Photocatalytic Reduction Activity Research Articles

A coordinatively saturated cobalt complex as a new kind catalyst for efficient electro- and photo-catalytic hydrogen production in purely aqueous media

It has been shown that coordinatively unsaturated complexes can catalyze hydrogen production via an unstable hydride intermediate. Herein we present a new kind of water soluble catalyst based on a coordinatively saturated cobalt complex, [(phen)2Co(CN)2]⋅ClO41 that is formed by the reaction of 1,10-phenanthroline (phen), Co(ClO4)2·6H2O and tetracyanoethylene (TCNE). Under photoirradiation with blue light (λmax = 469 nm) in air, together with [Ru(bpy)3]Cl2 and ascorbic acid in a pH 5.5 aqueous solution, 1 possesses photocatalytic activity for water reduction to hydrogen with an initial turnover number (TON) of 1232H2 per mol of catalyst at first 10 h, and this activity is sustained for at least 70 h. This can be attributed to that oxidative quenching by 1 (kq = 1.69 × 1010 M−1 s−1) dominates over reductive quenching to [Ru(bpy)3]Cl2 by ascorbic acid (kq = 1.55 × 1010 M−1 s−1). Additionally, 1 electrocatalyze hydrogen generation from a neutral water with a turnover frequency (TOF) of 1113.1 mol of hydrogen per mole of catalyst per hour (mol H2/mol catalysts/h) at an overpotential (OP) of 838 mV. We hope this can afford a new method in proton or water reduction catalysis using coordinatively saturated complexes in purely aqueous media.

Surface engineering of TiO2-MWCNT nanocomposites towards tuning of functionalities and minimizing toxicity

Multiwall carbon nanotubes were coupled with titanium dioxide (in different mole ratios of titanium and carbon) at the nano-scale, using a simple sonochemical and calcination process. The titanium dioxide-multiwall carbon nanotubes nanocomposites were for the first time surface modified with an innovative biotechnology-based reaction by using laccase to activate and covalently graft gallic acid dimers/oligomers/polymers on the nanocomposite surface in order to impart new functionalities and to minimize the nanocomposites’ toxicity. Structure of the titanium dioxide-multiwall carbon nanotubes, before and after surface modification, was investigated with X-ray powder diffraction, infrared, and UV-visible diffuse reflectance spectroscopy analysis, and scanning electron microscopy. The results indicated preferential formation of anatase titanium dioxide on one hand and covalent grafting of gallic acid dimers/oligomers/polymers functionalities on the nanocomposite surface, on the other. After modification, the antioxidant activity was analyzed using 2,2-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and photocatalytic activity toward the liquid-phase degradation of methylene blue in aqueous solution under both UV and visible light irradiation. Up to 98% antioxidant activity of the surface modified nanocomposites was established after 24 h of incubation, whereas non-modified nanocomposite induced the formation of the ABTS•+ radicals. In addition, 1.3-2.8-fold reduction in photocatalytic activity was achieved, depending on the irradiation. Accordingly, the gallic acid dimers/oligomers/polymers modified titanium dioxide-multiwall carbon nanotubes appear to simultaneously exhibit photocatalytic activity with an ability to scavenge free radicals, and can thus be considered as engineered nanoparticles with low toxicity.

Anomalous Photocatalytic Activity of Nanocrystalline γ-Phase Ga2O3 Enabled by Long-Lived Defect Trap States

Semiconductor photocatalysis has emerged as an efficient and sustainable advanced oxidation process for wastewater treatment and other environmental remediation and forms the basis for water splitting and solar-to-fuel conversion. Nanocrystalline metal oxides are particularly promising photocatalysts because of their efficiency, stability, and low toxicity. However, the influence of the crystal structure and defects on the photocatalytic activity of these polymorphic materials is still poorly understood. In this work we investigated the structural dependence of the photocatalytic activity of nanocrystalline Ga2O3. We demonstrate that metastable cubic-phase γ-Ga2O3 prepared from colloidal nanocrystals exhibits an anomalously high photocatalytic activity, which rapidly decreases upon thermally induced transformation to monoclinic β-Ga2O3. Using steady-state and time-resolved photoluminescence measurements we showed that the reduction in photocatalytic activity upon annealing is accompanied by a decrease in ...

