Investigation of photo-electrochemical response of iron oxide/mixed-phase titanium oxide heterojunction toward possible solar energy conversion

Photocatalysts are part of key strategies to enable green fuel. Photocatalysis and water splitting could be a promising solution to challenges associated with the intermittent nature of sunlight as a huge energy source on Earth. In this study, photo-electrochemical performance and behavior of mixed-phase titanium oxide and iron oxide heterojunction (Ti-TiO_x (High-voltage)-FeO_x electrode) are compared to the photo-electrochemical performance and behavior of titanium oxide nanotubes with the rutile phase and iron oxide heterojunction (TiO_x-nanotubes (H₂SO₄/KF)-FeO_x electrode). The results of photo-electrochemical experiments show that the application of stabilization potential and the presence/absence of dissolved oxygen could not be considered as significant factors affecting the photo-electrochemical properties of the Ti-TiO_x (High-voltage)-FeO_x and TiO_x-nanotubes (H₂SO₄/KF)-FeO_x electrodes. The Ti-TiO_x (High-voltage)-FeO_x electrode shows an anodic photo-electrochemical response in wavelengths shorter than 530 nm and cathodic photo-electrochemical response in wavelengths longer than 530 nm. However, the Ti-nanotubes (H₂SO₄/KF)-FeO_x electrode consistently exhibits the anodic photo-electrochemical response. Both of the prepared heterojunctions are further characterized through Scanning Electron Microscopy, Energy-dispersive X-ray Spectroscopy, Diffuse Reflectance UV–Vis Spectroscopy, X-ray Diffraction, and Attenuated Total Reflectance Spectroscopy methods. These experiments show that despite different morphologies observed in SEM imaging data, the deposited iron oxide layers on both mixed-phase titanium oxide and titanium oxide nanotubes share the same hematite phase structure. However, only iron oxide electro-deposited on the surface of the mixed-phase titanium oxide, which contains both anatase and rutile phases, with vacant sites of oxygen, exhibits un-expected anodic and cathodic photo-electrochemical responses. Furthermore, according to the results of the characterization and photo-electrochemical investigations, the different chemical environment of mixed-phase titanium oxide, and the possible formation of different types of heterojunction structures in mixed-phase titanium oxide and iron oxide, in contrast to the titanium oxide nanotubes and iron oxide, might be considered the possible discernible reasons for the observed different photo-electrochemical responses. This paper sheds new light on photo-electrochemistry of iron oxide/mixed-phase titanium oxide heterojunction for possible solar energy conversion.     

Dye sensitized solar cells incorporating obliquely deposited titanium oxide layers

Porous films of titanium oxide were deposited by oblique reactive electron beam evaporation. Both as-deposited and annealed samples of these films were structurally characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The as-deposited films were found to consist of amorphous columns with a very fine structure while the annealed films consisted of polycrystalline anatase columns whose fine structure showed evidence of conglomeration. These films were sensitized with a photoactive dye and implemented into a dye sensitized solar cell (DSSC) configuration as the electron collecting electrode. Solar cells incorporating annealed titanium oxide films fabricated at deposition angles between 60° and 75° produced higher short current densities than conventional DSSC based on colloidal TiO₂ film measured under the same conditions. The best performing solar cell incorporating annealed titanium oxide films was found to have a photoelectric conversion efficiency of 4.1%.     

Enhanced photocatalytic hydrogen production activity of noble metal free MWCNT-TiO2 nanocomposites

Nanocomposite photocatalysts, MWCNT-TiO₂ were prepared by hydrothermal method. The photocatalysts were characterized by X-ray diffraction, Transmission electron microscopy (TEM), Raman spectroscopy, UV-Visible diffuse reflectance spectroscopy and photoluminescence (PL) spectroscopy to understand the crystal structure, morphology, and optical properties. The catalyst synthesis parameters such as calcination temperature and loading of MWCNTs were optimized for better hydrogen (H₂) production in 5 vol% glycerol aqueous solution under UV-visible light irradiation. Among the prepared nanocomposites, 0.1 wt% CNT loaded TiO₂ calcined at 450 °C for 2 h showed the highest H₂ production rate of 8.8 mmol g^?1 h^?1. This higher H₂ production rate obtained can be ascribed to effective utilization of the photo generated electrons and holes for redox reactions.     

