Photocatalytic H2 production over RuO2@ZnS and RuO2@CuS nanostructures

Photocatalytic hydrogen production is a promising approach of sustainable economy, because a use of sunlight and water to produce a fuel will solve a problem of fossil fuels depletion. Metal sulfides are well known photocatalysts in water splitting process, but in absence of sacrificial electron donor they undergo a photocorrosion. In this paper we studied a possible strategy to protect the sulfide photocatalysts and to improve its photostability by a deposition of small amount of ruthenium oxide at surface of sulfides. Nanocrystalline zinc sulfide and copper sulfide were prepared in a hydrothermal way and have been functionalized by RuO₂. As prepared photocatalysts showed good activity towards hydrogen formation. Modification of sulfides with ruthenium oxide had a few positive effects: it expanded a light absorption range by photocatalysts, enhanced the photocatalytic activity towards H₂ formation, improved a photostability in comparison with neat ZnS and CuS as well as protected from the electronic and structural changes within semiconductors due to irradiation.     

Green synthesis of well dispersed TiO2/Pt nanoparticles photocatalysts and enhanced photocatalytic activity towards hydrogen production

Deposition of Pt NPs with preferred dispersion and morphologies on TiO₂ have been the focus of studies in photocatalytic and photoelectrochemical hydrogen production. Green synthesis of TiO₂/Pt NPs nanocomposites with narrow size distribution of Pt NPs still remain a challenge. Herein, we report that sucrose is highly efficient for the preparation of well-dispersed TiO₂/Pt NPs photocatalysts. Moreover, the sucrose could act as an electron donor, showing higher hydrogen production activity under simulated sunlight than pure water. The as-synthesized photocatalysts have been characterized by techniques of transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX), and diffuse reflectance spectroscopy (DRS). Compared with TiO₂/Pt NPs photocatalysts prepared through conventional photodeposition, the photocatalysts as prepared showed higher photocatalytic efficiency. Moreover, the catalyst could be reused easily without apparent degradation of their original photocatalytic activities. This approach presents a promising and low-cost strategy to improve the photocatalytic performance of TiO₂ from biomass.     

Organosilica-assisted superhydrophilic oxygen doped graphitic carbon nitride for improved photocatalytic H2 evolution

The regulation of surface wettability and heteroelement doping have been proved to be effective strategies to enhance photocatalytic H₂ evolution activity of graphitic carbon nitride (CN) based photocatalysts. Herein, we report, for the first time, an organosilica assisted method was adopted to synthesize the superhydrophilic oxygen doped graphitic carbon nitride (O–CN). The presence of organosilica induced simultaneous oxygen-containing groups grafting and oxygen doping within carbon nitride substrate. The grafted oxygen-containing groups improved the surface hydrophilicity and water adsorption. Oxygen doping tailored electronic structure and localized electron distribution, contributing to extended visible light harvesting and elevated photoelectric conversion efficiency. As a result, the H₂ generation rate of O–CN photocatalyst was 5.4 times higher than that of pristine CN photocatalyst attributed to the formation of hydrophilic groups and the oxygen doping.     

Solar-based photoreduction of methyl orange using zeolite supported photocatalytic materials

A new class of novel photocatalysts has been prepared by supporting TiO₂ on the zeolite matrix by various routes of synthesis. Different transition metals like cobalt, nickel, and ruthenium have been incorporated in these photocatalysts, alongwith molybdenum based heteropolyacid (HPA) to improve the photocatalytic activity of these materials. Photoreduction of methyl orange under solar radiation was compared with photoreduction in presence of artificial visible light illumination to evaluate their photocatalytic activity. The quantity of methyl orange photoreduced by the cobalt containing photocatalyst was about 2.40 mg/g of TiO₂ under the influence of sunlight as compared to 4.111 mg/g of TiO₂ under artificial visible light illumination. However, the efficiency of the photocatalyst is high as compared to P25 TiO₂ under solar light (0.508 mg/g of TiO₂). The high photocatalytic activity of these materials is due to the synergistic effect of incorporation of transition metals in combination with TiO₂ and HPA supported by the zeolite matrix. These materials are being evaluated for photocatalytic water splitting.     

