Density functional theory (DFT) in connection with ultrasoft pseudopotential (USP) and generalized gradient spin-polarized approximations (GGSA) is applied to calculate the adsorption energies and structures of monolayer-adsorbed InN on the TiO(2) anatase (101) surface and the corresponding electronic properties, that is, partial density of states (PDOS) for surface and bulk layers of the TiO(2) anatase (101) surface and monolayer-adsorbed InN, to shed light on the possible structural modes for initial photoexcitation within the UV/vis adsorption region followed by fast electron injection through the InN/TiO(2) interface for an InN/TiO(2)-based solar cell design. Our calculated adsorption energies found that the two most probable stable structural modes of monolayer-adsorbed InN on the TiO(2) anatase (101) surface are (1) an end-on structure with an adsorption energy of 2.52 eV through N binding to surface 2-fold coordinated O (O(cn2)), that is, InN-O(cn2), and (2) a side-on structure with an adsorption energy of 3.05 eV through both N binding to surface 5-fold coordinated Ti (Ti(cn5)) and In bridging two surface O(cn2), that is, (O(cn2))(2)-InN-Ti(cn5). Our calculated band gaps for both InN-O(cn2) and (O(cn2))2-InN-Ti(cn5) (including a 1.0-eV correction using a scissor operator) of monolayer-adsorbed InN on the TiO(2) anatase (101) surface are red-shifted to 1.7 eV (730 nm) and 2.3 eV (540 nm), respectively, which are within the UV/vis adsorption region similar to Gratzel's black dye solar cell. Our analyses of calculated PDOS for both surface and bulk layers of the TiO(2) anatase (101) surface and monolayer-adsorbed InN on the TiO(2) anatase (101) surface suggest that the (O(cn2))(2)-InN-Ti(n5) configuration of monolayer-adsorbed InN on the TiO(2) anatase (101) surface would provide a more feasible structural mode for the electron injection through the InN/TiO(2) interface. This is due to the presence of both occupied and unoccupied electronic states for monolayer-adsorbed InN within the band gap TiO(2) anatase (101) surface, which will allow the photoexcitation within the UV/vis adsorption region to take place effectively, and subsequently the photoexcited electronic states will overlap with the unoccupied electronic states around the lowest conduction band of the TiO(2) anatase (101) surface, which will ensure the electron injection through the InN/TiO(2) interface. Finally, another thing worth our attention is our preliminary study of double-layer-adsorbed InN on the TiO(2) anatase (101) surface, that is, (O(cn2))(2)-(InN)(2)-Ti(cn5), with a calculated band gap red-shifted to 2.6 eV (477 nm) and a different overlap of electronic states between double-layer-adsorbed InN and the TiO(2) anatase (101) surface qualitatively indicated that there is an effect of the thickness of adsorbed InN on the TiO(2) anatase (101) surface on both photoexcitation and electron injection processes involved in the photoinduced interfacial electron transfer through InN/TiO(2). A more thorough and comprehensive study of different la...

Ab initio molecular dynamics simulations accompanied by a Fourier transform of the dipole moment (aligned perpendicular to the surface) autocorrelation function are implemented to investigate the temperature-dependent infrared (IR) active vibrational modes of CH3C(β)C(α)(ads) and I(ads) when coadsorbed on an Ag(111) surface at 200 and 400 K, respectively. The analytic scheme of the Fourier transform of a structural coordinate autocorrelation function is used to identify two distinguishable IR active peaks of C(β)C(α) stretching, which are characterized by two types of dynamic motion of adsorbed CH3C(β)C(α)(ads) at 200 K, namely, the motion of the tilted CC(β)C(α) axis and the motion of the stand-up CC(β)C(α) axis. These two recognisable IR active peaks of C(β)C(α) stretching are gradually merged into one peak as a result of the dominant motion of the stand-up CC(β)C(α) axis as the temperature increases from 200 to 400 K. The calculated intensities of the IR active peaks of the asymmetrical deformation mode of CH3 and the asymmetrical stretching mode of CH3, with their dynamic dipole moments nearly perpendicular to the CC(β)C(α) axis, become relatively weak; however, the symmetrical deformation mode of CH3 and the symmetrical stretching mode of CH3, with their dynamic dipole moments randomly directed away from the CC(β)C(α) axis, will not have direct correspondence between the intensities of their IR active peaks and the angle between the Ag(111) surface and the CC(β)C(α) axis as the temperature increases from 200 to 400 K. Finally, the increased flipping from the motion of the tilted CC(β)C(α) axis to the motion of the stand-up CC(β)C(α) axis followed by its diffusion, resulting from the increasing temperature from 200 to 400 K or even higher, seems to be the initial event that initiates the alkyne self-coupling reaction that leads to the final production of H3CCCCCCH3.

