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Facile preparation and formation mechanism of three low valent transition metal oxides in supercritical methanol

Published online by Cambridge University Press:  18 April 2016

Shuangming Li ,
Zhe Zhang ,
Shengnan Jiang ,
Xin Ge ,
Jie Zhang ,
Wenxiu Li  and
Sansan Yu
Shuangming Li
Affiliation:
College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China; and Key Laboratory of Chemical Separation Technology of Liaoning Province, Shenyang University of Chemical Technology, Shenyang 110142, China
Zhe Zhang
Affiliation:
College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
Shengnan Jiang
Affiliation:
College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
Xin Ge
Affiliation:
College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
Jie Zhang
Affiliation:
College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
Wenxiu Li
Affiliation:
Key Laboratory of Chemical Separation Technology of Liaoning Province, Shenyang University of Chemical Technology, Shenyang 110142, China
Sansan Yu*
Affiliation:
College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China; and Key Laboratory of Chemical Separation Technology of Liaoning Province, Shenyang University of Chemical Technology, Shenyang 110142, China
*
a)Address all correspondence to this author. e-mail: ssyu@syuct.edu.cn

Abstract

Three important low valent transition metal oxides were synthesized in supercritical methanol by using inorganic metal salts as precursors. X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and x-ray photoelectron spectroscopy were applied to analyze the composition, structure, and morphology of the products. Results showed that Cu2O, MoO2, and V2O3 were obtained successfully under a supercritical condition of 240 °C and 9.0 MPa. MoO2 and V2O3 displayed sphere-like morphology with average particle sizes of 20–40 and 20–50 nm, respectively. Cu2O particles displayed edge-truncated cubic morphology with a particle size of 2.5 μm. Formation mechanism proposed that high valent metal oxides (CuO, MoO3, and V2O5) were formed firstly in supercritical methanol by the decomposing of precursors and then reduced to target products by free hydroxyl anions. In addition, methanol performed important roles not only as a reaction medium but also as a reducing agent under supercritical fluid conditions.

Keywords

low valent transition metal oxidesinorganic precursorssupercritical alcoholspreparationformation mechanism

Information

Type
Articles
Information
Journal of Materials Research , Volume 31 , Issue 10 , 28 May 2016 , pp. 1440 - 1447
Copyright
Copyright © Materials Research Society 2016 

I. INTRODUCTION

In recent years, low valent transition metal oxide (LVTMO) materials, e.g., Cu2O, MoO2, V2O3, and so on, have attracted much interest due to their enhanced catalytic abilities, novel electrical, and conduction properties and unique magnetic properties.Reference Kuo and Huang1–Reference Zhao, Wang, Li, Zhang, Yan and Huo4 These materials have been applied in various fields, including catalysis, sensors, batteries, high density magnetic recording materials, and electrochemistry.Reference Koo, Xiong, Slater, Prakapenka, Balasubramanian, Podsiadlo, Johnson, Rajh and Shevchenko5–Reference Xu, Wang, Yu, Tian, Zhao and Zhang9

