Facet-Dependent Gas Adsorption Selectivity on ZnO: A DFT Study

Abstract

Semiconductor-based gas sensors are of great interest in both industrial and research settings, but poor selectivity has hindered their further development. Current efforts including doping, surface modifications and facet controlling have been proved effective. However, the “methods-selectivity” correlation is ambiguous because of uncontrollable defects and surface states during the experiments. Here, as a case study, using a DFT method, we studied the adsorption features of commonly tested gases—CH2O, H2, C2H5OH, CH3COCH3, and NH3—on facets of ZnO(0001¯), ZnO(101¯0) and ZnO(101¯1). The adsorption energies and charge transfers were calculated, and adsorption selectivity was analyzed. The results show ZnO(0001¯) has obvious CH2O adsorption selectivity; ZnO(101¯0) has a slight selectivity to C2H5OH and NH3; and ZnO(101¯1) has a slight selectivity to H2, which agrees with the experimental results. The mechanism of the selective adsorption features was studied in terms of polarity, geometric matching and electronic structure matching. The results show the adsorption selectivity is attributed to a joint effort of electronic structure matching and geometric matching: the former allows for specific gas/slab interactions, the latter decides the strength of the interactions. As the sensing mechanism is probably dominated by gas–lattice interactions, this work is envisioned to be helpful in designing new sensing material with high selectivity.

Theoretical Study on Copper Adsorption on ZnO Surfaces

“…For the study of Cu adsorption on ZnO surfaces, we employed slab model with 4 × 4 surface unit cell consisting of 7 layers and a vacuum layer of 15 Å separating the slabs. This vacuum thickness was chosen based on previous studies of similar systems [42][43][44][45] where negligible phonon-phonon interactions were observed. The top four layers of ZnO bilayers and the adatom H and Cu are allowed to relax while the bottom three bilayers of ZnO are fixed at their bulk positions to imitate the bulk substrate.…”

“…Depositing ZnO on Cu surfaces can significantly improve catalytic activity. For example, studies show that ZnO on Cu(111) increases methanol synthesis reaction rates compared to Cu(110) due to the higher concentration of corner and edge atoms on Cu(111), these sites offer more reactive locations for species adsorption [45][46][47][48][49][50]74]. The adsorption calculations below explore the most stable adsorption sites for Cu on various ZnO surfaces (Zn-and O-terminated).…”

“…In the calculations below, the optimized structures with obtained lattice constants were used for both zinc blend and wurtzite ZnO. Previous studies found that ZnO prevents agglomeration of Cu particles, that leads to the large Cu surface area needed for industrial applications [22,[41][42][43][44][45][46]. Therefore, we examined the stability of systems with atomic Cu atom adsorbed on different sites (Hollow, Bridge, Top of Zn, and Top of O) for both Oterminated and Zn-terminated ZnO(0001) and ZnO(111) as shown in table 2.…”

Back to the Basics: Probing the Role of Surfaces in the Experimentally Observed Morphological Evolution of ZnO

“…Combining density functional theory (DFT) with ab initio atomistic thermodynamics can be a good strategy to overcome such experimental drawbacks and allow the investigation of exposed surfaces and morphological evolution of different materials. This knowledge is also key to discovering and controlling the properties of materials with tunable multifunctionalities and more [ 1 , 2 , 3 , 4 , 5 , 6 ].…”

“…In addition to SnO 2 and TiO 2 , ZnO has been investigated and extensively used in a variety of technological applications. Due to its high activity, environment-friendly nature, and low cost, it has a wide range of properties that can be applied in optics, electronics, catalysis, and gas sensing [ 3 , 7 , 8 , 9 ]. These multifunctional properties of ZnO are known to be strongly dependent on its morphology.…”