Introduction

Tellurium is a material of special interest in technology due to its special physical and chemical properties. Tellurium exhibits catalytic activity (Fujiwara et al. 1992), photoconductivity (Ufimtsev et al. 1997) and interesting piezoelectric and thermoelectric properties (Gerlach and Grosse 1979; Cooper 1974). It has been demonstrated by solution synthesis, that tellurium precipitates in nearly spherical nanoparticles whose crystal structure is trigonal. These nanoparticles have the capability to grow nanowires, nanorods and nanobelts (with hexagonal crystalline phase) in a vapor phase reactor, where tellurium nanoparticles serve as seeds (Furuta et al. 1975). On the other hand, it is widely known that the preparation of a NaHTe solution, where elemental tellurium is used as precursor, requires a strong reduction condition supported by an inert atmosphere due that tellurium is tetra-positive in valence and is insoluble. Additionally, telluride ion is highly reactive with oxygen (Gautam and Rao 2004), this reactivity promotes the formation of tellurates with the exposition of the sample to the laboratory environment. In this context, sodium borohydride has proved to be a strong reducer and partially dissolves the tellurium. Otherwise, the NaHTe reaction produces sodium borates, considered as contaminants; however, these borates are easily removed by decanting. To obtain a tellurium deposition by chemical methods, it is necessary solve the above problems.

In the present report, we found that the preparation of NaHTe complex under basic conditions is an excellent precursor for adherent, uniform and free-borates tellurium deposition. This synthesis has the advantage of be realized completely under ambient conditions. Finally, we also found that the reaction between H2Te gas (obtained by the decomposition of NaHTe by the H2SO4 injection) and NH4OH aqueous solution triggers the formation of tellurium nanostructures such as nanorods and “Y” structures, which are obtained only by hydrothermal synthesis and using a surfactant.

Experimental

Materials

Tellurium powder (−200 mesh, 99.8 % Aldrich); sodium borohydride (NaBH4, mesh, 99.8 % Aldrich), ammonium hydroxide (35 %, J.T. Baker).

Synthesis of NaHTe in ammonia solution (NaHTe:NH3)

The preparation of sodium hydrogen telluride in ammonia solution (NaHTe:NH3) was based on a method reported by Gu et al. (2008), where 204.2 mg (1.6 mmol) of tellurium powder and 157.8 mg (4.2 mmol) of NaBH4 were loaded in a 50 mL Erlenmeyer flask. Afterwards, 20 mL of NH4OH (164 mmol) solution was added and the mixture was heated up to 80 °C under N2 atmosphere during 15 min of constant stirring. Tellurium nanostructured thin films were deposited by dripping of NaHTe:NH3 on glass substrate under laboratory conditions and the depositions were analyzed by X-ray diffraction.

Synthesis of NaHTe:NH3 using H2Te as telluride ion precursor

According to the reaction presented by Gautam and Rao (2004) and Zheng et al. (2004), the synthesis of NaHTe produces a secondary phase of sodium borate (Na2B4O7) consisting of a whitish sediment. As an effort to reduce the secondary phases such as sodium borate and tellurates during the synthesis, samples of NaHTe:NH3 were prepared by employing a method proposed by Bao et al. (2004). In 20 mL of an aqueous solution of NaHTe, 2 mL (36.7 mmol) of sulfuric acid were injected to obtain H2Te. Hydrogen telluride was transferred to a solution of NH4OH (20 mL, 0.1 M) through a bubbler by using N2 flux. The solution acquired the characteristic purple tonality of NaHTe solution; nevertheless, 2 min later, solution loses stability and was turned transparent, and tellurium particles were flocculated on the vessel bottom. The flocculate was analyzed by transmission electron microscopy (TEM).

Characterization

Morphology and size of tellurium nanostructures were analyzed by TEM using a JEOL 2010 microscope, operated with 100,000× and 400,000× magnification. The samples were dropped on 300 mesh Lacey carbon grid, dried under laboratory conditions, and then introduced to the microscope vacuum chamber. X-ray diffraction (XRD) measurements were done on deposited NaHTe:NH3 in a Phillips x´Pert diffractometer at normal incidence, using monochromatic Cu Kα radiation.

Results and discussion

TEM analysis

Although the NaHTe:NH3 samples were not stable, the collected products allowed us to observe a variety of crystalline structures such as rounded tellurium nanoparticles (Fig. 1) and nanostructures such as nanorods (Fig. 2), Y-like nanostructures (Fig. 3), and bigger crystallites produced by the subsequent aggregation of these nanostructures (inset of Fig. 3).