Some methods of preparing transition metal oxides have been reported over the past few years including wet chemical, solvothermal, thermal decomposition, reverse micelle, and chemical deposition method.Reference Srivastava, Anu Prathap and Kore10–Reference Sikhwivhilu, Pillai and Hillie15 Cu2O, for example, has been prepared mainly by wet chemical technique.Reference Kuo and Huang1,Reference Wang and Li16–Reference Giannousi, Sarafidis, Mourdikoudis, Pantazaki and Dendrinou-Samara19 This method, however, requires complex synthesis systems composed of solvents, precursors, reducing agents, surfactants, stabilizers, and other additives, which increase the operation complexities and the subsequent separation difficulties. Furthermore, the method is not environmentally friendly due to the usage of toxic reducing agents and the production of hazardous chemical wastes. There are some very different methods for the synthesis of MoO2 and V2O3. Ramana et al. reported the formation of V2O3 nanocrystals directly from V2O5 thin films at 600 °C.Reference Ramana, Utsunomiya, Ewing and Becker20 Zheng et al. prepared V2O3 nanopowder by reducing the precursor (NH4)5[(VO)6(CO3)4(OH)9]·10H2O in H2 atmosphere at high temperature.Reference Zheng, Zhang, He, Fu and Lei21 Gharbi et al. prepared V2O3 particles with particle size of 2.6–5.2 μm via hydrothermal method by using a mixture of V2O5, 1,6-diaminohexane, and H2O.Reference Sediri and Gharbi22 Su et al. prepared V2O3 nanopowder through thermal decomposition of the oxalate by using V2O5 and concentrated sulfuric acid as starting material.Reference Zhang, Sun, Lou, Liu, Li and Su23 Shi et al. used phosphomolybdic acid as a precursor and mesoporous silica KIT-6 as a hard template to prepare mesoporous crystalline MoO2 materials in a 10% H2 atmosphere via nanocasting strategy.Reference Shi, Guo, Corr, Shi, Hu, Heier, Chen, Seshadri and Stucky24 Yang et al. synthesized MoO2 through the reduction of MoO3 with ethanol vapor at 400 °C.Reference Yang, Gao, Tang, Wu and Holze25 From the above references, it is known that various methods have been applied to synthesize Cu2O, V2O3, and MoO2 material. However, there is still not a common approach to prepare the above three LVTMOs. In addition, large-scale production of LVTMOs is limited due to the relatively high operational costs, substantial use of toxic and expensive organic agents, and safety and environmental issues.

Due to the advantages of efficiency, environmental friendliness, and its uniqueness, supercritical fluid (SCF) has attracted much interest as an alternative method for materials synthesis.Reference Veriansyah, Kim, Min and Kim26–Reference Cansell and Aymonier29 In recent years, one of the most important applications of SCF is the micronization of solid materials at mild temperatures, such as preparation of metal nano/microparticles. Until now, various nano/micrometals, metal oxides, and composites have been prepared in SCF, in which water and alcohols (MeOH and EtOH) were mostly used as solvents. Chang et al.Reference Chang, Chang, Lo, Tzing and Ling30 reported the formation of Ag nanoparticles, nanowires, and nanobanners from Ag2O in supercritical water. Ziegler et al.Reference Ziegler, Doty, Johnston and Korgel31 reported the synthesis of copper (Cu) nanocrystals from CuNO3·2.5H2O and Cu(CH3COO)2·H2O in supercritical water with an addition of 1-hexanethiol. Sue et al.Reference Sue, Suzuki, Suzuki, Arai, Hakuta, Hayashi and Hiaki32 reported the synthesis of nickel (Ni) particles from Ni(HCOO)2·2H2O and Ni(CH3COO)2·4H2O in supercritical water mixed with formic acid. Aymonier's group had synthesized various metal-based nanoparticles, e.g., nitrides (Cr2N, Co2N, Fe4N, Cu3N, Ni3N), metal (Cu), and oxides (Al2O3, TiO2, Ga2O3, CeO2, MnFe2O4, Fe3O4)Reference Desmoulins-Krawiec, Aymonier, Loppinet-Serani, Weill, Gorsse, Etourneau and Cansell27,Reference Marre, Cansell and Aymonier33–Reference Pascu, Marre, Aymonier and Roig35 in SCFs including ammonia and CO2/alcohol/H2 mixtures. Kim's group had also synthesized the nanoparticles of Ni, Ag, Cu, Co, CeO2, Fe3O4, and Li4Ti5O12 etc using batch and continuous supercritical alcohols process.Reference Veriansyah, Kim, Min and Kim26,Reference Shin, Lee, Shin, Kim and Lee28,Reference Kim, Kim, Veriansyah, Won Kang and Kim36–Reference Nugroho, Kim, Chang, Chung and Kim39 Although Huang et al.Reference Liu, Zhang, Yi, Huang, Liao, Li, Xiao and Tao40 had prepared V2O3 nanopowders by supercritical ethanol, the organic precursor VOC2O4 should be synthesized first. To date, however, there is no report about the preparation of MoO2 in supercritical alcohols.

