Coadsorption of Formic Acid and Hydrazine on Cu ... - ACS Publications

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Coadsorption of Formic Acid and Hydrazine on Cu(110) SingleCrystal Surfaces Yunxi Yao,† Jonathan Guerrero-Sánchez,‡ Noboru Takeuchi,†,‡ and Francisco Zaera*,† †

Department of Chemistry, University of California, Riverside, California 92521, United States Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Apartado Postal 14, Ensenada, Baja California Código Postal 22800, México

‡

S Supporting Information *

ABSTRACT: The chemistry of coadsorbed formic acid and hydrazine on Cu(110) surfaces was characterized both experimentally, by temperature-programmed desorption (TPD) and Xray photoelectron spectroscopy (XPS), and theoretically, via density functional theory (DFT) calculations. It was found that the two reactants interact with each other via hydrogen bonds and that this modifies their individual thermal chemistry on the surface in two main ways: by stabilizing a HCOOH:N2H4 adduct, which desorbs molecularly at around 240 K, and by slightly delaying the decomposition of the hydrazine to higher temperatures and shifting the selectivity of that step from dehydrogenation and formation of N2Hx(ads) species to scission of the N−N bond and ammonia production. The coadsorbed formic acid was determined to react at higher temperatures than hydrazine, in chemistry not affected by the latter, which is no longer present on the surface at that stage. One interesting aspect of this chemistry revealed by the DFT calculations is that formic acid may preferentially H-bond on top of adsorbed hydrazine rather than directly attach to the copper surface. The implications of these results to atomic layer deposition (ALD) processes are discussed.

1. INTRODUCTION

Here we report results from our study of the interaction of formic acid with hydrazine on copper surfaces, specifically on a Cu(110) single-crystal plane. Our interest in this system stems from a recent report on the sequential use of formic acid and hydrazine, together with an appropriate copper metalorganic precursor, as a way to deposit thin copper films on solid substrates using a so-called atomic layer deposition (ALD) process,10 by which the overall stoichiometric reaction is split in three self-limiting and complementary steps to control the rate of film growth at a monolayer scale. The sequence of reactions proposed in that process includes an early formation of a copper(II) formate, which is then readily reduced to copper metal by a subsequent treatment with hydrazine.10 It has been previously reported that an aqueous solution of copper(II) formate undergoes rapid reduction to copper metal at ambient temperature upon treatment with hydrazine hydrate,11 but the analogous reaction on solid surfaces has not been demonstrated. Our study was aimed to assess the viability of the proposed ALD mechanism.

The reactions of Brønsted−Lowry acids with bases in solution are ubiquotous in chemistry and are introduced and discussed in most general chemistry textbooks. Such reactions on solid surfaces, on the other hand, have been characterized to a much lesser extent. On metals in particular, it is often difficult to isolate molecular species in ionic states because the metal acts as an electron “bath” capable of neutralizing such charged species, and as a consequence, the detection of deprotonated acids or protonated bases is rarely possible on such substrates. It is interesting to note, however, that zwitterionic forms of some adsorbed amino acids have been isolated and characterized under ultrahigh vacuum (UHV) conditions,1−3 Even in cases where proton transfer is not observed, the coadsorption of acids and bases on surfaces can lead to the formation of hydrogen bonds: this is the type of interaction often cited as responsible for the formation of complex selfassembled layers on weakly interacting surfaces such as graphene or coinage metals.4−6 Hydrogen bonding is also believed to play a pivotal role in the way chiral modifiers such as cinchona alkaloids bestow chirality to catalysts based on transition metals such as platinum or palladium.7−9 Given the wide range of chemistry that involves hydrogen bonding, it would be useful to develop a better understanding of such interaction among adsorbed species. © XXXX American Chemical Society

Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift Received: February 21, 2018 Revised: May 3, 2018

