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 using Pt-loaded TiO2 photocatalysts

A series of synthesised TiO₂-based and commercial photocatalysts were modified by Pt photodeposition and a study made of their photocatalytic activity in hydrogen production. The modified commercial photocatalysts were Evonik P25, Kronos vlp7000 and Hombikat UV-100, and the other modified photocatalysts were synthesised by our group using sol–gel and sol–gel hydrothermal processes (SG400, SG750 and HT). Pt weight percentages used in the study were 0.5, 1.0 and 2.1 wt.% (Pt/TiO₂). The photocatalysts were extensively characterised by X-ray diffraction (XRD), UV–vis diffuse reflectance, Brunauer–Emmett–Teller (BET) surface area measurement, transmission electron microscopy (TEM), scanning electron microscopy (SEM–EDX), Fourier transform infrared spectroscopy (FTIR) and laser light dispersion. Methanol (25% vol.) was used as sacrificial agent over the 8 h of the hydrogen production tests and measurements were taken of the final concentrations of formaldehyde and formic acid as well as initial and final TOC. Photoactivity of all photocatalysts increased in the presence of Pt. The most efficient of the synthesised photocatalysts was SG750 and of the commercial photocatalysts P25. Maximum production of SG750 was 1846 μmol h⁻¹ at 1.0 wt.% Pt and its production per surface unit was notably higher than that of P25.     

Efficient photocatalytic hydrogen production from formic acid on inexpensive and stable phosphide/Zn3In2S6 composite photocatalysts under mild conditions

Developing an efficient, stable and low-cost photocatalytic hydrogen production from formic acid is a daunting challenge and has attracted the intense interest of many of researchers. In this paper, we report the synthesis of novel composite photocatalysts (Ni₂P/Zn₃In₂S₆ (ZIS6) and MoP/ZIS6) and their catalytic performance for H₂ production reaction from formic acid under visible light irradiation, in which Ni₂P and MoP were used as cocatalysts to enhance hydrogen generation activity of ZIS6. The photocatalytic hydrogen production rates of the optimized 1.5 wt% Ni₂P/ZIS6 (45.73 μmol·h⁻¹) and 0.25 wt% MoP/ZIS6 (92.69 μmol·h⁻¹) were 3.5 times and 7.2 times higher than that of the pure ZIS6 (12.88 μmol·h⁻¹), respectively. The apparent quantum efficiency at wavelength λ = 400 ± 10 nm for the two photocatalysts was about 1.8% and 6.4%, respectively. Significantly, it was found that the remarkable improvement of hydrogen production performance is attributed to the introduction of the phosphide cocatalysts, which can serve as a charge separation center and an active site for photocatalytic hydrogen production from the decomposition of formic acid. The reaction mechanism of photocatalytic hydrogen production from formic acid was also proposed.     

CuCr2O4/TiO2 heterojunction for photocatalytic H2 evolution under simulated sunlight irradiation

CuCr₂O₄/TiO₂ heterojunction has been successfully synthesized via a facile citric acid (CA)-assisted sol-gel method. Techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectrum (UV-vis DRS) have been employed to characterize the as-synthesized nanocomposites. Furthermore, photocatalytic activities of the as-obtained nanocomposites have been evaluated based on the H₂ evolution from oxalic acid solution under simulated sunlight irradiation. Factors such as CuCr₂O₄ to TiO₂ molar ratio in the composites, calcination temperature, photocatalyst mass concentration, and initial oxalic acid concentration affecting the photocatalytic hydrogen producing have been studied in detail. The results showed that the nanocomposite of CuCr₂O₄/TiO₂ is more efficient than their single part of CuCr₂O₄ or TiO₂ in producing hydrogen. The optimized composition of the nanocomposites has been found to be CuCr₂O₄·0.7TiO₂. And the optimized calcination temperature and photocatalyst mass concentration are 500 °C and 0.8 g l⁻¹, respectively. The influence of initial oxalic acid concentration is consistent with the Langmuir model.     

