07) 1908–1911www.elsevier.com/locate/matlet

Materials Letters 61 (20

Synthesis of LaCrO3 films using spray pyrolysis technique

Yinzhu Jiang, Jianfeng Gao ⁎, Mingfei Liu, Yanyan Wang, Guangyao Meng

Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei, 230026, PR China

Received 3 April 2006; accepted 27 July 2006Available online 17 August 2006

Abstract

LaCrO3 thin films on electrolyte yttria-stabilized zirconia (YSZ) substrates were prepared by spray pyrolysis technique in the temperaturerange of 600–750 °C using lanthanum and chromium nitrates as precursors. Thin films obtained at 600–650 °C appear to be a mixture of cubic La(OH)CrO4 and cubic LaCrO4 phases, which transforms to pure cubic LaCrO4 with the substrate temperature increasing to 700–750 °C. Afterbeing annealed at 900 °C for 2 h, all films convert to a single cubic LaCrO3 phase. The change of Cr2p spectra in X-ray photoelectronspectroscopy (XPS) analysis shows the similar phase transformation process. Reaction processes with respect to the substrate temperature wereproposed according to X-ray diffraction (XRD) and XPS analysis. The surface morphology of the films was found to depend strongly on thesubstrate temperature, which would be the deciding factor of the film growth mechanisms.© 2006 Elsevier B.V. All rights reserved.

Keywords: Lanthanum chromites; Phase transformation; Film morphology; Growth mechanism

1. Introduction

Due to their high redox stability and conductivities, lanthanumchromites, LaCrO3, have been widely investigated for applica-tions as interconnect materials and potential anode and cathodeelectrocatalysts for solid oxide fuel cells (SOFCs) [1–3]. It hasalso been well established that the substitution on A and B siteswith alkali earth (Ca, Mg, Sr, etc.) and transition metal elements(Ni, Cu, etc.) respectively, will provide considerable modifica-tions of their electronic as well as their catalytic properties [4–6].Recently, these LaCrO3 and LaCrO3-basedmaterials were used asthe anode materials in SOFCs fed with hydrocarbon fuels [7–10].They are expected to solve the problems in conventional Ni-basedanode, such as poor redox cycling stability and coking. It is wellrecognized that, in addition to the composition of an anodestrongly influencing the performance of anode, the morphology(or microstructure), directly related with the process applied forfabrication, is another critical factor impacting the electrocatalyticproperty of anode. Commonly, anodes are prepared by conven-tional ceramic processes, such as screen sprinting, dip-coating,spin-coating and so on. These techniques usually include the

⁎ Corresponding author. Tel.: +86 551 3601700; fax: +86 551 3607627.E-mail address: jfgao@ustc.edu.cn (J. Gao).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.07.153

synthesis of anode powders, deposition of film and followingsintering. The performance of as-prepared anode is affected to agreat extent by adopted technique, slurry or suspension properties(starting powders and viscosity) and sintering temperature. In ourprevious papers [11–13], preparation of yttria-stabilized zirconia(YSZ) and dopedCeO2 (DCO) films by aerosol-assisted chemicalvapor deposition (AACVD) or spray pyrolysis technique has beenreported, which offers an attractive way for the formation of thinfilms due to the simplicity of the apparatus and good productivityof this technique on a large scale. Besides these, spray pyrolysistechnique, which prepares films directly from precursors, is muchsimpler and more controllable as compared with the conventionalceramic processes.

In the present work, LaCrO3 thin films were prepared by spraypyrolysis technique on YSZ electrolyte substrates at 600–750 °C,using lanthanum and chromium nitrates as precursors. The struc-tural, compositional, surface morphology transformation and thegrowth process with respect to the substrate temperatures wereinvestigated in detail, and reaction processes and growth mecha-nisms were also primarily discussed.

2. Experimental procedure

Thin film deposition using spray pyrolysis technique involvesthe spraying of a metal salt solution onto a heated substrate. An

Fig. 1. XRD patterns of LaCrO3 films on YSZ substrates at different temperatures.(a) 600 °C; (b) 650 °C; (c) 700 °C; (d) 750 °C and (e) deposited at 650 °C andannealed at 900 °C for 2 h.

