Journal of Colloid and Interface Science 357 (2011) 95–100

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science

www.elsevier .com/locate / jc is

Preparation, characterization and catalytic property of CuO nano/microspheresvia thermal decomposition of cathode-plasma generating Cu2(OH)3NO3

nano/microspheres

Zhi-Kun Zhang, Deng-Zhu Guo ⇑, Geng-Min ZhangKey Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, People’s Republic of China

a r t i c l e i n f o

Article history:Received 8 December 2010Accepted 28 January 2011Available online 4 February 2011

Keywords:CuOCu2(OH)3NO3

SpherePlasma electrolysisAmmonium perchlorate

0021-9797/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcis.2011.01.102

⇑ Corresponding author. Fax: +86 10 62765112.E-mail address: guodz@pku.edu.cn (D.-Z. Guo).

a b s t r a c t

CuO nano/microspheres with a wide diametric distribution were prepared by thermal decomposition ofCu2(OH)3NO3 nano/microspheres formed in a simple asymmetric-electrode based cathodic-plasma elec-trolysis. The morphological, componential, and structural information about the two kinds of sphereswere characterized in detail by SEM, TEM, EDX, XPS and XRD, and the results revealed that the morphol-ogy of the spheres were well kept after the componential and structural transformation fromCu2(OH)3NO3 into CuO. The TGA/DSC study showed that the CuO nano/microspheres could be exploredto be a promising additive for accelerating the thermal decomposition of ammonium perchlorate (AP).Combining with the current curve and emission spectrum measured in the plasma electrolysis, formationmechanism of the Cu2(OH)3NO3 spheres was also discussed.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

As an old research subject, glow discharge electrolysis (GDE)could date back to more than a 100 years ago [1]. It was conven-tionally performed in a gas/liquid dual-phase system, in whichtwo asymmetric electrodes were placed close to or in the electro-lyte, and the plasma was ignited on the surface of the working elec-trode under a high cell potential. Many works have already beenconducted in order to obtain a comprehensive understanding ofthe GDE [2–4], and it is found that the main influencing factorsin the formation of the plasma are applied potential, geometriesof electrodes, temperature of the electrolyte, properties of the elec-trolyte, and its flow dynamics [5]. It is well known that an obviousfeature of the GDE process is the non-Faraday behavior [2,6], thatis, the chemical yield around the discharge electrode is much high-er than those calculated from the Faraday law. Meanwhile, plentyof energetic species, such as –OH and H2O2, are generated in theelectrolyte during the GDE process. The abundant reactions andunstable states involved in the GDE make it widely investigatedfor promising applications in metal surface engineering [7–11],decontamination of aqueous pollutants [12–14], hydrogen evolu-tion [15] and material synthesis [16–18] up to now.

As a typical p-type semiconductor with a considerable narrowband gap (�1.21 eV), cupric oxide (CuO) have been of particularinterests to researchers due to its potential applications in

ll rights reserved.

microelectromechanical systems (MEMS) [19], field effect transis-tors [20], gas sensors [21,22], field emissions [23,24] and catalysis[25,26]. In order to optimize its catalytic performance and utiliza-tion efficiency, great efforts have been focused on the fabricationsof different CuO nanostructures, such as nanorods [27], nanoflow-ers [28] and nanoparticles [29]. Among these shapes, nanoparticlesor nanospheres are more attractive because of their specific mor-phology and high surface-to-volume ratio, which are believed tobe favorable for the catalytic applications. For this purpose, severalsynthetic strategies have been developed. For example, Xu andcoworkers [30] prepared CuO pricky microparticles with a widesize range by a hydrothermal reaction. Zhang et al. [31]obtained monodisperse CuO nanospheres via gas-phase oxidationof Cu2O nanospheres that were prepared by a low temperaturesolution-phase approach, and found that the gas sensors basedon the as-prepared CuO nanospheres exhibit high sensitivity andexcellent selectivity. However, the complicated technology andpollutional chemicals involved in these preparation methods ofCuO nano/microparticles greatly confine their popularization.

