Bimetallic PtNi/g-C3N4 nanotubes with enhanced photocatalytic activity for H2 evolution under visible light irradiation

Bimetallic PtNi-decorated graphitic carbon nitride (g-C₃N₄) nanotubes were prepared through calcining the mixture of urea and thiourea in the presence of Pluronic F127, followed by deposition of bimetallic PtNi nanoparticles (NPs) via chemical reduction. It is found that the photocatalytic activity of PtNi/g-C₃N₄ nanotubes is strongly dependent on the molar ratio of Pt/Ni and the highest activity is observed for Pt₁Ni₁/g-C₃N₄. Under visible light (λ > 420 nm) irradiation, the H₂ generation rate over Pt₁Ni₁/g-C₃N₄ nanotubes is 104.7 μmol h^?1 from a triethanolamine (10 vol%) aqueous solution, which is higher than that of Pt/g-C₃N₄ nanotubes (98.6 μmol h^?1) and is about 47.6 times higher than that of pure g-C₃N₄ nanotubes. The cyclic photocatalytic reaction indicates that our Pt₁Ni₁/g-C₃N₄ nanotubes function as a stable photocatalyst for visible light-driven H₂ production. The effect of bimetallic PtNi NPs in the transfer and separation of photogenerated charge carriers occurring in the excited g-C₃N₄ nanotubes was investigated by performing photo-electrochemical and photoluminescence measurements. Our results reveal that bimetallic PtNi could replace Pt as a promising cocatalyst for photocatalytic H₂ evolution with better performance and lower cost.     

TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation

The CdS/TiO₂ composites were synthesized using titanate nanotubes (TiO₂NTs) with different pore diameters as the precursor by simple ion change and followed by sulfurization process at a moderate temperature. Some of results obtained from XRD, TEM, BET, UV–vis and PL analysis confirmed that cadmium sulfide nanoparticles (CdSNPs) incorporated into the titanium dioxide nanotubes. The photocatalytic production of H₂ was remarkably enhanced when CdS nanoparticles was incorporated into TiO₂NTs. The apparent quantum yield for hydrogen production reached about 43.4% under visible light around λ = 420 nm. The high activity might be attributed to the following reasons: (1) the quantum size effect and homogeneous distribution of CdSNPs; (2) the synergetic effects between CdS particles and TiO₂NTs, viz., the potential gradient at the interface between CdSNPs and TiO₂NTs.     

Efficient photocatalytic hydrogen evolution on amorphous MoSx-modified CdS/KTaO3 heterojunctions under visible light irradiation

Constructing heterostructures with efficient charge separation is a promising route to improve photocatalytic hydrogen production. In this paper, MoS_x/CdS/KTaO₃ ternary heterojunction photocatalysts were successfully prepared by a two-step method (hydrothermal method and photo deposition method), which improved the photocatalytic hydrogen evolution activity. The results show that the rate of hydrogen evolution for the optimized photocatalyst is 2.697 mmol g^?1·h^?1under visible light, which is 17 times and 2.6 times of the original CdS (0.159 mmol g^?1 h^?1) and the optimal CdS/KTaO₃(1.033 mmol g^?1 h^?1), respectively, and the ternary photocatalyst also shows good stability. The improvement on photocatalytic hydrogen evolution performance can be attributed to the formation of heterojunction between the prepared composite materials, which effectively promotes the separation and migration of photo-generated carriers. Amorphous MoS_x acts as an electron trap to capture photogenerated electrons, providing active sites for proton reduction. This provides beneficial enlightenment for hydrogen production by efficiently utilizing sunlight to decompose water.     

Hollow structured PtNix alloy as cocatalyst of CdS for hydrogen generation under visible light irradiation

PtNi_x hollow nanoparticles (HNPs) as a cocatalyst was synthesized by a galvanic replacement method at room temperature, and then was loaded on CdS. The hollow structure of PtNi_x alloy was confirmed by TEM and STEM. Photocatalytic hydrogen production reaction was performed for PtNi_x/CdS catalyst under 300 W Xe lamp (λ ≥ 420 nm). It is noteworthy that PtNi_0.5/CdS shows the highest hydrogen evolution activity of 2.9 mmol/h with QE = 51.24% at 420 nm, which is higher than that of situ-photodeposited Pt onto CdS (0.57 mmol/h). The enhancement of hydrogen evolution performance of PtNi_0.5/CdS could be attributed to the porous shell and hollow structure of PtNi_0.5 NPs, as well as the strong electronic coupling effect between Pt and Ni. Besides, the sheet structure of CdS in some degree promoted the hydrogen production rate. Therefore, the PtNi_x HNPs could be a promising cocatalyst of CdS for solar driven hydrogen evolution.     

