The role of HNO3 in the electrochemical deposition of dendritic PbTe microstructures

Panpan Dong , Xuemei Wang , Shifeng Li and Yonghong Ni *
College of Chemistry and Materials Science, Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, 189 Jiuhua Southern Road, Wuhu, 241002, PR China. E-mail: niyh@mail.ahnu.edu.cn; Fax: +86 553 3869303

Received 18th November 2017 , Accepted 9th January 2018

First published on 10th January 2018


Dendritic PbTe microstructures were successfully obtained by a simple potentiostatic electrodeposition route from an electrolyte consisting of Pb(NO3)2, Na2TeO3 and HNO3 at room temperature. Different from Xiao's report (Electrochimica Acta, 2006, 52, 1101), in which the effects of the deposition potential and original concentrations of Pb and Te sources on the film composition and microstructure of PbTe obtained from an acidic nitrate bath were investigated, our research mainly focused on the discussion of the role of HNO3 in the formation of PbTe dendrites. Experiments uncovered that some experimental parameters, including the deposition potential, the time and the additive, only slightly affected the formation of PbTe dendrites in the presence of HNO3 at a certain concentration. Also, no PbTe dendrites were obtained under the same deposition conditions after HNO3 had been replaced by other strong acids such as HCl, H2SO4 and HClO4, implying that HNO3 played a crucial role in the formation of dendritic PbTe microstructures. Obviously, the present work is of great significance in directing the fast electrodeposition of dendritic PbTe microstructures in practical applications.


1. Introduction

Lead telluride (PbTe) with a rock salt structure and a narrow band gap of 0.31 eV at 300 K has been drawing extensive research interest in chemistry and materials science owing to its important thermoelectric properties, such as high figure of merit and melting point (∼900 K), low vapor pressure and good chemical stability, and wide applications in solar cells, biological imaging, telecommunication and field-effect transistors.1–10 Over the past two decades, many methods have been developed for the synthesis of PbTe micro/nano-structures with various morphologies, including nanospheres, nanotubes, nanorods, nanowires, nanoboxes, nanoparticles/nanocrystals, and microstructures with X-shaped flowers and hierarchical dendrites.11–16 Furthermore, Liu and coworkers employed well-defined ultrathin single-crystalline Te nanowire patterns as templates to prepare macroscale ordered ultrathin telluride nanowire films (Ag2Te, Cu2Te, and PbTe) via the chemical transformation technology.17 Zhu et al. hydrothermally synthesized unique PbTe hopper (skeletal) crystals with high hierarchy in large quantities without the assistance of any surfactants or capping agents.18

Among the various methods to prepare PbTe materials, the electrochemical deposition route has been paid much attention due to its simplicity, speediness, and low cost. The electrochemical deposition of PbTe films on various substrates has been extensively studied in the literature to date. Also, under the assistance of different additives, dendritic PbTe microstructures could be obtained.19–21 Under the assistance of tartaric acid, for instance, Li et al. galvanostatically deposited highly symmetrical PbTe dendrites which consisted of orderly and regular particles at a current density of 3.33 mA cm−2 for 30 min.19 Moreover, thicket-like PbTe microstructures20 and feather-like PbTe dendrites21 were obtained on a Cu plate via a potentiostatic model in the coexistence of disodium salt of ethylenediaminetetraacetic acid (Na2EDTA) and tartaric acid, and in the presence of tartaric acid at a deposition potential of −0.2 V for 5 min at room temperature, respectively. In 2006, Xiao and coworkers investigated the electrodeposition of PbTe thin films from an acidic nitrate bath using e-beam evaporated gold thin films on silicon as the substrate and Pb(NO3)2 and TeO2 as the Pb and Te sources, and found that PbTe dendrites could be formed from the system with 0.01 M HTeO2+, 0.05 M Pb2+ and 1 M HNO3 at a deposition potential below −0.15 V.22 However, it was a pity that they merely investigated the effect of various deposition conditions, such as the deposition potential and the original concentrations of Pb and Te sources, on the film composition and microstructure. The role of HNO3 in the formation of PbTe dendrites was not mentioned. Later, our research found that perfect PbTe dendrites with distinguishable secondary, tertiary, and quaternary branches could be deposited during the electrodeposition of feather-like PbTe microstructures after a suitable amount of HNO3 was introduced into the system of tartaric acid under the same deposition conditions,21 implying that HNO3 possibly played an important role in the formation of PbTe dendrites.

