Epitaxial electrodeposition of thallic oxide on single crystal gold

Electrochimica Acta 45 (2000) 3233 – 3239 www.elsevier.nl/locate/electacta

Epitaxial electrodeposition of thallic oxide on single crystal gold Alexey A. Vertegel, Mark G. Shumsky, Jay A. Switzer * Uni6ersity of Missouri-Rolla, Department of Chemistry and Graduate Center for Materials Research, Rolla, MO 65409 -1170, USA Received 15 November 1999; received in revised form 14 January 2000

Abstract Thin films of thallic oxide Tl2O3 (Ia3, a=1.0534 nm) were electrodeposited on the three low-index surfaces of single crystal Au (Fm3m, a=0.4079 nm). Epitaxial films of Tl2O3 with a strong out-of-plane (100) texture were obtained in the case of the (100) and (111) Au substrates, while the film on the (110) Au substrate did not show a preferred out-of-plane orientation. Studies of the in-plane orientation of the films revealed that the large lattice mismatch between Au and Tl2O3 is accommodated by the formation of the coincidence lattices on (100) and (111) Au. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Epitaxy; Electrodeposition; Thallic oxide; Single crystal

1. Introduction Epitaxial films can be prepared by a variety of methods, which usually include vapor deposition onto a single crystal substrate. As a rule, the substrate is chosen to have the crystal structure and lattice parameters similar to those of the film. Recently, we reported that epitaxial films of d-Bi2O3 can be electrodeposited from an alkaline tartrate solution onto single crystal Au [1], although the lattice mismatch between d-Bi2O3 and Au is 35.4%. We also showed that high quality epitaxial films of PbS can be grown on (100) single crystal Au substrates [2], though the lattice mismatch in this case is even higher (45.5%). In both cases, the large mismatch is accommodated by forming coincidence lattices, in which the structure of the films is rotated in relation to the structure of the gold substrate. Here, we consider the electrodeposition of thallic oxide (Tl2O3) onto (100), * Corresponding author. Tel.: +1-573-3414383; fax: +1573-3412071. E-mail address: [email protected] (J.A. Switzer)

(111) and (110)-oriented single crystal Au substrates. The crystal structures of Au (cubic, Fm3m, a= 0.4079 nm) and Tl2O3 (cubic, Ia3, a= 1.0534 nm) are shown in Fig. 1. The lattice mismatch in the Au/Tl2O3 system is high. Based on a simple comparison of the lattice parameters, the mismatch is 158%. Since the bcc bixbyite structure of thallic oxide can also be described as a doubled fcc fluorite structure with one quarter of the oxygen atoms missing, it may be more germane to this study to compare the lattice parameter of Au with one half of the lattice parameter of thallic oxide. In this case, the lattice mismatch is 29.1%. The 29.1% mismatch may be more appropriate for calculating strain in this system, and for comparing these results to other epitaxial systems. We show in this work that thallic oxide can be grown epitaxially on the (100) and (111) surfaces of gold despite the high mismatch. Tl2O3 is a degenerate n-type semiconductor with an optical bandgap of 1.4 eV [3]. This optical transition is indirect, while the first direct transition occurs at 2.2 eV [4]. Since the Fermi level is located in the conduction

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band, the optically measured bandgap is shifted by the Moss-Burstein effect to be much larger than the intrinsic bandgap, which varies between 0.5 and 0.66 eV [5]. Thin films of thallium (III) oxide have been used to enhance the electrocatalytic activity of electrode materials and to inhibit the photodegradation of small bandgap n-type semiconductors [6,7]. Due to the high optical transparency and electrical conductivity they can be used in heterojunction photovoltaic cells [8,9]. The electrodeposition of Tl2O3 on various polycrystalline substrates has been studied quite extensively [10–14] and has been used for the preparation of PbaTlbOc ceramic superlattices [15–18] and defect chemistry Tl2O3 – x superlattices [19]. Electrodeposited Tl2O3 films were also used as the substrates for electrochemical growth of materials with a large lattice parameter (e.g. Ag(Ag3O4)2NO3) [20]. Several methods have been used to achieve epitaxial growth by electrochemical methods, including underpotential electrodeposition [21] and electrochemical atomic layer epitaxy [22]. Underpotential deposition of Tl2O3 has been studied by Markovic and Adzic [23]. They showed that strong and sharp UPD peaks are observed in the cases of the (100) and (111)-oriented substrates, but not in the case of the (110)-oriented substrate. They concluded that these UPD peaks result from the formation of anhydrous Tl2O3 via the oxidation of TlOH species adsorbed on the surface of the substrate. They also suggested a 2D mechanism for the

