Jiuqing Wen

Photocatalysis fundamentals and surface modification of TiO2 nanomaterials

Abstract As a green and sustainable technology, heterogeneous photocatalysis using semiconductors has received much attention during the past decades because of its potential to address energy and environmental problems. Among various semiconductors, TiO 2 has been regarded as the best and most widely investigated photocatalyst in the past 10 years. Based on the fundamentals of photocatalysis and surface chemistry of TiO 2 nanomaterials, we herein summarize and discuss the achievements in the different surface modification strategies employed to date such as surface doping and sensitization, construction of surface heterojunctions, loading of nano-sized co-catalysts, increase in the accessible surface areas, and usage of surface F effects and exposure of highly reactive facets. Especially, the interesting synergistic effects of these different surface modification strategies deserve more attention in the near future. Studying these important advances in photocatalysis fundamentals, and surface chemistry and modification may offer new opportunities for designing highly efficient TiO 2 -based and non-TiO 2 -based photocatalysts for solar fuel production, environmental remediation, organic photosynthesis, and other related fields such as solar cell device fabrication, thermal catalysis, and separation and purification.

Design and fabrication of semiconductor photocatalyst for photocatalytic reduction of CO 2 to solar fuel

The shortage of fossil fuels and the disastrous pollution of the environment have led to an increasing interest in artificial photosynthesis. The photocatalytic conversion of CO2 into solar fuel is believed to be one of the best methods to overcome both the energy crisis and environmental problems. It is of significant importance to efficiently manage the surface reactions and the photo-generated charge carriers to maximize the activity and selectivity of semiconductor photocatalysts for photoconversion of CO2 and H2O to solar fuel. To date, a variety of strategies have been developed to boost their photocatalytic activity and selectivity for CO2 photoreduction. Based on the analysis of limited factors in improving the photocatalytic efficiency and selectivity, this review attempts to summarize these strategies and their corresponding design principles, including increased visible-light excitation, promoted charge transfer and separation, enhanced adsorption and activation of CO2, accelerated CO2 reduction kinetics and suppressed undesirable reaction. Furthermore, we not only provide a summary of the recent progress in the rational design and fabrication of highly active and selective photocatalysts for the photoreduction of CO2, but also offer some fundamental insights into designing highly efficient photocatalysts for water splitting or pollutant degradation.摘要近年来, 严 重的化石燃料短缺以及环境污染问题使得人工光合作用引起了科研工作者的广泛关注, 光催化转换CO2成为有价值的太阳能燃料被认为是解决能源危机以及环境问题的最好的方法之一. 有效地控制半导体表面的催化反应以及光生载流子是制备高活性以及高选择性半导体CO2还原光催化剂的关键因素, 至今, 研究人员已经提出了许多策略来增强光催化转换CO2的活性以及选择性. 本文在分析提高光催化效率和选择性限制因素的基础上, 尝试从几个不同方面总结了近些年来提高光催化CO2还原效率的方法以及它们的设计原理, 包括增强半导体可见光响应、 促进光生电子空穴分离、 提高CO2的吸附和活化、 加速CO2还原的动力学以及抑制不良反应等方面. 因此, 本文不仅系统地总结了近年来高活性高选择性光催化CO2还原光催化剂的设计进展, 而且为高效光解水产氢和污染物降解光催化剂的设计提供了重要参考.

Enhanced photocatalytic H2 evolution over noble-metal-free NiS cocatalyst modified CdS nanorods/g-C3N4 heterojunctions

In this report, CdS nanorods/g-C3N4 heterojunctions loaded with a noble-metal-free NiS cocatalyst were for the first time fabricated by an in situ hydrothermal method. The as-synthesized heterostructured photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy, UV-visible spectroscopy, nitrogen absorption, photoluminescence (PL) spectra, transient photocurrent responses and electrochemical impedance spectroscopy (EIS) measurements. Their photocatalytic activity for hydrogen production was evaluated using an aqueous solution containing triethanolamine under visible light (λ ≥ 420 nm). The results clearly demonstrated that the ternary hybridization of the NiS cocatalyst, 1D CdS nanorods and 2D g-C3N4 nanosheets is a promising strategy to achieve highly efficient visible-light-driven photocatalytic H2 evolution. Among all the photocatalysts employed, the ternary hybrid g-C3N4–CdS–9% NiS composite materials show the best photocatalytic performance with a H2-production rate of 2563 μmol h−1 g−1, which is 1582 times higher than that of the pristine g-C3N4. The enhanced photocatalytic activity was ascribed to the combined effects of NiS cocatalyst loading and the formation of the intimate nanoheterojunctions between 1D CdS nanorods and 2D g-C3N4 nanosheets, which were favorable for promoting charge transfer, improving the separation efficiency of photoinduced electron–hole pairs from the bulk to the interfaces and accelerating the surface H2-evolution kinetics. This work would not only provide a promising photocatalyst candidate for applications in visible-light H2 generation, but also offer a new insight into the construction of highly efficient and stable g-C3N4-based hybrid semiconductor nanocomposites for diverse photocatalytic applications.

