5.2.1. Hydrothermal Method

As mentioned above, the hydrothermal method is capable of synthesizing various inorganics with different morphologies, including the nanofibrous materials. For example, Yang et al. [97] once reported the fabrication of a novel TiO2 nanofibers with a shell of anatase nanocrystals based on the hydrothermal process. Actually, the whole fabrication process included three steps: First, the H2Ti3O7 nanofibers were obtained from the anatase TiO2 particles and NaOH solutions via hydrothermal method. After that, the as-prepared H2Ti3O7 nanofibers were treated using a dilute acid solution under certain hydrothermal condition to generate the anatase nanocrystal shell on the outside. Finally, the H2Ti3O7 phase was converted to TiO2(B) phase after a heat treatment while the anatase nanocrystal shell remained unchanged. Owing to the well-matched phase interfaces, which ensures the charge transfer across the interfaces, the recombination of electron-hole pairs was effectively suppressed and the corresponding photoactivity was significantly enhanced. Most importantly, they demonstrated that these nanofibrous photocatalysts possess specific surface areas similar to the commercial P25 powder, and the fibril morphology endowed them with a good recyclability from water, which is critically important in practical applications. Recently, our group also carried out a series of studies on the synthesis of nanofibrous photocatalysts via employing the hydrothermal method such as the bimetallic AuPd alloy nanoparticles deposited on MoO3 nanowires [98]. As shown in Figure 19, MoO3 nanowires were firstly prepared from Mo powder and H2O2 via the hydrothermal method. Then, the as-prepared MoO3 nanowires were used as the substrates to synthesize the MoO3/Au-Pd bimetallic alloy nanowires via a simple chemical reduction method. As expected, the MoO3/Au–Pd bimetallic alloy nanowires exhibited a good photocatalytic degradation performance for trichloroethylene (TCE) under the driving of visible light. Similarly, the composite nanowires could be easily separated from the reaction slurry in a short time after the reaction.

**Figure 19.** Schematic illustrating the band structure of MoO3/Au-Pd composite photocatalyst and the possible reaction mechanism. Adapted with permission from Reference [98]. Copyright (2018) Elsevier.

#### 5.2.2. Electrospinning Method

Electrospinning is considered as a promising way to synthesize nanofibers with several advantages, such as easy operation, low cost, and scalable [99–101]. In general, there are four major parts in an electrospinning device: (i) an electrical power supplier, (ii) a metallic needle, (iii) syringes with the

polymer solution, and (iv) a conductive collector. Meanwhile, several process parameters, such as the polymer-based solution concentration, the viscosity of solution, the flow rate of the syringe driver, and the electric field power, could also be well-regulated to manipulate the morphology of fibers. During the electrospinning process, the solution is injected through a metallic needle via a syringe with a constant pump speed. At the same time, a voltage is applied on the metallic needle; therefore, the solution droplet will be charged, and then a Taylor cone will be generated when the electronic force is enough to overcome the surface tension. Following this, a liquid jet is formed between the grounded collector and the needle. The generated jets will be stretched by an electrostatic repulsion force until it reaches the collector; meanwhile, the solvent will rapidly evaporate during this process. Finally, the jets are solidified and the corresponding nanofibers are collected on the collector [100].

As for the applications of photocatalysis, high specific surface area is required to provide more active sites for the redox reaction. More specifically, electrospun nanofibers as forefront fibrous materials have attracted considerable research attention in the area of photocatalysis due to its several advantages of large surface area, extremely high aspect ratio, and ease of functionalization [102,103]. For example, Zhang et al. [104] reported the fabrication of a flexible and hierarchical mesoporous TiO2 nanoparticle (TiO2 NP) modified TiO2 nanofiber composites via the combination of electrospinning and in situ polymerization method. At first, flexible TiO2 nanofibers were prepared via the electrospinning and the subsequently consuming process with the dopant of yttrium. After that, the as-prepared TiO2 nanofibers were used as a template for the incorporation of TiO2 NPs by utilizing a bifunctional benzoxazine as the carrier through a calcination process in the N2 atmosphere. The as-prepared membranes exhibited remarkable photocatalytic activity towards organic dyes in water; moreover, it could be reused well via simply rinsing with water, and without time-consuming separation procedures owing to the long aspect ratio and good mechanical property of the composite nanofibers. In recent years, our group has carried out several works on the design of electrospun nanofibrous photocatalysts [105–109]. As a representative sample, a BiOCl0.3/BiOBr0.3/BiOI0.4/PAN composite fibrous catalyst was fabricated via combining the electrospinning and sol-gel method [109]. As shown in Figure 20, the obtained composite photocatalyst exhibited a typical fibril structure with a good uniformity, and the corresponding field emission transmission electron microscope (FE-TEM) image demonstrated a highly crystalline structure in the composite fibers with a clear lattice spacing relating to the (112) plane of BiOCl, the (110) plane of BiOBr, and the (200) plane of BiOI; therefore, a heterojunction structure was generated via a close contact of the composite semiconductors. After a visible-light driven photocatalysis performance evaluation, it was found that the obtained BiOCl0.3/BiOBr0.3/BiOI0.4/PAN fiber displayed the highest photocatalytic degradation performance of TCE. Moreover, it was concluded that the improved visible-light driven photocatalytic activity is caused by the interfacial contact of a heterojunction and the inhibition of the recombination rate of the electron–hole pairs.

