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.     

Visible-light-driven ZnIn2S4/CdIn2S4 composite photocatalyst with enhanced performance for photocatalytic H2 evolution

ZnIn₂S₄/CdIn₂S₄ composite photocatalysts (x = 0–1) were successfully synthesized via a hydrothermal route. Compositions of ZnIn₂S₄/CdIn₂S₄ composite photocatalysts were optimized according to the photocatalytic H₂ evolution rate. XRD patterns indicate the as-prepared samples are mixtures of hexagonal and cubic structures. FESEM and TEM images show that the as-prepared samples are composed of flower-like microspheres with wide distribution of diameter. There is obviously distinguishing distribution of Zn, Cd elements among the composite architectures. UV–vis absorption spectra of different compositions exhibit that absorption edges of ZnIn₂S₄/CdIn₂S₄ composites slightly move towards longer wavelengths with the increment of CdIn₂S₄ component. A typical time course of photocatalytic H₂ evolution from an aqueous Na₂SO₃ and Na₂S solution over unloaded and PdS-loaded ZnIn₂S₄/CdIn₂S₄ composite photocatalyst is carried out. The initial activity for H₂ evolution over 0.75 wt% PdS-loaded sample is up to 780 μmol h⁻¹. And the activity of unloaded sample also reaches 490 μmol h⁻¹ with consistent stability.     

Enhanced photocatalytic H2 production from glycerol solution over ZnO/ZnS core/shell nanorods prepared by a low temperature route

A series of ZnO/ZnS core/shell nanorods with different ZnS/ZnO molar ratios was synthesized via a new water bath route. The nanorods have a diameter of about 100 nm and a length ranging from a few hundred nanometers to several micrometers. They are formed by coating ZnO nanorod with a layer of porous ZnS shell mainly consisting of crystals which are about 12 nm in diameter. The results showed that the deposition thickness of the ZnS shell layer strongly affected the morphologies, surface area, structure, photo absorption and photocatalytic performance of the ZnO/ZnS core/shell nanorods. The as-prepared ZnO/ZnS core/shell nanorods exhibited a higher photocatalytic activity for H₂ evolution from the glycerol/water mixtures compared with the ZnO nanorods under the same conditions. The maximum H₂ production was 2608.7 and 388.4 μmol h⁻¹ gcat⁻¹ under UV and solar-simulated light irradiation and the corresponding quantum efficiencies were 22% and 13%, respectively. The deposition thickness of the ZnS shell and the interaction between the ZnO rod and ZnS shell and the core/shell structure with n-p heterojunction substantially influence the optical and catalytic performance of the ZnO/ZnS core/shell nanorods.     

Oxide content optimized ZnS–ZnO heterostructures via facile thermal treatment process for enhanced photocatalytic hydrogen production

The ZnS–ZnO heterostructured photocatalysts are synthesized by thermal treatments of the ZnS materials at various thermal processing temperatures (150 °C – 550 °C) with controlling O₂ partial pressures (7.8 kPa – 33.8 kPa). The ZnS–ZnO composite structure shows much higher photocatalytic hydrogen production than those from the ZnS and ZnO pure substances. This phenomenon is mainly caused by effective charge separation between the photoexcited electrons and holes. The thermal oxidation of ZnS materials proceeds at temperatures higher than 500 °C. In addition to the thermal processing temperature, O₂ partial pressure is also chosen for an experimental variable in order to control the atomic composition minutely. The ZnS–ZnO photocatalyst composite fabricated at 500 °C under 16.9 kPa of O₂ partial pressure shows the highest hydrogen production rate of 494.8 μmol g⁻¹ h⁻¹ under 1 sun irradiation condition, and it is 37 times higher than that (13.5 μmol g⁻¹ h⁻¹) from the ZnS pure substance. At this optimized production rate, the Zn/S/O atomic compositions are measured as 45.9/46.9/7.2 (XPS) and 53.3/42.1/4.6 (ICP-AES), respectively.     

