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1Â§, Qi-Rui Ma. 1Â§, Yu-Qing Zhao. 1. , Biao Liu. 1. , Meng-Qiu Cai. 1,2*. 1School of Physics and electronics Science, Hunan University, Changsha, Hunan ... bring about selective growth in the specified surface-termination is still a challenge. ... p
Dec 31, 2008 - Department of Chemistry, 1101 UniVersity AVenue, UniVersity of Wisconsin, Madison, ...... (35) Jackson, S. T.; Nuzzo, R. G. Appl. Surf. Sci.
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J. Phys. Chem. C 2009, 113, 1526–1535
Influence of Surface Termination and Electronic Structure on the Photochemical Grafting of Alkenes to Carbon Surfaces Paula E. Colavita,†,‡ Bin Sun,† Xiaoyu Wang,† and Robert J. Hamers*,† Department of Chemistry, 1101 UniVersity AVenue, UniVersity of Wisconsin, Madison, Wisconsin 53706-1396, and School of Chemistry, Trinity College Dublin, College Green, Dublin 2, Republic of Ireland ReceiVed: July 5, 2008; ReVised Manuscript ReceiVed: NoVember 5, 2008
The ultraviolet-initiated photochemical grafting of n-alkenes to surfaces of amorphous carbon, diamond, and carbon nanofibers has recently emerged as a way to impart new surface properties to these materials. Recent studies have shown that grafting on these materials is initiated by a novel photoelectron ejection process, evidenced by a strong dependence of reaction efficiency on the electron affinity of the reactant molecules. Yet, the role of different surface functional groups and the resulting changes in valence band density of states and surface work function have not been determined previously. Understanding the influence of surface carbonyl groups is particularly significant because CdO groups increase the surface work function but are also known to catalyze certain electron-transfer processes at carbon surfaces. Here, we use infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) to investigate how changes in surface termination (H-termination vs O-termination) and surface annealing impact the valence electronic structure and work function of the samples, and how these changes influence the grafting of alkenes to the surfaces. Four different n-alkenes bearing terminal groups were used in order to identify the role of the molecular electron affinity, using alkenes terminated with -NHCOCF3, -NHCOO(tert-butyl), -COOMe, and -CH3 groups. By using narrowly spaced interdigitated electrodes of amorphous carbon, we are able to directly detect the UV-induced photoemission. Our results show that O-termination of amorphous carbon surfaces enhances the photochemical grafting yield compared with H-termination of surfaces, contrary to what has been found on diamond surfaces. Surfaces annealed to increase the amount of sp2-hybridized carbon, and therefore being most metallic in character, have the highest reactivity. The changes in reactivity are explained in terms of the changes in valence electronic structure of the samples and the influence of oxygen on the photoelectron emission process that initiates the grafting reaction. 1. Introduction While it is widely established that terminal alkenes can be grafted to surfaces using ultraviolet light (4.9 eV),1-12 the mechanism by which this reaction occurs remains controversial. The reaction on semiconductor surfaces has been widely attributed to an excitonic mechanism, in which the presence of excess holes in the valence band promotes reaction with the electron-rich CdC group of the alkenes.1,13,14 However, recent studies have shown that alkenes can also be grafted to materials with metallic electronic structures, including glassy carbon,15,16 and carbon nanofibers17,18 and that pronounced differences in reactivity are observed between 1-alkenes bearing different functional groups at the distal end; these observations suggest that a different mechanism must control the reactivity of these materials.19-21 By comparing experimental measurements of reaction efficiency with density functional calculations of molecular electronic structure, we recently showed that the reactivity of different molecules toward carbon surfaces is correlated with the electron affinity of the reactant molecules and that molecules bearing good electron-accepting groups grafted readily, while molecules that are poor electron acceptors react slowly or not at all.8,19,20,22 On this basis, we proposed that photochemical * To whom correspondence should be addressed. E-mail: [email protected] wisc.edu. † University of Wisconsin. ‡ Trinity College Dublin.
grafting can occur via a new “internal photoemission” mechanism, in which the grafting process is initiated by the UVinduced photoemission of electrons from the solid surfaces directly into the acceptor levels of adjacent reactive fluids. However, little is known about how the electronic properties of the solid surface affect the photochemical reactivity toward n-alkenes. The role of surface oxidation is particularly significant because on diamond surfaces oxygen reduces photoemission of electrons,23 while on graphitic surfaces the process of surface carbonyl groups is known to enhance the rate of certain electrontransfer processes to redox couples in solution.24-26 Here, we use X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), Fourier-transform infrared spectroscopy (FTIR), and direct measurements of the UV-induced photoelectric current to investigate how the photochemical grafting of alkenes to carbon surfaces is influenced by the electronic properties of the surface. We use amorphous carbon as a substrate because its valence electronic structure can be easily modified by annealing and by exposure to atomic oxygen, thereby yielding oxidized surface groups.27 The use of amorphous carbon also makes it possible to directly measure the photoelectron emission into the highly insulating reactant fluids through the use of interdigitated electrode arrays. Our results show that even though surface oxygen groups increase the surface work function, they enhance the overall reactivity compared with H-termination, contrary to what has been shown on H- and O-terminated diamond surfaces.6,28 Our results
10.1021/jp805933h CCC: $40.75 2009 American Chemical Society Published on Web 12/31/2008
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Figure 1. IRRAS spectra of the organic layers obtained for (a) TFAAD, (b) tBOC, (c) UA-Me and (d) 1-dodecene on (A) O-terminated and (B) annealed surfaces after 15 h illumination time. The results obtained on H-terminated surfaces are reported for comparison on trace C. Spectra were baseline-corrected and offset for clarity. The structures of the alkenes are also shown.