In this work, we introduce a simple and productive plasma-electrolysis approach to synthesize Cu2(OH)3NO3 nano/micro-spheres with a highly round and smooth morphology. Theelectrolysis is conducted in an asymmetric-electrode system undera low cell potential of 100 V, using a Cu wire as the mother cathodeand a nontoxic NH4NO3 aqueous solution as the electrolyte. The as-prepared Cu2(OH)3NO3 spheres are then thermally decomposed intoCuO spheres, which exhibit excellent catalytic performance in accel-erating the thermal decomposition of ammonium perchlorate (AP).

96 Z.-K. Zhang et al. / Journal of Colloid and Interface Science 357 (2011) 95–100

2. Materials and methods

2.1. Preparation of Cu2(OH)3NO3 nano/microspheres in plasmaelectrolysis and their transformation into CuO species

Fig. 1 is a sketch of the apparatus for the plasma-electrolysispreparation of Cu2(OH)3NO3 nano/microspheres. A Cu metal wire(99%) and a Pt sheet were used as the mother cathode and anode,respectively. The anode had a much larger surface area than that ofthe cathode, and both of them were immerged in a 3 M NH4NO3

solution. The cell was driven with a home-made DC regulatedpower supply and the voltage was increased from open circuit po-tential (OCP) at a ratio of 1 V/s. The plasma was ignited at the cath-ode under about 25 V, accompanying with an abrupt increase ofthe cell potential to a high value (such as 90 V). The potentialwas kept at 100 V for the stable plasma discharge thereafter. Itwas found that during the electrolysis process, the solution becameturbid and some precipitation accumulated at the bottom of thecell. After the electrolysis, the precipitation was purified with re-peated centrifugation and ultrasonic dispersion in deionized water,and then dried in air. The productivity of the sample is estimated to100 mg/h.

In order to transform the Cu2(OH)3NO3 nano/microspheres intoCuO species, the as-prepared sample were heated at 300 �C for 1 hunder atmosphere condition with a heating rate of 2 �C min�1.

2.2. Characterizations

A scanning electron microscope (SEM: FEI Quanta 600F FEG envi-ronmental) and a transmission electron microscope (TEM: TecnaiG20) were employed to characterize the morphologies of the sam-ples. Energy dispersive X-ray spectroscopy (EDX) and X-ray photo-electron spectroscopy (XPS: AXIS Ultra instrument) were hired forcomposition analysis. X-ray Diffraction (XRD: Rigaku Dmax/2400)analysis was performed using Cu Ka1 (k = 0.15406 nm) at a scanningratio of 4� min�1. Thermogravimetric analysis (TGA) was carried outin a SDT Q600 system (Thermal Analysis Inc., USA) with a heatingrate of 10 �C min�1. The current of the cell was monitored with aKeithley 2400, and the emission spectrum of the glow was detectedwith a fiber optic spectrometer (HR4000, Ocean Optics Co. Ltd., USA).

2.3. Measurement of the catalytic activity of CuO nano/microspheres

The catalytic activity of the as-prepared CuO spheres was inves-tigated via their performance in the thermal decomposition ofammonium perchlorate (AP), which is widely used as a commonoxidizer in composite solid propellants. The CuO spheres were

Fig. 1. A sketch of the apparatus for the plasma-electrolysis preparation.

thoroughly mixed with AP at a mass ratio of 1:49 by grinding themin an agate mortar. For a comparison, pure AP without CuO wasalso put into the measurement. The thermal decomposition analy-sis of the AP–CuO mixture and pure AP were performed byemploying simultaneous thermogravimetry–differential scanningcalorimetry (TG–DSC) in a SDT Q600 thermal analysis system(Thermal Analysis Inc., USA) with a heating rate of 10 �C min�1 un-der a flow of nitrogen, covering a temperature from 200 �C to500 �C.

