Article pubs.acs.org/Langmuir
Studies on the Effect of Solvents on Self-Assembled Monolayers Formed from Organophosphonic Acids on Indium Tin Oxide Xin Chen, Erwann Luais, Nadim Darwish, Simone Ciampi, Pall Thordarson, and J. Justin Gooding* School of Chemistry, University of New South Wales, Sydney NSW 2052, Australia S Supporting Information *
ABSTRACT: The preparation of self-assembled monolayers (SAMs) of organophosphonic acids on indium tin oxide (ITO) surfaces from different solvents (triethylamine, ethyl ether, tetrahydofuran (THF), pyridine, acetone, methanol, acetonitrile, dimethyl sulfoxide (DMSO), or water) has been performed with some significant differences observed. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and contact angle measurement demonstrated that the quality of SAMs depends critically on the choice of solvents. Higher density, more stable monolayers were formed from solvents with low dielectric constants and weak interactions with the ITO. It was concluded low dielectric solvents that were inert to the ITO gave monolayers that were more stable with a higher density of surface bound molecules because higher dielectric constant solvents and solvents that coordinate with the surface disrupted SAM formation.
1. INTRODUCTION Surface functionalized glass, metal, metal oxides, and semiconductors have shown tremendous applications in electronics1,2 and biological devices3 due to the ability of these modified surfaces to couple the external environment to their electronic (current−voltage responses and electrochemistry) and optical (local refractive index and surface plasmon energy) properties.4,5 However, glass lacks conductivity, while most metal, metal oxides, or semiconductor materials efficiently quench fluorescence or have limited optical transparency properties. Thus, a substrate is needed that is both conductive and transparent but can also be modified in a well-defined way. One such material is indium tin oxide (ITO). ITO is widely used for applications in optoelectronics, often being the transparent conductive coatings in plasma, touch, and liquid crystal displays, as well as solar cells and organic light emitting diode (OLED) devices.6,7 Its high conductivity permits the characterization of ITO with a variety of analytical techniques.8,9 Unlike metals, the optical transparency of ITO provides opportunities for studies involving fluorescence and possibly even super-resolution fluorescence microscopy.10−13 Various phosphonic acids have recently been developed for the modification of oxide surfaces such as ITO. 14−16 Phosphonic acids are believed to create a more robust modification layer compared to those created from chemisorbed carboxylic acids, especially after annealing to maximize the number of P−O bonds to the metal oxide lattice.17,18 The initial publications in this area have focused on the modification of ITO with phosphonic acids for improving OLEDs19−21 and a few papers that characterize the phosphonic acid monolayer formation per se.22−25 Although there are some investigations about the adsorption kinetics and adsorption condition of © 2012 American Chemical Society
organophosphonic acids on different oxide surfaces such as Al2O3,26−29 there is almost no research related to the influence of self-assembly condition on the qualities of self-assembled monolayers (SAMs) assembled on ITO (e.g., compactness, electron-transfer, and permeation). We have recently assessed the role of the underlying ITO substrate on the quality of SAMs formed and showed that amorphous smooth surfaces gave higher quality SAMs than rough crystalline substrates.30 Halik and co-workers also revealed the effect of the length of organophosphate molecules on the quality of SAMs on ITO using n-alkylphosphonic acid.31 Herein, we turn our attention to solvent effects on the SAM formation, which is thus far unexplored (Scheme 1). The modification of gold with alkanethiol self-assembled monolayers teaches us that the order of SAMs formed on gold depends strongly on the nature of the solvent employed.32,33 It is therefore possible that the solvent could also be one of the most important factors that affects the qualities of SAMs assembled on ITO. By means of investigating the effect of solvents on phosphonic acid self-assembly on ITO, we can obtain some guidelines for performing the modification with the optimal procedure. A series of organophosphonic acids, different in their functional group and length, were chosen to prepare SAMs from solvents with different dielectric constants. We then investigate how the solvent affects the quality of these SAMs by contact angle, XPS, STM, and electrochemistry. The relationship between solvent parameters (dielectric constant and interaction ability) and the effect of terminal groups on the Received: March 9, 2012 Revised: May 16, 2012 Published: May 23, 2012 9487
dx.doi.org/10.1021/la3010129 | Langmuir 2012, 28, 9487−9495
Langmuir
Article
Scheme 1. Schematic of the Self-Assembly Process in Different Solvents
Table 1. Contact Angle and Electrochemical Evaluation of SAMs Prepared from the Four Phosphonic Acid Derivatives from the Different Solventsa
ΔEp is the potential difference between the position of the oxidation peak and the reduction peak in cyclic voltammograms (CV). The lower the value of ΔEp, the greater the number of defects in the SAM. CV measurements were recorded in 0.1 mM K3Fe(CN)6 containing 0.2 M KCl in MilliQ water, at 100 mV/s.