provide new insights into how the surface chemical composition affects the photochemical reactivity of carbon, in both amorphous and crystalline forms, toward alkenes. These results can likely be generalized to other materials to achieve a more comprehensive understanding of UV-induced grafting of alkenes to surfaces. 2. Experimental Section Chemicals. Absolute ethanol (Aaper), electronic grade methanol (Fisher), and chloroform (Fisher) were used without further purification. The following compounds were used for surface modification: 1-dodecene (Sigma), trifluoroacetic acid protected 10-aminodec-1-ene (TFAAD, Chemical Synthesis Services), 10N-Boc-Amino-dec-1-ene (tBOC, Astatech, Inc.), and methyl 10undecenoate (UA-Me, Sigma), as depicted in Figure 1. All of the compounds were used as neat liquids. The tBOC and TFAAD molecules were purified by vacuum distillation. Sample Preparation. Details of the carbon film deposition and sample preparation have been previously described.19,20 Briefly, 50 nm amorphous carbon films were sputtered using a direct current (DC) magnetron system at an Ar pressure of 3 mTorr. The films were sputtered on Ti metallic layers that
ensured excellent adhesion properties and provided optical enhancement for IRRAS measurements (100 nm Ti layers were used for IRRAS experiments). Amorphous carbon films were modified after deposition via (a) H-termination, (b) rapid thermal annealing, and (c) Htermination followed by ozone treatment (O-termination). Htermination was carried out in a 13.56 MHz inductively coupled plasma, at H2 pressure of 10 Torr, for 10 min, as previously described. Annealing was carried out using an annealing lamp at 625 °C for 30 s under 5% H2/N2 atmosphere. The Oterminated samples were obtained by placing the H-terminated samples under a UV lamp for 40 min in air. The carbon substrates were placed inside a nitrogen-purged reaction chamber sealed with a quartz window and illuminated with a mercury lamp having a primary emission line at 254 nm. A thin liquid film of the neat n-alkene was in contact with the substrates,, and a quartz slide was placed over the liquid to prevent evaporation. After running the reactions, samples were rinsed using chloroform, ethanol, and methanol and finally dried under nitrogen flow. Characterization. IRRAS spectra were collected on a Bruker FTIR spectrometer (Vector 33 and Vertex 70) equipped with a
1528 J. Phys. Chem. C, Vol. 113, No. 4, 2009 VeeMaxII variable angle specular reflectance accessory and a wire grid polarizer. Spectra were collected using p-polarized light at 80° incidence from the surface normal; a bare carbon sample was used as a reference in all cases, and 500 scans at 4 cm-1 resolution were collected for both background and sample. The spectra presented in the figures were baseline-corrected and offset for clarity. Typical root mean square (rms) and peak-topeak noise levels were below 2 × 10-5 and 1 × 10-4 absorbance units, respectively. XPS characterization was performed on an ultrahigh-vacuum system, with 8 × 10-10 Torr base pressure, equipped with a monochromatized Al KR source (1486.6 eV nominal energy) and a multichannel array detector. An analyzer resolution of 0.1-0.2 eV and a 45° takeoff angle were used for all spectra. Atomic ratios were determined by fitting absorption peaks to Voigt functions (Igor Pro) after Shirley background correction,29 and normalizing the peak area ratios by the corresponding atomic sensitivity factors (C ) 0.296; N ) 0.477; O ) 0.711; F ) 1.0).30 UPS characterization was carried out using a He(I) emission lamp (21.2 eV) and analyzer resolution of 0.05-0.1 eV. The sample was oriented at a takeoff angle of 75° (from the surface plane) and biased -4.50 to -9.00 V with respect to the spectrometer: negative biasing ensured that the sample vacuum level was higher in energy than that of the analyzer. Spectra at progressively higher biases were collected until the high binding energy cutoff was observed to converge; the spectrum thus obtained was used for the work function calculations. Energies are referenced to the sample Fermi level, which was determined by measuring the Ta clips in direct contact with the sample. The spectra were normalized by the total integrated intensity in order to facilitate comparison. 3. Results 3.1. Covalent Functionalization of Carbon Surfaces. The carbon substrates were covered by a thin film of the different liquid alkenes and illuminated with 254 nm light for 15 h under inert atmosphere. The organic layers obtained in this way were characterized via IRRAS spectroscopy and, in the case of TFAAD, also via XPS. Parts a-d of Figure 1 show the IRRAS spectra of the organic layers obtained after photochemical grafting for 15 h on (A) O-terminated and (B) annealed carbon surfaces respectively for the four n-alkenes used in our experiments; the previously reported IRRAS spectra for the same molecules obtained on H-terminated surfaces (C) are reported on the same figures for comparison purposes. Infrared peak assignments of organic layers of these same molecules have been discussed in previous work,18,19,21 and detailed peak assignments can be found in the Supporting Information. Figure 1 shows that there are remarkable differences in reactivity between different carbon surfaces for each of the four alkenes examined. Since all of the alkenes have the same number of -CH2 groups along their backbone and differ only by the terminal group (except for 1-dodecene which has an additional methylene unit), the intensity associated with the CH2 peaks (∼2930 and ∼2858 cm-1) would be expected to be similar if they displayed similar surface density. Although molecular orientation effects also play a role in the intensity of IRRAS peaks, we have previously shown that the net absorbance of the -CH2 stretching peaks can be used to provide a reasonable estimate of the molecular surface coverage.19,20 Table 1 summarizes the absolute and relative values of the net absorbances of the -CH2 asymmetric stretch (2930 cm-1), calculated from the spectra in Figure 1; relative values are all referenced to that of TFAAD.