CuS,NiS as co-catalyst for enhanced photocatalytic hydrogen evolution over TiO2

TiO₂ photocatalysts loaded CuS and NiS as co-catalyst were prepared by hydrothermal approach and characterized by XRD, UV–visible DRS, BET, XPS, SEM and TEM. When TiO₂ was loaded MS as co-catalyst, it showed higher photocatalytic activities for splitting water into hydrogen in methanol aqueous solution under 500 W Xe lamp. Among the photocatalysts with various compositions, the maximum evolution of H₂ obtained from 5 wt% CuS–5 wt% NiS–TiO₂ sample was about 800 μmol h⁻¹, which was increased up to about twenty-eight times than that of TiO₂ alone. It was proven that CuS, NiS can act as effective dual co-catalysts to enhance the photocatalytic H₂ production activity of TiO₂.     

Effects of general zero-valent metals power of Co/W/Ni/Fe on hydrogen production with H2S as a reductant under hydrothermal conditions

Hydrogen production from water with S²⁻ as a reductant under hydrothermal conditions is an effective new method, in which S²⁻ is oxidized into S₂O₃²⁻, SO₃²⁻ and SO₄²⁻. However, the reactor wall had great effect on hydrogen production as large amount hydrogen is produced with Hastelloy C-22 reactor while almost no hydrogen generated in SUS 316 reactor. Therefore, the influence of main components of Hastelloy C-22 reactor (Co/W/Ni) and SUS 316 reactor (Fe) on hydrogen production with H₂S as the reductant was investigated. The results showed that Fe had negative effect, whereas W, Co and Ni had significant positive effect on improving hydrogen production. These results provided a possible explanation for no hydrogen generated with SUS 316 reactor，and some suggestions for improving hydrogen production. The highest hydrogen production of 199 mL (2 times than the control) was obtained with 4.00 mmol Co, 4.00 mmol W, and 1.00 mmol Ni.     

Noble-metal-free CuS/CdS composites for photocatalytic H2 evolution and its photogenerated charge transfer properties

CuS/CdS composites have been successfully prepared by a simple hydrothermal and cation exchange method. Even without noble-metal cocatalyst, the prepared CuS/CdS composites exhibited enhanced photocatalytic H₂ evolution activity. CuS content had a great influence on photocatalytic activity and an optimum amount of CuS was determined to be ca. 3 mol%, at which the CuS/CdS displayed the highest photocatalytic activity, giving an H₂ evolution rate of 332 μmol g⁻¹ h⁻¹, exceeding that of pure CdS by 3.5 times. The results of SPV (surface photovoltage) and SPC (surface photocurrent) revealed that photogenerated electrons were captured by CuS loaded. TPV (transient photovoltage techniques) indicated that photogenerated charges lifetime in CdS, was prolonged with CuS loaded. Those are the main reasons for the improvement of photocatalytic H₂ evolution.     

Stable photocatalytic hydrogen evolution from water over ZnO–CdS core–shell nanorods

Stability and efficiency are important to realize the practical applications of photocatalysts for photocatalytic hydrogen evolution from water splitting. ZnO–CdS core–shell nanorods with a wide absorption range were designed and synthesized by a two-step route. The ZnO–CdS core–shell nanorods exhibit stable and high photocatalytic activity for water splitting into hydrogen in the presence of S²⁻ and SO₃²⁻ as sacrificial reagents. Furthermore, the photocatalytic activity and stability of ZnO–CdS core–shell nanorods/RuO₂ co-catalyst is superior to that of ZnO–CdS core–shell nanorods/Pt co-catalyst. The merits of stable ZnO and CdS, core–shell and nanorod structures employed are considered to contribute to the favorable photocatalytic hydrogen evolution of ZnO–CdS core–shell nanorods.     

