Formation of Thick Aminophenyl Films from Aminobenzenediazonium

Apr 8, 2014 - Aminobenzenediazonium Ion in the Absence of a Reduction Source. Bradley M. Simons, Joshua Lehr,. †. David J. Garrett,. ‡ and Alison ...
1 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Formation of Thick Aminophenyl Films from Aminobenzenediazonium Ion in the Absence of a Reduction Source Bradley M. Simons, Joshua Lehr,† David J. Garrett,‡ and Alison J. Downard* MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand ABSTRACT: Aminophenyl films, electrografted to conducting substrates from a solution of the corresponding diazonium ion, are a useful platform for building up functional surfaces. In our hands, reproducible preparation of aminophenyl films via electrografting is difficult, suggesting competing grafting pathways. To investigate the grafting process without the possibility of reduction of the diazonium ion by the substrate, we have used a spin-coated and cured SU-8 substrate that is nonconducting and very smooth (rms surface roughness 0.43 nm). After in situ formation of the aminobenzenediazonium ion (50 mM) in acidic solution, the substrate was added to the solution in the presence and absence of reducing agents (hypophosphorous acid and iron powder). At short reaction times, the films prepared with and without reducing agent have the same thickness and composition (as revealed by X-ray photoelectron spectroscopy). However, in the presence of a reducing agent, films reach a limiting thickness of 7−8 nm after 10 min, whereas, in the absence of a reducing agent, strong film growth continues, giving a film thickness of 14 nm after 120 min. This behavior contrasts with that of other diazonium ions which, in the absence of an applied potential, a reducing agent, or a reducing substrate, give only very thin films after long reaction times.

■

INTRODUCTION Since first reported by Lyskawa and Bélanger in 2006,1 layers of covalently attached aminophenyl (AP) groups grafted via diazonium ion chemistry have found wide application in the preparation of functional surfaces. Examples include the use of AP films for amide coupling of redox molecules to glassy carbon (GC),2 protein (biotin) to silicon,3 vertically aligned carbon nanotubes (CNTs) to GC4 and Si,5 and antibodies to screen-printed carbon electrodes.6 An alternative approach is to convert the AP film to a diazonium-terminated layer for subsequent immobilization of species including organic compounds,7 CNTs,7,8 nanoparticles,7,9 graphene flakes,7 and organometallic species.10 Other applications of AP films range from anchoring polymers to surfaces11−13 to immobilizing Sn nanoparticles14 and to improving the performance of graphite anodes in microbial fuel cells.15 In their initial report, Lyskawa and Bélanger described selective formation of the monodiazonium ion by addition of 1 equiv of sodium nitrite to an acidic aqueous solution of pphenylenediamine, followed by electroreduction in the diazotization solution to give the grafted AP layer (“in situ” grafting).1 Scheme 1 shows the generally accepted sequence of steps for in situ grafting. After diazotization, reduction of the diazonium ion generates an aryl radical that attacks the substrate, forming a covalent bond to the surface. Two other methods have since been reported for direct grafting of AP and other aryl films.16 Some substrates can reduce diazonium ions, yielding the corresponding aryl radical. Simple immersion of the substrate into the diazotization solution leads to a grafted film. Carbon powder (Vulcan © 2014 American Chemical Society

Scheme 1. In Situ Formation of Aminobenzenediazonium Ion (ABD) and Subsequent Grafting of an AP Film

XC72R17,18) and CNTs8 are examples of substrates that have been modified with AP layers by that route. Alternatively, insulating substrates that cannot reduce diazonium ions can be modified by adding a chemical reducing agent, typically Fe powder or hypophosphorous acid.11 The method was used to graft AP groups to a commercial cation exchange membrane19 and has also been used with in situ diazotization of pphenylenediamine in the presence of vinyl monomers to attach polymers to a range of substrates via an AP anchor layer.11 In addition to the above grafting methods, for some aryldiazonium ion derivatives there is evidence that films can be grafted without a reduction step. We have previously reported that surface films grow very slowly in nitrobenzenediazonium ion solution under conditions where there is no added reducing agent and electron transfer from the substrate is not possible.20 Similarly, Deniau and co-workers have reported that very thin films are grafted to gold substrates immersed in solutions of nitro-, carboxy-, bromo-, (thiomethReceived: March 31, 2014 Published: April 8, 2014 4989

