Hongxian Han

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.

Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generations

Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States

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.

Photocatalytic Overall Water Splitting Promoted by an α–β phase Junction on Ga2O3

When Alpha met Beta: a tuneable α-β surface phase junction on Ga(2)O(3) can significantly improve photocatalytic overall water splitting into H(2) and O(2) over individual α-Ga(2)O(3) or β-Ga(2)O(3) surface phases. This enhanced photocatalytic performance is mainly attributed to the efficient charge separation and transfer across the α-β phase junction.

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.

A Tantalum Nitride Photoanode Modified with a Hole-Storage Layer for Highly Stable Solar Water Splitting**

Photoelectrochemical (PEC) water splitting is an ideal approach for renewable solar fuel production. One of the major problems is that narrow bandgap semiconductors, such as tantalum nitride, though possessing desirable band alignment for water splitting, suffer from poor photostability for water oxidation. For the first time it is shown that the presence of a ferrihydrite layer permits sustainable water oxidation at the tantalum nitride photoanode for at least 6 h with a benchmark photocurrent over 5 mA cm(-2) , whereas the bare photoanode rapidly degrades within minutes. The remarkably enhanced photostability stems from the ferrihydrite, which acts as a hole-storage layer. Furthermore, this work demonstrates that it can be a general strategy for protecting narrow bandgap semiconductors against photocorrosion in solar water splitting.

Achieving overall water splitting using titanium dioxide-based photocatalysts of different phases

Titanium dioxide (TiO2) is regarded as the benchmark semiconductor in photocatalysis, which possesses a suitable band structure and makes the overall water splitting reaction thermodynamically possible. However, photocatalytic overall water splitting (POWS) (2H2O → 2H2 + O2) can only take place on rutile but hardly on anatase and brookite TiO2. So obtaining the POWS on TiO2-based photocatalysts has remained a long-standing challenge for over 40 years. In this work, we found that the POWS on anatase and brookite TiO2 becomes feasible under prolonged UV light irradiation. Further investigation by means of electron spin resonance spectroscopy (EPR) and transient infrared absorption–excitation energy scanning spectroscopy (TRIRA-ESS) reveals that both kinetics and thermodynamics factors contributed to unique POWS activity for different phases of TiO2. Kinetically the process of photocatalysis differs on different phases of TiO2 due to the intermediates (˙OH radical for anatase and brookite TiO2, peroxy species for rutile TiO2) that are formed. Thermodynamically there are many trapped states lying near the valence band of anatase and brookite but not for rutile TiO2, which reduce the overpotential for water oxidation. These findings develop our understanding of why some semiconductors are inactive as POWS photocatalysts despite having thermodynamically suitable band structures for the proton reduction and water oxidation reactions.

Effects of Zn2+ and Pb2+ dopants on the activity of Ga2O3-based photocatalysts for water splitting

Zn-doped and Pb-doped β-Ga2O3-based photocatalysts were prepared by an impregnation method. The photocatalyst based on the Zn-doped β-Ga2O3 shows a greatly enhanced activity in water splitting while the Pb-doped β-Ga2O3 one shows a dramatic decrease in activity. The effects of Zn(2+) and Pb(2+) dopants on the activity of Ga2O3-based photocatalysts for water splitting were investigated by HRTEM, XPS and time-resolved IR spectroscopy. A ZnGa2O4-β-Ga2O3 heterojunction is formed in the surface region of the Zn-doped β-Ga2O3 and a slower decay of photogenerated electrons is observed. The ZnGa2O4-β-Ga2O3 heterojunction exhibits type-II band alignment and facilitates charge separation, thus leading to an enhanced photocatalytic activity for water splitting. Unlike Zn(2+) ions, Pb(2+) ions are coordinated by oxygen atoms to form polyhedra as dopants, resulting in distorted surface structure and fast decay of photogenerated electrons of β-Ga2O3. These results suggest that the Pb dopants act as charge recombination centers expediting the recombination of photogenerated electrons and holes, thus decreasing the photocatalytic activity.

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.

Nitrogen-doped layered oxide Sr5Ta4O15−xNx for water reduction and oxidation under visible light irradiation

Development of a photocatalyst with wide visible light absorption is of vital importance in solar-chemical energy conversion. In this work, we introduce a new nitrogen-doped layered oxide, Sr5Ta4O15−xNx, which exhibits a significantly extended absorption edge compared with the undoped oxide Sr5Ta4O15. The extension of the visible light absorption has been ascribed to the substitution of nitrogen for oxygen atoms as well as the formation of Ta–N bonds, which was confirmed by X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Judged by the first principle calculation, the N 2p states mixed with pre-existing O 2p states shift the valence band maximum upward and result in wide visible light absorption. Band structure analysis combined with UV-Vis diffuse reflectance spectrum (DRS) and Mott–Schottky (M–S) measurement shows that the conduction and valence bands of Sr5Ta4O15−xNx are sufficient for water reduction and oxidation, respectively. The photocatalytic water splitting performances of Sr5Ta4O15−xNx are strongly related to the deposited cocatalyst. With an optimized cocatalyst, the Sr5Ta4O15−xNx shows both H2 and O2 evolution activities under visible light irradiation using CH3OH and AgNO3 as scavengers respectively. Following the optimized cocatalyst deposition of the Sr5Ta4O15−xNx, the cocatalyst-modified nitrogen-doped tantalum-based layered oxides Sr2Ta2O7−xNx and Ba5Ta4O15−xNx also exhibit activities for both the water splitting half reactions. This work demonstrates that the nitrogen-doped tantalum-based layered oxides may be a new type of potential photocatalyst with wide visible light absorption for solar water splitting.