a
0.5 M K2CO3 in 2:1 methanol/Milli-Q water mixture for 20 min to remove any residual organic contaminants. The ITO substrates were then rinsed with copious amounts of Milli-Q water, dried, and finally stored in inert nitrogen. 2.3. Preparation of SAMs. SAMs on ITO surface were prepared as described previously.30 Self-assembling molecules (4-aminophenylmethylphosphonic acid, phenylphosphonic acid, n-octadecylphosphonic acid, and 16-phosphonohexadecanoic acid) were adsorbed onto clean ITO surfaces for 24 h from a 1 mM solution in one of the following solvents: triethylamine, ethyl ether THF, pyridine, acetone, methanol, acetonitrile, DMSO or water. The surfaces were rinsed with copious amounts of the corresponding solvent to remove loosely adsorbed species, dried in nitrogen, and then annealed at 200 °C for 24 h under N2 to promote the formation of stable covalent bonding. The annealed SAM modified surfaces were then rinsed with copious amounts of Milli-Q water to remove possible multilayers and weakly bound molecules. Subsequently, the surfaces were dried under nitrogen. All the films were prepared immediately prior to use. 2.4. Contact Angle Goniometry. Contact angles with Milli-Q water were measured using the sessile drop method using a Rame-Hart 100-00 goniometer. Each contact angle value reported here is the average of at least three measurements per substrate from multiple substrates. The reproducibility of the contact angle measurements for
formation and electrochemical stability of SAMs are all reported.
2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. 4-Aminophenylmethylphosphonic acid, phenylphosphonic acid, n-octadecylphosphonic acid, 16-phosphonohexadecanoic acid, potassium carbonate (K2CO3), potassium chloride (KCl), potassium ferricyanide (K3Fe(CN)6), triethylamine, ethyl ether, tetrahydrofuran (THF), pyridine, acetone, methanol, acetonitrile (ACN), and dimethyl sulphoxide (DMSO) were obtained from Sigma-Aldrich (Sydney, Australia). Chemicals such as sodium perchlorate (NaClO4) and lead dichloride (PbCl2) were of reagent grade obtained from Ajax Chemicals, Sydney. ITO substrates were obtained from SPI USA (6471-AB, 15−30 ohm cm) and have the surface characteristics of low crystallinity and low roughness we described previously that give high quality monolayers by self-assembly of phosphonates.30 All reagents were used as received, and aqueous solutions were prepared with Milli-Q water (18 MΩ cm, Millipore, Sydney, Australia). 2.2. Pretreatment of the Electrodes. Commercial ITO (SPI, USA) coated on coverslips were used, which we previously showed gave the best quality SAMs.30 The substrates were first cleaned in an ultrasonicator with methanol for 10 min, followed by treatment with 9488
dx.doi.org/10.1021/la3010129 | Langmuir 2012, 28, 9487−9495
Langmuir
Article
Figure 1. Cyclic voltammograms of (a) phenylphosphonic acid SAM and (b) n-octadecylphosphonic acid SAM formed from different solvents versus bare ITO surface. CV measurements were recorded at a scan-rate of 100 mV/s in 0.1 mM K3Fe(CN)6 aqueous solution containing 0.2 M KCl. Peak separations were estimated from background subtracted cyclic voltammograms. different substrates treated under the same conditions was estimated using multiple repeated measurements. 