Photocatalytic Reduction Activity Research Articles

In-situ DRIFT investigation of photocatalytic reduction and oxidation properties of SiO2@\u03b1-Fe2O3 core-shell decorated RGO nanocomposite

In this work, SiO2@α-Fe2O3 core-shell decorated RGO nanocomposites were prepared via a simple sol-gel method. The nanocomposites were prepared with different weight percentages (10, 30, and 50 wt %) of the SiO2@α-Fe2O3 core-shell on RGO, and the effects on the structural and optical properties were identified. The photocatalytic reduction and oxidation properties of the nanocomposites in the gas phase were assessed through the reduction of CO2 and oxidation of ethanol using in-situ diffuse-reflectance infrared fourier transform spectroscopy (DRIFT). The prepared nanocomposite with (30 wt %) of SiO2@α-Fe2O3 showed superior photocatalytic activity for the gas phase reduction of CO2 and oxidation of ethanol. Enhancement in the activity was also perceived when the light irradiation was coupled with thermal treatment. The DRIFT results for the nanocomposites indicate the active chemical conversion kinetics of the redox catalytic effect in the reduction of CO2 and oxidation of ethanol. Further, the evaluation of photoelectrochemical CO2 reduction performance of nanocomposites was acquired by linear sweep voltammetry (LSV), and the results showed a significant improvement in the onset-potential (–0.58 V) for the RGO (30 wt %)-SiO2@α-Fe2O3 nanocomposite.

Design of porous double-shell Cu2O@CuCo2O4 Z-Scheme hollow microspheres with superior redox property for synergistic photocatalytic degradation of multi-pollutants

Hollow double-shell Cu2O@CuCo2O4 (HDS-Cu2O@CuCo2O4) Z-Scheme photocatalyst with a porous structure was designed and successfully synthesized by anchoring Cu2O nanoparticles (NPs) on the surface of CuCo2O4 hollow spheres in situ. HDS-Cu2O@CuCo2O4 exhibited remarkable photocatalytic activity towards chlorotetracycline (CTC) oxidation and nitrobenzene (NB) reduction under visible light irradiation. Total organic carbon (TOC) removal of 63% was achieved over HDS-Cu2O@CuCo2O4 for CTC degradation, which is 1.7 and 2.1 times higher than that of pristine Cu2O and CuCo2O4. Moreover, the photocatalytic activity for both oxidation and reduction were simultaneously improved in CTC + NB mixed system owing to the presence of the synergistic photocatalytic effect. The superior photocatalytic performance of HDS-Cu2O@CuCo2O4 hybrid system should be mainly attributed to the unique hollow structure as well as the Z-scheme charge transfer pathway, which not only improve the utilization of visible light but also enhance the separation efficiency of photoexcited charge carriers. The proposed strategy provides an insight into the application of hollow Z-Scheme photocatalyst in environmental remediation.

Preparation of worm-like SnS2 nanoparticles and their photocatalytic activity

In this work, worm-like SnS2 nanoparticles were prepared by a hydrothermal process. And then their photocatalytic activity was explored for the restoration of effluent containing Cr (VI). The results indicate that the product obtained is hexagonal SnS2 nanoparticles and adopts a worm-like morphology with length of approximate 6 nm and diameter of 3 nm. Moreover, the as-prepared SnS2 nanoparticles possess high photocatalytic activity for the visible light-driven reduction of Cr (VI). When the concentration of Cr (VI) is 4 × 10−4 mol/L, 99% Cr (VI) can be reduced over the as-prepared SnS2 nanoparticles within 30 min under visible irradiation.

SrTaO2N co-doped with La/Zr as promising photocatalysts for water reduction under visible light illumination

La/Zr co-doping preserves visible light absorption of SrTaO2N, prohibits the formation of defects, improves surface hydrophilicity and charge separation conditions, all of which contribute to an enhanced photocatalytic activity for water reduction.