Effects of anions on the photocatalytic H2 production performance of hydrothermally synthesized Ni-doped Cd0.1Zn0.9S photocatalysts

A series of Ni²⁺ doped Cd_0.1Zn_0.9S photocatalysts were prepared with different salt by hydrothermal method. The prepared photocatalysts were characterized by XRD, UV–Vis, BET and SEM. The effects of SO₄²⁻, CH₃COO⁻, Cl⁻ and NO₃⁻ anions on the photocatalytic hydrogen production performance of these photocatalysts were examined. Experimental results showed that the photocatalysts prepared with acetates and chlorides have the highest hydrogen production activity, and their initial hydrogen production rate reaches to 76.52 μmol/h and 80.75 μmol/h under visible-light irradiation with the apparent quantum yield of 12.30% and 14.36% at 420 nm without any noble metal loading, respectively. The absence of noble metal is propitious to reduce the cost of photocatalyst preparation.     

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

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

Photodeposited-metal/CdS/ZnO heterostructures for solar photocatalytic hydrogen production under different conditions

In this study, photocatalytic hydrogen production over metal-incorporated CdS and ZnO (M/CdS/ZnO) nanocomposites under simulated solar light illumination was investigated. M/CdS/ZnO samples were synthesized by photodepositing a metal into CdS/ZnO powders. All photocatalysts showed increased hydrogen production with an increase in the light exposure time. The M/CdS/ZnO samples exhibited better hydrogen production yields than the CdS/ZnO nanocomposites, which in turn showed higher hydrogen production yields than pure ZnO did. The hydrogen production yields of the CdS/ZnO samples increased as the CdS/ZnO weight ratio increased from 0.01 to 0.10. However, they decreased with further increases in CdS loading, although the light absorption edges of the CdS/ZnO samples were further extended to the visible region. Pt/CdS/ZnO and Pd/CdS/ZnO exhibited similar hydrogen production yields, which were higher than the Ni/CdS/ZnO yield. The hydrogen production yield of Pt (0.5%)/CdS/ZnO was higher than that of Pt (0.1%)/CdS/ZnO. Notably, the hydrogen production yield of CdS/Pt/ZnO was lower than that of Pt/CdS/ZnO. Among three different electron donors (Na₂S + Na₂SO₃, methanol, and lactic acid solutions), the Na₂S + Na₂SO₃ solution led to the highest hydrogen production yield. A tentative mechanism for photocatalytic hydrogen production over M/CdS/ZnO nanocomposites under solar light irradiation, using a Na₂S + Na₂SO₃ solution as an electron donor, was proposed. In summary, M/CdS/ZnO photocatalysts can be utilized efficiently for photocatalytic hydrogen production with solar light exposure through proper control of operating parameters.     

Photocatalytic hydrogen evolution with simultaneous degradation of organics over (CuIn)0.2Zn1.6S2 solid solution

Using various organics as electron donor, (CuIn)_0.2Zn_1.6S₂ microsphere solid solution prepared via hydrothermal method as photocatalyst, hydrogen production by anaerobic photocatalytic reforming organics were researched. The photocatalytic hydrogen production activity was notably enhanced in the presence of the organic electron donors. Formic acid was found to be the most efficient sacrificial agent among methanol, glucose, triethanolamine and formic acid. The effects of initial formic acid concentration on hydrogen generation were investigated. When the initial formic acid concentration was 10 vol%, the photocatalytic activity reached the highest. The average activity in initial 10 h can amount to 144 μmol h⁻¹. The possible mechanism of photocatalytic reaction for hydrogen production with simultaneous formic acid degradation was discussed preliminary.     

Sonochemistry synthesis of Bi2S3/CdS heterostructure with enhanced performance for photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution from water splitting is an efficient, eco-friendly method for the conversion of solar energy to chemical energy. A great number of photocatalysts have been reported but only a few of them can respond to visible-light. Metal sulfides, a class of visible-light response semiconductor photocatalysts for hydrogen evolution and organic pollutant degradation, receive a lot of attention due to their narrow band gaps. Herein, we report the sonochemical synthesis of Bi₂S₃/CdS nanocrystal composites with microsphere structure at mild temperature. The phases of Bi₂S₃ and CdS can be observed obviously in HRTEM image. The heterostructure consisting of the two species of nanocrystals plays a key role in separating photo-generated charge carriers. Photocatalytic activities for water splitting are investigated under visible-light irradiation (λ > 400 nm) and an enhanced photocatalytic activity is achieved. The initial rate of H₂ evolution is up to 5.5 mmol h⁻¹ g⁻¹ without resorting to any cocatalysts.     

