In Situ Raman Microdroplet Spectroelectrochemical Investigation of Cuscn Electrodeposited on Different Substrates

nanomaterials

Article In Situ Raman Microdroplet Spectroelectrochemical Investigation of CuSCN Electrodeposited on Different Substrates

Zuzana Vlˇcková Živcová *, Milan Bouša, MatˇejVelický, Otakar Frank and Ladislav Kavan

J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Dolejškova 2155/3, 182 23 Prague, Czech Republic; [email protected] (M.B.); [email protected] (M.V.); [email protected] (O.F.); [email protected] (L.K.) * Correspondence: [email protected]

Abstract: Systematic in situ Raman microdroplet spectroelectrochemical (Raman-µSEC) characterization of copper (I) thiocyanate (CuSCN) prepared using electrodeposition from aqueous solution on

various substrates (carbon-based, F-doped SnO2) is presented. CuSCN is a promising solid p-type inorganic semiconductor used in perovskite solar cells as a hole-transporting material. SEM characterization reveals that the CuSCN layers are homogenous with a thickness of ca. 550 nm. Raman spectra of dry CuSCN layers show that the SCN− ion is predominantly bonded in the thiocyanate resonant form to copper through its S−end (Cu−S−C≡N). The double-layer capacitance of the CuSCN layers ranges from 0.3 mF/cm2 on the boron-doped diamond to 0.8 mF/cm2 on a glass-like carbon. In situ Raman-µSEC shows that, independently of the substrate type, all Raman vibrations from CuSCN

and the substrate completely vanish in the potential range from 0 to −0.3 V vs. Ag/AgCl, caused by the formation of a passivation layer. At positive potentials (+0.5 V vs. Ag/AgCl), the bands

Citation: VlˇckováŽivcová, Z.; Bouša, corresponding to the CuSCN vibrations change their intensities compared to those in the as-prepared, M.; Velický, M.; Frank, O.; Kavan, L. dry layers. The changes concern mainly the Cu−SCN form, showing the dependence of the related In Situ Raman Microdroplet vibrations on the substrate type and thus on the local environment modifying the delocalization on Spectroelectrochemical Investigation the Cu−S bond. of CuSCN Electrodeposited on Different Substrates. Nanomaterials Keywords: CuSCN; hole-transport material; carbon; in situ Raman spectroelectrochemistry 2021, 11, 1256. https://doi.org/ 10.3390/nano11051256

Academic Editor: Alessandra Alberti 1. Introduction The architecture of both solid-state dye-sensitized solar cell (ss-DSSC) [1–3] and Received: 26 March 2021 perovskite solar cell (PSC) [4,5] requires the hole-transport layer, conducting the photo- Accepted: 4 May 2021 Published: 11 May 2021 generated holes to a positive terminal of the cell. The advantages of ss-DSSCs over the traditional liquid-junction DSSCs are (i) prevention of dye desorption, (ii) absence of liquid

Publisher’s Note: MDPI stays neutral leakage or evaporation and (iii) a facile device encapsulation and stacking in series [3]. The with regard to jurisdictional claims in first materials that made it possible to replace liquid electrolytes were organic hole-transport −3 2 −1 −1 published maps and institutional affil- materials: triphenyl diamine with a hole mobility of about 10 cm V s [6] and 0 0 0 iations. 2,2 ,7,7 -tetrakis-(N,N-di-p-methoxyphenylamine)-9,9 -spirobifluorene (spiro-OMeTAD) with a mobility of the amorphous phase of ~10−5 cm2V−1s−1 [7]. Another very promising solid p-type inorganic semiconductor, used in photovoltaic devices (ss-DSSC [8–15] and PSC [5,16–19]) as a hole-transport material, is a copper(I) thiocyanate (CuSCN). Due to its advantageous electrical properties (wide bandgap of ~3.9 eV, hole mobility Copyright: © 2021 by the authors. 2 −1 −1 Licensee MDPI, Basel, Switzerland. 0.001–0.1 cm V s )[20–22], chemical stability and optical transparency [23], CuSCN is This article is an open access article a good alternative to the more expensive and less-stable spiro-OMeTAD particularly if distributed under the terms and CuSCN is interfaced to a layer of reduced graphene oxide [19,24]. This finding illustrates conditions of the Creative Commons that the CuSCN/carbon junction is of primary interest for applications in photovoltaics. − Attribution (CC BY) license (https:// The SCN ion may coordinate to a metal atom (M) through either the S or the N donor creativecommons.org/licenses/by/ sites. As a result, thiocyanate (M−S−C≡N), isothiocyanate (M−N=C=S), or bridging (M– 4.0/). SCN–M’) linkage isomers are possible [25,26]. Electrodeposition from an aqueous solution

