Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2021 Jan 6;125(2):1361–1367. doi: 10.1021/acs.jpcc.0c09382

Structural Evolution of Ga–Cu Model Catalysts for CO2 Hydrogenation Reactions

Jian-Qiang Zhong 1, Shamil Shaikhutdinov 1,*, Beatriz Roldan Cuenya 1
PMCID: PMC7830109  PMID: 33510828

Abstract

graphic file with name jp0c09382_0010.jpg

We studied the initial stages of Ga interaction with the Cu(001) surface and environment-induced surface transformations in an attempt to elucidate the surface chemistry of the Cu–Ga catalysts recently proposed for CO2 hydrogenation to methanol. The results show that Ga readily intermixes with Cu upon deposition in vacuum. However, even traces of oxygen in the gas ambient cause Ga oxidation and the formation of two-dimensional (“monolayer”) Ga oxide islands uniformly covering the Cu surface. The surface morphology and the oxidized state of Ga remain in H2 as well as in a CO2 + H2 reaction mixture at elevated pressures and temperatures (0.2 mbar, 700 K). The results indicate that the Ga-doped Cu surface under reaction conditions exposes a variety of structures including GaOx/Cu interfacial sites, which must be taken into account for elucidating the reaction mechanism.

1. Introduction

Gallium-containing alloys and intermetallic compounds have recently received significant attention in catalysis, in particular, for industrially important processes such as selective hydrogenation of alkynes (on Pd–Ga)1 and CO2 hydrogenation to methanol (on Ni–Ga).2,3 The methanol synthesis was found to proceed with relatively high activity and selectivity on Ni5Ga3/SiO2 catalysts even at an ambient pressure. The latter is important for the development of small-scale devices operating at low pressures using solar-generated hydrogen. On the basis of theoretical considerations, the reactivity has been attributed to the formation of Ga-rich metallic sites.2 On the other hand, Ga is prone to oxidation,4 thus resulting in a Ga oxide shell, surrounding a metallic core of nanoparticles (NPs),5 which can only be reduced in hydrogen at high temperatures.

The discovery of the Ni–Ga catalysts triggered studies on other Ga-promoted 3d metal catalysts, in particular, Cu,6 which is a crucial component of the industrial Cu/ZnO/Al2O3 catalyst for methanol production. A strong promotional effect of Ga was observed on SiO2-supported Cu NPs. The addition of Ga increased the methanol formation rate without a significant change in the reverse water–gas shift reaction. The results suggested that Ga promotes Cu by increasing its methanol selectivity, likely by creating new active sites for methanol formation without modifying its oxidation state, which remains mostly metallic under the reaction conditions. However, the formation of a Ga2O3 phase that activates CO2 has been used to explain the infrared spectra.6 Nevertheless, surface structures and surface reactions on Ni–Ga and Cu–Ga nanoparticulate systems remain far from being well understood. In this respect, “surface science” studies employing surface-sensitive techniques applied to well-defined model systems may provide substantial insights into atomic structures and the surface chemistry of the Ga-based catalysts and aid in understanding the promotional effects of Ga in hydrogenation reactions.

To date, most of the model studies performed on Ga–3d metal compounds were primarily focused on the growth of Ga oxide thin films because of its application in various disciplines related to microelectronics, solar cells, and gas sensors.7 Upon exposure to oxygen at 300 K, an amorphous Ga oxide thin film forms on the CoGa(110) and CoGa(001) surfaces. At high temperatures (above 700 K), it transforms into an ordered structure assigned to a thin film of β-Ga2O3.8,9 A more recent in situ surface X-ray diffraction study10 of CoGa(001) complemented with density functional theory calculations revealed that the ultrathin oxide film consists of an oxygen ion double layer containing the basic building block of β-Ga2O3. Ultrathin films of Ga2O3 were also grown on a Ni(100) substrate by oxidation of a ∼15 Å-thick Ga film prepared by physical vapor deposition.11,12 Upon O2 exposure at room temperature, an amorphous Ga oxide layer formed on top of the metallic Ga layer. Subsequent annealing at 700 K in vacuum led to the formation of a well-ordered oxide film tentatively assigned to the γ’-Ga2O3 phase exposing the (111) surface.11 Interestingly, the diffraction pattern observed on this film is very similar to that reported for a Ga oxide film formed on the CoGa(110) surface,8 pointing to the similar structural motif of the Ga oxide films on both metal surfaces.