利用Ni(OH) x 助催化剂修饰提高g-C 3 N 4 纳米片/WO 3 纳米棒Z型纳米体系的可见光产氢活性的研究

Abstract Novel WO 3 /g-C 3 N 4 /Ni(OH) x hybrids have been successfully synthesized by a two-step strategy of high temperature calcination and in situ photodeposition. Their photocatalytic performance was investigated using TEOA as a hole scavenger under visible light irradiation. The loading of WO 3 and Ni(OH) x cocatalysts boosted the photocatalytic H 2 evolution efficiency of g-C 3 N 4 . WO 3 /g-C 3 N 4 /Ni(OH) x with 20 wt%defective WO 3 and 4.8 wt%Ni(OH) x showed the highest hydrogen production rate of 576 μmol/(g–h), which was 5.7, 10.8 and 230 times higher than those of g-C 3 N 4 /4.8 wt%Ni(OH) x , 20 wt%WO 3 /C 3 N 4 and g-C 3 N 4 photocatalysts, respectively. The remarkably enhanced H 2 evolution performance was ascribed to the combination effects of the Z-scheme heterojunction (WO 3 /g-C 3 N 4 ) and loaded cocatalysts (Ni(OH) x ), which effectively inhibited the recombination of the photoexcited electron-hole pairs of g-C 3 N 4 and improved both H 2 evolution and TEOA oxidation kinetics. The electron spin resonance spectra of • O 2 − and • OH radicals provided evidence for the Z-scheme charge separation mechanism. The loading of easily available Ni(OH) x cocatalysts on the Z-scheme WO 3 /g-C 3 N 4 nanocomposites provided insights into constructing a robust multiple-heterojunction material for photocatalytic applications.

Earth-abundant NiS co-catalyst modified metal-free mpg-C3N4/CNT nanocomposites for highly efficient visible-light photocatalytic H2 evolution.

In the present work, the earth-abundant NiS co-catalyst modified mesoporous graphite-like C3N4 (mpg-C3N4)/CNT nanocomposites were prepared via a two-step strategy: the sol-gel method and the direct precipitation process. The mpg-C3N4/CNT/NiS composite photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis absorption spectroscopy, photoluminescence spectroscopy (PL), photoelectrochemical (PEC) and electrochemical impedance spectra (EIS) experiments. The photocatalytic H2-production activity over the composite catalysts was also evaluated by using an aqueous solution containing triethanolamine under visible light (λ≥ 420 nm). The results showed that the loading of earth-abundant NiS co-catalysts onto metal-free mpg-C3N4/CNT nanocomposites can remarkably enhance their photocatalytic H2-production activity. The optimal loading amount of NiS on metal-free mpg-C3N4/CNT nanocomposites was about 1 wt%. The as-obtained mpg-C3N4/CNT/1% NiS ternary composite photocatalyst exhibits the best H2-evolution activity with the highest rate of about 521 μmol g(-1) h(-1) under visible light (λ≥ 420 nm), which is almost 148 times that of a pure mpg-C3N4/CNT sample. The enhanced photocatalytic activity can be mainly attributed to the synergistic effect of effectively promoted separation of photo-generated electron-hole pairs and enhanced H2-evolution kinetics. The co-loading of nanocarbon materials and earth-abundant co-catalysts onto metal-free mpg-C3N4 photocatalysts offers great potential for practical applications in photocatalytic H2 evolution under visible light illumination.

Efficient visible-light photocatalytic H2 evolution over metal-free g-C3N4 co-modified with robust acetylene black and Ni(OH)2 as dual co-catalysts

