Article pubs.acs.org/Langmuir
Surface Chemistry of Isopropoxy Tetramethyl Dioxaborolane on Cu(111) Brendan P. Miller,† Octavio J. Furlong,†,‡ and Wilfred. T. Tysoe†,* †
Department of Chemistry and Biochemistry, and Laboratory for Surface Studies, University of Wisconsin−Milwaukee, Milwaukee, Wisconsin 53211, United States ‡ INFAP/CONICET, Universidad Nacional de San Luis, Ejercito de los Andes 950, 5700 San Luis, Argentina ABSTRACT: The surface chemistry of isopropoxy tetramethyl dioxaborolane (ITDB), tetramethyl dioxaborolane (TDB), and 2-propanol is studied on a clean Cu(111) single crystal using temperature-programmed desorption (TPD). 2-Propanol is found to have two competing reactions on the copper surface. Dehydration results in water and propene formation, and dehydrogenation results in the formation of acetone and hydrogen. ITDB directly adsorbed on the surface reacts completely and does not molecularly desorb. TDB and 2propanol decompose desorbing mainly 2,3-dimethyl 2-butene and acetone, respectively. Both of those products desorb above room temperature and are present in TPDs of ITDB. An additional acetone desorption peak was observed for ITDB at higher temperatures than acetone desorption from 2-propanol. This higher temperature peak at ∼391 K was attributed to two acetone molecules forming from the tetramethyl end group resulting from a stronger bound surface species in ITDB compared to TDB despite their identical end groups. The copper surface seems to be reactive enough toward ITDB at room temperature that a potential boron-containing tribofilm could be produced for copper−copper sliding contacts. Despite their similarities, ITDB and TDB have different surface species present at room temperature, so their tribological properties will be investigated in the future.
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INTRODUCTION The push for greener lubricants has steered the focus away from lubricant additives that contain halogens, sulfur, and phosphorus. Boron-containing compounds can provide more environmentally benign alternatives since the borate esters that are used as additives react with water to form environmentally benign boric acid. The tribological chemistry of a particular lubricant additive depends critically on the nature of the substrate so that a good lubricant additive for one type of surface may not be applicable to another. The lubrication of sliding copper−copper interfaces in electrical motors1−4 provides a particular challenge due to the requirement for a lubricious, yet conductive, tribofilm. Gas-phase lubricants based on water vapor have been used to reduce friction and wear, but they tend to lead to asymmetric wear rates and failure at higher temperatures.5,6 Because of their low toxicity, boron-containing molecules have been proposed as potential lubricants. Different types of boron-containing compounds (borates,7−9 boron nitrides,10,11 boron carbides,12 borides13) have shown good tribological properties including friction and wear reduction. Philippon et al. have shown that a borate glass is formed under vacuum when rubbing an iron surface with trimethylborate, which partially digested the iron oxide but not the metallic iron and led to a significant reduction in friction.9 However, one of the main drawbacks with borate esters is their © 2012 American Chemical Society
instability in air and low solubility in the base oil. Organic borate esters have been proposed to increase solubility and have shown good antiwear properties,8 and the addition of nitrogen to borates has been observed to increase stability and tribochemically form boron nitride.14 The following investigates the surface chemistry of a borate ester, isopropoxy tetramethyl dioxaborolane (ITDB), on a Cu(111) surface to determine whether a reactive film can form near room temperature to potentially provide a tribofilm needed for lubrication of the sliding copper−copper contact in an electric motor. These experiments will provide the background information for eventually investigating the frictional properties in ultrahigh vacuum (UHV).15−19 ITDB contains both an alkoxy group and a bridging −O−C−C−O− group. The latter is included to improve the stability of ITDB toward reaction with water. In order to disentangle the chemistry of the bridging tetramethyl end group versus the isopropoxy end group, tetramethyl dioxaborolane (TDB), and 2-propanol were also studied to assist in the analysis of the surface reaction pathways. TDB possesses the same structure as ITDB except for a hydrogen termination in place of the Received: January 18, 2012 Revised: March 8, 2012 Published: March 26, 2012 6322
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RESULTS TPD experiments of 2-propanol at different exposures were performed on a Cu(111) surface. Exposures in the following are given in Langmuirs (L) (1 L = 1 × 10−6 Torr s), where the dosing pressure was not corrected for ionization gauge sensitivity. The profiles in Figure 2 display the 45 amu signals,
isopropoxy end group of ITDB. Figure 1 shows the structures of the molecules used in this work.
