Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (1): 123-128

The article information

Xiao-jiao Deng, Xiang-yu Kong, Xi-ling Xu, Hong-guang Xu, Wei-jun Zheng
Photoelectron Spectroscopy and Density Functional Calculations of TiGen- (n=7-12) Clusters
Chinese Journal of Chemical Physics, 2016, 29(1): 123-128
http://dx.doi.org/10.1063/1674-0068/29/cjcp1511232

Article history

Received on November 15, 2015
Accepted on December 29, 2015
Photoelectron Spectroscopy and Density Functional Calculations of TiGen- (n=7-12) Clusters
Xiao-jiao Deng, Xiang-yu Kong, Xi-ling Xu, Hong-guang Xu , Wei-jun Zheng     
Dated: Received on November 15, 2015; Accepted on December 29, 2015
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Part of the special issue for "the Chinese Chemical Society's 14thNational Chemical Dynamics Symposium".
Author: Hong-guang Xu, E-mail: xuhong@iccas.ac.cn
Wei-jun Zheng, E-mail: zhengwj@iccas.ac.cn, Tel.: +86-10-62635054,FAX: +86-10-62563167
Abstract: The growth pattern and electronic properties of TiGen- (n=7-12) clusters were investigated using anion photoelectron spectroscopy and density functional theory calculations. For both anionic and neutral TiGen clusters, a half-encapsulated boat-shaped structure appears at n=8, and the boat-shaped structure is gradually covered by the additional Ge atoms to form Gen cage at n=9-11. TiGe12- cluster has a distorted hexagonal prism cage structure. According to the natural population analysis, the electron transfers from the Gen framework to the Ti atom for TiGen-/0 clusters at n=8-12, implying that the electron transfer pattern is related to the structural evolution.
Key words: Photoelectron spectroscopy    Density functional theory    Germanium clusters    
Ⅰ. INTRODUCTION

Experimental and theoretical studies suggested that the doping of transition metals (TMs) can stabilize the cage structures of germanium-based clusters and tailor their properties [1, 2, 3, 4, 5]. Ti-doped germanium clusters have attracted much attention because they may be used to produce cluster-assembled materials of special electronic and magnetic properties [6]. It has been proposed that Ti-Ge binary alloys may be developed for dental materials [7]. It has also been found that strain-released hybrid multiplayer Ge-Ti nanomembranes can form anode materials with both high conductivity and high storage capacity,therefore,enhance the performance of lithium batteries [8]. Investigating the structural and electronic properties of Ti-doped germanium clusters may provide valuable information for developing cluster-assembled materials as well as their applications in electronics,biomedicine,and energy storage. Kumar and Kawazoe conducted theoretical calculations on a number of TM-doped germanium clusters and predicted TiGe16 cluster to be a Frank-Kasper polyhedron structure with a large gap between its highest-occupied-orbital (HOMO) and lowest-unoccupied-orbital (LUMO) [9]. In addition to the TiGe16 cluster,the density functional theory (DFT) calculations of Bandyopadhyay et al. on TiGen (n=14-20) suggested that TiGe18 cluster also has enhanced stability [10]. The ground state structure of TiGe12 cluster is proposed to be a remarkably stable pseudoicosahedron by Tang et al. [11]. More recently,the DFT calculations of TiGen (n=1-20) clusters of Kumar et al. suggested that the Ti atom is encapsulated in the Gen cage when n is larger than 9 and the most stable structure of TiGe12 cluster is a relaxed hexagon [12]. Compared with the theoretical studies of TiGen clusters,the experimental investigation of TiGen clusters is quite rare except that Nakajima and co-workers investigated TiGen clusters using mass spectrometry and anion photoelectron spectroscopy,and probed their stability via their reactivity to H2O adsorption [2, 13].

Previously,we have investigated the structural,electronic and magnetic properties of small size TiGen- (n=2-6) clusters using anion photoelectron spectroscopy and DFT calculations [14]. The results displayed that the most stable structures of these small clusters can be considered as a Ti atom substituting one of the Ge atoms in the corresponding Gen+1 cluster or a Ti atom capping a Gen cluster,and the Ti atom is inclined to interact with more Ge atoms. In order to get more information regarding the structural evolution and electronic properties of larger TiGen clusters,in this work,we investigated the TiGen- (n=7-12) clusters using mass-selected anion photoelectron spectroscopy experiments combined with DFT calculations.

