Dalton Transactions View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
PAPER
Cite this: Dalton Trans., 2014, 43, 4901
View Journal | View Issue
Ethylenediamine-modified oriented MCM-41 at the electrode surface, cobalt adsorption ability and electrochemical performance† Mohammad Rafiee,*a Babak Karimi,*a Simin Arshia and Hojatollah Valib Mesoporous silica thin films (MCM-41) functionalized with ethylenediamine groups were electrochemically fabricated on electrode surfaces. These ligand functionalized film were a promising matrix for the immobilization of cobalt ions and preparation of cobalt complexes covalently bound to the MCM-41 support. The constructed MCM-41 were characterized by TEM, EDS and TGA analysis. This method yields uniform thin films with hexagonal mesochannels aligned and accessible to electrode surface. Welldefined electrode responses were, therefore, observed for the anchored complexes which made the electrochemical analysis of the structure possible as well. Voltammetric studies revealed the reactivity of
Received 26th August 2013, Accepted 6th December 2013
the covalently bound complexes differed significantly from the dissolved ones. The anchored complexes
DOI: 10.1039/c3dt52343h
preferred to be in their oxidized form which inhibits formation of oxygen adducts. The covalently bound complexes had relatively good leaching stability with good catalytic performance towards hydrogen per-
www.rsc.org/dalton
oxide reduction.
Introduction Organic–inorganic hybrid materials that combine structural properties of a rigid inorganic framework with chemical activity and property of organic moieties have drawn enormous attention in several scientific disciplines during the past two decades.1 The most well-known representative class of these hybrid materials includes ordered mesoporous silica structures.2 Derivatization with organofunctional groups, especially by exploiting the versatility of sol–gel chemistry and reactive OH groups at the surface, provides easy access to organically modified silica-based materials.3 Another more important characteristic of ordered mesoporous structures is their large surface area, around 1000 m2 g−1, owing to the presence of nanometer-sized channels with controllable and narrow size distribution. Owing to their robust porosity and potential for modification, ordered mesoporous structures have emerged as potential supports for the heterogenization of homogeneous substrates and especially for homogeneous catalysts.4 Immobilization of metal complexes is one of the widely studied
a Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Gava-Zang, Zanjan 4513766731, Iran. E-mail: [email protected], [email protected]; Fax: +98-241-4153232 b Department of Anatomy & Cell Biology, McGill University, 3640 University St., Montréal, Quebec H3A 0C7, Canada † Electronic supplementary information (ESI) available: TGA analysis and the analysis of the leaching stability of the electrodes. See DOI: 10.1039/c3dt52343h
This journal is © The Royal Society of Chemistry 2014
examples of heterogenization on mesoporous silica structures. Ligand-grafted mesoporous silicas have attracted much attention as promising supports to enhance the binding affinity of metal ions. The immobilization of metal complexes was often performed through coordination of metal ions with the supported ligands.5 The designed hybrid silica supported organometallics have been used widely in a variety of applications such as heterogeneous catalysts (as the major concern)6 and electrochemistry.7 Moreover, the area of electrochemistry concerned with transition metal complexes has significant contributions in the various electrochemical studies and applications.8 The ability of electron transfer of many organometallic compounds renders them attractive for electrochemical applications. Alternatively, electrochemical methods, especially cyclic voltammetry (CV), have been used to obtain information about their structural features.9 Notwithstanding such significant results on electrochemical investigation and application of supported organometallic compounds there is still scope for the study of these mutual concepts. The objective of the current study is to investigate the electrochemical behaviour of cobaltammine complexes in solution and in the supported form in the interior of ordered mesoporous silica (MCM-41) at the electrode surface. Electro-Assisted Self Assembly (EASA)10 was used to deposit ethylenediamine containing MCM-41 and MCM-41 with accessible channels on the electrode surfaces and to potentially form anchored cobaltammine complexes inside the channels. Cobaltammine complexes have remarkable electron transfer ability11 which may be due to the
Dalton Trans., 2014, 43, 4901–4908 | 4901
View Article Online
Paper
Dalton Transactions
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
stabilization of a number of oxidation states by amine ligands.12 The study of cobalt complexes was carried out as a typical example of first-row transition metals.13 Moreover, cobaltammine complexes are of interest for their reversible adduct formation with molecular oxygen and the importance of their desired adducts.14
Experimental Materials Tetraethoxyorthosilicate 99% (TEOS), cetyltrimethylammonium bromide 98% (CTAB), ethylenediamine (En), potassium nitrate and hydrochloric acid 37% from Merck. Cobaltous nitrate hexahydrate and N-[3-(trimethoxysilyl)propyl]ethylenediamine 97% (EnPTS) were purchased from Sigma-Aldrich and Fluka respectively, all of the materials were used as received. Instrumentation The characterization of the film was performed using the following instruments: NETZSCH STA 409 PC for thermal gravimetric analysis (TGA); Philips CM200 transmission electron microscope (TEM) equipped with a Gatan Ultrascan 1000 Model 895 CCD camera and EDAX Genesis energy-dispersive X-ray spectrometer. All electrochemical measurements were performed at room temperature using an Autolab PGSTAT-101 monitored by the Electrochemical System Software (Eco Chemie). Experiments were carried out in a classical three-electrode undivided cell configuration with a platinum counter electrode and a glassy carbon working electrode (surface area = 3.14 mm2). The potentials were measured versus an Ag/AgCl reference electrode (all of the electrodes were manufactured by AZAR Electrode Company, Urmia, Iran). A Metrohm pH-meter 691 was used for measuring solution pH. To remove oxygen from solution during the experiments, nitrogen gas with a purity of 99.999% was used for 5 minutes. Preparation of mesoporous silica films on electrode Sol–gel solution composition for preparation of En-functionalized mesoporous silica was 15 mL ethanol 7 mL H2O and 8 mL aqueous solution of 0.2 M NaNO3 and 0.2 M HCl to which 10.1 mM (TEOS + EnPTS) and 3.26 mM CTAB were added under stirring. Then the solution pH was adjusted to the value of 3. The sol–gel solution was aged for 2.5 h under stirring before deposition. A cathodic potential of −2.2 V was applied for 20 s then the electrode removed and immediately rinsed with distilled water. Preparation of pure silica film was achieved on the basis of previously reported procedures using only TEOS as silica precursor.10 The electrodeposited surfactant-templated film was then dried overnight in an oven at 130 °C. Extraction of the surfactant template was carried out in acidic ethanol solutions under moderate stirring for 10 min.10 Several aqueous solutions containing the various ratios of Co2+ to En were used for grafting of cobalt onto the film.
4902 | Dalton Trans., 2014, 43, 4901–4908
Fig. 1 CVs of 2.0 mM Co2+ in the presence of various concentrations of En, En concentrations from (a) to (d) are: 2.0, 4.0, 6.0 and 12.0 mM respectively. Supporting electrolyte 0.15 M KNO3, scan rate 100 mV s−1.
Results and discussion Homogeneous study The voltammetric behaviour of cobalt and its complexes was studied in aqueous solution in order to obtain detailed information about the electrochemistry of the cobaltammine complexes and to compare its electrochemical behaviour to the supported ones. Fig. 1 shows the cyclic voltammograms (CVs) of 2.0 mM of Cobalt II (Co2+) in an aqueous 0.15 M KNO3 solution with the continuous addition of En under a nitrogen atmosphere. In the selected range and the whole potential window of the desired aqueous solution no redox peaks were observed for Co2+ in the absence of En. After the addition of En, one anodic peak (A1) and its cathodic counterpart (C1) appeared and their height increased with increasing concentration of En up to 6.0 mM. No significant increase in height of either A1 and C1 peaks were observed in the presence of higher En concentration confirming that the major (only) electroactive complex of Co2+ is likely to be Co(En)32+. The voltammograms also demonstrated the relatively large formation constant between Co2+ ions with En to give Co(En)32+. The formation constant of the electroactive complex (Co(En)32+) were obtained on the basis of the “molar ratio method”.15 One measurement was made with a Co2+ analytical concentration (CM) and a large excess of En to yield the anodic peaks of I0. The amount of excess En was adjusted so that the dissociation of the complex is negligible and the concentration of the Co(En)32+ can be considered as CM. Thereafter, the A1 peak current of a series of solutions with different En-to-Co2+ concentration ratios (constant CM) were measured as In. The ratios of In/I0 yield to equilibrium concentration of electroactive complex and then the equilibrium concentrations of Co2+ and En for each concentration of En. The conditional formation constant of the complexation reaction was calculated as 1.32 × 108 M−1 (considering the equilibrium concentrations of the reaction components) which is in good agreement with the previously reported results.11
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Dalton Transactions
Paper
Scheme 1 Proposed mechanism for the electron transfer of cobaltammine complex and its oxygen adduct. Fig. 2 (I) The positive going scan CVs of Co(En)32+ in the presence of oxygen with time at pH 3.70, time intervals 2 min from (a) to (i), (II) the negative going scan CVs after 30 minutes ( j) first cycle and (k) second cycle, scan rate: 100 mV s−1.