2.5. Electrochemical Measurements. All electrochemical measurements were performed with a BAS-100B electrochemical analyzer (Bioanalytical System Inc., Lafayette, IL) and a conventional three-electrode system, comprising an ITO working electrode, a platinum wire as the auxiliary electrode, and a Ag/AgCl 3.0 M NaCl electrode (CH Instrument, USA) as reference. The electrochemical cell was a purpose made cell, which was assembled with constant area O-ring on the working ITO electrodes with the size of ring (surface area) at 0.636 cm2. All potentials were reported versus the Ag/AgCl reference electrode at room temperature. The electrochemical stability experiments were carried out by applying different potentials at each self-assembled layer in 0.2 M KCl aqueous solution. The experiment was performed by poising the electrode at designated potentials starting from 0 mV to 1000 mV or from 0 mV to −1000 mV. Each potential was applied for 5 min. All cyclic voltammograms (before and after holding different potentials) were performed using 2 cycles between −300 mV and 800 mV versus Ag/AgCl with scan rate of 100 mV/s in an aqueous solution of 0.1 mM ferricyanide containing 0.2 M KCl. For monolayer defect mapping studies, a solution containing 1 mM of PbCl2 in 0.2 M of NaClO4 was used to deposit Pb by holding the potential at −0.3 V for 2 min on the SAMs-modified ITO surfaces. 2.6. XPS Measurements. X-ray photoelectron spectra were collected on an EscaLab 250Xi spectrometer with a monochromated Al Kα source (1486.6 eV), hemispherical analyzer, and multichannel detector. The spectra were accumulated at a takeoff angle of 90° at a pressure of less than 10−8 mbar. The pass energy for the survey scan is 100 eV and for the narrow scan 20 eV. The step size for the survey scan is 1.0 eV and for the narrow scan is 0.1 eV. Survey spectra (0− 1300 eV) were obtained, followed by high-resolution scans of C 1s, O 1s, N 1s, and P 2p regions. The spectra were calibrated on the In3d5/2 peak (444.7 eV). Spectra were analyzed using Avantage 4.54 software. 2.7. STM Measurements. Mapping the deposited lead on different monolayer-modified surfaces with STM gives an indication on the quality of the monolayer. The defects in the SAM provides areas for the deposition of lead to take place, which, in turn, can be imaged by STM. The STM measurements were performed using Multimode 8 (Bruker) in air with a Pt−Ir tip. SAMs from different solvents without and with lead were recorded in constant-current mode with a tip current of 1 nA and a bias voltage of 0.1 V.
with either a hydrophobic or a hydrophilic distal moiety and surfaces terminated with methyl, amino, or carboxyl derivative distal groups. We also chose phenyl or alkyl phosphonate derivatives such that the impact of π-stacking between molecules forming the SAMs or significant van der Waals forces could be explored. Once the resultant SAMs were formed, they were evaluated using contact angle measurements to probe surface disorder, their ability to impede electron transfer between redox species in solution and the underlying ITO electrode was used as a measure of any defects (henceforth known as electrochemical blocking experiments), their chemical composition was explored using XPS, and electrochemical blocking experiments were used to assess the stability of the SAMs under different applied potentials. Thicknesses of the SAMs, as estimated using XPS and the attenuation of the In 3d peak showed that, under the conditions by which the SAMs were formed, they were monolayers with no significant multilayer formation. Initially, the SAMs formed from the different organophosphonic acid derivatives from different solvents were investigated using water contact angle (see Table 1). The water contact angle on cleaned ITO was