Hybridizing MoS2 and C60 via a van der Waals heterostructure toward synergistically enhanced visible light photocatalytic hydrogen production activity

Molybdenum disulfide (MoS₂) as a representative transition-metal dichalcogenide (TMD) has been extensively used as a noble-metal-free cocatalyst for photocatalytic hydrogen (H₂) production, but suffers from poor photocatalytic activity due to the catalytic inactivity of its basal plane. Herein, by bounding another metal-free cocatalyst, C₆₀, with MoS₂, we report the first MoS₂-C₆₀ hybrid featuring a van der Waals heterostructure prepared via a facile and eco-friendly solid-state mechanochemical route. C₆₀ bounding onto the edge of MoS₂ nanosheets leads to the decreases of both the number of layers and the size of MoS₂ nanosheets, as well as a negative shift of the conduction band minimum along with a positive shift of valance band maximum relative to the bulk MoS₂ and MoS₂ ball-milled without C₆₀ (MoS₂-BM). Under the optimized weight ratio of MoS₂:C₆₀ (1:1) in the raw mixture subject to ball-milling, MoS₂-C₆₀ hybrid containing 2.8 wt% C₆₀ shows an exceptional visible light photocatalytic H₂ production rate of 6.89 mmol h^?1 g^?1 in the presence of a photosensitizer Eosin Y (EY), which is significantly enhanced relative to the bulk MoS₂ and pristine C₆₀, both of which show almost no photocatalytic H₂ activity. Thus, the synergistic enhancement of photocatalytic activities of both MoS₂ and C₆₀ is revealed.     

Photocatalytic H2 evolution under visible light from aqueous methanol solution on NaBixTa1−xO3 prepared by spray pyrolysis

Solid–solution NaBi_xTa_1−xO₃ photocatalyst powders were prepared by spray pyrolysis from aqueous and polymeric precursor solutions. The effects of Bi ions and additives in the precursors on the photocatalytic activities of the resulting catalysts were investigated by measuring hydrogen evolution rate from aqueous methanol solution under visible light irradiation (λ > 415 nm). The effects of NiO co-catalyst on hydrogen evolution rate are also tested. Hydrogen evolution rate was enhanced almost 20 times (to 1355 μmol g⁻¹ h⁻¹), with an induction period of 1 h, compared with a photocatalyst prepared by hydrothermal method because of the compositional uniformity and unique surface morphology with large BET surface area of the photocatalyst. Bi ions in the photocatalyst were confirmed by XPS to be mainly present as Bi⁵⁺. Polymeric additives to the precursors and NiO co-catalyst resulted in maximized hydrogen evolution rate when used at 300 mol% and 0.2 wt.%, respectively. The composition of the photocatalyst was optimized at NaBi_0.07Ta_0.93O₃.     

In-situ growth of CdS quantum dots on g-C3N4 nanosheets for highly efficient photocatalytic hydrogen generation under visible light irradiation

Well dispersed CdS quantum dots were successfully grown in-situ on g-C₃N₄ nanosheets through a solvothermal method involving dimethyl sulfoxide. The resultant CdS–C₃N₄ nanocomposites exhibit remarkably higher efficiency for photocatalytic hydrogen evolution under visible light irradiation as compared to pure g-C₃N₄. The optimal composite with 12 wt% CdS showed a hydrogen evolution rate of 4.494 mmol h⁻¹ g⁻¹, which is more than 115 times higher than that of pure g-C₃N₄. The enhanced photocatalytic activity induced by the in-situ grown CdS quantum dots is attributed to the interfacial transfer of photogenerated electrons and holes between g-C₃N₄ and CdS, which leads to effective charge separation on both parts.     