formation of the UPD Tl2O3 films. However, because of the low thickness of the UPD films, no structural evidence for the formation of epitaxial Tl2O3 was presented. Epitaxial quantum dots of Cd(Se,Te) have been electrodeposited on evaporated Au (111) films [24]. Epitaxial CdS nanocrystals have been prepared on graphite by a hybrid electrochemical/chemical method [25]. However, little work has been done on the electrodeposition of epitaxial semiconductor films with thicknesses in the micrometer range. Lincot et al. reported the electrodeposition of epitaxial CdTe on InP(111) and ZnO on GaN (0001) [26,27]. Epitaxial films of PbO2 have been deposited on TiO2 and SrTiO3 substrates by a photoelectrochemical method [28]. Single-crystal Bi films were deposited on Au-covered, single crystal Si (100) by electrodeposition with subsequent annealing at 268°C [29].

2. Experimental section The deposition solution was prepared by dissolving 13.32 g (0.05 mol) of TlNO3 (reagent grade, purchased from Aldrich) in 500 ml of 5M NaOH. HPLC-grade water (Aldrich) was used to prepare the 5M NaOH. The working electrodes consisted of electropolished (100), (111) and (110) Au single crystals purchased from Monocrystals Company, with a diameter of 10 mm and a thickness of 1 mm. A gold wire was fitted around the edge of a crystal to serve as the electrical contact during deposition. The counter electrode consisted of a chromel alloy wire. A constant anodic current density of 5.0 mA/cm2 was applied to the working electrode with an EG&G Princeton Applied Research Model 273A potentiostat/galvanostat for a period of 25 s, giving a nominal Tl2O3 film thickness of :0.15 mm. Approximately 1.2 mm of the film is deposited for every coulomb/cm2 of charge that is passed. The deposition was performed at room temperature. X-ray diffraction (XRD) experiments were carried out with a Scintag XDS 2000 diffractometer using Cu Ka radiation. The 2u scans were performed with a step size of 0.03° and a data acquisition time of 1 s per point. Azimuthal scans were obtained by the use of a texture goniometer accessory with a step size of 1° and a data acquisition time of 1 s per point.

3. Results and discussion Fig. 1. Crystal structures of Au and Tl2O3. Au is face centered cubic, space group Fm3m, with a lattice parameter of 0.4079 nm. Tl2O3 is body centered cubic, space group Ia3, with a lattice parameter of 1.0534 nm. The Au and O atoms are dark, and Tl atoms are light.

Fig. 2 shows the Bragg – Brentano x-ray diffraction (XRD) patterns for the electrodeposited Tl2O3 films on single crystal Au substrates. A strong (100) orientation is observed for the films deposited onto Au(100). The film grown on Au(111) also has a (100) preferred

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Fig. 2. X-ray diffraction patterns for Tl2O3 films electrodeposited on (A) (100); (B) (111); and (C) (110) single crystal Au. Only the (100) family of planes are observed for Tl2O3 in the X-ray patterns of the films deposited on the (100) and (111) Au substrates. The diffraction pattern of the film deposited on the (110) Au substrate does not show a preferred orientation.