To the best of our knowledge, such studies are missing for the Ga–Cu system. Actually, the interest in the Ga–Cu interaction is still largely driven by the use of liquid Ga-based alloys in low-temperature soldering in microelectronics.13 The Ga–Cu equilibrium phase diagram is quite complex. At least 10 different phases were found.14 Among those, the CuGa2 intermetallic phase is the dominant phase formed at Ga/Cu interfaces at room temperature, which induces the wetting behavior of liquid Ga via stable metallic bonds to Cu.15 Note also that this phase is the most stable at relatively low temperatures (below 550 K), a typical temperature range of the CO2 hydrogenation reaction.

In order to get more insights into the Ga–Cu interaction, in particular, the Ga–Cu surfaces and their transformations under catalytically relevant conditions, here we prepared planar model systems by physical vapor deposition of Ga onto a Cu(001) substrate. The surface structures were studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and near-ambient pressure X-ray photoelectron spectroscopy (NAP–XPS). The results suggest that metallic Ga readily intermixes with the Cu surface upon deposition. However, even traces of oxygen in the gas ambient cause oxidation of Ga and the formation of two-dimensional Ga oxide nanoislands uniformly covering the Cu surface. The morphology and the oxidized state of Ga remain in a pure H2 atmosphere as well as in the CO2 + H2 reaction mixture. Obviously, the Ga-doped Cu surface under CO2 hydrogenation reaction conditions exposes a variety of surface structures, particularly GaOx/Cu interfacial sites, which have to be taken into account for elucidating the reaction mechanism.

2. Methods and Materials

The experiments were carried out in an ultrahigh vacuum (UHV) chamber equipped with LEED/AES (from Specs), STM (Omicron), and a quadrupole mass spectrometer (QMS, from Hiden). The Cu(001) single crystal (MaTeck GmbH) was mounted onto a sample holder plate having a hole to ease sample heating by electron bombardment from the backside of the crystal using a tungsten filament. The temperature was measured using a chromel–alumel thermocouple clamped at the edge of the Cu crystal. The surface was cleaned by several cycles of Ar+ sputtering and UHV annealing at 800 K, until the LEED shows sharp diffraction spots and no impurities were detected by AES. Gallium (Aldrich, 99.999%) was deposited using an electron-beam assisted evaporator (Focus EFM3) from a BN crucible placed in a Mo liner. All STM images were obtained in vacuum at room temperature using Pt–Ir tips.

NAP–XPS measurements were performed in another UHV chamber (Specs) equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) and a hemispherical analyzer (Phoibos 150 NAP). The samples were transferred between the two UHV setups using a vacuum suitcase having a background pressure below 10–7 mbar.

3. Results and Discussion

Figure 1 shows the STM images obtained on the Cu(001) surface after Ga deposition at room temperature as a function of the deposition time. Note that a clean Cu substrate was prepared prior to each deposition. At low Ga coverages, that is, when the ratio of the Ga (at 1070 eV) and Cu (at 921 eV) peaks in AES spectra (see Figure S1 in the Supporting Information) is below 0.1, the STM images revealed square-shaped islands randomly distributed on the terraces (Figure 1a–c). Interestingly, there is no preferential nucleation at step edges. The island density (Figure 1a,b) and then the island size (Figure 1b–d) both increase with the increasing deposition time, indicating that the islands are pure Ga in nature. Accordingly, the square shape of the islands points to the formation of an epitaxial Ga(001) layer. The height of the islands is virtually identical to that measured for the monolayer steps separating the Cu(001) terraces. The latter can, in principle, be explained by the atomic radius of metallic Ga being only slightly smaller than that of Cu (130 and 135 pm, respectively), thus making them hardly distinguishable in the topographic STM images. On further increasing the coverage (Figure 1e), the islands coalesce and the surface becomes uniformly covered with a few similarly oriented rectangular islands. The latter seem to be the precursors for the formation of much larger structures imaged like flat-lying “sticks” several hundreds of nanometers long and only about 1.5 nm high (Figure 1f). Based solely on the STM images, one can tentatively conclude that Ga grows on the Cu(001) surface in the Stranski–Krastanov mode, that is, first forming an epitaxial Ga layer on top of which three-dimensional structures start to grow at further Ga deposition.

