Effect of Adsorbed Donor and Acceptor Molecules on Electron

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Effect of Adsorbed Donor and Acceptor Molecules on Electron Stimulated Desorption: O2/TiO2(110) Zhen Zhang and John T. Yates, Jr.* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904

ABSTRACT The role of band bending on the efficiency of charge transfer across the TiO2(110) single crystal surface has been measured in ultrahigh vacuum, in the absence of a wide range of surface site inhomogeneities, surface impurities and solvent effects, and particle size effects. The adsorption of the Cl2 (electron acceptor) molecule and the O2 (electron acceptor) molecule have been found to enhance hole transport from TiO2 to 18O2 molecules adsorbed on oxygen vacancy sites, increasing the rate of electron stimulated desorption (ESD) of 18O2. This confirms that O2-ESD is hole mediated. Conversely, adsorption of CH3OH, a donor molecule, reduces the transfer rate for holes to the adsorbed O2, reducing its rate of ESD to near zero. The maximum effect of donor and acceptor molecules occurs near 1 monolayer coverage. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

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In photocatalysis by semiconductors, electron-acceptor or electron-donor molecules are known to enhance photocatalytic activity.7-14 Most experiments are carried out under conditions where the photoreaction is studied on polycrystalline substrates where influences of particle size and surface morphology are uncontrolled. In addition, solvent effects may influence these results, and in the case of aqueous media, oxidation processes due to OH radicals are often observed.9 In the experiment to be reported here, a TiO2(110) single crystal, quantitatively dosed in ultrahigh vacuum with acceptor or donor molecules, is used as the substrate. The novel experiments reported here are carried out under ideal conditions compared to those done on powdered TiO2 and in solvent. A simple O2 desorption reaction, induced by hole transfer, is employed to monitor the kinetic effect of variable coverages of donor and acceptor molecules. The desorption of O2 from TiO2(110), induced by electronic excitation of the TiO2, is known to be mediated by holes produced either by photoexcitation or by electron excitation.15,16 An O2 molecule, adsorbed on oxygen-vacancy defect sites, is desorbed by photons with energy above the bandgap (∼3.1 eV).17 Theoretical considerations18 support the involvement of holes as do experiments where a hole-trapping molecule was shown to suppress the rate of O2 photodesorption.15 It is confirmed on the basis of recent experiments16 that the electron stimulated desorption (ESD) of O2 from TiO2 is also hole mediated. Therefore, at different coverages of electron-donor and electron-acceptor molecules, we use the initial rate of 18O2 ESD as a monitor of

he enhancement of the efficiency of charge transfer between an electronically excited semiconductor and a molecule in contact with the semiconductor surface is centrally important for influencing the efficiency of photochemical reactions at surfaces. In addition, charge transport within photovoltaic devices will be strongly influenced by the presence of electron donor or acceptor molecules at interfaces.1,2 When adsorbed electron-acceptor or electrondonor molecules are used to modify the efficiency of charge transport at semiconductor surfaces, these effects may be caused by band bending,3-6 where the gradient of the potential energy of electrons and holes in the near surface region beneath the surface is modified by the adsorbed donor or acceptor molecule. Figure 1 schematically shows the adsorption of an acceptor molecule on a semiconductor surface. As the molecule approaches the surface to chemically bond, an unfilled level shifts downward in energy and broadens as a result of interactions with the semiconductor. A Helmholtz layer on the semiconductor surface is formed, and both the conduction and valence bands bend upward. When electron-hole pair excitation in the semiconductor occurs, the upward band bending due to the acceptor molecule on the surface will diminish the probability of electron transport from the solid to the interface and to an adsorbed molecule; conversely, hole transport will be enhanced to an adsorbed molecule. For an adsorbed donor molecule, the opposite band bending will occur, and electron transport will be enhanced and hole transport will be diminished to an adsorbed molecule. Thus adsorbed donor or acceptor molecules can influence the efficiency of charge transfer to an adsorbed molecule from the semiconductor, due to band bending effects. Electronhole pairs may be excited either by electron impact or by photons.

r 2010 American Chemical Society

Received Date: June 3, 2010 Accepted Date: June 28, 2010 Published on Web Date: July 01, 2010

