Formation of Copper Nanoparticles on ZnO Powder by a Surface

Article pubs.acs.org/JPCC

Formation of Copper Nanoparticles on ZnO Powder by a SurfaceLimited Reaction Hsuan Kung and Andrew V. Teplyakov* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Formation of copper nanoparticles on zinc oxide (ZnO) powder is studied using a common chemical vapor deposition precursor, copper hexafluoroacetylacetonate vinyl trimethyl silane, Cu(hfac)(VTMS). This process is investigated by high-vacuum Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The growth was found to be promoted by exposing ZnO powder to the gas-phase water, and the intensity of the hydroxyl groups stretching signatures decreases after the powder is exposed to the copper deposition precursor. Vibrational spectroscopy results support the reaction on both polar and (101̅0) surfaces of the powder, and XPS confirms that the copper deposition takes place and identifies Cu(I) species as the main copper species on the surface of ZnO powder. The mechanism of the reaction includes the elimination of hfac ligand that reacts with surface hydrogen present in hydroxyl groups, and this surface-limited process stops when the surface runs out of available hydrogen. SEM is used to visualize the formation of copper-containing nanoparticles on ZnO(101̅0) and ZnO(0001̅) surfaces and defects. The mechanism for the initial stages of the deposition is proposed based on the computational investigation consistent with the experimental results. This general approach can be used to design a range of copper catalysts supported on ZnO with a high degree of control over the amount of copper deposited and the desired size distribution of the nanoparticles produced.

1. INTRODUCTION

Two common approaches to prepare Cu/ZnO catalysts are wet chemistry16−18 and vacuum-based techniques.19,20 Sol−gel, or coprecipitation procedures, are usually used as wet catalyst preparation routes. These approaches are robust and reproducible and do not require anaerobic atmosphere; however, the materials prepared by these methods normally contain a wide range of surface sites, and following their transformations at a molecular and atomic level is difficult. On the other hand, vacuum deposition can be used to provide a well-defined material for in situ observation and control over copper deposition at the atomic level and the ability to follow the changes in oxidation state of copper over the course of catalytic processes. Here, we will adopt a high-vacuum approach to produce the desired material and determine its properties. We used a common copper precursor, copper hexafluoroacetylacetonate vinyl trimethylsilane, Cu(hfac)(VTMS), as a copper source. Cu(I) and Cu(II) diketonate precursors are often used in chemical vapor deposition (CVD) processes and atomic vapor deposition (ALD) processes. In general, ALD processes are more effective in reducing surface contamination compared to the CVD process, which often poses surface contamination problems because of the reactions of the precursor ligands at CVD conditions.21,22 Perrine and Teplyakov23 have developed a method to utilize Cu(hfac)-

Transition metal oxides are used in a variety of applications because of their wide range of chemical and physical properties.1,2 Noble metals, such as palladium and platinum, are commonly supported by metal oxides in catalytic materials.3,4 As an example of a cheap, versatile, and reasonably environmentally friendly transition metal, copper has shown its outstanding catalytic potential in selected heterogeneous processes, and one of the most common copper support materials for catalytic reactions is zinc oxide. For example, Cu/ZnO is a very important catalyst for methanol synthesis, methanol steam reforming, and hydrogen production reactions on the industrial scale.5−7 Because of these properties, a number of studies have focused on possible active sites of Cu/ZnO catalyst.7−9 However, because of different preparation methods and reaction conditions, understanding the behavior of the reactive sites on Cu/ZnO materials at the molecular level still remains a challenge. Nevertheless, some of the properties found a general consensus in science and engineering communities. For example, the oxidation state of copper plays a major role in the catalytic reactions. Much attention has been paid to copper(I) because it can be used in either oxidation or reduction steps. Previous studies showed that Cu+ species are active sites for CO adsorption and hydrogenation10,11 and these copper sites may be stable at the Cu/ZnO interface.12,13 It was also suggested that metallic copper is not as reactive; however, its presence may promote the reactivity of Cu+ species present in the same system.14,15 © 2013 American Chemical Society

Received: October 5, 2013 Revised: December 20, 2013 Published: December 23, 2013 1990

dx.doi.org/10.1021/jp409902c | J. Phys. Chem. C 2014, 118, 1990−1998

The Journal of Physical Chemistry C

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nanoparticles formed on commercially available ZnO powder surfaces using a reaction of copper deposition precursor with reducing surface sites in a self-limiting process.

