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Perspective

Integrating Ab Initio Simulations and X-ray Photoelectron Spectroscopy: Towards A Realistic Description of Oxidized Solid/Liquid Interfaces Tuan Anh Pham, Xueqiang Zhang, Brandon C. Wood, David Prendergast, Sylwia Ptasinska, and Tadashi Ogitsu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01382 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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Integrating Ab Initio Simulations and X-ray Photoelectron Spectroscopy: Towards A Realistic Description of Oxidized Solid/Liquid Interfaces Tuan Anh Pham,∗,†,⊥ Xueqiang Zhang,∗,‡,¶,⊥ Brandon C. Wood,∗,† David Prendergast,∗,§ Sylwia Ptasinska,∗,‡,k and Tadashi Ogitsu∗,† Quantum Simulations Group, Lawrence Livermore National Laboratory, Livermore, CA 94551, United States, Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, United States, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, United States, Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, CA 94720, United States, and Department of Physics, University of Notre Dame, Notre Dame, IN 46556, United States E-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]

Abstract

calculations of explicit interfaces with ambientpressure X-ray photoelectron spectroscopy and direct spectroscopic simulations. We illustrate the benefit of this combined approach towards insights into native oxide chemistry at prototypical InP/water and GaP/water interfaces. This example suggests a more general roadmap for obtaining a realistic and reliable description of the chemistry of complex interfaces by combining state-of-the-art computational and experimental techniques.

Many energy storage and conversion devices rely on processes that take place at complex interfaces, where structural and chemical properties are often difficult to probe under operating conditions. A primary example is solar water splitting using high-performance photoelectrochemical cells, where surface chemistry, including native oxide formation, affects hydrogen generation. In this perspective, we discuss some of the challenges associated with interrogating interface chemistry, and how they may be overcome by integrating high-level first-principles

Graphical TOC Entry

∗

To whom correspondence should be addressed Quantum Simulations Group, Lawrence Livermore National Laboratory, Livermore, CA 94551, United States ‡ Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, United States ¶ Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, United States § Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, CA 94720, United States k Department of Physics, University of Notre Dame, Notre Dame, IN 46556, United States ⊥ Contributed equally to this work †

XPS

Intensity (Arb. units)

H2O/InP(001)

H2O*

534

M-O*H-M

533

532

M-O*H

531

M-O*-M

530

529

Binding energy (eV)

7

Band edge analyses InP

|VBM| (eV)

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The Journal of Physical Chemistry Letters

θ=1.5 ML

6 5

δ(2x4) θ=0 ML

4 3

Vacuum

ACS Paragon Plus Environment

1

Water

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Understanding and controlling the chemistry of complex solid/liquid interfaces is essential for discovering and optimizing emerging energy materials, including photoelectrochemical cells (PEC). These devices, which produce hydrogen via solar water splitting in a process long considered the “Holy Grail” of electrochemistry, 1–3 require efficient and stable operation. This remains a challenging task, as many of the most efficient photoabsorbing materials, such as IIIV semiconductors, are unstable under harsh operating conditions at the solid/liquid interface in a PEC device. 4 On the other hand, metal oxides, although being more stable, possess transport properties and band gaps that are unfavorable for achieving high solar-to-hydrogen conversion efficiency. 5,6 In recent years, significant efforts have been directed toward improving the stablity and efficiency of photoabsorbing materials by interfacing the photoelectrode absorbers with catalysts and/or protective layers. 5,7 One important aspect of interface chemistry that has been discussed at length in the context of certain PEC semiconductors is the possible role of oxidation. Native oxidation occurs spontaneously on the surfaces of many non-oxide PEC photoelectrodes under operation, and can also be introduced deliberately during processing to alter electrochemical properties. 5,8–14 The various negative and positive impacts of these oxides continue to be the subject of much debate. The insulating nature of oxides can inhibit charge transfer and introduce additional kinetic barriers to redox processes; 8 for instance, in the case of GaP, it has shown that water-splitting reactions may be limited by the formation of a thin Ga2 O3 oxide layer due to insufficient band bending and unfavorable band alignment at the GaP/Ga2 O3 interface. 15 Oxide-related defect states in III-V semiconductors might also act as traps that can induce corrosion processes. 9 At the same time, it has been suggested that oxides can aid hydrogen evolution, possibly due to improved kinetics for hydrogen transport. 10–12,16 Oxide films have also been investigated as a potential strategy for stabilizing surfaces of photocorrosive electrode materials. 5,13,14

