Electrochemical Self-assembly Research Articles

EM and CT synergistic SERS enhancement in Si/TiO2/Ag heterostructures via local interfacial effect

The Si/TiO2/Ag heterostructures with high SERS sensitivity were prepared by sol–gel and electrochemical self-assembly. The electromagnetic (EM) and charge transferring (CT) synergistic mechanism for SERS enhancement was explored by FDTD. The “localized interfacial effect (LIE)” of metal–semiconductor heterostructures can result in the CT between them and the charge carriers’ redistribution, which can facilitate the probe molecules’ adsorption and generate the 3D volume-enhancement of electromagnetic fields, therefore improve the SERS performance effectively. Furthermore, the morphology of Ag was flexibly regulated by changing the applied voltage, and the optimal SERS performance was achieved. With the R6G as a probe molecule, the ultra-low concentration of 10-13 M can be detected. The EF is estimated to be about 1.75 × 1011, and the RSD is about 4.3 %. The photocatalytic effect of Si/TiO2/Ag heterostructure can realize the SERS substrates’ UV self-cleaning and recyclable use with high stability.

Highly ordered Si-based ZnO/Ag bilayer structure for recyclable SERS substrates with ultra-sensitivity

Compositing semiconductors with noble metal is considered as an effective strategy for improving SERS enhancement attributed to synergistic contribution of LSPR and CT mechanisms. In this work, highly ordered ZnO/Ag bilayer structures on p-type silicon wafers are prepared by photolithography and electrochemical self-assembly. The synergistic effect of EM and CT for SERS enhancement is identified by the Raman, PL spectra and 3D FDTD simulation. Flower-like Ag deposited on coarse ZnO films exhibits high SERS sensitivity. The ultra-low concentration of R6G molecules as low as 10−15 M can be detected, and the EF is estimated to be 8.08 × 1013. Due to the excellent photocatalytic property of ZnO, the bilayer structures possess steady and effective UV-cleaning performance. After being UV irradiated for 10 min, this SERS substrate can be re-used for many times. And the detecting RSD is about 4.11 %. The multi-functional ZnO/Ag bilayer structures with a wide direct bandgap has the prominent capacity to provide an efficient charge transfer path to possess trace detection ability for many probe molecules.

MOFs-functionalized regenerable SERS sensor based on electrochemistry for pretreatment-free detection of serum alkaline phosphatase activity

Alkaline phosphatase (ALP), widely distributing in the human body, associates with many liver and bone diseases. Herein, we designed a reusable surface-enhanced Raman scattering (SERS) sensor via electrochemical self-assembly of Au nanoparticles (AuNPs) and zinc-based metal organic frameworks (Zn-MOFs) at anodic aluminum oxide (AAO) membrane for detecting ALP activity. The detection is based on the sensitive SERS response of the newly synthesized 2-mercaptoquinone (2-MBQ) modified on AuNPs surface towards ascorbic acid (AA) formed from the dephosphorylation of ascorbic acid 2-phosphate (AA2P) in the presence of ALP. Benefitting from the sieving function of Zn-MOFs and superiority of SERS, the developed sensor exhibited strong anti-interference performance and wide detection range from 1 to 300 U/L with a detection limit of 0.38 U/L, which allowed the direct detection of ALP activity in serum without any pretreatment. Besides, we regenerated the used sensor through the electrochemical oxidation of the reduced probe molecule 2-mercaptohydroquinone (2-MHQ). The regenerated sensor exhibited good maintenance of sensitive response to ALP, in favor of improving the usage efficiency and cutting the test cost. This work provides a promising platform for early diagnosis of ALP-related diseases and paves the way towards developing regenerable and pretreatment-free SERS sensors.

Electrochemical Self-Assembled Gold Nanoparticle SERS Substrate Coupled with Diazotization for Sensitive Detection of Nitrite.

The accurate determination of nitrite in food samples is of great significance for ensuring people’s health and safety. Herein, a rapid and low-cost detection method was developed for highly sensitive and selective detection of nitrite based on a surface-enhanced Raman scattering (SERS) sensor combined with electrochemical technology and diazo reaction. In this work, a gold nanoparticle (AuNP)/indium tin oxide (ITO) chip as a superior SERS substrate was obtained by electrochemical self-assembled AuNPs on ITO with the advantages of good uniformity, high reproducibility, and long-time stability. The azo compounds generated from the diazotization-coupling reaction between nitrite, 4-aminothiophenol (4-ATP), and N-(1-naphthyl) ethylenediamine dihydrochloride (NED) in acid condition were further assembled on the surface of AuNP/ITO. The detection of nitrite was realized using a portable Raman spectrometer based on the significant SERS enhancement of azo compounds assembled on the AuNP/ITO chip. Many experimental conditions were optimized such as the time of electrochemical self-assembly and the concentration of HAuCl4. Under the optimal conditions, the designed SERS sensor could detect nitride in a large linear range from 1.0 × 10−6 to 1.0 × 10−3 mol L−1 with a low limit of detection of 0.33 μmol L−1. Additionally, nitrite in real samples was further analyzed with a recovery of 95.1−109.7%. Therefore, the proposed SERS method has shown potential application in the detection of nitrite in complex food samples.