Photocatalytic H2 production from acetic acid solution over CuO/SnO2 nanocomposites under UV irradiation

The CuO/SnO₂ composites have been prepared by the simple co-precipitation method and further characterized by the XRD, FESEM and Raman spectroscopy. The photocatalytic H₂ production from acetic acid (HAc) solution over CuO/SnO₂ photocatalyst has been investigated at room temperature under UV irradiation. Effects of CuO loading, photocatalyst concentration, acetic acid concentration and pH on H₂ production have been systematically studied. Compared with pure SnO₂, the 33.3 mol%CuO/SnO₂ composite exhibited approximately twentyfold enhancement of H₂ production. The H₂ yield is about 0.66 mol-H₂/mol-HAc obtained under irradiation for prolonged time. The Langmuir-type model is applied to study the dependence of hydrogen production rate on HAc concentration. A possible mechanism for photocatalytic degradation of acetic acid over CuO/SnO₂ photocatalyst is proposed as well. Our results provide a method for pollutants removal with simultaneous hydrogen generation. Due to simple preparation, high H₂ production activity and low cost, the CuO/SnO₂ photocatalyst will find wide application in the coming future of hydrogen economy.     

TiO2 nanotubes coupled with nano-Cu(OH)2 for highly efficient photocatalytic hydrogen production

Noble-metal-free Cu(OH)₂/TNTs (TNTs: TiO₂ nanotubes) nanocomposite photocatalysts were successfully prepared by loading nano-Cu(OH)₂ on TNTs via a hydrothermal-precipitation process. These were then characterized in terms of morphology and physicochemical properties by employing TEM, XRD, XPS, BET, UV–Vis DRS and PL. The effects of Cu(OH)₂ loading, amount of catalyst on the photocatalytic hydrogen production performance of Cu(OH)₂/TNTs were investigated in detail in aqueous methanol solution under UV irradiation. The results show that, compared with pure TNTs, the TNTs loaded with highly dispersed 8 wt% Cu(OH)₂ exhibited remarkably improved activity for hydrogen production (the largest quantity of evolved hydrogen was ca. 14.94 mmol h⁻¹ g⁻¹ catalyst) with good photostability. This high activity is attributed to the strong synergistic function of Cu(OH)₂/TNTs, including suitable potential of Cu(OH)₂/Cu (E⁰ = −0.222 V) between conduction band (−0.260 V) of TNTs and the reduction potential of H⁺/H₂ (E⁰ = 0.000 V), a unique tubular microstructure of TNTs coated with nano-Cu(OH)₂, large BET specific surface area and high dispersion of Cu(OH)₂. Furthermore, a process mechanism for methanol/water decomposition over Cu(OH)₂/TNTs is proposed to understand its high activity.     

Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S + Na2SO3 solution

The photocatalytic hydrogen production in the sacrificial S²⁻–SO₃²⁻ anions was investigated with ZnO in the addition of metal sulfides containing Ag₂S, CuS, Fe₂S₃, and NiS. In the absence of metal sulfides, the photocatalytic H₂ evolution using ZnO was observed with 255 μmol g⁻¹. The CuS amount and the concentrations of S²⁻ and SO₃²⁻ ions were optimized. It was found that ternary component semiconductor CuS/ZnS/ZnO was formed during the photocatalytic hydrogen production in the aqueous Na₂S + Na₂SO₃ solution. The photocatalytic hydrogen evolution with CuS/ZnS/ZnO in the 0.4 M Na₂S–0.4 M Na₂SO₃ solution was more than about 8.5 times better compared with those obtained with only ZnO. The CuS clusters on the surface of ZnS/ZnO seem to play an important role on the separation for electron–hole pair and the enhancement of H₂ production. Nano-sized ZnS/ZnO photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

Photocatalytic hydrogen production over CuO and TiO2 nanoparticles mixture

Cheap and efficient photocatalysts were fabricated by simply mixing TiO₂ nanoparticles (NPs) and CuO NPs. The two NPs combined with each other to form TiO₂/CuO mixture in an aqueous solution due to the opposite surface charge. The TiO₂/CuO mixture exhibited photocatalytic hydrogen production rate of up to 8.23 mmol h⁻¹ g⁻¹ under Xe lamp irradiation when the weight ratio of P25 to CuO was optimized to 10. Although the conduction band edge position of CuO NPs is more positive than normal hydrogen electrode, the TiO₂/CuO mixture exhibited good photocatalytic hydrogen production performance because of the inter-particle charge transfer between the two NPs. The detailed mechanism of the photocatalytic hydrogen production is discussed. This mixing method does not require a complicated chemical process and allows mass production of the photocatalysts.     

