Fabrication and Characterization of Surfaces Modified with

Article

Fabrication and Characterization of Surfaces Modified with Carboxymethylthio Ligands for Chelate Assisted Trapping of Copper John Onyango Adongo, Tilmann Neubert, Guoguang Sun, Silvia Janietz, Iver Lauermann, Klaus Rademann, and Joerg Rappich ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05131 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 20, 2017

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ACS Applied Materials & Interfaces

Fabrication and Characterization of Surfaces Modified with Carboxymethylthio Ligands for Chelate Assisted Trapping of Copper John O. Adongo1*, Tilmann J. Neubert1, Guoguang Sun2, Silvia Janietz3, Iver Lauermann,4 Klaus Rademann5 and Jörg Rappich1

1.

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Si-Photovoltaik, Kekuléstrasse. 5, 12489 Berlin, Germany.

2.

Leibnitz-Institut für Analytische Wissenschaften-ISAS-e.V., Berlin, Schwarzschildstr. 8, 12489 Berlin, Germany.

3.

Fraunhofer-Institut für Angewandte Polymerforschung, Geiselbergstr. 69, 14476 PotsdamGolm, Germany.

4.

Helmholtz-Zentrum Berlin, PVcomB, Schwarzschildstr. 3, 12489 Berlin, Germany.

5.

Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Strasse. 2, 12489 Berlin, Germany.

KEYWORDS: Electrografting, Diazonium, Chelating, Copper, IRSE, EQCM, Raman, XPS

ACS Paragon Plus Environment

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ABSTRACT: Metal ion chelating property was conferred onto silicon (Si) and gold (Au) surfaces by direct electrografting of 4-[(carboxymethyl)thio]benzenediazonium cation (4CMTBD). Infrared spectroscopic ellipsometry (IRSE) showed presence of characteristic phenyl and carbonyl vibrational bands on the functionalized surfaces as a proof of existence of surface bound organic units of 4-[(carboxymethyl)thio]benzene (4-CMTB). The loss of diazonium group (N≡N+) upon electrografting of 4-CMTBD was investigated using IR spectroscopy. A faradaic efficiency of about 18.8 – 20.0 % was realized in mass deposition experiments for grafting 4CMTB on Au surface using electrochemical quartz crystal microbalance (EQCM) technique. Raman spectroscopy performed on the Si-(4-CMTB) surface after treatment with copper (Cu) ion solution provided evidence of metal ion chelation based on an observed v(Cu-O) peak at about 487 cm-1 and a v(Cu-S) signal at about 267 cm-1. The binding of Cu ions by the chelating ligands also caused a red-shift of about 10 cm-1 in the Raman spectrum of Si-(4-CMTB)-Cu surface within the spectral region characteristic of v(C-O) signal. X-ray photoelectron spectroscopy (XPS) investigations showed indications of Cu(II) ion species chelated by the surface bound carboxymethylthio ligands. The functionalized surface Si-(4-CMTB) constitutes an alternative metal ion chelating surface that may potentially be developed for applications in trace level trapping of Cu ions.

1.

Introduction

Materials that incorporate organic ligands to exploit reversible affinity or binding interactions have emerged as promising candidates for metal ion sequestration or recognition.1,2 Organic chelating ligands are particularly known to bind metal ions in solution more rapidly compared to non-chelating ligands due to the chelate effect. This has fostered the exploration of pre-organized metal ion coordinating ligands for a wide range of applications.3 These include bio-imaging