Recently, we have reported the synthesis of Cu powder and Cu2O microcrystals in sub- and supercritical methanol (T c = 239.3 °C and P c = 8.084 MPa) using copper oxide and inorganic copper salts as precursors.Reference Yu, Li, Ge, Niu, Zhang, Xu and Li41,Reference Li, Ge, Jiang, Peng, Zhang, Li and Yu42 We demonstrated that zero-valent copper could be prepared by introducing CuO in supercritical MeOH (250 °C, 9.0 MPa). Octahedral and cubic Cu2O microcrystals were obtained by introducing Cu(NO3)2 in subcritical MeOH (230 °C, 7.8 MPa). In this study, however, it was found that Cu2O with edge-truncated cubic morphology could be formed when Cu(NO3)2 was introduced in supercritical MeOH (240 °C, 9.0 MPa). In addition, the other two important LVTMOs (MoO2 and V2O3) were also synthesized by using the same process. This technique may be applied to the preparation of other LVTMOs and scaled up using a flow type reactor, which is important for a wide range of potential applications such as catalysis, sensors, electronics, and recording materials.

II. MATERIAL AND METHODS

Cupric nitrate trihydrate (Cu(NO3)2·3H2O), ammonium hexamolybdate ((NH4)6Mo7O24·4H2O), and ammonium metavanadate (NH4VO3) of analytical reagent (AR) grade were used as inorganic precursors. Methanol of AR grade was used as solvent for the SCF. All of the above reagents were purchased from Sinopharm Chemical Reagent Co, Ltd. (Shanghai, China) and were used without further purification.

A. Synthesis

Preparation of three LVTMO was carried out according to a procedure similar to that previously reported.Reference Yu, Li, Ge, Niu, Zhang, Xu and Li41,Reference Li, Ge, Jiang, Peng, Zhang, Li and Yu42 Typically, a certain quantity of inorganic metal salts were introduced into a high pressure tank reactor. After being tightly sealed, the air inside was replaced with N2 for several minutes and then the N2 was exhausted under vacuum for 10 min. 30 mL of methanol was then discharged into the reactor. The reactor was heated to 240 °C within 15 min and kept for 15 min under the pressure of 9.0 MPa. Upon the completion of the reaction, the reactor was then quenched with cold water. The resulting powders were filtrated and washed with deionized water and methanol several times. The obtained products were then dried under vacuum.

B. Characterization

The phase composition of the products was identified by x-ray powder diffraction (XRD), which was carried out by using Cu Kα1 radiation on an Empyrean X-ray generator (PANalytical B.V., Almelo, The Netherlands). The X-ray intensity was measured over a diffraction 2θ angle from 20° to 80° with a step size of 2° min−1. The microscopic feature of the products was observed by scanning electron microscopy (SEM; S-4800 FESEM, Hitachi Ltd., Tokyo, Japan) and transmission electron microscopy (TEM; TECNAI-G2 F20, FEI Company, Hillsboro, Oregon). For the SEM measurements, the dried products were mounted on specimen studs and sputter-coated with a thin film of gold to prevent charring, and then the gold-coated surface was scanned at various magnifications by using scanning electron microscope. Samples for TEM evaluation were prepared by placing a drop of the products which were crushed and dispersed in ethanol on a holey carbon TEM grid and by drying the products under vacuum. X-ray photoelectron spectroscopy (XPS; ESCALAB250, Thermo Fisher Scientific, Waltham, Massachusetts) was used to analyze the surface composition of transition metal oxides powders. Dynamic light scattering (DLS, Zetasizer Nano ZS90, Malvern Instruments Ltd., Malvern, United Kingdom) was used to measure the diameter and size distribution of synthesized particles.

III. RESULTS AND DISCUSSION

Figure 1 shows a typical XRD pattern of the products of Cu2O, MoO2 and V2O3 synthesized in scMeOH and the peaks of the reference code [standard card, The Joint Committee on Powder Diffraction Standards (JCPDS)]. It was found that the position of peaks matched well with that of the standard substances for each oxide. Seven peaks at 2θ = 29.58, 36.44, 42.33, 52.48, 61.41, 73.56, and 77.41° shown in Fig. 1(a) are corresponded to face-centered cubic Cu2O structure, and the diffraction peaks of Cu2O are indexed to PDF 0782076 with a = b = c = 4.267 Å. For MoO2, the diffraction peaks at 2θ = 26.07, 36.99, 41.58, 53.53, 60.26, and 66.63° [see Fig. 1(b)] are indexed to PDF 0731249 with monoclinic p21/c (14) space group and a = 5.611, b = 4.856, c = 5.628 Å. While the diffraction peaks of V2O3 are found at 2θ = 24.35, 33.05, 36.30, 41.40, 49.85, 54.05, 63.20, 65.30, and 70.80° [see Fig. 1(c)] and are indexed to PDF 0840317 with rhombohedral $r\minus3c$ (167) space group and a = 4.932, b = 4.932, c = 13.991 Å, respectively. The XRD diffraction peaks are sharp with high intensity, indicating the high quality of the crystalline of the three synthetic substances formed in the supercritical condition.

