Fabrication of NiTe films by transformed electrodeposited Te thin films on Ni foils and their electrical properties

Abstract

The compact nickel telluride (NiTe) thin film was prepared by two simple steps. High-density tellurium (Te) film with vertical pillar morphology (Te source) was synthesized by a rapid and convenient electrodeposition method without any external agent for the first time. Ni_xTe as the ohmic contact layer material was obtained by a low temperature heat treatment of a Te thin film deposited on a Ni substrate. The structure, morphology, optical and electrical properties of the as-prepared thin films were examined. The current voltage (I–V) characteristic shows that all of the Ni_xTe/Ni samples have already formed ohmic contacts. The NiTe/Ni also shows smaller contact resistance than other samples, which suggests it has potential for application in solar cells as a flexible Ni substrate.

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1. Introduction

Tellurium (Te), an important p-type narrow band gap semiconductor, has been widely studied because of its unique and useful properties, such as photoconductivity, piezoelectricity, thermoelectricity, nonlinear optical responses and photoelectricity.^1–5 At present, Te nanocrystals of different morphology such as: nanorods, nanowires, nanobelts, nanotubes, nanoplates, flower-like and feather-like have been successfully synthesized by several methods, such as electrochemical deposition, hydrothermal methods, solvothermal methods, physical vapor deposition and microwave-assisted synthesis.^6–17 It is well known that, for a device in practical applications, ordering of nanostructures assembled into arrays on a conducting substrate is required. However, the fabrication of aligned Te pillars on conducting substrates remains challenging. Compared with other methods, electrochemical deposition has the advantages such as high material incorporation, simple technique and low energy consumption.¹⁸ Therefore, the electrochemical deposition method has been widely used by many researchers to fabricate Te nanocrystals.^13,19 In addition, Te nanocrystals have become a subject of intensive research, as a promising template to fabricate tellurides with various novel nanostructures. Zhu et al. synthesized Ag₂Te nanoribbons of the same shape and with a similar size through the reaction of Ag with tri-wing Te nanoribbons as the precursor and template.²⁰ Furthermore, Te can react readily with other elements to generate a wealth of functional materials such as NiTe,²¹ Bi₂Te,^3,5 CdTe,²² PbTe,²³ ZnTe,²⁴ and HgCdTe.²⁵ These Te nanostructure materials have wide applications in diverse fields such as thermoelectric material, light emitting diodes, solar cells, and microwave devices infrared detector.

Nickel telluride (NiTe), an important intermetallic compound with a nickel-arsenide-type (NiAs type) crystal structure, has attracted great interest because of its electric, magnetic, crystallographic (helical chain conformation) and thermodynamic properties.^26–30 In 1938, Tengnér asserted the existence of a continuous solid-solution range between the compounds NiTe and NiTe₂. He suggested that NiTe (NiAs type) was transformed to NiTe₂ (Cd(OH)₂ type).³¹ It is important to point out that NiTe can be used as an ohmic contacting layer of II–VI semiconductors used for their distinctive electrical transport property.³² Traditionally, the method used to prepare NiTe is the direct combination of the Ni and Te elements in evacuated silica tubes at high temperature (600 °C) for several hours.⁷ In addition, Zhang et al. have prepared NiTe nanocrystallites by a hydrothermal method with hydrazine hydrate as a reducing agent.³³ Nevertheless, the previously mentioned methods for preparing NiTe have some disadvantages, such as toxicity, high energy consumption and relatively long reaction times. Thus, environmentally friendly, energy saving and convenient methods should be sought for commercial production.

In this work, a simple method to prepare NiTe was explored. Firstly, a simple one-step, environmentally friendly and template free electrochemical deposition method to deposit a Te thin film at room temperature without any external agent was used. Subsequently, the NiTe thin film was obtained by calcinating the as-prepared Te thin film (Te source) on a Ni substrate (Ni source) at 350 °C. It is worth mentioning that there are three advantages using this method to prepare chalcogenide compounds: (1) selection of a single Te source electrolyte avoids the complications involved with the codeposition of the elements and impurities, (2) using a graphite plate instead of a conventional platinum (Pt) as the counter electrode for Pt, the graphite may dissolve into the electrolyte and thus, influence the reaction of the working electrode,³⁴ (3) compared to other research,^35–38 crystalline products can be obtained by a thermal transformation step. Furthermore, the electrical properties of NiTe/Ni indicate that it would have a potential application as a back contacting layer in solar cells.

