NiS2助剂修饰g-C3N4光催化剂的简易合成及光催化制氢性能增强研究

g-C3N4是一种新型的稳定的半导体光催化材料,它可以通过热缩聚法、固相反应法、电化学沉积法和溶剂热法等制备.g-C3N4禁带宽度约为2.7 eV,吸收边在460 nm左右,具有合适的导带位置,可用作可见光响应制氢的光催化材料,但在实际应用中g-C3N4光催化性能较低,其原因可归纳为:(1)g-C3N4在吸收光子产生电子和空穴对后,光生载流子的传输速率较慢,容易在体相或表面复合,致使g-C3N4的量子效率较低;(2)材料在合成过程中易于结块,使g-C3N4的比表面积远小于理论值,严重削弱了g-C3N4光催化材料的制氢性能.目前已有很多关于g-C3N4改性的报道,但一些方法对材料的处理过程耗时较长或者合成过程较难控制.用助剂改性是提高光催化制氢活性的半导体材料的主要策略之一.合适的助剂可改进电荷分离和加速表面催化反应,从而提高光催化剂的制氢活性.虽然稀有金属或贵金属,如铂、金和银可大大提高g-C3N4的制氢速率,但由于其昂贵和稀缺性,因而应用严重受限.因此,开发成本低、储量丰富、高性能助剂来进一步提高制氢性能具有重要意义.NiS2来源丰富、价格低廉.它可在酸性和碱性的环境保持相对较高的稳定性,且其表面电子结构表现出类金属特性.但它较难与半导体光催化剂形成强耦合和界面,通常需要水热等条件下合成.实验表明,g-C3N4表面存在着大量的含氧官能团及未缩合的氨基基团,为表面接枝提供了丰富的反应活性位点,因而可利用g-C3N4表面均匀分布的含氧官能团等和Ni2+结合,再原位与S2?反应,从而在g-C3N4上负载耦合紧密的NiS2助剂,进一步提高复合材料的光催化制氢活性.本文采用低温浸渍法制备了NiS2/g-C3N4光催化剂.NiS2助剂在温和的反应条件下与g-C3N4光催化剂复合,可以防止催化剂结构的破坏,同时使得助剂均匀地分散,并紧密结合在催化剂表面,从而大大提高光催化剂的制氢性能.该样品制备过程为:(1)通过水热处理制备含氧官能团和较大比表面积的g-C3N4;(2)添加Ni(NO3)2前驱体后,Ni2+离子由于静电作用紧密吸附在g-C3N4表面;(3)在80oC加入硫代乙酰胺(TAA),可在g-C3N4的表面紧密和均匀形成助剂NiS2.表征结果证实成功制备NiS2纳米粒子修饰的g-C3N4光催化剂.当Ni含量为3 wt%,样品表现出最大的制氢速率(116μmol h?1 g?1),明显高于纯g-C3N4.此外,对NiS2/g-C3N4(3 wt%)的样品进行光催化性能的循环测试结果表明:该样品在可见光照射下可以保持一个稳定的、有效的光催化制氢性能.根据实验结果,我们提出一个可能的光催化机理:即NiS2促进了物质表面快速转移光生电子,使g-C3N4光生电荷有效分离.基于NiS2具有成本低和效率高的优点,因而有望广泛应用于制备高性能的光催化材料.

Facile synthesis and enhanced photocatalytic H2-evolution performance of NiS2-modified g-C3N4 photocatalysts

NiS2 is a promising cocatalyst to improve the photocatalytic performance of g-C3N4 for the production of H2. However, the synthesis of the NiS2 cocatalyst usually requires harsh conditions, which risks destroying the microstructures of the g-C3N4 photocatalysts. In this study, a facile and low-temperature (80 °C) impregnation method was developed to prepare NiS2/g-C3N4 photocata-lysts. First, the g-C3N4 powders were processed by the hydrothermal method in order to introduce oxygen-containing functional groups (such as–OH and–CONH–) to the surface of g-C3N4. Then, the Ni2+ions could be adsorbed near the g-C3N4 via strong electrostatic interaction between g-C3N4 and Ni2+ ions upon the addition of Ni(NO3)2 solution. Finally, NiS2 nanoparticles were formed on the surface of g-C3N4 upon the addition of TAA. It was found that the NiS2 nanoparticles were solidly and homogeneously grafted on the surface of g-C3N4, resulting in greatly improved photocatalytic H2 production. When the amount of NiS2 was 3 wt%, the resultant NiS2/g-C3N4 photocatalyst showed the highest H2 evolution rate (116.343μmol h?1 g?1), which is significantly higher than that of the pure g-C3N4 (3μmol h?1 g?1). Moreover, the results of a recycling test for the NiS2/g-C3N4(3 wt%) sample showed that this sample could maintain a stable and effective photocatalytic H2-evolution performance under visible-light irradiation. Based on the above results, a possible mechanism of the improved photocatalytic performance was proposed for the presented NiS2/g-C3N4 photocatalysts, in which the photogenerated electrons of g-C3N4 can be rapidly transferred to the NiS2 nanoparticles via the close and continuous contact between them; then, the photogenerated electrons rapidly react with H2O adsorbed on the surface of NiS2, which has a surficial metallic character and high catalytic activity, to produce H2. Considering the mild and facile synthesis method, the presented low-cost and highly efficient NiS2-modified g-C3N4 photocatalysts would have great potential for practical use in photocatalytic H2 production.