Electrodeposition of silver telluride thin films from non-aqueous baths

Electrochimica Acta 49 (2004) 2243–2248

Electrodeposition of silver telluride thin films from non-aqueous baths Ruizhi Chen1 , Dongsheng Xu∗ , Guolin Guo, Linlin Gui State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Physical Chemistry, Peking University, Beijing 100871, PR China Received 2 June 2003; received in revised form 26 December 2003; accepted 5 January 2004

Abstract We describe a method for preparation of crystalline silver telluride films by cathodic deposition from dimethyl sulfoxide (DMSO) solutions containing 0.1 M NaNO3 , 5.0 mM AgNO3 and 3.5–7.0 mM TeCl4 . X-ray diffraction data indicated that the deposited silver telluride films could be adjusted from Ag excess and stoichiometric monoclinic Ag2 Te to hexagonal Ag7 Te4 by increasing the concentration of TeCl4 in the electrolyte or lowering the deposition potential. The Ag2 Te film is gray and the Ag7 Te4 film is dark blue-gray and mirror like adhered strongly to the substrates. Scanning electron microscopy images show that Ag2 Te films were formed with globular grains with average diameters of more than 1 ␮m. In contrast, Ag7 Te4 film consists of triangles characteristic of a (1 1 1) single-crystal with a hexagonal structure in average sizes of about 0.4 ␮m. The X-ray photoelectron spectra (XPS) indicated that the binding energies deviation of Te3d in Ag7 Te4 is less than that in Ag2 Te, which is consistent with the apparent valences of Te in Ag2 Te and Ag7 Te4 . Finally, the cathodic deposition reactions were studied by cyclic voltammetry. © 2004 Elsevier Ltd. All rights reserved. Keywords: Electrodeposition; Thin films; Silver telluride; Ag7Te4; DMSO

1. Introduction Thin films of semiconductors have been the focus of extensive research in the past two decades, due to their important applications in fabricating various devices. Interest in the electrodeposition of semiconductor films has depended on the expectation that this technique will present several advantages over other techniques for film processing, namely low-temperature process, large active areas, arbitrary substrate shape, controllable film thickness and morphology [1]. Silver telluride is known as a typical example of mixed ionic-electronic conductivity in a solid. The low-temperature phase of monoclinic silver telluride is a semiconductor with a narrow band-gap, high carrier mobility and low lattice thermal conductivity, whereas its high-temperature phase gives rise to superionic conductivity [2–5]. Perfectly stoichiometric Ag2 Te has negligible magnetoresistance, but a ∗

Corresponding author. Fax: +86-10-62753937. E-mail address: [email protected] (D. Xu). 1 Present address: The high school affiliated to Renmin University of China. 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.01.004

large positive magnetoresistance effect has been observed in both Ag-rich (n-type) and Te-rich (p-type) silver telluride bulk samples or thin films [6–8]. With remarkable magnetoresistance effect at room temperature, the absence of saturation, and its linear field dependence down to 10 Oe [7,8], n-type Ag2+␦ Te could be a promising material for application in a wide range of magnetic field sensors. Bulk Ag2 Te was traditionally prepared by heating a mixture of high-purity Ag and Te in stoichiometric ratio in an evacuated quartz tube above the melting point of Ag2 Te [8,9]. The reaction between Ag and Te in liquid ammonia produced a mixture of Ag2 Te with 5–10% Ag7 Te4 [10]. Nanocrystalline silver tellurides Ag2 Te and Ag7 Te4 were prepared by high-intensity ultrasonic irradiation of a stoichiometric mixture of tellurium and AgNO3 in an ethylenediamine or an ethanol bath [11]. Thin films of Ag2 Te have been mainly synthesized via vacuum techniques [8,12,13]. Synthesis of silver telluride thin films by electrodeposition has never been reported. Gore and Pandey have reported a cathodic electrodepositing technique for CdTe films in ethylene glycol solution of CdCl2 and TeCl4 [14]. We have previously shown that,

