In-situ photo-deposition CuO1−x cluster on TiO2 for enhanced photocatalytic H2-production activity

CuO_1?x cluster-modified TiO₂ (CuO_1?x/TiO₂) photocatalysts were prepared by an in-situ photoreduction deposition of Cu on TiO₂ powder support using copper acetate as a Cu source. The prepared samples without any Pt co-catalyst present an especially high photocatalytic H₂-evolution activity under solar light irradiation with 5% glycerol as sacrificial agent. The optimal CuO_1?x/TiO₂ catalyst with only 1 wt% CuO_1?x exhibits a high activity of 1725 μmol h^?1 g^?1 for H₂ evolution, which reaches 120 times that of TiO₂. The high photocatalytic activity of H₂ production is attributed to the highly dispersed CuO_1?x nano clusters on the surface of the TiO₂. In addition, Pt/CuO_1?x/TiO₂ was also prepared by loading Pt on CuO_1?x/TiO₂ sample, and its photocatalytic hydrogen evolution activity is enhanced 1.8 times compared with that of Pt/TiO₂ for overall water splitting reaction under solar light, demonstrating that a small amount CuO_1?x wondrously improves the photocatalytic activity of Pt/TiO₂ for overall water splitting reaction. This paper reports an economic and simple approach to prepare a photocatalyst with high hydrogen-production activity.     

Fabrication of anatase TiO2 tapered tetragonal nanorods with designed {100}, {001} and {101} facets for enhanced photocatalytic H2 evolution

In this study, anatase TiO₂ nanorods with exposed high-energy {100} and {001} facets and low-energy {101} facets were fabricated in the presence of surfactants cetyltrimethylammonium bromide, didecyldimethylammonium bromide, and ammonia via a facile hydrothermal method without the erosive reagent hydrofluoric acid. The particle size and morphology were mainly tuned by regulating the hydrothermal temperature. When the temperature was increased from 150 °C to 180 °C and 200 °C, the length of the nanorods decreased from 700-1000 nm to 400–500 nm and 100–200 nm, respectively. Concurrently, the edges and tops of the truncated tetragonal pyramid of the TiO₂ nanorods became blurry and flattened. The synthesized typical TiO₂ nanorods were then used as photocatalysts, and their performance during the direct generation of H₂ from water was evaluated. The TiO₂ nanorods obtained at 150 °C successfully produced high amounts of H₂ evolution (281.36 μmol) in the presence of methanol as a sacrificial agent under ultraviolet light irradiation for 4 h. The outstanding photocatalytic activity of the nanorods was mainly ascribed to the formation of surface heterojunctions in the edges and corners between adjacent high-energy {001} or {100} facets and low-energy {101} facets. The formed heterojunctions could facilitate charge separation through preferential carrier flow toward the specific facets.     

High visible-light photocatalytic hydrogen evolution of C,N-codoped mesoporous TiO2 nanoparticles prepared via an ionic-liquid-template approach

A simple and novel method was developed to fabricate carbon and nitrogen codoping mesoporous TiO₂ (CNMT-x) through evaporation induced self-assembly by using an ionic liquid simultaneously as carbon, nitrogen sources and a mesopore creator. These CNMT-x samples were fully characterized by a series of spectroscopic and analytical techniques, such as small- and large-angle X-ray diffraction (XRD), N₂ adsorption–desorption isotherms, transmission electron microscopy (TEM), Raman, ultraviolet–visible (UV–vis) diffuse reflectance and X-ray photoelectron (XPS) spectroscopies. The obtained results showed that CNMT-0.75 catalysts exhibited the optimal photocatalytic hydrogen generation activity in water/methanol sacrificial reagent system under visible light irradiation, which was highly superior to those of conventionally prepared C-doped and commercial TiO₂. The superior photocatalytic performances observed for CNMT-x could be attributed to the synergistic effects of C,N-codoping with high surface area of mesostructured TiO₂.     