Nickel‐Doped Porous ZnO Nanosheets Functionalized with CuInS2 Nanoparticles: An Efficient Photocatalyst for Chromium (VI) Reduction

AbstractHeterojunction nanocomposites comprising Nickel‐doped porous ZnO nanosheets decorated with CuInS2 nanoparticles (CuInS2/Ni−ZnO) were prepared through a facile two‐step process. Compared with pure ZnO nanosheets, the Ni doping modifies the energy band structure and narrows the bandgap from 3.1 eV to 2.4 eV, thus enhancing the absorption of visible light. The production of high‐quality CuInS2/Ni−ZnO nanosheets with well‐matched band energy alignment facilitates effective charge transportation and separation. Benefiting from these favorable properties, the as‐prepared CuInS2/Ni−ZnO heterojunction nanosheets exhibit higher photocatalytic activity for reduction of Cr (VI) than CuInS2 and Ni−ZnO in the presence of visible light. The CuInS2/Ni−ZnO‐5 (2 % Ni) composite shows the best photocatalytic activity, which is 7.2 and 5.4 times higher than that of the Ni−ZnO and CuInS2. 98 % of Cr (VI) (50 mg/L) can be reduced within 25 min. Moreover, the activity and structure of the CuInS2/Ni−ZnO catalyst are maintained after five cycles. Combining theoretical calculations and experimental results, a possible photocatalysis mechanism of the CuInS2/Ni−ZnO heterojunction nanocomposites is proposed.

Metal-free azo-bridged porphyrin porous organic polymers for visible-light-driven CO2 reduction to CO with high selectivity.

Two nitrogen-rich azo-bridged porphyrin porous organic polymers (Azo-Por-Bpy-POP and Azo-Por-Dadp-POP) with high surface areas were prepared by coupling 5,10,15,20-tetra(p-nitrophenyl)-porphyrin with the aromatic amines of 2,2'-bipyridine-5,5'-diamine (Bpy) and diaminodiphenyl (Dadp). Azo-Por-Bpy-POP and Azo-Por-Dadp-POP display high photocatalytic reduction activity for CO2 to CO under visible-light irradiation without a sacrificial reagent or metal co-catalyst. Azo-Por-Bpy-POP exhibits the highest photoreduction for CO2 with CO as the only carbonaceous reduction product with a production rate of 38.75 μmol g-1 h-1. Theoretical investigations indicate a stronger electrostatic interaction between CO2 and Azo-Por-Bpy-POP than Azo-Por-Dadp-POP, which favors CO2 photoreduction.

Co-MOF as an electron donor for promoting visible-light photoactivities of g-C3N4 nanosheets for CO2 reduction

A possible mechanism for boosting the visible-light photoactivities of graphitic carbon nitride (g-C3N4) nanosheets for CO2 reduction via coupling with the electron donor Co-metal-organic framework (MOF) is proposed in this study. Specifically, Co-MOF as an electron donor is capable of transferring the photogenerated electrons in the lowest unoccupied molecular orbital (LUMO) to the conduction band of g-C3N4 to facilitate charge separation. As expected, the prepared Co-MOF/g-C3N4 nanocomposites display excellent visible-light-driven photocatalytic CO2 reduction activities. The CO production rate of 6.75 µmol g−1 h−1 and CH4 evolution rate of 5.47 µmol g−1 h−1 are obtained, which are approximately 2 times those obtained with the original g-C3N4 under the same conditions. Based on a series of analyses, it is shown that the introduction of Co-MOF not only broadens the range of visible-light absorption but also enhances the charge separation, which improves the photocatalytic activity of g-C3N4 to a higher level. In particular, the hydroxyl radical (•OH) experiment was operated under 590 nm (single-wavelength) irradiation, which further proved that the photogenerated electrons in the LUMO of Co-MOF can successfully migrate to g-C3N4. This work may provide an important strategy for the design of highly efficient g-C3N4-based photocatalysts for CO2 reduction.

Water-Soluble 3D Covalent Organic Framework that Displays an Enhanced Enrichment Effect of Photosensitizers and Catalysts for the Reduction of Protons to H2.

