Electrolytic deposition of niobium oxide films

May 1998

Materials Letters 35 Ž1998. 188–193

Electrolytic deposition of niobium oxide films I. Zhitomirsky Israel Institute of Metals, Technion-Israel Institute of Technology, Haifa 32000, Israel

1

Received 25 September 1997; accepted 1 October 1997

Abstract Cathodic electrolytic deposition of Nb 2 O5 films on platinum substrates was performed via hydrolysis by use of an electrogenerated base of the NbCl 5 salt dissolved in water in the presence of hydrogen peroxide. The deposits were characterized by XRD, TGrDTA, SEM and Auger methods. The crystallization behavior of the deposits and precipitated gels was compared. A possible mechanism of electrodeposition is discussed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Electrodeposition; Niobium oxide; Deposit; Gel; Hydrogen peroxide; Film

1. Introduction

that generate OHy include reduction of water, nitrate ions and dissolved oxygen:

Thin films of niobium oxide w1–5x and complex niobates w6–8x are of considerable interest for electrochemical, catalytic and electronic applications. Recently, much attention has been focused on the use of Nb-doped PZT thin films for nonvolatile memory device applications w9,10x. Various methods have been applied for preparation of niobium oxide films, including chemical vapor deposition w1x, sol– gel w2,3x, reactive dc magnetron sputtering w4x and electrodeposition w11x. The formation of ceramic films by cathodic electrodeposition has received considerable attention during recent years w11–25x. In the cathodic deposition process a metal ion or complex is hydrolyzed by electrogenerated base to form a deposit on the cathodic substrate w12,13x. Possible cathodic reactions

2H 2 O q 2eym H 2 q 2OHy

Ž 1.

y y y NOy 3 q H 2 O q 2e m NO 2 q 2OH

Ž 2.

O 2 q 2H 2 O q 4eym 4OHy

Ž 3.

1 Tel.: q972-4-8294477; fax: q972-4-8235103; e-mail: [email protected].

The cathodic electrodeposition process presents difficulties for the formation of some oxides such as TiO 2 and Nb 2 O5 . The major problem with electrodeposition of these oxides is related to the use of water Žreactions Ž1. – Ž3.. for base generation. Indeed, titanium and niobium salts immediately react with water to form precipitates. Other difficulties in electrodeposition of Nb 2 O5 were discussed in w11x. Lee and Crayston w11x utilized superoxide and tertiary alcohols to generate OHy ions. Electrodeposition of niobium oxide was achieved on a dropping mercury electrode, but no deposition was achieved on more convenient electrodes w11x. Peroxoprecursor method w13x was designed in order to solve problems associated with the electrodeposition of titania. The perox-

00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 2 4 8 - 6

I. Zhitomirskyr Materials Letters 35 (1998) 188–193

ocomplex of titanium is stable under certain conditions in water and has a cationic character. Hydrolysis of the peroxocomplex by a electrogenerated base resulted in deposition of a corresponding peroxocompound, and thermal decomposition resulted in the formation of titania w13,17,18x. Experiments performed in aqueous and mixed solutions demonstrated formation of titania films on various substrates w13,17,18,20x. This approach has been further expanded to formation of complex titanates w18,23–25x and composites w20x. It was pointed out that similar chemical reactions underlie the formation of powders, precipitated by the addition of alkali to a solution containing peroxocomplexes, and the formation of a deposit from the same solution by the use of an electrogenerated base w24–26x. It had been supposed that the peroxoprecursor method can be utilized for electrodeposition of other materials, including niobium oxides and complex niobates w13,26x. In the present work the possibility of cathodic electrodeposition of niobium oxide films on platinum substrates from aqueous solutions containing hydrogen peroxide has been demonstrated. The experimental results of a study of niobium oxide deposits and powders precipitated from the same solutions are reported.

2. Experimental procedures As starting materials niobium chloride NbCl 5 ŽFluka Chemie AG. and hydrogen peroxide Ž30 wt% in water, Carlo Erba Reagenti. were used. Freshly prepared aqueous solutions, contained 0.005 M NbCl 5 and 0.05 M H 2 O 2 were used for electrodeposition. After aging at 208C for 3 days these solutions became increasingly cloudy due to sol formation. This was followed by the precipitation of a yellow gel. The gel was washed with deionized water and dried at room temperature. Rectangular Pt specimens Ž40 = 50 = 0.1 mm. were used as cathodic substrates. The electrochemical cell for deposition in a galvanostatic regime included the cathodic substrate centered between two parallel platinum counterelectrodes. Electrodeposition experiments were performed at 18C. Cathodic deposits were obtained at a constant current density

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of 20 mArcm2 . Deposition times were up to 60 min. Obtained deposits were washed with water in order to prevent Cl impurity from the solution. After drying at room temperature the deposits were removed from the substrates and subjected to X-ray and TGrDTA studies. The obtained deposits were studied using a scanning electron microscope ŽJeol, JSM-840.. The phase content was determined by X-ray diffraction with a diffractometer ŽPhillips, PW-1820. using monochromatized Cu K a radiation. Thermal analysis was carried out in air between room temperature and 8008C at a heating rate of 108Crmin using a computer-controlled thermoanalyzer ŽSetaram, TGA92.. Thin films were also studied by use of a scanning Auger microscope ŽPerkin-Elmer, PHI model 590A..

