# One-Pot Hydrothermal Synthesis of Thin Tellurene Nanosheet and Its Formation Mechanism

Authors: Mei Gao; Xinwei Wang; Yuanze Hong; Yongji Shi; Dengkui Wang; Dan Fang; Xuan Fang; Zhipeng Wei
Journal: Journal of Nanomaterials
 (2019)

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## Abstract

Two-dimensional thin tellurene (Te) was effectively synthesized by a one-pot hydrothermal reduction process. It had the hexagonal phase with good crystallization, the extent of radiation absorption (400-850 nm), and a 1.37 eV band gap. Based on the evolution of the crystal structure and morphology, the formation mechanism of Te nanomaterials has been speculated by the time-dependent reactions as the nucleation of nanosized clusters first, then self-assembly into uniform nanowires, and, finally, formation into the nanosheets in the presence of polyvinylpyrrolidone (PVP). It is expected that thin Te nanosheets can further be used in the field of photoelectric applications.


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## Body

## 1. Introduction

Since the discovery of graphene, two-dimensional (2D) nanosheets have aroused the concern of researchers because they can be used as interconnected and functional units due to especial physical properties and potential applications in optoelectronic, thermoelectric, and electromechanical nanodevices, etc. [1–4]. Te nanomaterials have a peculiar chiral-chain crystal lattice structure and intriguing features such as superior carrier mobility [5–7], strong light absorption capacity [6, 8], high ductility [9], and good environmental stability [1]. Various Te nanostructures have been successfully synthesized using ionic liquid microwave-assisted synthesis [10], vapor phase growth [11, 12], solution phase synthesis [13–16], and surfactant-assisted growth [17, 18]. A number of synthesis methods favor the 1D nanostructure due to its inherent structural anisotropy. In these prior 1works, the morphology-selective synthesis of 1D Te nanomaterials suggested that the surfactant-assisted modulation of the growth rates of relevant crystalline planes could be utilized to precisely control the reaction kinetics [18–20], such as polyvinylpyrrolidone (PVP) or cetyltrimethylammonium bromide (CTAB). Inspired by the above synthesized methods, surfactant-assisted modulation can potentially be used to synthesize other nanostructures of Te nanomaterials. In contrast to various synthesis methods of 1D Te nanostructures, much less is known about the 2D Te nanostructures, their processing schemes, and their related properties. For example, Xie et al. [21] have synthesized 2D nonlayered Te nanocrystals through the liquid phase stripping (LPE) technique, which show a wide photoresponse range extending the visible region from the ultraviolet depending on the strong time and cycle stability for the on/off switching behaviors. Yashun et al. [22] have researched desorption/ionization properties of small molecules by assisted laser desorption ionization time-of-flight mass spectrometry for Te nanosheets synthesized via the hydrothermal reaction. They exhibit good UV light absorption, low matrix ion interference, and high desorption ionization efficiency. Amani et al. [23] have reported that quasi-2D Te nanosheets with the thickness of 12 nm show high hole mobilities of 450 and 1430 cm2 V-1 s-1 at 300 K and 77 K, respectively, from a solution synthesis. Yixiu et al. [24] effectively regulated and controlled the size from 50 to 100 μm and thickness from 10 to 100 nm of Te nanosheets by varying the amount ratio of sodium citrate and PVP solution during the hydrothermal process. However, until now, there have been very few literature reports on the formation process and mechanism of 2D Te nanostructures. Nevertheless, understanding the formation process of 2D Te nanomaterials is of great significance for its future properties and applications. Therefore, it is highly desirable to find out the direct experimental evidence to reveal the formation mechanism of 2D Te nanosheets.Herein, we have synthesized relatively thin (~5 nm) and uniform 2D Te nanosheets via a one-pot hydrothermal reaction with the use of PVP. The synthesized 2D Te nanosheets are confirmed to be a pure hexagonal phase and show good crystallinity. The 2D Te nanosheets have the extent of radiation absorption (400-850 nm) and a 1.37 eV band gap. Furthermore, the formation mechanism of 2D Te nanosheets is proposed based on the evolutionary evidence of morphology and structure during the time-dependent reaction process.