Covalent organic frameworks (COFs) are emerging porous polymers that have 2D or 3D long-range ordering. Currently available COFs are typically insoluble or decompose upon dissolution, which remarkably restricts their practical implementations. For 3D COFs, the achievement of noninterpenetration, which maximizes their porosity-derived applications, also remains a challenge synthetically. Here, we report the synthesis of the first highly water-soluble 3D COF (sCOF-101) from irreversible polymerization of a preorganized supramolecular organic framework through cucurbit[8]uril (CB[8])-controlled [2 + 2] photodimerization. Synchrotron X-ray scattering and diffraction analyses confirm that sCOF-101 exhibits porosity periodicity, with a channel diameter of 2.3 nm, in both water and the solid state and retains the periodicity under both strongly acidic and basic conditions. As an ordered 3D polymer, sCOF-101 can enrich [Ru(bpy)3]2+ photosensitizers and redox-active polyoxometalates in water, which leads to remarkable increase of their photocatalytic activity for proton reduction to produce H2.

Photocatalytic reduction of Cr(VI) by iron tungstosilicate under visible light

In this study, visible-light-active iron tungstosilicate (FeSiW) was fabricated via solvothermal method. Fe3+ was applied as counter ion to precipitate with silicotungstic acid (H4SiW12O40) in methanol solution. It can be observed from FT-IR spectra that the Keggin structure of SiW12O404− was well maintained in FeSiW. Besides, the effect of different initial molar ratios of Fe3+ to H4SiW12O40 was investigated (from 4:1 to 4:4). By photoelectrochemical characterizations, such as photocurrent response and Mott-Schottky (M-S) plots, easiest generation of electron-hole pairs and lowest charge transfer resistance can be observed on FeSiW with initial molar ratio of 4:3. Moreover, FeSiW with 4:3 ratio also displayed highest activity for photocatalytic reduction of Cr(VI), despite of the largest band gap (ca. 2.71 eV). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the optimized FeSiW were estimated to be +0.03 V and 2.74 V vs NHE, respectively, which were thermodynamically feasible for Cr(VI) reduction and H2O oxidation. Even under acidic condition (pH 3), the optimized photocatalyst displayed good stability in cyclic runs. Therefore, the present study opens a new way for heterogeneous polyoxometalates as efficient photocatalyst for environmental application.

Construction of two-dimensionally relative p-n heterojunction for efficient photocatalytic redox reactions under visible light

Precisely modulating the interfacial contact of heterojunction photocatalysts and regulating the spatial transfer of photoexcited charge carriers are the core factors to promote the photocatalytic efficiency for energy and environmental applications. Herein, novel 2D/2D relative p-n heterojunction CdS/g-C3N4 with Z-scheme (or S-scheme) was fabricated by in-situ growing ultrasmall 2D CdS nanoflakes on the surface of 2D g-C3N4 nanosheets. The optimized 2D/2D heterojunction structure with proper band configuration and suitable heterojunction interface can improve photoexcited charge carriers separation spatially and transfer to the ideal reaction sites. Therefore, 2D/2D CdS/g-C3N4 composites show high photocatalytic reduction (H2 evolution) and oxidation (selective oxidation of 5-hydroxymethylfurfural into 2,5-diformylfuran) activities, which are about 26.8 times and 3.51 times higher than individual g-C3N4 and are about 4.4 times and 2.39 times higher than individual CdS, respectively. The mechanisms including direction and inner impetus of the photoexcited charge separation and transfer in the composite photocatalyst are proposed and verified by experimental and computational analyses. This investigation highlights the promising scope to intelligently regulate the spatial separation and transfer of photoexcited charge carriers and rationally construct new relative p-n junction or Z-scheme photocatalysts for energy and environmental applications.