(VTMS) precursor to cleanly form copper nanoparticles on silicon single crystalline surfaces terminated with hydrogen in a self-limiting surface process. VTMS was shown to leave the surface immediately upon precursor adsorption at room temperature on a number of different substrates.24−26 Pirolli and Teplyakov have also shown that, even for a very reactive clean Si(100)-2×1 surface in ultrahigh vacuum, 934 eV) and characteristic shake-up satellites.50 However, the metallic copper (Cu0) and Cu1+ are not easily distinguished based on the Cu 2p XPS spectra alone.20,50 To understand the oxidation state of copper species formed on the ZnO powder surfaces, we recorded Cu L3M45M45 Auger spectra. This approach is sufficiently sensitive to differentiate between Cu0 and Cu1+.20 Parts b and c of Figure 4 follow the Auger spectra for clean and water-predosed ZnO

Figure 6. (a) SEM image of ZnO powder. (b) Geometric structure of the ZnO crystals in a powder material.

Figure 4. X-ray excited Cu LMM Auger spectra of (a) ZnO powder background; (b) copper deposition onto a clean ZnO powder, without predosing water; (c) copper deposition on a ZnO powder following water predosing; (d) copper foil standard.

Figure 7. SEM images before (a, b) and after (c, d) copper deposition onto a water-predosed ZnO powder. (b) and (d) are the highresolution images of (a) and (c), respectively. Most ZnO structures are hexagonal wurtzite. Cu nanoparticles are clear to be identified (c), and the average particle size is ∼30 nm (d).

powder reacted with Cu(hfac)VTMS. This figure also compares the obtained spectra with the same spectral region of a clean ZnO powder material (Figure 4a). As expected, in all cases the spectra are dominated by the features of the ZnO material itself.51 However, in addition to those signals, a feature indicating the presence of copper-containing species is recorded in spectra b and c of Figure 4. This feature is very clearly present, and its intensity is increased substantially following

copper deposition precursor reaction with ZnO powder preexposed to water. Here, the presence of Cu1+ was confirmed by the feature at 916.8 eV (c) compared to Cu0 signature at ∼918 eV recorded for a copper standard in Figure 4d. It should be noted that this result does not completely exclude the possible presence of metallic copper on a surface of ZnO powder. Once the copper layer or nanostructures are formed on a surface (as will be further investigated below), it is very likely that the outer layer of copper contains mostly Cu1+ (either as a result of the presence of remaining hfac ligands or as a result of partial oxidation). However, because the sampling depth of XPS is limited to a few nanometers, it is possible that the underlying layers of copper are metallic, as was the case in our previous studies.23 In the studies performed on functionalized silicon surfaces, metallic copper (Cu0) was clearly the dominant species following deposition.23 However, Cu2+ was the major species following the formation of the nanoparticles in a similar process on the silicon crystal covered with silicon oxide. It is indeed possible that the use of oxide substrate in place of a clean semiconductor promotes oxidation of the outer layer of deposited copper or affects the mechanism of copper deposition. At the same time, analysis of the F 1s XPS features shown in Figure 5 indicates the presence of a single peak corresponding to hfac ligand; however, the hfac reaction on functionalized silicon substrates was much more complex, leading to several surface species including the formation of Si− F on silicon surfaces. To confirm that the single feature observed in the F 1s XPS spectra is consistent with the chemical environment of fluorine within the hfac ligand, we

Figure 5. F 1s XPS spectra of the ZnO powder following copper deposition (a) without predosing water and (b) with predosing water. 1993



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Scheme 1. Copper Nanoparticles Deposition on OH-Terminated ZnO Surface Following Cu(hfac) Exposure

