Single-step fabrication of phase-controllable nanocrystalline TiO2 films for enhanced photoelectrochemical water splitting and dye-sensitized solar cells

We introduce a single-step procedure for growing a phase-controllable bilayer-structured TiO₂ film directly onto transparent conductive oxide glass by precipitation from hydrolysis of TiCl₄ in acid solution containing sulfate ions. The obtained bilayer-structured film with anatase nanoparticles in the inner layer which provide high surface area, and an outer layer of larger rutile particles for incident light scattering. In both the water splitting and the dye-sensitized solar cells under AM 1.5 simulated solar light, the bilayer-structured film outperformed the single layer-structured films with either anatase or rutile TiO₂ alone by at least 50%.     

Photoelectrochemical water splitting on highly smooth and ordered TiO2 nanotube arrays for hydrogen generation

To improve the photoelectrochemical (PEC) water splitting efficiency for hydrogen production, we reported the fabrication of lotus-root-shaped, highly smooth and ordered TiO₂ nanotube arrays (TiO₂ NTs) by a simple and effective two-step anodization method. The TiO₂ NTs prepared in the two-step anodization process (2-step TiO₂ NTs) showed better surface smoothness and tube orderliness than those of TiO₂ NTs prepared in one-step anodization process (1-step TiO₂ NTs). Under illumination of 100 mW/cm² (AM 1.5, simulated solar light) in 1 M KOH solution, water was oxidized on the 2-step TiO₂ NTs electrode with higher efficiency (incident-photon-to-current efficiency of 43.4% at 360 nm and photocurrent density of 0.90 mA/cm² at 1.23 V_RHE) than that on the 1-step TiO₂ NTs electrode. The effective photon-to-hydrogen conversion efficiency was found to be 0.18% and 0.49% for 1-step TiO₂ NTs and 2-step TiO₂ NTs, respectively. These results suggested that the structural smoothness and orderliness of TiO₂ NTs played an important role in improving the PEC water splitting application for hydrogen generation.     

Physically and chemically synthesized TiO2 composite thin films for hydrogen production by photocatalytic water splitting

TiO₂ thin films have been synthesized by radio-frequency magnetron sputtering and sol–gel method to study the hydrogen generation by photocatalytic water splitting under visible light irradiation. Photoelectrochemical cell with chemical bias, involving photo-anode in form of TiO₂ film deposited on conducting indium tin oxide (ITO) film and Pt as cathode, is developed. The effect of conducting ITO layer on photo-voltage is studied by varying the thickness of ITO films. Constant H₂ generation rate is obtained for long period of time by both the TiO₂ films because of the separated evolution of H₂ and O₂ gas, thus eliminating the back-reaction effect. Sputter-deposited film as compared to sol–gel-synthesized film showed better H₂ generation rate, mainly explained in terms of the higher visible light absorption achieved by oxygen vacancies created in the TiO₂ film by the energetic target ions during deposition in pure Ar gas pressure.     

TiO2 photoanodes for electrically enhanced water splitting

Photoelectrochemical properties of self-organized TiO₂ nanotubes formed by electrochemical anodization of titanium sheets and their working mechanism are investigated. Formation and growth of self-organized nanotubes were carried out by Titanium anodization in acid electrolyte containing fluorides. Annealing of the samples was performed in order to increase the crystallinity of the material. Also some results obtained with samples annealed in Nitrogen atmosphere are presented. Electrochemical Impedance Spectroscopy was used to give an interpretation of the main charge transfer processes that occur at the nanotube/electrolyte interphase. The use of glycerol as hole scavenger was considered in order to improve the photoelectrochemical performance of the samples.     

TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation

The CdS/TiO₂ composites were synthesized using titanate nanotubes (TiO₂NTs) with different pore diameters as the precursor by simple ion change and followed by sulfurization process at a moderate temperature. Some of results obtained from XRD, TEM, BET, UV–vis and PL analysis confirmed that cadmium sulfide nanoparticles (CdSNPs) incorporated into the titanium dioxide nanotubes. The photocatalytic production of H₂ was remarkably enhanced when CdS nanoparticles was incorporated into TiO₂NTs. The apparent quantum yield for hydrogen production reached about 43.4% under visible light around λ = 420 nm. The high activity might be attributed to the following reasons: (1) the quantum size effect and homogeneous distribution of CdSNPs; (2) the synergetic effects between CdS particles and TiO₂NTs, viz., the potential gradient at the interface between CdSNPs and TiO₂NTs.     

