High Aspect Ratio Tio2 Nanowires Tailored in Concentrated Hcl Hydrothermal Condition for Photoelectrochemical Water Splitting
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RSC Advances This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. This Accepted Manuscript will be replaced by the edited, formatted and paginated article as soon as this is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/advances Page 1 of Journal7 Name RSC Advances Dynamic Article Links ► Cite this: DOI: 10.1039/c0xx00000x www.rsc.org/xxxxxx ARTICLE TYPE High aspect ratio TiO2 nanowires tailored in concentrated HCl hydrothermal condition for photoelectrochemical water splitting Jinzhan Su*, Liejin Guo Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX 5 DOI: 10.1039/b000000x TiO2 nanowire/nanorod array grown on FTO substrate by hydrothermal reaction have attracted great attentions because of their favorable application in DSSC and quantum dot solar cells. In this paper, discrete vertically aligned TiO2 nanowire arrays with their length as long as 7.2 µm and their diameter less than 100 nm were successfully synthesized on FTO substrate by hydrothermal method. The influence 10 of hydrothermal precursor composition on the morphologies of the grown wires was investigated. The photoelectrochemical water splitting performance for nanowires with different lengths were studied. The Manuscript best photoelectrochemical performance was observed for the longest array without compact layer. The growth mechanism in the concentrated HCl was also proposed. The TiO2 nanowire array developed in this paper provides an optimal structure for those energy harvesting application requiring long and 15 discrete nanowires as electron collector. 1. Introduction conductive substrates were synthesized via the hydrolysis of 22 TiCl4 under the co-existence of amino acid catalysts. Some Nanostructural titanium oxide based material (e.g. nanowires, 23 50 methods like dc reactive magnetron sputtering, pulsed laser nanotube or nanoporous structure) has gained intense attention deposition (PLD),24 CVD25 were also adopted to deposited TiO due to their unique chemical and physical properties.1 The 2 nanorods. Hierarchically TiO2 nanoarrays was synthesized by 20 utilization of such advanced nanostructure on dye-sensitized solar acid vapor oxidation of metallic Ti film.26 Another type of cells,2 quantum-dot solar cells,3 photoelectrochemical water hierarchically TiO was reported by Chen27 who grew TiO splitting,4 gas sensor5 and batteries6 boost the development of 2 2 55 nanowires on electrospun titania fiber by hydrothermal treatment nanostructural materials. Compared to nanoparticles, one in Ti(OBu) –HCl–H O solution. dimensional nanorod/nanowire/nanotube arrays provide un- 4 2 It was until 2008 that high quality oriented single-crystalline TiO2 25 interrupted electrical pathways and promote the accessibility of nanowires on substrates of transparent conducting (TCO) glass Accepted holes to the electrolyte, as a result improving the electron transfer were reported by Feng10 and Liu11 by hydrothermal method. rate7 and efficiency in energy harvest systems. Numerous 60 Their techniques were then used by many groups to growth TiO semiconductor materials in one dimensional form have been 2 nanowire/nanorod for DSSC28, quantum dot29 and perovskite30 investigated for energy conversion.8 solar cells While most reports use the same synthesis strategy 30 TiO is the first discovered semiconductor material for solar fuel 2 without optimization. Tuning the nanorod structures such as the conversion and remain one of the most important candidates nanorod thickness, length, and density are very good candidates because of its excellent chemical stability, photocorrosion 65 for further improving the efficiency. For example, hierarchical resistance, and low cost.9 Before the reported of hydrothermal TiO 31,32 with enhanced photovoltaic performance have been synthesized TiO nanowire arrays in 2008,10,11 most of the 2 2 reported. While decreasing the diameter and increasing the length 35 reported one-dimensional TiO nanostructures were synthesized 2 remain a promising way to further improve the light harvesting in the form of disoriented nanowires/nanotubes or oriented arrays and conversion performance of TiO2 nanowires/nanorods. The on nontransparent or nonconductive substrates using various 11 70 TiO nanorod reported by Liu possess a relatively large synthesis techniques. Disoriented nanowires/nanotube was 2 diameter of 100nm and short length. The diameter of TiO2 synthesized by hydrothermal treatment of TiO2 nanoparticles in 10 12 13 14 nanowires reported by Feng is thinner but compactly packed