Electrochemical preparation and characterization of brain-like nanostructures of Y2O3

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Electrochemical preparation and characterization of brain-like nanostructures of Y2O3 Mustafa Aghazadeh1,*, Mojtaba Hosseinifard1, Mohammad Hassan Peyrovi2, Behrouz Sabour2 (1. Department of Chemistry, Islamic Azad University, Shar-e-Ray Branch, Tehran, Iran; 2 Department of Chemistry, Faculty of Science, Shahid Beheshti University, G. C., P. O. Box 19396-4716, Tehran, Iran) Received 20 December 2012; revised 7 February 2013

Abstract: Nanostructured Y2O3 was successfully prepared via a two-step and template-free method. Firstly, yttrium hydroxide precursor was galvanostatically grown on the steel substrate from chloride bath by direct and pulse current deposition modes. Direct current deposition was carried out at the constant current density of 0.1 A/dm2 for 600 s. The pulse current was also performed at a typical on-time and off-time (ton=1 s and toff=1 s) with an average current density of 0.05 A/dm2 (Ia=0.05 A/dm2) for 600 s. The obtained hydroxide films were then scraped from the substrates and thermally converted into final oxide product via heat-treatment. Thermal behaviors and phase transformations during the heat treatment of the hydroxide powder samples were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The final oxide products were characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results showed that the well-crystallized Y2O3 with brain- and sphere-like morphology were achievable via pulse and direct deposition modes, respectively. It was concluded that pulse current cathodic electrodeposition offered a facile route for preparation of nanostructured Y2O3. Keywords: cathodic electrodeposition; heat-treatment; electrochemical growth; yttrium oxide; brain-like nanostructure; rare earths

Synthesis of metal oxides at nanoscale with morphologies such as particle, wire, tube, sphere and rod has recently found interesting research area in nanotechnology. This is associated with their nanosize dependent novel physico-chemical properties, enhancing their effective surface area and maximizing their utilization. Among the metal oxides, yttrium oxide, Y2O3, has found various useful technological applications due to its thermal, optical and chemical stability and excellent mechanical properties such as high strength and fracture toughness. For example, it is widely used in red-lightemitting phosphors[1], yttrium-aluminum garnets (YAG)[2,3], cathode material for Li batteries[4] and yttria stabilized zirconia (YSZ) thin films[5–7]. In the recent years, various physical and chemical routes including chemical precipitation[8–10], alkalide reduction[11], solgel[12,13], combustion[14], pyrolysis[15], solvothermal[16–19] and hydrothermal[20–26] syntheses have been employed for the synthesis of nanoparticles, nanotubes, nanorods, nanowires and nest-like structures of Y2O3. Besides, electrochemical synthesis, i.e., cathodic electrodeposition has also been applied as an attractive technique in the synthesis of yttrium oxide and hydroxide thin films[27–35]. The most successful development principle in this clean and inexpensive technique has been mentioned as a powerful control on the structural composition, adhesion, thickness and microstructure of the deposited films via

parameters including the applied current and potential, bath concentration, pH, temperature and additives such as surfactant and supporting electrolyte[31–35]. For example, Lee and Tak synthesized Y2O3 films on indium-tinoxide (ITO) via cathodic electrodeposition of yttrium hydroxide followed by heat-treatment[28]. In thermal analysis experiments, they observed that the as-deposited hydroxide film was transformed into α-Y(OH)3 phase at about 673 K and then into Y2O3 at 723 K. Zhitomirsky and Petric[29] reported cathodic deposition of Y2O3 from the aqueous baths of Y(NO3)3 and YCl3 on Ni substrates. They found that the amount of hydroxide deposit could be controlled by changing the current density, deposition time and yttrium salt concentration. Recently, Tondo et al.[35] electrodeposited Y2O3, Y2O3/Co3O4 and Y2O3/Au composite coatings on AISI 430 plates from the chloride salts dissolved in hydroalcoholic solutions containing chitosan as binder. However, all of the above works[27–35] have mainly focused on the preparation of thin, adhesive and track-free Y2O3 films or coatings on various substrates without considering the structure of the obtained products at nanoscale. In fact, to our knowledge, there is no research on the synthesis and characterization of Y2O3 nanostructures via this method. Thus, cathodic electrodeposition of Y2O3 nanostructures is an interesting subject to study. On the other hand, due to the nanosize dependent physico-chemical properties of metal oxides,