In order to ascertain the role of HNO3, in the present work, we designed a simple potentiostatic electrodeposition route to successfully obtain dendritic PbTe microstructures at a suitable deposition potential for 5 min at room temperature, employing a Cu plate as the working electrode and a mixed system of Pb(NO3)2, Na2TeO3 and HNO3 as the electrolyte. It was found that in the presence of HNO3 with certain concentrations some experimental parameters only slightly affected the formation of PbTe dendrites, including the deposition potential, the time and the complexant. Also, no PbTe dendrites were produced under the same deposition conditions after HNO3 was replaced by other strong acids such as H2SO4, HCl and HClO4, implying that HNO3 indeed played a crucial role in the formation of dendritic PbTe microstructures. Obviously, it is of great significance to direct the fast electrodeposition of dendritic PbTe microstructures in practical applications.

2. Experimental section

All reagents were analytically pure, purchased from Shanghai Chemical Company and used without further purification. The solutions were prepared with twice-distilled water.

2.1 Preparation of dendritic PbTe nanostructures

In a typical experimental procedure, a conventional three-electrode cell was used, employing a Pt wire as the counter electrode, a saturated Ag/AgCl electrode as the reference electrode, and a pure Cu plate (99.99%, 1.0 cm2) as the working electrode. 0.5 mmol of Pb(NO3)2 (99 wt%) and 1.0 mmol of Na2TeO3 (98 wt%) were dissolved in twice-distilled water, then 1.0 mol L−1 HNO3 was added dropwise into the solution till the white precipitate was dissolved. Finally, twice-distilled water was added to give an electrolyte of 30 mL volume. PbTe dendrites were successfully obtained at room temperature by potentiostatic electrolysis with a potential of −0.25 V for 5 min without any additives.

To investigate the influence of HNO3 on the formation of PbTe dendrites, the above deposition process was repeated through replacing 1.0 mol L−1 HNO3 with 0.5 mol L−1 H2SO4, 1.0 mol L−1 HCl and 1.0 mol L−1 HClO4 with the same volume, respectively.

2.2 Characterization

The X-ray diffraction (XRD) pattern of the electrodeposited product was obtained using a Shimadzu XRD-6000 X-ray diffractometer (Cu Kα radiation, λ = 0.154060 nm), employing a scanning rate of 0.02° s−1 and 2θ ranges from 20° to 80°. The field emission scanning electron microscopy (FESEM) images and energy dispersive spectrum (EDS) of the final product were taken using a Hitachi S-4800 field emission scanning electron microscope, employing an accelerating voltage of 5 kV or 15 kV (15 kV for EDS). Transmission electron microscopy and high resolution transmission electron microscopy (TEM/HRTEM) were carried out using a JEOL-2010 transmission electron microscope, employing an accelerating voltage of 200 kV. The cyclic voltammogram (CV) measurements were carried out on a CHI660D electrochemical workstation.

2.3 Photocatalytic activity of PbTe dendrites for the degradation of methylene blue (MB)

To investigate the photocatalytic activity of as-deposited PbTe dendrites, methylene blue (MB) solution was selected as the pollution model. In a typical experiment, 20 mg PbTe dendrites was added into 50 mL of 10 mg L−1 MB solution. The mixed system was magnetically stirred in the dark for 60 min to reach the sorption–desorption equilibrium. Subsequently, the mixed system was irradiated by simulated sunlight for different durations. The concentration change of MB dye was monitored using an UV-vis spectrometer (Metash UV-6100S, Shanghai).

3. Results and discussion

3.1 Structure and morphology characterization

Fig. 1a depicts the XRD pattern of the final product deposited at a potential of −0.25 V for 5 min without the assistance of any additive. The main diffraction peaks are in good agreement with those of the standard cubic PbTe form by comparison with the data of PDF card files no. 78-1905 (see Fig. 1a, pattern below). In addition, several weak diffraction peaks centered at ∼24.8°, 43.6° and 45.3°, respectively, can be attributed to hexagonal Cu7Te4 (PDF no. 89-2402).23 Further evidence of the formation of PbTe comes from the EDS analysis of the final product. Fig. 1b exhibits the EDS analysis of the final product. The strong Pb and Te peaks can be readily observed. The C peak can be attributed to the adhesive tape and the O peak should come from the physical adsorption of oxygen in air.
image file: c7ce01997a-f1.tif
Fig. 1 Powder X-ray diffraction pattern (a) and EDS analysis (b) of the product deposited from the Pb(NO3)2–Na2TeO3–HNO3 system at a potential of −0.25 V for 5 min at room temperature.