dx.doi.org/10.1021/la501217n | Langmuir 2014, 30, 4989−4996

Langmuir

Article

The aryldiazonium ion was formed in situ by dropwise addition of an aqueous solution of 0.1 M NaNO2 to an equal volume of a 0.1 M solution of the required arylamine in 0.5 M HCl. The solution was left to react for 5 min, and then either the substrate was added or the reducing agent was added followed immediately by the substrate. The reducing agent was HPA at a final concentration of 0.6 M or 9 mol equiv of Fe (as a powder). Substrates were immersed in the unstirred grafting solution for 40 min, unless otherwise stated. After modification, the substrates were rinsed and sonicated for 5 min in water. Patterned AZ1518 photoresist (if present) was removed from the grafted samples by successive sonication in acetone (10 min) and isopropyl alcohol (5 min). The same series of cleaning steps were used for nonpatterned substrates. Microscopy, XPS, and Water Contact Angle Measurements. AFM images were obtained with a Dimension 3100 and Nanoscope IIIa controller (Digital Instruments, Veeco, Plainview, NY). Silicon cantilevers (Tap300Al-G, Budget Sensors, Bulgaria) with a resonant frequency of typically 200−400 kHz were used in tapping mode. Topographical images were obtained with a scan rate of 0.5 Hz with set point voltage, amplitude, scan size, and feedback control parameters optimized for each scan. All images were processed with Nanoscope Analysis software provided by Digital Instruments. Film thicknesses were measured by imaging the boundary between modified and unmodified areas of patterned SU-8 substrates. The average height of a 5 μm × 2 μm area on each side of the boundary was calculated, giving the film thickness as the difference between the two heights. Images were obtained from three regions of a sample, and three measurements of the film thickness were obtained from each image, giving a total of nine measurements. Unless stated otherwise, reported film thicknesses are an average of the nine measurements. Average rms surface roughness was calculated from AFM measurements on n areas (2 μm × 2 μm) across duplicate samples; unless stated otherwise, n = 10. Note: our usual method for measuring the thickness of grafted films could not be used with the SU-8 substrate. That method involves completely removing a section of film by scratching with an AFM tip and profiling across the scratch using AFM.26,27 The SU-8 substrate was too soft for this procedure; a significant but variable amount of substrate, in addition to film, was removed by the AFM tip, making accurate measurement of the film thickness impossible. Optical microscopy images were obtained using an Olympus BX30 microscope with digital image capture (Olympus, Center Valley, PA). Static water contact angles were measured by placing a 1 μL droplet of water from a microsyringe onto the substrate surface mounted on an illuminated stage with directional control. A digital image of the droplet was recorded using a macrolens and camera (Edmund Scientific). The contact angle was measured from the digital image using ImageJ v1.45s software (NIH, Bethesda, MD) with a drop analysis plug-in.28 Multiple droplets were imaged per sample, and an average value was reported. XPS data were obtained from SU-8 substrates using a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα source operating at 60−80 W. A charge neutralizer was used, and most of the sample surface was masked with carbon tape and aluminum foil to reduce the effect of charge build-up. Wide scans were recorded with a step size of 1 eV and pass energy of 160 eV; for narrow scans the corresponding parameters were 0.1 and 20 eV, respectively. Peak positions were referenced to aromatic carbon (present in SU-8 photoresist) at 284.7 eV.

yl)-, and methoxybenzenediazonium salts in the absence of a reducing agent.21 They argue that reduction of the diazonium ion via electron transfer from the substrate should not be possible and propose that films form through attack on the substrate by diazonium cations and aryl cations arising from heterolytic dediazoniation reactions.21 It should be noted that these studies have been undertaken in conditions where diazoate formation is not expected. Aryldiazonium ions form the corresponding diazoates at high pH22 and have been shown to graft spontaneously to glassy carbon, gold, iron, and iron oxide substrates.23,24 Although AP films prepared by in situ grafting have been widely applied, in our hands the amount of film deposited during electrografting has poor reproducibility compared with films grafted from other diazonium derivatives; furthermore, cyclic voltammograms of the grafting solution have surprisingly poorly defined reduction peaks. These observations raise the question of whether more than one grafting pathway might contribute significantly to film formation. To address this question, we have examined the grafting of AP films in the absence of a reduction source, that is, in the absence of an applied potential, a reducing substrate, and a chemical reducing agent. To ensure that reduction by the substrate is not possible, films are grafted to a nonconducting polymer. Using water contact angle measurements, atomic force miscrosopy (AFM), and X-ray photoelectron spectroscopy (XPS), we have compared films grafted in the absence and presence of added reducing agent after in situ preparation of the diazonium ion from p-phenylenediamine in acidic solution.

