Electric Charge of Single Au Atoms Adsorbed on TiO2(110) and

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Electric Charge of Single Au Atoms Adsorbed on TiO2(110) and Associated Band Bending Zhen Zhang,† Wenjie Tang,‡ Matthew Neurock,†,‡ and John T. Yates, Jr.*,† †

Department of Chemistry and ‡Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904, United States ABSTRACT: The influence of charge transfer from single Au atoms to TiO2(110) on the photostimulated desorption (PSD) of 18O2 has been studied using a measurement of the rate of hole transport which mediates PSD. Band bending effects observed experimentally and theoretically by density functional theory indicate that Auδ+ atoms are present. The presence of atomically dispersed Au on the TiO2(110) surface with a constant coverage of preadsorbed 18 O2 depresses the photoinduced hole transport rate from the bulk to the TiO2 surface and decreases the 18O2 PSD yield. This indicates that single Au atoms donate a fraction of an electron to the surface, causing downward band bending. DFT calculations show that ∼0.2 electron transfers from single Au atoms to the O2/TiO2(110) surface and the valence and conduction bands bend ∼0.6 eV downward. With increasing Au coverage, the positive charge per Au atom decreases due to the formation of small Au clusters.

1. INTRODUCTION The Au/metal oxide system (such as Au/TiO2) has been extensively studied due to the unique catalytic activity of Au nanoparticles deposited on metal oxides, in contrast to the noble character of bulk Au.1
3 Different mechanisms have been proposed to elucidate the origin of the catalytic activity, such as the quantum size effect for small Au clusters,3,4 the presence of lowcoordinate Au atoms,5,6 the influence of dual support
Au catalytic sites at the Au particle perimeter,7
9 and the charge state of the Au nanoparticles.10
13 Among the above mechanisms, the electronic interaction between gold and metal oxide and the charge state of Au is an important topic. Both cationic and anionic Au clusters can be prepared on metal oxide surfaces.13
15 Most of the efforts on understanding charge have focused on its influence on the catalytic activity of the Au cluster itself. Little work has been done on the influence of the Au charge state on the electronic behavior of the metal oxide support surface. This paper deals with the electronic charge of single Au atoms adsorbed on the TiO2(110) surface and the band bending in TiO2 caused by electron transfer from Au to TiO2. Previous experiments16,17 showed that the adsorption of electron donor (CH3OH) or acceptor (Cl2 or O2) molecules on the TiO2(110) surface will change the carrier concentration near the surface while causing downward or upward band bending. When an electron
hole pair in a semiconductor is excited by incident photons, the downward/upward band bending increases the probability of electron/hole transport from the bulk to the surface, thereby influencing the rate of a photodesorption reaction at the surface. The rate of O2 desorption, which is excited by the holes in the valence band induced by incident electrons or UV photons,16
26 has been used as a probe to monitor the rate of hole transport following photoexcitation and to deduce the sign of the band bending effect. r 2011 American Chemical Society

In this study, we combine photostimulated desorption (PSD) of 18O2 and density functional theory (DFT) calculations to probe the influence of the adsorbed Au atom charge on the photoexcited process.

2. EXPERIMENTAL METHODS AND DFT CALCULATIONS 2.1. Experimental Methods. The experiments were carried out in a stainless ultrahigh vacuum (UHV) chamber with a base pressure below 3  10
11 mbar.16,18,27 A clean TiO2(110)-(1  1) sample (Princeton Scientific, 7 mm  7 mm  1 mm) with ∼8% surface oxygen vacancy density was prepared by cycles of Ar+ sputtering and annealing at 900 K in UHV, and the (1  1) character was confirmed by low-energy electron diffraction. Adsorption of 18O2 (99% isotopically pure) was carried out using an absolutely calibrated capillary array doser,28 keeping the TiO2(110) sample at 89 K. The shielded Au evaporation source was made by tightly wound Au wires (99.998% purity) on a W filament supported on a pair of separately degassable W leads.28 Constanttemperature evaporation of Au onto the TiO2(110) crystal at 89 K is reproducibly achieved over many weeks at a constant source temperature of ∼1290 K monitored by I/V measurements on the Au atom doser assembly,28 which is located about 15 cm from the TiO2 crystal. Au deposition in the submonolayer regime could be achieved reproducibly in these experiments using a shutter to interrupt the Au deposition as has been used for other metals.29 The Au signal was measured by Auger electron spectroscopy (AES) using a single-pass cylindrical mirror analyzer (PHI model 10-155) with a primary electron energy of 2 keV. UV light from a Received: July 15, 2011 Revised: October 10, 2011 Published: November 15, 2011 23848

