Photocatalyst For Water Oxidation Research Articles

Enhanced photoelectrochemical activity of Co-doped β-In2S3 nanoflakes as photoanodes for water splitting.

This work is primarily focused on indium sulfide (β-In2S3) and cobalt (Co)-doped β-In2S3 nanoflakes as photoanodes for water oxidation. The incorporation of cobalt introduces new dopant energy levels increasing visible light absorption and leading to improved photo-activity. In addition, cobalt ion centers in β-In2S3 act as potential catalytic sites to promote electro-activity. 5 mol% Co-doped β-In2S3 nanoflakes when tested for photoelectrochemical water splitting exhibited a photocurrent density of 0.69 mA cm−2 at 1.23 V, much higher than that of pure β-In2S3.

Enhanced solar light driven activity of p-n heterojunction for water oxidation induced by deposition of Cu2O on Bi2O3 microplates

As an important half reaction in solar-driven water splitting, it is more challenging to develop low-cost and highly efficient photocatalysts for water oxidation. The enhancement of sunlight harvesting and inhibition of charge-carrier recombination are keys to fabricating efficient semiconductor-based photocatalysts for energy conversion from solar light to chemicals. Herein, we reported highly dispersive Cu2O/Bi2O3 composites prepared by a facile and benign synthetic route, where n-type Bi2O3 microplates and nano-sized p-type Cu2O were coupled together to construct heterojunctions to improve the transportation efficiency of photoinduced charge carriers, benefited from the intimate interactions at the interfaces between Bi2O3 and Cu2O. The electrochemical properties of charge-transportation and population of charge carriers were investigated in the heterojunctions. The hybrid materials exhibit both enhanced photocatalytic performances in water oxidation and photodegradation of dyes compared with sole Bi2O3 or Cu2O under artificial solar light irradiation. The initial O2 evolution rate of the heterojunction system is 1.4- and 8-fold higher than the pure Bi2O3 and Cu2O, respectively. This study provides new protocols for synthesizing novel hybrid materials with insights into heterojunction-based photocatalysis for green energy production and wastewater purification.

Direct Growth of BiVO4/Bi2S3 Nanorod Array on Conductive Glass as Photocatalyst for Enhancing the Photoelectrochemical Performance

Photoelectrochemical water splitting is one of the promising methods to generate hydrogen fuels. Designing photocatalysts for accelerating four-electron transfer water oxidation is necessary to improve photoelectrochemical water splitting. Bismuth vanadate is one of the efficient photocatalysts for water oxidation, but the short diffusion length limits its photocatalytic ability. This work first proposes synthesis of bismuth sulfide/bismuth vanadate nanorod array on conducting glass via solution method and hydrothermal reaction to enhance visible-light absorption and provide efficient charge-transfer with type II heterojunction and one-dimensional transfer path. Bismuth sulfide coverage on bismuth vanadate nanorod array is simply tuned by varying thiourea amounts in hydrothermal solution. Higher photocurrent density of 1.43 mA/cm2 are obtained for bismuth sulfide/bismuth vanadate electrode than that for bismuth vanadate electrode (0.25 mA/cm2) at 1.23 V versus reversible hydrogen electrode, measured under...

Improved photocatalytic activity of rutile TiO2 for organic oxidation through Ag nanoparticles and borate anions

Rutile TiO2 (RT) is usually used as photocatalyst for water oxidation, but seldom for organic oxidation, probably due to the slow reduction of O2. Herein we report a mutual effect of Ag nanoparticles and borate anions greatly increasing the photocatalytic activity of RT for phenol oxidation in neutral aqueous solution. RT was home-made, followed by calcination at 200―800 °C. With a given RT, the rates of phenol oxidation on the addition of Ag and borate were increased and decreased, respectively. In the presence of both Ag and borate, interestingly, the rate of phenol oxidation was larger than that in the presence of Ag. A (photo)electrochemical measurement revealed that Ag and borate, in contact with the irradiated RT, could catalyze O2 reduction and phenol oxidation, respectively. Since the electron and hole transfer processes would promote each other, the positive effect of Ag is further enhanced in the presence of borate. This work demonstrates that RT could be developed as a promising photocatalyst for organic oxidation through a surface co-catalyst.

Visible-Light-Responsive 2D Cadmium-Organic Framework Single Crystals with Dual Functions of Water Reduction and Oxidation.

