Structure and Reactivity of Molecularly Adsorbed Ammonia on the

Article pubs.acs.org/JPCC

Structure and Reactivity of Molecularly Adsorbed Ammonia on the ZrB2(0001) Surface Kedar Manandhar,† Weronika Walkosz,‡ Yuan Ren,† Shigeki Otani,§ Peter Zapol,‡ and Michael Trenary*,† †

Department of Chemistry, University of Illinois at Chicago, 845 W Taylor Street, Chicago, Illinois 60607, United States Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States § National Institute for Material Science, 1-1 Namiki, Tsukuba-shi, Ibaraki 305-0044, Japan ‡

S Supporting Information *

ABSTRACT: X-ray photoelectron spectroscopy (XPS) and reflection absorption infrared spectroscopy (RAIRS) have been used to study the structure of molecularly adsorbed ammonia on the ZrB2(0001) surface and its subsequent dissociation. Spectra were obtained as a function of ammonia exposure to the surface at 95 K and as a function of annealing temperature following exposures at 95 and 300 K. The infrared peak positions do not vary with exposure, from the lowest submonolayer coverages to thick multilayers and are at the same values as those of solid ammonia. This indicates that the NH3 molecules have high enough mobility at 95 K to aggregate into hydrogenbonded clusters with the same structure as that of solid ammonia. The peak positions match those of solid ammonia much better than those calculated for an isolated ammonia molecule adsorbed on top of a Zr atom, which was found to be the most stable binding site. Although aggregation into clusters at low temperatures implies a weak interaction with the substrate, a minor dissociation channel may exist, even at 95 K, as indicated by XPS through the appearance of a small N 1s peak at 397.5 eV attributed to atomic nitrogen in addition to a N 1s peak at 401.9 eV due to molecularly adsorbed NH3. Ammonia completely dissociates to atomic nitrogen and hydrogen for adsorption temperatures of 300 K and above. The hydrogen product of dissociation desorbs between 535 and 555 K, as indicated by the disappearance of a Zr−H vibration at 999 cm−1.

■

INTRODUCTION The adsorption and decomposition of ammonia on transition metal surfaces is of considerable interest because it relates to industrial catalytic processes such as ammonia synthesis.1 In some cases, ammonia adsorbs molecularly at low temperatures and desorbs without dissociation upon annealing.2−10 On the Fe(110), (100), and (111)11,12 and Ru (112̅1)13,14 surfaces, which are metals used as catalysts in the synthesis of ammonia, a chemical transformation of adsorbed ammonia takes place below 290 K, followed by dissociation to atomic nitrogen and hydrogen at higher temperatures. Dissociation of ammonia was also reported on the Ni(110) surface.7,15,16 Here we investigate the adsorption of ammonia on the (0001) surface of zirconium diboride, a metallic compound whose surface chemical properties are largely unknown but are nonetheless relevant to important potential applications. Zirconium diboride is a layered compound with a hexagonal crystal structure in which graphite-like boron layers alternate with close-packed zirconium layers.17 In principle, the (0001) surface of ZrB2 can be terminated with either a Zr or B layer but previous studies have indicated that Zr termination is thermodynamically more stable.18−21 The surface chemistry of ZrB2 has been studied in only a few previous cases. The interaction of ZrB2(0001) with hydrogen has been studied with reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT)22 and high-resolution electron energy loss spectroscopy (HREELS), with the latter technique also used to study the interaction with O2 and CO.23 Recently, we used X-ray photoelectron spectroscopy (XPS) and DFT to © 2014 American Chemical Society

investigate the buildup of surface nitrogen through dissociative adsorption of ammonia at room temperature and above.24 The work reported here differs from the previous one in focusing on lower temperatures where molecular adsorption occurs. In comparison to other metal surfaces, the interaction of ammonia with ZrB2 is similar to that of unreactive surfaces in that at low temperatures the molecules adopt a structure that is remarkably similar to that of solid ammonia, as indicated by RAIRS, and yet is similar to reactive metals in that it undergoes dissociative adsorption for high temperature exposures. In this respect, ZrB2(0001) is similar to Ru(0001) where RAIRS revealed molecular adsorption at 80 K,6 but decomposition for ammonia exposures with the surface at 350−500 K.25−27 As Ru is the most effective catalyst for ammonia synthesis, these properties are consistent with an earlier suggestion that ZrB2 might have useful catalytic properties.28 The chemistry of ammonia on ZrB2(0001) is also of interest because this surface has recently been proposed as a viable substrate for GaN epitaxial growth using ammonia as a nitrogen source. This application is primarily motivated by the good lattice constant and thermal expansion matches between the ZrB2(0001) surface and GaN.29−32 GaN has been grown successfully on ZrB2(0001) using molecular beam epitaxy Special Issue: John C. Hemminger Festschrift Received: June 1, 2014 Revised: July 12, 2014 Published: July 14, 2014 29260

dx.doi.org/10.1021/jp505406r | J. Phys. Chem. C 2014, 118, 29260−29269

The Journal of Physical Chemistry C

Article

(MBE),31 pulsed-laser deposition,32 and metal organic chemical vapor deposition (MOCVD),33 with adsorption and decomposition of ammonia particularly relevant to the latter growth method where the N and Ga precursors are typically ammonia and trimethyl gallium (TMG), respectively. In MOCVD and MBE of GaN on ZrB2, the growth sequence has been found to be important, and the substrates were exposed to the nitrogen sources before the metal sources.33,34 Besides growth sequence, the substrate temperature during nitrogen-source exposure is critical. At higher temperatures, a reaction occurs between nitrogen and ZrB2 resulting in changes to the chemical composition and lattice structure at the interface. Wang et al.34 reported formation of a hexagonal BN surface layer and Tomida et al.33 and Manandhar et al.24 reported formation of a ZrxByNz alloy at the interface. Therefore, when a GaN film is grown at high temperature, it is not possible to achieve as good a lattice match, as suggested by an ideal GaN/ZrB2(0001) interface and a lower growth temperature is desirable. To achieve this goal, better information on the dissociation temperature and decomposition mechanism of NH3 and other precursor gases on ZrB2(0001) is needed. The present work on molecular adsorption of ammonia at low temperatures therefore complements our recent work on the thermal decomposition of TMG35 and the higher temperature deposition of N from NH3 decomposition on the ZrB2(0001) surface.24