Metal-free hydrogen evolution over defect-rich anatase titanium dioxide

Commercial anatase phase titanium dioxide was annealed under various gases (hydrogen, nitrogen, argon, and air) to induce the formation of defects. While annealing in the absence of oxygen there was a notable increase in the concentration of paramagnetic defects as measured by Electron Paramagnetic Resonance (EPR) and X-ray Photoelectron Spectroscopy (XPS). The presence of these defects increased the metal-free photocatalytic activity of the samples towards hydrogen evolution from photocatalytic methane steam reforming (MSR) under UV illumination. Catalyst activity was stable for over 42 h while illuminated owing to the regeneration of Ti³⁺ defects by UV photoexcitation, but rapidly decayed in the dark. The high concentration of unique Ti³⁺ defect sites generated during annealing catalyze hydrogen evolution, avoiding the need for precious metal cocatalysts, while anatase lacking these defects is inactive. This work shows that the implementation of defect-rich anatase TiO₂ provides new catalytic pathways for hydrogen generation from photocatalytic methane steam reforming.     

Thin film n-titanium oxide photoanodes for photoelectrochemical production of hydrogen

The semiconductor photoanodes made of thin film titanium oxide were prepared at room temperature by anodization of titanium plates in the hydrofluoric acid solution at direct bias. The influence of the change of the titanium oxide film growth conditions (concentration of hydrofluoric acid, voltage, duration of anodization process) and subsequent heat treatment of films on the photocurrent and current–voltage characteristics of photoelectrodes were investigated. The influence of the electrolyte concentration change on photoelectrochemical behaviour of thin film titanium oxide photoanodes was investigated.     

Properties of spray deposited titanium dioxide thin films and their application in photoelectrocatalysis

Thin films of titanium dioxide were deposited onto optically transparent, electrically conducting substrates (fluorine doped tin oxide on glass). The two oxide layers, SnO₂ and TiO₂, were deposited sequentially by spray pyrolysis. TiO₂ films of up to 800 nm thickness were prepared by varying the quantity of sprayed solution (titanyl acetylacetonate in methanol), at a growth rate of 0.15 nm/s.The effect of film thickness on the structural, optical and photoelectrochemical properties of TiO₂ films was studied. Scanning electron microscopy showed that the polycrystalline anatase films were compact. The grain size increased up to 1100 nm with increase in film thickness, whereas the crystallite size remained constant (40 nm) as shown by X-ray diffraction. The films had a transmittance of more than 70% in the visible region.Junctions of the semiconducting films with aqueous electrolytes were rectifying and photoactive. Films of 330 or 600 nm were thick enough to exhibit maximum photoelectrochemical response for light of a wavelength of 313 or 365 nm, respectively. Under depletion conditions, an IPCE (incident photon to current conversion efficiency) of 0.8 for a 330 nm thick film at 313 nm was obtained.Oxalic acid degradation under UVA light and under sunlight, applying electrical bias, was demonstrated using these electrodes.     

Growth from solution of CdTe films by conversion of chemically deposited cadmium oxide hydroxide films

Cadmium oxide hydroxide films were grown on glass substrates by the chemical bath deposition technique and subsequently converted to CdTe films by immersion in a stable alkaline aqueous solution containing Te ions. The chemical deposition of cadmium oxide hydroxide films was based on CdCl₂ as the source of Cd ions and sodium citrate as the complexing agent. The solution containing Te ions was prepared employing hydroxymethane sulfinic acid as reducing agent. Both types of films were analyzed by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The results show that cadmium oxide hydroxide films are nanocrystalline with cubic structure and they were successfully converted by ionic exchange to crystalline CdTe films with cubic structure.     

Kinetic study of the hydrogen oxidation reaction on sub-stoichiometric titanium oxide-supported platinum electrocatalyst in acid solution

The kinetics and mechanism of the hydrogen oxidation reaction were studied in 0.5 mol dm⁻³ HClO₄ solution on an electrode based on titanium oxide with Magneli phase structure-supported platinum electrocatalyst applied on rotation Au disk electrode. Pt catalyst was prepared by impregnation method from 2-propanol solution of Pt(NH₃)₂(NO₂)₂ and sub-stoichiometric titanium oxide powder. Sub-stiochiometric titanium oxide support was characterized by X-ray diffraction and BET techniques. The synthesized catalyst was analyzed by TEM technique. Based on Tafel-Heyrovsky-Volmer mechanism the corresponding kinetic equations were derived to describe the hydrogen oxidation current-potential behavior on RDE over the entire potential region. The polarization RDE curves were fitted with derived polarization equations according to proposed model. The fitting shows that the HOR on Pt proceeds most likely via the Tafel-Volmer (TV) pathway in the lower potential region, while the Heyrovsky-Volmer (HV) pathway is operative in the higher potential region. It is pointed out that Tafel equation that has been frequently used for the kinetics analysis in the HOR, can not reproduce the polarization curves measured with high mass-transport rates. Polarization measurements on RDE revealed that the Pt catalyst deposited on titanium suboxide support showed equal specific activity for the HOR compared to conventional carbon-supported Pt fuel cell catalyst.     