Multidimensional Ti3C2Tx MXene Architectures via Interfacial Electrochemical Self-Assembly.

An effective pathway to build macroscopic scale functional architectures bearing diverse structural dimensions is one of the critical challenges in the two-dimensional (2D) MXene research area. Unfortunately, assembling MXene without adhesive binder is largely limited due to its innate brittle nature and the relatively weak inter-flake van der Waals contact, in contrast to other mechanically compliant 2D materials such as graphene. Herein, an electrochemical self-assembly of pure Ti3C2Tx MXenes is presented for functional multidimensional MXene structures, effectively driven by layer-by-layer spontaneous interfacial reduction at metal template surfaces and subsequent defunctionalization. A three-dimensional open porous aerogel as well as 2D highly stacked thin film structures could be readily obtained in this approach, along with largely enhanced electrical properties induced by spontaneous removal of charge-trapping oxygen functional groups. Accordingly, supercapacitors and electromagnetic interference shielding films based on the multidimensional assembly demonstrate excellent performances.

Electrochemical self-assembly of bunch-shaped ZnO nanowire without catalysts

There are many types of metal oxide nanostructure and one of them is zinc oxide (ZnO). Despite of vast field of application for this material, the fabrication process of nanostructure (ZnO) is rather costly or complicated due to exotic equipment or reagent required. To the leading of our understanding, there are no details about synthesizing the bunch-shaped ZnO nanowire patterns on silicon substrate. A series based on aqueous medium composition: (x) Zn (NO3)2 + 0.1 NaOH, where 0.005 ⩽ x ⩽ 0.2 mol% is prepared using chemical bath method. The distribution and size of ZnO nanostructure was determined using FESEM spectroscopy. Meanwhile, EDX spectra show the presence of ZnO nanostructures in all samples. These researches generate new knowledge on embodiment of ZnO nanostructures on silicon thin film which may useful for finding material with high optical quality for diverse application in photonic devices.

(Invited) Mechanistic Studies for Electrochemical Self-Assembly of Cuscn/Stilbazolium Dye Hybrid Thin Films

We have established electrochemical self-assembly (ESA) of inorganic / organic hybrid thin films in which the inorganic is CuSCN, known as a wide bandgap p-type semiconductor, whereas the organic is 4-N,N-dimethylamino-4’-N’-methylstilbazolium chromophore (abbreviated as DAS+) as its salt with tosylate (DAST) known to exhibit second-order nonlinear optical property for terahertz emitters. The CuSCN/DAS hybrid thin films by ESA show concerted photoluminescence based on energy transfer from inorganic CuSCN to organic DAS, making us anticipate its potential use in optoelectronic devices [1]. Understanding the mechanism of ESA to obtain tuning knob to maximize its functionality is therefore important.In our previous study, switching of dye loading mechanism has been suggested, depending on DAS concentration (C DAS) in the bath [2]. With low C DAS, the loading is limited by diffusion so that DAS is entrapped within CuSCN crystal grains, while surface reaction of hybridization begins to limit the dye loading with high C DAS, resulting in formation of unique nanostructures as well as phase separation of inorganic and organic domains. The border for the switching of dye loading mechanism was found as the ratio between bulk concentrations of dye and the [Cu(SCN)]+ complex, C DAS/C comp = ca. 1/31 in case of DAS+ [3].In this study, electrochemical analysis has been performed employing rotating disk electrode (RDE) under variation of bulk concentration of dye (DAST and CNST = 4-cyano-4’-(N’-methyl)stilbazolium tosylate for comparison) to verify the proposed mechanism. CNS+ has a strongly electron withdrawing -CN group and therefore bears a much larger dipole moment of 23.5016 Debye than that of DAS+ (10.1442 Debye) as determined by DFT calculation. Thus, a much stronger interaction of CNS+ with CuSCN (essentially with SCN- ion) is expected than DAS+.The overall behavior of CNS+ has been found exactly the same as DAS+ such as changing the crystal orientation of β-CuSCN to lay down its c-axis and phase transition from β- to α-CuSCN. However, such transitions were found for smaller concentrations of CNS+ than DAS+ because of its stronger affinity with CuSCN. The switching dye loading mechanism and associated change of the hybrid structure has also been observed for CNS. Fig. 1 shows plots of amount of dye loaded into the hybrid thin films against the dye concentrations in the electrolytic bath for DAS+ and CNS+. The steep slopes in low dye concentration range represent diffusion limited loading, whereas the moderate slopes are those limited by surface reaction. The bending points, C DAS = 60 μmol dm-3 and C CNS = 200 μmol dm-3, indicate the switching borders for C comp = 2.5 mM. Because of the increased stability of adsorption of CNS+ on CuSCN surface, diffusion limited loading, thus all dyes reaching the electrode surface buried inside the film, continues to operate up to this high dye concentration. The increased stability of CuSCN/CNS surface complex also resulted in about 3 times more efficient dye uptake in the surface reaction limited regime. These results nicely confirm the validity of the proposed mechanism both for DAS+ and CNS+, and that the switching border can be varied by the stability of CuSCN/dye complex. Full analysis should be able to determine C CNS/C comp ratio as the switching border.[1] K. Uda et al. ACS Omega, 4, 4056-4062 (2019).[2] Y. Tsuda et al., Monatshefte für Chemie, 148, 845-854 (2017).[3] Y. Tsuda et al., J. Electrochem. Soc. 166(9) B3096-B3102 (2019). Figure 1