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DOI: 10.1021/acs.jpcc.8b01804 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2. EXPERIMENTAL AND THEORY DETAILS The experiments reported here were all carried out in a UHVbased instrument equipped with several surface-sensitive techniques, as described in previous publications.12−14 Briefly, the temperature-programmed desorption (TPD) data were acquired by using an Extrel C-50 quadrupole mass spectrometer with an electron-impact ionizer enclosed in a stainless-steel box with a 7 mm diameter aperture for the selective sampling of the gases that desorb from the front surface of the solid samples. Acquisition of the TPD data is carried out via a personal computer interface, in a setup capable of collecting data for up to 15 different masses in a single TPD run. The raw data were deconvoluted when needed by using a well-established procedure.15 The X-ray photoelectron spectroscopy (XPS) data were acquired by using a 50 mm radius hemispherical electron energy analyzer (VSW HAC 5000), set at a constant pass energy of 50 eV (for a total resolution of approximately 1 eV), together with an aluminum-anode (hν = 1486.6 eV) X-ray source. The integrated XPS signal intensities were corrected for the relative sensitivities of the electron energy analyzer to the different elements16 and then calibrated to a reference saturation coverage of 0.65 ML for a saturation layer of HCOOH on Cu(110).17−20 The polished Cu(110) single crystal, a disk approximately 10 mm in diameter and 1 mm in thickness, was mounted on an on-axis manipulator capable of x−y−z−θ motion and of resistive heating and liquid-nitrogen cooling by wedging it in between thin tantalum wires attached to copper rods, which were in turn connected to the main body of the manipulator.21 The temperature of the crystal was followed by using a K-type thermocouple wedged into a hole drilled on the side of the crystal and controlled by a homemade proportional−integral− derivative (PID) circuit; a constant heating rate of 5 K/s was used for the TPD experiments. The surface of the crystal was cleaned before each TPD and XPS experiment by sequential cycles of Ar+ ion sputtering (using a Perkin-Elmer PHI 04-300 ion gun and an ion energy of 2 kV) and annealing at 1100 K. Both formic acid and anhydrous hydrazine were purchased from Sigma-Aldrich (reagent grade, ≥95 and 98% purity, respectively) and distilled in situ within the gas manifold via several freeze−pump−thaw cycles right before use. Dosing of the sample was done by backfilling of the chamber using leak valves, and is reported in Langmuirs (1 L = 1 × 10−6 Torr/s), uncorrected for differences in ion gauge sensitivities. The pressure in the main UHV chamber was measured by using a nude ion gauge. The quantum mechanics calculations were carried out using periodic density functional theory (DFT) as developed in the PWscf code of the Quantum Espresso package,22 with added van der Waals interactions with empirical dispersion corrections (DFT-D2)23,24 and ultrasoft pseudopotentials. The generalized gradient approximation (GGA) was used for the exchangecorrelation energy. The electron states were expanded in plane waves with an energy cutoff of 35 Ry (475 eV). A 3 × 4 surface unit cell was used in most instances, 7 atomic layers in thickness, and with a 20 Å vacuum space in the z direction added. The Brillouin zone integration was done using a kpoints grid of 4 × 6 × 1. Additional calculations were carried out with 3 × 2 surface unit cells, 4 atomic layers in thickness, 20 Å vacuum space, and a k-points grid of 3 × 5 × 1 to compare with reported values for hydrazine adsorption.

3. RESULTS The general thermal chemistry of formic acid and hydrazine coadsorbed on the Cu(110) single-crystal surface was first explored by TPD. A typical set of data from those experiments is provided in Figure 1, in that case for a surface sequentially

Figure 1. Temperature-programmed desorption (TPD) data from a Cu(110) single-crystal surface dosed sequentially with 3.0 L of hydrazine and 3.0 L of formic acid at 150 K. Data are provided for the most relevant amus to help with the identification of the desorbing products.

exposed to 3.0 L of N2H4 and 3.0 L of HCOOH at 150 K, and a contrast with the results from the individual adsorbed components is highlighted in Figure 2. For reference,

Figure 2. Comparison of TPD results with formic acid and hydrazine coadsorbed on Cu(110) at 150 K as a function of the order of dosing. Traces for each of the two reactants when adsorbed alone, by themselves, are provided as well for reference. The data correspond to the desorption of (from left to right): H2 (2 amu), NH3 (17 amu), N2 (28 amu, after deconvolution of the N2H4andCO2 contributions), N2H4 (29 amu), CO2 (44 amu), and HCOOH (46 amu). 3.0 L exposures were used in all cases.

monolayer saturation for hydrazine and formic acid on clean Cu(110) is attained after exposures of approximately 4.0 L25 and 3.0 L,18,20 respectively. Many atomic mass units (amus) were followed in order to identify the nature of the desorbing products; Figure 1 reports the data from the most relevant ones. Several general features seen in that figure are worth highlighting to guide our discussion of the surface chemistry seen in these systems. To begin with, some molecular desorption of the reactants from condensation was detected at the onset of the temperature ramp, below 200 K. This B


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formic acid (red) alone are also provided for reference (as already referred to above). The different panels report the desorption of the main products observed, namely (from left to right), H2 (2 amu), NH3 (17 amu), N2 (28 amu, after deconvolution of the N2H4 and CO2 contributions), N2H4 (29 amu, chosen instead of the 32 amu trace because it is more intense and displays less interferences with other species), CO2 (44 amu), and HCOOH (46 amu). It can be seen in this figure that the order of adsorption does not, by and large, affect the final result, but it does induce some minor changes. Perhaps the main difference is that molecular desorption of both species in the feature around 240 K occurs at higher temperatures (240 versus 230 K), and the yields are higher, if hydrazine is adsorbed first. In addition, the threshold temperature for NH3 and N2 production may be lower (approximately 370 versus 400 K) in that case as well. The reasons for the differences seen with dosing order were explored with the aid of quantum mechanics calculations. Figure 3 reports the calculated energetics for the reaction