## 2. Materials and Methods

### 2.1. Synthesis of Tellurium Nanosheets

All the experimental procedures were carried out in a draught cupboard. Similar to a previous report [18], tellurium nanosheets were synthesized through a one-pot hydrothermal reaction. Sodium tellurite (Na2TeO3) powder, polyvinylpyrrolidone (PVP, K-30, Mw=40000), ammonium hydroxide solution (NH3·H2O), and hydrazine monohydrate (N2H4·H2O) are of analytical grade and have not been further purified. The Te nanosheet synthesis was performed as follows: PVP (1.5 g) and Na2TeO3 (0.046 g) were added to double-distilled water (16 mL) in a 50 mL beaker to form a homogeneous solution by magnetic stirring at room temperature. Then, the NH3·H2O (1.66 mL) and N2H4·H2O (0.838 mL) were successively dissolved in the above solution. Afterwards, the solution was transferred to a 25 mL stainless steel autoclave lined with tetrafluoroethylene and placed in an oven. The reaction was carried out at 180°C with different reaction times from 1 h to 4 h. Then, the autoclave was naturally cooled to room temperature, and the precipitate obtained was washed with deionized water and alcohol by centrifugation at 4000 rpm for 5 min. After that, a silver-gray sample was obtained and then dried at 60°C for at least 12 h. Finally, the precipitate was ground using a mortar and pestle to obtain a fine powder.


### 2.2. Characterizations

Morphology and structure of Te nanomaterials were studied using an FEI Quanta 200 F Scanning Electron Microscopy (SEM), and the elemental composition was analyzed using energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) measurement of synthetic samples was obtained using the D8-Focus powder diffractometer made by the Bruker company in Germany. The Raman spectrum was measured on a high-resolution confocal Raman microscope (Horiba Lab Ram Hr Evolution) at room temperature with an excitation wavelength of 532 nm. An Atomic Force Microscope (AFM) image was recorded using a dimension AFM (Multimode 8, Bruker, Germany) at room temperature. UV-vis spectra were obtained in the range of 200-1100 nm using a UV-vis absorbance spectrometer (UV-2450, Japan Shimadzu).


## 2.1. Synthesis of Tellurium Nanosheets

All the experimental procedures were carried out in a draught cupboard. Similar to a previous report [18], tellurium nanosheets were synthesized through a one-pot hydrothermal reaction. Sodium tellurite (Na2TeO3) powder, polyvinylpyrrolidone (PVP, K-30, Mw=40000), ammonium hydroxide solution (NH3·H2O), and hydrazine monohydrate (N2H4·H2O) are of analytical grade and have not been further purified. The Te nanosheet synthesis was performed as follows: PVP (1.5 g) and Na2TeO3 (0.046 g) were added to double-distilled water (16 mL) in a 50 mL beaker to form a homogeneous solution by magnetic stirring at room temperature. Then, the NH3·H2O (1.66 mL) and N2H4·H2O (0.838 mL) were successively dissolved in the above solution. Afterwards, the solution was transferred to a 25 mL stainless steel autoclave lined with tetrafluoroethylene and placed in an oven. The reaction was carried out at 180°C with different reaction times from 1 h to 4 h. Then, the autoclave was naturally cooled to room temperature, and the precipitate obtained was washed with deionized water and alcohol by centrifugation at 4000 rpm for 5 min. After that, a silver-gray sample was obtained and then dried at 60°C for at least 12 h. Finally, the precipitate was ground using a mortar and pestle to obtain a fine powder.


## 2.2. Characterizations

Morphology and structure of Te nanomaterials were studied using an FEI Quanta 200 F Scanning Electron Microscopy (SEM), and the elemental composition was analyzed using energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) measurement of synthetic samples was obtained using the D8-Focus powder diffractometer made by the Bruker company in Germany. The Raman spectrum was measured on a high-resolution confocal Raman microscope (Horiba Lab Ram Hr Evolution) at room temperature with an excitation wavelength of 532 nm. An Atomic Force Microscope (AFM) image was recorded using a dimension AFM (Multimode 8, Bruker, Germany) at room temperature. UV-vis spectra were obtained in the range of 200-1100 nm using a UV-vis absorbance spectrometer (UV-2450, Japan Shimadzu).