3. Results and discussion

Fig. 2a shows a typical I–t curve of the electrolysis process,which can be divided into two regions. Region I is the conventionalelectrolysis region. In this region, the current increases with theincreasing cell voltage according to the Ohmic law. Since the twoelectrodes are asymmetric, IR loss is mainly concentrated at thesmall-size cathode, and the released heat rapidly raises the tem-perature of the ambient solution, generating plenty of vapor bub-bles. The vapor bubbles then mix with the electrolytic gasbubbles, forming a vapor/gas sheath around the cathode and sepa-rating it from the solution. The voltage drop therefore primarily oc-curs within this sheath, ionizing the inside atom species andcausing the plasma. Region II is the plasma electrolysis region, inwhich the current mostly keeps at a low value. The fluctuation ofthe current is caused by the inhomogeneous burst of the plasmaaround the cathode [32]. Interestingly, the current drops sharplyfrom the peak value to a transient negative one at the beginningof this region. This may be explained by the truth that the free elec-trons abruptly diffuse to the cathode just when the plasma is ig-nited. It is found that the formation of the vapor/gas sheath is akey factor for the generation of the plasma. When the thicknessand pressure of the sheath exceed a threshold value, plasma is gen-erated accompanying with a sudden increase of the cell voltage toa high value and a decrease of the current to a minimal one. Addi-tionally, the threshold for the plasma is related to the temperatureof the solution. A lower threshold potential for the generation ofthe plasma is always observed at a higher solution temperature.

Since the large electric field strength exists within the vapor/gassheath, free electrons in that sheath are intensively accelerated tohigh energy, impacting the atom species and exciting them. Theexcited atom species then drop down to the stable energy leveland radiate photons. In this sense, the color of the plasma glow de-pends on the nature of the electrode and solution [33]. Fig. 2bshows the emission spectrum of the plasma using Cu wire cathode.Comparing with the standard spectral lines [34], several Cu charac-teristic lines are found, as labeled in the figure. This indicates thatthe Cu atoms are sputtered or ablated from the cathode into theplasma region. Meanwhile, characteristic lines of hydrogen atomsat k = 656.2 nm, H2O at k = 809.7 nm, and oxygen atoms atk = 454.9 nm and k = 777.5 nm are also detected, suggesting thatthe species of the solution are also involved in the plasma. How-ever, further consideration should be given to the peak atk = 793.4 nm in the long-wavelength band, which may be relatedto the NHþ4 species. A digital image of the light-blue plasma glowand the as-formed precipitation are shown in the left and right in-sets of Fig. 2b, respectively. The blue–green color of the precipita-tion denotes its Cu2+ components.

After purification, the precipitation is deposited for character-ization. Fig. 3a is a SEM image of the sample showing that theas-formed spheres have a widely distributed diameters rangingfrom sub-100-nm to hundreds of micrometers. The average diam-eter of the spheres is about 200 nm, and this value is believed todepend on the electrolysis parameters. In our observation, thesmall-size spheres are apt to agglomerate with each other. This

Fig. 2. (a) A typical I–t curve of the electrolysis process and (b) the emission spectrum of the plasma using Cu wire cathode. The left and right insets show the digital image ofthe glow and the as-formed precipitation, respectively.

Fig. 3. SEM images of (a) the overall spheres sample, (b) a intact sphere and (c) a broken sphere. (d) EDX spectrums of the sphere-on-substrate (red line) and pristine Sisubstrate without spheres (blue line). (e) The peak fitted narrow scan Cu 2p3/2 XPS spectrum of the spheres. (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)

Z.-K. Zhang et al. / Journal of Colloid and Interface Science 357 (2011) 95–100 97

Fig. 4. (a) XRD pattern of the spheres and standard spectrum of Cu2(OH)3NO3 fromfile JCPDS 15-0014. (b) A low-magnification TEM image and the corresponding EDpattern of a typical sphere with a diameter of 80 nm. (c) TG curve of the as-formedCu2(OH)3NO3 spheres. The insets show the digital images of the sphere samplebefore and after the thermal treatment.

