Synthesis, characterization, and optical properties of Te, Te/TeO₂ and TeO₂ nanostructures via a one-pot hydrothermal method

Abstract

Herein, tellurium (Te), tellurium dioxide (TeO₂) and Te/TeO₂ nanostructures were successfully synthesized via a one-pot hydrothermal route using TeCl₄ as a tellurium source. The obtained products were characterized by XRD, SEM, TEM, FT-IR, EDS, and DRS. The effects of different parameters such as temperature, time, surfactant, solvent etc. on the morphology, particle size and product type of the as-synthesized nanostructures were investigated. Based on the XRD results, it was found that the production of Te increased by increasing the reaction temperature. Also, pure Te nanorods were obtained in the presence of ethylene glycol, and when using ammonia pure TeO₂ microstructures were formed. Providing favorable conditions for the formation of pure Te and pure TeO₂ results in the self-assembly of nanoparticles to form 1-D structures and 3-D nanostructures, respectively, but providing conditions for the formation of a mixture of Te/TeO₂ leads to the formation of 0-D nanostructures. These results are significant because the obtained mixture can easily transform to 0-D nanostructures of TeO₂ and Te.

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1. Introduction

Tellurium (Te) is a p-type semiconductor with a direct narrow band gap energy of 0.35 eV at room temperature.¹ Te and Te based chalcogenides as significant semiconductor materials have a wealth of useful properties including nonlinear optical responses, photoconductivity, and thermoelectric properties, which result in their potential applications in electronic and optical electronic devices.^1–5 Additionally, Te is considered an important material for high-efficiency photoconductors,⁶ CO and NO₂ gas sensors^7,8 and removal of mercury ions.⁹ Various Te nanostructures such as nanotubes,¹⁰ nanowires¹¹ and nanorods¹² have been prepared by many approaches such as electrochemical deposition,¹³ microwave,¹⁴ hydrothermal,¹⁵ and physical evaporation methods.¹⁶ Nevertheless, the preparation of Te nanostructures via wet chemistry routes is currently being used. For example, Cao et al. synthesized single-crystalline Te nanowires under hydrothermal conditions at 130 °C and 170 °C.¹⁷ Te nanotubes and nanorods have been prepared via a hydrothermal method at 120 °C for 12 h by Zhu et al.¹⁸ The synthesis of trigonal Te (t-Te) nanorods, nanowires and nanobelts at 150 °C under refluxing conditions in ethylene glycol and diethylene glycol for 2 h was reported by Gautam and Rao.¹⁹ Xia and Mayers^20,21 have successfully synthesized a series of uniform Te nanowires through the reduction of H₆TeO₆ or TeO₂ in different solvent systems by N₂H₄·H₂O and a refluxing process. Besides Te, TeO₂ is an excellent optical crystal with elastic behavior, good non-linear optical properties and electrical conductivity.^22–26 Nowadays, TeO₂ nanowires are applied as transparent chemical gas sensors.^26–28 Many chemical processes involving sol–gel,²⁹ thermal oxidation of Te microtubes,³⁰ thermal evaporation of Te microstructures³¹ and transformation of sodium tellurite (Na₂TeO₃) to TeO₂ in an acid solution³² have been introduced to fabricate TeO₂ nanostructures. However, there are some other challenges for the preparation of these materials that require more investigations in this scope. For instance, tellurium inherently tends to form 1-D structures and there are many reports on it.^18,19,33,34 So, presenting a simple route for synthesizing the other morphologies of these materials is significant. In this study, we presented a simple route for the preparation of Te/TeO₂ nanoparticles that can be easily transformed to 0-D nanostructures of Te and TeO₂. Te, Te/TeO₂ and TeO₂ nanostructures were synthesized by a facile hydrothermal route using TeCl₄ as a tellurium precursor. Additionally, the effects of temperature, surfactant, time, solvent, NaOH and ammonia on the final product morphologies and qualities were investigated. The as-synthesized products were extensively characterized by techniques such as X-ray diffraction (XRD), energy dispersive spectrometry (EDS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and diffused reflectance UV-visible spectroscopy (DRS).

