV. Its conductivity depends on three basic parameters which are doped metal ion level, and oxygen vacancies. In addition, TCO possess decent thermal and chemical stability since during photoanode formation, high temperature treatments (around 450 °C) and the harsh cell environment (due to the electrochemical reaction formation) are utilized. Huge amount of researches over the last thirty years have been performed on TCOs. Among them, fluorine-doped tin oxide (FTO) and indium tin oxide (ITO) are the most widely used photoelectrodes as they are believed to be the best settlement in the manufacturing process, optical and electrical functions. 21

Wide Bandgap Metal Oxide Semiconductor Materials: TiO2 and ZnO are mainly used photo-anodic nano-materials for DSSC [24 - 27]. In the DSSC, TiO2 nanomaterials in DSSC are considered to be ideal materials for electron transfer and prolong electron lifetime expectancy in the photo-anode, resulting in improvement in solar cell efficiency. The wide bandgap semiconductors deposited on TCO are often referred to as photoanodes, being the backing material on which the dye-molecules (i.e., it captures the light photon) are tied up. The Dye is the main source of photo-generated carriers in DSSCs, which are collected at the TCO layer. ZnO, CdSe, CdS, WO3, Fe2O3, SnO2, Nb2O5, and Ta2O5 can be alternate options for DSSC [28]. The earliest photoanode material is made from nanocrystalline titanium dioxide (TiO2). From the practical application point, TiO2 is an ideal candidate due to a low-cost, high surface area, robustness, abundance and non-toxicity material. Titanium dioxide is the naturally existed oxide form of titanium, and its chemical formula is TiO2. Its high refractive index is mainly responsible for its wide applications such as paints, food coloring, cosmetics, toothpastes, polymers, and other instances. Its main three polymorphisms are Rutile (tetragonal), Anatase (tetragonal) and Brookite (orthorhombic). Anatase and Rutile crystal structures are shown in figure 6.

Figure 6 – Anatase, Rutile crystal structure of TiO2 [29].

Nonetheless, Rutile and Anatase phases are responsible for titania extensive applications. Although both of them have similar tetragonal crystal structure, Rutile and anatase contain six atoms and twelve per unit cell respectively. In both structures, the main building block formed in a titanium atom surrounded by six oxygen atoms in almost distorted octahedral configuration. The octahedral stacking brings about threefold coordinated oxygen atoms. Anatase has pyramid-like crystals which is stable at low temperatures. The rutile form has needle-like crystals and is syntheiszed at high temperature processes. The density of anatase is 3.89 g/cm3 whereas rutile is 4.26 g/cm3. The amount of light absorbed by the rutile in the near – UV region is approximately 4% of whole solar spectrum. As the energy is absorbed by titania, the electron is excited from valence band to conduction band of titania, where holes in the valence band serves as strong oxidants cutting the long-term

22 endurance of the DSSCs. The brookite crystalline form of TiO2, is difficult to obtain and not commonly studied for the DSSCs. The band gaps for anatase and rutile are 3.2 eV and 3.0 eV respectively [30]. The position of the conduction band edge is the key condition for electron injection efficiency and is slightly below than the LUMO of most of the ruthenium dyes. In addition to this, titania possess high dielectric constant (ɛ = 80 for anatase phase). Furthermore, refractive index of titania is suitable (n = 2.5 for anatase) for inducing an efficient diffused scattering of the light, which increases the light absorption. It is well known that Zinc Oxide (ZnO) is a promising substitute for TiO2 used in the photoanode of DSSC [24]. ZnO has similar conduction band position with titania and its band gap is 3.37 eV. The ruthenium- based dyes developed for TiO2 should work for a DSSC consisted of ZnO photoelectrodes. However, ZnO is not stable for dye absorption compare to TiO2 , resulting in less popularity in DSSC[26]. In this reason, we choose TiO2 in our research. The Ruthenium-Complex Sensitizers (Dye): The Sensitizing agent (Dye) forms the core of the DSSC: sunlight is utilized to pump electrons from a lower energy level to a higher energy level of the dye, shaping a potential difference, which can be used to produce electricity. Ruthenium (Ru) complexes are one of the widely used sensitizers employed in DSSCs, because of its easily tunable photo-electrochemical functions and the preferable oxidation states of the metal. When dye molecules absorb exact wavelength photon, an excited electron jumps from its HOMO (the highest occupied molecular orbital) to LUMO (the lowest unoccupied molecular orbital). Grätzel and his colleagues published N719 dye in 1997 [31], and black dye in 2001 [32]. These two ruthenium base dyes have been extensively studied for DSSC purposes. Chemical structures for N719 and black dyes are shown in Figure 7.

Figure 7 – The Black dye and the N719 dye, both ruthenium-based [33]

- - The Iodide/ Triiodide (I /I3 ) Redox Couple Electrolytic Solution: In DSSC, electrolytic solutions regenerate the dye molecules via supplying electron to the holes of the dye and become the oxidized form of the electrolyte species which later receive an electron from the counter electrode, thus it completes the circle. Electrolytes are divided into three types including liquids, quasi-solids, and solids depending on their viscosity. The Counter Electrode (platinum electrode): A counter electrode is a core linkage of the electrical circuit through the reduction of triiodide into iodide. To prepare a counter electrode, Catalysis reaction is needed to produce platinum thin layers through reducing platinum ions. As a result, the surface of the TCO/glass substrate is covered with a thin layer of platinum (Pt). The preparation of Pt layer onto substrate is thoroughly discussed in experimental part. Pt has a good chemical durability to the different electrolytes and conductive properties so that it is widely applied as a counter electrode in DSSCs.

DSSC I-V curve and characterization techniques A power and current of an external load can be produced when a solar cell device is illuminated. Several essential parameters have to be introduced to clarify and totally comprehend the DSSC. The DSSC’ I-V curve characterizes its energy conversion capability at certain conditions (temperature and light intensity level). It can be used to describe the performance of solar cells. When a DSSC was tested for I/V curve measurement and solar cell performance, the maximum current can be measured and it is called the short circuit current (ISC). Despite this, the maximum voltage that is formed when current is disabled, known as the open circuit voltage (VOC) which can be measured. In the I-V characteristic, the maximum power point (PMAX) is the maximum point in the graphic of current and voltage as illustrated in figure 8 (next page). For any intermediate resistance value, the DSSC produces potential voltage ranges from 0 and VOC, and each equivalent current satisfies the relation V = RLI (where RL stands for the external load). In general, current density is preferred in order to have a generally comparable amount, rather than current.

Figure 8 – I-V characteristic of a solar cell 24

One of the main parameter to describe the DSSC performances in addition to the define current density (IMP) and maximum voltage (VMP) is fill factor (FF). Fill factor is characterized as the maximum power point ratio to the product between open circuit voltage and short circuit current:

Green area I × V Fill factor = = FF = MP MP (12) Blue area ISC × VOC

In the typical solar cell, the fill factor equals 1 and the shape of I and V curve shows a perfectly rectangular. The power density is expressed in equation13:

PMAX = FF ×VOC × ISC (13)

Photoelectric conversion efficiency (PCE, ɳ) is equal to the ratio of the maximum power to the incident light power Pin,. Pin is a power of the incident light at a temperature of 25 °C , which is measured in W/m2 or 1 sun, with the surface area of the solar cell [m2]). That is:

P V I FF ɳ = PCE = MAX = oc SC (14) Pin Pin

Incident photons to current conversion efficiency or Incident light spectrum by the quantum efficiency QE (E) is related to the current density, it is characterized as the possibility of electron excitation formation, hν, when a photon absorbs energy. Using the Quantum efficiency, the current density can be expressed as follow:

JSC = q ∫ 푏푠(E) QE (E) E (15) Where bs (E) represents the flux density of incoming photons or the number of incident photons per time unit and with an energy between E and E + dE), q is the elementary charge. The ability to adjust morphology at the nanoscale opens up a wide range of opportunities in controlling the charge transfer in DSSC. In fact, fast and direct transport in one-dimensional or quasi-one-dimensional nanostructures, such as nanowires, nanorods and nanofibers, can be carried out with a suitably broad exposed area of dye-sensitized typical of mesoporous layers. The incident photon- to-current conversion efficiency (IPCE) is defined as

IPCE = LHE ∙ φinjection ∙ ηcollection (16) where LHE is the light harvesting efficiency. φinjection is the electron injection efficiency from the excited dye molecule to conduction band of the TiO2 and ηcollection is the electron collection efficiency. The External Quantum efficiency (EQE, also known as Incident photons to current conversion efficiency or IPCE) can be characterized

Number of charges collected by the solar cell EQE = IPCE = Number of photons of a given energy shining (17) on the solar cell from outside

The IPCE measurement is critical to get data about the DSSC photoelectrical response, which allows us to know wavelength ranges the dye molecules are able to absorb photons and efficiently generate injected electrons. Open Circuit Voltage Decay is a common approach to examine the recombination kinetics in DSSC. The cell maintains a constant level of illumination under open conditions until a stable voltage level is reached. After that, the light is suddenly switched off and the DSSC is measured as a function of time [34]. Since all photogenerated electrons cannot be collected by the electrodes under an open-circuit voltage condition, they recombine at a substantially regular rate, which decrease the photovoltage. Thus, the cutback of VOC depends only on charge recombination, and can be associated to the electron lifetime through the following equation:

푘 푇 푑푉 τ = – 퐵 ( 푂퐶)-1 (18) 푞 푑푡 where kBT is the thermal energy. However, we also use another technique to find electron lifetime, that is Electrochemical Impedance Spectroscopy. Electrochemical Impedance Spectroscopy (EIS) can be also used to study the recombination and transport properties in DSSC. A sinusoidal voltage with variable frequencies is applied to obtain impedance. Bode plot is a graph of the frequency response of a system. Hendrik Wade Bode developed it in 1930s. Bode plots are a very useful method for expressing the gain and phase of a system as a function of frequency. In the typical Bode phase impedance representation in the DSSC, three main peaks can be observed in the following figure 9.

Figure 9 – typical Bode diagram of a DSSC

The first peak in a high frequency range (>100 Hz) gives information on the charge transfer at the Pt/electrolyte interface. The peak in the middle frequency range (1- 26

100 Hz) corresponds to the information of the charge transport/recombination at the photoanode /electrolyte interface. The peak at low frequency (below 1 Hz) represents the charge diffusion into the electrolyte [35]. The EIS preliminary curves can be fitted using an equivalent circuit model to get data about transport and charges recombination. In the modeling circuit: the resistance Rs represents the TCO series resistance, the parallel CCE//RCE refers to the counter electrode/electrolyte interface and the impedance Zph models the photoanode behavior. As stated in [36] the photoanode impedance Zph was modelled through a transmission line. RT is equal to the transport resistance and RCT represents the charge transfer resistance which is related to charge recombination. The constant phase element (CPE) Qμ indicates all form of the electrochemical capacitance. Analogous electrochemical capacitance Cμ is:

1/β (1/β) -1 Cμ = (Qμ) (RCT) (19)

The effective electron lifetime τ, the diffusion coefficient ,D, and the diffusion length L can be calculated according to the fitting parameters with the help of the following equation:

1/β τ = (RCT Qμ) (20)

푑2 D = (21) 푅푇퐶휇

L = (D τ)1/2 (22)

Where d is the paste (oxide) thickness.

Figure 10 – Charge dynamics timescale and different forward, reverse processes in the DSSCs (Adopted from ref. [37])

Figure 10 describes parallel electron transfer processes, which appear on the condition of rivalry and various time characteristics between them. In figure 27

10(right), when light photon occurs, the dye molecule is excited in a few femtoseconds and the TiO2 conduction band absorbs the electron from LUMO in the sub picosecond range. In order to ensure enough time for the preferable process to happen, nanosecond-ranged relaxation of excited states is less speedy than its injection. The HOMO of the dye is then injected again by an electron from iodide (I-) in the microsecond domain. Two main processes: electron percolation across the - - photoanode structure and collection of electrons by the oxidized I (which forms I3 ) within milliseconds are preceded by this process. It is necessary to reach the time durability of these processes to achieve desired conversion efficiencies in DSSCs. Figure 10 (left) indicates two processes in the DSSCs. One is electron forward steps (black lines) with their corresponding time scales. Red dash lines indicate the recombination steps with their corresponding time scales[21, 38. To obtain the energy conversion efficiency, the electron motion time in the conduction band of titania has to be lower than the electron life time. It means that electron can reach the working electrode until recombination happenings. The recombination processes include the photogenerated electrons in the conduction band of titania returns to fill up either the holes of oxidized electrolyte species or the holes of the excited dyes. The electron transport performance is usually characterized by the diffusion length, Ln, which is inversely proportional to the production of electron life time and the diffusion coefficient. On the assumption of parameter greater than the TiO2 film thickness, the entire photo-generated electrons will be collected statistically [37, 39].

-2 Ln = τnDn (23)

1.1.3 Photocatalytic Water splitting Due to the lack of continuous availability of solar light [40–43], the study of converting solar energy into other forms of energy for continuous supply becomes crucial. Therefore, water splitting research becomes intensive [44–47]. In watersplitting, solar energy can be directly converted into the simple chemical energy form of hydrogen [48 – 50] since hydrogen possesses high-energy density and acts as a green energy carrier. When It is used in a fuel cell, water is the only by-product which is environmentally benign. British scientist J · B · S · Haldane as early as in 1923 [51] proposed the concept of a photocatalytic hydrogen production. There is no such a natural existed hydrogen on our planet, though the universe is rich in hydrogen. As fossil fuels, water or biomass can be further utilized to produce hydrogen or other chemical fuels. Hydrogen is considered a good energy carrier since it can be generated from water, and also when it burns with oxygen, it not only produce water, but also generates heat or energy. The process is considered to be very clean as it does not emit any pollutants. Then resulting water can reutilize to return either nature or our daily lives. Water is the main source of hydrogen and it is attainable as the Planet Earth is rich in it. Thus we are able to cut down fossil fuel imports dependence. Hydrogen,

28 being comfortable to store and carry, can be used as a fuel for fuel cell vehicles and is a suitable candidate future transportation. One million tons of hydrogen produce per year mainly for the chemical, oil refining, metal and electronics applications [52]. For instance, the ammonia consumes a rich amount of hydrogen for making fertilizers. Currently Hydrogen can be produced typically by thermal production process. It means it consumes huge fossil fuels and also emitted Carbon dioxide emissions. There are some different approaches for hydrogen production. However, one of the best way is photocatalytic water splitting or photoelectrochemical water splitting. It is similar to a solar cell; the only difference is to change the energy of the sun into a chemical bond formation instead of converting directly to electric power. The photoelectrochemical cell consists of three main components: the anode, cathode and electrolyte (aqueous media). At the anode the water is oxidized to generate oxygen through oxygen evolution reaction (OER) and at the cathode hydrogen ion is reduced to hydrogen gas via a hydrogen evolution reaction (HER). Furthermore either or both of cathode and anode can be a photoactive semiconductor, which can absorb light. Another idea is utilized to split water via connecting p-n junction solar cell in parallel with photoelectrochemical cell which not only avoids the complicated manufacturing process, but also reduce system costs [53]. Although extensive researches have been conducted using a lot of semiconductors configurations, there is still a gap to reach the targeted goals from point of the efficiency and stability. Inspired by the first study conducted by Fujishima and Honda [54], who demonstrated photoassisted water splitting using titania as a photoanode material under UV irradiation in 1972, broad photoactive materials including inorganic and organic dyes have been explored [55-57]. Below the figure 11 shows the main processes in a photocatalytic reaction. The first step is absorption of photons by a semiconductor which led to form electron– hole pairs. In the second step charge separation and migration of photogenerated carriers occur. In the final step on the surface of semiconductor both oxygen and hydrogen evolution reactions proceed. As far as the surface catalytic reaction is concerned, surface active sites and surface area are critical [53]. Thermodynamically, the decomposition of water into H2 and O2 is a endothermic reaction (positive value: 237.2 kJ/mol) which requires additional energy as shown in equation (24):

0 0 ΔG = −nFΔE = +237.2 kJ/mol H2 (24)

where: F – is the Faraday’s constant (F = 96485 C/mol), N – indicates the number of transferred electrons (n = 2) ΔE0 – is the standard potential of the electrochemical cell (ΔE0 = 1.229 V).

Figure 11 – Main steps of photocatalytic water splitting [2]

ΔG = 237.2 kJ / mol is an Gibbs free energy required to split of a molecule of water into hydrogen and oxygen, which relates to ΔEo ≈ 1.23 eV per electron transforming to the Nernst equation under the standard conditions. It means a minimum energy of 1.23 eV per electron should be supplied by the photo-catalyst to split water. This process can be written in the following two half-reactions:

+ + Water oxidation: H2O + 2h → ½ O2 + 2H (HER) (25)

+ - Water reduction: 2H + 2e → H2 (ORE) (26)

Overall reaction: H2O → ½ O2 + H2 ΔG = + 237.2 kJ/mol (27)

The minimum energy per electron is 1.23 eV, which should be provided by the photocatalyst to decompose the water. Bandgap (Eg ) is the main parameter that defines the light-harvesting ability of an absorber, as photons alone with energies more than the bandgap are able to excite the electrons in the valence band to the conduction band. The excess energy or the difference in the energy of the absorbed photon and the band gap energy (퐸 − 퐸푔), is lost as phonon. The bandgap value is required to reach the minimum energy needed to drive the reaction, but absorption coefficient of the semiconducting materials is another parameter to show how efficiently a photocatalyst can harness the solar spectrum in addition to the band gap. One of the crucial points that need to be taken into consideration is the intrinsic loss (Eloss) associated with the solar energy conversion process to quantify the optimal minimum band gap value. These losses have connection with the fundamental loss caused by thermodynamics because of non-ideality (kinetic losses) in the conversion process [53, 54]. The former loss is the result of the second law of thermodynamics.

In fact, the following equation shows the bandgap energy (Eg) corresponding to the change in internal energy, which is related to the Gibbs energy change:

ΔG = ΔU + PΔV – TΔS (28) where U, P, V, T and S indicate the internal energy, pressure, volume, temperature and entropy, correspondingly. When the semiconductor absorbs photon, more and more excited states can be created in addition to ground states, increasing the entropy of the ensemble. ΔSmix represents the entropy change because of the mixing of the excited state with the ground state. A volume change (ΔVmix) is also caused by the mixture of excited and ground states. However, this is not true for the ideal chemical system (ΔVmix = 0). Thus, the band gap energy should be greater than the available work under ideal conditions (Gibbs energy change per electron), at least Eloss = TΔSmix with a minimum of 0.3-0.5eV. In reality, Eloss reaches higher values (roughly 0.8 eV) as a result of kinetic losses and due to non-ideality (overpotential at the anode and cathode, reduction in resistance at the electrolyte, recombination electron- hole pairs recombination). Therefore, in order to maximize chemical conversion efficiency, materials commonly used as photoelectrodes in Photoelectrochemical cells (PEC) require a band gap of 2.0 to 2.25 eV [6, 53, 54]. The photocatalytic working principle is very simple, as shown figure 12, at pH=1 with ideal semiconductor materials under illumination. CB, VB and Eg are + conduction band, valence band and band gap of semiconductor. (H /H2) and (O2/H2O) are redox couples of water.

