Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water

Efficient Cu incorporated TiO₂ (Cu–TiO₂) photocatalysts for hydrogen generation were fabricated by four methods: in situ sol–gel, wet impregnation, chemical reduction of Cu salt, and in situ photo-deposition. All prepared samples are characterized by good dispersion of Cu components, and excellent light absorption ability. Depending on the preparation process, hydrogen generation rates of the as-prepared Cu–TiO₂ were recorded in the range of 9–20 mmol h⁻¹ g_catalyst⁻¹, which were even more superior to some noble metal (Pt/Au) loaded TiO₂. The various fabrication methods led to different chemical states of Cu, as well as different distribution ratio of Cu between surface and bulk phases of the photocatalyst. Both factors have been proven to influence photocatalytic hydrogen generation. In addition, the Cu content in the photocatalyst played a significant role in hydrogen generation. Among the four photocatalysts, the sample that was synthesized by in situ sol–gel method exhibited the highest stability. High efficiency, low cost, good stability are some of the merits that underline the promising potential of Cu–TiO₂ in photocatalytic hydrogen generation.     

Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO

TiO₂ photocatalyst with deposited CuO (CuO-TiO₂) was synthesized by the impregnation method using P25 (Degussa) as support, and exhibited high photocatalytic hydrogen generation activity from methanol/water solution. A substantial hydrogen evolution rate of 10.2 ml min⁻¹ (18,500 μmol h⁻¹ g⁻¹_catalyst) was observed over this efficient CuO-TiO₂ with optimal Cu content of 9.1 mol% from an aqueous solution containing 10 vol% methanol; this improved hydrogen generation rate is significantly higher than the reported Cu-containing TiO₂, including some Pt and Pd loaded TiO₂. Optimal Cu content of 9.1 mol% provided maximum active sites and allowed good light penetration in TiO₂. Over this efficient CuO-TiO₂, the hydrogen generation rate was accelerated by increasing the methanol concentration according to Freundlich adsorption isotherm. However, the photocatalytic hydrogen generation rate was suppressed under long time irradiation mainly due to accumulation of by-products, reduction of CuO and copper leaching, which requires further investigation.     

Photocatalytic hydrogen production with aid of simultaneous metal deposition using titanium dioxide from aqueous glucose solution

The photocatalytic hydrogen production with aid of simultaneous metal deposition using TiO₂ was investigated in biomass glucose solution. Because the hydrogen production was very trace with pure TiO₂, the simultaneous metal deposition was applied into the glucose solution. The photocatalytic H₂ production activity with TiO₂ was significantly enhanced by simultaneous metal deposition for Au and Pd. The experimental factors such as glucose concentration, metal ion concentration and reaction temperature were investigated. The photocatalytic hydrogen production increased with increasing the concentration of glucose, and it followed Langmuir–Hinshelwood mechanism. Under the optimal conditions, the photocatalytic hydrogen generations from aqueous glucose solution with in-situ Au and Pd deposited TiO₂ were about 203 and 362 times larger compared with those observed with pure TiO₂. The enhanced photocatalytic activity could be explained in terms of reduced electron hole recombination via electron transfer from conductance band of TiO₂ to metal.     

The use of products from CO2 photoreduction for improvement of hydrogen evolution in water splitting

CuO/TiO₂ photocatalysts were prepared and shown to enhance the rate of CO₂ photoreduction and the production of total organic carbon (TOC), including HCOOH, HCHO and CH₃OH. Resulting TOC could act as electron donors for enhancing visible light hydrogen evolution from Pt/TiO₂ photocatalysts. The impacts on CO₂ photoreduction were investigated including the effect of Cu dopant, pH, irradiation time and using Na₂SO₃ as a sacrificial agent, and those on hydrogen evolution was also studied including TOC concentration and Pt doping. The CO₂ photoreduction mechanisms with respect to pH and CO₂ reduction potentials were discussed. CuO/TiO₂ and Pt/TiO₂ photocatalysts were characterized by X-ray diffraction, Raman spectroscopy and diffuse reflection UV-vis spectrophotometry. Both photocatalysts showed a visible light response in comparison with pure TiO₂. The photocatalytic experiments and FT-IR spectra indicated that photoproduct desorption was the rate-limiting step in the CO₂ photoreduction.     