Polyaniline-TiO2 composite photocatalysts for light-driven hexavalent chromium ions reduction

In order to develop efficient photocatalysts, great efforts have been made to reduce hexavalent chromium to trivalent chromium. The photocatalytic efficiency of this reduction depends largely on the adsorption and diffusion of hexavalent chromium ions on the surface of the photocatalyst. In this paper, polyaniline-TiO2 composite can effectively improve the photocatalytic reduction performance and stability of hexavalent chromium ion. The effect of polyaniline (PANI) thickness on Cr(VI) activity and stability of photocatalytic reduction was studied by adjusting the content of PANI on Mesoporous TiO2 (MT) surface. Under the irradiation conditions, the reaction results showed that the reduction rate was 100%, and the maximum reaction rate reached 0.62 min−1 when the PANI modification was 3.0%. Moreover, the results showed that the reduction performance remained 100% after ten cycles. The main reason is that the PANI modified on the surface of TiO2 is rich in positively charged amino group, which can efficiently adsorb the reactant Cr(VI), and make the product Cr(III) leave the reaction interface quickly, thus ensuring the performance of photocatalyst.

A Mechanism Investigation of how the Alloying Effect Improves the Photocatalytic Nitrate Reduction Activity of Bismuth Oxyhalide Nanosheets

AbstractAlloyed bismuth oxyhalides have been widely used in photocatalytic water treatment processes due to their superior photocatalytic activity. However, there is no report on the removal of NO3− for alloyed bismuth oxyhalides, and the mechanism of the effect of alloying on NO3− reduction activity is still unclear. In this work, we prepared a series of BiOClnBr1‐n (0≤n≤1) with different Cl/Br ratios but with controlled similar morphologies, and explored their catalytic activities on reducing NO3− under visible light irradiation. Density functional theory investigations revealed that the formation energy for an oxygen (O) vacancy on the surface of BiOClnBr1‐n alloys is clearly reduced in comparison with the monohalide, illustrating that more O vacancies can be produced on the surface of BiOClnBr1‐n alloys. A high concentration of O vacancies not only promotes the adsorption of NO3− but also enhances the separation efficiency of the photogenerated electron‐hole (e–h) pairs, with both being beneficial to enhance the photocatalytic activity of BiOClnBr1‐n alloys for NO3− reduction. These new discoveries not only promote the design and development of new photocatalysts with excellent NO3− reduction properties, but also help to understand the relationship between alloying effects and semiconductor photocatalytic properties.

Z-scheme SnFe2O4-graphitic carbon nitride: Reusable, magnetic catalysts for enhanced photocatalytic CO2 reduction

The challenge in developing suitable photocatalysts for converting CO2 to solar fuels is to achieve effective CO2 adsorption capacity and high charge separation efficiency. In this report, we demonstrate the construction of a Z-scheme photocatalyst composed of coupling graphitic carbon nitride (g-C3N4) and SnFe2O4 semiconductors (denoted as SFO-CN) based on theoretical calculations and the characterization of their performance in photocatalytic CO2 reduction. The Z-scheme SFO-CN composite shows an enhanced photocatalytic activity in the reduction of CO2 to CO, yielding a CO evolution rate of 7.56 μmol/g/h without any cocatalyst and sacrifice reagent, which is 2.2 times higher than that of CN alone (3.45 μmol/g/h). Additionally, the SFO-CN catalyst can be easily separated from its aqueous dispersions for recycled usage due to its room temperature ferromagnetism. Characterizations revealed that the enhanced photocatalytic reduction activity of SFO-CN can be ascribed to the following unique characteristics: (1) SFO promotes the CO2 adsorption on the catalyst surface; (2) the Z-scheme charge transfer efficiently enhances the separation of the electron-hole pairs and maintains the high reducibility of electrons in the SFO conduction band. This study creates new opportunities for SFO and other ferrimagnetic spinel-type complex oxides-based Z-scheme system for solar fuel generation.