Understanding the effect of partial N3−-to-O2− substitution and H+-to-K+ exchange on photocatalytic water reduction activity of Ruddlesden–Popper layered perovskite KLaTiO4

Ruddlesden–Popper perovskites, HLaTiO4 and HLaTiO4−xNx were prepared by proton exchange of layered KLaTiO4 synthesized by a solid-state reaction and KLaTiO4−xNx obtained by nitridation, respectively. The influence of nitridation time on crystal structure, morphology, and absorption wavelength and optical band-gap energy of KLaTiO4 crystals was studied. According to the XRD and SEM results, the crystal structure and plate-like morphology of the parent oxide were roughly retained even after nitridation at 800°C for 10h. The absorption edge wavelength of the KLaTiO4 crystals was found to be at about 350nm (Eg=3.54eV), while the absorption edge wavelength of the KLaTiO4−xNx crystals (after 10h nitridation) was about 586nm (Eg=2.12eV). To investigate the effect of partial N3−-to-O2− substitution and H+-to-K+ exchange on photocatalytic water reduction activity of Ruddlesden–Popper layered perovskite KLaTiO4, the photocatalytic activity for water reduction half-reaction over Pt-photodeposited KLaTiO4, KLaTiO4−xNx, HLaTiO4, and HLaTiO4−xNx was evaluated under simulated solar light. Among all the samples, Pt-photodeposited KLaTiO4 exhibited the highest photocatalytic activity for H2 evolution. In contrast, Pt-photodeposited KLaTiO4−xNx showed a low photostability and photocatalytic activity for H2 evolution due to the negative impact of the defective layer and reduced titanium species. In addition, perovskite oxynitride [LaTiO4−xNx]− nanosheets were successfully fabricated by a mechanical exfoliation (sonication) of the KLaTiO4−xNx crystals. The colloidal suspension of the oxynitride nanosheets showed a Tyndall effect, implying their good dispersion and stability in water.

Low temperature gel-combustion synthesis of porous nanostructure LaFeO3 with enhanced visible-light photocatalytic activity in reduction of Cr(VI)

LaFeO3 was synthesized via a simple gel-combustion method at the calcination temperatures of 200–400°C. The X-ray diffraction characterization indicated that the products synthesized at 200–400°C were all pure perovskite phase LaFeO3. The scanning electron microscopy images revealed that the LaFeO3 synthesized at 200°C had a porous nanostructure, but became a solid nanostructure at 400°C. The nitrogen adsorption measurements suggested that the specific surface area of LaFeO3 increased with the lowering of the calcination temperature. The UV–vis diffuse reflection spectra showed that the LaFeO3 synthesized at 200°C exhibited the strongest absorption of visible-light. The photocatalytic experiments demonstrated that as the calcination temperature was lowered, the resultant LaFeO3 exhibited obviously enhanced photocatalytic activity in reduction of Cr(VI) in aqueous solution under visible-light (wavelength longer than 420nm) irradiation. This work can enrich our knowledge on the synthesis and photoabsorption properties of porous nanostructure LaFeO3, and contribute to the application of LaFeO3 nanomaterials in treating the wastewaters contaminated by highly toxic and intractable Cr(VI).

Strontium titanates with perovskite structure as photo catalysts for reduction of CO2 by water: Influence of co-doping with N, S & Fe