An efficient photocatalytic system containing Eosin Y, 3D mesoporous graphene assembly and CuO for visible-light-driven H2 evolution from water

In the present work, 3D mesoporous graphene assembly was fabricated in a hydrothermal process using triethylenetetramine molecules as cross-linkers. And CuO nanoparticles were introduced in the graphene assembly via in-situ photodeposition. Then, a photocatalytic system containing Eosin Y as a sensitizer, graphene assembly as a supporter material and electron transfer channel, and CuO nanoparticles as an active center of H₂ evolution from water was prepared. Meanwhile, photocatalytic hydrogen evolution from water over the as-prepared photocatalytic system was explored under visible irradiation. Furthermore, for practical purposes, the durability of the photocatalytic system was also studied. And the photocatalytic mechanism was preliminarily discussed. The experimental results indicate that the as-prepared photocatalytic system is an efficient photocatalyst for visible-light-driven H₂ evolution from water. The rate of H₂ evolution over the photocatalytic system is up to 5.85 mmol g^?1 h^?1 under optimal conditions, which is 2.3 times higher than that over reduced graphene oxide loaded with CuO. The 3D porous graphene assembly plays an important role in the photocatalytic process. It can not only efficiently enhance the electron transfer in the photocatalytic system, but also result in fast diffusion of sacrificial reagent and timely release of H₂ bubbles. This work provides us with new possibility for designing an efficient Pt-free visible photocatalyst for H₂ evolution from water.     

Hydrogen production from sulphide wastewater using Ce3+–TiO2 photocatalysis

Cerium (Ce³⁺) doped TiO₂ powder was synthesized by a sol-gel method and characterized by Transmission Electron Microscope (TEM), X-ray Diffraction (XRD), UV–Vis Diffuse Reflectance Spectroscopy (UV-DRS), Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The Ce³⁺ doping strongly reduced the band gap of the TiO₂ from 3.2 eV (UV) to 2.7 eV (visible region). The photocatalytic activity of Ce³⁺ doped TiO₂ catalysts was evaluated by hydrogen production from sulphide wastewater under visible light illumination. The photocatalytic production of H₂ was studied in a batch recycle tubular photocatalytic reactor. The results show that 0.4% Ce³⁺–TiO₂ suspended in 500 mL of simulated sulphide wastewater irradiated at 150 W visible lamp produced maximum H₂ of 6789 μmol h^?1. It was noticed that the Ce³⁺ doped TiO₂ performs well than Nano TiO₂ and P25 TiO₂ photocatalysts.     

Lab scale optimization of various factors for photocatalytic hydrogen generation over low cost Cu0.02Ti0.98O2-δ photocatalyst under UV/Visible irradiation and sunlight

We have demonstrated earlier that maximum H₂ generated @ 1.167 l/h/m² over Cu_0.02Ti_0.98O_2-δ photocatalyst with apparent quantum efficiency, AQE of 7.5% and solar fuel efficiency, SFE of 3.9% under sunlight. With an aim to scale-up the solar photocatalytic hydrogen process to pilot plant, optimization studies at lab scale as well as in upscaled photoreactors were performed over Cu_0.02Ti_0.98O_2-δ, photocatalyst under UV/visible and sunlight. Cu_0.02Ti_0.98O_2-δ was synthesized by facile sol-gel route and characterized by relevant techniques. Several operational parameters were investigated in order to finalize the conditions which are most favourable for photocatalytic hydrogen yield. Factors such as photocatalyst loadings, v/v concentration of sacrificial reagent, replacement of methanol by industrial waste glycerol, role of different configuration of light source with reactor, effect of stirring during the photocatalytic reaction, effect of fluctuations of solar flux at hourly basis, illumination area on hydrogen yield were studied. Contribution of each factor in determining the hydrogen yield was quantified. Relative standard deviation in hydrogen yield as a function of each factor was estimated. Our findings suggest that in addition to catalyst loadings and sacrificial reagent, improved dispersion of photocatalyst obtained by stirring the reaction mixture in horizontal geometry resulted in enhanced H₂ yield. Hydrogen yield obtained at lab scale can be appropriately extrapolated with respect to illumination area instead of weight of photocatalyst. A relative standard deviation (RSD) of ± 3.82% and ± 4.53% in H₂ yield was calculated for sunny and cloudy days in time zone of 10.30–16.30 h IST. Deviation of H₂ yield was more on cloudy days and beyond 16:30 h. These studies have provided a daily window of 11:00–15:00 h to be utilized throughout the year for a commercial scaled up process, prohibiting the illumination during less productive hours of the day for shaping the improved economics of solar hydrogen generation. Our results obtained at lab scale would be useful to perform sunlight driven scaled –up photocatalytic process using low cost visible light efficient photocatalyst, Cu_0.02Ti_0.98O_2-δ.     