■

EXPERIMENTAL SECTION

Reagents and Materials. Hydrochloric acid (Sigma-Aldrich), 1,4diaminobenzene (Sigma-Aldrich), 4-(aminomethyl)aniline (Aldrich), hypophosphorus acid (HPA; Riedel De Haën AG), sodium nitrite (Sharlau Chemie, Spain), 99.5% acetone (Mallinckrodt Chemicals), 99.5% isopropyl alcohol (Sigma-Aldrich), and Fe powder (BDH, England) were used as received. Ultrapure water (Satorius AG, Germany), resistivity >18 MΩ cm, was used for all aqueous solutions and cleaning steps. SU-8 substrates were fabricated on clean pieces of silicon wafer (Virginia Semiconductor, Inc.). The wafer was protected with AZ1518 photoresist (Microchemicals, Germany) and cut into 13 mm × 13 mm samples. Photoresist was removed by sonication in acetone (5 min) followed by isopropyl alcohol (5 min), and the Si samples were then exposed to oxygen plasma at 100 W rf power for 2 min (Emitech K1050X plasma asher, Emitech, U.K.). SU-8 2005 (Microchemicals, Germany) was spin-coated at 2000 rpm for 1 min onto Si pieces and soft baked for 60 s at 95 °C on a hot plate. An MA6 mask aligner (Suess Microtec, Germany) was used to irradiate the SU-8 wafers for 60 s with a 350 W ultraviolet lamp. Postexposure, the SU-8 was baked for a further 60 s at 95 °C and hard baked for 60 min at 200 °C on a hot plate. Patterning SU-8 Substrates with AZ1518 Photoresist. SU-8 substrates were patterned using conventional UV photolithography. AZ1518 photoresist was spin-coated onto the cured SU-8 substrate at 3000 rpm for 30 s on a PWM32-PS-R790 spinner system (Headway Research Inc., Garland, TX) and soft baked for 60 s at 95 °C. The photoresist was exposed for 10 s through a chrome-on-glass photomask with an MA6 mask aligner operating in vacuum mode. The substrate was then developed by immersion in a 2% solution of tetramethylammonium hydroxide ((TMA)OH; AZ MIF326 developer, Microchemicals, Germany) for 25 s, rinsed with H2O, and dried with a stream of N2 gas. Modification of SU-8 Substrates. SU-8 substrates either nonpatterned or patterned with photoresist were modified with an AP film following the general method described by Mevellec et al.25

■

RESULTS AND DISCUSSION The substrate used for this study was required to be nonconducting, to be stable to extensive sonication in media used in the experimental procedures (aqueous acid, acetone, acetonitrile, (TMA)OH, and isopropyl alcohol), and to have subnanometer surface roughness to allow measurement of low nanometer film thickness by AFM. Initial experiments established that SU-8, an epoxy-based negative photoresist,29 spin-coated and cured on a silicon wafer, meets these 4990

dx.doi.org/10.1021/la501217n | Langmuir 2014, 30, 4989−4996

Langmuir

Article

Table 1. Water Contact Angle (CA), Film Thickness, and Surface Roughness for SU-8 Modified with AP Films in the Presence and Absence of HPA and Fe Powdera sample

CA (deg)

sb

bare SU-8 ABD with HPA ABD with Fe powder ABD, no reducing agent 1,4-diaminobenzene blankf

87.9 74.6 75.2 73.2

2.1 1.5 1.0 1.2

thickness (nm) 7.4 8.4 7.4 0.12

sc

rms roughness (nm)

s

0.3 1.0 0.2 0.04

0.43 2.8 2.2 3.2 0.59

0.08d 0.3c 0.3e 0.6d 0.08d

a Reaction time 40 min. bn = 8; four drops were measured across two samples. cn = 9. dn = 10. en = 12. fA 50 mM concentration of 1,4diaminobenzene in 0.25 M HCl.

requirements. AFM measurements confirmed that the SU-8 substrates could be reproducibly prepared with rms surface roughness of 0.43 nm (standard deviation (s) of 0.08 nm). To compare AP film formation in the presence and absence of reducing agent, initial experiments employed water contact angle measurements to monitor surface changes. After in situ preparation of aminobenzenediazonium ion (ABD) in HCl solution, nonpatterned SU-8 substrates were immersed in the solution for 40 min, with or without addition of HPA and Fe powder as reducing agents. The samples were cleaned by sonication and dried, and water contact angles were measured. The data in Table 1show that, within error, all samples have the same contact angle after reaction with ABD, both in the presence and in the absence of reducing agents. The decrease in water contact angle compared with that of untreated SU-8 substrate is consistent with the presence of an AP film which is expected to be less hydrophobic than SU-8, thus suggesting that all surfaces have been modified with an AP film. To examine the grafted films in more detail, film thicknesses were measured. Prior to grafting, a photoresist layer (AZ1518) was patterned onto the SU-8 substrate by UV photolithography. The patterned substrate was then modified in the diazonium ion solution, after which the AZ1518 photoresist was removed from the surface by sonication in acetone and isopropyl alcohol. During this step, any occluded or loosely associated material should also be removed from the SU-8 surface, leaving only strongly attached film. AFM imaging of the boundary between a modified (grafted) and unmodified (nongrafted) area enabled the film thickness to be measured. Prior to use of the photoresist-patterned substrates, the effect of removal of AZ1518 on the SU-8 surface was examined by depositing, developing, and then removing the photoresist, but in the absence of the grafting step. AFM measurements showed a small increase in rms surface roughness from 0.43 nm (s = 0.08 nm, n = 10) to 0.54 nm (s = 0.09, n = 9) after removal of AZ1518 photoresist in the initial developing (patterning) step, consistent with a small amount of photoresist residue remaining on the surface or some damage to the SU-8 surface. However, after removal of the patterned AZ1518 photoresist, the roughness of the masked SU-8 areas was the same as that of unmodified SU-8 (0.42 nm, s = 0.06 nm, n = 9), and AFM scans across the two areas of SU-8 (the area that was exposed in the first developing step and the area that was exposed after removal of the patterned photoresist) showed no height difference. These results confirm that any changes to the SU-8 surface after removal of AZ1518 are very small and are insignificant compared with the changes observed after grafting the AP film (see below). Grafting reactions were carried out using an SU-8 substrate patterned with photoresist in the presence and absence of HPA and Fe powder. After removal of the photoresist, the surfaces

were imaged by AFM. The 3D image of Figure 1a clearly shows the presence of a film on areas of the surface not masked by