The morphology of the as-deposited product under the current experimental conditions was characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). A low-magnification FESEM image is shown in Fig. 2a. Plenty of orderly dendrites of 10–20 micrometers in length can be easily found. Fig. 2b depicts an enlarged FESEM image of one dendrite. One can clearly see that the central trunk is made up of orderly and regular star-like particles with sizes of 300–400 nm. The secondary, tertiary, and quaternary branches can also be seen. All the secondary branches are almost parallel to each other and consist of regular trigonal particles with sizes of about 100–200 nm. Fig. 2c depicts a TEM image of a sub-branch, which was broken from a dendrite during ultrasonication. The dendritic structure of the product is clearly visible, which confirms the result of FESEM observations. A HRTEM image is inserted in Fig. 2c. The clear lattice fringes imply the good crystallinity of the dendrites. The neighboring plane is measured to be ∼0.33 nm, which is close to 0.3227 nm of the (200) plane of cubic PbTe. Obviously, the above characterization results prove that the product deposited from the HNO3 system under the current potentiostatic conditions has no difference from that deposited from the tartaric acid system by the galvanostatic model at a current density of 1.0 mA m−2 for 60 min or 3.33 mA m−2 for 30 min.19 Also, the above fact reveals that additives are not indispensable in the formation of PbTe dendrites through the electrodeposition route.


image file: c7ce01997a-f2.tif
Fig. 2 (a) Low-magnification and (b) high-magnification FESEM images, and (c) TEM image of the final product deposited from the Pb(NO3)2–Na2TeO3–HNO3 system at a potential of −0.25 V for 5 min at room temperature. The inset in (c) is a HRTEM image.

3.2 The role of HNO3

To ascertain the role of HNO3 in the formation of PbTe dendrites, PbTe was also deposited from the Pb(NO3)2–Na2TeO3–H2SO4, Pb(NO3)2–Na2TeO3–HCl and Pb(NO3)2–Na2TeO3–HClO4 systems at a potential of −0.25 V for 5 min at room temperature, respectively. As shown in Fig. 3, no dendrites similar to those exhibited in Fig. 2a were deposited from the above three systems, implying that HNO3 is indeed indispensable in the formation of PbTe dendrites. During the experiments, it was found that the white precipitate appeared when mixing Pb(NO3)2 and Na2TeO3 solutions, which was attributed to the production of PbTeO3. Here, the pH was 9.36 in a 30 mL mixed system of Pb(NO3)2 and Na2TeO3. After dropping HNO3, H2SO4, HCl and HClO4 with the same volume, the pH values were in turn 1.47, 1.66, 1.56 and 1.54. Markedly, the four systems have close pH values. However, different phenomena could be observed during the addition of various acids. The white PbTeO3 precipitate was fully dissolved in HNO3, and only partly dissolved in the other three acids, which caused different concentrations of Pb2+ and HTeO2+ in the four systems. Under the same deposition conditions, different concentrations of Pb2+ and HTeO2+ led to different depositions and growth rates of PbTe.
image file: c7ce01997a-f3.tif
Fig. 3 FESEM images of the products deposited from various systems at a potential of −0.25 V for 5 min at room temperature: (a) Pb(NO3)2–Na2TeO3–H2SO4, (b) Pb(NO3)2–Na2TeO3–HCl and (c) Pb(NO3)2–Na2TeO3–HClO4.

Fig. 4 compares the cyclic voltammogram (CV) curves of Pb(NO3)2 and Na2TeO3 in the four acidic systems. Fig. 5 exhibits the CVs of the glassy carbon electrode in 1.0 M HNO3, 0.5 mmol Pb(NO3)2 + 1.0 M HNO3 and 1.0 mmol Na2TeO3 + 1.0 M HNO3, respectively. One can readily find that the CV curve in HNO3 medium obviously differs from the curves in the other three acids. In the HNO3 system, a pair of marked redox peaks appears in the range of −0.55–−0.3 V. According to the CV curve of the system consisting of Pb(NO3)2 and HNO3 shown in Fig. 5b, the above redox peaks should be attributed to the oxidation and reduction of lead. Another obvious difference is located in the range of −0.2–0.2 V. In the CV curve of the system containing 1.0 mmol Na2TeO3 + 1.0 M HNO3 (see Fig. 5c), a reduction peak appears in the range of −0.2–−0.1 V, which is attributed to the reduction of HTeO2+ to Te.19 In the other three acids, a new strong reduction peak appears at about 0.1 V besides the above reduction peak at −0.2–−0.1 V. According to Xiao's report,22 the reduction peak at ∼0.1 V should be ascribed to the electrochemical formation of H2Te as an intermediate species (see eqn (1)).