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containing SCN− and Cu2+ ions is one of the low-temperature deposition techniques for CuSCN layers [23,27–34] in addition to spin-coating [24], drop-casting [35] or spray deposition [36]. This technique allows the CuSCN layer deposition on conductive substrates, such as the commonly used conducting glass (Fluorine doped Tin Oxide/FTO, or Indium Tin Oxide/ITO), carbon substrates (glass-like carbon/GC, boron-doped diamond/BDD, highly ordered pyrolytic graphite/HOPG), or metals. Since these photovoltaic systems are composed of several individual layers (sandwich architecture), which form interface transitions, it is necessary to know the materials’ structural changes at different applied voltages to improve device stability and efficiency. In situ Raman spectroelectrochemistry (SEC) is a useful characterization method combining Raman spectroscopy and electrochemistry, which allows monitoring of the structural and electronic changes of the studied material with the applied potential. Ra- man spectra (changes in frequencies and intensities) are influenced by the double-layer charging (electrochemical doping) and by Faradaic processes on the electrode surface [37]. A commonly used in situ Raman SEC setup is a closed electrochemical cell with both the electrolyte solution and the sample contained inside, which only allows the application of potential across the whole electrochemically active area of the immersed sample. Such a “macro setup” has further disadvantages, such as the attenuation of the Raman intensity due to the optical window and the need for a flawless sealing of the electrical contact to the working electrode. An in situ Raman SEC arrangement using a microdroplet electrochemical cell (Raman-µSEC) can be employed to avoid these drawbacks [38–40]. The µSEC allows highly localized electrochemical studies on spatially heterogeneous surfaces (typical microdroplet diameter of 10–20 µm), such as two-dimensional materials [38–40], where only a well-defined microscale area of the sample in contact with the microdroplet is affected by the applied potential. The electrochemical behavior of quasi-metallic or semiconducting carbon materials, such as polycrystalline doped diamond with strong and nearly isotropic tetrahedral sp3 bonding, or carbons with sp2 hybridization, such as disordered glass-like carbon or highly oriented pyrolytic graphite, are described in the literature [41–49]. Compared to sp2 materials, doped diamonds have a wider electrochemical potential window in aqueous media and they are (electro)chemically more stable. The first in situ Raman SEC study [45] of polycrystalline boron-doped diamond films with varying sp2 carbon content and boron doping performed in a closed electrochemical cell showed the stability of the sp3 diamond lattice and incorporated boron atoms, while specific spectral changes of the sp2 carbon under cathodic and anodic treatment were observed. The spectroelectrochemical behavior of sp2 carbon materials (graphite, graphene, carbon nanotubes, or fullerenes) is accompanied by changes in the Raman frequencies and intensities, described in detail in refs. [37,41,50–52]. In our previous work [19], we performed an in situ Raman spectroelectrochemistry of spin-coated CuSCN layers prepared on gold and GC substrates. Herein, we present an in- depth advanced microdroplet SEC investigation of CuSCN layers electrodeposited from an aqueous solution at room temperature on different substrates: (i) carbon-based materials (BDD, GC and HOPG) with varied amounts of surface oxygen groups on sp2 carbon and (ii) carbon-free FTO with a high concentration of surface oxygen-containing groups. Such a combination of various substrate properties enables us to study the structural and electrochemical properties of the electrodeposited CuSCN layers in detail.

2. Materials and Methods Two different types of conductive substrates, i.e., carbon-based (BDD, GC, HOPG) and FTO conducting glass, were used to prepare the CuSCN layer by electrodeposition from an aqueous electrolyte solution following the methodology in ref. [23]. Polycrystalline BDD ﬁlm was deposited on fused silica in an ASTeX 5010 (Seki Technotron, Tokyo, Japan) series microwave plasma-enhanced chemical vapor deposition reactor [45]. The BDD ﬁlm was grown in a conventional CH4/H2 plasma and doping was induced by trimethyl boron gas B(CH3)3. The growth conditions were as follows: B/C ratio in the gas phase Nanomaterials 2021, 11, 1256 3 of 12

1000 or 2000 ppm (designated as BDD or BDD 2000, respectively), pressure 47.7 mbar, temperature 720 ◦C, methane content 0.8% and deposition time 60 min. Glass-like carbon (GC 3000C) with a resistivity of 400 µOhm cm was purchased from Good fellow Cambridge Ltd. (Huntingdon, UK). HOPG substrate was prepared as a thin sheet, peeled off using an adhesive Scotch® double sided tape (3M Company, Maplewood, MN, USA) from the bulk crystal (12 × 12 × 2 mm3, ZYB Grade, Momentive Performance Materials Quartz, Inc., Strongsville, OH, USA) and glued onto the glass. FTO with the resistivity of 15 Ohm/sq (TEC 15, Libbey-Owens-Ford Glass Co., Toledo, OH, USA) was used as a carbon-free substrate. The electrodeposition of CuSCN layers was performed potentiostatically at a potential of −0.5 V vs. Ag/AgCl (3 M KCl) for 30 min in a three-electrode cell at room temperature. In this arrangement, the substrate was the working electrode and a Pt mesh was used as the counter electrode. The electrolyte solution was mixed from cupric sulfate pentahydrate (99.9%, Sigma-Aldrich, St. Louis, MO, USA) and potassium thiocyanate (99.0%, Sigma-Aldrich) as precursors and triethanolamine (TEA; 99.0%, Sigma-Aldrich) as a chelating reagent for Cu(II) cations. The molar ratio [Cu2+]:[TEA] was 1:10 and the 2+ concentration of Cu was 0.01 M. Before electrodeposition, the CuSO4/TEA complex solution was mixed with 0.1 M KSCN solution, stirred for 1 h and then stored for 24 h. All Raman spectra (ex situ and in situ) were excited using a 514 nm line of an Ar+ laser (laser power of 1 mW at the sample) and recorded by a LabRAM HR spectrometer (Horiba Jobin-Yvon, Kyoto, Japan) interfaced with an Olympus microscope (100× −1 objective). The spectrometer was calibrated using the F1g mode of Si at 520.2 cm . The Raman peaks were fitted using Voigt lineshapes. The surface morphology of the samples was investigated employing field emission scanning electron microscopy (FESEM, S-4800 Hitachi, Tokyo, Japan). SEM images were acquired at an acceleration voltage of 5 kV and a working distance of 9–11 mm. The thickness of the electrodeposited layer on carbon substrates was measured by profilometry (Dektak 150, Veeco Instruments Inc., Plainview, NY, USA) over a scratch made by a glass tip. Cyclic voltammetry (CV) was performed in a closed three-electrode cell under Ar atmosphere (working electrode: CuSCN layer, counter electrode: Pt mesh, reference electrode: Ag/AgCl in 3 M KCl). The electrolyte solution was aqueous 0.5 M KCl (Sigma-Aldrich) saturated with CuSCN (pH 6). Electrochemical measurements were carried out using an Autolab PGSTAT128N potentiostat (Metrohm AG, Herisau, Switzerland) controlled by GPES4 software. In situ SEC characterization was performed in a microdroplet electrochemical cell. The microdroplet of 6 M LiCl aqueous electrolyte solution was expelled through a microcapillary containing the reference (Ag/AgCl wire) and counter (Pt wire) electrodes. The interface between the sample surface and microdroplet serves as the working electrode. Note that high electrolyte concentration prevents water evaporation and reduces solution resistance.

