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We report a systematic theoretical study on the growth pattern and electronic properties of Zn12O12-assembled material using density-functional theory within a generalized gradient approximation. Our results show that assembly can form by attaching a Zn12O12 cage on a hexagonal site. A Zn12O12 cage should combine with eight hexagons in adjacent eight Zn12O12 cages, respectively, forming more stable assemblies. As the assembly process continues, we find that the Zn12O12 cages form a new three-dimensional nanoporous ZnO phase with a rhombohedral lattice framework. The Zn12O12 cage structure in the phase is preserved, and the Zn−O bond lengths between Zn12O12 monomers are slightly larger than those in the isolated Zn12O12 cage and the bulk wurtzite ZnO phase. The band analysis reveals that this new phase is a semiconductor with large gap value. Because of the nanoporous character of this new phase, it could be used for heterogeneous catalysis, molecular transport, and so on.

aMotivated by the recent realization of two-dimensional (2D) nanomaterials as gas sensors, we have investigated the adsorption of gas molecules (SO 2 , NO 2 , HCN, NH 3 , H 2 S, CO, NO, O 2 , H 2 , CO 2 , and H 2 O) on the graphitic GaN sheet (PL-GaN) using density functional theory calculations. It is found that among these gases, only SO 2 and NH 3 gas molecules are chemisorbed on the PL-GaN sheet with apparent charge transfer and reasonable adsorption energies. The electronic properties (especially the electric conductivity) of the PL-GaN sheet showed dramatic changes after the adsorption of NH 3 and SO 2 molecules. However, the strong adsorption of SO 2 on the PL-GaN sheet makes desorption difficult, which precludes its application to SO 2 sensors. Therefore, the PL-GaN sheet should be a highly sensitive and selective NH 3 sensor with short recovery time. Furthermore, the adsorption of NO (or NO 2 ) molecules introduces spin polarization in the PL-GaN sheet with a magnetic moment of about 1 m B , indicating that magnetic properties of the PL-GaN sheet are changed obviously. Based on the change of magnetic properties of the PL-GaN sheet before and after molecule adsorption, the PL-GaN sheet could be used as a highly selective magnetic gas sensor for NO and NO 2 detection.

Ag−Au bimetallic clusters have demonstrated extreme sensitivity, which can be theoretically explained by the conductivity change of the clusters induced by the absorption process, to molecules such as CO, H 2 S, and so forth. Recently, a 13-atom alloy quantum cluster (Ag 7 Au 6 ) has been experimentally synthesized and characterized. Here, the adsorption of CO, HCN, and NO on the Ag 7 Au 6 cluster was investigated using density functional theory calculations in terms of geometric, energetic, and electronic properties to exploit its potential applications as gas sensors. It is found that the CO, HCN, and NO molecules can be chemisorbed on the Ag 7 Au 6 cluster with exothermic adsorption energy (−0.474 ∼ −1.039 eV) and can lead to finite charge transfer. The electronic properties of the Ag 7 Au 6 cluster present dramatic changes after the adsorption of the CO, HCN, and NO molecules, especially its electric conductivity. Thus, the Ag 7 Au 6 cluster is expected to be a promising gas sensor for CO, HCN, and NO detection. ■ INTRODUCTIONThe environmental gas monitoring and controlling is now recognized as an important issue for our safety and health. Much research has been focused on the development of suitable gas-sensitive materials for continuous monitoring and setting off alarms for hazardous chemical vapors present beyond specified levels. 1−4 Chemical gases such as CO, HCN, and NO are highly toxic to human beings and animals as they inhibit the consumption of oxygen by body tissues. They are colorless, odorless, and tasteless, and thus human beings do not have timely alertness to their presence. For example, exposure levels of 100 ppm of HCN which would result in death are about 1 h or less in some cases, while exposure levels of 500 ppm of HCN are within 15 min. 5 Higher concentration levels will result in faster onset of symptoms or death. Therefore, effective methods for sensing these three toxic gases are highly desired.Bimetallic nanoclusters (also called "nanoalloys" 6 ) are of great interest from both the fundamental and the technological points of view not only because of their new degrees of freedom for understanding their electronic and geometric properties of clusters but also because of their potential applications in catalysis, 7−11 optics, 12−14 nanoelectronics, 15 and sensing. 16 In particular, silver−gold (Ag−Au) clusters have been investigated extensively from a computational point of view 17−27 as well as experimentally. 28−37 Their unique physicochemical properties depend on the shape and structure of the clusters, the surface segregation of the clusters, and the alloying extent or atomic distribution in nanoclusters. Recently, Ag−Au clusters are investigated as potential catalysts or sensors for removal and detection of toxic molecules. In this regard, adsorption of toxic molecules on Ag−Au clusters has attracted several theoretical and experimental studies. 38−45 Experimental investigations on CO adsorption on Au n Ag m (n = 10−45, m = 0, 1, 2) clusters at 140 K indicate that the CO molecule a...