Figure 12 – The HER and OER for overall water splitting

The simplest way saying, when UV and / or visible light of sunlight shines to semiconductor photocatalyst, the semiconductor absorbs photon and creates

31 electron-hole pairs. In detail, the excited electron jumps from valence band to conduction band. This is the so-called "photo-excited" semiconductor phase. Bandgap is the difference between the maximum valence band and the minimum conduction band. Ideally, semiconductors not only have a bandwidth greater than 1.23 V, but also have more negative conduction band relative to the water reduction potential,and more positive valence bands boundaries relative to water oxidation potential. Another thermodynamic precondition is the position of the band edge. Actually, for the oxidation reaction to be driven, the free movement of holes from the photoelectrode to the interface between the semiconductor and the solution is a prerequisite. The top edge of the valence band has to be more positive than the oxidation potential of O2 / H2O as seen in figure 12. Likewise, the reduction reaction happens as the bottom edge of the conduction band is more negative than the + reduction potential of H / H2. Figure 13 shows the band structure and bandgap values of some semiconductors including narrow and wide bandgap materials. For our research, SrTiO3 [58– 62] has been selected to study due to its high chemical stability in aqueous solution, photocorrosion resistance, and relatively high conduction band energy position, which is beneficial for hydrogen production. Although this metal oxide (Eg > 3.2 eV) is able to harvest solely a tiny portion of the solar spectrum (less than 4%), its band gap can be easily engineered to visible light range via both metal and not metal doping.

Figure 13 – Band gap values (in eV) and band edge positions for selected semiconductors [53]

Although, narrow bandgap materials (e.g. Fe2O3) are able to absorb visible light, their bandgap energy positions are not aligned well to split water or drive the

32 water reduction and oxidation reactions. Therefore, in most of the cases, they are used to construct to tandem cell structures to use for water splitting reaction. Generally, various types of overall water splitting techniques that have been developed include: particulate system [63], Z-scheme [64, 65], and photoelectrochemical cell [66]. In the photoelectrochemical cell for overall water splitting, the particulate photocatalysts are assembled as a photoanode and a photocathode electrode film. However, in the photoelectrochemical cell for a water splitting system, one of the difficulties is to fabricate a stable photoelectrode under the exposure of strong solar irradiation. As usual, the photoelectrode film is fabricated by the doctor blading technique. This method, however, produces a photoelectrode film that is not stable for long- term use photoelectrochemical cells under strong solar irradiation. The wide band gap photocatalysts primarily can be the semiconductors with d0 and d10 electronic configurations. The biggest family of wide band gap photocatalysts is Metal Oxides with d0 Electronic Configuration. The further possible categories are Group 4 metal oxides, Group 5 metal oxides and other metal oxides with d0 electronic configuration. After the discovery of the Honda-Fujishima effect in 1972 TiO2 (Group 4 metal oxides) has become the most remarkable photocatalyst. Under UV irradiation water is splitted by a photoelectrochemical cell usage with aid of certain 4+- external bias. Numerous other oxide photocatalysts containing Ti such as SrTiO3, Sr3Ti2O7, Sr4Ti3O10, La2Ti2O7, Ba-doped La2Ti2O7, KLaZr0.3Ti0.7O4 and La4CaTi5O17 were explored extensively after the TiO2 success. They all have one common feature, which is the layered perovskite structures and wide band gaps [6, 53]. To design efficient solar hydrogen production photoelectrochemical cell (PEC), PEC must meet all of the following requirements: 1. low cost 1) Material and the earth's abundance 2) Device manufacturing 3) System 2. High efficiency 1) Light absorption efficiency 2) Conversion efficiency 3) Low over potential for reduction/oxidation of water (catalysts) 3. Stability 1) Chemical stability in aqueous environment 2) Electrical stability

Until now, large band gap semiconductors have been intensively studied for photocatalytic water splitting due to their robustness and suitable bandgap energy positions. However, these materials are suffered from harnessing less portion of the solar spectrum. The following formula helps us to calculate the theoretical maximum photocurrent Jmax which is proportional to solar-hydrogen conversion efficiency (STH):

Jmax = q ∫ Фλ [1 – exp(-αλd)] dλ (29)

In the above formula, λ , q, d and αλ represent wavelength, electron charge, sample thickness, and absorption coefficient under the photon flux of the AM1.5G solar spectrum. Taking into account of conversion losses, reflection losses and other losses, obtaining 10% of solar-to-hydrogen (STH) conversion efficiency is very challenging. Four main efficiency measures for PEC cells are solar-to-hydrogen (STH) conversion efficiency, applied bias photon-current efficiency (ABPE), external quantum efficiency or incident photon current efficiency (IPCE), and internal quantum efficiency or absorbed photons to current efficiency (APCE). STH efficiency is commonly used to evaluate PEC device performance and is expressed in the follwung way:

[(H2 production rate) × (Gibbs free energy per H2)] STH = (30) [Incident energy]

Equation (31) can be applied to calculate ABPE:

퐽 ×(1.23−푉 ) ABPE = 푝ℎ 푏 (31) 푃푡표푡푎푙

In this formula, Jph is a photocurrent density as an bias Vb is applied. Ptotal indicates the total incident solar light power. IPCE defines the photocurrent generation per incident photon flux under an certain irradiation wavelength. A STH efficiency can be evaluated via applying the IPCE data over the total solar spectrum in the two electrode system. However, applying IPCE data obtained in three-electrode system under a bias to estimate STH is not considered to be a valid method. Even if it is, it is still considered to be a useful approach to find out the PEC materials properties. The IPCE is expressed as by equation (32):

퐽 ×ℎ푐 IPCE = 푝ℎ (32) 푃푚표푛표×휆

Where Jph is the photocurrent density, h is Planck's constant, c is the light speed, Pmono is the power of calibration and monochromatic illumination, and λ is the wavelength of monochromatic light.

1.2 Nanostructured materials, 1D nanostructured materials and methods

One of the most important tasks of the XXI century is to develop nanostructured materials in various fields. Nanotechnology is an advanced technique which apply to manipulate the substance at the level of an atomic, molecular, and supramolecular.

The most acceptable definition for nanotechnology is manipulation of atoms and molecules precisely for a certain technological goal, which is called as molecular nanotechnology. Nanotechnology plays an important functions in most of science fields. These fields cover surface science, organic chemistry, molecular biology, semiconductor physics, energy storage, microfabrication, and molecular engineering. Its applications are broad to extend from conventional device physics to molecular self-assembly [2], and also from synthesis of nanomaterial’s to direct atomic or molecular level manipulations. Nanostructured material (NSM) is a material with one of the dimension less than 100 nm [67]. On the other hand, nanotechnology is the development of near- atom or nanoscale materials, components, devices and / or systems. This technology mainly involves fabricating, measuring, modeling, imaging, and manipulating matter at the nanoscale. Nanoscience is considered to be highly multidisciplinary fields, including physics, chemistry, biology, engineering, and some other disciplines. For more than two decades, significant progress has been made in designing, analyzing, and fabricating nanoscale materials and devices, and this trend will continue for a few more decades in various fundamental studies and in research and development fields [67, 68]. The field of nanoscience includes the development or research of sub-fields of materials with unique properties and nanoscale dimensions [6, 67, 68].

Figure 14 – Classification of Nanomaterials by pictorial representation and expected properties ( density of states, DOS) (a) 0D spheres and clusters, (b) 1D nanofibers, wires, and rods, (c) 2D films, plates, and networks, (d) 3D nanomaterials

NSM, the subject of nanotechnology, is classified as low-dimensional materials, including building blocks with sub-micron or nano-scale dimensions in at least one direction, and exhibits size effects. Gleiter introduced the first classification 35 idea of NSMs in 1995 [69] and Skorokhod explained it in 2000 [70]. However, in the Gleiter and Skorokhod proposed scheme, due to no inclusion of zero dimension (0D), one dimension (1D), two dimensions (2D), and three dimensions (3D) structures such as fullerenes, nanotubes, and nanoflowers, this proposed classification was not adopted. Later, in the Pokropivny and Skorokhod [71] classification scheme in Figure 14, NSM classification includes 0D, 1D, 2D and 3D nanostructures. 0D: all the dimensions of the materials are at the nanoscale (no dimensions, or 0-D, are larger than 100 nm). The common representation is nanoparticle, which are spheres or clusters and it includes oxides, metal, semiconductor, fullerene moecules arrays (or quantum dot arrays), core-shell quantum dots, hollow spheres and nano lenses etc. All of the dimensions of 0D structure or quantum dot are reduced to the nanoscale (i.e. three-dimensional quantization). While electrons and holes are confined in all three directions, the nanomaterials have high energy gap (Eg means the energy needed by an electron to leave the atom). This quantum confinement of electrons in the three-dimensional quantum dots gives these dots a good property in dealing with light effectively and making it an ideal candidate to form devices such as solar cells [72 - 75]. Therefore the electrons are fully confined in 3D space or No electron delocalization (freedom to move) occurs. The energy formula for this case is expressed as the below:

2 2 휋 ℏ 2 2 2 En = [ ] (푛 +푛 +푛 ) (33) 2푚퐿2 푥 푦 푧

In equation (33), ℏ is equal to h/2π, where h is Planck’s constant. L is equal to the width of deep potential well and nx, ny and nz are principal quantum numbers in the three dimensions x, y, and z. m represents the mass of the electron. Since L is inversely proportional to En, the separation of energy levels is getting bigger with decrease of the dimensions of the nanostructures (figure 14 for 0, 1, 2, 3 Ds). 1D: an object with at least one dimension in the range 100 nm, it covers nanofibers, nanowires, nanorods and nanotubes. The quantum wire is also belong to 1D structure. Here the electrons are confined in two dimensions which simply means that two directions are reduced to nanometer range and one dimension remains. For 1D, electron confinement and delocalization coexist. Electron confinement occurs in 2-D, whereas delocalization takes place along the long axis of the nanowire / rod / tube. The energy

2 2 휋 ℏ 2 2 En = [ ] (푛 +푛 ) (34) 2푚퐿2 푥 푦

2D: 2D nanomaterials usually have plate-like shapes including monolayer, multilayer, self-assembled, and plates. Quantum well is the 2D structure where one of the three dimensions is in the range of nanoscale (1 – 100 nm) while the other two dimensions remain on their actual size (i.e. electrons are confined in one dimension). Nanowires, nanorods, nanotubes, and thin films are all classified as (2D) structures.

For 2D, electron confinement and delocalization coexist as 1D. In the case of 2D, the conduction electrons will be confined across the thickness but delocalized in the plane of the sheet. And energy is formulated as below:

2 2 휋 ℏ 2 En = [ ] (푛 ) (35) 2푚퐿2 푥

3D: it is considered as a bulk nanomaterial that is not confined to the nanoscale in any dimension. All three dimensions of the nanomaterials are greater than 100 nm and they include nanocomposite nanohybrids, micro and mesoporous hybrid, nanometer-sized grains. The material has a nanocrystalline structure or the presence of nano-scale features. The bulk is 3D structure which means that electrons in the conduction band and holes in the valence band are moving freely in all space's dimensions. For 3D nanomaterials the electrons are fully delocalized 1D nanostructured materials In the last ten years, 1D NSMs have aroused an increasing interest owing to their unique properties in research and developments, and demonstrated potential applications from nanoelectronics to nanophotonics. 1D NSMs are commonly agreed upon as the perfect systems to be explored to find a large number of novel phenomena at the nano-scale. They are believed to play a crucial role in fabricating electronic, and optoelectronic with nanoscale dimensions. A research work done by Iijima [76] played a significant role and drew the public attention to the 1D NSMs field. The 1D NSMs have a weighty impact on nanoelectronics, nanodevices, nanocomposite materials, alternative energy, and national security [77]. Figure – 15 indicates the fast growth of publication quantities in electrospinning. According to the figure, one estimates the number of groups actively engaged in research and development of nanowire materials and devices [78]. As seen figure recent decade there are rapid growth of nanofiber studies by electrospinning methods. Being a 1D-nanostructured materials, nanofibers research interest increases year by year (the data 2017). There are two types of approaches for synthesis of nano materials and fabrication of nano structures. One is a Top-Down and the other is a Bottom-Up approaches. Top-Down approach refers to reducing or minimizing a bulk material to get nano sized particles. To produce two dimensional nanostructures on the substrate, suitable precursor gasses are applied to precisely deposit material simultaneously with assistance of focused ion beams. This method can be used to create sub-100nm nanostructures. To create nanostructures via top down approach, atomic force microscope tip is used as a nanoscale "write head" to transfer molecules from tip to create a resist, then an etching process is applied to eliminate to create nanostrutures in combination with a top-down method. Bottom-up refers to create nanostructures or devices via a self-assembly. Bottom-up is usually used to produce nanoscale devices in combination with top-down method. Nature has the ability to create the nanostructures via long time evolution and self-assembly processes [80, 81]. Based on the concepts of supramolecular chemistry, and molecular recognition, molecules are self-assembled to create useful conformation or nanostructures. A Dip 37 pen nanolithography is developed to create desired nanoscale patterns using atomic force microscope tips that transfer a chemical molecules from the AFM tip onto the surface with help of nanolithography program [82]. The method fits into the subfield of nanolithography.

Figure 15 – An annual increase in the number of publications related to the 1-D nanostructure research by electrospinning methods [79]

1.2.1 1D nanostructured materials for Dye-sensitized Solar Cell Electron transport is one of the major steps in the DSSC. One-dimensional (1- D) structures including nanowires and nanofibers transfer electron faster than nanoparticles because of their unique electron properties. They have potential applications in electronics, photonics, and other related fields [82 – 84]. The factor which lowers electron transport in nanoparticle based porous photo-anode electrodes is called as “trap-limited diffusion”. The axial length of 1D nanomaterials is usually in the scale of hundreds of nanometers to tens of micrometers. They can scatter light effectively like the larger nanoparticles mentioned above [85, 86]. The PV device is usually a semiconductor diode, where the incident light is absorbed by the semiconductor with a bandgap smaller than the photon energy (Eg < hv). Once the absorbed photons generate pairs of electron and hole, these carriers will be separated by the internal electric field in the p–n junction. There are mainly two kinds of photovoltaic cells: DSSC [87] and quantum dot-sensitized solar cells (QDSSCs) [88]. The major difference between these two cells lies in the different materials used for light absorption. 38

Photosensitive dyes are utilized in DSSCs and small bandgap semiconductor quantum dots are used in QDSSCs [89 - 91]. In both cases, due to their unique light scattering effect and electron transport highway, 1D materials exhibit their potential as an active material. Therefore, the 1D nanomaterial based DSSCs is discussed in detail. During 1988-1991, Gratzel's team first reported DSSCs which is much cheaper than silicon based solar cells [2; 92]. The use of one-dimensional nanomaterials is beneficial in terms of the low-cost structure, which makes such practical use possible and the potential for increased efficiency. 1D material may scatter more light to improve the light trapping efficiency. Fortunately, 1D TiO2 nanorods or nanotubes provide the pathway for photoelectrons go directly to the anode, leading to more efficient separation of electrons and holes [93][94]. In addition, the overall resistance of the cell decreases due to the lower resistivity of the "electron (e-) expressway"; therefore, the corresponding fill factor of the cell will also increase [95]. Moreover, to improve efficiency, the electron collection time can be further reduced by introducing another semiconductor housing with well-aligned energy bands. The conduction band potential of the shell should be lower than the potential of the core, which facilitates the injection of electrons into the core and helps reduce electron recombination by the introduced potential gradient. For example, an energy barrier of 75-150 meV exists between the ZnO core and the TiO2 shell, which helps to suppress the electron / hole recombination at the surface [96]. In such a core-shell structured photoanode, the decorative shell's physical dimensions or distribution should be well designed to achieve high performance cell [97]. It is also noted that special attention should be given to the control of the crystalline phase in the preparation of these 1D nanomaterials to improve the electron collection efficiency. However, the anode completely based on 1D materials cannot yield enhanced efficiency because of the relatively lower dye load which is a result of smaller surface area than nanoparticles, except for the double-surfaced nanotubes, which can have comparable surface areas with nanoparticles [98, 99]. Another recent development is the use of high electron mobility semiconductors NWs, anodes, such as zinc oxide [100], stannic oxide [99] and gallium nitride [101]. Stannic oxide and gallium nitride have high electron mobility and stability in electrolyte solutions; therefore, they are expected to be used in anode assemblies in DSSCs. Although the DSSC's efficiency is still low compared to traditional photovoltaic cells, all the recent developments in DSSC, especially the use of 1D nanomaterials at the anode, have demonstrated the potential to produce a future low cost and efficient energy harvesting device [102].