Strontium titanates, SrTiO3, Sr3Ti2O7 and Sr4Ti3O10, with perovskite structure, have been modified by co-doping with N, S and Fe and characterized by X-ray diffraction (XRD), Diffuse reflectance Spectroscopy (DRS), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDXA), X-ray Photoelectron Spectroscopy (XPS) and Photo Luminescence Spectroscopy (PLS) techniques. XRD, EDXA and XPS results substantiate the incorporation of the three dopants into the titanate matrices. Based on the XPS binding energy data, it is inferred that doped nitrogen in SrTiO3 and Sr4Ti3O10 phases occupies O2− ion sites by substitution and interstitial sites in Sr3Ti2O7. Fe as Fe3+ and S as cationic S6+ occupy Ti4+ ion sites in the respective lattice in all three titanates. Additional energy levels created by the dopants within the band gap result in narrowing of the band gap, as revealed by DRS data and thus enable visible light absorption by the modified titanates. Significant reduction in the intensity of photo luminescence emission lines of modified titanates indicates that doping retards the recombination rates of charge carriers, enhancing their life time. Changes in the photo-physical properties brought out by modification with dopants improve the activity for photo catalytic reduction of CO2 vis-a-vis pristine titanates. Both Sr3Ti2O7 and Sr4Ti3O10, perovskites with layered structure, display higher activity when compared to simple perovskite, SrTiO3. Layered structure facilitates faster transport of charge carriers and the interlayer space could function as oxidation/reduction reaction sites, leading to the separation of charge carriers. Amongst the two layered perovskites, double layered Sr3Ti2O7 displays higher activity compared to triple layered Sr4Ti3O10 since the conduction band minimum of Sr3Ti2O7 is at higher negative energy level vis-à-vis that for Sr4Ti3O10 and the inter layer spacing is appropriate for CO2 reduction and splitting of water as well.

One-pot synthesis of manganese porphyrin covalently functionalized graphene oxide for enhanced photocatalytic hydrogen evolution

In this paper, graphene oxide (GO) sheets covalently functionalized with (5,10,15,20-tetraphenyl) porphinato manganese(III) (MnTPP) has been successfully synthesized and tested as a photocatalyst for hydrogen evolution from water under UV-vis light irradiation. The obtained sample was systematically characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis, Fourier transform infrared (FTIR), and Raman spectroscopy. The results show that the MnTPP moiety has been successfully grafted on the graphene oxide surface to form MnTPP modified GO (GO-MnTPP). The fluorescence quenching and photocurrent enhancement of GO-MnTPP confirm that the rapid electrons transfer from photoexcited the MnTPP moiety to the GO sheets. The platinized GO-MnTPP exhibits enhanced photocatalytic activity for water reduction to produce hydrogen. Moreover, with the assistance of polyvinyl pyrrolidone (PVP), the photocatalytic activity is further improved because of aggregation prevention of the GO-MnTPP nanocomposite. This study provides a facile method to build porphyrin-graphene-based photocatalysts for solar energy conversion.

Photocatalytic reduction behavior of hexavalent chromium on hydroxyl modified titanium dioxide

The selective adsorption of hexavalent chromium (Cr2O72− (Cr(VI))) and the desorption of trivalent chromium (Cr3+(Cr(III))) at the surface of photocatalysts are very important factors in determining the photocatalytic reduction activity of Cr(VI). Here, a homogeneous anchoring of hydroxyl groups on TiO2 was achieved via a simple soaking method in alkaline solution. The increase of surface hydroxyl groups was confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectra (FTIR). Moreover, the zeta potential analysis revealed the positive charged TiO2 surface in the acid system shown a significant improvement. The surface positive charges had the selective adsorption for Cr(VI) and desorption for Cr(III). Further experimental results revealed that such selective adsorption process played important roles in the photocatalytic reduction of Cr(VI). Finally, a process of selective adsorption/desorption promoted photocatalytic reduction of Cr(IV) was proposed in detail.

Openmouthed β-SiC hollow-sphere with highly photocatalytic activity for reduction of CO2 with H2O

A novel hollow spherical 3D structure of β-SiC with an open mouth was successfully fabricated by an environmentally friendly approach starting from ethylsilicate interaction with the P123 and glucose. The proposed growth mechanism of SiC hollow spheres was revealed step by step with SEM and XRD. When the as-prepared SiC was applied for the photocatalytic reduction of CO2 with pure water, it was found to be highly active for the conversion CO2 into mainly CH4 hydrocarbon products due to its unique electronic structure, hollow morphology and high BET surface area. Moreover, the photocatalytic activity of the hollow spherical SiC can be greatly improved by loading Pt cocatalyst. The optimal 2.0wt% Pt loading led to a stable CH4 evolution as high as 16.8μmol/g/h (or 376.4μl/g/h) with the simulated solar light irradiation, which is higher than that of many reported metal oxides (Pt/TiO2, CdS/WO3, Zn2GeO4, CeO2 and g-C3N4/NaNbO3 et al.) under similar experimental conditions. This work provides a new strategy for the architecture of thermally and chemically stable non-metallic carbide with unique hollow spherical structure to be a potential nonmetallic photocatalyst for CO2 reduction into CH4.

