Solid Solution Photocatalysts Research Articles

Germanium-substituted Zn2TiO4 solid solution photocatalyst for conversion of CO2 into fuels

Photocatalytic CO2 reduction conjugated with H2O oxidation is regarded as a promising artificial photosynthesis system because it can simultaneously solve energy and environment problems. Here, a novel solid solution, Zn2Ti1−xGexO4 (0 ≤ x ≤ 0.15), with high photocatalytic activity for the reaction was successfully synthesized via a facile molten salts route. The Zn-based solid solutions with size about 200 nm have a homogeneous inverted cubic spinel structure (Fd3m) and a continuously modulated band gap with the Ge content. For the CO2 reduction reaction with H2O under simulated solar irradiation, the Zn2Ti1−xGexO4 solid solutions display not only high activity for the conversion of CO2 into CH4 and CO fuels, but also long-term stability (>60 h of catalytic reaction). Experimental results and theoretical calculations indicated that the conduction and the valence bands of the cubic spinel Zn2TiO4 are positively shifted by introducing Zn2GeO4 with a pseudocubic inverse spinel structure, but the band gaps of solid solutions are simultaneously modulated with the introduction of germanium. Nevertheless, the Zn2TiO4 affords a light-carrier effective mass and strong electron delocalization by forming a solid solution with Zn2GeO4, which is beneficial to improving migration of photogenerated electrons and holes. As a synergistic result of band gap narrowing and high carrier diffusion, good conversion efficiencies for production of solar fuels through the reactions of CO2 reduction with H2O are achieved.

One-step preparation of Bi4O5BrxI2−x solid solution with superior photocatalytic performance for organic pollutants degradation under visible light

Highly efficient photocatalyst is the demand for controlling environmental pollution, especially the toxic and refractory organic contaminants. Herein, a series of bismuth-rich Bi4O5BrxI2−x solid solution photocatalysts were prepared through a one-step solvothermal method by adjusting the molar ratio of Br/I, which can effectively degrade organic pollutants of phenol and Rhodamine B (RhB). The structure and morphology of the photocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM). The results indicated that Bi4O5BrxI2−x solid solution photocatalysts were successfully synthesized. The obtained samples showed superior photocatalytic activities toward phenol and RhB under visible light irradiation. Besides, the photocatalytic performances can be optimized by changing the ratio of Br/I. When the ratio of Br/I was 1:1, the obtained Bi4O5BrxI2−x solid solution exhibited the highest photocatalytic performance, which was nearly 6.4 and 8.5 times higher than that of pure Bi4O5Br2 and Bi4O5I2 for phenol degradation, respectively. The enhancement can be ascribed to the high efficient separation of photogenerated electron-hole pairs in Bi4O5BrxI2−x solid solution, which was confirmed by the corresponding optical and photoelectrochemical characterizations.

Photocatalytic overall water splitting on isolated semiconductor photocatalyst sites in an ordered mesoporous silica matrix: A multiscale strategy

Photocatalytic overall water splitting (OWS) in a stoichiometric ratio has attracted increasing attention for the realization of a sustainable, environmentally friendly future. However, this reaction exhibits sluggish kinetics due to efficiencylimitations of the involved steps, including photon absorption, electron transfer, and the reactions that occur at triple-phase boundary regions. Herein, we report a general multiscale strategy to address this challenge by designing a model composite catalyst with a high loading density of isolated Bi0.5Y0.5VO4 nanocrystals, as building blocks, dispersed in a hexagonally ordered mesoporous silica matrix. In contrast to the well-recognized heterojunction formed between different semiconductors, we show that confined growth favours the formation of isolated quaternary solid-solution photocatalysts (Bi0.5Y0.5VO4), which can further interface with the insulating silica to overcome temperature limitations and exhibit enhanced photon absorption and electrochemical and mass transfer properties due to the transparent periodic porous structure of silica and the as-formed small nanocrystals with high crystallinity and a passivated surface. When the semiconductor photocatalyst is incorporated with the inert silica insulator, this nanoarchitecture does not inhibit the OWS activity but actually delivers a 10-fold higher OWS activity than bulk Bi0.5Y0.5VO4 prepared by the conventional solid-state method.