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These often contradictory roles highlight the fact that much is left to be understood regarding the nature and specific impacts of the surface oxidation at complex semiconductor/water interfaces. Many of the key knowledge gaps relate to the electronic properties of the interfaces, most notably the effects on the absolute energetic alignment of the semiconductor valence band maximum (VBM) and conduction band minimum (CBM) with respect to the various redox processes (e.g., hydrogen evolution versus corrosion) that might occur in the device. 1 While it is generally understood that these band positions depend on the details of the semiconductor interface, 17 the specific relationship between the electronic properties of oxidized PEC surfaces and their chemical constituents remains enigmatic. Realization of this correlation would open up opportunities not only for tuning interfacial chemistry to achieve highly efficient PECs but also for quantitative predictions of photoelectrode/water interfacial structure based on macroscopic observations such as experimental band edge measurements. In principle, obtaining the necessary understanding of chemical and electronic properties of interfaces requires experimental probes that can achieve atomic sensitivity under operating conditions, which currently presents significant technical challenges. 18 Band edge measurements yield information on the electronic structure of the photoelectrodes, but provide little understanding on the chemistry at the photoelectrode/liquid interface. In this regard, recent development of ambient pressure X-ray photoelectron spectroscopy (APXPS) offers a promising way for investigating chemical species and intermediates of solid/liquid interfaces, as this technique directly probes chemical processes near surfaces at elevated vapor pressures, which approximate in situ conditions that cannot easily be achieved with standard measurements. 19,20 In practice, however, definitive XPS identification of chemically similar species can be extremely challenging, particularly for complex amorphous oxides where intrinsic configurational heterogeneity significantly broadens


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and overlaps key spectral features. For wellcharacterized surfaces in vacuum or gas-phase adsorbents, experimental standards may aid in the interpretation of photoemission spectra; however, such reference standards are likely to be far less reliable for these more complex interfacial systems and intermediate species. In addition to difficulties in the preparation of standard samples, definitive fingerprinting of interfacial XPS spectral signatures using experimental references tends to be limited to cases in which discrete changes in formal oxidation state are encountered. On the other hand, subtler changes in local chemical environment may produce distinctions that are unique to the surface chemistry and cannot be unambiguously identified using existing bulk or molecular standards. In this regard, experiments can benefit from additional information derived from simulation and modelling, since recent advances in highperformance computing and theoretical techniques now make it possible to computationally probe the chemistry and electronic properties of the photoelectrode/water interface more explicitly. 16,21–38 In particular, first-principles molecular dynamics (FPMD) simulations allow for accessing complex interfacial chemical processes, 39 while high-level electronic structure theory, such as many-body perturbation theory within the GW approximation, 40 offers a promising way to compute accurate properties of interfaces, due to its ability to treat the electronic structure of semiconductors and aqueous solutions on the same footing. 41–49 In recent years, these techniques have advanced to the point where quantitative accuracy is realistically achievable on modern computing platforms. Nevertheless, the length and time scales accessible to first-principles simulations continue to constrain the configurational complexity. Perhaps more importantly, the exact results obtained from first-principles techniques depend strongly on the precise details of the model. Such details are particularly difficult to intuit for systems with a high degree of structural and/or chemical complexity, such as kinetically stabilized surface oxides. For these systems, the predictive power of simulations becomes critically dependent on experimental in-

sights. The rapid developments in advanced computational and experimental methods, as well as the mutual interdependence of the two approaches, afford a unique opportunity to directly integrate advanced first-principles methods with APXPS experiments for interrogating complex interfaces. One way to achieve this is to compute XPS core-binding energies directly from first-principles based on proposed interface models. This allows for direct comparison between chemical processes of theoretical models and those of realistic photoelectrode/water interfaces, while providing a more complete interpretation of the experimental photoemission spectra. By taking advantage of a close feedback loop between computational and experimental tools, one can hope to fully elucidate the photoelectrode chemistry (e.g., oxidation), as well as its relationship to the observed electronic properties (e.g., band edges) during operation. In this perspective, we provide a roadmap for how high-level electronic structure theory may be combined with APXPS and other in situ experimental data as a first step towards probing surface chemistry at a complex solid/liquid interface. We illustrate the specific procedure by applying it to a demonstration PEC system, focusing on the nature and role of native surface oxides of GaP and InP in water. Notably, these materials can be alloyed for highly efficient PEC photocathodes but have persistent stability issues. 4,7 In addition to a better understanding of the interfacial chemistry, we show how the approach can offer insight into the specific correlation between surface composition and band edges of photoabsorbers in liquid water, an important consideration for functionality and stability of a PEC device. The theoretical framework is based on FPMD simulations in conjunction with GW calculations, and leverages recently developed algorithms that now make it possible to obtain reliable quantitative predictions of band edge positions within full interface simulations. In our demonstration system, model InP and GaP surfaces representing different degrees of oxidation are employed to investigate the influence of sur-


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-2

-3

Energy (eV)

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Figure 1: Models of InP/water interfaces considered in this work: Pristine reconstructed δ(2 × 4)(001) (left) and hydroxylated surfaces (right).