R. Chen et al. / Electrochimica Acta 49 (2004) 2243–2248

(212)

(322) (412)

(312) (321)

(f) 7.0mM

(331)

(222)

The cathodical deposits were analyzed by XRD. Silver telluride thin films can be obtained at potentials between −0.50 and −0.70 V from DMSO solutions containing 0.1 M NaNO3 , 5.0 mM AgNO3 , and various concentrations of TeCl4 . When the concentration of AgNO3 was higher than 5.0 mM, excess Ag was obviously detected in the deposited films, which was difficult to control by changing the potentials. Fig. 1 shows the XRD patterns of the deposited films from electrolytes containing different concentrations of TeCl4 (from 3.5 to 7.0 mM) at a constant potential of −0.60 V. When the concentration of TeCl4 was kept at 3.5, 4.0 or 4.5 mM, the deposited films were monoclinic Ag2 Te, as shown in Fig. 1a–c. The diffraction peaks at2θ = 28.18, 29.92, 31.04, 38.88, 40.1, 41.3 and 42.28◦ can be indexed to (111), (−211), (−210), (−113), (−303), (310) and (021) of monoclinic Ag2 Te, respectively, which are well agreement with the standard diffraction data of powder monoclinic Ag2 Te (JCPDS standard 12–695). Apart from the Ag2 Te phase, Ag (1 1 1) diffraction peak was detected in Fig. 1a and b, and gradually disappeared with an increase of the concentration of TeCl4 . These deposited films are gray in color with good adhesion to the substrates. When the

(e) 6.0mM

+

(-113) ×

o+

(-303) (310)

×

o o

(-210)

+

(-211)

(c) 4.5mM

o

+

+o

o

(021)

o

(d) 5.0mM (111)

Tellurium(IV) chloride (TeCl4 , 99%, ACROS), silver nitrate (AgNO3 , 99.9%) and DMSO (A.R.) were used without further purification. Sodium nitrate (NaNO3 ) (A.R.) was recrystallized before used. The indium-doped tin oxide (ITO) covered glass substrates with a sheet resistance of about 20
cm were cleaned ultrasonically in 0.1 M NaOH, double distilled water, acetone and then rinsed in double distilled water. Silver telluride films were cathodiclly deposited from DMSO solutions containing 0.1 M NaNO3 , 5.0 mM AgNO3 and TeCl4 with various concentrations in a range of 3.5–7.0 mM. NaNO3 was used as supporting electrolyte. Here, DMSO is both an electrochemical solvent, in which it is easy to dissolve TeCl4 to form (2DMSO·TeCl3 )+ ·Cl− , and a weak complexing agent for Ag+ [16]. The glass cell was immersed in an oil bath at a temperature of 80 ± 1 ◦ C. The electrodeposition process was performed potentiostatically with continuously stirring, using a three-electrode system with an ITO substrate as a working electrode, an Ag/AgCl as a reference electrode and a platinum plate as a counter electrode. All of the potentials are expressed versus. Ag/AgCl. After deposition for 10–20 min, the substrate was removed from the electrolyte, first rinsed with hot DMSO (80 ± 1 ◦ C), followed by ethanol, and then dried in air at room temperature. Cyclic voltammetry measurements were carried out using a CHI660A Potentiostat/Galvanostat (CH Instruments Inc., China). All the cyclic voltammograms were made in unstirred solutions at the temperature of 80 ± 1 ◦ C using ITO/glass as the working electrode. The phase identification was characterized by X-ray diffraction (XRD), which was performed on a rotating anode Rigaku (Japan) X-ray diffractometer using Ni-filtered Cu K␣ radiation. The X-ray tube was operated at 40 kV and 100 mA. Surface morphologies of the films were studied by a JEOL JSM-5600LV scanning electron microscopy (SEM), operated at 10 kV. X-ray

3. Results and discussion

(112)

2. Experimental

photoelectron spectrum (XPS) of the film was recorded on a VG-ESCALAB 5 using Al K␣ as the exciting source. The anode operated at 9 kV and 18.5 mA and the chamber pressure was kept at 10−8 Torr.

