Zaicheng Sun

Effect of defects on photocatalytic activity of rutile TiO2 nanorods

To further understand the effect of structural defects on the electrochemical and photocatalytic properties of TiO2, two synthetic approaches based on hydrothermal synthesis and post-synthetic chemical reduction to achieve oxygen defectimplantation were developed herein. These approaches led to the formation of TiO2 nanorods with uniformly distributed defects in either the bulk or on the surface, or the combination of both, in the formed TiO2 nanorods (NRs). Both approaches utilize unique TiN nanoparticles as the reaction precursor. Electron microscopy and Brunauer-Emmett-Teller (BET) analyses indicate that all the studied samples exhibit similar morphology and similar specific surface areas. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) data confirm the existence of oxygen defects (VO). The photocatalytic properties of TiO2 with different types of implanted VO were evaluated based on photocatalytic H2 production. By optimizing the concentration of VO among the TiO2 NRs subjected to different treatments, significantly higher photocatalytic activities than that of the stoichiometric TiO2 NRs was achieved. The incident photon-to-current efficiency (IPCE) data indicate that the enhanced photocatalytic activity arises mainly from defect-assisted charge separation, which implies that photo-generated electrons or holes can be captured by VO and suppress the charge recombination process. The results show that the defective TiO2 obtained by combining the two approaches exhibits the greatest photocatalytic activity enhancement among all the samples.

Synthesis of Carbon Dots with Multiple Color Emission by Controlled Graphitization and Surface Functionalization

Multiple-color-emissive carbon dots (CDots) have potential applications in various fields such as bioimaging, light-emitting devices, and photocatalysis. The majority of the current CDots to date exhibit excitation-wavelength-dependent emissions with their maximum emission limited at the blue-light region. Here, a synthesis of multiple-color-emission CDots by controlled graphitization and surface function is reported. The CDots are synthesized through controlled thermal pyrolysis of citric acid and urea. By regulating the thermal-pyrolysis temperature and ratio of reactants, the maximum emission of the resulting CDots gradually shifts from blue to red light, covering the entire light spectrum. Specifically, the emission position of the CDots can be tuned from 430 to 630 nm through controlling the extent of graphitization and the amount of surface functional groups, uf8ffCOOH. The relative photoluminescence quantum yields of the CDots with blue, green, and red emission reach up to 52.6%, 35.1%, and 12.9%, respectively. Furthermore, it is demonstrated that the CDots can be uniformly dispersed into epoxy resins and be fabricated as transparent CDots/epoxy composites for multiple-color- and white-light-emitting devices. This research opens a door for developing low-cost CDots as alternative phosphors for light-emitting devices.

Surface Defects Enhanced Visible Light Photocatalytic H2 Production for Zn-Cd-S Solid Solution

In order to investigate the defect effect on photocatalytic performance of the visible light photocatalyst, Zn-Cd-S solid solution with surface defects is prepared in the hydrazine hydrate. X-ray photoelectron spectra and photoluminescence results confirm the existence of defects, such as sulfur vacancies, interstitial metal, and Zn and Cd in the low valence state on the top surface of solid solutions. The surface defects can be effectively removed by treating with sulfur vapor. The solid solution with surface defect exhibits a narrower band gap, wider light absorption range, and better photocatalytic perfomance. The optimized solid solution with defects exhibits 571 μmol h(-1) for 50 mg photocatalyst without loading Pt as cocatalyst under visible light irradiation, which is fourfold better than that of sulfur vapor treated samples. The wavelength dependence of photocatalytic activity discloses that the enhancement happens at each wavelength within the whole absorption range. The theoretical calculation shows that the surface defects induce the conduction band minimum and valence band maximum shift downward and upward, respectively. This constructs a type I junction between bulk and surface of solid solution, which promotes the migration of photogenerated charges toward the surface of nanostructure and leads to enhanced photocatalytic activity. Thus a new method to construct highly efficient visible light photocatalysts is opened.

Se & N co-doped carbon dots for high-performance fluorescence imaging agent of angiography

Both long wavelength excitation and emission are highly desired for fluorescent carbon dots in bio-related applications. According to semiconductor theory, heteroatom doping is an effective way to tune the band gap of a semiconductor. In this work, we developed an exceedingly simple and mild one-pot method to synthesize selenium and nitrogen co-doped carbon dots (Se&N-CDs) by using low cost citric acid, ethane diamine and selenium powder as the starting materials. After doping the Se element into N-CDs, the absorption of Se&N-CDs was expanded from UV to the visible light region (∼497 nm). More important is that the Se&N-CDs show a strong green light emission with a high quantum yield of ∼52%, attributed to the new Se-state introduced in the Se&N-CDs due to the low electronegativity of Se. Furthermore, we successfully demonstrate that Se&N-CDs have an excellent performance as a dye for fluorescein fundus angiography (FFA). After injection, a clear vascular image in the retina and detailed images of the capillary bed were exhibited in the FFA image. This will provide a great advantage for bio-imaging and diagnosis.