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.     

Highly efficient CuO incorporated TiO2 nanotube photocatalyst for hydrogen production from water

Highly dispersed CuO was introduced into TiO₂ nanotube (TNT) made by hydrothermal method via adsorption-calcination process or wet impregnation process, to fabricate CuO incorporated TNT photocatalysts (CuO-TNT) for hydrogen production. It was found that CuO-TNT possessed excellent hydrogen generation activity, which was constantly vigorous throughout 5 h reaction. Depending on the preparation method, hydrogen evolution rates over CuO-TNT were founded in the range of 64.2-71.6 mmol h⁻¹ g⁻¹_catalyst, which was much higher than the benchmark P25 based photocatalysts, and even superior to some Pt/Ni incorporated TNT. This high photocatalytic activity of CuO-TNT was mainly attributed to the unique 1-D tubular structure, large BET surface area and high dispersion of copper component. Compared to wet impregnation, adsorption-calcination process was superior to produce active photocatalyst, since it was prone to produce photocatalyst with more highly dispersed CuO.     

Efficient photocatalytic hydrogen generation by Pt modified TiO2 nanotubes fabricated by rapid breakdown anodization

Photo-assisted hydrogen generation studies of platinum loaded titanium (IV) oxide nanotubes suspended in ethanol–water mixture were carried out at room temperature. The TiO₂ nanotubes synthesized by rapid breakdown anodization technique were loaded with Pt nanoparticles by chemical reduction of aqueous chloroplatinic acid solution using sodium borohydride. The chemisorption (active) surface area of the synthesized nanocomposites for hydrogen was measured by pulse chemisorption method using temperature programmed desorption reduction oxidation equipment and found to decrease with increase in platinum loading in the range 1–10 wt%. The platinum supported nanotube composites were characterized for phase and morphology by XRD, TEM and SEM. The hydrogen generated by the photocatalytic reduction of water from water–ethanol mixture at different wavelengths of incident light, using the Pt-TiO₂ nanocomposite photocatalyst, was determined by using a proton exchange membrane based hydrogen meter. The highest hydrogen generation efficiency was observed at 1–2.5 wt% of Pt loading. The maximum photocatalytic hydrogen generation of 0.03 mol/h/g of Pt-TiO₂ was observed with a 64 W UV light source (λ = 254 nm). The photoluminescence property of the Pt loaded TiO₂ has been correlated with the hydrogen generation efficiency and the reaction mechanism briefly discussed.     

Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite

The photocatalytic hydrogen production from aqueous methanol solution was investigated with ZnO/TiO₂, SnO/TiO₂, CuO/TiO₂, Al₂O₃/TiO₂ and CuO/Al₂O₃/TiO₂ nanocomposites. A mechanical mixing method, followed by the solid-state reaction at elevated temperature, was used for the preparation of nanocomposite photocatalyst. Among these nanocomposite photocatalysts, the maximal photocatalytic hydrogen production was observed with CuO/Al₂O₃/TiO₂ nanocomposites. A variety of components of CuO/Al₂O₃/TiO₂ photocatalysts were tested for the enhancement of H₂ formation. The optimal component was 0.2 wt% CuO/0.3 wt% Al₂O₃/TiO₂. The activity exhibited approximately tenfold enhancement at the optimum loading, compared with that with pure P-25 TiO₂. Nano-sized TiO₂ photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

Improvement of photocatalytic hydrogen generation from CdSe/CdS/TiO2 nanotube-array coaxial heterogeneous structure