 
HTeO2+ (aq) + 5H+ (aq) + 6e → H2Te (aq) + 2H2O,   E° = +0.121 V (vs. NHE)(1)


image file: c7ce01997a-f4.tif
Fig. 4 CVs of the glassy carbon electrode in various strong acids containing 0.5 mmol Pb(NO3)2 and 1.0 mmol Na2TeO3: (a) 1.0 mol L−1 HNO3, (b) 0.5 mol L−1 H2SO4, (c) 1.0 mol L−1 HCl, and (d) 1.0 mol L−1 HClO4.

image file: c7ce01997a-f5.tif
Fig. 5 CVs of the glassy carbon electrode in solutions of (a) 1.0 M HNO3, (b) 0.5 mmol Pb(NO3)2 + 1.0 M HNO3 and (c) 1.0 mmol Na2TeO3 + 1.0 M HNO3.

Distinctly, the deposition environment of PbTe in HNO3 differs from those in the other three acids, which should be an important reason to cause the formation of PbTe dendrites.

In previous reports,19–21,23 some coordinative reagents were used to tune the morphologies of the electrodeposition products. In the present work, we also investigated the influences of some complexants such as tartaric acid and citric acid on the deposition of PbTe dendrites in the current HNO3 medium. Fig. 6 displays the FESEM images of the deposited products after introducing 1.0 mmol and 3.0 mmol of tartaric acid and citric acid, respectively. Although the deposited products present some differences, including the size decrease and the slight change of the outlines with the increase in the amount of complexants from 1.0 to 3.0 mmol, the dendritic structures are well preserved in all the products. The above facts indicate that the influence of the additive can be ignored under the current conditions. This further confirms that HNO3 plays a crucial role in the electrodeposition of PbTe dendrites.


image file: c7ce01997a-f6.tif
Fig. 6 FESEM images of the products deposited from the Pb(NO3)2–Na2TeO3–HNO3 system in the presence of 1.0 mmol and 3.0 mmol of tartaric acid (a and b) and citric acid (c and d), respectively.

Furthermore, the original concentration of HNO3 had a marked influence on the electrodeposition of PbTe dendrites. When no HNO3 was added, the electrodeposition process could not occur under the same deposition conditions. When the concentration of HNO3 was 0.25 mol L−1, flake-like products were deposited (see Fig. 7a). Upon increasing the concentration of HNO3 to 0.5 mol L−1, dendritic products were obtained (see Fig. 7b). On continuously raising the concentration of HNO3, dendritic products were still generated. Obviously, the low HNO3 concentration is unfavorable for the formation of PbTe dendrites. In our previous discussion, the formation of PbTe dendrites is attributed to the presence of abundant free Pb2+ and HTeO2+ ions in HNO3 medium. When a small amount of HNO3 was added, the white PbTeO3 precipitate could merely be partly dissolved. Few Pb2+ and HTeO2+ ions were released, so no dendritic structures were formed. After more HNO3 was introduced, more Pb2+ and HTeO2+ ions existed in the system. Thus, dendritic structures were successfully deposited.


image file: c7ce01997a-f7.tif
Fig. 7 FESEM images of the products deposited from the system containing various HNO3 concentrations: (a) 0.25 mol L−1 and (b) 0.5 mol L−1.

3.3 Influences of the deposition potential and time on the formation of PbTe dendrites

In the electrodeposition process of PbTe, the deposition potential and time are two important factors. Fig. 8 depicts the FESEM images of the products deposited at various potentials for 5 min. When the deposition potential was −0.1 V, no obvious product was deposited (see Fig. 8a). After reducing the potential to −0.15 V, abundant dendrites were formed (see Fig. 8b). However, some dendrites had no marked sub-structures. When lower deposition potentials were employed, such as −0.3 V, dendrites with hierarchical substructures were generated (see Fig. 8c). Obviously, the lower deposition potential was favorable to the formation of PbTe dendrites. Furthermore, the experiments showed that no dendrites were deposited after the Cu substrate was replaced by an ITO substrate under the same deposition conditions (see Fig. 8d), implying that the conductivity of the substrate could affect the formation of dendritic PbTe microstructures. In our previous works,20,21 we had found that the current density could strongly influence the morphology of the final product. In this work, Cu has a better conductivity than ITO. After the Cu plate was replaced by ITO, the current density markedly varied due to their different conductive abilities. Thus, the morphology of the final product was changed.
image file: c7ce01997a-f8.tif
Fig. 8 FESEM images of the products deposited from the present Pb(NO3)2–Na2TeO3–HNO3 system at various potentials for 5 min: (a) −0.1 V, (b) −0.15 V and (c) −0.3 V. (d) A typical FESEM image of the product deposited on an ITO substrate from the present Pb(NO3)2–Na2TeO3–HNO3 system at −0.25 V for 5 min. The inset in (d) is an enlargement of the FESEM image.