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diagnostics, bio-medical/biological sciences, metal ion separation for environmental protection, radioactive waste recovery and catalysis research.4-11 The selective binding of charged analyte species to receptor elements on planar semiconducting surfaces is a phenomenon that has fueled research in the development of sensing systems that combine semiconductor electronics with ligand-analyte interactions.12-19 Copper is an essential metal in biological systems, however, at concentrations beyond threshold limits, not only can it decimate aquatic life forms but also become highly toxic to humans when present in drinking water.20,21 The heavy metal is capable of binding onto certain organic ligands via coordination mechanisms. Electrochemical grafting of aryl diazonium derivatives have successfully been used to introduce robust layers of various organic functional groups onto metallic and semiconducting surfaces.22 A range of interesting approaches for immobilizing heavy metals ions (HMI’s) on a variety of organically modified substrate materials have received tremendous attention in recent years. Strategies involving functionalization of substrates with large molecular weight oligomers and peptides via diazonium grafting routes for sensing and/or extraction of HMI pollutants have been reported.23–28 Some of these methods involve introduction of chelating groups in more than a single step. However, a relatively simpler onestep quick grafting of low molecular weight chelating molecule may not only present some cost reduction advantages towards devising kits for trace level HMI extraction but can also enable the fabrication of relatively thinner layers with optimal surface grafting, excellent chelation efficiency and increased surface sensitivity for sensing applications. Silicon is one of the most abundant materials on the earth’s crust and its suitable surface chemistry has motivated organic functionalization efforts towards developing wide range of applications.


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The scope of this study explores a one-step functionalization strategy for introducing carboxymethylthio (CMT) chelating groups via direct electrografting of the diazonium cation 4CMTBD onto Si and Au surfaces. The covalent binding of Cu ions via chelation by CMT ligands on the modified Si surface is also qualitatively investigated and characterized. The molecule 4CMTB contains sulphur and oxygen donor atoms in a typical bidentate chelating ligand orientation suitable for metal ion binding (see figure 1). The findings of this study raise the possibility of such modified surfaces playing a role in developing a variety of important applications for rapid trace level recognition, uptake and extraction copper from aqueous solutions.

Figure 1. Idealized schematic for (i) surface functionalization via cathodic electroreduction of 4CMTBD and (ii) copper ion chelation by carboxymethylthio (CMT) ligands. 2.

Materials and methods

2.1

Substrate pre-treatment

Glass substrate with 200 nm Au layer were used as the working electrode for functionalization after cleaning by performing five initial potential cycles in 0.1 M H2SO4 electrolyte solution. A three-electrode assembly reaction cell with clean Au wire as the counter electrode and the Ag/AgCl (saturated KCl) reference electrode was used in the electrografting procedure. For EQCM deposition experiments, AT-cut Au-QCM crystals (5 MHz, Fil-Tech Inc.) were used as


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received from the manufacturer. The recorded frequency change, ∆, was converted to corresponding mass change, ∆, using the Sauerbrey-equation.29 ∆

∆

= −

------ equation (1)

where A = Area in cm2, 1/Cf´ = 17.668 ng/Hz. The faradaic efficiency for the electrografting process was determined by dividing the actual measured mass deposited, ∆mEQCM, by the theoretically calculated mass based on charge flow, ∆mmax. =

∆() ∆()

=

∆() (.) (.)

------ equation (2)

where M is the molar mass of the monomer, z is the number of transferred electrons, F = Faraday’s constant and Q represents the quantitative electrical charge thereof as described in reference.30 A wet chemical etching procedure was employed to prepare the reactive atomically flat hydrogen terminated Si surface Si(111)-H. Silicon wafers (with oxide films ~180 nm) were cut and immersed in 5% HF solution to remove the surface oxide layer. This Si wafer was then reoxidized in ozone for 15 min. Finally, the sample was dipped in 40% NH4F for about 4 minutes. A characteristic sharp narrow IR absorption peak at 2083 cm-1 observed in its IRSE spectrum confirmed the presence of the reactive Si–H surface bonds. The prepared Si substrate was immediately used in cathodic electrografting procedure. 2.2

Surface functionalization and characterization

In all the electrografting reactions, anhydrous acetonitrile (99.8% - Sigma-Aldrich) was used as the solvent. The conducting salt was 0.1 M tetrabutylammonium tetrafluoroborate TBABF4,


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(electrochemical

grade,

99.0%,

Sigma-Aldrich).