(211)

using a non-aqueous dimethyl sulfoxide (DMSO) bath containing 0.1 M NaNO3 , 5.0 mM AgNO3 and 6.0 mM TeCl4 , stoichiometric monoclinic Ag2 Te nanowires could be prepared by cathodic electrodeposition in the pores of the anodic alumina templates [15]. We identified that the composition of the nanowires was controlled continuously from Ag-rich to Te-rich mainly with the concentration of TeCl4 in the solutions. The present paper is devoted to electrodeposition of silver telluride films from DMSO solutions. We also found that silver telluride films can be electrodeposited with a controlled composition from Ag-rich to Te-rich by adjusting the concentration of TeCl4 or the deposition potential. Importantly, a high-quality Ag7 Te4 film with 0.3–0.5 ␮m single-crystalline grains was formed when the concentration of TeCl4 increased to 6.0–7.0 mM.

Intensity (arb. units)

2244

(b) 4.0mM (a) 3.5mM

20

25

Ag

30

35

40

45

2θ (Degree) Fig. 1. XRD patterns of the silver telluride films deposited from DMSO solutions containing various concentrations of TeCl4 at a potential of −0.60 V vs. Ag/AgCl. The diffraction peaks of: (×) ITO, In (c); (䊊) Ag2 Te; and (+) Ag7 Te4 .

R. Chen et al. / Electrochimica Acta 49 (2004) 2243–2248

Fig. 3 shows the XRD patterns of the films deposited at the fixed concentration of TeCl4 of 5.0 mM. At a potential between −0.50 and −0.55 V, the deposited film was monoclinic Ag2 Te (Fig. 3a), in contrast to the hexagonal Ag7 Te4 film deposited at a potential between −0.65 and −0.70 V (Fig. 3c). Beyond −0.70 V, the film was black and amorphous. Both monoclinic Ag2 Te and hexagonal Ag7 Te4 coexisted in the film deposited at the middle potential range around −0.60 V (Fig. 3b). Fig. 4a gives the SEM image of the Ag2 Te film deposited at −0.60 V and 4.0 mM TeCl4 for 10 min. The film have monoclinic structure by XRD, and adhered well to the substrate in appearance as described above. It can be seen that this film consists of nearly spherical grains with diameters of more than 1 ␮m. Fig. 4b shows the SEM image of the Ag7 Te4 film deposited at −0.60 V and 7.0 mM TeCl4 for 10 min. The Ag7 Te4 film consists of triangles characteristic of a (1 1 1) single-crystal with a hexagonal structure. These triangular single-crystallites grew uniformly with average sizes of about 0.4 ␮m and well-connected on the surface of the film. The obtained Ag2 Te and Ag7 Te4 films were further characterized by XPS. To our knowledge, the XPS binding energies of Ag and Te in both Ag2 Te and Ag7 Te4 have never been reported. Fig. 5 presents the XPS survey spectrum of the Ag2 Te film scanned from 1 to 1160 eV. All the peaks were identified and attributed to Ag, Te, C, and O. The presence of carbon and oxygen were attributed to the atmospheric exposure of the film. Accurate binding energies (Eb ) were obtained by repeatedly scanning over narrow energy ranges containing the peaks, and measured at the mid-waves. The narrow spectra of Ag3d and Te3d are shown in Fig. 6a and b.

o o

(b) -0.60V ×

×

(111)

20

30

35

40

45

2θ (Degree) Fig. 2. XRD patterns of the Ag2 Te films deposited at different potentials. Plating solution: 0.1 M NaNO3 , 5.0 mM AgNO3 and 4.0 mM TeCl4 dissolved in DMSO. (×) The diffraction peaks of ITO.