condensation was minimized by performing the uptake at 150 K, a temperature close to that where sublimation of both reactants takes place under a vacuum.20,25 The first desorbing feature associated with species adsorbed in the first monolayer is seen at approximately 230 K. That feature is detected in many of the TPD traces, in particular in those corresponding to hydrazine (29 and 32 amu) and formic acid (46 amu), an observation that suggests molecular desorption. On the other hand, no equivalent peaks were detected in cases where the Cu(110) surface was exposed to either hydrazine25 of formic acid20 alone, as shown in Figure 2 (fourth and sixth panels from the left). It appears that the coadsorption of both compounds leads to a new intermolecular interaction that stabilizes a new adsorption state, a point that is discussed in more detail below. The next desorption feature seen in the TPD data in Figure 1 is the peak at 420 K in the 17, 28, and 29 amu traces. The most likely interpretation of these results is that they reflect the chemistry of hydrazine disproportionation into ammonia (17 amu) and molecular nitrogen (28 amu). That reaction is in fact also observed with hydrazine alone adsorbed on Cu(110) (Figure 2, second and third panels from the left), in which case the threshold temperature is lower (330 K).25 It should be indicated that these reactions appear to take place over a relatively wide range of temperatures, as the 17 and 29 amu traces display at least two distinct peaks (the second centered at 460 K). The signal seen in the 29 amu trace is somewhat puzzling, given that it is not accompanied by the corresponding peaks in the 32 amu trace expected if it were to be associated with N2H4. It may be that some radical species, N2Hx• (x = 1, 2, or, most likely, 3), resulting from partial dehydrogenation of the adsorbed hydrazine, is ejected from the surface at this stage. For reference, NH2• was seen to desorb from thermal activation of N2H4 adsorbed alone on clean Cu(110).25 It is also interesting to note that no H2 desorption is seen at this stage of the thermal activation of the N2H4 + HCOOH coadsorbed system, suggesting that the hydrazine disproportionation is close to stoichiometric (3N2H4 → 4NH3 + N2). The final temperature range where desorbing products are seen is around 465 K. The most prominent feature there, by far, is that of the 44 amu trace, which corresponds to CO2 desorption from formic acid decomposition, the same as that seen with pure HCOOH on Cu(110).20 Similar peaks (albeit with lower intensities) are seen in the 16, 28, 45, and 46 amu traces. The first two are due to CO2 cracking in the electronimpact ionizer of the mass spectrometer, but the last two indicate that some HCOOH molecular desorption also takes place at this temperature (also seen with formic acid alone).20 H2 (2 amu) is produced at this stage as well. Because the TPD behavior seen in this temperature range is similarly independent of the presence or absence of coadsorbed N2H4, we conclude that by this temperature all species resulting from hydrazine adsorption and decomposition may have desorbed from the surface, and that the hydrogen produced may predominantly originate from formic acid decomposition. It should also be noted that hydrogen production in this case is reaction limited, as the production and desorption of H2 from hydrogen-saturated Cu(110) takes place at much lower temperatures, around 300−350 K.26−30 The effect of the order of dosing was tested next. Figure 2 shows the results from TPD experiments where the N2H4 and HCOOH were adsorbed either in that order (as in Figure 1; golden traces) or in reverse (formic acid first; green traces). The data for surfaces dosed with either hydrazine (blue) or

Figure 3. Density-functional theory (DFT) calculations of the energetics of adsorption of formic acid and hydrazine, individually and together, on Cu(110) surfaces. The red values correspond to energies for each system relative to the clean Cu(110) surface plus the two molecules free in the gas phase (the top-left system), whereas the light-green numbers are the energies associated with the transitions indicated by the arrows. Evidence is provided for the favorable formation of hydrogen-bonded HCOOH:N2H4 adducts in both the gas phase and on Cu(110), and for the reasons for the differences seen in the TPD experiments in adsorbate uptake upon reversing the dosing order.

scheme (additional images with details of the adsorption geometries are provided in Figures S1, S2, and S3, Supporting Information). Several facts relevant to the thermal chemistry described above can be extracted from these simulations. To begin with, it is clear that the complexation of hydrazine with formic acid in the gas phase is exothermic because of the formation of two hydrogen bonds (d(COH···N) = 1.60 Å and d(CO···HN) = 2.11 Å). The calculated energy released by that step is 0.75 eV (72 kJ/mol; Figure 3, top-left step), larger than what was reported in an early paper (50.6 kJ/ C


