Synthesis of Ni₃S₂ and NiSe nanoparticles encapsulated in carbon shell and coating these onto stainless steel surfaces by RAPET

Abstract

Nickel sulfide and nickel selenide nanoparticles encapsulated in carbon shells are synthesized by the thermal decomposition of mixtures of nickel acetate and sulfur or selenium via a RAPET (Reaction under Autogenic Pressure at Elevated Temperatures) technique at 750 °C for 3 h. The synthesized products are systematically characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and the encapsulation of Ni₃S₂ and NiSe nanoparticles by a carbon shell is confirmed by transmission electron micrographs (TEM). Such materials have pausible applications in photo voltaic devices, magnetic devices, in catalysis and as electrodes for batteries. Coated carbon on the surface of Ni₃S₂ and NiSe nanocrystals acts as a protective layer, making them thermally as well as chemically stable. Additionally, the possibility of coating a pristine stainless steel plate with Ni₃S₂–C core-shell composite layers is also demonstrated.

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1. Introduction

Nanosized metal chalcogenide materials have a wide range of important applications, which include solar cells, photo voltaic devices, catalysis, semiconductors, magnetic devices and battery electrode materials.^1–3 The ability to generate nanoscale structures of metal chalcogenide materials offers new opportunities to optimize and improve their properties. Among metal chalcogenide materials, the transition metal sulphides show more than one reduction potential plateau in Li-ion conductive electrolytes. Among the various candidates for cathode materials, metal sulphides are known to be promising materials because of their high theoretical capacity, good electronic conductivity and high energy capacity.^4–15 The nickel sulphide system has found promising applications in Li-ion batteries, providing cathode capacity of 580 mAh g⁻¹.¹⁶. In addition, the nickel sulphide system contains a number of phases, including Ni₃S₂, Ni_3+xS₂, Ni₄S_3+x, Ni₆S₅, Ni₇S₆.^17–20 Traditionally, nickel sulphides were synthesized using a variety of methods, such as the reaction of Ni metal nanoparticles supported on graphitized carbon with a H₂S/H₂ gas mixture at 600 °C.²¹ Yu and his coworkers have demonstrated the synthesis of thin films of Ni₃S₂via a soft solution chemical route.²²

Nickel selenide semiconductors exhibit interesting electronic and magnetic properties, and have found applications in the field of materials science that have attracted considerable research attention over the last ten years.^23–31 Due to the valence electronic configuration of Ni (3d⁸4s²) and the small difference in electronegativity between Ni (c = 1.9) and Se (c = 2.4), nickel and selenium can form a variety of nickel selenides, including non-stoichiometric compounds.³² At room temperature there are three stable phases: NiSe₂, Ni_{1 − x}Se (nickel content can vary from 1.00 to 0.85 relative to 1 Se), and Ni₃Se₂. The physical and chemical properties and applications of nickel selenide systems are usually determined by their composition, phase structure, and morphology.^33–35 Routes for the synthesis of nickel selenides include: solid-state synthesis, elemental direct reactions, ultrasonic synthesis and mechanical alloying.³⁶ The solvothermal technique has also been used for the synthesis of nickel selenide nanoparticles using organic solvents, such as pyridine or ethylenediamine, at around 180 °C. However, these methods often need toxic precursors, like metallorganic reagents, or multiple steps for the formation of crystalline products.³⁷

In this current synthesis, nanophase composite structures, consisting of a nano Ni₃S₂ or NiSe in a carbon shell, are developed. According to the literature, there are no reports demonstrating the synthesis of carbon shell encapsulated Ni₃S₂ and NiSe nanocrystals. These hybrid materials can not only combine the advantages of nickel sulphide or nickel selenide nanoparticles and carbon, but also may result in new properties which might have potential applications in nanoscale electronic devices and catalysis. It is reasonable to imagine that the carbon acts as a protective layer for Ni₃S₂ and NiSe nanocrystals, making them thermally as well as chemically stable.

