Surface science and chemical studiesof NiO/single crystal TiO2heterostructure photocatalysts

Abstract

Photocatalysis on semiconductor metal oxide surfaces has attracted considerable attention as a sustainable environmentally friendly method for water/air purification and hydrogen production by water splitting. Among semiconducting metal oxides TiO2 has been intensively investigated as a promising photocatalyst candidate. However, despite many efforts, its photocatalytic activity is far from a practical level mainly due to inefficient charge carrier separation and resulting charge carrier recombination. An advantageous strategy to address this issue is the development of heterostructures by coupling to a metal to form a Schottky junction or to metal oxides to create a p-n junction at their interface in order to prevent the recombination by vectorial charge carrier separation at these energy junctions. On the other hand it was revealed over the past decade that crystal facets play a decisive role in trapping of charge carriers and thus photocatalytic redox reactions. Thus, selective deposition of metal or metal oxides onto specific facets would enhance the photocatalytic activity by improving charge separation. To achieve higher activities, two methods, the supercritical fluid chemical deposition route and the photodeposition method, were investigated to deposit selectively p-type NiO onto specific facets of ntype TiO2 single crystalline nanoparticles to establish a p-n junction. The resulting NiO/TiO2 nanocrystals were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), N2 sorption measurements, UV-visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The heterojunction photocatalysts showed higher photocatalytic efficiency than pure TiO2 for the decomposition of organic dyes. Particularly, 0.1-0.25 wt % of NiO was the optimal loading amount, showing the highest activity. To elucidate the role of crystal facets of TiO2 and the effect of selective deposition of NiO, rutile (001), rutile (110), anatase (001), and anatase (101) surfaces with different surface states were prepared and their electronic properties were systematically compared by XPS and UPS measurements. Furthermore, water adsorption onto the different surfaces were also investigated. Regardless of surface stoichiometry, the Fermi level position of the anatase (001) surface is situated higher than that of the anatase (101) surface in energy while that of the rutile (001) surface is located lower than that of the rutile (110) surface. This can explain why photo-generated electrons and holes preferentially migrate to the (101) and (001) facets on TiO2 anatase crystals, respectively. Work function values of these oriented surfaces vary depending upon the surface states related to distribution and amount of oxygen vacancies as well as adsorbed oxygen peroxo species on the surface. In order to experimentally determine energy band alignments, interface experiments were performed by stepwisely depositing NiO onto above well-defined oriented TiO2 surfaces. The enhanced photocatalytic activity of NiO/TiO2 heterostructure nanoparticles were rationalized on the basis of the obtained band alignments. The information of electronic properties of different oriented TiO2 under various surface states would provide a new insight to construct the optimal energy band alignment of the heterostructure system with TiO2. In addition, the concept of heterojuction nanocrystals where co-catalysts are selectively deposited should find practical application to purify the environment and to sustainably produce renewable hydrogen.