Donge Wang

Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis

Since the 1970s, splitting water using solar energy has been a focus of great attention as a possible means for converting solar energy to chemical energy in the form of clean and renewable hydrogen fuel. Approaches to solar water splitting include photocatalytic water splitting with homogeneous or heterogeneous photocatalysts, photoelectrochemical or photoelectrocatalytic (PEC) water splitting with a PEC cell, and electrolysis of water with photovoltaic cells coupled to electrocatalysts. Though many materials are capable of photocatalytically producing hydrogen and/or oxygen, the overall energy conversion efficiency is still low and far from practical application. This is mainly due to the fact that the three crucial steps for the water splitting reaction: solar light harvesting, charge separation and transportation, and the catalytic reduction and oxidation reactions, are not efficient enough or simultaneously. Water splitting is a thermodynamically uphill reaction, requiring transfer of multiple electrons, making it one of the most challenging reactions in chemistry. This Account describes the important roles of cocatalysts in photocatalytic and PEC water splitting reactions. For semiconductor-based photocatalytic and PEC systems, we show that loading proper cocatalysts, especially dual cocatalysts for reduction and oxidation, on semiconductors (as light harvesters) can significantly enhance the activities of photocatalytic and PEC water splitting reactions. Loading oxidation and/or reduction cocatalysts on semiconductors can facilitate oxidation and reduction reactions by providing the active sites/reaction sites while suppressing the charge recombination and reverse reactions. In a PEC water splitting system, the water oxidation and reduction reactions occur at opposite electrodes, so cocatalysts loaded on the electrode materials mainly act as active sites/reaction sites spatially separated as natural photosynthesis does. In both cases, the nature of the loaded cocatalysts and their interaction with the semiconductor through the interface/junction are important. The cocatalyst can provide trapping sites for the photogenerated charges and promote the charge separation, thus enhancing the quantum efficiency; the cocatalysts could improve the photostability of the catalysts by timely consuming of the photogenerated charges, particularly the holes; most importantly, the cocatalysts catalyze the reactions by lowering the activation energy. Our research shows that loading suitable dual cocatalysts on semiconductors can significantly increase the photocatalytic activities of hydrogen and oxygen evolution reactions, and even make the overall water splitting reaction possible. All of these findings suggest that dual cocatalysts are necessary for developing highly efficient photocatalysts for water splitting reactions.

Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4

Charge separation is crucial for increasing the activity of semiconductor-based photocatalysts, especially in water splitting reactions. Here we show, using monoclinic bismuth vanadate crystal as a model photocatalyst, that efficient charge separation can be achieved on different crystal facets, as evidenced by the reduction reaction with photogenerated electrons and oxidation reaction with photogenerated holes, which take place separately on the {010} and {110} facets under photo-irradiation. Based on this finding, the reduction and oxidation cocatalysts are selectively deposited on the {010} and {110} facets respectively, resulting in much higher activity in both photocatalytic and photoelectrocatalytic water oxidation reactions, compared with the photocatalyst with randomly distributed cocatalysts. These results show that the photogenrated electrons and holes can be separated between the different facets of semiconductor crystals. This finding may be useful in semiconductor physics and chemistry to construct highly efficient solar energy conversion systems.

Crystal Facet Dependence of Water Oxidation on BiVO4 Sheets under Visible Light Irradiation

Monoclinic BiVO(4) crystals with preferentially exposed (040) facets were hydrothermally synthesized by using a trace amount of TiCl(3) as the directing agent; this function was confirmed by X-ray diffraction patterns (XRD) and high-resolution transmission electron microscopy (HRTEM). The effects of the directing agent TiCl(3) and the pH values applied during synthesis have been studied, and the optimized BiVO(4) sample with highly exposed (040) facet could be obtained by using 1.2 at.% of TiCl(3) as the directing agent at a pH value of 2. Some complementary techniques were also applied to exclude the effects of the structural and physical property changes, such as surface area and hydrophilicity. The photocatalytic activity of oxygen evolution on BiVO(4) is found to be proportionally correlated with the exposed surfaces of the (040) facet. It is assumed that the active sites with a BiV(4) structure on the exposed (040) facet is assigned to be responsible for the high activity of O(2) evolution.