Figure 8. Possible structures formed following absorption of Cu(hfac)VTMS on fully and partially OH-terminated ZnO surface represented by a Zn20O20 cluster model. Side and top views of adsorbed species on ZnO(1010̅ ) substrate are presented.

performed DFT calculations and predicted binding energies of all six fluorine atoms to be close to 689 eV.52 If a fluorine atom was bound to Zn or Cu, its chemical shift would be expected to move down to ∼684 eV.53 The observation of a single F 1s XPS feature is also consistent with our IR spectrum, clearly indicating the presence of intact hfac entities as a dominant vibrational signature. The

similar intensity of the F 1s features following the reaction of clean and water-exposed ZnO powder suggests that on both surfaces the amount of hfac ligands is similar, which is consistent with the mechanism of copper deposition proposed below. 3.3. Confirmation of the Nanoparticle Formation by SEM. To understand the nature of copper deposition on ZnO 1994



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powder material, we performed a scanning electron microscopy investigation. Figure 6a shows the image of the ZnO powder material used in the study. As expected for the most common ZnO structure of hexagonal wurtzite, three lower index orientations on observed hexagonal particles are ZnO(0001), ZnO(0001̅), and ZnO(101̅0), as highlighted in Figure 6b. Figure 7 compares ZnO powder before and after copper deposition. Very pronounced small particles are clearly identified in SEM images obtained following copper deposition. Figure 7d confirms that copper nanoparticles are actually formed on different surface faces of ZnO as well as on defect sites (such as the border of two ZnO(101̅0) planes, for example). Overall, this is not surprising, because the presence of hydroxyl groups is very likely on all those surfaces and defects, and the role of these sites in initiating the copper deposition process is confirmed spectroscopically, as summarized above. The average size of the particles observed in SEM experiments was estimated to be ∼30 nm (a histogram of particle size distribution is included in the Supporting Information section). However, it is possible that the presence of substantially smaller nanoparticles is not detectable in our experiments. 3.4. DFT Investigation of the Mechanism of the Initial Steps in Copper Deposition and Analysis of the Vibrational Spectra Obtained Following the Completion of the Deposition Process. Understanding the results of the spectroscopic studies summarized in the previous sections can be aided tremendously by a computational investigation. DFT calculations can provide plausible explanations for the initial steps of copper deposition and also yield vibrational characteristics of the surface species present on a surface following copper deposition. The main proposed steps of the deposition based on the experimental data provided above are summarized in Scheme 1. The two key observations suggested by the experimental studies indicate that hfac ligands of a copper deposition precursor should react with surface hydrogen-containing functionality (in order to desorb hfacH and to remove an unwanted hfac ligand) and that copper nanoparticle formation is promoted if the surface concentration of hydroxyl groups is increased. To examine both statements, the reaction of a Cu(hfac) fragment was investigated on a cluster representing a partially (isolated hydroxyl groups) and fully (all possible surface sites for hfac interaction are saturated with dissociated water) hydroxyl-terminated ZnO surfaces. These two surfaces were simply created by adding hydroxyl groups to the zinc atoms and hydrogen to oxygen atoms of a fully optimized cluster representing a clean (101̅0) surface of ZnO. First, these two types of hydroxylated structures were optimized and then used to study the interaction with Cu(hfac) fragments. Structure I in Figure 8 represents the Cu(hfac)VTMS molecule and partially hydroxylated ZnO surface that were optimized separately, without their interaction. Further studies omitted the possibility of VTMS reaction with the surface (as it is nonessential as described above) and focused on the structural changes of the Cu(hfac) fragment upon its adsorption. Structures II and III correspond to two different possible orientations of the Cu(hfac) fragment as it attaches to the surface. The energy difference between these two structures is only 13 kJ/mol, as summarized in Figure 9. Structure III is slightly more stable than structure II, as would be expected, because in this case the hfac ligand is stabilized by a neighboring zinc atom. However, structure II represents the