高分散超薄二硫化钼/硫、氮共掺碳复合材料的电催化析氢性能

Abstract Ultrathin small MoS2 nanosheets exhibit a higher electrocatalytic activity for the hydrogen evolution reaction. However, strong interactions between MoS2 layers may result in aggregation; together with the low conductivity of MoS2, this may lower its electrocatalytic activity. In this paper we present a method that we developed to directly produce solid S, N co-doped carbon (SNC) with a graphite structure and multiple surface groups through a hydrothermal route. When Na2MoO4 was added to the reaction, polymolybdate could be anchored into the carbon materials via a chemical interaction that helps polymolybdate disperse uniformly into the SNC. After a high temperature treatment, polymolybdate transformed into MoS2 at 800 °C for 6 h in a N2 atmosphere at a heating rate of 5 °C/min, owing to S2– being released from the SNC during the treatment (denoted as MoS2/SNC-800-6h). The SNC effectively prevents MoS2 from aggregating into large particles, and we successfully prepared highly dispersed MoS2 in the SNC matrix. Electrochemical characterizations indicate that MoS2/SNC-900-12h exhibits a low onset potential of 115 mV and a low overpotential of 237 mV at a current density of 10 mA/cm2. Furthermore, MoS2/SNC-900-12h also had an excellent stability with only ∼2.6% decay at a current density of 10 mA/cm2 after 5000 test cycles.

A highly active and durable iron/cobalt alloy catalyst encapsulated in N-doped graphitic carbon nanotubes for oxygen reduction reaction by a nanofibrous dicyandiamide template

Exploration of competitive electrocatalysts to replace Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cells is one of the most promising strategies to confront the energy and environmental crises. Herein, we highlighted an FeCo alloy catalyst encapsulated in N-doped graphitic carbon nanotubes (FeCo@N-GCNT-FD) as a highly efficient non-precious electrocatalyst for the ORR. The FeCo@N-GCNT-FD catalyst exhibits a positive onset (0.96 V vs. RHE) and half-wave potential (0.88 V vs. RHE) as well as 5.6 times the specific activity of commercial Pt/C at 0.70 V in alkaline media. The excellent catalytic behavior of FeCo@N-GCNT-FD is attributed to the structural properties, including a large surface area, and the synergistic effect of FeCo alloy and N-GCNT, which guarantee a large number of accessible catalytic sites and rapid mass-transfer kinetics. Theoretical calculations further confirm that the strong synergistic and electronic effects, especially the FeCo-NG sites, provide a favorable local coordination environment and electronic structure and a lower oxygen absorbance energy. The improvement of ORR activity and durability of the catalyst by the synergistic and electronic effects between the metal and carbon provides a versatile approach for tuning the catalytic performance of non-noble electrocatalysts.

Constructing CdS/Cd/doped TiO 2 Z-scheme type visible light photocatalyst for H 2 production

Constructing Z-scheme type photocatalyst is an efficient way to improve the charge separation efficiency and enhance the photocatalytic activity. In this report, the Cd:TiO2 nanoparticles are prepared via the sol-gel route and employed as a starting material. When it was reduced by NaBH4 at 300°C, the surface oxygen vacancies were produced and Cd2+ was reduced into metal Cd0 nanoparticle (denoted as R-Cd:TiO2). Subsequently, the formed R-Cd:TiO2 was treated with thioureain the hydrothermal reaction. Through the decomposition of thiourea, the oxygen vacancies were refilled by S2− from thiourea to form S:TiO2/TiO2 (d-TiO2) and Cd was partially converted into CdS to form CdS/Cd/d-TiO2 composite. The formed CdS/Cd/d-TiO2 composite exhibits improved photocatalytic activity. Under visible light irradiation (λ>400 nm), the H2 production rate of CdS/Cd/d-TiO2 reaches 119 μmol h−1 with 50 mg of photocatalyst without any cocatalyst, which is about 200 and 60 times higher than that of S:TiO2/TiO2 (0.57 μmol h−1), CdS (2.03 μmol h−1) and heterojunction CdS/d-TiO2 (2.17 μmol h-1) materials, respectively. The results illustrate that metal Cd greatly promotes the charge separation efficiency due to the formation of Z-scheme type composite. In addition, the photocatalytic activity in the visible light region was dramatically enhanced due to the contribution of both CdS and d-TiO2. The method could be easily extended to other wide bandgap semiconductors for constructing visible light responsive Z-scheme type photocatalysts.摘要构建Z型光催化体系是提高光生电荷分离效率和光催化活性的一种有效途径. 本文通过溶胶凝胶方法制备了Cd掺杂的TiO2纳米颗粒, 并通过一步NaBH4固相热还原的方式在材料表面可控地引入氧空位(VO), 同时掺入的Cd2+可被还原为金属Cd0纳米粒子(即R-Cd:Ti〇2). 进一步将获得的R-Cd:TiO2材料与硫脲热水反应, 材料表面VO的可被S2−替代同时部分金属Cd0硫化, 从而获得CdS/Cd/d-TiO2 Z型光催化复合材料. 研究结果表明该Z型光催化复合材料具有优异的模拟太阳光及可见光区光催化活性和稳定性. 通过实验分析证明掏建这种全固态金属-无机半导体Z型光催化复合材料, 金属介质层显著促进了光生电荷的分离与迁移; 此外, 由于CdS和d-TiO2在可见光区的光吸收作用, 该CdS/Cd/d-TiO2 Z型光催化复合材料在可见光区的光催化活性获得了显著增强.