Zn2x(CuIn)1−xS2 photocatalysts synthesis by a hydrothermal process using H4EDTA as complexing agent

The study of the Zn_2x(CuIn)_1−xS₂ (0 ≤ x ≤ 1) solid solutions formation by hydrothermal synthesis using ethylenediaminetetraacetic acid (H₄EDTA) as a complexant and surfactant agent is reported for the first time. Different synthesis parameters were varied: the H₄EDTA concentration, the initial pH value, the Tu concentration, the duration and temperature of autoclaving process. The as obtained powders were characterized by X-ray powder diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and UV/vis/NIR diffuse reflection spectroscopy (DRS). The XRD and EDX results show that, with a careful adjustment of the reaction conditions, especially of the H₄EDTA concentration and initial pH value of the precursor solution, a mixture of solid solutions with sphalerite type structure can be obtained, which transforms into a single phase solid solution after heat treatment. The particles, as revealed by SEM investigations, have nanoporous hexagonal microplates morphology, about 1 μm thick and several microns in diameter. The Cu_0.159In_0.111Zn_1.778S₂ photocatalyst obtained by this method presents photocatalytic activity for hydrogen evolution from aqueous solutions containing S²⁻ ions as sacrificial agent, even without cocatalyst.     

A new photosensitive coordination compound [RuL(bpy)2](PF6)2 and its application in photocatalytic H2 production under the irradiation of visible light

A new organic–inorganic photosensitive coordination compound [RuL(bpy)₂](PF₆)₂ (to represent by TM1) had been synthesized by reaction of L (L = 2-hydroxyl-5-(imidazo-[4,5-f]-1,10-phenanthrolin) benzoic acid) with bipyridyl ruthenium, and further characterized by UV–vis, IR, NMR MS and CV. The target photocatalyst 6 wt% TM1-0.5 wt% Pt-TiO₂ (Ⅰ) was obtained by sensitization of Pt-loaded TiO₂ with TM1. The H₂ production activity of target photocatalyst Ⅰ was systematically evaluated by the reaction of photocatalytic H₂ production from water under visible light irradiation. The maximum H₂ evolution of 386.7 μmol in irradiation 3 h and H₂ production rate of 2578 μmol · h⁻¹ · g⁻¹ was detected under the optimal conditions with pH 5, target photocatalyst Ⅰ 50 mg and 5% sacrificial reagent TEOA (v/v).     

Structure characteristics of CdS/H1.9K0.3La0.5Bi0.1Ta2O7 and photocatalytic activity for hydrogen evolution under visible light

Visible-light-driven CdS/HKLBT photocatalyst was prepared by ion exchange of Cd²⁺ in aqueous Cd(CH₃COO)₂ solutions, then by sulfurization in aqueous N₂H₈S solutions. The characterization by XRD, SEM, HRTEM and XRS revealed that CdS nanoparticles exist both on the surface and in the interlayer of HKLBT. The composite CdS/HKLBT showed higher photocatalytic activity for hydrogen evolution (504.2 μmol/h) than that of pure CdS (187.3 μmol/h), even than that of 0.5 wt%Pt/CdS (496.0 μmol/h) under visible light (λ > 400 nm) in the presence of lactic acid as sacrificial reagent. The enhancement of photocatalytic activity is attributed to the strong contact between CdS and HKLBT in CdS/HKLBT as well as the effective separation of photogenerated carrier in CdS through electron rapid injection into CB of HKLBT.     