Colavita et al. TABLE 1: Net Absorbance of the ν(CH2)a Peak at 2930 cm-1, Determined from the IRRAS Spectra Reported in Figure 1a net absorbances ν(CH2) TFAAD dodecene UA-Me tBOC
a The values between parentheses are the ratio of the net absorbance with respect to the TFAAD absorbance for the same surface postdeposition treatment.
Figure 2. Reaction rate of TFAAD on O-terminated, annealed, and H-terminated amorphous carbon surfaces. The reaction progress is followed via XPS by calculating the AF(1s)/AC(1s) as a function of reaction time. Weighed exponential fits have been added to the graph to guide the eye. The annealed surfaces graft TFAAD at a faster rate than Oand H-terminated surfaces.
On the basis of the net absorbances reported in Table 1, TFAAD grafts with similar yields on all three of the carbon surfaces investigated; in fact, the net absorbances associated with the -CH2 asymmetric stretch differ by 10% at most. The infrared profile of the TFAAD layer is also very similar for all of the surfaces investigated, except for a broad shoulder around 1800 cm-1 for O-terminated carbon, which we assign to oxidized species on the O-terminated surface (vide infra); this suggests that the structure of the TFAAD layer is independent of the type of carbon treatment introduced prior to grafting. All of the other alkenes display significant differences in coverage on the three surfaces. As previously reported, H-terminated surfaces lead to layers with high coverage only in the case of TFAAD.20 Annealed carbon yields organic layers with almost identical surface density for 1-dodecene, tBOC, and UA-Me; these three molecules graft to approximately 30% of the final coverage of TFAAD. Finally, O-terminated surfaces display slightly higher coverages than H-terminated surfaces, in particular in the case of tBOC which grafts to approximately 74% of the TFAAD coverage. The IRRAS results suggest that annealed carbons display higher reactivity toward the UV-induced photochemical grafting of terminal alkenes, since it is the only postdeposition treatment that leads to the formation of organic layers for all of the molecules examined. The presence of a fluorinated group on TFAAD allowed us also to carry out XPS measurements in order to understand if the higher reactivity toward a wider range of alkenes also corresponded to a higher reaction rate. Figure 2a shows the evolution of the AF(1s)/AC(1s) ratio for the reaction carried out on (A) annealed and (B) O-terminated carbon films; the previously reported grafting rate on H-terminated carbon is also displayed on the same graph for comparison.20 Although the AF(1s)/AC(1s) ratio has been shown in previous work to provide
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Figure 4. XPS in the C(1s) and O(1s) regions of (A) O-terminated and (B) annealed amorphous carbon. The individual contributions to the C(1s) peak were obtained via fitting procedures and are reported under the curves.
Figure 3. XPS in the C(1s), N(1s), and O(1s) regions of TFAAD layers grafted for 3.5 h on (A) O-terminated, (B) annealed, and (C) H-terminated amorphous carbon surfaces. The spectra were normalized on the basis of the C(1s) integrated intensity in order to facilitate comparison.
limited quantitative information beyond the grafting of a first TFAAD molecular layer (because of the inability in distinguishing between C atoms from the substrate and those from the organic layer), this ratio still provides a suitable means of following the rate during the initial stages of the reaction when the C(1s) signal arises primarily from the substrate and the contribution from the molecular backbone is small.10,19,20 Figure 2 shows that annealed surfaces lead to a faster rise in the AF(1s)/ AC(1s) ratio than O- and H-terminated surfaces. Parts a-c of Figure 3 show the XPS spectra in the C(1s), N(1s), and O(1s) regions of TFAAD functionalized surfaces after approximately 3.5 h illumination. No differences were found in either the relative area ratios or the peak profiles between TFAAD layers grafted on the three surfaces, thus indicating that the structure of the final TFAAD layers is the same, in agreement with the IRRAS spectra (Figure 1a). In summary, our results indicate that, with the exception of TFAAD, the yield of the photochemical reaction between amorphous carbons and n-alkenes is highly dependent not only on the specific terminal group attached to the alkene, as shown in previous work,19,22 but also on the particular postdeposition treatment of the amorphous carbon films. Annealed carbon displays higher reactivity toward all of the molecules examined, and this enhanced reactivity translates into both a wider range of grafted n-alkenes and a faster rate of functionalization, as