In the presence of oxygen a new irreversible cathodic peak (C2) appeared at more negative potentials and its height increased with time as the currents of A1/C1 redox peak decreased (Fig. 2(I)). After 30 min, the C1 cathodic peak disappeared and the C2 peak reached its maximum value. At the end of the reaction, the negative going scan half-cycles showed only one cathodic peak (C2), whereas in the positive going halfcycle the A1 peak appears again. The multi-cyclic voltammograms of the solution showed the appearance of the C1 peak as the counterpart of A1 at the second cycle (Fig. 2(II)). It should be noted that no redox peak was detected when the less negative switching potential (−0.6 V vs. Ag/AgCl) was employed. Fig. 1 and 2 clearly demonstrate adduct formation between the cobaltammine complex and molecular oxygen as reported previously in the literature.14 The appearance of the reductive peak (C2) in the negative going scan demonstrated the cobalt nucleolus of this adduct has a higher oxidative state (Co3+). This is likely due to the charge transfer causing oxidation of Co2+ with concomitant reduction of dioxygen during the adduct formation. This voltammetric behaviour also demonstrates that the adduct, in which both cobalt and the reduced form of oxygen are in their reductive forms (Co2+ and peroxo or superoxo),14 is not stable and undergoes a decomposition reaction (Scheme 1, eqn (4)) to form the initial cobaltammine complex and the reduced form of oxygen. The voltammetric data confirms the release of the reduced form of oxygen (likely O22−) is electroactive and also undergoes electron transfer (Scheme 1, eqn (5)). Both the voltammetric data at various scan rates and the hydrodynamic voltammetric data (RDE) at various rotation rates are in good agreement with the evidences of an ECE mechanism.16 The E and C nomenclature are used for electron transfer and chemical reaction, respectively.17 The following mechanism is proposed for the complex formation and electron transfer of cobaltammine complex in the presence of O2. The formation of cobaltammine complexes and redox activity of these complexes encouraged us to further investigate
This journal is © The Royal Society of Chemistry 2014
the possibility of immobilizing En followed by complexation with Co2+ and the electron transfer of the complex. Electrochemical deposition of En-functionalized MCM-41 There are many published articles on the anchoring of metalcomplexes onto oxides (from macroporous materials to zeolitic structures) and their catalytic applications.18 However, key issues with many of the electrochemical studies and applications are the insulating character of some of the oxides and their particulate structure. Complicated electron transfer, limitation of mass and/or coupled charge transfer through these structures made their electrochemical applications difficult.19 Even there is a claim saying that electron transfer of the anchored electroactive groups inside the channels is not possible for insulating supports such as silica.14 Recently EASA was proposed as a method to produce mesoporous silica thin films on electrode surfaces with hexagonal mesochannels perpendicular to the electrode surface to provide good accessibility and mass transport capabilities.10 This method has been developed for one-pot synthesis of amine functionalized films with a morphology that induced metal ion (Cu2+) absorptivity on the film and its related modified electrode.20 We have also demonstrated that the anchored electroactive group on the oriented mesochannels have a simpler electron and mass transfer regime.21 Considering the above discussion, we reasoned that functionalization of such unique structures with appropriate ligand moiety might offer versatile mesoporous structures for the electrochemical study and application of supported electroactive complexes. Preparation of En-functionalized thin films was achieved by employing EASA, under potentiostatic conditions (−2.2 V for 20 s), on Glassy Carbon (GC) electrodes in the presence of TEOS and EnPTS as a silica precursor and CTAB as a structure-directing agent. Film permeability and surface coverage was examined by voltammetry with Fe(CN)63− as an electroactive probe. The voltammetric currents were negligible for the electrode with as-synthesised thin film on its surface (Fig. 3, curve a) indicating the crack-free structure of the film. After extraction of CTAB (Fig. 3, curve b), the voltammetric current of the modified electrode with the En-functionalized silica structure
Dalton Trans., 2014, 43, 4901–4908 | 4903
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Paper
Fig. 3 CVs of 1.0 mM Fe(CN)63− on EnSE (a) before and (b) after extraction of surfactant, (c) on bare electrode and (d) on unfunctionalized silica film modified electrode; supporting electrolyte 0.15 M KNO3, scan rate: 100 mV s−1.
(denoted as EnSE) suggested a porous structure with good electrode accessibility. The voltammetric currents of Fe(CN)63− on EnSE were even higher than that of the bare electrode (Fig. 3, curve c), whereas the voltammetric currents of Fe(CN)63− on the modified electrode with pure silica film (SiE, Fig. 3, curve d) with essentially the same mesoporous structure was significantly less than the bare electrode. The difference in electrochemical behaviour of the modified electrodes for this charge species is evidence for the incorporation of positively charged protonated En groups in the structure of EnSE. Likewise, the thermogravimetric analysis (TGA) confirms the incorporation of En groups as an organic moiety in the structure of the films (Fig. S1†). After screening and examination of several EnTS to TEOS ratios, the maximum loading of functional En groups in the structure was attained using 8% EnPTS–TEOS in the sol solution. Attempts to prepare a thin silica film with higher En loading resulted in disordered porous films, which confirms previous reports for the amine containing silica precursor.20,21 However, this loading, together with the well-known chelating ability of En, holds promise for supporting complexes and further applications. In the next stage, EnSE was used for the formation of supported cobaltammine complexes and thus the prepared EnSE was immersed in a solution containing Co2+ (in all of the experiments the solutions contained 0.15 M KNO3 as supporting electrolyte). As clearly shown in Fig. 4(I) EnSE did not show much electrochemical response to the Co2+ solution (curve a). This observation may be due either to inefficient complexation of Co2+ with EnSE or the electro-inactive character of the Co2+– EnSE species. To further clarify this issue, a similar experiment was performed in the presence of 1.0 equivalent of dissolved En under otherwise the same experimental conditions on the basis that En might enhance the formation of the cobaltammine complex. However, the electrochemical behaviour of this modified electrode was essentially the same as that obtained
4904 | Dalton Trans., 2014, 43, 4901–4908
Dalton Transactions
Fig. 4 (I) CVs of EnSE in (a) solution containing 2.0 mM Co2+ and (b) solution containing 2.0 mM Co2+ and 2.0 mM En; (II) the CV of EnSE after impregnation with Co2+ in solution containing 2.0 mM of En (c) at initial time, (d) positive going scan and (e) negative going scan after 5 minutes; supporting electrolyte 0.15 M KNO3, scan rate: 100 mV s−1.