3. Results 3.1. Structural and Electrochemical Characterization of CuSCN Layers SEM was used to study the morphological properties of the electrodeposited CuSCN thin layer on various substrates. Figure1A–D display the plan view SEM micrographs of the dense fine-crystalline morphology of the uniform structure of fabricated CuSCN layers on four different substrates. The thickness of the CuSCN layer deposited on the FTO substrate (CuSCN/FTO) determined from a cross-sectional SEM image is approximately 550 nm, comparable to the thickness of CuSCN layers deposited on carbon-based substrates determined from stylus profilometry (approx. 450 ± 40 nm), when the uncer- tainty stemming from the surface scratching is taken into account. Surface morphology comparison of the CuSCN layers prepared by spin-coating from diethyl sulfide solution in our previous work [19] and layers prepared by electrodeposition in this work shows that surface morphology of the electrodeposited CuSCN layers differs significantly from layers prepared by spin-coating; the crystal size of the electrodeposited layers is significantly larger with a well-defined orientation. The root-mean-square (RMS) roughness values of individual CuSCN layers were determined using profilometry. As expected, RMS values Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 13

Nanomaterials 2021, 11, 1256 4 of 12 prepared by spin-coating; the crystal size of the electrodeposited layers is significantly larger with a well-defined orientation. The root-mean-square (RMS) roughness values of areindividual higher forCuSCN CuSCN layers layers were prepared determined by electrodeposition using profilometry. (RMS As increasesexpected, fromRMS 19values nm forare CuSCN/BDDhigher for CuSCN to 30 nmlayers for prepared CuSCN/FTO; by electrodeposition see Figure1) compared (RMS increases to the CuSCN from layers19 nm preparedfor CuSCN/BDD by spin-coating to 30 nm (RMS for CuSCN/FTO; for CuSCN/GC see isFigure approximately 1) compared 9 nm to [the19]). CuSCN layers prepared by spin-coating (RMS for CuSCN/GC is approximately 9 nm [19]).

Figure 1. SEM images of CuSCN electrodeposited on (A) boron-doped diamond (CuSCN/BDD), Figure 1. SEM images of CuSCN electrodeposited on (A) boron-doped diamond (CuSCN/BDD), (B) glass-like carbon (CuSCN/GC), (C) highly ordered pyrolytic graphite (CuSCN/HOPG) and D) (B) glass-like carbon (CuSCN/GC), (C) highly ordered pyrolytic graphite (CuSCN/HOPG) and D) F-doped SnO glass (CuSCN/FTO). The inset of chart (D) shows the cross-sectional view. F-doped SnO22 glass (CuSCN/FTO). The inset of chart (D) shows the cross-sectional view. Ex situ Raman spectra in the range of 100–2400 cm−1 of the pristine dry CuSCN Ex situ Raman spectra in the range of 100–2400 cm−1 of the pristine dry CuSCN layers layers electrodeposited on various substrates (BDD, GC, HOPG, FTO), as well as spectra electrodeposited on various substrates (BDD, GC, HOPG, FTO), as well as spectra of bare of bare substrates with their characteristic Raman features [45,49,53,54] are shown in substrates with their characteristic Raman features [45,49,53,54] are shown in Figure 2A. Figure2A . Raman spectrum of single-crystal graphite (sp2 carbon) exhibits only one Raman spectrum of single-crystal graphite (sp2 carbon) exhibits only one narrow peak at narrow peak at ~1580 cm−1 (E phonon, designated as the G mode) in this range [55], −1 2g ~1580 cm (E2g phonon, designated as the G mode)2 in this range [55], while polycrystalline while polycrystalline graphite2 or amorphous sp carbon (like GC) have an additional−1 peak graphite or −amorphous1 sp carbon (like GC) have an additional peak at ~1350 cm (A1g at ~1350 cm (A1g phonon, D mode) [54], whose intensity reflects the effective crystallite phonon, D mode) [54], whose intensity reflects the effective crystallite size La [56]. The size L [56]. The Raman spectrum of diamond (sp3 carbon) exhibits a narrow natural single Ramana spectrum of diamond (sp3 carbon) exhibits a narrow natural single line at 1332 line at 1332 cm−1 [57] identified as the three-fold degenerate, zone-center optical mode at cm−1 [57] identified as the three-fold degenerate, zone-center optical mode at the Γ point the Γ point and additional broad, weak-intensity peaks near 1580 cm−1, which indicate and additional broad, weak-intensity peaks near 1580 cm−1, which indicate the presence the presence of a small amount of sp2 impurities in the films [58]. Bands corresponding to of a small amount of sp2 impurities in the films [58]. Bands corresponding to the vibrations the vibrations associated with the increased boron concentration in BDD start to appear at associated with the increased boron concentration in BDD start to appear at ~500 and ~500 and ~1225 cm−1. This is accompanied by an increase in electrical conductivity and ~1225 cm−1. This is accompanied by an increase in electrical conductivity and transition transition from semimetallic to metallic character of BDD (B content in diamond lattice from semimetallic to metallic character of BDD (B content in diamond lattice > 3 × 1020 cm−3 > 3 × 1020 cm−3 [59]). At the same time, the characteristic diamond line at 1332 cm−1 −1 downshifts,[59]). At the attenuatessame time, and the exhibitscharacteristic an asymmetric diamond Fano-likeline at 1332 line cm shape downshifts, [45,60]. Theattenuates bands ofand graphitic exhibits or an amorphous asymmetric carbon Fano-like impurities line shape in BDD [45,60]. (depending The bands on of the graphitic film quality) or amor- are −1 locatedphous carbon at 1335 impurities cm−1 (D band), in BDD 1580 (depending cm−1 (G on band), the film 1610 quality) cm−1 (D are band) located and at 1520 1335 cm cm−1 −1 −1 −1 (tetrahedral(D band), 1580 amorphous cm (G band), carbon) 1610 [45 cm,49]. (D The band) Raman and bands 1520 cm of pristine (tetrahedral FTO areamorphous located −1 atcarbon) 480, 500 [45,49]. and 1080The cmRaman−1 [53 bands]. Independently of pristine FTO of the are substrate located at type, 480, the 500 Raman and 1080 spectra cm of[53]. CuSCN Independently layers prepared of the substrate by electrodeposition type, the Raman exhibit spectra similar of CuSCN Raman layers features prepared as the CuSCNby electrodeposition layers prepared exhibit by spin-coating similar Raman in our fe previousatures as work the [CuSCN19]. In the layers higher prepared frequency by regionspin-coating (2000–2300 in our cm previous−1), the electrodeposited work [19]. In the CuSCN higher layers frequency exhibit region the peaks (2000–2300 of ν (C cm≡N)−1), ν ≡ as stretchingthe electrodeposited mode, which CuSCN is very layers sensitive exhibit to the the local peaks environment of as(C N) of stretching the system mode, as well which as to theis very nature sensitive of the cation to the group local [environment61] in thiocyanate of the resonant system form as well of the as SCNto the− ion. nature The of weak the peakcation at group 2115 cm[61]− 1inis thiocyanate assigned to resonant the Cu− formNCS isomerof the SCN (Cu−+ −ion.:N ≡TheC− weakS−) and peak the at most 2115 intensecm−1 is assigned peak at 2175 to the cm Cu−1−toNCS the isomer Cu−SCN (Cu+ isomer−:N≡C− (CuS−) +and−− theS− Cmost≡N). intense In the peak spin-coated at 2175 CuSCNcm−1 to the layers Cu− fromSCN ourisomer previous (Cu+− work−S−C≡ [N).19], In the the most spin-coated intense peak CuSCN of the layers Cu− fromSCN our isomer pre- wasvious red-shifted work [19], due the tomost the differentintense peak morphology of the Cu of−SCN the spin-coated isomer was layersred-shifted [62]. In due the to low- the frequencydifferent morphology region of 100–1000 of the spin-coated cm−1, the presence layers [62]. of the In thiocyanatethe low-frequency SCN− resonantregion of form 100– is documented by the Raman peaks corresponding to the stretching modes of ν(Cu−S) at 205 cm−1, ν(Cu−N) at 241 cm−1 and ν(C−S) at 746 cm−1 and to the δ(SCN) bending