orientation. A random orientation is observed for the film on the (110) oriented Au substrate. Rocking curves were performed on the (400) reflections of Tl2O3 (see Fig. 3). The measured values for the full-width at half maximum (FWHM) were 3.4°, 3.2° and 11.6° for the (100), (111) and (110) oriented substrates, respectively. In all three cases, FWHM values for the films are higher than those for the substrates (0.6° for the (200), (111) and (220) reflections of the (100), (111) and (110) single crystal Au substrates, respectively). The FWHM values for the films on the (100) and (111) Au substrates are much less than those usually obtained for polycrystalline non-textured samples (ca. 10°), while the FWHM value for the rocking curve of Tl2O3 on the (110) Au is characteristic of a polycrystalline sample. It should be noted that 2u scans probe only the out-of-plane orientation of the films. In the case of Tl2O3, a preferential (100) orientation can be achieved

even for films grown on polycrystalline substrates because the growth in the [100] direction is kinetically preferred for thallic oxide in certain experimental conditions [11]. To show that the films truly have grown epitaxially on the (100) and (111) Au substrates it is necessary to show both in-plane and out-of-plane orientation. Another question to be addressed is how the lattice mismatch between Tl2O3 and Au lattices is accommodated. In order to substantiate the in-plane orientation of the film and determine the epitaxial relationship between the film and substrate, azimuthal (f) scans of the (222) reflection of Tl2O3 were accomplished along with f scans of the (111) and (200) reflections for the (200) and (111) Au substrates, respectively. The azimuthal scans shown on Fig. 4 were acquired for the samples tilted so that the angle, x, between the plane of the goniometer and a vector normal to the plane of a substrate was 54.4°. This angle corresponds to the angle between the {111} and {100}

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plane families in a cubic lattice. In the case of the Tl2O3 film on the (100)-oriented substrate (Fig. 4a), the expected four-fold symmetry is observed for both film and substrate, indicating a strong in-plane orientation of the film. The average FWHM values for the azimuthal scans were 0.4° and 5.2° for Au and Tl2O3, respectively. As one can see from Fig. 4a, the (222) reflections of Tl2O3 are rotated 45 degrees with respect to the (111) reflections of Au. This means that though the film and the substrate adopt the same (100) out-ofplane orientation, the structure of thallic oxide is rotated 45 degrees with respect to the structure of gold in the plane of the substrate. An interface model consistent with this rotation is (2 ×2)Tl2O3(100)//((7/2) 2× (7/2) 2)R45° Au (100) (see Fig. 5a). For this coincidence lattice, the mismatch is reduced to +4.4%. Fig. 4b shows the azimuthal scans of the (222) Tl2O3 and (200) Au reflections for the film on the (111)-oriented substrate. In this case, twelve-fold symmetry is observed for the film, again showing an in-plane orientation. It should be noted that a four-fold symmetry should be detected for a (111) reflection of a (100)-oriented epitaxial film. Thus, the observation of the 12fold symmetry indicates the presence of the three equivalent types of in-plane orientations for Tl2O3. The formation of three equivalent types of orientation can be explained taking into account the different symmetry of the (111) and (100) planes in a cubic lattice: a 60° rotation of a (100) plane positioned over a (111) plane results in the equivalent relative positions of the two layers. Fig. 5b shows the schematic plot of the coinci-

dence lattice formed by Tl2O3 on the (111) single crystal Au. The interface model describing this coincidence lattice is (1× 1) Tl2O3 (100)//(4× 2 3 R30°) Au (111). The value of the lattice mismatch is now different for the [100] and [010] crystallographic directions of Tl2O3, and consists of −8.7% and +5.4%, respectively. Our X-ray diffraction data are in agreement with the previous UPD studies of Markovic and Adzic [23]. They observed sharp UPD peaks corresponding to the formation of epitaxial Tl2O3 films on Au(100) and Au(111), while for the (110) oriented substrate the film was not epitaxial and the UPD peaks were not sharp. One possible reason for the absence of epitaxy for Au(110) substrate is insufficient symmetry of the (110) atomic plane, which has a rectangular unit mesh. The coincidence lattices can be formed by relatively small fragments of symmetrical Tl2O3(100) and Au(100) or Au(111) planes, but the matching units between any low index atomic plane of Tl2O3 and Au(110) are too large and probably cannot be formed due to kinetic reasons. SEM micrographs for the films deposited onto the (100), (111) and (110) Au substrates are shown in Fig. 6. No ordered features characteristic of epitaxial films can be seen in case of the (100) and (111) substrates, though the observed Tl2O3 crystals are more uniform in size and shape as compared to those on the (110)-oriented substrate. The small dots observed in all three micrographs are Tl2O3 nuclei forming on the larger Tl2O3 crystals, suggestive of progressive nucleation. Their presence gives a possible explanation for the

Fig. 3. Rocking curves for Tl2O3 films electrodeposited on (A) (100); (B) (111) and (C) (110) single crystal Au. The slight shift in position for the rocking curves is due to miscut of the single crystal substrates.