The atomic-scale structure of LaCrO3-NaTaO3 solid solution photocatalysts with enhanced electron population.

Visible light sensitization of sodium tantalate (NaTaO3), a highly UV-active material, is critical for realizing its practical application in photocatalytic water splitting under solar light. Double doping of a half-filled transition metal together with another metal for cationic charge balance is a promising way of sensitizing NaTaO3 to visible light. One fundamental issue is that the atomic-scale structure of such doubly doped NaTaO3 is not yet fully understood. In this study, we doubly doped NaTaO3 with La3+ and Cr3+ through a solid-state route. The occupation preference of La3+ in a doubly doped system was particularly studied by the extended X-ray absorption fine structure technique. We revealed the substitution of La3+ for Na+, and Cr3+ for Ta5+, forming a LaCrO3-NaTaO3 solid solution. We then showed that doping NaTaO3 with La3+ and Cr3+ appreciably increased the population of electrons photoexcited by either visible light or UV light. Photoactivation of the doubly doped NaTaO3 with visible light produced a population of electrons comparable to that under UV light. The charge compensation scheme of double doping with La3+ and Cr3+ is shown here to be a good option for the sensitization of NaTaO3 to visible light.

One-pot solvothermal synthesis of MoS2-modified Mn0.2Cd0.8S/MnS heterojunction photocatalysts for highly efficient visible-light-driven H2 production

Development of low-cost, highly efficient and stable CdS-based solid solution photocatalysts is of great significance towards photocatalytic H2 production. Herein, a solvothermal process has been used to fabricate MnxCd1-xS-based products, which can transform into novel heterojunctions consisting of nanorod-like Mn0.2Cd0.8S solid solution and nanoparticle-like α-MnS once the x value is higher than 0.20, and the resulted Mn0.2Cd0.8S/MnS heterojunction containing 38 mol% α-MnS demonstrates an optimum composition ratio with the best H2 production activity (335 μmol h−1), which is 1.95 times higher than that (171 μmol h−1) of the single CdS under visible light (λ ≥ 420 nm) irradiation. After modified with MoS2 cocatalyst via a one-pot solvothermal process, those MoS2-Mn0.2Cd0.8S/MnS composites show significantly improved photocatalytic performance, and the 15 wt% MoS2-Mn0.2Cd0.8S/MnS achieves the best H2 production activity (995 μmol h−1), which is 2.97 times higher than that (335 μmol h−1) of the pristine Mn0.2Cd0.8S/MnS with the optimum composition ratio and also higher than that (868 μmol h−1) of the 1.0 wt% Pt-Mn0.2Cd0.8S/MnS. The well aligned energy band structures and the intimate contacts among Mn0.2Cd0.8S, α-MnS and MoS2 facilitate the photogenerated electron transferring from the nanoparticle-like α-MnS to the nanorod-like Mn0.2Cd0.8S and then to the nanoflake-like MoS2, thus promoting the charge separation and providing more active sites for H2 production reaction. This study not only presents a rare example of binary Mn0.2Cd0.8S/MnS heterojunction photocatalyst consisting of Mn0.2Cd0.8S solid solution and α-MnS, but also paves a new way to explore highly efficient and noble metal-free photocatalytic H2 production system for solar energy conversion.

One-step facile synthesis and high H2-evolution activity of suspensible CdxZn1-xS nanocrystal photocatalysts in a S2-/SO32- system.