2. Experimental

2.1. Materials and physical measurements

All of the chemicals used in the experiments were of analytical grade and used as received without further purification. TeCl₄, ethylenediamine, ethylene glycol, ammonia, sodium dodecyl sulfate (SDS), polyethylene glycol (PEG-6000), Na₂EDTA·2H₂O and cetyl trimethyl ammonium bromide (CTAB) were purchased from Merck Company. X-ray diffraction (XRD) patterns were recorded by a Philips-X’PertPro, X-ray diffractometer using Ni-filtered Cu Kα radiation in a scan range of 10 < 2θ < 80. The energy dispersive spectrometry (EDS) analysis was studied by a X-Max oxford, Philips microscope. Scanning electron microscopy (SEM) images were obtained on a IGMA-VP equipped with an energy dispersive X-ray spectrometer. A GC-2550 TG gas chromatograph (Teif Gostar Faraz Company, Iran) was used for all chemical analyses. Transmission electron microscopy (TEM) images were obtained on a Philips EM208S transmission electron microscope with an accelerating voltage of 100 kV. Fourier transform infrared (FT-IR) spectra were recorded on a Magna-IR, spectrometer 550 Nicolet with 0.125 cm⁻¹ resolution in KBr pellets in the range of 400–4000 cm⁻¹. The diffused reflectance UV-visible spectrum (DRS) of the sample was recorded by an Ava Spec-2048TEC spectrometer.

2.2. Preparation of Te, Te/TeO₂ and TeO₂ nanostructures

In a typical procedure, 0.1 g of TeCl₄ and 0.007 g of CTAB were dissolved in 30 ml distilled water with stirring at room temperature for 5 min. Subsequently, reagents were loaded into an autoclave (200 ml) and the autoclave was kept at 200 °C for 8 h and then was allowed to cool down to room temperature naturally. The obtained precipitate was centrifuged and washed with absolute ethanol and distilled water several times and was dried at 70 °C for 5 h (sample A-8). For investigation of the effect of temperature, time, surfactant and solvent as well as pH several experiments were performed at different conditions and the reaction terms are listed in Table 1. For the production of TeO₂ nanoparticles, samples B-SDS and B-PEG were annealed at 400 °C for 1 h. Due to the toxicity properties of Te and especially H₂Te, all of the steps of the experiments were conducted in a glove box to prevent direct contact with the body.

Table 1 Experimental conditions used for the preparation of the as-synthesized nanostructures

Sample no.	Time (h)	Surfactant	Temperature (°C)	Solvent	OH^− source	H^+ source	Reducing agent	Product
A-8	8	CTAB	200	Water	—	—	—	Te/TeO2
A-12	12	CTAB	200	Water	—	—	—	Te/TeO2
A-16	16	CTAB	200	Water	—	—	—	Te/TeO2
B-SDS	12	SDS	200	Water	—	—	—	Te/TeO2
B-PEG	12	PEG	200	Water	—	—	—	Te/TeO2
B-EDTA	12	Na2EDTA	200	Water	—	—	—	Te/TeO2
C-160	12	CTAB	160	Water	—	—	—	TeO2
C-180	12	CTAB	180	Water	—	—	—	TeO2
D-EG	12	CTAB	200	EG	—	—	—	Te
E-NaOH	12	CTAB	200	Water	NaOH	—	—	Te/TeO2
E-NH3	12	CTAB	200	Water	NH3	—	—	TeO2
E-HCl	12	CTAB	200	Water	—	HCl	—	Te/TeO2
F-N2H4	12	CTAB	200	Water	—	—	N2H4	Te

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3. Results and discussion

Fig. 1a–c provide a comparison of typical XRD patterns of Te/TeO₂ nanostructures derived from the different reaction conditions for samples A-8 (prepared at 200 °C for 8 h in the presence of CTAB), B-PEG (prepared at 200 °C for 12 h in the presence of PEG) and B-EDTA (prepared at 200 °C for 12 h in the presence of Na₂EDTA), respectively. As can be seen in all three cases a mixture of the tetragonal phase of TeO₂ (space group: P4₁2₁2, JCPDS no. 76-0679 with cell constants a = b = 4.8052 Å, c = 7.6021 Å) and hexagonal phase of Te (space group: P3₁21, with cell constants a = b = 4.4572 Å, c = 5.9290 Å and JCPDS no. 86-2268) was obtained and TeO₂ is the major product.