98 Z.-K. Zhang et al. / Journal of Colloid and Interface Science 357 (2011) 95–100

may be attributed to the fact that those spheres are large in num-ber and have high specific surface energy, and they agglomerate tominimize the free energy of the integral system. Fig. 3b shows ahigh-magnification SEM image of a typical sphere with a diameterof 2.2 lm. One can see that the sphere has highly round andsmooth morphology. Fig. 3c shows the SEM image of a brokensphere (diameter = 6.8 lm) with a novel hole on the surface, andit can be observed from the hole that the sphere seems to be con-sisted of abundant small crystals. The EDX spectrum of the spheres(red line) is shown in Fig. 3d. It should be mentioned that in ourexperiment, the sample was deposited on a Si substrate for theEDX characterization, thus the spectrum of the pristine Si substratewithout sample is also presented (blue line). One can concludefrom the EDX results that the main components of the spheresare Cu, O and N. Fig. 3e shows the peak fitted narrow scan XPSspectrums of the sample. The peak at binding energy 935.1 eV iswell in line with Cu 2p3/2 state in Cu–OH and Cu–NO3 compoundsreported by previous works [35,36].

Further structural information about the spheres was character-ized by employing the XRD and TEM technique. Fig. 4a shows theXRD pattern of the spheres. Comparing with the standard spectrum(JCPDS 15-0014) given in this figure, it can be seen that the spheresgenerated in the plasma electrolysis are composed of copper ni-trate hydroxide (Cu2(OH)3NO3). This is in agreement with theXPS observation. Fig. 4b is a low-magnification TEM image show-ing a typical sphere with a diameter of 80 nm and its electron dif-fraction (ED) pattern. The well aligned ED spots reveal a relativelysimple crystal structure in the sphere. Two major diffraction spotsin the pattern are indexed to be (0 �3 1) and (�1 0 �2) crystalplane along [�6 1 3] zone axis of Cu2(OH)3NO3. Interestingly, it isfound that the crystal structure of a sphere is related to its size.A small-size (mostly sub-100 nm) sphere always has a homoge-neous crystalline phase with a well discrete ED pattern, while acomplicated ED pattern with several spot series is mostly foundon a large-size sphere, suggesting the polycrystalline structure ex-isted in this sphere. This is in agreement with the SEM observationof the broken sphere above.

Fig. 4c shows the TG curve of the as-formed Cu2(OH)3NO3

spheres. The total weight loss in the whole temperature range of20–700 �C is 33.35%, which is in good agreement with the theoret-ical value of Cu2(OH)3NO3 (33.6%). Meanwhile, most of the weightloss occurs within a narrow temperature range of 225–280 �C, dueto the intensive one-step transformation of the Cu2(OH)3NO3 intoCuO [37]. The digital images of the insets show the alteration ofthe sample color before (light-green) and after (black) the thermaltreatment. The phenomenon further confirms the transformation.

The formation mechanism of the Cu2(OH)3NO3 spheres via theplasma electrolysis is considered to be ‘‘melting–quenching’’process, as schematically shown in Fig. 5. In the plasma region,the current flow concentrates at certain surface spots of the Cucathode due to the electrothermal instability [38], and the releasedJoule heating rapidly increases the plasma temperature near thesespots. Since the electroconductivity of the plasma is exponentiallyproportional to its temperature, the current and the correspondingJoule heating are both repeatedly enlarged, resulting in the localmelting of the Cu cathode. The small molten metal cluster thenyields to be spherical shape in order to minimize the surface en-ergy and it falls or sputters down from the mother Cu cathode.Accordingly, the nano/microsphere is formed via the quenchingof the metal cluster in the electrolyte. The quenching process iscritical to the structures of the spheres. A small-size sphereis mostly well crystallized in its integrity, while a large-size oneis apt to form polycrystalline structure because of the local differ-ence of temperature or pressure in this sphere during the quench-ing process. Importantly, the NO�3 species in the NH4NO3 solutionare also incorporated into the components of the spheres by the

quenching, and this phenomenon can be explored to have promis-ing application in controllable synthesis of various metallic oxidenanostructures.