■

EXPERIMENTAL METHODS The experiments were performed in a stainless steel ultra high vacuum (UHV) chamber with a base pressure of ∼1 × 10−10 Torr that has been described elsewhere.36 The ZrB2 crystal growth and surface preparation have been described elsewhere.22,37 Procedures for mounting the crystal and for cleaning the surface and techniques used for analyzing the cleanness of the surface have also been described previously.22,24 All XPS spectra were taken at a 15° angle between the analyzer and the surface normal using Mg Kα radiation and a constant analyzer pass energy of 50 eV corresponding to a nominal resolution of 1.32 eV. The RAIRS spectra were acquired using a resolution of 4 cm−1 and 1024 scans using a mercury cadmium telluride (MCT) detector. The RAIRS experimental set up, calibration of the XPS system and details about the X-ray source operation are the same as in previous experiments.22,24 In cases where the crystal was annealed above 95 K or room temperature (RT), it was cooled back down to 95 K or RT before acquiring the spectra. All background spectra were also acquired at either 95 K or RT. The ammonia (experimental grade) was purchased from Matheson and was used without further purification. Gas exposures are given in units of Langmuir (L), where 1 L = 1 × 10−6 Torr s, and are based on the rise in chamber pressure during the exposure. However, because a gas doser was used, the effective exposures at the crystal surface were much higher than implied by the values given in Langmuir units, and therefore serve only to provide relative exposures. For the annealing experiments, the sample was heated to the desired temperature and held there for ∼2 min.

Figure 1. XPS spectra in the (a) N 1s and (b) Zr 3d/B 1s regions as a function of annealing temperature following a 0.2 L NH3 exposure to ZrB2(0001) at 95 K. (c) Plots of relative concentration of nitrogen (black line with solid squares), zirconium (red line with open squares), and boron (blue line with open circles) as a function of annealing temperature.

K. Two N 1s peaks, a large one centered at 401.9 eV and a weak one centered at 397.5 eV, are apparent and are accompanied by significant suppression of the Zr 3d and B 1s emission. The position of the 401.9 eV peak was unchanged upon annealing up to 115 K. After annealing to 129 K, the 401.9 eV peak shifted −0.4 to 401.5 eV (Figure 1a). Upon annealing to 139 K, a small new peak at 399.3 eV appeared, and after annealing further to ∼289 K, only the peak at ∼397.5 eV remained. Figure 1c is a plot of relative concentrations (RCs) of boron, zirconium and nitrogen as a function of annealing temperature, which provide a semiquantitative estimate of the elemental composition of the surface based on integrated peak areas and XPS sensitivity factors.38 For the clean surface, the RCs of

■

RESULTS XPS. Figure 1 displays XPS spectra of the N 1s (a) and B 1s/ Zr 3d (b) regions as a function of annealing temperature following a 0.2 L NH3 exposure to the ZrB2(0001) surface at 95 29261



Article

Figure 2. RAIR spectra as a function of ammonia exposure at 95 K showing the intense (a) δs(NH3) and (b) νas(NH3) peaks and spectra (c) and (d) in the same regions with an expanded intensity scales that reveal weaker peaks. (e) Plot of RAIRS intensity of the 1077 (black line with open squares), 1215 (red line with filled squares), and 1887 cm−1 peaks (blue line with open circles) vs exposure.

at 401.9 eV gradually decreased to 18%, largely due to the decrease in the 401.9 eV peak. The RC of nitrogen showed a further significant decrease upon annealing to 139 and 225 K. The total RCs of nitrogen based on both N 1s peaks after annealing from 139 to 289 K were basically unchanged at ∼7%, implying that the remaining adsorbates are surface species, rather than part of a multilayer. Furthermore, the sharp decrease in the RC of nitrogen from 115 to 139 K indicates molecular desorption of ammonia in this temperature range. RAIRS. Figure 2 shows RAIR spectra obtained as a function of NH3 exposure to the ZrB2 surface at 95 K. Figure 2a,b shows the spectral regions with the most intense peaks, which for a 0.01 L exposure occur at 1077 and 3381 cm−1 and shift by ∼+2 cm−1 for higher exposures. A full spectrum from 800 to 4000

boron, zirconium, and nitrogen were 58, 42 and 0%, respectively. Although the stoichiometry implies that the RCs of B and Zr for the clean surface should be 67 and 33%, respectively, because the top layer is Zr, with the first boron layer below the surface, the B/Zr ratio from the RCs is slightly lower than would be obtained for a homogeneous distribution of the two elements over the escape depth of the photoelectrons. The RC of nitrogen (based on the sum of the 401.9 and 397.5 eV peak areas) was calculated to be ∼59%, while those of Zr and B were 16 and 25%, respectively. The RC of nitrogen associated with the 397.5 eV peaks was unchanged at ∼4%, from 95 to 225 K. Annealing the sample in steps to 129 K gradually increased the RCs of Zr and B to 37 and 45%, respectively, and that of nitrogen based on the large N 1s peak 29262



Article

Figure 3. RAIR spectra as a function of annealing temperature following a 0.2 L ammonia exposure at 95 K showing the intense (a) δs(NH3) and (b) νas(NH3) peaks and spectra (c) and (d) in the same regions with expanded intensity scales that reveal weaker peaks. (e) Plot of RAIRS intensity of the 1077 (black line with open squares), 1215 (red line with filled squares), and 1890 cm−1 peaks (blue line with open circles) vs annealing temperature. The left and right arrows in the legend identify which intensity axis is associated with the different data points.

cm−1 is given in the SI. The intensity scale is expanded in Figure 2c,d to reveal additional weak peaks at 1215, 1407, 1494, 1590, 1648, 1887, 3209, and 3291 cm−1, which do not shift with increasing exposure. In Figure 2e, plots of the intensities of the 1077, 1215, and 1887 cm−1 peaks as a function of exposure reveal that the intensities increase rapidly for exposures less than about 0.1 L, followed by a slower increase and that all three peaks show parallel behavior. A similar plot at even lower exposures is given in the SI. Figure 3 shows RAIRS spectra as a function of annealing temperature following a 0.2 L ammonia exposure to the ZrB2 surface at 95 K. As before, Figure 3a,b are the spectral regions with the most intense peaks and Figure 3c,d shows spectra with an expanded intensity scale revealing

weaker peaks. The spectra for a 0.2 L exposure at 95 K in Figures 2 and 3 were obtained in separate experiments and are nearly identical, which thus demonstrates the high degree of reproducibility of the RAIRS results. Figure 3e is a plot of the intensities of only the 1077, 1215, and 1890 cm−1 peaks as a function of annealing temperature, although all peaks behaved similarly. Upon annealing from 100 to 129 K, the decrease in intensity of all the peaks was very small, whereas large intensity decreases occur upon annealing above 129 K. Figure 4a shows RAIR spectra as a function of annealing temperature following a 0.1 L ammonia exposure at 300 K. The peak at 998 cm−1 is attributed to a Zr−H stretch vibration as essentially the same result was observed when the ZrB2(0001) 29263



