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TiO2 presents a broad application prospect in the field of degradation of environmental pollutants. However, the development of TiO2 photocatalyst has been limited by two factors, namely, narrow spectral response range and low quantum efficiency. Surface coating can selectively change the physical and chemical properties of core particles, whereas the synergistic action between core and shell can optimize material performance. In this paper, surface modification of titanium dioxide (TiO2) by dopamine polymerization was used to improve its photocatalytic performance. Among polydopamine (PDA)-modified TiO2, TiO2@PDA-0.4 mg/mL-2 h showed the best degradation rate of Rhodamine B (RhB): the degradation rate reached 99.28% and 77.40% after 1.5 h of ultraviolet–visible (UV–vis) and visible (Vis) light irradiation, respectively. Compared with the uncoated TiO2, the degradation rate has shown improvement, especially under Vis light irradiation, with the photocatalytic performance improving by 35.04%. Absorption, emission, and impedance spectra showed that surface modification of TiO2 with suitable thickness of PDA effectively increased light absorption and promoted separation of photogenerated carriers. High-performance liquid chromatography–mass spectrometry revealed the differences in photocatalytic degradation mechanism of TiO2@PDA to RhB under UV–vis and Vis light irradiation. Compared with those under UV–vis light irradiation, the intermediate products under Vis light irradiation are relatively simple. TiO2@PDA generated more electrons, which interacted with H2O to produce large amounts of ·OH and holes under UV–vis light irradiation in comparison with that under Vis light irradiation. In addition to deacetylation, deamination, and decarboxylation by hydroxylation, excess ·OH in the photocatalytic process would attack benzene rings directly to produce N,N-diethylacetamide.

The excellent adhesion property of polydopamine (PDA) provides a new way to fixing nanoparticles. This study investigated the effects of PDA/ZnO nanocomposite as an electron transport layer (ETL) material on the performance of polythieno[3,4-b]-thiophene/benzodithiophene:[6,6]-phenyl-C70-butyric acid methyl ester bulk heterojunction solar cell. On one hand, the strong adhesive property of PDA to ZnO nanoparticles is beneficial to the formation of a uniform ZnO thin film, which is favorable for electron transfer. On the other hand, an energy level mismatch exists between PDA and ZnO, which hinders electron transfer from ZnO to ITO. Upon optimization, filling factor and photoelectric conversion efficiency are improved by 23.2% and 13.8%, respectively, when the single ZnO layer is replaced by PDA/ZnO nanocomposite as the ETL. Open circuit voltage resulted in 9.8% enhancement from 0.61 V to 0.67 V. However, the short circuit current density is reduced.

Nitrogen-doped graphene quantum dots (N-GQDs) with prime fluorescence properties were synthesized through a simple one-pot hydrothermal method by using citric acid as carbon source and ammonia (A-GQDs) or urea (U-GQDs) as nitrogen source. Both A-GQDs and U-GQDs exhibited a graphene-like structure, but their average particle size is about ten times different. A-GQDs exhibited an evident excitation-dependent photoluminescence (PL) behavior. By contrast, the emission of U-GQDs can be divided into two regions with the development of excitation wavelength: a wavelength-independent emission peak at approximately 540 nm (green) with the excitation wavelength from 400 nm to 500 nm and an excitation-dependent emission peak with the excitation wavelength below 400 nm. Ammonia was introduced into citric acid polycondensator in the form of five membered ring and urea in the form of seven membered ring, respectively, which further affected the subsequent crystallization and surface functionalization. At this time, the structural instability of the nitrogen-containing doped ring itself, coupled with the progress of carbonization, mostly forms a stable nitrogen-containing ring structure, which is likely to be pyrrolic nitrogen or graphitized nitrogen. Therefore, the differences in particle size and nitrogen-containing doped rings partially between A-GQDs and U-GQDs lead to the differences in optical properties. Finally, U-GQDs with high quantum yield (QY) of 54 % were obtained at the molar ratio of 9:10 (citric acid/urea) by optimizing the reaction conditions.

Graphene-based hydrogels (GH) have exhibited prominent performances in wastewater treatment. Herein, Special-shaped ZnO tetrapods and TiO2 nanosheets were introduced to the preparation of GH via a one-step hydrothermal reduction method. The ZnO-GH and TiO2-GH composite hydrogel with a uniform 3D network structure and good mechanical properties can be used as an adsorbent for printing and dyeing wastewater. The optimum doping amount of ZnO and TiO2 in the composite hydrogel and the adsorption regularity to rhodamine B (RhB) with different concentrations were discussed. The cyclic adsorption performance and adsorption kinetics were briefly discussed. When the doping ratio of ZnO to graphene oxide (GO) is 5:8, ZnO-GH has the best adsorption rate, which is 71.7%. The best degradation rate appears at a ZnO doping ratio of 1:2, which also shows the best total removal rate of RhB of approximately 82.20%. When the doping ratio of TiO2 to GO is 3:4, the photocatalytic degradation rate is 30.3%, and the total removal rate of RhB reaches 99.1%, which indicates that RhB can be almost completely removed by TiO2-GH through adsorption and degradation. After five cycles, the adsorption capacity of ZnO-GH and TiO2-GH for the low RhB concentration is still good. The addition of ZnO and TiO2 improves not only the adsorption and degradation properties of the composite hydrogel but also its recycling performance as a result of the increase in mechanical stability.