Kuniaki Murase

Structural and Electrical Characterizations of Electrodeposited p-Type Semiconductor Cu2O Films

The p-type semiconductor cuprous oxide (Cu 2 O) film has been of considerable interest as a component of solar cells and photodiodes due to its bandgap energy of 2.1 eV and high optical absorption coefficient. We prepared Cu 2 O films on a conductive substrate by electrodeposition at 318 K from an aqueous solution containing copper sulfate and lactic acid. The structural and electrical characterizations of the resulting films were examined by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption measurements, and the Hall effect measurement, respectively. The resistivity varied from 2.7 × 10 4 to 3.3 X 10 6 Ω cm, while the carrier density was from 10 1 2 to 10 1 4 cm - 3 and the mobility from 0.4 to 1.8 cm 2 V - 1 s - 1 , depending on the preparation conditions, i.e., solution pH and deposition potential. The carrier density was sensitive to the atomic ratio of Cu to O in the films and the mobility to the grain size.

Recovery of rare metals from scrap of rare earth intermetallic material by chemical vapour transport

Abstract A dry process for recovery of rare metals from sludges of Sm 2 Co 17 , Nd 2 Fe 14 B and LaNi 5 intermetallic compounds was investigated using chemical vapour transport along a given temperature gradient. Chlorine and aluminium chloride were used as a chlorinating agent and a transporting agent respectively, and the rare metal chlorides were chemically transported as vapour complexes, e.g. RAl n Cl 3+3 n (R = rare earth), Mal n Cl 2+3 n (M = Co, Ni). Rare earth chlorides were concentrated in the deposits in the higher temperature zone (800–900 °C) while cobalt and nickel chlorides were in the lower temperature zone (500–700 °C). Purity of each of the recovered chlorides was more than 99%. Chlorides of other metals, such as iron, copper, zirconium, and aluminium, were condensed at the outlet of the reactor (below 350 °C) without any contamination for the recoveries. After transport reaction for 6 h, yields of nickel and cobalt were more than 99%, whereas those of lanthanum, samarium, neodymium and dysprosium were lower, i.e. 27%, 39%, 59% and 68%, owing to the relatively low formation rate of RAl n Cl 3+3 n complexes. If the transport reaction lasts for a longer time, the yields of rare earths can be improved.

A concept of dual-salt polyvalent-metal storage battery

In this work, we propose and examine a battery system with a new design concept. The battery consists of a non-noble polyvalent metal (such as Ca, Mg, Al) combined with a positive electrode already well-established for lithium ion batteries (LIBs). The prototype demonstrated here is composed of a Mg negative electrode, LiFePO4 positive electrode, and tetrahydrofuran solution of two kinds of salts (LiBF4 and phenylmagnesium chloride) as an electrolyte. The LIB positive-electrode materials such as LiFePO4 can preferentially accommodate Li+ ions; i.e., they work as a “Li pass filter”. This characteristic enables us to construct a septum-free, Daniel-battery type dual-salt polyvalent-metal storage battery (PSB). The presented dual-salt PSB combines many advantages, e.g., fast diffusion of Li+ ions in the positive electrode, high cyclability, and a high specific capacity of lightweight polyvalent metals. The concept is expected to allow the design of many combinations of dual-salt PSBs having a high energy density and high rate capability.

Self-assembly of ionic liquid (BMI-PF6)-stabilized gold nanoparticles on a silicon surface: chemical and structural aspects.

Ultrafine monodisperse gold nanoparticles (AuNPs) were synthesized by an elegant sputtering of gold onto 1- n-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF(6)) ionic liquid. It was found that the BMI-PF(6) supramolecular aggregates were loosely coordinated to the gold nanoparticles and were replaceable with thiol molecules. The self-assembly of BMI-PF(6)-stabilized AuNPs onto a (3-mercaptopropyl)trimethoxysilane (MPS)-functionalized silicon surface in 2D arrays, followed by dodecanethiol (DDT) treatment, have been demonstrated using X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and contact angle measurements. DDT treatment of tethered AuNPs revealed two types of interactions between AuNPs and the MPS-functionalized surface: (a) AuNPs anchor through Au-S chemisorption linkage resulting in strong immobilization and (b) some of the AuNPs are supported by physisorption, driven by BMI-PF(6). The attachment of these particles remains unchanged with sonication. The replacement of BMI-PF(6) aggregates from physisorbed AuNPs with DDT molecules advances the dilution of their interaction with the MPS-functionalized surface, and they subsequently detach from the silicon surface. The present finding is promising for the immobilization of ionic liquid-stabilized nanoparticles, which is very desirable for electronic and catalytic device fabrication. Additionally, these environmentally friendly AuNPs are expected to replace conventional citrate-stabilized AuNPs.