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The

salt

4-

[(carboxymethyl)thio]benzenediazonium tetrafluoroborate, as the source of 4-(CMTBD), was prepared as per procedures in previous reports.31 The presence of the diazonium group (N≡N+) confirmed by the characteristic sharp IR absorption peak at around 2256 cm-1. A potentiostat was used as the current source in a three-electrode assembly to perform cathodic electrochemical reduction leading to electrografting via cyclic voltammetry at a scan rate of 100 mV/s. The corresponding chronoamperometric deposition experiments on the respective substrates were also conducted by applying appropriate constant voltages in stepwise manner while monitoring the current change over time. An IRSE spectrometer (TENSOR 37, Bruker, Germany), fitted with a mercury – cadmium – telluride (MCT) detector and a Bruker 55 Fourier-transform Infrared (FTIR) was used to investigate the presence of characteristic vibrational bands of grafted 4-CMTB units on the surfaces upon loss of the diazonium groups during electroreduction reaction. The functionalized Si(111) and Au surfaces were labeled: Si-(4-CMTB) and Au-(4-CMTB) respectively. 2.3

Copper ion treatment / complexation

A copper ion solution (965 ppm) was prepared by dilution of weighed anhydrous copper(II)chloride (CuCl2, 99.0 %, Sigma-Aldrich) in hot water (~ 55 oC). The functionalized surface Si-(4-CMTB) was then treated with this solution by shaking using orbital shaker (Neolab, GmbH; DOS-20S) at 200 revolutions per minute for 5 minutes. The reaction chamber was then sealed with a perforated transparent parafilm and left to soak for about 48 hours. The Cu ion solution was later carefully drained off and the treated surface Si-(4-CMTB)-Cu cleaned in ultrasonic bath for 5 minutes, after which the surface was dried with a stream of nitrogen gas.


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2.4

Surface characterization after copper ion treatment

Raman spectrometer fitted with a charge-coupled device (CCD) detector was used to acquire spectra of the Si-H, Si-(4-CMTB) and Si-(4-CMTB)-Cu surfaces in order to investigate surface functionalization and Cu binding. A laser excitation source of 632.8 nm with an incident power of 60 µW.µm-2 was used. A Raman spectrum of hydrated CuCl2 samples was also obtained for comparison purposes. X-Ray photoelectron spectroscopy (source: Mg Kα 1253.6 eV) was used to experimentally derive overview spectra of Si-H, Si-(4-CMTB) and Si-(4-CMTB)-Cu surfaces. Further detailed core level C1s, S2p, Cu2p and Si2p signals were obtained and analyzed for surface characterization. The raw signals were fitted using voigt functions. The Si2p signal was studied in signal attenuation experiments to investigate peak intensity variation among Si-H, Si-(4-CMTB) and Si-(4-CMTB)-Cu surfaces. 3.

Results and discussion

3.1. Cyclic voltammetry (CV) and Chronoamperometry (CA) The CV’s and corresponding CA’s obtained for the electrografting of 4-CMTBD on Si(111)-H and Au surfaces are shown in figures 2(a – d). Electrochemical cathodic reduction of the diazonium cations on both Si and Au surfaces is observed by the characteristic first broad reduction cathodic waves as illustrated in the CV graphs in figures 2a and 2b respectively. The disappearance of the reduction wave in the subsequent cycles is due to blockage of electron transfer resulting from the modification of the substrate’s electrode surface by the electrografted units of 4-CMTB. Figures 2c and 2d shows the CA graphs that reveal characteristic sharp drops in currents indicating rapid electroreduction of diazonium cations 4-CMTBD, which is followed by steep decay in currents that represents subsequent fast blocking of the electrode surface due to


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surface modification via electrografting of 4-CMTB units on both the Si and Au surfaces respectively.

Figure 2. (a,b) Cyclic voltammograms of 5 mM 4-CMTBD electroreduction on Si(111)-H and Au surfaces in ACN + 0.1 M TBABF4 (scan rate = 100 mV/s) and (c,d) the corresponding chronoamperometric curves respectively. On the Si(111)-H surface, the reduction of 4-CMTBD begins at about +0.24 V, reaching a peak at about -0.10 V (see figure 2a). A solution containing ACN + 0.1 M TBABF4 with no diazonium salt labeled ‘blank’ shows no reduction peak in the CV on Si (see the hatched green line figure 2a). This confirms that the observed reduction peak is due to the reduction of 4-CMTBD species which subsequently modifies the surface as illustrated in figure 1(i). On Au surface, the reduction of 4-CMTBD molecules starts at an onset potential of about +0.42 V and reaches a peak at about at +0.14 V on during cathodic scan. The ‘blank’ solution shows no reduction peak