Article

probably due to the coadsorption of hydrogen and nitrogen. The main peak at 998 cm−1 is accompanied by a shoulder at 1004 cm−1, the possible origin of which is discussed elsewhere.22

■

DISCUSSION Analysis of Dissociation of NH3 from XPS Data. XPS was used to analyze the species formed on the ZrB2 surface from dissociation of ammonia as a function of annealing temperature. The RC of nitrogen, which was 59% at 95 K, was reduced by a factor of about three upon annealing to 129 K. This RC change was accompanied by a −0.4 eV shift in the 401.9 eV peak to 401.5 eV (Figure 1a). A N 1s binding energy of 401.5 eV agrees well with that of molecular ammonia on metal surfaces.39 It is noteworthy that in the NH3/Pt(111) system, the N 1s BE of multilayer ammonia was 0.6 eV higher than that of monolayer ammonia.40 Also in our recent XPS study of TMG on ZrB2, we observed that the C 1s BE of multilayer TMG was ∼0.3 eV higher than that of the surface layer.35 Therefore, the 401.9 eV peak in Figure 1a is assigned to multilayer ammonia. The peak at 397.5 eV agrees well with the measured BEs of atomic nitrogen on semiconductor and metal surfaces.39,41,42 The fact that it is visible even for exposure at 95 K suggests that there is some dissociation even at this low temperature. The BE of the very weak peak at 399.3 eV was associated with NH2 or NH in previous studies.38,40,41 These BEs also match the positions of overlapping N 1s peaks obtained from exposure of ammonia to the ZrB2 surface at temperatures of 660 K or higher.24 Two N 1s peaks at 397.6 and 399.2 eV were attributed to nitrogen bonded to zirconium and to boron, respectively, in a ZrxByNz alloy. Formation of a ZrxByNz alloy here is not possible for at least two reasons. First, the 399.3 eV peak in the 139 and 225 K spectra disappears following the 289 K anneal, suggesting that it is unrelated to the reaction that occurs at the much higher temperatures used earlier.24 Second, in the RAIRS spectrum taken after annealing to 139 K, peaks of molecularly adsorbed ammonia were still observed implying that dissociation all the way to atomic N and H does not occur. The results from a theoretical calculation showed that the lowest barrier in the stepwise dissociation of

Figure 4. (a) RAIR spectra following a 0.1 L exposure of NH3 to ZrB2(0001) at 300 K and following annealing to the indicated temperatures. (b) Plot of peak area as a function of annealing temperature.

surface was exposed to H2(g) at 300 K. The peak position remains unchanged as does the peak width as the annealing temperature is increased. Figure 4b is a plot of the peak intensity as a function of annealing temperature. A small decrease in intensity began at around 485 K and the peak disappeared completely between 535 and 555 K (Figure 4a,b). The approximately 55 K lower hydrogen desorption temperature observed here compared with our earlier report22 is

Table 1. Vibrational Modes (cm−1) of Ammonia Observed in This Experiment and in the Literaturea

a

NH3/substrate; vibrational mode

δs(NH3)

δas(NH3)

this work (ZrB2(0001) NH3/ZrB2(0001), theory43 solid ammonia; IR49 solid ammonia; IR51 solid ammonia; IR52 solid ammonia; IR50 HOPG; RAIRS45 Ag(111); RAIRS2 Au(111); RAIRS3 Pt(111); RAIRS44 Pt(111); RAIRS5 Pt(111); HREELS40 Pt(111); HREELS46 GaAs(100); HREELS41 Ni(110); HREELS7 Pd(111)/Mo(110); HREELS10 Ru(001); HREELS47

1077 1090 1060 1057 1058 1057 1076 1079 1103 1076 1080−1199 1088 (1L), 1230 (mL) 1090 1210 1100−1400 1100 (lc), 1050 (1L), 1100 (mL) 1155−1085

1590−1648 1534 1646 1650 1650 1650 1652 1652 1626 1632 1526−1617 1533 (1L), 1607 (mL) 1600 (lc), 1630 (hc) 1600 1500−1650 1610 (2 mL), 1625 (mL) 1560

νs(NH3) 3209 3334 3223 3210 3210 3210 3299 3210 3284 3150−3250 3155−3214 3240 (lc), 3150 (hc) 3200 3200−3350 3200−3500, 3360 3270

νas(NH3) 3381 3481 3378 3375 3378, 3370 (Sh) 3374 3380 3377 3394 3381 3325 3340 (lc), 3320 (hc) 3340 3300−3400 3400

1L = first layer, mL = multilayer, lc = low coverage, and hc = high coverage. 29264