Eg=2.44 eV

-4

Eg=1.25 eV

SHE -5

-6 (110) (001)

GaP

(110) (001)

InP

Figure 2: Band edge positions of pristine In(Ga)P(110) and (001) surfaces in vacuum, as computed with the G0 W0 approximation. The G0 W0 results were obtained with the wavefunctions obtained with DFT and the PBE exchange-correlation functional. Experimental results are presented by blue lines. 52,53 The potential of the standard hydrogen electrode (SHE) is indicated by the dashed line.

face oxide species on the band edges, which is shown to be significant due to the change in the surface polarization. These theoretical results are combined with experimental band edge measurements to probe and compare the general chemical properties of model semiconductor surfaces in water. Finally, we outline a procedure for how APXPS experiments can be closely integrated with direct first-principles core-binding energy calculations of semiconductor/water interfaces to provide a more complete interpretation of the surface chemistry, including the contribution of specific oxygen-derived surface species to the photoelectron spectrum. In the case of III-V semiconductors, the results reveal clear and interesting differences in the structure and chemistry of native oxides on GaP and InP photoelectrodes at the interface with water, demonstrating the power of the integrated theory-experiment approach. We conclude by highlighting opportunities for improving and extending this approach towards a comprehensive and realistic description of complex interfaces under device operation conditions. Before discussing the results, we briefly summarize the theoretical models employed in this work. Specifically, to capture the structure and chemistry of GaP and InP photoelectrodes under ideal and working conditions, we considered pristine GaP(001) and InP (001) surfaces in vacuum, as well as different models of oxidized surfaces in direct contact with liquid water. For the former, we focus on the mixeddimer δ(2×4) In- and Ga-rich (001) reconstruction that is generally preferred under vacuum conditions, 50 while also making comparisons to non-polar (110) surfaces in order to address the

role of surface polarization. For the surfaces fully in contact with liquid water, we considered two model systems that represent two extreme limits of oxidation extent, which were generated using FPMD simulations. The mixeddimer δ(2×4) reconstruction with zero oxygen coverage was used to simulate the low oxidation limit. For the high oxidation limit, we used a fully hydroxylated Ga- and In-rich (001) surface derived from our prior investigations. 16,36 This model is based on an oxygen coverage of θ = 1.5 ML, consisting of surface M –OH–M bridges along h110i with M –OH dangling bond atop structures (M ≡ In or Ga). As we will show below, the consideration of model systems representing extreme limits of the oxidation degree allows for infering how the realistic surface chemistry envolves under operation. Representative snapshots of mixed-dimer δ(2×4) and hydroxylated surfaces in contact with liquid water are shown in Fig. 1 for InP. Further details of the model systems can be found in the Supporting Information and our previous publications. 9,16,36,51 Relationship Between Surface Oxidation and Electronic Properties. The relation between electronic properties and surface oxidation can be established based on a combination of highlevel electronic structure theory and estab-


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lished experimental band edges. Experimentally, the measured absolute band edges (VBM and CBM) of pristine semiconductor surfaces vac in vacuum (EVBM/CBM ) correspond to the ionization potential and the electron affinity, and are typically measured using photoemission experiments. On the other hand, band edges wat of semiconductors in liquid water (EVBM/CBM ) are usually obtained from capacitance-voltage measurements of the semiconductor/water interface (see Supporting Information), where the wat vac difference between EVBM/CBM and EVBM/CBM represents the collective effect of water on the band edges of the semiconductor surface. 54 In vac our calculations, EVBM/CBM are computed using many-body perturbation theory within the G0 W0 approximation 55 following the procedure wat described in Ref. 33,56. To compute EVBM/CBM , the effects of liquid water were taken into account by considering the variation in the electrostatic potential at the interface, as described more fully in Ref. 33. This method has the advantage of treating the electronic structure of the semiconductor and liquid water on equal footing, 43–46 and has previously demonstrated predictive capability for interfaces. 33 Further details of band edge calculations are described in the Supporting Information. To baseline the discussion of surface oxidation in water and to validate the predictive accuracy of our theoretical approach, we first summarize the first-principles results for pristine InP and GaP surfaces in vacuum, for which the structural and electronic properties are established. 52,53 We find that the G0 W0 approach accurately describes the electronic properties of both systems, yielding band gaps of 2.44 eV and 1.25 eV for GaP and InP, respectively. These values are in good agreement with experimental values of 2.26 eV and 1.35 eV. 52,53 Furthermore, the predicted VBM/CBM for InP (110) and δ(2×4) (001) surfaces are consistent with available experiments, yielding errors less than 0.15 eV as demonstrated in Fig. 2. 52,53 Good agreement between theory and experiment is also found for the GaP (110) surface 52,53 (experimental band positions for δ(2×4) GaP (001) surface are not available for comparison). This is a significant improvement over the conven-

tional density functional theory (DFT) calculations with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, 57 which produce sizable errors in the computed band gap (about 50%) and band edge positions (about 0.6–0.7 eV). The results highlight the importance of surface structure and composition in determining the photoelectrode band edges, even in the absence of liquid water. For instance, δ(2×4) (001) surfaces exhibit band edges closer to vacuum than (110) surfaces by 0.7–1.0 eV for both GaP and InP. This shift in the electronic energy levels can be qualitatively explained based on the linear variation of the surface potential associated with the change in the surface dipole moment: 17 ∆Vsurf = −4πµsurf /S0 ,