in the absence of added En (Fig. 4(I), curve b). This observation highlights our opinion that the in situ addition of an equivalent amount of En before the formation of Co2+–EnSE species would not improve the electrochemical performance of the modified electrode. Interestingly, by removing the electrode (from the above mentioned solution), washing with distilled water and immersing in a solution containing only En, the electrodes exhibited one pronounced anodic peak (A′1) accompanied with its cathodic counterpart (C′1). The midpoint potential of these redox peaks were in good agreement with the dissolved cobaltammine complex (Co(ED)32+). In addition, the peak currents did not decrease with time, but rather showed a considerable increase at 15 min (Fig. 4(II)). It can be seen from curve d in Fig. 4(II) that in the positive going scan CVs, the cathodic current appeared at more negative potentials than the midpoint of the cobaltammine complex (Emid1) and reached its maximum value after only 5 minutes. The negative going scan CVs (Fig. 4(II), curve e) did not show any oxidative current at the starting potentials (the more positive values than Emid1). This voltammetric data demonstrates that the anchored cobalt nucleus converted to a higher oxidation state (Co3+) over a relatively short time period. These results suggest that the preferred stabilization of one oxidative state (here the oxidized form) occurs via heterogenization. Voltammetric analysis in the presence of air indicated that the anchored complex has the same electrochemical behaviour in both the absence and presence of dissolved O2 and adduct formation with molecular oxygen was not observed. This behaviour might be explained by considering that oxidation of anchored cobaltammines is faster than adduct formation with O2. An alternate explanation is that the oxidized form of the cobaltammine species has little or no affinity for dissolved molecular oxygen.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Dalton Transactions
Paper
Fig. 6 The CVs of CASE in (I) solution containing 0.15 M KNO3 and (II) solution containing 0.15 M KNO3 and 0.2 M En, the CVs from (a) to (d) recorded at 1, 10, 30 and 60 min respectively, scan rate: 100 mV s−1.
Fig. 5 (a) EDS analysis of the scratched film from EnSE after impregnation with Co2+ and En; (b) and (c) top view TEM images of the film at different magnification.
Another extrapolation that can be made from the abovementioned observations is that EnSE displays a relatively high affinity for adsorption of dissolved Co2+ ions. As is shown in Fig. 5, the EDS analyses of the scratched film clearly demonstrated the presence of cobalt in the film. TEM images of the film also showed the uniform thickness of the film and hexagonal pattern of the channels. After impregnation with Co2+ and En and washing with deionized water, the film still retained its original ordered porous structure despite the relatively alkaline basic nature of the En solutions. Considering the well-defined electrochemical response of anchored cobaltammine complexes, the leaching stability of the electrodes was further studied by voltammetry. The CVs of EnSE after impregnation with Co2+ and En, i.e. functionalization with the cobaltammine complex (denoted as CASE) were recorded over time in a blank solution containing only 0.15 M of KNO3 as the supporting electrolyte. The signals of CASE decreased at 30 and 60 min to less than 40% and 15% of the initial values, respectively (Fig. 6(I)). More detailed studies showed that potential cycling (the number of CVs that have been used for monitoring the loading over time) accelerated the leaching of the complexes. Moreover, for the CVs of CASE in the KNO3 solution, the ratio of anodic-to-cathodic peak currents (A′1/C′1) was considerably less than unity (Fig. 6(I), curves b and c). It can, therefore, be concluded that in neutral aqueous solutions, and partially in the absence of En, the reduced form of the anchored cobaltammine complex was not stable owing to its low formation constant.11 Therefore, the reduction of anchored cobaltammine complexes resulted in decomposition and faster leaching. In the next stage, the leaching stability of CASE was examined in En containing
This journal is © The Royal Society of Chemistry 2014
solutions where there was a significant enhancement in the stability of CASE. When the concentration of En was optimized for maximum stability, the leaching of cobaltammine complexes in their reduced form minimizes En concentration within 1.0 to 10.0 mM. Results obtained from the voltammetric analysis have been compared in Fig. 6. As is shown in Fig. 6(II), the currents of the CASE electrode in the 0.1 to 10 mM En solutions decreased by less than 5% of the initial value after 1 hour and thereafter a slight and continuous decrease was observed. Fig. 6(II) shows that at longer times, the CVs of CASE electrodes exhibited an additional cathodic shoulder (at more negative potential than C′1). Although the appearance of this shoulder was not completely reproducible, its height was independent of the presence of O2 and inversely proportional to En concentration. The most probable mechanism for the appearance of this peak is the interaction of the hydroxyl groups in the silica channels with the anchored cobaltammine complexes.22 After analysing the above results the following mechanism (Scheme 2) is proposed for the functionalization of the EnSE with the cobaltammine complexes. It is also encouraging to compare the possibility of the absorption of dissolved Co2+ and cobaltammine complex (Co(ED)32+) on parent silica film (SiE) with the above proposed pattern. With the same justification for EnSE, the SiE did not show any voltammetric response in the Co2+ solution. By removing the electrode and immersing it in an En solution, the electrode showed a considerable current, but decreased significantly and reached background currents in less than 15 min. The adsorption of the cobaltammine complexes on SiE was also investigated by immersing the electrode in a cobaltammine solution (containing 1 : 3 Co2+ to En) for 20 min. This electrode (CASiE) showed voltammetric responses comparable to CASE, but leaching of CASiE increased considerably more than CASE. The electrode current attained 40% of the initial value in the En solution within 30 min and background after 80 min (Fig. S2 and S3†). It is also worth noting that CASiE did
Dalton Trans., 2014, 43, 4901–4908 | 4905
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Paper
Scheme 2
Dalton Transactions
Proposed mechanism of the construction of EnSE and formation of cobaltammine complex on its surface.
not exhibit any significant current after the first CV cycle in the solution containing only KNO3 (and in the absence of En). Electrochemical and catalytic performances Voltammetric analysis, at various scan rates, was also used to acquire qualitative information on the mechanism of electron transfer through the channels of CASE. As shown in Fig. 7, the voltammetric currents increased by increasing the scan rate of CVs. Plotting of peak currents versus scan rate did not show a linear trend. Plotting of the peak currents versus square root of scan rate (v1/2) showed a positive deviation. Fig. 7(II) shows that the normalized CVs did not superimpose. Normalized CVs were obtained by dividing the current of the CVs by the square root of the scan rate. For diffusion-controlled reactions, the peak current is proportional to v1/2, whereas in the adsorption-controlled reaction it is proportional to v. Hence, it is suggested that on the basis of the simplified model proposed by Scholz–Lovric there is an intermediate mechanism for the electron transfer through insulating porous solid supports.21,23
The diffusion-like behaviour of voltammograms is related to the diffusion of electrons through the film. The relatively high concentrations of anchored electroactive groups made electron percolation through the film easier and dictate an adsorption-controlled behaviour to the system especially at very low scan rates. The loading of anchored complexes was estimated after determination of the Faradaic charge transfer through the electrode in a voltammetric experiment that was recorded at a low scan rate (for example the scan rates less than 50 mV s−1, Fig. 7, curve a).21 The results indicated a loading of 8.6 × 10−9 mol cm−2 (mol per cm2 for geometric area of electrode) in the film. Finally, the catalytic performance of the CASE electrode was investigated for the electrochemical reduction of H2O2. H2O2 is a well-known oxidizing agent and as an electron mediator has a relatively slow rate of electron transfer in both chemical and electrochemical systems. H2O2 is not only an essential mediator for environmental, pharmaceutical and industrial applications, but is also a by-product of several important reactions during oxygen reduction.24 Fig. 8 shows the increase in height of C′1, the disappearance of the A′1, and potential shifts to more negative values in the presence of H2O2 which are in good agreement with the diagnostic criteria of the mediated reduction of H2O2 at CASE.16 It is also apparent from this figure that the direct electron transfer of H2O2 was accelerated at the surface of CASE so that the reduction currents of H2O2 (C3) increased considerably at the surface of CASE (comparing its current at the surface of EnSE). As curve e in Fig. 8 demonstrates, considering the amperometric measurements, CASE shows a short response time (within 5 s), high sensitivity and good stability for H2O2 reduction.
Conclusions Fig. 7 (I) CVs and (II) normalized CVs of CASE in solution containing 0.15 M KNO3 and 0.2 M En at various scan rates; scan rates from (a) to (e) are 50, 100, 200, 500 and 1000 mV s−1 respectively.
