Deqiang Zhao

Preparation of a Microspherical Silver-Reduced Graphene Oxide-Bismuth Vanadate Composite and Evaluation of Its Photocatalytic Activity

A novel Ag-reduced graphene oxide (rGO)-bismuth vanadate (BiVO4) (AgGB) ternary composite was successfully synthesized via a one-step method. The prepared composite was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Brunauer-Emmett-Teller (BET) surface area measurement, Raman scattering spectroscopy, and ultraviolet-visible diffuse-reflection spectroscopy (UV-vis DRS). The results showed that bulk monoclinic needle-like BiVO4 and Ag nanoparticles with a diameter of approximately 40 nm formed microspheres (diameter, 5–8 μm) with a uniform size distribution that could be loaded on rGO sheets to facilitate the transport of electrons photogenerated in BiVO4, thereby reducing the rate of recombination of photogenerated charge carriers in the coupled AgGB composite system. Ag nanoparticles were dispersed on the surface of the rGO sheets, which exhibited a localized surface plasmon resonance phenomenon and enhanced visible light absorption. The removal efficiency of rhodamine B dye by AgGB (80.2%) was much higher than that of pure BiVO4 (51.6%) and rGO-BiVO4 (58.3%) under visible light irradiation. Recycle experiments showed that the AgGB composite still presented significant photocatalytic activity after five successive cycles. Finally, we propose a possible pathway and mechanism for the photocatalytic degradation of rhodamine B dye using the composite photocatalyst under visible light irradiation.

New insights into Ag-doped BiVO4 microspheres as visible light photocatalysts

This study describes the synthesis of Ag–bismuth vanadate (Ag–BiVO4) microspheres, a highly efficient visible light photocatalyst for the degradation of methylene blue, via a one-step hydrothermal method. Multiple characterization techniques showed that bulk, monoclinic, needle-like BiVO4 and Ag nanoparticles (50 nm diameter) formed microspheres (3–7 μm diameter) with a uniform size distribution. Compared with pure BiVO4, the Bi–Ag microspheres showed significantly enhanced absorption of visible light (480 to 700 nm) during measurement of UV-Vis diffuse reflectance. Electron paramagnetic resonance measurements indicated that Ag doping enhanced photocatalytic performance because it facilitated separation and transfer of photo-generated electrons and electron holes. This study provides a cost-effective approach for synthesizing Bi–Ag microspheres with enhanced photocatalyst performance for environmental and energy applications.

Synthesis of carbon-doped BiVO4@multi-walled carbon nanotubes with high visible-light absorption behavior, and evaluation of their photocatalytic properties

In order to realize high efficiency visible-light absorption and electron–hole separation of bismuth vanadate (BVO), we synthesized carbon-doped BVO (C-BVO) with high visible-light absorption behavior. We used polyvinylpyrrolidone K-30 as a template and L-cysteine as the carbon source in a one-step hydrothermal synthesis method, and then obtained the carbon-doped BVO@multi-walled carbon nanotubes (C-BVO@MWCNT) by a two-step method. The carbon nanotubes were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, specific surface area, electron spin resonance, and transient photocurrent responses. The XRD analysis confirmed that all photocatalysts were in the same crystal form with a single monoclinic scheelite structure. Combining this with the other characterization results, we showed that the carbon element was successfully doped in BVO and the resulting C-BVO was successfully coupled with multi-walled carbon nanotubes. The removal ratio of rhodamine B by C-BVO@MWCNT was much higher than those by BVO and C-BVO under visible-light irradiation. Recycling experiments verified the stability of C-BVO@MWCNT, which was proved to offer excellent adsorption, strong visible-light absorption behavior, and high electron–hole separation efficiency. Such properties are expected to be useful in practical applications.