The fabrication and characterization of CdSe/CdS/TiO₂ nanotube-array coaxial heterogeneous structure that has potential applications in photocatalytic water splitting and toxic pollutants degradation are investigated. CdSe(top)/CdS(under) double-layer is conformally deposited onto TiO₂ nanotubes by successive ionic layer adsorption and reaction (SILAR) and electrochemical atomic layer deposition (ECALD), respectively, for the CdS under layer and the CdSe top layer. Such double sensitized TiO₂ nanotubular photoelectrode exhibits significant enhancements in photoconversion efficiency, visible light response, and efficient hydrogen generation. The detailed synthesis process and the surface morphology, phase structure, elemental analysis, and photoelectrochemical properties of the resulting films with the CdSe/CdS/TiO₂ nanotube-array coaxial heterogeneous structure are discussed. The photoconversion efficiency of 9.47% and hydrogen generation rate of 10.24 ml h⁻¹ cm⁻² were observed. Both values are a 7-fold enhancement compared with that of the pure TiO₂ nanotube. The as-prepared photoelectrode presents potential application for industrialized photocatalytic hydrogen generation in the future.     

The design of a hierarchical photocatalyst inspired by natural forest and its usage on hydrogen generation

A novel photocatalyst was designed from the inspiration of natural forest's high efficient on light harvesting and energy conversion. This novel “forest-like” photocatalyst was successfully synthesized by a facile continuously-conducted three steps methods: electrospinning TiO₂ nanofiber acts as the trunks, hydrothermal growth ZnO nanorods on the surface of TiO₂ nanofiber acts as the branches, while photodeposition of Cu nanoparticles on the surface of TiO₂ nanofiber and ZnO nanorods act as the leaves. This novel photocatalyst demonstrated higher photocatalytic hydrogen generation rate than most of semiconductor catalysts and many newly developed catalysts such as Pt/TiO₂ catalyst and artificial leaves Pt/N–TiO₂ catalyst in a water/methanol sacrificial reagent system under the light irradiation as a result of its enhanced light absorption ability, enlarged specific surface area promoting mass transfer and providing more reaction sites and its potential on anti-recombination of electrons and holes. Meanwhile, it is interesting to note that the photocatalytic hydrogen generation activity has a liner relationship with the hierarchy of materials, which means higher hierarchy materials display higher photocatalytic hydrogen generation activity. It is reasonable to believe that this natural mimic photocatalyst without noble metals will benefit the energy generation and novel materials development.     

CuO@NiO core-shell nanoparticles decorated anatase TiO2 nanospheres for enhanced photocatalytic hydrogen production

Core-shell structured co-catalyst has been created much attention in photocatalytic hydrogen production due to their efficient electron-hole pair separation, suppression of surface back reaction and long term stability. Here, we report the preparation of CuO@NiO hierarchical nanostructures as a co-catalyst deposited on TiO₂ nanospheres for enhanced photocatalytic hydrogen generation. The formation of ultrathin NiO shell over the CuO core was confirmed by TEM analysis. Fabricated core-shell nanostructured CuO@NiO over TiO₂ nanospheres was studied for hydrogen evolution under the direct solar light and it showed a high rate of H₂ production of 26.1 mmol. h⁻¹. g⁻¹_cat. It was scrutinized that the rate of hydrogen production was improved with shell thickness and co-catalyst loading. Systematic investigation on CuO@NiO co-catalyst loading, pH of the medium and glycerol concentration for augmented H₂ production. The recorded rate of hydrogen production is almost six folds greater than that of pristine TiO₂. In the view of largescale synthesis for alternative energy storage applications, the composited photocatalyst was made of by simple mixing method, which could be scaled up without any loss in photocatalytic activity. Further, the stability test of photocatalyst for continuous use found that 82% of initial photocatalytic activity is retained even after three days.     

