ZnO(101̅0) Surface Hydroxylation under Ambient Water Vapor - The

Article pubs.acs.org/JPCB

ZnO(101̅0) Surface Hydroxylation under Ambient Water Vapor John T. Newberg,*,† Chris Goodwin,† Chris Arble,† Yehia Khalifa,† J. Anibal Boscoboinik,‡ and Sana Rani† †

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States

‡

S Supporting Information *

ABSTRACT: The interaction of water vapor with a single crystal ZnO(101̅0) surface was investigated using synchrotron-based ambient pressure X-ray photoelectron spectroscopy (APXPS). Two isobaric experiments were performed at 0.3 and 0.07 Torr water vapor pressure at sample temperatures ranging from 750 to 295 K up to a maximum of 2% relative humidity (RH). Below 10−4 % RH the ZnO(101̅0) interface is covered with ∼0.25 monolayers of OH groups attributed to dissociation at nonstoichiometric defect sites. At ∼10−4 % RH there is a sharp onset in increased surface hydroxylation attributed to reaction at stoichiometric terrace sites. The surface saturates with an OH monolayer ∼0.26 nm thick and occurs in the absence of any observable molecularly bound water, suggesting the formation of a 1 × 1 dissociated monolayer structure. This is in stark contrast to ultrahigh vacuum experiments and molecular simulations that show the optimum structure is a 2 × 1 partially dissociated H2O/OH monolayer. The sharp onset to terrace site hydroxylation at ∼10−4 % RH for ZnO(101̅0) contrasts with APXPS observations for MgO(100) which show a sharp onset at 10−2 % RH. A surface thermodynamic analysis reveals that this shift to lower RH for ZnO(1010̅ ) compared to MgO(100) is due to a more favorable Gibbs free energy for terrace site hydroxylation.

1. INTRODUCTION Zinc oxide (ZnO) is one of the more extensively studied metal oxides in surface science1 and has use in a number of applications including solar cells, lasers, transistors, catalysis, photocatalysis, gas sensors, piezoelectric devices, and photodetectors.2−11 Because numerous applications occur under ambient or humid conditions, it is important to understand the extent to which the ZnO interface adsorbs and/or dissociates water. Many factors can affect the tendency of water to dissociate on metal oxide surfaces, including surface structure, surface defects, the strength of surface/water interactions, and the corresponding water/water interactions.12 ZnO exhibits a hexagonal structure (Figure 1a) with nonpolar ZnO(101̅0) representing the sides of the hexagon and the top and bottom representing the polar Zn-terminated (0001) and O-terminated (0001)̅ surfaces.1 Each surface produces a different chemical environment to interact with adsorbed water, although roughly ∼80% of ZnO particle surfaces are composed of the ZnO(101̅0) facets13 making it the most heavily studied termination in both single crystal and powder form. The ZnO(1010̅ ) termination, in contrast to its polar counterparts, is electrostatically stable and well-ordered with few surface defects.14−16 Figure 1b shows a side view of the (101̅0) termination depicting slightly tilted ZnO “dimers” at the interface formed by 3-fold coordinated Zn and O atoms. Early water adsorption studies by Zwicker and Jacobi17 on single crystal ZnO(101̅0) used temperature-programmed desorption (TPD), ultraviolet photoelectron spectroscopy (UPS), and low energy electron diffraction (LEED). The © XXXX American Chemical Society

Figure 1. (a) ZnO hexagonal crystal structure. (b) Side view of (101̅0) termination.

conclusions from this study under cryogenic and ultrahigh vacuum (UHV) conditions were that water in the submonolayer to monolayer range was adsorbing molecularly predominantly at Zn sites. It was later shown by Meyer et al.18 using LEED, scanning tunneling microscopy (STM), He-atom scattering (HAS), and He-atom TPD under UHV that the Special Issue: Miquel B. Salmeron Festschrift Received: April 8, 2017 Revised: July 5, 2017 Published: August 11, 2017 A