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at +0.14 V providing a confirmation that the observed reduction in the first cathodic wave for the solution containing 4-CMTB molecules relates to electroreduction of 4-CMTBD on Au surface that leads to grafting (see figure 2b). The conventionally accepted aryldiazonium electrografting mechanism on Si(111)-H surface involves silyl and aryl diazonium radical coupling and can also lead to formation of closely packed surface bound phenyl layers.32-35 The existence of Au-Carbon bond resulting from grafting of diazonium salts has previously been elucidated using Surface Enhanced Raman Scattering (SERS) technique.36 Surface functionalization via electrografting route has a significant advantage of producing more robust grafted layers in a quicker manner as opposed to other non-electrochemically initiated methods that generally require exceedingly longer reaction times. 3.2. EQCM measurements and Faradaic efficiency on Au surface As depicted in the EQCM measurement corresponding mass change during electrodeposition of 4-CMTBD as calculated from the measured frequency change using equation 2 increases rapidly in the first cycle (see figure 3a). The change in mass per unit area ∆m* = ∆m/A (see equation 1) in subsequent cycles gradually becomes much smaller. The mass gain per cycle decreases in proportion to resultant current flow in the subsequent scans. This shows passivation of electrode surface due to grafting of the 4-CMTB molecules generated during the cathodic electrochemical reduction process. The CV recorded for the electroreduction of 4-CMTBD on Au surface is reproducible in EQCM measurements (see figure 3b). A plot of charge flow in relation to the corresponding increases in deposited mass for each of the first four cycles is shown in figure 3c.


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Figure 3. (a) Cyclic massogram and (b) voltammogram of electroreduction of 5 mM 4-CMTBD in ACN + 0.1 M TBABF4 on Au-QCM-chip at a scan rate of 100 mV/s. (c) A plot of deposited mass per unit area (∆mEQCM) versus respective cumulative electrical charge Q for the first 4 CV cycles during electrodeposition of 4-CMTB. (d) A plot of computed faradaic efficiency, ηF, and (e) A plot of deposited mass per area, ∆mEQCM, during electrodeposition of 4-CMTB for the first 6 CV cycles Au-QCM-chip. The greatest increase in mass deposition occurs during the first cycle as the greatest charge transfer occurs therein. However, charge transfer per cycle diminishes in successive scans. This behavior indicates that most of the diazonium cations 4-CMTBD are reduced and subsequently deposited within the first cycle. The first grafted layer(s) subsequently impede electron transfer


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in subsequent scans and hence increasingly inhibit the charge transfers. This corresponds to reduction in deposited mass recorded in subsequent cycles (see figure 3c). However, the respective overall gradients for mass changes versus charge in each of the cycles remain almost the same. This is an indication that the deposition efficiencies for each of cycles are comparable since the ratio ∆mEQCM/Q is proportional to ηF (see equation 2). The faradaic efficiency ηF of the grafting process as calculated by equation 2 and illustrated in figure 3d reaches about 18.8 % after the first cycle and increases to about 20 % by the 6th cycle. Although lesser currents are recorded in the subsequent cycles, it is at the same time used for the reduction of the residual diazonium cations to produce fewer radical species that possibly attack the already grafted layers. In the first cycle the mass gain is highest. In the second cycle, the mass drops to about a quarter of the total mass deposited in the first cycle (see figure 3e). 3.3. Infrared spectroscopy Infrared spectroscopic ellipsometry (IRSE) has become one of the most powerful spectroscopic tools applied in the analysis of ultra-thin surface bound organic layers.37 Figure 4 shows the transmission mode FTIR spectrum of the powder sample of the prepared aryl diazonium salt of 4-CMTBD mixed with KBr pellets (orange line) plotted together with the IRSE spectra of the functionalized Si and Au surfaces (blue and red lines respectively). The IRSE spectra are normalized against the respective non-functionalized Au and Si substrates.