4906 | Dalton Trans., 2014, 43, 4901–4908
The objective of this study was to extend and evaluate the application of EASA for fabricating porous silica films at an
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Dalton Transactions
Fig. 8 CVs of (a) bare GC electrode, (b) EnSE and (c) CASE in 1.0 mM solution of H2O2 (d) CV of CASE in the absence of H2O2; scan rate 250 mV s−1; (e) amperometric response at a rotating modified CASE (the rotation speed is 500 rpm) held at −800 mV for successive addition of 500 µL H2O2 (5 mM) to 50 mL 0.15 M KNO3 and 2.0 mM En.
electrode surface. En functionalized MCM-41, with its unique morphology, aligned channels and uniform thickness, has been constructed at an electrode surface. The En containing MCM-41 acts as an appropriate matrix for the immobilization of Co2+. The most interesting finding was that adsorbed Co2+ did not leach considerably in the En solution, but rather increased the adsorption of En ligands. Adsorption of additional En ligands yields the formation of electroactive cobaltammine complexes. Uniform thickness of the film and well-defined texture provides excellent electrochemical response and made the voltammetric study of the complexes possible. Voltammetric data show that the behaviour of anchored cobaltammine complexes is different from the dissolved complexes. The well-defined electrode response and solution accessibility of the anchored complexes provided good electrocatalytic performances toward H2O2 reduction. The results of this study will serve as the basis for future investigation of the incorporation of electroactive complexes on electrode surfaces and electrode modification. Moreover, it has gone some way towards acquiring accurate electrochemical response and enhancing our understanding about the supported complexes and organometallic compounds.
Acknowledgements The authors acknowledge financial support from the Iran National Science Foundation (INSF) and Dr Kelly Sears (McGill University) for his helpful discussions and reading of the manuscript.
Notes and references 1 G. Kickelbick, Hybrid Materials. Synthesis, Characterization, and Applications, ed. G. Kickelbick, Wiley-VCH, Weinheim, 2007.
This journal is © The Royal Society of Chemistry 2014
Paper
2 G. A. Ozin, A. C. Arsenault and L. Cademartiri, Nanochemistry: A Chemical Approach to Nanomaterials, RSC, London, 2009. 3 S. Sayen and A. Walcarius, J. Electroanal. Chem., 2005, 581, 70. 4 F. Hoffmann, M. Cornelius, J. Morell and M. Froba, Angew. Chem., Int. Ed., 2006, 45, 3216. 5 J. A. Bae, K.-C. Song, J.-K. Jeon, Y. S. Ko, Y.-K. Park and J.-H. Yim, Microporous Mesoporous Mater., 2009, 123, 289. 6 S. Carboni, C. Gennari, L. Pignataro and U. Piarulli, Dalton Trans., 2011, 40, 4355; J. Guzman and B. C. Gates, Dalton Trans., 2003, 3303; B. Karimi, A. Zamani, S. Abedi and J. H. Clark, Green Chem., 2009, 11, 109; B. Karimi, A. Zamani and J. H. Clark, Organometallics, 2005, 24, 4695; A. Ghosh and R. Kumar, Microporous Mesoporous Mater., 2005, 87, 33; Y. Luo and J. Lin, Microporous Mesoporous Mater., 2005, 86, 23; M. S. Saraiva, N. L. Dias Filho, C. D. Nunes, P. D. Vaz, T. G. Nunes and M. J. Calhorda, Microporous Mesoporous Mater., 2009, 117, 670; J. H. Clark, D. J. Macquarrie and S. J. Tavener, Dalton Trans., 2006, 4297; N. Linares, A. E. Sepulveda, M. C. Pacheco, J. R. Berenguer, E. Lalinde, C. Najera and J. Garcia-Martinez, New J. Chem., 2011, 35, 225. 7 H.-J. Im, C. E. Barnes, S. Dai and Z. Xue, Microporous Mesoporous Mater., 2004, 70, 57; A. Walcarius, N. Luthi, J.-L. Blin, B.-L. Su and L. Lamberts, Electrochim. Acta, 1999, 44, 4601; G. Grigoropoulou, P. Stathi, M. A. Karakassides, M. Louloudi and Y. Deligiannakis, Colloids Surf., A, 2008, 320, 25; D. Perez-Quintanilla, I. del Hierro, M. Fajardo and I. Sierra, Microporous Mesoporous Mater., 2006, 89, 58. 8 D. Pletcher and C. J. Pickett, The electrochemistry of transition-metal complexes, RSC, London, 1983. 9 W. E. Geiger, Instructional Examples of Electrode Mechanisms in Transition Metal Complexes, in Laboratory Techniques in Electroanalytical Chemistry, ed. P. Kissinger and W. R. Heineman, Marcel-Dekker, NY, 2nd edn, 1996. 10 A. Walcarius, E. Sibottier, M. Etienne and J. Ghanbaja, Nat. Mater., 2007, 6, 602; A. Goux, M. Etienne, E. Aubert, C. Lecomte, J. Ghanbaja and A. Walcarius, Chem. Mater., 2009, 21, 731. 11 J. Vasilevskis and D. C. Olson, Inorg. Chem., 1971, 10, 1228; X. Ji, M. C. Buzzeo, C. E. Banks and R. G. Compton, Electroanalysis, 2006, 18, 44; E. Norkus, A. Vaskelis, A. Griguceviciene, G. Rozovskis, J. Reklaitis and P. Norkus, Transition Met. Chem., 2001, 26, 465; S. Teller, Y. Marcus and Y. TurYan, Polyhedron, 1997, 16, 1047; A. Domenech, F. Torres and J. Alarcon, J. Solid State Chem., 2004, 8, 127. 12 B. E. Douglas, J. Chem. Educ., 1952, 29, 119. 13 A. E. King, Y. Surendranath, N. A. Piro, J. P. Bigi, J. R. Long and C. J. Chang, Chem. Sci., 2013, 4, 1578. 14 C. Comuzzi, A. Melchior, P. Polese, R. Portanova and M. Tolazzi, Inorg. Chem., 2003, 42, 8214; J. F. Diaz, K. J. Balkus Jr., F. Bedioui, V. Kurshev and L. Kevan, Chem. Mater., 1997, 9, 61.
Dalton Trans., 2014, 43, 4901–4908 | 4907
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Paper
15 J. D. Ingle and S. R. Crouch, Spectrochemical Analysis, Prentice-Hall, New Jersey, 1988. 16 J. M. Saveant, Elements of Molecular and Biomolecular Electrochemistry, Wiley-VCH, New Jersey, 2006. 17 W. E. Geiger, in Inorganic Reactions and Methods, ElectronTransfer and Electrochemical Reactions; Photochemical and Other Energized Reactions, ed. J. J. Zuckerman and A. P. Hagen, Wiley VCH, USA, p. 128. 18 M. Tada and Y. Iwasawa, in Modern Surface Organometallic Chemistry, ed. J.-M. Basset, R. Psaro, D. Roberto and R. Ugo, Wiley-VCH, Weinheim, 2009, ch. 10. 19 G. Inzelt, in Encyclopedia of Electrochemistry, Vol. 10 Modified Electrodes, ed. M. Fujihira, I. Rubinstein and J. F. Rusling, Wiley-VCH, Weinheim, 2007, ch. 9;
4908 | Dalton Trans., 2014, 43, 4901–4908
Dalton Transactions
20 21
22 23 24
A. Domenech, Electrochemistry of Porous Materials, Taylor and Francis, Boca Raton, 2010. M. Etienne, A. Goux, E. Sibottier and A. Walcarius, J. Nanosci. Nanotechnol., 2009, 9, 2398. M. Rafiee, B. Karimi, S. Farrokhzadeh and H. Vali, Electrochim. Acta, 2013, 94, 198; M. Rafiee, B. Karimi, Y. Abdossalami Asl and H. Vali, Analyst, 2013, 138, 1740. R. Trujillano, F. Villain, C. Louis and J.-F. Lambert, J. Phys. Chem. C, 2007, 111, 7152. M. Lovric and F. Scholz, J. Solid State Electrochem., 1997, 1, 108. P. Niethammer, C. Grabher, A. T. Look and T. J. Mitchison, Nature, 2009, 459, 996; R. Noyori, M. Aoki and K. Sato, Chem. Commun., 2003, 1977.