Template-free precursor-surface-etching route to porous, thin g-C3N4 nanosheets for enhancing photocatalytic reduction and oxidation activity

Precursor hydrothermal pre-treatment protocol was used to yield porous and thin g-C3N4 nanosheets for enhanced photocatalytic H2 evolution and NO removal.

Enhanced photocatalytic reduction activity of uranium(vi) from aqueous solution using the Fe2O3-graphene oxide nanocomposite.

Photocatalytic technologies are a potential solution for remediation of radioactive wastewater, including the reduction of radioactive hexavalent uranium, which is commonly found in wastewater from the nuclear industry. In this study, Fe2O3-graphene oxide composites were synthesized by an easy and scalable impregnation method as a catalyst for the reduction of U(vi). X-ray photoelectron spectroscopy analysis and high-resolution transmission electron microscopy images of this composite clearly showed that the Fe2O3 nanoparticles exist in the layered structure of graphene oxide. The photocatalytic activity of the Fe2O3-graphene oxide composite was evaluated by the reduction of U(vi) to U(iv) in aqueous solution under visible light. The results showed that the photocatalytic process of the Fe2O3-graphene oxide composite was always faster than that of the Fe2O3 nanoparticles. Moreover, the experimental kinetic data for the catalytic process followed a pseudo-first-order model. The stability of the Fe2O3-graphene oxide composites was studied over successive experiments, with the photocatalytic reduction efficiency of U(vi) decreasing to 76.0% after four cycles. Based on these experimental results, the enhanced photocatalytic activity and stability of Fe2O3-graphene oxide composites can be attributed to the improved adsorption properties of U(vi) at GO and the electron transfer from iron oxide to GO.

ZnFe2O4 deposited on BiOCl with exposed (001) and (010) facets for photocatalytic reduction of CO2 in cyclohexanol

ZnFe2O4-BiOCl composites were prepared by both hydrothermal and direct precipitation processes and the structures and properties of the samples were characterized by various instrumental techniques. The samples were then used as catalysts for the photocatalytic reduction of CO2 in cyclohexanol under ultraviolet irradiation to give cyclohexanone (CH) and cyclohexyl formate (CF). The photocatalytic CO2 reduction activities over the hydrothermally prepared ZnFe2O4-BiOCl composites were higher than those over the directly-precipitated composites. This is because compared to the directprecipitation sample, the ZnFe2O4 nanoparticles in the hydrothermal sample were smaller and more uniformly distributed on the surface of BiOCl and so more heterojunctions were formed. Higher CF and CH yields were obtained for the pure BiOCl and BiOCl composite samples with more exposed (001) facets than for the samples with more exposed (010) facets. This is due to the higher density of oxygen atoms in the exposed (001) facets, which creates more oxygen vacancies, and thereby improves the separation efficiency of the electron-hole pairs. More importantly, irradiation of the (001) facets with ultraviolet light produces photo-generated electrons which is helpful for the reduction of CO2 to ∙CO 2 - . The mechanism for the photocatalytic reduction of CO2 in cyclohexanol over ZnFe2O4-BiOCl composites with exposed (001) facets involves electron transfer and carbon radical formation.