The development of new metal-organic frameworks (MOFs) with dual functions of both water reduction and oxidation under visible-light irradiation is highly desirable for promising solar water splitting, but is not yet reported. Herein, a cadmium-based MOF (denoted as "Cd-TBAPy") single crystal with a 2D layered framework by employing 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H4 TBAPy) as an organic linker is reported, which exhibits good visible-light absorption with edge of ≈600 nm. The Mott-Schottky (M-S) measurement and UV-vis analysis integrally reveal that the Cd-TBAPy is an n-type semiconductor with a bandgap of ≈2.15 eV whose conduction and valence band are estimated to be -0.05 and 2.10 eV, respectively. Together with loading of Pt or CoPi cocatalyst, the Cd-TBAPy is active for both water reduction and oxidation in the presence of scavengers under visible-light irradiation. Especially, the optimized apparent quantum efficiency for O2 evolution reaches 5.6% at 420 nm, much higher than that of previous MOF-based photocatalysts reported so far. This is thought to be the first MOF that functions as a photocatalyst for both water reduction and oxidation under visible light, demonstrating the intriguing future of MOF materials in solar-to-chemical energy conversion.

Atomic-scale understanding of the electronic structure-crystal facets synergy of nanopyramidal CoPi/BiVO4 hybrid photocatalyst for efficient solar water oxidation

The observation of a spontaneous electron enrichment at the apexes of bismuth vanadate (BiVO4) nanopyramidal arrays is achieved by performing synchrotron-based angular dependent X-ray absorption spectroscopy. In accordance with density function theory calculations, the formation of (112) facets enriched apexes is proposed. Such intrinsically electron-enriched (112) facets at the apexes produce a potential facilitating the diffusion of photoexcited holes, which not only mitigates the recombination with the photoexcited electrons, but also provide an active reaction site for the photoassisted selective growth of cobalt phosphate (CoPi), a well known and highly active co-catalyst for water oxidation at the apex of BiVO4 nanopyramids. Benefiting from the element-resolved properties of synchrotron-based X-ray spectroscopy, 3d electrons on the vanadium site are directly resolved by resonant inelastic X-ray scattering measurements. Due to this unique morphology, the charge at the apex is expected to induce a strong interaction with CoPi, as a result of the metal-to-ligand charge transfer spectroscopy feature observed from the cobalt site. Substantial evidences are collected by means of comprehensive soft X-ray spectroscopy techniques with high spectral resolution revealing the atomic-scale origin and the nature of the synergy as well as the strong correlation between its unique structural properties and the electronic structure of this novel highly-ordered hybrid photocatalyst and its significantly improved photoelectrochemical performance.

N-doped carbon dots@layer facilitated heterostructure of TiO2 polymorphs for efficient photoelectrochemical water oxidation

Abstract A cocktail strategy was developed to fabricate sandwich nanostructure of hydrogenated C-doped anatase TiO2 nanocrystals/N-doped carbon dots@layer/rutile TiO2 nanorod array as an efficient photocatalyst for photoelectrochemical water oxidation. The one-dimensional, single crystalline nature of the innermost rutile TiO2 nanorod facilitates charge transport. The middle N-doped carbon dots@layer offers dual functionalities, with the N-doped carbon dots as the photosensitizer to harvest long wavelength lights and the N-doped carbon layer as the conductive layer for fast charge transport toward the current collector. The outermost hydrogenated C-doped anatase TiO2 nanocrystals serve to increase the visible light absorption and to form a type II heterostructure with the innermost rutile TiO2 nanorods to enahnce the charge separation. An enhancement of 86% in photocurrent density was achieved at 1.23 V (vs. RHE) under illumination of simulated sun light of 100 mW/cm2 by the sandwich nanostrcuture, as compared to that of the plain rutile TiO2 nanorod array. The enhancement was boosted to 228% under visible simulated sun light (λ > 400 nm) illumination. The photoconversion efficiency and photocurrent density retention rate were significantly improved from 0.23% and 60% for the plain rutile TiO2 nanorod array to 0.65% and 84% for the sandwich nanostructure, respectively.