3. Results and discussion The experiments revealed the formation of deposits on Pt substrates. X-ray analysis was made on fresh deposits and gels and those thermally treated in air at different temperatures for 1 h. X-ray diffractograms of fresh deposits and gels exhibited their amorphous nature ŽFigs. 1 and 2.. In Fig. 1 it can be seen that in the temperature region of 100–5008C the deposits remained amorphous. In contrast, XRD data for the gels ŽFig. 2. indicate that small reflections appear at 4008C, which become clearer and sharp at higher temperatures. Deposits and gels heated at 600 and 7008C exhibit well defined peaks attributed to a pseudohexagonal Nb 2 O5 structure w27x. It is in this regard that crystallization of the pseudohexagonal Nb 2 O5 at 6008C from peroxo–citrato–niobium gels was reported in w28x. The pseudohexagonal structure was also reported for Nb 2 O5 powders prepared by hydrothermal H 2 O 2 oxidation of metallic Nb w29x and by the sol–gel process w3x. Fig. 3 shows an assemblage of TGrDTA curves for the obtained deposits and gels. For the deposits the total weight loss in the temperature region up to 8008C was about 24% of the initial sample weight, although essentially most of the weight loss occurred below 2008C ŽFig. 3a.. The DTA curve exhibits a broad endotherm around 1308C and a broad exothermic peak at ; 5958C. A sharp reduction of sample weight was observed up to ; 2008C for gel samples

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Fig. 1. X-ray diffraction patterns of deposits: as prepared Ža. and after thermal treatment at different temperatures for 1 h: 100 Žb., 200 Žc., 300 Žd., 400 Že., 500 Žf., 600 Žg. and 7008C Žh..

Fig. 3. TGrDTA data for deposits Ža. and gels Žb. obtained at a 108Crmin heating rate.

ŽFig. 3b. and no appreciable weight change was recorded after 3008C. The total weight loss in the temperature region up to 8008C was about 32%. The DTA curve for the gel samples exhibits a broad endotherm around 1308C and a sharp exothermic peak at ; 5558C. Observed endothermic peaks are associated with the weight losses. The exothermic peaks are considered to be due to crystallization of Nb 2 O5 and are in agreement with the X-ray data. Typical Auger spectrum of the surface of the as-prepared film revealed Nb, O and C, but did not revealed Cl as a surface contaminant ŽFig. 4a.. At this point it is important to mention that chloride ions can be removed from solutions through an oxidoreduction process w30x: 3H 2 O 2 q 2HCl ´ 4H 2 O q O 2 q Cl 2

Fig. 2. X-ray diffraction patterns of precipitated gels: as prepared Ža. and after thermal treatment at different temperatures for 1 h: 100 Žb., 200 Žc., 300 Žd., 400 Že., 500 Žf., 600 Žg. and 7008C Žh..

˚ sputtering peaks of Pt were obAfter 5500 A served in addition to those of Nb and O ŽFig. 4b.. Thin deposits Ž0.2–0.3 m m. were crack free and adhered well to the substrates. However, cracking was observed when the deposit thickness exceeded ; 0.5–1 m m, as shown in Fig. 5. The spots on the

I. Zhitomirskyr Materials Letters 35 (1998) 188–193

˚ sputterFig. 4. Auger spectra of the surface Ža. and after 5500 A ing Žb. of as-prepared film.

photomicrograph can be attributed to defects in the green deposits, which probably resulted from the gas evolution during electrodeposition. Further experi-

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ments are necessary to improve the deposit morphology. An important point to be discussed is the electrochemical mechanism of film formation. Several previous investigations have been undertaken on the electrolytic deposition of ZrO 2 , La 2 O 3 , PbO and TiO 2 and other materials w13,16–18,20,23–26,31,32x via the corresponding hydroxides or peroxides. It should be mentioned that various positively charged species of Zr, La, Pb, Ti exist in acidic solutions w33–35x. When the pH is increased, the corresponding hydroxides or peroxides will precipitate, which are soluble in a strongly basic medium owing to the existence of anionic species w33–35x. The production of OHy in the cathodic reactions ŽEqs. Ž1. – Ž3.. increases the local pH, resulting in deposition of hydroxides or peroxides w31x. Therefore, it is important to obtain stable species w13,18,20,26x in the starting solutions and the desired rate of OHy generation w13,26,31,36x for their deposition or co-deposition w18,20,23–25x at the cathode. Previous experiments have shown the importance of the peroxoprecursor method w13,17,18,26,31x, which allowed one to solve the problem of titania deposition and to find experimental conditions for the formation of com-

Fig. 5. SEM picture of an as-prepared film.