In this present work, we describe a convenient one-step RAPET process, which does not require either a solvent or a surfactant, for the synthesis of Ni₃S₂–C and NiSe–C composites. Moreover, considering the plausible applications of such materials in catalysis, semiconductors, magnetic devices and battery electrode materials, where powder needs to be in a surface layer form, the in situ coating of the Ni₃S₂–C composite onto pieces of pristine stainless steel plates by employing RAPET process, is effectively demonstrated.

2. Experimental

Ni(C₂H₃O₂)₂·4H₂O (Sigma-Aldrich, 99.9%), S and Se (Sigma-Aldrich, >99% pure) were used as received. The synthesis of nickel sulfide and nickel selenide nanoparticles encapsulated in a carbon shell (Ni₃S₂–C and NiSe–C, respectively) was carried out by the thermal reaction of a mixture of Ni(C₂H₃O₂)₂·4H₂O and S or Se. The 5 mL closed reactor was assembled from stainless steel Letlok parts (manufactured by the HAM-LET Co., Israel). For the synthesis of Ni₃S₂–C nanoparticles, 0.4 g of Ni(C₂H₃O₂)₂·4H₂O and 0.114 g of sulfur were introduced into the reactor at room temperature. The filled reactor was closed, placed in a furnace and heated at a heating rate of 10 °C min⁻¹ to 750 °C and held for 3 h. The reaction took place under the autogenic pressure of the precursors. The Letlok reactor was gradually cooled (∼1–2 h) to room temperature and a black powder was obtained. For the synthesis of NiSe–C nanoparticles, 0.4 g of Ni(C₂H₃O₂)₂·4H₂O and 0.28 g of selenium were introduced into the reactor at room temperature and further followed the above synthetic strategy. In the current study, the RAPET technique yielded 44 and 48 wt% recovery of the starting reaction mixture of Ni₃S₂–C and NiSe–C, respectively.

Similar experiments under controlled conditions were carried out for the synthesis of Ni₃S₂–C product coating on stainless steel plate (SSP) surfaces. The supporting SSP is an iron-based alloy, which contains at least 10.5% chromium (Cr) with carbon. SSPs were polished with 1 mm diamond paste finish, cut into 50 mm × 50 mm pieces and cleaned in alcohol in an ultrasonic bath prior to deposition. In the typical experiment, 0.4 g of Ni(C₂H₃O₂)₂·4H₂O, 0.114 g of sulfur and a few small, clean SSPs were introduced into the reactor at room temperature. Similar reaction conditions to those described above were employed for the coating of Ni₃S₂–C product on SSP surfaces. The weight of the SSP was measured before and after the RAPET reaction and it was found that the amount of Ni₃S₂–C material coated onto the SSP was around 40 mg. The adherence of Ni₃S₂–C product on SSP was examined by rubbing the SSP after the reaction with paper, and found that the product did not peel off the SSP during rinsing, drying, weighing and sample preparation for analysis.

2.2. Characterization

The XRD patterns of Ni₃S₂–C and NiSe–C were recorded using a Bruker D8 diffractometer with Cu-Kα radiation. C, H analysis was carried out on an Eager 200 CE instrument and an EA 1110 Elemental Analyzer. Sample preparation for C, H analysis typically requires 1–2 mg of sample per analysis, which was weighed in a standard aluminum capsule. The morphologies of the as-prepared samples were studied using a SEM. TEM studies were carried out on a JEOL 2000 electron microscope with a 80 kV accelerating voltage. High-resolution TEM (HRTEM) images were taken using a JEOL 2010 with a 200 kV accelerating voltage. Samples for the TEM and HRTEM measurements were obtained by placing a drop of the suspension from the as-sonicated reaction product in ethanol onto a carbon-coated copper grid, followed by drying under argon atmosphere to remove the solvent.