Synthesis of LaFeO3/Bi3NbO7 p-n heterojunction photocatalysts with enhanced visible-light-responsive activity for photocatalytic reduction of Cr(Ⅵ)

LaFeO3/Bi3NbO7 p-n heterojunction photocatalysts were prepared successfully through an in-situ growth method. The phase structure, optical property, morphology and microstructures of the obtained samples were studied by X-Ray diffractometer (XRD), Ultraviolet–visible absorption spectrum (UV–vis DRS), photoluminescence (PL) spectroscopy, transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and scanning electron microscope (SEM). The visible light responsive photocatalytic activity of as-obtained samples were investigated by the photo-reduction of Cr (Ⅵ). The effects of preparation conditions (proportion of BNO/LFO and temperature) on the activity were studied. Results showed that the as-obtained BNO-LFO-200-5 composite behaved the best photocatalytic activity (with rate constant of 0.0212 min−1) among all the obtained samples. The rate constant was about 42.4-times and 17.7-times as that of pristine BNO (0.0005 min−1) and LFO (0.0012 min−1), respectively. The enhanced photocatalytic property of the as-prepare LaFeO3/Bi3NbO7 composites can be ascribed to the formation of p-n junction between BNO and LFO, which significantly increased the separation efficiency of photo-excited electron-holes.

A wide visible light active photocatalyst Mg5Ta4O15-xNy for water oxidation and reduction

More than 1.6 eV band gap reductions have been realized on Mg5Ta4O15 by nitrogen doping with its pseudobrookite structure maintained. The nitrogen doped Mg5Ta4O15, i.e. Mg5Ta4O15-xNy, shows broad absorption in the visible light region and promising photocatalytic activity for both water reduction and oxidation reactions under visible light illumination (λ ≥ 400 nm). Apparent quantum efficiency as high as ~0.90% has been achieved at 420 ± 20 nm on Mg5Ta4O15-xNy which is comparable to a number of active metal oxynitrides photocatalysts. Apart from visible light absorption, Nitrogen doping on Mg5Ta4O15 is also accompanied by improved surface hydrophilicity and a negative shift of flat band potential. This simple strategy for band gap engineering can be extended to other metal oxides which may open new playgrounds in the design and development of efficient visible light active photocatalysts.

Enhanced photocatalytic reduction activity of BiOCl nanosheets loaded on β-Bi2O3

BiOCl nanosheets were loaded on the surface of β-Bi2O3 and its photocatalytic reduction activity was investigated. The composite displays excellent photocatalytic reduction activity. The NH3 yield rate of β-Bi2O3/BiOCl composite is nearly 25 times as that of pure BiOCl under Xenon lamp irradiation. The β-Bi2O3/BiOCl heterojunction presents superior photocatalytic reduction activity for the degradation of Cr(VI) ions, and Cr(VI) ions are totally removed in 18 min. The results of photocurrent and electrochemical impedance spectroscopy measurements reveal that the formation of β-Bi2O3/BiOCl heterojunction contributes to reduce the recombination of the photogenerated electrons and holes. The process of photocatalytic reduction activity over composite is explained by Z-scheme mechanism.

A Covalent Triazine-Based Framework Consisting of Donor-Acceptor Dyads for Visible-Light-Driven Photocatalytic CO2 Reduction.

Photocatalytic conversion of CO2 into value-added chemical fuels is a promising approach to address the depletion of fossil energy and environment-related concerns. Tailor-making the electronic properties and band structures of photocatalysts is pivotal to improve their efficiency and selectivity in photocatalytic CO2 reduction. Herein, a covalent triazine-based framework was developed containing electron-donor triphenylamine and electron-acceptor triazine components (DA-CTF). The engineered π-conjugated electron donor-acceptor dyads in DA-CTF not only optimized the optical bandgap but also contributed to visible-light harvesting and migration of photoexcited charge carriers. The activity of photocatalytic CO2 reduction under visible light was significantly improved compared with that of traditional g-C3 N4 and reported covalent triazine-based frameworks. This study provides molecular-level insights into the mechanism of photocatalytic CO2 reduction.