(1)

where µsurf is the surface dipole moment component normal to the surface plane for a unit cell of area S0 ; a negative µsurf implies a positive ∆Vsurf and band positions further from the vacuum, and vice versa. According to the electronegativity relation χP = 2.19 > χIn/Ga = 1.78/1.81, electrons are extracted from In/Ga, resulting in a positive µsurf for the In/Ga rich δ(2×4)(001) surfaces. The band edges therefore move closer toward the vacuum level than for the non-polar (110) surfaces. Accordingly, effects of surface polarization are critical in the determination of the electronic properties of pristine GaP/InP surfaces. We now turn to the relation between electronic properties and surface oxidation for InP and GaP in the presence of liquid water. We note that our computational approach probes the band edges in the bulk region of photoelectrode materials where the bulk band gap is recovered; accordingly, the computed CBM and VBM follow the same trend in the presence of liquid water, reflecting the band edge evolution obtained in differential capacitance measurements. 59 We find the effects of water on the band edges are significant, as demonstrated in Fig. 3. Focusing on the VBM, the level for pristine δ(2×4) GaP(001) and InP(001) surfaces is shifted toward the vacuum level by 0.8–1.0 eV


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The Journal of Physical Chemistry Letters 7

7

InP 5

GaP

θ=1.5 ML

6

|VBM| (eV)

|VBM| (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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δ(2x4) θ=0 ML

4

θ=1.5 ML

6 5

δ(2x4) θ=0 ML

4

3

3

Vacuum

Water

Vacuum

Water

Figure 3: Absolute values of the valence band maximum of GaP/InP in vacuum (mixed-dimer δ(2 × 4) surfaces), and in the presence of liquid water, including mixed-dimer δ(2 × 4) (θ = 0, magenta solid line) and hydroxylated (θ = 1.5 monolayer (ML), green solid line) surfaces. Results computed for the corresponding surfaces with a monolayer of water molecules and hydroxyls in vacuum are presented by magenta and green dashed lines, respectively. All theoretical results were obtained with the G0 W0 approximation using the PBE wavefunction. Experimental results obtained for pH=0–14 and neutral water are represented by filled boxes and the solid blue lines, which were deduced from Wrighton et al., 58 as discussed in more detail in the Supporting Information. InP(001) δ(2×4) surfaces with a monolayer of adsorbed water molecules, as well as hydroxylated surfaces with water removed. These surfaces are broadly representative of configurations with a monolayer of molecularly adsorbed (undissociated) water molecules and a monolayer of dissociated water molecules, respectively. Calculated band edges of these models in vacuum are represented by the dashed magenta and green lines in Fig. 3, respectively. Interestingly, the overall trend in the band edge shifts is similar to what was found for the full interface with liquid water (solid lines, Fig. 3). Accordingly, the photoelectrode band edges are largely determined by these first monolayers of adsorbed water molecules or hydroxyl groups. This behavior can be understood by considering that adsorbed water molecules on the mixeddimer surfaces introduce a net positive surface dipole, resulting in VBMs closer to the vacuum. On the other hand, the dense oxygen coverage on the hydroxylated surfaces leads to a negative surface dipole and shifts band edges in the opposite direction by a similar amount. The presence of the full interface with liquid water preserves this overall picture, and consequently leads to similar trends in the band edge positions. At the same time, the effect of the full interface is also quantitatively important, contributing on the order of 0.5–0.8 eV to the band

when the effect of liquid water is taken into account, as represented by the difference between the red and magenta solid lines in Fig. 3 for each materials. The pristine GaP(001) and InP(001) surfaces in water can be compared with the corresponding fully hydroxylated surfaces in water with an oxygen coverage of θ = 1.5 ML (solid green lines) to assess the effect of surface oxidation. In the latter case, the band edges are again shifted with respect to the pristine surface in vacuum, but in the opposite direction. Note that the hydroxylated surfaces in water can be interpreted as the limit of full surface water dissociation, with all available sites covered with hydroxyl; on the other hand, the pristine surface can be interpreted as the limit of exclusively molecularly adsorbed water that remains intact. 36 We conclude that three factors are important for determining the band edge positions of a photoelectrode in liquid water: surface polarity, the interface with water, and the extent of surface oxidation. However, these effects are closely interconnected, and together form the basis of the relationship between surface composition and electronic structure. While our simulations include full solid/liquid interfaces, to gain additional insights into the effects of interfaces with water on the calculated band edges, we also considered GaP(001) and