Figure 9. Energy diagram of the predicted reaction pathways for Cu(hfac)VTMS on (a) partially and (b) fully OH-terminated ZnO(1010̅ ) surface.

surface species most amenable for the elimination of the hfac ligand if a nearby hydrogen atom is available. Once the structures of the type represented by structure III are formed, further hfac elimination becomes more difficult, because hfac ligand starts interacting with the other available surface sites. Structure IV indicates the possibility of hfacH formation by transferring a hydrogen atom from the surface to form an hfacH species that is still interacting with the surface. The small energy difference presented in Figure 9 between structures IV and V simply reflects the removal of weakly bound molecular hfacH once this molecule is formed on a surface. The sequence of structures VI, VII, and VIII offers a very similar reaction pathway on a fully hydroxylated ZnO surface: adsorption of the Cu(hfac) fragment, removal of a hydrogen from a nearby functionality, and hfacH desorption. It should be pointed out that these reaction pathways offer a glimpse only into the initial steps of copper deposition. Following these initial steps of the reaction, the interaction of the next incoming Cu(hfac) fragment will be influenced by the copper already deposited, possibly in a dramatic way; however, as long as the surface hydrogen is available, this second fragment could still undergo a reaction similar to the first to grow nanoparticles observed in our studies. It is apparent that, after the reaction stops, there are still hfac ligands present on the surface. They can be bound to the outer layer of the copper nanoparticles or directly to the ZnO surface, if there is no surface hydrogen available for the formation and removal of hfacH. Thus, surface species similar to structure III can be formed and stabilized by bonding to the open surfacereactive sites. Once formed, these species can obviously also undergo further surface transformations, especially at elevated temperatures, likely a variety of surface decompositions, whose rich chemistry is outside of the present study, with the exception of several general observations noted below. It can be added that, compared to the partially OHterminated ZnO material, Cu(hfac) on the fully OH-terminated ZnO (structure VII) exhibited more substantial stabilization (ΔE between structures VI and VII) than that observed on partially hydroxylated surface (ΔE between structures I and III), as summarized in Figure 9. This result would again support the idea that the hydroxyl groups on the surface prevent 1995



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interacting with Cu(hfac)VTMS following water predosing (Figure 10a) and without predosing water (Figure 10b) were very similar, but not identical. For example, predosing water results in higher intensity of the vibrational features. At the first site, this observation is obvious, because predosing water promotes copper deposition. However, it is important to realize that, in the case of infrared spectroscopy studies, we observe the completion of a self-terminating surface reaction, when no surface hydrogen is available for hfacH elimination and only remaining hfac ligands are observed stabilized on the surface, fully consistent with the XPS studies of the F 1s spectral range presented previously. It is likely that the intensity difference that we observe in infrared simply reflects the slightly higher surface area available on a ZnO surface covered with nanoparticles formed. Spectra (c) and (e) in Figure 10 give examples of spectral features that would be expected for an hfac ligand strongly bound to the surface, and these features are also compared to those for a single isolated Cu(hfac) species in spectrum (f). Table 1 shows possible assignments for the observed spectral features. Of course, it should be mentioned that the exact arrangement of surface hydroxyl species (for example, predicted for structures II and III) and consequently their vibrational assignment is outside of the scope of this study. As was discussed in section 3.1, hfac has a very characteristic set of absorption features around 1500 cm−1 for CO/CC stretching vibrations. Structure III (Figure 10d) and Cu(hfac) on ZnO (Figure 10e) are very consistent with the spectra observed experimentally, and this signifies the interaction of the hfac ligand with open surface sites (either copper atoms of the copper nanoparticles or Zn atoms of the ZnO substrate). On the other hand, the spectra predicted for structure II (Figure 10c) and Cu(hfac) species (Figure 10f) have a set of very closely spaced absorption features below 1200 cm−1 that were not observed in the experimental studies. This also indicates that the observed peaks of CO/CC stretch features correspond to the hfac ligand asymmetrically and strongly bound to the surface. Again, this observation confirms that