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The Journal of Physical Chemistry C mol, not including van der Waals interactions)31 but similar to those reported more recently for formic acid−ammonia complexes;32 in both cases, two hydrogen bonds were identified as well. The individual adsorption steps for formic acid and hydrazine were also found to be exothermic, releasing 0.77 and 1.38 eV (74 and 133 kJ/mol), respectively (Figure 3, bottom left). Interestingly, recent reported calculation for formic acid on Cu(111) puts the adsorption energy at either 0.48 or 0.66 eV (46 or 64 kJ/mol, respectively), values close to what we obtained if we did not include any van der Waals contribution (as was the case in those reports).33,34 Regarding the adsorption of hydrazine, previous calculations on Cu(110) yielded values of 1.34 and 0.82 eV (129 and 79 kJ/mol) with and without van der Waals interactions added, respectively,35,36 the former being close to our value. These comparisons point to the key role that van der Waals interactions play in this chemistry. The results from our calculations most relevant to the experiments reported here are the adsorption energies provided in the right-hand side of Figure 3. Three scenarios were considered. In the first, the coadsorption of the two molecules was forced on adjacent sites to allow for hydrogen bonding (Figure S1, Supporting Information); the result is a slight increase in energy, about 0.04 eV (4 kJ/mol) from the case where the two molecules are two sites apart (Figure 3, bottom row). The final energy of these coupled coadsorption systems is −2.10 eV (−203 kJ/mol) with respect to a clean Cu(110) surface plus both molecules free in the gas phase, or −1.35 eV (−130 kJ/mol) compared to the case where the gas molecules are coupled as an adduct in the gas phase. Interestingly, approximately the same energy (−2.05 eV, −198 kJ/mol) was calculated for a case where formic acid hydrogen-binds on top of an adsorbed hydrazine molecule (Figure 3, right center). In this case, d(COH···N) = 1.61 Å and d(CO···HN) = 1.88 Å, and clear electron density is seen in between the two molecules, in particular in the regions associated with where the hydrogen bonds are expected (Figure S4, Supporting Information). The reported configuration here is the one that yielded the lowest energy, but other arrangements were considered as well, by adsorbing hydrazine on different surface sites and by rotating the adsorbed hydrazine with respect to the surface and the formic acid on top; some energy differences were obtained but not significant enough to affect the general trends reported here. By contrast, the opposite hydrogen-bonding of hydrazine to adsorbed formic acid proved to be much less stable, only 0.32 eV (31 kJ/ mol) more stable than the HCOOH:N2H4 adduct in the gas phase and much less stable than either of the two coadsorbed options, by approximately 1 eV (100 kJ/mol). This may explain why much less uptake was seen in the 240 K TPD peak when the Cu(110) surface was dosed with formic acid first than when the dosing order was reversed (Figure 2). One final point should be made regarding these DFT calculations, and it is that the energetics of the adsorption is affected by the surface coverages of the two adsorbates. Because of the slight repulsion between molecules, the energy of their adsorption side by side decreases to 1.96 eV (189 kJ/mol) per pair at a coverage of a quarter of a monolayer of each. On the other hand, the configuration with the formic acid molecules adsorbed on top of chemisorbed hydrazine becomes more stable, yielding an adsorption energy of 2.16 eV (208 kJ/mol). The COH···N hydrogen bond becomes stronger than in

the gas-phase dimer, and additional CO···HN bonds are formed (Figures S5 and S6, Supporting Information). An additional TPD test was performed to probe the potential formation of HCOOH:N2H4 adducts, both adsorbed on the surface and condensed above it. The left panel of Figure 4

Figure 4. Low-temperature region of the TPDs obtained with HCOOH + N2H4-dosed Cu(110) surfaces, highlighting the behavior of the desorption of the hydrogen-bonded complex formed by the two species, the ∼240 K peak. Left: TPD from a surface sequentially dosed with 150 L of HCOOH + 150 L of N2H4 at 150 K. Shown are the traces for 2, 14, 16, 17, 28, 29, 32, 44, 45, and 46 amu. Right: Comparison of HCOOH (46 amu) traces obtained with 3.0 L of HCOOH + 3.0 L of N2H4 (bottom, blue trace), 3.0 L of N2H4 + 3.0 L of HCOOH (middle, green), and 150 L of HCOOH + 150 L of N2H4 (top, red).

displays results from a TPD obtained from a thick condensed layer built up by sequentially dosing formic acid first (150 L) and hydrazine afterward (150 L as well) at 150 K. The traces for several amus in the low-temperature regime are shown to help identify the molecular species that desorb from both of the condensed layers, at 190 K, and the first monolayer, at 240 K. No formic acid evolves from the multilayer, but that may be because of the low sublimation energy of that compound; multilayer desorption has been reported in past TPD experiments to occur around 155 K.37 Hydrazine, on the other hand, is detected in large quantities at 190 K. Interestingly, the peak at 240 K, associated with the desorption of the stabilized adduct adsorbed on the first layer, shows only a limited yield; this adduct must only form at the interface near the Cu(110) surface, in the first adsorbed monolayer. The total amount of formic acid retained in these experiments, using large gas doses, is only about twice the yield seen in the experiment where 3.0 L of hydrazine was followed by only 3.0 L of formic acid (Figure 4, right panel; red versus green traces). Again, that sequence of exposures was shown to be more effective at the formation of the HCOOH:N2H4 adsorbed species than the reverse formic acid-first experiment (Figure 4, right panel, blue trace). It is also worth pointing out that the adduct formation is seen even if multilayers of the first adsorbate (formic acid in the example of Figure 4) are made before adding the second: the surface species appear to be sufficiently mobile to reach the metal or the first adsorbed monolayer in all cases. The TPD studies were complemented with data from XPS characterization of the surface as a function of annealing temperature. Figure 5 displays the C 1s (left panel), O 1s (middle), and N 1s (right) XPS traces obtained for a surface D