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DOI: 10.1021/jz1007559 |J. Phys. Chem. Lett. 2010, 1, 2185–2188

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near unity, so that the exposure needed for the full effect places roughly 1 ML of acceptor or donor molecules on the surface. Thus, we conclude that Cl2 and O2, which are both electronegative adsorbates, increase the availability of holes necessary for ESD of O2. The opposite effect is observed for CH3OH, which decreases the availability of holes necessary for O2 desorption. In this work, it is not known whether at 85 K, Cl2 and CH3OH adsorb as undissociated molecules. Figure 3 shows a schematic diagram of the potential energy of electrons in TiO2 as a function of distance beneath the surface for two cases involving adsorbed Cl2 and adsorbed CH3OH. The electronegative Cl2 molecule will be associated with upward band bending over a large depth on the order of hundreds of Angstroms,3 as a result of the accumulation of negative charge within the near surface region of the semiconductor TiO2 toward the surface.19 The adsorption of CH3OH is associated with downward band bending in the semiconductor as a result of its electropositive character. This is consistent with density functional theory (DFT) calculation that the charge is transferred from the adsorbed CH3OH to below the Ti 5c sites.20 Previous ultraviolet photoelectron spectroscopy experiments21 also showed that CH3OH adsorbs on Ti 5c sites via an oxygen lone pair. Both the downward band bending effect and the adsorbate dipole effects are related to the big decrease of work function (∼1.4 eV) upon CH3OH adsorption on TiO2(110).20,22,23 One may recall from our previous O2 photon stimulated desorption (PSD) experiment15 that CH3OH can also act as a hole trap, which may also contribute in the present ESD experiment, to decrease the surface hole concentration and thus decrease the O2 desorption rate. The difference of electron and hole recombination kinetics between ESD and PSD has been discussed before.16 From the previous kinetics analysis,16 the hole concentration is constant and huge in the ESD experiment compared to the PSD experiment. So the hole scavenger effect of CH3OH would not predominate and will not influence the O2 ESD compared to the PSD experiment. Thus, hole transport from the TiO2 to the adsorbed O2 molecule is enhanced by upward band bending (caused by adsorption of either Cl2 or O2), leading to enhancement of the ESD of O2. In the case of the electron-donor molecule, CH3OH, the opposite band bending effect is observed, leading to diminution of the rate of ESD of O2. The presence of charge carriers in TiO2 single crystals (due to natural impurities and defects) and the long screening length make a band bending model attractive. Previous experiments4,24-26 also showed that O2 is an electron scavenger on TiO2(110), and the adsorption of O2 caused the valence band shift due to the upward band bending, confirming the results reported here, where our use of oxygen isotopes allows clear discrimination of this effect. In summary, these experiments, done for the first time on a single crystal under well-controlled conditions, free of TiO2 particle size effects, extensive surface structure inhomogeneities, and solvent and impurity effects, clearly demonstrate the role of band bending caused by adsorbed electron-acceptor and electron-donor molecules. Such band bending effects influence the efficiency of hole and electron transport across the TiO2 surface. The observations show that the use of

Figure 1. Schematic diagram shows the adsorption of an acceptor molecule (A) on a semiconductor surface.