Facile fabrication of Bi2O3/TiO2-xNx nanocomposites for excellent visible light driven photocatalytic hydrogen evolution

Mesoporous Bi₂O₃/TiO_2−xN_x nanocomposites (BiNT) were synthesized by soft chemical template free homogeneous co-precipitation technique. XRD, XPS, TEM, UV-Vis DRS and photoluminescence studies were adapted to determine the structural, electronic and optical properties. The photocatalytic activities of the catalysts were evaluated for water splitting to generate clean hydrogen fuel under visible light irradiation (λ ≥ 400 nm). BiNT-400 catalyst showed highest results towards hydrogen production (198.4 μmol/h) with an apparent quantum efficiency of 4.3%. The pronounced activity of BiNT-400 sample towards hydrogen production was well consistent with high crystallinity, large surface area, proper excitation by N doping and Bi₂O₃ sensitization.     

Photoelectrochemical water splitting for hydrogen generation on highly ordered TiO2 nanotubes fabricated by using Ti as cathode

Sonication assisted anodization of titanium in a fluorinated ethylene glycol and water electrolyte using Ti itself as a cathode is investigated. The prepared anodic film has a highly ordered nanotube-array surface architecture. The resulting TiO₂ nanotubes at potential 20-40 V have various diameters (30-100 nm), tube length (3-12 μm) and wall thicknesses (6-15 nm). The tube diameter and wall thickness are increased with the anodization time while the overall length of the nanotube arrays is controlled by the duration of the anodization time. In addition, apart from the anodization time, formation of nanotubes is governed by the distance and supplied voltages between the two electrodes, for a given electrolyte. The crystal structure and surface morphology of the annealed anodic films are investigated by XRD and SEM, respectively. The corresponding photoelectrochemical water splitting efficiency (PCE) was calculated under UV light. Our results show a very high PCE under UV (315-400 nm, 100 mW/cm²) irradiation. The maximum value of PCE for hydrogen generation obtained was 29% which is one of the best results reported in literature [1].     

Photocatalytic H2 production from water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled Pt/TiO2 nanocrystal photocatalyst

Sensitized photocatalytic production of hydrogen from water splitting is investigated under visible light irradiation over mesoporous-assembled titanium dioxide (TiO₂) nanocrystal photocatalysts, without and with Pt loading. The photocatalysts are synthesized by a sol–gel process with the aid of a structure-directing surfactant and are characterized by N₂ adsorption–desorption analysis, X-ray diffraction, UV–vis spectroscopy, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray analysis. The dependence of hydrogen production on the type of TiO₂ photocatalyst (synthesized mesoporous-assembled and commercial non-mesoporous-assembled TiO₂ without and with Pt loading), the calcination temperature of the synthesized photocatalyst, the sensitizer (Eosin Y) concentration, the electron donor (diethanolamine) concentration, the photocatalyst dosage and the initial solution pH is systematically studied. The results show that in the presence of the Eosin Y sensitizer, the Pt-loaded mesoporous-assembled TiO₂ synthesized by a single-step sol–gel process and calcined at 500 °C exhibits the highest photocatalytic activity for hydrogen production from a 30 vol.% diethanolamine aqueous solution with dissolved 2 mM Eosin Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic activity for hydrogen production are 3.33 g dm⁻³ and 11.5, respectively.     

Highly efficient hematite films via mid-/ex-situ Sn-doping for photoelectrochemical water oxidation

In this study, we report the facile fabrication of Sn-doped hematite film via mid-situ and ex-situ doping methods for efficient photoelectrochemical (PEC) performances. The morphology of Sn-doped Fe₂O₃ films was varied with Sn precursor in the mid-situ doping process. The addition of SnCl₂ rendered a smooth-surfaced and well-distributed nanorod morphology, but SnCl₄ gave a deformed nanorod structure covered with layered coalescence of SnO₂ particles. The former demonstrated much higher photoelectrochemical performances as photoanodes than the latter. The photocurrent can be further improved by surface modification with SnCl₄ through spin-coating method. The effects of Sn precursors on the morphology, surface characteristics and the PEC properties of the photoanodes are investigated.     

Single crystalline TiO2 nanorods with enhanced visible light activity for solar hydrogen generation

Single crystalline TiO₂ nanorods and polycrystalline nanotubes were fabricated with same length to investigate the effects of their nanostructures on photocatalytic properties for splitting water. In order to enhance the visible light absorbance, TiO₂ nanorods and nanotubes were sensitized with semiconductor nanoparticles such as CdS, CdSe, and CdS/CdSe, and compared in viewpoint of solar hydrogen generation. It was observed that single-crystalline nanorods showed superior photocatalytic properties to polycrystalline nanotubes, and also the potential level of the nanorods with rutile phase was measured as lower than that of the nanotubes with mixture of anatase and rutile. Further improvement of photo-conversion efficiency was obtained by subsequent heat treatments of the sensitized photoelectrodes. It turns out that the improvement is attributed to the improved crystallinity and the increased size of the nanoparticles during the post-annealing treatments. It was demonstrated that TiO₂ nanorods with lower potential level and a single crystalline phase on FTO glass were advantageous for effective charge injection from the sensitized nanoparticles and transport without recombination lost at grain boundaries.     