3. Results and discussion

3.1. PXRD and elemental (C and H) analysis

The XRD pattern of carbon-encapsulated nickel sulfide synthesized by the thermally-decomposed mixture of Ni(C₂H₃O₂)₂·4H₂O and sulfur at 750 °C in a closed Letlok reactor under argon atmosphere is presented in Fig. 1a. All the main peaks can be indexed indisputably to Ni₃S₂ [powder diffraction file (PDF) # 00-044-1418]. The absence of graphite peaks indicates the possibility that carbon is present only as an amorphous carbon. The diffraction peaks at 2θ = 21.7, 31.1, 37.7, 44.3, 49.7 and 55.1° are assigned to the (101), (110), (003), (202), (113), (211) and (112) planes of Ni₃S₂ respectively. The average crystallite size for Ni₃S₂–C particles was calculated as ca. 30 nm using the Debye–Scherrer equation, which correlates well with our TEM and SEM observations, as described in the following section. The observed extra minute peaks in the Ni₃S₂ XRD pattern belong to NiS (solid circle) and Ni_xS₆ (circle). An analogous multiphase diffraction pattern is previously reported for the synthesis of Ni₃S₂via ball milling,¹⁶ which provided good electrochemical properties as an attractive cathode material for a rechargeable lithium battery.

Fig. 1 PXRD pattern of (a) nickel sulfide–carbon composite and (b) nickel selenide–carbon composite.

Fig. 1b illustrates a representative XRD pattern for NiSe–C composite. All the main peaks, cell parameters and peak intensities can be indexed indisputably to NiSe [powder diffraction file (PDF) # 01-89-7160]. The diffraction peaks at 2θ = 32.8, 44.5, 49.5, 59.2 and 60.9° are assigned to the (101), (102), (110), (103) (201) and (202) planes of NiSe, respectively. The average crystallite size for NiSe nanocrystals was calculated as ca. 31.4 nm using the Debye–Scherrer equation.

The calculated elemental %wt of C, H, O, S and Ni in the precursor mixture are 17.09%, 5.0%, 45.7%, 11.4% and 20.9%, respectively. We determined the carbon and hydrogen content in the Ni₃S₂–C sample with an elemental [C, H, N and S] analyzer. The measured amount of carbon in the Ni₃S₂–C sample is 1.0 wt% and the amount of hydrogen is reduced to 0.5%, while the amount of sulfur is 12 ± 3% and remained constant before and after the reaction. The calculated elemental %wt of C, H, O, Se and Ni in the reaction mixture of [Ni(acetate)₂·4H₂O] and Se are 14.6%, 4.2%, 39.0%, 24.0% and 17.9%, respectively. The measured amount of carbon in the NiSe–C sample is 0.2 wt%, while the amount of hydrogen is reduced to 0.01%. It is clear that the amounts of carbon and hydrogen in the Ni₃S₂–C and NiSe–C sample are reduced, as compared with the precursors, because gases such as CO₂, C_xH_y (hydrocarbons) and/or H₂ are formed during the decomposition of the precursors. These gases are liberated upon the opening of the closed Letlok cell as a result of excess pressure.^38–41 The total amount of sulfur available before the reaction is 114 mg and no elemental sulfur is detected after the reaction, as it is consumed during the formation of Ni₃S₂–C products. The thermo-gravimetric analysis (TGA) of Ni₃S₂–C and NiSe–C composites in an air atmosphere at a heating rate of 10 °C min⁻¹ were carried out to assess the amount of carbon. A weight loss of approximately 1.4% occurred between room temperature and 450 °C. This weight loss has been tentatively attributed to the gradual loss of adsorbed hydrogen initially and carbon burning above 300 °C. This obtained weight loss is consistent with the C, H, N, S analysis of Ni₃S₂–C composite, considering the loss of both adsorbed hydrogen and surface coated carbon. The measured amount of carbon in the NiSe–C sample by TGA is within experimental error, due to the insignificant amount of carbon (0.2 wt%). However, such a minute amount of carbon around the NiSe nanoparticles is indeed envisaged in the TEM analysis of the NiSe–C sample (Fig. 3d).