Article

NH3 to adsorbed N and H atoms is the first step to produce NH2 and H on the ZrB2(0001) surface, but the barrier for subsequent NH2 dissociation to NH and H is also small, suggesting that NH2 may not be a stable surface species.43 The calculated barrier for NH dissociation to N and H is higher, but there is no clear indication from the present RAIRS data for its presence on the surface. If the 399.3 eV peak observed with XPS is due to NH2 or NH, then one or both of these moieties are not detectable with RAIRS on this surface. Annealing between 139 and 289 K caused the intensity of the 397.5 eV peak to increase, while the 399.3 and 401.5 eV peaks disappeared (Figure 1a), suggesting dissociation of the remaining NH3 and possible NHx species on the surface to N and H atoms. The conclusion that complete dissociation to N and H atoms occurs by 300 K is further supported by the RAIRS data for ammonia exposure at 300 K (Figure 4a). The RAIRS spectra reveal only the ∼998 cm−1 peak assigned to a Zr−H stretch, as reported earlier.22,23 After annealing the ammonia multilayer deposited at low temperature to 289 K, the RC of nitrogen is ∼7%, which is ∼3% less compared to the maximum RC of nitrogen on the ZrB2(0001) surface obtained by exposing to ammonia at RT.24 For both hydrogen and nitrogen, the 3-fold hollow sites on the ZrB2(0001) surface are energetically favored. From our recent work it is known that hydrogen atoms on the ZrB2(0001) surface are mobile, even at 95 K.22 A slightly larger RC of nitrogen when ammonia is exposed at RT is probably due to the reordering of hydrogen to form more dense islands, thus, making more 3-fold sites available for nitrogen adsorption. Assignment and Analysis of RAIRS Spectra. The RAIR spectra in Figures 2 and 3 consist of two very intense peaks at 1077 and 3381 cm−1 along with multiple relatively weak peaks. The spectra can be compared with previous RAIRS studies of ammonia on Pt(111),5,44 Au(111),3 highly ordered pyrolytic graphite (HOPG),45 and Ag(111).2 Further comparison can be made to high resolution electron energy loss spectroscopy (HREELS) studies of NH3 on Pt(111),40,46 GaAs(100),41 Ni(110),7 Ru(001),47 and Pd(111)10 and to transmission IR studies of NH3 on silicon surfaces.48 In particular, our spectra are in good agreement with IR spectra of solid ammonia49−52 (Table 1) and of NH3 on weakly interacting surfaces like Ag(111)2 and HOPG,45 indicating that the structure of the overlayer, from submonolayer to multilayer, on the ZrB2 surface is similar to that of solid ammonia. The peaks at 1077 and 1590−1648 cm−1 correspond to symmetric and asymmetric deformations, δs(NH3) and δas(NH3), respectively. Compared to the peak at 3381 cm−1, the one at 3209 cm−1 was relatively weak (∼1/95 of the 3381 cm−1 peak height) (Figure 2b,d). For gas phase ammonia, a weak peak at 3414 cm−1 and a very strong peak at 3337 cm−1 correspond, respectively, to νas(NH3) and νs(NH3) vibrations.53 In thin films of crystalline ammonia formed on KBr at 83 K, Reding and Hornig observed an intense peak at ∼3380 cm−1 and a weak peak at ∼3220 cm−1.49 After careful analysis, they assigned the higher and lower frequency peaks to νas(NH3) and νs(NH3), respectively. A complete reversal was observed in the relative intensity of νs(NH3) and νas(NH3) when ammonia changes from the gas to the solid phase. In several other studies of solid ammonia and in a RAIRS study of NH3 on the Ag(111) surface, the authors assigned weaker peaks at ∼3215 cm−1 and intense peaks at ∼3280 cm−1 to νs(NH3) and νas(NH3), respectively.2,50−52,54 Similarly, in HREELS studies of NH3 on GaAs(100),41 νas(NH3) was very intense compared to νs(NH3), and on

Pt(111),46 νas(NH3) was larger than νs(NH3). Therefore, we assign the strong peak at 3381 and the weak peak at 3209 cm−1 observed here, to νas(NH3) and νs(NH3), respectively. Observation of δas(NH3) and νas(NH3) here suggest that the C3 axis of the ammonia molecule is tilted with respect to the surface normal. If the C3 axis of ammonia were perpendicular to the surface, then δas(NH3) and νas(NH3) would not be dipole allowed and therefore not observable with RAIRS. In addition to comparison with previous experimental studies of the vibrational frequencies of ammonia in the solid state and on surfaces, Table 1 also contains theoretical values for NH3 on the ZrB2(0001) surface. These results were obtained with firstprinciples density functional theory (DFT) calculations using the projector augmented wave (PAW) method in the VASP implementation.55,56 The surface calculations were performed with the optimized bulk lattice parameters (a = 3.179 Å and c = 3.548 Å) using a symmetric slab consisting of seven atomic layers of ZrB2. The theoretical method used is described in more detail in our previous study22 of hydrogen on the ZrB2(0001) surface and the optimized geometries and adsorption energies for NH3 and its decomposition products NH2, NH, N, and H as well as the activation energies for the decomposition steps are presented elsewhere.43 The calculations indicate that NH3 bonds through the N atom to a Zr atom in an atop geometry with the 3-fold axis perpendicular to the surface with a binding energy of 0.95 eV. As only one molecule per surface unit cell was included in the calculations, they do not account for the hydrogen bonding interactions that are present in solid ammonia. Such interactions generally lead to red shifts in stretching modes, and the fact that the experimental νas(NH3) value measured here matches that of solid ammonia much better than the calculated value for an isolated NH3 molecule adsorbed at a Zr site indicates that the molecules do not have the calculated structure on ZrB2(0001). The calculations also indicate that there is no barrier to NH3 dissociation, implying that molecular adsorption of NH3 should not occur. The clear experimental evidence for molecular adsorption may indicate that the formation of hydrogenbonded clusters on the surface stabilizes NH3 against dissociation. The NH2 dissociation product has a binding energy of 4.33 eV and occupies a bridge site between two Zr atoms and NH occupies a hollow site between three Zr atoms. The calculated frequencies of NH2 on ZrB2(0001) are νas(NH2) = 3378, νs(NH2) = 3209, and δ(NH2) = 1443 cm−1, while for NH, ν(NH) = 3462 cm−1. Although N−H stretch values do not necessarily preclude NH2 or NH on the surface, the fact that the 1076 cm−1 peak accompanies the observed N−H stretch peak at 3382 cm−1 is only consistent with adsorbed NH3. Assignments of the remaining weak peaks seen in Figures 2 and 3, which are not fundamentals, are listed in Table 2. The peak at 3291 cm−1 is assigned to the overtone of δas(NH3), which was observed at 3290 ± 10 cm−1 in IR studies of solid ammonia and of NH3 on the Ag(111) surface.2,49 In the spectra for exposures of ∼0.05 L and higher, a broad peak at 1215 cm−1 is revealed. White and co-workers reported peaks at 1210 and 1230 cm−1 in their HREELS studies of ammonia on the Pt(111) and GaAs(100) surfaces, respectively.40,41 They attributed these peaks to δs(NH3) of NH3 multilayers. However, in this case it is more likely due to one of the combination bands listed in Table 2 based on the following considerations. The XPS results shown here in Figure 1a,b implied that a 0.2 L ammonia exposure resulted in a multilayer 29265