Electrochemical Self-Assembly of CuSCN/4-Cyano-4’-(N’-methyl)Stilbazolium Hybrid Thin Films

Electrochemical self-assembly (ESA) of CuSCN/4-cyano-4’-(N’-methyl)stilbazolium (CNS+) hybrid thin films has been performed and analyzed in comparison with CuSCN/4-(N,N-dimethylamino)-4’-(N’-methyl)stilbazolium (DAS+) hybrid system. Due to the presence of strongly electron-withdrawing -CN group, CNS+ has a much higher dipole moment than DAS+ to exhibit stronger affinity to CuSCN. CNS+ therefore gave strong impact to the crystal growth of CuSCN with its smaller concentration than DAS+, such as changing crystal orientation, creating unique nanostructure and phase transition from β- to α-CuSCN. CNS+ has been found to obey the same mechanism as DAS+ for its loading into the film. The limit for diffusion limited loading was pushed up to its bulk concentration of 200 μmol dm-3, much higher than that for DAS+ (60 μmol dm-3). Also, the efficiency of dye uptake in the surface reaction limited loading was enhanced about 3 times due to the improved stability of CuSCN/CNS surface complex.

Electrochemical Self-Assembly of CuSCN/4-Cyano-4’-(N’-Methyl)Stilbazolium Hybrid Thin Films

Electrochemically self-assembled (ESA) CuSCN/4-(N,N-dimethylamino)-4’-(N’-methyl)stilbazolium (DAS) hybrid thin film was found to exhibit concerted photoluminescent (PL) property based on energy transfer from CuSCN to DAS [1]. The amount of co-precipitated DAS (P DAS) during ESA linearly increased on increasing DAS concentration (C DAS) in the bath but changes its slope due to switching of dye loading mechanism from diffusion limited to surface reaction limited [2]. The full analysis revealed the switching border to be dependent on the ratio between the concentration of CuSCN precursor complex, [Cu(SCN)]+ (C comp), and C DAS as C comp/C DAS = ca. 31 [3]. When the dye loading operates as surface reaction limited, phase separation of CuSCN and DAS in nano-scale occurs to exhibit efficient PL. Thus, finding an alternative chromophore with higher or lower affinity with CuSCN than DAS should result in shifting of the switching border with smaller or larger C comp/C dye than the above indicated value for DAS. In this study, we employed 4-cyano-4’-(N’-methyl)stilbazolium (CNS) for ESA with CuSCN. CNS has a strong dipole moment due to the presence of strongly electron withdrawing cyano group and thus is expected to exhibit a higher affinity to CuSCN than DAS.CuSCN/CNS hybrid thin films were obtained by cathodic electrolysis with various C CNS while fixing C comp = 2.5 mM. P CNS linearly increased upon increase of C CNS and changes its slope at around 200 μM, far larger than the switching point observed for DAS (around 60 μM). The steep slope for the low C CNS range should be determined by the diffusion of CNS to yield its diffusion coefficient D CNS = 1.49×10-6 cm2 s-1, slightly larger than that found for DAS (D DAS = 1.25×10-6 cm2 s-1). On the other hand, about 3 times larger P CNS for given dye concentrations and steeper slope of surface reaction limited regime for CNS than those found for DAS indicate higher adsorption stability of CNS than DAS to CuSCN surface. Soaking the CuSCN/CNS hybrid thin films electrodeposited at C CNS = 60 and 800 μM to represent the dye loading as diffusion and surface reaction limited, respetively, into dimethylacetamide resulted in partial and total extraction of CNS to reveal entrapment and phase-separation of CNS, just like the case of CuSCN/DAS. The maximum P CNS appears to be saturated when C CNS > 800 μM for CNS aggregate to occupy close to 30% of the total film volume.The present results already nicely support the validity of the switching of ESA mechanism, being common for both DAS and CNS. Smaller C comp/C CNS for the switching border and larger stability constant for surface complex formation are expected for CNS than DAS from full analysis.[1] K. Uda et al., ACS Omega, 4, 4056-4062 (2019).[2] Y. Tsuda et al., Monatsh. Chem., 148, 845 (2017).[3] Y. Tsuda et al., J. Electrochem. Soc., 166, B3096-B3102 (2019). Figure 1

