Sugang Meng

Design of a direct Z-scheme photocatalyst: Preparation and characterization of Bi2O3/g-C3N4 with high visible light activity

A direct Z-scheme photocatalyst Bi2O3/g-C3N4 was prepared by ball milling and heat treatment methods. The photocatalyst was characterized by X-ray powder diffraction (XRD), UV-vis diffuse reflection spectroscopy (DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) surface areas, photoluminescence technique (PL), and electron spin resonance (ESR) technology. The photocatalytic activity was evaluated by degradation of methylene blue (MB) and rhodamine B (RhB). The results showed that Bi2O3/g-C3N4 exhibited a much higher photocatalytic activity than pure g-C3N4 under visible light illumination. The rate constants of MB and RhB degradation for Bi2O3(1.0wt.%)/g-C3N4 are about 3.4 and 5 times that of pure g-C3N4, respectively. The migration of photogenerated carriers adopts a Z-scheme mechanism. The photoexcited electrons in the CB of Bi2O3 and photogenerated holes in the VB of g-C3N4 are quickly combined, so the photoexcited electrons in the CB of g-C3N4 and holes in the VB of Bi2O3 participate in reduction and oxidation reactions, respectively. O2(-), OH and h(+) are the major reactive species for the Bi2O3/g-C3N4 photocatalytic system.

Novel approach to enhance photosensitized degradation of rhodamine B under visible light irradiation by the ZnxCd1-xS/TiO2 nanocomposites.

In order to exploit efficient photosensitizers with appropriate electronic states to enhance the transfer of electrons, ZnxCd1-xS/TiO2 nanocomposites were first synthesized by a simple hydrothermal method. The samples were characterized by X-ray diffraction, transmission electron microscopy, diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, electron spin resonance, and photoluminescence techniques. The results showed that the composite of the two inorganic semiconductors largely enhanced the photosensitized degradation of rhodamine B (RhB) under visible light irradiation (420 nm<λ<800 nm). These photocatalytic reactions were driven mainly by the light absorption of RhB molecules and to a lesser extent by the excitation of ZnxCd1-xS. They were supposed to arise mainly from the electron transferred from the adsorbed dye in its singlet excited state to the conduction band of ZnxCd1-xS and TiO2. Such a heterogeneous photocatalytic reaction has much significance in the degradation of organic pollutants in ordinary photocatalysis.

ZnO photonic crystals with enhanced photocatalytic activity and photostability

ZnO photonic crystals (ZnO-PCs) with large area and high quality were prepared by a facile auto-forced impregnation method. The resulting ZnO-PCs exhibited remarkable photocatalytic performance and photocorrosion inhibition compared with porous and commercial nanoparticle ZnO.

Structuring β-Ga2O3 Photonic Crystal Photocatalyst for Efficient Degradation of Organic Pollutants

Coupling photocatalysts with photonic crystals structure is based on the unique property of photonic crystals in confining, controlling, and manipulating the incident photons. This combination enhances the light absorption in photocatalysts and thus greatly improves their photocatalytic performance. In this study, Ga2O3 photonic crystals with well-arranged skeleton structures were prepared via a dip-coating infiltration method. The positions of the electronic band absorption for Ga2O3 photonic crystals could be made to locate on the red edge, on the blue edge, and away from the edge of their photonic band gaps by changing the pore sizes of the samples, respectively. Particularly, the electronic band absorption of the Ga2O3 photonic crystal with a pore size of 135 nm was enhanced more than other samples by making it locate on the red edge of its photonic band gap, which was confirmed by the higher instantaneous photocurrent and photocatalytic activity for the degradation of various organic pollutants under ultraviolet light irradiation. Furthermore, the degradation mechanism over Ga2O3 photonic crystals was discussed. The design of Ga2O3 photonic crystals presents a prospective application of photonic crystals in photocatalysis to address light harvesting and quantum efficiency problems through manipulating photons or constructing photonic crystal structure as groundwork.

