Improved Photocatalytic Activity via n-Type ZnO/p-Type NiO Heterojunctions

Abstract

The design and construct pn heterojunction to reduce the recombination rate of photogenerated electron-hole pairs can effectively improve photocatalytic activity. In this study, ZnO/NiO heterojunctions were fabricated by annealing a Zn/Ni metal organic framework precursor synthesized via coprecipitation. The effects of the precursor annealing temperature on the microstructure, morphology, and optical properties of the ZnO/NiO nanocomposites were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-vis absorption spectroscopy. The results showed that the nanocomposite was composed of hexagonal wurtzite ZnO and cubic NiO, with the former being the dominant phase. Large ZnO nanoparticles were attached to small NiO nanoparticles, and a pn heterojunction interface was formed. The photodegradation performance of the nanomaterials was evaluated by monitoring the degradation of RhB under irradiation by ultraviolet light. The ZnO/NiO nanocomposites exhibited excellent photocatalytic activity when the annealing temperature was 550 °C. The photodegradation mechanism was also analyzed in detail, revealing that the heterojunction between the n-type ZnO and the p-type NiO played an important role in impeding the recombination of photogenerated electron-hole pairs and improving the photocatalytic efficiency.

Synthesis of mesoporous composites based on α-Fe2O3/NiO nanowires for the photocatalytic degradation of rhodamine B dye

“…Table 3 Comparison of the photocatalytic performance between the current works with the previously reported results 16,21,26,27,41,[45][46][47][48][49][50]55 The results show that the photocurrent density decreases in the initial cycles but then stabilizes in several on-off cycles, with a minimum overrun at the start or the end of the on-off cycle, demonstrating that the direction of electron diffusion is free of grain boundaries, which can create traps to impede the electron movement and slow down photocurrent generation. 66,67 Furthermore, the improved photocurrent response of the S2-based a-Fe 2 O 3 /NiO composite under light illumination demonstrates a higher separation efficiency of photo-induced electron-hole pairs and a lower recombination rate in such hybrid structures.…”

“…Thus, the photocatalyst based on the α-Fe 2 O 3 /NiO composite in this work exhibits satisfactory stability and recyclability, and the removal efficiency was 92% even after the fifth cycle, outperforms the previously reported metal oxide based heterostructure and commercial TiO 2 (P25), as presented in Table 3. 16,21,26,27,41,45–50,55…”

“…It is well known that various properties of the individual metal oxides can be significantly improved using a p/n junction model of two different metal oxides via creating an inner electric field, which promotes the electron/hole separation and opens up a new research area for photocatalysis, optoelectronics, electrical, and biological applications. 9,26,27 The fabrication of NiO (p-type) nanosheet-coated (n-type) BVO was recently reported by Peng et al, 16 and the as-prepared NiO/BVO heterojunction exhibited significantly increased photocatalytic activity. The increased dye adsorption ability of the composite material was ascribed to the substantial specific surface area of the NiO nanosheets.…”

“…Thus, the photocatalyst based on the a-Fe 2 O 3 /NiO composite in this work exhibits satisfactory stability and recyclability, and the removal efficiency was 92% even after the fifth cycle, outperforms the previously reported metal oxide based heterostructure and commercial TiO 2 (P25), as presented in Table 3. 16,21,26,27,41,[45][46][47][48][49][50]55 It is well known that the active species formed during photocatalysis are critical for the breakdown of RhB. Therefore, scavengers such as benzoquinone (BQ) 2 mM, isopropanol (IPA) 10 mM, and ammonium oxalate (AO) 10 mM were added to the RhB solution as the scavengers of holes (h + ), hydroxyl radicals ( OH), and superoxide radicals ( O 2 À ).…”

“…It is well known that various properties of the individual metal oxides can be significantly improved using a p/n junction model of two different metal oxides via creating an inner electric field, which promotes the electron/hole separation and opens up a new research area for photocatalysis, optoelectronics, electrical, and biological applications. 9,26,27…”