PCCP View Article Online PAPER View Journal | View Issue Comparison of photocatalytic and transport properties of TiO2 and ZnO nanostructures Cite this: Phys. Chem. Chem. Phys., 2015, 17, 7775 for solar-driven water splitting† ab ab a a Simelys Herna´ndez,* Diana Hidalgo, Adriano Sacco, Angelica Chiodoni, Andrea Lamberti,ab Valentina Cauda,a Elena Tressoab and Guido Saraccob Titanium dioxide (TiO2) and zinc oxide (ZnO) nanostructures have been widely used as photo-catalysts due to their low-cost, high surface area, robustness, abundance and non-toxicity. In this work, four TiO2 and ZnO-based nanostructures, i.e. TiO2 nanoparticles (TiO2 NPs), TiO2 nanotubes (TiO2 NTs), ZnO nanowires (ZnO NWs) and ZnO@TiO2 core–shell structures, specifically prepared with a fixed thickness of about 1.5 mm, are compared for the solar-driven water splitting reaction, under AM1.5G simulated sunlight. Complete characterization of these photo-electrodes in their structural and photo-electrochemical properties was carried out. Both TiO2 NPs and NTs showed photo-current saturation reaching 0.02 and Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 0.12 mA cmÀ2, respectively, for potential values of about 0.3 and 0.6 V vs. RHE. In contrast, the ZnO NWs and the ZnO@TiO2 core–shell samples evidence a linear increase of the photocurrent with the applied potential, reaching 0.45 and 0.63 mA cmÀ2 at 1.7 V vs. RHE, respectively. However, under concen- trated light conditions, the TiO2 NTs demonstrate a higher increase of the performance with respect to the ZnO@TiO2 core–shells. Such material-dependent behaviours are discussed in relation with the different Received 15th December 2014, charge transport mechanisms and interfacial reaction kinetics, which were investigated through electro- Accepted 11th February 2015 chemical impedance spectroscopy. The role of key parameters such as electronic properties, specific DOI: 10.1039/c4cp05857g surface area and photo-catalytic activity in the performance of these materials is discussed. Moreover, This article is licensed under a proper optimization strategies are analysed in view of increasing the efficiency of the best performing www.rsc.org/pccp TiO2 and ZnO-based nanostructures, toward their practical application in a solar water splitting device. Open Access Article. Published on 17 febrúar 2015. Downloaded 27.9.2021 13:21:33. 1. Introduction and oxygen is still an open challenge. It has been found2 that the photochemical water-splitting reaction can be catalyzed by With increasing concern over the fossil fuel depletion and the over 140 metal oxides, perovskites and oxynitrides, and the environmental degradation, energy is one of the greatest issues principal controlling factors of the photocatalysis activity have that humanity will be facing in the coming years. Hydrogen, been identified. Nevertheless many questions concerning the present in the water molecules, is an efficient energy carrier molecular mechanisms of water reduction and oxidation and and is also environmentally friendly: therefore using solar the charge transfer dependence on the electronic and structural energy to split water into oxygen and hydrogen (also called properties have not been completely solved yet, and the ideal ‘‘artificial photosynthesis’’) is a key point towards the development semiconducting photocatalyst has still to be identified. At the of sustainable and renewable energy devices. same time, research efforts focused on proposing artificial More than 40 years after the pioneering work of Fujishima photosynthesis devices have been recently greatly increased in and Honda,1 the search for suitable semiconductors to be number and importance, but functional prototypes with convenient employed for the water dissociation into molecular hydrogen efficiencies have still to be fabricated.3 The H2 photocatalytic generation involves three main steps: a Center for Space Human Robotics (IIT@POLITO), Istituto Italiano di Tecnologia, (i) absorption of photons (with energy higher than the semi- C.so Trento 21, 10129, Torino, Italy. E-mail: [email protected]; conductor band gap (Eg) and consequent generation of electron– Tel: +39 0110904774/3418 hole (eÀ–h+) pairs in the semiconductor), (ii) excited charge carrier b Department of Applied Science and Technology (DISAT), Politecnico di Torino, separation and migration within the semiconductor, and (iii) C.so Duca degli Abruzzi 24, 10129, Torino, Italy † Electronic supplementary information (ESI) available: (i) Long-time (12 h) I–t surface reaction of the carriers with water molecules. To provide curves of both TiO2 NTs and ZnO@TiO2 photoelectrodes and (ii) FESEM images the water splitting, the bottom of the semiconductor conduction after PEC tests of the four studied