Figure 1. SU-8 modified with an AP film in the presence of HPA. (a, b) AFM images. (c) Line profile obtained from the area indicated in image b. Reaction time 40 min.

photoresist during reaction with ABD. By scanning over an area that includes both bare and film-coated surfaces (indicated in the topographical image of Figure 1b), the line profile shown in Figure 1c was obtained. The profile is the average of approximately 50 scans and shows a film thickness of approximately 7.4 nm. Table 1 lists the film thickness and surface roughness for films grafted in the presence and absence of a reducing agent. Surprisingly, after 40 min of reaction time, all films grafted from ABD solution with and without a reducing agent have a thickness close to 8 nm and similar rms roughness values (∼2.2−3.2 nm). The films are clearly multilayered: an AP group attached in a perpendicular orientation to the surface has a height close to 0.6 nm, and hence, the films contain many layers. As a control experiment, a patterned SU-8 substrate was immersed in a solution of 50 mM 1,4-diaminobenzene in 0.25 M HCl in the absence of both sodium nitrite and reducing agent. After 40 min of reaction time, a 0.12 nm thick layer was deposited on the surface with an rms roughness (0.59 nm) similar to that of patterned SU-8 after removal of the photoresist (0.54 nm). This layer of material may result from a small amount of reaction of 1,4-diaminobenzene with residual epoxy groups on the SU-8 surface or may simply be due to physisorption of 1,4-diaminobenzene or adventitious impurities on the surface. Importantly, it is clear that thick films are 4991

dx.doi.org/10.1021/la501217n | Langmuir 2014, 30, 4989−4996

Langmuir

Article

deposited only after addition of sodium nitrite, confirming that the diazonium ion is necessary for film growth. The results described above, obtained from water contact angle and film thickness measurements, demonstrate that no reducing agent, either added to the solution or in the form of a reducing substrate, is required to graft films from the acidic ABD solution to SU-8 substrates. Furthermore, the results suggest that, after a 40 min reaction time, the AP films grafted to SU-8 in the presence of both reducing agents are essentially the same. For convenience, only HPA was used as a reducing agent in subsequent experiments. The composition of films grafted to nonpatterned SU-8 for 40 min in the presence and absence of HPA was probed by XPS. The data listed in Table 2 also include those for an Table 2. XPS Data for Unmodified SU-8 and AP-Modified SU-8a Calculated from Survey Spectra and Narrow Scans concn (atom %) sample SU-8 AP (with HPA) AP (no HPA) a

C b

82.1 (82.1c) 81.3b (81.5c) 81.6b (81.3c)

O

N

b

16.6 (16.6c) 12.8b (12.8c) 13.3b (13.2c)

Si

Cl

b

b

3.2 (3.1c) 3.3b (3.1c)

1.4 (1.3c) 1.6b (1.6c) 1.0b (1.4c)

Figure 2. XPS spectra in the N 1s core level region for SU-8 modified with an AP film (a) in the presence of HPA and (b) in the absence of HPA. Reaction time 40 min.

1.1b 0.8b

terminal N, respectively. It is possible that a second N signal (from the second diazonium N) contributes to the peak at 405.6 eV.33 Hence, it is tentatively suggested that the AP film grafted in the absence of HPA reducing agent incorporates a very small amount of diazonium ion. The similarity of films grafted in the presence and absence of added reducing agent raises the question of whether the reducing agent plays any role in the system. To investigate this question, films were grafted to SU-8 in the presence and absence of HPA for times ranging from 5 to 120 min. Figure 3 shows plots of the average film thickness (determined from AFM measurements) vs reaction time in the presence (Figure 3a) and absence (Figure 3b) of HPA. Each datum point represents an individual sample, and the uncertainty is the standard deviation of film thickness measurements for that sample, determined as described in the Experimental Section. Clearly, the film growth behavior is different for the two systems, and only for relatively short grafting times (up to 40 min) are the film thicknesses similar. With added reducing agent, the film grows to its maximum thickness of 7−8 nm in approximately 10 min, whereas, in the absence of a reducing agent, the film thickness continues to increase at a significant rate, reaching 14 nm after 120 min of grafting time. Parts c and d of Figure 3 show the rms film roughness vs grafting time for films prepared with and without HPA, respectively. The data reveal two important results, first, that, for both preparation conditions, the film roughness increases as the film thickness increases and, for films of similar thicknesses, the roughness values are very similar and, second, that at all thicknesses these films are much rougher than those electrografted to smooth glassy carbon like substrates. For example, nitrophenyl films, approximately 6 nm thick, electrografted to a smooth glassy carbon like substrate (pyrolyzed photoresist film, PPF) had an rms roughness value only slightly higher than that of the substrate (0.46 and 0.40 nm for the film and substrate, respectively).34 The AP film thickness in the presence of HPA is consistent with a recent report describing grafting of nitrophenyl groups