Fig. 1 XRD patterns of the as-synthesized products in water at 200 °C and (a) in the presence of CTAB for 8 h (sample A-8), (b) in the presence of PEG for 12 h (sample B-PEG), (c) in the presence of Na₂EDTA for 12 h (sample B-EDTA) and (d) the sample B-PEG-400.

These results show that by employing the mentioned synthetic route without additional reducing agent, the Te nanostructures can be prepared. In other words, the proposed reactants (TeCl₄ and CTAB) and the hydrothermal method can reduce some of the Te⁴⁺ to Te with no reductant such as NaBH₄ and hydrazine. When the experiment was performed in the presence of PEG (sample B-PEG) and Na₂EDTA (sample B-EDTA) as surfactants, the amount of Te in the mixture was increased (Fig. 1b and c) which can be related to the regenerative properties of these surfactants. Fig. 1d shows the XRD pattern of sample B-PEG after annealing at 400 °C for 1 h (sample B-PEG-400) and as can be seen only the pure tetragonal phase of TeO₂ with JCPDS no. 76-0679 was synthesized. The strong and sharp peaks reveal the good crystallinity of the obtained products. EDS analysis was employed to investigate the chemical composition and purity of the as-synthesized product. Fig. 2 shows a typical EDX spectrum of the as-synthesized product at the reaction conditions for sample B-PEG. As can be seen, only Te and O peaks exist in the as-synthesized product which indicates that the obtained product is without any impurity. Therefore, both XRD and EDX analyses depicted that Te/TeO₂ nanostructures were successfully obtained via the current synthetic route.

Fig. 2 EDX spectrum of the as-synthesized Te/TeO₂ in the presence of water and PEG at 200 °C for 12 h (sample B-PEG).

The morphology of the products was characterized by SEM. According to the SEM images, the obtained products in our experiment are sphere-like nano and micro particles, nanofibers and nanorods. Fig. 3a shows different scales of the SEM images of the product obtained in the presence of CTAB as the surfactant and water as the solvent at 200 °C for 8 h (sample A-8). As shown, in this condition the particles conglomerate and this process needs more time to form the noted structures completely. Therefore, the reaction time was increased to 12 h (sample A-12) and 16 h (sample A-16). Fig. 3b shows different magnifications of the sample prepared at 12 h (sample A-12) and it is clear that formation of spherical nanoparticles with the size range of 10–20 nm has been completed but the rod-like nanostructures are still growing. For sample A-16 obtained at 16 h (Fig. 3c), the growth of both the rod-like and spherical nanostructures was well completed and the mixture of spherical nanoparticles and three-dimensional rod-like nanostructures can be seen and the nanoparticles are well self-assembled to form 3D-rod-like nanostructures. Also, it can be observed that with the increase of reaction time from 12 to 16 h, the size and agglomeration of the nanoparticles increased. Therefore, 12 h was chosen as the optimum reaction time for preparing products with fine particle size and better uniformity. For investigation of surfactant effects on the morphology and size of nanostructures the three surfactants SDS, PEG and Na₂EDTA (samples B-SDS, B-PEG and B-EDTA, respectively) were utilized instead of CTAB. Using SDS instead of CTAB resulted in the formation of spherical nanoparticles (Fig. 4a) but the particle size was smaller compared to using CTAB as the surfactant (Fig. 3b). Also, in the presence of PEG (sample B-PEG), spherical nanoparticles were formed (Fig. 4b) and the size of the spherical nanostructures was bigger compared to using CTAB and SDS. SDS and CTAB are ionic surfactants and so it is obvious that they have very different effects on the morphology of the final products compared to PEG as a neutral surfactant. When the Na₂EDTA was employed as the surfactant (sample B-EDTA), the morphology was changed and fiber-like nanostructures were synthesized (Fig. 4c). Fig. 4d and e show SEM images of samples B-SDS-400 and B-PEG-400 obtained by calcination of samples B-SDS and B-PEG at 400 °C for 1 h, respectively. As can be seen, in both conditions, TeO₂ nanoparticles were obtained and the most homogenous and the finest particles were obtained by calcination of sample B-SDS.