For a CdS-based photocatalyst, both the photocorrosion resistance and the rapid H2-production reaction are highly required for improving its photocatalytic H2-production performance. In this study, a facile strategy was reported to simultaneously realize an improved photocorrosion resistance and rapid interfacial H2-evolution reaction of CdxZn1-xS solid-solution photocatalysts in a sulfur-rich S2-/SO32- solution. Here, the suspensible CdxZn1-xS nanocrystal photocatalysts are prepared by a one-step co-precipitation route through the direct introduction of Zn2+/Cd2+ mixing ions in a sulfur-rich Na2S-Na2SO3 solution, and the resultant CdxZn1-xS nanocrystals (ca. 5 nm) display a suspensible structure owing to the numerous and selective adsorption of S2-/SO32- on the surface of these CdxZn1-xS nanocrystals. It is found that the bandgap structure of CdxZn1-xS (from 2.25 to 3.52 eV) nanocrystals can be easily controlled by adjusting the Cd2+/Zn2+ molar ratio. The photocatalytic experimental results suggested that the suspensible CdxZn1-xS nanocrystal photocatalysts clearly displayed an excellent photocatalytic H2-production performance, and the suspensible Cd0.6Zn0.4S nanocrystals exhibit the highest photocatalytic H2-generation performance of 717.19 μmol h-1, a value higher than that of the sole CdS (320.99 μmol h-1) and ZnS (5.89 μmol h-1) by a factor of 2.2 and 121.8 times, respectively. Based on the experimental results, a possible S2- active site-mediated mechanism accounted for the high H2-production activity of the suspensible CdxZn1-xS nanocrystals, namely the numerous adsorbed S2- ions not only function as efficient hole scavengers to rapidly consume the photogenerated holes, resulting in an improved photocorrosion resistance of suspensible CdxZn1-xS nanocrystals, but also serve as effective H+-capturing active sites to accelerate the interfacial H2-production reaction. Meanwhile, an optimum bandgap structure of suspensible CdxZn1-xS nanocrystals is also extremely required for promoting the photocatalytic H2-production activity. This research may provide advanced insights for developing stable and high-activity photocatalytic materials.

Solid phase fabrication of Bismuth-rich Bi3O4Cl Br1− solid solution for enhanced photocatalytic NO removal under visible light

Abstract A new bismuth-rich Bi3O4ClxBr1−x solid solution photocatalyst was firstly synthesized through a solid phase conversion method. The X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), high-resolution transmission electron microscopy (HRTEM), UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL) were used to characterize the physico-chemial properties of samples. Results demonstrated that Bi3O4Cl0.5Br0.5 had a higher activity than that of Bi3O4X (X = Cl, Br) for photocatalytic NO removal. The valence band XPS and photoelectrochemical analyses, theoretical calculation and scavenger trapping experiment indicated that Bi3O4Cl0.5Br0.5 had higher photocatalytic oxidation ability and effective prohibition the recombination of photo-induced charged carriers. This study suggested that solid solution should be effective approach to improve the photocatalytic activity of bismuth-rich photocatalysts for air purification.

Photocatalytic degradation of H2S aqueous media using sulfide nanostructured solid-solution solar-energy-materials to produce hydrogen fuel

H2S is a corrosive, flammable and noxious gas, which can be neutralized by dissolving in alkaline media and employed as H2-source by utilizing inside semiconductor-assisted/photochemical reactors. Herein, through a facile hydrothermal route, a ternary nanostructured solid-solution of iron, zinc and sulfur was synthesized in the absence and presence of Ag-dopant, and applied as efficient photocatalyst of hydrogen fuel production from H2S media. The effect of pH on the photocatalyst performance was scrutinized and the maximum activity was attained at pH=11, where HS− concentration is high. BET, diffuse reflectance and photoluminescence studies indicated that the ternary solid-solution photocatalyst, in comparison to its solid-solvent (ZnS), has a greater surface area, stronger photon absorption and less charge recombination, which justify its superiority. Moreover, the effect of silver-dopant on the photocatalyst performance was examined. The investigations revealed that although silver could boost the absorption of photons and increase the surface area, it could not appreciably enhance the photocatalyst performance due to its weak influence on retarding the charge-recombination process. Finally, the phenomenon was discussed in detail from mechanistic viewpoint.