Mechanistic Studies for Electrochemical Self-Assembly of CuSCN/Stilbazolium Dye Hybrid Thin Films

We have found electrochemical self-assembly (ESA) of inorganic / organic hybrid thin films in which the inorganic is CuSCN, known as a wide bandgap p-type semiconductor, whereas the organic is 4-N,N-dimethylamino-4’-N’-methylstilbazolium chromophore (abbreviated as DAS) as its salt with tosylate (DAST) known to exhibit second-order nonlinear optical property for terahertz emitters. DAS is also known to yield a layered inorganic-organic hybrid crystal in a (DAS)(Cu5I6) composition [1]. Strong dipole-dipole interaction of the DAS chromophores aligned between the CuI layers results in a spontaneous photocarrier generation and ambipolar transport in a single absorber solar cell. Thus, CuSCN/DAS hybrid thin films can be attractive alternatives for such opto-electrical applications, when ordered arrangement of DAS chromophore is achieved during ESA.In our previous study, switching of dye loading mechanism has been suggested, depending on DAS concentration (C DAS) in the bath [2]. In low C DAS, the loading is limited by diffusion so that DAS is entrapped within CuSCN crystal grains, while surface reaction of hybridization begins to limit the dye loading in high C DAS, resulting in formation of unique nanostructures as well as phase separation of inorganic and organic domains.In this study, electrochemical analysis by employing rotating disk electrode (RDE) has been performed to verify the mechanism of ESA and also to explore the limit of DAS loading. Electrodeposition of CuSCN undergoes as limited by transport of 1 : 1 complex between Cu2+ and SCN- ions, namely [Cu(SCN)]+ species, near 100% Faradic efficiency with marginal influence by the presence of DAS. Thus, the rate of CuSCN precipitation, P CuSCN, is always proportional to the concentration of the complex (C comp), and also to ω 1/2 (ω = angular speed of rotation of RDE) therefore P CuSCN can be expressed Eq. (1) from Levich equation. P CuSCN = J/F = 0.62×C comp×D comp 2/3×ν -1/6×ω 1/2 ··· (1)On the other hand, the rate of DAS precipitation, P DAS, should also be proportional to ω 1/2 under the regime of diffusion limited loading with a given C DAS, and is independent of CuSCN formation rate. P DAS = 0.62×C DAS×D DAS 2/3×ν -1/6×ω 1/2 ··· (2)Plots of P DAS against C comp for variation of C DAS were examined, and indeed such a trend as predicted from the above-mentioned model was found (Fig. 1). In the high C comp range, P DAS is independent of C comp and appears proportional to C DAS, because of the diffusion limited loading mechanism. When C comp goes below certain concentration, P DAS changes proportionally to C comp, while its slope also increase as dependent on C DAS. The relationship between the switching C comp and C DAS yields C DAS / C comp = 1/31 as the border for the switching of the mechanism [3], whereas the dependence of the slope on C DAS in the low C comp range should reveal the stoichiometry of surface complex between CuSCN and DAS. When the rate surface complex formation, P DAS become proportional to the P CuSCN, assuming n of DAS complexes with single CuSCN site yields the equilibrium as the right and the surface is expected to be always equilibrated such equation, P CuSCN + nC DAS ⇄ P DAS ··· (3)Stability constant K should be expressed like Eq. (4), K = P DAS/P CuSCN × C DAS n ··· (4)Eq. (4) can be re-written by inserting Eq. (1), P DAS/C comp= 0.62×D comp 2/3×ν -1/6×ω 1/2 ×K×CDAS n ··· (5)Plotted slope of Fig. 1 in the low C comp range (P DAS/C comp) against C DAS and did fitting with n as variable and then resulted n = 0.6. It means approximately one DAS coordinating two CuSCNs and stability constant K can be calculated as 6.78×102 mol−0.5 cm2.5.[1] Elena Cariati et al., Adv. Mater., 13, 1665-1668 (2001).[2] Yuki Tsuda et al., Monatshefte für Chemie, 148, 845-854 (2017).[3] Yuki Tsuda et al., J. Electrochem. Soc. 166(9) B3096-B3102 (2019). Figure 1