materials. See DOI: 10.1039/c4cp05857g band must be in a more negative energy position with respect to This journal is © the Owner Societies 2015 Phys. Chem. Chem. Phys., 2015, 17, 7775--7786 | 7775 View Article Online Paper PCCP 15,16 the reduction potential of water to produce H2;andthetopofthe been grown by anodic oxidation on Ti foils, while ZnO nano- valencebandmustbemorepositivethantheoxidationpotential wires (ZnO NWs) have been obtained on a FTO seeded substrate 17 of water to produce O2. Furthermore, the photo-catalyst must be using a hydrothermal route. Finally, ZnO@TiO2 core–shell stable in aqueous solutions under photo-irradiation. The total structures have been fabricated on FTO by covering the ZnO 18,19 amount of generated H2 molecules is determined by the amount NWs with sol–gel synthesized TiO2 nanoparticles. In particular, of excited electrons at the water/photo-catalyst interface capable of the TiO2 nanoparticle-based films, the ZnO nanowires and the reducing water. Charge recombination and separation–migration ZnO@TiO2 core–shell structures have already demonstrated processes are the two most important competitive processes that promising photocatalytic properties for the water splitting 14,18 largely affect the efficiency of the photocatalytic reaction. Charge reaction. The ZnO@TiO2 core–shell heterostructures offer À + recombination reduces the number of e –h pairsbyemitting some advantages: the TiO2 shell functions as a protective layer light or generating phonons. Efficient charge separation, fast charge to reduce the ZnO degradation and the multi-dimensional contact carrier transport and limited bulk/surface charge recombination are permits to fully utilize the heterojunction between the two semi- thus fundamental characteristics of an optimal semiconductor conductors, which exhibits very favorable electron-transfer proper- photocatalyst material. ties that are beneficial to an effective separation of the photo- 1 À + 18,20,21 Since 1972, titanium dioxide (TiO2) has been the most generated e –h pairs. For what concerns the TiO2 NTs commonly studied material for photocatalysis. It exhibits an fabricated in our laboratory, they areemployedforthefirsttimein appropriate band gap of about 3.2 eV, together with high photo- thisworkforthesolarwatersplittingreaction. catalytic efficiency, good chemical and optical stability, optimal In general, due to the broad range of dimensions and 4 environmental and biological compatibility. Zinc oxide (ZnO) has thicknesses of fabricated TiO2 and ZnO nanostructures, and also been largely considered because of its band gap energy, because of the different testing operative conditions, a direct 5 which is comparable to TiO2, with the energy levels located comparison of both transport properties and performance of almost at the same positions, its higher electron mobility and photoactive electrodes, between our materials and those 6 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. lifetime, relatively lower production costs and easy fabrication reported in the literature, is not straightforward. Thus, in this under a variety of nanostructures such as nanowires, nanoribbons, paper, the aim is to compare the transport and photo-catalytic 7 nanobelts, nanocombs, nanospheres, nanofibers, nanotetrapods. properties of four different photoelectrodes based on TiO2 and To date TiO2 and ZnO have been close to be ideal photocatalysts. ZnO nanostructures, specifically prepared in order to have the They are relatively inexpensive and they provide photo-generated samethicknessandthesameactivearea.Thestudiedelectrodes holes with high oxidizing power due to their wide band gap energy. are based on (i) mesoporous films of TiO2 NPs, (ii) TiO2 NTs, Unfortunately their solar-to-hydrogen efficiency is limited by (iii) ZnO NWs and (iv) 1D ZnO@TiO2 core–shell nanostructures. the high band gap and the many electron–hole recombination The thickness of the four photoelectrodes has been fixed at centers;8 moreover, ZnO has the disadvantage of a facile dissolution about 1.5 mm and the active area to about 4 cm2, in order to This article is licensed under a under UV light irradiation in aqueous solution.9 reliably compare the electronic and PEC properties of these Different routes have been adopted for enhancing the TiO2 materials under the same operative conditions, for the sunlight- and ZnO photocatalytic performances. Based on the fact that activated water splitting reaction. The morphological and optical Open Access Article. Published on 17 febrúar 2015. Downloaded 27.9.2021 13:21:33. size, shape and also defects significantly affect the final photo- properties of these nanostructures are also presented and discussed. catalytic activity, the optimization of the morphology