The Cu2(OH)3NO3 spheres were transformed into CuO speciesby heating at 300 �C for 1 h. The XRD spectrum of the thermallytreated sample is shown in Fig. 6a. One can see that the spectrumwell fits the standard data of CuO (JCPDS 45-0937), suggesting theabsolute transformation of the spheres from Cu2(OH)3NO3 intoCuO. Fig. 6b is a low-magnification SEM image of the CuO spheresshowing that their spherical shape and size range are well keptafter the transformation. Interestingly, the surface morphologiesof the spheres are found to exhibit two different types. One isthe smooth type that is similar to the as-formed Cu2(OH)3NO3

sphere in the plasma electrolysis, as representatively shown inFig. 6c, while the other is the bumpy type with furrows shown inFig. 6d. It is inferred that during the heating process, the compo-nents change, as well as the gas generation (NO and H2O) in the

Fig. 5. A schematic diagram for the formation of the Cu2(OH)3NO3 nano/micro-spheres in the plasma electrolysis.

Fig. 6. (a) XRD spectrum of the spheres after thermal treatment under 300 �C for1 h and standard spectrum of CuO from file JCPDS 45-0937. SEM images of (b) theoverall, (c) a typical smooth and (d) bumpy CuO sphere.

Z.-K. Zhang et al. / Journal of Colloid and Interface Science 357 (2011) 95–100 99

transformation lead to the mass reduction and reconfiguration ofthe spheres, yielding this bumpy morphology.

Cupric oxide is considered to be an effective additive for thedecomposition of AP among the different kinds of transition metalor metal oxide, and has received extensive interests [26,39]. Mean-while, it is well known that the nanostructure is superior than bulkin catalysis, hence good catalytic activity can be expected on theas-prepared CuO nano/microspheres in our experiment. Fig. 7shows the TGA/DSC curves of the AP–CuO mixture and pure AP.It can be observed from the TGA results in Fig. 7a that only onemass loss is detected in both of the samples, while the additionof CuO spheres reduces the ending decomposition temperature ofAP from 413 �C to 348 �C. The DSC curves of the AP–CuO mixtureand pure AP are shown in Fig. 7b. For pure AP, an endothermic peakat 244 �C appears due to the crystallographic transformation of APfrom orthorhombic to cubic without mass loss [40]. The AP thenundergoes two decomposition processes with the increase of thetemperature, including a low temperature peak at 320 �C and ahigh temperature peak at 453 �C. The low temperature peak is rel-ative to the partial decomposition of AP that includes several reac-tions, i.e. transfer of proton in the AP subsurface, generation of NH3

and HClO4, decomposition of HClO4 and its reaction with NH3,while the high temperature peak is attributed to the thorough dis-sociation and sublimation of AP into gaseous HClO4 and NH3 [40].For the AP–CuO mixture, a same endothermic peak at 244 �C is alsofound, which suggests that the CuO has little impact on the crystal-lographic transformation of AP. Nevertheless, comparing with thatof pure AP, the decomposition process of the AP–CuO mixtureis greatly simplified, that is, the original two peaks in the

Fig. 7. (a) TG, (b) DSC curves of pure AP (blue line) and AP–CuO mixture with 2 wt% CuOthe reader is referred to the web version of this article.)

decomposition process of pure AP almost blend into one largepeak. Moreover, the high temperature peak of AP shifts from453 �C to a much lower value of 344 �C in the presence of CuOspheres, indicating the positive accelerating effect of the CuOspheres in the thermal decomposition of AP.

