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.     

Structural properties of CuO/TiO2 nanorod in relation to their catalytic activity for simultaneous hydrogen production under solar light

CuO was introduced into porous TiO₂ nanorod through impregnation method. Before the impregnation step, TiO₂ nanorod was hydrothermally synthesized from TiO₂ powder in aqueous NaOH solution and followed by thermal treatment at 450 °C. The structures and properties of impregnated samples were characterized using various techniques, including XRD, BET, XAS, TEM, and UV-DRS. Their photocatalytic performance on simultaneous hydrogen production from pure water and aqueous methanol solution was also investigated under solar light. It was found that CuO/TiO₂ nanorod possessed a high surface area, good photocatalytic property and excellent hydrogen generation activity. Incorporation of Cu ions into the lattice framework of anatase TiO₂ nanorod enhanced the efficiency in visible region at 438–730 nm. Moreover, the XAS results showed that some Cu ions formed solid solution in the TiO₂ nanorod (Cu_xT_1−xO₂). However, the excessive incorporation of Cu ions did not improve any ability of anatase TiO₂ nanorod for production of hydrogen from pure water splitting. This could be due to the excessive CuO agglomeration at outside-pores which blocked the sensitization of TiO₂ nanorod. Only 1% Cu/TiO₂ nanorod was found to be a remarkable and an efficient photocatalyst for hydrogen production under solar light from both pure water and sacrificial methanol splitting. The highest rate of hydrogen production of 139.03 μmol h⁻¹ g_catalyst⁻¹ was found in sacrificial methanol which was 3.24% higher than in pure water.     

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.     

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.     

Cobalt doped TiO2: A stable and efficient photocatalyst for continuous hydrogen production from glycerol: Water mixtures under solar light irradiation

Glycerol is the main by-product during the trans-esterification of vegetable oils to biodiesel. In this study, we investigate the process of photocatalytic hydrogen production from glycerol aqueous solution, with the use of cobalt doped TiO₂ photocatalyst under solar light irradiation. Cobalt doped TiO₂ photocatalysts are prepared by impregnation method and these catalysts are characterized by XRD, EDAX, DRS, TEM, EPR and XPS techniques. DRS studies clearly show the expanded photo response of TiO₂ into visible region on impregnation of Co²⁺ ions on surface of TiO₂. XPS studies also show change in the binding energy values of O1s, Ti 2p and Co 2p, indicating that Co²⁺ ions are in interaction with TiO₂. Maximum hydrogen production of 220 μ mol h⁻¹ g⁻¹ is observed on 2 wt% cobalt doped TiO₂ catalysts in pure water under solar irradiation. A significant improvement in hydrogen production is observed in glycerol: water mixtures; and maximum hydrogen production of 11,021 μ mol h⁻¹ g⁻¹ is obtained over 1 wt% cobalt doped TiO₂ in 5% glycerol aqueous solutions. Furthermore, to evaluate some reaction parameters such as cobalt wt% on TiO₂, glycerol concentration, substrate effect (alcohols) and pH of the solution on the hydrogen production activity are systematically investigated. When the catalysts are examined under UV irradiation, a 3–4 fold increase in activity is observed where this activity seems to decrease with time; however, a continuous activity is observed under solar irradiation on these catalysts. The decreased activity could be ascribed the loss of cobalt ions under UV irradiation, as evidenced by EDAX and TEM analysis. A possible explanation for the stable and continuous activity of cobalt doped TiO₂ photocatalysts under solar irradiation is proposed.     

Hydrogen production from water splitting under UV light irradiation over Ag-loaded mesoporous-assembled TiO2-ZrO2 mixed oxide nanocrystal photocatalysts

Hydrogen production from the photocatalytic water splitting reaction is very attractive because it is an environmentally friendly process, where hydrogen is produced from two abundantly renewable sources, i.e. water and solar energy, with the aid of photocatalysts. TiO₂ is the most widely investigated photocatalyst; however, it alone still exhibits low performance to photocatalytically produce hydrogen. Hence, the aim of this work focused on the enhanced photocatalytic hydrogen production over Ag-loaded mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalysts under UV light irradiation. The TiO₂-ZrO₂ mixed oxides with various TiO₂-to-ZrO₂ molar ratios were synthesized by a sol-gel process with the aid of a structure-directing surfactant, followed by Ag loading via a photochemical deposition method. The influences of photocatalyst preparation parameters, i.e. calcination temperature, phase composition, and Ag loading, were studied. The results revealed that the mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalyst with a TiO₂-to-ZrO₂ molar ratio of 93:7 calcined at 500 °C exhibited the highest photocatalytic hydrogen production activity, and the Ag loading of 0.5 wt.% further greatly enhanced the photocatalytic activity of such TiO₂-ZrO₂ mixed oxide photocatalyst.     