Multiple light scattering and nanotip effect of hierarchical sea urchin-like W18O49 boosting photocatalytic hydrolysis of ammonia borane

The morphology of catalysts is a key factor influencing their photocatalytic performance. Herein, we prepared three W₁₈O₄₉ photocatalysts with different morphologies of sea urchin (W₁₈O₄₉SU), nanorod (W₁₈O₄₉NR) and hollow sphere (W₁₈O₄₉HS), and evaluate their performances in photocatalytic hydrolysis of ammonia borane (AB). Remarkably, W₁₈O₄₉SU exhibited an impressive photocatalytic H₂ production rate with the highest TOF of 53.1 min^?1 under visible light, which is 10.4 and 7.5 times higher than those of W₁₈O₄₉HS and W₁₈O₄₉NR, respectively. The comprehensive characterizations and control experiments revealed that the unique sea urchin-like morphology enabled the strong light harvesting by multiple light scattering, fast separation of photogenerated charges and directional electron enrichment by nanotip effect, which collaboratively boost the photocatalytic activity. Moreover, the enhancement effect of sea urchin-like morphology is again approved well on Nb₂O₅ photocatalysts. This work opens a paradigm to synergistically improve light harvesting and charge separation efficiency through rational design of photocatalyst nanostructure.     

TiO2 photocatalyst with single and dual noble metal co-catalysts for efficient water splitting and organic compound removal

Photocatalysts can be used both for air cleaning and solar energy harvesting through water splitting. However, pure TiO₂ photocatalysts are often inefficient and therefore co-catalysts are needed to improve the yield. To achieve this goal, we prepared TiO₂ and deposited Pt, Ir and Ru co-catalysts on its surface. Two base TiO₂ nanoparticles were used: P25 and rutile TiO₂ synthesized via hydrothermal method. Co-catalysts were deposited by wet impregnation technique using single element and a combination of two elements (Pt and Ir or Pt and Ru), followed by annealing in either air or H₂/Ar. Annealing in reducing atmosphere increased the photocatalytic activity of oxidation of isopropanol compared to annealing in air. We demonstrated a clear influence of the co-catalysts on the photocatalytic degradation of isopropanol and on electrochemical water-splitting reaction. The platinum-containing samples showed the best HER activity.     

Engineering highly efficient photocatalysts for hydrogen production by simply regulating the solubility of insoluble compound cocatalysts

A series of heterostructured composites composed of insoluble copper (II) compounds (CuX) loaded on P25, in which the CuX possess different solubility products (K_sp), have been fabricated to compare the charge separation efficiencies and photocatalytic hydrogen production activities. The results indicate that the K_sp of CuX in the as-prepared photocatalysts strongly correlates with the charge separation efficiencies and photocatalytic activities for hydrogen production. The as-optimized Cu₂(OH)₂CO₃/P25 photocatalyst shows an excellent photocatalytic activity for hydrogen production with an apparent quantum efficiency up to 31.9%, far exceeding that of bare P25 by 485 times. An innovative strategy for constructing highly efficient insoluble compound-semiconductor heterostructured photocatalysts is proposed, where regulating the reduction potential (φ) of the insoluble compounds can simultaneously control both the separation efficiency of photogenerated charge carriers and the reduction ability of the transferred electrons. This design strategy shows an obvious advantage that changing K_sp through selecting right Xⁿ⁻ can easily modulate the φ of the insoluble compounds to significantly enhance the photocatalytic activity of the heterostructured photocatalyst. The results reported here not only inspire us to engineer highly efficient photocatalysts by the utilization of insoluble compounds as cocatalysts, but also offer an innovative possibility for the enhanced separation of photogenerated charge carriers in solar cells.     

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⁻).     

High efficiency photocatalytic CO2 reduction realized by Ca2+ and HDMP group Co-modified graphitic carbon nitride

The development of highly efficient and visible-light responsive carbon nitride (CN) photocatalysts is desirable to address energy shortages and environmental pollution challenges. Herein, we synthesized novel 2-hydroxy-4,6-dimethylpyrimidine (HDMP) group and Ca²⁺ co-modified carbon nitride (CN) photocatalyst (CN-CAA) using a facile in situ copolymerization procedure employing urea and calcium acetylacetonate (CAA) as precursors. The HDMP group and Ca²⁺ co-modification contributed to increased electron density and modulated electronic structure, resulting in extended visible light harvesting and accelerated separation and migration of photoinduced charge carriers. Benefiting from the enhanced visible light utilization and improved photoexcited carriers separation and transportation, the CN-CAA exhibited significantly elevated visible-light-driven photocatalytic activity for CO₂ reduction. This work provided a new insight into the photocatalytic performance promotion of CN through molecular engineering and metal ions incorporation co-modification.     