This journal is © The Royal Society of Chemistry 2014
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
PAPER
Cite this: Dalton Trans., 2014, 43, 4901
View Journal | View Issue
Ethylenediamine-modified oriented MCM-41 at the electrode surface, cobalt adsorption ability and electrochemical performance† Mohammad Rafiee,*a Babak Karimi,*a Simin Arshia and Hojatollah Valib Mesoporous silica thin films (MCM-41) functionalized with ethylenediamine groups were electrochemically fabricated on electrode surfaces. These ligand functionalized film were a promising matrix for the immobilization of cobalt ions and preparation of cobalt complexes covalently bound to the MCM-41 support. The constructed MCM-41 were characterized by TEM, EDS and TGA analysis. This method yields uniform thin films with hexagonal mesochannels aligned and accessible to electrode surface. Welldefined electrode responses were, therefore, observed for the anchored complexes which made the electrochemical analysis of the structure possible as well. Voltammetric studies revealed the reactivity of
Received 26th August 2013, Accepted 6th December 2013
the covalently bound complexes differed significantly from the dissolved ones. The anchored complexes
DOI: 10.1039/c3dt52343h
preferred to be in their oxidized form which inhibits formation of oxygen adducts. The covalently bound complexes had relatively good leaching stability with good catalytic performance towards hydrogen per-
www.rsc.org/dalton
oxide reduction.
Introduction Organic–inorganic hybrid materials that combine structural properties of a rigid inorganic framework with chemical activity and property of organic moieties have drawn enormous attention in several scientific disciplines during the past two decades.1 The most well-known representative class of these hybrid materials includes ordered mesoporous silica structures.2 Derivatization with organofunctional groups, especially by exploiting the versatility of sol–gel chemistry and reactive OH groups at the surface, provides easy access to organically modified silica-based materials.3 Another more important characteristic of ordered mesoporous structures is their large surface area, around 1000 m2 g−1, owing to the presence of nanometer-sized channels with controllable and narrow size distribution. Owing to their robust porosity and potential for modification, ordered mesoporous structures have emerged as potential supports for the heterogenization of homogeneous substrates and especially for homogeneous catalysts.4 Immobilization of metal complexes is one of the widely studied
a Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Gava-Zang, Zanjan 4513766731, Iran. E-mail: [email protected], [email protected]; Fax: +98-241-4153232 b Department of Anatomy & Cell Biology, McGill University, 3640 University St., Montréal, Quebec H3A 0C7, Canada † Electronic supplementary information (ESI) available: TGA analysis and the analysis of the leaching stability of the electrodes. See DOI: 10.1039/c3dt52343h
This journal is © The Royal Society of Chemistry 2014
examples of heterogenization on mesoporous silica structures. Ligand-grafted mesoporous silicas have attracted much attention as promising supports to enhance the binding affinity of metal ions. The immobilization of metal complexes was often performed through coordination of metal ions with the supported ligands.5 The designed hybrid silica supported organometallics have been used widely in a variety of applications such as heterogeneous catalysts (as the major concern)6 and electrochemistry.7 Moreover, the area of electrochemistry concerned with transition metal complexes has significant contributions in the various electrochemical studies and applications.8 The ability of electron transfer of many organometallic compounds renders them attractive for electrochemical applications. Alternatively, electrochemical methods, especially cyclic voltammetry (CV), have been used to obtain information about their structural features.9 Notwithstanding such significant results on electrochemical investigation and application of supported organometallic compounds there is still scope for the study of these mutual concepts. The objective of the current study is to investigate the electrochemical behaviour of cobaltammine complexes in solution and in the supported form in the interior of ordered mesoporous silica (MCM-41) at the electrode surface. Electro-Assisted Self Assembly (EASA)10 was used to deposit ethylenediamine containing MCM-41 and MCM-41 with accessible channels on the electrode surfaces and to potentially form anchored cobaltammine complexes inside the channels. Cobaltammine complexes have remarkable electron transfer ability11 which may be due to the
Dalton Trans., 2014, 43, 4901–4908 | 4901
View Article Online
Paper
Dalton Transactions
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
stabilization of a number of oxidation states by amine ligands.12 The study of cobalt complexes was carried out as a typical example of first-row transition metals.13 Moreover, cobaltammine complexes are of interest for their reversible adduct formation with molecular oxygen and the importance of their desired adducts.14
Experimental Materials Tetraethoxyorthosilicate 99% (TEOS), cetyltrimethylammonium bromide 98% (CTAB), ethylenediamine (En), potassium nitrate and hydrochloric acid 37% from Merck. Cobaltous nitrate hexahydrate and N-[3-(trimethoxysilyl)propyl]ethylenediamine 97% (EnPTS) were purchased from Sigma-Aldrich and Fluka respectively, all of the materials were used as received. Instrumentation The characterization of the film was performed using the following instruments: NETZSCH STA 409 PC for thermal gravimetric analysis (TGA); Philips CM200 transmission electron microscope (TEM) equipped with a Gatan Ultrascan 1000 Model 895 CCD camera and EDAX Genesis energy-dispersive X-ray spectrometer. All electrochemical measurements were performed at room temperature using an Autolab PGSTAT-101 monitored by the Electrochemical System Software (Eco Chemie). Experiments were carried out in a classical three-electrode undivided cell configuration with a platinum counter electrode and a glassy carbon working electrode (surface area = 3.14 mm2). The potentials were measured versus an Ag/AgCl reference electrode (all of the electrodes were manufactured by AZAR Electrode Company, Urmia, Iran). A Metrohm pH-meter 691 was used for measuring solution pH. To remove oxygen from solution during the experiments, nitrogen gas with a purity of 99.999% was used for 5 minutes. Preparation of mesoporous silica films on electrode Sol–gel solution composition for preparation of En-functionalized mesoporous silica was 15 mL ethanol 7 mL H2O and 8 mL aqueous solution of 0.2 M NaNO3 and 0.2 M HCl to which 10.1 mM (TEOS + EnPTS) and 3.26 mM CTAB were added under stirring. Then the solution pH was adjusted to the value of 3. The sol–gel solution was aged for 2.5 h under stirring before deposition. A cathodic potential of −2.2 V was applied for 20 s then the electrode removed and immediately rinsed with distilled water. Preparation of pure silica film was achieved on the basis of previously reported procedures using only TEOS as silica precursor.10 The electrodeposited surfactant-templated film was then dried overnight in an oven at 130 °C. Extraction of the surfactant template was carried out in acidic ethanol solutions under moderate stirring for 10 min.10 Several aqueous solutions containing the various ratios of Co2+ to En were used for grafting of cobalt onto the film.
4902 | Dalton Trans., 2014, 43, 4901–4908
Fig. 1 CVs of 2.0 mM Co2+ in the presence of various concentrations of En, En concentrations from (a) to (d) are: 2.0, 4.0, 6.0 and 12.0 mM respectively. Supporting electrolyte 0.15 M KNO3, scan rate 100 mV s−1.
Results and discussion Homogeneous study The voltammetric behaviour of cobalt and its complexes was studied in aqueous solution in order to obtain detailed information about the electrochemistry of the cobaltammine complexes and to compare its electrochemical behaviour to the supported ones. Fig. 1 shows the cyclic voltammograms (CVs) of 2.0 mM of Cobalt II (Co2+) in an aqueous 0.15 M KNO3 solution with the continuous addition of En under a nitrogen atmosphere. In the selected range and the whole potential window of the desired aqueous solution no redox peaks were observed for Co2+ in the absence of En. After the addition of En, one anodic peak (A1) and its cathodic counterpart (C1) appeared and their height increased with increasing concentration of En up to 6.0 mM. No significant increase in height of either A1 and C1 peaks were observed in the presence of higher En concentration confirming that the major (only) electroactive complex of Co2+ is likely to be Co(En)32+. The voltammograms also demonstrated the relatively large formation constant between Co2+ ions with En to give Co(En)32+. The formation constant of the electroactive complex (Co(En)32+) were obtained on the basis of the “molar ratio method”.15 One measurement was made with a Co2+ analytical concentration (CM) and a large excess of En to yield the anodic peaks of I0. The amount of excess En was adjusted so that the dissociation of the complex is negligible and the concentration of the Co(En)32+ can be considered as CM. Thereafter, the A1 peak current of a series of solutions with different En-to-Co2+ concentration ratios (constant CM) were measured as In. The ratios of In/I0 yield to equilibrium concentration of electroactive complex and then the equilibrium concentrations of Co2+ and En for each concentration of En. The conditional formation constant of the complexation reaction was calculated as 1.32 × 108 M−1 (considering the equilibrium concentrations of the reaction components) which is in good agreement with the previously reported results.11
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Dalton Transactions
Paper
Scheme 1 Proposed mechanism for the electron transfer of cobaltammine complex and its oxygen adduct. Fig. 2 (I) The positive going scan CVs of Co(En)32+ in the presence of oxygen with time at pH 3.70, time intervals 2 min from (a) to (i), (II) the negative going scan CVs after 30 minutes ( j) first cycle and (k) second cycle, scan rate: 100 mV s−1.