suggested by IRRAS and XPS data, respectively. It is interesting to note that, except for TFAAD, the yield of the photochemical reaction on annealed carbon appears to be independent of the type of n-alkene chosen for the functionalization; this lack of selectivity resembles more closely the behavior of n-alkenes on Si under UV illumination,31-33 than that on H-terminated diamond.8 3.2. Composition and Valence Structure of the Amorphous Carbon Surfaces. To understand what differences in composition and electronic structure are introduced by the postdeposition treatments, we characterized the surface composition and the valence band properties using XPS and UPS after rapid thermal annealing and O-termination treatments. Figure 4 shows the XPS spectrum of (A) O-terminated and (B) annealed carbon in the C(1s) and O(1s) regions (C and O were the only elements detected in XPS survey scans); details of the XPS spectrum of as-deposited and H-terminated carbon can be found in a previous publication.20 The C(1s) spectrum of the O-terminated sample shows an asymmetric broad line which is characteristic of amorphous carbon.34,35 The spectrum was fitted with four components centered at 284.2, 284.9, 285.9, and 288.4 eV. The first two peaks are separated by 0.7 eV, which is consistent with the separation between sp2 and sp3 carbon in disordered graphite and other amorphous carbon samples.20,34,35 The two contributions at higher binding energies can be attributed to the presence of oxidized carbon species. This assignment is supported by the presence of a large O(1s) absorption peak at 532.4 eV, which indicates a high surface oxygen content. The AO(1s)/AC(1s) ratio of this sample was indeed found to be 0.22, a higher value than AO(1s)/AC(1s) ratios of 0.09-0.16 previously reported for as-deposited samples and much higher than the ∼0.05 value found on H-terminated samples.20 It has been shown that in particular gas oxidation treatments tend to favor the formation of carbonyl (ketone, aldehyde, quinone, and anhydride) and phenolic groups vs the formation of carboxylates.36 Although the presence of carboxyl groups cannot be excluded, this preference is also evident from the XPS data in Figure 4, where the broad oxidized carbon component at 285.9 eV, typically attributed to carbonyl and phenolic groups, dominates over the component at 288.4 eV, usually attributed to carboxyl groups.37-40 The amount of oxidized species on the surface is sufficiently high to be also evident in the IRRAS spectrum of O-terminated
1530 J. Phys. Chem. C, Vol. 113, No. 4, 2009
Colavita et al.
Figure 5. IRRAS spectrum of O-terminated carbon referenced against a H-terminated background sample. A broad peak centered at ∼1800 cm-1 suggests that surface oxidized groups are present after ozone treatment. The spectrum has been baseline corrected for clarity.
samples referenced against a H-terminated background, shown in Figure 5. The IRRAS spectrum shows a very broad peak centered at 1798 cm-1 that is typical of oxidized carbon surfaces in which carbonyl-containing groups such as anhydrides, quinones, and ketones have been generated.36,41-44 Also, negative peaks at 3009 and 2960 cm-1 associated with CsH stretching vibrations indicate that upon O-termination the amount of surface hydrogen is reduced from that of the H-terminated reference sample. The XPS spectrum of an annealed sample, shown in trace B of Figure 4, displays a fairly symmetric C(1s) line instead, dominated by a single peak contribution at 283.9 eV. Amorphous carbons, including our sputtered films, usually display broad asymmetric C(1s) lines;34,35 the presence of a highly symmetric line therefore suggests that the sample undergoes graphitization during annealing. This is often observed after thermal treatment of amorphous carbons, which tends to increase the sp2/sp3 carbon ratio.45-49 The rapid annealing under H2/N2 atmosphere has also the effect of reducing the oxygen content in the carbon film, as evidenced by the fact that the AO(1s)/AC(1s) ratio decreases from a typical value >0.0920 to 0.04. We have shown in previous studies that photoemission of electrons from valence states into the liquid phase is an important step in the photochemical reaction of carbons with terminal alkenes.6,7,19,20 For this reason we characterized the amorphous carbon surfaces using UPS; Figure 6 shows He I spectra of the annealed, O-terminated, and H-terminated carbon surfaces. As discussed in previous work, the H-terminated sample displays a single broad peak with maximum intensity at approximately 8.1 eV which was assigned to p-σ orbitals. After O-termination the contribution from these p-σ states is still observed at approximately the same energy; in addition, a new prominent feature appears at 4.6 eV. On the basis of previous studies on oxidized pyrocarbon and carbon blacks, and in agreement with the increase in oxygen content observed in the XPS and IRRAS spectra of O-terminated samples, we attribute this peak to O 2p states of oxygen-containing species.50 The annealed sample does not display prominent peaks, suggesting that the contribution of p-σ states is lower with respect to the H- and O-terminated samples and that oxidized species are not present in significant amounts. Emission in the 0-3 eV region, close to the Fermi energy (EF), which arises from p-π states in amorphous carbons (see Figure 6 inset), is enhanced in the annealed sample, and the onset of the valence emission occurs at <0.1 eV below EF. This demonstrates that thermal treatments increase the density of p-π states, in agreement with previously reported UPS spectra of
Figure 6. He(I) UPS spectra of the (A) O-terminated and (B) annealed amorphous carbon samples; the spectrum of (C) H-terminated carbon is reported for comparison. The inset shows details of the region near the Fermi energy; a linear extrapolation of the sharp threshold at high binding energy to zero intensity gives the surface work function. Spectra have been normalized by the total integrated intensity.