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0.0 ML and 1.5 ML represented by the theoretical models. Our conclusion is also consistent with other XPS experiments 61,62 that find a mixture of non-dissociated and dissociated adsorbed water on the GaP(001) and InP(001) surfaces in the presence of water vapor. The G0 W0 calculations also indicates significant differences between GaP and InP in the hydrogen-bond network at their interfaces with liquid water. As shown in Fig. 3, when compared to the corresponding results obtained for the full interfaces with bulk water (solid green and magenta lines), band edges of the δ(2 × 4) surface with a monolayer of adsorbed water molecules (dashed magenta lines), as well as those of the hydroxylated surface (dashed green lines) consistently show that the deviations are different for GaP and InP. Specifically, these deviations are notably larger for GaP (by ∼0.3 eV). Such an observation suggests that GaP band edges are affected more strongly by interfacial water, and conversely, that the perturbation introduced by the semiconductor surfaces on interfacial water molecules is stronger for GaP. The above conclusion reflects key aspects of our previous FPMD simulations on GaP/water and InP/water interfaces. 16 In particular, we reported that water at the interface with GaP(001) shows significant density perturbations from the bulk limit, whereas the effect of InP(001) on water is much weaker. This difference was attributed to higher covalent character in the Ga–O surface bonds, which results in a more rigid hydrogen-bond network near the interface. 16 The relatively larger impact of GaP on the properties of interfacial water implies a stronger direct interaction with the semiconductor surface, which is reflected in the band edges of Fig. 3. Interfacial Chemistry from X-ray Photoelectron Spectroscopy. Although the band edge analysis provides insight into the overall extent of surface oxidation, an additional probe is required for more specific identification of oxygen-bearing surface chemical species. In previous studies, we found that surface oxygen atoms can exist in several coordination environments, which were found to interchange dynam-

edge shift. This is likely due to the partial mitigation of the surface dipole due to hydrogen bonding with the liquid water. Overall, our analysis highlights the importance of dipolar effects on electronic band edges of the photoelectrodes in water that stem from complex chemistry involving adsorbed and dissociated water molecules at the solid/liquid interface. We note that the importance of surface dipoles on the electronic band edges of photoelectrodes and, by extension, performance has also been discussed in the literature. 60–63 For example, it has been shown that the surface dipole leads to a downward movement of the GaP band edges when in contact with water vapor (by about 0.17 eV in water pressure of 10−5 mbar 61 ). This has important implications for performance, since it reduces the offset between the CBM and the water reduction potential, thereby benefiting the use of GaP as a photocathode for hydrogen evolution. Interfacial Chemistry from Band Edge Analysis. Having demonstrated the surface oxidation-band edge correlation, the G0 W0 calculations can be combined with experimental band edge determinations to predict chemistry of GaP/water and InP/water interfaces. In particular, band edges of GaP and InP in water is known to vary with respect to the pH following the Nernst equation, 58,64,65 resulting in a wide range of experimental VBMs, i.e., 4.72– 5.52 eV and 4.88–5.68 eV below vacuum in the pH range of 0–14 for GaP and InP surfaces, respectively, as represented by the filled boxes in Fig. 3. When compared to the experimental band edges at neutral pH that corresponds to the simulation condition, our calculations show that theoretical VBMs of δ(2×4) and hydroxylated surfaces straddle the experimental data, indicating that the realistic surfaces are likely to be a mixed of the two model surfaces that represent two extreme limits of oxidation extent. As a consequence, this observation suggests that non-dissociated and dissociated adsorbed water coexist on the GaP(001) and InP(001) surfaces, and that water dissociation leads to the formation of surface oxygen and hydroxyl groups with an overall surface oxygen coverage between the two extreme limits of


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The Journal of Physical Chemistry Letters Binding energy (eV)

InP(001)/H2O

534

533

H2O*

GaP(001)/H2O



Binding energy (eV)