Reaction time 40 min. bFrom survey spectrum. cFrom narrow scans.

unmodified SU-8 sample. Most importantly, in contrast to the unmodified substrate, both modified samples show a measurable N 1s signal, and there are no significant differences between the elemental compositions of the films prepared with and without a reducing agent. The N:C ratio for the modified samples is lower than predicted for an AP film. This observation is attributed to the presence of an AP film which is thinner than the XPS sampling depth, leading to a contribution to the C signal from the underlying SU-8 surface. Consistent with this observation, the modified samples show a strong O signal originating from the epoxy and ether groups of the SU-8 substrate. The presence of small amounts of Cl in the AP-modified samples is assumed to originate from the HCl medium used for the grafting reactions and may be present as a Cl− counterion for protonated amine groups. Si, present in all samples, is most likely due to contamination from the Si substrate underlying the SU-8 layer. Figure 2 shows narrow scan N 1s core level spectra of the modified samples, and Table 3 lists the binding energies and [N] (%) for each peak. The major peak for both samples occurs at 399.9 eV and is assigned to amine groups and, if present, azo groups.18,30 The peak at 402.4 eV in both samples is assigned to protonated amine31 and that at 406.0 eV to nitrite.32 A small amount of nitrite is presumably present as a counterion to protonated amine groups. The latter assignment is supported by the observation that when only 0.5 mol equiv of NaNO2 to ABD is used during in situ preparation of the diazonium ion, the peak at 406 eV is absent from XPS spectra. For the sample prepared in the presence of HPA, the peak assigned to nitrite is broad (Figure 2a), possibly indicating two closely spaced peaks. The spectrum of the AP film grafted in the absence of added reducing agent is best fitted with a low-intensity fourth peak at 403.5 eV. This binding energy matches that reported for benzenediazonium ion: binding energies of 403.8 and 405.1 eV have been assigned to the N attached to the phenyl ring and the 4992

dx.doi.org/10.1021/la501217n | Langmuir 2014, 30, 4989−4996

Langmuir

Article

Table 3. XPS Data for the N 1s Core Level Region for AP-Modified SU-8a

a

sample

binding energy (eV)

[N] (%)

binding energy (eV)

[N] (%)

AP (with HPA) AP (no HPA)

399.9 399.9

75.5 75.9

402.2 401.9

10.7 7.6

binding energy (eV) 403.5

[N] (%)

binding energy (eV)

[N] (%)

3.4

405.6 406.0

13.8 13.3

Reaction time 40 min.

spontaneous grafting through reduction by the substrate, electron transfer from the substrate to diazonium ions in solution leads to film growth. Electron transfer and hence film growth slow as the insulating layer deposits on the electrode, and eventually the layer reaches a thickness at which electron transfer is negligible. In the case of nitrophenyl films grafted to glassy carbon and PPF at open-circuit potential, film growth was shown to then continue at a very low rate through a substrate-independent mechanism, possibly involving a cation intermediate.20 For AP films grafted to SU-8 in the presence of HPA, electron transfer from the substrate is not involved in film growth, and hence, the film growth rate must be related to the concentration of active modifier in solution. Addition of HPA is assumed to rapidly convert diazonium ions to the corresponding radicals throughout the whole solution. Only those aminophenyl radicals generated near the substrate will contribute to film growth. Others will be deactivated by coupling with other radicals and by reacting with components of the solution. Thus, the concentration of aminophenyl radicals is expected to decrease with time, decreasing the film growth rate, which eventually reaches zero. The long-term continued strong film growth in the absence of a reducing agent indicates that either the active modifier is stable on this time scale or the modifier is being generated throughout the reaction time. These possibilities will be discussed below. The difference in surface roughness between electrografted films and the present films can also be explained by the mode of film growth. For electrografted films, as the film grows, electron transfer will occur most rapidly across the thinnest areas of film, and hence, aryl radicals will be generated at the greatest rate where the film is thinnest. This will lead to fast film growth at the thin areas of film, effectively resulting in uniform film thickness. On the other hand, when the active modifier is generated throughout the solution, film growth will be most rapid at the most accessible regions, such as protrusion in the films, leading to high surface roughness. The high surface roughness of AP films grafted both in the presence and in the absence of a reducing agent is thus consistent with the presence of the active modifier throughout the reaction solution. As mentioned in the Introduction, a conducting substrate can act as a reducing agent to promote grafting of films from diazonium ion solutions, and even in the absence of a reducing substrate or added reducing agent, there is slow growth of thin surface films from diazonium ion solutions.20,21 To confirm that SU-8 does not promote film growth and hence that the rapid growth of thick AP films is indeed unusual, we examined the grafting of (aminomethyl)phenyl films on SU-8. Immersion of SU-8 samples in a solution of (aminomethyl)benzenediazonium ion (prepared in situ) for 40 min under the same conditions as for grafting AP films gave a film thickness of 1.4 nm (s = 0.7 nm, n = 12) in the absence of a reducing agent and 8.2 nm (s = 1.1 nm, n = 12) in the presence of HPA. The low film thickness in the absence of a reducing agent clearly demonstrates that SU-8 itself does not act as a reducing substrate. The film formed by (aminomethyl)benzenediazonium ion under these conditions is typical of

