Article pubs.acs.org/JPCC
Initial and Final State Effects in the Ultraviolet and X‑ray Photoelectron Spectroscopy (UPS and XPS) of Size-Selected Pdn Clusters Supported on TiO2(110) F. Sloan Roberts,† Scott L. Anderson,*,† Arthur C. Reber,‡ and Shiv N. Khanna*,‡ †
Department of Chemistry, University of Utah, 315 S. 1400 E., Salt Lake City, Utah 84112, United States Department of Physics, Virginia Commonwealth University, 701 W. Grace St., Richmond, Virginia 23284, United States
‡
S Supporting Information *
ABSTRACT: Photoelectron spectroscopy is a powerful tool for investigating the electronic structure of supported clusters, especially when initial state and final state contributions to the electron binding energy can be separated. We have performed a combined experimental and theoretical study of ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) using atomically size-selected Pdn clusters on rutile TiO2(110). Theoretical investigations allow for the UPS and XPS shifts to be split into initial state and final state contributions. In XPS, the occupation of the 4d orbital of Pd controls the initial state shift offering information about the hybridization of the cluster, while the size and the charging of the cluster controls the final state shift. In UPS, we evaluate two methods for calculating the final state shift in periodic unit cells and find that both methods give reasonable results for pristine TiO2; however, using a p-type dopant fails when two separate donor−acceptor pairs are present. The observed UPS shifts can be described by combining the surface dipole and the final state shifts. Metallic contacts to the semiconductor surface result in band alignment between the metallic contact and the cluster, shifting the Fermi level to lie just below the conduction band of the TiO2. Information about the charge state and hybridization of the cluster are revealed by separating the initial and final state effects.
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INTRODUCTION Understanding how the electronic structure changes with the number of atoms in size-selected supported clusters is a valuable tool for identifying features for optimizing catalytic activity. Our groups1−4 and others5−26 have shown that the unique electronic properties of the clusters play a role in enhancing catalyst activity. A common method to investigate the electronic structure of surfaces is photoelectron spectroscopy (PES). This includes X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). These surface analysis tools are well-known;27−29 however, difficulties remain when interpreting the experimental spectra as there are a number of effects that play a role in the photoelectron energy.30−41 In photoelectron spectroscopy, the surface is exposed to ionizing electromagnetic radiation, and the binding energy of electrons in the sample are measured using the energy of the incident photon and the kinetic energy spectra of the photoelectrons. Photoelectron experiments ideally produce spectra with well-defined peaks in the electron binding energy that correspond to orbital energies. The direct measurement of orbital energies is confounded because the energies of the ejected photoelectrons depends on the energy difference between the N electron initial neutral state of the system and the N − 1 electron ionized final state. The electron binding energy can then be understood through contribution from the initial state of the system, such as the effect of the © 2015 American Chemical Society
screening of the nucleus by the valence electrons on the core states of the atom in XPS, the surface dipole generated by adsorbates or other features of the surface, and final state contributions involving the ability of the hole produced by ionization to be screened by its chemical environment. In this paper, we have performed a combined experimental and theoretical study to determine what physical features contribute to the initial and final state effects in the XPS and UPS of sizeselected Pd clusters supported on rutile TiO2(110). The initial state contribution of the electron binding energy comes from the position of the orbital relative to the vacuum level of the system. A common example is in the XPS of core electrons of the metal versus metal oxide system where the measured electron binding energies from the metallic elemental system will be significantly lower than those in the metal oxide system. In the metal−oxide system, the metal loses a significant amount of charge density to the oxygen and this reduces the screening of the nucleus, which lowers the energy of the core state and increases the electron binding energy. These initial state shifts can be on the order of several electron volts (eVs), and with an energy resolution on the order of meV, these shifts may signify a change in the oxidation state of the metal. This is Received: December 9, 2014 Revised: February 16, 2015 Published: February 19, 2015 6033
DOI: 10.1021/jp512263w J. Phys. Chem. C 2015, 119, 6033−6046
Article
The Journal of Physical Chemistry C
of a home-built VUV lamp.45 In addition to UPS, the UHV chamber is also equipped with X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), and a differentially pumped mass spectrometer positioned behind a skimmer cone for temperature-programmed desorption (TPD). The UPS lamp is a doubly differentially pumped, windowless hollow cathode capillary discharge lamp, with discharge typically maintained at +3 kV and 60 mA, in ultrahigh purity helium cleaned by passage through liquid-nitrogen-cooled molecular sieves. Under these conditions, the dominant VUV emission is the HeI line at 21.2 eV, with the HeII line (40.8 eV) emission minimized by running the discharge region pressure (measured at the exhaust) at 650 mTorr. Under these conditions, the second pumping region was at ∼7.0 × 10−4 Torr, and the He flux into the main UHV chamber raised its pressure to 2.0 × 10−7 Torr. In order to efficiently detect low energy electrons for work function determination, the sample was biased to −15.0 V. Each UPS scan took about 3.5 min to obtain, with