Ⅱ. EXPERIMENTAL AND THEORETICAL METHODS

The experiments were conducted on a home-built apparatus equipped with a laser vaporization supersonic cluster source,a time-of-flight mass spectrometer,and a magnetic-bottle photoelectron spectrometer,which has been described elsewhere [15]. The TiGen- (n=7-12) cluster anions were generated in the laser vaporization source by laser ablation of a rotating translating disk target (13 mm diameter,Ti:Ge mole ratio 1:4) with the second harmonic of a nanosecond Nd:YAG laser (Continuum Surelite Ⅱ-10). The typical laser power used in this work is about 10 mJ/pulse. Helium gas with ~4 atm backing pressure was allowed to expand through a pulsed valve (General Valve Series 9) into the source to cool the formed clusters. The generated cluster anions were mass-analyzed with the time-of-flight mass spectrometer. The cluster anions of interest were selected with a mass gate,decelerated by a momentum decelerator,and crossed with the beam of another Nd:YAG laser (Continuum Surelite Ⅱ-10,266 nm) at the photodetachment region. The electrons from photodetachment were energy-analyzed by the magnetic-bottle photoelectron spectrometer. The photoelectron spectra were calibrated with the spectra of Cu- and Pb- taken at similar conditions. The resolution of the magnetic-bottle photoelectron spectrometer was about 40 meV at electron kinetic energy of 1 eV.

For the TiGen-/0 clusters,geometry optimizations were performed using DFT with Becke's three-parameter and Lee-Yang-Parr's gradient-corrected correlation hybrid functional (B3LYP) [16, 17, 18] and 6-311+G(d) basis sets as implemented in the Gaussian 03 program package [19]. For all clusters,a large amount of initial structures were taken into accounts at all possible spin states. These initial structures were constructed by Ti-capping or Ti-substituting of pure Gen clusters or based on the structures of TM-doped Gen clusters reported in the literature [12, 20, 21, 22, 23, 24, 25, 26, 27]. All geometries were optimized without any symmetry constraint. Harmonic vibrational frequencies were calculated to make sure that the structures correspond to real local minima,and the zero-point vibrational energy corrections were included for the relative energies of isomers. The natural population analysis (NPA) of TiGen-/0 (n=7-12) clusters were conducted with the nature bond orbital (NBO) version 3.1 program [28, 29, 30, 31, 32, 33, 34] implemented in the Gaussian 03 program package.

Ⅲ. RESULTS A. Experimental results

The photoelectron spectra of the TiGen- (n=7-12) clusters taken with 266 nm photons are shown in Fig. 1,and the vertical detachment energies (VDEs) and adiabatic detachment energies (ADEs) of these clusters estimated from photoelectron spectra are listed in Table Ⅰ. The VDEs were estimated from the maxima of the first peaks. The ADE of each cluster was determined by drawing a straight line along the leading edge of the first peak to cross the baseline of spectrum and adding the instrument resolution to electron binding energy (EBE) value at the crossing point. Our spectra are in agreement with previous measurement of TiGen- (n=7-17) reported by Nakijima and coworkers [2] except that our experimental spectra show better resolution.

FIG. 1 Photoelectron spectra of TiGen- (n=7-12) clusters recorded with 266 nm photons.
TABLE Ⅰ Experimentally observed VDEs and ADEs from the photoelectron spectra of TiGen- (n=7-12).

As shown in Fig. 1,there are four resolved peaks centered at 2.97,3.47,3.84,and 4.04 eV,and an unresolved peak over 4.27 eV in the spectrum of TiGe7-. With respect to the spectrum of TiGe8-,four peaks centered at 3.32,3.70,3.91,and 4.27 eV and a shoulder at \mbox{3.53 eV} can be observed. As for TiGe9-,it has two major peaks centered at 3.15 and 3.91 eV,as well as two barely noticeable peaks at 3.40 and 4.37 eV. TiGe10- has a major peak centered at 3.35 eV,a shoulder at 3.12 eV,a broad peak at 3.96 eV,and the onset of a high EBE peak above 4.3 eV. The spectral features of TiGe11- are very broad with three barely discernible peaks centered at 3.5,3.7,and 3.9 eV respectively. The spectral features of TiGe12- are also very broad. It has a major peak centered at ~3.8 eV and a shoulder at ~3.5 eV.

B. Theoretical results

The typical low-lying isomers for the TiGen- (n=7-12) clusters obtained from the DFT calculations are presented in Fig. 2 with the most stable ones on the left. Their theoretical VDEs and ADEs are summarized in Table Ⅱ. The VDE is defined as the energy difference between the neutral and anion both at the equilibrium structure of the anion,whereas the ADE is the energy difference between the neutral and the anion with the neutral relaxed to the nearest local minimum using the anionic structure as initial structure. The Cartesian coordinates of the low-lying isomers are available in the supplementary material.