25

30

o+

+ +

(-210)

(-211)

+

(a) -0.52V

25

o

(-113) (-303) (310) (021)

o

(a) -0.50V

o

+ +o

(b) -0.55V

20

(312) (321)

(112)

(211)

Intensity (arb. units)

(-113)

¡×

Ag

(-303) (310) (021)

×

(-210)

(c) -0.60V

(-211)

(d) -0.65V

(111)

Intensity (arb. units)

(212)

(c) -0.68V

(322) (412)

(e) -0.70V

(331)

(222)

concentration of TeCl4 increased to 6.0 and 7.0 mM (Fig. 2e and f), the XRD patterns of the resulting films are quite different from that of monoclinic Ag2 Te. All the diffraction peaks appeared at2θ = 22.72, 24.84, 28.54, 29.24, 33.98, 34.88, 35.16, 39.81, 41.38 and 41.60◦ could be indexed on the basis of hexagonal Ag7 Te4 (JCPDS standard 18–1187) as indicated in Fig. 1f. No diffraction peak of Te or Ag2 Te was detected. These Ag7 Te4 films are dark blue-gray in color, mirror-like and adhered strongly to the substrates. Further, when the concentration of TeCl4 was in the middle, 5.0 mM, the XRD pattern of the deposited film indicated that both monoclinic Ag2 Te and hexagonal Ag7 Te4 existed in the film. In appearance, it is interesting that this film was gray in part of the area, and dark blue-gray in other area, and it is rough and easily peeled off. This result implies that Ag2 Te and Ag7 Te4 cannot be grown uniformly with each other. We further study the effect of the deposition potential on both the composition and the structure of the films. Fig. 2 shows the XRD patterns of the films deposited at different potentials from −0.50 to −0.70 V (interval: 0.05 V) and at the fixed concentration of TeCl4 of 4.0 mM. The current densities during the deposition stabilized in a range of −2.7 to −3.0 mA cm−2 and the deposited films are gray in color and adhered well to the substrates. The XRD patterns in Fig. 2 confirmed that all these films were monoclinic Ag2 Te and no hexagonal Ag7 Te4 phase existed. The Ag (1 1 1) diffraction peak was also detected in these films, and the relative intensity of Ag (1 1 1) decreased with lowering the potential from −0.50 to −0.70 V, indicating a decreasing of excess Ag in the film.

2245

35

40

45

2θ (Degree) Fig. 3. XRD patterns of the Ag2 Te films deposited at different potentials. Plating solution: 0.1 M NaNO3 , 5.0 mM AgNO3 and 5.0 mM TeCl4 dissolved in DMSO. The diffraction peaks of: (×)ITO; (䊊) Ag2 Te; and (+) Ag7 Te4 .

R. Chen et al. / Electrochimica Acta 49 (2004) 2243–2248

Intensity (arb. units)

2246

Ag 3d5/2 Ag 3d3/2

364

(a)

Intensity (arb. units)

Auger α (eV) = AlKα (1486.6 eV) − AgMNN + Ag3d5/2 (1)

AgMNN

TeMNN

380

Te 3d3/2

572

576

580

584

588

Binding Energy (eV)

Fig. 6. Close-up surveys for Ag3d and Te3d cores of the electrodeposited silver telluride films.

the standard values in elemental Te [17] and Ag [18]. Thus, it can be concluded that the electrodeposits are compound, but not elemental Ag and Te. The deviation of Auger α (Ag) in Ag2 Te and Ag7 Te4 are almost the same, which show that the valences of Ag in these compounds are similar. Meanwhile, the Eb (Te3d) deviation in Ag7 Te4 is less than that in Ag2 Te, which is consistent with the apparent valence of Te in Ag7 Te4 . Cyclic voltammetry was used to study the cathodic deposition process of silver telluride. Fig. 7a presents the cyclic voltammogram of a solution containing 0.1 M NaNO3 and 5.0 mM AgNO3 . Silver deposition is evident by the appearance of a reduction peak at +0.14 V. Fig. 7b shows a scan in a solution containing 0.1 M NaNO3 and 5.0 mM TeCl4 . The reduction of Te4+ to Te by Eq. (2) begins at about −0.13 V and reaches a peak current at −0.54 V. The XRD identification of the film electrodeposited from this solution at around −0.3 V confirms the formation of hexagonal Te. Cyclic voltammogram of a solution containing both AgNO3 and TeCl4 is shown in Fig. 7c. In this figure, the first reduction wave between −0.1 and −0.3 V is mainly caused by the reduction of Ag+ to Ag, and companying the