Lead Bismuth Oxyhalides PbBiO2X (X = Cl, Br) as Visible-Light-Responsive Photocatalysts for Water Oxidation: Role of Lone-Pair Electrons in Valence Band Engineering

We show that layered oxyhalides PbBiO2X (X = Cl, Br, I) with a Sillen-type structure possess band levels appropriate for visible-light-induced water splitting. Under visible light, PbBiO2Cl and PbBiO2Br with band gap (BG) of 2.51 and 2.48 eV, respectively, stably oxidized water to O2 in the presence of an Fe3+ electron acceptor. A comparison with structurally related SrBiO2Cl and BaBiO2Cl (BG = 3.55 and 3.54 eV) combined with DFT calculations revealed a significant interaction between O 2p and Pb 6s orbitals leading to the upward shift of the valence band maximum in PbBiO2X as compared with (Sr,Ba)BiO2Cl. Z-scheme water splitting into H2 and O2 has been demonstrated using PbBiO2Cl as an O2-evolving photocatalyst, coupled with an appropriate H2-evolving photocatalyst in the presence of an Fe3+/Fe2+ redox mediator.

Synthesis of monoclinic BiVO4 nanorod array for photoelectrochemical water oxidation: Seed layer effects on growth of BiVO4 nanorod array

Bismuth vanadate is one of promising photocatalysts for water oxidation. Synthesizing bismuth vanadate nanorods is important to achieve efficient water oxidation with efficient carrier transport routes. Depositing the seed layer is necessary as the support for growing bismuth vanadate nanorods on the fluorine-doped tin oxide glass. Different configurations of seed layers are fabricated by spin-coating bismuth vanadate precursors on the fluorine-doped tin oxide glass and then drying the layers using different processes. The smallest onset potential of 0.57 V versus reversible hydrogen electrode and the highest photocurrent density of 0.39 mA/cm2 at 1.23 V versus reversible hydrogen electrode are obtained for the optimized bismuth vanadate electrode, which is prepared using the vacuum oven to dry the seed layer, under air mass 1.5 global illumination in the electrolyte containing 0.5 M sodium sulfate and 0.1 M phosphate buffer, due to the well-defined nanorod structure with the largest gaps in-between for more efficient light illumination, less charge recombination, and more effective electrolyte diffusion. The smallest charge-transfer resistance is also attained for the optimized bismuth vanadate electrode evaluated using the electrochemical impedance spectroscopy. The results provide an alternative way for designing the morphology of photocatalyst by carefully controlling the drying rate for making the seed layers.

Spontaneous Formation of Noble- and Heavy-Metal-Free Alloyed Semiconductor Quantum Rods for Efficient Photocatalysis.

Quasi-1D cadmium chalcogenide quantum rods (QRs) are benchmark semiconductor materials that are combined with noble metals to constitute QR heterostructures for efficient photocatalysis. However, the high toxicity of cadmium and cost of noble metals are the main obstacles to their widespread use. Herein, a facile colloidal synthetic approach is reported that leads to the spontaneous formation of cadmium-free alloyed ZnSx Se1-x QRs from polydisperse ZnSe nanowires by alkylthiol etching. The obtained non-noble-metal ZnSx Se1-x QRs can not only be directly adopted as efficient photocatalysts for water oxidation, showing a striking oxygen evolution capability of 3000 µmol g-1 h-1 , but also be utilized to prepare QR-sensitized TiO2 photoanodes which present enhanced photo-electrochemical (PEC) activity. Density functional theory (DFT) simulations reveal that alloyed ZnSx Se1-x QRs have highly active Zn sites on the (100) surface and reduced energy barrier for oxygen evolution, which in turn, are beneficial to their outstanding photocatalytic and PEC activities.

Dual Z-scheme g-C3N4/Ag3PO4/Ag2MoO4 ternary composite photocatalyst for solar oxygen evolution from water splitting

Semiconductor-based solar-driven photocatalytic water splitting has been considered as one of the most promising solutions to solve the problem of fossil-based energy crisis, while the development of advanced photocatalytic materials for high-performance oxygen evolution from water splitting is the biggest challenge we are facing. We report the fabrication of novel g-C3N4/Ag3PO4/Ag2MoO4 ternary composite materials and the exploration of heterostructure materials for water oxidation under LED illumination. The hybridization of three semiconductors has been confirmed by microscopic study, chemical and structural analyses. Enhanced oxygen-producing activity over the obtained ternary composite photocatalysts was observed. The reasons responsible for the enhanced oxygen-evolving performance can be ascribed to the improved light absorption toward visible light, faster charge separation and charge transportation, as well as more powerful water oxidation capability originating from the in-situ construction of dual Z-scheme-type channels under visible light irradiation. The key role of in-situ formed metallic Ag as the electron mediator is suggested based on the theoretical and experimental results. The successful synthesis of fascinating ternary water oxidation photocatalysts provides new insights into the development of novel all-solid-state Z-scheme photocatalytic systems for energy and environmental applications.