Efficient hydrogen production by composite photocatalyst CdS–ZnS/Zirconium–titanium phosphate (ZTP) under visible light illumination

A novel composite CdS–ZnS/Zirconium–titanium phosphate (ZTP) photocatalyst working under visible light was successfully prepared by a two-step sulfidation procedure. The photocatalytic activity of the cadmium sulfide–zinc sulfide supported composite catalyst was evaluated toward hydrogen energy production in the presence of hole scavenger, sulfide (S²⁻) and compared with the activity of neat CdS, ZnS, ZTP, CdS–ZnS, CdS/ZTP and ZnS/ZTP without using any co-catalyst. The photocatalysts were characterized by X-ray diffraction (Small Angle X-ray diffraction Studies and Broad-Angle X-ray Diffraction studies), N₂ adsorption–desorption, diffuse reflectance UV–vis spectroscopy (DRUV-vis), photoluminescence (PL) studies, SEM/EDX, X-ray photoelectron spectroscopic (XPS) studies, transmission electron microscopy (TEM) etc. Amongst all the catalysts, 5CdS–ZnS/ZTP showed highest results toward hydrogen production (2142.7 μmol) with an apparent quantum efficiency of 9.6% under visible light illumination.     

Self-assembled ionic liquid synthesis of nitrogen-doped mesoporous TiO2 for visible-light-responsive hydrogen production

Nitrogen-doped mesoporous TiO₂ has been synthesized by a simple solvent evaporation-induced self-assembly method using a nitrogen-containing ionic liquid concurrently as a nitrogen source and mesoporous template. After being evaporated and subsequently calcined at various temperatures (300–900 °C), the synthesized samples were thoroughly characterized by X-ray diffraction (XRD), Raman, small-angle X-ray scattering patterns (SAXS), N₂ adsorption-desorption isotherms, X-ray photoelectron (XPS) and UV–Vis diffuse reflectance (UV–Vis DR) spectroscopies. The obtained results suggest that the calcination temperature greatly influences the crystallization of TiO₂, formation of mesoporous structure, specific surface area and N-doping amounts. Among the fabricated photocatalysts, the samples calcined at 600 °C exhibit superior photocatalytic performance for hydrogen production in water/methanol solution under visible light illumination if compared to other synthesized samples and commercial TiO₂ (Degussa P25). The finding is possibly due to the synergy of more N-doping amounts on the well-defined mesoporous TiO₂ with highly anatase crystal phase and moderate surface area in the catalysts.     

TiO2–SiO2 mixed oxides: Organic ligand templated controlled deposition of titania and their photocatalytic activities for hydrogen production

A simple approach to the controlled deposition of titania with different particle sizes on silica surface has been developed by impregnation of an organic titania precursor followed by calcination. Among the several Ti-complexes tested, the templating effect of titanium phthalocyanine dichloride resulted in silica-supported titania with enhanced photocatalytic activity for photosplitting of water under UV light irradiation. The titania–silica materials were characterized by Powder X-ray diffraction (XRD), UV–Vis diffuse reflectance spectra (DRS), nitrogen adsorption studies, and Raman spectroscopic studies. The photocatalytic activity for hydrogen production is maximum at an optimal particle size wherein surface and volume recombination is minimized.     

Thermally stable Pt/Ti mesh catalyst for catalytic hydrogen combustion

In this study, platinum (Pt) supported on titanium (Ti) mesh catalysts for catalytic hydrogen combustion were prepared by depositing Pt as a thin-layer on metallic or calcined Ti mesh. The Pt thin-layer could be stabilized as uniformly distributed, near nano-sized particles on the surface of calcined Ti mesh by exposing the freshly sputtered Pt to hydrogen. Temperatures between 478 and 525 °C were reached during hydrogen combustion and could be maintained at a hydrogen flow rate of 0.4 normal liter (Nl)/min for several hrs. It was determined that Ti mesh calcination at ≥900 °C formed an oxide layer on the surface of Ti wires, which prevented significant Pt aggregation. X-ray photoelectron spectroscopy revealed that the surface of Ti mesh was fully converted to TiO₂ at ≥900 °C. Raman spectroscopy showed that the majority of TiO₂ was present in the rutile phase, with some minor contribution from anatase-TiO₂. The calcined Ti support was stable through all investigations and did not indicate any signs of degradation.     