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It is also worth mentioning that our previous studies [10] showed that the mechanism of formation of Tl2O3 films on polycrystalline substrates involves three-dimensional growth, rather than two-dimensional growth. The SEM studies of the films deposited on the single crystal Au substrates presented here suggest the 3Dgrowth mechanism as well. This seems to contradict the results of Markovic and Adzic [23], who proposed a 2D film formation basing on their UPD studies. However,

Fig. 4. (A) Azimuthal scans for the (222) reflection of Tl2O3 (2u=29.35°) and the (111) reflection of Au (2u= 38.18°) for the film deposited on the (100)-oriented substrate. (B) Azimuthal scans for the (222) reflection of Tl2O3 and the (200) reflection of Au (2u=44.39°) for the film deposited on the (111)-oriented substrate. The scans are acquired for the samples tilted so that the angle x between the normal to the sample surface and the plane of goniometer was 54.4°, thus corresponding to the angle between the {100} and {111} plane families. Azimuthal scans for Au are shown in bold lines.

absence of the ordered features on the epitaxial films. Nucleation is favored in the connections between two or more crystals (see Fig. 7). Thus, the shape of the growing crystals is determined by the surface interactions between them, and does not correspond to the ‘ideal’ octahedral shape of the (100)-oriented cubic crystals.

Fig. 5. (A) Schematic of the coincidence lattice, formed by thallic oxide on the (100)-oriented gold. The mismatch is +4.4%. (B) Schematic of the coincidence lattice, formed by the (100) oriented thallic oxide on the (111)-oriented gold. Because of the different symmetry of the (100) and (111) planes in a cubic lattice, the mismatch depends on the crystallographic direction of Tl2O3. The mismatch is − 8.7% in the [100] direction and + 5.4% in the [010] direction. Note that rotation of the Tl2O3 adlattice 60° with respect to Au results in the same structure. The atoms of the adlattice are shown atop of the substrate atoms though they may actually occupy the hollow sites. The Au and O atoms are dark, Tl atoms are light.

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Fig. 7. Higher magnification SEM micrograph of the Tl2O3 film on the (100)-oriented Au substrate. Note that smaller crystals occur on the borders between the larger ones.

mechanism changes from 2D to 3D. A good estimation for the critical thickness for the systems with the lattice mismatch of ca. 5% is several nanometers [31]. After relaxation of the strain, the mismatch is accommodated by misorientaion. Thus, it is likely that the formation of Tl2O3 films on the (100) and (111) Au involves 2D growth for the first several nanometers, which then changes to 3D growth for the rest of the film.

Acknowledgements This work was supported by Office of Naval Research grant N00014-96-1-0984, National Science Foundation grants CHE-9816484 and DMR-9704288, the University of Missouri Research Board, and the Foundation for Chemical Research.

Fig. 6. SEM micrographs of Tl2O3 films deposited on Au single crystal substrates with different orientations. (A) Tl2O3 on the (100)-oriented Au substrate; (B) Tl2O3 on the (111)-oriented Au substrate; (C) Tl2O3 on the (110)-oriented Au substrate.

it should be noted that UPD deals with the formation of single layers of a film. This first step, which involves the rearrangement of the absorbed TlOH species, is indeed likely to occur according to the 2D mechanism. However, in case of the coincidence lattices formed by Tl2O3 on the (100) and (111) Au substrates, the mismatch between the film and the substrate consists of several percent and, for the first layers of the film, should be accommodated by strain. Theory predicts [30] that as soon as the film achieves a certain critical thickness, strain relaxation occurs and the growth

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