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gas-phase adduct +0.42 and +0.44, respectively. It is also interesting to point out that the gains in oxygen charge density at an adduct coverage of a quarter of a monolayer were calculated to be +0.45 and +0.42 electrons, respectively, which means that bonding between the HCOOH and N 2 H 4 molecules become stronger at higher coverages. Heating the surface to 250 K induces the molecular desorption of HCOOH:N2H4 adducts, leaving behind only a fraction of a monolayer of the two compounds or of moieties produced by their decomposition. Although it is difficult to fully quantify the coverages of the adsorbed species based on the areas of these XPS spectra, it is worth mentioning that the intensity of the N 1s XPS peak at this stage is comparable to that obtained with hydrazine on clean Cu(110), in which case the coverage has been estimated to be approximately 0.2 ML (1 ML = 1 adsorbed molecule per copper surface atom).25 The C 1s XPS signal is much weaker and therefore much less reliable for the estimation of the coverage of the surface species originating from formic acid, but it is consistent with a similar coverage (∼0.2 ML).20 The C 1s, O 1s, and N 1s XPS BEs at this point are 288.1, 531.8, and 400.3 eV, respectively (Figure 5, fourth traces from top). This transitory state evolves further after heating to 300 K, at which point the XPS peak intensities decrease further and their positions shift to values of 287.9, 531.2, and 399.7 eV, respectively (Figure 5, third traces from top). The large red shift of the N 1s XPS peak is consistent with a transition from a molecular amine (hydrazine or ammonia) to a NH2(ads) species,39−42 and therefore suggests that the scission of the N− N bond in adsorbed hydrazine may take place at relatively low temperatures; this would justify the production of ammonia seen in the TPD traces. Interestingly, it was not possible to identify this step by XPS with hydrazine adsorbed alone on Cu(110), where the formation of N2Hx(ads) species was proposed instead.25,43 The difference in decomposition pathways appears to be promoted by the interaction with the coadsorbed formic acid, which may stabilize the adsorbed hydrazine species. By 400 K, most of the N 1s XPS signal disappears, because of the N2, NH3, and N2Hx• desorption seen in the TPD experiments, but the signals in the C 1s and O 1s traces are still significant; those only go away after heating to 500 K (Figure 5, top two traces). The behavior of the adsorbed HCOOH extracted from these XPS data is similar to that reported for formic acid adsorbed alone on clean Cu(110), suggesting that the coadsorbed hydrazine does not affect it much. In fact, the formic acid conversion, manifested mainly by the desorption of HCOOH, CO2, and H2 at 465 K, happens after all nitrogen-containing products have been removed from the surface. The fact that formic acid affects the thermal chemistry of adsorbed hydrazine but hydrazine does not modify that of adsorbed formic acid advocates again for the formation of an adduct on the surface with the HCOOH on top, as discussed above and shown to be stable in the right-center panel of Figure 3. One final piece of evidence for the formation of the HCOOH:N2H4 adduct on the Cu(110) surface comes from the stoichiometry derived from the XPS data. Figure 6 shows that, for all temperatures above 200 K, the C:O:N atomic ratios are close to 1:2:2, as expected from a 1:1 formic acid-hydrazine adduct. The one case where a significant deviation is seen is at 400 K, at which point there are more carbon and oxygen than nitrogen atoms on the surface. This is because 400 K is a temperature high enough to induce the

Figure 5. C 1s (left panel), O 1s (center), and N 1s (right) X-ray photoelectron spectroscopy (XPS) data for 20 L of N2H4 + 20 L of HCOOH adsorbed sequentially on a Cu(110) surface. Results are shown as a function of annealing temperature. Reference traces are also provided at the bottom for the clean surface and for 10 L of HCOOH or 20 L of N2H4 adsorbed alone on the clean Cu(110).