Switching on wide visible light photocatalytic activity over Mg4Ta2O9 by nitrogen doping for water oxidation and reduction

Band gap engineering on wide band gap semiconductor photocatalysts is of critical significance in improving their solar energy utilization. Here, we have successfully realized large band gap reduction and visible light photocatalytic water splitting on a wide band gap semiconductor Mg4Ta2O9 by nitrogen doping. More than 2.6 eV reductions in band gap have been achieved after doping nitrogen into Mg4Ta2O9, which strikingly red-shifts its absorption edge from less than 300 nm to as far as 600 nm. The nitrogen doped Mg4Ta2O9, i.e. Mg4Ta2O9-xNy, shows promising photocatalytic activities for water reduction and oxidation under visible light illumination (λ ≥ 400 nm). Apparent quantum efficiency as high as ∼1.57% has been achieved at 420 ± 20 nm which is comparable to a number of active metal oxynitride photocatalysts. Photoelectrochemical analysis suggests that this band gap reduction stems mainly from an uplift of valence band edge position along with nitrogen doping. Our band gap engineering on a compact corundum type compound implies that nitrogen doping can be applicable to a wider range of metal oxides, not necessarily limited to those with an ‘open’ crystal structure (tunnels or layers, etc.) reported in the literatures, thereby opening new possibilities in searching and developing visible light active photocatalysts.

Non-noble metal thickness-tunable Bi2MoO6 nanosheets for highly efficient visible-light-driven nitrobenzene reduction into aniline

High efficiency and selectivity is significant for photocatalytic organic transformation. In this work, the efficient photocatalytic reduction of nitrobenzene into aniline is achieved on non-noble metal thickness-tunable bismuth molybdate nanosheets prepared by the simple hydrothermal reaction. The photocatalytic activity of Bi2MoO6 strongly depends on the nanosheet thickness. The Bi2MoO6 monolayer displays remarkably enhanced photocatalytic activity for selective reduction of nitrobenzene than bilayer, multilayers and bulk Bi2MoO6 due to the unique properties of 2D materials with the large fraction of surface atoms and the ultrafast charge separation. The obtained conversion rate of 487.5 μmol g−1. h-1 (>95% conversion and >99% selectivity within 60 min reaction) on Bi2MoO6 monolayers is 5 times higher than that of Bi2MoO6 bulk, far exceeding most of common photocatalysts up to date. This work provides a thickness dependent nanosheet concept in the design of newly highly efficient catalysts for the photocatalytic reduction conversion of nitroarenes to anilines.

InVO4/β-AgVO3 Nanocomposite as a Direct Z-Scheme Photocatalyst toward Efficient and Selective Visible-Light-Driven CO2 Reduction.

Photocatalytic CO2 reduction to solar fuel is a promising route to alleviate the ever-growing energy crisis and global warming. Herein, to enhance photoconversion efficiency of CO2 reduction, a series of direct Z-scheme composites consisting of β-AgVO3 nanoribbons and InVO4 nanoparticles (InVO4/β-AgVO3) are prepared via a facile hydrothermal method and subsequent in situ growth process. The prepared InVO4/β-AgVO3 composites exhibit enhanced photocatalytic activity for reduction of CO2 to CO under visible-light illumination. A CO evolution rate of 12.61 μmol·g-1·h-1 is achieved over the optimized 20% In-Ag without any cocatalyst or sacrificial agent, which is 11 times larger than that yielded by pure InVO4 (1.12 μmol·g-1·h-1). Moreover, the CO selectivity is more than 93% over H2 production from the side reaction of H2O reduction. Significantly, based on the results of electron spin resonance (ESR) and in situ irradiated XPS tests, it is proposed that the synthesized InVO4/β-AgVO3 catalysts comply with the direct Z-scheme transfer mechanism. Significantly improved photocatalytic activities for selective CO2 reduction could be primarily ascribed to effective separation of photoinduced electron-hole pairs and enhanced reducibility of photoelectrons at the conduction band of InVO4. This work provides a new insight for constructing highly efficient photocatalytic CO2 reduction systems toward solar fuel generation.