Recent advances in visible light-driven water oxidation and reduction in suspension systems

Abstract In order to solve the shortage of sustainable energy and the related concern about combustion of fossil fuels, converting the most abundant solar energy into chemical fuels becomes one of the most promising choices to provide the everlasting and environmentally friendly energy vector along with the minimum impact on environment. Among the methods of converting solar energy into chemical fuels, there is a significant interest in the renewable hydrogen production by photocatalysts from abundant water under visible light irradiation. Therefore, the development of efficient photocatalysts for water reduction and oxidation in a suspension system is the footstone for the development of solar energy conversion. In this review, the fundamental theory of photocatalysis and key factors affecting photocatalysis will be introduced first. Then the new materials development covering inorganic materials (oxides, nitrides and sulfides), carbon-based photocatalysts, and semiconductor-coordination compound photocatalysts developed over the past 10 years will be addressed with discussion about dominating factors in the photochemical process. This review would provide a comprehensive reference to exploring the efficient and novel materials working for the solar energy conversion to affordable and sustainable fuels. Finally, the perspective of the technology is also discussed.

Conformal coating of ultrathin metal-organic framework on semiconductor electrode for boosted photoelectrochemical water oxidation

Exploring efficient photocatalysts for water oxidation is highly demanded because of their significant role in artificial photosynthesis. The research of metal-organic frameworks (MOFs) based photoelectrochemical (PEC) water splitting is promising but still in its infancy due to the challenge to fabricate high-quality MOFs photoelectrode. Here an Fe2O3/Fe-based MOF core/shell nanorod is prepared for efficient PEC water oxidation via a facile surfactant-assisted solvothermal method. With polyvinylpyrrolidone (PVP) molecules as intermedia, an ultrathin MOF shell of several nanometers thick can be controllably grown on the surface of Fe2O3 nanorod. Charge dynamical behaviors study by ultrafast transient absorption spectroscopy (TAS) and photoelectrochemical impedance spectroscopy (PEIS), reveal that the ultrathin MOF shell is crucial to promote the charge separation by providing a cascade band level, but also accelerate the hole injection efficiency via exposing more accessible Fe-oxo cluster active sites. Accompanied with good visible-light response, the Ti-doped Fe2O3/NH2-MIL-101(Fe) photoanode delivers a boosted photocurrent density of 2.27 mA cm−2 at 1.23 V vs. RHE, which is about 2.3 folds of that of pristine Fe2O3. This study provides new insights into the rational design of semiconductor/MOFs hybrid materials for photo/photoelectrochemical catalysis.

DFT study on water oxidation on nitrogen-doped ceria oxide

Heteroatom doping is effective to regulate electronic structure and improve the performance of photocatalyst. Herein, water oxidation on nitrogen-doped CeO2(1 1 0) was investigated by using density functional theory calculation to understand the role of nitrogen doping. The results show that the optimal pathway of water oxidation on perfect and nitrogen-doped CeO2(1 1 0) is H2O → OH + H → O + 2H; O + 2H + H2O → OOH + 3H → O2 + 4H, and the rate-limiting step of whole water oxidation process is O + 2H + H2O → OOH + 3H. The presence of nitrogen dopant in CeO2(1 1 0) induces new characteristic peak of impurity level, and reduces the activation energies of water oxidation, especially for the rate-limiting step of OO bond formation. The reverse reaction of O and H to OH that is easily occur on perfect CeO2(1 1 0) is effectively prohibited on nitrogen-doped counterpart. Additionally, nitrogen dopant makes the rate-limiting step easier than that on oxygen-defective CeO2(1 1 0) from the kinetic perspective. Therefore, nitrogen-doped CeO2 is expected to be a promising photocatalyst for water oxidation.