FIG. 1.

XRD patterns of the Cu2O (a), MoO2 (b), and V2O3 (c) particles synthesized in scMeOH (240 °C, 9.0 MPa).

Figure 2 shows the SEM and TEM images of Cu2O, MoO2, and V2O3 materials synthesized in scMeOH. The Cu2O particles in Figs. 2(a) and 2(b) dispersed well and displayed regular edge-truncated cubic morphology with approximately an average diameter of 2.5 μm, while MoO2 and V2O3 displayed spherical morphology with a particle size distribution of 20–40 and 20–50 nm, respectively [see Figs. 2(d), 2(e), 2(g) and 2(h)]. It is apparent that the particle size of Cu2O is much larger than that of MoO2 and V2O3, possibly due to the greater solubility of Cu(NO3)2 in supercritical methanol than that of (NH4)6Mo7O24 and NH4VO3. Under the condition of ambient temperature and atmospheric pressure, Cu(NO3)2 can be well dissolved in methanol while (NH4)6Mo7O24 and NH4VO3 are almost insoluble. It is well known that the solubilities of SCFs are much better than that of normal solvents. Therefore, it can be admitted that the solubility of Cu(NO3)2 in supercritical methanol is also greater than that of (NH4)6Mo7O24 and NH4VO3. According to the study reported by Cansell et al., greater solubility of the precursor could result in larger precursor concentration, which might lead to the formation of product particles with larger size in supercritical alcohol system.Reference Pessey, Garriga, Weill, Chevalier, Etourneau and Cansell43 In addition, the average grain sizes of the particles were calculated to be around 33 and 25 nm according to half width of the strongest diffraction peak using Debye–Scherrer formula for the obtained MoO2 and V2O3 samples, respectively. This is in agreement with that analyzed by TEM. Figure 3 shows the typical sizes and particle size distributions of synthesized samples as measured by DLS. The average sizes are 3 μm, 46 nm, and 50 nm for Cu2O, MoO2, and V2O3, respectively [Figs. 3(a), 3(d) and 3(g)]. The sizes of the particles determined by DLS are slightly larger than those determined by SEM and TEM, mainly because the DLS technique gives the mean hydrated diameter of the core of particles surrounded by solvated layers, which are affected by the concentration and viscosity of the solution.Reference Selim, Lee, Kim, Xing, Kang, Chang and Guo44,Reference Selim, Xing, Choi, Chang, Guo and Kang45

FIG. 2.

SEM and TEM images of the Cu2O (a–c), MoO2 (d–f), and V2O3 (g–i) particles synthesized in scMeOH (240 °C, 9.0 MPa) for 15 min (a, b, d, e, g, h) and 45 min (c, f, i), (b) is the detailed morphology of the zone marked in (a).

FIG. 3.

Particle size distributions of Cu2O (a–c), MoO2 (d–f) and V2O3 (g–i) synthesized in scMeOH for different times.

Furthermore, under constant reaction conditions, the reaction time is extended to 30 and 45 min to investigate the influences of time on particle morphologies and size distributions. As shown in Fig. 3, there are no obvious changes in size distributions for three products within the given time range. Figs. 2(c), 2(f) and 2(i) also illustrate that the increase of reaction time has no great influence on the shapes of products. Only partial small particles of cuprous oxide increased slightly [see Fig. 2(c)].