FIG. 2 Low-lying isomers of TiGen- (n=7-12) clusters. The relative energies to the most stable isomers are shown.
TABLE Ⅱ Relative energies,VDEs and ADEs of the low energy isomers of the TiGen- (n=7-12) obtained by DFT calculations. Multiplicities are 2.

With respect to TiGe7-,isomer 7A can be regarded as a Ge atom capping the TiGe6 pentagonal bipyramid with the Ti atom at the vertex,and its calculated VDE (2.70 eV) is in reasonable agreement with the experimental value (2.97 eV). Isomers 7B and 7C are much less stable than 7A by 0.50 and 0.64 eV in energy,respectively. Thus,we suggest that 7A is the dominant structure observed in our experiments.

The most stable isomer (8A) of TiGe8- can be viewed as a half-endohedral structure with the Ti atom locating in a boat-shaped Ge8 framework,and its theoretical VDEs of 8A (3.15 eV) is in agreement with the experimental value (3.32 eV). Isomer 8B can be seen as two Ge atoms capping on a TiGe6 pentagonal bipyramid. The energies of 8B and 8C are much higher than that of 8A by 0.57 and 0.66 eV,respectively. Therefore,we suggest that 8A is the major structure contributed to the experimental spectrum.

For TiGe9-,the most stable isomer 9A can be viewed as a Ge atom capping the boat-shaped structure of TiGe8- (8A). Isomer 9B can be obtained by connecting a Ge3 triangular to a TiGe6 pentagonal bipyramid,and its energy is higher than 9A by 0.21 eV. Isomer 9C is less stable than isomer 9A by 0.50 eV,thus the existence of 9C can be ruled out. The theoretical VDEs of isomers 9A and 9B are 2.97 and 3.31 eV,respectively,both close to the experimental value (3.15 eV). Therefore,isomer 9A is suggested to be the most probable one detected in our experiments,but the existence of isomer 9B cannot be ruled out.

As for TiGe10-,the structures of isomers 10A and 10B can be described as two Ge atoms connecting to the different locations of the boat-shaped structure of TiGe8- (8A),and isomer 10B is higher than isomer 10A in energy by only 0.15 eV. The calculated VDE of isomer 10A (3.20 eV) is in good agreement with the experimental value (3.12 eV). That of isomer 10B is calculated to be 3.38 eV. Isomer 10C is much less stable than 10A by 0.56 eV in energy. Thus,we suggest that isomer 10A is the major species in our experiments and isomer 10B may contribute to the higher EBE peaks in the experimental spectrum.

For TiGe11-,both isomers 11A and 11B can be regarded as three Ge atoms connecting to the top of the boat-shaped structure of TiGe8- (8A). The calculated VDE of 11A (3.80 eV) is in reasonable agreement with the experimental value (3.5 eV). Isomer 11B is higher in energy than isomer 11A by 0.25 eV and its theoretical VDE (3.04 eV) is much lower than the experimental value. Isomer 11C is less stable than 11A by 0.32 eV. Therefore,isomer 11A is the most likely structure of TiGe11- in our experiments.

The most stable structure of TiGe12- (12A) is a distorted hexagonal prism structure with the Ti atom at the center. Isomer 12B can be seen as four Ge atoms connecting to the top of boat-shaped structure of TiGe8- (8A). The calculated VDEs of 12A (\mbox{3.43 eV}),12B (3.40 eV) and 12C (3.40 eV) are all consistent with the experimental value (3.5 eV). Isomer 12B is higher than that isomer 12A by only 0.10 eV while isomer 12C is higher than 12A by 0.24 eV. Therefore,we suggest that 12A is the dominant structure observed in our experiments,but the existence of 12B cannot be ruled out.

We have also investigated the structures of neutral TiGen (n=7-12) clusters and displayed them in Fig. 3. It can be seen that the most stable structures of the neutral TiGen (n=7-12) clusters are similar to those of the TiGen- anions,except that those of TiGe11 and TiGe12 are different from their anionic counterparts. The most stable structure of neutral TiGe11 cluster (11A') is similar to the second stable structure of TiGe11- anion (11B). For neutral TiGe12,the most stable structure (12A') is similar to the second stable structure of TiGe12- anion (12B),while the second stable structure (12B') is similar to the most stable one of TiGe12- anion (12A). Isomer 12B' is higher in energy than 12A' by only 0.08 eV. The most stable structures of the neutral TiGen (n=7-12) clusters found in this work are slightly different from those reported in Ref.[12]. In Ref.[12],the ground state structure of TiGe8 can be seen as a Ge atom capping a TiGe7 hexagonal bipyramid,which is different from the half-encapsulated boat-shaped structure in this work. We found that the most stable structures of TiGen (n=9-12) clusters are all derived from the boat-shaped structure of TiGe8. In Ref.[12],the ground structures of TiGe9 and TiGe10 can also be seen as deriving from the boat-shaped structure of TiGe8,but they are slightly distorted compared with the most stable structures in this work. While the ground structure of TiGe11 in Ref.[12] can be described as a basket-shaped structure. Also,the ground state structure of neutral TiGe12 reported previously is a distorted hexagonal prism,which is similar to the most stable structure of anionic TiGe12- (12A) and the second stable structure of neutral TiGe12 (12B') in this work. Such different results are probably because we used a larger basis set in this work.