It was traditionally believed that the decomposition of AP wascaused by the electron transfer from NHþ4 to ClO�4 , thus the electrontransfer between AP and nanocrystals domains the catalytic behav-ior in the presence of nanostructure additives [41]. However, since

spheres (red line). (For interpretation of the references to color in this figure legend,

100 Z.-K. Zhang et al. / Journal of Colloid and Interface Science 357 (2011) 95–100

the AP is a typical dielectric, suggestion was then put forward thatthe AP decomposition process is dominated by a proton transferrather than the electron transfer, and assumptions with regard tothe accelerating mechanism of CuO in catalysis had been well built[40]. Based on these previous works, we infers that the O�2 specieson the surface of CuO spheres, as well as the O�2 species generatedfrom decomposition of AP, act as the proton traps through theirreaction with NHþ4 for the decomposition. Additionally, the largesurface area and the interspace caused by the nonuniformity ofCuO nano/microspheres benefit the adsorption of NHþ4 on the sur-face of CuO spheres. These are all believed to account for the sim-plified process and high efficiency in the thermal decomposition ofAP by using the as-prepared CuO spheres as an additive.

4. Conclusion

In summary, CuO nano/microspheres are fabricated via thermaldecomposition of Cu2(OH)3NO3 spheres, which are generated inasymmetric-electrode based plasma electrolysis under a lowpotential of 100 V. The plasma provides specific conditions forthe local melting of Cu mother cathode, and the Cu2(OH)3NO3

spheres are formed by the following quenching of the molten Cuclusters in the NH4NO3 solution. The preparation process is simple,nontoxic and productive. Importantly, the as-prepared Cu2(OH)3-

NO3 spheres can be easily transformed into CuO species withoutdeformation, and the large surface area and the interspaces ofthe CuO nano/microspheres are found to have positive catalytic ef-fect in the thermal decomposition of AP. The phenomenon that thechemical species of the electrolyte are incorporated into the finalproduction by the plasma electrolysis is believed to further drawmuch interest in the controllable synthesis of various metallicoxide nanostructures.

Acknowledgment

This work was financially supported by the National NaturalScience Foundation of China (Grant No. 60971002).

References

[1] J. Gubkin, Ann. Phys. 32 (1887) 114.[2] A.R. Denaro, A. Hickling, J. Electrochem. Soc. 105 (1958) 265.

[3] Y. Kanzaki, M. Hirabe, O. Matsumoto, J. Electrochem. Soc. 133 (1986) 2267.[4] T. Cserfalvi, P. Mezei, J. Anal. At. Spectrom. 9 (1994) 345.[5] J. Garbarz-Olivier, C. Guilpin, J. Electroanal. Chem. 91 (1978) 79.[6] A. Hickling, M.D. Ingram, J. Electroanal. Chem. 8 (1964) 65.[7] M. Aliofkhazraei, A. Sabour Roohaghdam, Electrochem. Commun. 9 (2007)

2686.[8] T. Paulmier, J.M. Bell, P.M. Fredericks, Thin Solid Films 515 (2007) 2926.[9] T. Pauporté, J. Finne, A. Kahn-Harari, D. Lincot, Surf. Coat. Technol. 199 (2005)

213.[10] M. Aliofkhazraei, A. Sabour Rouhaghdam, A. Heydarzadeh, H. Elmkhah, Mater.

Chem. Phys. 113 (2009) 607.[11] M.A. Béjar, R. Henríquez, Mater. Des. 30 (2009) 1726.[12] L. Wang, J. Hazard. Mater. 171 (2009) 577.[13] J. Gao, J. Yu, Y. Li, X. He, L. Bo, L. Pu, W. Yang, Q. Lu, Z. Yang, J. Hazard. Mater.