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.     

Hydrogen production over metal-loaded mesoporous-assembled SrTiO3 nanocrystal photocatalysts: Effects of metal type and loading

Mesoporous-assembled SrTiO₃ photocatalysts with different loaded metal co-catalysts (Au,Pt, Ag, Ni, Ce, and Fe) synthesized by the single-step sol–gel method with the aid of a structure-directing surfactant were tested for the photocatalytic activity of hydrogen production from a methanol aqueous solution under both UV and visible light irradiation. The Au, Pt, Ag, and Ni loadings had a positive effect on the photocatalytic activity enhancement, whereas the Ce and Fe loadings did not. The best loaded metal was found to be Au due to its electrochemical properties compatible with the SrTiO₃-based photocatalyst and its visible light harvesting enhancement. A 1 wt.% Au-loaded SrTiO₃ photocatalyst exhibited the highest photocatalytic hydrogen production activity with a hydrogen production rate of 337 and 200 μmol h⁻¹ g_cat⁻¹ under UV and visible light irradiation, respectively. The hydrogen diffusivity from the liquid phase to the gas phase also significantly affected the photocatalytic hydrogen production efficiency. An increase in the hydrogen diffusability led to an increase in the photocatalytic hydrogen production efficiency.     

Facile synthesis of noble-metal free polygonal Zn2TiO4 nanostructures for highly efficient photocatalytic hydrogen evolution under solar light irradiation

Designing of noble-metal free and morphologically controlled advanced photocatalysts for photocatalytic water splitting using solar light is of huge interest today. In the present work, novel polygonal Zn₂TiO₄ (ZTO) nanostructures have been synthesized by citricacid assisted solid state method for the first time and synthesized nanostructures were characterized by using various techniques like PXRD, UV-Vis-DRS, PL, FT-IR, BET, FE-SEM and TEM for their structural, optical, chemical, surface and morphological properties. The PXRD and UV-Vis-DRS analysis show the existence of cubic and tetragonal phases. FE-SEM and TEM results confirm the formation of polygonal ZTO nanostructures. Synthesised ZTO nanostructures have been potentially applied for solar light-driven photocatalytic hydrogen evaluation from water splitting and compare the photocatalytic activity with synthesized conventional Zn₂TiO₄ and commercially available TiO₂, ZnO photocatalysts. A high rate of 529 μmolh^?1g^?1 solar light-driven photocatalytic H₂ evolution has been achieved by using a small amount (5 mg) of polygonal Zn₂TiO₄ nanostructures from glycerol-water solution. The enhanced photocatalytic performance of the polygonal Zn₂TiO₄ nanostructures compare to conventional Zn₂TiO₄ under solar light irradiation is due to the large surface area and low recombination rate. However having the same bandgap, the polygonal Zn₂TiO₄ nanostructures have shown enhanced photocatalytic performance than that of commercially available TiO₂, ZnO photocatalysts.     

Size- and composition-dependent photocatalytic hydrogen production over colloidal Cd1-xZnxSe nanocrystals

Colloidal nanocrystals (NCs) have emerged as a new kind of photocatalysts for solar hydrogen production due to the tunable optical and photoelectrical properties. Herein, size- and composition-tunable alloyed Cd_1-xZn_xSe NCs were successfully prepared via a one-step hot injection method for photocatalytic hydrogen production under visible light irradiation. By prolonging the reaction time, CdSe NCs with the varied particle sizes were firstly fabricated. It is found that the driving force derived from the difference between conduction band position of CdSe NCs and water reduction potential played a key role in determining the photocatalytic performance. The larger driving force from smaller particle size would give rise to a faster electron transfer and better photocatalytic activity. Furthermore, a series of alloyed Cd_1-xZn_xSe NCs with different compositions were prepared. With the increased zinc amount, the photocatalytic activity of Cd_1-xZn_xSe NCs was initially increased and then decreased. Cd_1-xZn_xSe with the moderate Zn content exhibited the best photocatalytic hydrogen production. It is inferred that the photocatalytic performance of Cd_1-xZn_xSe NCs has a close relationship with the driving force and crystal structure. The present study can provide a guidance to develop efficient nanocrystal photocatalysts by simply controlling the particle size and composition.     