Fig. 3 SEM images of the as-synthesized products in the presence of CTAB at 200 °C for (a) 8 h (sample A-8), (b) 12 h (sample A-12) and (c) 16 h (sample A-16).

Fig. 4 SEM images of the as-synthesized products in water at 200 °C for 12 h and different surfactants: (a) SDS (sample B-SDS), (b) PEG (sample B-PEG), (c) Na₂EDTA (sample B-EDTA), (d) B-SDS-400 and (e) B-PEG-400.

One of the other crucial parameters that can affect the morphology and particle size of final products in the hydrothermal route is the temperature effect. So for investigation of the temperature effect, two experiments were designed at two different temperatures of 160 °C (sample C-160) and 180 °C (sample C-180) in the presence of water as the solvent and CTAB as the surfactant for 12 h (Fig. 5a and b, respectively). As can be seen in Fig. 5a, by reducing the temperature from 200 °C to 160 °C (sample C-160), the morphology was changed and microstructures instead of spherical nanoparticles (sample A-12, Fig. 3b) were obtained. When the reaction temperature was set at 180 °C, three-dimensional sphere-like nanostructures were prepared from a self-assembling of nanoparticles (Fig. 5b). The particle size was also increased compared to the nanoparticles obtained at 200 °C (sample A-12, Fig. 3b).

Fig. 5 SEM images of the products in the presence of CTAB for 12 h and (a) at 160 °C in water (sample C-160), (b) at 180 °C in water (sample C-180) and (c) at 200 °C in ethylene glycol (sample D-EG).

An increase of the reaction temperature always results in an increase of the rate of reaction, therefore at relatively high temperatures more nuclei will form before the growth process, which causes the formation of more particles with smaller diameters.³⁵ The obtained results from the SEM images depict that the temperature has an important effect on the morphology and size of nanostructures in the hydrothermal route and for reaching the desired nanostructure morphology and size, the temperature should be properly adjusted. To investigate the solvent effect, ethylene glycol (sample D-EG) was used instead of water (sample A-8) and in this condition uniform nanoparticles were self-assembled to form 3D-rod-like nanostructures (Fig. 5c). Various solvents have different polarities, solubilities and coordination capabilities and so are effective in the process of cluster formation.³⁶ In this experiment (sample D-EG), using ethylene glycol instead of water led to an increase in the reducing ability and this could be a good reason for the formation of rod-like shaped Te nanostructures instead of Te/TeO₂ nanoparticles. Fig. 6a and b show XRD patterns of samples C-180 (obtained at 180 °C in water) and D-EG (obtained at 200 °C in EG as a solvent), respectively.

Fig. 6 XRD pattern of the as-synthesized product in the presence of CTAB for 12 h and (a) at 180 °C in water (sample C-180) and (b) at 200 °C in ethylene glycol (sample D-EG).

All the reflection peaks of Fig. 6a can be indexed to the pure tetragonal phase of TeO₂ and the peaks of Fig. 6b are indexed to the pure hexagonal phase of Te. As the XRD results show, TeO₂ has been formed at low temperatures (sample C-180, Fig. 6a) and with increasing temperature the Te phase can also be produced (sample A-12, Fig. 1b) without additional reducing agent.

The effects of NaOH, ammonia and HCl on morphology were also investigated. In the presence of NaOH (sample E-NaOH) the morphology did not change and nanoparticles with an average size of about 10–20 nm were obtained (Fig. 7a) but using ammonia (sample E-NH₃) led to a change in the morphology and microstructures were formed (Fig. 7b). When the experiment was performed using 1 ml of HCl (1 M) (sample E-HCl) agglomeration occurred and the obtained particles were not homogenous and fused to each other (Fig. 7c).

