Thermodynamics of Hydrogen Adsorption and Incorporation at the

Article pubs.acs.org/JPCC

Thermodynamics of Hydrogen Adsorption and Incorporation at the ZnO(101̅0) Surface Hugh F. Wilson*,†,‡ and Amanda S. Barnard† †

CSIRO Virtual Nanoscience Lab, 343 Royal Pde, Parkville 3052, Victoria, Australia School of Applied Sciences, RMIT University, Melbourne 3001, Victoria, Australia

‡

ABSTRACT: Hydrogen, as a surface adsorbate and a bulk impurity, plays an important role in determining the electronic and catalytic properties of zinc oxide, but the nature of the interaction between these two species remains poorly understood. In this work we study the thermodynamics of hydrogen adsorption on zinc oxide’s (1010)) surface. We find that the adsorption of hydrogen on the oxygen sites only, as observed in several previous experiments, is metastable and consider the thermodynamics of exchange of hydrogen between surface and bulk as an explanatory mechanism. We propose that the hydrogen termination of zinc oxide at room temperature is strongly dependent on the hydrogen concentration of the interior and thus that surface reactions involving hydrogen may proceed in an unreliable way depending on the history of the sample, particularly for ZnO in nanostructures.

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INTRODUCTION Zinc oxide is a highly versatile material presenting a combination of unusual properties that makes it ideal for many applications. Aside from its main existing industrial uses as a white pigment, in sunscreen, in the vulcanization of rubber, as an antibacterial agent, and in heterogeneous catalysis for many industrially important reactions, ZnO has emerging applications in transparent electronics, photovoltaics, gas sensing,1,2 and the degradation of pollutants by photocatalysis.3 Hydrogen is an extremely common impurity in ZnO in both interstitial and substitutional forms as well as molecular interstitial form,4 and its presence both on the surface and within the bulk plays a dominant role in determining many of its key physical and chemical properties. In the bulk, hydrogen is unavoidably present, regardless of fabrication method, as an n-type dopant5 and is the dominant species that controls its electronic properties. In catalytic applications, H on the ZnO surface plays an important role in many reactions the including synthesis of methanol, the hydrogenation of unsaturated hydrocarbons, and the water−gas shift reaction. ZnO nanostructures have also been proposed as sensors for molecular hydrogen1,2,6 and materials for hydrogen storage.7 Zinc oxide under ambient conditions forms a wurtzite structure, with crystals most commonly presenting some combination of (1010), (1120), (0001), and (0001) surfaces. The (1010) and (1120) surfaces are nonpolar and significantly more stable than the two polar surfaces and hence comprise the majority of the surface area of most ZnO structures,8−10 including ∼80% for ZnO powders11 and the vast majority for nanowires. The (1010) surface is more stable9,10 and consists of © XXXX American Chemical Society

an array of surface Zn−O dimers, with each surface atom bonded to one surface- and two second-layer atoms of the opposite type, leaving one potential adsorption site per surface atom. Nonpolar ZnO surfaces are highly reactive when exposed to atomic hydrogen and much less so for molecular H2.12,13 Early IR spectroscopy experiments in the 1960s14 showed the adsorption of H on ZnO powders at 303 K to produce both Zn−H and O−H stretching vibrations. Heating above 353 K while maintaining the H2 atmosphere resulted in both peaks decreasing in intensity while maintaining a constant ratio. Wöll and coworkers have carried out a comprehensive set of experimental studies of atomic hydrogen adsorption on ZnO(1010)15,16 utilizing a combination of helium atom scattering, high-resolution electron energy loss spectroscopy, and scanning tunneling microscopy. Two (1 × 1) adsorption configurations were observed. The first was a low-temperature configuration (200 K)that was assigned to the heterolytic adsorption of hydrogen on both sides of each ZnO dimer (labeled 2 ML-both in Figure 1). Upon heating to 300 K this is replaced by the second configuration, assigned to the adsorption of hydrogen on the oxygen side only of each dimer (which we label 1 ML-O in Figure 1). Traeger et al.17 used nuclear reaction analysis to study the concentration of surface and subsurface hydrogen at (1010), (0001), and (0001) surfaces. Exposure of the (1010) surface to Received: September 4, 2015 Revised: October 21, 2015

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DOI: 10.1021/acs.jpcc.5b08628 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

the surface, the presence of either the unterminated reactive cation sites or the loosely bound zinc-mounted hydrogen atoms may either promote or inhibit a given reaction. Understanding the atomic-scale thermodynamics of hydrogen on the nonpolar ZnO surfaces is thus of high importance for optimizing the use of ZnO in many applications. In this work, we compute surface-phase diagrams of hydrogen on the ZnO(1010) surface using density functional theory thermodynamic methods and predict that the configuration in which only oxygen sites are terminated is metastable relative to the heterolytic termination of both surface species under all thermodynamic conditions. We propose that this configuration is formed at room temperature in a kinetically limited manner by the insertion of hydrogen from the zinc side of the dimer to form a positively charged bulk interstitial in a process whose thermodynamic favorability depends strongly on defect concentration within the bulk.