DOI: 10.1021/acs.jpcb.7b03335 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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least 3 times prior to the start of experiments. Collecting APXPS spectra in the O 1s region back-to-back under constant RH showed no evidence of X-ray illumination induced hydroxylation. In order to minimize the possible effects of beam damage from extensive X-ray illumination, spectral acquisition times were limited to less than 3 min, the sample position was periodically moved to different spots, and the Xray exposure was turned off between spectra when changing temperatures. During isobaric experiments XPS spectra were mostly collected in the O 1s region every few kelvin while decreasing the sample temperature. Periodic spectra were also collected in the C 1s region and showed measurable but minimal C 1s contamination under ambient water vapor (Figure S2). A depth profiling experiment was performed by collecting O 1s spectra at different kinetic energies from ∼55 to 260 eV by varying the X-ray energy. XPS spectral analyses were performed using peak fitting software (CasaXPS, version 2.3.16) with GL (20) peak shapes and linear background subtraction. The full width at half-maximum (fwhm) was constrained to 1.70 ± 0.05 eV for both Ox and OH O 1s peaks which were separated by 1.5 eV. We found that this fitting procedure fits the data best for both isobars from low to high RH. If the constraints were relaxed, this led to large variations in Ox and OH peak areas as a function of increasing RH. In order to obtain quantitative insight into hydroxylation of ZnO(101̅0), the thickness of the hydroxyl layer (tOH) is calculated from the intensity ratio of the hydroxyl overlayer peak (IOH) relative to the bulk oxide peak (IOx) determined by fitting the experimental O 1s XPS spectra and using eq 1:27

monolayer structure formed a favorable hydrogen-bonded network at the ZnO(101̅0) interface. The network consisted of alternating molecular and dissociated water in a 2 × 1 superstructure, resulting in the coexistence of H2O and OH species on the surface under UHV. This result was confirmed by Wang et al.19 using TPD and high resolution electron energy loss spectroscopy (HREELS). While significant effort on a fundamental level has been directed to investigate the interaction of water with the ZnO(101̅0) interface, there is still little known about the molecular level effects of water on this termination under ambient conditions. The studies up to now have been under conditions where desorption predominates. The extent to which hydroxylation occurs under ambient conditions where both adsorption and desorption are occurring concomitantly remains unknown. It is essential to explore the interfacial chemistry as a function of relative humidity (RH) in order to elucidate the true interfacial surface composition under relevant ambient conditions (in the environment) and under operando conditions (in catalysis and active devices). Herein we investigate the surface chemistry of ZnO(101̅0) for the first time using synchrotron-based ambient pressure XPS (APXPS). APXPS provides insight into how oxide surfaces interact with water vapor at pressures in the Torr range.20,21 APXPS has been used to investigate the interaction of water vapor with various metal oxide surfaces under ambient conditions, including TiO2,22 SiO2/Si(111),23 SiO2/Si(100),24 Cu2O,25 Al2O3,25 α-Fe2O3(0001),26 MgO(100)/Ag(100),27,28 Fe3O4(001),29 FeO/Au(111),30 Al2O3/NiAl(110),31 GeO2/ Ge(100), 24,32 LaMO 3 perovskites, 33,34 LaCeO 2 (111), 35 LiNbO3,36 and LaFeO3.37 Herein we report on results of isobar experiments performed at two different pressures (0.07 and 0.3 Torr) and temperatures from 295 to 750 K up to a maximum RH of 2%. The extent to which ZnO(101̅0) dissociates water (hydroxylation) is quantitatively assessed under adsorption−desorption conditions in the presence of water vapor.

⎛ I λ N ⎞ tOH = λOH cos(ϕ) ln⎜1 + OH Ox Ox ⎟ IOxλOHNOH ⎠ ⎝

(1)

where ϕ = 20° is the angle between the sample normal and electron detector, N is oxygen atom density, and λ is the inelastic mean free paths (IMFPs) calculated with the NIST IMFP database40 at the various kinetic energies using and the TPP-2M predictive formula for ZnO and Zn(OH)2.