FIG. 3 Low-lying isomers of neutral TiGen (n=7-12) clusters. The relative energies to the most stable isomers are shown.
Ⅳ. DISCUSSION

As shown in Fig. 3,the Ti atom is half-encapsulated by a boat-shaped Ge8 framework at n=8,and the opening of the boat-shaped structure is gradually covered by the additional Ge atoms to form Gen cage from n=9 to 11. At n=8-12,the endohedral structures of both anionic and neutral TiGen clusters are more stable than their exohedral structures. For anionic and neutral TiGe12 cluster,the Ti atom is completely encapsulated by the Ge12 cage,in which the anionic TiGe12 has a distorted hexagonal prism cage structure while the neutral TiGe12 has a Ge4 capped boat-shaped structure. The structural evolution found in this work agrees with the adsorption reactivity of neutral TiGen (n=7-16) towards H2O [2],in which the reactivity generally decreases with an increasing number of Ge atoms from n=7-11,and the relative reactivity is lost at n=12.

We also investigated the electronic and magnetic properties of the most stable structures of anionic and neutral TiGen (n=7-12) clusters using NPA. As shown in Table Ⅲ,for both anionic and neutral TiGen clusters size with n=8-12,the negative charge on Ti atom increases significantly,suggesting that there is more electron transfer from the Ge atoms to the Ti atom. This also indicates that the electron transfer from the Gen framework to the Ti atom is strongly related to the structural evolution of TiGen clusters,especially related to the formation of endohedral structures. In addition,this electron transfer pattern is similar to those of CoGen-/0 (n=2-11) [26] and VGen-/0 (n=3-12) [27] clusters,but different from those of the CuGen (n=2-13) [20],and NiGen (n=1-20) [32] clusters,in which the electron transfers from the Cu or Ni atom to the Gen framework.As shown in Table Ⅲ,the total magnetic moments of the anion TiGen- (n=7-12) clusters are 1 μB. As for the neutral TiGen clusters,our calculations show that the multiplicities of the most stable isomers of TiGe7 to TiGe10 are 3,while those of TiGe11 and TiGe12 are 1; thus,the total magnetic moments are 2 μB for sizes of n=7-10,and 0 μB for n=11 and 12. The magnetic moment of TiGe11 and TiGe12 quench is possibly because there are more electrons transfer from the Ge atoms to the Ti atom and stronger connection between the Ge atoms and Ti atom. For both anionic and neutral TiGen (n=7-12) clusters,the total magnetic moments are contributed by both Ti and Ge atoms.

TABLE Ⅲ NPA charge of the Ti atom,atomic magnetic moments of the Ti atom (μTi),total magnetic moments (μt) in the most stable isomers of the anionic and neutral TiGen (n=7-12) clusters.
Ⅴ. CONCLUSION

The structural,electronic and magnetic properties of anionic and neutral TiGen (n=7-12) clusters were investigated using anion photoelectron spectroscopy in combination with density functional theory calculations. A half-endohedral boat-shaped structure emerged at n=8,then the opening of the boat-shaped structure is gradually capped by the additional Ge atoms to form Gen cage at n=9-11. TiGe12- cluster has a distorted hexagonal prism cage structure. The electron transfer from the Gen cage to the Ti atom increases at n=8-12,suggesting that the electron transfer pattern is related to the structural evolution,especially to the formation of endohedral structure. For both anionic and neutral TiGen (n=7-12) clusters,the total magnetic moments are contributed by both Ti and Ge atoms.

Supplementary materials: The Cartesian coordinates of the low-lying isomers are available.

Ⅵ. ACKNOWLEDGMENTS

Wei-jun Zheng acknowledges the Knowledge Innovation Program of the Chinese Academy of Sciences (No.KJCX2-EW-H01) and Hong-guang Xu acknowledges the National Natural Science Foundation of China (No.21103202) for financial support. The theoretical calculations were conducted on the ScGrid and DeepComp 7000 of the Supercomputing Center,Computer Network Information Center of the Chinese Academy of Sciences.

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