Silver deposited holmium hydroxide nanowires for synthesis of aniline from visible light reduction of nitrobenzene

Abstract Holmium hydroxide nanowires were prepared by a hydrothermal method, while silver was deposited on the surface of holmium hydroxide nanowires by a photoassisted deposition method. Holmium hydroxide nanowires and silver deposited holmium hydroxide nanowires were characterized by different techniques such as XRD, PL, XPS, UV–vis, TEM and BET surface area measurements. The photocatalytic performance of holmium hydroxide nanowires and silver deposited holmium hydroxide nanowires was measured by studied the visible light reduction of nitrobenzene to aniline. The results reveal that the form of deposited silver is a metallic silver and it well dispersed on the surface of holmium hydroxide nanowires. Band gap of holmium hydroxide nanowires was decreased from 2.92 to 2.64 eV by depositing of silver. Weight percent of deposited silver plays important role in control band gap and photocatalytic activity of holmium hydroxide nanowires. 0.3 wt% silver deposited holmium hydroxide has the lowest band gap and the highest photocatalytic activity for reduction of nitrobenzene.

Synthesis of hierarchical bismuth-rich Bi4O5BrxI2-x solid solutions for enhanced photocatalytic activities of CO2 conversion and Cr(VI) reduction under visible light

Bismuth oxyhalides (BiOX, X=Br, I) photocatalysts are rarely applied for photocatalytic reduction reaction withthe photo-induced electron, as this is impeded by their low conduction band. As a widely used approach for enhancing the photocatalytic reduction activity, bismuth-rich strategy results the bismuth content of BiOX photocatalysts increasing. In this paper, a solid solutions of bismuth-rich Bi4O5BrxI2-x were prepared applying the molecular precursor method. Bi4O5BrxI2-x were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), element mapping, Brunauer–Emmett–Teller surface analysis (BET), UV–vis diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS). The obtained photocatalytic data showed that Bi4O5BrxI2-x solid solutions had higher photocatalytic activities than Bi4O5Br2 and Bi4O5I2. At an optimal ratio of x=1, the Bi4O5BrI photocatalyst showed the highest photocatalytic reduction activity for CO2 conversion (22.85μmolh−1g−1 CO generation, AQE was 0.372 at 400nm) and Cr(VI) removal (88%). CO2 adsorption data and CO2 temperature programmed desorption (CO2-TPD) revealed that Bi4O5BrI exhibited the highest chemical adsorption ability of CO2 molecules Photocurrent and electrochemical impedance (EIS) spectroscopy demonstrated the enhanced photo-induced carrier separation efficiency of Bi4O5BrI. These mechanistic studies suggest that Bi4O5BrxI2-x solid solutions are excellent photocatalysts for solar fuel generation and environmental remediation.

Facile synthesis of Bi2S3 nanoribbons for photocatalytic reduction of CO2 into CH3OH

The bismuth sulfide (Bi2S3) nanoribbons with different length-to-width ratio are successfully prepared using a simple solvothermal technique. Their crystal structures, morphologies, as well as the optical absorption are characterized by powder X-ray diffraction, transmission electron microscope, UV–vis diffuse-reflection spectroscopy and nitrogen adsorption. The electron microscopy observation shows that the molar ratio of Bi and S source greatly influences the size of the as-prepared Bi2S3 nanoribbons and, thereby, the length-to-width ratio. The Bi2S3 nanocatalysts with the largest length-to-width ratio exhibits the highest activity for photocatalytic reduction of CO2 into methanol because it has the most negative potential of conduction band, the largest BET specific surface area and lowest recombination of photogenerated charge carriers.

Facet-Dependent Photocatalytic N2 Fixation of Bismuth-Rich Bi5O7I Nanosheets

Bismuth-rich bismuth oxyhalides (Bi-O-X; X = Cl, Br, I) display high photocatalytic reduction activity due to the promoting conduction band potential. In this work, two Bi5O7I nanosheets with different dominant facets were synthesized using either molecular precursor hydrolysis or calcination. Crystal structure characterizations, included X-ray diffraction patterns (XRD), field emission electron microscopy and fast Fourier transformation (FFT) images, showed that hydrolysis and calcination resulted in the dominant exposure of {100} and {001} facets, respectively. Photocatalytic data revealed that Bi5O7I-001 had a higher activity than Bi5O7I-100 for N2 fixation and dye degradation. Photoelectrochemical data revealed that Bi5O7I-001 had higher photoinduced carrier separation efficiency than Bi5O7I-100. The band structure analysis also used to explain the underlying photocatalytic mechanism based on the different conduction band position. This work presents the first report about the facet-dependent photocatalytic performance of bismuth-rich Bi-O-X photocatalysts.