Concerted Photoluminescence of Electrochemically Self-Assembled CuSCN / Stilbazolium Dye Hybrid Thin Films

Whether they are inorganic or organic, the key words for creation of new functional materials should be (1) solution, (2) low temperature, and (3) rare-element-free for sustainable development. As a leading example, we explore electrochemical self-assembly (ESA) of inorganic / organic hybrid thin films. In addition to the interests to study evolution of unique hybrid structures, concerted new functionalities are anticipated due to intimate interaction between the inorganic and organic constituents. Herein, we report ESA of CuSCN / 4-(N,N-dimethylamino)-4’-(N’-methyl)stilbazolium (DAS) hybrid thin films and its concerted photoluminescence (PL) behavior [1]. Hybrid thin films of crystalline CuSCN/DAS in three distinctively different nanostructures were obtained by ESA from a single pot containing all the chemical ingredients. Adsorption of DAS during the electrochemical precipitation of CuSCN result in significant change of film morphology, crystal structure and its orientation, as DAS “occluded” into crystal grains of β-CuSCN, DAS phase separated nano-“haircomb” shape β-CuSCN, and DAS phase-separated in nano-“scale” shape α-CuSCN, by simply increasing DAST concentration in the bath [2]. Their optical properties for UV-vis-NIR absorption, photoluminescence (PL), and PL excitation (PLE) spectra were examined between 77 and 298 K, in comparison with solution and solid powder of DAS tosylate (DAST). While all the fluorescent dyes we previously found to hybridize with CuSCN underwent quenching due to hole injection from dye excited state to the valence band of CuSCN, DAS presented the first and only example to exhibit PL when combined with CuSCN [3]. DAST exhibited a strong exciton-phonon coupling to weaken, broaden, and red shift PL at room temperature, so that it inversely is strongly enhanced, sharpened, and blue-shifted at 77 K. However, the PL of the same dye in the hybrid thin film shows a slight red shift and not much intensified. Dielectric environment as well as ordered alignment of DAS greatly stabilize exciton against thermalization loss by suppressing twisted intramolecular charge transfer (TICT) and exciton-phonon coupling. The smallest temperature dependence, thus to be interpreted as the strongest exciton stabilization, was found for the “haircomb” β-CuSCN/DAS hybrid. The PLE spectra of the “scale” α-CuSCN/DAS hybrid show a prominent sharp peak at 380 nm and another double-peak between 400 and 450 nm by cooling temperature. The former corresponds to the band-edge absorption of CuSCN, while the latter being similar to the LMCT absorption of [Cu(SCN)]+ complex. It has been confirmed that those peaks in PLE spectra indicate the contribution of CuSCN thin film without DAS for the generation of PL. These contributions of the light absorption by CuSCN to the PL from DAS clearly indicate energy transfer from CuSCN to DAS as a concerted PL mechanism. Concerted PL by energy transfer from CuSCN to DAS has also been found to occur efficiently, especially for the “scale” α-CuSCN/DAS with the largest contact area due to interpenetrating hybrid nanostructure in the smallest domain size. Those hybrid thin film having the concerted PL mechanism (Fig. 1) can possibly make these materials useful as LEDs.[1] K. Uda et al., ACS Omega, 2019, 4 (2), 4056-4062.[2] Y. Tsuda et al., Chem., 2017, 148, 845-854.[3] K. Uda et al., ECS Trans., 2019, 88, 323-333. Figure 1

Surface-Oxygen Induced Electrochemical Self-Assembly of Mesoporous Conducting Polymers for Electrocatalysis

Porous polymers have immense potential in catalysis, energy conversion and storage, separation sciences and life sciences due to their high surface area and high diffusion flux. Developing porous polymers with micro and mesoscale porosity with long-range order is challenging and involves multistep templated approaches. Here we demonstrate a simple surface-oxygen induced electropolymerization route to directly obtain self-assembled porous polymers of polyparaphenylene (PPP) and PPP based copolymers in ionic liquids. By combining experimental and theoretical studies, we show that surface oxygen on Cu changes the orientation and assembly of benzene which then results in a change in electropolymerization mechanism leading to a self-assembled porous structure with porosity between 2 and 5 μm. Furthermore, with controlled experimental parameters, bicontinuous conducting polymers with porosity of >10 μm are obtained. The porous conducting polymers show absorption of light in the visible range which was also used as an efficient electrode for investigation of the photoelectrochemical oxygen evolution reaction.