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

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

Highly efficient photocatalytic hydrogen evolution by using Rh as co-catalyst in the Cu/TiO2 system

Cu/TiO₂ was modified by adding Rh as co-catalyst and used as a highly efficient photocatalyst for the hydrogen evolution reaction. A low amount of Rh was loaded onto Cu/TiO₂ by the deposition-precipitation with urea (DPU) method to observe the effect on the hydrogen production displayed by different samples. The Rh–Cu/TiO₂ oxide structure exhibited a remarkably high photocatalytic hydrogen evolution performance, which was about twofold higher than that of the Cu/TiO₂ monometallic photocatalyst. This outstanding performance was due to the efficient charge carrier transfer as well as to the delayed electron-hole recombination rate caused by the addition of Rh. The influence of the different parameters of the photocatalyst synthesis and reaction conditions on the photocatalytic activity was investigated in detail. Hydrogen evolution was studied using methanol, ethanol, 2-propanol and butanol as scavengers with an alcohol:water ratio of 20:80. The methanol-water system, which showed the highest hydrogen production, was studied under 254, 365 and 450 nm irradiation; Rh–Cu/TiO₂ showed high photocatalytic activity with H₂ production rates of 9260, 5500, and 1940 μmol h^?1 g^?1, respectively. The Cu–Rh/TiO₂ photocatalyst was active under visible light irritation due to its strong light absorption in the visible region, low band gap value and ability to reduce the electron (e^?) and hole (h⁺) recombination.     

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

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

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

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

The role of CuO in promoting photocatalytic hydrogen production over TiO2

Low cost semiconductor photocatalysts that can efficiently harvest solar energy to generate H₂ from water or biofuels will be critical to future hydrogen economies. In this study, low cost CuO/TiO₂ photocatalysts (CuO loadings 0–15 wt.%) were prepared, characterized and evaluated for H₂ production from ethanol–water mixtures (80 vol.% ethanol, 20 vol.% H₂O) under UV excitation. TEM, XRF, EDAX, EPR, Raman, TGA, XPS and Cu L-edge NEXAFS data showed that at CuO loadings <5 wt.%, Cu(II) was highly dispersed over the TiO₂ support, possibly as a sub-monolayer CuO species. At higher loadings, CuO crystallites of diameter 1–2 nm were identified. The photocatalytic activity of CuO/TiO₂ photocatalysts was highly dependent on the CuO loading, with 1.25 wt.% CuO being optimal (H₂ production rate = 20.3 mmol g⁻¹ h⁻¹). Results suggest that sub-monolayer coverages of Cu(II) or CuO on TiO₂ are highly beneficial for H₂ generation from ethanol–water mixtures and support the development of a sustainable H₂ economy.     

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

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

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

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

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

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

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

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

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.     

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.     

Facile fabrication of Bi2O3/TiO2-xNx nanocomposites for excellent visible light driven photocatalytic hydrogen evolution

Mesoporous Bi₂O₃/TiO_2−xN_x nanocomposites (BiNT) were synthesized by soft chemical template free homogeneous co-precipitation technique. XRD, XPS, TEM, UV-Vis DRS and photoluminescence studies were adapted to determine the structural, electronic and optical properties. The photocatalytic activities of the catalysts were evaluated for water splitting to generate clean hydrogen fuel under visible light irradiation (λ ≥ 400 nm). BiNT-400 catalyst showed highest results towards hydrogen production (198.4 μmol/h) with an apparent quantum efficiency of 4.3%. The pronounced activity of BiNT-400 sample towards hydrogen production was well consistent with high crystallinity, large surface area, proper excitation by N doping and Bi₂O₃ sensitization.     

Hydrogen production from water splitting under visible light irradiation using sensitized mesoporous-assembled TiO2–SiO2 mixed oxide photocatalysts

This work focused on hydrogen production from the photocatalytic water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled TiO₂–SiO₂ mixed oxide photocatalysts, of which the mesoporous-assembled TiO₂–SiO₂ mixed oxides with various TiO₂-to-SiO₂ molar ratios were synthesized by a sol–gel process with the aid of a structure-directing surfactant. The effects of SiO₂ content, calcination temperature, and phase composition of the mixed oxide photocatalysts were investigated. The experimental results showed that the TiO₂–SiO₂ mixed oxide photocatalyst with the TiO₂-to-SiO₂ molar ratio of 97:3 and calcined at 500 °C provided the maximum photocatalytic hydrogen production activity. The characterization results supported that the 0.97TiO₂–0.03SiO₂ mixed oxide photocatalyst (with the suitable SiO₂ content of 3 mol%) possessed superior physicochemical properties for the photocatalytic reaction as compared to the pure TiO₂, particularly higher specific surface area, lower mean mesopore diameter, higher total pore volume, and lower crystallite size, which led to an enhanced photocatalytic activity.