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Figure 1. Schematic depiction of the studied hydrogen adsorption configurations on the ZnO(1010) surface in the 2 × 2 surface supercell. The oxygen and zinc sides of the surface dimers are shown in red and gray, respectively, with the sites of the hydrogen atoms denoted by the light-colored circles.

METHODS Total energies and vibrational contributions to the free energy were computed using the VASP plane-wave density functional theory code.26 Pseudopotentials of the PAW type27 and the exchange-correlation function of Perdew, Burke, and Ernzerhof28 were used. Kohn−Sham orbitals were expanded in a basis set with a cutoff of 500 eV, and k-points were sampled at a density of 8 × 8 in the unit cell, corresponding to 4 × 4 in the 2 × 2 four-dimer supercell used here for all calculations. The slab used was four bilayers thick, consisting of 64 atoms in the 2 × 2 supercell, with a vacuum spacing of 18 Å between periodic images. Careful convergence tests were carried out on cutoff, kpoint sampling, and slab thickness. Optimized geometries of two of the slabs are shown in Figure 2. Lattice constants used were the same as in our previous work9 (a = 3.2862 Å, c = 5.3005 Å).

atomic hydrogen at temperatures of 30−100 °C was found to result in a surface hydrogen concentration half that found by exposure of a cleaned surface to water vapor, leading the authors to assign the observed termination to the 1 ML-O configuration. Exposure of a (1010) slab to atomic hydrogen was also found to result in a substantial increase (0.11 to 0.22% over 50 min at room temperature) of hydrogen concentration at a depth of 100 Å. The two hydrogen adsorption configurations previously mentioned (1 ML-O and 2 ML-both) have also been considered by theoretical works using a variety of methods.18−20 Zapol18 used Hartree−Fock calculations to study hydrogen adsorption in both configurations, finding adsorption on both sides of the dimer (2 ML-both) to have a more favorable adsorption energy per atom than adsorption only on the O side (1 ML-O). Wander and Harisson19 used B3LYP hybrid functional calculations to study the geometry and electronic structure of (only) the heterolytically terminated 2 ML-both configuration. Martins et al.21 used high-level CCSD calculations to compare the energetics of the 2 ML-both and molecular H2 adsorption configurations on small ZnO cluster models. Most recently, Usseinov et al. 20 used hybrid PBE0 calculations to study hydrogen adsorbed on both sides of the dimer and on the O side at several different coverages. An important discrepancy is found between the Zapol and Usseinov results, with Zapol predicting a larger adsorption energy per atom for the 2 ML-both configuration and Usseinov et al. predicting the opposite. Extensive theoretical work has also been carried out on the formation and diffusion energies of hydrogen in bulk ZnO, as an interstitial,22−24 and in substitutional sites.25 In light of the existing theoretical and experimental work, it remains unclear whether the 1 ML oxygen-terminating or 2 ML heterolytic adsorption configuration will occur under any given thermodynamic conditions. The relative stability of these two configurations has important implications for processes involving this surface; the 1 ML-O configuration is known to be metallic at the surface,15 while the 2 ML-both termination is insulating, like the bare surface. For heterogeneous catalysis at

Figure 2. Optimized geometries of the 1 ML-O (left) and 2 ML-both (right) slabs. Slabs are terminated on top and bottom. Zinc atoms are shown in gray, oxygen atoms in red, and hydrogen atoms in white.

We computed vibrational contributions to the free energy in a quasi-harmonic approximation using a finite displacement method. Phonons were restricted to those that occur within the (2 × 2) surface supercell. We considered seven possible adsorption configurations for hydrogen on the ZnO(1010) surface, with hydrogen bonded to either the zinc or oxygen side of the dimer or to both. The structure set included structures studied in previous works as well as a number not previously considered. The energies and vibrational modes of all structures were computed in a (2 × 2) surface supercell consisting of four Zn−O dimers. The structures are depicted schematically in Figure 1 with their labels. For the 1 ML-both and 0.5 ML-O structures, three B


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The Journal of Physical Chemistry C terminations are possible depending on which two out of four dimers are terminated; the configuration displayed in each case was found to be more stable by tenths of an electronvolt than the alternatives due to more favorable placement of charges.