TiO2 nanotubes for hydrogen generation by photocatalytic water splitting in a two-compartment photoelectrochemical cell

Highly ordered TiO₂ nanotube arrays for hydrogen production have been synthesized by electrochemical anodization of titanium sheets. Under solar light irradiation, hydrogen generation by photocatalytic water splitting was carried out in the two-compartment photoelectrochemical cell without any external applied voltage. The hydrogen gas and oxygen generated on Pt side and on TiO₂ nanotubes side respectively were efficiently separated. The effect of anodization time on the morphology structures, photoelectrochemical properties and hydrogen production was systematically investigated. Due to more charge carrier generation and faster charge transfer, a maximum photoconversion efficiency of 4.13% and highest hydrogen production rate of 97 μmol h⁻¹cm⁻² (2.32 mL h⁻¹cm⁻²) were obtained from TiO₂ nanotubes anodized for 60 min.     

Enhancement of photocatalytic hydrogen production by liquid phase plasma irradiation on metal-loaded TiO2/carbon nanofiber photocatalysts

Enhanced hydrogen production by photocatalytic decomposition was assessed using liquid phase plasma over metal-loaded photocatalysts. Effects of irradiation of the liquid phase plasma were evaluated in the photocatalytic hydrogen production of hydrogen. Carbon nanofiber was introduced as photocatalytic support for the Ni-loaded TiO₂ photocatalyst. The influence of addition of organic reagents into water on hydrogen evolution was also evaluated. The photocatalytic decomposition by irradiation of the liquid phase plasma without photocatalyst produced some hydrogen evolution. The rate of hydrogen evolution was improved by the metal loading on the TiO₂ surface. The carbon nanofiber acted as a useful photocatalytic support for the fixation of TiO₂. Hydrogen evolution was enhanced by the Ni loading on the TiO₂ nanocrystallites supported on the carbon nanofiber support. Hydrogen evolution was increased significantly by the addition of organic reagents, which acted as a type of sacrificial reagent promoting photocatalysis.     

Enhanced photocatalytic hydrogen evolution from reduced graphene oxide-defect rich TiO2-x nanocomposites

Solar-driven photocatalytic hydrogen generation by splitting water molecules requires an efficient visible light active photocatalyst. This work reports an improved hydrogen evolution activity of visible light active TiO_2-x photocatalyst by introducing reduced graphene oxide via an eco-friendly and cost-effective hydrothermal method. This process facilitates graphene oxide reduction and incorporates intrinsic defects in TiO₂ lattice at a one-pot reaction process. The characteristic studies reveal that RGO/TiO_2-x nanocomposites were sufficiently durable and efficient for photocatalytic hydrogen generation under the visible light spectrum. The altered band gap of TiO_2-x rationally promotes the visible light absorption, and the RGO sheets present in the composites suppresses the electron-hole recombination, which accelerates the charge transfer. Hence, the noble metal-free RGO/TiO_2-x photocatalyst exhibited hydrogen production with a rate of 13.6 mmol h^?1g^?1_cat. under solar illumination. The appreciable photocatalytic hydrogen generation activity of 947.2 μmol h^?1g^?1_cat with 117 μAcm^?2 photocurrent density was observed under visible light (>450 nm).     

Enhanced hydrogen generation from methanol aqueous solutions over Pt/MoO3/TiO2 under ultraviolet light

The photocatalytic activity in hydrogen production from methanol reforming can be significantly enhanced by Pt/MoO₃/TiO₂ photocatalysts. Compared with Pt/P25, the photocatalytic activity of optimized Pt/MoO₃/TiO₂ shows an evolution rate of 169 μmol/h/g of hydrogen, which is almost two times higher than that of Pt/P25. XRD and Raman spectra show that MoO₃ are formed on the surface of TiO₂. It is found that with the bulk MoO₃ just formed, the catalyst shows the highest activity due to a large amount of heterojunctions and the high crystallinity of MoO₃. The HRTEM image showed a close contact between MoO₃ and TiO₂. It is proposed that the Z-scheme type of heterojunction between MoO₃ and TiO₂ is responsible for the improved photocatalytic activity. The heterojunction structure of MoO₃/TiO₂ does not only promote the charge separation, but also separates the reaction sites, where the oxidation (mainly on MoO₃) and reduction (on TiO₂) reactions occurred.     

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.     

