Junwang Tang

Visible-light driven heterojunction photocatalysts for water splitting – a critical review

Solar driven catalysis on semiconductors to produce clean chemical fuels, such as hydrogen, is widely considered as a promising route to mitigate environmental issues caused by the combustion of fossil fuels and to meet increasing worldwide demands for energy. The major limiting factors affecting the efficiency of solar fuel synthesis include; (i) light absorption, (ii) charge separation and transport and (iii) surface chemical reaction; therefore substantial efforts have been put into solving these problems. In particular, the loading of co-catalysts or secondary semiconductors that can act as either electron or hole acceptors for improved charge separation is a promising strategy, leading to the adaptation of a junction architecture. Research related to semiconductor junction photocatalysts has developed very rapidly and there are a few comprehensive reviews in which the strategy is discussed (A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253–278, K. Li, D. Martin, and J. Tang, Chinese Journal of Catalysis, 2011, 32, 879–890, R. Marschall, Advanced Functional Materials, 2014, 24, 2421–2440). This critical review seeks to give an overview of the concept of heterojunction construction and more importantly, the current state-of-the art for the efficient, visible-light driven junction water splitting photo(electro)catalysts reported over the past ten years. For water splitting, these include BiVO4, Fe2O3, Cu2O and C3N4, which have attracted increasing attention. Experimental observations of the proposed charge transfer mechanism across the semiconductor/semiconductor/metal junctions and the resultant activity enhancement are discussed. In parallel, recent successes in the theoretical modelling of semiconductor electronic structures at interfaces and how these explain the functionality of the junction structures is highlighted.

Mechanism of Photocatalytic Water Splitting in TiO2. Reaction of Water with Photoholes, Importance of Charge Carrier Dynamics, and Evidence for Four-Hole Chemistry

We show for the first time that the photogenerated hole lifetime in TiO 2 is a strong determinant of the ability of TiO 2 to split water. Hole lifetimes were measured using transient absorption spectroscopy over a range of excitation intensities. The lifetimes of the holes were modulated by the use of exogenous scavengers and were also found to vary systematically with the excitation intensity. In all cases the quantum yield of oxygen production is found to be linked to the light intensity used, ranging from below 1 sun equivalent to nearly 1 sun equivalent. We also provide evidence that oxygen production requires four photons for each molecule of oxygen, which is reminiscent of the natural photosynthetic water-splitting mechanism. This in turn suggests a mechanism for oxygen production which requires four-hole chemistry, presumably via three, as yet unidentified intermediates. It is also shown that at excitation densities on the order of 1 sun, nongeminate electron-hole recombination limits the quantum yield significantly.

Highly Efficient Photocatalytic H2 Evolution from Water using Visible Light and Structure‐Controlled Graphitic Carbon Nitride

The major challenge of photocatalytic water splitting, the prototypical reaction for the direct production of hydrogen by using solar energy, is to develop low-cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported. The g-C3N4 exhibits an extraordinary hydrogen-evolution rate (ca. 20 000 μmol h−1 g−1 under full arc), which leads to a high turnover number (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5 % under visible light, which is nearly an order of magnitude higher than that observed for any other existing g-C3N4 photocatalysts. Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen-evolution rate is significantly enhanced.

Visible Light-Driven Pure Water Splitting by a Nature-Inspired Organic Semiconductor-Based System

For the first time, it is demonstrated that the robust organic semiconductor g-C3N4 can be integrated into a nature-inspired water splitting system, analogous to PSII and PSI in natural photosynthesis. Two parallel systems have been developed for overall water splitting under visible light involving graphitic carbon nitride with two different metal oxides, BiVO4 and WO3. Consequently, both hydrogen and oxygen can be evolved in an ideal ratio of 2:1, and evolution rates in both systems have been found to be dependent on pH, redox mediator concentration, and mass ratio between the two photocatalysts, leading to a stable and reproducible H2 and O2 evolution rate at 36 and 18 μmol h(-1) g(-1) from water over 14 h. Our findings demonstrate g-C3N4 can serve as a multifunctional robust photocatalyst, which could also be used in other systems such as PEC cells or coupled solar cell systems.

Cu2O/Reduced Graphene Oxide Composites for the Photocatalytic Conversion of CO2

A facile one-step microwave-assisted chemical method has been successfully used for the synthesis of Cu2O/reduced graphene oxide (RGO) composites. Photocatalytic CO2 reduction was then investigated on the junction under ambient conditions. The RGO coating dramatically increases Cu2O activity for CO2 photoreduction to result in a nearly six times higher activity than the optimized Cu2O and 50 times higher activity than the Cu2O/RuOx junction in the 20th hour. Furthermore, an apparent initial quantum yield of approximately 0.34 % at 400 nm has been achieved by the Cu2O/RGO junction for CO2 photoreduction. The photocurrent of the junction is nearly double that of the blank Cu2O photocathode. The improved activity together with the enhanced stability of Cu2O is attributed to the efficient charge separation and transfer to RGO as well as the protection function of RGO, which was proved by XRD, SEM, TEM, X-ray photoelectron spectroscopy, photo-electrochemical, photoluminescence, and impedance characterizations. This study further presents useful information for other photocatalyst modification for efficient CO2 reduction without the need for a noble-metal co-catalyst.