1.2.2 1D nanostructured materials for Photocatalytic water splitting In addition to photovoltaic cells, photoelectrochemical cells (PEC) are another type of solar energy harvesting system [87]. One of the major problems with using solar cells to generate energy is the dependence on geography and run time. Thus, the generated energy is stored as electricity in the battery, but the storage cannot last for a long time with moderate

39 efficiency. In contrast, hydrogen is considered as one of the best choices for storing electrical energy in the form of a chemical bond with higher efficiency and longer storage times, which is also a substitute for liquid fuels and has a relatively clean combustion [103]. Using photocatalyts to split water is one of the method to produce hydrogen. Again, with 1D nanostructured materials, the recombination losses of electrons are highly suppressed and photon absorption is greatly enhanced due to internal scattering between nanostructures. The use of TiO2 and ZnO semiconductor materials as anode in the solar hydrogen production is well summarized elsewhere [104, 105]. It is noticeable that even though a high efficiency of 16.25% (efficiency at certain wavelength range) was gained in the ultraviolet (UV) range of 320–400 2 nm by using 45-μm TiO2 nanotubes under 100 mW/cm illumination [106], the overall solar-to-hydrogen conversion is relatively low considering the small fraction (~5%) of the UV light energy in the solar energy. It is worth noting that, similar to DSSCs, 1D nanomaterials in the anode can also be integrated with other materials to increase efficiency [107, 108]. For example, 3.7% nitrogen-doped ZnO nanorods exhibit increased IPCE compared to pure ZnO nanorods [109], but have an effective absorption range below 400 nm in the wavelength range. Low IPCE at a wavelength of > 400 nm results in a low conversion efficiency of about 0.15%. 1D nanomaterials with the smaller bandgap such as ferrous oxide (1.9–2.1 eV; [109], CuO/Cu2O (1.2– 1.9 eV; [110], tungsten nitride (2.2 eV; [111]) as well as other III–V and II–VI compound semiconductors are attractive for the anode materials here [112, 113]. We focused on 1D nanomaterials for both solar conversion study which are DSSC and Water Splitting in upper parts. As a summary, recently more and more researches have been focused on one-dimensional (1D) nanostructures including nanorods, nanowires, and nanotubes for DSSC/PEC photoanodes since their horizontal dimensions are not to reduce the distance for holes to reach the surface of the photoanodes, but their longitudinal axis also increase optical lengths and electron transport [114]. There is still a huge urge to explore more 1D nanomaterial design and fabrication to further increase the solar hydrogen production efficiency. Nanomaterials are the major building blocks of solar energy conversion devices. Among the nanostructured solar energy conversion systems and devices, binary and ternary metal oxides are the most widely used and have a promising future in this field. The interaction between light and materials is critically dependent on the frequency (and hence, energy) of the light. For all the applications involving light-matter interactions, it is thus important to identify the energy distribution of light as a function of light wave frequency. This frequency range is known as the spectrum of light. Sunlight has a complex spectrum extending from the ultraviolet range into the infrared range, originating from the generation and propagation processes of sunlight. Therefore, figure 16 shows our motivation and research direction of our goal. Some binary and ternary metal oxides are photoactive and are used for photocatalytic activities in solar cells, water splitting and other solar conversion reaction. Synthetic methods for binary and ternary metal oxide photo-catalysts emphasize green reaction processes. The bandgaps of metal oxides with d0 metal

40 ions are constructed from 2p orbital of oxygen and nd orbital of a metal cation. Its conduction band energy position is usually more negative than the zero potential of hydrogen ions. The bandgap of metal oxides is belong to wide band gap semiconductor which absorbs ultraviolet spectrum. So far, titanium dioxide is usually synthesized via sol-gel methods, but strontium titanate is not easy to synthesize due to stoichiometric ratio balance of the three elements. Even though binary metal oxides with d0, d10, and f0 metal ions show efficient photocatalytic activity, their ternary oxides have been widely studied and proven to have the same photocatalytic effects.

Figure 16 – Solar energy conversion to electricity and hydrogen gas

Synthesis of ternary oxide or higher metal oxides NWs including pervoskite NWs is more challenging over binary metal oxides since it requires a preservation of stoichiometric balance between all compositions and certain reaction conditions such as high temperatures, pressures and special precursors. Urban et al. [115] synthesized single-crystalline perovskite ternary oxide (BaTiO3 and SrTiO3) NWs using metal-alkoxide precursors in the presence of surfactants, oleic acid. This synthesis approach was developed based on the synthesis technique for monodisperse perovskite NCs developed by O’Brien et al. [116].

1.2.3 Sol-gel and Electrospinning methods How we could get these 1D structured materials for the solar cell and photocatalytic water splitting? Here, we introduce an Electrospinning and sol-gel methods which were developed to produce 1D nanomaterials for our research purposes. This approach provides an eco-friendly path to fabricate pure 1D NSMs. The term "sol gel" first appeared in the late 19th century. It usually refers to the low temperature method using inorganic precursors, which can produce ceramics and glass with better purity and uniformity over conventional methods [117].

A sol-gel technique is a chemical synthesis approach which has been widely utilized for the fabrication of metal and metal oxide nanomaterials. In this synthesis, hydrolysis and gelation are major steps for the formation of an oxide network and finally removal of the solvent is realized via heating or calcination at high temperatures. The sol - gel process (figure–17) includes steps of hydrolysis, gelation, precipitation, and hydrothermal treatment [119]. The controlled hydrolysis of metal-organic compounds (alkoxides) in an organic solvent is a critical step in sol-gel process [118].

Figure 17 – Illustration of different stages and routes of the sol-gel process [119]

Experimental parameters which affect size distribution and stability of metal and metal oxide are dopant introduction, heat treatment and properly selection of surfactants, and polymer matrix architecture. In addition, the basic chemistry of the sol–gel process including different reactivity’s of the network forming and its modifying components is important [117, 118]. To synthesize 1D nanomaterials, electrospinning approach is convenient technique to produce them using sol-gel incorporated polymer fibers [120 -- 123]. It is an most common technique to produce nanofibers from various polymers or inorganic/organic hybrid nano-composites by applying electrical field [82, 124 - 126]. At the end of the 16th century, William Gilbert started to explain the behavior of magnetic and electrostatic phenomena in the electrospinning process. He examined that when a suitably electrically voltage was applied to a droplet of water a cone shape of small droplets was ejected from the tip of the cone. It is the first experimental observation of electrospraying. Several research groups in the early 1990s (notably that of Reneker and Rutledge who popularized the name electrospinning for the process) demonstrated 42 nanofibers using electrospinning polymers. From then on, the number of publications has been increased each year [127]. Working principle of Electrospinning is to apply electric field to polymer solutions to generate fiber diameters at the nanoscale. As a polymeric liquid droplet is subject to high voltage, the liquid gets charged and stretched as the surface tension is overcome by electrostatic repulsion. As the critical point of the two force balance is reached, a liquid stream erupts from the needle of syringe which is called as the Taylor cone. The stream cannot be produced as the cohesion energy among the polymeric solution is strong. Figure – 18 diagram explains the fiber formation.

Figure 18 – Diagram showing fiber formation by electrospinning [128]

The polymeric jet solution is further elongated to deposit on the counter electrode [129]. The elongation resulting from this bending instability leads to nanoscale fibers [130]. The solution properties is important in electrospinning process (table 2). Small fibers are usually generated by electrospinning at a high voltage, and at a low liquid flow rate as the distance between collector and the tip is fixed [131]. An electrospinning set up includes a spinneret (a syringe needle mounted on the syringe), a direct current power supply, a syringe pump, and a grounded collector. In this fabrication process, first, a polymer solution is loaded into the syringe. This liquid is pumped from the needle tip at a constant speed by a syringe pump under certain bias applied by power supply [132]. To produce the stable fibers, the parameters including the distance between the two electrodes, applied bias, flow rate of the solution from the pump and viscosity of the solution are critical to apply. In our research, we used a combined approach of electrospinning with sol-gel. It will be introduced in next section.

Table 2 – The list of process conditions and solution parameters affects to electrospun nanofibers.

Parameters Effect to diameter of the fibers Process conditions: (V > Vc) Applied voltage* ↓ (H) Distance to collector* ↓ (Q) Flow rate* ↑ (I) Current ↓ Solution parameters: (η) Viscosity* ↑ (ᵏ) Conductivity* ↓? (ᵋ ) Dielectric constant of liquid ↑ (ᵧ) Surface tension ↑ * most important variables / ↑ diameter of the fibers increases / ↓ diameter of the fibers decreases / ? not always

Conclusions for section 1 To summarize this section: Nanomaterials are the important components for creating solar energy conversion devices and they have been explored for this purpose in the following three ways. (1) They can be assembled as donors-acceptors to mimic photosynthesis. (2) They can be utilized to produce using semiconductor-assisted photocatalysis (3) They are important nanostructured semiconductor materials in solar cells. Among the nanostructured solar energy conversion systems and devices, binary and ternary metal oxides have been extensively studied and have a promising future in this field.

2 EXPERIMENTAL, RESULTS AND DISCUSSION FOR DYE SENSITIZED SOLAR CELL

This section covers experimental synthesis of nanomaterials, and their characterizations. Moreover, the details of DSSC fabrication and its IV curve testing are also discussed. In the DSSC, the study of the transport properties of electrons in the two different photoelectrodes is our focus. In this research, single-wall carbon nanotubes (SWCNT) and multi-wall carbon nanotubes (MWCNT) were applied to fabricate 1D core-shell structured TiO2 nanocomposite materials. Titania is shell for both core-shell nanostructures. Core materials were MWCNT (TiO2@MWCNTs) and SWCNT (TiO2@SWCNTS) respectively. A combination of electrospinning and sol-gel approach was applied to fabricate the above two different core-shell nanostructures. The core-shell materials were further utilized to study fundamental physics of electron transport in photo-anodes of DSSC for the first time. Characterization techniques applied for the above synthesized nanostructures include field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), and X-ray photon spectroscopy (XPS). Results of I-V curve (by solar cell performance measurement) and bode plot (by electrochemical impedance spectroscopy) were measured to illustrate electron transfer properties of DSSC.

1.2 Experimental section for Dye-Sensitized Solar Cell

Materials for fabrications of nanofibers The following chemicals were purchased from Sigma Aldrich: polyvinylpyrrolidone (PVP, average M.W. 1 300 000), ethanol (99.5%), titanium (IV) isopropoxide and multi-walled carbon nanotube (MWCNT, 96% Metallic). Single-walled carbon nanotube (SWCNT, 95% Pure Semiconducting) was purchased from Nano Integris. FTO glass and surlyn 1702 were purchased from Solaronix. Two glass syringes and coaxial needle (20 – gauge) were obtained from kd Scientific. Water was purified through by distillation and filtration (18.2 MΩ). These above materials are used to synthesize 1D nanofibers. In these core-shell nanostructures of TiO2 with carbon nanotubes, two different types of carbon nanotubes were selected to study their charge transfer mechanism in the DSSCs. Therefore, it is essential to give a brief introduction on carbon nanotubes. Carbon nanotubes (CNTs)’s invention in 1991 has led to new research opportunity for materials scientist to develop functional composite materials and revolutionary era in material science and engineering history. They possesses fascinating properties including electronic, magnetic and mechanical. Furthermore, its weight is only one-sixth of the steel, but it is mechanically as 100 times strong as steel. Therefore it is important component to reinforce almost any material. Its electric and thermal conductivity are better than copper [133]. The aspect ratio of its diameter to length diameter is large. Usually the diameter of most single wall CNT

45 is close to 1 nm. A SWNT is a cylindrical tube formed from one-atom-thick layer of graphite called graphene. The wrap or formation of the graphene sheet as CNT can be characterized by a pair of indices (n,m). The n and m represent the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If the value of m is equal to zero, the nanotube is called armchair nanotubes. If m takes integers rather than zero, it is called chiral. The tube diameter can also be estimated from the value of n and m as shown in the following formula (Equation 36).

훼 d = √(푛2 + 푛푚 + 푚2 = 78.3 √((푛 + 푚)2 − 푛푚)pm, (36) 휋 where a = 0.246 nm. SWCNT properties are to some degree determined by the (n,m) values so that there exist various types of SWNTs. Furthermore, its band gap also varies from zero to 2 eV and its electrical conductivity also can be varied between metallic or semiconducting regions. Multi-walled nanotubes (MWNTs) are another type of CNTS made up of multiple layers of graphene. The MWCNTs structures are divided into two models. The Russian Doll model indicates concentric cylinders formed from layers of graphite, which is most common structure in MWCNT. For example, a (0, 8) SWNT is layered within a larger (0, 17) SWCNT. The Parchment model indicates a single piece of graphite rolled by itself. The interlayer distance of multi-layered nanotubes is around 3.4 Å which is similar to that of graphene layers in graphite. Each layer can be either metallic or semiconducting SWNTs. A Whole MWCNT is normally a zero-gap metal [134]. Figure 19 shows three different carbon nanotubes which are Multiwall, single wall and double wall carbon nanotubes. There are two of them used for our research purpose, which are MWCNT and SWCNT.

Figure 19 – MWCNT, DWCNT and SWCNT [135]

The reason why a carbon nanotube selected as an ideal material for electron mobility enhancement in DSSC is that it has both metallic and semi-conductive features [136-139].

Titanium dioxide is a naturally existed oxide form of titanium, chemical formula of TiO2 [140]. Titanium dioxide has eight polymorphisms that are rutile, anatase, brookite and other six forms exist. We selected anatase titania (tetragonal crystal structure) for our study as seen in figure 6 in chapter 1 since charge recombination in anatase form is ten times slower than that in rutile form and its grain sizes are relatively smaller in anatase phase [141]. Titania related properties are listed in table 3.

Tabe 3 – The properties of TiO2

Properties formulas of Chemicals TiO2 Molar mass 79.866 g/mol Appearance White solid Odor odorless

Density 3.78 g/cm3 (Anatase) Melting point 1843 °C (3,349 °F; 2116 K) Boiling point 2972 °C (5,382 °F; 3245 K) Solubility in water insoluble Band gap 3.26 eV (Anatase, 380 nm) Magnetic susceptibility (χ) +5.9·10−6 cm3/mol

Refractive index (nD) 2.488(Anatase)

2.1.1 Fabrications of titania based nanofibers Firstly, two kinds of core and shell solutions were prepared separately by sol- gel method. Then the two solutions were loaded to two different syringes connected through coaxial needles. After immobilization of the two syringes onto the syringe pump, certain voltage was further applied between two electrodes of tips and collectors to produce the fibers at optimum experimental conditions. Briefly, 0.01g of MWCNT was added to 10 ml of 5.7% w/w PVP in ethanol solution [142] and the mixed solution was further sonicated under ultrasonication ( the Q500 Sonicator and bath sonicator) to disperse the MWCNT in the PVP solution. It is labeled as a core solution. The experimental procedure was shown in in figure 20 (a, b). Here sonication and crushing are so important, otherwise it will not be fully dispersed in solution. Before ultrasonication and mixing, another important step is that carbon nanotubes should be crushed by mortar which lead to good smash particles. It is also essential to obtain core-shell structures in electrospinning case.

The ultra-sonication leads to the homogeneous solution of CNT as seen in figure 21(c). It is difficult to obtain homogenously dispersed solution without applying high power ultrasonication (figure 21(d)). It influences the fiber quality with the bad decorations.

a b

Figure 20 (a, b) – left, Q500 Sonicator and right, bath sonicator sonicated to disperse the MWCNT in the PVP solution

c d

Figure 21 – sonication of the CNT solution in (c) fully homogeneous and (d) nonhomogeneous

In the preparation of shell solution, 0.42g of TIP was added to a vial containing 10.6 ml of 5.7% w/w PVP and 0.202g of acetic acid. Likewise, same procedures were applied to make core and shell solutions of SWCNT. Table 4 and table 5 provide the recipe (actual used amounts of all components) for core and shell solution of SWCNT and MWCNT.

Table 4 – MWCNT solution mixture for core solution Core solution mixture Shell solution mixture MWCNT 0.00885 g AA 0.2 g TIP 0.4 g ethanol 5 ml ethanol 5 ml PVP 0.175 g PVP 0.175 g 48

Table 5 – SWCNT solution mixture for core solution

Core solution mixture Shell solution mixture SWCNT 0.00885 g AA 0.2 g TIP 0.4 g ethanol 5 ml ethanol 5 ml PVP 0.175 g PVP 0.175 g

We prepared all solutions according to the above calculated ratio. Electrospinning After preparation of the both core and shell solutions, electrospinning is applied to generate nanocomposite fiber. Figure 22 shows the electrospinning equipment setup for producing core-shell nanofibers.

Figure 22 – (a) Whole Electrospinning process view include high voltage power supply, syringe pump, syringes with needle, collector. (b) Double syringe settle in syringe pump. The lower is real setup and SW/MWCNT materials

Double syringe unique method was set up as shown in Figure 22 (b) to produce the core-shell nanocomposite electrospun fibers. The core line of the needle is connected to CNT solution and shell line contains TiO2 precursor’s solution. The core needle size of syringe is 20 gouge which is equal to 0.91 mm (figure 23). The coaxial needle was connected to the two syringes which contain core and shell solutions respectively.

Figure 23 – Coaxial needle and its top down view

In most of the experiments, the distance between the needle and the collector was fixed to 15cm and applied voltage between the two electrodes was 20KV. Aluminum foil was used as a collector for productions of electrospun fibers. The flow rate of pump for the core and shell solution was 1.5ml/h for the whole electrospinning process. Each two or one and half hour we collected electrospun fiber from the aluminum foil and replaced it by new one. After collecting sufficient electrospun nanocomposite fibers (figure 24, left), we carried out calcination subsequently. Calcination Calcination process was allowed to crystalize inorganic nanocrystal to grow as assembled structure and simultaneously decompose the nanocomposite fibers via burning polymer components. In this process, the electro spun nanocomposite fibers was placed in the furnace until the temperature reached 450ºC and then, it was kept there for additional 2 hours (figure 24, right). Its purpose is to remove the polymer and get titania nanoparticles. When those experimental processes were finished we were going to check the integrity of the fibers. The resulting core-shell nanofibers were further characterized by scanning electron microscope, X-ray diffraction and X-ray photon spectroscopy techniques. After confirming the products with the above techniques, next step we used these nanomaterials to make DSSC device for performance test. The produced 50 core-shell fibers from calcinations were two types. One is TiO2@MWCNTs and the other is TiO2@SWCNTS.

Figure 24 – (left) taking out the fiber tissue & (right) calcination steps in furnace with gas

2.1.2 Assembly of device The chemicals obtained from Sigma Aldrich were: Tert-butylpyridine, guanidinium thiocyanate, 1-butyl-3-methyl imidazolium iodide (BMII), acetic acid, ethanol, N-719 dye (Ruthenium-based dyes (C58H86N8O8RuS2)), Terpineol, Tetrabutylammonium hydroxide, and Iodine (I2, 99.999% purity). List of chemicals purchased from Fluka includes: Acetonitrile, Nitric Acid, valeronitrile, H2PtCl6, TiCl4, Ethyl Cellulose (#46070, 5-15 mPa·s and #46080, 30-50 mPa·s). Fluorine doped tin oxide (SnO2: F, FTO) glass (4 and 2.2 mm thick, 10 and 15Ω resistance) and surlyn 1702 (25 µm thick, it is a sealing spacer) were purchased from Solaronix. For making device, calculated amounts of chemicals were shown in table 6 (page 54) following literature [143]. Paste preparation procedure A Paste was used to prepare porous titania photoanodes for DSSC, which was prepared by following the procedures in the literature [143, 144, 145]. Briefly, First, 10 wt% ethanol solution of two celluloses (ethyl cellulose (EC) powders, EC (5–15 mPas) and EC (30–50 mPas)), were prepared. Then calculated amount of the above cellulose solutions (4.5g of 10 wt% EC (5-15) and 3.5g of 10 wt% EC (30-50) ) were transferred to a rotavap flask containing 1.6 g of MWCNT-TiO2 nano-composite fibers and 6.49 g of terpineol (anhydrous). The mixture solution was further added with 8 ml of ethanol to keep total volume of 28 ml. A uniform solution of the above mixture was obtained via sonication and hot stirring. A rotary evaporator was used to remove the solvents under the condition of 40 °C and 120 mbar [143]. As seen figure 25(b) the water bath kept constant temperature via heating and the flask rotated also with the plastic tube circulate the cooling water, the evaporated solvents from vacuum were dipped into stationary flask in the ice bath. Final product in the round flask should be a viscous paste, which were ready for preparation of porous titania photoelectrode (figure 25 in page 53). Similar process was repeated to make SWCNT-TiO2 nano-composite paste. We added 1.6 g SWCNT instead of MWCNT, and other sings were exact same.