Figure 1.

Figure 1

Open in a new tab

STM images of Ga deposited on Cu(001) at 300 K at increasing deposition time (a–f). The height profiles along the lines are shown below the images. Tunneling parameters (bias and current) are (a) 0.5 V, 0.05 nA, (b) 1.5 V, 0.08 nA, (c) 0.3 V, 0.05 nA, (d) 1.0 V, 0.2 nA, (e) 0.7 V, 0.2 nA, and (f) 0.38 V, 0.29 nA.

To shed more light on the atomic structure of the Ga/Cu(001) surface, we studied the adsorption of CO as a probe molecule, commonly used for mono- and bimetallic surfaces.16,17Figure 2a shows the TPD spectra obtained on the clean Cu(001) surface as a function of CO exposure (in Langmuir (L), 1 L = 10–6 Torr × s). One desorption peak is observed that gains in intensity and shifts to lower temperatures with increasing CO dosage.

Figure 2.

Figure 2

Open in a new tab

TPD spectra of CO adsorbed at 130 K on a clean Cu(100) surface (a) and a Ga/Cu(100) surface (b) at CO exposures (in L) as indicated. The heating rate was 2 K/s. The inset shows STM images (300 nm × 300 nm) of the samples studied.

The spectra changed considerably after Ga deposition (Figure 2b). Based on the STM images (see the inset in Figure 2b and also Figure 1c), and assuming that all deposited Ga atoms form Ga(001) monolayer islands, Ga covers about 50% of the Cu surface in this sample. Therefore, one anticipates the desorption signal from uncovered Cu(001) to be attenuated by a factor of two. Accordingly, all other signals should be associated with CO adsorption on the Ga(001) surface. However, the spectra showed a different picture (Figure 2b). At the lowest CO exposure, the peak appears at 162 K, which is 16 K lower than that for the clean Cu(001) surface at the same dosage. With the increasing CO exposure, this signal gains inintensity, shifts to a lower temperature, and saturates at about 1 L. Concomitantly, a new feature shows up at 137 K at higher dosages, and a weak signal appears at around 180 K. It may well be, however, that the former one solely represents a “cut-off tail” of CO desorption having a maximum at the lower temperature.

Therefore, in contrast to the STM-based model of Ga(001) islands surrounded by pristine Cu(001), the TPD results point to the absence of a pure Cu(001) surface. Apparently, the Ga adatoms intermix with the Cu surface at room temperature, probably in the same manner as observed for Zn–Cu systems (ref.18 and the references therein). The conclusion is further supported by STM results on the samples exposed to oxygen.

Figure 3 shows the room-temperature STM images of the Ga/Cu(001) surface after annealing in UHV and then in 10–7 mbar of O2 at several different temperatures as indicated. Clearly, the islands become larger at 400 K (Figure 3a,b), probably via an Ostwald ripening mechanism, which, in turn, implies the high diffusivity of surface atoms.19 The morphology does not change much after exposure to oxygen at 400 K. The surface reconstructs and becomes considerably rougher at 500 K and remains as such after annealing at higher oxidation temperatures (600–700 K). However, the islands and the terraces show a virtually identical morphology. This finding points to an uniform distribution of Ga in the initial sample, both within islands and on terraces, in full agreement with the above-presented TPD results showing the absence of CO desorption signals of pure Cu(001).

Figure 3.

Figure 3

Open in a new tab

Room-temperature STM images of Ga on Cu(100) as-deposited (a), after annealing first in UHV at 400 K (b), and then in 1 × 10–7 mbar of O2 at 400 K (c), 500 K (d), 600 K (e), and 700 K (f) for 5 min each. (g) The zoomed-in image of (f). (h) Topography profile along the dashed line indicated in (g). The scale bar in (a–f) is 75 nm and that in (g) is 10 nm. The corresponding LEED patterns (at 123 eV) are shown as insets. Tunneling parameters (bias and current) are (a) 1.0 V, 0.2 nA, (b) 1.0 V, 0.2 nA, (c) 1.0 V, 0.1 nA, (d) −0.5 V, 0.06 nA, (e) 0.5 V, 0.05 nA, (f) 1.0 V, 0.05 nA, and (g) 0.5 V, 0.1 nA.