Article

The high resolution of RAIRS allows us to measure small frequency shifts as a function of exposure and annealing temperature. The observed shifts are no greater than ∼2 cm−1 for the δs(NH3) and νas(NH3) peaks in Figures 2a,b and 3a,b. Much larger shifts with increasing coverage are often observed due to dipole−dipole coupling for molecules such as CO.58 An essentially constant peak frequency with coverage often occurs when molecules aggregate into clusters, even at the lowest coverages, because the local density and structure is invariant as the overall coverage increases. Ammonia−ZrB2 Surface Interaction versus NH3−NH3 Interaction. The XPS results of Figure 1 reveal that a 0.2 L NH3 exposure produces an ammonia multilayer as implied both by the 401.9 eV BE of the dominant peak and the fact that there is a large intensity decrease of the N 1s peak upon annealing to 139 K, a temperature that appears to be high enough to desorb the multilayer but to leave an adsorbed monolayer on the surface. A similar conclusion is reached from the RAIRS data where, even for the lowest exposure at 95 K of 0.01 L in Figure 2, the peaks are essentially at the same positions as for multilayer ammonia, which also match the positions for crystalline NH3. As shown in the SI, for an even lower exposure of 0.001 L, the δs(NH3) is still observed at ∼1077 cm−1 and the peak has a ΔR/R value comparable to that seen for δs(NH3) for submonolayer coverages of ammonia on other metal surfaces.5,6 In the RAIR spectrum taken after annealing to 139 K (Figure 3c,d), the peaks that remain are δs(NH3) at ∼1074 cm−1, δas(NH3) at 1590−1648 cm−1, and νas(NH3) at 3380 cm−1. For the 0.01 L spectra in Figure 2a−c, only these three peaks are visible. This implies that, for an exposure of ∼0.01 L, the ammonia coverage is less than a monolayer and that all the molecules could therefore be bonded directly to the surface if they were uniformly distributed across the surface. In contrast, for a much higher exposure of 0.2 L exposure, XPS indicates that the RC of zirconium was only 16%, which is 26% (42−16 = 26) lower than that of clean ZrB2. This implies that the ammonia multilayer had a thickness approximately equal to the photoelectron escape depth. Therefore, it is reasonable to assume that a 0.01 L exposure corresponds to an overall coverage in the submonolayer range. An understanding of the unusual observation here that the RAIR spectra are essentially the same for submonolayer and multilayer ammonia requires consideration of alternative interpretations. One possibility is that there is no activation barrier for complete dissociation to atomic nitrogen and hydrogen on the bare surface so that once a monolayer of N and H forms, the surface is passivated against further dissociation. In this scenario, the first NH3 molecules to impinge on the surface lead to no observable RAIRS peaks, and that the submonolayer results are for NH3 on the passivated surface. Indeed some dissociation at low temperature is indicated by the barely discernible XPS peak in Figure 1 at 397.5 eV. However, there are two aspects of the RAIR spectra that preclude this possibility. First, as shown by the spectra in Figure 4, NH3 dissociation at room temperature yields a Zr−H stretch vibration at 998 cm−1. As shown in the SI, a close examination of the spectral region from 800 to 1200 cm−1 for nominal exposures of 0.001 to 1.0 L reveals no peaks other than δs(NH3) at ∼1077 cm−1. Second, if the ammonia must first dissociate to passivate the surface before molecular adsorption can occur, then there should be a threshold exposure before any NH3(ads) peaks appear. Such a threshold would be revealed in

Table 2. Combination and Overtone Bands of Ammonia on the ZrB2(0001) Surfacea NH3 vibration (cm−1) observed 1215

calculated

1407

1220 1236 1440

1494

1547

1887 3291

1904 3180−3296

assignment νp + ρr(NH3) δs(NH3) + 3 × ν6 ν6 = 53 cm−1 [32] 2 × ρr(NH3); ρr(NH3) = 720 cm−1 νs(ZrB2 − N) + ρr(NH3) νs(ZrB2 − N) = 827 cm−1 δs(NH3) + νs(ZrB2 − N) 2 × δas(NH3)

ν6, parallel torsional oscillation in ammonia lattice; ρr(NH3), rocking vibration; νs(ZrB2−N), substrate−nitrogen stretch; νp, ZrB2 surface phonon (∼500 cm−1). a

but for exposures from 0.05 to 1 L, the 1215 cm−1 peak appears in the RAIR spectra as a broad and very weak shoulder on the δs(NH3) peak at 1077 cm−1 (Figure 2c). Furthermore, after exposure at 95 K, the intensity of this peak was essentially unchanged upon annealing up to 129 K (Figure 3c,e), which is in contrast to the XPS results (Figure 1a−c) where desorption of multilayer ammonia after annealing to ∼129 K occurred. Even for exposures that are too low to give true multilayers, three-dimensional NH3 clusters could still form for overall submonolayer coverages. The RAIR spectra of such clusters would resemble the IR spectra of solid ammonia. In the HREEL studies of NH3 on Pt(111), Sun et al.40 and Sexton and Mitchell46 reported the rocking vibration, ρr(NH3), at ∼720 cm−1. This vibration is not observed here as our spectral range is limited by the IR detector used to above 800 cm−1. Assuming ρr(NH3) is also at ∼720 cm−1 here, 1215 cm−1 is a reasonable value for a combination band of ρr(NH3) and a ZrB2 substrate phonon (νp) at ∼500 cm−1.22,23 Reding and Horning reported a broad weak peak at ∼1207 cm−1 in their IR study of ammonia deposited on KBr at 83 K.49 They attributed this peak to a combination of the intense δs(NH3) mode and the second overtone of an infrared active fundamental at 53 cm−1. The 53 cm−1 vibration was assigned to a parallel torsional oscillation of the ammonia lattice. Hence, as indicated in Table 2, there are two plausible assignments of the weak 1215 cm−1. We assigned the peak at 1407 cm−1 to the overtone of ρr(NH3). In vibrational studies of ammonia and its dissociation products on surfaces and isolated in matrices, the scissors mode of NH2, δ(NH2), was reported at 1483−1520 cm−1,7,41,57 the same range where the peak observed here at 1494 cm−1 occurs. In the annealing experiment results in Figure 3c, this weak peak is not observed after annealing to 139 K. However, a peak possibly due to NH2, was observed with XPS at 399.3 eV only after the sample was annealed to 139 K (Figure 1a). As the 1494 cm−1 peak is observed for exposure at 95 K in Figures 2 and 3 and is seen to grow along with the other ammonia peaks in Figure 2, we assume it is due to NH3 rather than NH2. In the vibrational studies of NH3 on semiconductor and metal surfaces by several groups, a ∼827 cm−1 peak was assigned to a substrate−nitrogen stretch.40,41 Assuming the νs(ZrB2−N) stretch is also at ∼827 cm−1, the 1494 cm−1 peak matches closely with a combination of νs(ZrB2−N) and ρr(NH3) at ∼720 cm−1. We attribute a peak at 1887 cm−1 to a combination of δs(NH3) at 1077 cm−1 and νs(ZrB2 surface−N) at 827 cm−1. The increase of the 1887 cm−1 peak intensity as the 1077 cm−1 peak increases (Figures 2c,e and 3c,e) supports this assignment. 29266



