Applied Science and Convergence Technology 2020; 29(5): 108-112
Published online September 30, 2020
Copyright © The Korean Vacuum Society.
Jae Moon Yia, So Jeong Parka, Choong Kyun Rheea, and Youngku Sohna,b,*
aDepartment of Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea
bDepartment of Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea
Correspondence to:E-mail: email@example.com
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
Cr(VI) reduction has been used to diminish the high toxicity of the Cr(VI) ion in the environment. Herein, electrochemical (EC) reduction was demonstrated using Au NP-loaded Ti electrodes functionalized with 2,2':6',2''-terpyridine-4'-thiol (STpy). Cyclic voltammetry tests were performed and these revealed Cr(VI) reduction potentials around –0.55, –0.35, and –0.55 V (vs Ag/AgCl) for bare Ti, Au NP-loaded Ti, and STpy-functionalized Au NP-loaded Ti (Ti-AuNP-STpy) electrodes, respectively. The reduction potential and Cr(VI) reduction performance varied with Ti oxidation state as well as surface functionalization. The EC Cr(VI) reduction using the Ti-AuNP-STpy electrode was found to have the best reduction performance. In addition, for future potential applications, the surface plasmon resonance response was also demonstrated for Cr(III) and Cr(VI) sensing over the Au-STpy surface. Thus, these novel electrode systems provide very useful information for developing improved detoxification methodologies.
Keywords: Cr(VI) reduction, Electrochemical reduction, Terpyridine-derivatized Au NPs, Cyclic voltammetry, Surface plasmon resonance
Decontamination of hexavalent chromium Cr(VI) is a major environmental challenge [1–3]. Low limit-of-detection Cr(VI) ion sensing and safer treatments methods are actively being researched both in industry and academia [4–7]. Various safer Cr(VI) treatment methods have thus far been developed, including adsorption removal, photocatalytic reduction, and electrochemical reduction methods [8–23]. As an example of adsorption–reduction removal of Cr(VI), mesoporous polydopamine/TiO2 composite nanospheres have been prepared for the adsorption of Cr(VI) ions via electrostatic interactions followed by reduction to Cr(III) . The design of a material with suitable surface charge and electron-transfer properties is important for adsorption–reduction treatments [10–12]. Photocatalytic reduction of Cr(VI) to the less toxic Cr(III) ions using existing catalysts has been extensively been studied. The photocatalysts used include TiO2-impregnated ceramic hollow fibers , TiO2/g-C3N4 microspheres/reduced graphene oxide, AgI/TiO2 , Zn-Al-layered double hydroxide and TiO2 composites , TiO2-coated cellulose acetate monolithic structures , hollow TiO2 from polystyrene@TiO2 , and CoO
Motivated by the abundant literature, to further develop the EC method and obtain new information, we demonstrated the EC reduction of Cr(VI) ions using a newly developed EC electrode consisting of terpyridine-derivatized AuNPs on Ti. It was found that the terpyridine-derivatized surface played a positive role in the electrochemistry. This new methodology could be employed for improving EC reduction methods as well as other related applications.
The following chemicals were used as received: 2,2':6',2''-terpyridine-4'-thiol (STpy, 95%, Shanghai iChemical), potassium chromate (K2CrO4, 99.0%, Ducksan Pure Chem.), sulfuric acid (H2SO4, 95-97%, Emsure), and dimethyl sulfoxide (DMSO, GR 99%, Kanto Chem.). As-received Ti sheets (MSL Tools Store, China) were cleaned repeatedly by ultrasonication in acetone, isopropyl alcohol, and water and then dried under an IR lamp. The cleaned Ti sheets (0.5 cm × 2.0 cm) then underwent thermal treatment at 400 °C for 2 h to obtain oxidized Ti (TiO
The morphologies of the electrodes were examined using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). The concentration of Cr(VI) ions was examined using UV–visible absorption spectrometry (SCINCO NeoSys-2000 UV–Vis spectrophotometer) before and after electrochemical testing. Top monitor changes in the chemical states of the electrodes before and after the EC experiments, X-ray photoelectron spectroscopy was carried out (Thermo Scientific K-Alpha+ X-ray photoelectron spectrometer, equipped with a monochromated Al Kα X-ray source and hemispherical energy analyzer). Cyclic voltammetry and electrochemical (EC) amperometry testing was conducted using a typical three-electrode system with a counter Pt wire, a Ag/AgCl reference, and a WPG100 Potentiostat/Galvanostat (WonATech Co., Ltd.). The electrolytes used were 0.1 M H2SO4 (aq.), 1 (and 10) mM Cr(VI)/0.1 M H2SO4 (aq.), and 1 (and 10) mM Cr(VI)/0.1 M H2SO4 (aq.) solutions. The working electrodes tested were bare Ti, Ti-AuNP, Ti-AuNP-STpy, bare TiO
A key aspect of this study was the preparation of the Ti-AuNP-STpy electrode, which is depicted in Fig. 1, and described in the Experimental Details above. In our method for the preparation of the Ti-AuNP-STpy electrode, Au NP-loaded Ti sheets are immersed in a 1 mM STpy DMSO solution for 6 h and then carefully rinsed. During the immersion, Au-S covalent bond formation occurs between the Au NPs and STpy ligands . This newly developed electrode system is expected to function via strong interactions between three N atoms in the STpy ligands and guest Cr(VI) ions.