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1000 cm−1, the presence of the thiocyanate SCN− resonant form is documented by the Ra- man peaks corresponding to the stretching modes of ν(Cu−S) at 205 cm−1, ν(Cu−N) at 241 −1 − cm−1 andmode ν(C at−S) 431 at cm746 cm(indicative−1 and to the of predominant δ(SCN) bending thiocyanate mode at S 431binding). cm−1 (indicative The overlapping of −1 predominantpeaks in thiocyanate the higher-frequency S−binding). region The overlapping (same as 746 peaks cm peakin the in higher-frequency the low-frequency re- region, Table S1 in the Supporting Information) were ﬁtted with Voigt lineshapes to resolve their gion (same as 746 cm−1 peak in the low-frequency region, Table S1 in the Supporting In- complex spectral structure (Table S2). Additional Raman bands with small intensity appear formation) were fitted with Voigt lineshapes to resolve their complex spectral structure at ~2159 cm−1 corresponding to the bridged –SCN– [26] and at ~2167 cm−1 assigned to (Table S2). Additional Raman bands with small intensity appear at ~2159 cm−1 correspond- asymmetric ν(CN) vibration, which shifts due to the varying local environment of the ing to the bridged –SCN– [26] and at ~2167 cm−1 assigned to asymmetric ν(CN) vibration, SCN− ion (see below). which shifts due to the varying local environment of the SCN− ion (see below).

Figure 2. Raman spectra (A) and cyclic voltammograms (B) of CuSCN layers electrodeposited on (i) Figure boron-doped2. Raman spectra diamond (A) and (CuSCN/BDD, cyclic voltammograms blue trace), (ii)(B)glass-like of CuSCN carbon layers (CuSCN/GC, electrodeposited green on trace), (iii) i) boron-doped diamond (CuSCN/BDD, blue trace), ii) glass-like carbon (CuSCN/GC, green trace), highly ordered pyrolytic graphite (CuSCN/HOPG, yellow trace) and (iv) FTO glass (CuSCN/FTO, iii) highly ordered pyrolytic graphite (CuSCN/HOPG, yellow trace) and iv) FTO glass red trace) substrate. The reference Raman spectra of bare substrates are shown in black. The spectra (CuSCN/FTO, red trace) substrate. The reference Raman spectra of bare substrates are shown in black. Theare excitedspectra byare 514 excited nm laserby 514 radiation nm laser and radiat offsetion for and clarity. offset Thefor clarity. electrolyte The solutionelectrolyte used solu- in chart B tion usedwas in 0.5 chart M KCl B was sat. 0.5 CuSCN, M KCl scansat. CuSCN, rate was scan 100 mV/s.rate was 100 mV/s.

CyclicCyclic voltammograms voltammograms of the ofCuSCN the CuSCN layers layerselectrodeposited electrodeposited on different on different substrates substrates (BDD,(BDD, GC, HOPG, GC, HOPG, FTO) in FTO) 0.5 M in KCl 0.5 sat. M CuSCN KCl sat. were CuSCN recorded were over recorded the potential over the range potential − from −range0.3 V to from +0.5 V0.3 vs. V Ag/AgCl to +0.5 Vto vs.illustrate Ag/AgCl their toelectrochemical illustrate their behavior electrochemical (Figure 2B). behavior (Figure2B ). The CVs show the typical behavior of a p-type semiconductor [63]. In the The CVs show the typical behavior of a p-type semiconductor [63]. In the cathodic direc- cathodic direction no presence of capacitive charging can be seen, while with the increasing tion no presence of capacitive charging can be seen, while with the increasing positive positive potential the anodic current increases due to electrochemical reactions occurring potential the anodic current increases due to electrochemical reactions occurring at the at the electrode surface with variations depending on the substrate type [45]. The double- electrode surface with variations depending on the substrate type [45]. The double-layer layer capacitance of CuSCN layers on various substrates determined at 0 V, normalized capacitance of CuSCN layers on various substrates determined at 0 V, normalized to the to the projected geometric surface area, increases from 0.3 mF/cm2 for CuSCN/BDD and projected geometric surface area, increases from 0.3 mF/cm2 for CuSCN/BDD and 0.4 0.4 mF/cm2 for CuSCN/HOPG to 0.6 mF/cm2 for CuSCN/FTO and 0.8 mF/cm2 for mF/cm2 for CuSCN/HOPG to 0.6 mF/cm2 for CuSCN/FTO and 0.8 mF/cm2 for CuSCN/GC. CuSCN/GC. The capacitances of bare substrates (FTO 30 µF/cm2 [19], GC 20 µF/cm2 [19], The capacitances of bare substrates (FTO 30 μF/cm2 [19], GC 20 μF/cm2 [19], BDD < 12 BDD < 12 µF/cm2 [45] and HOPG 6 µF/cm2 [64]) are at least one order of magnitude μF/cm2 [45] and HOPG 6 μF/cm2 [64]) are at least one order of magnitude lower than the lower than the capacitance of the CuSCN layers and therefore do not signiﬁcantly affect capacitance of the CuSCN layers and therefore do not significantly affect the measure- the measurement. ment. 3.2. In Situ Raman Microdroplet Spectroelectrochemistry (Raman-µSEC) 3.2. In situ Raman Microdroplet Spectroelectrochemistry (Raman-μSEC) A series of in situ Raman spectra of the CuSCN layers electrodeposited on various Asubstrates series of in (FTO, situ GC,Raman BDD, spectra HOPG) of the as aCuSCN function layers of the electrodeposited applied potential on (anodic-blackvarious substratestraces, (FTO, cathodic-gray GC, BDD, traces,HOPG) 0 as V vs.a function Ag/AgCl-pink of the applied traces) potential is shown (anodic-black in Figures3–6 and traces,compared cathodic-gray with thetraces, (ex situ)0 V Ramanvs. Ag/AgCl-pink spectra of dry traces) CuSCN is layersshown on in the Figures respective 3–6 substratesand compared(bottom with spectrum the (ex situ) in each Raman ﬁgure). spectra The of general dry CuSCN spectroelectrochemical layers on the respective trends sub- in Raman stratesspectra (bottom are spectrum comparable in each with figure). those The for general the spin-coated spectroelectrochemical CuSCN layers trends from our in Ra- previous man spectrawork, whichare comparable were measured with those in the for traditional the spin-coated macro-SEC CuSCN arrangement layers from [our19]. previ- A complete ous work,attenuation which were of all measured Raman vibrations in the traditional from CuSCN, macro-SEC as well arrangement as that of the[19]. substrate, A com- was plete attenuationobserved in of the all potential Raman vibrations range from from 0 V CuSCN, to −0.3 as V vs.well Ag/AgCl as that of forthe all substrate, four samples. was observedIn the potential in the potential range from range 0 Vfrom to +0.5 0 V Vto vs.–0.3 Ag/AgCl, V vs. Ag/AgCl the bands for all corresponding four samples. to the CuSCN vibrations change their relative intensities compared to those in the dry layers.