Preparation of the dye (N-719) solution To prepare 0.5mM N-719 dye (50 ml), 0.0297g of the N-719 was added to 1:1 volumetric ratio of acetonitrile and butyl alcohol to obtain complete dissolved solution [143]. Electrolyte preparation An electrolyte solution was prepared in a mixture of acetonitrile and valeronitrile (volume ratio, 85:15). The concentrations of the chemicals in the electrolyte solution were 0.6 M BMII, 0.03M I2, 0.10 M Guanidinium thiocyanate and 0.5 M 4-tert- butyl pyridine [143]. The actual used amount of the above prepared electrolyte solution per device test was 1ml.

(a)

(b)

(

(c)

Figure 25 – (a) Sonication and hot stirring to obtain uniform suspension (b) Rotary evaporator (the condition of 40 ºC and 120 mbar) (c) high pressure heater

Table 6 – Measurement of each chemicals by volume and mass

Name of chemicals By mass (g) By volume (ml) TiCl4(stock solution) 49.3 #46070 ethyl cellulose solution (in ethanol) 0.45 #46080 ethyl cellulose solution (in ethanol) 0.35 Terpineol 6.49 6.95

TiCl4(for both baths) 0.8 N-719 dye (for 10 ml electrolyte bath) 0.00594 Acetonitrile (for 10 ml electrolyte bath) 5 Tert-butyl alcohol (for 10ml electrolyte bath) 5 H2PtCl6 (in ethanol) 0.0042 BMII 0.16 g I2 0.00761 Guanidinium Thiocyanate 0.0118 4-tert-butylpyridine 0.0676 Acetonitrile 0.85 Valeronitrile 0.12 0.15

Doctor blending (assembling device) After the preparation of these above important stock solutions and necessary materials, we started to build DSSC devices using prepared pastes of TiO2@MWCNTs and TiO2@SWCNTS. The steps of device building as below (the real picture shown in end pages of dissertation): (1). Prepare the TCO (FTO) glass, 1 inch × 1 inch. (2). Make detergent solution and Sonicate the FTO glass in the detergent water. (3). Make sure FTO conductive side by voltmeter. (4). Rinse the FTO glass with water and ethanol. (5). Remove organic contaminates from FTOs via the air plasma treatment for one minute (Air plasma system;plasma cleaner PDC-001, HARRICK PLASMA). (6). Put in the TiCl4 solution and heat at 80 degree for 30 min (7). Rinse with ethanol. (8). Conductive side face up and keep it clean area. (9). place the conductive side and apply the paste by doctor blending, layer depending on thickness (figure 26). A profilometer was used to confirm the real thickness of the titania mesoporous layer in the DSSC. (10). Dry it for 3 min. (11). Heat at 120 degree for 10 min (figure 27 a). (12). Repeat it until thickness get 13 micron (step 7 to 9) (figure 27 b). (13). Heat it at 500 degree for 30 min (14). Put in the TiCI4 solution at 80 degree for 30 min. (15). Heat it at 500 degree for 30 min. 53

(16). Cool down till 80 degree then immerse it Dye solution for 24 h. (17). Prepare Pt coated glass. (18). Prepare copper tape (19). Prepare to cut the spacer size

Figure 26 – (a) pasting by doctor blending. (b) After result of by doctor blending

Figure 27 – (a), heating and drying. (b) Different layer thickness of SW/MWCNT- TiO2 paste

(20). Put the spacer by heating, dropped the electrolyte. (21). Covered with the Pt coated glass, conductive side faced to the paste, and make sure do not shadow the paste area. (22). Put the clipper.

Moreover, the assembling solar cell done by above steps, and please see the whole steps real picture in the appendix A. We prepared two types of Solar cells using two different types of core-shell nanomaterials of TiO2@MWCNTs and TiO2@SWCNTS. Please see the final solar cell images as seen figure 28(a) before testing and its schematic cross-sectional image (figure 28 (b)).

Figure 28 – (a) real image and (b) schematic cross-sectional image of DSSC device based on CNT-TiO2 nanostructures

2.2 Dye-sensitized solar cell results and discussion

2.2.1 Characterizations of fibers To fabricate TiO2@MWCNTs and TiO2@SWCNTS nano-composite fibers, pre-made solutions of core and shell in two separate syringes were connected by a coaxial needle. The produced nanocomposites were further calcined to produce the desired core-shell nanomaterials. They were further characterized by several techniques to confirm the materials we wanted. A scanning electron microscopes (SEM) is an electron microscope to observe small nanostructures (as small as 1 nanometer). The surface morphology and textures of obtained nanostructures were characterized by using SEM. Scanning electron microscope (SEM) SEM is a powerful instrument to examine nanoscale objects. It provides useful information on morphology, surface and 3 D structures. In combination of Energy dispersive spectroscopy, we can also get compositional information using elemental analysis. The working principle of SEM relies on secondary electrons are emitted from the specimen surface when the specimen is irradiated with a fine electron beam (called an electron probe). Topological image of the nanostructures can be created by two-dimensional scanning of the electron probe over the surface from the detected secondary electrons. A filament is heat up by applying electric current to emit in the traditional SEM. On contrary, in the FESEM, emission is obtained by placing the filament in a huge electrical potential gradient of a field emission gun

(see the figure 29). FESEM easily attain a cleaner image, less electrostatic distortions and spatial resolution less than 1 nanometer under field emission gun.

Figure 29 – Difference of filament in SEM and FESEM

In our case, we used a field emission scanning electron microscope (HITACHI S-4300SE/N series SEM). Basic information of the scanning electron microscope are described below. Filament Type: Cold-cathode field emission; Detectors: SE/BSE/EBSD; Image Resolution: Secondary Electron = 1.5 nm; Specimen Stage: Motion range = 50 x 100 mm; Tilt angle = 5Â° to + 60 Â°; Rotation = 360Â° (continuous); Max Specimen Size: 19Ã—102 mm (height x dia); Oxford HKL Electron Backscatter Diffraction (EBSD) system for crystal texture analysis and orientation image. This instrument is ideal for high-resolution imaging, crystal orientation and Z-contrast imaging. After electrospinning, as-produced fibers collected from aluminum foil were scanned to observe morphology. Figure 30 clearly [146] indicates that uniform nano-fibers were formed. The diameters of the fibers were between 100-200nm. The operation condition of SEM for his characterization is WD15.9 mm, 5.0KV × 900. This image for MWCNT-TiO2 is similar to the image of the SWCNT-TiO2 (not shown here). As-produced nano-fibers were further calcined at 450 Celsius to obtain MWCNT-TiO2 and SWCNT-TiO2 nano-composite fibers. During the calcination, PVP polymer scaffolds were decomposed to remove from the final core-shell nanomaterials. The titanium isopropoxide precusors were crystalized to form anatase titania form.

Figure 30 – SEM image of MWCNT-TiO2 nanocomposite fibers before calcination

The main factors affecting the calcination process: calcination temperature, gas phase composition, the thermal stability of the compound and so on. Therefore, our calcination process took place in CVD furnace (THERMO SCIENTIFIC/ LINDBERG BLUE M). Figure 31 and figure 32 represent FSEM images of TiO2@MWCNTs and TiO2@SWCNTS nano-composite fibers respectively.

Figure 31 – SEM image of MWCNT-TiO2 nano-composite fibers after calcination [146]

After calcination the diameter was reduced to range of 50-100 nm. As seen above images the core-shell nanostructures preserve the smooth morphology and structural integrity. For the carbon encapsulation in the titania we carried out more verifications. For this, Energy dispersive spectroscopy (EDS, EDAX Mapping) and X-ray photon spectroscopy (XPS) were applied to further characterize the above core-shell nanostructures. For the EDS and EDS Mapping we used HITACHI S- 4300SE/N series SEM again since it has this capability.

Figure 32 – SEM image of SWCNT-TiO2 nano-composite fibers after calcination

Energy dispersive spectroscopy (EDS) The Energy Dispersive Spectroscopy (EDS) or Energy Dispersive X-ray (EDX or EDXS) is used to analyze characteristic X-ray spectra obtained from the samples by applying the X-rays. The principle of EDS is shown in figure 33. Energy Dispersive X-ray Spectroscopy is to provide spectra of x-rays which is obtained from an element or substrate which expose to a high energy electron beam. As an electron falls from a higher binding energy electron level into the hole, an x-ray is released. The emitted x-ray energies for elements are varied from element to element with certain spectral lines. The obtained spectral lines from sample are identified via comparing to the database of known elemental spectra [148]. EDAX (or EDS) is an x-ray spectroscopic method for determining elemental compositions. The relationship between the wavelengths of the characteristic X-ray emitted from the element and its atomic number Z.

Z = λ-1/2 (37)

Therefore, we know energy relation to wavelength as equation (3) or E = 12.4/ λ, from there it was found that energy levels in electron shells varied in discrete fashion with atomic number. 58

Figure 33 – Principle of EDS

Carbon concentration in the titania and CNT nanostructures was found to be 27.7% from EDS measurement as shown in figure 34. EDS analysis proved successful encapsulation of CNT in core-shell structures of CNT and TiO2.

Figure 34 – EDS spectrum for MWCNT-TiO2 nano composite [146]

The EDX technique has a capability of providing elemental mapping which indicates the local presence of different elements in the nanofiber. For this analysis, 59 backscatter images were required to get atomic number contrast. Here the positions of specific elements emitting characteristic x-rays within an inspection field can be indicated by unique colors.

Figure 35 – EDAX mapping of elemental distribution of Ti, O, and C. in diameters of 50-100 nm fibers, length of fiber about several micron

In figure 35 the elemental mapping images of Ti, O, and C clearly showed that nanofibers were composed of TiO2 and CNTs. Color-coded elements were consistent with the nanofiber structures. The elemental mapping is great technique to show fibers compositions with local positions and phases with varying composition. X-ray photon (Photoelectron) spectroscopy (XPS) The working principle of X-ray photoelectron spectroscopy (XPS) is similar to EDS. In brief, electrons emerged from the sample and accelerated in the magnetic field when a sample absorb photons of a particular energy. The electronic states of atoms in the sample can be obtained from the kinetic energy analysis of electrons emitted from the surface yields. XPS is a very useful technique for analyzing surface properties of the nanomaterials around 10nm. The technique is comprehensively described by Briggs and Seah [149, 150]. Concisely, in XPS surface analysis, the sample is placed under X-rays with a well-defined energy. The emitted electrons from the interaction of the X-rays with the nucleus of the different atoms present in the sample with a well- defined kinetic energy, EK, are measured and calculated using the following equation (38):

Ek = hv−Eb−Φ (38)

In the equation (38), hv indicates the X-ray photon energy; Eb represents the electron binding energy which is determined by materials properties. Φ is the work function that is the energy needed for the electron to free itself from the surface. The total applied energy is equal to the sum of the kinetic energy of emitted electrons and their binding energy. The kinetic energies of the photoelectrons emanating from different types of atoms and orbitals in the samples are related to binding energies. Thus, through separating the electrons with different kinetic energy in an analyzer, the information of atoms in the surface layer including their charges, bomding and concentration can be obtained (figure 36).

Figure 36 – Basic components of a monochromatic XPS system [151]

The survey spectrum (figure 37) clearly indicates that the main compositions of nano-composite fibers are Ti, O, and C after the calcination step. The peak at 284eV represents carbon binding energy peaks in the depth profile analysis which performed after removal of certain layers. Figure 38 proved that binding energy of carbon with TiO2 in the tight XPS spectrum was different from carbon binding energy from atmospheric carbon and TiO2. This is clear evidence for the successful encapsulation of MWCNT and SWCNT in TiO2 as core-shell nano-composite fibers. So we made 1D uniformly core-shell nanofibers using Electrospinning and Sol- gel methods, the diameter of fibers was between 50-100 nm and length approximately several microns, the core-shell form and its decoration of elements conformed by SEM, EDS, and XPS. For imaginary, when we assemble DSSC it forms a film on the top of FTO glass which is explained in figure 39 (that view of CNT-TiO2 films or electrodes).

Figure 37 – XPS survey spectrum MWCNT-TiO2 nano-composite fiber [146]

Figure 38 – Tight spectrum for carbon in MWCNT-TiO2 nano-composite fiber [146]

Figure 39 – Imagining figure for TiO2 with CNT (SWCNT or MWCNT)

Also we made the device by these 1D core-shell nanomaterials. Therefore, in the next step, we will use these fibers to examine the different capabilities of CNTs for electron transport (analysis of device). Analysis and discuss will presented in next part.

2.2.2 Discussion of experimental results (analysis of device) Two different types of DSSC devices were built to compare photovoltaic performance using the TiO2@MWCNTs and TiO2@SWCNTS nano-composites. At the same time the device performances were explained based on the electron transport properties of MWCNT and SWCNT in the core-shell nanocomposite fibers. To explain the DSSC device performances, studies of I-V curve and bode plot were carried out. To prevent the electron trapping center in the photoanodes of DSSCs, core-shell structures were applied to prevent the direct contact of carbon nanotubes with electrolytes. Covering CNTs with TiO2 shell is a great strategy to provide easy electron pathways to transfer the injected electron from dye external circuit through avoiding the recombination of the electron with electrolyte species. We have proved that we have successfully performed 1D core-shell nanofibers. In this part, we have to check its electron transport property. Before that, we explain some instruments and methods for that. I-V curve / electrochemical impedance A Solar cell generates electricity when solar light illuminates on it. The output current is determined by the potential of the cell as well as intensity of the incident light. Current-potential curve was produced to calculate the device performance (also called I-V curve).

Electrochemical impedance spectroscopy (EIS) is a useful technique to determine the contribution of electrode or electrolytic processes in these systems. Electrochemical impedance is acquired via supplying an AC potential to the cell. The trapping and detrapping, recombination, transport models of photoinjected electrons have been analyzed by the data of electrical impedance spectroscopy. Here, we used a Model chi6011E Series Electrochemical Analyzer / Workstation (CH Instruments), which is designed for general purpose electrochemical measurements which computerized potentiostat/ galvanostat. It also called Potentiostat. LCS-100 solar simulators were used to irradiate a certain area of DSSC device for mimicking standard solar spectrum. The LCS-100 models have a standard with an AM1.5G filter to fulfill the Class A Spectral Match specifications. A standard power of AM 1.5G filter is 100 mW/cm-2. Further, we combine these EIS and solar simulator to do I-V measurement and inside impedance process. Our DSSC device was placed under the LCS -100 solar simulator and connected to Model chi6011E Series EIS by four wires connector to test cell performance. Then it will be analyzed by laptop or personal computer in room condition as seen actual figure 40(left top LCS, right top EIS, below DSSC device), and further more analysis results will explained in the following discussion.

Figure 40 – DSSC device in Chi 6011e potontiostat with solar simulator

The principal block diagram of the EIS instrument shown in figure 41.

Figure 41 – the principal block diagram of the EIS

All impedance measurements were performed under a bias and light illumination of 100 mW/cm2 from a solar light simulator at open circuit condition. Impedance measurements of cells was recorded in a certain frequency range with an AC amplitude of 10 mV. The purpose to obtain those curves is to measure the performance of our solar cells. The setup is explained in the above on how to use the Chi 6011e instrument electrochemical analyzer introduction. The excel file attached would have the experimental data and trials we had including calculation on the spreadsheet.

Figure 42 – I-V curve for MWCNT-TiO2 and SWCNT-TiO2 nanocomposite fibers [146] 65

It can be observed from (figure 42) that SWCNT-TiO2 based DSSC showed better photovoltaic performance than that of MWCNT-TiO2 nano-composite based DSSC. Furthermore, SWCNT in the DSSC produced a higher short circuit current of 0.21 mA while MWCNT had a short circuit current of 0.069 mA in core-shell nano-composite materials. A short circuit current generated by SWCNT (Isc =0.21 mA) is three times high as that of the cell with MWCNT (Isc = 0.069 mA). To interpret our data, we used impedance analyzer to find out the life times for the two different devices. Figure 43 shows bode plots for two different DSSCs made of SWCNT and MWCNT nano-composites (figure 43).

Figure 43 – Bode plot for MWCNT-TiO2 and SWCNT-TiO2 nanocomposite fibers [146]

Equation (39) is applied to calculate the electron lifetime for both devices:

-1 τ = (2πfmax) (39) where fmax is maximum frequency in the bode plot [152].

An electron lifetime for MWCNT-TiO2 nano-composite was found to be 0.1082 ms while SWCNT-TiO2 nano-composite had an electron lifetime of 10.8 ms (table 7). The high electron life time from SWCNT is consistent with its relatively high device performance, which indicates that SWCNT provides faster electron

66 transfer relative to multiwall carbon nanotube. A reasonable explanation was ascribed to higher lifetime which brought to better cell performance [153].

Table 7 – Max frequency and electron lifetime for MWCNT and SWCNT nanocomposite fibers.

DATA MWCNT SWCNT Max frequency from Bode plot 1470 Hz 14.68 Hz Electron lifetime 0.108 ms 10.8 ms

Figure 44 and figure 45 showed schematic explanations for electron transfer pathways in the two different solar cells. Furthermore, there are highly presence of recombination and trapping centers in the DSSC made from MWCNT-TiO2 nanocomposite fibers than that of SWCNT-TiO2 nano-composite fibers. In the SWCNT- TiO2 nano-composite fibers, since the energy position of semiconducting single wall carbon nanotubes was well-aligned with the conduction band of titania, the electron transport is faster and no recombination centers were avoided. However, in the DSSC made of TiO2 @ MWCNT, metallic MWCNT easily forms recombination and trapping centers, thus resulting in low short circuit current. Thus, in our DSSC device study, SWCNT based DSSC offered larger diffusion length which interpreted higher short circuit.