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Figure 4. IR spectra of the diazonium salt 4-CMTBD (orange line), normalized tan ѱ spectra of Si-(4-CMTB) surface (blue line) and of Au-(4-CMTB) surface (red line). The spectrum of the of 4-CMTBD salt reveals IR absorption bands due to phenyl ring (Ph), carbonyl (C=O) and diazonium (N≡N+) groups at about 1560, 1720 and 2256 cm-1 respectively. The IRSE spectra of the functionalized surfaces Si-(4-CMTB) and Au-(4-CMTB) shows the disappearance of the diazonium groups at 2256 cm-1 which is due to the loss of dinitrogen (N2) during the electrografting surface modification reaction. The IRSE absorption peaks due to phenyl ring and carbonyl vibrations remain present on the surfaces after the electrografting process though somewhat shifted to ~ 1590 and ~ 1726 cm-1 respectively. The shift in the carbonyl absorption bands of the modified surfaces with respect to the carbonyl signal of 4CMTBD salt is only about 6 cm-1, which is not so much significant compared to the more pronounced phenyl ring shifts of approximately 30 cm-1 evidently observed for Si-(4-CMTB) and Au-(4-CMTB) surfaces with respect to corresponding phenyl ring signal of the 4-CMTBD salt. The shifts in the phenyl ring absorption peaks to higher wavenumbers observed after surface


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electrografting most likely arise due alteration of the nature of binding on the phenyl rings. These changes in phenyl binding can be attributed to the resultant loss of previously bonded diazonium (N≡N+) groups and subsequent grafting to the substrate surface. The observed shifts therefore can attest to the actual binding of the phenyl groups to the substrate via proposed radical mechanisms cited earlier in the text. The infrared spectroscopic data discussed herein therefore provide evidence of successful surface functionalization in which pendant carboxymethylthio ligand groups are introduced onto the surface along with the substrate bound phenyl groups. 3.4. Raman spectroscopy Raman spectroscopy has increasingly become an important tool for characterization of many organometallic species.38 Raman spectra of Si-H, Si-(4-CMTB) and Si-(4-CMTB)-Cu surfaces are plotted to illustrate the binding Cu ions (see figure 5). A strong signal at 487 cm-1 and a weak sharp signal at 267 cm-1 are evident in the spectrum of Si-(4-CMTB)-Cu surface (red line). In addition, a red shift in the range 1050 – 1100 cm-1 is equally conspicuous. Raman reference fingerprints of non-functionalized Si-H surface (orange line) and hydrated CuCl2 (cyan line) is devoid of any peaks at 487 cm-1, 267 cm-1 or ~ 1080 cm-1. The Raman shift at 487 cm-1 characterizes Cu-O vibrations.39-41 The signal at 267 cm-1 is ascribed to the Cu-S vibration typically observed in chelate environments.42,43 Given the relatively weaker v(Cu-S) signals compared to v(Cu-O) signal, it may be suggested that chelation of copper on the Si-(4-CMTB)Cu surface happens predominantly via oxygen atoms of the carboxylate ligands while to a lesser extent with the sulfur atoms of the thioether linkage.


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Figure 5. Raman spectra of Si-(4-CMTB)-Cu (red line), Si-(4-CMTB) (blue line), Si-H (orange line) and hydrated CuCl2 sample (cyan line). Thioether groups are relatively less soluble in water than carboxylic acid groups therefore the Cu ions in water would more likely associate with carboxylic acid groups to a greater extent hence leading to formation of more Cu-O bonds compared to Cu-S bonds. This is perhaps a factor that contributes to the observed weaker v(Cu-S) signal in comparison to the more pronounced v(CuO) signal. A comparison of the Raman spectra of Si-(4-CMTB) and Si-(4-CMTB)-Cu surfaces reveal a red shift in the region from 1050 to 1100 cm-1. The signals in this range are typical signature bands that characterize v(C-O) signal of the carboxylic acid groups.44 The interpretation here is that the v(C-O) signal on the Si-(4-CMTB) surface appears at around 1080 cm-1 while the signal of C-O bonds coordinated to Cu in the Si-(4-CMTB)-Cu surface are shifted closer to 1070 cm-1. The delocalization of electron densities from oxygen atoms of carboxylate ligand groups into the