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ADSORPTION ENERGIES Computed adsorption energies per hydrogen atom for each configuration are shown in Table 1. All adsorption energies are Table 1. Total Adsorption Energy Per Hydrogen Atom, Vibrational Entropy Difference to the Clean Surface Per Atom, and Implied ΔG of Adsorption at 300 K for Each of the Hydrogen Adsorption Configurations Studied configuration

ΔE (eV/atom)

2 ML-both 1 ML-O 1 ML-Zn 1 ML-both 0.5 ML-O 0.5 ML-both 0.25 ML-O

−2.49 −2.13 −0.55 −2.57 −2.14 −2.44 −2.54

ΔS (eV/K/atom) −8.58 −5.83 −1.72 −1.25 −1.37 −2.17 +3

× × × × × × ×

10−5 10−5 10−4 10−4 10−4 10−4 10−5

ΔG @300 K (eV) −2.519 −2.145 −0.599 −2.606 −2.185 −2.502 −2.528

Figure 3. Atomic structures for dimers in the 1 ML-O, 1 ML-Zn and 2 ML-both configurations. H, O and Zn atoms are depicted in white, red and gray. Bader charges are shown in blue and bond lengths in black.

given in electronvolts per hydrogen atom, relative to the reference states of atomic hydrogen and the bare surface. Negative energies imply that adsorption is exothermic. In agreement with previous studies, adsorption on the Zn side of the dimer is found to be less favorable by far than other configurations, with an adsorption energy of −0.55 eV/atom for the 1 ML-Zn structure. Adsorption on the oxygen sites only is substantially more favorable, with an energy of adsorption of −2.13 eV per atom for the 1 ML-O structure. Adsorption on both sides of the dimer in the 2 ML-both configuration is found, however, to be more favorable than adsorption on either side, at −2.49 eV/atom. Vibrational contributions are found to play a small role at surfaces close to room temperature but do not change the ordering of the energies. The vibrational contributions tend to favor structures containing Zn−H bonds over structures containing O−H bonds. Figure 3 shows the computed atomic structures of the dimers in the four configurations with (1 × 1) periodicity (bare, 1 MLO, 1 ML-Zn, and 2 ML-both). Bond lengths for the dimer and adsorbate bonds are shown, as are effective charges on each atom computed by the method of Bader.29

Table 2. Adsorption Energies Per Atom (Relative to Atomic Hydrogen and the Bare (1010) Surface) for Hydrogen on the ZnO(1010) Surface in the 1 ML-Zn, 1 ML-O, and 2 MLboth Adsorption Configurations, Calculated by the Present and by Several Previous Methods method present work (PBE) present work (HSE06) Zapol (HF + corr) Zapol (HF) Usseinov et al. (PBE0) Wander and H(B3LYP)

Eads(1 ML-O) [eV/atom]

Eads(1 ML-both) [eV/atom]

−2.13 −2.02 −1.80 −0.95 −4.0

−2.49 −2.51 −2.87 −2.92 −3.14 −2.56

Table 3. Adsorption Energy (in eV per H atom relative to atomic H) for Hydrogen on ZnO for Coverages of 0.25, 0.5, and 1 Monolayer, with Hydrogen Adsorbed on O Atoms

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COMPARISON TO PREVIOUS CALCULATIONS The adsorption energies of the 1 ML-O and 2 ML-both configurations have been studied by several previous authors using hybrid and Hartree−Fock methods. A comparison of our computed energies to those of previous theoretical works is shown in Table 2. The stability of the 2 ML-both configuration matches the predictions of Zapol18 but is in conflict with the PBE0 results of Usseinov et al.,20 which predicts a higher adsorption energy per atom for the 1 ML-O than the 2 MLboth configuration. Another surprising discrepancy between the present results and those of Usseinov is shown in Table 3, which compares the effect of increasing coverage on adsorption energy in configurations where only hydrogen atoms are terminated. In Usseinov et al.’s results for structures with hydrogen adsorbed only on O, the adsorption energy per atom increases steadily from −2.7 to −4.0 eV as the coverage increases from 0.25 to 1 ML, a behavior that is at odds with what is seen in most