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532

531

M-O*H-M M-O*H

530

529

534

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532

H2O* M-O*H-M

M-O*-M

M-O*H

531

530

529

M-O*-M

Figure 4: The O 1s photoemission spectra (red circles) recorded for InP(001) (left) and GaP(001) (right) at a water vapor pressure of 5 mbar. Peak assignments (vertical lines) for different surface species were derived from first-principles: surface oxygen bridge bonds (M –O*–M , magenta), atop hydroxyl (M –O*H, blue), bridge hydroxyl (M –O*H–M , green), and molecularly adsorbed water (H2 O*, maroon), where M ≡ In or Ga, and ∗ indicates the oxygen atom from which the photoelectrons are emitted. The experimental spectra (red circles) were fitted with four components using Gaussian/Lorentzian hybrid functions and calculated binding energies (see Suporting Information for details of the fitting procedure). sion that the GaP(001)/water interface shows more rigid hydrogen-bond network than its InP counterpart, which was initially deduced from FPMD simulations and the combined theoryexperiment band edge analysis. This apparent discrepancy turns out to be easily understood once the O 1s spectra are properly interpreted with aid from first-principles theory, highlighting the strength of an integrated experimenttheory approach to studying interfacial chemistry. To assist the interpretation of the experimental photoemission spectra, first-principles calculations of oxygen binding energies (BEs) were carried out for the oxidized GaP(001) and InP(001) surfaces in water, particularly for the hydroxylated interfaces as they provide a more complete set of surface oxide configurations. In addition, to elucidate oxygen speciation, the individual oxygen atoms contributing to the computed BEs were categorized according to the same coordination scheme described before, consisting of bridge oxygens, bridge and atop hydroxyls, and adsorbed water molecules. We emphasize that our computational procedure is applicable to the full semiconductor/water interface, thereby allowing for direct comparison between the experimental spectra and the calculated BEs (see Supporting Information). Ac-

ically in FPMD simulations: M –O–M , M –OH– M , M –OH, and H2 O. 9,16 This suggests a more complex surface speciation at the semiconductor/water interface, which can be probed more directly using APXPS experiments. Specifically, we recorded the O1s spectra of the semiconductor surfaces at room temperature, with the water vapor pressure adjusted from ultrahigh vacuum (UHV) to 5 mbar. The reproducibility of the XPS spectra were carefully checked; specifically, each experimental conditions, we repeated XPS measurements for at least three times. The O1s signal measured for InP and GaP at the highest pressure is presented in Fig. 4. Clear differences are visible in the shape of the photoemission spectra obtained for InP(001)/water and GaP(001)/water interfaces. Most notably, the spectrum of GaP is much broader and exhibits more complex features within its overall shape. Such an observation is consistent with previous XPS studies of GaP(001) and InP(001) surfaces at the interface with water, 61,62 and implies fundamentally different chemistry occurring at the two interfaces, as we have suggested. However, at first glance, the relative breadth of the O 1s spectrum for GaP compared to InP appears counterintuitive to our earlier conclu-


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ing task in the case of complex systems. The breadth of the overall photoemission spectrum of GaP(001) in Fig. 4 is therefore not attributable to thermal broadening alone and does not necessarily indicate a more fluid interchange between surface species. In addition, we verified that the shape is not merely an artifact of the band bending effects, as examination of the spectral shift of GaP and InP before and after the introduction of water vapor indicates that the variations in the band bending are relatively small (∼0.1 eV). Rather, it reflects the much more significant chemical differences between oxygen species on GaP, which lead to a larger collective spread in the corresponding BEs. In particular, a large separation between photoemission peaks obtained for oxygen atoms belonging to atop and bridge hydroxyls in the GaP spectrum implies that M – O bond strengths in M –OH and M –OH–M are significantly different for the GaP surface, in contrast to InP where these bond strengths are much more similar. This indicates that the interchange of atop and bridge hydroxyls is likely to be more kinetically limited for GaP compared to InP. Accordingly, the core-binding energy calculations also support the interpretation of a more rigid, well-defined network structure at the GaP/water interface predicted by the G0 W0 calculations. Finally, it is worth discussing possible effects of surface structure orientation on the binding modes of water molecules. Our current and previous studies 70 of the GaP(001) and GaP(111) surfaces indicate that the binding mode of water molecules are not sensitive to surface orientation. For example, we find that the shape of the O 1s spectra are similar, showing a consistent position of the main peak at 531.7±0.1 eV. However, this similarity may stem from the fact that these are both polar surfaces; accordingly, further investigation of other surface orientations with different characters, e.g., non-polar surface, is needed in order to provide a solid conclusion on the surface orientation dependence of water behavior at the interface with III-V semiconductors. Conclusions and Outlook. The combination of experimental measurements with high-level