To get information about the chemical states of Cu and Ga, we employed XPS. The “as-deposited” Ga/Cu(001) samples were transferred to another UHV setup using a vacuum suitcase (base pressure < 10–7 mbar). Figure 4 shows the XPS spectra of the Ga 2p3/2, Cu 2p3/2, and O 1s core levels as well as the Cu LMM Auger transition region that allows to distinguish Cu(I) and Cu(0) species. The Cu 2p3/2 peak at 932.5 eV in combination with the Auger line at 568.1 eV are fingerprints of metallic Cu, expected for the “as-deposited” Ga/Cu(001) surface. A small O 1s signal detected at 530.7 eV (Figure 4b) suggests that Ga deposits are partially oxidized during sample transfer due to the well-known high affinity of Ga to oxygen. Indeed, a prominent shoulder appears in the Ga 2p3/2 region at 1117.5 eV in addition to the main peak at 1116.4 eV (Figure 4a) corresponding to oxidized and metallic Ga, respectively.4,20 After exposure to 10–7 mbar of O2 at 400 K, the 1117.5 eV signal gains in intensity, while the 1116.4 eV signal attenuates. The oxidation seems to be completed at 500 K as no more changes are detected after exposure at 600 and 700 K, in agreement with the STM results showing no further morphological changes (see Figure 3). However, during these surface transformations, Cu remains metallic (Figure 4b,c) although the presence of O atoms on Cu(001) cannot be excluded. Indeed, in addition to the main peak at 530.7 eV associated with the Ga oxide phase, there is a weak signal at 529.8 eV, which was observed on the Ga-free Cu(001) surface (Figure S2 in the Supporting Information). Another weak state at 532.3 eV is tentatively assigned to traces of carbonates (CO32–) (see also Figure S3 for the C1s region) and/or hydroxyls (OH) caused by the reaction of Ga oxide with residual CO2 and water molecules in the vacuum background.

Figure 4.

Figure 4

Open in a new tab

XPS spectra of the Ga/Cu(100) surface measured at 300 K in UHV before (black) and after annealing in 1 × 10–7 mbar of O2 at 400, 500, 600, and 700 K as indicated in the same color code in all panels. In addition to the Ga 2p, O 1s, and Cu 2p core level regions, the Cu LMM Auger lines are also shown. The typical morphology of the sample before the XPS measurements is shown in Figure 3a.

Therefore, the combined STM and XPS results clearly show an oxygen-induced phase separation of the initially bimetallic Ga–Cu surface into a Ga oxide and a pure Cu surface, ultimately resulting in surface roughening (Figure 3). It is interesting to note that the corrugation amplitude measured on oxidized samples by STM (Figure 3h) is close to or slightly below the monoatomic step height of a substrate, suggesting that either Ga oxide forms an ultrathin film or the image contrast is actually dominated by an electronic rather than a topographic contribution. Figure 5 shows the STM images of two oxidized samples at Ga coverages differing by a factor of two. Apparently, depressions (dark spots) in these images correlate with the initial Ga coverage and can be attributed to the Ga oxide phase. Moreover, such spots are elongated and oriented parallel to the two main crystallographic orientations of Cu(001). This finding points to good epitaxial relationships between the GaOx layer and the Cu(001) surface underneath. Yet, the atomic resolution of the GaOx layer could not be obtained by STM.

Figure 5.

Figure 5

Open in a new tab

STM images (150 nm × 150 nm) of the two Ga/Cu(001) samples at submonolayer coverages obtained after oxidation in 10–7 mbar of O2 at 700 K. The Ga coverage of the sample shown in (b) is double that of the sample shown in (a). Tunneling parameters: (a) 1.0 V, 0.1 nA and (b) 0.3 V, 0.1 nA.

