Pt-C_(3)N_(4)/BiOCl S型异质结应用于光催化CO_(2)还原的理论计算研究

A DFT Study on S-Scheme Heterojunction Consisting of Pt Single Atom Loaded G-C_(3)N_(4) and BiOCl for Photocatalytic CO_(2) Reduction

光催化CO_(2)还原制备可再生的碳氢燃料为缓解温室效应、解决能源短缺问题提供了一个可行的办法。然而,单一组分光催化剂的CO_(2)还原活性非常低。一是因为光生载流子的快速复合导致光子效率很低。二是因为CO_(2)的活化需要较高的能垒。对此,研究人员作出了许多改进以提高CO_(2)还原性能。例如,发展S型异质结可以增强载流子的分离和光催化剂的氧化还原能力,引入金属单原子助催化剂可以优化反应热力学。因此,协同利用S型异质结和金属单原子修饰将能同时促进载流子的转移和CO_(2)还原反应过程。本文构建了由单原子Pt负载的g-C_(3)N_(4)和BiOCl组成的Pt-C_(3)N_(4)/BiOCl异质结模型。用密度泛函理论计算研究了其光催化性能,包括几何结构和电子性质的探索、CO_(2)转化过程的模拟。差分电荷密度结果表明g-C_(3)N_(4)中的电子转移至BiOCl,这是由于g-C_(3)N_(4)的费米能级比BiOCl的费米能级高。由此在g-C_(3)N_(4)/BiOCl异质结的界面处形成了由g-C_(3)N_(4)指向BiOCl的内建电场。在光照下,g-C_(3)N_(4)/BiOCl复合物中载流子的转移路径符合S型机制。具体而言,BiOCl导带的光生电子与g-C_(3)N_(4)价带的光生空穴复合,而g-C_(3)N_(4)导带的电子与BiOCl价带的空穴得以保留。在g-C_(3)N_(4)的空隙中添加Pt原子后,g-C_(3)N_(4)的功函数减小,由此增大了g-C_(3)N_(4)和BiOCl的费米能级差异。结果,有更多的电子从Pt-C_(3)N_(4)转移至BiOCl,内建电场的强度增大。这有利于Pt-C_(3)N_(4)/BiOCl S型异质结的电荷转移。此外,反应自由能计算结果表明,g-C_(3)N_(4)/BiOCl异质结上CO_(2)还原反应的限速步骤是CO_(2)氢化生成COOH,其能垒为1.13 eV。Pt原子修饰后,限速步骤变为CO氢化生成HCO,其能垒为0.71 eV。这些结果表明Pt单原子的引入能够增强界面电场、降低能垒,从而提高CO_(2)还原活性。本工作为构建金属原子修饰的S型异质结光催化剂以实现高效的CO_(2)还原提供了理论指导。

Photocatalytic CO_(2) reduction to renewable hydrocarbon fuels provides a feasible protocol for alleviating the greenhouse effect and addressing energy shortage.However,the CO_(2) reduction activity of a single-component photocatalyst is very low because of two problems.One is the fast recombination of photogenerated charge carriers,which leads to low photon efficiency,while the other is the large energy barrier to CO_(2) activation.There have been considerable research efforts to develop photocatalysts with improved CO_(2) reduction performance.For example,step-scheme(Sscheme)heterojunctions have been developed to improve charge carrier separation and enhance the redox abilities of photocatalysts.Single-atom metals have also been applied cocatalysts to optimize the reaction thermodynamics.Thus,the synergy between Sscheme heterojunctions and single-atom metal cocatalysts is anticipated to promote both charge carrier transfer and CO_(2) reduction reaction processes.In this study,a Pt-C_(3)N_(4)/BiOCl heterojunction photocatalyst is modeled,composed of singleatom Pt-loaded g-C_(3)N_(4) and BiOCl,and its photocatalytic properties are studied using density functional theory calculations.Its structure and electronic property are explored,and the process of CO_(2) conversion is also simulated.The charge density difference results show that electrons in g-C_(3)N_(4) are transferred to BiOCl owing to the higher Fermi level of g-C_(3)N_(4) than that of BiOCl.Therefore,an interfacial electric field from g-C_(3)N_(4) to BiOCl is established at the g-C_(3)N_(4)/BiOCl interface.Under light irradiation,charge carrier transfer in the g-C_(3)N_(4)/BiOCl composite is consistent with the S-scheme mechanism.Specifically,the photogenerated electrons in the CB of BiOCl recombine with the photogenerated holes in the VB of g-C_(3)N_(4),while the photogenerated electrons in the CB of g-C_(3)N_(4) and the photogenerated holes in the VB of BiOCl are retained.After the loading of Pt atom at each sixfold cavity of g-C_(3)N_(4),the work function of g-C_(3)N_(4) decreases,thereby enlarging the difference between the Fermi levels of the two semiconductors.Consequently,more electrons are transferred from Pt-C_(3)N_(4) to BiOCl,and the strength of the interfacial electric field is increased.This enhanced electric field is beneficial to the S-scheme charge transfer in Pt-C_(3)N_(4)/BiOCl heterojunctions.Besides,based on the calculated variation in reaction energy,the rate-limiting step involved in CO_(2) reduction on g-C_(3)N_(4)/BiOCl heterojunction is the hydrogenation of CO_(2) to COOH,which has an energy barrier of 1.13 eV.After Pt loading,the hydrogenation of CO to HCO is the rate-limiting step and the corresponding energy increase is 0.71 eV.These results manifest that the introduction of Pt single-atom cocatalysts improves the CO_(2) reduction performance of g-C_(3)N_(4)/BiOCl S-scheme photocatalysts by strengthening the interfacial electric field and reducing the energy barrier.This study provides guidance for constructing metal-atom-incorporated S-scheme heterojunction photocatalysts to realize efficient CO_(2) reduction.