In the presence of oxygen a new irreversible cathodic peak (C2) appeared at more negative potentials and its height increased with time as the currents of A1/C1 redox peak decreased (Fig. 2(I)). After 30 min, the C1 cathodic peak disappeared and the C2 peak reached its maximum value. At the end of the reaction, the negative going scan half-cycles showed only one cathodic peak (C2), whereas in the positive going halfcycle the A1 peak appears again. The multi-cyclic voltammograms of the solution showed the appearance of the C1 peak as the counterpart of A1 at the second cycle (Fig. 2(II)). It should be noted that no redox peak was detected when the less negative switching potential (−0.6 V vs. Ag/AgCl) was employed. Fig. 1 and 2 clearly demonstrate adduct formation between the cobaltammine complex and molecular oxygen as reported previously in the literature.14 The appearance of the reductive peak (C2) in the negative going scan demonstrated the cobalt nucleolus of this adduct has a higher oxidative state (Co3+). This is likely due to the charge transfer causing oxidation of Co2+ with concomitant reduction of dioxygen during the adduct formation. This voltammetric behaviour also demonstrates that the adduct, in which both cobalt and the reduced form of oxygen are in their reductive forms (Co2+ and peroxo or superoxo),14 is not stable and undergoes a decomposition reaction (Scheme 1, eqn (4)) to form the initial cobaltammine complex and the reduced form of oxygen. The voltammetric data confirms the release of the reduced form of oxygen (likely O22−) is electroactive and also undergoes electron transfer (Scheme 1, eqn (5)). Both the voltammetric data at various scan rates and the hydrodynamic voltammetric data (RDE) at various rotation rates are in good agreement with the evidences of an ECE mechanism.16 The E and C nomenclature are used for electron transfer and chemical reaction, respectively.17 The following mechanism is proposed for the complex formation and electron transfer of cobaltammine complex in the presence of O2. The formation of cobaltammine complexes and redox activity of these complexes encouraged us to further investigate
This journal is © The Royal Society of Chemistry 2014
the possibility of immobilizing En followed by complexation with Co2+ and the electron transfer of the complex. Electrochemical deposition of En-functionalized MCM-41 There are many published articles on the anchoring of metalcomplexes onto oxides (from macroporous materials to zeolitic structures) and their catalytic applications.18 However, key issues with many of the electrochemical studies and applications are the insulating character of some of the oxides and their particulate structure. Complicated electron transfer, limitation of mass and/or coupled charge transfer through these structures made their electrochemical applications difficult.19 Even there is a claim saying that electron transfer of the anchored electroactive groups inside the channels is not possible for insulating supports such as silica.14 Recently EASA was proposed as a method to produce mesoporous silica thin films on electrode surfaces with hexagonal mesochannels perpendicular to the electrode surface to provide good accessibility and mass transport capabilities.10 This method has been developed for one-pot synthesis of amine functionalized films with a morphology that induced metal ion (Cu2+) absorptivity on the film and its related modified electrode.20 We have also demonstrated that the anchored electroactive group on the oriented mesochannels have a simpler electron and mass transfer regime.21 Considering the above discussion, we reasoned that functionalization of such unique structures with appropriate ligand moiety might offer versatile mesoporous structures for the electrochemical study and application of supported electroactive complexes. Preparation of En-functionalized thin films was achieved by employing EASA, under potentiostatic conditions (−2.2 V for 20 s), on Glassy Carbon (GC) electrodes in the presence of TEOS and EnPTS as a silica precursor and CTAB as a structure-directing agent. Film permeability and surface coverage was examined by voltammetry with Fe(CN)63− as an electroactive probe. The voltammetric currents were negligible for the electrode with as-synthesised thin film on its surface (Fig. 3, curve a) indicating the crack-free structure of the film. After extraction of CTAB (Fig. 3, curve b), the voltammetric current of the modified electrode with the En-functionalized silica structure
Dalton Trans., 2014, 43, 4901–4908 | 4903
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Paper
Fig. 3 CVs of 1.0 mM Fe(CN)63− on EnSE (a) before and (b) after extraction of surfactant, (c) on bare electrode and (d) on unfunctionalized silica film modified electrode; supporting electrolyte 0.15 M KNO3, scan rate: 100 mV s−1.
(denoted as EnSE) suggested a porous structure with good electrode accessibility. The voltammetric currents of Fe(CN)63− on EnSE were even higher than that of the bare electrode (Fig. 3, curve c), whereas the voltammetric currents of Fe(CN)63− on the modified electrode with pure silica film (SiE, Fig. 3, curve d) with essentially the same mesoporous structure was significantly less than the bare electrode. The difference in electrochemical behaviour of the modified electrodes for this charge species is evidence for the incorporation of positively charged protonated En groups in the structure of EnSE. Likewise, the thermogravimetric analysis (TGA) confirms the incorporation of En groups as an organic moiety in the structure of the films (Fig. S1†). After screening and examination of several EnTS to TEOS ratios, the maximum loading of functional En groups in the structure was attained using 8% EnPTS–TEOS in the sol solution. Attempts to prepare a thin silica film with higher En loading resulted in disordered porous films, which confirms previous reports for the amine containing silica precursor.20,21 However, this loading, together with the well-known chelating ability of En, holds promise for supporting complexes and further applications. In the next stage, EnSE was used for the formation of supported cobaltammine complexes and thus the prepared EnSE was immersed in a solution containing Co2+ (in all of the experiments the solutions contained 0.15 M KNO3 as supporting electrolyte). As clearly shown in Fig. 4(I) EnSE did not show much electrochemical response to the Co2+ solution (curve a). This observation may be due either to inefficient complexation of Co2+ with EnSE or the electro-inactive character of the Co2+– EnSE species. To further clarify this issue, a similar experiment was performed in the presence of 1.0 equivalent of dissolved En under otherwise the same experimental conditions on the basis that En might enhance the formation of the cobaltammine complex. However, the electrochemical behaviour of this modified electrode was essentially the same as that obtained
4904 | Dalton Trans., 2014, 43, 4901–4908
Dalton Transactions
Fig. 4 (I) CVs of EnSE in (a) solution containing 2.0 mM Co2+ and (b) solution containing 2.0 mM Co2+ and 2.0 mM En; (II) the CV of EnSE after impregnation with Co2+ in solution containing 2.0 mM of En (c) at initial time, (d) positive going scan and (e) negative going scan after 5 minutes; supporting electrolyte 0.15 M KNO3, scan rate: 100 mV s−1.
in the absence of added En (Fig. 4(I), curve b). This observation highlights our opinion that the in situ addition of an equivalent amount of En before the formation of Co2+–EnSE species would not improve the electrochemical performance of the modified electrode. Interestingly, by removing the electrode (from the above mentioned solution), washing with distilled water and immersing in a solution containing only En, the electrodes exhibited one pronounced anodic peak (A′1) accompanied with its cathodic counterpart (C′1). The midpoint potential of these redox peaks were in good agreement with the dissolved cobaltammine complex (Co(ED)32+). In addition, the peak currents did not decrease with time, but rather showed a considerable increase at 15 min (Fig. 4(II)). It can be seen from curve d in Fig. 4(II) that in the positive going scan CVs, the cathodic current appeared at more negative potentials than the midpoint of the cobaltammine complex (Emid1) and reached its maximum value after only 5 minutes. The negative going scan CVs (Fig. 4(II), curve e) did not show any oxidative current at the starting potentials (the more positive values than Emid1). This voltammetric data demonstrates that the anchored cobalt nucleus converted to a higher oxidation state (Co3+) over a relatively short time period. These results suggest that the preferred stabilization of one oxidative state (here the oxidized form) occurs via heterogenization. Voltammetric analysis in the presence of air indicated that the anchored complex has the same electrochemical behaviour in both the absence and presence of dissolved O2 and adduct formation with molecular oxygen was not observed. This behaviour might be explained by considering that oxidation of anchored cobaltammines is faster than adduct formation with O2. An alternate explanation is that the oxidized form of the cobaltammine species has little or no affinity for dissolved molecular oxygen.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Dalton Transactions
Paper
Fig. 6 The CVs of CASE in (I) solution containing 0.15 M KNO3 and (II) solution containing 0.15 M KNO3 and 0.2 M En, the CVs from (a) to (d) recorded at 1, 10, 30 and 60 min respectively, scan rate: 100 mV s−1.