Effects of Calcination Temperature on the Physical Properties and Hydrogen Evolution Activities of La5Ti2Cu(S1‐xSex)5O7 Photocatalysts

AbstractLa5Ti2Cu(S1‐xSex)5O7 (LTCS1‐xSexO) solid solutions are found to function as visible‐light‐driven photocatalysts to evolve H2 from aqueous solutions containing sacrificial electron donors. However, this photocatalytic activity is reduced with increasing Se concentrations because of excessive particle growth during calcination at high temperatures. In the present study, the physical properties and photocatalytic H2 evolution activities of LTCS1‐xSexO (0 ≤ x ≤ 0.6) solid solution photocatalysts synthesized by solid‐state reactions at varying temperatures are assessed. It is found that the photocatalyst particle sizes are reduced upon lowering the calcination temperature. In addition, the calcination temperature resulting in the highest photocatalytic H2 evolution rates for NiS‐loaded LTCS1‐xSexO is shown to become lower with increasing Se content. The H2 evolution activity of LTCS1‐xSexO (0.2 ≤ x ≤ 0.6) is improved several‐fold by optimizing the calcination temperature because the excessive growth of particles is avoided. The activity of these materials is further improved by coloading Pt and NiS cocatalysts. This work demonstrates the importance of controlling the particle size of narrow bandgap LTCS1‐xSexO oxysulfoselenides so as to effectively utilize visible light during photocatalytic H2 evolution.

Biomolecule-assisted hydrothermal synthesis of ZnxCd1−xS nanocrystals and their outstanding photocatalytic performance for hydrogen production

To achieve scalable applications in solar hydrogen production, it is necessary to develop visible-light-responsive photocatalysts that are highly efficient, cost-effective, stable and environmentally-benign. Here narrow bandgap Zn–Cd–S solid solution photocatalysts (Eg = 2.11–2.53 eV) were prepared via a facile and green hydrothermal strategy under mild conditions. Amazingly, over the naked Zn0.5Cd0.5S photocatalyst, an extraordinarily high H2 production activity in Na2S–Na2SO3 aqueous solution is achieved up to 18.3 mmol h−1 g−1 with an apparent quantum efficiency of 73.8% per 50 mg under 420 nm light irradiation, which, to our knowledge, outperforms cocatalyst-free metal sulfide photocatalysts previously reported to date. Such super high performance arises from the enhanced visible-light-absorption capacity, suitable conduction and valence band potential together with the facilitated charge transport in Zn–Cd–S solid solutions. This work may open an avenue for the green preparation of inexpensive photocatalysts for solar H2 production.

Control the energy band potential of ZnMgO solid solution with enhanced photocatalytic hydrogen evolution capacity

In this study, we prepared a novel hierarchic nanorod ZnMgO solid solution photocatalyst, and in the crystal, part of Zn atoms were replaced by Mg atoms. Experimental results and theoretical calculation data firstly indicated that the energy band structure of the ZnO can be tuned with the Mg elements doping in it. Especially, during this process, the conduction band (CB) potential of ZnMgO solid solution moved to more negative site gradually with the Mg content increasing, which is an important characteristic for n-type water splitting photocatalyst. When the mole ratio of Mg in the reaction solution reach to 50%, corresponding end-product ZnMgO showed a much more negative CB potential (−0.46V vs NHE) than pure ZnO (−0.05V vs NHE), and the photocatalytic for hydrogen evolution product of ZnMgO solid solution increased dramatically than pure ZnO (from near zero of ZnO to 1103.9μmol/g in 4h). Furthermore, in the future, based on the ZnMgO solid solution prepared in this study, more visible-light-responsive solid solution with suitable water splitting band structure, such as ZnMgON and ZnMgOS etc., can be expected.

Enhancement of the H2 evolution activity of La5Ti2Cu(S1−xSex)5O7 photocatalysts by coloading Pt and NiS cocatalysts

Coloading of Pt and NiS cocatalysts enhanced the hydrogen evolution activity of La5Ti2Cu(S1−xSex)5O7 (0 ≤ x ≤ 0.6) solid solution photocatalysts more effectively than the individual loadings of each of the two cocatalysts.