Figure 44 – Scheme for electron transport in SWCNT-TiO2 nanocomposite fiber [146]

Figure 45 – Scheme for electron transport for MWCNT-TiO2 nanocomposite fiber [146] As seen figure 42 a higher open current voltage, Uoc and lower short circuit currents, Isc were found in our experiments for both SW and MWCNT –TiO2. For this explanation, we tested more and took the standard, Integrated Solar Simulator & I-V Measurement System, which from Photo Emission Tech., Inc. I-V measurement systems from 1, 3, 5, 15, up to 20 Amps and up to 60V Capability Systems. The model characteristic specification as seen table 8 and the whole instrument seen in figure 46.

Tabel 8 – PET Solar Simulator Energy Output Table (Square Output Beam)

PET SS Model # SS50 Square Beam: Original Size(cm) 5cm Original Beam Area (cm2) 25 Normal Intensity at working plane 1000 (w/m2) (1sun) Concentrated Nominal Intensity At working Plane for 6250 2cm×2cm Device(w/m2) (6.25 Suns) Concentrated Nominal Intensity At Working Plane for 25000 1cm× 1cm Device (w/m2) (25.0 Suns) Concentrated Nominal Intensity At Working Plane for 100000 0.5cm ×0.5cm Device (w/m2) (100.0 Suns)

Figure 46 – Integrated Solar Simulator & I-V Measurement System

We explain that 1D nanofibers give good electron transport for each interface, but short circuit current is not good. Even that compare usual DSSC they give higher open-circuit voltage (Voc), it approximately reached near 900mV to 1000mV, and short-circuit current (Isc) was lower, it was 1400µA (1.4 mA) as seen figure 47. (Fill Factor (FF) was between 0.62 -0.75, Efficiency was very low)

Figure 47 – two different solar cells I-V curve, such as SWCN-TiO2 and MWCNT- TiO2 via Solar simulator SS50. 69

Why the short-circuit current (Isc) was lower and open-circuit voltage was higher (Voc)? We considered the follwoing several reasons: first, the assembling of room conditions, second, Thickness condition: in our project the paste thickness that was measured by Profile meter was 30 microns, it was higher than required thickness. More especially third, Electron transfer expose and spacer adhesion: between the anode and cathode was directly connected or shorted (reason from our spacer was not good adhesion, there has wrinkle in figure 48).

Figure 48 – the paste and wrinkle of spacer on the FTO glass

Other possibility was not full coverage of some CNTs with TiO2 shell so that they exposed partially to electrolyte, resulting in blockage of successful transport of electrons to external circuit. These above results causes some lower short- circuit currents to occur. In this study we have investigated the property of DSSCs with MWCNT-TiO2 and SWCNT-TiO2 1D core-shell nanofibers. The MWCNT-TiO2 nanofiber have a larger defect structure than the SWCNT-TiO2. A short circuit current obtained from SWCNT-TiO2 is 5-6 times larger than that of the DSSC based on MWCNT-TiO2. This result is illustrated based on the two studies: one is high electron density and the other is an effective lifetime in the SWCNT-TiO2 cell. The effective electron lifetime in SWCNT-TiO2 is approximately two times larger than that in MWCNT- TiO2. The photovoltage of DSSC with the SWCNT-TiO2 array has been almost two times more than that of the cell with powdered titania. It can be related the high electron transport density in CNT-TiO2, which can have high electron quesi-Femmi level.

Conclusions for section 2 DSSC has been attracted broad attention due to its advantages including device assembly in ambient conditions, cost-effective fabrication, and decent photovoltaic efficiency. To obtain higher DSSC efficiency, it is important to better understand fundamentals of DSSC. To this end, unique coaxial electrospinning and sol-gel approach combined technique was developed to synthesize 1-D core-shell nanofibers of CNTs and TiO2. Diameters of the core-shell SWCNT-TiO2 and 70

MWCNT-TiO2 nanocomposite fibers varied from 50 to100 nm. Presence of CNT in the core-shell structure was further proved by EDS and XPS. Electron transport properties of both SWCNT-TiO2 and MWCNT-TiO2 in DSSCs were further investigated by electrochemical impedance analyzer. Resulted I-V and bode plots were further applied to explain the cell performances via finding out their diffusion length. It was obtained that SWCNT-TiO2 based DSSC produced larger short circuit current of 0.21 mA than that of MWCNT-TiO2 based DSSCs. This findings was ascribed to semi-conductive properties of SWCNT which was consistent with higher electron lifetime of SWCNT (10.8 ms). This study indicates that SWCNT is a promising material for DSSC application [146]. Simultaneously, a combination of sol-gel and electrospinning approach is not only applied to fabricate the core shell- nanostructures consisting of carbon nanotube and titania, but also can be extended to other types of core-shell nanomaterials including metal/metal oxide and metal oxide/metal oxide nanomaterials. They will have potential applications from photovoltaics to sensors [154 –156].

3 EXPERIMENTAL, RESULTS AND DISCUSSION FOR PHOTOCATALYTIC WATER SPLITTING

The synthesis of perovskite nanomaterials with controlled size, proper morphology, and highly crystalline structure is challenging when conventional methods are used [157]. Preparing ternary metal oxides with different morphologies in 1D network systems is even more challenging. To select strontium titanate for photocatalytic hydrogen evolution study, the main reason is that it has suitable conduction band position to produce hydrogen, decent stability in aqueous media under long solar irradiation, and tunable band gap for apply in visible light range via doping approaches. Therefore, our research focused on developing a porous 1D Strontium Titanate nanofibers (STO-NFs or SrTiO3-nanofibers) by combining the electrospinning technique with the sol-gel method, where a polymer is used as a nanotemplate. In addition, the growth behavior of STO-NFs, including crystallite size and the diameter of the fiber, was also investigated under different precursor concentration and calcination temperature conditions. This study provides an important understanding of crystallization growth of SrTiO3 in a polymeric nanotemplated system. The effect of calcination temperature and precursor concentration on the fabricated STO-NFs photocatalytic activities was also evaluated by H2 production from water splitting under UV irradiation.

3.1 Experimental section for Photocatalytic Water splitting

3.1.1 Fabrications of Strontium Titanate nanofibers Materials Polyvinylpyrrolidone (average M.W. 1,300,000), chloroplatinic acid hexahydrate (38–40% Pt), ethanol (99.5%) were purchased from Fisher Scientific. Strontium titanium alkoxide – DSRTI50 was purchased form Gelest Inc., and chloroplatinic acid hexahydrate and commercial strontium titanate nanopowder (<100 nm particle size) from Sigma Aldrich. All chemicals were used as received without any further purification. Water of 18.2 MΩ-cm resistivity was used. Two fabrication methods, particle transfer [158] and nanorod fabrication [159], have been developed to improve the electrode stability. Due to the complexity of these methods [160] and economic viability of the photoelectrochemical cell, the use of a particulate system is beneficial for the hydrogen production from water splitting [161, 162]. The perovskite nanomaterials are promising photocatalysts in a particulate system. SrTiO3 is our study material, which we have to understand the properties for further discussion, so let us see the strontium titanate atomic structure. SrTiO3 is belong to perovskite family of the ABO3 and possess a cubic structure (space group Pm3m) with a lattice parameter of 0.3905 nm. The crystal structure is sketched in figure 49. The Ti4+ ions is located in the center of cubic units via sixfold coordinating with O2- ions, whereas the Sr2+ ions occupies the corners of the unit cell [163]. Hence, SrTiO3 has mixed ionic-covalent bonding properties. The physical properties of SrTiO3 as seen in table 9.

Figure 49 – Atomic structure of SrTiO3. (The sizes of the spheres representing the atoms are arbitrary and are not related to atomic radii)

Table 9 – Summary of the physical properties of SrTiO3.

Property Value Lattice parameter at RT (nm) 0.3905 Atomic density (g/cm3) 5.12 Melting point (°C) 2080 Mohs hardness 6 Dielectric constant ( ɛ0) 300 Thermal conductivity (W/m.K) 12 Coefficient of thermal expansion (Å/°C) 9.4×10-6 Refractive index 2.31-2.38

STO-NFs synthesis: STO-NFs were synthesized by a sol-gel assisted electrospinning method. To prepare the precursor solution, the desired amount (0.25–1.5 ml) of strontium titanium alkoxide (DSRT) was added to 3 ml ethanol and then the solution was stirred at 400 rpm for 10 min. For the polymeric solution, 9.3 wt.% of polyvinylpyrrolidone (PVP) was dissolved in ethanol under stirring condition and kept for 24 hours prior to use. The precursor solution was prepared, then added to 5 ml of PVP solution and stirred for 30 min. A 10 ml glass syringe equipped with a 21-gauge needle was used to electrospun the polymer-inorganic composite solution. The electrospinning process was conducted at room temperature at a voltage of 16 kV with a flow rate of 1.2 ml/h at 15 cm distance from the needle tip to the collector. A circular Aluminum foil of 20 cm diameter was used as a nanofiber collector and

73 the collector was replaced every 1.5 h during the process. The schematic and real pictures are listed in figure 50. The produced polymer-inorganic composite nanofibers were then calcined to produce STO-NFs in a furnace (Thermolyne1400 M) at different calcination temperatures (400–800◦C) for 2 hours at a heating rate of 10◦C per min. Differential scanning calorimetry (DSC) and Thermogravimetric analysis (TGA) analysis of the composite nanofibers were performed prior to the calcination to select the calcination temperature range. The STO-NFs were characterized by X-ray diffraction (XRD) and SEM. XRD was performed to determine to the crystallinity and crystallite size of the STO-NFs and the surface morphology of the STO-NFs was investigated by SEM.

Figure 50 – the picture of (a) schematic (b) and real electrospinning process

Pt photodeposition: Pt was photodeposited onto the STO-NFs surface after the calcination of polymer-inorganic composite nanofibers [164, 165]. Among the various hydrogen evolution co-catalysts, Pt was selected to study for co-catalyst effect on hydrogen evolution. In the photocatalytic hydrogen evolution, Pt is considered to be the best cocatalyst since it can reduce the overpotential at the interface of semiconductor and solution, thus improving charge separation of photocatlytic generated electron/hole pairs in the strontium titanate photocatlyst. So, a 0.15 g sample of synthesized STO- NFs and a calculated amount of chloroplatinic acid hexahydrate were mixed in 40 ml ethanol to obtain the desired weight percentage (0.3–2 wt.%) of Pt on STO-NFs. The solution was then sonicated for 20 min. A quartz reactor was used to expose the continuously stirred solution to UV light for 1.5 h. Next, the reaction mixture was centrifuged 3 times to remove the unreacted Pt precursor and was followed by vacuum filtration to separate the particles. Figure 51 explains schematically all of the platinum deposition procedures. Then the filtered particles were dried overnight at 120◦C in a vacuum oven to obtain Pt-loaded STO-NFs (Pt/STO-NFs).

Figure 51 – For Pt photodeposition steps setup one by one from (1) to (3)

3.1.2 Experimental setup for hydrogen evolution The prepared Pt/STO-NFs were subjected to photocatalytic performance tests by mixing 0.05 g of Pt/STO-NFs with 40 ml of methanol-water solution (5:7 volumetric ratio) and followed by sonication for 20 min. Since the band gap of strontium titanate is 3.2 eV which is belong to UV region, the photocatalytic performance was evaluated under UV light irradiation (wavelength: 320nm). Before UV irradiation, argon gas was purged through the reactor for 30 min to remove the dissolved oxygen and air from the reactor. Purged conditions were confirmed by gas chromatography (GC) analysis. The reactor was exposed to a mercury lamp (100 W UV light, 365 nm) and H2 evolution data was collected at different time points. For each sample collection, 500 µl of gas was injected into the GC to evaluate the H2 composition. All steps schematically are introduced in figure 52. 75

Gas chromatograph (GC) is an analytical instrument which is broadly applied to measure gas amounts in the scientific research lab and industrial lab. Principle of gas chromatography is briefed as follows: Either gas or liquid sample was injected into the instrument which carries by a gas stream into a separation tube known as the "column." (Helium or nitrogen is usually used as the so-called carrier gas). The various components are separated inside the column. The different detectors were applied to measure the quantity of the components that exit the column. To find out the sample and its concentration, a standard sample with known concentration is measured to create a calibration curve. The standard sample peak retention time (appearance time) and area tells us the test sample and its concentration. The principle picture as seen Figure 53.

Figure 52 – Hydrogen evolution and hydrogen testing installation chart

Figure 53 – GC principle picture (the real picture seen in figure 52, right below)

So far we explained the experiment needed materials, experimental setup and hydrogen evaluation. The whole real work roadmap structure is sketched by figure 54. Moreover, the figure 55 shows schematic structure of experiments. These two figures give you our research step direction and give you easy imaginary understanding. Once it was clear we will switch to next part results and characterizations.

Figure 54 – Roadmap structure of the experiment

Figure 55 – Schematic structure of experiments [166]

3.2 Photocatalytic Water splitting results and discussion

Characterization: Differential scanning calorimetric(DSC) measurements were carried out using a Q20 (TA Instruments, Delaware, USA) instrument. Thermogravimetric analysis (TGA) was performed using a i1000 (supplied by Instrument Specialist Inc). The surface morphological images were obtained using a field emission scanning 77 electron microscope(FESEM) Hitachi S-4300 E/N and a high resolution transmission electron microscope (HRTEM) H-9500 (300 kV). The diffraction patterns for STO-NFs were collected on a Rigaku Ultima III powder diffractometer. Before explaining in detail, we should have a detailed understanding of these devices. Differential scanning calorimetry (DSC) DSC technique developed in 1960 were commercially available at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy in 1963. Basic working principle of DSC is to measure the difference in heat flow to the sample and a reference at the same temperature with a change of temperature. The heat change indicates phase transitions and chemical reactions. The reference can be an inert material such as alumina, or just an empty aluminum pan. The temperature of both the sample and reference rises at a constant rate. The Differential Scanning Calorimeter is operated at constant pressure so that the relationship between heat flow and enthalpy changes can be express in the following equation:

(dQ/dt)p = dH/dt (40)

Here dH/dt is the heat flow measured in mcal/sec. The heat flow difference between the sample and the reference is:

ΔdH/dt = (dH/dt)sample – (dH/dt)reference (41)

It takes either positive or negative values. ΔdH/dt is positive in an endothermic process since the sample absorbs heat and (dH/dt)sample is greater than (dH/dt)reference. However, in an exothermic process, this is vice versa. This reaction releases heat to the surroundings and dH/dt is negative. Endothermic process indicates a transition which absorbs energy. Exothermic process indicates a transition which releases energy. This technique is a common technique in polymer physics which is applied to study what happens to polymers/samples upon heating. DSC provides useful information on thermal transitions of a polymer/sample( the changes that take place on heating) including the melting point of a crystalline polymer, the glass transition temperature of amorphous polymers and crystallization mechanism. During the DSC measurement, the sample and reference are required to keep at the same temperature or the external energy is applied to maintain zero temperature difference between the sample and the reference. If a thermal event occurs in the sample during the heating process, the system will transfer heat to or from the sample pan to keep the same temperature by the computers system. The transferred or heat flow difference will be recorded. There are two basic types of DSC instruments which are power compensation DSC and heat-flux DSC [167 – 169]. As shown in figure 56, two pans are placed on the top of heaters and whole process will be controlled by a computer system which control heating process. One pan contain weighed sample and the other is a reference pan which is empty. The heating media can be used different gases such as nitrogen and argon gas or air. 78

Figure 56 – DSC instruments

Thermogravimetric analysis, TGA is to measure the mass change of a sample with temperatures. It consists of a sample pan which hangs off a hook and a tare pan which are connected by a microgram balance arm (figure 57 and figure 58).

Figure 57 – Schematic of the balance arm located inside a TGA machine.

Briefly, TGA is applied to determine the composition of a material or its thermal stability when the sample is heat up to 1000oC. A curve of mass of the sample with temperature is obtained. According to the change in mass at the different temperatures, the mass loss information is found, which can be correlated with the processes of decomposition, reduction, or evaporation. During the process, it is also possible to increase a mass of a sample due to oxidation or absorption. In this process, a microgram balance is used to track the mass changes in the sample. Temperature change is monitored by a thermocouple. The curves from TGA can be 79 further analyzed to find the important information on the sample [170]. To interpret the data in the curve, all of the possible reaction process including decomposition, evaporation , oxidation/reduction at the specific temperatures has to be considered.

Figure 58 – TGA installation for our research (in TTU)

XRD (X-ray diffraction) is considered to be a non-destructive tool to obtain crystal information of the materials. It is very important tool for materials characterization. When a beam of radiation X-rays interacts with a material, the beam is diffracted by different crystal planes of the material which generate X-ray diffraction spectra. The instrument is called diffractomer. In our research, Rigaku diffractometer was used (figure 59).

Figure 59 – Rigaku Ultima III powder diffractometer (XRD)

X-ray diffraction spectra were attained by scanning a 2θ range of 20–77◦ with certain rate and step size. The X-ray source was Cu Kα radiation (λ = 1.5418 Å) with an anode voltage of 40 kV and a current of 44 mA. Diffraction intensities were

80 recorded on a scintillation detector after being filtered through a Ge monochromator. The sample used was a powder mount and the obtained data was analyzed through the software JADE v9.1. In this software, crystallite size inside the STO-NFs was estimated by using the Scherrer formula:

d = Kλ/B cos θ, (42) where d is the crystallite size, K is the Scherrer constant (0.9), λ is the wavelength of Cu Kα radiation (average of Kα1 and Kα2), B is the full width at half maximum intensity of the peak after deducting the instrumental line broadening (Al2O3was used as a standard for instrumental line broadening), and θ is the diffraction angle. To synthesize STO-NFs, a prepared solution of PVP and DSRT at a calculated ratio was used in the electrospinning process to produce composite nanofibers. The produced nanofibers were then subjected to DSC and TGA analysis before calcination to acquire information regarding calcination temperature. TGA was performed in the presence of air while DSC was performed under a nitrogen environment. In the DSC thermogram (figure 60), one endothermic peak is detected at 430◦C, indicating the decomposition temperature of PVP in presence of DSRT. In figure 61, the TGA thermogram shows three major weight loss segments corresponding to 25–190◦C, 190–350◦C, and 350–650◦C. The respective weight losses are (a) 13.91 wt.% due to ethanol, n-butanol and methoxypropanol evaporation, (b) 62.6 wt.% due to the decomposition of the organic part of DSRT or PVP, and (c) 5.32 wt.% due to decomposition of organic residue and conversion of ◦ DSRT to SrTiO3. After 650 C, the weight of the sample becomes constant, which indicates that the organic residue is removed and DSRT is converted to SrTiO3.