cordingly, this method provides standards for interpreting photoemission spectra that are far more representative of the in situ environment under which the measurements are performed, while simultaneously accounting for the configurational complexity of the dynamical interfaces. The computed BEs of photoelectrons emitted from oxygen atoms belonging to the bridge bonds (M –O*–M ), atop (M –O*H) and bridge (M –O*H–M ) hydroxyls, and molecularly adsorbed water (H2 O*) are presented by blue vertical lines in Fig. 4 for both InP/water and GaP/water interfaces, together with fitted experimental spectra using the calculated BEs. Our analysis shows that the contribution to the spectra at lower binding energies (530-532 eV) are attributed to water dissociation products, whereas the broad peak around 533 eV is attributed to molecular water molecules on the semiconductor surfaces. The contribution to this peak stems mostly from water molecules in the first monolayer, but may also originate from those belonging to water clusters on the surface. We find that the photoelectron BEs follow the order H2 O* > O*H > M –O*–M for both materials; such an ordering is found to be in agreement with the assignment obtained in previous in-situ XPS studies of water dissociative adsorption onto metal surfaces 19,66,67 and Ga-based surface compounds. 68–70 Interestingly, the calculated BEs patterns obtained for oxygen configurations on GaP and InP are significantly different, particularly in the relative BEs rising from different hydroxyl groups. Specifically, it is shown that the photoelectron BEs obtained for oxygen belonging to the atop and bridge hydroxyls are rather close in energy for InP (less than 0.2 eV), together contributing to the main experimental peak. In contrast, these two peaks are well separated by almost 0.8 eV for GaP, leading to a much wider BEs pattern. This is consistent with the experimental observation, showing that the GaP surface yields a broader spectrum with more complex features. Accordingly, BEs patterns obtained from calculations can greatly assist in interpretation and assignment of photoemission spectra obtained from experiments, which is a challeng-


9


first-principles methods can provide valuable insight into the role of surface oxidation on InP and GaP photoabsorbers. For instance, our results clearly indicate that surface polarity, the interface with water, and the extent of surface oxidation are all important factors in determining band edges. Theoretical calculations of XPS reference spectra and band edges can be combined with analogous experiments to establish these factors in the context of the specific surface chemistry. When combined with APXPS and band edge measurements, the theoretical calculations can be used to deduce that dissociated and non-dissociated adsorbed water coexist on the GaP and InP surfaces, altering surface dipolar properties. Notable differences between GaP and InP are also observed, with the band edges of GaP exhibiting much higher sensitivity to the presence of water. Differences between the two surfaces are also reflected in the O 1s XPS spectra in the presence of water, which are broader for GaP than for InP. Using simulations of model interfaces, we trace the origin of this behavior to the higher rigidity of the water hydrogen-bond network at the GaP/water interface. The importance of the intrinsic surface polarity in the determination of the photoelectrode band edges demonstrated here indicates that surface treatment is critical for improving PEC efficiency. In this regard, careful engineering of the photoelectrode surface may result in a well-adapted band alignment with their native oxide layers and the water oxidation/reduction potentials, thereby benefiting water-splitting performance. Beyond these specific insights into GaP and InP oxidation, the recipe illustrated here— using experimental insights to refine models, and in turn using modeling data to inform experimental interpretation—can be viewed as a more general roadmap for investigations of wider classes of interfaces. High-level firstprinciples methods are employed to understand the specific relationship between surface chemistry and key electronic (e.g., band edge) and spectroscopic (e.g., XPS) features at the interface, based on model systems that represent features present in the real system. The reference electronic structures and spectra of these

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models are then compared with in situ experimental probes of these same quantities at the complex interface, with the similarities and differences used to infer how the surface chemistry evolves under operation. For example, APXPS experiments can be used to probe not only the chemistry but also the electronic properties of solid/water interfaces, as demonstrated in recent studies, 71,72 thereby providing a direct comparison with theoretical calculations. This combined theory-experiment approach offers a promising way for investigating surface morphology and chemical composition, which is important to guide future strategies for improving device performance through interface engineering. As first-principles methods advance to access larger and longer simulations, and as resolution and sensitivity of X-ray probes simultaneously continue to improve, these methods will become capable of probing interfaces with increasing complexity. In this event, the combination of computational and experimental approaches will certainly play an increasingly important role in interfacial science. Whereas historically, solid/liquid simulations have often focused on simplified models using singlemolecule or monolayer interface descriptions, computational developments are also likely to make explicit interface simulations more routine. 73 This can offer a qualitatively or quantitatively different view of surface chemistry and electronic properties, as we will illustrate in the example covered within this perspective. On the experimental side, equally important is the role of in situ probes, which are beginning to provide a far more complete description of reactive interfaces than can be accessed using conventional surface-science probes under vacuum conditions. 18 Nevertheless, the approach we outline here is not without limitations when considering actual operating conditions, motivating further developments in the community. For instance, additional physical factors should be considered to improve the realism of the simulation and characterization environments. For instance, in the case of electrochemical interfaces, voltage and photobias play a critical role under oper-


10

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include interface structure, 21,23,24,29,30,80–83 mass transport, 16 and key reaction pathways, 24,84–86 all of which are needed to develop comprehensive and rational approaches for controlling (photo)electrochemical processes through interface engineering.