Visible light response photocatalytic water splitting over CdS-pillared zirconium–titanium phosphate (ZTP)

With an attempt to extend the light absorption towards the visible range and inhibit the rapid recombination of excited electrons/holes, a new type photocatalysts, cadmium sulfide intercalated zirconium–titanium phosphate (CdS–ZTP) was synthesized. The photocatalysts were characterized by small angle X-ray diffraction studies (SAXS), N₂ adsorption–desorption studies, diffused reflectance UV–vis (DRUV–vis) spectroscopic analysis, photoluminescence studies (PL), scanning electron microscopic/energy dispersive spectroscopic (SEM/EDS), X-ray photoelectron spectroscopic (XPS) studies etc. The samples exhibit a unique property of optical absorption in UV and visible regions with a wavelength, λ ≤ 450 nm followed by a clear long tail up to 700 nm. The pillared materials showed excellent activity for UV–visible light driven hydrogen production from photocatalytic splitting of water without using any co-catalyst. The photocatalytic activity of this cadmium sulfide pillared catalyst, as well as that of neat cadmium sulfide powder, was monitored for the visible light-induced evolution of hydrogen from water in the presence of hole scavenger, sulfide (S²⁻).     

Development and investigation of sol–gel solutions for the formation of TiO2 coatings

We show a technological approach for the sol–gel processing of stabilized xerogel colloidal titanium oxide films. Glycerol was used as a drying control additive agent. Glycerol helped in stabilizing the solution. The thermal transformation of a xerogel film was studied by differential thermal analysis (DTA) and powder X-ray diffraction. The optical index of the annealed coatings was evaluated using UV–VIS–NIR spectrophotometry. The results showed that a nanocrystalline titania anatase film of high optical quality (n=2.34 at 600 nm) can be obtained by the sol–gel process.     

Mechanochemical interaction of titanium powder with organic liquids

The formation of the nanocrystalline cubic titanium-based phase under high energy ball milling of α-Ti (hcp) in the presence of liquid hydrocarbons (toluene or n-heptane) used as a process control agent (PCA) has been studied by X-ray diffraction, X-ray photoelectron spectroscopy and chemical analysis in detail. The state of the PCA during ball milling has been examined by IR-spectroscopy, densitometry and refractometry. It has been shown that the formation of the nanocrystalline structure and the generation of deformation stacking faults in hcp Ti as well as the appearance of the titanium-based amorphous phase take place at the early stage of ball milling. The fcc nanocrystalline phase which the nucleation sites are stacking faults is found to be formed under longer-term (up to 50 min) ball milling when the amorphous phase vanishes. The atomic ratio of H/C in as-milled samples agrees well with that in toluene and n-heptane. The ball milling in toluene leads to more particle dispersivity than that in n-heptane. Most probably, the fcc phase is metastable titanium carbohydride with deficiency of carbon and hydrogen in comparison with stable titanium carbohydrides. The metastable titanium carbohydride transforms to stable titanium carbide and stable titanium carbohydride at low-temperature (550 °C) annealing under vacuum and argon respectively. In the latter case the β-Ti phase precipitates too. The milling has caused no changes of the PCA structure and physical properties. The mechanism of the mechanochemical interaction of titanium with PCA is considered. It involves repeated fracture, adsorption of the PCA molecules on newly formed surfaces and cold welding of powder particles. As a result, the absorption of carbon and hydrogen inside the titanium particles takes place.     

Synthesis and electrochemical evaluation of an amorphous titanium dioxide derived from a solid state precursor

Titanium oxides are an important class of lithium-ion battery electrodes owing to their good capacity and stability within the cell environment. Although most Ti(IV) oxides are poor electronic conductors, new methods developed to synthesize nanometer scale primary particles have achieved the higher rate capability needed for modern commercial applications. In this report, the anionic water stable titanium oxalate anion [TiO(C₂O₄)₂]²⁻ was isolated in high yield as the insoluble DABCO (1,4-diazabicyclo[2.2.2]octane) salt. Powder X-ray diffraction studies show that the titanium dioxide material isolated after annealing in air is initially amorphous, converts to N-doped anatase above 400 °C, then to rutile above 600 °C. Electrochemical studies indicate that the amorphous titanium dioxide phase within a carbon matrix has a stable cycling capacity of ∼350 mAh g⁻¹. On crystallizing at 400 °C to a carbon-coated anatase the capacity drops to 210 mAh g⁻¹, and finally upon carbon burn-off to 50 mAh g⁻¹. Mixtures of the amorphous titanium dioxide and Li₄Ti₅O₁₂ showed a similar electrochemical profile and capacity to Li₄Ti₅O₁₂ but with the addition of a sloping region to the end of the discharge curve that could be advantageous for determining state-of-charge in systems using Li₄Ti₅O₁₂.     