annealed carbons,49,51 thus further indicating that the sp2/sp3 content increases upon annealing. The emission in the 0-3 eV region is instead depressed in the O-terminated sample, when compared to H-terminated carbon. This result suggests that surface oxidation reduces the carbon density of states in the π region, a behavior that has been previously observed for various forms of carbon.52-54 The onset of the valence emission in O-terminated carbon remains, as for the H-terminated sample, at ∼0.5 eV below EF. The sharp emission threshold at high-binding energy in the UPS spectra was fit to a line and extrapolated to zero-intensity in order to find the energy of the high-binding energy cutoff (EHBC). This value was then subtracted from the incident photon energy (Ephoton) of 21.2 eV in order to obtain a value for the work function (φ) of the carbon surfaces via φ ) Ephoton EHBC.55,56 This procedure resulted in a work function of 4.3 and 5.1 eV for the annealed and O-terminated samples. The value found for the annealed surface is close to that of graphite (4.4-4.7 eV)53,57-59 and glassy carbon (4.2 eV),52,60 once again suggesting that the carbon film undergoes graphitization upon annealing. The oxidized surface, on the other hand, has a higher work function than that of the as-deposited film (4.8 eV),20 in agreement with previous literature on the effects of surface oxidation of carbon materials.23,52-54 Oxidation typically leads to an increase in work function due to the introduction of CsO surface dipoles in which the O negative charge is on the vacuum side of the interface.23,52-54 Figure 7 summarizes the UPS results, showing the location of the Fermi energy and the valence-band onset as observed in vacuum photoemission. To facilitate later discussion, we also show the energies of the electron acceptor levels of the four molecules investigated, based on prior gas-phase theoretical calculations of vertical electron affinities and including an estimated 1.5 eV stabilization due to dielectric screening in the liquid.20 The XPS and UPS results show that annealed and
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Figure 7. Summary of UPS measurements of work function and valence-band onset for H-terminated, annealed, and O-terminated surfaces. Also shown are the estimated energies of the electron acceptor levels of the four alkenes investigated, based on calculations of vertical ionization energies and estimated 1.5 eV solvent relaxation, from ref 20.
O-terminated carbon surfaces display very different compositions and valence electronic structures. Annealed carbon has a AO(1s)/AC(1s) area ratio nearly identical to that of H-terminated carbon (representing only trace amounts of surface oxygen), but annealing induces loss of hydrogen, increasing the relative amount of sp2 carbon in graphitic form. O-termination increases the surface oxygen content, as expected, and introduces contributions from O 2p states arising from oxidized groups on the surface. The work function of both the annealed (4.3 eV) and O-terminated (5.1 eV) samples are higher than that reported for H-terminated carbon (4.0 eV), in agreement with previously reported effects of thermal and oxidation treatments. 3.3. Direct Mmeasurement of Photoemission at the OTerminated Carbon/Liquid Interface. We have previously shown that the UV-induced grafting of n-alkenes to carbon surfaces is initiated by the photoemission of electrons from the carbon into the adjacent fluids.6,8,19,20,22 Both the ability to emit electrons from the surface and the ability to accept electrons by the organic molecule must be considered in order to explain the observed reactivity. The UPS results presented in section 3.2 show that the barrier to electron emission into vacuum is larger on the O-terminated than the H-terminated surfaces investigated previously and is also higher than the 4.9 eV photon energy used to drive the reaction. Thus, it is surprising that O-terminated surfaces are more reactive than H-terminated ones, as shown in Figure 1b. As discussed below, this trend is also opposite to that observed on diamond surfaces.6,61 To understand whether photoemission could be detected directly at the Oterminated carbon/organic interfaces, we carried out photocurrent experiments using interdigitated carbon electrode arrays (IDAs) and compared them to those obtained under the same conditions with H-terminated carbon. We used a previously reported setup20 in which amorphous carbon IDAs were sputter-deposited onto glass slides using photolithography; this process produces carbon electrodes in close proximity on a planar sample, facilitating measurement of photocurrent through the highly resistive alkenes. A small drop of the appropriate alkene was placed on top of the electrodes and covered with a quartz slide and a Teflon mask in order to define the geometric area illuminated by the UV lamp. A small bias voltage of 0.04 V was applied to the electrodes, and the system was allowed to stabilize for at least 2 h in the dark in an argon atmosphere prior to any measurements. Parts a and b of Figure 8 show the photocurrent profiles
Figure 8. Photocurrent measurements carried out using interdigitated electrode arrays (IDAs) covered by a thin film of the liquid alkene: (a) photocurrents obtained on (s) O-terminated and ( · · · · ) H-terminated IDAs using TFAAD; (b) photocurrents obtained on (s) O-terminated and ( · · · ) H-terminated IDAs using tBOC. Photocurrents were timeadjusted such that t ) 0 indicates the time at which UV illumination starts.
obtained using TFAAD and tBOC respectively on (A) Oterminated and (B) H-terminated IDAs when illuminated with 254 nm light. No photocurrent could be detected with 1-dodecene, presumably due to the very high resistance of the neat liquid. In Figure 8, a photocurrent is clearly observed on both O-terminated and H-terminated samples. In the case of TFAAD, O-termination slightly decreases the photocurrent, but otherwise has little effect; this is in agreement with our results showing that for TFAAD the reaction rate and yield are similar on both O-terminated and H-terminated samples, despite the difference in work function measured via UPS. Figure 8b shows that the photocurrents obtained using tBOC display a very different profile depending on whether the O- or the H-terminated surfaces are used. While the photocurrent on H-terminated electrodes display an immediate rise upon switching the UV light on, the O-terminated surfaces show a delayed response, typically of 60 s, and a long rise time of several minutes, but, at steady-state, the photocurrent magnitude is not statistically different from that observed on H-terminated surfaces. These results indicate that photoemission still occurs on O-terminated surfaces with the consequent generation of charged species in the liquid phase. However, the increased reactivity of O-terminated with respect to H-terminated carbon toward tBOC is likely due to a different mechanism of generation of charged species, as suggested by the significant changes in the photocurrent rising edge.