Photocatalytic hydrogen production from aqueous methanol solution using titanium dioxide with the aid of simultaneous metal deposition

The photocatalytic hydrogen generation from aqueous methanol solution using TiO₂ photocatalyst was investigated with the aid of simultaneous metal deposition. The photocatalytic hydrogen evolution with pure TiO₂ was very small. The simultaneous deposition for various metals was therefore evaluated. As a result, the additions of Au and Cu ions were effective for the improvement of photocatalytic hydrogen production. Methanol concentration and metal ion concentration were optimized for the system. The optimal methanol concentrations were 90 and 80 vol% in the case of addition of Au and Cu ions, respectively. Under the optimal conditions, the photocatalytic hydrogen production using TiO₂ photocatalyst with the aid of simultaneous Cu and Au deposition were approximately 25 and 120 times larger than those obtained with bare TiO₂.     

TiO2 nanosheets loaded with Cu: A low-cost efficient photocatalytic system for hydrogen evolution from water

Rutile TiO₂ nanosheets were prepared by a simple solvothermal process, and Cu was loaded on the surface of TiO₂ nanosheets using the in situ photo-deposition method. Meanwhile, photocatalytic H₂ evolution from water over the as-prepared TiO₂ nanosheets loaded with Cu was explored using methanol as a sacrificial reagent. The results indicate that the TiO₂ nanosheets loaded with Cu is an efficient photocatalyst under UV irradiation. During the first 5 h, a rate of H₂ evolution of approximately 22.1 mmol g⁻¹ h⁻¹ was achieved under optimal conditions. Furthermore, for practical purposes, the photocatalytic hydrogen evolution was studied as a function of content of Cu, pH of solution, concentration of methanol and dosage of photocatalyst, respectively. At last, the photocatalytic mechanism was preliminarily discussed.     

Carbon-incorporated TiO2 microspheres: Facile flame assisted hydrolysis of tetrabutyl orthotitanate and photocatalytic hydrogen production

We report a novel and facile energy-saving method to prepare microspherical carbon-incorporated titania powders by flame assisted hydrolysis of tetrabutyl orthotitanate. The as-prepared samples were fully characterized by XRD, SEM, XPS, UV-Vis absorption spectra and photocatalytic hydrogen production. Anatase TiO₂ can be obtained directly without any post heat treatment. The as-prepared TiO₂ is formed of microspheres with sizes in a range of 0.5∼2.0 μm. XPS measurement shows the presence of carbon species which come from the incomplete combustion of organic compounds. Enhanced photocatalytic hydrogen production rates were observed for the as-prepared samples. A maximum hydrogen production rate was 8.1 μmol h⁻¹, which was 1.8 times larger than that of Degussa P25. The improved photocatalytic activity is attributed to enhanced light absorption behavior, which is caused by carbon incorporation and microspherical structure. This work demonstrates a novel and efficient strategy to synthesize microspherical anatase TiO₂ photocatalyst without any special equipment or setup.     

Remarkable promotion of photocatalytic hydrogen evolution from water on TiO2-pillared titanoniobate

TiO₂-pillared titanoniobate TiO₂/HTiNbO₅ as an efficient photocatalyst was prepared via an exfoliation–restacking route. The as-prepared nanohybrid is mesoporous with a high specific surface area of 171 m²/g and a gallery height of 1.55 nm. Under a 300 W Xe lamp irradiation, the nanohybrid exhibited a high photocatalytic activity of 219 μmol/h/(g cat) in splitting water into hydrogen, which is 12 times as high as its parent HTiNbO₅ (18 μmol/h/g) and 24 times as TiO₂ (9 μmol/h/g). Enlarged surface area and effective electronic coupling between the host and the guest components contribute to the high photocatalytic activity of TiO₂/HTiNbO₅. Its photocatalytic activity was further improved through platinizing, and 5 wt% Pt-loaded TiO₂/HTiNbO₅ gave a remarkable hydrogen evolution rate of 4735 μmol/h/g. A photoexcitation model of the semiconductor–semiconductor pillared photocatalyst was proposed based on the results of XPS and UV–vis.