Figure 1. Schematic representation of isopropoxy tetramethyl dioxaborolane (ITDB) and tetramethyl dioxaborolane (TDB).
Alcohols have been previously studied on metal and oxygencovered metal surfaces.20−23 Dehydration, dehydrogenation, and even complete decomposition of 2-propanol and other alcohols have been observed. Dehydration of 2-propanol leads to water and propene desorption well below room temperature. Dehydrogenation results in surface isopropoxy species and is promoted on oxygen-covered surfaces. Further heating leads to a β-hydride elimination reaction resulting in acetone and hydrogen desorption. Gellman et al.24 observed, using Fouriertransform infrared (FTIR) and temperature-programmed desorption (TPD), that 1-propanol forms propoxide species on Cu(111), which is stable to above room temperature, eventually resulting in propanal desorption. On Cu(110), Bowker et al. did not observe any dehydration of 2-propanol but only dehydrogenation resulting in acetone and hydrogen desorption above room temperature.23 The following explores the surface chemistry and decomposition products of ITDB, TDB, and 2-propanol on atomically cleaned copper surfaces under UHV conditions by means of TPD experiments.
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Figure 2. TPD profiles at 45 amu of increasing exposures of 2propanol on a clean Cu(111) single crystal at ∼100 K collected at a heating rate of 3.0 K/s. Note the signal of the three lowest exposures are multiplied by ten.
the most intense mass fragment of 2-propanol in our mass spectrometer, which additionally does not include contributions from any of the observed decomposition products. With increasing exposure, the spectra show a peak growing in intensity at 197 K followed by another peak appearing at high coverages at 157 K. A small peak is observed at ∼270 K saturating at low exposures. Note that for exposures of 0.25− 0.72 L, the intensity has been multiplied by a factor of 10 for clarity. Figure 3 shows the corresponding 58 amu signal, which
EXPERIMENTAL SECTION
The equipment used for TPD experiments has been described in detail elsewhere.25 Briefly, TPD data were collected in a UHV chamber equipped with a Dycor quadrupole mass spectrometer interfaced to a computer that allowed up to five masses to be sequentially monitored in a single experiment. Temperature dependent X-ray photoelectron spectra were collected with a Mg Kα X-ray power of 250 W and a cylindrical mirror analyzer pass energy of 50 eV. B 1s, O 1s, and C1s signals were monitored at various temperatures. The sample could be cooled to ∼90 K by thermal contact to a liquid-nitrogen-filled reservoir and resistively heated to ∼1200 K. Once in UHV, the Cu(111) single crystal was cleaned using a standard procedure which consisted of Argon ion bombardment (∼1 kV, ∼2 μA/cm2) while heating at ∼550 K and subsequent annealing cycles up to ∼850 K. The cleanliness of the samples was monitored using Auger spectroscopy. After dosing, the sample was rapidly moved to within about 2 mm from the front of the mass spectrometer before collecting each TPD profile to minimize electron-beam effects from the ionizer filament. All compounds were dosed at a sample temperature of 100 K and heated to ∼550 K. The isopropoxy tetramethyl dioxaborolane, ITDB (Aldrich, 98% purity), tetramethyl dioxaborolane, TDB (Aldrich, 97%), and 2propanol (Aldrich, 99.5%) were transferred to glass bottles and attached to the gas-handling systems of the vacuum chambers where several freeze−pump−thaw cycles were performed to purify the sample. The cleanliness was monitored using mass spectroscopy.