Figure 2 shows the morphologies of bare Ti, Ti-AuNP-STpy, Ti-AuNP after Cr(VI) reduction, and Ti-AuNP-STpy after Cr(VI) reduction, all obtained via SEM. The image of the AuNP-loaded Ti sheet displays a uniform distribution of Au NPs before and after STpy functionalization. The surface morphology is clearly different before and after Au sputter deposition. For the Ti-AuNP and Ti-AuNP-STpy electrodes after EC Cr(VI) reduction, the morphology is markedly changed compared to that of the intact Ti-AuNP-STpy sample. The Au NP aggregate, forming a large Au island. Electric-field-assisted Au NP aggregation has been reported in the literature . When Au aggregates, it is plausible that the EC efficiency may be reduced.
Figure 3 shows cyclic voltammetry (CV) curves for bare the Ti, Ti-AuNP, and Ti-AuNP-STpy electrodes acquired between –0.8 and +0.2 V (vs. Ag/AgCl) at a scan rate of 100 mV/s in blank 0.1 M H2SO4, 1 mM Cr(III)/0.1 M H2SO4, and 1 mM Cr(VI)/0.1 M H2SO4 electrolyte solutions. In blank 0.1 M H2SO4, no redox peaks are observed. Instead, a current density (CD) increase is seen at negative potentials. For the Ti-AuNP and Ti-AuNP-STpy electrodes, the CD increases more sharply above –0.3 V. The sharp CD increase is attributed to the hydrogen evolution reaction (HER) [26,27]. For the 1 mM Cr(III)/0.1 M H2SO4 electrolyte, no reduction peak occurs in the negative potential region as expected, and HER CD is also observed. In the 1 mM Cr(VI)/0.1 M H2SO4 electrolyte, peaks are generated in the CV curves of all three electrodes, and this is attributed to the reduction of Cr(VI) to Cr(III). The Cr(VI) reduction potentials are observed at approximately –0.55, –0.35, and –0.55 V (vs Ag/AgCl) for the bare Ti, Au NP-loaded Ti, and STpy functionalized Au NP-loaded Ti electrodes, respectively. A lower negative potential is demonstrated for the Ti-AuNP electrode compared to those for the other two electrodes, although the CD is relatively lower. The less negative (or more positive) potential for Ti-AuNP could be due to the activity of the Au NPs. Similar observations have been reported in the literature . In the CV curves acquired at various scan rates—100, 200, 300, 400, and 500 mV/s—the reduction potential does not significantly shift toward more negative potentials as the scan rate increases. However, the corresponding CD levels increase with scan rate. The CD intensities of the electrodes at a given scan rate increase in the order Ti-AuNP < Ti-AuNP-STpy < bare Ti. The bare Ti electrode generates the highest CD, but the signal is much broader than those found in the other two electrodes. The CD increase is observed to be linear with the square root of the scan rate, which is typical of a diffusion-controlled reaction process [7,16].