the efficiency of hole transfer from the substrate to the adsorbed 18O2 molecule, causing 18O2 desorption. Figure 2a shows the observations of 18O2 desorption caused by ESD, where an electron-acceptor molecule (Cl2), has been adsorbed at 85 K following the adsorption of 18O2 onto bridge-bonded oxygen (BBO) vacancy sites on the prepared surface. It is seen that the initial rate of 18O2 desorption is greatly enhanced by the adsorption of Cl2. This enhancement of desorption rate persists as ESD reduces the coverage of 18O2 as seen in the tail of the O2 ESD curve. For the partial Cl2 coverage shown here (9.8 1013 molecules cm-2), a factor of 3.7 enhancement of the initial rate of 18O2 desorption is observed. Full coverage of Cl2 results in a 25-fold enhancement of the 18O2 desorption rate. Figure 2b shows similar observations of the 18O2 ESD yield when an electron-donor (or hole-trap) molecule (CH3OH) is adsorbed at 85 K after 18O2 adsorption. A decrease in the initial yield of 18O2 is observed as the rate of hole transfer to adsorbed O2 molecules is diminished by downward band bending caused by the donor molecule. The decrease in 18O2 yield continues to zero at high CH3OH coverage. Figure 2c shows the effect of the electron-acceptor molecule, O2, on itself. Here, after exposure to 18O2, 16O2 is added to the surface. It is observed that a factor of 2.3 increase in 18 O2 yield occurs when 16O2 is coadsorbed. Figure 2d summarizes these experiments for the adsorption of Cl2 and CH3OH surface modifier molecules as a function of their exposure at 85 K. Within the experimental error, the major effect of the acceptor or donor molecules occurs over a range of coverage equivalent to ∼1 monolayer (ML) (or ∼5 1014 molecules cm-2 exposure). At 85 K, the sticking coefficient of both Cl2 and CH3OH will probably be


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Figure 2. ESD (100 eV) measurements by quadrupole mass spectrometry (QMS) for 18O2 on 18O2/TiO2(110) surfaces in the presence of (a) Cl2 (green curve, Cl2 exposure: 9.8 1013 molecules cm-2), (b) CH3OH (red curve, CH3OH exposure: 1.3 1014 molecules cm-2), and (c) 16O2 (blue curve, 16O2 exposure: 1.0 1015 molecules cm-2). The ESD measurement of preadsorbed 18O2 (18O2 exposure: 3.8 1013 molecules cm-2) is also shown in the black curve in panels a, b, and c as the reference. All the gas dosing and ESD experiments were done at 85 K. (d) Plots of the initial ESD yield of 18O2 (as shown in the dashed circles in panels a and b) as a function of Cl2 or CH3OH exposure. The electron bombardment is rapidly initiated at 0 s, and the first point in the O2 desorption curve is measured within 0.2 s.

by low-energy electron diffraction (LEED) and Auger spectroscopy. About 8-10% of an ML of BBO vacancies on TiO2(110) were generated due to the annealing in vacuum at 900 K.28 These sites are active for the O2 adsorption.15 Adsorption of 18 O2 (99% isotopically pure), 16O2, Cl2 (Sigma-Aldrich, >99.5%), and CH3OH (Fisher Scientific, 99.9%) were carried out using an absolutely calibrated capillary array doser,29 maintaining the TiO2(110) at 85 K. A 100 eV incident electron beam was used for the ESD experiment. During ESD, the incident electron flux was kept at 5.1 1013 electrons cm-2 s-1, which was monitored by a picoammeter (Keithley 6487) biased at þ10 V to eliminate effects of secondary electron emission. The 18O2 desorption was detected by a line-of-sight quadrupole mass spectrometer (QMS) (UTI-100C) with internal O2 ionization by 70 eV electrons. The experiment measures the rate of 18O2 desorption, and only the first point, measured in 0.2 s acquisition time, is used to measure the initial rate of 18O2 ESD. Thus, the effect of extensive electron bombardment on O2 or on the donor/acceptor molecules is avoided.

Figure 3. Schematic diagram of band bending and its effect on hole transport causing increasing or decreasing 18O2 ESD yield by adsorption of Cl2 (acceptor) or CH3OH (donor) molecules.

surface modifier molecules that produce band bending effects in TiO2 can be employed to significantly influence charge transport across the interface.

AUTHOR INFORMATION Corresponding Author:

EXPERIMENTAL METHODS

*To whom correspondence should be addressed. E-mail: johnt@ virginia.edu.

The detailed experimental method and equipment have been described before.16,27 The ultrahigh vacuum system base pressure is 2 10-11 mbar. By Arþ sputtering followed by annealing at 900 K in vacuum, an impurity-free TiO2(110)-1 1 surface is reproducibly produced, which was characterized


ACKNOWLEDGMENT We acknowledge with thanks the support of the Department of Energy, Office of Basic Energy Sciences, under DOE Grant Number DE-FG02-09ER16080.

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Effect of Adsorbed Donor and Acceptor Molecules on Electron

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