Synthesis of multiphase Au/Cd0.6Zn0.4S/ZnS photocatalysts for improved photocatalytic performance

Novel composite photocatalysts consisting of a cadmium and zinc sulfide solid solution (Cd_0.6Zn_0.4S) and zinc sulfide (ZnS) nanoparticles were successfully prepared by a simple hydrothermal treatment of suspended Cd_0.3Zn_0.7S at 120 °C. The as-obtained materials were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and UV-VIS diffuse reflection spectroscopy. The obtained photocatalysts were tested in the photocatalytic evolution of hydrogen from a Na₂S/Na₂SO₃ aqueous solution under visible light irradiation (λ = 450 nm). It is shown that the hydrothermal treatment of Cd_0.3Zn_0.7S at 120 °C increases the activity by a factor of 7.5 due to the phase transformations of the solid solution with the formation of the multiphase Cd_0.6Zn_0.4S/ZnS sample. The deposition of gold on the surface of Cd_0.6Zn_0.4S/ZnS leads to a further increase in activity: the achieved photocatalytic activity and quantum efficiency (450 nm) for 1%Au/Cd_0.6Zn_0.4S/ZnS are 17.4 mmol g⁻¹ h⁻¹ and 42.6%, respectively. This excellent performance is found to be attributable to the transformation of Cd_1-xZn_xS from the cubic to the hexagonal phase during the hydrothermal treatment. Additionally, photoelectrodes based on Cd_0.6Zn_0.4S/ZnS and FTO were synthesized and tested in a two-electrode cell. A high value of the photocurrent equal to 0.5 mA/cm² is achieved for the Cd_0.6Zn_0.4S/ZnS/FTO electrode. An investigation by means of impedance spectroscopy reveals the longer lifetime of photogenerated charge carriers in the Cd_0.6Zn_0.4S/ZnS/FTO photoelectrode if to compare with Cd_0.3Zn_0.7S/FTO system.     

Noble metal-free cobalt oxide (CoOx) nanoparticles loaded on titanium dioxide/cadmium sulfide composite for enhanced photocatalytic hydrogen production from water

In this present paper, cobalt oxide (CoO_x) is studied as an effective cocatalyst in a photocatalytic hydrogen production system. CoO_x-loaded titanium dioxide/cadmium sulfide (TiO₂/CdS) semiconductor composites were prepared by a simple solvothermal method and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), and X-ray photoelectron spectroscopy (XPS). Photocatalytic hydrogen production was studied using the as-synthesized photocatalysts in aqueous solution containing sodium sulfide (Na₂S)/sodium sulfite (Na₂SO₃) as hole scavengers under visible light irradiation (λ > 400 nm). The optimal cobalt content in CoO_x-loaded TiO₂/CdS composite is determined to be 2.1 wt% and the corresponding rate of hydrogen evolution is 660 μmol g⁻¹ h⁻¹, which is about 7 times higher than TiO₂/CdS and CdS photocatalysts under the same condition. Visible light-driven photocurrents of the semiconductor composites were further measured on a photoelectrochemical electrode, revealing that the photocorrosion of CdS can be prevented due to the presence of TiO₂–CoO_x.     