**Figure 20.** SEM image of pristine PAN fibers (**a**) and BiOClx/BiOBry/BiOIz composite fibers (**b**). (**c**) FE-TEM image of BiOClx/BiOBry/BiOIz fibers. (**d**) Lattice-resolved image for BiOClx/BiOBry/BiOIz nanofibers. (**e**) Schematic indicating the photocatalytic degradation of TCE. Adapted with permission from Reference [109]. Copyright (2016) Elsevier.

### *5.3. Nanosheets (2D)*

Semiconductor nanosheets are typical 2D nanomaterials and have attracted significant attention in the research area of photocatalysis for their larger surface area and tunable structures. Up to now, a great deal of semiconductor nanosheets have been synthesized via various strategies for different applications. The hydrothermal process is one of the most used strategies for the preparation of 2D semiconductor photocatalysts for the application of wastewater treatment [110]. Through the hydrothermal process, various nanosheets derived from a single semiconductor or multi-semiconductors could be synthesized. For example, Chen et al. [111] prepared TiO2-based nanosheets (TNS) via the alkaline hydrothermal treatment of commercial P25. They reported that the as-prepared TNS exhibited much higher specific surface area and much stronger adsorption for crystal violet molecules than the raw P25. Furthermore, the TNS could be effectively regenerated using a H2O2-assisted photocatalysis process, showing great potential for dealing with the high-chroma dye wastewater. Besides TiO2, various nanosheets derived from different semiconductors could also be fabricated using a hydrothermal process, such as the WO3 nanosheet/K+Ca2Nb3O10<sup>−</sup> ultrathin nanosheet synthesized by Ma et al. [112] via a facile hydrothermal assembly of WO3 nanosheets and ultrathin K+Ca2Nb3O10<sup>−</sup> nanosheets. They demonstrated that the composite nanosheets possess 2D/2D heterojunctions and display remarkably enhanced photocatalytic activity compared to the pristine WO3 and K+Ca2Nb3O10<sup>−</sup> nanosheets, which were mainly caused by the strongly coupled hetero-interfaces that provided more active sites for reactions and band structure. Additionally, some other methods, such as the solvothermal or photo-reduction methods, could also be employed for the preparation of composite nanosheets. For example, Wang et al. [113] fabricated the Bi12O15Cl6 nanosheets with a narrowed band gap via a simple and facile solvothermal method followed by a simple thermal treatment. The obtained Bi12O15Cl6 nanosheets exhibited a good photocatalytic degradation performance of bisphenol A solely under the driving of visible light, and the reaction rate of the composite nanosheets was 13.6 and 8.7 times faster than those of BiOCl and TiO2 (P25), respectively. In addition, the as-prepared Bi12O15Cl6 nanosheets possessed good stability and recyclability during the photocatalytic process. As shown in Figure 21, our group recently developed a Pt/BiOI composite nanosheet via a photo-reduction method in ambient conditions [114], where the as-prepared Pt/BiOI composites exhibited a flower-like structure and could effectively photocatalytically degrade the rhodamine B and phenol under visible-light irradiation (λ > 420 nm), where the degradation rate was superior to that of pure BiOI. Moreover, it was found that the content of Pt in the composite plays a vital important role on the photoactivity, and it was found that the optimal ratio of Pt to BiOI in the composite was 3%. It was concluded that the enhanced photocatalytic activity of the Pt/BiOI composite was caused by the superior electron transfer ability with the presence of an appropriate amount of Pt.

**Figure 21.** (**a**,**b**) SEM images of the obtained Pt/BiOI composite nanosheets. (**c**,**d**) HR-SEM images of BiOI nanosheets and Pt/BiOI composite nanosheets, respectively. (**e**–**g**) HR-TEM images of Pt/BiOI composite nanosheets. (**h**) Element mappings of Pt, Bi, O, and I for the Pt/BiOI composite nanosheets. Adapted with permission from Reference [114]. Copyright (2017) Elsevier.