Hydrogen production from water splitting under UV light irradiation over Ag-loaded mesoporous-assembled TiO2-ZrO2 mixed oxide nanocrystal photocatalysts

Hydrogen production from the photocatalytic water splitting reaction is very attractive because it is an environmentally friendly process, where hydrogen is produced from two abundantly renewable sources, i.e. water and solar energy, with the aid of photocatalysts. TiO₂ is the most widely investigated photocatalyst; however, it alone still exhibits low performance to photocatalytically produce hydrogen. Hence, the aim of this work focused on the enhanced photocatalytic hydrogen production over Ag-loaded mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalysts under UV light irradiation. The TiO₂-ZrO₂ mixed oxides with various TiO₂-to-ZrO₂ molar ratios were synthesized by a sol-gel process with the aid of a structure-directing surfactant, followed by Ag loading via a photochemical deposition method. The influences of photocatalyst preparation parameters, i.e. calcination temperature, phase composition, and Ag loading, were studied. The results revealed that the mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalyst with a TiO₂-to-ZrO₂ molar ratio of 93:7 calcined at 500 °C exhibited the highest photocatalytic hydrogen production activity, and the Ag loading of 0.5 wt.% further greatly enhanced the photocatalytic activity of such TiO₂-ZrO₂ mixed oxide photocatalyst.     

Surface treated TiO2 nanorod arrays for the improvement of water splitting

Water splitting is widely employed for the hydrogen production for its abundant sources of water and sunlight. The TiO₂ nanostructures are the most promising materials because of their properties of the non-toxicity and relatively low cost. Surface treatments with TiCl₄ solution and titanium butoxide solution are applied on the TiO₂ nanorod arrays respectively. On the surface of the TiO₂ nanorods, TiO₂ nanoparticles are prepared through hydrolysis of TiCl₄ and homogeneous phase of TiO₂ synthesized with assist of second hydrothermal synthesis in titanium butoxide, resulting in the increase of the surface area of the TiO₂. Comparing with that of the original TiO₂ nanorod arrays, the incident photon-to-electron conversion efficiency (IPCE) of the TiO₂–TiCl₄ and TiO₂–H₂O samples is greatly enhanced by 25% and 250% in the ultraviolet region, respectively. The obviously enhanced activity is due to the larger surface structure after treatments, which could contribute to the improved performance in the water splitting. These surface treatments provide an efficient way to regulate the properties of the TiO₂ nanorod arrays for their extensive applications in the solar device for the hydrogen production.     

Optimization of surface charge transfer processes on rutile TiO2 nanorods photoanodes for water splitting

Electrochemical impedance spectroscopy (EIS) has been performed to investigate the photocatalytic properties of titanium dioxide nanorods in a photoelectrochemical water splitting system. A two-channel transmission line model has been proposed to interpret the frequency response of the main charge transfer processes that occur at nanorod/electrolyte and platinum/electrolyte interfaces. EIS was then employed to determine that the dramatic effect of the annealing treatment on the photocurrent density had its origin on a poor charge transfer at the titania/electrolyte interface. X-ray photoelectron spectroscopy and thermogravimetry measurements have been used to prove the relevance of the presence of chlorine coming from the synthesis process of TiO₂ nanorods.     

Self-organized TiO2 nanotube arrays with uniform platinum nanoparticles for highly efficient water splitting

Highly efficient water splitting electrode based on uniform platinum (Pt) nanoparticles on self-organized TiO₂ nanotube arrays (TNTAs) was prepared by a combination of multi-step electrochemical anodization with facile photoreduction process. Uniform platinum (Pt) nanoparticles with an average diameter of 8 nm are distributed homogeneously on nanoporous top layer and underneath TiO₂ nanotube wall. In comparison to pristine TNTAs, Pt@TNTAs show substantially enhanced photocurrent density and the incident photon-to-current conversion efficiency (IPCE) in the entire wavelength window. The maximum photocurrent density and IPCE from the optimized Pt@TNTAs photoelectrode (Pt, ～1.57 wt%) were about 24.2 mA cm⁻² and 87.9% at 350 nm, which is much higher than that of the pure nanotubes sample (16.3 mA cm⁻² and 67.3%). The resultant Pt@TNTAs architecture exhibited significantly enhanced photoelectrochemical activities for solar water splitting with hydrogen evolution rate up to 495 μmol h⁻¹ cm⁻² in 2 M Na₂CO₃ + 0.5 M ethylene glycol under the optimal external bias of −0.3 V_SCE.     