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In the potential range from 0 V to +0.5 V vs. Ag/AgCl, the bands corresponding to the CuSCN vibrations change their relative intensities compared to those in the dry layers. The disappearance of the Raman bands at negative potentials is most likely due to the formation of a passivation layer, which changes the surface reflectivityreflectivity and causescauses thethe Raman scattering scattering from from bulk bulk phonons phonons to decrea to decrease,se, as observed as observed previously previously (e.g. for (e.g., n-GaAs) for n- −1 [65].GaAs) In [65the]. high-frequency In the high-frequency region regionof 2000–2300 of 2000–2300 cm−1, we cm observe, we observe several several types of types major of changesmajor changes when approaching when approaching +0.5 V. +0.5 In the V. Incase the of case a CuSCN of a CuSCN layer on layer FTO on substrate, FTO substrate, three −1 clearlythree clearly resolved resolved peaks, peaks,located located at 2159, at 2167 2159, and 2167 2175 and cm 2175−1 are cmseen inare Figure seen in3E Figureor Figure3E −1 7A.or Figure While7 theA. Whilelowest thefrequency lowest peak frequency (at 2159 peak cm− (at1, assigned 2159 cm to the, assigned ν(CN) vibration to the ν of(CN) the − -SCNvibration− bridging of the unit) -SCN doesbridging not undergo unit) doesany recognizable not undergo change any recognizable upon the increasing change upon positivethe increasing potential, positivethe other potential, two high-frequency the other two components high-frequency switch components their relative switch intensities: their −1 ν − therelative intensity intensities: of the thepeak intensity at 2175 of cm the−1 ( peakν(CN) at vibration 2175 cm of( the(CN) Cu vibration−SCN isomer) of the decreases Cu SCN −1 isomer) decreases and the intensity of the− peak1 at 2167 cm increases. An increase−1 in the and the −intensity1 of the peak at 2167 cm increases. An increase in the 2167 cm peak intensity2167 cm is peakobserved intensity in the is observedcase of the in BDD the case substrates of the BDD also substrates(Figure 4E also or Figure (Figure 7B4E for or Figure7B for BDD, Figures S1 and S2 for BDD 2000). An upshift by 2 cm −1 is observed BDD, Figures S1 and S2 for BDD 2000). An upshift by 2 cm−1 is observed in the case of the in the case of the ν(CN) vibration of the Cu-SCN isomer on GC substrate, however, the ν(CN) vibration of the Cu-SCN isomer on GC substrate, however, the changes are not so changes are not so pronounced. The abovementioned behavior for CuSCN/FTO is in pronounced. The abovementioned behavior for CuSCN/FTO is in contrast with that of the contrast with that of the CuSCN layer grown on the HOPG substrate, for which only a CuSCN layer grown on the HOPG substrate, for which only a significant narrowing significant narrowing (∆FWHM is 3.8 cm−1) of the peak at 2175 cm−1 is observed when (ΔFWHM is 3.8 cm−1) of the peak at 2175 cm−1 is observed when the potential is increased the potential is increased to +0.5 V. A certain level of linewidth narrowing is present also to +0.5 V. A certain level of linewidth narrowing is present also for the other substrates for the other substrates (Table S2) and it correlates with their roughness, reflecting the (Table S2) and it correlates with their roughness, reflecting the increased sizes of the crys- increased sizes of the crystallites. tallites.