Again, we used CO as the probe molecule for the oxidized GaOx/Cu(001) surfaces. The TPD spectra measured on two samples of different coverages as well as on pure Cu(001), all exposed to 1 × 10–7 mbar of O2 at 700 K for 5 min for direct comparison, are shown in Figure 6. (Note that no other molecules, e.g., CO2 and H2O, were detected during sample heating to 400 K in these experiments.) The Ga-free Cu(001) sample after oxidation, in essence, does not adsorb CO (Figure 6a), probably due to O adatoms in fourfold hollow sites21 affecting CO adsorption in the on-top sites.22 The formation of Ga oxide islands enhances the CO uptake that scales with the Ga coverage (Figure 6b,c), indicating that CO adsorbs primarily on the GaOx surface. It is interesting to note that the desorption temperature at CO saturation (167 K) is considerably higher than that on the “as-prepared” bimetallic Ga/Cu(001) surface (153 K, see Figure 2b), although metals typically adsorb CO more strongly than their oxides. On the other hand, the much lower intensities of these signals on both the Ga/Cu(001) and GaOx/Cu(001) surfaces as compared to pure Cu(001) (cf Figures 2 and 6) indicate that CO adsorption on the Ga-containing surfaces most likely occurs on low-coordinated sites such as the step edges and corners present in abundance on these surfaces, rather than on the regular terrace sites.

Figure 6.

Figure 6

Open in a new tab

CO TPD spectra measured on (a) Cu(100) surface and (b, c) two submonolayer Ga/Cu(100) surfaces, all after annealing in 1 × 10–7 mbar of O2 at 700 K. The corresponding STM images of the samples shown in (b) and (c) are shown in Figure 5a,b, respectively. CO exposures at 130 K are indicated (in L). The heating rate was 2 K/s. Note that the y-axis scale is by a factor of 5 smaller than in the TPD graphs shown in Figure 2 for the same samples in (a) and (c) before oxidation.

In the next step, we examined the thermal stability of the GaOx/Cu(001) surfaces under reducing conditions by exposing the samples to 10–6 mbar of H2 at elevated temperatures, 400–700 K. The XPS results shown in Figure 7 reveal only minor changes. As expected, Cu remains metallic. Some intensity gain of the Cu 2p and Cu LMM signals can be assigned to the removal of the O adatoms as evidenced by the disappearance of the weak O 1s signal at 529.8 eV. Surprisingly, the Ga 2p3/2 and O 1s signals attributed to the GaOx overlayer remain virtually identical in the H2 atmosphere. A small shift (about 0.3 eV) toward higher binding energies (BEs) observed for the Ga 2p3/2 state correlates with the disappearance of the 532.3 eV signal in the O1s region and can be assigned to the removal of adventitious impurities on the Ga oxide surface, most likely carbonates, and associated with this charge redistribution.

Figure 7.

Figure 7

Open in a new tab

XPS spectra measured in UHV at 300 K after annealing the GaOx/Cu(001) surface stepwise in 10–6 mbar of H2 at the indicated temperatures. The oxidation of this sample was monitored by XPS and is shown in Figure 4, while the morphology is shown in Figure 3a.

To study the surface transformations of GaOx/Cu(001) films at higher pressures, we used NAP–XPS (Figure 8). However, the spectra measured in 0.18 mbar of pure H2 at 700 K revealed no features, which would be indicative of the partial Ga oxide reduction, and were, in fact, virtually identical to those measured in 10–6 mbar of H2 (Figure 7). Furthermore, adding CO2 to H2 to mimic CO2 hydrogenation reaction conditions caused no changes in the spectra (Figure 8a–c). Moreover, characterization of this sample with an STM showed the same morphology as before the reactions in the pure H2 and CO2 + H2 mixture. All these findings clearly show that once the Ga oxide layer has been formed, it remains very stable even in a hydrogen-rich atmosphere.

Figure 8.

Figure 8

Open in a new tab

(a–c) NAP–XPS spectra measured on a GaOx/Cu(001) film in 0.18 mbar of H2 and in the 0.18 mbar of H2 + 0.06 mbar of CO2 mixture, both at 700 K. (d) STM image of the sample after the NAP–XPS experiments. Tunneling parameters are bias 1.0 V and current 0.08 nA.

3.1. General Discussion

The experimental results presented above may be rationalized as follows. Upon physical vapor deposition of Ga onto Cu(001) at room temperature, Ga adatoms readily intermix within the topmost Cu layer. The surface appears homogeneous, and no preferential Ga adsorption on the step edges or other structural defects is observed. Although the precise mechanism of surface alloying is yet unknown, it probably proceeds via a place-exchange mechanism23 and/or trapping of the Ga adatoms with the Cu adatoms diffusing on the Cu(001) surface, thus resulting in monolayer-thick islands of a similar composition as the terraces. In principle, Cu and Ga form a stable intermetallic compound at 300 K, namely, CuGa2, which in the (001) orientation has a square unit cell like Cu(001). However, the surface lattice constant of CuGa2(001) is 2.83 Å, that is, much larger than that of Cu(001) (= 2.55 Å), which could be clearly observed by LEED if really formed. In addition, its formation would cause certain strain due to the substantial lattice mismatch and would be thermodynamically unfavorable.