Fig. 1
figure 1

TEM micrography of a powder obtained from NaHTe:NH3 sample using H2Te as telluride ion precursor

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Fig. 2
figure 2

TEM micrography of nanorods observed in a powder obtained from NaHTe:NH3 sample using H2Te as telluride ion precursor

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Fig. 3
figure 3

TEM micrography of Y-type nanostructures observed in a powder obtained from NaHTe:NH3 sample using H2Te as telluride ion precursor. The inset exhibits higher crystallites formed by the nanostructure aggregation

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The mean diameter measured for the rounded nanoparticles displayed in Fig. 1 was 10 nm. The nanorods showed in Fig. 2 have a mean width of 15 nm, practically without appreciable variation and their length varies from 60 to 158 nm. Gautam and Rao (2004) observed the formation of these nanostructures through a hydrothermal synthesis of NaHTe, employing sodium dodecylbenzenesulfonate as surfactant, and as oxidation inhibitor. In this work no surfactant was used, since our intention was prepared a tellurium deposition; nevertheless, the simple reaction between H2Te and ammonia solution has produced tellurium nanoparticles that have operated like seeds for the growth of nanorods (Fig. 4), and the formation of Y-like nanostructures (Furuta et al. 1975). These nanostructures also may be formed by the aggregation of nanorods as shown in the micrograph of Fig. 2. Electron diffraction analysis was realized on the nanostructure showed on the micrography of Fig. 4 and the result was the hexagonal structure of tellurium (Rabadanov et al. 2000).

Fig. 4
figure 4

TEM micrography of Y-type nanostructure observed in a powder obtained from NaHTe:NH3 sample using H2Te as telluride ion precursor. The micrography exhibits a tellurium nanoparticle that serves as seed for the growth of a “Y” structure

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X-ray diffraction analysis

The first test looking for tellurium deposition was through the preparation of a colloidal solution with characteristics similar to an ink by the flocculation of tellurium nanoparticles in a NaHTe solution. Initially, we extracted the solid phase, including the insoluble whitish sediment on the flask bottom and further analysis by X-ray diffraction, indicated that this sediment is composed of several phases of sodium borate, such as reported by Gautam and Rao (2004) and is shown in Fig. 5. These phases represent a pollution source for the deposition and is the predominant phase in the X-ray diffraction pattern showed in Fig. 5.

Fig. 5
figure 5

X-ray diffraction pattern of a powder sample obtained by the flocculation of solid phase of a NaHTe solution including the whitish sediment on bottom flask, secondary product of NaHTe synthesis

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Figure 6 corresponds to X-ray diffraction pattern for a sample prepared by the flocculation of solid phase from a NaHTe solution, with no borates on the flask bottom and under laboratory conditions. In spite of the number of diffraction peaks for the different phases of sodium borate were reduced, we did not observe any diffraction peak generated by tellurium or a tellurium based compound, such as oxides and tellurates given the strong reactivity between the ion telluride and oxygen.

Fig. 6
figure 6

X-ray diffraction pattern of a powder sample obtained by the flocculation of solid phase of a NaHTe solution without the whitish sediment on bottom flask

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The NaHTe:NH3 solution resulted with a dark-purple tonality and with a higher viscosity and stability compared with a standard NaHTe solution. Tellurium deposition was significantly easier, because inert atmosphere, flocculation and cleaning of solid phase were not required. A simple synthesis in ammonium hydroxide solution allowed to produce a solution with the characteristics of an ink and the volatility of ammonia also allowed the rapid evaporation of aqueous media, resulting the nanostructured tellurium deposition with high adherence and uniformity.

Despite the simplicity of this synthesis, we must note that sodium borates phases were not removed once tellurium powder and sodium borohydride were dissolved in ammonia solution, due to forming a viscous and darker solution. For this reason, we try to gasify an aqueous ammonium hydroxide solution with H2Te, but the product proved to be unstable.

The diffraction pattern of a tellurium nanostructure deposition is displayed in Fig. 7, it is clear that we have obtained a tellurium thin film highly starting from a modified NaHTe solution with a good purity degree and uniformity, just dripping the NaHTe:NH3 solution onto a substrate under ambient conditions. Besides, this synthesis rich in ammonia created an environment favorable to protect the telluride ions of the presence of environmental oxygen, factor which favors the formation of oxides and tellurates.

Fig. 7
figure 7

X-ray diffraction pattern of a nanostructured tellurium deposition, realized by the dropping of NaHTe:ammonia solution

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X-ray diffraction analysis indicated that the diffraction peaks showed in Fig. 7 correspond to the hexagonal phase of tellurium [10] with a preferred orientation on the (101) direction, corroborating the results obtained by electron diffraction in section “X-ray diffraction analysis”. Although the phase of borates was not removed, their presence in the diffraction pattern has been significantly reduced.

Conclusions

We have prepared a uniform and highly adherent tellurium deposition by a relatively simple process that does not require strict inert atmosphere, starting from a highly reactive compound with oxygen, such as NaHTe and without needing any organic stabilizer. Synthesis of tellurium powder with NaBH4 in aqueous NH4OH solution resulted in a purple hue darker solution, with higher viscosity compared to a NaHTe standard solution, but highly stable under laboratory conditions. According to TEM analysis, the solution contains various types of tellurium nanostructures, these nanostructures are nearly spherical nanoparticles, nanorods and structures of type “Y”, nanostructures traditionally obtained by chemical vapor techniques.