Hydrogen production from water splitting over Eosin Y-sensitized mesoporous-assembled perovskite titanate nanocrystal photocatalysts under visible light irradiation

The photocatalytic water splitting is a promising process for producing H₂ from two abundant renewable sources of water and solar light, with the aid of a suitable photocatalyst. In this work, a combination of sensitizer addition and noble metal loading was employed to modify perovskite photocatalysts in order to achieve the enhancement of photocatalytic H₂ production under visible light irradiation. The dependence of the H₂ production on type of mesoporous-assembled perovskite titanate nanocrystal photocatalysts (MgTiO₃, CaTiO₃, and SrTiO₃), calcination temperature of photocatalyst, Pt loading, type and concentration of electron donor (diethanolamine, DEA; and triethanolamine, TEA), concentration of sensitizer (Eosin Y, E.Y.), photocatalyst dosage, and initial solution pH, was systematically studied. The experimental results showed that the 0.5 wt.% Pt-loaded mesoporous-assembled SrTiO₃ nanocrystal synthesized by a single-step sol-gel method and calcined at 650 °C exhibited the highest photocatalytic H₂ production activity from a 15 vol% DEA aqueous solution with dissolved 0.5 mM E.Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic H₂ production activity were found to be 6 g/l and 11.6, respectively.     

Facile synthesis of AuPd/g-C3N4 nanocomposite: An effective strategy to enhance photocatalytic hydrogen evolution activity

AuPd bimetallic nanoparticle (NP) modified ultra-thin graphitic carbon nitride nanosheet photocatalysts were synthesized via photochemical deposition-precipitation followed by hydrogen reduction. The crystal structure, chemical properties, and charge carrier behavior of these photocatalysts were characterized by X-ray diffraction (XRD), surface photovoltage spectroscopy (SPS), transient photovoltage spectroscopy (TPV), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and UV-Vis diffuse-reflectance spectroscopy (DRS). Photocatalytic H₂ evolution experiments indicate that the hydrogen treated AuPd nanoparticles can effectively promote the separation efficiency of electron-hole pairs photo-excited in the g-C₃N₄ photocatalyst, which consequently promotes photocatalytic H₂ evolution. The 1.0 wt% AuPd/g-C₃N₄ (H₂) composite photocatalyst showed the best performance with a corresponding photocatalytic H₂ evolution rate of 107 μmol h^?1. The photocatalyst can maintain most of its photocatalytic activity after four photocatalytic experiment cycles. These results demonstrate that the synergistic effect of light reduction and hydrogen reduction of AuPd and g-C₃N₄ help to greatly improve the photocatalytic activity of the composite photocatalyst.     

Enhanced photocatalytic H2 production activity of Ag-doped Bi2WO6-graphene based photocatalysts

Ag-doped Bi₂WO₆-graphene based photocatalysts were found to exhibit hydrogen production activity. The performance of Bi₂WO₆-graphene based photocatalysts were investigated and optimized in this study. The activity can be further improved by Ag-doping. The morphology, surface chemistry, and phase structure of the photocatalysts were investigated by Field emission scanning electron microscopy, Transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectra, and X-ray diffraction. UV–vis diffuse reflectance spectroscopy and zeta potential were measured to study the optical properties, bandgap and dispersion stability of the photocatalysts. The effects of forming Bi₂WO₆-graphene contact and Ag doping on the light absorption, band gap, dispersion stability, and photocatalytic H₂ production performance of the composite photocatalysts were evaluated. The improved photocatalytic performance is mainly owing to the Ag doping and high electrical conductivity of graphene.     