Oxidation of the resulting bimetallic surface may occur at room temperature even from traces of oxygen in the UHV background, but it accelerates in 10–6 mbar of O2 with increasing annealing temperature and is fully completed at about 500 K. In oxygen ambient, the bimetallic surface undergoes transformation into the Ga oxide phase and the O-covered Cu(001) surface. This phase separation and oxide segregation are clearly observed by STM, where Ga oxide is imaged as rectangular-shaped domains oriented parallel to the main crystallographic directions of the Cu(001) surface. The apparent depth of such domains measured by STM is close to or even less than the interlayer distance in Cu, suggesting that electronic effects dominate the image contrast. In addition, one should also consider the model of Ga oxide islands embedded into Cu(001) as previously observed, for example, for NiO(001) layers on Ag(001).24 High-resolution STM studies in combination with tunneling spectroscopy would help to validate or discard such a scenario.

The determination of the oxide stoichiometry and the oxidation state of Ga by XPS is not a trivial task in the case of ultrathin oxide films, and this is even more challenging for embedded oxide layers. In addition, the absolute BEs of the Ga core levels reported in the literature for metallic Ga and Ga oxides often deviate. For example, “thick” Ga layers deposited onto a gold foil exhibited a Ga 2p3/2 shift to 1117.1 eV and further to 1118.6 eV upon complete oxidation resulting in a stoichiometry close to Ga2O3,25 whereas ref.4 reports 1116.4 and 1118.8 eV for Ga and Ga2O3 samples, respectively. Therefore, the shift caused by oxidation amounts to 1.5 and 2.4 eV in these studies. In our case, the value is 1.1 eV for the oxidized samples (1117.5 vs 1116.4 eV, see Figure 4), which increases to 1.5 eV in hydrogen ambient (Figure 7) presumably due to the removal of surface carbonates. It should be also mentioned that Ga oxide thin films grown by different deposition techniques are commonly assigned to the Ga2O3 phase,7 which may, however, exist in several different crystal modifications.

What is most striking is that the Ga oxide phase readily formed in oxygen ambient is extremely stable even at elevated pressures of hydrogen (∼0.2 mbar) at 700 K. This finding suggests that once the Ga oxide has been formed, it will remain stable in catalytic hydrogenation reactions, as exemplified here for the CO2 hydrogenation conditions. This may also explain the recent results for the Ni–Ga powder catalysts5 demonstrating that the reduction temperature is critical for the catalytic performance, which can be associated with the presence or absence of the Ga oxide phase.

4. Conclusions

In this work, we studied the initial stages of the Ga interaction with the Cu(001) surface and surface transformations in different gaseous atmospheres in order to get insights into the surface chemistry of the Cu–Ga catalysts recently proposed for CO2 hydrogenation to methanol. The results show that Ga readily intermixes with Cu upon deposition in vacuum. However, even traces of oxygen in the gas ambient cause Ga oxidation and the formation of two-dimensional Ga oxide nanoislands uniformly covering the Cu surface. Such a morphology and the chemical state of Ga remains in pure H2 as well as in the CO2 + H2 reaction mixture. It, therefore, appears that the Ga-promoted Cu surface under reaction conditions exposes a variety of GaOx/Cu interfacial sites, which may influence the reaction mechanism.

Acknowledgments

The work was supported by the European Research Council (Grant ERC-725915, OPERANDOCAT) and the German Research Foundation (DFG) under Germany’s Excellence Strategy—EXC 2008—390540038—“UniSysCat.” J.Q.Z. thanks the AvH Foundation for a postdoctoral fellowship.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.0c09382.

  • Auger spectra of the clean Cu(001) and “as-deposited” Ga/Cu(001) surfaces, comparison of the O1s XPS spectra on oxidized Cu(001) and Ga/Cu(001) surfaces to differentiate O species, and C1s region in the XPS spectra of the Ga/Cu(100) and GaOx/Cu(001) surfaces (PDF)

The authors declare no competing financial interest.