Electrochemical Self-Assembly of a 3D Interpenetrating Porous Network PEDOT-PEG-WS2 Nanocomposite for High-Efficient Energy Storage

Like interchange bridges used in traffic, 3D interpenetrating porous network (3D IPN) nano-/micromaterials are of great significance in the field of energy storage. Here, we developed a 3D IPN poly(3,4-ethyleenedioxythiophene)-poly(ethylene glycol)-WS2 (PEDOT-PEG-WS2) nanocomposite through the electrochemical self-assembly of EDOT and WS2 nanosheets with the assistance of PEG. The scanning electron microscopy, energy dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy results explicitly demonstrated the formation of the 3D IPN PEDOT-PEG-WS2 nanocomposite. The electrochemical results indicated that the PEDOT-PEG-WS2 nanocomposite had high specific capacitance of 236.5 mF cm–2 at 5 mV s–1, which was about 3.1 times higher than that for PEDOT-PEG (77.5 mF cm–2). The symmetric supercapacitors based on PEDOT-PEG-WS2 presented a high specific energy of 78 W h m–2 at 900 W m–2 and high cycling stability of 91% after 5000 cycles. It was proposed that the enhanced performances were attributed to th...

Switching of Dye Loading Mechanism in Electrochemical Self-Assembly of CuSCN/4-(N,N-dimethylamino)-4′- (N′-methyl)Stilbazolium Hybrid Thin Films

Electrochemical self-assembly of CuSCN/4-(N,N-dimethylamino)-4′-(N′-methyl)stilbazolium (DAS) hybrid thin films has been carried out on systematic variation of bulk concentrations of [Cu(SCN)]+, DAS tosylate (DAST) and changing their flux density by angular speed of rotation of the rotating disk electrode. Switching of DAS loading mechanism dependent on the DAST concentration in the bath has been verified and quantified. The switching occurs at the concentration ratio of the precursors DAST/[Cu(SCN)]+ = ca. 1/31, molar ratio of the product DAS/CuSCN = ca. 1/48, below which the loading becomes diffusion limited for DAS to be occluded in the CuSCN grains, whereas surface reaction for formation of complex between CuSCN and DAS begins to limit the DAS loading when the relative concentration of DAST exceeds this border, resulting in a phase-separated precipitation of CuSCN and (DAS)(SCN) aggregate in unique nanostructures.

Electrodeposition of Zn-Terephthalate Metal-Organic Framework Thin Films

Electrochemical self-assembly (ESA) of metal-organic framework (MOF) thin films of Zn2+ ions and terephthalic acids (TPA) in a layered structure and with a composition of Zn3(OH)4(TPA)•6H2O was made possible by addition of TPA into the bath for electrodeposition of ZnO, namely, O2-saturated aqueous solution of ZnCl2. The deposition of Zn-TPA MOF, however, resulted in a strong passivation of the electrode towards O2 reduction to limit the electrochemical growth. Interestingly, chemical precipitation of Zn-TPA MOF was found to take over the film growth to achieve a thickness of about 2 µm in the extended electrolysis, in spite of the negligibly small current. Due to decrease of TPA concentration in the solution bulk, the electrode finally recovered the activity for the O2 reduction, which, however, resulted in the electrodeposition of ZnO to be overlaid on the Zn-TPA MOF. The electrodeposited Zn-TPA MOF was well adherent to the substrate.

Switching of Dye Loading Mechanism in Electrochemical Self-Assembly of CuSCN/DAS Hybrid Thin Films

Electrochemical self-assembly of CuSCN/4-(N,N-dimethylamino)-4’-(N’-methyl)stilbazolium (DAS) hybrid thin films has been carried out on systematic variation of bulk concentrations of [Cu(SCN)]+, DAS tosylate (DAST) and changing their flux density by angular speed of rotation of the rotating disk electrode. Switching of DAS loading mechanism dependent on the DAST concentration in the bath has been verified and quantified. The switching occurs at the concentration ratio [Cu(SCN)]+/DAST = ca. 40, above which the loading becomes diffusion limited for DAS to be occluded in the CuSCN grains, whereas the second order reaction rate of complex formation between CuSCN surface sites and DAS begins to limit the DAS loading when the relative concentration of DAST exceeds this border, resulting in a phase-separated precipitation of CuSCN and (DAS)(SCN) aggregate in unique nanostructures.