Synthesis and photocatalytic activity of hexagonal phase NaYF4:Ho3+@TiO2 core–shell microcrystals

A β-NaYF4:Ho3+@TiO2 core–shell microcrystal photocatalyst was synthesized using a hydrothermal method followed by hydrolysis of tetra-n-butyl titanate (TBOT), with polyvinylpyrrolidone K-30 (PVP) as the coupling agent. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, fluorescence and Raman spectrometry, ultraviolet-visible diffuse reflectance spectroscopy, and electron spin resonance were used to characterize the photocatalyst. It was found that the cores hexagonal phase NaYF4:Ho3+ microcrystals were evenly coated by a TiO2 shell and that the average β-NaYF4:Ho3+ microcrystal length was roughly 9 μm with a diameter of about 3.8 μm. After doping which caused a slight increase, the average thickness of the TiO2 shells was about 50 nm. Because Ho3+-single-doped β-NaYF4:Ho3+@TiO2 displays strong visible absorption peaks at 450, 537, and 642 nm, we chose 450, 532, and 633 nm to be the excitation wavelengths. We found ultraviolet emission bands at 290 nm (5D4 → 5I8) and 389 nm (3K7/5G4 → 5I8). Also, the energy transfer from β-NaYF4:Ho3+ to anatase TiO2 was confirmed. Rhodamine B (RhB) was used as a model pollutant to investigate the photocatalytic activity of β-NaYF4:Ho3+@TiO2 microcrystals under Xe lamp (500 W) irradiation. This investigation showed an advanced visible-light-driven catalyst, with about 67% of the RhB decomposed after 10 h. Compared with the blank experiment, the efficiency was obviously improved. Recycling experiments showed that the β-NaYF4:Ho3+@TiO2 composite still presented significant photocatalytic activity after four successive cycles. Finally, we investigated the β-NaYF4:Ho3+ upconversion (UC) mechanism and the β-NaYF4:Ho3+@TiO2 core–shell microcrystal visible-light-responsive photocatalytic mechanism. Understanding the visible-light-responsive photocatalytic mechanism will help to improve the structural design and functionality of this new type of catalytic material.

Synthesis of Bi2S3/BiVO4 Heterojunction with a One-Step Hydrothermal Method Based on pH Control and the Evaluation of Visible-Light Photocatalytic Performance

The band gaps of bismuth vanadate (BiVO4) and bismuth sulfide (Bi2S3) are about 2.40 eV and 1.30 eV, respectively. Although both BiVO4 and Bi2S3 are capable of strong visible light absorption, electron–hole recombination occurs easily. To solve this problem, we designed a one-step hydrothermal method for synthesizing a Bismuth sulfide (Bi2S3)/Bismuth vanadate (BiVO4) heterojunction using polyvinylpyrrolidone K-30 (PVP) as a structure-directing agent, and 2-Amino-3-mercaptopropanoic acid (l-cysteine) as a sulfur source. The pH of the reaction solution was regulated to yield different products: when the pH was 7.5, only monoclinic BiVO4 was produced (sample 7.5); when the pH was 8.0 or 8.5, both Bi2S3 and BiVO4 were produced (samples 8.0 and 8.5); and when the pH was 9.0, only Bi2S3 was produced (sample 9.0). In sample 8.0, Bi2S3 and BiVO4 were closely integrated with each other, with Bi2S3 particles formed on the surface of concentric BiVO4 layers, but the two compounds grew separately in a pH solution of 8.5. Visible-light photocatalytic degradation experiments demonstrated that the degradation efficiency of the Bi2S3/BiVO4 heterojunction was highest when prepared under a pH of 8.0. The initial rhodamine B in the solution (5 mg/L) was completely degraded within three hours. Recycling experiments verified the high stability of Bi2S3/BiVO4. The synthesis method proposed in this paper is expected to enable large-scale and practical use of Bi2S3/BiVO4.