Figure 60 – DSC analysis of polymer-inorganic composite fiber at 10◦C/min [171]

Figure 61 – TGA analysis of polymer-inorganic composite fiber at 10◦C/min [171]

Based on the TGA and DSC analysis, the synthesized fibers were heated from 500 to 800◦C for 2 hours in the presence of air. The crystal structure of the SrTiO3 was confirmed by XRD (figure 62). The XRD patterns of STO-NFs matched with the PDF no.97-018-2762. The crystalline peak of STO-NFs was observed near 500◦C and very clear crystallite observed at 800 ◦C, but at 400 ◦C no crystal spectra peaks was observed.

Figure 62 – XRD patterns of STO-NFs after calcined at a temperature range of 400–800◦C [171] 82

The SEM images of the produced fiber before and after calcination are shown figure 63 and 64 in the next page. The SEM showed the fibers had smooth and homogeneous surfaces, and the structural integrity of the fibers was retained after calcination.

Figure 63 – SEM images of Polymer-inorganic composite fiber before calcination [171]

Figure 64 – SEM images of STO-NFs after calcination [171] 83

After calcination at 700°C for 2 hours, the diameter of the fiber obtained at different DSRT concentration was calculated using SEM images (figure 65 in the next page). The diameters of STO-NFs for 5.88 vol.%, 11.11 vol.%, and 15.78 vol.% DSRT concentration were 91–122 nm, 54–116 nm, and 88–161 nm, respectively. Although the diameter range of the synthesized STO-NFs is broad at a specific DSRT concentration, a relatively larger diameter of STO-NFs is obtained at a higher DSRT concentration. However, the concentration has more effect on the surface morphology of the STO-NFs rather than the diameter of the nanofibers. At low DSRT concentrations, the crystallite size in the STO-NFs is comparatively larger than the crystallite size of nanofibers produced at higher concentrations. The SEM images also show that the fibers merged together to form a branch structure at low concentrations, whereas at high concentrations (>11.11 vol.%), the fibers remained separate and maintained their individual fibrous integrity (figure 66).

Figure 65 – SEM images of STO-NFs at different DSRT concentration. (a) 5.88 vol.%, (b) 11.11 vol.%, and (c) 15.78 vol. % [171]

Figure 66 – SEM images of STO-NFs. (a) 5.88 vol.% DSRT concentration, (b) 15.78 vo.% DSRT concentration [171]

The crystallite sizes of STO-NFs for three different crystal planes were calculated from XRD spectra and plotted at different DSRT concentrations from 3.03 to 15.78 vol.%, with a fixed calcination temperature of 700◦C (figure 67). It was observed that the crystallite size decreases with increasing DSRT concentration until12 vol. % and becomes less responsive beyond that point. For the (110) plane, the crystallite size decreases from 77 to 58 nm while the DSRT concentration increases from 3.03 to 15.78 vol. %. Figure 68 (page 86) shows the effect of calcination temperature on crystallite size of STO-NFs at a fixed precursor concentration of 11.11 vol.%. The calcination temperature was varied from 500 to 800◦C to investigate the dependence of STO crystallite size on temperature. It was observed that the crystallite size monotonically grows as calcination temperature increases, which was expected according to literature [172]. The temperature effect on particle size was calculated for three different crystal planes: (110), (200), and (220). For the (110) plane, the crystallite size changed from 51 to 100 nm when the calcination temperature increased from 600 to 800◦C.

Figure 67 – Effect of DSRT concentration on crystallite size of STO-NFs at a constant calcination temperature of 700◦C [171]

Furthermore, the STO-NFs synthesized at different calcination temperatures were tested for H2 evolution under UV irradiation. It was found that the H2 production rate increases with increasing calcination temperature. As shown in figure 69, the H2 production rate of STO-NFs calcined at 800◦C is 0.135 mmol/g-h, which is around 2.5 times higher than that of the STO-NFs calcined at 600◦C.

Figure 68 – Effect of calcination temperature on crystallite size of STO-NFs at a constant DSRT concentration of 11.11 vol. %. [171]

Figure 69 – H2 production rate for STO-NFs from 41.6 vol.% methanol aqueous solution at different calcination temperatures, with a fixed DSRT concentration of 11.11 vol.% [171] 86

High calcination temperature improves the crystallinity of the STO-NFs, which facilitates better charge separation, and results in an enhanced H2 production rate at 800°C [173]. As the calcination temperature increases, STO-NFs not only achieved a high degree of crystallinity, but also a large crystallite size. To investigate the effect of either crystallinity or crystallite size on photocatalytic activity, H2 evolution measurements were carried out for different STO-NFs that were synthesized by changing precursor concentrations at a constant calcination temperature. For more clarity, the above content can also be displayed in the following figure. Figure 70 illustrated the charge separation and grain boundary reduction.

Figure 70 – Grain boundary reduction and Charge seperation

Figure 71 shows that the H2 production rate of STO-NFs produced at a 5.88 vol. % DSRT concentration is 0.141 mmol/g-h, which is 3 times larger than that of STO-NFs produced at a 15.78 vol.% DSRT concentration. This concentration study indicates that the photocatalytic activity of STO-NFs increases with increasing crystallite size. Townsend et al. also reported similar findings for the suspended free particle system, where the H2 production rate enhancement factor for larger particles (60 nm) was around 6 times more than that of the smaller particles (6.5 nm) [174]. 87

The summary of the H2 production rate with different crystallite size of STO-NFs produced by changing calcination temperature and precursor concentration. From the table, it can be concluded that the ideal conditions for synthesis of STO NFs are ◦ 800 C calcination temperature and 5.88 vol.% DSRT concentration to maximize H2 production rate.

Figure 71 – H2 production rate for STO-NFs from 41.6 vol.% methanol aqueous solution at different DSRT concentrations with a fixed calcination temperature of 800◦ C. [171]

Further, Pt was photodeposited as a cocatalysts on the STO-NFs to enhance the H2 production. Figure 72 (a) shows the HRTEM image of Pt/STO-NFs. The HRTEM lattice image (figure 72 (b)) shows two distinct interplanar spacing of 0.276 nm and 0.226 nm, which are assigned to the SrTiO3 crystal plane (110), and the Pt crystal plane(111), respectively [175 - 178]. In water splitting, the Pt cocatalyst has a large work function and provides catalytic surface active sites, which improves the surface charge separation and enhances the catalytic ability for proton reduction [179]. The H2 evolution tests of STO-NFs, synthesized at a calcination temperature of 800°C and a DSRT concentration of 11.11 vol.%, were performed with different Pt loading (0.3–2 wt.%) to determine the optimum weight percentage. Figure 73 shows that Pt/STO-NFs (0.5 wt.% Pt) exhibits the highest H2 production rate, which is consistent with literature [180,181]. Simultaneously, a parallel H2 evolution test was carried out for the 0.5 wt.% Pt-loaded commercial STO-NPs. It was found that the H2 production rate of Pt/STO-NFs is 1.14 mmol/g-h, which is approximately 2 times higher than that of Pt/STO-NPs (figure 74).

Figure 72 – (a) HRTEM image of Pt/STO-NF. (b) – HRTEM lattice image showing interplanar spacing of SrTiO3 and Pt. [171]

Figure 73 – H2 production rate of Pt/STO-NFs with different Pt loading. (STO-NFs were synthesized at a calcination temperature of 800◦C and a DSRT concentration of 11.11 vol.% to generate plot) [171]

Figure 74 – H2 production rate comparison for Pt/STO-NFs and commercial Pt/STO-NPs with 0.5 wt.% Pt loading (STO-NFs were synthesized at a calcination temperature of 800◦C and a DSRT concentration of 11.11 vol.% to generate plot) [171]

The reasons for enhanced H2 production of STO-NFs are that SrTiO3 nanoparticles in the nanofibers are inter-connected, and have well-ordered and aligned SrTiO3 particle assembly (figure 75). The well-ordered inter-connected particle assembly in the STO-NFs may facilitate the charge separation of electron- hole pairs and provide a suitable platform for the vectorial transport of photogenerated charge carriers through grain boundaries [182]. In contrast, STO- NPs are easily aggregated at a disordered fashion in their loose and random states, which could negatively affect the photo-catalytic activities of hydrogen evolution [183].

Figure 75 – Schematic of STO-NFs photocatalytic reaction mechanisms. (S represents a sacrificial agent and S+ is an oxidized form of the sacrificial agent) [171] 90

The stability test of the Pt/STO-NFs in the photocatalytic water spitting was carried out at 3 consecutive cycles of 250 min each. After each cycle, argon gas was purged through the reactor to drive out the accumulated H2.

Figure 76 – Photocatalytic performance of Pt/STO-NFs (0.5 wt.% Pt) [171]

Figure 76 shows that the H2 production performance of the Pt/STO-NFs is consistent in each cycle without losing any photocatalytic activity. This indicates that Pt/STO-NFs are stable and promising for photocatalytic water splitting. Photoluminescence (PL) provides useful information on material’s optical properties which is a simple nondestructive analysis technique. A PL spectra is produced after any matter absorbs photon of electromagnetic radiation. In principle, as the light is directed to the surface of the object, where it is absorbed, the process of the photoexcitation process takes place. During the photoexcitation process the valence electrons in the material jump to a higher electronic state, and will then release energy, (photons) as it relaxes and returns to back to a lower energy level. The emission of light or luminescence through this process is called as PL[184]. To explain charge separation enhancement of our 1D strontium titanate nanofibers over their free particles , we further took the above mentioned STO- NFs, Pt doped STO-NFs and STO-NPs to check their PL spectra for confirmation of recombination rate. This study offered understanding on electron transfer in these nanomaterials. Figure 77 shows morphology images of those STO nanomaterials. The intensities of PL spectra gives us which materials involve strong charge recombination issues. The results of PL explained that commercial STO-NPs has the highest recombination and the highest intensity peak. On the contrary, Pt/STO-NFs has low recombination rate. 91

(a) (b) (c)

Figure 77 – SEM images of commercial STO-NPs (a), STO-NFs (b) and STEM image of Pt/STO-NFs (c)

Moreover, the obtained PL spectra indicated photoluminescence of STO between 440 nm to 525 nm for the above three types of STO samples (it is not shown here). The PL spectra peak was found approximately at a 455 nm. The STO-NPs (2.0 × 107 a.u)’s intensity was much higher than the intensity of STO-NFs (8.0 × 105 a.u). It was learned that the 1D nanofiber had low recombination compare to other dimension materials, which explained why it has high photocatalytic hydrogen evolution rate. This photoluminescence results explained that 1D nanofibers have the ability to transmit electron faster relative free nanoparticle system. This results also indicates that the STO-NPs have much more defect density, or it is due to the different disorder or defects located at different crystal planes.

Conclusions for section 3 Water splitting is an important technique to convert solar energy into a potential chemical energy form of hydrogen, which is considered a clean energy carrier. SrTiO3 is one of the most promising photocatalysts for hydrogen production from water splitting. In this study, an electrospinning technique in combination with sol- gel method was used to synthesize 1D STO-NFs. The crystal growth behaviors of SrTiO3, including diameters of the nanofibers and crystallite size, were studied under different pre-cursor concentrations and calcination temperatures. The crystallite size of SrTiO3 decreased with increasing precursor concentrations (3.03–15.78 vol.%), but it increased with increasing calcination temperatures in the range of 600–800◦C. The photocatalytic activity of different STO-NFs (based on crystallite size) was also evaluated and showed that the H2 production rate increased with increasing STO- NFs crystallite size for both calcination temperature and pre-cursor concentration cases. In this study, the ideal conditions to synthesize STO-NFs were found to be 800°C calcination temperature and 5.88 vol. % DSRT concentration based on photocatalytic activity. The H2 production rate of Pt/STO-NFs was 1.14 mmol/g-h, which was approximately 2 times higher than Pt/STO-NPs. This synthesis method

92 could be used to fabricate other inorganic photocatalytic 1D structure materials [171]. Structural shape and size of the photocatalysts have significant effect on its photoactivity [185].

CONCLUSION

1. A core-shell nanostructure of TiO2@SWCNT ( semiconductive type, electron lifetime τ = 10.8 ms or diffusion length) provided larger electron life time than that of TiO2@MWCNT( metallic property, lifetime τ = 0.108 ms). For the First time core-shell nanostructured titania with a different property of MWCN and SWCNT and their Fundamental physics of electron transport mechanisms were investigated for DSSCs. It is first time to confirm that SWCNT and MWCNT lifetimes for DSSCs and found that Electron transport is faster in a 1D core-shell nanostructure comparing to typical titanium dioxide anode materials in DSSC;

2. As a result, a study of the 1D core-shell nanocrystal shows that the diameter of the shelf nanocrystals was between 50 nm and 100 nm. These core-shell nanostructurs were fabricated by a uniquely developed technique which is a combination of two methods: sol-gel, and electrospinning. In this technique, unique two syringe electrospinning method was developed to produce core-shell nanostructured nanofiber for DSSC study. A High open-circuit voltage of the solar cell was achieved as a result of the use of the core-shell method.

3. 1D SrTiO3 nanostructures were fabricated using a single precursor by using the method of electrospinning and sol-gel method for the first time. In the first time, crystal growth mechanism for assembled SrTiO3 crystals was investigated and it is found that the precursor concentration and calcination temperature are crucial to obtain assembled strontium titanate nanocrsytals with different sizes. This techniques has a potential to scale-up production.

4. As a result of the experiment, it was established that the increase in the size of the crystal of the strontium titanium oxide increases the hydrogen splitting volume. For the first time, the concentration of a precursor of strontium has a significant effect on the efficiency of hydrogen evolution.

REFERENCES

1 Md Moniruddin, Baurzhan Ilyassov, Xiao Zhao, Eric Smith, Timur Serikov, Niyazbek Ibrayev, Ramazan Asmatulu, Nurxat Nuraje. Recent progress on perovskite materials in photovoltaic and water splitting applications//Material Today Energy.-2017, Vol. 7 –P. 246-259 2 O’Regan B. and Gratzel M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films/ // Nature. – 1991. – Vol. 353(6346). – P. 737–740. 3 Worldometers // http://www.worldometers.info/world-population/ 4 U.S. Energy Information Administration, International Energy Outlook 2017, September 2017, https://www.eia.gov/outlooks/ieo/. 5 IEA (International energy agency)// https://www.iea.org/weo2017/ 6 Nuraje N. Green photo-active nanomaterials // published by RSC, UK. – 2016. – P.14-30. 7 Solar photovoltaic electricity empowering the world: https://www.greenpeace.org/archive-international/en/publications/reports/Solar- Generation-6/ 8 Crabtree, G.W. and Lewis, N.S., Solar Energy Conversion // Physics Today. – 2007. – Vol. 60(3). – P.37. 9 John A. Duffie, William A. Beckman. Solar Engineering of Thermal Processes( Fourth edition). Wiley. – 2013. 10 The Sun as energy source: http://klimat.czn.uj.edu.pl/enid/Climate_Change_classes_ss/ss_Energy_from_the_ Sun_6ev.html 11 Moniruddina Md., Ilyassov Baurzhan, Seliverstova Evgeniya, Bakranov Nurlan, Ibrayev Niyazbek, Nuraje Nurxat. Bioinspired study of energy and electron transfer in photovoltaic system // Journal of Experimental Nanoscience. Taylor & Francis. – 2017. – P. 1 – 12. 12 Hagfeldt A. and Gratzel M. Molecular photovoltaics // Accounts of Chemical Research. – 2000. – Vol. 33(5). – P. 269–277. 13 Gratzel M. Photoelectrochemical cells // Nature. – 2001. – Vol. 414(6861). – P. 338–344. 14 Bach U., Lupo D., Comte P., Moser J. E., Weissortel F., Salbeck J., Spreitzer H. and Gratzel M. Solid-state dye sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies // Nature. – 1998. – Vol. 395(6702). – P. 583–585. 15 He H., Zhang C., Liu T., Cao Y., Wang N. and Guo Z. Thermoelectric- photoelectric composite nanocables induced a larger efficiency in dye-sensitized solar cells // Journal of Materials Chemistry A. – 2016. – Vol. 4 (24). – P. 9362– 9369. 16 Liu T., Hou J., Wang B., Bai F., Chen H., Gao L., Cao Y., He H., Wang J., Wang N., Cao G. and Guo Z. Correlation between the in-plain substrate strain