1532 J. Phys. Chem. C, Vol. 113, No. 4, 2009 The photocurrent data presented in Figure 8a,b appear to reflect the grafting behavior of TFAAD and tBOC observed using IRRAS and XPS, demonstrating that (1) photoemission still occurs at the O-terminated carbon/organic interface, despite the increased work function associated with oxidation, and (2) grafting of tBOC on O-terminated and H-terminated surfaces might follow a different mechanism, as revealed through the different temporal profile of the photocurrent. 4. Discussion In previous work we showed that UV light (254 nm) can be used to functionalize H-terminated amorphous carbon.19,20 We found that, similarly to the case of H-terminated diamond,6-8 the mechanism involves a photoelectrochemical reaction that is initiated by the direct photoemission of electrons from the solid into the adjacent alkene. We showed that the electron affinity of the alkene is a crucial factor controlling the yield and that the presence of functional groups that impart positive electron affinity to the alkenes facilitates the charge separation process by capturing surface photoemitted electrons and leaving a positively charged surface. Figure 7 shows the previously estimated energies of the electron acceptor energies for the four alkenes, showing the TFAAD is an unusually good electron acceptor, while 1-dodecene is a very poor acceptor. While the energy of the alkene acceptor level controls the photoelectron ejection, it was proposed that the resulting positively charged surface then reacts with the olefin groups leading to covalent grafting of n-alkenes. The importance of the photoemission process was demonstrated by showing that molecules with negative electron affinities that graft poorly (or not at all) can be induced to graft to the surface by using a molecular mediator such as TFAAD to “seed” the reaction. In this case grafting a small amount of a molecule with positive EA facilitated electron ejection and greatly enhanced the ability of otherwise poorly grafting molecules to link to the surface.19,20 Our previously reported results, therefore, indicated that the electronic properties at the carbon/organic interface could be somewhat tuned in order to promote or inhibit reactivity. While the previous work addressed the role of the electronic structure of the alkene, the new work presented here demonstrates that such tuning can be achieved by modifying the properties of the carbon surface. These findings and how they relate to our previously proposed mechanism of enhancement will be discussed in the following sections. 4.1. Grafting of n-Alkenes on Annealed Carbon. A first result that emerges from our experiments is that annealed films are the most reactive surfaces, when compared to H- and O-terminated carbon. This enhanced reactivity is reflected: (a) on the enhanced grafting rate observed for TFAAD on annealed vs H- or O-terminated carbon and (b) on the observation that annealed carbon grafts a wider range of alkenes than H- and O-terminated surfaces. A second important result is that, once again, the reactivity of TFAAD is much higher than that of the other three molecules examined, in agreement with the trends observed on H-terminated surfaces. UPS data for annealed carbon show that while annealing slightly increases the work function (from 4.0 to 4.3 eV20), it also increases the density of states (DOS) close to the Fermi energy, effectively changing the onset of photoemission from -4.5 eV on H-terminated to -4.4 eV on annealed carbon (with respect to vacuum). Since photoemission into a liquid is accompanied by local relaxation associated with the solvent polarization, electron emission into a dielectric fluid is usually energetically favorable compared with photoemission in vacuum.
Colavita et al. We previously estimated that the acceptor level of TFAAD lies 0.7 eV below the vacuum level in the liquid phase (vertical EA value).20 This suggests that annealing changes the barrier to electron photoemission from the carbon valence band (as measured from the onset of photoemission) into the TFAAD acceptor level only slightly, from 3.8 eV on H-terminated to 3.7 eV on annealed surfaces, and that in both cases this barrier is lower than the 4.9 eV photon energy. However, the significant increase in the DOS close to the Fermi energy that results from annealing (a consequence of graphitization upon heating) would be expected to increase the quantum yield for photoemission after annealing,62 in agreement with our observations. These results all suggest that the grafting of TFAAD to H-terminated and annealed carbon likely occurs via the same mechanism and that this process is more efficient when the graphitic content of the films is increased by annealing due to the increase in density of states near the Fermi energy. Finally, the IRRAS net absorbance data also suggest that the coverage of TFAAD films does not differ significantly between H-terminated and annealed surfaces, as it would be expected since the final thickness of the layers is controlled ultimately by the escape depth of the photoelectron.19 In a previous study we found that the trifluoroacetamide group of TFAAD can degrade with extensive UV exposure, facilitating radical reactions beyond a first monolayer. atomic force microscopy (AFM) measurements showed that these layers self-terminated at a thickness of ∼5 nm, corresponding to a thickness of approximately three molecular layers. The other three molecules investigated also show the highest reactivity on annealed surfaces, compared to H- and Oterminated surfaces. Their surface coverages (as estimated by FTIR) are all very similar and are ∼30% of the values observed for TFAAD after 15 h reaction time. This suggests that the layers of UA-Me, tBOC, and 1-dodecene are all 1 molecular layer in thickness. Yu and Downard16 had previously observed similar trends in the photochemical grafting of terminal alkenes on pyrolyzed resist films, a highly graphitic form of disordered carbon. They observed that a perfluorinated 1-decene readily grafted forming multilayers, whereas hydrogenated terminal alkenes with a methyl, an ester, and a carboxy group at the distal end grafted to a self-limiting coverage approximately 3-4 times smaller. Also in their case the most reactive molecule is an organofluoride, which can form reactive radical anions in the liquid phase upon photoelectron ejection from the