Visible-light Photocatalytic CO2 Reduction Research Articles

Graphene-Zn0.5Cd0.5S nanocomposite with enhanced visible-light photocatalytic CO2 reduction activity

Carbon dioxide (CO2) extent in the atmosphere has gradually increased because of excessive activities such as burning fossil fuels and deforestation. Converting CO2 into value-added hydrocarbon fuels over semiconductors by using solar energy is thus essential to reduce the greenhouse effect. In this study, novel hierarchical graphene–Zn0.5Cd0.5S (xG-ZCS) nanocomposites were constructed through a straight forward dual-step hydrothermal approach at 180 °C. The prepared composites exhibited enhanced visible-light photocatalytic transformation of CO2 to methanol (CH3OH). The 2G-ZCS (2 wt% graphene-Zn0.5Cd0.5S) composite produced the highest amount of CH3OH (approximately 1.96 μmol g−1 h−1), that is almost 98 times greater than pure ZCS nanospheres (Zn0.5Cd0.5S). The increased photocatalytic CO2 reduction could be as a result of graphene, which served as an exceptional electron receiver and carrier, sinking the reconsolidation of charge mobility and enhance the catalytic property. The electrochemical impedance spectroscopy and transient photocurrent analysis was analyzed to demonstrate the proposed photocatalysis mechanism.

Efficient self-assembly synthesis of LaPO4/CdS hierarchical heterostructure with enhanced visible-light photocatalytic CO2 reduction

Root nodule-like LaPO4/CdS hierarchical heterostructures were constructed via in-situ growth of LaPO4 microspheres on the surface of CdS nanorods to remarkably enhance CO2 reduction under visible light. The self-assembly strategy allows intimate interfacial contact between LaPO4 and CdS, greatly facilitating the separation and transfer of photo-generated charge, which lifetime is increased from 18.6 ns to 48.5 ns. Benefiting from the synergistic effect of the enhanced CO2 adsorption and promoted visible light absorption, the optimized LaPO4/CdS photocatalyst shows excellent CO2 reduction performance with CO generation rate of 960 μmol⋅h−1⋅g−1, which is 3.9 times greater than the pure CdS nanorods. Moreover, considerable stability can be observed in LaPO4/CdS heterostructure photocatalysts. The current work provides a simple self-assembly approach to build 3D-1D semiconductor composites for artificial photosynthesis.

Electronic and nanostructure engineering of bifunctional MoS2 towards exceptional visible-light photocatalytic CO2 reduction and pollutant degradation

With recently increasing environmental issues and foreseeable energy crisis, it is desirable to design cheap, efficient, and visible-light responsive nano-photocatalyst for CO2 conversion and pollutant degradation. Herein, we report a flower-like of MoS2-based hybrid photocatalyst with high efficiency through nanostructure and electronic structure engineering. Nanostructure control is used to fabricate MoS2 in to flower-like nanosheets (NSs) with large surface active area. Then MoS2 is coupled with conduction-band edge matched tin dioxide (SnO2) and decorated with Ag nanoparticles for suitable work function to create a unique cascade band alignment electronic structure to facilitate photoexcited charge transfer. It is shown that the amount-optimized nanocomposite of SnO2/Ag/MoS2 exhibits exceptional visible-light photocatalytic activities for conversion of carbon dioxide (CO2) to methane (CH4), approximately one order of magnitude enhancement than original MoS2 with the apparent quantum efficiency 2.38% at 420 nm. Similarly, the optimized sample also shows high activities for 2,4-diclorophenol, Methylene-Blue, Rhodamine-B and Methyl-Orange degradation as compared to pure MoS2. It is clearly demonstrated through combination of hydroxyl radical evaluation, photoelectrochemical and electrochemical impedance, that the enhanced photoactivities are attributed to the increased specific surface area, optimized band alignment for charge transfer and suppressed recombination. Our current work provides feasible routes for further research.