Fig. 7 SEM images of the products in the presence of CTAB for 12 h, at 200 °C in water and (a) NaOH (sample E-NaOH), (b) NH₃ (sample E-NH₃), (c) HCl (sample E-HCl) and (d) N₂H₄ (sample F-N₂H₄).

In order to investigate the reducing agent effect, hydrazine was used (sample F-N₂H₄). As shown in Fig. 7d, three-dimensional rod-like nanostructures mixed with nanoparticles were obtained. Fig. 8 shows the TEM images of sample E-NaOH and as can be seen, spherical nanoparticles with sizes of about 10–20 nm were obtained and these findings can confirm the SEM results. XRD patterns of samples E-NaOH, E-NH₃, E-HCl and F-N₂H₄ are presented in Fig. 9a–d. As can be seen, in the presence of 1 ml of NaOH (1 M) (sample E-NaOH), a mixture of TeO₂ and Te was obtained and TeO₂ is the major product. The same result was obtained in the absence of NaOH (sample A-12). When the experiment was performed using 1 ml of NH₃ (1 M) (sample E-NH₃) pure tetragonal phase TeO₂ was produced. The XRD pattern of the sample obtained using HCl (sample E-HCl) revealed the presence of some Te in the final products. Fig. 9d shows the XRD pattern of the product obtained in the presence of hydrazine (sample F-N₂H₄) and as shown, in this condition, pure Te was prepared.

Fig. 8 TEM images of the products in the presence of CTAB for 12 h, at 200 °C in water and NaOH (sample E-NaOH).

Fig. 9 XRD pattern of the as-synthesized product in the presence of CTAB for 12 h, at 200 °C in water and (a) NaOH (sample E-NaOH), (b) NH₃ (sample E-NH₃), (c) HCl (sample E-HCl) and (d) N₂H₄ (sample F-N₂H₄).

Our SEM and XRD results (summarized in Scheme 1) show that providing favorable conditions for the formation of pure tellurium results in the self-assembly of nanoparticles to form 1-D structures, which is in good accordance with other similar studies for tellurium preparation.^37,38 Generally, trigonal tellurium structures have a tendency to form 1-D structures.^39,40 Trigonal structures of tellurium are an anisotropic crystalline structure consisting of atomic spiral chains with covalent bonds. Crystals grow only along the c axis and lead to the formation of 1-D structures (Fig. 10). Moreover, our results show that by providing desirable conditions for the preparation of pure tellurium dioxide, nanoparticles can self-assemble to form 3-D nanostructures and microstructures but providing conditions for the formation of a mixture of tellurium and tellurium dioxide leads to the formation of zero dimensional nanostructures. These results are significant because the obtained mixture can easily transform into tellurium dioxide (by annealing) and into pure tellurium (by reduction under gaseous conditions) and in such a manner, 0-D nanostructures of tellurium and tellurium dioxide can be easily synthesized.

Scheme 1 Schematic diagram of the formation process of the as-synthesized samples.

Fig. 10 Crystal structure of trigonal tellurium.

The mechanism of TeO₂/Te nanostructure formation in hydrothermal conditions and the presence CTAB can be described as follows:

TeCl₄ + 2H₂O → TeO₂ + 4HCl (1)

When TeCl₄ reacts with water (with or without CTAB), it will immediately form TeO₂, and as a result the product appears as bulk material. So, to obtain nanostructures, the hydrolysis rate should be controlled. In our work, by changing conditions such as pH and so on via the hydrothermal route, Te/TeO₂ was prepared. In other words, the hydrolysis process, nucleation mechanism and growth changed and zero-dimensional Te/TeO₂ nanostructures were prepared which by calcining can be transformed to TeO₂. Some research has been done into controlling the hydrolysis rate to prepare nanostructures and some solutions were presented.^39,40 For example, Sun et al.³⁹ by performing the reaction in mild acidic conditions, controlled the hydrolysis process and prepared TiO₂ nanostructures.