2. Experimental section

2.1 Preparation of the Te pillars film

Before performing the experiment, the Ni foil (1.5 × 4 cm²) was pretreated in isopropanol, acetone, anhydrous ethanol and double distilled water, respectively, using an ultrasonic cleaner for 30 min. Degreased Ni substrates were dried under N₂. The electrochemical experiments were carried out in a single compartment three electrode cell in an air atmosphere. A cleaned Ni foil, a saturated silver/silver chloride (Ag/AgCl) electrode and a graphite plate were used as the working electrode, reference electrode and counter electrode, respectively. The Te thin film was deposited in an acidic electrolytic bath containing 0.01 M sodium tellurite (Na₂TeO₃) and 1 M nitric acid (HNO₃). Na₂TeO₃ and HNO₃ were analytical grade and were used as purchased without further purification. Double distilled water was used as the solvent in the experiment. In the electrochemical experiment, the applied potential was optimized at −0.30 V (versus Ag/AgCl) and the deposition time was 300 s. The reaction temperature was kept constant at room temperature. Subsequently, the as-prepared Te thin film was washed with double distilled water and dried under N₂.

2.2 Preparation of NiTe film

Firstly, the Te thin film obtained was put in a porcelain boat, and then the porcelain boat was placed at the center of a muffle furnace, which was full of N₂. The muffle furnace was heated up to different temperatures (250 °C, 300 °C, 350 °C, 400 °C) at a rate of 25 °C min⁻¹. After a reaction time of 30 min, the furnace was switched off and allowed to cool down to room temperature naturally. NiTe film was obtained when the annealing temperature was 350 °C.

2.3 Characterization

Electrochemical studies were made using a potentiostat (CHI electrochemical workstation) with three electrodes. The crystal structures were identified using a Rigaku D/max-2500 X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 Å). XRD measurement was conducted in the scanning range between 20° and 80°. A Jeol JSM-6700 field emission scanning electron microscope (FESEM) was utilized to investigate the morphology and dimensions of the as-prepared samples. Energy dispersive X-ray analysis (EDX) of the NiTe film was performed with an EDAX Genesis 2000 system (FEI Inc) installed on the Philips XL 30 ESEM imaging instrument. The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) pattern were taken using a Jeol JEM-2200FS with an accelerating voltage of 200 kV. A Shimadzu UV-3150 double-beam spectrophotometer was used to characterize the optical absorption spectra at room temperature. A Keithley 2400 SourceMeter was used to characterize the electronic properties of all samples.

3. Results and discussion

Cyclic voltammetry was used to study the electrochemical reactions in the electrolyte solution. Fig. 1 shows the cyclic voltammogram of Te⁴⁺ at room temperature. The potential was changed between +0.30 V and −0.60 V (versus Ag/AgCl) and the scan rate was 100 mV s⁻¹. One cathodic peak was observed. The cathodic current began to increase gradually at about an applied potential of −0.18 V versus Ag/AgCl. When the potential was more negative than −0.45 V and more positive than −0.15 V, no products were obtained on the Ni substrates. A visible dark gray film appeared on the surface of the Ni foil at a potential of about −0.30 V versus Ag/AgCl. It could be attributed to the reduction of TeO₃²⁻. The mechanism of Te electrodeposition in an acidic solution is described as follows:

TeO₃²⁻ + 6H⁺ + 4e⁻ → Te + 3H₂O

Fig. 1 Cyclic voltammogram of a Ni substrate in the electrolyte solution containing Na₂TeO₃ (0.01 M) + HNO₃ (1 M) with a scan rate of 100 mV s⁻¹; inset: change in the current density during potentiostatic electrodeposition at −0.30 V versus Ag/AgCl within 300 s.

The change of current density with time is shown in Fig. 1 (inset) during the direct current potentiostatic electrodeposition at −0.30 V (versus Ag/AgCl) for 300 s. It shows that the cathodic current density increases quickly initially and then decreases gradually. The change of current density within the initial 100 s corresponds to the Te nucleation on the Ni foil surface. During electrodeposition, the first process is fast ionization. Subsequently, the charge is redistributed under the action of the voltage. The current was reduced with the extension of the deposition time, which is attributed to the increases of the resistivity of the deposited film. After uniform nucleation is attained on the entire surface, the current density became constant and the film growth starts.

Fig. 2 shows the XRD pattern of the Te thin film on the Ni substrate. Besides the diffraction peaks from the Ni substrate denoted by the black solid circles, the peaks of the sample can be clearly observed at 27.57°, 38.24° and 45.96° as shown in Fig. 2, which can be indexed to the (101), (102), and (003) planes of the hexagonal phase Te (JCPDS card no. 36-1452). No other phase can be detected by XRD, which reveals that pure Te is obtained. In addition, the preferred orientation (101) and peaks with small full-width at half-maximum are indicative of good crystallization of the samples.