Plasmonic Ag nanoparticles decorated SrTiO3 nanocubes for enhanced photocatalytic CO2 reduction and H2 evolution under visible light irradiation

Herein, Ag decorated SrTiO3 nanocomposites (ASTO) were fabricated by solvothermal strategy. The resultant ASTO nanocomposites possess the broad plasmonic resonance absorption for visible light, achieving the obvious visible light photocatalytic CO2 reduction and H2 evolution performance. CO, CH4 and H2 were detected for CO2 reduction, but the photocatalytic system exhibits a high CO selectivity. The A0.5STO achieves the largest CO generation amount of 80.24 μmol·g−1. Moreover, the ASTO nanocomposite also shows an enhanced visible light photocatalytic H2 evolution rate of 264.5 μmol·g−1·h−1. The remarkable photocatalytic reduction performance should belong to the improved visible light absorption capacity benefitted from the localized surface plasmon resonance, and the improved electron extraction and transfer between Ag and STO in ASTO nanocomposite. Admittedly, the abundant reactive electrons over ASTO nanocomposites achieved efficient photocatalytic CO2 reduction and H2 evolution performance under visible light. Finally, photocatalytic reduction mechanism was investigated. The work provides a feasible controlled synthesis approach to fabricate plasmonic Ag decorated perovskite nanocomposite, achieving efficient visible light photocatalytic CO2 reduction and H2 generation.

Visible-Light Photocatalytic Reduction of CO2 to Formic Acid with a Ru Catalyst Supported by N,N'-Bis(diphenylphosphino)-2,6-diaminopyridine Ligands.

Visible-light photocatalytic CO2 reduction is carried out by using a RuII complex supported by N,N'-bis(diphenylphosphino)-2,6-diaminopyridine ("PNP") ligands, an unprecedented molecular architecture for this reaction that breaks the longstanding domination of α-diimine ligands. These competent catalysts transform CO2 into formic acid with high selectivity and turnover number. A proposed mechanism, with combined electron transfer and catalytic cycles, models the experimental rate of formic acid production.

Synthesis of Z-scheme α-Fe2O3/g-C3N4 composite with enhanced visible-light photocatalytic reduction of CO2 to CH3OH

In this study, a series of α-Fe2O3/g-C3N4(FCN) photocatalysts for CO2 reduction at 0.6 MPa to produce CH3OH is synthesized via hydrothermal method and the FCN hybrid shows greater absorption of visible light than pure g-C3N4 due to the introduction of α-Fe2O3 (narrow bandgap). The Z-scheme heterojunction promotes photogenerated electron and hole separation and suppresses the recombination of photoexcited electron-hole pairs. Benefiting from this unique structure, the optimized FCN(40:60) hybrid shows a CH3OH evolution rate of 5.63 μmol g−1 h−1 without any sacrifice reagent and cocatalyst, which is 2.9 times higher than that of pure g-C3N4(1.94 μmol g−1 h−1). The excellent activity is ascribed to a Z-scheme transfer mechanism, which is proposed according to the band structure between g-C3N4 and α-Fe2O3. The results gained here may provide some illuminating insights for the design of new types of Z-scheme photocatalysts.

Synthesis and enhanced visible light photocatalytic CO2 reduction of BiPO4-BiOBr x I1-x p-n heterojunctions with adjustable energy band.

A series of novel BiPO4–BiOBrxI1−x p–n heterojunctions were successfully prepared by a facile solvothermal method. The morphology, structure and optical properties of photocatalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and ultraviolet visible diffuse reflectance spectroscopy. The visible light photocatalytic activities of BiPO4–BiOBrxI1−x heterojunctions were investigated by photocatalytically reducing CO2. After 4 hours of irradiation, the 5% BiPO4–BiOBr0.75I0.25 heterojunction showed the highest photocatalytic activity with the yields of CO and CH4 up to 24.9 and 9.4 μmol gcat−1 respectively. The improved photocatalytic activity may be due to the formation of BiPO4–BiOBrxI1−x p–n heterojunctions which can effectively restrict the recombination rate of the photoexcited charge carriers. Moreover, the energy band structure of BiPO4–BiOBrxI1−x heterojunctions could be easily adjusted by changing the mole ratio of I and Br. The possible mechanism of the enhancement of the photocatalytic performance was also proposed based on experimental and theoretical analysis. The present study may provide a rational strategy to design highly efficient heterojunctions with an adjustable energy band for environmental treatment and energy conversion.