S-doped ZnO nanorods on stainless-steel wire mesh as immobilized hierarchical photocatalysts for photocatalytic H2 production

S-doped ZnO nanorods were grown on stainless steel mesh as immobilized hierarchical photocatalysts for hydrogen production. Properties of the photocatalysts were investigated by field-emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), photoinduced current, and photocatalytic hydrogen evolution test. Effects of polymer additive and doping on the surface texture, surface property, and H₂ production performance of the photocatalysts were studied. Polyethyleneimine helps the growth of nanorods on the entire surface of wire mesh. Photocatalytic H₂ production activity of the photocatalysts changes with dopant content and surface texture modification. Due to increased surface area of the hierarchical photocatalyst, enhanced light trapping and liquid flow among wire-mesh, the highest hydrogen evolution rate of 3640 μmol g⁻¹ h⁻¹ is obtained. The photocatalytic activity of photocatalyst remained at 87% of its original performance after five cycles.     

Pd-Gardenia-TiO2 as a photocatalyst for H2 evolution from pure water

The photocatalytic activity for H₂ evolution from pure water over Pd loaded TiO₂ prepared by gardenia extract (Pd-Gardenia-TiO₂) is systematically investigated. The as-prepared photocatalysts are characterized by X-ray diffraction, high resolution transmission electron microscopy, Fourier transform infrared spectra, and X-ray photoelectron spectroscopy. Gardenia extract functions as reducing and stabilizing agents simultaneously. The mean size of the as-prepared Pd nanoparticles is in the range of 2.3 ± 0.5 nm based on TEM images. The Pd-Gardenia-TiO₂ catalyst exhibits good photocatalytic activity for H₂ evolution (93 μmol · h⁻¹ · g⁻¹), which is much higher than that of Pd photodeposited on TiO₂. Possible factors for its photocatalytic activity from pure water are also investigated.     

High photocatalytic hydrogen production from methanol aqueous solution using the photocatalysts CuS/TiO2

Photocatalysts CuS/TiO₂ for hydrogen production were synthesized by hydrothermal method at high temperature and characterized by XRD, UV–visible DRS, XPS, EDX, SEM and TEM. When TiO₂ was loaded with CuS, it showed photocatalytic activities for water decomposition to hydrogen in methanol aqueous solution under 500 W Xe lamp. Among the photocatalysts with various compositions, the one with 1 wt% CuS-loaded TiO₂ showed the maximum photocatalytic activity for water splitting, which indicated CuS could improve the separation ratio of photoexcited electrons and holes. What's more, the amounts of the produced hydrogen was about 570 μmol h⁻¹, which had exceeded pure titania (P25) 32 times. In the present paper, it is proven that CuS can act as an effective co-catalyst to enhance the photocatalytic H₂ production activity of TiO₂.     

Hydrogen production by photoreforming of formic acid in aqueous copper/TiO2 suspensions under UV-simulated solar radiation at room temperature

A new photocatalytic system consisting of two subsystems – Cu⁰/Cl⁻/H⁺/UV_ssr and Cu^II/H⁺/TiO₂/formic acid/UV_ssr – is proposed as a tool to produce hydrogen by reforming an organic species. Formic acid is used as hole scavenger during the experimental runs. An experimental campaign is carried out to demonstrate that the systems can generate hydrogen and to assess how the generation rate depends upon experimental conditions such as copper and TiO₂ loads, chloride and proton ions, and formic acid concentrations. A strong dependence of the Cu⁰/Cl⁻/H⁺/UV_ssr subsystem reactivity upon copper load, chloride concentration and pH is observed. The investigation on the complete system, starting from zero-valent copper, indicates that the addition of TiO₂ and formic acid to the Cu⁰/Cl⁻/H⁺/UV_ssr subsystem does not result into any gain in terms of hydrogen produced and, renders the system totally unreactive under some conditions. On the other hand, when a complete system, starting from cupric ions, is adopted, hydrogen production is observed also for prolonged reaction times with a surplus of hydrogen production with respect to that generated by the Cu⁰/Cl⁻/H⁺/UV_ssr subsystem with the same Cu⁰ starting load and in absence of TiO₂ and sacrificial agent.     