Total energy calculations based on (1) density functional theory (DFT) in connection with ultrasoft pseudopotential and generalized gradient spin-polarized approximation (GGSA) and (2) the partial structural constraint path minimization (PSCPM) method have been used to investigate the energetically more favorable pathway for methylene ( CH 2) insertion into the Ag–CF 3 bond followed by β-fluoride elimination to generate an isolated CH 2= CF 2( g ) above the Ag(111) surface. The diffusion of the fcc-hollow site of CF 3( ads ) toward the bridge site of CH 2( ads ) is proposed as an energe*tically more favorable path for CH 2 insertion into the Ag–CF 3 bond to form the bridge site of CH 2 CF 3( ads ) on the Ag(111) surface. Then we proceed with β-fluoride elimination to form an isolated CH 2= CF 2( g ) and the bridge site of F (ads) on the Ag(111) surface. Our calculated energy barrier for β-fluoride elimination is 0.715 eV higher than that for CH 2 insertion on the Ag(111) surface. These calculated results imply that β-fluoride elimination rather than CH 2 insertion on the Ag(111) surface controls the CH 2= CF 2( g ) formation rate as observed from temperature-programmed reaction (TPR) experimental data. Finally, we attribute these different energy barriers to the different transition state structures — largely distorted seven-centered versus less distorted four-centered — involved in these two different processes.

ABSTRACT:Recently, Chiang's research group successfully carried out the temperature programmed reaction (TPR) spectroscopy under ultrahigh-vacuum conditions to probe the reaction pathways of adsorbed methyl (CH 3(ads) ) and trifluoromethyl (CF 3(ads) ) with coadsorbed methylene (CH 2(ads) ) via the CH 2 insertion reaction, leading to the formation of adsorbed 1,1,1-trifluoro-ethyl (CF 3 CH 2(ads) ) and ethyl (CH 3 CH 2(ads) ) on the Ag(111) surface. Additionally, the authors found it feasible for CF 3 CH 2(ads) to proceed the subsequent ␤-fluoride elimination on the Ag(111) surface to produce 1,1 difluoroethylene (CF 2 ACH 2(g) ) but difficult for CH 3 CH 2(ads) to proceed ␤-hydride elimination to form ethylene (CH 2 ACH 2(g) ) on the Ag(111) surface. To elaborate on this noticeably different reactivity between ␤-hydride and ␤-fluoride eliminations on the Ag(111) surface we performed total energy calculations based on density functional theory in connection with ultrasoft pseudopotential and generalized gradient spin-polarized approximation, and partial structural constraint path minimization to establish the energetically more favorable pathways for the CH 2 insertion into AgOCX 3 (XAH and F) bonds followed by ␤-X elimination to generate an isolated CH 2 ACX 2(g) on the Ag(111) surface. Following our proposed reaction pathways, namely, the diffusion of the fcc-hollow site of CX 3(ads) toward the bridge site of CH 2(ads) through the CH 2 insertion and the subsequent ␤-X elimination to form both isolated CH 2 ACX 2(g) and hcp-hollow site of X (ads) on the Ag(111) surface, our calculated energy barrier for ␤-hydride elimination is significantly larger than (ϳ0.661 eV) that for ␤-fluoride elimination. This unusual high-energy barrier prohibits ␤-hydride elimination of CH 2 CH 3(ads) to form an isolated CH 2 ACH 2(g) on the Ag (111) surface and explains what we observed from the TPR experimental data. We also attribute this much higher energy barrier to forming a largely distorted seven-center ring transition state structure with a larger distortion of the AgOC (␣) H 2 C (␤) H 3 OAg and a less stable AgOH bond on the Ag(111) surface. Finally, our calculated energy barrier for ␤-fluoride elimination to form CH 2 ACF 2(g) is 0.687 eV, in very good agreement with the experimental data.

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