XPS was used to analyze elemental and chemical composition on the surface of synthesized materials. Figure 4 presents the XPS survey spectra of the three products of Cu2O, MoO2, and V2O3 [Figs. 4(a), 4(d), and 4(g)] and curve fitting of Cu 2p, Mo 3d, V 2p, and O 1s. In Fig. 4(b), XPS spectrum of Cu 2p with the binding energy ranging from 928 to 968 eV exhibits 10 peaks by curve fitting, in which the peaks centered at 932.06 and 951.85 eV are related to Cu+ (Cu2O). Peaks at 933.94, 935.84, 941.83, 944.14, 953.93, 956.03, 962.1, and 963.81 eV are attributed to Cu2+, which indicates that partial oxidation has occurred on the surface of the Cu2O particles during storage. The O 1s peak in Cu2O sample is split into three peaks with binding energies of 529.54, 530.70, and 531.57 eV, which are ascribed to CuO, Cu2O and –C=O, respectively [Fig. 4(c)].

FIG. 4.

XPS survey spectra of Cu2O (a), MoO2 (d), V2O3 (g) and curve fitting of Cu 2p (b), Mo 3d (e), V 2p (h) and O 1s of Cu2O (c), MoO2 (f) and V2O3 (i), respectively.

The Mo 3d peak of MoO2 sample, as shown in Fig. 4(e), can be fitted into five peaks centered at the binding energies of 229.32, 231.00, 232.44, 234.18, and 235.60 eV. The peaks at 229.32, 231.00 and 234.18 eV are attributed to Mo4+ (MoO2), while the peaks at 232.44 and 235.60 eV are related to Mo6+ (MoO3), respectively. Figure 4(f) shows that the O 1s peak of MoO2 sample is fitted into three peaks with binding energies of 529.79, 530.51, and 531.60 eV, corresponding to MoO2, MoO3, and –C=O, respectively.

As shown in Fig. 4(h), V 2p peak of V2O3 sample is split into six peaks, and the binding energies of 512.97 and 521.77 eV, 515.17 and 523.08 eV, 516.54, and 524.22 eV are attributed to V2O3, V2O5, and V2O4, respectively. Four peaks at 529.49, 530.28, 531.65, and 532.99 eV split from O 1s peak of V2O3 sample, are ascribed to V2O5, V2O3, –C=O and H2O, respectively [Fig. 4(i)]. Therefore, it is suggested that element V on the surface of the product existed in +3, +4, and +5 valence states.

On the basis of XRD and XPS results, the reaction mechanism available during the synthesis of low valence transition metal oxides (Cu2O, MoO2, and V2O3) in the reducing atmosphere of supercritical alcohols is elucidated and described in Fig. 5. In our previous work,Reference Yu, Li, Ge, Niu, Zhang, Xu and Li41 Cu(NO3)2 was used as precursor to synthesize Cu in sub- and supercritical methanol, and the XRD results showed that the mixture of CuO (i.e., high valence metal oxide) and Cu2O (i.e., low valence metal oxide) was obtained firstly under 250 °C and 9 Mpa for 0.25 h. Further, with the extension of time (from 0.25 to 1.0 h), CuO and Cu2O disappeared successively while Cu was formed as the final product. Therefore, according to the above results, we speculate that high valence metal oxides are formed firstly and then reduced to low valence metal oxides in the systems of sub- and supercritical alcohol when inorganic metal salts are used as precursors. In addition, the previous literature indicated that hydroxide ion (OH−) could be released from supercritical methanol.Reference Veriansyah, Kim, Min and Kim26,Reference Choi, Veriansyah, Kim, Kim and Kang37,Reference Andanson, Bopp and Soetens46 Accordingly, it can be proposed that methanol is decomposed into free OH− and alkyl cations when it reaches supercritical condition. Meanwhile, inorganic metal salts are decomposed to the corresponding high valence metal oxides (CuO, MoO3 and V2O5) under high temperature and high pressure reaction condition. The high valence metal oxides are then reduced to low valence metal oxides by the free OH−, which was considered as a strong reducing agent in some reports.Reference Veriansyah, Kim, Min and Kim26,Reference Choi, Veriansyah, Kim, Kim and Kang37,Reference Sawyer and Roberts47 In addition, the O 1s peaks ascribed to –C=O are observed in all the XPS analyses of three products, indicating that methanol is oxidized to related aldehyde in the process.Reference Gardner, Singamsetty, Booth, He and Pittman48 It is illustrated that the supercritical methanol played a role of a solvent as well as a reducing agent.

FIG. 5.