H2 evolution under visible light irradiation on La and Cr co-doped NaTaO3 prepared by spray pyrolysis from polymeric precursor

Photocatalysts of Na_1−xLa_xTa_1−xCr_xO₃ and NaTa_1−xCr_xO₃ were prepared by spray pyrolysis from aqueous and polymeric precursor solution. Apart from the contribution of La³⁺ ions co-doped into NaTa_1−xCr_xO₃ on the BET surface area and the surface morphology by preventing crystal growth, this co-doping contributed to the increased Cr³⁺ concentration by partially tuning the electron configuration from A⁺B⁵⁺O₃ to (A⁺A′³⁺)²⁺(B⁵⁺B′³⁺)⁴⁺O₃ in the lattice of the photocatalyst. Na_1−xLa_xTa_1−xCr_xO₃ prepared from polymeric precursor solution reduced the induction period to 33% and enhanced the hydrogen evolution rate 5.6-fold to 1467.5 μmol g⁻¹ h⁻¹ compared with the equivalent values of NaTa_1−xCr_xO₃ prepared from aqueous precursor. The optimum amounts of dopant and additives comprising the polymeric precursor to maximize the hydrogen evolution rate were x = 0.003 and 300 mol%, respectively.     

Effects of charge balance with Na on SrTiO3:Mo prepared by spray pyrolysis for H2 evolution under visible light irradiation

Photocatalyst powders of SrTi_1−xMo_xO₃ and Sr_1−2xNa_2xTi_1−xMo_xO₃ were prepared by spray pyrolysis for hydrogen evolution for the first time from aqueous methanol solution under visible light irradiation. The co-doping of Mo⁶⁺/Na⁺ ions resulted in increase of BET surface area and pore volume, and formation of unique morphology with wrinkled, furrowed and porous surface, without significant distortion of lattice structure of host material. The hydrogen evolution rate of Sr_1−2xNa_2xTi_1−xMo_xO₃ photocatalyst was enhanced up to 1115.8 μmol g⁻¹ h⁻¹ with an induction period of 1 h under visible light irradiation, which was 1.5 times higher than that of SrTi_1−xMo_xO₃. The co-dopant Na⁺ ion contributed to the charge balance in the host material by compromising the excess positive charge of Mo⁶⁺, which was effective for enhancing the hydrogen evolution rate. The optimum composition of photocatalyst corresponding to the maximum hydrogen evolution rate was Sr_1−2xNa_2xTi_1−xMo_xO₃ (x = 0.004).     

Preparation of novel SrTiO3:Rh/Ta photocatalyst by spray pyrolysis and its activity for H2 evolution from aqueous methanol solution under visible light

SrTiO₃:Rh/Ta powder was prepared by spray pyrolysis from polymeric precursors containing citric acid and ethylene glycol. Co-doping Ta into SrTiO₃:Rh increased the presence of Rh³⁺ in the host material. The retention of balanced charges by the substitution of two Ti⁴⁺ ions in the host material by one ion each of Rh³⁺ and Ta⁵⁺ enhanced hydrogen evolution from aqueous methanol solution under visible light irradiation (λ > 415 nm) by 3.5 times (to 531 μmol h⁻¹) and reduced the induction period by 50% (to 1 h), when compared with SrTiO₃:Rh. Thorough mixing of the multi-component spray pyrolysis precursor solution resulted in highly dispersed Rh ions and porous photocatalyst particles, which showed enhanced hydrogen evolution rate.     

Hydrogenated CdS nanorods arrays/FTO film: A highly stable photocatalyst for photocatalytic H2 production

To improve the photocorrosion of CdS nanorod arrays (CdS NRAs), we have designed a simple and facile treatment method of in-situ hydrogenation to fabricate CdS@SnS/SnO₂ heterostructure on fluorine-doped tin oxide glass, which is a highly photostable hydrogenated CdS-based film photocatalyst (CdS NRAs-H₂). Over a 25-h long time irradiation, the total photocatalytic hydrogen production of hydrogenated CdS NRAs is almost 2.0 times higher than that of un-hydrogenated CdS NRAs. Moreover, the average hydrogen production rate of CdS NRAs-H₂ can steadily maintain at 23.75 μmol cm^?2 h^?1 with 102% of retention rate after 5 reaction cycles, while they are only 6.13 μmol cm^?2 h^?1 with 30% of retention rate for un-hydrogenated common CdS NRAs. The photocatalytic mechanism on enhanced activity and stability for hydrogenated CdS NRAs photocatalyst is also investigated and discussed in detail.     