Rimelike Structure-Inspired Approach toward in Situ-Oriented Self-Assembly of Hierarchical Porous MOF Films as a Sweat Biosensor

Surface-supported metal-organic framework (MOF) films hold fantastic promises for viable scientific applications, particularly in sensors and electronic devices. However, slow diffusion, limited mass transfer, and low conductivity hinder the industrial application of MOFs. Herein, we propose a rime-inspired MOF film based on a kind of beautiful natural landscape. To mimic rime architecture, we compare and conclude the intrinsic similarity between natural biomineralization and electrochemical self-assembly of MOFs and used an anodic-induced approach to producing rime-structured MOF architecture. Interestingly, the MOF film with space-filling macro-meso-micropores exhibits remarkable electrochemical sensing performances for simultaneous determination of lactate and glucose, including high sensitivity, excellent selectivity, and a wide linear range in a wide range of pH values. Moreover, this rime-inspired system is able to be applied as a biofunctional human perspiration sensing platform. Our work opens a new horizon for poring on biomimetic rime concept to explore specifically structured MOFs with more diverse functionalities.

Switching of Dye Loading Mechanism in Electrochemical Self-Assembly of Cu­SCN-D­AST Hybrid Thin Films

We have found electrochemical self-assembly (ESA) of inorganic / organic hybrid thin films in which the inorganic is CuSCN, known to be a wide bandgap p-type semiconductor, whereas the organic is 4-N,N-dimethylamino-4’-N’-methylstilbazolium chromophore (abbreviated as DAS+) as its salt with tosylate (DAST) is known to exhibit second-order nonlinear optical property for terahertz emitters [1]. DAS+ is also known to yield a layered inorganic-organic hybrid crystal in a (DAS)(Cu5I6) composition [2]. Strong dipole-dipole interaction of the DAS+ chromophores aligned between the CuI layers results in a spontaneous photocarrier generation and ambipolar transport in a single absorber solar cell. Thus, CuSCN/DAS hybrid thin films can be attractive alternatives for such opto-electrical applications, when ordered arrangement of DAS+ chromophore is achieved during ESA. In our previous study, switching of dye loading mechanism has been suggested, depending on DAS+ concentration in the bath [1]. In low DAS+ concentration range, the loading is limited by diffusion so that DAS+ is entrapped within CuSCN crystal grains, while surface reaction of hybridization begins to limit the dye loading in high DAS+ concentration range, resulting in formation of unique nanostructures as well as phase separation of inorganic and organic domains. In this study, electrochemical analysis by employing rotating disk electrode (RDE) has been performed to verify the mechanism of ESA and also to explore the limit of DAS+ loading.Electrodeposition of CuSCN undergoes as limited by transport of 1 : 1 complex between Cu2+ and SCN- ions, namely {[Cu(SCN)]+} species, near 100% Faradic efficiency with marginal influence by the presence of DAS+. Thus, the rate of CuSCN growth is always proportional to the concentration of the active species, and also to ω1/2 (ω = angular speed of rotation of RDE). On the other hand, the rate of DAS+ precipitation, v(DAS)(SCN), should also be proportional to ω1/2 under the regime of diffusion limited loading with a given [DAS+], and is independent of CuSCN formation rate. When the rate surface complex formation begins to limit the rate of DAS+ precipitation, the amount of DAS+ loading should become proportional to the rate of CuSCN deposition, if a second order rate law as shown below holds,v(DAS)(SCN) (mol s-1 cm-2) = k[DAS+][site] (1)where, k (mol-1 cm3 s-1) is the second order reaction rate constant for formation of surface complex and [site] is the surface concentration of newly formed CuSCN sites, which should be expressed as,[site] (mol cm-2) = A × 0.62 × {[Cu(SCN)]+} × D{[Cu(SCN)]+}2/3 × ν(methanol, 298 K)-1/6 × ω1/2 (2)which is simply derived from Levich equation for diffusion limited electrodeposition of CuSCN, multiplied by a proportionality constant, A (s), to express the activity of the surface site.The amount of DAS+ loaded into the film electrodeposited for a given [DAS+] under variation of {[Cu(SCN)]+} in the bath was examined, and indeed such a trend as predicted from the above-mentioned model was found (Fig. 1). In the high {[Cu(SCN)]+} range, DAS+ loading is independent of {[Cu(SCN)]+} and appears proportional to [DAS+], because of the diffusion limited loading mechanism. When {[Cu(SCN)]+} goes below certain concentration, DAS+ loading changes proportionally to {[Cu(SCN)]+}, for a given [DAS+], but is also proportional to [DAS+] for a given {[Cu(SCN)]+}, as expected for the surface reaction limitation expressed by Eq. (1). The switching from surface reaction to diffusion limitation occurs at the lower {[Cu(SCN)]+} when [DAS+] goes lower, as recognized by the shift of the border between {[Cu(SCN)]+} dependent and independent parts.[1] Yuki Tsuda et al., Monatshefte für Chemie, 148, 845-854 (2017) [2] Elena Cariati et al., Adv. Mater., 13, 1665-1668 (2001). Figure 1