Structural Organization of Gold Nanoparticles onto the ITO Surface and Its Optical Properties as a Function of Ensemble Size

Self-assembly of citrate-stabilized gold nanoparticles (AuNPs) onto an optically transparent indium tin oxide (ITO) surface followed by neutralization of these particles using dodecanethiol as a surfactant have been demonstrated. X-ray photoelectron spectroscopic (XPS) studies revealed the partial removal of citrate ions from the immobilized AuNPs, which advances the dilution of electrostatic attraction between AuNPs and the APS (amino-terminated monolayer)-functionalized ITO surface. The resultant AuNPs restore their mobility to some extent and form small ensembles. Some of the immobilized AuNPs were completely removed from the surface due to neutralization, as confirmed by XPS studies. Interparticle distance and size of ensembles were manipulated by consecutive cycles of immobilization and neutralization of AuNPs. Controlled nanostructural fabrication progression, which leads to two-dimensional lateral growth of AuNPs, provides a method for systematically shifting the surface plasmon resonance band based on the increase in plasmon coupling among the closely placed AuNPs of an ensemble. The magnitude of shift increases with the size of ensemble. This manipulated chemical strategy offers a convenient and simple method to tune the optical properties of materials on a nanoscale.

Rare earth separation using a chemical vapour transport process mediated by vapour complexes of the LnCl3AlCl3 system

Abstract Mutual separations of rare earth chlorides in the PrEr, PrSm and PrNd binary systems and the PrGdEr ternary system were performed using chemical vapour transportation of vapour complexes LnAlnCl3+3n, (Ln, rare earths). Heavier rare earth chlorides were more readily transported and concentrated in the deposits in lower temperature zones while lighter ones were selectively deposited in higher temperature zones. Good separation characteristics (efficiency and purity) were observed for the resulting LnCl3 deposits by optimization of temperature gradients for the selective deposition of LnCl3 and by repeating the transport reaction. The separation factors, expressed as atomic ratios for the resulting chlorides, Pr:Er and Pr:Sm, were equal to 10.8 and 2.3 respectively.

Electrodeposition of CdTe Films from Ammoniacal Alkaline Aqueous Solution at Low Cathodic Overpotentials

Cathodic electrodeposition of CdTe films was studied using aqueous ammonia-alkaline electrolytic baths (pH 10.7; temperature 343 K) in which Cd(II) and Te(IV) species were dissolved to form Cd(NH{sub 3}){sub 4}{sup 2+} and TeO{sub 3}{sup 2{minus}} ions, respectively. From the solution, 60 mM Cd(II)-10 mM Te(IV)-4.0 M NH{sub 3}-1.0 M NH{sub 4}{sup +} (M = mol dm{sup {minus}3}), a flat and smooth polycrystalline CdTe film (thickness, ca. 1 {micro}m) with nearly stoichiometric composition was deposited at a constant cathode potential, ranging from {minus}0.70 to {minus}0.30 V vs. SHE, whereas dendrite CdTe accompanying elemental cadmium was obtained at {minus}0.80 V. The deposition behavior was fully explained by an underpotential deposition mechanism taking the calculated redox potentials of Te{sup 0}/Te{sup IV}O{sub 3}{sup 2{minus}} and Cd{sup 0}/Cd{sup II}(NH{sub 3}){sub 4}{sup 2+} pairs into consideration. During electrodeposition of nearly stoichiometric crystalline CdTe, the current density was decreasing monotonously.

Potential‐pH Diagram of the Cd ‐ Te ‐ NH 3 ‐ H 2 O System and Electrodeposition Behavior of CdTe from Ammoniacal Alkaline Baths

A potential‐pH diagram of the system was constructed based on diagrams of the and systems and discussed in connection with the redox behavior of an ammonia‐alkaline CdTe electrolytic bath with pH 10.7. CdTe has a wide domain of stability throughout the acidic and alkaline regions, and the redox behavior was well explained with the diagram. The diagram indicated that the cathodic electrodeposition of CdTe occurs across a domain of stability of tellurium metal, i.e., at lower potentials than the deposition potential of bulk Te and higher than that of bulk Cd, with respect to the bath with pH < ca. 11.5, while in the higher pH region, CdTe is expected to deposit directly from Te(IV) and Cd(II) ions. The deposition mechanism is considered as follows: (i) deposition of tellurium layer followed by (ii) an immediate underpotential deposition of Cd on it, which prevents the bulk deposition of tellurium. It can be considered that the stoichiometric CdTe is more easily electrodeposited from alkaline baths, since the domain for tellurium metal is narrower in the alkaline region compared to the conventionally employed acidic region with pH 0–2. Therefore, the bulk deposition of elemental tellurium is less apt to occur from an alkaline bath.