hemispherical analyzer pass energy of 2.95 eV and collection area set to 1.1 mm diameter to ensure that signal was collected only from the cluster deposition spot. The TiO2(110) (Commercial Crystal Laboratories) substrate was glued to a tantalum backing plate using UHV compatible, high temperature cement (Aremco, Ceramabond 571) and initially annealed in UHV to 1050 K for 1 h. This heat treatment has been shown to create oxygen defects on the surface, and throughout the bulk of the crystal, turning it blue and making it conductive enough to ensure that Pdn+ clusters are immediately neutralized upon deposition and to allow photoelectron spectroscopy with minimal charging.46−48 In essence, the O-vacancies act as electron donors, such that the heat-treated TiO2 is a heavily n-doped material. Prior to each experiment, the TiO2(110) support was cleaned by sputtering with 1 kV Ar+ for 20 min and then subsequently annealed to 900 K to restore surface order. XPS and UPS spectra were then collected to verify the cleanliness of the surface, i.e., no carbon, Pd, or other adsorbates were present. The surface density of oxygen vacancies was determined by titration with water and found to be the amount to vacancies in ∼8% of the surface unit cells.49 The vacancy density of the TiO2(110) affects the shape of the UPS spectrum, giving rise to midgap states in the same energy range as states arising from deposited Pdn. To allow subtraction of signal from the TiO2(110) support, UP spectra were taken every day following the sputter and anneal cycles, prior to cluster deposition. These as-prepared spectra were found to be highly reproducible from day to day, suggesting that the surface preparation was also reproducible. Following preparation of the substrate, Pdn+ clusters (n < 26 atoms) were deposited at approximately 1 eV/atom onto the TiO2(110) crystal at room temperature with deposition currents ranging between 100 and 1000 pA depending on the cluster size, corresponding to between 3 and 30 min deposition times. The cluster spot size on the TiO2(110) support is defined by passing the beam through a 2 mm diameter mask/ ion lens just before deposition, resulting in a uniform Pdn spot as profiled with XPS. Only 10% of a monolayer (ML) (1.53 × 1014 Pd atoms) was deposited in order to ensure adequate spacing between clusters and to minimize the probability of clusters landing close enough together to aggregate. The evidence regarding other sintering or ripening processes is discussed below. Following deposition, XP and UP spectra
not merely an effect based on the oxidation state of the atom because localized orbitals such as 3d and 4d orbitals screen core levels much more effectively than delocalized 4s and 5s orbitals, so the initial state shift is also sensitive to the hybridization of the atom in addition to the oxidation state. The initial state shifts also include changes in the work function of the surface due to changes in the surface dipole. Initial state effects are rich with information about the chemical state of an atom near the surface. Final state effects also change the electron binding energy due to differences in the energy of the sample with one fewer electron. When the electron is removed, it leaves behind a hole that is stabilized to a varying degree depending on the chemical environment. If this electronic relaxation occurs before the ejected photoelectron is completely decoupled from the system, then the electron binding energy is affected. This effect is especially important in the photoelectron spectroscopy of sizeselected clusters deposited on surfaces; the ionization of a single atom bound to a surface will result in a localized hole with little ability for the atom to electronically relax, while in a cluster of many atoms, the hole produced after ionization may be delocalized over the cluster lowering the final state energy and the electron binding energy. Recent work has shown that there are indeed ways to experimentally begin to separate these two effects, mainly via Auger electron spectroscopy.31,37,42 As the first core level electron is ejected from the atom, a higher level electron can drop down to fill that hole, with the resulting energy being released by ejecting another electron. The second photoelectron is then originating from the “final state” of the first system (the system with the hole present). This is a useful technique but can only be applied correctly to certain experimental systems, and this method may not be applied to Pd on TiO2. The challenge is to deconvolute the initial and final state effects to understand the origin of the changes in the photoelectron spectrum. In this paper, we investigate the photoelectron spectroscopy of atomically size-selected Pdn clusters deposited on rutile TiO2(110) and analyze the results within density functional theory in an effort to separate the initial and final state effects in the observed PES.5,6,40 We performed XPS and UPS experiments on Pdn/TiO2(110) (n = 1−25) system, and we show that the core and valence levels (XPS and UPS) are similarly affected. We have found the geometric and electronic structures of Pdn n = 1−7 clusters on pristine TiO2, on top of an O vacancy, and adjacent to an O vacancy. We next used the comparison between experiment and theory to evaluate our method of simulating XPS and UPS. We calculated the initial state and final state XPS shifts for all of these clusters, and determine whether the initial state XPS shifts are controlled by the net charge of the cluster or the hybridization between the 4d and 5s orbitals of the Pd cluster. We also test the hypothesis that the cluster states align with the metallic contact eliminating one source of initial state shift in the UPS spectra. We evaluated our method for correcting the dependence of the work function on the coverage of the cluster, and two methods for determining the final state shift in UPS on supported clusters in a periodic unit cell.
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EXPERIMENTAL SECTION The cluster deposition beamline and UHV chamber (base pressure