Supplementary Material

jp0c09382_si_001.pdf (798.6KB, pdf)

References

  1. Armbrüster M.; Behrens M.; Cinquini F.; Föttinger K.; Grin Y.; Haghofer A.; Klötzer B.; Knop-Gericke A.; Lorenz H.; Ota A.; et al. How to Control the Selectivity of Palladium-based Catalysts in Hydrogenation Reactions: The Role of Subsurface Chemistry. ChemCatChem 2012, 4, 1048–1063. 10.1002/cctc.201200100. [DOI] [Google Scholar]
  2. Studt F.; Sharafutdinov I.; Abild-Pedersen F.; Elkjær C. F.; Hummelsho̷j J. S.; Dahl S.; Chorkendorff I.; No̷rskov J. K. Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol. Nat. Chem. 2014, 6, 320. 10.1038/nchem.1873. [DOI] [PubMed] [Google Scholar]
  3. Sharafutdinov I.; Elkjær C. F.; Pereira de Carvalho H. W.; Gardini D.; Chiarello G. L.; Damsgaard C. D.; Wagner J. B.; Grunwaldt J.-D.; Dahl S.; Chorkendorff I. Intermetallic compounds of Ni and Ga as catalysts for the synthesis of methanol. J. Catal. 2014, 320, 77–88. 10.1016/j.jcat.2014.09.025. [DOI] [Google Scholar]
  4. Surdu-Bob C. C.; Saied S. O.; Sullivan J. L. An X-ray photoelectron spectroscopy study of the oxides of GaAs. Appl. Surf. Sci. 2001, 183, 126–136. 10.1016/S0169-4332(01)00583-9. [DOI] [Google Scholar]
  5. Gallo A.; Snider J. L.; Sokaras D.; Nordlund D.; Kroll T.; Ogasawara H.; Kovarik L.; Duyar M. S.; Jaramillo T. F. Ni5Ga3 catalysts for CO2 reduction to methanol: Exploring the role of Ga surface oxidation/reduction on catalytic activity. Appl. Catal. B: Environ. 2020, 267, 118369. 10.1016/j.apcatb.2019.118369. [DOI] [Google Scholar]
  6. Medina J. C.; Figueroa M.; Manrique R.; Rodríguez Pereira J.; Srinivasan P. D.; Bravo-Suárez J. J.; Baldovino Medrano V. G.; Jiménez R.; Karelovic A. Catalytic consequences of Ga promotion on Cu for CO2 hydrogenation to methanol. Catal. Sci. Technol. 2017, 7, 3375–3387. 10.1039/C7CY01021D. [DOI] [Google Scholar]
  7. Pearton S. J.; Yang J.; CaryIV P. H.; Ren F.; Kim J.; Tadjer M. J.; Mastro M. A. A review of Ga2O3 materials, processing, and devices. Appl. Phys. Rev. 2018, 5, 011301 10.1063/1.5006941. [DOI] [Google Scholar]
  8. März A.; Franchy R. Oxidation of CoGa(110). Surf. Sci. 2000, 466, 54–65. 10.1016/S0039-6028(00)00679-8. [DOI] [Google Scholar]
  9. Franchy R.; Eumann M.; Schmitz G. Elemental steps in the growth of thin β-Ga2O3 films on CoGa(100). Surf. Sci. 2001, 470, 337–346. 10.1016/S0039-6028(00)00878-5. [DOI] [Google Scholar]
  10. Stierle A.; Streitel R.; Nolte P.; Vlad A.; Costina I.; Marsman M.; Kresse G.; Lundgren E.; Andersen J. N.; Franchy R.; Dosch H. Real time observation of ultrathin epitaxial oxide growth during alloy oxidation. New J. Phys. 2007, 9, 331–331. 10.1088/1367-2630/9/9/331. [DOI] [Google Scholar]
  11. Jeliazova Y.; Franchy R. Growth of ultra-thin Ga and Ga2O3 films on Ni(100). Surf. Sci. 2003, 527, 57–70. 10.1016/S0039-6028(02)02681-X. [DOI] [Google Scholar]
  12. Jeliazova Y.; Franchy R. Vibrational properties of ultrathin Ga2O3 films grown on Ni(100). Surf. Sci. 2002, 502-503, 51–57. 10.1016/S0039-6028(01)01897-0. [DOI] [Google Scholar]