dosed with 20 L of N2H4 + 20 L of HCOOH at 150 K. Additional data are provided for the clean Cu(110) surface (bottom traces) and for a surface dosed with 10 L of HCOOH or 20 L of N2H4 alone (second and third from bottom, respectively) for reference. Adsorption of hydrazine alone at 150 K leads to a symmetric N 1s XPS centered at 401.5 eV (third-from-bottom trace, right panel). The binding energy (BE) value previously reported for a monolayer is 400.2 eV,25 but there seems to be a blue shift in all peaks at higher exposures, possibly because of either final state screening or charging effects on the condensed monolayer. Addition of 20 L of HCOOH to the hydrazine-dosed surface at 150 K results in a shift in the N 1s feature to 402.2 eV and a decrease in its intensity, due to shielding by the HCOOH added layers, and also in the growth of additional C 1s and O 1s XPS peaks at 289.8 and 533.2 eV, respectively (fourth traces from the bottom). Heating to 200 K facilitates the desorption of all condensed layers, and affords the probing of the HCOOH:N2H4 complex that we have proposed forms in the first monolayer. At that stage, the C 1s, O 1s, and N 1s XPS peaks are centered at BEs of 289.2, 532.3, and 401.5 eV, respectively (Figure 5, fourth traces from the bottom). Of particular significance in these spectra is the fact that the feature for the O 1s photoelectrons loses the asymmetric shape associated with the two types of oxygen atoms in HCOOH, the CO and COH moieties;20 at 200 K, the electronic properties of the oxygen pair seem to be more similar, potentially because of the hydrogen bonds they make with the adsorbed hydrazine. DFT calculations of the Lowdin charges38 associated with these complexes favor the configuration where formic acid binds on top of the adsorbed hydrazine, as shown in the right-center panel of Figure 3, rather than the one where both molecules are coadsorbed side by side (Figure 3, bottom right): in the first case, the carboxylic and hydroxylic oxygen atoms are estimated to gain charge densities equivalent to +0.39 electrons in both cases (relative to the 6 valence electrons in free oxygen atoms), respectively, making the two equivalent in the XPS traces, whereas in the second case the values are +0.26 and +0.36 (indicating a clear difference). For reference, in free HCOOH, the numbers are +0.36 and +0.43, and in the HCOOH:N2H4 E


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Figure 8. Comparison of TPD results with DCOOD and N2H4 coadsorbed on Cu(110) at 150 K as a function of the order of dosing. These data are to be contrasted with the results with HCOOH reported in Figure 2.

Figure 6. C, O, and N atomic surface coverages on Cu(110) as a function of annealing temperature for Cu(110) dosed sequentially with 20 L of N2H4 + 20 L of HCOOH, estimated from the XPS peak areas after proper calibration.

Similar isotope effects were already reported with HCOOH alone, suggesting again that coadsorbed hydrazine does not significantly affect this high-temperature formic acid decomposition channel.20 It is interesting to note, though, that the desorption of all three hydrogen isotopologues, H2 (2 amu), HD (3 amu, although limited), and D2 (4 amu), is detected at 480 K, but only HCOOH, and no DCOOD (48 amu), is seen at that temperature. It would appear that the adsorbed formic acid undergoes complete H−D exchange on the surface at lower temperatures. The thermal chemistry of the coadsorbed hydrazine also displays some changes upon switching HCOOH for DCOOD. Both ammonia and molecular nitrogen are still made, and no obvious kinetic effect is seen in their desorption traces. However, ammonia in this case is made via the incorporation of a deuterium (not a regular hydrogen) atom, to make NH2D (18 amu). No further isotope scrambling is seen (no NHD2  19 amu, data not shown, or ND3 20 amu signals were detected), indicating that the reaction taking place is likely to be a simple sequence of N−N bond scission and D addition (from DCOOD decomposition) steps. These events could probably take place sequentially but may in principle also occur in a concerted way within the hydrogen (deuterium)-bonded DCOOD:N2H4 complex that forms on the surface.

decomposition of hydrazine and the desorption of the resulting products but below what is needed to promote the decomposition of formic acid. Finally, further insight into the mechanism of the surface chemistry of these systems in the high-temperature (400−500 K) regime was obtained from TPD experiments with deuterium-labeled formic acid. Data from a typical run, in this case from a Cu(110) surface dosed sequentially with 2.0 L of N2H4 and 2.0 L of DCOOD, are provided in Figure 7, and a

4. DISCUSSION The TPD and XPS experiments and the DFT calculations reported above were designed to obtain a molecular-level understanding of the mechanism of the reaction of formic acid and hydrazine coadsorbed on copper surfaces. As stated in the Introduction, our motivation for this work was a report on the possible sequential use of those two reactants with metal (copper) precursors to grow thin solid metallic films by ALD.10 After deposition of an appropriate metal precursor on the surface, typically a metalorganic complex,44−47 the substrate in this protocol is exposed to formic acid in order to (presumably) displace the ligands from the original metal complex and to form the corresponding metal formate. Such chemistry is easy to envision in the liquid phase but may not be trivial at solid surfaces.48,49 Certainly, we have seen here, and before,17,20,50,51 that the chemisorption of formic acid on copper metal is weak; virtually no formate is produced from thermal activation of such an adsorbate. Part of the difficulty stems from the unfavorable deprotonation of adsorbed formic acid on the clean surface. However, that step can be greatly enhanced by the addition of atomic oxygen on the surface, in which case a layer of formate

Figure 7. TPD data from a Cu(110) single-crystal surface dosed sequentially with 2.0 L of N2H4 and 2.0 L of DCOOD at 150 K. These data are analogous to those for HCOOH in Figure 1.

