Abstract: Spintronic devices employing both semiconducting and ferromagnetic properties of materials are to be realized by doping of high spin transition metal ions to semiconductors such as ZnO. Transition metal oxides are also interesting as electrocatalysts for electrochemical conversion of renewable electricity to chemical energy for its storage. In this study, we have attempted microwave (MW)-assisted hydrothermal synthesis of Co, Mn-doped ZnO nanoparticles. Aqueous precursor solutions containing ZnCl 2 and CoCl 2 or MnCl 2 at various ratios in a total concentration of 0.2 M were basified with NaOH to ca. pH 13, and subjected to a 2.45 GHz MW[1,2] to promote reaction at 160°C for 30 min. The products were characterized by FE-SEM, XRD and UV-Vis. Mesoporous electrodes of ZnO and Zn 0.92 Co 0.08 O were fabricated by doctor blading method. The green-colored Co-doped samples exhibit characteristic absorption peaks at around 560, 610 and 650 nm due to d-d transition of Co(II) and a red shift of the bandgap absorption onset. Phase separation of Co(OH) 2 has also been suggested by XRD and FE-SEM images for x 〉 0.1, so that doping up to about 10% was apparently possible. Systematic shift of the three major XRD peaks of Wurtzite ZnO towards low angles was recognized upon increasing x , suggesting substitution of the lattice Zn 2+ ions with Co 2+ . Lattice constants were calculated as a = b = 3.252 Å, c = 5.204 Å for ZnO, whereas they were enlarged to 3.277, 5.244 Å, respectively, for x = 0.20. CVs of ZnO and Zn 0.92 Co 0.08 O electrodes in a neutral electrolyte are compared in Fig. 1. Besides the clear decrease of cathodic charging and discharging currents, Co-doping results in an anodic wave at around + 1 V due to Co 2+ → Co 3+ , followed by a large increase of irreversible anodic current, as compared to the non-doped sample. Chronoamperograms for a few minutes confirmed this current enhancement and gas bubble evolution from the anode due to catalytic oxidation of water (2H 2 O → O 2 + 4H + + 4e - ).   [1] Y. Hirai, K. Furukawa, H. Sun, Y. Matsushima, K. Shito, A. Masuhara, R. Ono, Y. Shimbori, H. Shiroishi, M. S. White & T. Yoshida, “Microwave-assisted hydrothermal synthesis of ZnO and Zn-terephthalate hybrid nanoparticles employing benzene dicarboxylic acids” Microsyst Technol (2017). doi: 10.1007/s00542-017-3392-y [2] H. Sun, L. Sun, T. Sugiura, M. S. White, P. Stadler, N. S. Sariciftci, A. Masuhara & T. Yoshida, “Microwave-assisted hydrothermal synthesis of struture-controlled ZnO nanocrystals and their properties in dye-sensitized solar cells” Electrochemistry , in press. Figure 1

Abstract: 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. N 2 -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 Cu 2+ , 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. References: T. Iwamoto et al., J. Phys. Chem. C, 118, 16581-16590 (2014). Y. Tsuda et al., Microsys. Tech., DOI 10.1007/s00542-017-3394-9. Figure 1

Abstract: We present a nonlinear impedance spectroscopy technique and demonstrate its ability to directly measure nonlinear processes including electron-hole recombination and space charge effects in organic-semiconductor-based diodes and MIS capacitors. The method is based on Fourier analysis of the measured higher harmonic current response to an AC voltage signal. Characterization of the higher harmonic response allows nonlinear impedance spectroscopy to measure material and device properties over a wide range of frequencies, which would otherwise be impossible using conventional impedance spectroscopy. As the higher harmonic signals are purely a product of nonlinear processes, they are independent of the linear device capacitance and resistance. This allows space charge and recombination effects to be investigated at several orders of magnitude higher frequency without fitting to an equivalent circuit model.