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The Journal of Physical Chemistry C broad-band Hg lamp was transferred to the sample by a fiber optic cable with a water filter to remove the IR component. The wattage of UV power received by the crystal was 1.4  10
3 W/ cm2 in the photon energy of 1.1
5.4 eV, and the UV beam covered the 0.5 cm2 crystal. For each experiment, the TiO2(110) surface was exposed to 18O2 with the same exposure (8.64  1013 molecules/cm2) prior to the deposition of various amounts of Au. The (O2 + Au)/TiO2 surface then was irradiated with the UV light. The rate of desorption of 18O2 induced by UV excitation was monitored by line-of-sight quadrupole mass spectrometry (QMS) (UTI-100C). The TiO2(110) sample was kept at 89 K during the Au and O2 deposition and the PSD experiment. After each experiment, the sample was cleaned by sputter-annealing. 2.2. DFT Calculations. The single-crystal TiO2(110) surface was simulated with a (2  5) supercell slab with four layers of O
Ti
O structure (∼12.5 Å in thickness) along with a 10 Å vacuum along the z-direction to separate the slab surfaces. The Ti and O atoms in the bottom two layers were frozen in their lattice positions to mimic the bulk substrate, whereas the top two layers were allowed to fully relax. One of the bridge-bonded oxygen (BBO) atoms was removed from the surface to create a bridgebonded oxygen vacancy (BBOV). All DFT calculations were carried out using the Vienna ab initio Simulation Package (VASP).30 Valence electrons were described with Kohn
Sham single-electron wave functions and expanded in a plane wave basis with an energy cutoff of 400 eV.31 Core electrons were treated by pseudopotentials with the projector augmented wave method.32,33 The PW91 generalized gradient approximation functional was applied to evaluate the exchange-correlation energy.34 To better describe on-site Coulomb interactions, the DFT+U method was used with the suggested U value of 4.0 eV from previous references.35,36 The Brillouin zone was sampled with a (2  2  1) k-point mesh.37 Spin polarization was considered in all cases and applied where required. Geometries were considered to be optimized when the force on each atom was less than 0.03 eV/Å. The charge around each atom was evaluated by the Bader analysis.38,39 Our calculated band gap (∼1.8 eV) for a TiO2 slab with DFT +U is consistent with the 1.5
2 eV band gaps reported by others36,40,41 in the literature for TiO2 slabs but still much lower than the bulk values for TiO2 reported experimentally.42 The band gaps for thin oxide films, however, are known to be markedly lower than those of the bulk. The calculated band gap for the bulk TiO2 is ∼3 eV, which is consistent with experimental results.

3. RESULTS AND DISCUSSION 3.1. Growth of Au on TiO2(110). To test the stability and flux of the homemade Au evaporation source, AES has been employed to monitor the Au AES peak (NVV, 69 eV) evolution with the deposition time (corrected to Au coverage) on the clean TiO2(110) surface at 89 K. Figure 1 shows the Au AES peak-to-peak height as a function of the deposition time. The data were normalized to the primary electron flux, which made the data directly comparable. The AES data can be well fitted linearly with the deposition time, indicating that the Au flux is stable and the Au signal is proportional to the deposition time. The linear behavior excludes the transition from two-dimensional (2D) to three-dimensional (3D) growth of Au on TiO2(110) at 89 K in the Au coverage region below 0.7 ML (monolayer). Although the Au would thermodynamically prefer 3D growth at high temperature, deposition on the surface at 89 K eliminates the Au diffusion

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Figure 1. Au AES peak (NVV, 69 eV) evolution with the deposition time on the clean TiO2(110) surface at 89 K (peak-to-peak heights used for AES).