Tetragonal β-In2S3: Partial ordering of In3 + vacancy and visible-light photocatalytic activities in both water and nitrate reduction

A highly crystallized tetragonal β-In2S3 was synthesized with a partial ordering of In3+ vacancy confirmed by Rietveld refinements, which was therefore considered as an intermediate phase between conversional cubic and tetragonal β-In2S3. Benefiting from the high crystallinity and hierarchical morphology, it exhibited an intrinsic visible light photocatalytic activity in both water and nitrate reduction. The optimized H2 evolution rate was 442μmol/h/g and the nitrate conversion rate was as high as 3.26mgN/h. Apparent quantum yields of water splitting were also studied from 400 to 650nm with an optimal value of 1.45% at 400nm.

Fabrication of TiO2/Co-g-C3N4 heterojunction catalyst and its photocatalytic performance

A novel heterojunction photocatalyst with well-dispersed TiO2 nanoparticles on Co-Doped g-C3N4 has been successfully fabricated by an in situ generation solvothermal method. The photocatalytic performance of the as-prepared TiO2/Co-g-C3N4 catalyst was evaluated by the degradation of phenol in the presence of Cr6+ under simulated sunlight irradiation. The catalyst exhibited markedly enhanced photocatalytic activity for degradation of phenol but obviously decreased photocatalytic activity for reduction of Cr6+ compared with common TiO2/g-C3N4 catalyst. The mechanism for this photocatalytic performance was proposed.

Facile Synthesis of g-C₃N₄ Nanosheets/ZnO Nanocomposites with Enhanced Photocatalytic Activity in Reduction of Aqueous Chromium(VI) under Visible Light.

Graphitic-C3N4 nanosheets (CN)/ZnO photocatalysts (CN/ZnO) with different CN loadings were successfully prepared via a simple precipitation-calcination in the presence of exfoliated C3N4 nanosheets. Their morphology and structure were thoroughly characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS) and photoluminescence spectra (PL). The results showed that hexagonal wurzite-phase ZnO nanoparticles were randomly distributed onto the CN nanosheets with a well-bonded interface between the two components in the CN/ZnO composites. The performance of the photocatalytic Cr(VI) reduction indicated that CN/ZnO exhibited better photocatalytic activity than pure ZnO under visible-light irradiation and the photocatalyst composite with a lower loading of CN sheets eventually displayed higher activity. The enhanced performance of CN/ZnO photocatalysts could be ascribed to the increased absorption of the visible light and the effective transfer and separation of the photogenerated charge carriers.

Hierarchical CdIn2S4 microspheres wrapped by mesoporous g-C3N4 ultrathin nanosheets with enhanced visible light driven photocatalytic reduction activity

In this investigation, a series of hierarchical CdIn2S4/g-C3N4 nanocomposites were firstly synthesized by a facile one-pot hydrothermal strategy, wherein the mesoporous g-C3N4 nanosheets were in-situ self-wrapped onto CdIn2S4 nanosheets. Systematic characterization by XRD, FT-IR, UV⿿vis DRS, SEM, TEM, HAAF-STEM, XPS, photoelectrochemical tests were employed to analyze the phase structure, chemical composition, morphology and photocatalytic mechanism. The application, including photo-redox reaction and photocatalytic water splitting, were used to estimate the photocatalytic activity of as-obtained CdIn2S4/g-C3N4 nanocomposites. The results indicate that CdIn2S4/g-C3N4 heterostructures exhibit more efficient improvement of the photocatalytic performances towards photo-reduction of 4-NA to corresponding 4-PDA and photocatalytic H2 generation from water splitting than these counterparts as results of construction of intimate interfacial contact, which would promote the separation of photo-generated holes and electrons. Meanwhile, benefitting from the excellent surface wrap, the CdIn2S4/g-C3N4 nanocomposites possess notable enhanced photocatalytic stability. This research may provide a promising way to fabricate highly efficient photocatalysts with excellent stability and expand the application of CdIn2S4 in fine chemical engineering.