In this work, a novel g-C3N4/acetylene black (AB)/Ni(OH)2 ternary composite photocatalyst with dual robust electron co-catalysts was successfully synthetized using a facile two-step strategy: ultrasonic dispersion treatment and a subsequent precipitation process. The photocatalytic H2-production activity over the composite photocatalyst was also evaluated using an aqueous solution containing triethanolamine under visible light (λ ≥ 420 nm). For the first time, it was revealed that the robust AB can be utilized as a co-catalyst to significantly enhance the photocatalytic H2-evolution activity of g-C3N4. The results also demonstrated that the ternary g-C3N4/AB/Ni(OH)2 nanocomposite exhibited enhanced photocatalytic H2-evolution activity as compared to bulk g-C3N4 and binary hybrids. The g-C3N4–0.5% AB–1.0% Ni(OH)2 (weight ratio) composite shows the highest H2 evolution rate of 240 μmol g−1 h−1 under visible light irradiation, which is 320, 100 and 3.31 times higher than that of pure g-C3N4, g-C3N4–0.5% AB and g-C3N4–1.0% Ni(OH)2, respectively. It is believed that the excellent synergetic effect between the robust AB and Ni(OH)2 as dual electron co-catalysts on the surface of g-C3N4 can achieve the effectively promoted separation of photo-generated electron–hole pairs and enhance the following H2-evolution kinetics, thus resulting in a significant enhancement of the photocatalytic H2 evolution activity over g-C3N4. It is expected that the combination of nano-carbons such as AB and other earth-abundant co-catalysts can become a general strategy to improve the H2-evolution activity over various kinds of conventional semiconductors.

Constructing Multifunctional Metallic Ni Interface Layers in the g-C3N4 Nanosheets/Amorphous NiS Heterojunctions for Efficient Photocatalytic H2 Generation

The construction of exceptionally robust and high-quality semiconductor-cocatalyst heterojunctions remains a grand challenge toward highly efficient and durable solar-to-fuel conversion. Herein, novel graphitic carbon nitride (g-C3N4) nanosheets decorated with multifunctional metallic Ni interface layers and amorphous NiS cocatalysts were fabricated via a facile three-step process: the loading of Ni(OH)2 nanosheets, high-temperature H2 reduction, and further deposition of amorphous NiS nanosheets. The results demonstrated that both robust metallic Ni interface layers and amorphous NiS can be utilized as electron cocatalysts to markedly boost the visible-light H2 evolution over g-C3N4 semiconductor. The optimized g-C3N4-based photocatalyst containing 0.5 wt % Ni and 1.0 wt % NiS presented the highest hydrogen evolution of 515 μmol g-1 h-1, which was about 2.8 and 4.6 times as much as those obtained on binary g-C3N4-1.0%NiS and g-C3N4-0.5%Ni, respectively. Apparently, the metallic Ni interface layers play multifunctional roles in enhancing the visible-light H2 evolution, which could first collect the photogenerated electrons from g-C3N4, and then accelerate the surface H2-evolution reaction kinetics over amorphous NiS cocatalysts. More interestingly, the synergetic effects of metallic Ni and amorphous NiS dual-layer electron cocatalysts could also improve the TEOA-oxidation capacity through upshifting the VB levels of g-C3N4. Comparatively speaking, the multifunctional metallic Ni layers are dominantly favorable for separating and transferring photoexcited charge carriers from g-C3N4 to amorphous NiS cocatalysts owing to the formation of Schottky junctions, whereas the amorphous NiS nanosheets are mainly advantageous for decreasing the thermodynamic overpotentials for surface H2-evolution reactions. It is hoped that the implantation of multifunctional metallic interface layers can provide a versatile approach to enhance the photocatalytic H2 generation over different semiconductor-cocatalyst heterojunctions.

Metal-free carbon nanotube–SiC nanowire heterostructures with enhanced photocatalytic H2 evolution under visible light irradiation

In this report, metal-free multi-walled carbon nanotube (MWCNT)–SiC nanowire 1D–1D nanoheterostructures were successfully synthesized by an in situ chemical reaction between MWCNTs and silicon powder. A vapor–liquid–solid (VLS) mechanism was found to be responsible for in situ growth of SiC nanowires along MWCNTs. The structure, morphology and composition of the as-obtained MWCNT–SiC 1D–1D samples were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA) and UV-vis absorption spectroscopy. The H2 evolution photoactivities of the resultant MWCNT–SiC nanoheterostructures under visible light irradiation were also investigated. Results showed that the metal-free MWCNT–SiC 1D–1D nanoheterostructures exhibited the highest H2 evolution rate among all samples, up to 108 μmol g−1 h−1, which was 3.1 times higher than that of pure SiC without MWCNTs. It suggests that the H2 evolution activity enhancement of the MWCNT–SiC 1D–1D nanocomposites under visible light irradiation is mainly attributed to the synergistic effects of enhanced separation efficiency of photogenerated hole–electron pairs at the MWCNT–SiC interfaces, improved crystallinity, unique 1D–1D nanoheterostructures and increased visible light absorption. The present work not only gives new insights into the underlying photocatalysis mechanism of the metal-free MWCNT–SiC 1D–1D nanoheterostructures but also provides a versatile strategy to design 1D–1D nanocomposite photocatalysts, with great potential applications in photocatalytic H2 generation or environmental pollutant degradation.