on the TiO2 surface, preventing the formation of 3D islands. DFT analysis of the Au diffusion energy on various types of prepared TiO2(110) surfaces43 yields diffusion barriers from 0.4 to 0.7 eV, precluding Au surface diffusion at 89 K. Studies of Au+ ion deposition (which quickly neutralize to neutral Au atoms) on TiO2(110) also indicate that single Au atoms are immobile at 115 K.44 Au atom surface diffusion and nucleation occurs at higher temperatures. For example, Parker et al.45 found that the critical Au coverage necessary for the transition from 2D to 3D growth increases with decreasing temperature in the range from 293 to 153 K. Matthey et al.43 also found by scanning tunneling microscopy that single atomic-height Au clusters or single Au atoms nucleate homogeneously at room temperature on the reduced or oxidized TiO2(110) surface. In our experiments, the Au deposition flux is estimated to be 0.0045 ( 0.001 ML/min, by means measuring the attenuation of the O AES signal (KLL, 510 eV) by increasing Au coverages. The Au coverage has been calibrated in Figure 1 accordingly. The maximum Au coverage used in our experiment is 0.54 ML on the TiO2(110) surface, which corresponds to 5.5  1014 Au atoms/cm2. The monolayer coverage of Au (1.04  1015 Au atoms/ cm2) is based on the DFT calculations, which show that Au can adsorb on both the Ti5c and BBO sites on the TiO2(110). 3.2. Influence of Au Adatom Coverage on 18O2 PSD Yield. Following the exposure to a constant amount of 18O2 (8.64  1013 molecules/cm2) onto TiO2(110) at 89 K, various coverages of Au (0
0.54 ML) were deposited to investigate the influence of Au on the rate of 18O2 PSD by UV light. Figure 2a shows samples of the measurement by QMS of the 18O2 yields during the UV excitation in the presence of Au on the 18O2/TiO2(110) surface. Similar to the previous electron-stimulated desorption (ESD)16
18 and PSD20,21,46 experiments, the 18O2 PSD yield quickly reaches the maximum (indicated in the dashed circles) within the QMS sampling time of 0.2 s after the shutter for the UV light is opened. We employ the initial yield of photodesorbing O2 as a metric of the initial efficiency of O2 photodesorption at constant O2 coverage. The initial yield of 18O2 versus Au coverage on the 18O2/TiO2(110) surface has been plotted in Figure 2b. At low Au coverage, the initial 18O2 PSD yield decreases quickly and attains the half-value of the yield for the Auuncovered 18O2/TiO2(110) surface at ∼0.16 ML of Au coverage. 23849

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Figure 2. (a) QMS measurement of the 18O2 yields during the UV excitation in the presence of Au (coverage 0, 0.13, and 0.29 ML) on the 18 O2/TiO2(110) surface. The initial PSD yields are indicated by the dashed circles. (b) Initial yield of 18O2 versus Au coverage on the 18O2/TiO2(110) surface.

Figure 3. Structural models of (a) the clean TiO2(110) surface with BBOVs, (b) TiO2(110) with O2 adsorbed at a BBOV, (c) TiO2(110) with Au adsorbed near Ti5c sites (the sites between Ti5c and the BBOV site) and O2 adsorbed on a BBOV, and (d) TiO2(110) with Au adsorbed on a BBO and O2 adsorbed on a vicinal BBOV. The Au atoms and Ti atoms are shown in yellow and gray, respectively, whereas the O in the lattice and adsorbed O are shown in pink and red, respectively.

With a further increase of the Au coverage, the 18O2 PSD yield continues to decrease more slowly. The 18O2 data versus Au coverage data can be fit empirically to the Au coverage by a firstorder exponential function. 3.3. Au Charge State and Associated Band Bending on TiO2(110) Surface. The 18O2 PSD yield, caused by holes produced in TiO2 by the incident UV photons, is proportional to the flux of holes reaching adsorbed O2 molecules at the surface. The decrease of the initial 18O2 PSD yield indicates the depression of the hole flux to the surface. That means the Au on the 18O2/TiO2(110)

Figure 4. Calculated total DOS of TiO2(110) with (a) BBOVs, (b) O2 adsorption on a BBOV, (c) Au adsorption near Ti5c sites (the sites between Ti5c and a BBO site) and O2 adsorption on a BBOV, and (d) Au adsorption on a BBO and O2 adsorption on a vicinal BBOV. The corresponding structural models are indicated in Figure 3.

surface acts as an electron donor, bending the surface electron energy bands downward. Similar behavior is observed for CH3OH, which is well-known as an electron donor molecule for modifying O2 photodesorption on TiO2(110).16 While O2 dissociation can also occur47 and act to reduce the O2 PSD signal, this is not likely to occur since Au, which is deposited after O2 adsorption, is immobile at 89 K and unlikely to find the isolated O2 species on the surface. DFT calculations have been employed to investigate the energy band shift caused by the Au. Figure 3 shows the structural models of TiO2(110) with BBOVs (Figure 3a) and O2 and Au adsorption on different sites (Figure 3b
d) used in the DFT calculations. The corresponding valence band (VB) and conduction band (CB) structures of TiO2(110) near the Fermi level (EF) are 23850