Improving the photocatalytic H2 evolution activities of TiO2 by modulating the stabilizing ligands of the nanoscale Ti8O8-cluster precursors

Titanium-oxo clusters (TOCs) are important model complexes and potential precise single-source precursors of TiO₂ materials. We report here a series of TiO₂ samples derived from TOCs with the same wheel-like Ti₈O₈ clustercore but different stabilizing ligands. Our results demonstrate that the ligands on TOCs not only influence the phase-transition during themolysis, but more importantly tune the photocatalytic H₂ evolution activities of the obtained TiO₂ materials. Through 500 °C calcination, the nanosized Ti₈O₈ cluster stabilized by organic benzoate ligands gives rise to a TiO₂ sample with H₂ production rate of 611 μmol/h/g. When the stabilizing ligands are changed to inorganic SO₄^2?, a significant improvement of H₂ evolution activity has been achieved, resulting in a 7-times higher value of 4527 μmol/h/g. This work provides a new strategy to produce TiO₂ with enhanced and tunable photocatalytic activities from titanium-oxo cluster precursors.     

Construction of Z scheme system of ZnIn2S4/RGO/BiVO4 and its performance for hydrogen generation under visible light

A series of Z scheme systems are constructed in three ways and the photocatalytic H₂ evolution activities are evaluated under visible light irradiation. The Z scheme system can be constructed by loading ZnIn₂S₄ onto BiVO₄. The H₂ evolution is successfully realized under the visible light without providing a sacrificial agent, and the activity is greatly improved when the graphene is acted as a solid electron mediator. The best Z scheme system of 1.0La-ZnIn₂S₄/1.5RGO/1.0RuO₂/BiVO₄ (1:5) is obtained. Its photocatalytic H₂ evolution activity and the A.Q.Y. reach 4.1 μmol g^?1 h and 0.8%, respectively. Furthermore, its property is characterized by various analysis techniques, such as XRD, Raman, SEM, BET and PL, and the catalytic mechanism is also discussed.     

Edge-sulfonated graphene-decorated TiO2 photocatalyst with high H2-evolution performance

Graphene as a prospective cocatalyst can obviously promote the photocatalytic H₂-evolution performance of photocatalysts by rapidly transferring photogenerated electrons. For an efficient graphene-modified photocatalytic system for H₂ evolution, the fast H₂-evolution reaction is as important as the rapid photoelectron transfer via graphene. In this paper, edge-sulfonated graphene (rGO-SO₃H) with high H⁺-adsorbed activity was successfully synthesized by the formation of covalent bonds between graphene and benzenesulfonic acid through a diazotization reaction, which couples with TiO₂ nanoparticles to prepare rGO-SO₃H/TiO₂ photocatalyst for accelerating H₂-evolution reaction. The results showed that the rGO-SO₃H/TiO₂ displayed the highest H₂-production rate of 197.1 μmol h^?1 g^?1 as high as 5.38, 2.81, and 3.40 times of TiO₂, rGO/TiO₂, and SO₃H/TiO₂, respectively. The improved efficiency of rGO-SO₃H/TiO₂ can be attributable to the synergetic action of graphene as a photoelectron cocatalyst and sulfonate ions as H⁺-adsorbed active sites. This study provides a new insight for the efficient hydrogen-evolution graphene-based photocatalysts.     