contrasting of the results obtained as a function of the dosing order is provided in Figure 8 (together with reference data for DCOOD adsorbed alone on the Cu(110) surface). The results are in general quite similar to those recorded with regular HCOOH (compare the data in Figures 1 versus 7 and in Figures 2 versus 8), but there are also some significant differences worth highlighting. For one, there seems to be a kinetic isotope effect operative in the decomposition of the formic acid, as the corresponding peaks, in particular the 44 amu trace for CO2, shift to higher temperatures (from 460 K with HCOOH to 480 K with DCOOD). The same TPD peak temperature increase is seen in the peaks for HCOOH (46 amu) and H2 (2 amu), an indication of a common rate-limiting step for both molecular desorption and dehydrogenation steps. F


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of formate ions to yield carbon dioxide is still a likely side reaction (even if it may require higher temperatures). Our study has provided some new insights into the surface chemistry of hydrazine on copper surfaces and in the presence of coadsorbed formic acid that hopefully can shed additional light on how the ALD chemistry may proceed. As mentioned in the previous paragraph, we start from the important observation that thermal activation of hydrazine, coadsorbed with formic acid (on Cu(110)), yields both NH3 and N2. Hydrazine thermal conversion and disproportionation is not surprising, and has in fact been discussed in the past in connection with other processes. Indeed, hydrazine has long been considered a potential fuel, as a way to store hydrogen, and is known to decompose on metal catalysts at relatively low temperatures, as seen here. Several studies have focused on the reaction conditions and on the nature of the surface needed to favor either incomplete (to ammonia and molecular nitrogen, the chemistry reported here) or complete (to molecular nitrogen and molecular hydrogen) decomposition.67 In terms of the incomplete decomposition that has been seen here and that is likely to participate in ALD, it is interesting to highlight the stabilization of N2H4 on the surface by the coadsorption of HCOOH, manifested in our data by the new TPD peak for the adduct at 240 K. This pairing of both adsorbates is due to hydrogen bonding, and even if it takes a slightly different form during ALD processes (where the surface intermediates are more likely to involve formate species), it is still likely to be a factor there as well. In our experiments, it was also seen that the decomposition of the nitrogen-containing adsorbates becomes delayed in temperature and shifts toward the production of ammonia rather than N2Hx(ads) species, as was seen on clean Cu(110). This is an important observation that points to a change in relative selectivity from the preferential scission of N−H to the dominance of the breaking of N−N bonds on the surface: ammonia is suggested to be one of the main byproducts in reactions where hydrazine is used as the reducing agent (eqs 1−4). As of the way by which formic acid stabilizes adsorbed hydrazine on the surface, our DFT calculations offer a tantalizing answer: that this is because formic acid may adsorb, via hydrogen bonding, on top of the adsorbed hydrazine. This may occur instead of coadsorption of both molecules directly on the surface, although both modalities are possible. Experimental support for this conclusion comes from the differences in yields in the TPD experiments performed as a function of the order of dosing: the desorption yield of the HCOOH:N2H4 adduct at 240 K is significantly larger if the hydrazine is dosed first, followed by exposure to formic acid (as compared to adding the adsorbates the other way around, Figures 2 and 8), and even if large HCOOH doses are used, there is a limit to its uptake on the surface in terms of the TPD peak at 240 K (Figure 4). Also, the N 1s XPS signal from adsorbed hydrazine is shielded significantly upon addition of formic acid to the surface (Figure 5), indicating the positioning of the latter on top of the former. Importantly, formic acid seems to be more stable and to decompose after N2H4, in reactions not affected by it; this is one reason why it is unlikely that the decomposition of formic acid to CO2 formation, a step that is detected here, is directly associated with the reduction of the metal ions.

species is indeed produced. Presumably, the fact that metalorganic compounds with metal centers in cationic states are used for the ALD processes provides oxidized metal species in the first step amenable to the formation of formate compounds in the second (upon formic acid addition). We could not test this step directly here, but much is known about the formation of the formate intermediates on oxygen-dosed copper surfaces.19,20,52−57 The last step in the proposed ALD process for metal deposition using formic acid and hydrazine is the exposure of the metal formate that presumably forms on the surface in the second step to hydrazine. In this reaction, hydrazine is expected to act as a reducing agent. There are indeed precedents for the role of hydrazine as an agent to chemically reduce metal salts from solution to recover precious metals or make metal nanoparticles.58 One proposed reaction for the reduction of Cu2+ ions dissolved in an aqueous solution is59 Cu 2 + + 2OH− + 2N2H4 → Cu 0↓ + N2 + 2NH3 + 2H 2O

(1)

In other examples exhibiting some commonality with our system, Ni catalysts have been prepared in solution via the reduction of nickel acetate with hydrazine,60 and nickel salts have also been removed from metal effluents this way.61,62 Silver nanoparticles have been made from reduction of silver salts for optical sensor applications,63 and copper nanoparticles have been obtained via reduction of either chitosan-complexed copper ions64 or cupper sulfate.65 In all of those cases, reactions similar to eq 1 were proposed (without the participation of the acetate anion in the first case). However, such chemistry is not viable under our vacuum conditions, as there is no aqueous environment to provide hydroxide anions. Instead, it is conceivable that the organic-acid conjugated base (the formic anion in our case) may play the role of the hydroxide, for an overall reaction given by Cu 2 + + 2HCOO− + 2N2H4 → Cu 0↓ + N2 + 2NH3 + 2HCOOH