For comparison with the Ti surface, the oxidized Ti surface was also tested. Figure 4 shows cyclic voltammetry (CV) curves for bare the TiO
The Cr(VI) reduction performances of the bare Ti, Ti-AuNP, Ti-AuNP-STpy, bare TiO
X-ray photoelectron spectroscopy (XPS) was carried out on the Ti-AuNP-STpy, Ti-AuNP-STpy (immersed in the Cr6+ solution), and Ti-AuNP-STpy (after the EC tests in the Cr6+ solution) electrodes and the results are displayed in Fig. 6. The survey XP spectra include peaks that are assigned to Ti (Ti Support), Au (overlayer), C (functionalized STpy), and S (S of STpy or sulfate ions) species, as expected. Peaks assigned to Cr species were detected in the spectra for the Ti-AuNP-STpy electrodes measured whilst immersed in the Cr6+ solution and after EC testing. In the Ti 2p spectrum of the as-prepared Ti-AuNP-STpy electrode, the Ti 2p signal is weak, due to thick overlayer of Au and terpyridine. For the Ti-AuNP-STpy electrode in the Cr6+ solution, two strong peaks in the Ti 2p spectrum are observed, at 464.7 and 458.9 eV, suggesting a spin-orbit splitting of 5.8 eV and an assignment to Ti 2p1/2 and Ti 2p3/2 levels, respectively, of Ti(IV) species . A smaller signal is observed at a BE of 454.0 eV, assigned to the Ti 2p3/2 peak of metallic Ti(0) . The increase in the Ti 2p XPS signal upon immersion of the electrode into the Cr(VI) solution is due to the fact that some Au NP aggregation occurs during the interaction between Cr(VI) and the terpyridine ligand. Three Lewis basic N groups of STpy are expected to strongly interact with the Lewis acidic Cr(VI) ions . Therefore, the Ti support surface is exposed outwards because of Au NP aggregation. After EC testing, the Ti 2p signal assigned to metallic Ti(0) showed no critical change, remaining at a BE of 454.0 eV, while the Ti 2p signals of the Ti(IV) species shifted to a 0.5-eV higher BE. This indicates that surface Ti species may interact with Cr species during the EC. In the Au 4f spectrum of the as-prepared Ti-AuNP-STpy electrode, the BEs of the Au 4f7/2 and 4f5/2 electrons are observed at 84.2 and 87.8 eV, respectively, with a spin-orbit splitting of 3.6 eV. This BE position is attributed to the attraction between the metallic Au and thiol. Upon immersing the electrode in the Cr6+ solution, the Au 4f signal is diminished as a result of the aggregation of Au NPs during the interaction between Cr(VI) and the terpyridine ligand, as mentioned above in discussing the Ti 2p XPS. After the EC test, the Au 4f signal is markedly diminished, which is attributed to the removal from the surface of the Au NPs during the 2 h reduction test. The O 1s spectrum of the as-prepared Ti-AuNP-STpy electrode features the O 1s BE at 531.7 eV, a peak that is attributed to surface-adsorbed oxygen . Upon immersing the electrode in the Cr6+ solution, the O 1s BE appears at 530.6 eV, and this signal is attributed to lattice oxygen of Ti(IV) oxide species. The appearance of lattice oxygen is a result of aggregation of overlayer Au NPs, as mentioned above. After the EC testing, the O 1s peak at 530.6 eV remained unchanged but the peak at 532.3 eV was significantly enhanced, due to an increase in surface OH species such as Ti-OH. As a consequence, the Ti 2p BE was shifted to a higher BE position after the EC tests, as mentioned above. In the Cr 2p XP spectrum of the Ti-AuNP-STpy electrode dipped in the Cr6+ solution, two peaks are clearly observed at BEs of 578 and 587.5 eV, attributed to Cr 2p3/2 and Cr 2p1/2 peaks, respectively, of Cr(III) . These Cr 2p peaks of Cr(III) reflect the fact that the terpyridine ligand plays a significant role in the reduction of Cr(VI) to Cr(III). After the EC testing, the Cr 2p BE shifted to a higher BE position, possibly due to some interaction with sulfate residues. The corresponding S 2p signals appear at approximately 169.3 eV. The S 2p peak at 168.5 eV for the other two samples is assigned to thiol in the STpy-functionalized electrodes .
We demonstrated surface plasmon resonance (SPR) sensing performance to briefly show that the newly developed system can be applied to sensing using SPR . Figure 7 shows the SPR signal response over a STpy-functionalized Au film chip upon introducing Cr(VI) and Cr(III) ions. Interestingly, a strong SPR response was observed for both Cr(VI) and Cr(III). However, the SPR signal did not change significantly when Cr(VI) was converted to Cr(III) and vice versa. This indicates that once the Cr ion has strongly interacted with the terpyridine ligand, no substitutional change occurs. When water is introduced, however, the signal quickly returns to its original levels. This indicates that an adsorption–desorption equilibrium may occur when changing between Cr ion solutions and deionized water. This preliminary result indicates that the STpy-functionalized Au chip can be used for Cr ion sensing based on SPR. Further investigations should be carried out, however.
Toward the safer electrochemical treatment of toxic Cr(VI) in solution, bare Ti, Ti-AuNP, Ti-AuNP-STpy, bare TiO
This research was supported by Research Scholarship of Chungnam National University.