Table 1The binding energy (eV) of core levels for LaCrO3 films deposited at varioustemperatures

T/°C

600 650 700 750 900

O1 s (eV) 530.1 530.15 529.95 530.05 529.15531.45 531.7 531.50 531.5532.80 533.1 533.0

La3d5/2 (eV) 835.05 835.0 834.95 834.9 834.6Cr2p3/2 (eV) 578.1 577.9 578.0 575.8 575.7

579.6 579.6 579.4 576.95578.35579.7

1909Y. Jiang et al. / Materials Letters 61 (2007) 1908–1911

ultrasonic atomizer (Model 402A, YuYue Company, China) wasused for the generation of aerosols.

Firstly, quantified lanthanum and chromium nitrates (purityN99.0%, Sinopharm Chemical, China) were mixed to form awater solution of precursors. The total metal concentration wasfixed to be 0.05 mol/l. The solution was ultrasonically atomized(exciting frequency=1.7 MHz) to produce mists in the sizerange of 10–20 μm. Then the mists were carried by argon andmixed with oxygen in the nozzle region, subsequently sprayedonto the heated YSZ electrolyte substrate. The substrates wereheated by contact with an electric furnace and their temperatureswere controlled between 600 and 750 °C by a thermocouplewire set on the bottom of the substrates. The substrate wasprepared by pressing high purity 8%Y2O3 YSZ powders (purityN99.0%, Farmeiya (Jiujiang) Advanced, China) into disk formand following sintering at 1450 °C for 3 h, resulting in a dense

Fig. 2. Typical XPS spectrum of LaCrO3 film deposited at 700 °C.

disk with an approximate density 97%, 13 mm in diameter andapproximately 1 mm in thickness. Before the deposition, thesubstrates have been sanded with different grades of sand paper(180–3000 grit) and then polished with diamond grinding paste.

The typical deposition parameters are listed below:

• Carrier gas (argon) flow rate: 1700 sccm• Oxygen flow rate: 1500 sccm• Substrate temperature: 600–750 °C• Nozzle diameter: ∼1.5 mm• Distance between substrate and nozzle: ∼5 cm• Diameter of the spray cone: ∼15 mm• Feed rate: ∼0.25 ml/min• Deposition time: 120 min

Microstructure of the as-deposited films was examined by X-ray diffraction (XRD) (CuKa radiation, k=0.15418 nm). Filmmorphology was investigated by scanning electron microscopy(SEM; Hitachi X-650). The elemental composition of the as-deposited films was determined by X-ray photoelectron spec-troscopy (XPS) (Escalab MKII).

Fig. 3. XPS spectra of the Cr2p level in the LaCrO3 system deposited at differenttemperatures. (a) 600 °C; (b) 650 °C; (c) 700 °C; (d) 750 °C and (e) deposited at650 °C and annealed at 900 °C for 2 h.

1910 Y. Jiang et al. / Materials Letters 61 (2007) 1908–1911

3. Results and discussion

3.1. XRD patterns

Fig. 1 shows the XRD patterns of thin films deposited at varioussubstrate temperatures and annealed ones after heat treatment at 900 °C(heating rate: 2 °C/min, cooling rate: about 3 °C/min). It is observedthat the films deposited in the substrate temperature range (600–650 °C) are mostly a mixture of cubic La(OH)CrO4 and cubic LaCrO4

phases. When the substrate temperature goes above 700 °C, theintensity and the amount of the peaks representing cubic La(OH)CrO4

decrease greatly and cubic LaCrO4 comes to dominate. Moreover, bycomparing the width of the diffraction peaks, crystallinity of the film isimproved with increasing deposition temperature. Upon annealing at

Fig. 4. SEM photographs of LaCrO3 thin films deposited on YSZ substrates deposite

900 °C, all the as-prepared films convert to single cubic phase ofLaCrO3. Compared with cases using the other techniques, such assolid-phase reaction, coprecipitation and sol–gel, the phase-formationtemperature of cubic LaCrO3 was largely reduced by above 200 °C[14,15], the reason for which may be the introduction of the small mistsin the size range of 10–20 μm.

3.2. XPS analysis

In order to determine the valence state of ions, core levels ofXPSweremeasured for all films deposited at various temperatures. Fig. 2 illustratesa typical XPS spectrum of the film deposited at 700 °C. The bindingenergy data of each element are listed in Table 1. The values of La3dbinding energy indicate a +3 ion is always in all the films obtained at

d at various temperatures: (a) 600 °C; (b) 650 °C; (c) and (d) 700 °C; (e) 750 °C.