Article

νas(NH3) was quite strong. XPS clearly shows formation of multilayer ammonia following an exposure of 0.2 L to the ZrB2 surface at 95 K. The position of the RAIRS peaks from submonolayer to multilayer did not shift, indicating that NH3 molecules formed small clusters even at the lowest coverages. Desorption of the ammonia multilayer starts between 115 and 129 K leaving only a surface layer after annealing to ∼139 K. After annealing up to 225 K, the intensity of the 397.5 eV peak corresponding to N atoms on the surface did not change and the RC of atomic nitrogen was calculated to be ∼4%. The low dissociation temperature observed here agrees well with the low calculated barrier for the first step in the dissociation of NH3 to eventually form adsorbed N and H on the surface. The final calculated RC of nitrogen on the surface is ∼7%. The hydrogen on the surface desorbs between 535 and 555 K.

a plot of peak intensity versus exposure. An extrapolation to zero peak intensity would intercept the exposure axis at the exposure needed to just passivate the surface. As shown in the SI, the peak intensity extrapolates to zero at zero exposure. For these reasons, we conclude that the submonolayer coverages correspond to molecular adsorption on the bare surface. Either the minor dissociation channel implied by XPS is not real or it is so minor as to not be manifested in the RAIRS data. It is possible that some dissociation occurs at defects such as step sites, which has no influence on molecular adsorption on the nondefective areas of the ZrB2(0001) surface. Scanning tunneling microscopy has shown that the HfB2(0001) surface has a structure consisting of wide Hf-terminated terraces separated by steps with heights equal to one lattice constant along the c axis.59 It can be assumed that the ZrB2(0001) surface has the same structure, with a relatively small fraction of step sites. We thus conclude that NH3 adsorbs molecularly on the Zr-terminated terraces at even the lowest coverages. The lack of significant peak shifts in the RAIR spectra in Figure 2 along with the absence of any strong new peaks as the coverage transitions from submonolayer to multilayers has important implications for the nature of the molecule−surface interaction. Similar vibrational spectra of submonolayer to multilayer ammonia were observed on Ag(111)2 and HOPG.45 Another indication that the RAIR spectra reported here correspond to crystalline ammonia is that the fwhm of both the δs(NH3) and the νas(NH3) peaks are the same for all exposures at ∼9 cm−1. A similar fwhm of 11 cm−1 was reported for both δs(NH3) and νas(NH3) in the IR spectrum of crystalline ammonia prepared on ZnSe at 100 K, as measured with an experimental resolution of 4 cm−1.51 Formation of crystalline ammonia for all exposures implies that there is a high mobility of the NH3 molecules at the deposition temperature and that the hydrogen bonding interactions between ammonia molecules is stronger than the barriers to NH3 diffusion. Although this is similar to the cases where the ammonia− surface interactions are weak, ammonia generally desorbs without dissociation from such surfaces. Although the ZrB2(0001) and Ru(0001) surfaces are similar in that NH3 adsorbs molecularly at low temperature and dissociatively at elevated temperatures, there are important differences in the details of the low-temperature RAIR spectra. On the Ru(0001) surface, the spectra evolve with coverage and it is possible to resolve separate peaks corresponding to NH3 molecules bound to Ru atoms versus NH3 molecules that are part of a multilayer. This implies that the NH3−Ru interaction is strong enough to prevent the formation of crystalline hydrogen-bonded clusters of NH3 molecules at low coverages and the molecules instead occupy definite Ru sites. Thus, ZrB2(0001) displays unique surface chemistry in that ammonia weakly adsorbs at low temperatures, suggesting an unreactive surface, but it is highly effective at dissociating ammonia at higher temperatures.

■

ASSOCIATED CONTENT

S Supporting Information *

Additional results, including a full RAIR spectrum from 800 to 4000 cm−1 of ammonia on the ZrB2(0001) surface following a nominal 0.2 L exposure at a surface temperature of 95 K, spectra at exposures as low 0.001 L, and plots of peak intensity versus exposure for low exposures of 0.001 to 0.025 L. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: (312) 996-0777. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences-Materials Science under Contract No. DE-AC02-06CH11357. K.M. and M.T. acknowledge partial support from the National Science Foundation under grant CHE-1012201.

■

REFERENCES

(1) Jennings, J. R. Catalytic Ammonia Synthesis. Fundamentals and Practice.; Plenum Press: New York, 1991. (2) Szulczewski, G. J.; White, J. M. Infrared Spectroscopy and Photodesorption of Condensed Phase Ammonia. Surf. Sci. 1998, 406, 194−205. (3) Deng, X.; Friend, C. M. Selectivity Switch for Nitrogen Functionalization of Styrene on Au(111). Surf. Sci. 2008, 602, 1066−1071. (4) Kim, C. S.; Bermudez, V. M.; Russell, J. N. Interaction of Ammonia with Hydrogen on Al(111). Surf. Sci. 1997, 389, 162−176. (5) Herceg, E.; Mudiyanselage, K.; Trenary, M. Reversible Hydrogenation of Surface N Atoms To Form NH on Pt(111). J. Phys. Chem. B 2005, 109, 2828−2835. (6) Rodriguez, J. A.; Kevin Kuhn, W.; Truong, C. M.; Wayne Goodman, D. A FT-IRAS Study of Ammonia Adsorbed on Ru(0001). Surf. Sci. 1992, 271, 333−339. (7) Bassignana, I. C.; Wagemann, K.; Küppers, J.; Ertl, G. Adsorption and Thermal Decomposition of Ammonia on a Ni(110) Surface: Isolation and Identification of Adsorbed NH2 and NH. Surf. Sci. 1986, 175, 22−44. (8) Gland, J. L.; Sexton, B. A.; Mitchell, G. E. Ammonia Adsorption on the Ag(110) Surface. Surf. Sci. 1982, 115, 623−632. (9) Ceyer, S. T.; Yates, J. T., Jr Ammonia Adsorption on the Ag(311) Surface. Surf. Sci. 1985, 155, 584−595.