## 3. Results and Discussion

The morphology of as-synthesized typical Te samples at 180°C for 4 h is characterized by SEM in Figure1(a). The typical Te sample exhibits a relatively regular sheet shape with a uniform smooth surface, which shows the diameter of 4 μm, indicating that the synthesized Te nanosheets is relatively large. In order to further characterize the chemical composition, the energy dispersive X-ray spectrum (EDS) was used (see Figure 1(b)). From the spectrum, we can see a large amount of Te and a small amount of C, O, and N from PVP and N2H4·H2O precursors, in which the mass ratio of Te is 95.42%, confirming the formation and the purity of the Te nanomaterials. The typical powder diffraction pattern (XRD) and Raman spectrum are taken from as-synthesized Te nanosheets in Figures 1(c) and 1(d). Figure 1(c) shows that all X-ray diffraction peaks match well with a hexagonal phase (t-Te) with lattice parameters a=4.46Å and c=5.92Å (JCPDS 36–1452) [20, 25]. All corresponding peaks for the Te products are sharp and well defined indicating that a pure phase crystalline was formed. Compared to other peaks, the relatively intensive (101) peak, strong reflection peak of (h00), and weak reflection peaks of (l10) and (k10) indicate the preferred growth of Te nanomaterials [26–28]. Further, the intrinsic vibration information of Te nanosheets is quantitatively recorded by Raman spectroscopy in Figure 1(d). It shows three active Raman phonon modes at 92, 121, and 141 cm-1, corresponding to the E1-TO, A1, and E2 modes, respectively. Among them, the A1 mode is attributed to the chain expansion mode due to each atom movement in the basal plane. And the other two E modes are separated into the predominately bond-bending E1-TO mode from the rotation of the a-axis and the bond-stretching E2 mode from an asymmetric stretching mainly along the c-axis [11, 17]. These sharp, broad Raman peaks of Te indicating synthesized productions are highly crystalline and pure in accordance with EDS and XRD results.Figure 1
(a) SEM image, (b) EDS analysis, (c) XRD pattern, and (d) Raman spectrum of the Te nanosheets.
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(d)To further investigate the morphology and thickness of Te nanosheets, AFM is performed, as shown in Figure2. A representative sample deposited on a Si/SiO2 substrate is measured, which shows a relatively uniform large irregular nanosheet with the diameter of the lateral size of about 3.7 μm, as shown in Figure 2(a). And the topographic height is approximately 5.4±0.1 nm that can be identified as eight layers in Figure 2(b) [24]. The above analysis also confirms that the synthesized Te nanosheet is ultrathin, agreeing well with the SEM analysis.Figure 2
(a) The AFM image and (b) the corresponding height image of the Te nanosheets.
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(b)The UV-vis absorption performance of Te nanosheets is obviously shown in Figure3(a). Te nanosheets exhibit a strong characteristic absorption peak at 634 nm, in which the absorption peak in the range from 400 to 850 nm can ascribe that the valence band (p-lone-pair VB3) translates to the conduction band (p-antibonding CB1) [29–31]. Meanwhile, the optical band gap (Eg) can be obtained from the intercept of αhν1/2 to hν (α is the optical absorption coefficient, h is the Planck constant, and v is the photon frequency). The band gap of Te nanosheets is 1.37 eV, as shown in Figure 3(b). This value is much higher than the 0.34 eV of bulk Te, which is indicative of the smaller thickness of the Te nanosheets [6, 22]. The suitable Eg value also suggests that it has potential applications in the field of infrared light optoelectronic devices.Figure 3
(a) The UV-vis spectra and (b) the bandgap diagram of Te nanosheets.
(a)
(b)The formation process of Te nanosheets is studied in detail based on the evolution of the crystal structures and morphologies of the products with different reaction time during the one-pot hydrothermal reaction. The XRD patterns of as-synthesized powder samples of Te products with different reaction times from 1 h to 4 h are shown in Figure4. These samples appear as similar peak positions and patterns, which are classified as the hexagonal phase according to the JCPDS data (No. 36-1452). This phenomenon indicates the crystal structure and growth trend of Te products that is not affected by the time variation. When the reaction time is below 1 h, no diffraction peaks are detected for Te precursors, explaining that there is no crystallization in this reaction time. When the reaction time is after 2 h, the weak characteristic peaks are exhibited corresponding to the (101), (110), and (201) planes of the Te, in which the large FWHMs and weak intensities of these peaks demonstrate that Te products tend to crystallize. As the reaction time increases from 2 h to 4 h, the peak patterns of Te products become more obvious and their crystallinity gradually increases. However, sharp and intense peaks of Te products appear, and impurity peaks of reaction intermediates weaken or disappear gradually. These phenomena could be attributed to the formation of Te products with high crystallinity and purity.Figure 4