Figure 3. Film thickness (a, b) and rms roughness (c, d) versus grafting time for AP films on SU-8. (a, c) Grafting in the presence of HPA and (b, d) grafting in the absence of HPA.

to a poly(methyl methacrylate) substrate.35 A film with approximately four layers and a thickness of less than 10 nm was formed after 30 min of reaction of 20 mM nitrobenzenediazonium ion in the presence of HPA. The film thickness vs time behavior in the presence of HPA is also qualitatively similar to that observed for diazonium-derived electrografted films,36 and films grafted spontaneously at opencircuit potential at conducting substrates,20 but the origin of the behavior must be different. During electrografting or 4993

dx.doi.org/10.1021/la501217n | Langmuir 2014, 30, 4989−4996

Langmuir

Article

Scheme 2. Film Growth Mechanism Based on That Proposed by Pinson and Co-Workers37

example, ABD may disproportionate in the absence of a reducing agent, generating an aryl radical and an AP oxidation product. To account for the continued film growth, the disproportionation reaction rate would have to be relatively slow. Another possibility is that grafted AP groups may reduce ABD ions at the film−solution interface, generating aryl radicals that couple to the film. A catalytic or partly catalytic reaction mechanism can also be considered, following the proposals of Pinson and co-workers.37 Scheme 2 shows that, after an initial generation of radicals, film formation is self-sustaining, providing the redox reactions required in pathways A, B, and C can proceed. The unusual grafting behavior of ABD may be attributed to an ability to participate in the redox reactions required to sustain catalytic film growth. Future detailed examination of the grafting reaction will attempt to elucidate the important reaction pathway(s).

that found for all other aryldiazonium derivatives which have been investigated at nonreducing substrates.21 The results also establish that, in the presence of a reducing agent, (aminomethyl)benzenediazonium ion behaves similarly to ABD, generating a film of similar thickness on the SU-8 substrate. Hence, together these experiments confirm that the rapid and sustained AP film growth at the nonconducting SU-8 substrate in the absence of a reducing agent is unusual for aryldiazonium salts. The mechanism for grafting AP films to nonconducting substrates in the absence of a reducing agent will be investigated in future work; however, some preliminary comments can be made. It is reasonable to assume that, on adding reducing agent to the ABD solution, there is rapid and quantitative conversion to the aryl (and possibly diazenyl) radical, and that film growth stops after approximately 10 min because radicals have either reacted with the substrate or been deactivated by reactions in solution. The sustained reaction in the absence of a reducing agent establishes that ABD is not rapidly and quantitatively converted to either radicals or aryl cations, both of which are highly reactive. While thermally or photolytically induced heterolytic decomposition of the diazonium ion at a moderate rate could lead to steady film growth via aryl cations,21 this route seems unlikely with the cationic ammonium substituent. The ease of oxidation of AP groups1 raises the question of whether intra- or intermolecular redox reactions could account for the film growth behavior. For

■

CONCLUSION In situ preparation of ABD in aqueous acid medium leads to sustained growth of an AP film on the nonconducting SU-8 polymer substrate in the absence of added reducing agent. After a reaction time of 40 min, the film thickness is 7−8 nm, and after 120 min, the film thickness has reached 14 nm thick and shows a significant continued growth rate. In contrast, under the same reaction conditions but in the presence of added reducing agent, the AP film reaches its limiting thickness of 7− 8 nm after 10 min. To our knowledge, the strong growth of 4994

dx.doi.org/10.1021/la501217n | Langmuir 2014, 30, 4989−4996

Langmuir

Article

grafted films in the absence of a reducing agent (or reducing substrate) is, to date, unique to ABD. Our findings have two practical consequences for formation of AP films from ABD: first, to prepare thick films, no reducing agent should be added to the diazonium ion solution, and second, when “electrografting” AP films from in situ prepared ABD, three processes will contribute to film formation, spontaneous substrateindependent grafting, reduction by the substrate at open circuit potential, and cathodic electroreduction. All reactions need to be considered for reproducible preparation of AP films.