Electrodeposition of Sb2S3 light absorbers on TiO2 nanorod array as photocatalyst for water oxidation

Artificial photosynthesis provides a direct approach toward addressing energy and environmental issues by producing chemical fuels such as hydrogen for fuel cells. A photoanode composed of the composite of titanium dioxide (TiO2) nanorod and antimony sulfide (Sb2S3) nanoparticles was proposed in this work. The TiO2 nanorod array is made on a fluorine-doped tin oxide glass by using the hydrothermal reaction, and the Sb2S3 nanoparticles were deposited on the TiO2 nanorod array by using the electrodeposition method with different depositing currents. The electrodeposition method is more time-saving as compared with the commonly used chemical bath reaction method. The Sb2S3 was selected as the coating material due to its broader light absorption range and suitable band-edge positions relating to those of TiO2 for forming a type II heterojunction, aiming to improve the poor visible light absorption and low quantum efficiency of TiO2. A higher photocurrent density of 0.33 mA cm−2 (measured at 1.23 V vs. reversible hydrogen electrode) is obtained for the TiO2/Sb2S3 photoanode in a 0.5 M Na2SO4 aqueous solution under the air mass 1.5 global illumination, comparing to that of 0.017 mA cm−2 for the TiO2 electrode measured under the same conditions. This successful combination of two n-type semiconductors as the photocatalyst for achieving efficient water oxidation provides a conceptual blueprint for designing the composite with complementary optical and electrical properties.

Improving the Photo-Oxidative Performance of Bi2MoO6 by Harnessing the Synergy between Spatial Charge Separation and Rational Co-Catalyst Deposition.

It has been reported that photogenerated electrons and holes can be directed toward specific crystal facets of a semiconductor particle, which is believed to arise from the differences in their surface electronic structures, suggesting that different facets can act as either photoreduction or photo-oxidation sites. This study examines the propensity for this effect to occur in faceted, plate-like bismuth molybdate (Bi2MoO6), which is a useful photocatalyst for water oxidation. Photoexcited electrons and holes are shown to be spatially separated toward the {100} and {001}/{010} facets of Bi2MoO6, respectively, by facet-dependent photodeposition of noble metals (Pt, Au, and Ag) and metal oxides (PbO2, MnO x, and CoO x). Theoretical calculations revealed that differences in energy levels between the conduction bands and valence bands of the {100} and {001}/{010} facets can contribute to electrons and holes being drawn to different surfaces of the plate-like Bi2MoO6. Utilizing this knowledge, the photo-oxidative capability of Bi2MoO6 was improved by adding an efficient water oxidation co-catalyst, CoO x, to the system, whereby the extent of enhancement was shown to be governed by the co-catalyst location. A greater oxygen evolution occurred when CoO x was selectively deposited on the hole-rich {001}/{010} facets of Bi2MoO6 compared to when CoO x was randomly located across all of the facets. The elevated performance exhibited for the selectively loaded CoO x/Bi2MoO6 was ascribed to the greater opportunity for hole trapping by the co-catalyst being accentuated over other potentially detrimental effects, such as the co-catalyst acting as a recombination medium and/or covering reactive sites. The results indicate that harnessing the synergy between the spatial charge separation and the co-catalyst location on the appropriate facets of plate-like Bi2MoO6 can promote its photocatalytic activity.

Hollow titania spheres loaded with noble metal nanoparticles for photocatalytic water oxidation

A synthesis procedure for fabrication of hollow TiO2 spheres of mixed anatase/rutile composition loaded with noble metal nanoparticles (Au, Pt, Pd) is proposed. The materials demonstrated to be functioning photocatalysts for water oxidation. In particular nanoparticles loaded with Pd and Pt showed good catalytic activity in comparison to commercial TiO2 P25.

Nonaqueous synthesis of metal cyanamide semiconductor nanocrystals for photocatalytic water oxidation.

Herein, we report nonaqueous synthesis of metal cyanamide semiconductor nanocrystals, including Ag2NCN nanorods (NRs), ZnNCN NRs and PbNCN nanoparticles. The as-prepared Ag2NCN NRs with a band gap of 2.35 eV are applied as photocatalysts for water oxidation. Their oxygen evolution rate (280.7 μmol h-1 g-1) is much higher than that of Ag2NCN microcrystals (24.0 μmol h-1 g-1).

The first example of an oxide semiconductor photocatalyst consisting of a heptavalent cation: visible-light-induced water oxidation on M3ReO8

Re(vii)-based M3ReO8 (M = Y, La, Nd, Sm, Eu, Gd, Dy, Yb) semiconductors have been found to be visible-light-responsive photocatalysts for water oxidation.

Iron(iii)-based metal–organic frameworks as oxygen-evolving photocatalysts for water oxidation

Development of economical and highly efficient catalysts to achieve visible light-induced water oxidation is a potential approach to mitigate energy shortage and global warming.