Hexagonal Zn1−xCdxS (0.2 ≤ x ≤ 1) solid solution photocatalysts for H2 generation from water

A series of hexagonal Zn1−xCdxS photocatalysts with variable composition (0.2 ≤ x ≤ 1) is synthesized by a solvothermal method.

ChemInform Abstract: Solid Solution Photocatalyst with Spontaneous Polarization Exhibiting Low Recombination Toward Efficient CO2 Photoreduction.

AbstractIn view of efficient separation of photogenerated carriers, growth of a polar GaN:ZnO solid solution single crystal along its polarization axis is performed by molten salt ion exchange (space group P63mc, powder XRD).

Ni2P loading on Cd0.5Zn0.5S solid solution for exceptional photocatalytic nitrogen fixation under visible light

In this paper, transition metal phosphide (TMP) was used as cocatalyst to enhance the photocatalytic activity of Cd0.5Zn0.5S solid solution for nitrogen fixation under visible light irradiation. The Ni2P/Cd0.5Zn0.5S photocatalyst was characterized by X-ray diffraction patterns (XRD), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (DRS) and X-ray photoelectron spectroscopy (XPS). The photocatalytic data showed that Ni2P/Cd0.5Zn0.5S had higher activity than pure Cd0.5Zn0.5S for photocatalytic dinitrogen (N2) fixation in the absence of any scavengers. After 1h visible light irradiation (λ>400nm), the concentration of NH3 reached to 101.5μmolL−1. And the quantum efficiency at 420nm monochromatic light reached 4.32%, which much higher than other semiconductors. Time-resolved PL spectra, photocurrent and electrochemical impedance (EIS) spectroscopy revealed that higher photo-induced carrier separation efficiency of Ni2P/Cd0.5Zn0.5S. This work introduced Cd0.5Zn0.5S solid solution photocatalyst to reduce nitrogen, and offered a greenly, economically and efficiently way for nitrogen fixation.

Solid Solution Photocatalyst with Spontaneous Polarization Exhibiting Low Recombination Toward Efficient CO2 Photoreduction.

Decreasing the recombination of photogenerated carriers is a major challenge for efficiently converting solar energy into chemical energy by photocatalysis. Here, we have demonstrated that growth of a polar GaN:ZnO solid solution single crystal along its polarization axis is beneficial to efficient separation of photogenerated carriers, owing to the periodic potential barriers and wells generated from the periodically positive and negative atom arrangements in crystal structure. Local charge imbalance caused by replacing Ga(3+) with Zn(2+) leads to a polarization vector in the {0 0 0 1} planes of GaN:ZnO solid solution, thus forming a 1 D electron transport path along [2 1‾ 1‾ 0] in the {0 0 0 1} planes of GaN:ZnO solid solution to decrease recombination. Shorting the hole-transport distance by synthesizing porous nanoplates can further decrease recombination under the polarization field and improve the performance of polar photocatalyst in photoreduction of CO2 into CH4 .

Fabrication of BiOBrxI(1-x) photocatalysts with tunable visible light catalytic activity by modulating band structures.

A series of BiOBrxI1−x solid solutions were explored as novel visible light-sensitive photocatalysts. These BiOBrxI1−x solid-solution photocatalysts grew into two-dimensional nanoplates with exposed (001) facets and possessed continuously modulated band gaps from 2.87 to 1.89 eV by decreasing the Br/I ratio. The photocatalytic activities of these samples were measured, and the samples exhibited visible light-driven activities for the degradation of Rhodamine B (RhB). In particular, BiOBr0.8I0.2 exhibited the highest activity for the degradation of RhB. This result could be attributed to the balance between the effective light absorption and adequate redox potential. Additionally, investigations into the photocatalytic mechanism showed that the photodegradation of RhB over BiOBr0.8I0.2 solid-solution photocatalysts involved direct holes oxidation, in which the reaction that dominated during photocatalysis was determined by the potential of the valence band. Furthermore, a high stability in the photocatalytic activity of BiOBr0.8I0.2 was demonstrated by the cycling photocatalytic experiment and long-term irradiation, which might offer opportunities for its practical application as a catalyst.