■

SUMMARY The adsorption and dissociation of NH3 on the ZrB2(0001) surface was studied using XPS and RAIRS. At 95 K, ammonia mainly adsorbs molecularly, while a few percent, presumably initially bound to step sites, dissociate to N and H atoms, as indicated by the presence of an XPS peak at 397.5 eV, characteristic of atomic nitrogen. Consistent with vibrational frequencies observed for solid crystalline ammonia and ammonia weakly adsorbed on other surfaces, the symmetric N−H stretch, νs(NH3), peak was very weak, while that of 29267



Article

(33) Tomida, Y.; Nitta, S.; Kamiyama, S.; Amano, H.; Akasaki, I.; Otani, S.; Kinoshita, H.; Liu, R.; Bell, A.; Ponce, F. A. Growth of GaN on ZrB2 Substrate by Metal-Organic Vapor Phase Epitaxy. Appl. Surf. Sci. 2003, 216, 502−507. (34) Wang, Z. T.; Yamada-Takamura, Y.; Fujikawa, Y.; Sakurai, T.; Xue, Q. K.; Tolle, J.; Kouvetakis, J.; Tsong, I. S. T. Effect of Nitridation on the Growth of GaN on ZrB2(0001)/Si(111) by Molecular-Beam Epitaxy. J. Appl. Phys. 2006, 100, 033506. (35) Manandhar, K.; Trenary, M.; Otani, S.; Zapol, P. Dissociation of Trimethylgallium on the ZrB2(0001) Surface. J. Vac. Sci. Technol., A 2013, 31, 061405. (36) Kang, D. H.; Trenary, M. Surface Chemistry of Ethylenediamine (NH2CH2CH2NH2) on Pt(1 1 1). Surf. Sci. 2000, 470, L13−L19. (37) Otani, S.; Korsukova, M. M.; Mitsuhashi, T. Preparation of HfB2 and ZrB2 Single Crystals by the Floating-Zone Method. J. Cryst. Growth 1998, 186, 582−586. (38) Moulder, J.; Stickle, W.; Sobel, P.; Bomben, E. Handbook of Xray Photoelectron Spectroscopy; Physical Electronics: Eden Prairie, 1995. (39) Schwaner, A. L.; Pylant, E. D.; White, J. M. Electron-Induced Surface Chemistry: Synthesis of NHx Fragments on Ag(111). J. Vac. Sci. Technol., A 1996, 14, 1453−1456. (40) Sun, Y. M.; Sloan, D.; Ihm, H.; White, J. M. Electron-Induced Surface Chemistry: Production and Characterization of NH2 and NH Species on Pt(111). J. Vac. Sci. Technol., A 1996, 14, 1516−1521. (41) Zhu, X.-Y.; Wolf, M.; Huett, T.; White, J. M. Laser-Induced Interaction of Ammonia with GaAs(100). I. Dissociation and Nitridation. J. Chem. Phys. 1992, 97, 5856−5867. (42) Bozso, F.; Avouris, P. Photoemission Studies of the Reactions of Ammonia and N atoms with Si(100)-(2 × 1) and Si(111)-(7 × 7) Surfaces. Phys. Rev. B 1988, 38, 3937−3942. (43) Walkosz, W.; Manandhar, K.; Zapol, P.; Trenary, M. A DFT Study of the Decomposition of Ammonia on the ZrB2(0001) Surface. Manuscript to be published. (44) Villegas, I.; Weaver, M. J. Infrared Spectroscopy of Model Electrochemical Interfaces in Ultrahigh Vacuum: Interfacial Cation Solvation by Ammonia on Pt(111). Surf. Sci. 1996, 367, 162−176. (45) Bolina, A. S.; Brown, W. A. Studies of Physisorbed Ammonia Overlayers Adsorbed on Graphite. Surf. Sci. 2005, 598, 45−56. (46) Sexton, B. A.; Mitchell, G. E. Vibrational Spectra of Ammonia Chemisorbed on Platinum (111): I. Identification of Chemisorbed States. Surf. Sci. 1980, 99, 523−538. (47) Parmeter, J. E.; Wang, Y.; Mullins, C. B.; Weinberg, W. H. Electron Energy Loss Spectroscopy of Ammonia on Ru(001). J. Chem. Phys. 1988, 88, 5225−5236. (48) Dillon, A. C.; Gupta, P.; Robinson, M. B.; Bracker, A. S.; George, S. M. Ammonia Decomposition on Silicon Surfaces Studied Using Transmission Fourier Transform Infrared Spectroscopy. J. Vac. Sci. Technol., A 1991, 9, 2222−2230. (49) Reding, F. P.; Hornig, D. F. The Vibrational Spectra of Molecules and Complex Ions in Crystals. V. Ammonia and DeuteroAmmonia. J. Chem. Phys. 1951, 19, 594−601. (50) Ferraro, J. R.; Sill, G.; Fink, U. Infrared Intensity Measurements of Cryodeposited Thin Films of NH3, NH4HS, H2S, and Assignments of Absorption Bands. Appl. Spectrosc. 1980, 34, 525−533. (51) Holt, J. S.; Sadoskas, D.; Pursell, C. J. Infrared Spectroscopy of the Solid Phases of Ammonia. J. Chem. Phys. 2004, 120, 7153−7157. (52) Binbrek, O. S.; Anderson, A. Raman Spectra of Molecular Crystals. Ammonia and 3-Deutero-Ammonia. Chem. Phys. Lett. 1972, 15, 421−427. (53) Barker, E. F. Perpendicular Vibrations of the Ammonia Molecule. Phys. Rev. 1939, 55, 657−662. (54) Wood, B. E.; Roux, J. A. Infrared Optical Properties of Thin H2O, NH3, and CO2 Cryofilms. J. Opt. Soc. Am. 1982, 72, 720−728. (55) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (56) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558−561. (57) Milligan, D. E.; Jacox, M. E. Matrix-Isolation Infrared Spectrum of the Free Radical NH2. J. Chem. Phys. 1965, 43, 4487−4493.