* Corresponding author: Mustafa Aghazadeh (E-mail: [email protected]; Tel.: +98-21-820631116) DOI: 10.1016/S1002-0721(12)60273-7

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which enhance their effective surface area and also develop their utilization in many fields, synthesis of nanostructured yttrium oxide is valuable. Thus, in our pervious works, we applied cathodic electrodeposition for preparation of the yttrium oxide nanostructures and reported that various nanostructures such as nanospheres[36], nanoparticles[37,38], nanorods[39] and nanocapsules[40] of Y2O3 are achievable by this method. For this purpose, Y(OH)3 was first galvanostatically grown on a steel cathode by electrogeneration of base in chloride medium. Then it was thermally converted into final oxide product via heat-treatment at 873 K. Furthermore, it was reported that various nanostructures of metal oxides including MnO2 nanowires[41], La2O3 nanoplates[42], ZrO2 ultrafine nanoparticles, nanospheres and nanocapsules[43,44], Gd2O3 nanorods[45] and Co3O4 nanosheets[46] can be easily prepared using this technique. Here we reported, for the first time, pulsed galvanostatic cathodic electrodeposition of yttrium oxide from chloride bath. Also, the brain-like nanostructures of Y2O3 were prepared for the first time by this method and then characterized by the XRD, IR and SEM techniques. For comparison, direct current cathodic electrodeposition of Y2O3 at the current density of –0.1 A/dm2 was carried out under the same deposition conditions.

conditions. The experiments were performed by an electrochemical workstation system (Potentiostat/Galvanostat, Model: BHP-PGS 2066). After electrodeposition, the deposits were dried at room temperature for 48 h and then scraped from the steel electrodes for further characterization and analysis. For obtaining the oxide products, thermal annealing on the obtained hydroxide powders was conducted in dry air atmosphere at 873 K at a heating rate of 10 ºC/min for 3 h.

1 Experimental

2.1 Hydroxide deposit formation mechanism

1.1 Chemicals Y(NO3)3·6H2O (Merck) was used as received. All chemicals were of analytical grade and used without further purification. All solutions were prepared using double distilled water. Aqueous solution of 0.005 mol/L Y(NO3)3·6H2O was prepared for electrodeposition. 1.2 Sample preparation An aqueous 0.005 mol/L YCl3·6H2O was used as the electrolyte. The electrochemical cell included a stainless steel cathode (316 L, 0.1 m×0.05 m×0.005 m) centered between the two parallel graphite counter electrodes. Prior to each deposition, the steel substrates were given a galvanostatically electropolishing treatment at a current density of 0.05 A/dm2 for 300 s in a bath (343 K) containing 50 vol.% phosphoric acid, 25 vol.% sulfuric acid and balanced deionized water. The deposition experiments were performed at the modes of direct current (DC) and pulse current (PC) depositions with the equal applied charge (q=60 C). Direct current electrodeposition was carried out at the constant current density of 0.1 A/dm2 for 600 s. The pulse current was performed at a typical on-time and off-time (ton=1 s and toff=1 s) with an average current density of 0.05 A/dm2 (Ia=0.05 A/dm2). The deposition time at PC mode was 1200 s. Both the DC and PC electrodeposition experiments were done at RT

1.3 Characterization The crystal structure of the prepared hydroxide and oxide powders was determined by X-ray diffraction (XRD) with a diffractometer (Phillips, PW-1800) using monochromatized Cu Kα radiation at a scanning speed of 0.5 (º)/min. Thermogravimetric analysis (TGA) and thermal behavior of the hydroxide powders prepared by the DC and PC depositions were investigated by means of differential scanning calorimetry (DSC, STA-1500). Morphology of the oxide products was examined using a scanning electron microscope (Philips 515). FT-IR spectra were obtained by means of a Bruker Vector 22 FT-IR spectrometer.