O1s

Te3p

Ag3p

Intensity (arb. units)

Ag3d

Te3d

The characteristic energies of Te and Ag, that is, Te3d, and Auger α of Ag in both Ag2 Te and Ag7 Te4 deviate from

376

Te 3d5/2

568

The XPS spectrum of the Ag7 Te4 film was similar to that of Ag2 Te except the binding energies. The Eb values of Te3d, Ag3d, AgMNN, and Auger parameter α for the two compounds are listed in Table 1. The Auger parameter α was determined according to Eq. (1), which was the key factor to determine the valence of Ag when the Eb (Ag3d) cannot be differentiated.

372

Binding Energy (eV)

(b) Fig. 4. Top view SEM images of: (a) Ag2 Te film and (b) Ag7 Te4 film electrodeposited at the potential of −0.60 V vs. Ag/AgCl from solutions containing 4.0 mM TeCl4 , and 7.0 mM TeCl4 , respectively. The scale bars are 1 ␮m.

368

C1s

Table 1 The binding energies (Eb ) of Ag and Te in the Ag2 Te and Ag7 Te4 film detected by XPS

200

400

600

800

1000

Binding Energy (eV) Fig. 5. Typical XPS survey spectrum of the electrodeposited silver telluride films.

Eb (eV)

Te3d5/2

Te3d3/2

Ag3d5/2

AgMNN

Auger α

Ag2 Te Ag7 Te4 Elemental Ag17 and Te18

571.6 572.2 573.1

581.8 582.4 583.3

368.0 368.3 368.3

1131.7 1131.8 1134.7

722.9 723.1 720.2

R. Chen et al. / Electrochimica Acta 49 (2004) 2243–2248

-2

i (mA cm )

1.0 0.5 0.0 cycle 1 cycle 2

-0.5

the strong reactivity of silver and tellurium ((GAg2 Te)298 = −81.66 kJ mol−1 [19] ). At an appropriate potential range, stoichiometric Ag2 Te was formed. Further beyond this potential range, the deposition rate of Te increased, and the excess Te resulted in another stable compound with hexagonal structure, Ag7 Te4 (Ag/Te = 1.75). This process is expressed by Eq. (5). Te4+ + 4e → Te

(2)

2Ag+ + Te + 2e → Ag2 Te

(3)

1.0

2Ag + Te → Ag2 Te

(4)

0.5

7Ag+ + 4Te + 7e → Ag7 Te4

(5)

-1.0

(a)

-0.4

0.0

0.4

0.8

-2

i (mA cm )

2247

0.0

4. Conclusions

-0.5 -1.0

cycle 1 cycle 2

-1.5

(b)

-2.0

-1.0

-0.5

0.0

0.5

1.0

4

2

-2

i (mA cm )

3

1 0 -1

-3 -4 (c)

cycle 1 cycle 2

1

-2 2 -1.0

-0.5

0.0

0.5

1.0

Potential /V (vs.Ag/AgCl)

Fig. 7. Cyclic voltammogram for ITO/glass electrode in DMSO solution containing: (a) 0.1 M NaNO3 and 5.0 mM AgNO3 ; (b) 0.1 M NaNO3 , and 5.0 mM TeCl4 , and (c) 0.1 M NaNO3 , 5.0 mM AgNO3 and 5.0 mM TeCl4 , at the temperature of 80 ± 1 ◦ C. Scan rate = 50 mV s−1 .