Figure 3. In situ Raman-µSEC spectra of the CuSCN layer (spectral regions of 160–300, 410–510, 720–780, and 2020–2220 cm–1 Figure 3. In situ Raman-μSEC spectra of the CuSCN layer (spectral regions of 160–300, 410–510, 720–780, and 2020–2220 in charts (A–C,E), respectively) electrodeposited on FTO substrate (spectral region of 1000–1200 cm–1 in chart (D) as a cm–1 in charts (A), (B), (C), and (E), respectively) electrodeposited on FTO substrate (spectral region of 1000–1200 cm–1 in functionchart (D) ofas thea function applied of potential the applied (vs. potential Ag/AgCl; (vs. shown Ag/AgCl; on the shown left-side on the chart) left-side in 6 Mchart) LiCl. in The6 M measurementLiCl. The measurement sequence wassequence as follows; was as the follows; ﬁrst spectrum the first wasspectrum acquired was at acquired 0 V (bottom at 0 pinkV (bottom trace) andpink the trace) last and spectrum the last was spectrum acquired was again acquired at 0 V (topagain pink at 0 trace).V (top Thepink sequence trace). The of positivesequence potentials of positive (from potent +0.1ials V (from to +0.5 +0.1 V) isV visualizedto +0.5 V) is by visualized black traces by andblack the traces negative and potentialthe negative of − potential0.3 V is visualized of –0.3 V is by visual the grayized trace.by the The gray reference trace. The (ex reference situ) Raman (ex situ) spectrum Raman of spectrum the dry CuSCN of the layerdry CuSCN on the FTOlayer substrate on the FTO is shown substrate for comparisonis shown for (bottom comparison red trace). (bottom Spectra red trace). are offset Spectra for clarity; are offset the intensityfor clarity; scale the isintensity identical scale for all is theidentical charts. for all the charts.

Nanomaterials 2021, 11, 1256 7 of 12 Nanomaterials 20212021,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 1313

Figure 4. In situ Raman-µSEC spectra of the CuSCN layer (spectral regions of 160–300, 410–510, 720–780, and 2020–2220 cm–1 Figure 4. InIn situsitu Raman-μSECSEC spectraspectra ofof thethe CuSCNCuSCN layerlayer (spectral(spectral regionsregions ofof 160–300,160–300, 410–510,410–510, 720–780,720–780,–1 andand 2020–22202020–2220 in charts–1 (A–C,E), respectively) electrodeposited on the BDD substrate (spectral region of 1000–1700 cm in chart (D) as a– cmcm–1 inin chartscharts ((A),), ((B),), ((C),), andand ((E),), respectively)respectively) electrodepositedelectrodeposited onon thethe BDD substrate (spectral region of 1000–1700 cm– 1function of the applied potential (vs. Ag/AgCl; shown on the left-side chart) in 6 M LiCl. The measurement sequence was 1 in chart (D) as a function of the applied potential (vs. Ag/AgCl; shown on the left-side chart) in 6 M LiCl. The measure- as in follows;chart (D the) as ﬁrsta function spectrum of the was applied acquired potential at 0 V (bottom(vs. Ag/AgCl; pinktrace) shown and on thethe lastleft-side spectrum chart) was in 6 acquired M LiCl. The at − measure-0.3 V (top ment sequence was as follows; the first spectrum was acquireded atat 00 VV (bottom(bottom pinkpink trace)trace) andand thethe lastlast spectrumspectrum waswas gray trace). The sequence of positive potentials (from +0.1 V to +0.5 V) is visualized by black traces. The reference (ex situ) acquiredacquired atat −−0.30.3 VV (top(top graygray trace).trace). TheThe sequencesequence ofof positivepositive potentiapotentialsls (from(from +0.1+0.1 VV toto +0.5+0.5 V)V) isis visualized by black traces. Raman spectrum of the dry CuSCN layer on the BDD substrate is shown for comparison (bottom blue trace). Spectra are The reference (ex situ) Raman spectrum of the dry CuSCN layer on the BDD substrate is shown for comparison (bottom blueoffset trace). for clarity; Spectra the are intensity offset for scale clarity; is identical the intensityintensity for all scalescale the charts. isis identicalidentical forfor allall thethe charts.charts.

Figure 5. In situ Raman-µSEC spectra of the CuSCN layer (spectral regions of 160–300, 410–510, 720–780, and 2020–2220 cm–1 Figure 5. InIn situsitu Raman-μSECSEC spectraspectra ofof thethe CuSCNCuSCN layerlayer (spectral(spectral regionsregions ofof 160–300,160–300, 410–510,410–510, 720–780,720–780, andand 2020–22202020–2220 in charts–1 (A–C,E), respectively) electrodeposited on the GC substrate (spectral region of 1210–1750 cm–1 in chart (D) as–1 a cmcm–1 inin chartscharts ((A),), ((B),), ((C),), andand ((E),), respectively)respectively) electrodepositedelectrodeposited onon ththee GCGC substratesubstrate (spectral(spectral regionregion ofof 1210–17501210–1750 cmcm–1 ininfunction chartchart ((D of)) asas the aa functionfunction applied potentialofof thethe appliedapplied (vs. Ag/AgCl;potentialpotential (vs.(vs. shown Ag/AgCl;Ag/AgCl; on the shsh left-sideown on the chart) left-side in 6 Mchart) LiCl. in The6 M measurementLiCl. The measurement sequence sequencesequencewas as follows; waswas asas the follows;follows; ﬁrst spectrum thethe firstfirst spectrumspectrum was acquired waswas acquiredacquired at 0 V (bottom atat 00 V (bottom pink trace) pink and trace) the and last the spectrum last spectrum was acquired was acquired at −0.3 at V −−(top0.30.3 VV gray (top(top trace). graygray trace). Thetrace). sequence TheThe sequencesequence of positive ofof positivepositive potentials potentialspotentials (from +0.1 (from(from V to+0.1+0.1 +0.5 VV V)toto is+0.5+0.5 visualized V)V) isis visuvisu byalizedalized black byby trace blackblack and tracetrace the negativeandand thethe negativepotentials potentials (from −0.3 (from V to −−0.3−0.30.1 VV V)toto –0.1 are–0.1 visualized V)V) areare visualizedvisualized by the gray byby thethe trace. graygray The tractrac referencee.e. TheThe referencereference (ex situ) (ex(ex Raman situ)situ) Raman spectrumRaman spectrumspectrum of dry CuSCN ofof drydry CuSCN layer on the GC substrate is shown for comparison (bottom green line). The Electrolyte solution 6 M LiCl. Spectra CuSCNlayer on layer the GC on substratethe GC substrate is shown is for shown comparison for comparison (bottom (b greenottom line). green The line). Electrolyte The Electrolyte solution 6solution M LiCl. 6 SpectraM LiCl. are Spectra offset are offset for clarity; the intensity scale is identical for all the charts. arefor offset clarity; for the clarity; intensity the scaleintensity is identical scale is foridentical all the for charts. all the charts.