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valence shells of Cu2+ species leading to formation of Cu-O coordinate bonds possibly weakens the C-O bond to causing a shift in the v(C-O) signal. The observed red / bathochromic shift acts as proof of involvement of carboxylates in chelation of Cu. 3.5. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron overview spectra of Si-(4-CMTB) and Si-(4-CMTB)-Cu reveal the presence of sulphur atoms (S2s - 228 eV and S2p - 164 eV) that emanate from carboxymethylthio groups on the modified surfaces (see figure 6). It can also be noted from the spectra of the modified surfaces: Si-(4-CMTB) and Si-(4-CMTB)-Cu, that the C1s peak is approximately more than three-fold as intense in comparison to the corresponding Si2p and Si2s signals of the respective spectra. In contrast, the Si-H spectrum shows a C1s peak that is markedly less intense than the corresponding Si2p and Si2s characteristic signals. The C1s signal on the Si-H bare surface represents adventitious carbon contamination that possibly originates from brief exposure to air or due to substrate solution processing before the actual XPS measurement. The extra-intense C1s peaks on the modified surfaces coexisting with characteristic sulphur and silicon substrate peaks in the overview XPS spectra provides proof of existence of grafted organic layers containing 4-CMTB units on Si surface (see figures 6b,c).


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Figure 6. XPS Overview spectra for (a) Si-H, (b) Si-(4-CMTB) and (c) Si-(4-CMTB)-Cu. In the modified surfaces, there is a slight overlap of a plasmon loss signal with the S2p peak in the region ~ 163 - 170 eV as illustrated in the overview XPS chart. The spectrum of Si-(4CMTB)-Cu surface also confirms presence of Cu on the surface i.e. Cu3p ~ 78 eV having exposed the Si-(4-CMTB) surface to aqueous CuCl2 solution for chelation reaction. The Cu2s peak also overlaps with a Si plasmon loss signal at ~ 122 eV and therefore not clearly discerned in the Si-(4-CMTB)-Cu spectra. A detailed XPS spectra of C1s signals of Si-(4-CMTB), Si-(4-CMTB)-Cu indicate the presence of carboxylate groups on the surfaces by the characteristic peaks in the region ~ 290.0 – 288.5 eV (see figures 7a,b). The C1s signal related to carboxylate peak notably shifts to lower binding energies by ~ 0.5 eV upon chelation of Cu ions on the Si-(4-CMTB) surface. This characterizes the participation of carboxylate ligands in the actual binding to Cu. Upon chelating Cu on the surface, the carboxylic acid groups are no longer free. The formation of multiple five-membered chelate ring systems on the surface is perhaps an important factor that contributes to the observed


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shift in the C1s signal to a lower binding energy. Bound systems such as chelates typically have lower energies than sum of their constituent parts i.e. the ligands and the metal ions. This is what holds the system together as energy is released upon creation of a more stable bound state - the chelate. Therefore, without neglecting other factors, it is imperative that energy required for ejecting C1s electrons from the carbon atoms of carboxylates already bound to Cu would predictably be lower compared to the case of free carboxyl groups.

Figure 7. Core level XPS spectra: (a) C1s Si-(4-CMTB), (b) C1s of Si-(4-CMTB)-Cu, (c) S2p of Si-(4-CMTB) and (d) S2p of Si-(4-CMTB)-Cu surfaces. The S2p signals of both Si-(4-CMTB) and Si-(4-CMTB)-Cu samples encompass a Si plasmon loss signal and therefore a comparison can still provide a distinction between the two surfaces


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(see figures 7c,d). The Si-(4-CMTB)-Cu surface is treated with warm aqueous Cu ion solution while the Si-(4-CMTB) surface acts as a control since it is only treated with warm deionised water. A comparison of the two spectra reveals emergence of a distinctly broadened shoulder peak or ‘hump’ on the S2p spectrum of Si-(4-CMTB)-Cu surface in the region ~ 167 eV. The appearance of the broadened shoulder peak or ‘hump’ which to some extent narrows the main peak at 164 eV, suggests that a fraction of sulphur atoms on the surface are bound to Cu ions. It can be concluded the observed extra broadening which seems to create a the ‘hump’ or shoulder peak as evident in the S2p spectrum of Si-(4-CMTB)-Cu surface results from the formation of Cu-S coordinate bonds.