Hydrogen production from glucose degradation in water and wastewater treated by Ru-LaFeO3/Fe2O3 magnetic particles photocatalysis and heterogeneous photo-Fenton

An optimized Ru-doped LaFeO₃ photocatalyst was coupled with magnetic Fe₂O₃ particles and was tested in the photocatalytic hydrogen production from glucose degradation under visible light irradiation. The catalysts were successfully synthesized by solution combustion synthesis using citric acid as organic fuel. Complete glucose degradation and hydrogen production of 5460 μmol/L was obtained after 4 h of irradiation using composite containing 67 wt% of Ru-doped LaFeO₃. After seven cycles of reuse, the photocatalytic activity did not change, evidencing the high stability of the magnetic photocatalyst, which can be recovered from the photoreactor using an external magnetic force. The recyclable sample was finally tested in the treatment of real wastewater from cherries washing process, and a very high hydrogen production (12344 μmol/L) was achieved. Finally, the possibility to couple the photocatalytic process (used for the production of hydrogen) with a heterogeneous photo-Fenton process was investigated in order to mineralize the unconverted organics in the wastewater coming from the photoreactor.     

Photocatalytic H2 evolution on a novel CaIn2S4 photocatalyst under visible light irradiation

A novel visible-light-driven photocatalyst CaIn₂S₄ was synthesized using a facile hydrothermal method followed by a post-calcination process. The influence of the calcination temperature and time on the activities of the photocatalyst was investigated. CaIn₂S₄ exhibits optical absorption predominantly in visible region with an optical band gap of 1.76 eV. Considerable activity for hydrogen evolution from pure water was observed without any sacrificial agents or cocatalysts under visible light irradiation. The maximum hydrogen evolution rate achieved was 30.92 μmol g⁻¹ h⁻¹ without obvious deactivation of the photocatalytic activity for four consecutive runs of 32 h.     

Stability of an H2-producing photocatalyst (Ru/(CuAg)0.15In0.3Zn1.4S2) in aqueous solution under visible light irradiation

Efficient photocatalytic water-splitting systems require stable photocatalysts that have photocatalytic activity with repeated consecutive use. This study investigated H₂ production under visible light irradiation with an Ru/(CuAg)_0.15In_0.3Zn_1.4S₂ photocatalyst and KI as an electron donor. In addition, the stability and reusability of the catalyst were evaluated over multiple cycles of H₂ production and catalyst regeneration. The results show that sintering temperature influenced the crystallinity and photocatalytic activity, as indicated by the X-ray diffraction analyses and H₂ production rates. In particular, the catalyst sintered at 873 K yielded the highest quantum yield of 4.6% at 420 ± 5 nm of wavelength. After seven consecutive reaction cycles, the quantum yield decreased from 4.6% to 3.0% at the end of the seventh cycle. The decrease probably occurred because (1) particles of the catalyst underwent pronounced aggregation, which led to the increase in particle size; and (2) a release of significant metal ions was observed during H₂ production, which led to a loss of the catalyst mass and potential changes in the photocatalytic activity. This study will help facilitate a search of stable photocatalysts for water splitting.     

Electrospun Pt/TiO2 hybrid nanofibers for visible-light-driven H2 evolution

One-dimensional (1D) Pt/TiO₂ hybrid nanofibers (HNFs) with different concentrations of Pt were fabricated by a facile two-step synthesis route combining an electrospinning technique and calcination process. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) results showed that the Pt nanoparticles (NPs) with the size of 5–10 nm were well dispersed in the TiO₂ nanofibers (NFs). Further investigations from the UV–Vis diffuse reflectance (DR) and X-ray photoelectron spectroscopy (XPS) analysis revealed that some Pt ions were incorporated into the TiO₂ lattice as Pt⁴⁺ state, which contributed to the visible light absorption of TiO₂ NFs. Meanwhile, the Pt²⁺ ions existing on the surface of Pt NPs resulted in the formation of Pt–O–Ti bond at Pt NPs/TiO₂ NFs interfaces that might serve as an effective channel for improving the charge transfer. The as-electrospun Pt/TiO₂ HNFs exhibited remarkable activities for photocatalytic H₂ evolution under visible light irradiation in the presence of l-ascorbic acid as the sacrificial agent. In particular, the optimal HNFs containing 1.0 at% Pt showed the H₂ evolution rate of 2.91 μmol h⁻¹ and apparent quantum efficiency of 0.04% at 420 nm by using only 5 mg of photocatalysts. The higher photocatalytic activity could be ascribed to the appropriate amount of Pt ions doping and excellent electron-sink effect of Pt NPs co-catalysts.     

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.