95 and electrocatalytic activity of strontium ruthenate thin films in dye-sensitized solar cells // Journal of Materials Chemistry A. – 2016. – Vol. 4 (28). – P. 10794–10800. 17 Chen H., Liu T., Ren J., He H., Cao Y., Wang N. and Guo Z. Synergistic carbon nanotube aerogel - Pt nanocomposites toward enhanced energy conversion in dye-sensitized solar cells // Journal of Materials Chemistry A. – 2016. – Vol. 4(9). – P. 3238–3244. 18 Yella A., Lee H.-W., Tsao H. N., Yi C., Chandiran A. K., Nazeeruddin M. K., Diau E.W.-G., Yeh C.-Y., Zakeeruddin S. M., and Gr¨atzel M. Porphyrin- sensitized solar cells with cobalt (II/III)–based redox electrolyte exceed 12 percent efficiency // Science. – 2011. – Vol. 334(6056). – P. 629. 19 Baruah S. and Dutta J. Hydrothermal growth of ZnO nanostructures // Science and Technology of Advanced Materials. – 2009. – Vol. 10(1). – P. 013001. 20 National renewable energy laboratory // https://www.nrel.gov/pv/national-center-for-photovoltaics.html 21 Ardo S. and Meyer G. J. Photodriven Heterogeneous Charge Transfer with Transition Metal Compounds Anchored to TiO2 Semiconductor Surfaces // Chem. Soc. Rev. – 2009. – Vol.38. – P.115–164. 22 Koops S. E., O’Regan B. C., Barnes P. R. F., and Durrant J. R. Parameters Influencing the Efficiency of Electron Injection in Dye-Sensitized Solar Cells // J. Am. Chem. Soc. – 2009. – Vol. 131(13). – P 4808–4818. 23 Feldt S. M., Lohse P. W., Kessler F., Nazeeruddin M. K., Gratzel M., Boschloo G., and Hagfeldt A. Regeneration and Recombination Kinetics in Cobalt Polypyridine Based Dye-sensitized Solar Cells, Explained using Marcus Theory // Phys. Chem. Chem. Phys., – 2013. – Vol. 15. – P. 7087–7097. 24 Ilyassov B., Ibrayev N., and Nuraje N. Hierarchically assembled nanostructures and their photovoltaic properties // Materials Science in Semiconductor Processing. – 2015. – Vol. 40. – P. 885–889. 25 Kang S. H., Choi S. H., Kang M. S., Kim J. Y., Kim H. S., Hyeon T., and Sung Y. E. Nanorod-based dye-sensitized solar cells with improved charge collection efficiency // Advanced Materials. – 2008. – Vol. 20(1). – P. 54–58. 26 Aravind Kumar Chandiran, Mojtaba Abdi-Jalebi, Mohammad K. Nazeeruddin, and Michael Gratzel // Analysis of Electron Transfer Properties of ZnO and TiO2 Photoanodes for Dye-Sensitized Solar Cells // ACS NANO. – 2014. – Vol. 8(3). – P. 2261–2268. 27 Шабдан Е., Н. Нураже, К.К. Диханбаев. структурные и оптические свойства диоксида титана (TiO2), полученного магнетронным распылением в вакууме // Vestnik KazNRTU, – 2017. – Vol. N3 (121). – P. 634-638. 28 Hagfeldt A., and Grätzel M. // Chem. Rev. – 1995. – V49. – P 95. 29 Downs R. T., Hall-Wallace M., The American Mineralogist Crystal Structure Database // American Mineralogist. – 2003. – Vol. 88. – P 247-250. 30 Agrell H. G., Boschloo G., and Hagfeldt A. // Phys. Chem. B. – 2004. – Vol. 108. – P 12388. 31 Nazeeruddin Md. K., Zakeeruddin S. M., Humphry-Baker R., Jirousek M., Liska P., Vlachopoulos N., Shklover V., Fischer C. H., and Gratzel M. Acid- 96 base Equilibria of (2, 2 ’ -bipyridyl-4, 4 ’ -dicarboxylic acid) Ruthenium (II) Complexes and the Effect of Protonation on Charge-transfer Sensitization of Nanocrystalline Titania // Inorg. Chem. – 1999. – Vol. 38(26). – P 6298–6305. 32 Pechy P., Renouard T., Zakeeruddin S. M., Humphry-Baker R., Comte P., Cevey L., Liska P., Costa E., Shklover V., Spiccia L., Deacon B. G., Bignozzi C. A., and Gratzel M. Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO2-based Solar Cells // J. Am. Chem. Soc. – 2001. – Vol. 123(8). – P 1613–1624. 33 Gopal K. Mor, James Basham, Maggie Paulose, Sanghoon Kim, Oomman K. Varghese, Amit Vaish, Sorachon Yoriya and Craig A. Grimes. High- Efficiency Förster Resonance Energy Transfer in Solid-State Dye Sensitized Solar Cells // Nano Letters. – 2010. – Vol. 10 (7). – P 2387–2394. 34 A. Zaban, M. Greenshtein, J. Bisquert. ChemPhysChem. 4 (2003) 859. 35 Sacco A., Lamberti A., Quaglio M., Bianco S., Tresso E., Alexe- Ionescu A.L., Pirri C.F. Int. J. Photoenergy. – 2012. – Vol. 2012. – P 216780. 36 Bisquert J., Garcia-Belmonte G., Fabregat-Santiago F., Ferriols N. S., Bogdanoff P., Pereira E. C.. J. Phys. Chem. B. – 2000. – Vol.104. – P 2287. 37 Grätzel M. // Inorg. Chem. – 2005. – Vol. 44. – P 6841. 38 Ahmed M. El-Zohry. Exploring Organic Dyes for Grдtzel Cells Using Time-Resolved Spectroscopy. PhD thesis, Uppsala University, – 2015. 39 Жаңабаев З., Диханбаев Қ.К., Шабдан Е., Джунсбеков А.С., Оразова А.А. Титан қостотығы негізінде жасалған жұқа қабатты күн элементі //«Chaos and structures in nonlinear systems. Theory and experiment», Proceedings of the 10th international scientific conference, ISBN 978-601-04-2695-5. Almaty, June 16-18, – 2017. – P.169-172. 40 Bak T., Nowotny J., Rekas M., Sorrell C.C. Photo-electrochemical hydrogen generation from water using solar energy // Materials-related aspects, Int. J. Hydrogen Energy. – 2002. – Vol. 27. – P. 991 - 1022. 41 Kamat P.V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion // The Journal of Physical Chemistry C. – 2007. – Vol.111. – P. 2834 - 2860. 42 Roger I., Shipman M.A., Symes M.D. Earth-abundant catalysts forelectrochemical and photoelectrochemical water splitting // Nat. Rev. Chem. – 2017. – Vol. 1. – P. 0003. 43 Moniruddin M., Kudaibergenov S., Nuraje N. Chapter 7 hierarchicalnanoheterostructures for water splitting, in: green photo- activenanomaterials: sustainable energy and environmental remediation // R. Soc.Chem. – 2016. – P.142–167. 44 Chen X., Shen S., Guo L., Mao S.S. Semiconductor-based photocatalytichydrogen generation // Chem. Rev. – 2010. – Vol. 110. – P. 6503 – 6570. 45 Gratzel M. Photoelectrochemical cells // Nature. – 2001. – Vol. 414. – P. 338–344.

46 Higashi M., Abe R., Ishikawa A., Takata T., Ohtani B., Domen K. Z- schemeoverall water splitting on modified-TaON photocatalysts under visible light(<500 nm) // Chem. Lett. – 2008. – Vol.37. – P. 138 – 139. 47 Iwase A., Saito K., Kudo A. Sensitization of NaMO3 (M: Nb and Ta) photocatalysts with wide band gaps to visible light by Ir doping // Bull. Chem.Soc. Jpn. – 2009. – Vol.82. – P. 514 – 518. 48 Islam S.Z., Reed A., Wanninayake N., Kim D.Y., Rankin S.E. Remarkable enhancement of photocatalytic water oxidation in N2/Ar plasma treated, mesoporous TiO2 films // J. Phys. Chem. C – 2016. – Vol.120. – P. 14069 – 14081. 49 Islam S.Z., Rankin S.E. Hydrazine-based synergistic Ti(III)/N doping of surfactant-templated TiO2 thin films for enhanced visible light photocatalysis // Mater. Chem. Phys. – 2016. – Vol. 182. – P. 382 – 393. 50 Hou D., Hu X., Ho W., Hu P., Huang Y. Facile fabrication of porous Cr-doped SrTiO3 nanotubes by electrospinning and their enhanced visible-light- driven photocatalytic properties // J. Mater. Chem. A. – 2015. – Vol. 3. – P. 3935– 3943. 51 Foley Julie. H2: driving the future // IPPR. ISBN - 1-86030-176-2. – 2001. 52 Velu Subramani, Angelo Basile and T. Nejat Veziroglu. Compendium of hydrogen energy// WP. Elsevier. ISBN 978-1-78242-383-6. – 2015. 53 Kudo Akihiko and Miseki Yugo. Heterogeneous photocatalyst materials for water splitting // Chem. Soc. Rev. – 2009. – Vol. 38. – P 253–278. 54 Fujishima A., Honda K. Electrochemical photolysis of water at a semiconductor electrode // Nature. – 1972. – Vol. 238. – P. 37 – 38. 55 Youngblood W.J., Lee S.H.A., Maeda K., Mallouk T.E. Visible light watersplitting using dye-Sensitized oxide semiconductors // Acc. Chem. Res. – 2009. – Vol. 42. – P. 1966 – 1973. 56 Maeda K., Eguchi M., Lee S.-H.A., Youngblood W.J., Hata H., Mallouk T.E. Photocatalytic hydrogen evolution from hexaniobate nanoscrolls and calciumniobate nanosheets sensitized by Ruthenium(II) bipyridyl complexes // J. Phys.Chem. C. – 2009. – Vol. 113. – P. 7962 – 7969. 57 Islam S.Z., Reed A., Kim D.Y., Rankin S.E. N2/Ar plasma induced doping of ordered mesoporous TiO2 thin films for visible light active photocatalysis // Microporous Mesoporous Mater. – 2016. – Vol. 220. – P. 120 – 128. 58 Wang J., Li H., Li H., Yin S., Sato T. Preparation and photocatalytic activity of visible light-active sulfur and nitrogen co-doped SrTiO3 // Solid State Sci. – 2009. – Vol.11. – P. 182 – 188. 59 Iwashina K., Kudo A. Rh-doped SrTiO3 photocatalyst electrode showing cathodic photocurrent for water splitting under visible-light irradiation // J.Am. Chem. Soc. – 2011. – Vol. 133. – P. 13272 – 13275. 60 Asai R., Nemoto H., Jia Q., Saito K., Iwase A., Kudo A. A visible light respons iverhodium and antimony-codoped SrTiO3 powdered photocatalyst loaded withan IrO2 cocatalyst for solar water splitting // Chem. Commun. – 2014. – Vol. 50. – P. 2543 – 2546.

61 Chen W., Liu H., Li X., Liu S., Gao L., Mao L., Fan Z., Shangguan W., Fang W., Liu Y. Polymerizable complex synthesis of SrTiO3:(Cr/Ta) photocatalysts to improve photocatalytic water splitting activity under visible light // Appl. Catal. B: Environ. – 2016. – Vol. 192. – P. 145–151. 62 Kim H., Hwang D., Kim Y., Lee J. Highly donor-doped (110) layered perovskite materials as novel photocatalysts for overall water splitting // Chem. Commun. –1999. – P. 1077–1078. 63 Wang Q., Hisatomi T., Jia Q., Tokudome H., Zhong M., Wang C., Pan Z., Takata T., Nakabayashi M., Shibata N., Li Y., Sharp I.D., Kudo A., Yamada T., Domen K. Scalable water splitting on particulate photocatalyst sheets with a solar- to-hydrogen energy conversion efficiency exceeding 1% // Nat. Mater. – 2016. – Vol. 15. – P. 611–615. 64 Iwase A., Ng Y.H., Ishiguro Y., Kudo A., Amal R. Reduced graphene oxide as a solid-state electron mediator in Z-scheme photocatalytic water splitting under visible light // J. Am. Chem. Soc. – 2011. – Vol. 133. – P. 11054–11057. 65 Sayama K., Mukasa K., Abe R., Abe Y., Arakawa H. A new photocatalytic watersplitting system under visible light irradiation mimicking a Z- scheme mechanism in photosynthesis // J. Photochem. Photobiol. A: Chem. – 2002. – Vol. 148. – P. 71–77. 66 Matsumoto Y., Unal U., Tanaka N., Kudo A., Kato H. Electrochemical approach to evaluate the mechanism of photocatalytic water splitting on oxide photocatalysts // J. Solid State Chem. – 2004. – Vol. 177. – P. 4205–4212. 67 Vladimir Pokropivny, Rynno Lohmus, Irina Hussainova, Alex Pokropivny, Sergey Vlassov. Introduction to nanomaterials and nanotechnology. Tartu. – 2007. – P 192. 68 Yury Gogotsi. Nanomaterials handbook. Yaylor & Francis. – 2006. 69 Gleiter H. // Acta. Mater. – 2000. – Vol.48:1. 70 Skorokhod V, Ragulya A, Uvarova I. Physico-chemical kinetics in nanostructured systems. Kyiv: Academperiodica; 2001. p. 180. – . – Vol.– P 71 Pokropivny VV, Skorokhod VV // Mater Sci. Eng. C. – 2007. – Vol. 27. – P 990. 72 Brennan, kevin F. The physics of semiconductor. Cambridge University Press. New York, NY. – 1997. 73 Schubert. Quantum mechanics applied to semiconductor devices. 74 Neudeck and Pierret. Advanced semiconductor fundamentals. Peterson education, Inc. Upper Saddle River, NJ. – 2003. 75 Van Zeghbroeck, Bart J. Density of States Calculation. 76 Iijima S. // Nature. – 1991. – Vol.354. – P 56. 77 Kuchibhatla SVNT, Karakoti AS, Bera D, Seal S // Prog Mater Sci. – 2007. – Vol. 52. – P 699. 78 Spanier, Jonathan. "One-Dimensional Semiconductor and Oxide Nanostructures", Nanomaterials Handbook, – 2006.

79 GuolongWang, DemeiYu, Ajit D.Kelkar, LifengZhang. Electrospun nanofiber: Emerging reinforcing filler in polymer matrix composite materials // Progress in Polymer Science. – 2017. – Vol.75. – P 73-107. 80 Cao Guozhong. Wang Ying. Nanostructures and Nanomateials: Synthesis, Properies and Applications // World Scientific Publishing Company. – 2011. – P 596. 81 Edward L. Wolf. Nanophysics and Nanotechnology. Wily-VCH. ISBN: 978-3-527-61898-9. – 2008. – P 308. 82 Huang Y., Duan X., Wei Q., and Lieber C. M. Directed assembly of one-dimensional nanostructures into functional networks // Science. – 2001. – Vol. 291(5504). – P. 630. 83 Xia Y., Yang P., Sun Y., Wu Y., Mayers B., Gates B., Yin Y., Kim F., and Yan H. One-dimensional nanostructures: Synthesis, characterization, and applications // Advanced Materials. – 2003. – Vol. 15(5). – P. 353–389. 84 Taylor G. Electrically driven jets // Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences. – 1969. – Vol. 313(1515). – P. 453. 85 Tan B and Wu YY. Dye-sensitized solar cells based on anatase TiO2 nanoparticle/nanowire composites // Journal of Physical Chemistry B. – 2006. – Vol. 110(32). – P 15932–15938. 86 Wang ZS, Kawauchi H, Kashima T and Arakawa H. Significant influence of TiO2 photoelectrode morphology on the energy conversion efficiency of N719 dye-sensitized solar cell. Coordination Chemistry Reviews. – 2004. – Vol. 248(13–14). – P 1381–1389. 87 Gratzel M. Photoelectrochemical cells // Nature. – 2001. – V. 414(6861). – P 338–344. 88 Gratzel M. Solar energy conversion by dye-sensitized photovoltaic cells // Inorganic Chemistry. – 2005. – Vol. 44(20). – P 6841–6851. 89 Kamat PV. Quantum dot solar cells. Semiconductor nanocrystals as light harvesters // Journal of Physical Chemistry C. – 2008. – Vol. 112(48). – P 18737–18753. 90 Hotchandani S and Kamat PV. Charge-transfer processes in coupled semiconductor systems – photochemistry and photoelectrochemistry of the colloidal CdS–ZnO system // Journal of Physical Chemistry. – 1992. – Vol. 96(16). – P 6834– 6839. 91 Seabold JA, Shankar K, Wilke RHT et al. Photoelectrochemical properties of heterojunction CdTe/TiO2 electrodes constructed using highly ordered TiO2 nanotube arrays // Chemistry of Materials. – 2008. – Vol. 20(16). – P 5266– 5273. 92 Vlachopoulos N, Liska P, Augustynski J and Gratzel M.Very efficient visible-light energy harvesting and conversion by spectral sensitization of high surface-area polycrystalline titanium-dioxide films. Journal of the American Chemical Society. – 1988. – Vol. 110(4). – P 1216–1220.

100

93 Mor G K, Shankar K, Paulose M, Varghese OK and Grimes CA. Use of highly-ordered TiO2 nanotube arrays in dyesensitized solar cells // Nano Letters. – 2006. – Vol. 6(2). – P 215–218. 94 Enache-Pommer E, Boercker JE and Aydil ES. Electron transport and recombination in polycrystalline TiO2 nanowire dye-sensitized solar cells // Applied Physics Letters. – 2007. – Vol. 91(12). – P123116. 95 Jiu JT, Isoda S, Wang FM and Adachi M. Dye-sensitized solar cells based on a single-crystalline TiO2 nanorod film // Journal of Physical Chemistry B. – 2006. – Vol. 110(5). – P 2087–2092. 96 Law M, Greene LE, Radenovic A et al. ZnO–Al2O3 and ZnO–TiO2 core-shell nanowire dye-sensitized solar cells // Journal of Physical Chemistry B. – 2006. – Vol. 110(45). – P 22652–22663. 97 Kongkanand A, Tvrdy K, Takechi K, Kuno M and Kamat PV. Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe–TiO2 architecture // Journal of the American Chemical Society. – 2008. – Vol.130 (12). – P 4007–4015. 98 Bang JH and Kamat PV. Solar cells by design: Photoelectrochemistry of TiO2 nanorod arrays decorated with CdSe // Advanced Functional Materials. – 2010. – Vol. 20(12). – P 1970–1976. 99 Gubbala S, Chakrapani V, Kumar V and Sunkara MK. Band-edge engineered hybrid structures for dye-sensitized solar cells based on SnO2 nanowires // Advanced Functional Materials. – 2008. – Vol. 18(16). – P 2411–2418. 100 Law M, Greene LE, Johnson JC, Saykally R and Yang PD. Nanowire dye-sensitized solar cells // Nature Materials. – 2005. – Vol.4(6). – P 455–459. 101 Chen XY, Yip CT, Fung MK, Djurisic AB and Chan WK. GaN- nanowire-based dye-sensitized solar cells // Applied Physics a-Materials Science & Processing. – 2010. – Vol. 100(1). – P 15–19. 102 Han, Wang and Ho. One-dimensional nanostructured materials for solar energy harvesting // ICE. – 2011. – P.1-14. 103 Maugh TH. Two new routes to solar hydrogen // Science. – 1982. – Vol. 218(4572). – P 557. 104 Li Y and Zhang JZ. Hydrogen generation from photoelectrochemical water splitting based on nanomaterials // Laser & Photonics Reviews. – 2010. – Vol.4(4). – P 517–528. 105 Shankar K, Basham JI, Allam NK et al. () Recent advances in the use of TiO2 nanotube and nanowire arrays for oxidative photoelectrochemistry. Journal of Physical Chemistry C). – 2009. – Vol. 113(16). – P 6327–6359. 106 105 Paulose M, Shankar K, Yoriya S et al. Anodic growth of highly ordered TiO2 nanotube arrays to 134 μm in length. Journal of Physical Chemistry B. – 2006. – Vol.110(33). – P16179–16184. 107 Jin Z, Zhang X, Li Y, Li S and Lu G. 5.1% Apparent quantum efficiency for stable hydrogen generation over eosin-sensitized CuO/TiO2 photocatalyst under visible light irradiation // Catalysis Communications– 2007. – Vol. 8(8). – P 1267– 1273.