Schematic diagram of formation mechanism for Cu2O*, MoO2 and V2O3 in the supercritical methanol (* the formation mechanism of Cu2O has been reported by Ref. Reference Li, Ge, Jiang, Peng, Zhang, Li and Yu42).

Moreover, in the same system, cubic Cu2O microcrystals with exposed {100} facets were obtained at 230 °C and 7.8 MPa in our previous work,Reference Li, Ge, Jiang, Peng, Zhang, Li and Yu42 while edge-truncated cubic Cu2O with exposed {100} and {110} facets [see Fig. 2(b)] are formed under the condition of 240 °C and 9 MPa in this work. It was well documented that the morphology of Cu2O crystals was usually defined by polyhedra enclosed by {111}, {100}, and {110} facets.Reference Chen and Xue49 The different growth rate of these facets caused the shape evolution of the Cu2O polyhedrons. Furthermore, Xu et al. demonstrated that within a certain reaction condition, appropriate OH− concentration was beneficial to the exposure of {110} facets, which grew between any two adjacent {100} facets.Reference Wang, Jiao, Wu, Li, Zhou, Jiang and Xu50 Therefore, it can be suggested that the given concentration of OH− released from methanol at 240 °C and 9 MPa cause the selective exposure of both {100} and {110} facets, which then fabricate the edge-truncated cubic Cu2O.

The results of XPS (see Fig. 4) indicate that there is still a small amount of CuO, MoO3, V2O4, and V2O5 existing on the surface of the obtained products. While XRD results have clearly shown that synthesized materials are completely reduced in bulk under the reaction condition and presented as low valence oxides. It should be noted that low valence oxide of Cu2O, MoO2, and V2O3 are not very stable under an aerobic atmosphere. Therefore, during exposure to air, very small percentage of the material might be oxidized to high valence oxide,Reference Chan, Zhao, Hicks, Schatz and Van Duyne51–Reference Weber, Stork, Nakhal, Wessel, Reimann, Hermes, Muller, Ressler, Pottgen, Bredow, Dronskowski and Lerch53 which is attached to the surface of synthesized materials.

IV. CONCLUSIONS

A convenient method for preparing LVTMOs including Cu2O, MoO2, and V2O3 particles in a short time and at a low temperature by using supercritical technique has been developed. Edge-truncated cubic Cu2O with an average particle size of 2.5 μm, spherical MoO2 with a particle size distribution of 20–40 nm and spherical V2O3 with a particle size distribution of 20–50 nm were synthesized in scMeOH (240 °C, 9.0 MPa). It is indicated that the decomposition of metal precursors and the reduction occurred simultaneously. In addition, it is supposed that scMeOH could release hydroxyl anions, which processed reducing ability and the supercritical methanol acted as both a reaction medium and a reducing agent in the system. Furthermore, our synthesis process provided a generally simple and potentially scalable method for preparing low valence transition metal oxides.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial supports by the Program for Liaoning Excellent Talents in University, China (Grant No. LR2012012) and (Grant No. LJQ2012034), and the Scientific Research Fund of Liaoning Provincial Education Department, China (Grant No. L2015423).

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Figure 0

FIG. 1. XRD patterns of the Cu2O (a), MoO2 (b), and V2O3 (c) particles synthesized in scMeOH (240 °C, 9.0 MPa).

Figure 1

FIG. 2. SEM and TEM images of the Cu2O (a–c), MoO2 (d–f), and V2O3 (g–i) particles synthesized in scMeOH (240 °C, 9.0 MPa) for 15 min (a, b, d, e, g, h) and 45 min (c, f, i), (b) is the detailed morphology of the zone marked in (a).

Figure 2

FIG. 3. Particle size distributions of Cu2O (a–c), MoO2 (d–f) and V2O3 (g–i) synthesized in scMeOH for different times.

Figure 3

FIG. 4. XPS survey spectra of Cu2O (a), MoO2 (d), V2O3 (g) and curve fitting of Cu 2p (b), Mo 3d (e), V 2p (h) and O 1s of Cu2O (c), MoO2 (f) and V2O3 (i), respectively.

Figure 4

FIG. 5. Schematic diagram of formation mechanism for Cu2O*, MoO2 and V2O3 in the supercritical methanol (* the formation mechanism of Cu2O has been reported by Ref. 42).