Fig. 5 (a) EDS analysis of the scratched film from EnSE after impregnation with Co2+ and En; (b) and (c) top view TEM images of the film at different magnification.
Another extrapolation that can be made from the abovementioned observations is that EnSE displays a relatively high affinity for adsorption of dissolved Co2+ ions. As is shown in Fig. 5, the EDS analyses of the scratched film clearly demonstrated the presence of cobalt in the film. TEM images of the film also showed the uniform thickness of the film and hexagonal pattern of the channels. After impregnation with Co2+ and En and washing with deionized water, the film still retained its original ordered porous structure despite the relatively alkaline basic nature of the En solutions. Considering the well-defined electrochemical response of anchored cobaltammine complexes, the leaching stability of the electrodes was further studied by voltammetry. The CVs of EnSE after impregnation with Co2+ and En, i.e. functionalization with the cobaltammine complex (denoted as CASE) were recorded over time in a blank solution containing only 0.15 M of KNO3 as the supporting electrolyte. The signals of CASE decreased at 30 and 60 min to less than 40% and 15% of the initial values, respectively (Fig. 6(I)). More detailed studies showed that potential cycling (the number of CVs that have been used for monitoring the loading over time) accelerated the leaching of the complexes. Moreover, for the CVs of CASE in the KNO3 solution, the ratio of anodic-to-cathodic peak currents (A′1/C′1) was considerably less than unity (Fig. 6(I), curves b and c). It can, therefore, be concluded that in neutral aqueous solutions, and partially in the absence of En, the reduced form of the anchored cobaltammine complex was not stable owing to its low formation constant.11 Therefore, the reduction of anchored cobaltammine complexes resulted in decomposition and faster leaching. In the next stage, the leaching stability of CASE was examined in En containing
This journal is © The Royal Society of Chemistry 2014
solutions where there was a significant enhancement in the stability of CASE. When the concentration of En was optimized for maximum stability, the leaching of cobaltammine complexes in their reduced form minimizes En concentration within 1.0 to 10.0 mM. Results obtained from the voltammetric analysis have been compared in Fig. 6. As is shown in Fig. 6(II), the currents of the CASE electrode in the 0.1 to 10 mM En solutions decreased by less than 5% of the initial value after 1 hour and thereafter a slight and continuous decrease was observed. Fig. 6(II) shows that at longer times, the CVs of CASE electrodes exhibited an additional cathodic shoulder (at more negative potential than C′1). Although the appearance of this shoulder was not completely reproducible, its height was independent of the presence of O2 and inversely proportional to En concentration. The most probable mechanism for the appearance of this peak is the interaction of the hydroxyl groups in the silica channels with the anchored cobaltammine complexes.22 After analysing the above results the following mechanism (Scheme 2) is proposed for the functionalization of the EnSE with the cobaltammine complexes. It is also encouraging to compare the possibility of the absorption of dissolved Co2+ and cobaltammine complex (Co(ED)32+) on parent silica film (SiE) with the above proposed pattern. With the same justification for EnSE, the SiE did not show any voltammetric response in the Co2+ solution. By removing the electrode and immersing it in an En solution, the electrode showed a considerable current, but decreased significantly and reached background currents in less than 15 min. The adsorption of the cobaltammine complexes on SiE was also investigated by immersing the electrode in a cobaltammine solution (containing 1 : 3 Co2+ to En) for 20 min. This electrode (CASiE) showed voltammetric responses comparable to CASE, but leaching of CASiE increased considerably more than CASE. The electrode current attained 40% of the initial value in the En solution within 30 min and background after 80 min (Fig. S2 and S3†). It is also worth noting that CASiE did
Dalton Trans., 2014, 43, 4901–4908 | 4905
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Paper
Scheme 2
Dalton Transactions
Proposed mechanism of the construction of EnSE and formation of cobaltammine complex on its surface.
not exhibit any significant current after the first CV cycle in the solution containing only KNO3 (and in the absence of En). Electrochemical and catalytic performances Voltammetric analysis, at various scan rates, was also used to acquire qualitative information on the mechanism of electron transfer through the channels of CASE. As shown in Fig. 7, the voltammetric currents increased by increasing the scan rate of CVs. Plotting of peak currents versus scan rate did not show a linear trend. Plotting of the peak currents versus square root of scan rate (v1/2) showed a positive deviation. Fig. 7(II) shows that the normalized CVs did not superimpose. Normalized CVs were obtained by dividing the current of the CVs by the square root of the scan rate. For diffusion-controlled reactions, the peak current is proportional to v1/2, whereas in the adsorption-controlled reaction it is proportional to v. Hence, it is suggested that on the basis of the simplified model proposed by Scholz–Lovric there is an intermediate mechanism for the electron transfer through insulating porous solid supports.21,23
The diffusion-like behaviour of voltammograms is related to the diffusion of electrons through the film. The relatively high concentrations of anchored electroactive groups made electron percolation through the film easier and dictate an adsorption-controlled behaviour to the system especially at very low scan rates. The loading of anchored complexes was estimated after determination of the Faradaic charge transfer through the electrode in a voltammetric experiment that was recorded at a low scan rate (for example the scan rates less than 50 mV s−1, Fig. 7, curve a).21 The results indicated a loading of 8.6 × 10−9 mol cm−2 (mol per cm2 for geometric area of electrode) in the film. Finally, the catalytic performance of the CASE electrode was investigated for the electrochemical reduction of H2O2. H2O2 is a well-known oxidizing agent and as an electron mediator has a relatively slow rate of electron transfer in both chemical and electrochemical systems. H2O2 is not only an essential mediator for environmental, pharmaceutical and industrial applications, but is also a by-product of several important reactions during oxygen reduction.24 Fig. 8 shows the increase in height of C′1, the disappearance of the A′1, and potential shifts to more negative values in the presence of H2O2 which are in good agreement with the diagnostic criteria of the mediated reduction of H2O2 at CASE.16 It is also apparent from this figure that the direct electron transfer of H2O2 was accelerated at the surface of CASE so that the reduction currents of H2O2 (C3) increased considerably at the surface of CASE (comparing its current at the surface of EnSE). As curve e in Fig. 8 demonstrates, considering the amperometric measurements, CASE shows a short response time (within 5 s), high sensitivity and good stability for H2O2 reduction.
Conclusions Fig. 7 (I) CVs and (II) normalized CVs of CASE in solution containing 0.15 M KNO3 and 0.2 M En at various scan rates; scan rates from (a) to (e) are 50, 100, 200, 500 and 1000 mV s−1 respectively.