The XRD patterns of as-synthesized Te nanosheets with the different reaction times: (A) 1 h, (B) 2 h, (C) 3 h, and (D) 4 h.In order to further understand the evolution of morphologies, the SEM images of time-dependent Te products are shown in Figure5, corresponding to their crystallization. The SEM images show the morphologies of the typical products at certain reaction times. As the reaction time is less than 1 h, no solid products are found by the centrifugal treatment (4000 rpm/min), which could be that the Te precursors dissolved in solvents, that is, an incubation period of Te prior to nucleation in the initial stages of hydrothermal reaction. As the reaction time is greater than 1 h, numerous tiny incomplete nanoparticles are first observed indicating the formation of the Te nucleus (Figure 5(a)). At further elevation of the reaction time to 2 h, the uniform and straight nanowires are formed which have an average length and width of 1.3μm×40nm (Figure 5(b)). As the reaction proceeded to 3 h, a small number of nanosheets are generated in Figure 5(c), as indicated by the white arrows. The coexisting phenomenon of nanowires and nanosheets is shown. As the reaction progressed to 4 h, the nanosheets with a smooth surface are completely formed and the corresponding nanowires disappeared (see Figure 5(d)). Based on the evolution of crystallization and morphologies of Te products, these results are direct evidence to further illustrate the formation process of Te nanosheets in which Te products firstly form nanoparticles, then form nanowires, and eventually transform into nanosheets from noncrystalline to crystalline with the increase of time.Figure 5
SEM images of Te nanosheets synthesized with the different reaction times: (a) 1 h, (b) 2 h, (c) 3 h, and (d) 4 h.
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(b)
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(d)The formation mechanism of Te nanosheets could interpret the formed process of Te products in accordance with the above time-dependent evolution evidences of morphologies and crystallization, as shown in Figure6. In our reaction mechanism, the PVP with facet-specific binding energies is capable of playing an indispensable role in controlling shape evolution of Te nanoparticles [32–34]. The formation process of Te nanosheets can be made up of two main parts of nucleation and growth: in the initial reaction between 1 h and 2 h, some nanoparticles of Te are produced through the gathering of the primary seed nucleus by the reduction of Te4+ to the Te atom using hydrazine hydrate as a reductant. And this primary nucleus with high surface energies is thermodynamically metastable which prefers to aggregate into bigger nanoparticles for minimizing its interfacial energies [35, 36]. After that, with the increase of time, these Te nanoparticles can gradually dissolve into the PVP solution and release more Te nuclei with a hexagonal shape of polar and nonpolar facets. The polar planes of these nuclei have higher surface energies that it is more conducive to rapid growth through the adsorption of two end faces of the atoms. Because of their hexagonal structure and anisotropic growth tendency, the Te atoms prefer to grow along the polar c-axis to the kinetically driven typical one-dimensional nanowire. From 3 h to 4 h, more PVP molecules are adsorbed onto the polar facets of the Te nucleus, which inhibits the direction of growth along the c-axis and accelerates two-dimensional growth of the other two nonpolar facets from a gradually mature and thermodynamic-driven assembly to the final two-dimensional nanosheets at relatively high temperature and pressure [37]. During the reaction process, the main reaction formula is shown as follows:
(1)Na2TeO3+N2H4⋅H2O=Te+N2+2NaOH+2H2OFigure 6
The depiction illustration of the proposed formation mechanism for Te nanosheets.However, our understanding is limited for the formation mechanism of Te nanosheets based on the existing experimental evidences. Therefore, more detailed studies will be carried out to clarify the mechanism.


## 4. Conclusions

In summary, we have adopted a facile one-pot hydrothermal process to synthesize thin tellurium nanosheets, in which Na2TeO3 as the Te source and N2H4 as the reductant were dissolved into an alkaline PVP solution at 180°C for 4 h to synthesize Te nanosheets. The XRD analysis confirmed that Te nanosheets are a pure hexagonal phase. The images of SEM and AFM show that the Te nanosheets are thin and large and reveal the transformation of nanosheets from 0D nanoparticles, 1D nanowires, to 2D nanosheets. A possible formation mechanism is proposed to explain the formation of Te nanosheets according to the evolution evidences of structures and morphologies. The thin and uniform Te nanosheets are expected to be further used in photoelectric applications.



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