Corrosion resistance and electrical conductivity of plasma nitrided titanium

The continuous and dense Ti–N compound layers with a thickness ranging from 0.7 to 2.1 μm were formed on the titanium by plasma nitriding at 700 °C for different times with hollow cathode discharge assistance. Scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the nitrided layer. XRD and XPS results showed that the compound layer was mainly composed of Ti₂N phase. The corrosion current density of 4 h nitrided titanium was 0.016 μA/cm² (cathode) and 0.03 μA/cm² (anode), respectively. The electrical conductivity of samples was evaluated by means of the interfacial contact resistance (ICR). The value of 4 h nitrided titanium was 4.94 mΩ-cm² which was much lower than that of original titanium 26.25 mΩ-cm² under applied force of 150 Ncm^?2 after corrosion test. The results showed that the electrical conductivity and corrosion resistance of the titanium bipolar plates (BPs) were apparently improved with the formation of Ti₂N compound layer.     

Fabrication and characterization of TiO2 nanotube by hydrothermal method in the design of DSSC

In this work it was presented a direct formation of anatase TiO₂ nanotube by titanium powder using, as source material. TNT was prepared by hydrothermal synthesis method using with successive acid and heat treatment. This method proves a new route to fabricate DSSC with high electron to photon conversion efficiency. The phase identification of the samples was conducted with powder X-ray diffraction (XRD). The morphologies and structure of samples were observed through SEM and TEM. The results of the investigation showed that the anatase TiO₂ nanotubes have the outer diameters of around 10 nm. We fabricated DSSC with this well-aligned one-dimensional (1-D) titania nanotubes (TNT) on indium tin oxide (TCO) by doctor blading technique. The fabricated TNT photoelectrode showed 1.64% efficiency whereas the composite photoanode with the mixture of TNP + 30% TNT gives 2.06%. This method will surely be an exciting addition to the fast growth family of synthesis of oriented TiO₂ nanotubes.     

Preparation of new titanium oxy nitride based electro catalysts using an anhydrous sol-gel method for water electrolysis in acid medium

Titanium oxy nitride has been fabricated for the first time from anhydrous Sol-gel Method and used as anode for water electrolysis in acid medium. It is shown that the properties of this material are highly dependent on the calcination temperature. The conductivity increases from insulator (titanium dioxide type) to metallic (titanium nitride type) when the calcination temperature rose from 500 °C to 1000 °C. When the catalyst material is calcinated at 700 °C, the rutile structure of titanium dioxide is obtained and agrees well with those obtained with the TGA-DSC results. These results also show that the structure of the osbornite of titanium nitride appears at 1000 °C. The BET surface area also increases with the calcination temperature. The morphology of the calcinated samples gradually breaks into smaller conchoidal fracture particles when the sample preparation temperature increases. Electrochemical voltage–current polarization curves of the oxygen evolution reaction in sulphuric acid on these anodes electrodes confirm that the materials calcinated at 800 °C or even at higher temperatures exhibit a low Tafel slope and a high electrochemistry surface area which are both known as good values for the oxygen evolution in acid medium. The results of this study suggest that our titanium oxy nitride prepared by an anhydrous sol-gel method followed by calcination is an interesting anode catalyst for water electrolysis. Its performance for this reaction is compared to those of some noble metal oxides.     

Flexible TCO-free counter electrode for dye-sensitized solar cells using graphene nanosheets from a Ti–Ti(III) acid solution

A rapid and simple route to synthesize highly conductive graphene-based nanosheets for use as a flexible counter electrode in dye-sensitized solar cells is presented. The flexible counter electrode is free of transparent conductive oxide layer, i.e., TCO-free. A clean graphene with high quality is obtained by the chemical reduction of graphene oxide (GO) using titanium metallic powders in a hydrochloric acid solution. The Ti⁺³ ions that dissociated from metallic Ti particles in a hydrochloric acid solution result in a clean graphene material with no formation of TiO₂ nanoparticles, which are always present on graphene when only Ti⁺³ ions are used for the reduction, i.e., an anatase TiO₂ nanoparticle by-product will be always left on the graphene product when not using metallic Ti particles. The chemical reaction mechanisms for these differences are revealed in this report. The reduced materials are characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, thermo-gravimetric analysis, Fourier transform infrared spectrometry, UV–vis spectroscopy and X-ray photoelectron spectroscopy. The four-point probe method is also employed to characterize the surface conductivity of the graphene films. This high quality graphene film exhibits comparable or better performance than those obtained using conventional sputtered Pt counter electrode when used as a flexible counter electrode of dye-sensitized solar cells.