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Table 1. Charges of Au and Aux/TiO2(110) Surfaces from the Literaturea measurement Au charge

method

adsorption surface

ref


+

PES PES

Au/R-TiO2(110) Au/S-TiO2(110)

56 57




PES

Au/R-TiO2(110)

58

+

PES

Au/S-TiO2(110)

59

+

PES

Au/S-TiO2(110),

60

+,
, 0

DFT

Au/R-TiO2(110) Au1
, Au3
, Au5+, Au7+,

10

Au20, Au40, Au60/R-TiO2(110)

Figure 5. (a) Schematic diagram of downward band bending by Au and its effect on hole transport, causing decreasing 18O2 PSD yield. (b) Work function change calculated by DFT and the Macdonald and Barlow model vs Au coverage on the 18O2/TiO2(110) surface.

depicted in Figure 4. In the experiments, the 18O2 coverage is 4.32  1013 molecules/cm2 (assuming an O2 sticking coefficient of 0.548), which is about ∼8% of the BBO surface coverage on the TiO2(110) surface, and equivalent to the O-vacancy site density (∼8%) on the surface.48 Therefore, in the DFT calculation, we assume that each oxygen vacancy site has been occupied by one 18 O2 (Figure 3b
d). Since O2 is preadsorbed with a binding energy ∼
2.3 eV at the vacancy and Au is only deposited after the O2 has been adsorbed, a Au atom is less likely to replace the strongly adsorbed O2 at our experimental temperature. The O2 acts as an electron acceptor accepting ∼1 electron from the oxide. As shown in Figure 4b, the presence of O2 on the TiO2(110) surface shifts the VB and CB edges to higher energy and therefore bends the energy bands upward as a result of electron acceptance, which agrees well with the previous ESD and PES experimental results.16,17,49
51 The electronic shift for single Au atom adsorption on the 18O2/TiO2(110) surface has also been calculated (Figure 4c,d). The DFT results indicate that sites for the adsorption of a single Au are those sites near Ti5c sites (sites between Ti5c and a BBO site) (Figure 3c) or the BBO sites (Figure 3d). For the different sites considered in Figure 3c,d, we find an average theoretical charge on a single Au atom of ∼ +0.2 e. Contrary to the 18O2-induced upward band bending, parts c and d of Figure 4 show that in both cases the presence of Au shifts the VB and CB to lower energy and bends the energy band of the 18O2/TiO2(110) surface ∼0.6 eV downward, which corroborates the 18O2 PSD result that Au is an electron donor and causes downward band bending on the oxygen-precovered TiO2(110) surface, resulting in a reduction of the O2 PSD yield. The deposition of Au does not change the charge state of O2. O2 has a charge of
0.96 e on the structure in Figure 3b and charges of
0.97 and
0.98 e on the structures in parts c and d of Figure 3. When looking at the shift of the O2 partial density of states (DOS; not shown), it follows the same trend as found in the shifts in DOS for the oxide surface (Figure 4), which indicates that the work function change for the whole system is caused by the adsorbed Au; therefore, the oxide surface and O2 show similar shifts. The above discussion gives a qualitative description of the Auassociated band bending and its effect on the O2 photodesorption as shown in Figure 5a. The downward band bending caused by electropositive Au on the TiO2(110) surface depresses the

+ +

DFT DFT

Au/O-TiO2(110) Au/S-TiO2(110)

61 62

+,
, 0

DFT

Au
/R-TiO2(110),

63

Au+/O-TiO2(110), Au0/S-TiO2(110) +

DFT

Au7/O-TiO2(110)

64

+

PSD

Au/O-TiO2(110)

this work

+

DFT

Au/O-TiO2(110)

this work

a

R-TiO2(110) = reduced TiO2(110), S-TiO2(110) = stoichiometric TiO2(110), and O-TiO2(110) = TiO2(110) with adsorbed atomic or molecular O2.