(10) Xu, C.; Goodman, D. W. Adsorption of Ammonia and Its Influence on Coadsorbed Carbon Monoxide on Monolayer and Multilayer Palladium Epitaxially Grown on Mo(110). J. Phys. Chem. B 1998, 102, 4392−4400. (11) Erley, W.; Ibach, H. Vibrational Spectra of Ammonia Adsorbed on Fe(110). Surf. Sci. Lett. 1982, 119, L357−L362. (12) Grunze, M.; Bozso, F.; Ertl, G.; Weiss, M. Interaction of Ammonia with Fe(111) and Fe(100) Surfaces. Appl. Surf. Sci. 1978, 1, 241−265. (13) Dietrich, H.; Jacobi, K.; Ertl, G. Decomposition of NH3 on Ru(11−21). Surf. Sci. 1996, 352−354, 138−141. (14) Jacobi, K.; Wang, Y.; Fan, C. Y.; Dietrich, H. Adsorption and Thermal Dehydrogenation of Ammonia on Ru(11−21). J. Chem. Phys. 2001, 115, 4306−4313. (15) Jacobi, K.; Jensen, E. S.; Rhodin, T. N.; Merrill, R. P. AngularResolved Photoemission from NH3 on Ni(110). Surf. Sci. 1981, 108, 397−410. (16) Grunze, M.; Golze, M.; Driscoll, R. K.; Dowben, P. A. Ammonia Adsorption and Decomposition on a Ni(110) Surface. J. Vac. Sci. Technol. 1981, 18, 611−615. (17) Hoard, J. L.; Hughes, R. E. Elemental Boron and Compounds of High Boron Content: Structure, Properties, and Polymorphism. In The Chemistry of Boron and Its Compounds; Muetterties, E. L., Ed.; Wiley: New York, 1967; pp 26−254. (18) Yamamoto, K.; Kobayashi, K.; Kawanowa, H.; Souda, R. Difference in the Outermost Layer between TaB2(0001) and HfB2(0001). Phys. Rev. B 1999, 60, 15617−15620. (19) Aizawa, T.; Hayami, W.; Otani, S. Surface Phonon Dispersion of ZrB2(0001) and NbB2(0001). Phys. Rev. B 2001, 65, 024303. (20) Liu, P. L.; Chizmeshya, A. V. G.; Kouvetakis, J.; Tsong, I. S. T. First-Principles Studies of GaN(0001) Heteroepitaxy on ZrB2(0001). Phys. Rev. B 2005, 72, 245335. (21) Zhang, X.; Luo, X.; Li, J.; Han, J.; Han, W.; Hong, C. Structure and Bonding Features of ZrB2(0001) Surface. Comput. Mater. Sci. 2009, 46, 1−6. (22) Walkosz, W.; Manandhar, K.; Trenary, M.; Otani, S.; Zapol, P. Dissociative Adsorption of Hydrogen on the ZrB2(0001) Surface. Surf. Sci. 2012, 606, 1808−1814. (23) Aizawa, T.; Hayami, W.; Otani, S. Adsorption of H2, 2H2, O2, and CO on ZrB2(0001). J. Chem. Phys. 2002, 117, 11310−11314. (24) Manandhar, K.; Walkosz, W.; Trenary, M.; Otani, S.; Zapol, P. Dissociative Adsorption of Ammonia on the ZrB2(0001) Surface. Surf. Sci. 2013, 615, 110−118. (25) Egawa, C.; Nishida, T.; Naito, S.; Tamaru, K. Ammonia Decomposition on (1110) and (001) Surfaces of Ruthenium. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1595−1604. (26) Tsai, W.; Weinberg, W. H. Steady-State Decomposition of Ammonia on the Ruthenium(001) Surface. J. Phys. Chem. 1987, 91, 5302−5307. (27) Dietrich, H.; Jacobi, K.; Ertl, G. Coverage, Lateral Order, and Vibrations of Atomic Nitrogen on Ru(0001). J. Chem. Phys. 1996, 105, 8944−8950. (28) Aizawa, T.; Suehara, S.; Hishita, S.; Otani, S.; Arai, M. Surface Core-Level Shift and Electronic Structure on Transition-Metal Diboride (0001) Surfaces. Phys. Rev. B 2005, 71, 165405. (29) Kinoshita, H.; Otani, S.; Kamiyama, S.; Amano, H.; Akasaki, I.; Suda, J.; Matsunami, H. Zirconium Diboride (0001) as an Electrically Conductive Lattice-Matched Substrate for Gallium Nitride. Jpn. J. Appl. Phys. 2001, 40, L1280−L1282. (30) Liu, R.; Bell, A.; Ponce, F. A.; Kamiyama, S.; Amano, H.; Akasaki, I. Atomic Arrangement at the AlN/ZrB2 Interface. Appl. Phys. Lett. 2002, 81, 3182−3184. (31) Suda, J.; Matsunami, H. Heteroepitaxial Growth of Group-III Nitrides on Lattice-Matched Metal Boride ZrB2(0001) by Molecular Beam Epitaxy. J. Cryst. Growth 2002, 237, 1114−1117. (32) Kawaguchi, Y.; Kobayashi, A.; Ohta, J.; Fujioka, H. Characteristics of GaN/ZrB2 Heterointerfaces Prepared by Pulsed Laser Deposition. Jpn. J. Appl. Phys. 2006, 45, 6893−6896. 29268



Article

(58) Hollins, P.; Pritchard, J. Infrared Studies of Chemisorbed Layers on Single Crystals. Prog. Surf. Sci. 1985, 19, 275−349. (59) Perkins, C. L.; Singh, R.; Trenary, M.; Tanaka, T.; Paderno, Y. Surface Properties of Hafnium Diboride(0001) as Determined by Xray Photoelectron Spectroscopy and Scanning Tunneling Microscopy. Surf. Sci. 2001, 470, 215−225.

29269


Structure and Reactivity of Molecularly Adsorbed Ammonia on the

Recommend Documents