2 Results and discussion The electrochemical growth of Y(OH)3 from an additive free YCl3·6H2O (0.005 mol/L) aqueous electrolyte occurs at the cathode through a mechanism of base (OH–) electrogeneration[36,47]. In the aqueous chloride medium, the positively charged yttrium(III) species (i.e., Y3+, Y(OH)2+ and Y2(OH)24+) are expected, which predominantly exist as Y3+ ions[48]. These cations can be hydrolyzed by electrogenerated base and accumulate in the form of colloidal Y(OH)3 particles near the cathode. Thus the formation of Y(OH)3 deposit on the cathode surface from chloride medium can be explained on the basis of a two-step (i.e., electrochemical-chemical (EC)) mechanism as follows[36–39]: Electrochemical step (base electrogeneration): O2+2H2O+4e−→4OH− Eº=0.26 V vs. Ag/AgCl (1) − − 2H2O+2e →H2+2OH Eº=−1.08 V vs. Ag/AgCl (2) Chemical step (deposition formation): Y3+(aq)+3OH−(aq)+yH2O→[Y(OH)3·yH2O]↓ (3) Y3+(aq)+(3–x)OH−(aq)+xCl−+yH2O→ [Y(OH)(3–x)(Cl)x·yH2O]↓ (4) The reactions of (1) and (2) result in an increase of local pH at the cathode surface. By increasing the OH− concentration to the required conditions in the reactions of (3) and (4), yttrium hydroxide will form and deposit on the cathode. Recording the potential during the Y(OH)3 deposition from chloride bath showed −1.05 V and –0.94 V for the

Mustafa Aghazadeh et al., Electrochemical preparation and characterization of brain-like nanostructures of Y2O3

DC and PC modes, respectively. These values suggest that the reduction of water (reaction (2)) has a major role in the production of OH− at our experimental conditions. In fact, the electrochemical step proceeds via reaction of (2) at both modes. Variations of the pH at the cathode surface during the cathodic electrodeposition of Y(OH)3 from chloride bath at direct and pulse current modes are also shown in Fig. 1. These curves clearly indicate that pH of cathode surface is quickly increased at initial times of the deposition process due to base electrogeneration in the electrochemical step (reaction of (2)). It is worth noting that the rate of base generation in the DC mode is higher compared with the PC mode as one can obviously see from the slope of the pH curves in Fig. 1. During the off-times in the PC mode, the electrochemical reactions are off and no base is generated. With increasing of the pH at the cathode surface at longer deposition times, the conditions for the chemical step are ready and yttrium hydroxide starts to form and deposit on the cathode surface. Considering the solubility constant of Y(OH)3 (Ksp=[Y3+]·[OH–]3= 10–22.1), it is expected that the yttrium hydroxide will deposit from the 0.005 mol/L aqueous solution of YCl3·6H2O at pH of 7.8. In fact, at this time,

Fig. 1 Variations of cathode pH during the cathodic electrodeposition of Y(OH)3 from chloride bath at the DC (■) and PC (□) deposition modes

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the OH– produced from the electrochemical step starts to be consumed in the chemical step (i.e. reactions of (3) and (4)). Deposit formation via the reaction of (4) can result in the intercalation of chloride ions in the hydroxide deposit structure. Thus the yttrium hydroxide/hydroxide-chloride composition is expected for the obtained deposits. Fig. 1 shows that the chemical step is approximately starts at the times of 60 and 110 s after the starting of the experiments (starting points of deposition). Due to the low rate of base electrogeneration in the PC mode, these points are observed at longer times compared as with the DC mode. After these points, the electrogenerated base in electrochemical step is subsequently consumed at the chemical step and the slope of the pH curves is lowered (as seen in Fig. 1). Thus the amount of the pH on the cathode surface remains approximately constant. However, the level of pH on the cathode surface is mainly determined by the applied current density. The stages of Y(OH)3 electrodeposition from chloride bath including base electrogeneration (a), Y(OH)3 formation (b) and film deposition (c) on the cathode surface are schematically shown in Fig. 2. 2.2 Structural characterization The XRD patterns of the electrodeposited, and those heat-treated samples 873 K are shown in Fig. 3. The prepared sample in the DC mode (Fig. 3(1)) exhibits two broad peaks near 2θ=30° and 50° and is essentially amorphous. All diffraction peaks except (001) and (002) in XRD pattern of the deposited sample in the PC mode (Fig. 3(2)) can be assigned to the monoclinic phase of Y(OH)3 (JCPDS Card No. 21-1447)[16]. Based on Refs. [49,50], it can be said that the (001) and (002) peaks have resulted from the intercalation of chloride ions in the deposit structure during the electrodeposition. The intensity of these peaks can be related to the amount of the intercalated ions. These findings are completely in agreement with the proposed mechanism. It is worth noting that