probability of hole transport to the surface and hence decreases the rate of hole-induced O2 desorption. On the TiO2(110) surface, the Au-induced band bending is equal to the work function change, Δϕ. The metal-induced Δϕ can be described by the equations developed by Macdonald and Barlow52,53 in accordance with the earlier Topping model:54 Δϕ ¼
4πeμθσ=½1 þ αΛδðθσÞ3=2 

ð1Þ

where μ is the initial dipole moment induced by the Au adsorption (assuming rAu
TiO2 = 2.6  10
8 cm), θ is the Au coverage, σ is the saturated Au atom concentration on the TiO2(110) (1.04  1015 atoms/cm2), α is the Au polarizability (5.8  10
24 cm3),55 Λ is a constant (generally equal to 9), δ = θ
1/2 for immobile Au atoms on TiO2(110) at 89 K, and e is the elementary charge (4.8  10
10 esu). In this model, the charge transfer per Au atom between the adsorbate and substrate is not invariable with the coverage and is modified by the depolarization between neighboring polarized adsorbate atoms. However, in the low Au coverage range in this experiment, the number of electrons transferring from Au to TiO2 is almost constant. Using eq 1, the Au-induced Δϕ can be calculated, and the data for Δϕ (blue triangles) have been plotted in Figure 5b. The data show an almost linear decrease in ϕ with the Au coverage, where small depolarization effects are seen to result in slight curvature. Using a generalized site of Auδ+ (δ+ = +0.2 e) on which additional Au atoms are statistically arranged in accordance with the deposition of immobile Au atoms to make a planar Au cluster, the charges of the Au2 and Au3 clusters are calculated as Au2δ+ (δ+ = 0.1 e) and Au3δ+ (δ+ = 0.1 e). The cluster charges for ensembles of the various cluster distributions are used in Figure 5b to calculate the change in the work function or the band edge shift as the Au coverage increases. As the Au coverage becomes higher, the relative population of monomer decreases while the dimer and trimer populations increase. Consequently, the average charge of the Au atoms will decrease with increasing Au 23851

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The Journal of Physical Chemistry C coverage due to the Au cluster formation. This is seen by the more curved plot in Figure 4b (red squares). The curvature in Figure 4b qualitatively correlates with the curvature in 18O2 PSD yield for increasing Au coverage (Figure 2b) as caused by Aux clustering at higher coverages. There have been a number of experimental studies of the charge on Au atoms and Aux clusters of the TiO2(110) surfaces with the stoichiometric (S-TiO2), reduced (R-TiO2, TiO2 with oxygen vacancy), or O-rich (O-TiO2) TiO2(110) surfaces. In addition, DFT calculations are abundant for this system. Table 1 summarizes the result of this work from the literature. In general, on fully oxidized TiO2(110), Au+ is measured, whereas on substoichiometric TiO2(110), Au
is measured. For DFT calculations on fully oxidized TiO2(110), Au+ is found. These results agree with our findings, using the band bending concept as applied to hole-mediated O2 photodesorption.

4. CONCLUSIONS In summary, the charge transfer between Au atoms and TiO2(110) and its effect on the modification of the TiO2 band structure have been investigated, using O2 photodesorption and DFT calculations. (1) The PSD experiment for 18O2 on TiO2(110) shows that the presence of Au depresses the transport efficiency of holes to the surface and hence decreases the 18O2 PSD yield. (2) DFT calculation shows two sites for single Au atom adsorption: the sites near Ti5c and BBO sites. An average of ∼0.2 electron transfers from a single Au atom to TiO2. This effect is qualitatively related to downward band bending for Au adatoms on the O2/TiO2 surface. (3) Using DFT calculations, the DOS of TiO2 modified with O2 and single Au atoms has been studied. The adsorption of Au on O2/TiO2(110) shifts the VB and CB of TiO2 to lower energy (bending the VB and CB downward). (4) With increasing Au coverage, the average electron transfer from the Au to TiO2 decreases due to dimer and trimer Au cluster formation. The average charges of Au monomer, dimer, and trimer clusters are about +0.2 e, +0.1 e, and +0.1 e, respectively, on an O2/TiO2(110) surface. These results are consistent with the previous use of O2 PSD to monitor band bending in TiO2 due to charged adsorbate surface modifiers. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We acknowledge with thanks the support of the Department of Energy, Office of Basic Energy Sciences, under DOE Grant DE-FG02-09ER16080 and the Texas Advanced Computing Center for Teragrid resources. ’ REFERENCES (1) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (2) Haruta, M. Catal. Today 1997, 36, 153. (3) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647.

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