Mechanisms for ·O2- and ·OH Production on Flowerlike BiVO4 Photocatalysis Based on Electron Spin Resonance

Many studies have focused on the use of BiVO4 as a photocatalyst, but few have investigated the production of free radicals during the photocatalytic process. Following synthesis of flowerlike BiVO4 and characterization by X-ray diffraction (XRD), Raman spectroscopy, Scanning electron microscopy (SEM) Scanning electron microscopy (EDX), UV-Vis and XPS, we successfully prepared BiVO4. Then we used electron spin resonance (ESR) to determine the production and degradation of individual active free radicals, including the superoxide radical (·O2-) and the hydroxyl radical (·OH). In the first experiment, we used ESR to detect the signals of free radicals (·O2- and ·OH) under varying oxygen conditions. The results shown that in addition to production by ·O2-, ·OH could also be produced by oxidation of h+ to OH−. In the next experiment, we detected ·OH under varying pH to identify the result of the first experiment, and found that signal intensities increased with increasing pH, indicating the mechanism for ·OH production. Finally, we conducted a trapping experiment to examine free radical degradation mechanisms. We identified ·OH and h+ as the main active free radicals and showed the complete production about ·OH. These results improve current knowledge of free radical production mechanisms, which can be used to enhance the photocatalytic performance of BiVO4.

One-step synthesis of composite material MWCNT@BiVO4 and its photocatalytic activity

In this research, a composite material (MWCNT@BiVO4) was prepared using a one step hydrothermal method. The prepared composite material was characterized by energy-dispersive X-ray analysis, X-ray diffraction, scanning electron microscopy, EDS, UV-Vis diffuse-reflectance spectroscopy, electron spin resonance (ESR), X-ray photoelectron spectroscopy, and photoluminescence spectroscopy. The scanning electron microscopic images showed that MWCNTs were successfully embedded into BiVO4. MWCNT@BiVO4 showed a strong visible-light absorption capacity, high efficiency for electron–hole separation, and excellent stability. The degradation test of RhB was conducted under visible light irradiation. Compared with BiVO4 (K = −0.05657) and P25 (K = −0.03227), MWCNT@BiVO4 (K = −0.11894) realized the highest removal ratio of Rhodamine B (RhB) under visible light irradiation, therefore, MWCNT@BiVO4 might be promoted to practical applications. The stability of MWCNT@BiVO4 was also verified via recycling and reusing experiments. After 5 cycles, MWCNT@BiVO4 could still maintain the removal rate of RhB at 95.96%. In addition, this paper deduced the growth mechanism of MWCNT@BiVO4 and the degradation mechanism of RhB, proving that MWCNT@BiVO4 can be used in future practices.

Preparation of a Leaf-Like BiVO4-Reduced Graphene Oxide Composite and Its Photocatalytic Activity

We prepared a unique leaf-like BiVO4-reduced graphene oxide (BiVO4-rGO) composite with prominent adsorption performance and photocatalytic ability by a single-step method. Multiple characterization results showed that the leaf-like BiVO4 with average diameter of about 5 um was well dispersed on the reduced graphene oxide sheet, which enhanced the transportation of photogenerated electrons into BiVO4, thereby leading to efficient separation of photogenerated carriers in the coupled graphene-nanocomposite system. The characterization and experiment results also indicated that the outstanding adsorption ability of such composite was closely associated with the rough surface of the leaf-like BiVO4 and doped rGO. The surface photocurrent spectroscopy and transient photocurrent density measurement results demonstrated that the doped rGO enhanced separation efficiency and transfer rate of photogenerated charges. As a result, the BiVO4-rGO exhibited higher photocatalytic capacity toward the degradation of rhodamine B dye under visible-light irradiation compared with pure BiVO4 and P25.

Correction: One-step synthesis of composite material MWCNT@BiVO4 and its photocatalytic activity

Correction for ‘One-step synthesis of composite material MWCNT@BiVO4 and its photocatalytic activity’ by Deqiang Zhao et al., RSC Adv., 2017, 7, 33671–33679.