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The advances in cluster-assembled materials where clusters serve as building blocks have opened new opportunities to develop ever more sensitive gas sensors. Here, using density functional theory calculations, the structural and electronic properties of cluster-assembled nanowires based on M12N12 (M = Al and Ga) clusters and their application as gas sensors have been investigated. Our results show that the nanowires can be produced via the coalescence of stable M12N12 fullerene-like clusters. The M12N12-based nanowires have semiconducting electrical properties with direct energy gaps, and are particularly stable at room temperature for long enough to allow for their characterization and applications. Furthermore, we found that the CO, NO, and NO2 molecules are chemisorbed on the M12N12-based nanowires with reasonable adsorption energies and apparent charge transfer. The electronic properties of the M12N12-based nanowires present dramatic changes after the adsorption of the CO, NO, and NO2 molecules, especially their electric conductivity. However, the adsorption of NO2 on the Al12N12-based nanowire is too strong, indicating an impractical recovery time as NO2 sensors. In addition to this, due to reasonable adsorption energies, apparent charge transfer, change in the electric conductivity, and the short recovery time, the Al12N12-based nanowire should be a good CO and NO sensor with quick response as well as short recovery time, while the Ga12N12-based nanowire should be a promising gas sensor for CO, NO, and NO2 detection.

We report the results of density functional theory calculations on cluster-assembled materials based on M(12)N(12) (M = Al, Ga) fullerene-like clusters. Our results show that the M(12)N(12) fullerene-like structure with six isolated four-membered rings (4NRs) and eight six-membered rings (6NRs) has a T(h) symmetry and a large HOMO-LUMO gap, indicating that the M(12)N(12) cluster would be ideal building blocks for the synthesis of cluster-assembled materials. Via the coalescence of M(12)N(12) building blocks, we find that the M(12)N(12) clusters can bind into stable assemblies by either 6NR or 4NR face coalescence, which enables the construction of rhombohedral or cubic nanoporous framework of varying porosity. The rhombohedral-MN phase is energetically more favorable than the cubic-MN phase. The M(12)N(12) fullerene-like structures in both phases are maintained and the M-N bond lengths between M(12)N(12) monomers are slightly larger than that in isolated M(12)N(12) clusters and the bulk wurtzite phases. The band analysis of both phases reveals that they are all wide-gap semiconductors. Because of the nanoporous character of these phases, they could be used for gas storage, heterogeneous catalysis, filtration and so on.

Properties of gas molecules (NO, NH 3 , and NO 2 ) adsorbed on two-dimensional GaN with a tetragonal structure (T-GaN) are studied using first-principles methods. Adsorption energy, adsorption distance, Hirshfeld charge, electronic properties, electric conductivity, and recovery time are calculated. It is found that these three molecules are all chemisorbed on the T-GaN with reasonable adsorption energies and apparent charge transfer. The electronic properties of the T-GaN present dramatic changes after the adsorption of NO 2 and NO molecules, especially its electric conductivity, but NH 3 molecule hardly changes the electronic properties of the T-GaN. Furthermore, the recovery time of the T-GaN sensor at T = 300 K is estimated to be quite short for NO 2 and NO but very long for NH 3 . Moreover, the magnetic properties of the T-GaN are changed obviously due to the adsorption of NO (or NO 2 ) molecule. Therefore, we suggest that the T-GaN can be a prominent candidate for application as NO 2 and NO molecule sensors.