Highly efficient photocatalysts constructed by rational assembly of dual-cocatalysts separately on different facets of BiVO4

Cocatalysts play important roles in promoting the catalytic reactions of semiconductor photocatalysts. Especially, deposition of dual cocatalysts, i.e., oxidation and reduction cocatalysts, onto a semiconductor photocatalyst can significantly improve its photocatalytic activity due to the synergetic effect of rapid consumption of photogenerated electrons and holes. However, in most cases, the cocatalysts are randomly deposited onto the semiconductor photocatalysts, where the cocatalysts cannot function fully. Herein, based on the findings that photogenerated electrons and holes can be spatially separated onto the different facets of BiVO4, we have successfully prepared two types of photocatalysts (M/MnOx/BiVO4 and M/Co3O4/BiVO4, where M stands for noble metals) with reduction and oxidation cocatalysts selectively deposited onto the {010} and {110} facets of BiVO4 by a photo-deposition method. Remarkably enhanced photocatalytic activities were observed for such assembled photocatalysts in control experiments of photocatalytic water oxidation and photocatalytic degradation of methyl orange and rhodamine B. In-depth investigations show that the enhanced photocatalytic performances are due to not only the intrinsic nature of charge separation between the {010} and {110} facets of BiVO4, but also the synergetic effect of dual-cocatalysts deposited onto the different facets of BiVO4. This work further proves the feasibility of the general concepts for approaching efficient artificial photosynthesis systems, namely, engineering of crystal-based photocatalysts by selective deposition of suitable reduction and oxidation cocatalysts onto the different facets of light absorbing semiconductor crystals.

Photocatalytic oxidation of thiophene on BiVO4 with dual co-catalysts Pt and RuO2 under visible light irradiation using molecular oxygen as oxidant

Thiophene is one of the main sulfur-containing compounds in gasoline and difficult to be oxidized with the conventional oxidative processes. Herein for the first time we report that thiophene can be oxidized to SO3 on BiVO4 co-loaded with Pt and RuO2 co-catalysts (denoted as Pt-RuO2/BiVO4) under visible light irradiation with molecular oxygen as oxidant. The high activity of the catalyst can be achieved by only loading as low as 0.03 wt% of Pt and 0.01 wt% of RuO2 as dual co-catalysts on BiVO4. ESR measurements give the evidence that the active oxygen species (˙OH and O2˙−) generated by photocatalytic processes are involved in the photocatalytic oxidation of thiophene. The considerable enhancement of photocatalytic activity can be attributed to the simultaneous presence of the reduction and oxidation co-catalysts which are beneficial for the efficient separation and transfer of the photo-generated electrons and holes.

Visible light driven overall water splitting using cocatalyst/BiVO4 photoanode with minimized bias†

BiVO4 and many other semiconductor materials are ideal visible light responsive semiconductors, but are insufficient for overall water splitting. Upon loading water oxidation cocatalyst, for example Co-borate (denoted as CoBi) used here, onto BiVO4 photoanode, it is found that not only the onset potential is negatively shifted but also the photocurrent and the stability are significantly improved. And more importantly, PEC overall water splitting to H2 and O2 is realized using CoBi/BiVO4 as photoanode with a rather low applied bias (less than 0.3 V vs. counter electrode) in a two-electrode scheme, while at least 0.6 V is needed for bare BiVO4. This work demonstrates the practical possibility of achieving overall water splitting using the PEC strategy under a bias as low as the theoretical minimum, which is the difference between the flat band and proton reduction potential for a photoanode thermodynamically insufficient for water reduction. As long as the water oxidation overpotential is overcome with an efficient cocatalyst, the applied bias of the whole system is only used for that thermodynamically required for the proton reduction.