NiSe as an effective co-catalyst coupled with TiO2 for enhanced photocatalytic hydrogen evolution

Construction of semiconductor heterojunctions can effectively accelerate the separation of photo-induced charge carriers and thereby enhance photocatalytic activity. Here, NiSe was used as an effective co-catalyst to construct an active NiSe/TiO₂ heterojunction for improving the photocatalytic H₂ production of TiO₂. The resultant 10%NiSe/TiO₂ heterojunction exhibited 11 times higher photocatalytic H₂-production activity than that of bare TiO₂. The NiSe/TiO₂ heterojunction and the photo-reduction of partial Ni²⁺ to Ni⁰ notably accelerated the separation and transfer of photo-excited electron-hole pairs, and thus resulted in obvious improvement of H₂-evolution activity. This work holds promise for the application of NiSe in photocatalysis as a high-efficiency photocatalytic cocatalyst.     

Solar photocatalytic decomposition of Probenazole in water with TiO2 in the presence of H2O2

The photocatalytic decomposition of Probenazole in water using TiO₂/H₂O₂ under sunlight illumination is studied. The addition of H₂O₂ is effective for the improvement of photocatalytic decomposition of Probenazole with TiO₂. Furthermore, the operating conditions, such as photocatalyst dosage, temperature, pH, sunlight intensity and illumination time are also optimized. The kinetics of photocatalytic decomposition follow a pseudo–first–order kinetic law, and the rate constant is 0.129 min^?1. The activation energy (E_a) is 11.34 kJ/mol. The photocatalytic decomposition mechanism is discussed on the basis of molecular orbital (MO) simulation for frontier electron density.     

Enhanced H2 evolution of TiO2 with efficient multiple electrons transfer modified by tiny CuOx–NiO bimetallic oxides

CuO_x–NiO bimetallic oxides modified TiO₂ catalysts were prepared via a precipitation-photoreduction approach for photocatalytic H₂ production. 0.9%Cu0.1%Ni/TiO₂ exhibited the highest H₂ evolution rate (about 55.4 μmol/g/min) under UV–visible light irradiation (350–780 nm), which is higher than the sum of CuO_x/TiO₂ and NiO/TiO₂ catalyst. Cu species exhibited more reducing valence state including Cu⁰ when deposited on NiO/TiO₂ compared with deposited on TiO₂. The polyvalent Cu species lead to enhanced photocurrent and reduced transfer resistance confirmed by photoelectron chemical analysis. CuO_x–NiO modification of the TiO₂ surface is beneficial for the photoexcited electron transfer, which suppresses electrons recombination. The lifetime of photoexcited electrons was enhanced greatly from 1.16 (TiO₂) to 5.15 ns (CuO_x–NiO/TiO₂). A multiple electrons transfer mechanism for polyvalent bimetallic oxides co-modified TiO₂ was proposed based on careful analysis. This work demonstrated a polyvalent bimetallic oxides modified TiO₂ system with efficient photocatalytic H₂ production which can provide some insightful understanding for bimetallic metal oxides co-catalyst design and H₂ evolution mechanism.     