4906 | Dalton Trans., 2014, 43, 4901–4908
The objective of this study was to extend and evaluate the application of EASA for fabricating porous silica films at an
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Dalton Transactions
Fig. 8 CVs of (a) bare GC electrode, (b) EnSE and (c) CASE in 1.0 mM solution of H2O2 (d) CV of CASE in the absence of H2O2; scan rate 250 mV s−1; (e) amperometric response at a rotating modified CASE (the rotation speed is 500 rpm) held at −800 mV for successive addition of 500 µL H2O2 (5 mM) to 50 mL 0.15 M KNO3 and 2.0 mM En.
electrode surface. En functionalized MCM-41, with its unique morphology, aligned channels and uniform thickness, has been constructed at an electrode surface. The En containing MCM-41 acts as an appropriate matrix for the immobilization of Co2+. The most interesting finding was that adsorbed Co2+ did not leach considerably in the En solution, but rather increased the adsorption of En ligands. Adsorption of additional En ligands yields the formation of electroactive cobaltammine complexes. Uniform thickness of the film and well-defined texture provides excellent electrochemical response and made the voltammetric study of the complexes possible. Voltammetric data show that the behaviour of anchored cobaltammine complexes is different from the dissolved complexes. The well-defined electrode response and solution accessibility of the anchored complexes provided good electrocatalytic performances toward H2O2 reduction. The results of this study will serve as the basis for future investigation of the incorporation of electroactive complexes on electrode surfaces and electrode modification. Moreover, it has gone some way towards acquiring accurate electrochemical response and enhancing our understanding about the supported complexes and organometallic compounds.
Acknowledgements The authors acknowledge financial support from the Iran National Science Foundation (INSF) and Dr Kelly Sears (McGill University) for his helpful discussions and reading of the manuscript.
Notes and references 1 G. Kickelbick, Hybrid Materials. Synthesis, Characterization, and Applications, ed. G. Kickelbick, Wiley-VCH, Weinheim, 2007.
This journal is © The Royal Society of Chemistry 2014
Paper
2 G. A. Ozin, A. C. Arsenault and L. Cademartiri, Nanochemistry: A Chemical Approach to Nanomaterials, RSC, London, 2009. 3 S. Sayen and A. Walcarius, J. Electroanal. Chem., 2005, 581, 70. 4 F. Hoffmann, M. Cornelius, J. Morell and M. Froba, Angew. Chem., Int. Ed., 2006, 45, 3216. 5 J. A. Bae, K.-C. Song, J.-K. Jeon, Y. S. Ko, Y.-K. Park and J.-H. Yim, Microporous Mesoporous Mater., 2009, 123, 289. 6 S. Carboni, C. Gennari, L. Pignataro and U. Piarulli, Dalton Trans., 2011, 40, 4355; J. Guzman and B. C. Gates, Dalton Trans., 2003, 3303; B. Karimi, A. Zamani, S. Abedi and J. H. Clark, Green Chem., 2009, 11, 109; B. Karimi, A. Zamani and J. H. Clark, Organometallics, 2005, 24, 4695; A. Ghosh and R. Kumar, Microporous Mesoporous Mater., 2005, 87, 33; Y. Luo and J. Lin, Microporous Mesoporous Mater., 2005, 86, 23; M. S. Saraiva, N. L. Dias Filho, C. D. Nunes, P. D. Vaz, T. G. Nunes and M. J. Calhorda, Microporous Mesoporous Mater., 2009, 117, 670; J. H. Clark, D. J. Macquarrie and S. J. Tavener, Dalton Trans., 2006, 4297; N. Linares, A. E. Sepulveda, M. C. Pacheco, J. R. Berenguer, E. Lalinde, C. Najera and J. Garcia-Martinez, New J. Chem., 2011, 35, 225. 7 H.-J. Im, C. E. Barnes, S. Dai and Z. Xue, Microporous Mesoporous Mater., 2004, 70, 57; A. Walcarius, N. Luthi, J.-L. Blin, B.-L. Su and L. Lamberts, Electrochim. Acta, 1999, 44, 4601; G. Grigoropoulou, P. Stathi, M. A. Karakassides, M. Louloudi and Y. Deligiannakis, Colloids Surf., A, 2008, 320, 25; D. Perez-Quintanilla, I. del Hierro, M. Fajardo and I. Sierra, Microporous Mesoporous Mater., 2006, 89, 58. 8 D. Pletcher and C. J. Pickett, The electrochemistry of transition-metal complexes, RSC, London, 1983. 9 W. E. Geiger, Instructional Examples of Electrode Mechanisms in Transition Metal Complexes, in Laboratory Techniques in Electroanalytical Chemistry, ed. P. Kissinger and W. R. Heineman, Marcel-Dekker, NY, 2nd edn, 1996. 10 A. Walcarius, E. Sibottier, M. Etienne and J. Ghanbaja, Nat. Mater., 2007, 6, 602; A. Goux, M. Etienne, E. Aubert, C. Lecomte, J. Ghanbaja and A. Walcarius, Chem. Mater., 2009, 21, 731. 11 J. Vasilevskis and D. C. Olson, Inorg. Chem., 1971, 10, 1228; X. Ji, M. C. Buzzeo, C. E. Banks and R. G. Compton, Electroanalysis, 2006, 18, 44; E. Norkus, A. Vaskelis, A. Griguceviciene, G. Rozovskis, J. Reklaitis and P. Norkus, Transition Met. Chem., 2001, 26, 465; S. Teller, Y. Marcus and Y. TurYan, Polyhedron, 1997, 16, 1047; A. Domenech, F. Torres and J. Alarcon, J. Solid State Chem., 2004, 8, 127. 12 B. E. Douglas, J. Chem. Educ., 1952, 29, 119. 13 A. E. King, Y. Surendranath, N. A. Piro, J. P. Bigi, J. R. Long and C. J. Chang, Chem. Sci., 2013, 4, 1578. 14 C. Comuzzi, A. Melchior, P. Polese, R. Portanova and M. Tolazzi, Inorg. Chem., 2003, 42, 8214; J. F. Diaz, K. J. Balkus Jr., F. Bedioui, V. Kurshev and L. Kevan, Chem. Mater., 1997, 9, 61.
Dalton Trans., 2014, 43, 4901–4908 | 4907
View Article Online
Published on 09 December 2013. Downloaded by Carleton University on 24/06/2014 13:39:30.
Paper
15 J. D. Ingle and S. R. Crouch, Spectrochemical Analysis, Prentice-Hall, New Jersey, 1988. 16 J. M. Saveant, Elements of Molecular and Biomolecular Electrochemistry, Wiley-VCH, New Jersey, 2006. 17 W. E. Geiger, in Inorganic Reactions and Methods, ElectronTransfer and Electrochemical Reactions; Photochemical and Other Energized Reactions, ed. J. J. Zuckerman and A. P. Hagen, Wiley VCH, USA, p. 128. 18 M. Tada and Y. Iwasawa, in Modern Surface Organometallic Chemistry, ed. J.-M. Basset, R. Psaro, D. Roberto and R. Ugo, Wiley-VCH, Weinheim, 2009, ch. 10. 19 G. Inzelt, in Encyclopedia of Electrochemistry, Vol. 10 Modified Electrodes, ed. M. Fujihira, I. Rubinstein and J. F. Rusling, Wiley-VCH, Weinheim, 2007, ch. 9;
4908 | Dalton Trans., 2014, 43, 4901–4908
Dalton Transactions
20 21
22 23 24
A. Domenech, Electrochemistry of Porous Materials, Taylor and Francis, Boca Raton, 2010. M. Etienne, A. Goux, E. Sibottier and A. Walcarius, J. Nanosci. Nanotechnol., 2009, 9, 2398. M. Rafiee, B. Karimi, S. Farrokhzadeh and H. Vali, Electrochim. Acta, 2013, 94, 198; M. Rafiee, B. Karimi, Y. Abdossalami Asl and H. Vali, Analyst, 2013, 138, 1740. R. Trujillano, F. Villain, C. Louis and J.-F. Lambert, J. Phys. Chem. C, 2007, 111, 7152. M. Lovric and F. Scholz, J. Solid State Electrochem., 1997, 1, 108. P. Niethammer, C. Grabher, A. T. Look and T. J. Mitchison, Nature, 2009, 459, 996; R. Noyori, M. Aoki and K. Sato, Chem. Commun., 2003, 1977.
This journal is © The Royal Society of Chemistry 2014