NanomaterialsNanomaterials 20212021, 11, 11, x, 1256FOR PEER REVIEW 8 8of of 13 12 Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 13

Figure 6. In situ Raman-µSEC spectra of the CuSCN layer (spectral regions of 160–300, 410–510, 720–780, and 2020–2220 cm–1 Figure 6. In situ Raman-μSEC spectra of the CuSCN layer (spectral regions of 160–300, 410–510, 720–780, and 2020–2220 Figure 6. In situ Raman-μSEC spectra of the CuSCN layer (spectral regions of 160–300, 410–510, 720–780,–1 and 2020–2220 cmin–1 charts in charts (A–C (A,E),), ( respectively)B), (C), and electrodeposited(E), respectively) on electrodeposited the HOPG substrate on the (spectral HOPG regionsubstrate of 1550–1620 (spectral region cm in of chart 1550–1620 (D) as a cm–1 in charts (A), (B), (C), and (E), respectively) electrodeposited on the HOPG substrate (spectral region of 1550–1620 cmfunction–1 in chart of the(D) applied as a function potential of the (vs. applied Ag/AgCl; potential shown (vs. on Ag/AgCl; the left-side shown chart) on in the 6 M left-side LiCl. The char measurementt) in 6 M LiCl. sequence The meas- was cm–1 in chart (D) as a function of the applied potential (vs. Ag/AgCl; shown on the left-side chart) in 6 M LiCl. The meas- urementas follows; sequence the ﬁrst was spectrum as follows; was acquiredthe first spectrum at 0 V (pink was trace) acquired and the at 0 last V (pink spectrum trace) was and acquired the last atspectrum +0.5 V (top was black acquired trace). urement sequence was as follows; the first spectrum was acquired at 0 V (pink trace) and the last spectrum was acquired atThe +0.5 reference V (top black (ex situ)trace). Raman The reference spectrum (ex of situ) dry CuSCNRaman sp layerectrum on theof dry HOPG CuSCN substrate layer on is shownthe HOPG for comparison substrate is (bottomshown at +0.5 V (top black trace). The reference (ex situ) Raman spectrum of dry CuSCN layer on the HOPG substrate is shown foryellow comparison trace). Spectra(bottom are yellow offset trace). for clarity; Spectra the are intensity offset for scale clarity; is identical the intensity for all thescale charts. is identical for all the charts. for comparison (bottom yellow trace). Spectra are offset for clarity; the intensity scale is identical for all the charts.

Figure 7. Raman spectra of the electrodeposited CuSCN i) dry (ex situ) layers (bottom) and ii) in 6 M LiCl at +0.5 V vs. Figure 7. Raman spectra of thethe electrodepositedelectrodeposited CuSCNCuSCN (i)i) dry dry (ex (ex si situ)tu) layers (bottom) and ii) (ii) in in 6 6 M M LiCl LiCl at at +0.5 V V vs. vs. Ag/AgCl upon in situ Raman-μSEC measurement (top) on (A) FTO (red trace), (B) GC (green trace), (C) BDD (blue trace), Ag/AgCl upon upon in in situ situ Raman- Raman-μµSECSEC measurement measurement (top) (top) on on ( (AA)) FTO FTO (red (red trace), trace), ( (BB)) GC GC (green (green trace), trace), ( (CC)) BDD BDD (blue (blue trace), trace), (D) HOPG (yellow trace) substrates, in the region of SCN vibrations (2080–2200 cm−1). The fit envelopes and individual (D) HOPGHOPG (yellow(yellow trace)trace) substrates,substrates, inin thethe regionregion ofof SCNSCN vibrations vibrations (2080–2200 (2080–2200 cm cm−−1).). The The fit ﬁt envelopes and individual peak components, obtained using Voigt approximation, are shown in pink and gray, respectively. peak components, obtained using Voigt approximation,approximation, areare shownshown inin pinkpink andand gray,gray, respectively. respectively. Two processes affecting the electrodeposited CuSCN layer can thus, be recognized Two processes affectingaffecting thethe electrodeposited electrodeposited CuSCN CuSCN layer layer can can thus, thus, be be recognized recognized in in relation to the potential increase, manifested by the appearance of a new peak at 2167− inrelation relation to theto the potential potential increase, increase, manifested manifested by the by appearancethe appearance of a new of a peaknew atpeak2167 at cm21671 cm−1 (and/or change in relative intensity with the peak of the ν(CN) vibration of the cm(and/or−1 (and/or change change in relative in relative intensity intensity with the with peak the of peak the ν (CN)of the vibration ν(CN) vibration of the Cu −ofSCN the Cu−SCN isomer at −21751 cm−1) and the peak narrowing. The latter evidences an increased Cuisomer−SCN at isomer 2175 cm at 2175) and cm the−1) peakand the narrowing. peak narrowing. The latter The evidences latter evidences an increased an structuralincreased structural ordering (higher crystallinity and a smaller amount of defects) and/or an in- structuralordering (higherordering crystallinity (higher crystallinity and a smaller and amounta smaller of amount defects) of and/or defects) an and/or increased an ho-increased homogeneity of the crystallites, possibly due to the electrochemical dissolution of creasedmogeneity homogeneity of the crystallites, of the crystallites, possibly duepossibl to they due electrochemical to the electrochemical dissolution dissolution of smaller of smaller CuSCN particles. As for the former, the assignment of the new peak at 2167−1 cm−1 smallerCuSCN CuSCN particles. particles. As for theAs former,for the former, the assignment the assignment of the new of the peak new at peak2167 at cm 2167is cm not−1 is not straightforward. A clue can be found in the behavior of the lower frequency peaks, isstraightforward. not straightforward. A clue A can clue be can found be found in the in behavior the behavior of the of lower the lower frequency frequency peaks, peaks, espe- especially of the C-S stretching vibration at ~746−1 cm−1, which is also indicative of the especiallycially of the of C-S the stretching C-S stretching vibration vibration at ~746 at cm ~746, which cm−1, iswhich also indicative is also indicative of the Cu −ofSCN the Cu−SCN isomer [26,62]. Interestingly, this peak undergoes similar changes as the peak at−1 Cuisomer−SCN [26 isomer,62]. Interestingly, [26,62]. Interestingly, this peak undergoesthis peak undergoes similar changes similar as changes the peak as at the 2175 peak cm at, 2175 cm−1, i.e., shifts and appearance of asymmetry are observed. Due to its low intensity, 2175i.e., shifts cm−1, andi.e., appearanceshifts and appearance of asymmetry of asymmetry are observed. are Due observed. to its low Due intensity, to its low it isintensity, imprac-