■

(9) Liu, G. Z.; Luais, E.; Gooding, J. J. The Fabrication of Stable Gold Nanoparticle-Modified Interfaces for Electrochemistry. Langmuir 2011, 27, 4176−4183. (10) Lee, L.; Brooksby, P. A.; Downard, A. J. The Stability of Diazonium Ion Terminated Films on Glassy Carbon and Gold Electrodes. Electrochem. Commun. 2012, 19, 67−69. (11) Mevellec, V.; Roussel, S.; Tessier, L.; Chancolon, J.; MayneL’Hermite, M.; Deniau, G.; Viel, P.; Palacin, S. Grafting Polymers on Surfaces: A New Powerful and Versatile Diazonium Salt-Based OneStep Process in Aqueous Media. Chem. Mater. 2007, 19, 6323−6330. (12) Garcia, A.; Berthelot, T.; Viel, P.; Mesnage, A.; Jegou, P.; Nekelson, F.; Roussel, S.; Palacin, S. ABS Polymer Electroless Plating through a One-Step Poly(acrylic acid) Covalent Grafting. ACS Appl. Mater. Interfaces 2010, 2, 1177−1183. (13) Le, X. T.; Viel, P.; Sorin, A.; Jegou, P.; Palacin, S. Electrochemical Behaviour of Polyacrylic Acid Coated Gold Electrodes: An Application To Remove Heavy Metal Ions from Wastewater. Electrochim. Acta 2009, 54, 6089−6093. (14) Zeb, G.; Gaskell, P.; Le, X. T.; Xiao, X. C.; Szkopek, T.; Cerruti, M. Decoration of Graphitic Surfaces with Sn Nanoparticles through Surface Functionalization Using Diazonium Chemistry. Langmuir 2012, 28, 13042−13050. (15) Picot, M.; Lapinsonniere, L.; Rothballer, M.; Barrière, F. Graphite Anode Surface Modification with Controlled Reduction of Specific Aryl Diazonium Salts for Improved Microbial Fuel Cells Power Output. Biosens. Bioelectron. 2011, 28, 181−188. (16) Barrière, F.; Downard, A. J. Covalent Modification of Graphitic Carbon Substrates by Non-Electrochemical Methods. J. Solid State Electrochem. 2008, 12, 1231−1244. (17) Grondein, A.; Bélanger, D. Chemical Modification of Carbon Powders with Aminophenyl and Aryl-Aliphatic Amine Groups by Reduction of in Situ Generated Diazonium Cations: Applicability of the Grafted Powder towards CO2 Capture. Fuel 2011, 90, 2684−2693. (18) Lyskawa, J.; Grondein, A.; Bélanger, D. Chemical Modifications of Carbon Powders with Aminophenyl and Cyanophenyl Groups and a Study of Their Reactivity. Carbon 2010, 48, 1271−1278. (19) Le, X. T.; Viel, P.; Jegou, P.; Garcia, A.; Berthelot, T.; Bui, T. H.; Palacin, S. Diazonium-Induced Anchoring Process: An Application To Improve the Monovalent Selectivity of Cation Exchange Membranes. J. Mater. Chem. 2010, 20, 3750−3757. (20) Lehr, J.; Williamson, B. E.; Downard, A. J. Spontaneous Grafting of Nitrophenyl Groups to Planar Glassy Carbon Substrates: Evidence for Two Mechanisms. J. Phys. Chem. C 2011, 115, 6629−6634. (21) Mesnage, A.; Lefèvre, X.; Jégou, P.; Deniau, G.; Palacin, S. Spontaneous Grafting of Diazonium Salts: Chemical Mechanism on Metallic Surfaces. Langmuir 2012, 28, 11767−11778. (22) Hegarty, A. F. In The Chemistry of Diazonium and Diazo Groups; Patai, S., Ed.; John Wiley and Sons: Chichester, U.K., New York, 1978; Part 2, p 511. (23) Podvorica, F. I.; Kanoufi, F.; Pinson, J.; Combellas, C. Spontaneous Grafting of Diazoates on Metals. Electrochim. Acta 2009, 54, 2164−2170. (24) Griffete, N.; Herbst, F.; Pinson, J.; Ammar, S.; Mangeney, C. Preparation of Water-Soluble Magnetic Nanocrystals Using Aryl Diazonium Salt Chemistry. J. Am. Chem. Soc. 2011, 133, 1646−1649. (25) Mevellec, V.; Roussel, S.; Tessier, L.; Chancolon, J.; Mayne-L’ Hermite, M.; Deniau, G.; Viel, P.; Palacin, S. Grafting Polymers on Surfaces: A New Powerful and Versatile Diazonium Salt-Based OneStep Process in Aqueous Media. Chem. Mater. 2007, 19, 6323−6330. (26) Anariba, F.; DuVall, S. H.; McCreery, R. L. Mono- and Multilayer Formation by Diazonium Reduction on Carbon Surfaces Monitored with Atomic Force Microscopy “Scratching”. Anal. Chem. 2003, 75, 3837−3844. (27) Brooksby, P. A.; Downard, A. J. Electrochemical and Atomic Force Microscopy Study of Carbon Surface Modification via Diazonium Reduction in Aqueous and Acetonitrile Solutions. Langmuir 2004, 20, 5038−5045.

AUTHOR INFORMATION

Corresponding Author

*Phone: +64 3 364 2501. Fax: +64 3 364 2110. E-mail: alison. [email protected]. Present Addresses †

J.L.: Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, United Kingdom. ‡ D.J.G.: Department of Physics, The University of Melbourne, The David Caro Building, Gate 1 Swanston Street, Victoria 3010, Australia. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the MacDiarmid Institute for Advanced Materials and Nanotechnology. B.M.S. thanks the MacDiarmid Institute and J.L. and D.J.G thank the New Zealand Tertiary Education Commission for doctoral scholarships.