In the present work, PbTe dendrites were successfully deposited within 5 min. To investigate the influence of the deposition time on the formation of the dendrites, PbTe was also deposited within shorter deposition durations. As seen from Fig. 9a, when the deposition time was only 5 s, long feather-like products were deposited. After 20 s, the feather-like products further grew (see Fig. 9b). When a deposition time of 60 s was employed, the feather-like structures decreased and dendritic structures increased (see Fig. 9c). Upon further prolonging the deposition time to 90 s, dendrites with clearly distinguishable sub-structures were formed (see Fig. 9d).


image file: c7ce01997a-f9.tif
Fig. 9 FESEM images of the products deposited from the present Pb(NO3)2–Na2TeO3–HNO3 system at a potential of −0.25 V for various durations: (a) 5 s, (b) 20 s, (c) 60 s and (d) 90 s.

The above shape evolution experiments indicate that the hierarchical structure can be formed at the original stage of the electrodeposition under the current experimental conditions, and with prolonged deposition time, the further growth of feather-like structures causes the formation of PbTe dendrites with clearly distinguishable sub-structures.

3.4 Photocatalytic activity for the degradation of methylene blue

Semiconductor micro/nanostructures usually exhibit outstanding photocatalytic activity for the degradation of organic pollutants.24,25 In the present work, it was found that the as-deposited PbTe dendrites could also be used as the photocatalyst for the degradation of methylene blue (MB) under the irradiation of simulated sunlight. Fig. 10a exhibits the UV-vis absorption spectra of MB solution irradiated by simulated sunlight for various durations in the presence of 20 mg PbTe dendrites. One can clearly find that the absorption peak intensity of MB only slightly decreases after the system has been stirred in the dark for 60 min, implying that the adsorption between MB dye and PbTe dendrites can be ignored. Subsequently, the absorption peak intensity of MB gradually decreases with prolonged irradiation time. After 80 min, ∼85% of MB is degraded, indicating the good catalytic activity of PbTe dendrites. The inset shown in Fig. 10a depicts the linear correlation between ln(C0/C) and time. The rate constant is calculated to be 0.023 min−1.
image file: c7ce01997a-f10.tif
Fig. 10 (a) The UV-vis absorption spectra of 10 mg L−1 MB solution irradiated by simulated sunlight for various durations in the presence of 20 mg PbTe dendrites. The inset in (a) is the linear correlation between ln(C0/C) and time. (b) The degradation efficiency–time curves of MB dye in the presence of various scavengers under the same irradiation conditions.

To ascertain the photocatalytic mechanism of PbTe for degrading MB, three scavengers were introduced, including tert-butanol (TBA), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) and benzoquinone (BQ), which act as scavengers of the hydroxyl radicals (˙OH), the holes (h+), and the superoxide radicals (˙O2), respectively.26 As shown in Fig. 10b, when TBA and BQ were separately added into the system, the degradation efficiency of MB had a slight decrease, indicating that ˙OH radicals and ˙O2 had no obvious contribution to the degradation of MB. When EDTA-2Na was added, only ∼6% of the degradation efficiency was obtained, indicating that the h+ played a crucial role in the process of photodegradation of MB.

4. Conclusions

In summary, dendritic PbTe microstructures have been successfully electrodeposited on a Cu substrate through a potentiostatic model at a potential of −0.25 V for 5 min in the presence of HNO3. After HNO3 was replaced by other often-used strong acids, such as H2SO4, HCl and HClO4, however, dendritic PbTe microstructures could not be deposited under the same experimental conditions. Simultaneously, some factors affecting the formation of dendrites were also investigated, including the deposition potential, the concentration of HNO3 and the complexant. It was found that in the presence of HNO3 with a suitable amount the formation of PbTe dendrites was unrelated to complexants, and a low deposition potential was favorable to the formation of hierarchical PbTe dendrites. The time-dependent shape evolution experiments showed that feather-like PbTe microstructures could be rapidly formed at the original stage and further grew into PbTe dendrites with distinguishable substructures with prolonged deposition time. The present work proves that HNO3 plays a crucial role in the electrodeposition of PbTe dendrites. Furthermore, the photocatalytic experiments showed that the as-deposited PbTe dendrites could be used as the photocatalyst for the degradation of MB under the irradiation of simulated sunlight, and have potential applications in environmental treatment and protection.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21571005), the High School Leading Talent Incubation Programme of Anhui Province (gxbjZD2016010), the Innovation Foundation of Anhui Normal University (2017xjj104) and The Recruitment Program for Leading Talent Team of Anhui Province for the funding support.

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