method

0.25 ML-O

0.5 ML-O

1 ML-O

present work (PBE) Usseinov et al.20

−2.47 −2.70

−2.14 −3.62

−2.10 −4.0

systems. In the present results, by contrast, a less surprising behavior is seen, with the adsorption energy per atom decreasing slightly from −2.47 to −2.10 eV/atom. To determine whether discrepancy between the present results and those of Usseinov et al.20 is due to the difference between our DFT methodology and Usseinov’s hybrid methodology, we also used a hybrid methodology similar to that used by Usseinov, based in our case on the HSE06 functional. For these more computationally demanding calculations we used a slightly lower cutoff (400 eV rather than 500 eV) and a (1 × 1) unit cell, giving an effective k-point density half that used in the DFT-PBE calculations. An exact exchange fraction of 25% was used. The results of our HSE06 calculations were quite similar to the PBE results, shown in Table 2, with Eads −2.02 eV per hydrogen atom for the 1 ML-O configuration and −2.51 eV per hydrogen atom for 2 ML-both C



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DISCUSSION Given the apparent thermodynamic instability of the 1 ML-O phase observed at room temperature (although not at 200 K15), we are naturally led to suspect that kinetic effects lead to the formation of the 1 ML-O structure at room temperature by allowing the loss of zinc-bonded but not oxygen-bonded hydrogens from the 2 ML-both phase. The first possible kinetic route is the loss of hydrogen to the molecular gas phase, perhaps by the pairing up of hydrogen atoms from adjacent zinc atoms; however, this is implausible at room temperature due to the large distance between adjacent Zn-terminating H atoms in the 2 ML-both configuration (3.29 Å); the temperature at which the hydrogen is lost is also substantially below the temperature at which hydrogen desorbs from the ZnO(0001)-Zn terminated surface,15 and in any case the process is endothermic. The other possible kinetic route is the loss of hydrogen from the zinc atoms into the ZnO bulk. The formation of substitutional hydrogen requires the creation of an oxygen vacancy, which is likely to be energetically inaccessible at room temperature; however, interstitial hydrogen is known to be quite stable and yet highly mobile at room temperature. We therefore propose that interstitial hydrogen forms at room temperature by the insertion of zinc-mounted hydrogens from the 2 ML-both configuration but not from the oxygen-mounted hydrogen atoms, which are more closely bonded to their host atoms. This is also consistent with the nuclear reaction analysis work of Traeger,17 which shows the hydrogen concentration at a depth of 100 Å increasing when the surface is dosed. The calculations of Van de Walle5 give a formation energy of +1.07 eV for the neutral hydrogen interstitial H0i relative to the energy of molecular hydrogen. This makes the formation of a neutral interstitial from the zinc-bonded hydrogen in the 2 MLboth configuration endothermic with an energy of +1.67 eV. The formation of the positively charged interstitial H+i , on the contrary, varies depending on the Fermi energy, but for a Fermi energy at the bottom of the band gap, van de Walle predicts a formation energy of −1.84 eV relative to molecular hydrogen, corresponding to a −1.24 eV (exothermic) total energy of formation for the positive hydrogen interstitial from the zincbonded surface hydrogen. If the Fermi level lies in the middle of the 3.4 eV band gap this implies that the total formation of the hydrogen interstitial is endothermic with energy +0.45 eV. The value of the Gibbs free energy of formation ΔG of the H+i interstitial may be expected to differ from this value, however, due to two factors. The first is the shift in Fermi level with dopant concentration; hydrogen is a donor in the interstitial and substitutional sites, meaning that as the concentration of hydrogen (or other donor defects) increases the Fermi level will also increase, making the formation of the interstitial less favorable. The effect of the defect concentration term on the formation energy is always positive because no common p-type dopants exist31 and increases as the donor concentration increases. The second important effect is configurational entropy. The effect of the configurational entropy term on ΔG is always negative and increases with increasing H+i concentration. The net effect of these two terms is that ΔG increases as the concentration of H+i increases, going from negative values for small donor concentrations to positive values as the concentration gets larger. When ΔG passes through zero, the expected configuration of the surface will change from 1 ML-O to 2 ML-both as the loss of the zincmounted hydrogens ceases to be thermodynamically viable. We

adsorption on both Zn and O. We can thus dismiss the exchange-correlation functional as the origin of the discrepancy. The basis set remains a possible cause of discrepancy, as our calculations use a plane-wave basis set in comparison with the Gaussian basis sets used by Usseinov; care must be taken with Gaussian basis sets for the correct description of the metallic nature of the 1 ML-O surface. Further work may help to resolve this discrepancy. We also computed the energies of adsorption for the analogous hydrogen configurations on the ZnO (1120) surface. On this surface we studied only four structures: the bare surface, the termination of all Zn atoms (1 ML-Zn), the termination of all O atoms (1 ML-O), and the termination of all atoms of both species (2 ML-both). Results for these configurations are shown in Table 4 and compared with the Table 4. Computed Energies of Adsorption on the ZnO(1120) Surface, Compared with the Results of Wang et al.30 method present work (PBE) Wang30 (LDA)