■

REFERENCES

(1) Lyskawa, J.; Bélanger, D. Direct Modification of a Gold Electrode with Aminophenyl Groups by Electrochemical Reduction of in Situ Generated Aminophenyl Monodiazonium Cations. Chem. Mater. 2006, 18, 4755−4763. (2) Noel, J. M.; Sjoberg, B.; Marsac, R.; Zigah, D.; Bergamini, J. F.; Wang, A. F.; Rigaut, S.; Hapiot, P.; Lagrost, C. Flexible Strategy for Immobilizing Redox-Active Compounds Using in Situ Generation of Diazonium Salts. Investigations of the Blocking and Catalytic Properties of the Layers. Langmuir 2009, 25, 12742−12749. (3) Flavel, B. S.; Gross, A. J.; Garrett, D. J.; Nock, V.; Downard, A. J. A Simple Approach to Patterned Protein Immobilization on Silicon Via Electrografting from Diazonium Salt Solutions. ACS Appl. Mater. Interfaces 2010, 2, 1184−1190. (4) Garrett, D. J.; Flavel, B. S.; Shapter, J. G.; Baronian, K. H. R.; Downard, A. J. Robust Forests of Vertically Aligned Carbon Nanotubes Chemically Assembled on Carbon Substrates. Langmuir 2010, 26, 1848−1854. (5) Flavel, B. S.; Garrett, D. J.; Lehr, J.; Shapter, J. G.; Downard, A. J. Chemically Immobilised Carbon Nanotubes on Silicon: Stable Surfaces for Aqueous Electrochemistry. Electrochim. Acta 2010, 55, 3995−4001. (6) Qi, H. L.; Ling, C.; Ma, Q. Y.; Gao, Q.; Zhang, C. X. Sensitive Electrochemical Immunosensor Array for the Simultaneous Detection of Multiple Tumor Markers. Analyst 2012, 137, 393−399. (7) Viel, P.; Le, X. T.; Huc, V.; Bar, J.; Benedetto, A.; Le Goff, A.; Filoramo, A.; Alamarguy, D.; Noel, S.; Baraton, L.; Palacin, S. Covalent Grafting onto Self-Adhesive Surfaces Based on Aryldiazonium Salt Seed Layers. J. Mater. Chem. 2008, 18, 5913−5920. (8) Martin, C.; Crosnier, O.; Retoux, R.; Bélanger, D.; Schleich, D. M.; Brousse, T. Chemical Coupling of Carbon Nanotubes and Silicon Nanoparticles for Improved Negative Electrode Performance in Lithium-Ion Batteries. Adv. Funct. Mater. 2011, 21, 3524−3530. 4995

dx.doi.org/10.1021/la501217n | Langmuir 2014, 30, 4989−4996

Langmuir

Article

(28) Stalder, A. F.; Kulik, G.; Sage, D.; Barbieri, L.; Hoffmann, P. A Snake-Based Approach to Accurate Determination of Both Contact Points and Contact Angles. Colloids Surf., A 2006, 286, 92−103. (29) Microchem. SU-8 2000 Data Sheet. www.microchem.com/pdf/ SU-82000DataSheet2000_5thru2015Ver4.pdf (accessed 20 April 2014). (30) Toupin, M.; Bélanger, D. Thermal Stability Study of Aryl Modified Carbon Black by in Situ Generated Diazonium Salt. J. Phys. Chem. C 2007, 111, 5394−5401. (31) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer Corp.: Eden Prairie, MN, 1992. (32) Lebegue, E.; Madec, L.; Brousse, T.; Gaubicher, J.; Levillain, E.; Cougnon, C. Modification of Activated Carbons Based on Diazonium Ions in Situ Produced from Aminobenzene Organic Acid without Addition of Other Acid. J. Mater. Chem. 2011, 21, 12221−12223. (33) Finn, P.; Jolly, W. L. Nitrogen 1s Binding-Energies of Some Azide Dinitrogen, and Nitride Complexes of Transition-Metals. Inorg. Chem. 1972, 11, 1434−1435. (34) Gross, A. J.; Downard, A. J. Regeneration of Pyrolyzed Photoresist Film by Heat Treatment. Anal. Chem. 2011, 83, 2397− 2402. (35) Chehimi, M. M.; Lamouri, A.; Picot, M.; Pinson, J. Surface Modification of Polymers by Reduction of Diazonium Salts: Polymethylmethacrylate as an Example. J. Mater. Chem. C 2014, 2, 356−363. (36) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. Covalent Modification of Carbon Surfaces by Aryl Radicals Generated from the Electrochemical Reduction of Diazonium Salts. J. Am. Chem. Soc. 1997, 119, 201−207. (37) Doppelt, P.; Hallais, G.; Pinson, J.; Podvorica, F.; Verneyre, S. Surface Modification of Conducting Substrates. Existence of Azo Bonds in the Structure of Organic Layers Obtained from Diazonium Salts. Chem. Mater. 2007, 19, 4570−4575.

4996

dx.doi.org/10.1021/la501217n | Langmuir 2014, 30, 4989−4996