1911Y. Jiang et al. / Materials Letters 61 (2007) 1908–1911

different temperatures. However, variations of Cr ion valence (Cr5+ /Cr6+→Cr3+) always accompany the phase transformation (cubic La(OH)CrO4 and LaCrO4→cubic LaCrO3) as the substrate temperature goes up.Fig. 3 shows the Cr2p spectra of these samples. For the films obtained atlower substrate temperatures (600–700 °C), Cr2p level could be basicallydivided into two peaks assigned to Cr5+ (578.0 eV) and Cr6+ (579.5 eV)respectively, which are corresponding to themixed phase of La(OH)CrO4

and LaCrO4 in the XRD patterns. In the process of elevating the substratetemperature, the ratio of Cr5+ to Cr6+ obtained by peak fitting increases,indicating the transformation from La(OH)CrO4 to LaCrO4. When thesubstrate temperature rises to 750 °C, a small new peak assigned to Cr3+

(575.7 eV) appears and grows into a unique peak of Cr2p after annealingat 900 °C for 2 h. This result is consistent with the formation of cubicLaCrO3 phase suggested by the XRD patterns.

According to the analysis results on XRD and XPS patterns, thereaction from the precursors and the phase transformation of the filmsmay be summed up as follows. In the present employed substrate tem-perature range (600–750 °C), the nitrates would undergo the followingreaction:

2LaðNO3Þ3 þ 2CrðNO3Þ3 þ H2OðgÞ→LaðOHÞCrO4 þ LaCrO4

þ by� product

At lower substrate temperatures, cubic La(OH)CrO4 is the dominatingproduct and will transform to cubic LaCrO4 when the substratetemperature increases:

2LaðOHÞCrO4→2LaCrO4 þ H2O↑ þ 1=2O2↑

Upon annealing in air at high temperatures (N900 °C), LaCrO4

would decompose to yield perovskite LaCrO3:

LaCrO4→LaCrO3 þ 1=2O2↑

Consequently, a single cubic phase of LaCrO3 could be found in theannealed samples.

3.3. Morphology of LaCrO3 films

For fuel cell applications, the morphology and porosity of the anodefilms is just as important as the film purity and composition. Quitedifferent film morphologies are obtained at various temperatures evenusing the same nitrate solution (Fig. 4). The highly porous films (a)consisting of fine granules are obtained at 600 °C. The sizes of thegranules vary from 0.5 μm to 1.5 μm, existing in discrete agglomeratedclusters. There are large void channels between clusters, although it isimpossible to determine by SEM observation whether these voidsextend through the film thickness. When the substrate temperature risesto 650 °C, no obvious changes are observed from the plan view exceptfor the increase of grain size as well as the decrease in average pore size[16]. As increasing the substrate temperature to 700 °C, the film pos-sesses a much more uniform plan view, which contains a large numberof rose-like incorporated particles. The thickness of the film is esti-mated to be about 8 μm, with a high growth rate (∼4.0 μm/h). Fig. 4dshows an enlarged photograph of single rose-like incorporated particle.It is obvious that each particle consists of porous crystallites small than50 nm. When the substrate temperature increases to 750 °C, these rose-like particles appear to be sintered and the film becomes denser than theones obtained at lower substrate temperatures.

4. Conclusions

Spray pyrolysis technique was applied for the fabrication ofLaCrO3 thin films on electrolyte YSZ substrates in thetemperature range of 600–750 °C. Lanthanum and chromiumnitrates were used as precursors. X-ray examinations show thata mixture phase of cubic La(OH)CrO4 and cubic LaCrO4

appears on thin films obtained at a relatively low substratetemperature (600–650 °C), which will transform to pure cubicLaCrO4 phase when the substrate temperature increases to 700–750 °C. Single cubic LaCrO3 thin films are obtained afterannealing treatment at 900 °C for 2 h. XPS analysis indicatesthat the chromium ions are in a mixed valence state and the ratioof Cr5+ to Cr6+ obtained by peak fitting increases with elevatingthe substrate temperature. The reaction processes with respect tothe substrate temperature were proposed according to XRD andXPS analysis, and quite different film morphologies were ob-tained at various substrate temperatures.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China under contract No. 20271047.

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