Figure 8. XPS spectra: (a) Cu2p of Si-(4-CMTB)-Cu, (b) Si2p of Si-H, Si-(4-CMTB) and Si-(4CMTB)-Cu surfaces. Figure 8a shows the XPS Cu2p signal obtained from Si-(4-CMTB)-Cu surface. It reveals the two prominent Cu peaks: 2p1/2 at about 955 eV and 2p3/2 at about 935 eV, including the typical shakeup satellites at about 943 and 963 eV that characterize (+2) oxidation state of Cu species on the Si-(4-CMTB)-Cu surface.45 The shape of the satellite peaks closely resembles that of Cu2+


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species partly coordinated to acetate ligands i.e. Cu(Acac) which suggests the existence of Cu(II) species predominantly chelated by carboxylate ligands.46 A comparative analysis done at similar measurement parameters for the three surfaces: Si-H, Si(4-CMTB) and Si-(4-CMTB)-Cu reveals varying peak intensities in an intensity signal attenuation experiment using the Si2p peak (see figure 8b). Intensity attenuation of an XPS signal can be used to investigate the presence of surface overlayer thickness. The intensity of photoelectron Io, emitted at a depth -‘d or x’ below the surface attenuates to intensity I in accordance to the Beer - Lambert decay equation.47,48 This means that thicker and more densely packed overlayer would cause an in increase in signal attenuation resulting to a reduction of signal intensity. The non-functionalized Si-H surface has the highest peak intensity due to lack of overlayer grafted molecules. The Si-(4-CMTB) shows comparatively lower intensity than that of Si-H surface due to attenuation caused by the ultrathin organic overlayer containing grafted units of 4-CMTB. The Si-(4-CMTB)-Cu surface records the lowest intensity due to the extra attenuation caused by additional Cu-O and Cu-S on the surface resulting from Cu binding and uptake on the surface. In carboxymethylthio groups, the sulfur atom and the carbon atom of the methylene (-CH2-) group are both sp3 hybridized. This allows free bond rotations among the CH2-(CO), CH2-S, and S-Ph bonds. The suitable reorientations of these chelating CMT groups on the surface can act to facilitate uptake via complexation of Cu ions on the modified surface. The signal intensity attenuation experiment can therefore provide a means of distinguishing between functionalized surfaces that are metal ion packed from those that are not. 4.

Conclusion

This work explores direct electrografting of an aryl diazonium derivative containing carboxymethylthio groups (-S-CH2-COOH) as route for fabricating metal ion chelating surfaces


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for trapping of copper ions. Infrared spectroscopy confirmed the existence of grafted phenyl groups present along with the carbonyl groups of the pendant chelating ligands on the modified surfaces. The observed v(Cu-S) and v(Cu-O) Raman signals coupled with the red shift in the region of v(C-O) characterized the binding of Cu to the ligands on the modified surface upon treatment with Cu ion solution. Analysis of core level C1s, S2p, Cu2p, Si2p XPS signals characterized chelation and the uptake of Cu ions by the modified surfaces. Considering that such modified surfaces are capable of trapping Cu(II) ions at trace levels, they constitute important alternatives that can be applied in the development of functional materials or surfaces for Cu ion extraction. They may act as candidates of potential engineering interests for heavy metal ion sensing and/or extraction and therefore could be explored further. This study offers positive contributions in the fields of environmental protection, forensic diagnostics, bio-sensing, and mineral prospecting among other related disciplines. AUTHOR INFORMATION Corresponding Author * John Onyango Adongo [email protected] Author Contributions All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENT J.A appreciates financial support provided by the Deutscher Akademischer Austauchdienst – DAAD Germany in collaboration with the National Research Fund – NRF Kenya under a joint scholarship program.


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Fabrication and Characterization of Surfaces Modified with

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