ZnmIn2S3+m (m = 1–5, integer): A new series of visible-light-driven photocatalysts for splitting water to hydrogen

A new series of Zn_mIn₂S_3+m (m = 1–5, integer) photocatalysts was synthesized via a simple hydrothermal method. X-ray diffraction (XRD), Raman spectra, UV–vis–near-IR diffuse reflectance spectra (UV–vis), X-ray fluorescence (XRF) and scanning electron microscope (FESEM) were used to characterize these photocatalysts. These Zn_mIn₂S_3+m photocatalysts had a similar layered crystal structure. The absorption edge of Zn_mIn₂S_3+m shifted to shorter wavelength as the atomic ratio of Zn/In in the synthetic solution was increased (i.e. m increased from 1 to 5). Additionally, the morphology of Zn_mIn₂S_3+m greatly depended on the atomic ratio of Zn/In. The photocatalytic activity of Zn_mIn₂S_3+m was evaluated by photocatalytic hydrogen production from water under visible light. The Zn₂In₂S₅ product, with quantum yield at 420 nm determined to be 11.1%, had the highest photocatalytic activity among these Zn_mIn₂S_3+m (m = 1–5, integer) photocatalysts.     

Synthesis of (CuIn)xCd2(1−x)S2 photocatalysts for H2 evolution under visible light by using a low-temperature hydrothermal method

A new series visible-light driven photocatalysts (CuIn)_xCd_2(1−x)S₂ was successfully synthesized by a simple and facile, low-temperature hydrothermal method. The synthesized materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–Vis DRS). The results show that the morphology of the photocatalysts changes with the increase of x from 0.01 to 0.3 and their band gap can be correspondingly tuned from 2.37 eV to 2.30 eV. The (CuIn)_xCd_2(1−x)S₂ nanocomposite show highly photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃²⁻ and S²⁻ under visible light. Substantially, (CuIn)_0.05Cd_1.9S₂ with the band gap of 2.36 eV exhibits the highest photocatalytic activity even without a Pt cocatalyst (649.9 μmol/(g h)). Theoretical calculations about electronic property of the (CuIn)_xCd_2(1−x)S₂ indicate that Cu 3d and In 5s5p states should be responsible for the photocatalytic activity. Moreover, the deposition of Pt on the doping sample results in a substantial improvement in H₂ evolution than the Pt-loaded pure CdS and the amount of H₂ produced (2456 μmol/(g h)) in the Pt-loaded doping system is much higher than that of the latter (40.2 μmol/(g h)). The (CuIn)_0.05Cd_1.9S₂ nanocomposite can keep the activity for a long time due to its stability in the photocatalytic process. Therefore, the doping of CuInS₂ not only facilitates the photocatalytic activity of CdS for H₂ evolution, but also improves its stability in photocatalytic process.     

Free-standing CuS–ZnS decorated carbon nanotube films as immobilized photocatalysts for hydrogen production

Free-standing carbon nanotube films (CNTF) with entangled carbon nanotubes (CNT) were used as conductive supports for the preparation of CuS–ZnS/CNTF composite as immobilized photocatalysts for H₂ production. The surface morphology, crystalline property, surface chemistry, and optical properties of the CuS–ZnS/CNTF photocatalysts were investigated. The effects of forming CuS–ZnS heterojunction and conductive CNTF on the separation of photogenerated charges and photocatalytic hydrogen production activity of CuS–ZnS/CNTF photocatalysts were evaluated by the photocatalytic hydrogen production tests. Conductive CNT films can prevent the recombination of photogenerated electron–hole pairs. The deposition of CuS nanoparticles on the ZnS/CNTF leads to higher photocatalytic activity which can be attributed to the effective electron–hole separation. Introducing ZnS and CuS makes the photocatalyst surface more hydrophilic. The porous structure contributed to the effective contact between the sacrificing agents and the photocatalysts, leading to enhanced H₂ production activity.     

Photocatalytic hydrogen evolution from aqueous solutions of glycerol under visible light irradiation

Multiphase photocatalysts Pt/Cd_1−xZn_xS/ZnO/Zn(OH)₂ and single-phase photocatalysts Pt/Cd_1−xZn_xS were prepared by a two-step technique. The photocatalysts were characterized by a wide range of experimental techniques: X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) combined with energy-dispersive X-ray (EDX) spectroscopy, low-temperature N₂ adsorption/desorption, and UV/VIS spectroscopy. The photocatalytic activity was tested in a batch reactor in the reaction of hydrogen evolution from aqueous solutions of glycerol under visible light irradiation (λ > 420 nm). The highest achieved photocatalytic activity was 449 μmol H₂ per gram of photocatalyst per hour; the highest quantum efficiency was 9.6% (λ > 420 nm). The activity of the multiphase catalysts was shown to exceed that of the single-phase catalysts by a factor of 2.1, likely because of the heterojunctions between sulfides, oxides and hydroxides.