Fig. 2 Schematic of the stages of Y(OH)3 cathodic electrodeposition from yttrium chloride solution (a) Electrogeneration of base at the cathode surface via reduction reactions of water and/or dissolved oxygen molecules (electrochemical step); (b) Local increasing of the pH near the cathode surface and reaching the required conditions for the hydroxide deposition (chemical step); (c) film formation

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Fig. 3 XRD patterns of the electrodeposited hydroxide samples in the DC (a) and PC (b) modes and the oxide products obtained after the heat-treatment at 600 ºC

the XRD pattern of the hydroxide sample prepared by the DC mode shows a small broad peak at 2θ=10°, which can be resulted from chloride intercalation in the deposit structure. However, the intensity of this peak is very low, indicating the low amount of intercalated chloride under the DC deposition conditions. It is also clearly seen that the pulse current has a major effect on the crystal structure of the hydroxide deposit. For the annealed samples, the XRD patterns in Figs. 3(3) and (4) show a marked increase in the peak intensity and the formation of new peaks, as well as a decrease in the peak width, confirming the process of phase transformations into yttrium oxide. All peaks can be readily indexed to a pure cubic phase of Y2O3 (space group Ia3/(206 1)) with a lattice constant of a=106.04 nm (JCPDS 25-1200). In both samples, none of the XRD patterns shows any extra diffraction lines from any impurities for the final oxide products.

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changes and oxide formation during the heat treatment (Fig. 4(a)). For both samples (A and B), the low temperature endothermic peak at temperatures less than 423 K is related to the dehydration of free and physically absorbed water molecules associated with the prepared powders. Accordingly, the TG curves of both samples (Fig. 4(b)) show mass losses at this range of temperature, which are related to the following reaction (section 1 in Fig. 4(b)): [Y(OH)(3–x)(Cl)x ·yH2O]→[Y(OH)(3–x)(Cl)x]+yH2O (5) In the literature, the temperature of chloride removal from the deposit structure has been reported in the range of 473–623 K[51–53]. On the other hand, it has been found that the initially formed hydroxide phase transforms into α-Y(OH)3 at about 623 K[28,36]. Thus the following changes can be proposed for the peaks observed at the temperatures 473–673 K: [Y(OH)(3–x)(Cl)x ]→[α-Y(OH)(3–x)(Cl)x] (6) [α-Y(OH)(3–x)(Cl)x]→α-YO(OH)(3–x)+Clx/2 (7) Although this phase transformation is not associated with any mass loss, the TG curves show mass losses related to the removal of chloride ions from the deposit structure (section 2 in Fig. 4(b)). The mass losses of approximately 3.1 wt.% and 6.1 wt.% can be estimated for the samples A and B, respectively. It seems that the chloride ions have enough time for intercalation in the de-

2.3 Thermal behavior characterization To explore the mechanism of oxide formation during the heat treatment process, thermal behavior of the obtained Y(OH)3 powders was examined by differential scanning calorimetry (DSC). The results of thermal analysis (DSC and related TG) of the Y(OH)3 powders prepared by the DC and PC modes are presented in Fig. 4. The DSC curves in Fig. 4(a) show that both samples have approximately the same thermal behavior. The curves also exhibit three distinct endothermic peaks with maximums at 373, 688 and 758 K for both samples, indicating the three successive stages of physicochemical

Fig. 4 DSC (a) and related TG (b) curves of the prepared hydroxide powders by the DC (sample A) and PC (sample B) modes

Mustafa Aghazadeh et al., Electrochemical preparation and characterization of brain-like nanostructures of Y2O3