Eads(H− Zn)

Eads(1 ML-O) [eV/atom]

Eads(2 ML-both) [eV/atom]

−0.47

−2.01 −3.24

−2.52 −3.75

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previous plane-wave DFT-LDA results of Wang et al.30 The adsorption energy per atom in the 2 ML-both configuration is again found to be substantially higher than the adsorption energy per atom of the 1 ML-O configuration, in agreement with the results of Wang et al.

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SURFACE PHASE DIAGRAM Given the total energy and vibrational entropy values, we construct a surface phase diagram showing the most thermodynamically stable surface phase for a ZnO(1010) surface, which may exchange particles with a hydrogen reservoir at a given chemical potential and temperature, as shown in Figure 4. We find that only the bare, 1 ML-both, and 2 ML-

Figure 4. Surface phase diagram for H adsorption on the ZnO(1010) surface as a function of μ per hydrogen atom and temperature.

both surface phases are stable at any μ value; in particular, the 1 ML-O phase is not thermodynamically stable for any value of T or μ. This is somewhat surprising, given the consistent identification of the 1 ML-O surface phase in several experimental works.15−17 Vibrational effects play only a small role. D


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may be sufficient to cause a flip in the favorability of the conversion of the 2 ML-both to the 1 ML-O phase.

thus suggest the possibility that either the 1 ML-O or 2 ML-Zn surface phase may potentially be the thermodynamic ground state at room temperature, depending on the concentration of defects in the bulk and/or on the surface as well as band structure effects due to the surface. To determine the conditions under which the equilibrium surface termination could be expected to change from 2 MLboth to 1 ML-O is difficult due to the dependence of the Fermi level on a large number of different defect sites as well as the effect of nearby surfaces. An extremely rough first-order estimate to determine whether a sign change in ΔG is plausible at reasonable concentrations however is the entropy of the defect by the simple configurational formula TS = kBT log(n int /Nsites)

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CONCLUSIONS Our work suggests that the equilibrium surface termination of ZnO is dependent on the details of the defect distribution and band structure. This could have important practical consequences because both the saturation and the conductivity of the surface will vary depending on factors not typically controlled or measured in experiment. This effect could be especially important in nanostructured ZnO, where the large ratio of surface sites to bulk sites will lead to more rapid changes in interior concentration with surface hydrogen flux. If the equilibrium surface termination does indeed vary strongly and unpredictably due to effects happening in the bulk, this may have important applications for the use of ZnO in important catalytic reactions as well as in gas sensing applications. The 1 ML-O surface termination leaves the highly reactive Zn+ sites exposed, while the 2 ML-O surface termination gives a source of loosely bound hydrogen atoms, which may lead to the promotion of different types of reaction. Future work should also consider the role of water and other forms of oxygen in this process.

(1)

where nint is the number of hydrogen interstitials and Nsites is the density of possible sites, which we take as equal to the number of ZnO units (almost certainly an underestimate because several interstitial sites are close in energy5). The variation in the Fermi level we assume to be dependent on the number of donors that we assume to include only hydrogen, in both the interstitial and substitutional sites ⎛ n + nsub ⎞ Ef − E i = kT log⎜ int ⎟ N0 ⎠ ⎝

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(2) −3

where N0 is the intrinsic number of carriers, ∼10 cm for ZnO at 300 K, nsub is the number of substitutional hydrogen Hs present, and Ei is the intrinsic Fermi level that we locate in the middle of the band gap. The sum of these two terms gives the formation free energy of the positive interstitial from the 2 MLO configuration relative to the +0.45 eV internal energy difference when the Fermi level is in the middle of the band gap. The dependence of this free energy of formation on substitutional and interstitial hydrogen concentrations is shown in the inset of Figure 5. The formation energy is negative for 6

AUTHOR INFORMATION

Corresponding Author

*Tel: +61 3 9662 7349. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 5. Energetics of the 1 ML-O and 2 ML-both hydrogen surface phases in relation to the gas and interstitial hydrogen phases. Energy barrier heights are unknown and shown only as a guide to the eye. Inset: estimated formation energy of the positively charged interstitial as a function of concentration of interstitial and substitutional hydrogen based on the vastly simplified model described in the text.

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Thermodynamics of Hydrogen Adsorption and Incorporation at the

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