In the presence of EG and hydrazine as reducing agents Te⁴⁺ was reduced to Te completely and one-dimensional Te nanostructures were prepared. Ethylene glycol can be easily oxidized to acid and thus is able to reduce Te⁴⁺ to Te (sample D-EG, Fig. 6b). The mechanism of tellurium nanostructure formation in the presence of hydrazine can be described as follows:

N₂H₄ → N₂ + 4H⁺ + 4e (4)

TeCl₄ + 3H₂O → H₂TeO₃ + 4HCl (5)

4e + 4H⁺ + H₂TeO₃ → Te + 3H₂O (6)

The final reaction:

TeCl₄ + N₂H₄ → Te + N₂ + 4HCl (7)

Fig. 11a–c show the FT-IR transition spectra of samples A-12 (Te/TeO₂), E-NH₃ (TeO₂) and F-N₂H₄ (Te). In Fig. 11a and b, two obvious absorption bands around 775 and 665 cm⁻¹ are related to the vibration of Te–O bonds which are similar to the absorption bands of α-TeO₂.³² This indicated that the as-synthesized TeO₂ nanostructures might be α-TeO₂.³² The absence of remarkable peaks in Fig. 11c shows that no organic molecules related to precursors and surfactant remained on the surface of the Te nanostructures (sample A12) and ligands and surfactant were removed by washing and products were obtained without any organic impurity.

Fig. 11 FT-IR spectra of samples A-12 (Te/TeO₂), E-NH₃ (TeO₂) and F-N₂H₄ (Te).

The optical properties of the Te/TeO₂ nanostructures obtained from samples A-12 (Te/TeO₂), E-NH₃ (TeO₂) and F-N₂H₄ (Te) were characterized by UV-Vis absorption spectroscopy (DRS) (Fig. 12). It can be seen that it shows two typical absorption peaks located at about 295 and 360 nm for Te/TeO₂ (sample A-12), one absorption peak at 300 nm for TeO₂ (sample E-NH₃) and two peaks at 325 and about 650 nm for Te (sample F-N₂H₄).

Fig. 12 Diffuse reflectance spectrum of the as-synthesized nanostructures (samples A-12, E-NH₃ and F-N₂H₄).

Optical absorption spectra of Te have been studied by a few coworkers. Gautam et al.¹⁹ have reported an absorption band in the range of 250–350 nm due to the transition from the valence band (p-nonbonding triplet) to the conduction band (p-antibonding triplet), and a band in the 600–850 nm range due to the valence band (p-bonding triplet) to conduction band (p-antibonding triplet) transition. So (for Te/TeO₂, sample A-12) the first peak at 295 and the second peak at 360 nm can be related to TeO₂ and Te, respectively. Also, a broad peak at about 650 nm is attributed to Te nanostructures. These results are in good agreement with the literature.^2,12,40,41 According to our results, Te/TeO₂ shows better optical properties than each of Te or TeO₂ alone and this can be useful in optical applications. In comparison to other similar works, our method is simpler, faster, and more controllable. We have used TeCl₄ as a tellurium source to prepare the Te, Te/TeO₂ and TeO₂ nanostructures. In our experiment, when TeCl₄ was added in deionized water, a white precipitate due to the hydrolysis reaction was obtained. Controlling the rate of hydrolysis of TeCl₄ was critical in the formation of initial structures in the form of a precipitate. Hydrothermal reaction in the presence of CTAB led to the formation of the nanostructures.

4. Conclusions

In summary, Te, Te/TeO₂ and TeO₂ nanostructures with various shapes have been prepared via a facile hydrothermal method in which TeCl₄ was used as a tellurium source. The crystal structure, morphology, composition and optical properties of the as-synthesized products were extensively characterized by XRD, EDS, SEM, FTIR and DRS. The effect of temperature, reaction time, solvent, surfactant and NaOH on the morphology and purity of the final products was also studied. Results showed that all of the mentioned factors lead to various products from Te to TeO₂ with various shapes and sizes. Furthermore, our results showed that with using no additional reductant such as NaBH₄ in these hydrothermal conditions, some of the Te⁴⁺ can be reduced to Te to form a mixture of 0-D Te/TeO₂ nanostructures which can easily transform into TeO₂ and Te nanoparticles.

Acknowledgements

Authors are grateful to the council of the Iran National Science Foundation and University of Kashan for supporting this work by Grant No. (159271/265).

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