Light-induced confined growth of amorphous Co doped MoSx nanodots on TiO2 nanoparticles for efficient and stable in situ photocatalytic H2 evolution

Amorphous molybdenum sulfide (a-MoS_x) prepared by in situ photoreduction method with an abundance of exposed active sites has been identified as an efficient cocatalyst for catalyzing photocatalytic H₂ evolution reaction (HER). However, the intrinsic activity of the a-MoS_x cocatalyst toward HER is low due to the unfavorable electronic structures of the active sites. Herein, we report a facile light-induced method for the confined growth of transition metal (TM) doped MoS_x (a-TM-MoS_x) cocatalysts on TiO₂ nanoparticles and their catalytic activity for in situ photocatalytic HER. It is found that doping Co into a-MoS_x can greatly enhance the activity of resulted a-Co-MoS_x cocatalyst for photocatalytic H₂ evolution over TiO₂ among the transition metal dopants (Co, Ni, Fe, Cu, Zn) tested. The most efficient a-Co-MoS_x cocatalyst (Co/Mo = 1/4 and 4 mol% loading) loaded TiO₂ (TiO₂/a-Co-MoS_x) shows a H₂ evolution rate of 133.8 μmol h⁻¹, which is 3.3 times higher than that of a-MoS_x loaded TiO₂ (TiO₂/a-MoS_x). Moreover, the TiO₂/a-Co-MoS_x photocatalyst shows excellent recycling H₂ evolution stability. The characterization results reveal that a-Co-MoS_x cocatalyst can not only effectively capture the photogenerated electrons of TiO₂ to greatly enhance the separation efficiency of photogenerated charges but also significantly reduce the overpotential of HER due to the formation of highly active “CoMoS” sites, thus synergistically enhancing the catalytic activity of TiO₂/a-Co-MoS_x. Moreover, the light-induced growth of a-Co-MoS_x on TiO₂ is found to readily couple with the in situ photocatalytic HER. Therefore, this work provides a simple and efficient strategy for designing high-performance a-MoS_x-based cocatalysts for stable in situ photocatalytic H₂ evolution.     

Shining light on panchromatic ruthenium sensitizers towards dye-sensitized photocatalytic hydrogen evolution

Five new photocatalysts have been synthesized in order to extend the photo response upto visible range, by adsorbing MC113-MC117 ruthenium complexes on TiO₂-Pt composites. Highlight harvesting properties of these ruthenium complexes instigated us to evaluate for photocatalytic activity. The absorption curves of the synthesized photocatalysts extended up to 750 nm. Morphological studies of photocatalysts have been carried out using SEM and powder X-ray crystallography. Among all photocatalysts, MC113PC showed high photocatalytic activity i.e. 9474 TONs. IPCE and fluorescence quenching studies of the catalysts revealed the light harvesting nature and electron injection efficiency. The photocatalytic activity of MC photocatalysts were systematically screened at different pH and employing different sacrificial electron donors (SED) in order to obtain optimal photocatalytic performance.     

Simultaneous hydrogen production and decomposition of dissolved in alkaline water over composite photocatalysts under visible light irradiation

A process of simultaneous hydrogen production and H₂S removal has been investigated over a highly active composite photocatalyst made of bulk CdS decorated with nanoparticles of TiO₂, i.e. CdS(bulk)/TiO₂. The photocatalytic activity was evaluated for hydrogen production from aqueous electrolyte solution containing H₂S dissolved in water or alkaline solution under visible light irradiation. The rate of hydrogen production from the H₂S-containing alkaline solution was similar to the rate obtained from photocatalytic hydrogen production from water containing sacrificial reagents (Na₂S+Na₂SO₃) in the similar concentration. The isotope experiment was carried out with D₂O instead of H₂O to investigate the source of hydrogen from photocatalytic decomposition of H₂S dissolved in H₂O or alkali solution under visible light. Hydrogen originated from both H₂S and H₂O when the reaction solution contained H₂S absorbed in alkaline water.     

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.     