3.2 Morphology

The dry powders of Ni₃S₂–C and NiSe–C products were mounted separately on one side of the carbon tape, while the other side of the carbon tape was attached to the conducting Si substrate for the SEM morphological analysis. Fig. 2a shows the SEM image of the Ni₃S₂–C product at lower magnification. The Ni₃S₂–C product is an aggregation of small particles forming micron size particles. Fig. 2b and c show the higher magnification images of Ni₃S₂–C and NiSe–C products, respectively. The individual particles have a diameter of less than 100 nm and the agglomerated particles have a diameter in the range of 1–2 μm. From the SEM observations, it can be seen that the Ni₃S₂–C and NiSe–C products have mostly spheroidal morphologies. The detailed particle size and crystallinity of Ni₃S₂–C and NiSe–C products have been further probed using TEM.

Fig. 2 SEMs of the Ni₃S₂–C (a–b) and (c) NiSe–C products.

3.3. TEM and HRTEM measurements

Since the SEM images of the Ni₃S₂–C product mostly show the aggregation of particles, supplementary TEM measurements were carried out. The Ni₃S₂–C black product was dispersed in ethanol solution in an ultrasonic bath and a drop of the solution was mounted on the carbon coated copper grid, followed by drying. The low resolution TEM image (Fig. 3a) shows the poly-dispersed (<100 nm) dark Ni₃S₂ particles surrounded by faint carbon layers. Most of the Ni₃S₂ particles are bounded with carbon forming a Ni₃S₂–C composite. The bottom edge of the TEM image shows individual dark Ni₃S₂ particles surrounded by carbon, while the top portion shows thicker Ni₃S₂–C samples, and eventually overlapped particles and thus larger diameters. Tilley's synthesis of spherical-shaped Ni₃S₂ nanoparticles²¹ occurs in two steps. In the first step, reduction of an aqueous solution of Ni(NO)₃·6H₂O supported on carbon black in a hydrogen atmosphere was carried out for 3 h at 300 °C. The second step is the sulfuridization process, and the nickel nanoparticles formed in the first step are treated with 10% H₂S/H₂ gas mixtures in the temperature range of 300–600 °C. The current RAPET synthetic approach involves simpler precursors, Ni(C₂H₃O₂)₂·4H₂O and sulfur, and does not require any toxic solvents, or a H₂S/H₂ atmosphere, for the formation of Ni₃S₂ nanocrystals. In the TEM measurements of the Ni₃S₂–C product, the nature of the carbon is disordered, thus we focused on the Ni₃S₂ core. Fig. 3b depicts the HRTEM image of a crystalline Ni₃S₂ nanoparticle surrounded by amorphous carbon. The coated carbon is mostly disordered; reflecting the absence of any graphitic peaks in the PXRD pattern.

Fig. 3 TEMs of (a–b) carbon-encapsulated nanocrystals of Ni₃S₂, (c–d) TEM images of spherical NiSe–C nanoparticles at different magnifications.

Fig. 3c demonstrates that the core of NiSe nanoparticles is dark, while these particles are surrounded by faint carbon layers. The NiSe–C particles are around 30 nm diameter with ±10 nm deviation. The HRTEM (Fig. 3d) of the NiSe–C product shows that several 10 nm particles (indicated by arrows) further merged together to form an around 30 nm NiSe particle surrounded by amorphous carbon. The overall spherical shape of our NiSe nanoparticles differs from the micro spheres or nanowire-shaped NiSe particles obtained by Yadong's hydrothermal synthetic strategy.³⁶ The current RAPET synthetic approach involves Ni(C₂H₃O₂)₂·4H₂O and selenium, and does not require the solvents, toxic chemicals like Na₂SeO₃ or long reaction times generally required for hydrothermal processes. The NiSe nanowires or NiSe microspheres prepared by Yadong's synthetic strategy³⁶ have an average diameter of 50 nm and are larger than the spherical NiSe nanoparticles produced under RAPET reaction conditions.