ating conditions, 72 yet these are neglected in the roadmap we introduce here. On the simulation side, developments are underway to address this shortcoming, 74–76 but the methods are currently in their infancy. In addition, our investigation considered only the role of liquid water, yet actual operating electrolytes contain salt ions that can have a significant effect on performance. 77 Other shortcomings may be unique to either simulations or to experiments. For instance, first-principles simulations are typically limited to the immediate interfacial region operating under equilibrium conditions. However, many devices have behaviors that are connected to operation under non-equilibrium conditions, where dynamical changes in factors such as local concentration or activity must be considered. This requires integrating first-principles chemical simulations with broader mesoscale or device-level simulations. On the experimental side, APXPS does not provide any notable spatial resolution, and is also limited in temporal resolution. In this regard, it may be helpful to combine this technique with other advanced probes that can offer higher spatiotemporal fidelity. 78,79 The potential importance of these capabilities becomes evident when considering the possible role of local singularities and dynamical processes in promoting chemical processes. Although such features can be readily introduced in models, validation becomes difficult without appropriate experimental probes. For this reason, the roadmap introduced here avoids making specific connections between the surface chemistry and interfacial catalytic processes, focusing instead on properties such as band edges, which can be obtained via temporal and spatial averaging (i.e., equilibrium properties). Further developments in techniques with high spatiotemporal resolution could strengthen the connection between interface composition and chemical kinetics. These are a high priority for future research efforts as such approaches will offer ways to validate, and if necessary revise, the atomistic insights obtained from first-principles simulations of electrochemical interfaces that are particularly relevant for (photo)catalytic reactions. Examples

Acknowledgement Part of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. T.A.P. acknowledges support from the Lawrence Fellowship. T.O and B.C.W. are supported by the U.S. Deparment of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. X.Z. and S.P. acknowledge the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award Number DE-FC02-04ER15533 (NDRL no: 5152). Computational support was from the LLNL Grand Challenge Program. We thank Eric Schwegler and Giulia Galli for fruitful discussions.

Biographies Tuan Anh Pham is a Lawrence Postdoc Fellow in the Quantum Simulations Group and Materials Science Division at the Lawrence Livermore National Laboratory (LLNL). He received a Ph.D. in Chemistry at the University of California, Davis in 2014. He is also a recipient of the 2011 LLNL Graduate Scholar, 2014 UC Davis Outstanding Chemistry Dissertation Award, and 2017 PCTC Postdoctoral Fellow Award. His research interest includes the development and applications of first-principles techniques to validate, understand and predict materials properties for energy and enviromental applications. Xueqiang Zhang is currently a postdoctoral research associate in the Department of Chemistry at University of Illinois at UrbanaChampaign. He received a Ph.D in Physical Chemistry from the University of Notre Dame


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in 2016. His research interests include the design and mechanistic understanding of heterogeneous catalysis and photocatalysis, and the study of fundamental surface chemistry at vapor/solid interfaces using in-situ/operando spectroscopic techniques.

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phasis on energy-related materials, using advanced in situ techniques. Tadashi Ogitsu is a staff scientist in the Quantum Simulations Group and Materials Science Division at LLNL. He received his Ph.D. in Materials Science in 1994 from the University of Tsukuba in Japan. His research interests include high pressure physics, ultrafast non-equilibrium physics, and materials and interfaces for energy applications such as carbon aerogels for supercapacitor applications and photoelectrochemical hydrogen production. He is the LLNL Laboratory Lead for the HydroGEN Advanced Water Splitting Materials consortium, funded by the Fuel Cell Technologies Office within the DOE Office of Energy Efficiency and Renewable Energy.

Brandon C. Wood is a staff scientist in the Quantum Simulations Group and Materials Science Division at LLNL. He received a Ph.D. in Materials Science and Engineering from MIT in 2007. His primary research interests lie in the application of high-performance computing to ab initio and mesoscale simulations of complex interfaces for energy storage and conversion. He is the LLNL Laboratory Lead for the Hydrogen Storage Materials—Advanced Research Consortium (HyMARC), funded by the Fuel Cell Technologies Office within the DOE Office of Energy Efficiency and Renewable Energy.

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David Prendergast is a staff scientist at The Molecular Foundry, Lawrence Berkeley National Laboratory. He has been the Facility Director for the Theory of Nanostructured Materials Facility at The Molecular Foundry since 2012. He obtained a Ph.D. in Physics from University College Cork, Ireland (2002). His current research focuses on developments and applications of first-principles electronic structure theory in the context of energy-relevant phenomena in materials and chemistry. He has particular expertise in the simulation and interpretation of synchrotron X-ray spectroscopy measurements as a means of connecting characterization of complex interfacial systems to atomistic or molecular scale structural and dynamical models.

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