Cu(hfac) from directly absorbing to the ZnO reactive sites, stimulate the formation of the hfacH, and thus promote the formation of copper nanoparticles. An additional insight into the mechanism of nanoparticle formation can be gained by a brief analysis of the computationally predicted infrared spectra and the experimental results summarized in Figure 10. The IR spectra of the ZnO surface

Figure 10. Experimental infrared spectra of Cu(hfac)VTMS reaction with ZnO powder surface (a) following water predosing and (b) without predosing water are compared to the computationally predicted infrared spectra of (c) structure II, (d) structure III, (e) Cu(hfac) fragment on a Zn20O20 cluster representing (101̅0) surface of the ZnO powder and (f) gas-phase Cu(hfac) species. (Arrow indicates the position of the corresponding O−H stretching mode.) All computationally predicted spectra are given without additional scaling corrections.

Table 1. Approximated Assignment and Peak Positions for Species in Figure 10 (a)

(b)

3276

3280

1654 1616 1564 1531 1464 1346

1656 1600 1560 1529 1473 1409

1257 1236

1257 1226

1147 1097

1149 1099

(c) 3701 3700 3282 1589 1575 1475

1326 1302 1299 1230 1214 1193 1141 1117 1092

(d)

(e)

3840 3732 3648 3271 1702 1632 1602 1513

(f)

3277

3278

1611 1587 1488

1579 1577 1471

1370 1303 1285 1248

1297 1244

1308 1247

1154 1143 1112

1146 1116 1087

1145 1115 1095

1996

approximated assignment O−H stretch O−H stretch O−H stretch C−H (hfacH) O−H bend CO stretch/CC stretch CC stretch/C−H bend CO stretch/CC stretch /C−H bend

O−H bend O−H bend C−CF3 sym CF3 stretch O−H bend O−H bend CF3 stretch CF3 stretch CF3 stretch



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structures of the type of structure II are more amenable to hfac removal via hfacH elimination into a gas phase.

ASSOCIATED CONTENT

S Supporting Information *

XRD investigation, a histogram of particle size distribution for copper deposited on ZnO powder, and complete references 19 and 45. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSIONS This work examined the formation of copper nanoparticles on a ZnO powder. Deposition was achieved in high-vacuum conditions with the use of a Cu(hfac)VTMS precursor molecule. This surface-limited process involves elimination of the hfac ligand upon availability of surface hydrogen, while VTMS ligand is easily eliminated upon initial adsorption. The growth of the nanoparticles can be aided by pre-exposing ZnO powder with water gas. The formation of copper nanoparticles on different surfaces of ZnO powder was confirmed by SEM, and the oxidation state of the topmost layer was verified to be Cu(I) by XPS. In addition, XPS has suggested that the CF3 groups of the remaining surface-bound ligands are intact and should be amenable for removal by chemical or thermal treatments. DFT investigations justified the need for available surface hydroxyl groups during the initial steps of the deposition and supported the experimental infrared investigations by identifying the type of surface species observed. By knowing the initial steps of the deposition process, further chemical treatments can be used to control copper nanoparticle formation and to maintain the oxidation state of the topmost layer of copper, making this material amenable to catalytic applications. This combination of chemical control with the robust deposition chemistry, the controlled nanoparticle formation, and the desired Cu(I) oxidation state in a surfacelimited reaction make this approach a good candidate for catalytic applications in the future.

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AUTHOR INFORMATION

Corresponding Author

*Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716. Tel.: (302) 831-1969. Fax: (302) 831-6335. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This work was also partially supported by the National Science Foundation (CHE 1057374). The authors would like to thank Dr. Holt Bui with XPS spectra at the Surface Analysis Facility (Department of Chemistry and Biochemistry, University of Delaware), Professor Chaoying Ni and his student Mr. Chang Liu (the W. M. Keck Electron Microscopy Facility, University of Delaware) for their assistance with SEM measurements, and Professor Svilen Bobev and his student Mr. Nian-Tzu Suen for the XRD experiments used in the Supporting Information section. 1997



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Formation of Copper Nanoparticles on ZnO Powder by a Surface

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