B137 (2006) 431.[14] R. Amano, M. Tezuka, Water Res. 40 (2006) 1857.[15] Z. Yan, L. Chen, H. Wang, J. Phys. D: Appl. Phys. 41 (2008) 155205.[16] J. Guo, H. Wang, J. Zhu, K. Zheng, M. Zhu, H. Yan, M. Yoshimura, Electrochem.

Commun. 9 (2007) 1824.[17] S. He, Y. Meng, Thin Solid Films 517 (2009) 5625.[18] M. Tokushige, T. Nishikiori, Y. Ito, J. Appl. Electrochem. 39 (2009) 1665.[19] K. Zhang, Y. Yang, E.Y.B. Pun, R. Shen, Nanotechnology 21 (2010) 235602.[20] L. Liao, Z. Zhang, B. Yan, Z. Zheng, Q.L. Bao, T. Wu, C.M. Li, Z.X. Shen, J.X. Zhang,

H. Gong, J.C. Li, T. Yu, Nanotechnology 20 (2009) 085203.[21] A. Chowdhuri, V. Gupta, K. Sreenivas, R. Kumar, S. Mozumdar, P.K. Patanjali,

Appl. Phys. Lett. 84 (2004) 1180.[22] N.D. Hoa, N.V. Quy, H. Jung, D. Kim, H. Kim, S.-K. Hong, Sens. Actuators, B 146

(2010) 266.[23] J. Chen, S.Z. Deng, N.S. Xu, W. Zhang, X. Wen, S. Yang, Appl. Phys. Lett. 83

(2003) 746.[24] Y. Liu, L. Zhong, Z. Peng, Y. Song, W. Chen, J. Mater. Sci 45 (2010) 3791.[25] L. Chen, L. Li, G. Li, J. Alloys Compd. 464 (2008) 532.[26] S. Yang, C. Wang, L. Chen, S. Chen, Mater. Chem. Phys. 120 (2010) 296.[27] Q. Liu, Y. Liang, H. Liu, J. Hong, Z. Xu, Mater. Chem. Phys. 98 (2006) 519.[28] H. Zhang, J. Feng, M. Zhang, Mater. Res. Bull. 43 (2008) 3221.[29] R. Wu, Z. Ma, Z. Gu, Y. Yang, J. Alloys Compd. 504 (2010) 45.[30] Y. Xu, D. Chen, X. Jiao, J. Phys. Chem. B 109 (2005) 13561.[31] J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Chem. Mater. 18 (2006) 867.[32] A.L. Yerokhin, L.O. Snizhko, N.L. Gurevina, A. Leyland, A. Pilkington, A.

Matthews, J. Phys. D: Appl. Phys. 36 (2003) 2110.[33] K. Azumi, T. Mizuno, T. Akimoto, T. Ohmori, J. Electrochem. Soc. 146 (1999)

3374.[34] P.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, Chapman

& Hall Ltd., London, 1950.[35] N.S. McIntyre, S. Sunder, D.W. Shoesmith, F.W. Stanchell, J. Vac. Sci. Technol. 18

(1981) 714.[36] V.I. Nefedov, É.K. Zhumadilov, T.Y. Konitova, J. Struct. Chem. 18 (1977) 549.[37] L. Kong, X. Chen, G. Yang, L. Yu, P. Zhang, Appl. Surf. Sci. 254 (2008) 7255.[38] R. Rosa, L. Farrar, D. Trundnowski, J. Propul. Power 4 (1988) 466.[39] P.R. Patil, V.N. Krishnamurthy, S.S. Joshi, Propellants, Explos., Pyrotech. 33

(2008) 266.[40] V.V. Boldyrev, Thermochim. Acta 443 (2006) 1.[41] D.V. Survase, M. Gupta, S.N. Asthana, Prog. Cryst. Growth Charact. Mater. 45

(2002) 161.