1.1. Self-Assembled Nanostructured Membrane Photocatalysts for Wastewater Treatment
Liu et al. prepared titanium dioxide films via a layer-by-layer self-assembly technique to immobilize TiO
2 nanoparticles using polyurethane (PU) and increase the adsorption capacity of the photocatalyst
[1]. In addition, the photocatalytic performance and reusability of the films were investigated by the decomposition of MB under UV irradiation, and it was shown that the catalytic efficiency of the prepared films was still as high as 94.56% after five cycles and they could be reused six times without affecting the photocatalytic activity. Synthetic membranes are therefore promising candidate materials for wastewater treatment applications.
However, single nano-TiO
2 photocatalysts have low specific surface area, poor adsorption capacity for pollutants, and are easily agglomerated and difficult to recover, resulting in low catalytic efficiency for the photocatalytic degradation of low concentrations of organic matter. The researchers found that composite nano-TiO
2 materials can significantly improve the defects of single nano-TiO
2 materials which are prone to agglomeration. To overcome the defects of nano-TiO
2 particles, research and application of composite materials are gaining increasing attention
[2][3][4].
Zhang et al. successfully coated TiO
2 nanofibers onto ceramic hollow fiber membranes using a simple dip-coating technique to form TiO
2 nanofiber membranes with reticular morphologies, and they evaluated the performances of the TiO
2 nanofiber membranes in treating hyaluronic acid by monitoring the change in the total organic carbon values in water
[5]. After six cycles, there was no significant loss of activity of the TiO
2 nanofiber hollow membrane. Therefore, the TiO
2 nanofiber hollow membranes proposed possessed high stability during the removal of hyaluronic acid.
Bai et al. successfully synthesized a novel multifunctional carbon nanotube/ZnO/TiO
2 nanocomposite ultrafiltration membrane by hydrothermal synthesis and ultrasonic-assisted acid treatment
[6]. Chang et al. obtained self-assembled nanoporous Ti with a smooth surface and many folds by a hydrothermal method using metallic titanium foam as the raw material
[7]. A self-assembled layer of a strongly adherent 3D Na
2Ti
3O
7 nanowire network was grown on the surfaces of Ti particles and channels after alkaline hydrothermal treatment in a NaOH solution . The self-assembled TiO
2 nanowire networks were uniform, with lengths of 2–3 μm
[7]. The self-assembled nanowire network utilized the wastewater degradation device for two different types of dyes, RhB (20 mg/L) and MB (20 mg/L), both of which showed good photocatalytic properties after UV irradiation for 60 min only, reflecting its good degradation effects. The above results illustrate the high performance of self-assembled nanoporous Ti as photoelectrolytic electrode materials.
In addition, Fe
3O
4-based nanocomposites can be used as good Fenton-like catalysts for the degradation of organic pollutants in water
[8]. The introduction of magnetic Fe
3O
4 nanoparticles provides another advantage for nanostructured composites, and the magnetic properties of the prepared composites facilitate fast and easy separation during catalyst recovery and reuse
[9].
Wang et al. successfully prepared an Fe
3O
4/rGO/metal–organic framework (MOF) composite with a dispersed interlayer structure by a hydrothermal method and investigated the degradation performances of these composites on phenol
[10]. They found that the degradation of phenol was mainly dependent on the pH, and the degradation efficiency of phenol reached 80% within 2 min at pH = 3. The degradation rate of phenol decreased sharply when the pH was further reduced
[11]. The phenol was completely removed within 16 min for all pH conditions. By exploring the role of the catalyst components in Fenton-like reactions, they found that the excellent catalytic performances of the Fe
3O
4/rGO/MOF composites were mainly due to the synergistic effect of the porous MOF shell and the internal Fe
3O
4/rGO
[12]. The reusability of Fe
3O
4/rGO/MOF was tested by recovering the catalyst at the end of the reaction and reusing it in the next run. As shown, the catalytic activity of Fe
3O
4/rGO/MOF was maintained at 96% after five reuses.
2. Self-Assembled Nanostructured Membrane Photocatalysts for Photocatalytic Hydrogen Production
Photocatalyst modification using doped noble metal nanoparticles is an effective method to improve the photocatalytic performance. Dal’Acqua et al. prepared a multilayer composite by combining gold (Au) and titanium dioxide (TiO
2) nanoparticles (NPs) into self-assembled photocatalytic films (SAPFs)
[13], forming a composite with larger specific surface area compared to those of conventional nanostructured catalysts. This facilitated the maximization of the photocatalytic activity. In this structured photocatalyst, hydrogen is produced in the polymer/(TiO
2-Au) nanoparticle network and also in the body of the polymer/(TiO
2-Au) NP assembly. Hydrogen is readily produced in large quantities under radiation, and the amount of hydrogen produced by the structured photocatalyst increased linearly with increasing UV irradiation time. The SAPFs have great potential for renewable energy development due to their simple preparation process and excellent photocatalytic activity.
He et al. prepared nanoporous CoFe
2O
4 loaded with platinum and silver by dealloying
[14]. They showed that the hydrogen precipitation rate of the resulting sample was as high as 2.36 mmol/h/g under full-spectrum irradiation, which was 24 times that of CoFe
2O
4 without platinum or silver. The H
2-releasing activity did not decrease significantly after 32 h of continuous irradiation, indicating the excellent stability of this photocatalyst. The analysis showed that the silver NPs had a strong surface plasmon resonance (SPR) effect in visible light, resulting in effective visible light absorption. This effect expanded the range of light absorption and effectively improved light utilization, while Pt could act as an effective electron trap for electron–hole pair separation, effectively inhibiting electron and hole complexation. In addition, the simultaneous loading of Pt and Ag on CoFe
2O
4 produced a synergistic effect that contributed to its photocatalytic performance.
The morphological modification of photocatalysts and the construction of heterojunctions are considered to be the main means of significantly improving the performances of photocatalytic hydrogen evolution
[15][16].
Zhang et al. successfully synthesized ZnS nanocrystals with different morphologies using cysteine as the sulfur source at different heating temperatures using a template-free method and evaluated the photocatalytic performances of the samples
[17]. After a series of catalytic experiments, it was found that the inherent self-absorption and photon recirculation of photoluminescence played a key role in the photocatalysis . The photocatalytic activity was investigated by the degradation of RhB solutions. It is well known that the morphology of a material has an important influence on its properties. As shown by the photocatalytic results and the morphological images, ZnS-200 exhibits a simpler surface structure and better catalytic activity than ZnS-100 and ZnS-150, suggesting the idea that defect-rich edge states are advantageous in providing reactive sites and broadening the absorption range.
Bhirud et al. prepared hierarchical nanostructures of cubic-spinel-structured CdIn
2S
4 selectively by a hydrothermal method
[18]. The effects of surfactants on the morphology and microstructure of cadmium sulfide were investigated using polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide as surfactants. The cadmium sulfide samples prepared with PVP as the surfactant exhibited excellent photocatalytic activities, with a maximum hydrogen production rate of up to 3238 μmol/h.