Nanoscale Bi-based photocatalysts are promising candidates for visible-light-driven photocatalytic environmental remediation and energy conversion. However, the performance of bulk bismuthal semiconductors is unsatisfactory. Increasing efforts have been focused on enhancing the performance of this photocatalyst family. Many studies have reported on component adjustment, morphology control, heterojunction construction, and surface modification. Herein, recent topics in these fields, including doping, changing stoichiometry, solid solutions, ultrathin nanosheets, hierarchical and hollow architectures, conventional heterojunctions, direct Z-scheme junctions, and surface modification of conductive materials and semiconductors, are reviewed. The progress in the enhancement mechanism involving light absorption, band structure tailoring, and separation and utilization of excited carriers, is also introduced. The challenges and tendencies in the studies of nanoscale Bi-based photocatalysts are discussed and summarized.

Review on nanoscale Bi-based photocatalysts

A series of cerium ion-doped titanium dioxide (Ce 3+ –TiO2) catalysts with special 4 f electron configuration was prepared by a sol–gel process and characterized by Brunauer-Emmett-Teller method, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The photocatalytic activity of Ce 3+ –TiO2 catalysts was evaluated in the 2-mercaptobenzothiazole (MBT) degradation in aqueous suspension under UV or visible light illumination. The experimental results demonstrated that the overall photocatalytic activity of Ce 3+ –TiO2 catalysts in MBT degradation was signigicantly enhanced due to higher adsorption capacity and better separation of electron-hole pairs. The experimental results verified that both the adsorption equilibrium constant (Ka) and the saturated adsorption amount (Gmax) increased with the increase of cerium ion content. The results of XPS analysis showed that the Ti 3+ ,C e 3+ , and Ce 4+ ions reside in the Ce 3+ –TiO2 catalysts. The results of DRS analysis indicated that the Ce 3+ –TiO2 catalysts had significant optical absorption in the visible region between 400 and 500 nm because electrons could be excited from the valence band of TiO2 or ground state of cerium oxides to Ce 4 f level. In the meantime, the dependence of the electron-hole pair separation on cerium ion content was investigated by the PL analysis. It was found that the separation efficiency of electron-hole pairs increased with the increase of cerium ion content at first and then decreased when the cerium ion content exceeded its optimal value. It is proposed that the formation of two sub-energy levels (defect level and Ce 4 f level) in Ce 3+ –TiO2 might be a critical reason to eliminate the recombination of

for 2-mercaptobenzothiazole degradation in aqueous suspension for odour control

In the past few years, Ni(OH)2 has been found to be an effective cocatalyst for photocatalytic hydrogen evolution. Herein, we report that it can also be used to enhance the photoreduction of CO2 into chemical fuels. Vertically aligned Ni(OH) 2 nanosheets are deposited onto electrospinning TiO2 nanofibers via simple wet-chemical precipitation to manufacture TiO2/Ni(OH) 2 hybrid photocatalysts. The TiO2 nanofibers can direct the ordered growth of Ni(OH) 2 nanosheets, which have a thickness of 20 nm and uniformly cover the surface of the TiO2 substrate. The TiO2/Ni(OH) 2 hierarchical composite displays remarkably improved photocatalytic CO2 reduction activity compared to that displayed by pristine TiO2 fibers. The bare TiO2 can only produce methane and carbon monoxide (1.13 and 0.76 μmol h−1 g−1, respectively) upon CO2 photoreduction. After loading 0.5 wt% Ni(OH) 2, the methane yield increases to 2.20 μmol h−1 g−1, meanwhile the CO yield is unchanged. Interestingly, alcohols (methanol and ethanol) also appear as products, in addition to CH4 and CO, over the TiO2/Ni(OH) 2 hybrid, and the maximum yield is reached with 15 wt% Ni(OH) 2 loading (0.58 and 0.37 μmol h−1 g−1 for methanol and ethanol, respectively). This can be ascribed to an enhanced charge separation efficiency and higher CO2 capture capacity due to the presence of Ni(OH) 2. These results demonstrate that Ni(OH) 2 can not only improve the total CO2 conversion efficiency, but can also alter the product selectivity upon photocatalysis. This work opens a new pathway for achieving high-efficiency photocatalytic CO2 reduction with Ni(OH) 2 as a cocatalyst.

Hierarchical TiO2/Ni(OH)2 composite fibers with enhanced photocatalytic CO2 reduction performance

Constructing homojunction is more favorable to transfer and separation of charge carriers at the interface between structural units owing to the matching chemical and electronic structures, nevertheless it still is difficult to fabricate the morphology-controlled nonmetal homojunction. Herein, a nonmetal 2D/3D homojunction is constructed via the facile surface in-situ polymerization process, where 3D g-C3N4 (3D CC) microspheres tightly anchor on the surface of 2D g-C3N4 (2D CN) nanosheets. The obtained nonmetal 2D/3D CN/CC homojunction displays the dramatically enhanced photocatalytic performance for degrading tetracycline hydrochloride (TC-HCl) compared with single 2D CN nanosheets and 3D CC microspheres, mainly attributing to the improved transfer and separation efficiency of charge carriers resulted from synergetic effect of 2D-3D structural coupling and energy band controlling. Moreover, the important degradation pathway, intermediate products and photocatalytic mechanism are investigated in detail. This work develops a feasible exemplificative strategy for fabricating new morphology-controlled nonmetal homojunction to improve photocatalytic activity.

Construction of morphology-controlled nonmetal 2D/3D homojunction towards enhancing photocatalytic activity and mechanism insight

Heterogeneous photocatalysis has long been considered to be one of the most promising approaches to tackling the myriad environmental issues However, there are still many challenges for designing efficient and cost-effective photocatalysts and photocatalytic degradation systems for application in practical environmental remediation In this review, we first systematically introduced the fundamental principles on the photocatalytic pollutant degradation Then, the important considerations in the design of photocatalytic degradation systems are carefully addressed, including charge carrier dynamics, catalytic selectivity, photocatalyst stability, pollutant adsorption and photodegradation kinetics Especially, the underlying mechanisms are thoroughly reviewed, including investigation of oxygen reduction properties and identification of reactive oxygen species and key intermediates This review in environmental photocatalysis may inspire exciting new directions and methods for designing, fabricating and evaluating photocatalytic degradation systems for better environmental remediation and possibly other relevant fields, such as photocatalytic disinfection, water oxidation, and selective organic transformations

Qian Ni

Papers

BiOCl/TiO2 heterojunction network with high energy facet exposed for highly efficient photocatalytic degradation of benzene

Charge separation in branched TiO2 nanorod array homojunction aroused by quantum effect for enhanced photocatalytic decomposition of gaseous benzene