Enhancing hydrogen evolution of water splitting under solar spectra using Au/TiO2 heterojunction photocatalysts

An effective improvement of hydrogen evolution from water splitting under solar light irradiation was investigated using quantum dots (QDs) compounds loaded onto a Au/TiO₂ photocatalyst. First, Au/TiO₂ was prepared by the deposition-precipitation method, and then sulfide QDs were loaded onto the as-prepared Au/TiO₂ by a hydrothermal method. QDs were loaded onto Au/TiO₂ to enhance the energy capture of visible light and near-infrared light of the solar spectrum. The results indicated that the as-prepared heterojunction photocatalysts absorbed the energy from the range of ultraviolet light to the near-infrared light region and effectively reduced the electron-hole pair recombination during the photocatalytic reaction. Using a hydrothermal temperature of 120 °C, the as-prepared (ZnS–PbS)/Au/TiO₂ photocatalyst had a PbS QDs particle size of 5 nm, exhibited an energy gap of 0.92 eV, and demonstrated the best hydrogen production rate. Additionally, after adding 20 wt % methanol as a sacrificial reagent to photocatalyze for 5 h, the hydrogen production rate reached 5011 μmol g⁻¹ h⁻¹.     

Development of hydrogen production by liquid phase plasma process of water with NiTiO2/carbon nanotube photocatalysts

Hydrogen evolution by water photocatalysis using liquid phase plasma system was disserted over metal-loaded TiO₂ photocatalysts. Carbon nanotube was applied as a support for the metal-loaded TiO₂ nanocrystallites. Photocatalytic activities of the photocatalysts were estimated for hydrogen production from water. Hydrogen was produced from the photodecomposition of water by liquid phase plasma irradiation. The rate of hydrogen evolution was improved by the metal loading on the TiO₂ surface. TiO₂ nanocrystallites were incorporated above 40 wt% onto the carbon nanotube support. The carbon nanotubes could be applied 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 nanotube. Hydrogen evolution was increased apparently with addition of the alcohols which contributes as a kind of sacrificial reagent promoting the photocatalysis.     

Chlorophyll derivatives/MXene hybrids for photocatalytic hydrogen evolution: Dependence of performance on the central coordinating metals

Development of efficient photocatalytic hydrogen evolution reaction (HER) with illumination of visible light is challenging. In this work, five chlorophyll derivatives (M-Chls; M = H₂/Cu/Ni/Co/Zn) with different central ions in its cyclic tetrapyrrole ring including free base, copper, nickel, cobalt, and zinc were synthesized and employed as the effective visible-light harvester for efficient HER. In addition, two-dimensional (2D) noble metal-free co-catalyst Ti₃C₂T_x MXene was used as an excellent electron capturer due to its outstanding conductivity property. These M-Chls are modified on the surface of Ti₃C₂T_x MXene with 2D accordion-like morphology by means of a simple deposition process to form noble metal-free Chl/Ti₃C₂T_x-based photocatalysts for HER. It is found that the best HER performance as high as 49 μmol/h/g_cat was achieved with the Co-Chl@Ti₃C₂T_x hybrid, which was much higher than those of other M-Chl@Ti₃C₂T_x composites. This research provides a specific way to synthesize low-cost and environmentally friendly natural Chls for developing highly efficient photocatalytic HER through molecular engineering.     

Photocatalytic system with water soluble nickel complex of S,S′-bis(2-pyridylmethyl)-1,2-thioethane over CdS nanorods for hydrogen evolution from water under visible light

In this paper, we report a new nickel complex, [(bpte)NiCl₂] (bpte = S,S′-bis(2-pyridylmethyl)-1,2-thioethane) that can serve as a catalyst both for electrochemical and photochemical driven hydrogen production from water. As an electrocatalyst, [(bpte)NiCl₂] can electrocatalyze hydrogen generation from a neutral buffer with a turnover frequency (TOF) of 555.78 mol of hydrogen per mole of catalyst per hour (mol H₂/mol catalyst/h) at an overpotential (OP) of 837.6 mV. Together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H₂A) as a sacrificial electron donor, the nickel complex also can photocatalyze hydrogen evolution in heterogeneous environments and can work for 107 h. Under an optimal condition, the photocatalytic system can afford 24900 mol of H₂ per mole of catalyst during 83 h irradiation, with a TOF of 300H₂ per catalyst per hour. The average value of apparent quantum yield (AQY) is ～24% at 420 nm.