2. EXPERIMENTAL DETAILS Experiments were performed on a 10 mm × 10 mm × 0.5 mm ZnO(101̅0) single crystal (Marketech, roughness of 0). However, upon aggregation (formation of water dimer) an autocatalytic dissociation at MgO(100) terrace sites occurs for one of the water molecules within the dimer which is stabilized by the second molecularly adsorbed water.46,47 Moreover, this half dissociated water dimer formation occurs in the absence of any significant reaction barrier,47 making it truly autocatalytic. Thus, the sharp onset in hydroxylation observed on MgO(100) terrace sites at 10−2 % RH is attributed to water molecules going from monomers to dimers at terrace sites under ambient water vapor conditions. While a sharp onset in hydroxylation on Fe3O4(001) 29 and Al2O3/NiAl(110) 31 has also been observed at 10−2 % RH, additional mechanistic insight is still needed to explain why this occurs for these two interfaces. Density function theory (DFT), reactive force field molecular dynamics (RFFMD), and ab initio molecular dynamics (AIMD) simulations have been utilized to examine the interactions of water on ZnO(101̅0) from water monomers all the way up to multilayer water.18,45,48−63 Given we do not observe molecularly bound water under our experimental conditions and extensive hydroxylation at saturation coverage occurs in the range of 550−650 K, our interests are in understanding the initial fundamental interactions of water in the range of monomers up to 1 ML coverage in order to elucidate the mechanisms responsible for the sharp onset in hydroxylation. For water monomers at ZnO(101̅0) terrace sites both the molecular and dissociated forms are energetically stable (Ead < 0), although the molecular form is favored.58 The aggregation of water molecules at the ZnO(101̅0) has been investigated recently via DFT.45 Water dimers are energetically favorable for both the molecular and half dissociated forms, where the half dissociated is slightly favored. This suggests that both water monomers and dimers can exist in both the molecular and dissociated states on ZnO(101̅0), in contrast to MgO(100) terrace sites where dissociation is unstable for water monomers. Moreover, while for MgO(100) it is clear that there is a

T=

ΔH °

(

ΔS° − R ln

(Θ)n K ° p(1 − Θ)m

)

(2)

where T is the temperature, p is pressure, ΔH° and ΔS° are the enthalpy and entropy of terrace site hydroxylation, respectively, m and n are hydroxylation and dehydroxylation reaction orders, respectively, R is the gas constant, and K° is the standard state of p ° = 1 atm and Θ ° = 0.5 ML. Equation 2 allows for the analysis of isobar curves as a function of temperature. In order D

DOI: 10.1021/acs.jpcb.7b03335 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B to assess Θ as a function of RH (data in Figure 3b), we use the definition of RH: RH =

100p p0 (T )

ZnO(101̅0) suggests that there are other as yet unknown fundamental reaction steps occurring during the adsorption and desorption process. For example, water likely acts as a precursor and adsorbs molecularly prior to the onset in hydroxylation. Likewise, there are likely multiple steps that can occur in order for hydroxyl groups to recombine (dehydroxylation) and eventually desorb from the surface. The details of these fundamental steps cannot be extracted from the data herein. For the studies herein we were restricted to performing experiments up to a maximum of 2% RH due to technical limitations of sample cooling at the beamline. However, future studies with capabilities of cooling the sample further may yield some interesting results. Some of the early volumetric studies by Nagao and co-workers68−70 examining water on ZnO particles (which are mainly composted of ZnO(101̅0)) have shown that there is a sudden “jump” in molecular water coverage in the 20−30% RH range. This jump was attributed the sudden onset in lateral interactions in molecularly adsorbed water on an OH saturated surface.

(3)