Abstract: 1. Introduction Oxygen reduction reaction (ORR) has frequently been employed to electrodeposit metal oxide thin films [1-4]. ORR can often be limited by mass transport of oxygen because of its low solubility in water. It is therefore important to precisely grasp bulk concentration of O 2 and its diffusion in a wide temperature range. Optically transparent electrodes such as F-doped SnO 2 (FTO) coated glass is often used as the substrate for the purpose of thin film characterization and device fabrication. So it is important to understand the kinetics of such electrodes for the ORR. In this study, we have carried out hydrodynamic electrochemical analysis employing rotating disk Pt and FTO electrodes (RDEs) to quantify each of these limiting factors.   2. Experiment Commercial Pt RDE or a home-made RDE setup employing an FTO (Asahi-DU, 10 Ω/sq.) was used as the working electrode. 0.1 M KCl aqueous solution (pH 5.6) was used as the electrolyte to which a mixture of Ar and O 2 is bubbled through. The O 2 concentration was separately measured with an Ebara DIO-1350 sensor, based on fluorescence quenching, while the temperature was controlled by a thermostat in a range between 298 and 353 K.   3. Result and Discussion Care must be taken for the bulk concentration of O 2 in real electrochemical experimental systems, because the gas phase in a closed cell should be equilibrated with water vapor under a total pressure of 1 atm, and salting out effect by the electrolyte also need to be taken into account. The actual O 2 concentrations were measured in O 2 saturated water and 0.1 M KCl between 298 and 353 K and compared with the values calculated from the handbook data as shown in Fig. 1. Both the plots are quite nicely overlapping with the respective curves. It is then safe to take C O2 (III) as the bulk O 2 concentration for analysis of the electrochemical data measured in 0.1 M KCl. Diffusion coefficient ( D ) of O 2 was precisely determined from Levich analysis of diffusion limited plateau current at Pt RDE. The curves measured at 298 K are chosen an example. The plateau current indicates diffusion limit ( j D ) of the ORR, and is proportional to ω 0.5 (rad 0.5  s -0.5 ), as expressed by the following Levich equation, j d =0.62 nFD 2/ 3 v 1 /6 C * ω 1/2 (1) D was calculated as 1.92×10 -5 cm 2 s -1 for 298 K from the slope of Levich plot, which nicely compares with the literature value. The same analysis has been carried out for different temperatures. Arrhenius plot between D and 1/ T gave a reasonable straight line to yield frequency factor A d and activation energy E a as 8.41×10 -3 cm 2 s -1 and 15.0 kJ mol -1 , respectively. Having known the saturation concentration of O 2 and the diffusion coefficient, O 2 flux density can be precisely known for any given temperature, partial pressure and w . Blend of Butler-Volmer equation and Levich equation yields Koutecky-Levich (K-L) equation to describe current in the mixed regime, namely, where current is controlled both by charge transfer kinetics and mass transport, j -1 = j d -1 + j k -1 =(0.62 nFD 2/3 v -1/6 C * ) -1 ω -1/2 +( nFCk 0 exp(- a nF ( E - E eq )/ RT )) -1 (2) K-L plot, j -1 vs. ω -0.5 , was drawn from the J-V curve to determine a j k value from the intercept with the ordinate for a given overpotential. Extrapolation of the linear fit of log j k vs. overpotential for E – E eq =0 yields the standard charge transfer rate constant k 0 . Such analysis has been conducted for the Pt RDE over a wide range of temperature. A reasonably straight Arrhenius plot was obtained to indicate thermal activation of the charge transfer process with a frequency factor ( A k ) of 9.82×10 2 cm 2 s -1 and an activation energy of 53.2 kJ mol -1 . The same analysis has been attempted with the FTO RDE. However, its electrode kinetics was highly unstable. Simply degreased FTO was poor for the ORR and was not able to reach the diffusion limit. Electrochemical activation resulted in a reasonable activity to allow determination of k 0 as 1.82×10 -7 cm s -1 at 343 K, which is found to be 2 orders smaller than that of Pt (2.16×10 -5 cm s -1 ) at the same temperature. [1] T. Yoshida, K. Terada, D. Schlettwein, T. Oekermann, T. Sugiura, H. Minoura, Adv. Mater. 2000, 12, 1214. [2] T. Yoshida, T. Oekermann, K. Okabe, D. Schlettwein, K. Funabiki, H. Minoura, Electrochemistry 2002, 70, 470. [3] A. Goux, T. Pauporté, T. Yoshida, D. Lincot, Langmuir 2006, 22, 10 545. [4] T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauporté, D. Lincot, T. Oekermann, D. Schlettwain, H. Tada, D. Wöhrle, K. Funabiki, M. Matsui, H. Miura and H. Yanagi, Adv. Func. Mater. 2009, 19, 17. Figure 1

Abstract: 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)(Cu 5 I 6 ) 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 Cu 2+ 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 ω 1/2 (ω = angular speed of rotation of RDE). On the other hand, the rate of DAS + precipitation, v(DAS)(SCN), should also be proportional to ω 1/2 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 cm 3 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 × ω 1/2 (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

Abstract: 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