(2)

or, if starting from a Cu(I) precursor: 2Cu+ + 2HCOO− + 2N2H4 → 2Cu 0↓ + N2 + 2NH3 + 2HCOOH

(3)

No direct evidence exists yet, and none was acquired here, to directly support this chemistry, however. In fact, a competing reaction has been recently proposed on the basis of quantum mechanics calculations:66 2Cu+ + 2HCOO− + N2H4 → 2Cu 0↓ + 2NH3 + 2CO2 (4)

However, although this reaction is exothermic and quite viable, it does not seem likely to be central to the ALD processes because: (1) in this scheme not only the copper ions but also the hydrazine is reduced, to ammonia (it is the formate that acts as the reducing agent, something that it could presumably do by itself in the second step of the ALD cycle, before hydrazine addition); and (2) we now know from our surfacescience experiments, reported above, that hydrazine does decompose on copper surfaces to produce both ammonia and molecular nitrogen. On the other hand, the decomposition G



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5. CONCLUSIONS

Article

ASSOCIATED CONTENT

* Supporting Information S

TPD and XPS experiments, as well as DFT calculations, have been performed to help characterize the thermal chemistry that coadsorbed formic acid and hydrazine follow on copper surfaces. The TPD data show three main temperature regimes where the evolution of gas-phase products is detected: at about 240 K, where both molecular HCOOH and N2H4 are detected (presumably as a HCOOH:N2H4 adduct), around 420 K, at which point chemisorbed hydrazine decomposes into ammonia and molecular nitrogen, and at about 465 K, a regime dominated by the decomposition of formic acid to carbon dioxide. The order in which the two compounds are dosed on the surface affects these results only in terms of the yields of the molecular species evolving at 240 K (they are higher if N2H4 is dosed first) and of the threshold temperature for the decomposition of hydrazine (lower with the N 2H 4 + HCOOH dosing sequence). Isotope labeling of the formic acid (using DCOOD instead of HCOOH) leads to a delay in surface decomposition, due to a kinetic isotope effect, to an extensive H−D scrambling of the adsorbed formic acid before its high-temperature molecular desorption, and to the selective incorporation of deuterium from that formic acid into the NH2(ads) species resulting from N2H4 decomposition to produce labeled (NH2D) ammonia. The DFT calculations carried out here attest to the energetic stability gained by the formation of hydrogen-bonded HCOOH:N2H4 adducts, both in the gas phase and on the surface. In the latter case, however, a curious result was obtained: the two molecules may either coadsorb directly on the metal surface, or formic acid can bond on top of the adsorbed hydrazine (both configurations show similar energetics); addition of hydrazine on top of adsorbed formic acid proved to be much less stable. The occurrence of the second option, the H-bonding of HCOOH on top of N2H4, is further suggested by the yield difference of the HCOOH:N2H4 adduct versus dosing order mentioned above, as well as by the limited amount of HCOOH that can be added to the copper surface. XPS data are consistent with the interpretation of the TPD and DFT results provided above. Hydrogen bonding is manifested by the loss in the asymmetry of the O 1s XPS peaks, which indicates the disappearance of the different character of the carbonyl and hydroxo oxygen atoms in HCOOH. The shift seen in the N 1s XPS data upon heating the surface to 300 K supports the idea of the activation of the N−N bond in adsorbed hydrazine, to form the surface NH2(ads) species that may act as the precursor to ammonia production. Importantly, this is a change with respect to the surface chemistry of hydrazine alone on clean Cu(110), where significant dehydrogenation to produce N2Hx• species is observed. Finally, the C 1s and O 1s XPS peaks survive to higher temperatures, and only go away after heating to 500 K, after HCOOH decomposition to yield gas-phase CO2. One additional interesting observation deriving from a quantitative analysis of the XPS data is that the HCOOH:N2H4 ratio on the surface is stoichiometric (i.e., 1:1) throughout most temperatures; the only exception is seen at 400 K, where most of the N 1s XPS intensity is gone but where there are still significant amounts of carbon and oxygen on the surface.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b01804. Calculated adsorption geometries and isosurfaces of charge density for HCOOH + N2H4 coadsorbed on Cu(110) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Francisco Zaera: 0000-0002-0128-7221 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this project was provided by a grant from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering (MSE) Division, under Award No. DE-FG02-03ER46599. N.T. thanks DGAPAUNAM project IN100516 for partial financial support and DGAPA-UNAM for a scholarship for a sabbatical leave at the University of California, Riverside. Calculations were performed in the DGCTIC-UNAM supercomputing center, project LANCAD-UNAM-DGTIC-051.

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