posit structure during the off-times in the PC mod. This finding is completely in agreement with the results obtained from the structural analysis of hydroxide sample where the existence of intercalated chloride at both samples was clearly confirmed by the XRD patterns. The next endothermic peaks at 673–623 K are related to the removal of structural water and transformation from yttrium hydroxide to oxyhydroxide: α-Y(OH)(3–x)→YO(OH)(1–x)+H2O (8) The last endothermic peaks at 773 K for both samples show the complete removal of the structural water and the formation of well-crystallized final oxide product (i.e., Y2O3): 2YO(OH)(1–x)+1/2O2→Y2O3+(1–x)H2O (9) This phase transformation is also supported by the XRD patterns (Fig. 3), showing the crystalline cubic phase of Y2O3 at 873 K. From the TG diagrams in Fig. 5(b), the total mass losses of the samples A and B are estimated to be 24.7 wt.% and 30.2 wt.%, respectively. Approximately 8.9 wt.% and 11.7 wt.% of the total amount of the observed mass losses for the samples A and B, respectively, correspond to the physically adsorbed water and the rest to the intercalated chloride ions as well as structural water. 2.4 FT-IR analysis Figs. 5 and 6 show the IR spectra of the Y(OH)3 and Y2O3 products. The bands at about 3460 and 1640 cm–1 are due to the (O–H) stretching vibration of the adsorbed water and (O–H) deformation vibration, respectively. As seen in the spectra of oxide samples, the vibration intensity of OH group decreases markedly. However, vibrations still exist in the annealed sample due to moisture absorption during the testing. The peaks at about 1520 and 1440 cm–1 can be attributed to the carbonate group, which may also originate from air. For both hydroxide samples, the peak at approximately 670 cm–1 deals with the Y–O–H stretching mode. Following the heat treatment of hydroxide samples at 600 ºC, new bands ap-

Fig. 5 IR spectra of the hydroxide and oxide samples prepared via the DC electrodeposition

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Fig. 6 IR spectra of the hydroxide and oxide samples prepared via the PC electrodeposition

peared at 570, 450 and 420 cm–1 (insets of Figs. 5 and 6). These bands are characteristic of Y–O–Y stretching frequency of cubic Y2O3[23]. There are no Y–O–Y absorption bands in the Y(OH)3 samples. These results are consistent with the results of XRD and DSC-TG analyses, confirming the formation of crystalline Y2O3 after heattreatment of the Y(OH)3 powders prepared at 873 K. 2.5 Morphological characterization Figs. 7 and 8 show the surface morphology of the Y2O3 products resulting from the heat-treatment of the

Fig. 7 SEM images of the oxide product prepared by heat treatment of the cathodically grown hydroxide via the DC mode

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acid salt as the template and chlorine gas as the oxidant. This oxide product is composed of agglomerates with 500 nm in diameters, which resemble the cerebral cortex of the brain. Thus it can be said that the oxide sample prepared via the PC mode has brain-like structure. It is worth noting that a systematic study about the correlations between the pulse electrodeposition parameters and the structural and morphological properties of Y2O3 has been carried out in our lab and its report is being developed.

3 Conclusions

Fig. 8 SEM images of the oxide product prepared by heattreatment of the cathodically grown hydroxide via the PC mode

hydroxide powders. The oxide sample obtained from DC route (Fig. 7) displays completely sphere morphology at nanoscale. In fact, this sample comprised well-dispersed spherical particles with the average diameter of 1 μm. It is also clearly seen that these spheres have resulted from agglomeration of smaller spheres. The existence of small spherical particles confirms that the large spheres are built up of fine dispersed spherical agglomerates. As seen in the high magnification SEM image, these spherical grains are approximately 200 nm (Fig. 7(b)). It is worth noting that we previously reported the Y2O3 nanospheres prepared at the applied current density of –0.2 A/dm2 from chloride bath via the DC mode. With comparison of the observed morphologies in this work and Ref. [36], the current density of –0.1 A/dm2 is selected for the DC preparation of the  homogeneous and well-dispersed nanospheres of Y2O3 from chloride bath. The morphological characteristics of the oxide product obtained from the PC route are also shown in Fig. 8. This sample has rather different surface morphology than that obtained via the DC electrodeposition. The SEM images in Fig. 8(b) show that the prepared oxide in the PC mode has no spherical particles, rather it has elliptical shape particles with some convolutions on their surface. This type of morphology has only been reported for polyaniline by Zhu et al.[54] from the gas/solid reaction of aniline/citric

The nanospheres and brain-like nanostructures of Y2O3 were prepared from the heat-treatment of cathodically grown hydroxide samples at direct and pulse current modes, respectively. The structural changes including dehydration and physical changes of the yttrium hydroxide precursor during its conversion into oxide were detected by the DSC-TG analysis. The mechanism of hydroxide deposit formation and pH changes on the cathode surface, and chloride ions intercalation in the deposit structure during the DC and PC depositions were proposed and discussed. Based on the results, it was concluded that PC electrodeposition could be recognized as a facile method for the synthesis of Y2O3 nanostructures.

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