We present a systematic investigation of the microscopic mechanism of interface interaction, charge transfer and separation, as well as their influence on the photocatalytic activity of heterojunctions by a combination of theoretical calculations and experimental techniques for the g-C3N4–ZnWO4 composite. HRTEM results and DFT calculations mutually validate each other to indicate the reasonable existence of g-C3N4 (001)–ZnWO4 (010) and g-C3N4 (001)–ZnWO4 (011) interfaces. The g-C3N4–ZnWO4 heterojunctions show higher photocatalytic activity for degradation of MB than pure g-C3N4 and ZnWO4 under visible-light irradiation. Moreover, the heterojunctions significantly enhance the oxidation of phenol in contrast to pure g-C3N4, the phenol oxidation capacity of which is weak, clearly demonstrating a synergistic effect between g-C3N4 and ZnWO4. Interestingly, based on the theoretical calculations, we find that electrons in the upper valence band can be directly excited from g-C3N4 to the conduction band, that is, the W 5d orbital of ZnWO4, under visible-light irradiation, which should yield well-separated electron–hole pairs, with high photocatalytic performance in g-C3N4–ZnWO4 heterojunctions as shown by our experiment. The microcosmic mechanisms of interface interaction and charge transfer in this system can be helpful for fabricating other effective hetero-structured photocatalysts.

Enhanced visible-light photocatalytic activity of g-C3N4–ZnWO4 by fabricating a heterojunction: investigation based on experimental and theoretical studies

The effective separation and transport of photoinduced electron–hole pairs in photoanodes is of great significance to photoelectrochemical and catalytic performance. Here, a facile and effective two-step strategy is developed to fabricate double-shelled ZnO/CdS/CdSe porous nanotube photoanodes from ZnO nanorod arrays (NRAs). Surprisingly, after the process of the deposition of CdS and CdSe, the ZnO nanorod arrays are partially dissolved, resulting in the formation of ZnO/CdS/CdSe porous nanotube arrays (NTAs). By virtue of their unique porous nanotube structure and cosensitization effect, the ZnO/CdS/CdSe porous NTAs show superior photoelectrochemical water-splitting performance and organic-pollutant-degradation ability under visible light irradiation, as well as excellent long-term photostability.

Double-Shelled CdS- and CdSe-Cosensitized ZnO Porous Nanotube Arrays for Superior Photoelectrocatalytic Applications.

g-C3N4 decorated ZnO nanorod arrays for enhanced photoelectrocatalytic performance

ZnWO4/BiOI heterostructures with different constituents are synthesized via a chemical bath approach under mild conditions by tuning the Zn/Bi molar ratios. The obtained ZnWO4/BiOI heterostructures display high photocatalytic activities in degradation of MO and photocurrent response under visible light irradiation. Combining the experimental findings, first-principles calculations are used to investigate the surface geometry structures and the work functions of the (011) and (010) surfaces of the ZnWO4 phase and the (001) surface of the BiOI phase. The results show that the lattice and energy levels between the ZnWO4 and BiOI phases match well with each other to be capable of forming efficient ZnWO4/BiOI p–n heterojunction structures. This match promotes the separation and transfer of photoinduced electron–hole pairs at the interface, resulting in the excellent photocatalytic performance of the ZnWO4/BiOI heterostructures. Our findings show that the formation of a heterostructure would possess the excellent photocatalytic activities only if the lattice and energy level match between the two semiconductors was satisfied, which is of great importance for designing and developing more efficient heterostructured photocatalysts.

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ZnWO4/BiOI heterostructures with highly efficient visible light photocatalytic activity: the case of interface lattice and energy level match

The development of highly efficient and robust photocatalysts has attracted great attention for solving the global energy crisis and environmental problems. Herein, we describe the synthesis of a p-n heterostructured photocatalyst, consisting of ZnO nanorod arrays (NRAs) decorated with BiOI nanoplates (NPs), by a facile solvothermal method. The product thus obtained shows high photoelectrochemical water splitting performance and enhanced photoelectrocatalytic activity for pollutant degradation under visible light irradiation. The p-type BiOI NPs, with a narrow band gap, not only act as a sensitizer to absorb visible light and promote electron transfer to the n-type ZnO NRAs, but also increase the contact area with organic pollutants. Meanwhile, ZnO NRAs provide a fast electron-transfer channel, thus resulting in efficient separation of photoinduced electron-hole pairs. Such a p-n heterojunction nanocomposite could serve as a novel and promising catalyst in energy and environmental applications.

Enhanced Photoelectrocatalytic Activity of BiOI Nanoplate–Zinc Oxide Nanorod p–n Heterojunction

A high-quality inclusion-free ZnWO4 crystal 90 mm in diameter and with a mass of up to 8 kg was grown by the low thermal gradient Czochralski technique. Crystallographic properties of ZnWO4(010) cleaved surfaces were evaluated with reflection high energy electron diffraction (RHEED), and the electronic structure of the surface was studied using X-ray photoelectron spectroscopy (XPS). A system of Kikuchi lines was observed for cleaved ZnWO4(010) by RHEED confirming the high crystallographic state of the surface. The XPS valence-band and core-level spectra of ZnWO4(010) have been measured. The XPS measurements reveal that tungsten and zinc atoms are in the formal valences +6 and +2, respectively, on cleaved ZnWO4(010) surface. The 3.0 keV Ar+ bombardment of the surface causes partial transformation of tungsten ions from W6+ to lower valence states; however, no partial loss of oxygen atoms belonging to ZnO6 octahedra occurs due to this bombardment of the ZnWO4(010) surface because after this treatment zinc r...

Structural and Electronic Properties of ZnWO4(010) Cleaved Surface

Atomic force microscopy (AFM) is a useful instrument to explore the top-surface properties. The nano-morphology of ZnWO 4 (010) cleaved surface has been evaluated in the present study. Optical-quality ZnWO 4 crystal has been grown by the Low Thermal Gradient Czochralski technique. An atomically smooth surface can be obtained by mechanical cleaving under ambient atmospheric conditions due to high cleavage of ZnWO 4 crystal parallel the (010). Atomic steps with height of 0.57 nm were observed by AFM measurements. The steps contain mesodefect peaks with height of ∼1 nm located uniformly on the cleaved surface.

Evgeny N. Galashov

Papers

Structural and Electronic Properties of ZnWO4(010) Cleaved Surface

AFM study of atomic-flat terraces on ZnWO 4 (010) cleaved surface