Electrochemical Self-Assembly of Cu­SCN / Dye Hybrid Thin Films and Their Optical Properties

Introduction: Combination of inorganic and organic materials offers unlimited new opportunities. While “composites” stands for their physical mixtures, “hybrids” and “inorganic/organic compounds” are especially interesting for their new functionalities, as they are achieved by self-organization due to “chemistry” among the constituents. We have established electrochemical self-assembly (ESA) of inorganic/organic hybrid thin films from one pot containing all the ingredients. 1,2) In this work, we have employed cathodic electrodeposition of CuSCN as the test bed for the ESA with various organic dyes to understand its principle and evaluated optical properties of the product thin films. Experiments: CuSCN / dye hybrid thin films were cathodically electrodeposited according to the reaction, [Cu(II)(SCN)]+ + e- → Cu(I)SCN, in the presence of various organic dyes shown in Fig. 1. Photoluminescence (PL) and PL excitation (PLE) spectra were measured between 77 and 298 K on a Horiba Fluorolog-3 equipped with a high power Xe lamp excitation source and lq. N2-cooled PMT detector. Results and discussion: Anionic xanthene dyes which afford ESA with ZnO were not successful with CuSCN except FLNCS, whereas zwitter ionic RB and all the cationic dyes yielded colorful thin films as they were loaded into CuSCN. Soft and hard, acid and base (HSAB) principle nicely explains the behavior of FLNCS, as its –NCS is soft Lewis basic, finding stable coordination to soft Lewis acidic Cu(I) site of CuSCN, just like the hard –COO- being good with hard Zn(II) of ZnO. On the other hand, partial replacement of Cu+ ions with cationic dyes (RB with its ammonium moiety) can account for their successful ESA. These two mechanisms were confirmed by the decrease of dye loading in baths containing excess SCN− and Cu2+, respectively. RB, R6G, NB and DAST show strong PL both as their solutions and in solid states with their PLEs nicely matched with their absorption spectra, while that of the hybrid thin films was totally quenched except for DAST (CuSCN/DAS hybrid thin film). The energy diagram drawn for the energy levels of individual components suggest hole transfer from HOMO of photoexcited organic chromophore to valence band of inorganic CuSCN, except for NB. Hybridization of NB with CuSCN might have altered its energetic structure to exhibit photo-induced carrier generation of the hybrid thin films, thus suggesting usefulness of the hybrid thin films with RB, R6G and NB as light absorbers in solar cells. On the other hand, the DAS+ chromophore exhibits PL even when it is loaded into CuSCN, with its energy in between solution (= monomer) and powder of DAST. While PL of solution and powder become sharp, blue-shifted and intensified at 77 K, as typically expected by exciton confinement, that of the hybrid film is red-shifted and not much intensified, suggesting a strong excitonic interaction with CuSCN to offer a new type of luminescent material.

Selective hybridization of organic dyes with CuSCN during its electrochemical growth

Nine different organic dyes have been used for electrochemical self-assembly (ESA) of CuSCN/dye hybrid thin films. One of these dyes was added to the bath for cathodic electrodeposition of CuSCN, containing Cu2+ and SCN− ions in ethanol. While anionic xanthene dyes having a carboxylic acid group were not loaded into the film, resulting in colorless CuSCN thin films with unchanged morphology and crystal orientation with the c-axis of β-CuSCN perpendicular to the substrate, all the other dyes were loaded to yield colorful CuSCN/dye hybrid thin films with specific hybrid nanostructures and preferential orientation to lay down the c-axis in parallel with the substrate. The hard and soft, acid and base (HSAB) principle nicely explains the successful ESA of dyes having a soft Lewis basic –NCS group for stable coordination to soft Lewis acidic Cu(I) sites of CuSCN. This further explains why the xanthene dyes, having hard Lewis basic carboxylate, did not coordinate with the CuSCN during electrodeposition, but did yield ESA with hard Lewis acidic ZnO in our previous studies. On the other hand, partial replacement of Cu+ ions of CuSCN with cationic dyes was found to be another efficient loading mechanism of ESA. These two mechanisms were confirmed by ESA in bath containing excess SCN− and Cu2+ respectively.