The thermal decomposition of the precursor, Ni(C₂H₃O₂)₂·4H₂O, into Ni, CH₃COCH₃ and CO₂ takes place at 330 °C.⁴² The boiling points of the S and Se used in this study are 445 and 685 °C, respectively, and both elements are in the gaseous state at or above 750 °C. The acetone is further decomposed to elemental carbon at 750 °C as similarly described in the series of our previous papers.^43–52 It is therefore assumed that at 750 °C the cell contains Ni, C, O, H, as well as gaseous S or Se atoms. The second step involves the formation of the Ni₃S₂ or NiSe as a result of the reaction between nickel and S or Se. Although carbon atoms are also available, nickel carbide is not formed, because the reaction of metallic Ni with S and Se is so fast that it prevents the formation of the corresponding carbides. In addition to the kinetic argument, Ni₃S₂ and NiSe are also thermodynamically more stable than nickel carbide.⁵³ These products serve as seeds for the solidification of the carbon coating around them. Therefore, Ni₃S₂ and NiSe nanocrystals are coated with carbon in these RAPET reactions. An analogous dry RAPET technique has been previously employed successfully for the synthesis of a wide range of materials,⁵⁴ such as oxides, carbides, phosphides, nitrides, borides, sulfides, selenides, carbonaceous materials and core-shell nanostructures.

In a controlled experiment, the successful deposition of Ni₃S₂–C onto pieces of pristine stainless steel was carried out by employing a RAPET technique. The thermal decomposition of Ni(C₂H₃O₂)₂·4H₂O, S mixture was performed in the presence of tiny SSPs in a closed reactor at 750 °C for 3 h under autogenic pressure. The morphologies of the deposited Ni₃S₂–C on SSP obtained were primarily investigated by SEM measurements. Fig. 4a represents the flat SSP before starting the reaction, confirming its smooth and clean surface. The SEM image of the as-coated SSP is shown in Fig. 4b, evidence that the SSP is coated with submicron-sized Ni₃S₂–C crystals. The Ni₃S₂–C particle diameters are larger (>100 nm) compared to the Ni₃S₂–C particles (<100 nm) prepared without a SSP substrate. The adhesion of Ni₃S₂–C crystals on SSP is ascribed to preferential adsorption of Ni/S species on some special crystallographic planes of SSP, the so-called selective epitaxial vapor–solid growth mechanism. A similar phenomenon is observed for the coating of NiSe–C nanoparticles on SSP.

Fig. 4 (a) SEM image of the plane stainless steel plate (SSP), (b) SEM image of SSP coated with Ni₃S₂–C composite.

Yu and his co workers have deposited arrays of ZnO nanorods onto a stainless steel grid. In that method, the grid was initially coated with 10 nm thick Au film and the grid was placed inside the quartz boat containing the Zn powder, which was placed near the quartz boat containing the Se, and finally requires the constant gas flow of Ar + O₂.⁵⁵ The entire system was maintained under vacuum by employing a mechanical pump. They succeeded in the formation of ZnO nanorod arrays on the SS grid. Our deposition approach towards pristine SSPs requires only a readily available solid organometallic precursor, Ni(C₂H₃O₂)₂·4H₂O, and S or Se powders, and the cost of these chemicals is relatively low. The Ni₃S₂–C coating did not peel off the SSP during rinsing, drying, weighing and sample preparation for electron microscopy analysis. Moreover, the Ni₃S₂–C coated SSPs were immersed in water or ethanol and kept there for 10 min before taking them out. The coated Ni₃S₂–C particles did not disperse in a solvent or peel off the SSP substrate, further confirming the additional stability of the surface adhesion.

In summary, carbon encapsulated Ni₃S₂ and NiSe nanoparticles have been successfully synthesized by the RAPET method and the capability of depositing the Ni₃S₂–C composite on a SSP has been demonstrated. The method presented herein is a simple and efficient reaction for the direct preparation of core shell nanoparticles by a single step process, which can be extended to other inorganic sulfides and selenides.

Acknowledgements

We thank to the Bar-Ilan Research authority for providing us the scientific instrumental facilities to carry out this work.

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