Fig. 2 XRD pattern of the as-synthesized Te thin film on a Ni substrate.

Fig. 3(a) and (b) show FESEM images of the sample, which reveals that the Ni substrate is covered with large-scale pillars, whose lengths range from 400 nm to 500 nm. They are oriented vertically with respect to the Ni substrate surface. The surface morphology of the as-deposited sample clearly shows that the pillars have a compact and uniform structure, and the surface is rough. The TEM image of a single pillar in Fig. 3(c) shows that the Te pillar has a diameter of 250 nm and a length of 500 nm. The SAED image taken from a single pillar further confirms the well-crystallized, hexagonal phase structure of the pillar (Fig. 3(d)). Regular diffraction dots show the single crystal nature of the Te pillar. Surveys of the sample using SAED indicate that these Te pillars are consistently the preferred orientation (101).

Fig. 3 (a) Cross-sectional FESEM image, (b) tilted top-view FESEM image, (c) TEM image of a single Te pillar and (d) SAED pattern of the sample.

The UV-visible absorption spectrum for the as-deposited Te thin film is shown in Fig. 4. The UV-vis absorption bands are quite similar to those reported by Swan et al.⁴⁰ and Qian et al.⁴¹ As shown in Fig. 4, there are two absorption peaks which are located at 248 nm and 563 nm. The absorption peak centered at 248 nm is because of the allowed direct transition from the valence band (p-bonding triplet) to the conduction band (p-antibonding triplet), and the other absorption peak at 563 nm can be assigned to a forbidden direction transition.^39–41 However, further research is needed because the origin of this absorption peak is still not clear.⁴²

Fig. 4 UV-visible absorption spectrum of a Te thin film fabricated with the electrochemical deposition method.

To study the process of Te transformation into Ni_xTe in detail, a series of experiments were carried out from 200 °C to 400 °C. Fig. 5 displays the XRD patterns of the as-prepared samples which were annealed from 200 °C to 400 °C for 30 min in N₂. The results display the process whereby Te transforms into NiTe. As shown in Fig. 5, the phase transformation does not start when the annealing temperature is 200 °C. In addition, Fig. 5(c) shows that NiTe and Ni₃Te₂ appear with the disappearance of the Te phase when the Te film is sintered at 250 °C for 30 min. With increase of the annealing temperature, the diffraction peak intensity of NiTe changes, which indicates that the crystallinity has improved. At the same time, the diffraction peaks of Ni₃Te₂ weaken gradually until they disappear.

Fig. 5 XRD patterns of the samples prepared at different annealing temperatures for 30 min: (a) as-prepared Te, (b) 200 °C, (c) 250 °C, (d) 300 °C, (e) 350 °C, (f) 400 °C.

The result of Fig. 5(e) indicates that the diffraction peak position and relative intensities of the prepared samples are well matched with the standard powder diffraction file NiTe (JCPDS 89-2018). No diffraction peaks from other products were surveyed. Furthermore, the NiTe completely transforms into (NiTe₂ + Ni_2.86Te₂) when the annealing temperature is extended to 400 °C. In addition, the study of a series of annealing times at 350 °C is also carried out. Fig. 6 shows the XRD patterns of the samples prepared at 350 °C for different times (0–60 min). As shown in Fig. 6(b), the products are mainly Te and a mixture of some indistinct substances when the annealing time is 15 min. Fig. 6(c) shows that Te completely transforms into NiTe by extending the annealing time to 30 min. With increase of annealing time to 45 min and 60 min, the reaction products are mainly NiTe₂ and a mixture of Ni_2.86Te₂. Above all, annealing temperature and time prompt the reaction and the phase transition. Therefore, the optimum reaction conditions should be (350 °C, 30 min) to make sure that the products prepared were pure phase and well-crystallized NiTe.

Fig. 6 XRD patterns of the samples prepared at 350 °C for different times: (a) as-prepared Te, (b) 15 min, (c) 30 min, (d) 45 min, (e) 60 min.