Correlating long-lived photogenerated hole populations with photocurrent densities in hematite water oxidation photoanodes

Photogenerated charge carrier dynamics are investigated as a function of applied bias in a variety of different hematite photoanodes for solar water oxidation. Transient absorption spectroscopy is used to probe the photogenerated holes, while transient photocurrent measures electron extraction. We report a general quantitative correlation between the population of long-lived holes and the photocurrent amplitude. The yield of long-lived holes is shown to be determined by the kinetics of electron-hole recombination. These recombination kinetics are shown to be dependent upon applied bias, exhibiting decay lifetimes ranging from ca 5 μs to 3 ms (at −0.4 and +0.4 V versus Ag/AgCl, respectively). For Si-doped nanostructured hematite photoanodes, electron extraction and electron-hole recombination are complete within ∼20 ms, while water oxidation is observed to occur on a timescale of hundreds of milliseconds to seconds. The competition between electron extraction and electron-hole recombination is electron-density-dependent: the effect on recombination of applied bias and excitation intensity is discussed. The timescale of water oxidation is independent of the concentration of photogenerated holes, indicating that the mechanism of water oxidation on hematite is via a sequence of single-hole oxidation steps.

Facet engineered Ag3PO4 for efficient water photooxidation

The photooxidation of water using faceted Ag3PO4 was investigated, guided by theoretical modelling. Firstly, theoretical calculations were performed to predict the optimum morphology for solar energy conversion by probing the surface energies of three primary low index facets of Ag3PO4: {100}, {110} and {111}. It was elucidated that the {111} facet possessed considerably higher surface energy (1.65 J m−2) than either {110} or {100} (0.78 and 0.67 J m−2 respectively). We therefore attempted to fabricate Ag3PO4 crystals with {111} facets. Tetrahedral Ag3PO4 crystals, composed of {111} facets, were then successfully synthesised using a novel kinetic control method in the absence of surfactants. In comparison to rhombic dodecahedron {110} and cubic {100} structures, tetrahedral crystals show an extremely high activity for water photooxidation, with an initial oxygen evolution rate exceeding 6 mmol h−1 g−1, 10 times higher than either {110} or {100} facets. Furthermore, to the best of our knowledge it is the first time that the internal quantum yield for water photooxidation is almost unity at 400 nm, and greater than 80% from 365 to 500 nm, achieved by {111} terminated tetrahedrons. The excellent and reproducible performance is attributed to a synergistic effect between high surface energy and a small hole mass, leading to high charge carrier mobility and active surface reaction sites.

Dynamics of photogenerated charges in the phosphate modified TiO2 and the enhanced activity for photoelectrochemical water splitting

Phosphate modified nanocrystalline TiO2 (nc-TiO2) films were prepared by a doctor blade method, followed by post-treatment with monometallic sodium orthophosphate solution. The dynamic processes of the photogenerated charges from the resulting nc-TiO2 films were thoroughly investigated by means of transient absorption spectroscopy (TAS). It is shown that photogenerated holes in the un-modified TiO2 film exhibit the same dynamic decay process as its photogenerated electrons, in oxygen-free water of pH 7. However, photogenerated holes in the phosphate modified film display a slightly faster dynamic decay process than its photogenerated electrons, and photogenerated charges of the modified film have a much longer lifetime than those of the un-modified film. These differences are attributed to the surface-carried negative charges of nc-TiO2 resulting from the phosphate groups (–Ti–O–P–O−). Interestingly, the photoelectrochemical (PEC) experiments show that modification with an appropriate amount of phosphate could improve the photocurrent density of the nc-TiO2 film electrode by about 2 times, at a voltage of 0 V in the neutral electrolyte. Based on the TAS and PEC measurements of un-modified and phosphate modified nc-TiO2 films, with different conditions, it is suggested that the prolonged lifetime of photogenerated charges can be attributed to the negative electrostatic field formed in the surface layers. It is also responsible for the increase in activity for PEC water splitting and for the reported photocatalytic degradation of pollutants. The suggested mechanism would be applicable to other oxide semiconductor photocatalysts and to modification with other inorganic anions.

Biomolecule-assisted fabrication of copper doped SnS2 nanosheet–reduced graphene oxide junctions with enhanced visible-light photocatalytic activity

Novel copper (3 at%) doped SnS2 nanosheet–reduced graphene oxide (RGO) junctions were fabricated by a facile cysteine-assisted hydrothermal method. Superior visible-light-driven activity for methyl orange decomposition has been achieved. The average apparent rate of Cu-doped SnS2 nanosheet–RGO composites is more than 7 times higher than that of SnS2, and even 2 times higher than CdS nanoparticles, the benchmark material. Further characterization indicates that facilitating charge separation, fast electron transport and a large surface area together play key roles in the materials enhanced photoactivity. Due to their chemical stability, low cost and lower toxicity, Cu-doped SnS2–RGO composites show great potential as high-efficiency visible-light-driven photocatalysts for environmental remediation and energy conversion.

Fe2O3–TiO2 Nanocomposites for Enhanced Charge Separation and Photocatalytic Activity

Photocatalysis provides a cost effective method for both renewable energy synthesis and environmental purification. Photocatalytic activity is dominated by the material design strategy and synthesis methods. Here, for the first time, we report very mild and effective photo-deposition procedures for the synthesis of novel Fe2 O3 -TiO2 nanocomposites. Their photocatalytic activities have been found to be dramatically enhanced for both contaminant decomposition and photoelectrochemical water splitting. When used to decompose a model contaminant herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), monitored by both UV/Vis and total organic carbon (TOC) analysis, 10% Fe-TiO2 -H2 O displayed a remarkable enhancement of more than 200 % in the kinetics of complete mineralisation in comparison to the commercial material P25 TiO2 photocatalyst. Furthermore, the photocurrent is nearly double that of P25. The mechanism for this improvement in activity was determined using density functional theory (DFT) and photoluminescence. These approaches ultimately reveal that the photoelectron transfer is from TiO2 to Fe2 O3 . This favours O2 reduction which is the rate-determining step in photocatalytic environmental purification. This in situ charge separation also allows for facile migration of holes from the valence band of TiO2 to the surface for the expected oxidation reactions, leading to higher photocurrent and better photocatalytic activity.