A Theoretical Study on the Mechanism of Photocatalytic Oxygen Evolution on BiVO4 in Aqueous Solution

The oxygen evolution reaction (OER) is regarded as one of the key issues in achieving efficient photocatalytic water splitting. Monoclinic scheelite BiVO(4) is a visible-light-responsive semiconductor which has proved to be effective for oxygen evolution. Recently, the synthesis of a series of monoclinic BiVO(4) single crystals was reported, and it was found that the (010), (110), and (011) facets are highly exposed and that the photocatalytic O(2) evolution activity depends on the degree of exposure of the (010) facets. To explore the properties of and photocatalytic water oxidation reaction on different facets, DFT calculations were performed to investigate the geometric structure, optical properties, electronic structure, water adsorption, and the whole OER free-energy profiles on BiVO(4) (010) and (011) facets. The calculated results suggest both favorable and unfavorable factors for OER on the (010) and the (011) facets. Due to the combined effects of the above-mentioned factors, different facets exhibit quite different photocatalytic activities.

Visible light driven H2 production in molecular systems employing colloidal MoS2 nanoparticles as catalyst

Colloidal MoS(2) nanoparticles with diameters of less than 10 nm were prepared with a simple solvothermal method and demonstrated high efficiency in catalyzing H(2) evolution in Ru(bpy)(3)(2+)-based molecular systems under visible light.

Photocatalytic H2 production on Pt/TiO2–SO42− with tuned surface-phase structures: enhancing activity and reducing CO formation

Photocatalytic reforming of biomass is a promising way to produce hydrogen using renewable energy. Photocatalytic reforming of methanol on Pt/TiO2–SO42− as a model reaction of biomass reforming was investigated. Sulfated TiO2 (TiO2–SO42−) with a tunable surface phase was prepared by calcining commercially available titanium dioxide TiO2 (Degussa P25) with deposited sodium sulfate Na2SO4 as a modifier. Compared with P25, the as-prepared TiO2–SO42− with Pt co-catalyst shows an increase up to 6-fold in the activity for H2 production via photocatalytic reforming of methanol, and the CO (undesired product) concentration in the produced H2 is decreased by about two orders of magnitude. XRD patterns and UV Raman spectra clearly indicate that TiO2 depositing with Na2SO4 can significantly retard the phase transformation from anatase to rutile during calcination at elevated temperatures. It is proposed that both the phase composition and the high crystallinity of TiO2 contribute to the high H2 evolution activity. IR spectra of pyridine adsorption and the NH3-TPD profile show that the surface acid sites of the photocatalyst are greatly reduced after calcination at high temperatures. It is proposed that the decrease in the acidity of the samples might be responsible for the low CO selectivity.

Composite Sr2TiO4/SrTiO3(La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation

A composite Sr2TiO4/SrTiO3(La,Cr) heterojunction photocatalyst has been prepared by a simple in situ polymerized complex method. Upon Pt cocatalyst loading, this catalyst shows higher photocatalytic activity towards hydrogen production than individual SrTiO3(La,Cr) and Sr2TiO4(La,Cr) in the presence of methanol sacrificial reagent. Microscopic morphology studies show that well defined heterojunctions are formed by matching the lattice fringes of SrTiO3(La,Cr) and Sr2TiO4(La,Cr), and Pt was preferentially loaded on the surface of the Sr2TiO4(La,Cr) component in the composite Sr2TiO4/SrTiO3(La,Cr) photocatalyst. XPS and EPR analyses show that the composite photocatalyst also has the lowest amount of Cr6+ electron trapping sites. Band structure analysis by combining absorption spectroscopy and Mott–Schottky plots shows that, in the composite photocatalyst, the photogenerated electrons and holes tend to migrate from SrTiO3(La,Cr) to Sr2TiO4(La,Cr) and from Sr2TiO4(La,Cr) to SrTiO3(La,Cr), respectively. This kind of band structure can facilitate charge transfer and separation driven by the minor potential difference between the two components, which is further confirmed by the observation of long lived electrons in the time resolved FT-IR spectroscopic study. It is concluded that the superior photocatalytic activity of the composite heterojunction photocatalyst is due to efficient charge transfer and separation by well defined heterojunctions formed between SrTiO3(La,Cr) and Sr2TiO4(La,Cr), preferential loading of Pt nanoparticles on the Sr2TiO4(La,Cr) component, and the lowest amount of Cr6+ in the composite photocatalyst. The tailored design and synthesis of the composite heterojunction structure is a promising approach for the improvement of the photocatalytic activity of a photocatalyst.