reduction of Te4+ to Te. Comparing to Fig. 6a, the potential of Ag+ /Ag moves to a more negative value after TeCl4 was added in the solution, which is caused by the formation of the complex ion [AgCl4 ]3− . XRD results demonstrated that the main component of the film deposited at around −0.2 V was silver, accompanying the diffraction peaks of Ag2 Te, but no signal of Te was detected. This result implies that Ag2 Te begins to form by Eq. (3) almost at the potential of Te4+ /Te, and this cathodic reduction from Te to Ag2 Te is complete, which was induced by the formation of insoluble Ag2 Te (log Ksp = –71.7[2]). By making the potential more negative, the formation rate of Te, and further reduction to Ag2 Te increased. At the same time, the deposition rate of Ag decreased due to the diffusion control of Ag+ . Hence, the excess amount of Ag in the deposited film decreases. During this stage, another chemical reaction expressed by Eq. (4) can also occur driven by

Cathodic electrodeposition from DMSO solutions containing NaNO3 , AgNO3 and TeCl4 has resulted in crystalline thin films of Ag2 Te and Ag7 Te4 , both stoichiometric and with excess Ag. XRD analysis of the deposited films have revealed that increasing the concentration of TeCl4 in the electrolyte or lowering the deposition potential favored to the formation of hexagonal Ag7 Te4 phase. SEM images showed that the Ag7 Te4 film consisted of triangles characteristic of a (1 1 1) single-crystal with a hexagonal structure, in contrast to the ∼1 ␮m spherical grains of monoclinic Ag2 Te in the Ag2 Te film. It is worth noting that pure, hexagonal Ag7 Te4 is not easy to synthesize by common chemical methods. We expect that these crystalline thin films of Ag2 Te and Ag7 Te4 could be used for studies of transport properties and megnetoresistance behavior and in future applications such as thermoelectronical devices, miniature field sensors and high-density data storage.

Acknowledgements This work was supported by the Major State Basic Research Development Program. (Grant No. 2000077503).

References [1] K. Rajeshwar, Adv. Mater 4 (1992) 23. [2] E.A. Buketov, M.Z. Ugorets, A.S. Pashinkin, Russ. J. Inorg. Chem. 9 (1964) 292. [3] J. Schneider, H. Schulz, Z. Kristallogr. 203 (1993) 1. [4] W. Gorbachev, Semiconductor Compounds AI2 BVI —Metallurgy, Moscow, 1980. [5] M. Kobayashi, K. Ishikawa, F. Tachibana, H. Okazaki, Phys. Rev. B 38 (1988) 3050. [6] R. Xu, A. Husmann, T.F. Rosenbaum, M.L. Saboungi, Nature 390 (1997) 57. [7] H.S. Schnyders, M.L. Saboungi, T.F. Rosenbaum, Appl. Phys. Lett. 76 (2000) 1710. [8] I.S. Chuprakov, K.H. Dahmen, Appl. Phys. Lett. 72 (1998) 2165.

2248

R. Chen et al. / Electrochimica Acta 49 (2004) 2243–2248

[9] V. Damodara Das, D. Karunakaran, J. Appl. Phys. 66 (1989) 1822. [10] G. Henshaw, I.P. Parkin, G. Shaw, J. Mater. Sci. Lett. 15 (1996) 1742. [11] B. Li, Y. Xie, Y. Liu, J.X. Huang, Y.T. Qian, J. Solid State Chem. 158 (2001) 260. [12] V.D. Das, D. Karunakaran, J. Phys. Chem. Solids 46 (1985) 551. [13] M. Ohto, K. Tanaka, J. Vac. Sci. Technol. B 14 (1996) 3452. [14] R.B. Gore, R.K. Pandey, Thin Solid Films 164 (1988) 255.

[15] R.Z. Chen, D.S. Xu, G.L. Guo, L.L. Gui, J. Mater. Chem. 12 (2002) 2438. [16] D.W.A. Sharp, Inorganic Chemistry Series two, vol. 5, Transition Metals, Part 1, Butterworths, London, 1974. [17] R. Nyholm, N. Martensson, J. Phys. C 13 (1980) L297. [18] R. Romand, M. Roubin, J.P. Deloume, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 229. [19] K.C. Mills, Thermodynamic Data for Inorganic Sulphides, Selenides and Tellurides, Butterworth, London, 1974.