where p0(T) is given by the temperature dependent function of Wagner and Pruss.66 By substitution of calculated values of T from eq 2 into p0(T) in eq 3, RH values are calculated as a function of coverage Θ and then plotted as Θ versus RH. Results of the best fits to the data are shown as dashed lines in Figure 3, giving values of ΔH° = −50 kJ mol−1, ΔS° = −15 J mol−1 K−1, m = 0, and n = 0.2 for ZnO(101̅0) terrace site hydroxylation. Note that the changes in values of the reaction orders m and n significantly affect the “shape” of the top and bottom of the model fits, respectively, whereas changes the thermodynamics parameters (ΔH°, ΔS°) significantly affect the “separation” and the “shift” in the sharp onset to hydroxylation. Thus, while there are four parameters being fit to the data, these four parameters are sensitive to the nature of the isobar curves.65 Comparing the results of ZnO(101̅0) to MgO(100),65 it is apparent that the observed surface thermodynamics are quite different. The enthalpy of hydroxylation is more negative (exothermic) for ZnO(101̅0) terrace sites (−50 kJ mol−1) compared to MgO(100) (−40 kJ mol−1), whereas the entropy is less negative (−15 versus −50 J mol−1 K−1, respectively). The negative values for the entropy of hydroxylation are consistent with the large entropy of the initial state (gas phase water plus metal oxide site) compared to the smaller entropy of the final state (hydroxylated terrace site). At room temperature the Gibbs free energy for ZnO(101̅0) terrace site hydroxylation is calculated to be ΔG°(500 K) = ΔH° − TΔS° = −43 kJ mol−1. For MgO(100) terrace site hydroxylation ΔG°(500 K) = −15 kJ mol−1.65 Thus, terrace site hydroxylation is significantly more favorable for ZnO(101̅0) compared to MgO(100). Note that this comparison in ΔG° was calculated at 500 K because it is within the temperature range of both ZnO(101̅0) and MgO(100) experiments. Moreover, it was shown previously using a Clausius−Clapeyron analysis for MgO(100) that the enthalpy of hydroxylation was independent of temperature under the range of temperatures used during the APXPS experiments. It is this difference in thermodynamics that drives the observed shift in the sharp onset to hydroxylation from 10−2 % RH for MgO(100) to 10−4 % RH for ZnO(101̅0). This trend in experimentally observed thermodynamics via APXPS is consistent with molecular simulations which show more negative values for calculated adsorption energies per molecule on ZnO(101̅0) compared to MgO(100) from one water up to monolayer coverage. For example, the most favorable dissociated dimer on ZnO(101̅0) is about −1.0 eV 45 whereas for MgO(100) it is about −0.5 eV.47 Likewise, the most favorable partially dissociated monolayer for ZnO(101̅0) is about −1.2 eV 45 whereas for MgO(100) it is about −0.7 eV.67 The adsorption (forward) reaction order (m = 0) is similar for both ZnO(101̅0) and MgO(100), whereas the desorption (reverse) reaction order for ZnO(101̅0) (n = 0.2) is smaller than MgO(100) (n = 1). These results suggest that the kinetic mechanisms for these two surfaces under adsorption− desorption conditions are different. Since these reaction orders are for the overall reaction (e.g., initial and final states), having reaction order values of less than unity observed for

4. CONCLUSIONS We have investigated the interaction of a ZnO(101̅0) single crystal surface with water vapor under two different isobaric conditions using ambient pressure XPS. When the RH reached ∼10−4 %, there was a sharp onset in surface hydroxylation. This was observed for two independent isobars at 0.3 and 0.07 Torr of water vapor. We attribute this onset in hydroxylation to reaction at stoichiometric terrace sites leading to a saturated surface covered in OH groups with a thickness of ∼0.26 nm. Because there is no observed molecularly bound water under our experimental conditions, we suggest the interface is covered with a fully dissociated 1 × 1 monolayer. This contrasts with molecular simulations and UHV experiments which suggest a half dissociated 2 × 1 is the most stable monolayer structure. The reasons for this discrepancy are not fully understood, but it should be noted that under our experimental conditions water molecules are adsorbing, dissociating, recombining, and desorbing concomitantly, a scenario that is not currently mimicked by existing UHV experiments and molecular simulations. A surface thermodynamic analysis of terrace site hydroxylation reveals that the Gibbs free energy is more favorable for ZnO(101̅0) compared to MgO(100), which gives rise to the sharp onset in hydroxylation at 10−4 % RH for ZnO(101̅0) compared to 10−2 % RH for MgO(100).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b03335. O 1s spectra of sputter/annealed ZnO crystal; C 1s spectra in vacuum and in the presence of water vapor (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

John T. Newberg: 0000-0003-1576-4436 J. Anibal Boscoboinik: 0000-0002-5090-7079 Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.jpcb.7b03335 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS J.T.N. acknowledges funding from a University of Delaware Research Foundation Strategic Initiative grant. The National Synchrotron Light Source and the Center for Functional Nanomaterials, Brookhaven National Laboratory are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886.

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DOI: 10.1021/acs.jpcb.7b03335 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcb.7b03335 J. Phys. Chem. B XXXX, XXX, XXX−XXX