Synthesis and photochemical performance of morphology-controlled CdS photocatalysts for hydrogen evolution under visible light

Novel CdS nanomaterials were synthesized by a simple “one-pot” hydrothermal biomolecule-assisted method using glutathione (GSH) as the sulfur source and structure-directing reagent. Various morphologies of CdS photocatalysts, such as solid nanospheres (s-CdS), hollow nanospheres (h-CdS) and nanorods (r-CdS), were obtained by controlling only the hydrothermal temperatures. The X-ray diffraction patterns confirmed that all of the samples were typical hexagonal wurtzite CdS. It was found that the absorption edge of s-CdS was at 465 nm with a greater blue shift compared to that of h-CdS and r-CdS. The photocatalytic activity of s-CdS was superior to that of h-CdS and r-CdS under visible light. Photoluminescence measurements revealed their different photogenerated electron/hole recombination ability, which was in accordance with the order of s-CdS < h-CdS < r-CdS. The excellent photocatalytic activity of s-CdS was ascribed to the small sizes of sub-nanocrystallites, which make it easy for photoinduced electrons and holes on the solid sphere to migrate to the surface and react with water and the sacrificial agent quickly. It was crucial to control the temperature for preparing CdS photocatalysts via hydrothermal methods. The formation mechanism of different morphology might be due to complexation, S-C bond rupture, spherical aggregation and Ostwald ripening processes.     

Alloyed PdNi hollow nanoparticles as cocatalyst of CdS for improved photocatalytic activity toward hydrogen production

Photocatalytic hydrogen evolution has been regarded as an efficient method for H₂ production, in which the cocatalysts play a crucial role. In this work, two-dimensional (2D) snow-flake CdS was synthesized via a solvothermal method. PdNi hollow alloy with different compositions were synthesized by a galvanic replacement method, and decorated on CdS surface. The structural microscopic and spectroscopic analysis demonstrated that the formation of PdNi hollow nanoparticles (HNPs) and the decoration of PdNi HNPs on CdS (PdNi/CdS). Photocatalytic hydrogen evolution reaction was performed under visible light irradiation (λ ≥ 420 nm). Pd₁Ni₁/CdS exhibited higher photocatalytic H₂ generation rate about 54 mmol/h/g with a quantum efficiency of 63.97% at 420 nm, which was 1.7-fold higher than that of Pd/CdS (32.4 mmol/h/g). The high photocatalytic performance for Pd₁Ni₁/CdS was mainly attributed to the strong interaction between Pd₁Ni₁ HNPs and CdS, and the formation of unique hollow structure of PdNi alloy with porous nature which provided more active sites for H₂ evolution. Additionally, the synergistic effect between Pd and Ni, as well as the 2D morphology of CdS enhance the mobility of photo-generated charge carriers which minimize their recombination in turn enhance the photocurrent and photocatalytic performance of solar water splitting reaction.     

Effect of tri-doping on H2 evolution under visible light irradiation on SrTiO3:Ni/Ta/La prepared by spray pyrolysis from polymeric precursor

Tri-doped photocatalyst, SrTiO₃:Ni/Ta/La, was prepared by spray pyrolysis from aqueous and polymeric precursor solutions. The third dopant, La³⁺, contributed to the BET surface area and porous morphology by preventing crystal growth, and increased the Ni²⁺/Ni³⁺ ratio by affecting the electron configuration in the lattice structure, which is closely related to the hydrogen evolution rate. The hydrogen evolution rate of the tri-doped photocatalyst, SrTiO₃:Ni(0.2 mol%)/Ta(0.4 mol%)/La(0.3 mol%), was increased by about 60%–895.2 μmol g⁻¹ h⁻¹ from the value of 561.2 μmol g⁻¹ h⁻¹ for the co-doped photocatalyst, SrTiO₃:Ni(0.2 mol%)/Ta(0.4 mol%), and was further enhanced to 2305.7 μmol g⁻¹ h⁻¹ when a polymeric precursor was used instead of an aqueous precursor in spray pyrolysis. The optimum additive content for polymeric precursor solution was 300 mol%.     

Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light

The advantage of copper doping onto TiO₂ semiconductor photocatalyst for enhanced hydrogen generation under irradiation at the visible range of the electromagnetic spectrum has been investigated. Two methods of preparation for the copper-doped catalyst were selected – complex precipitation and wet impregnation methods – using copper nitrate trihydrate as the starting material. The dopant loading varied from 2 to 15%. Characterization of the photocatalysts was done by thermogravimetric analysis (TGA), temperature programmed reduction (TPR), diffuse reflectance UV-Vis (DR-UV-Vis), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Photocatalytic activity towards hydrogen generation from water was investigated using a multiport photocatalytic reactor under visible light illumination with methanol added as a hole scavenger. Three calcination temperatures were selected – 300, 400 and 500 °C. It was found that 10 wt.% Cu/TiO₂ calcined at 300 °C for 30 min yielded the maximum quantity of hydrogen. The reduction of band gap as a result of doping was estimated and the influence of the process parameters on catalytic activity is explained.     