101

108 Wang GM, Yang XY, Qian F, Zhang JZ and Li Y. Doublesided CdS and CdSe quantum dot co-sensitized ZnO nanowire arrays for photoelectrochemical hydrogen generation // Nano Letters. – 2010. – Vol. 10(3). – P 1088–1092. 109 Yang XY, Wolcott A, Wang GM, Sobo A, Fitzmorris RC, Qian F, Li Y and Zhang JZ. Nitrogen-doped ZnO nanowire arrays for photoelectrochemical water splitting // Nano Letters. – 2009. – Vol.9(6). – P 2331–2336. 110 Vayssieres L, Sathe C, Butorin SM, Shuh DK, Nordgren J and Guo JH. One-dimensional quantum-confinement effect in α-Fe2O3 ultrafine nanorod arrays. Advanced Materials. – 2005. – Vol.17 (19). – P 2320–2323. 111 Zhao X, Wang P and Li B. CuO/ZnO core/shell heterostructure nanowire arrays: Synthesis, optical property, and energy application // Chemical Communications. – 2010. – Vol. 46(36). – P 6768–6770 112 Chakrapani V, Thangala J and Sunkara MK. WO3 and W2N nanowire arrays for photoelectrochemical hydrogen production // International Journal of Hydrogen Energy. – 2009. – Vol. 34(22). – P 9050–9059. 113 Aryal K, Pantha BN, Li J, Lin JY and Jiang HX. Hydrogen generation by solar water splitting using p-InGaN photoelectrochemical cells // Applied Physics Letters. – 2010. – Vol. 96(5). – P 052110. 114 Seungho Cho, Ji-Wook Jang, Kun-Hong Lee, Jae Sung Lee. Research Update: Strategies for efficient photoelectrochemical water splitting using metal oxide photoanodes // APL Materials. –2014. 115 Lei Y., Asmatullu R. and Nuraje N.// ScienceJet. – 2015. – Vol. 4, – P 169-173. 116 Nuraje N., Kudaibergenov S and Asmatullu R. in Production of Fuels Using Nanomaterials, ed. Luque R. and Galu A. M., Taylor and Francis. – 2013. – Vol. – P 95-117. 117 Danks A. E., Hall S. R. and Schnepp Z. The evolution of ‘sol–gel’ chemistry as a technique for materials synthesis. // Mater. Horiz. – 2016. – Vol. 3. – P. 91-112. 118 Brinker C., Scherer George. Sol-Gel Science (1st Edition). The Physics and Chemistry of Sol-Gel Processing. // Academic Press. – 2013. – P 912. 119 https://en.wikipedia.org/wiki/Sol-gel 120 Lu Y., Huang J., Yu G., Cardenas R., Wei S., Wujcik E. K., and Guo Z. Coaxial electrospun fibers: Applications in drug delivery and tissue engineering // Wiley Interdisciplinary Reviews: Nanomedicine and Nanobiotechnology. – 2016. – Vol. 8(5). – P.654–677. 121 Qu H., Wei S., and Guo Z. Coaxial electrospun nanostructures and their applications // Journal of Materials Chemistry A. – 2013. – Vol.1 (38). – P. 11513– 11528. 122 Zhang L., Zhang Q., Xie H., Guo J., Lyu H., Li Y., Sun Z., Wang H., and Guo Z. Electrospun titania nanofibers segregated by graphene oxide for improved visible light photocatalysis // Applied Catalysis B: Environmental. – 2017. – Vol. 201. – P.470–478.

102

123 Huang J., Cao Y., Huang Z., Imbraguglio S. A., Wang Z., Peng X., and Guo Z. Comparatively thermal and crystalline study of poly(methyl- methacrylate)/polyacrylonitrile hybrids: Core–shell hollow fibers // Porous Fibers, and Thin Films, Macromolecular Materials and Engineering. – 2016. – Vol. 301(11). – P. 1327–1336. 124 Zeleny J. Instability of electrified liquid surfaces // Physical Review. – 1917. – Vol. 10(1). – P. 1–6. 125 Li D. and Xia Y. Fabrication of titania nanofibers by electrospinning // Nano Letters. – 2003. – Vol. 3(4). – P. 555–560. 126 Dang X., Yi H., Ham M.-H., Qi J., Yun D. S., Ladewski R., Strano M. S., Hammond P. T., and Belcher A. M. Virus templated self-assembled single- walled carbon nanotubes for highly efficient electron collection in photovoltaic devices // Nat Nano. – 2011. – Vol. 6(6). – P. 377–384. 127 Hench L L and West J K. Chemical processing of advanced materials, Wiley, New York. – 1992. 128 Joanna Gatford. The New Zealand Institute for Plant and Food Research Ltd. 2008. https://commons.wikimedia.org/wiki/File:Electrospinning_Diagram.jpg. Or https://en.wikipedia.org/wiki/Electrospinning. 129 Darrell H. Reneker, Alexander L. Yarin. Electrospinning jets and polymer nanofibers. // Polymer. – 2008. – Vol. 49. – P. 2387-2425. 130 Sangamesh Kumbar, Cato Laurencin, Meng Deng. Natural and Synthetic Biomedical Polymers. // Elsevier. – 2014. – P. 420. 131 Viness Pillay, Clare Dott, Yahya E. Choonara, Charu Tyagi, Lomas Tomar, Pradeep Kumar, Lisa C. du Toit, and Valence M. K. Ndesendo. A Review of the Effect of Processing Variables on the Fabrication of Electrospun Nanofibers for Drug Delivery Applications. // Journal of Nanomaterials. – 2013. – Vol. 2013. – P. 22. 132 Adnan Haider, Sajjad Haider, Inn-Kyu Kang. A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology. // Arabian Journal of Chemistry. – 2015. 133 Ahmad Aqel, Kholoud M.M. Abou El-Nour, Reda A.A. Ammar, Abdulrahman Al-Warthan. Carbon nanotubes, science and technology part (I) structure, synthesis and characterization // Arabian Journal of Chemistry. – 2012. – Vol. 5. – P 1-23. 134 Carbon nanotubes: https://en.wikipedia.org/wiki/Carbon_nanotube 135 Silvestre J., Silvestre N. & de Brito J. Review on concrete Nanotechnology //European Journal of Environmental and Civil Engineering. – 2016. – Vol. 20(4). 136 Yang L. and Leung W.W.-F. Application of a bilayer TiO2 nanofiber photoanode for optimization of dye-sensitized solar cells // Advanced Materials. – 2011. – Vol. 23(39). – P. 4559–4562. 137 Varghese O. K., Paulose M., and Grimes C. A. Long vertically aligned

103 titania nanotubes on transparent conducting oxide for highly efficient solar cells // Nat Nano. – 2009. – Vol. 4(9). – P. 592–597. 138 Lv M., Zheng D., Ye M., Xiao J., Guo W., Lai Y., Sun L., Lin C., and Zuo J. Optimized porous rutile TiO2 nanorod arrays for enhancing the efficiency of dye-sensitized solar cells // Energy and Environmental Science. – 2013. – Vol. 6(5). – P. 1615–1622. 139 Iijima S. and Ichihashi T. Single-shell carbon nanotubes of 1-nm diameter // Nature. – 1993. – Vol.363(6430). – P. 603–605. 140 Nowotny, Janusz, Oxide Semiconductors for Solar Energy Conversion: Titanium Dioxide // CRC Press. – 2011. – Vol. – P 156. 141 Yang Mi and Yuxiang Weng, Band Alignment and Controllable Electron Migration between Rutile and Anatase TiO2 // Scientific Reports. – 2015. – Vol. 5. – P 11482. 142 Yang L. and Leung W. W.-F. Electrospun TiO2 nanorods with carbon nanotubes for efficient electron collection in dye-sensitized solar cells // Advanced Materials. – 2013. – Vol. 25(12). – P.1792–1795. 143 Ito S., Murakami T. N., Comte P., Liska P., Gratzel C., Nazeeruddin M. K., and Gr¨atzel M. Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10% // Thin Solid Films. – 2008. – Vol. 516(14). – P. 4613–4619. 144 Shabdan Yerkin, Hanford Blake, Temirbayev Amirkhan, Dikhanbayev Kadyrjan, Nuraje Nurxat. Study of electron transport mechanism in Dye-sensitized solar cell with the effect of morphology, crystalline structure and electron mobility // The 2016 AIChE annual meeting, section: nanostructured / thin film photovoltaics.ISBN: 978-1-5108-3420-0. San Francisco. November 14. – 2016. – P. 151. 145 Shabdan Y., Dikhanbayev, K., Nuraje N. Study of Electron Transport Mechanism for Engineered Carbon Nanotube/TiO2 Nanofibers by Electrospinning // INESS 2017, 5th International Conference – on nanomaterials and Advanced Energy Storage Systems, Nazarbayev University, Astana, August 9-11, – 2017. – P 53. 146 Shabdan Y., Ronasi A., Coulibaly P., Moniruddin M. and Nuraje N. Engineered core-shell nanofibers for electron transport study in dye-sensitized solar cells // AIP Advances. – 2017. – Vol. 7. – P 065001-065008. 147 Argast, Anne and Tennis, Clarence F. A web resource for the study of alkali feldspars and perthitic textures using light microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy // Journal of Geoscience Education.– 2004. III. – Vol. 52 (3). – P 213-217. 148 Handbooks of Monochromatic XPS Spectra - Fully Annotated, PDF of Volumes 1 and 2, B.V.Crist, published by XPS International LLC, Mountain View, CA, USA– 2005. 149 Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, ed. J.T.Grant and D.Briggs, published by IM Publications, Chichester, UK. – 2003. 150 https://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy

104

151 Kern R., Sastrawan R., Ferber J., Stangl R., and Luther J. Modeling and interpretation of electrical impedance spectra of dye solar cells operated under open- circuit conditions // Electrochimica Acta . – 2002. – Vol. 47(26). – P.4213–4225. 152 Bisquert J., Fabregat-Santiago F., Mora-Ser´o I., Garcia-Belmonte G., and Gim´enez S. Electron lifetime in dye-sensitized solar cells: Theory and interpretation of measurements // The Journal of Physical Chemistry C. – 2009. – Vol. 113(40). – P.17278–17290. 153 Alippi C. A unique timely moment for embedding intelligence in applications // CAAI Transactions on Intelligence Technology. – 2016. – Vol. 1(1). – P. 1–3. 154 Jin H., Chen Q., Chen Z., Hu Y., and Zhang J. Multi-Leap Motion sensor based demonstration for robotic refine tabletop object manipulation task,” CAAI Transactions on Intelligence Technology. – 2016. – Vol. 1(1). – P. 104–113. 155 Zhang X., Gao H., Guo M., Li G., Liu Y., and Li D. A study on key technologies of unmanned driving // CAAI Transactions on Intelligence Technology. – 2016. – Vol. 1(1). – P. 4–13. 156 Nuraje N., Su K. Perovskite ferroelectric nanomaterials // Nanoscale. 2013. – Vol. 5. – P. 8752–8780. 157 Minegishi T., Nishimura N., Kubota J., Domen K. Photoelectrochemical properties of LaTiO2N electrodes prepared by particle transfer for sunlight-driven water splitting // Chem. Sci. – 2013. – Vol.4. – P. 1120–1124. 158 Li Y., Takata T., Cha D., Takanabe K., Minegishi T., Kubota J., Domen K. Vertically aligned Ta3N5 nanorod arrays for solar-Driven photoelectrochemical water splitting // Adv. Mater. – 2013. – Vol. 25. – P. 125–131. 159 Hierarchical Nanostructures for water splitting, in: N. Nuraje, R. Asmatulu, G.Mul (Eds.) Green photoactive nanomaterials: Sustainable energy and Environmental Remediation, – 2016. 160 Fabian D.M., Hu S., Singh N., Houle F.A., Hisatomi T., Domen K., Osterloh F.E., Ardo S. Particle suspension reactors and materials for solar-driven watersplitting // Energy Environ. Sci. – 2015. – Vol. 8. – P. 2825–2850. 161 Шабдан Е., Диханбаев Г., Серик Ж. солнечно-водородная энергетика // Венгрии, the scientific heritage. ISSN 9215-0365, –2017. – Vol.17. –P. 44-49. 162 Leapman R. D., Grunes L. A., And Fejes P. L., Phys. Rev. B. – 1982. – Vol. 26. – P 614 – 635. 163 Nuraje N., Dang X., Qi J., Allen M.A., Lei Y., Belcher A.M. Biotemplated synthesis of perovskite nanomaterials for solar energy conversion // Adv.Mater. – 2012. – Vol.24. – P. 2885–2889. 164 Konta R., Ishii T., Kato H., Kudo A. Photocatalytic activities of noble metal ion doped SrTiO3 under visible light irradiation // J. Phys. Chem. B. – 2004. – Vol.108. – P. 8992–8995. 165 Moniruddin Md., Nuraje Nurxat. Co-catalysts Incorporated Perovskites Nanofiber for Water Splitting photocatalysis. AIChE 2016. P 260.

105

166 Dean, John A. The Analytical Chemistry Handbook. New York: McGraw Hill, Inc. ISBN 0-07-016197-6. – 1995. – Vol. – P15.1–15.5. 167 Pungor, Erno. A Practical Guide to Instrumental Analysis. Florida: Boca Raton. – 1995. – Vol. – P181–191 168 Skoog, Douglas A., F. James Holler and Timothy Nieman. Principles of Instrumental Analysis (5 ed.) // New York. ISBN 0-03-002078-6. – 1998. – Vol. – P 805–808. 169 https://sites.google.com/a/iastate.edu/laboratory-10- thermogravimetric-analysis/home 170 Moniruddin Md, Afroz Khurshida, Shabdan Yerkin, Bizri Baraa, Nuraje Nurxat. Hierarchically 3D assembled strontium titanate nanomaterials for water splitting application // North-Holland. Applied Surface Science. – 2017. – Vol. 419. – P 886–892. 171 Macaraig L., Chuangchote S., Sagawa T. Electrospun SrTiO3 nanofibers for photocatalytic hydrogen generation // J. Mater. Res. – 2014. – Vol. 29. – P. 123–130. 172 Kudo A., Tanaka A., Domen K., Onishi T. The effects of the calcination temperature of SrTiO3 powder on photocatalytic activities // J. Catal. – 1988. – Vol. 111. – P. 296–301. 173 Townsend T.K., Browning N.D., Osterloh F.E. Nanoscale strontium titanate photocatalysts for overall water splitting, //ACS Nano. – 2012. – Vol. 6. – P. 7420–7426. 174 Xiang X., He W., Xie L., Li F. A mild solution chemistry method to synthesize hydrotalcite-supported platinum nanocrystals for selective hydrogenation of cinnamaldehyde in neat water // Catal. Sci. Technol. – 2013. – Vol. 3. – P. 2819 – 2827. 175 Zhang G., Sun S., Jiang W., Miao X., Zhao Z., Zhang X., Qu D., Zhang D., Li D., Sun Z. A novel perovskite srTiO3-Ba2FeNbO6 solid solution for visible light photocatalytic hydrogen production // Adv. Energy Mater. – 2017. – Vol. 7. – P. 1600932-n/a. 176 Bai H., Liu Z., Sun D.D. Facile fabrication of TiO2/SrTiO3 composite nanofibers by electrospinning for high efficient H2 generation // J. Am. Ceram. Soc. – 2013. – Vol. 96. – P. 942 – 949. 177 Zoontjes M.G.C., Han K., Huijben M., van der Wiel W.G., Mul G. The effect of Rh [small delta] + dopant in SrTiO3 on the active oxidation state of co- catalytic Pt nanoparticles in overall water splitting // Catal. Sci. Technol. – 2016. – Vol. 6. – P. 7793–7799. 178 Yang J., Wang D., Han H., Li C. Roles of cocatalysts in photocatalysis and photoelectrocatalysis // Acc. Chem. Res. – 2013. – Vol. 46. – P. 1900–1909. 179 Nuraje N., Lei Y., Belcher A. Virus-templated visible spectrum active perovskite photocatalyst // Catal. Commun. – 2014. – Vol. 44. – P. 68–72. 180 Puangpetch T., Sreethawong T., Yoshikawa S., Chavadej S. Hydrogen production from photocatalytic water splitting over mesoporous-assembled SrTiO3

106 nanocrystal-based photocatalysts // J. Mol. Catal. A: Chem. – 2009. – Vol. 312. – P. 97–106. 181 Choi S.K., Kim S., Lim S.K., Park H. Photocatalytic comparison of TiO2 nanoparticles and electrospun TiO2 nanofibers: effects of mesoporosity and inter particle charge transfer // J. Phys. Chem. C. – 2010. – Vol. 114. – P. 16475– 16480. 182 Liu Y., Xie L., Li Y., Yang R., Qu J., Li Y., Li X. Synthesis and high photocatalytic hydrogen production of SrTiO3 nanoparticles from water splitting under UV irradiation // Journal of Power Sources. – 2008. – Vol. 183. – P. 701 – 707. 183 Dikhanbaev K.K., Musabek G.K., Sivakov V.A., Shabdan Y., Bondarev A.I. thermoelectrically characteristics of ZnO:Al films obtained by thermal and magnetron sputtering // Eurasian Physical Technical Journal, ISSN 1811-1165, –2017. – Vol.14, No.2 (28). – P. 30- 35. 184 Suresh Sagadevan. Recent trends on nanostructures based solar energy applications: a review // Rev.Adv. Mater. Sci. – 2013. – Vol. 34. – P. 44-61. 185 Klimov VI (2006) Mechanisms for photogeneration and recombination of multiexcitons in semiconductor nanocrystals: Implications for lasing and solar energy conversion. Journal of Physical Chemistry B 110(34): 16827–16845

107

APPENDIX A: Table 10 DSSC device assembling steps with pictures

(1). Preparing the TCO (FTO) glass, 1 (2). Make detergent water and Sonicate inch in the detergent water × 1 inch.

(3). FTO face up (4). Rinsing with water and ethanol conductive side by tester

(5). Put in the air plasma conductive (6). Put in the TiCl4 side face up, solution and heat at 80 Air plasma degree for 30 min, system clean calculation conductive 1 minutes side face up

(7). (8). Conductive Rinsing side face up and with keep it clean area ethanol

(9). To the conductive side Put the (10). Dry for paste by doctor blending, layer depend 3 min on thickness

108

(11). Heat at 120 (12). Repeat it until thickness get 13 degree for 10 min micron (7 to 9)

(13). Heat it at (14). Put in the TiCI4 500 degree for 30 solution at 80 degree for min 30 min

(15). Heat it at (16). Cool 500 degree for 30 down till 80 min degree then immerse it Dye solution for 24 h. (17). (18). Prepare copper tape Prepare Pt coated glass

(19). (20). Put the spacer by Prepare to heating, Dipping the cut the electrolyte spacer size

(21). Put the Pt (22). Put the coated glass, clipper conductive side faced to the paste, and make sure do not shadow the paste area

109

APPENDIX B Table 11 Photocatalytic water splitting materials with pictures

Producing the fibers preparation sol-gel solution

Inorganic-polymer composite fiber

calcination in furnace

STO fiber Characterization results of STO

characterization result of Pt/ STO Scheme of hydrogen produce

Hydrogen testing in GC

110