Enhanced visible-light photocatalytic CO2 reduction performance of Znln2S4 microspheres by using CeO2 as cocatalyst

ZnIn2S4 flower-like microspheres decorated with CeO2 as cocatalyst have been successfully fabricated through a microwave-assisted hydrothermal method for visible-light carbon dioxide (CO2) photoreduction to produce methanol (CH3OH). All the composite photocatalysts exhibited a more excellent photocatalytic performance than bare CeO2 and ZIS, and reached the highest with a CH3OH evolution rate of 0.542 µmol g−1 h−1 when the content of CeO2 was 5 wt%. In the composites, the CeO2 with oxygen vacancies acted as a significant cocatalyst to efficiently trap the photogenerated electrons from ZIS and thus boosted the separation of photoinduced charge carriers, while the unique 3D structure of ZIS made it with a relatively large specific area, which could provide abundant active reaction sites and render the reactants and products diffuse more easily. This study verified the CeO2 as one of the ideal cocatalyst candidates for ZnIn2S4 in the application of photocatalytic CO2 reduction, and gave rise to an increasing interest in expanding potential applications of ZnIn2S4 based materials in the field of energy conversion.

Ball-flower like NiO/g-C3N4 heterojunction for efficient visible light photocatalytic CO2 reduction

A ball-flower like NiO/g-C3N4 heterojunction composite was synthesized via a hydrothermal deposition method combined with subsequent calcination route. The NiO/g-C3N4 heterojunction exhibited a superior performance in CO2 photoreduction. A maximum CO yields of 4.17 μmol/(h g-cat) had been obtained on 40% NiO/g-C3N4 composite, which was 2.5 and 7.6 times as high as the pure g-C3N4 and NiO respectively. The promotion mechanism could be ascribed to the perfect band matching and efficient internal charge transfer within the p-n junction, which results in high-efficiency separation of photogenerated electron-hole pairs, strong visible-light response and high specific surface area.

Nanocasting synthesis of chromium doped mesoporous CeO2 with enhanced visible-light photocatalytic CO2 reduction performance.

Chromium doped mesoporous CeO2 catalysts were synthesized via a simple nanocasting route by using silica SBA-15 as the template and metal nitrates as precursors. The effect of Cr doping concentration (5%, 10%, 15% and 20% of the initial Cr/(Cr+Ce) molar percentage) on the structures of these catalysts and their photocatalytic performances in reduction of CO2 with H2O were investigated. The results indicated that the introduction of Cr species could effectively extend the spectral response range from UV to visible light region (400-700nm) and improve the electronic conductivity for the mesoporous CeO2 catalysts which exhibited an enhanced photocatalytic activity in the reduction of CO2 with H2O when compared with the non-doped counterpart. The highest CO and CH4 yield of 16.2μmol/g-cat. and 10.1μmol/g-cat., respectively, were acquired on the optimal chromium doped CeO2 catalyst with the initial Cr(Cr+Ce) molar percentage of 15% under 8h visible-light irradiation, which were more than twice as high as that of bare CeO2. The remarkably increased photocatalytic performance should be attributed to the advantageous structural and compositional features of the chromium doped mesoporous CeO2.

Fabrication and enhanced CO2 reduction performance of N-self-doped TiO2 microsheet photocatalyst by bi-cocatalyst modification

Exploiting advanced photocatalyst to reduce carbon dioxide (CO2) into solar fuels is important. In particular, the combined improvement in visible-light harvesting ability, charge separation and utilization efficiency is essential to enhance photocatalytic CO2 reduction performance. Herein, N-self-doped anatase TiO2 (NT) microsheets with proper coexposed {101} and {001} facets were first prepared via simple hydrothermal method using HF as capping agent and TiN as precursor. Then, the exposed {101} and {001} facets of the as-prepared NT microsheets were further modified by simultaneous photodeposition of Ag and MnOx nanoparticles (NPs), respectively. The resulting Ag and MnOx co-modified NT microsheets showed good visible-light absorption ability, excellent charge separation, and utilization efficiency and consequently exhibited the highest visible-light photocatalytic CO2 reduction rate to methanol (0.53μmolh−1g−1). The enhanced photocatalytic activity is attributed to the synergistic modification of the visible-light harvesting ability of nitrogen doping, as well as charge separation and utilization efficiency by the combination of proper facet design and cocatalyst deposition together with Surface Plasmon Resonance (SPR) effect induced by Ag. TiO2 modification techniques may shed some light on the future development of advanced visible-light-driven photocatalysts.