as 40 NaOH solution, Electrospun, Biotemplating AAO(anodic nanowire bundles. Comparing these TiO nanorods, discrete alumina oxide)15 and ZnO nanorod template.16 2 nanorods with longer length and thinner diameter can derive to a TiO nanowire arrays on different substrates were also reported 2 Advances 75 better infiltration of both the light absorber and the hole such as Ti foil treated in concentrated NaOH aqueous solution17 transporting material. or annealed in oxygen flow18 or an argon stream with acetone.19 20 In this paper we report growth of well aligned discrete 45 Oriented needlelike titanium dioxide (TiO ) and TiO nanorod 2 2 nanowire arrays on FTO substrate with modified method based arrays21 were grown on glass substrate with TiCl and NaCl 3 on Feng’s technique. The nanowire arrays can be grown up to 7 aqueous solution. Short TiO2 nanorod arrays on transparent This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 1 RSC RSC Advances Page 2 of 7 μm while avoiding grown into compact bundles. The influence of integrated with a parabolic reflector, passing through an AM 1.5 different composition and growth mechanism were discussed. To filter and computer controlled monochromator with an Oriel demonstrate their application in solar water splitting, the TiO2 calibrated silicon photodiode used for detection. nanowires with different lengths were used as photoanodes. The 5 knowledge gained from this work will help understand the 3. Results and discussion growth mechanism and provide strategy to tailoring the nanowire 60 Well-aligned TiO2 nanowires were grown vertically on FTO morphology. This study demonstrate synthesis of TiO2 nanowire substrate by modified hydrothermal deposition based on Feng’s arrays with a long length and thin diameter, which are promising 10 applications in photoelectrochemical water splitting, dye- report, as described in the experimental section. Instead of densely packed nanowire array, discrete nanowire arrays were 10 sensitized solar cells and quantum-dot solar cells. grown by adjusting the composition of hydrothermal precursor. 2. Experimental 65 Figure 1a-d shows the nanowire arrays grown with 0 ml, 0.22 ml, 0.44 ml and 0.66 ml TiCl4 (99 wt%) added, respectively. While 2.1 Preparation of TiO2 nanoiwre arrays the amount of TiCl4 toluene solution (1M) added into the precursor, temperature and reaction time were kept as described The synthesis of the TiO nanowire arrays was performed by 2 in the experimental section. The left-sided insets are top views hydrothermal method. In a typical procedure, an anatase TiO2 70 and enlarged images. It was found that the wire length could be 15 seed layer was initially deposited on FTO coated glass(TEC 8, 8 well controlled by varying the amount of TiCl in the precursor. Ω per square) by spincoating using a solution prepared by 4 The lengths of the nanowires were measured to be about 0.7 μm, dissolving 1.7 ml Ti butoxide and 0.2 g poly(vinyl alcohol) (PVA) 2.9 μm, 7.2 μm and 26 μm, respectively. As shown in the insets in 5 ml deionized water and 5ml acetic acid, followed by 500 °C in Figure 1a-d, when the TiCl amount in the precursor increased, annealing for 2 h in air. The seed layer coated substrate was then 4 Manuscript 75 the diameters didn’t change much until 0.66 ml TiCl4 was added. 20 placed within a Teflon lined autoclave (23 ml), containing 2ml of For the samples without addition of TiCl , the diameters ranged titanium tetrachloride (1 M in toluene) and 8 ml of hydrochloric 4 from 21 nm to 65 nm with diameter of most wires at 42 nm. With acid (37 wt%). The concentration of TiCl was further adjusted 4 the volume of TiCl precursor increased to 0.22 ml and 0.44 ml, by adding certain amount of TiCl (Sigma, 99%) into the 4 4 the diameter range increased slightly to that from 23 nm to 101 precursor. The autoclave was then sealed and placed in an oven at 80 nm. While with 0.66 ml TiCl4 added, the diameters of the wires 25 170 °C for 48 h. The substrate was then rinsed with ethanol and increased to values between 43 nm and 220 nm. In addition, a dried in a nitrogen stream. All the TiO nanowire arrays were 2 dense packed nanorod layer(basal region, with thickness of 16 annealed at 500 °C for 0.5 h in air. μm) was grown before the growth of discrete nanowires (top 2.2 Structural and photoelectrochemical characterization region, 10 μm) as shown in figure 1(d). To the best of our 85 knowledge, the nanowire arrays with length as long as 7.2 µm X-ray diffraction (XRD) patterns were obtained on a PANalytical and their diameter less than 100 nm possess the highest aspect 30 X’pert MPD Pro X-ray diffractometer using Ni- filtered Cu Kα ratio among the reported TiO wire arrays.