surface, suggesting that electron photoemission might be the preferred grafting route if the molecule is an adequate electron acceptor. This striking resemblance with our results, despite the different carbon deposition and posttreatment methods used by their group and ours, suggests that such reactivity patterns are characteristic of highly graphitic surfaces. If electron photoemission was the process that controls the yield of the UV-induced reaction of these three alkenes on annealed carbon, we would expect to observe a reactivity trend that reflects the electron affinities of the remaining three n-alkenes. These values are very different and were found to be as follows, in decreasing order: UA-Me > tBOC > 1-dodecene.20 We note, however, that 1-dodecene, UA-Me, and tBOC all have similar energies for their highest occupied molecular orbitals,20 which correspond to the filled π bonds of the CdC group. Recent experiments on diamond have suggested that while the reactions are initiated by photoelectron emission, the actual rate-limiting step may be the reaction of a surface “hole” with the CdC group of the organic alkene.8 This process would then be akin to the exciton-initiated grafting of alkenes to H-terminated Si investigated previously.3,13,14,63 In those earlier
Photochemical Grafting of Alkenes to Carbon Surfaces studies it was proposed that absorption of light created electron-hole pairs in the (semiconducting) silicon, and that the valence-band holes initiated a reaction with the electronrich CdC group of the alkenes. These previous studies reported very similar yields independently of the alkene involved,32 and the reactions were self-terminating at a density approximately half that of a densely packed molecular layer.1,64 These characteristics are similar to those we observed on annealed carbon surfaces. Our recent experiments on diamond and amorphous carbon suggest that for many alkenes, the role of the UV excitation may be to create valence-band holes localized at the surface, yielding positively charged surface sites that will react with the CdC bonds.8,19 We propose that similar considerations may also hold in this case: the photoemission of electrons is not directly responsible for the reaction but may simply be an effective way of creating reactive holes at the surface that can subsequently react with the CdC groups of the organic alkenes. Previous studies have shown that dangling bonds and π defects are abundant in amorphous carbons;27,65,66 in particular, π defects in which a π state is half-filled are thermodynamically favored in amorphous carbons deposited at higher temperatures.27 These defect states are distributed across the carbon pseudogap and have been shown to be good traps for holes and electrons.27,67-69 These sites would be highly reactive, serving as preferred grafting sites, and via trapping of holes they could lead to the formation of electrophilic surface centers with high reactivity toward olefins, in analogy with the previously described silicon mechanism.13 4.2. Grafting of n-Alkenes on O-Terminated Carbon. Our results on O-terminated amorphous carbon surfaces indicate that surface oxidation does not prevent grafting of terminal alkenes, and in fact even facilitates it when compared to the H-terminated sample. Notably, this result is opposite to what has been observed on diamond, where surface oxidation greatly reduces the rate of alkene grafting compared with H-terminated diamond.6 However, diamond is an unusual example substrate because the H-terminated surface has negative electron affinity (i.e., the conduction band lies above the vacuum level) and is therefore an exceptionally good electron emitter, while the oxidized diamond surface has positive electron affinity and therefore has a significant barrier to photoelectron emission. On amorphous carbon our UPS data shows that oxidation increases the work function from 4.0 (H-terminated) to 5.1 eV (O-terminated) and also reduces the density of states near the Fermi energy. However, our photocurrent measurements in TFAAD show only a relatively small change in the magnitude of the photocurrent. As noted earlier, the acceptor level of liquid-phase TFAAD is estimated to lie ∼0.7 eV below the vacuum level;20 our work measurements place the Fermi energy of the O-terminated surface 5.1 eV below vacuum level. While amorphous carbon does not have a sharp valence-band edge, the onset of photoemission in the UPS spectrum of the O-terminated surface occurs from states 0.5 eV below EF. As shown in Figure 7, these measurements suggest that the minimum energy barrier to photoejection from valence states of the O-terminated surface into TFAAD acceptor levels is approximately 4.9 eV, equal (within experimental error) to the photon energy used in our experiments. Thus, our results suggest that it should be possible to directly photoemit electrons from O-terminated carbon into the adjacent TFAAD liquid but with less efficiency than on H-terminated diamond, in full agreement with the photocurrent measurements in Figure 7. However, molecules such as tBOC
J. Phys. Chem. C, Vol. 113, No. 4, 2009 1533 or 1-dodecene, for which the acceptor level was estimated to be at higher energies above the vacuum level would not be expected to graft using 4.9 eV photons. This suggests that a different mechanism might be responsible for their observed enhanced reactivity on O-terminated surfaces. In particular, in previous work we showed the grafting of lowreactivity alkenes on H-terminated surfaces could be greatly enhanced by “seeding” the surface with molecules possessing good electron accepting groups (high EA) such as the trifluoroacetamide group of TFAAD.19 We showed that by initially grafting trifluoracetamide groups, which display positive EAs, we were able to greatly enhance the reaction between Hterminated surfaces and unreactive terminal alkenes such as tBOC or UA-Me.19,20 We believe it likely that carbonyl groups of the amorphous carbon surface may act in a similar manner. Many species containing carbonyl groups are known indeed to have positive EAs in the gas phase. For instance, the gas-phase EAs of cyclic unsaturated dicarbonyls are typically >1 eV,70 that of aromatic ketones and aldehydes typically falls in the range of 0.3-1 eV,71,72 and several common saturated monocarbonyl compounds have been shown to also have positive EAs of a few millielectronvolts.73 