In situ synthesis of ZnO/ZnTe common cation heterostructure and its visible-light photocatalytic reduction of CO2 into CH4

The development of hierarchical superstructures and their application for the sake of clean and sustainable energy is an attractive research field. Here we reported in situ synthesis of common cation heterostructure via modification of zinc oxide (ZnO) by zinc telluride (ZnTe) photocatalyst through one-pot hydrothermal approach at reaction temperature of 180°C. The phase, crystal structure, morphology, composition, optical property and alignment of energy levels of the as-synthesized ZnO/ZnTe heterostructure have been thoroughly studied by different techniques. The heterostructure fabricated with different ZnO flower-like nanostructures (hundreds of nanometers for the rod length or sheet size of the petals and tens of nanometers for the corresponding diameter or thickness) exhibited different photocatalytic capability for reduction of carbon dioxide into methane under visible-light irradiation (λ≥420nm), which is mainly due to the different exposed crystal planes of ZnO and different surface area (15.0 and 5.6m2g−1 for sheet-like petals and rod-like petals, respectively). The obtained photocatalytic systems show good visible-light photoreduction capability even with ∼3.35% ZnTe in terms of atomic percentage, for which the solar energy conversion efficiency can reach ∼3.28% in the first 30min of photoreduction. The formation of heterojunction can facilitate charge transfer and thus improve the photocatalytic activity. In addition, the advantage of sharing common cations for a heterostructure is also briefly discussed.

A noble metal-free reduced graphene oxide–CdS nanorod composite for the enhanced visible-light photocatalytic reduction of CO2 to solar fuel

Solar-fuel production has attracted considerable attention because of the current demand to find alternative transportation fuels with particular emphasis on those fuels obtained photocatalytically from water and CO2. In this work, reduced graphene oxide (RGO)–CdS nanorod composites were successfully prepared by a one-step microwave-hydrothermal method in an ethanolamine–water solution. These composite samples exhibited a high activity for the photocatalytic reduction of CO2 to CH4, even without a noble metal Pt co-catalyst. The optimized RGO–CdS nanorod composite photocatalyst exhibited a high CH4-production rate of 2.51 μmol h−1 g−1 at an RGO content of 0.5 wt%. This rate exceeded that observed for the pure CdS nanorods by more than 10 times and was better than that observed for an optimized Pt–CdS nanorod composite photocatalyst under the same reaction conditions. This high photocatalytic activity was ascribed to the deposition of CdS nanorods onto the RGO sheets, which act as an electron acceptor and transporter, thus efficiently separating the photogenerated charge carriers. Furthermore, the introduction of RGO can enhance the adsorption and activation of CO2 molecules, which speeds up the photocatalytic reduction of CO2 to CH4. The proposed mechanism for the observed photocatalytic reaction with the RGO–CdS nanorod composite was further confirmed using transient photocurrent response and electrochemical impedance spectra. This work not only demonstrates a facile microwave-assisted hydrothermal method for fabricating highly active RGO–CdS nanorod composite photocatalysts, but also demonstrates the possibility of utilizing of an inexpensive carbon material as a substitute for noble metals in the photocatalytic reduction of CO2.

Photocatalytic reduction of carbon dioxide to formic acid, formaldehyde, and methanol using dye-sensitized TiO2 film

A bifunctionalized TiO2 film containing a dye-sensitized zone and a catalysis zone is designed for visible-light photocatalytic reduction of CO2 to chemicals continuously. Charge separation can be accomplished with electron transferring to catalysis zone and positive charge transforming to anode. Highly efficient conversion of CO2 to formic acid, formaldehyde, and methanol is achieved through the transferring electrons on conduction bands (CB) of TiO2. Reduction of CO2 and O2 evolution take place in separated solutions on different catalysts. The separated solution carried out in this photo-reactor system can avoid CO2 reduction products being oxidized by anode. The yields of reduction products were enhanced remarkably by external electrical power. This study provides not only a new photocatalytic system but also a potential of renewable energy source via carbon dioxide.