Photocatalytic reforming of glycerol for H2 evolution on Pt/TiO2: fundamental understanding the effect of co-catalyst Pt and the Pt deposition route

To fundamentally understand the effects of deposited Pt and its deposition route on the photocatalytic reforming (PR) of glycerol for H2 evolution over Pt/TiO2, several 1 wt% Pt/P25 (PT) samples were prepared by photo-deposition (PD, glycerol as hole scavenger) and impregnation–reduction deposition routes (IRD, NaBH4 or H2/Ar as reductant) using H2PtCl6 as the precursor. The samples were characterized by XRD, UV-Vis DRS, TEM and XPS, and the PR activities were examined and compared under ambient conditions. The formation of photo-induced charge carriers (CCs) over PT was measured by a photoluminescence technique using terephthalic acid as a probe molecule. The results indicated that the reforming activity depends on both the nature of the light harvesting of P25 and the characteristics of the co-catalyst Pt, including its chemical state, size, and the interaction with P25; Pt particles serve as the active sites for H2 evolution. Uniform Pt particles could be selectively and intimately deposited on P25 in the Pt(0) state via an in situ PD process (PT-S), while by IRD routes, Pt particles were randomly loaded on P25 with the surface in Pt(0) and the bulk in Pt(II/IV) states. Unlike the Pt chemical state, the Pt sizes were less impacted by the deposition routes and were about 2 nm. Compared to P25, a low generation efficiency of CCs was observed on platinized samples due to the covering of the photo-active sites by Pt. Pt(0) exhibits higher light shielding effect than Pt(II/IV). Meanwhile, the separation of CCs was promoted by Schottky barriers formed at the Pt–TiO2 interface. Photo-induced electrons could be trapped by the barriers and the process was favored by well-contacted Pt(0) and obstructed by the bulk Pt(II/IV) component. The promotion effect of Pt(0) prevails over its adverse effect. Thus, PT-S exhibited the highest PR activity as it only possesses Pt(0), demonstrating the advantage of the PD process. A control test suggests Pt with this kind of feature can only be achieved in a dilute suspension by the PD route.

Efficient utilization of photogenerated electrons and holes for photocatalytic selective organic syntheses in one reaction system using a narrow band gap CdS photocatalyst

In this study, a nanoparticle structure of CdS with cubic phase (CdS-G) was prepared by a facile solid-state reaction at room temperature for the first time. CdS-G can be used as a highly active photocatalyst for selective oxidation of p-methoxybenzyl alcohol (pMBA) to p-methoxybenzaldehyde (pMBAD) and reduction of nitrobenzene (NB) to aniline (AL) in a coupled reaction system under green mild reaction conditions through visible light irradiation. Compared with the counterparts prepared by the conventional precipitation method (CdS-P) and hydrothermal method (CdS-H), the photocatalytic performance of CdS-G is greatly improved owing to the unique features of the nanostructure, the high surface area, pore volume, visible light absorption and photoelectric properties. The yield of pMBAD (AL) over CdS-G is about 1.6 (5.2) and 1.9 (20.8) times higher than that over CdS-P and CdS-H, respectively. The CdS-G sample exhibits excellent selectivity and stability because its valence band (VB) and conduction band (CB) positions matched well with the redox potentials of pMBA/pMBAD and NB/AL. Furthermore, the photogenerated holes and electrons can be efficiently and directly reacted with pMBA and NB, respectively. The photocatalytic selective oxidation and reduction reaction is a synergistic reaction via producing and consuming protons. The photogenerated holes and electrons could be utilized thoroughly to produce pMBAD and AL, respectively. The molar ratio of pMBA and NB was 3 : 1, and the yield of pMBAD and AL could be successfully achieved at a theoretical ratio of 1 : 1. This work highlights the promising scope for selective organic synthesis in one reaction system under mild conditions using photogenerated electrons and holes directly and simultaneously.

Ultra-low content of Pt modified CdS nanorods: one-pot synthesis and high photocatalytic activity for H2 production under visible light

Noble metal-modified CdS is one of the most promising photocatalysts for solar H2 production due to its intrinsic band structure merits. It is highly desirable to develop an effective preparation route to pursue a high photocatalytic performance and to minimize the use of costly noble metals. For the first time, a simple and convenient one-pot solvothermal (OPS) method was developed to prepare platinized CdS nanorods (Pt/CdS-N) in this work. The formation of a hexagonal 1D structure CdS and the deposition of Pt(0) can be achieved simultaneously by the method, which is more efficient than the conventional post-deposition routes, such as photochemical reduction and impregnation–reduction methods, to enhance the photocatalytic activity of CdS-N for the H2 evolution reaction (HER). The H2 evolution rate (rH2) of CdS-N could be remarkably improved from 2.10 to 10.29 mmol h−1 g−1 by loading with only 0.06% Pt (wt%) under visible light irradiation (λ > 400 nm, 300 W Xe lamp). No deactivation of the sample was observed in cyclic experiments for 20 h reaction. This loading amount of Pt is substantially lower than the reported optimal values (commonly in the range of 0.5–2%) by more than one order of magnitude. A criterion of enhancement coefficient was proposed to identify the ideal loading amount of Pt. The result indicates that, considering the improvement efficiency of rH2 and the loading amount of Pt, this ultra-low amount of Pt is more practical than the optimal amount (determined to be 0.5%). The high and stable activity of Pt/CdS-N can be attributed to the hexagonal 1D structure of CdS and the high dispersion of Pt in the Pt(0) state. Besides Pt, the OPS method is also valid for the deposition of Pd or Ru on CdS and rH2 decreases in the order Ru/CdS-N (12.89) > Pt/CdS-N (10.29) > Pd/CdS-N (6.72 mmol h−1 g−1) with a loading amount of 0.06%. It reveals that the use of noble metal co-catalysts can be significantly reduced without unduly sacrificing the HRE efficiency. The developed OPS route provides a new insight into the preparation of highly efficient and stable chalcogenide photocatalysts for the HER.