As is known, NiTe can be obtained by hydrothermal and high temperature solid-state reactions,^43,44 which require high energy consumption and relatively long reaction times. In contrast, the reaction temperature and time are only 350 °C and 30 min in this study. There may be interdiffusion of Ni and Te and solid-phase chemical reactions may occur. The reaction to form NiTe can be shown in the following equation:

Ni + Te → NiTe

Based on the XRD result, a general trend of decreasing Ni content (Ni₃Te₂ → NiTe → NiTe₂) with respect to the increase in annealing temperature can be observed. Thus, it suggests that as-generated Ni_xTe may prevent further interdiffusion of Ni and Te, which is of benefit for photovoltaics because if too much metal diffuses out to the surface it can form defects and recombination centers.⁴⁵ As shown in Fig. 5(f) and 6(d and e), the increased annealing temperature or prolonged annealing time results in samples that include two substances, NiTe₂ and Ni_2.86Te₂. The reason for the appearance of Ni_2.86Te₂ may be because of some phase segregation but it needs to be investigated further.

The XRD, EDX and FESEM spectra of the NiTe thin film as as-deposited Te thin film are shown in Fig. 7. Fig. 7(a) and (b) show the cross-section and tilted top-surface FESEM images of the NiTe film, respectively. It is obvious that the morphology of NiTe is a dense thin film and the thickness is about 300 nm. Fig. 7(c) shows the XRD pattern of NiTe. All the diffraction peaks can be perfectly indexed to a pure hexagonal phase of NiTe (space group: P63/mmc), which is in good agreement with the standard XRD data (JCPDS 89-2018). The strong and sharp diffraction peaks indicate that the product is well crystallized. No peaks of impurities are detected, revealing that high purity NiTe films were fabricated in this experiment. In addition, NiTe has been peeled off from the Ni substrate before taking the EDX spectrum. As shown in Fig. 7(d), EDX analysis indicates that the sample is composed of Ni and Te and the atomic concentration of Ni and Te is 1.02 which is in agreement with the desired stoichiometric composition of NiTe. All of the results show that a NiTe thin film has been successfully prepared by simple calcination of a Te thin film deposited on a Ni substrate. According to the previous results, NiTe and Te are all hexagonal phase. In other words, the crystal structure has not changed during the transformation process.

Fig. 7 (a) Cross-sectional FESEM image, (b) tilted top-view FESEM image, (c) XRD and (d) EDX spectra of NiTe thin film.

To characterize the conductive properties of the Ni_xTe/Ni, the electrical contacts were made by attaching copper wires to the surface of the Te or Ni_xTe films with colloidal silver paste as the conductive glue. The schematic diagram is shown in the inset of Fig. 8. Fig. 8 shows the room temperature I–V characteristic of the sample contacted on Ni. The linear I–V curves indicate ohmic contacts. And the slope of the I–V curve reveals electroconductivity. As shown in Fig. 8, electroconductivity is significantly improved by transforming Te into Ni_xTe. When the annealing temperature is higher than 250 °C, all Ni_xTe/Ni samples show linear I–V curves, indicating that ohmic contacts have already been formed on Ni. Furthermore, it is evident that the contact resistivity of sample Ni_xTe/Ni firstly decreases and then increases with the increase of annealing temperature. It is well known that conductive properties depend on the materials. In our study, the heat treatment makes the material change (Te → Ni₃Te₂ → NiTe → NiTe₂). It is reported that NiTe₂ is suitable as a back contact layer for solar cells.⁴⁶ The slope of the I–V curve of the sample NiTe/Ni is higher than other samples. The results verify that NiTe/Ni possesses better ohmic contact characteristics.

Fig. 8 Current voltage (I–V) curves of Ni_xTe/Ni samples annealed at different temperatures. Inset: the structural schematic diagram of the electric properties measured.

4. Conclusions

In this study, the single crystalline hexagonal Te thin film on a Ni substrate has been prepared using an electrochemical deposition method at room temperature. A uniform growth with well-connected pillar morphology can be obtained at −0.30 V. XRD analysis showed that all the diffraction peaks in the pattern could be indexed to hexagonal Te, having the preferred orientation (101). Furthermore, it may provide a reference method for the preparation of Te thin film on a Ni foil by a template-free electrochemical deposition method. As-prepared Te thin film is demonstrated to be an ideal template to prepare other telluride nanocompounds through the successful synthesis of NiTe thin film. The transformation of phases with annealing temperature and time have been studied. The electrical properties of Ni_xTe/Ni show that ohmic contacts have already been formed and NiTe/Ni has better electrical properties than other samples. It is expected that these Ni_xTe films could be applied as back contact layers of solar cells on a flexible Ni substrate.

Acknowledgements

This work was financially supported by the Science and Technology Development Program of Jilin Province (no. 20110417), the National Natural Science Foundation of China (no. 51272086) and the Science and Technology Research Project of Heihe University (no. KJY201405). The authors would like to thank Dr Xiaoming Zhou and Dr Huizhen Yao for their help and discussion about the manuscript.

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