Photocatalytic H2 evolution from NaCl saltwater over ZnS1−x−0.5yOx(OH)y-ZnO under visible light irradiation

Photocatalytic hydrogen production was investigated over ZnS_1−x−0.5yO_x(OH)_y-ZnO using sulfide ion (Na₂S-Na₂SO₃) as an electron donor from NaCl saltwater. NaCl can affect markedly the activity for photocatalytic hydrogen production, depending on NaCl concentration. When NaCl concentration is lower, the activity is lower than that in pure water, whereas when NaCl concentration is higher, the activity is higher than that in pure water. NaCl decreases not only the surface charge of ZnS_1−x−0.5yO_x(OH)_y-ZnO but also the surface hydration. When ZnS_1−x−0.5yO_x(OH)_y-ZnO was impregnated with the electron donor (Na₂S-Na₂SO₃), ZnO was transformed partly into ZnS. The impregnated ZnS_1−x−0.5yO_x(OH)_y-ZnO exhibits higher activity than the non-impregnated one. The possible mechanisms were discussed.     

In2O3:Sn/TiO2/CdS heterojunction nanowire array photoanode in photoelectrochemical cells

The design of photoanode with highly efficient light harvesting and charge collection properties is important in photoelectrochemical (PEC) cell performance for hydrogen production. Here, we report the hierarchical In₂O₃:Sn/TiO₂/CdS heterojunction nanowire array photoanode (ITO/TiO₂/CdS-nanowire array photoanode) as it provides a short travel distance for charge carrier and long light absorption pathway by scattering effect. In addition, optical properties and device performance of the ITO/TiO₂/CdS-nanowire array photoanode were compared with the TiO₂ nanoparticle/CdS photoanode. The photocatalytic properties for water splitting were measured in the presence of sacrificial agent such as SO₃²⁻ and S²⁻ ions. Under illumination (AM 1.5G, 100 mW/cm²), ITO/TiO₂/CdS-nanowire array photoanode exhibits a photocurrent density of 8.36 mA/cm² at 0 V versus Ag/AgCl, which is four times higher than the TiO₂ nanoparticle/CdS photoanode. The maximum applied bias photon-to-current efficiency for the ITO/TiO₂/CdS-nanowire array and the TiO₂ nanoparticle/CdS photoanode were 3.33% and 2.09%, respectively. The improved light harvesting and the charge collection properties due to the increased light absorption pathway and reduced electron travel distance by ITO nanowire lead to enhancement of PEC performance.     

Photocatalytic hydrogen production using Fe2O3-based core shell nano particles with ZnS and CdS

Stability and efficiency of photocatalysts are important to realize the practical applications of them for photocatalytic hydrogen production from industrial sulfide effluent. Novel, magnetically separable core–shell nano photocatalysts viz., CdS/Fe₂O₃, ZnS/Fe₂O₃ and (CdS + ZnS)/Fe₂O₃ were prepared and their hydrogen evolution activity under visible light was examined. The XRD result shows that CdS and ZnS were very well coated on the surface of the iron oxide core shell particles. The HR-TEM result also confirms the core shell formation. (CdS + ZnS)/Fe₂O₃ evolved higher volume of hydrogen than the other catalysts. It is ascribed to rapid migration of excited electrons from (CdS + ZnS) toward Fe₂O₃ suppressing electron hole annihilation compared to other catalysts. The catalysts can be easily recovered from the reaction medium using external magnetic bar and so the photocatalyst can be reused without any mass loss. Hence, it can be a potential catalyst for recovery of hydrogen from industrial sulfide containing waste streams.     

All-solid-state rechargeable lithium batteries with Li2S as a positive electrode material

The Li₂S–Cu composite electrode materials were prepared by mechanical milling and applied to all-solid-state lithium cells using the Li₂S–P₂S₅ glass–ceramic electrolyte. The addition of Cu and the mechanical activation improved the electrochemical performance of Li₂S in all-solid-state cells. The In/Li₂S–Cu cells were charged and then discharged at room temperature, suggesting that Li₂S was utilized as a lithium source. The cells exhibited high discharge capacity of about 490 mAh g⁻¹ at the 1st cycle. The SEM and EDX analyses suggested that the amorphous Li_xCuS domain was partially formed by milling, and the domain played an important role in the improvement of capacity. The electrochemical reaction mechanism of the Li₂S–Cu composites was discussed on the basis of the mechanism of the S–Cu composite electrode.     

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.