H2 evolution under visible light irradiation from aqueous methanol solution on SrTiO3:Cr/Ta prepared by spray pyrolysis from polymeric precursor

SrTiO₃:Cr/Ta powders were prepared by spray pyrolysis from polymeric precursors. Effects of the amount of co-dopant and additives on the photocatalytic activity for hydrogen evolution from aqueous methanol solution under visible light irradiation (λ > 415 nm) were investigated. For the photocatalyst prepared by spray pyrolysis from polymeric precursor, the hydrogen evolution rate was increased by a factor of ∼100 and induction period was decreased by a factor of 8 compared with a photocatalyst prepared by solid state reaction. These enhancements result from increased roughness of surface, and the compositional uniformity which are intrinsic characteristics of spray pyrolysis. In addition, photocatalyst prepared by spray pyrolysis from polymeric precursor have large BET surface area and small amount of Cr⁶⁺ ion which is responsible for long induction period. It should be noted that the reduction of Cr⁶⁺ ion was achieved without hydrogen reduction process.     

Enhancement of activity of LaNi0.7Cu0.3O3 for photocatalytic water splitting by reduction treatment at moderate temperature

To accrue the full environmental benefit of hydrogen as an energy carrier, we need to develop new visible-light-active photocatalyst. Here, the LaNi_0.7Cu_0.3O₃, was used as a precursor and then treated at various temperatures (523–673 K) for 2 h under continuous H₂ flow. Among all the resultant photocatalysts, the LaNi_0.7Cu_0.3O₃ treated at 573 K, exhibited highest photocatalytic activity when exposed to visible-light irradiation. We characterized the samples by XRD, DRS, XPS and PL. It was found that a perovskite-related material with proper oxygen vacancies and Cu²⁺/Cu¹⁺ redox couples facilitated the enhancement in photocatalytic activity.     

Influence of different ruthenium(II) bipyridyl complex on the photocatalytic H2 evolution over TiO2 nanoparticles with mesostructures

H₂ production over dye-sensitized Pt/TiO₂ nanoparticles with mesostructures (m-TiO₂) under visible light (λ > 420 nm) was investigated by using methanol as electron donors. Experimental results indicate that three types of ruthenium(II) bipyridyl complex dyes (one binuclear Ru, two mononuclear Ru), which can be attached to Pt/m-TiO₂ with different linkage modes, show different photosensitization effects due to their different coordination circumstances and physicochemical properties. The dye tightly linked with m-TiO₂ has better durability but the lowest H₂ evolution efficiency, whereas the loosely attached dyes possess higher H₂ evolution efficiency and preferable durability. It seems that the dynamic equilibrium between the linkage of the ground state dye with TiO₂ and the divorce of the oxidization state dye from the surfaces plays a crucial role in the photochemical behavior during the photocatalyst sensitization process. It is helpful to improve the H₂ evolution efficiency by enhancing the electron injection and hindering the backward transfer. The binuclear Ru(II) dye shows a better photosensitization in comparison with mononuclear Ru(II) dyes due to its large molecular area, conjugation system, and “antenna effect”, which, in turn, improve the visible light harvesting and electron transfer between the dye molecules and TiO₂.     

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.     

SrS/CdS composite powder as a novel photocatalyst for hydrogen production under visible light irradiation

In this paper a novel SrS/CdS composite powders were prepared by coprecipitation method. The physicochemical properties of the photocatalysts were analyzed by XRD, UV–Vis, BET, PL and SEM. Photocatalytic hydrogen production results showed that these composite powders can work efficiently under visible light without loading noble metal, and it was found that the ratio of SrS/CdS equaling to 2/8 has the best performance among various SrS/CdS composite powders, and the hydrogen evolution rate amounted to 123 μmol/h under visible light irradiation. The apparent quantum yield for this photocalyst was calculated to be 2.85%, 4.59%, 9.63% at 420 nm, 440 nm and 480 nm respectively, and the apparent quantum yield under visible light was 5.83%. The reason for its high activity was analyzed.