Electrical Properties of CdTe Layers Electrodeposited from Ammoniacal Basic Electrolytes

CdTe is a promising material for solar cell application because its bandgap of 1.44 eV at room temperature is suitable for energy conversion from sunlight to electricity. In addition to dry processes such as screen printing and close-spaced sublimation, electrodeposition 1-3 has been investigated for the preparation of polycrystalline CdTe layers, and thin layered n-CdS/p-CdTe heterojunction solar cells have already been manufactured industrially. Although aqueous sulfate electrolytes with pH 1-2 have historically been studied for CdTe electrodeposition, we have proposed that ammoniacal basic aqueous electrolytes are also suitable, because basic solutions have a relatively high solubility of Te ~IV! species. 4-9 From the ammoniacal basic electrolytes, we successfully obtained smooth and flat polycrystalline CdTe deposits with a nearly stoichiometric composition at potentials positive to that of bulk-Cd deposition. 6 Furthermore, it turned out that the deposition rate was considerably increased by photoirradiation of the cathode surface during the electrodeposition. 10 The mechanism of the CdTe deposition is considered to be ~i! cathodic electrodeposition of surface tellurium atoms (TeO32 1 6H 1 1 4e ! Te(ads) 1 3H2O), followed by ~ii! an adsorption of Cd~II! ions on the tellurium, and ~iii! underpotential deposition of the Cd~II! ions to form CdTe @Cd(II) 1 Te(ads) 1 2e ! CdTe#. 9 It is known that electrodeposited CdTe from acidic sulfate electrolytes without intentional doping are n-type and that a heattreatment in air is necessary to obtain p-type. If as-deposited p-type CdTe can be obtained by electrodeposition from aqueous electrolytes without the heat-treatment process, it will lead to a less energyconsuming fabrication of p-n junction solar cells. Therefore, it is important to investigate the electrical properties, including conduction type, of electrodeposited CdTe. Generally, undoped CdTe with a nearly stoichiometric composition is known to have a high resistivity, or a low carrier density. We previously tried conventional MottSchottky plots using CdTe layers electrodeposited from an ammoniacal basic electrolyte to determine the majority carrier type and carrier density of the CdTe. However, reliable data could not be derived from the results, since the CdTe/electrolyte interface capacitance was almost constant and independent of the electrode potential. This is attributable to a high internal resistivity of the CdTe layer. In the present study, we tried to determine the majority carrier type of the electrodeposited CdTe layer by means of photoelectrochemical investigation using an aqueous solution containing sulfite ion as a hole scavenger and, then, resistivity and Hall effect measurements were carried out. The Hall effect measurement is a standard, reliable, and more direct method for obtaining fundamental electrical properties such as carrier type, carrier density, and mobility. Nevertheless, limited numbers of Hall effect measurements on electrodeposited materials have so far been carried out. As for the electrodeposited undoped CdTe, for example, the Hall effect for the deposits from an organic bath 11,12 has only been reported and there have been no papers regarding that from aqueous media. A possible reason for there being few reports is that the sample preparation for Hall effect measurements is difficult because the conducting substrate must be removed from the electrodeposited layer, while the CdTe layer is maintained intact. In the present work, we employed a method in which the CdTe layer was mechanically transferred from the conducting substrate onto a nonconductive epoxy resin without the formation of cracks. Experimental

Extraction and mutual separation of rare earths from concentrates and crude oxides using chemical vapor transport

Abstract A dry process for the separation of rare earths has been investigated using a chemical vapor transport reaction mediated by metal halide vapor complexes KRCl 4 (g) and RA n Cl 3+3 n (g) (R = rare earths), in which KCl and AlCl 3 act as complex formers, i.e. transporting agents. Rare earth concentrates of monazite and xenotime were used as raw materials as well as the crude oxides originating from monazite, bastnasite and ionic ores. The concentrates and crude oxides were chlorinated by Cl 2 gas at 1000°C. The resulting RCl 3 reacts with the complex formers KCl and AlCl 3 and was extracted as the vapor complexes, which selectively decompose along a well-controlled temperature gradient to give the RCl 3 . Heavier rare earth chlorides including YCl 3 were generally more readily transported and concentrated in the deposit at temperatures around 600–700°C, and lighter ones in higher temperature fractions at 800–900°C. Chlorides of other elements such as Th, U and P in the raw materials were generally concentrated in lower temperature fractions. Yields of individual rare earths after reaction for 82 h increased with increasing atomic number or decreasing ionic radius of the rare earth ion: 20%–30% for La; 50%–60% for Ce; 60%–70% for Pr and Nd; >80% for Gd-Lu and Y.