Xianliang Fu

Hydrogen production over titania-based photocatalysts

Because of their relatively high efficiency, high photostability, abundance, low cost, and nontoxic qualities, titania-based photocatalysts are still the most extensively studied materials for the photocatalytic production of hydrogen from water. The effects of the chemical and physical properties of titania, including crystal phase, crystallinity, particle size, and surface area, on its photoactivity towards hydrogen generation have been identified by various investigations. The high overpotential for hydrogen generation, rapid recombination of photogenerated electrons and holes, rapid reverse reaction of molecular hydrogen and oxygen, and inability to absorb visible light are considered the most important factors that restrict the photoactivity of titania, and strategies to overcome these barriers have been developed. These issues and strategies are carefully reviewed and summarized in this Minireview. We aim to provide a critical, up-to-date overview of the development of titania-based photocatalysts for hydrogen production, as well as a comprehensive background source and guide for future research.

In situ preparation of novel p-n junction photocatalyst BiOI/(BiO)2CO3 with enhanced visible light photocatalytic activity

Novel p-n junction photocatalysts BiOI/(BiO)2CO3 with different contents of BiOI were in situ synthesized by etching (BiO)2CO3 precursor with hydroiodic acid (HI) solution. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectrometry (FT-IR), energy-dispersive spectroscopy (EDS) and UV-vis diffuse reflectance spectroscopy (DRS) were employed to study the structures, morphologies and optical properties of the as-prepared samples. Under visible light (λ>420 nm), BiOI/(BiO)2CO3 hybrid displayed much higher photocatalytic activity than pure (BiO)2CO3 and BiOI for the degradation of methyl orange (MO). The increased photocatalytic activity of BiOI/(BiO)2CO3 could be attributed to the formation of the p-n junction between p-BiOI and n-(BiO)2CO3, which effectively suppresses the recombination of photoinduced electron-hole pairs. Moreover, the tests of radical scavengers confirmed that •O2- and h+ were the main reactive species for the degradation of MO.

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.

Electronic structure and optical properties of Ag3PO4 photocatalyst calculated by hybrid density functional method

The electronic structure and optical properties of Ag3PO4 were studied by hybrid density functional theory. The results indicated that the band gap is 2.43 eV, which agrees well with the experimental value of 2.45 eV. The conduction bands of Ag3PO4 are mainly attributable to Ag 5s and 5p states, while the valence bands are dominated by O 2p and Ag 4d states. The highest valence band edge potential was 2.67 V (vs. normal hydrogen electrode), which has enough driving force for photocatalytic water oxidation and pollutants degradation. The optical absorption spectrum showed that Ag3PO4 is a visible light response photocatalyst.

Ball milled h-BN: An efficient holes transfer promoter to enhance the photocatalytic performance of TiO2

High activity hexagonal-BN (h-BN)/TiO(2) composite photocatalysts were prepared by ball milling method. The structural and optical properties of the samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectra (DRS), and fluorescence emission spectra. The effect of the loading amount of h-BN and the ball milling time on the photocatalytic degradation of Rhodamine B (RhB) and methylene blue (MB) was investigated. The results indicated that the photocatalytic activity of TiO(2) could be improved substantially by coupling with a proper amount of milled h-BN. The optimal loading amount of h-BN was found to be 0.5 wt% and the milling time was 30 min. Under this condition, the photocatalytic removal efficiencies of TiO(2) for RhB and MB could be increased as high as 15 and 8 times. The role of the milling process and the mechanism for the enhancements was finally discussed in terms of creation of negatively charged h-BN surface and promotion the separation of photoinduced holes, respectively.

Urea-based hydrothermal growth, optical and photocatalytic properties of single-crystalline In(OH)3 nanocubes.

Nearly monodisperse single-crystalline In(OH)(3) nanocubes were successfully synthesized using In(NO(3))(3) x 4.5 H(2)O as indium source in the presence of urea and cetyltrimethyl ammonium bromide (CTAB) by a two-step hydrothermal process: the stock solution was heated at 70 degrees C for 24 h and then at 120 degrees C for 12 h. The structure and morphology of the resultant In(OH)(3) samples were determined by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results revealed that most of as-synthesized In(OH)(3) nanocubes were uniform in size, with the average edge length of approximately 700 nm. The influences of the reaction temperature, the reaction time, the mineralizer, and the surfactant on the morphology of the obtained products were discussed in detail. Room-temperature photoluminescence (PL) spectrum of the In(OH)(3) nanocubes showed a peculiar strong emission peak centered at 480 nm. Furthermore, the photocatalytic properties of the In(OH)(3) nanocubes were tested. It was found that In(OH)(3) exhibited not only higher activity for benzene removal, but also better H(2) evolution from water than the commercial Degussa P25 TiO(2).

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.

The preparation and characterization of composite bismuth tungsten oxide with enhanced visible light photocatalytic activity

A composite photocatalyst (Bi3.84W0.16O6.24–Bi2WO6) containing Bi, W and O elements was prepared by a facile hydrothermal method. Various characterization methods such as X-ray powder diffraction, UV-vis diffuse reflectance spectroscopy, scanning electron microscopy and transmission electron microscopy were employed to investigate the structure and optical properties. The activities of the samples were evaluated by the photocatalytic degradation of methylene blue under visible light irradiation. The results showed that when the pH of the precursor solution is 12.3, and a hydrothermal treatment at 140 °C for 20 h was used, the prepared sample shows the mixed phases of Bi3.84W0.16O6.24 and Bi2WO6. The Bi3.84W0.16O6.24–Bi2WO6 composite exhibited an enhanced photocatalytic activity compared with single Bi2WO6 or Bi3.84W0.16O6.24. The rate constant of Bi3.84W0.16O6.24–Bi2WO6 is about 5 times that of Bi2WO6. It is proposed that the increased photocatalytic activity may be attributed to the formation of a heterojunction between Bi3.84W0.16O6.24 and Bi2WO6, which suppresses the recombination of photoexcited electron–hole pairs.