Any of these groups would display even more positive EAs in the liquid phase, where the polarization of the solvent can stabilize charged species,74 typically by 1-3 eV depending on the solvent and ionic radius involved.75 This would suggest that radical anions of carbonyl containing species could be easily formed at the surface during UV illumination of the surface. Phenolic and alcohol groups, on the other hand, do not typically display positive electron affinities; however, their oxidation reactions are well-studied, and they are often used as hole scavengers in photochemical reactions. In fact, the oxidation potential of phenol is reported in the range of 1.0-1.5 eV vs SHE in aqueous solutions.76,77 Since the electron energy for the SHE is 4.44 eV below vacuum,78 the donor level of a phenolic group can be therefore estimated in the range of 5.4-5.9 eV below vacuum. This range overlaps the O-terminated carbon valence edge, which according to UPS measurements is positioned approximately at 5.6 eV below vacuum. Therefore, holes photogenerated in the carbon could be further stabilized by injection into the donor levels of phenolic and/or alcohol groups at the surface. The energy barrier for the creation of radical anions of carbonyl containing species should be lower than the 4.9 eV photons used to drive the reaction, based on literature values of EAs for these compounds. This strongly suggests that the formation of radical anions of carbonylated groups could facilitate the electron photoejection process, following a similar enhancement mechanism as that observed in the case of surfacebound trifluoroacetamide groups.19 Holes could also be trapped at the surface thanks to acceptor levels introduced by phenolic oxidized groups, also favoring surface reactions with electronrich olefin groups of the liquid terminal alkenes. Rosso et al.79 have recently shown that SiC surfaces displaying carbon-bound O-containing groups are thermally reactive toward terminal olefins; they suggested that the reaction could follow an acidcatalyzed Markownikoff addition mechanism, as previously observed on oxidized silicon80 and hydroxyl-rich silicates.81,82 The oxidized carbon surfaces used in our experiments should also increase their acidity after electron photoemission (positively charged) thus opening similar reaction pathways as those observed for SiC and acidic silanol groups. Photocurrent experiments carried out using tBOC show a clear increase in the rise times of the photocurrent and seem to support the idea that oxidative treatments introduce midgap donor and
1534 J. Phys. Chem. C, Vol. 113, No. 4, 2009 acceptor levels. An increase in the rise time in photocurrent and photoconductivity profiles is usually observed when the density of midgap states increases, due to the increased trapping of carriers.83-86 This phenomenon is observed in particular in disordered and amorphous semiconductors.87 The long rise times of several minutes observed, however, suggest that chemical transformations of the interfacial oxidized groups due to hole or electron capture might also contribute to the change in photocurrent profiles after carbon oxidation. Photoinduced reactions such as surface desorption or dimerization are known to occur at the surface of oxide semiconductors such as ZnO88-91 and TiO292-95 under UV illumination and cannot be excluded in the case of our O-terminated carbon. Finally, we note that while oxidation enhances the reactivity of amorphous carbon, it decreases the reactivity on H-terminated diamond. In the case of diamond, the band structure is very sensitive to surface termination. The 5.5 eV bandgap of diamond is substantially higher than the 4.9 eV photon energy so that little or no absorption occurs in the bulk. On H-terminated diamond, the C-H dipoles at the surface raise the energy of the valence and conduction bands relative to the vacuum level to the point where the conduction band of diamond lies above the vacuum level. Since the valence and conduction band energies are rigidly tied together, this energy-level alignment facilitates direct electron emission from the valence band (and possibly midgap states) to vacuum by reducing the energy barrier below the 4.9 eV photon energy.96 However, even small amounts of oxidation shift the band energies by 1 eV or more, inducing positive electron affinity and lead to an increased barrier to electron emission.97 On amorphous carbon, oxidation also leads to an increase in work function and decrease in density of states near the Fermi energy. However, in this case it appears that the ability of the surface CdO groups to trap electrons may be more important than the increased work function, leading to a net increase in reactivity despite the increase in work function. While the origin of these differences remain unresolved, it is clear that the valence electronic structure of the surface plays a very important role in controlling the overall reactivity toward alkenes. 5. Conclusions Our results show that amorphous carbons can display rich and complex photochemistry and provide new insights into the role of photoejection and charge separation in photochemical grafting of alkenes to surfaces of carbon. While the barrier to photoelectron emission from the solid into the acceptor levels of the alkenes is an important factor influencing the reactivity on different surfaces, our results show that specific surface groups (such as carbonyl groups) produced by surface oxidation with ozone can further enhance the reactivity, likely by locally capturing electrons in a manner similar to the “seeding” effect observed previously TFAAD.19 The net reactivity is a result of an interplay among the electronic properties of the liquid n-alkene, the electronic properties of surface groups, and the valence structure of the amorphous carbon used. Therefore wide variations and a great degree of tunability can be achieved in the photoinduced reactivity of amorphous carbons toward alkenes. While the work reported here focuses on the mechanistic aspects of photochemical grafting, understanding the factors controlling the reactivity is important to the successful practical use of amorphous carbons. The high stability of functionalized carbon surfaces, the ability to photopattern the surfaces with specific functional groups, and the ease with which the material
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