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plex compounds such as zirconium titanate w18,23x and PZT w24–26x. Owing to the complex chemistry of niobium species in aqueous solutions w30,37–40x difficulties are encountered in identification of the scheme for reactions which underlie the deposition process. It is known that the addition of hydrogen peroxide increases the solubility of niobium hydroxide at the low pH region w37,38x and clear solutions can be obtained. Niobium exists in these solutions as peroxocation Nb ŽOH . 4 ŽH 2 O 2 .q or NbO 2 ŽH 2 O 2 .q w30,37–39x. Maximal rate of the migration of niobium species toward the cathode in electrodialysis experiments w38x was achieved at a H 2 O 2 :Nb G 2. An ‘isoelectric range’ is characterized by minimum solubility, however niobium hydroxide is not precipitated in the presence of an excess of hydrogen peroxide w37x. According to w30,37x a yellow precipitate of niobium perhydroxide NbŽOH.5 ŽH 2 O 2 . or peroxo–niobic acid ŽHNbO4 P nH 2 O. can be obtained. At the high pH region the solubility of niobium hydroxide increases with increasing pH w37x. It is believed that peroxoanion complexes exist in this region w30,39,40x. It is reasonable to expect that by analogy to titania w13,17,18x, complex titanates w18,23–26x and other materials w31x the formation of niobium oxide deposits can be achieved via a peroxoprecursor. The use of a peroxo complex of Nb allows the problem of instability of Nb salts to be solved and water as solvent can be used. The mechanism of deposition can be envisioned w23,24x as hydrolysis of a cationic peroxocomplex of Nb by an electrogenerated base, formation of colloidal particles of peroxoprecursor ŽNbŽOH.5 ŽH 2 O 2 . or HNbO4 P nH 2 O w30,37x. near the cathode and their subsequent electrophoretic motion toward the cathode, resulting in the formation of the deposit. However, further investigations are necessary to get a better understanding of the mechanism of Nb 2 O5 deposition. It is in this regard that the results described in w41x indicate that cathodic deposition of WO 3 was achieved starting from anionic tungsten–peroxide species. It was pointed out that the peroxy-based route is completely different from deposition via a local pH increase, as a pH increase is detrimental to WO 3 deposition. Moreover, the performed experiments exhibited differences in X-ray and TGrDTA data for deposits and gels. Indeed, the

results presented in Figs. 1–3 indicate that in the case of deposits, crystallization from the amorphous phase starts at a higher temperature. Thermal analysis revealed different weight losses attributed to gradual decomposition of deposits and gels to form Nb 2 O5 . It should be noted that powder characteristics can be different due to an ageing process w42x or due to the different conditions of their precipitation w43x. It is in this regard that platinum is catalytic in the decomposition reaction of H 2 O 2 . It is supposed that the peroxocomplex of Nb forms in the starting solution, but Pt electrodes bring about the decomposition of excess H 2 O 2 . The results obtained have shown that the method of electrodeposition of ceramic oxide films via a peroxoprecursor developed in previous works can be extended to the deposition of Nb 2 O5 films from aqueous solutions. It is important to see that hydrogen peroxide suppresses hydrolysis of niobium ions, which are however able to co-precipitate with other ions by hydrolysis with ammonia solutions w44x. Therefore it is expected that thin films of complex niobates and Nb-doped films can be obtained via cathodic electrodeposition. Electrodeposition of Nb 2 O5 was performed on Pt substrates, which are important for electronic applications. Preliminary experiments have shown that the proposed approach is not restricted with regard to platinized silicon wafers to be used as substrates. 4. Conclusions The feasibility of cathodic electrodeposition of Nb 2 O5 films on Pt substrates has been demonstrated. Electrodeposition was achieved via hydrolysis by an electrogenerated base of NbCl 5 salt dissolved in water in the presence of hydrogen peroxide. The as-deposited films were amorphous. The crystallization behavior of the deposits and precipitated gels was studied and compared. The proposed peroxo precursor method allows new possibilities for the electrodeposition of complex niobates. References w1x T. Maruyama, T. Kanagawa, J. Electrochem. Soc. 141 Ž1994. 2868.

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