Hydrothermally stabilized Fe(III) doped titania active under visible light for water splitting reaction

Fe³⁺ doped TiO₂ photocatalysts were prepared by hydrothermal treatment for the photocatalytic water splitting to produce stoichiometric hydrogen and oxygen under visible light irradiation. It was found that hydrothermal treatment at 110 °C for 10 h was essential for the synthesis of highly stabilized Fe³⁺ doped TiO₂ photocatalysts. The synthesized photocatalysts were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) and BET surface area techniques. The doping of highly stabilized Fe³⁺ in the titania matrix leads to significant red shift of optical response towards visible light owing to the reduced band gap energy. Optimum amount of Fe³⁺ doped TiO₂, 1.0 wt% Fe/TiO₂, showed drastically improved hydrogen production performance of 12.5 μmol-H₂/h in aqueous methanol and 1.8 μmol-H₂/h in pure water, respectively. This Fe/TiO₂ photocatalyst was stable for 36 h without significant deactivation in the water splitting reaction.     

A ternary Ag/TiO2/CNT photoanode for efficient photoelectrochemical water splitting under visible light irradiation

A ternary Ag/TiO₂/CNT photoanode was prepared by grafting Ag nanoparticles on the surface of as-synthesized TiO₂/CNT nanocomposite for the photoelectrochemical (PEC) water splitting under visible light irradiance. The ternary composite photoanode was observed to generate four times higher photocurrent density compared to binary TiO₂/CNT nanocomposite under visible light irradiance. The Ag nanoparticles on the surface of nanocomposite act as a surface plasmon resonance (SPR) photosensitizer under visible light. The enhanced photocurrent density of Ag/TiO₂/CNT ternary photoanode is attributed to the increased light absorption in the visible region, decrease in band-bending and effective interfacial electron transfer due to the synergetic effect of Ag nanoparticles and CNTs. The enhanced charge transfer within the Ag/TiO₂/CNT was also confirmed by the electrochemical impedance spectroscopy. This work demonstrates a feasible route to improve the PEC performance of TiO₂ towards water splitting under sunlight irradiation.     

Effective water splitting using N-doped TiO2 films: Role of preferred orientation on hydrogen production

N-doped TiO₂ film with preferred (211) orientation, deposited by RF magnetron sputtering, was investigated for the water-splitting hydrogen production. It is found that the preferred crystal growth orientation of the films can be controlled by N₂ flow rate during the deposition. The results reveal that not only the N-doping, but also the preferred orientation (i.e., large percentage of exposed (211) facet), can effectively enhance the activity of TiO₂-based photocatalyst. With the increase of exposed (211) facets, the hydrogen production rates of N-doped TiO₂ films rise from 760 μmol H₂ h⁻¹ m⁻² to 4500 μmol H₂ h⁻¹ m⁻², indicating that high performance of TiO₂-based photocatalyst can be achieved by controlling the preferred orientation of the films.     

Improving hydrogen production via water splitting over Pt/TiO2/activated carbon nanocomposite

Mesoporous TiO₂/AC, Pt/TiO₂ and Pt/TiO₂/AC (AC = activated carbon) nanocomposites were synthesized by functionalizing the activated carbon using acid treatment and sol–gel method. Photochemical deposition method was used for Pt loading. The nano-photocatalysts were characterized using XRD, SEM, DRS, BET, FTIR, XPS, CHN and ICP methods. The hydrogen production, under UV light irradiation in an aqueous suspension containing methanol has been studied. The effect of Pt, methanol and activated carbon were investigated. The results show that the activated carbon and Pt together improve the hydrogen production via water splitting. Also methanol acts as a good hole scavenger. Mesoporous Pt/TiO₂/AC nanocomposite is the most efficient photocatalyst for hydrogen production compared to TiO₂/AC, Pt/TiO₂ and the commercial photocatalyst P25 under the same photoreaction conditions. Using Pt/TiO₂/AC, the rate of hydrogen production is 7490 μmol (h g catal.)⁻¹ that is about 75 times higher than that of the P25 photocatalyst.     

Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light

The advantage of copper doping onto TiO₂ semiconductor photocatalyst for enhanced hydrogen generation under irradiation at the visible range of the electromagnetic spectrum has been investigated. Two methods of preparation for the copper-doped catalyst were selected – complex precipitation and wet impregnation methods – using copper nitrate trihydrate as the starting material. The dopant loading varied from 2 to 15%. Characterization of the photocatalysts was done by thermogravimetric analysis (TGA), temperature programmed reduction (TPR), diffuse reflectance UV-Vis (DR-UV-Vis), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Photocatalytic activity towards hydrogen generation from water was investigated using a multiport photocatalytic reactor under visible light illumination with methanol added as a hole scavenger. Three calcination temperatures were selected – 300, 400 and 500 °C. It was found that 10 wt.% Cu/TiO₂ calcined at 300 °C for 30 min yielded the maximum quantity of hydrogen. The reduction of band gap as a result of doping was estimated and the influence of the process parameters on catalytic activity is explained.