Efficient visible light photocatalytic oxidation of NO with hierarchical nanostructured 3D flower-like BiOClxBr1−x solid solutions

Bismuth oxyhalides (BiOX, X = Cl, Br, I) as effective photocatalysts have attracted a wide attention for its outstanding performance on air purification with solar energy. In this work, a series of band-gap engineered BiOClxBr1−x homogeneous solid solution materials were explored as efficient visible-light induced photocatalysts. The BiOClxBr1−x solid solution photocatalysts with 3D hierarchical nanostructures have been successfully synthesized through a facile temple free hydrothermal method. The prepared BiOClxBr1−x possess a single tetragonal crystal structure without any secondary phases in spite of x varying from 0 to 1 indicating a homogeneous solid-solution characteristic. However, the band gaps of BiOClxBr1−x solid solutions are modulated from 3.30 to 2.75 eV by decreasing x from 1 to 0, which extends the light absorption from 375 nm to 450 nm. The obtained BiOCl0.5Br0.5 sample displays enhanced photocatalytic activity for the oxidation of NO, with a good stability under visible light. The existence of hydroxyl radicals and super oxygen radicals confirm its photocatalytic activities. The highly enhanced visible light photocatalytic activity of BiOCl0.5Br0.5 can be ascribed to the large specific surface area, hierarchical structures and the modified band structures. Furthermore, the modified band-gap adapts the balance between adequate redox potentials and effective visible light absorption. The present work provides novel insights into the design and synthesis of 3D hierarchical nanostructures with band-gap engineering to explore novel photocatalysts.

Facile synthesis of highly efficient nano-structured gallium zinc oxynitride solid solution photocatalyst for visible-light overall water splitting

Gallium-zinc oxynitride (GaZnON) solid solution is a photocatalyst capable of effective overall water splitting under visible light. In order to address the inefficiencies of the synthesis of GaZnON solid solution (e.g., 10+ h at 850°C under 250mlmin−1 NH3 flow), a facile technique is proposed. The technique utilizes crystalline Ga3+ and Zn2+ layered double hydroxides (LDHs) material as an atomic-level uniform precursor, and urea as the nanotemplated source of nitrogen. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis confirm the formation of wurtzite GaZnON in a 12min process through distribution of the uniform Ga3+ and Zn2+ LDHs precursor within the nanotemplate formed through urea pyrolysis. The structural, optical, and electrochemical properties of the prepared samples were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, and photoluminescence (PL) analysis. The newly synthesized photocatalyst consists of nanopores distributed uniformly through the surface and bulk of the solid solution particles; the presence of these nanopores improves the active surface area of the photocatalyst up to seven times, as compared to the one for traditionally prepared solid solution photocatalyst. The proposed technique is capable of controlling the composition of the final photocatalyst in a wide range of ZnO content ([Zn]/[Zn+Ga] up to 0.66). Apparent quantum yield (AQY) up to 2.5% at 420–440 was achieved by the photocatalyst with bulk [Zn]/[Zn+Ga]=0.32, loaded with 1wt% Rh nanoparticles. The effect of crystal defects and Zn content of the solid solution on the PL emission of the samples revealed that the GaZnON samples prepared with the LDHs precursor contain fewer crystal imperfections, which aids their water-splitting performance. The performance of the newly synthesized photocatalyst is among the highest reported in the open literature for photocatalysts loaded with a single co-catalyst. This photocatalyst has potential for future improvement through enhancement of the crystalline structure and improvement of the charge separation.

Substitution of Ce(III,IV) ions for Bi in BiVO4 and its enhanced impact on visible light-driven photocatalytic activities

The recombination of photogenerated charges can be greatly depressed by the charged crystal defects in the Bi1−xCexVO4+δ novel solid solution photocatalyst and both ˙OH and ˙O2− could be detected in the photoreaction.