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tical to reliably fit the 2167 cm−1 peak with more components for a precise comparison with the behavior of the 2175 cm−1 peak. Therefore, Figure S3 plots the correlation of the Raman shifts of the 746 and 2175 cm−1 peak structures, fitted as a single lineshape each. The fitted peak position is then an approximation of the center of mass of the structure. There is indeed a pronounced linear correlation, with the Pearson’s coefficient of 0.8. Hence, we surmise that the ‘new’ peak at 2167 cm−1 also belongs to a ν(CN) vibration of the Cu−SCN isomer, where changes in the bond strength take place upon positive bias. The different behavior of the peaks at the particular substrates points to a reorganization of the Cu−SCN units, depending on how strongly the Cu−SCN interacts with the surface—no changes were observed at HOPG, while all the other substrates induced peak shifts and/or intensity changes. The most pronounced changes observed on FTO indicate the oxygen on the surface, which is involved in the charge transfer, can locally modify the delocalization in the Cu−SCN, thereby influencing the molecular vibration. The high sensitivity of the C−N vibration to the charge distribution in the SCN− ion and its surroundings was also observed before [66]. In the case of the carbon substrates, these changes are also influenced by the presence of surface functional groups on the sp2 carbon (surface concentration of these groups is very small in HOPG compared to BDD and GC). The lower frequency regions in Figures3–6 (charts A–C) also exhibit pronounced changes in the positions or intensities of the Cu−S (205 cm−1) and Cu−N (241 cm−1) peaks. However, these cannot be reliably quantified due to the low intensity and broad nature of the bands. As in the case of spin-coated CuSCN on GC in our previous work [19], a new peak at 470 cm−1 appears at positive potentials with the simultaneous disappearance of the ν(SCN) band at 431 cm−1 (thiocyanate in the form of an S−bonding) in the case of electrodeposited CuSCN on GC (Figure5B). We assign the new peak at 470 cm −1 to the ν(NCS) bending mode, related to the presence of thiocyanate in the form of an N−bonding [61,67]. Charts D in Figures3–6 illustrate the spectroelectrochemical behavior of CuSCN on each substrate. In our earlier study [45] of bare BDD, we observed relative intensity changes of the sp2 carbon modes under the cathodic (intensity increase) and anodic (intensity decrease) treatment. We interpreted these changes as electrochemical charging and/or removal of the sp2 carbon surface layers [45]. Interestingly, we do not observe any such changes in the spectroelectrochemical behavior of the BDD substrates below the CuSCN layer (Figure3D or Figure S1D), indicating that the BDD is screened from the electrochemical action.

4. Conclusions We have studied an inorganic hole-transport material, copper (I) thiocyanate (CuSCN), electrodeposited from aqueous electrolyte solution into a thin layer on two types of conductive/semiconductive substrates; (i) carbon-based substrates (BDD, GC, HOPG) and (ii) carbon-free FTO conducting glass. SEM and Raman characterization evidence that electrodeposition from aqueous solution results in homogenous CuSCN layers (thickness of ca. 550 nm), predominantly in the form of the thiocyanate ion bonded to copper through its S−end (Cu−SCN bonding). The double-layer capacitance of CuSCN ranges from 0.3 mF/cm2 (CuSCN/BDD) to 0.8 mF/cm2 (CuSCN/GC). We carried out a comparative in situ Raman-µSEC study of CuSCN layers on all four substrates. A complete attenuation of all Raman vibrations from CuSCN, as well as those from the substrate, was observed at negative potentials (0 to −0.3 V vs. Ag/AgCl), caused by the formation of a passivation layer which changes the reﬂectivity of the surface. At positive potentials (0 to +0.5 V vs. Ag/AgCl), the bands corresponding to the CuSCN vibrations change their intensity compared to those in the dry layers (ex situ). The intensity of the strongest peak of the C≡N stretching vibration at 2175 cm−1 decreases and, at the same time, the intensity of another, initially unresolved peak at 2167 cm−1 increases. In cases where the individual components at 2175 and 2167 cm−1 are not resolved, the change in their relative intensities causes an apparent shift of the broad convoluted peak. The spectral changes are substrate- dependent: the largest changes are observed on FTO, whereas HOPG does not induce Nanomaterials 2021, 11, 1256 10 of 12

any. The shift and intensity variation of the peak composed of the 2175 and 2167 cm−1 bands strongly correlate with the shift of the C−S stretching vibration at 746 cm−1, thereby suggesting that the mode at 2167 cm−1 also originates from the Cu−SCN isomer. The position of this mode is inﬂuenced by the substrate, probably through local charges from the oxygen-bearing groups, which cause changes in the delocalization on the C−S bond. We also observed narrowing of the bands upon the application of positive potentials, which reﬂects an increased ordering of the crystal lattice.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/nano11051256/s1, Figure S1: Raman-µSEC spectra of the CuSCN layer electrodeposited on BDD 2000 substrate, Figure S2: Raman-µSEC spectra (experimental and fit) of the CuSCN layer electrodeposited on BDD 2000 substrate in the region of SCN vibrations, Figure S3: Correlation of the Raman shifts of the C−S stretching vibration with the C−N stretching vibration, Table S1: Fitting parameters of Raman ν (C−S) position of the CuSCN layers electrodeposited on GC, BDD, HOPG, FTO, Table S2: Fitting parameters of Raman νas(C≡N) position of the CuSCN layers electrodeposited on GC, BDD, HOPG, FTO. Author Contributions: Conceptualization, Z.V.Ž. and L.K.; methodology, Z.V.Ž.; formal analysis, Z.V.Ž., M.B. and M.V.; investigation, Z.V.Ž. and M.B.; resources, L.K. and O.F.; writing—original draft preparation, Z.V.Ž. and O.F.; writing—review and editing, O.F., L.K., M.V. and M.B.; supervision, L.K. and O.F. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Czech National Foundation, contract No. 18-08959S. The support by European Regional Development Fund; OP RDE; Project: “Carbon allotropes with rationalized nanointerfaces and nanolinks for environmental and biomedical applications” (No. CZ.02.1.01/0.0/0.0/16_026/0008382) is also acknowledged. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors would like to thank Václav Petrák for providing BDD substrates. Conflicts of Interest: The authors declare no conflict of interest.

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