Solvothermal synthesis of CdIn 2 S 4 photocatalyst for selective photosynthesis of organic aromatic compounds under visible light

Ternary chalcogenide semiconductor, cadmium indium sulfide (CdIn2S4), was prepared by a simple solvothermal method using ethylene glycol as a solvent, as well as indium chloride tetrahydrate (InCl3.4H2O), cadmium nitrate tetrahydrate [Cd(NO3)2.4H2O], and thiacetamide (TAA) as precursors. The resulted sample was subject to a series of characterizations. It is the first time to use CdIn2S4 sample as a visible light-driven photocatalyst for simultaneous selective redox transformation of organic aromatic compounds. The results indicate that the as-synthesized CdIn2S4 photocatalyst not only has excellent photocatalytic performance compared with pure In2S3 and CdS for the selective oxidation of aromatic alcohols in an oxygen environment, but also shows high photocatalytic redox activities under nitrogen atmosphere. A possible mechanism for the photocatalytic redox reaction in the coupled system was proposed. It is hoped that our current work could extend the applications of CdIn2S4 photocatalyst and provide new insights for selective transformations of organic compounds.

Integrating photonic bandgaps with surface plasmon resonance for the enhancement of visible-light photocatalytic performance

Intensifying the light harvesting and promoting the separation of photoinduced charge carriers are effective strategies to boost photocatalyst performance. Inspired by these insights, a hybrid photocatalyst was fabricated in this work by the deposition of Au nanoparticles (NPs) on a ZnO photonic crystal (ZnO-PC). The photonic band-gap of the ZnO-PC was tuned experimentally by Braggs law to couple the slow photon (SP) effect of the ZnO-PC with the surface plasmon resonance (SPR) of Au NPs. Transmission spectra results indicated that, when the SP effect of the ZnO-PC matched well with the SPR of Au NPs, the visible light absorption of Au NPs could be substantially amplified. The hybrid Au/ZnO-PC showed high photocatalytic activity for the degradation of RhB under visible light irradiation, and the degradation kinetic constant (1.42 h−1) is ca. 5.6-fold higher than that of the famous N doped TiO2 (TiO2−xNx, 0.22 h−1) and 24.8-fold higher than that of the commercial ZnO NPs (0.05 h−1). The synergistic effects of the SPR of Au, the SP effect of the ZnO-PC, and the heterostructures between ZnO and Au NPs account for the high photocatalytic performance, which can enhance the harvesting of visible light and promote the separation of charge carriers. A possible reaction mechanism was tentatively proposed based on the active species analysis result. The work not only provides an effective route to enhance photocatalytic efficiency, but also contributes to a better understanding of the role of PCs in the photocatalytic reaction.

Probing photonic effect on photocatalytic degradation of dyes based on 3D inverse opal ZnO photonic crystal

3D inverse opal ZnO photonic crystals (ZnO-PCs) with designed photonic bandgap (PBG) were prepared to study the pore size and slow photon effect on photocatalytic dye degradation. The PBGs of these ZnO-PC films were tuned experimentally by variation of the polystyrene sphere size of the opal templates. It was found that there is competition between the surface area and mass transport with increasing pore size during photocatalysis. ZnO-PCs exhibited higher photocatalytic activity under visible light irradiation, when the probe molecules were absorbed and well matched with the PBG. The enhancement could be attributed to intensified dye photosensitization as a result of the slow photon effect at the PBG edges, thus leading to a remarkable improvement in the light trapping. The present results provide useful information for developing high performance photocatalysts and photoanodes based on artificial photonic crystal design.