Figure 3. TPD profiles at 58 amu of increasing exposures of 2propanol on a clean Cu(111) single crystal at ∼100 K collected at a heating rate of 3.0 K/s.
is the parent mass of acetone, but is also present in the mass spectrum of 2-propanol. Similar peaks as seen in Figure 2 are detected at 197 and 157 K assigned to 2-propanol fragmentation but with lower intensities. A higher-temperature peak appears at ∼320 K, even at the lowest coverages and saturates in intensity at a 2-propanol exposure of 0.42 L. The 18 amu signal was also monitored, as shown in Figure 4, where a relatively low-temperature peak is observed at 205 K. This 6323
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in Figure 6 by following the 87 amu signal, representative of molecular ITDB. No signal is observed until exposures exceed
Figure 4. TPD profiles at 18 amu of increasing exposures of 2propanol on a clean Cu(111) single crystal at ∼100 K collected at a heating rate of 3.0 K/s. Figure 6. TPD profiles at 87 amu of increasing exposures of isopropoxy tetramethyl dioxaborolane on a clean Cu(111) single crystal at ∼100 K collected at a heating rate of 3.0 K/s.
feature grows in intensity with increasing exposure until 1.5 L, where it saturates. The 41 amu signal (not shown) depicts a low-temperature peak encompassing both the 197 and 205 K states. Finally, hydrogen (also not shown) was found to desorb at ∼350 K. The desorption spectra of TDB (tetramethyl dioxaborolane) at an exposure of ∼1.3 L are shown in Figure 5. In order to
1.0 L, where a peak at 228 K is seen growing in intensity with increasing dose. Figure 7 displays the corresponding 58 amu
Figure 7. TPD profiles at 58 amu of increasing exposures of isopropoxy tetramethyl dioxaborolane on a clean Cu(111) single crystal at ∼100 K collected at a heating rate of 3.0 K/s. Figure 5. TPD profiles following an exposure of 1.3 L of tetramethyl dioxaborolane on a clean Cu(111) single crystal at ∼100 K collected at a heating rate of 3.0 K/s. Profiles are shown for 43 amu (blue), 58 amu (green), 41 amu (red), and 69 amu (black).
signal (the parent mass of acetone and small fragment of ITDB and 2-propanol). Peaks can be observed at ∼327 and ∼391 K where the higher-temperature peak at ∼391 K is the most intense, approximately double that of the ∼327 K peak. Figure 8 shows the corresponding signals following 41 amu with increasing exposures of ITDB. A peak at ∼364 K grows with increasing exposure, saturating at ∼1.5 L. Figure 9 (69 amu) shows a similar high-temperature desorption peak saturating at an exposure of ∼1.5 L. A signal is observed in Figures 7 (58 amu), 8 (41 amu), and 9 (69 amu) at ∼229 K corresponding to ITDB desorption, in accord with the ionization fragmentation pattern. Water desorption was not observed at any exposure for ITDB.
show the surface chemistry, the 43, 58, 41, and 69 amu signals are displayed. A peak at 195 K is seen for each mass corresponding to the spectrometer ionizer fragmentation pattern of TDB. Another peak at ∼151 K is detected for each mass at higher coverages (not shown) corresponding to multilayer desorption of TDB. A higher-temperature state at 312 K is witnessed but only for 41 and 69 amu which, by analyzing the integrated intensities of the desorption profile at different masses, is attributed to 2,3-dimethyl 2-butene formation. The parent mass of dimethyl 2-butene at 84 amu is also observed at the correct proportional intensity to 41 and 69 amu signals (data not shown). Hydrogen was again seen to desorb at ∼350 K, while signals at 18 and 32 amu were not observed for any exposure of TDB. The desorption spectra of ITDB (isopropoxy tetramethyl dioxaborolane) from Cu(111) at different exposures are shown
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DISCUSSION 2-Propanol Chemistry on Cu(111). Since ITDB contains an isopropyl functional group, which could react by cleavage of the B−O or C−O bonds. The surface chemistry of 2-propanol is examined to assist with understanding this chemistry. It is proposed that dehydrogenation and dehydration can occur for 6324
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200 K.22,24 It is proposed here that reaction of 2-propanol with the copper surface has occurred well below room temperature. Rehydrogenation of surface isopropoxy species to 2-propanol at ∼270 K corroborates this assertion and confirms that the isopropoxy species has formed below this temperature. The data in Figure 4 indicate that, in addition to dehydrogenation, dehydration reactions also occur. Water26 and propene27 are observed in the literature to desorb from Cu(111) at