  13. Liu S.; McDonald S.; Gu Q.; Matsumura S.; Qu D.; Sweatman K.; Nishimura T.; Nogita K. Properties of CuGa2 Formed Between Liquid Ga and Cu Substrates at Room Temperature. J. Electron. Mater. 2020, 49, 128–139. 10.1007/s11664-019-07688-4. [DOI] [Google Scholar]
  14. Li J. B.; Ji L. N.; Liang J. K.; Zhang Y.; Luo J.; Li C. R.; Rao G. H. A thermodynamic assessment of the copper–gallium system. Calphad 2008, 32, 447–453. 10.1016/j.calphad.2008.03.006. [DOI] [Google Scholar]
  15. Cui Y.; Liang F.; Yang Z.; Xu S.; Zhao X.; Ding Y.; Lin Z.; Liu J. Metallic Bond-Enabled Wetting Behavior at the Liquid Ga/CuGa2 Interfaces. ACS Appl. Mater. Interfaces 2018, 10, 9203–9210. 10.1021/acsami.8b00009. [DOI] [PubMed] [Google Scholar]
  16. Rodriguez J. A.; Goodman D. W. The Nature of the Metal-Metal Bond in Bimetallic Surfaces. Science 1992, 257, 897. 10.1126/science.257.5072.897. [DOI] [PubMed] [Google Scholar]
  17. Vollmer S.; Witte G.; Wöll C. Determination of Site Specific Adsorption Energies of CO on Copper. Catal. Lett. 2001, 77, 97–101. 10.1023/A:1012755616064. [DOI] [Google Scholar]
  18. Liu B.-H.; Groot I. M. N.; Pan Q.; Shaikhutdinov S.; Freund H.-J. Ultrathin Zn and ZnO films on Cu(111) as model catalysts. Appl. Catal. A: Gen. 2017, 548, 16–23. 10.1016/j.apcata.2017.06.043. [DOI] [Google Scholar]
  19. Campbell C. T. Ultrathin metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties. Surf. Sci. Rep. 1997, 27, 1–3), 1-111. 10.1016/S0167-5729(96)00011-8. [DOI] [Google Scholar]
  20. Bourque J. L.; Biesinger M. C.; Baines K. M. Chemical state determination of molecular gallium compounds using XPS. Dalton Trans. 2016, 45, 7678–7696. 10.1039/C6DT00771F. [DOI] [PubMed] [Google Scholar]
  21. Yagyu K.; Liu X.; Yoshimoto Y.; Nakatsuji K.; Komori F. Dissociative Adsorption of Oxygen on Clean Cu(001) Surface. J. Phys. Chem. C 2009, 113, 5541–5546. 10.1021/jp808542z. [DOI] [Google Scholar]
  22. Gajdoš M.; Hafner J. CO adsorption on Cu(111) and Cu(001) surfaces: Improving site preference in DFT calculations. Surf. Sci. 2005, 590, 117–126. 10.1016/j.susc.2005.04.047. [DOI] [Google Scholar]
  23. Meyer J. A.; Behm R. J. Place-exchange as a mechanism for adlayer island nucleation during epitaxial growth and resulting scaling behavior. Surf. Sci. 1995, 322, L275–L280. 10.1016/0039-6028(95)90010-1. [DOI] [Google Scholar]
  24. Steurer W.; Surnev S.; Fortunelli A.; Netzer F. P. Scanning tunneling microscopy imaging of NiO(100)(1×1) islands embedded in Ag(100). Surf. Sci. 2012, 606, 803–807. 10.1016/j.susc.2012.01.012. [DOI] [Google Scholar]
  25. Grabau M.; Steinrück H.-P.; Papp C. Physical vapor deposition of Ga on polycrystalline Au surfaces studied using X-ray photoelectron spectroscopy. Surf. Sci. 2018, 677, 254–257. 10.1016/j.susc.2018.07.011. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

jp0c09382_si_001.pdf (798.6KB, pdf)

Articles from The Journal of Physical Chemistry. C, Nanomaterials and Interfaces are provided here courtesy of American Chemical Society

RESOURCES