Ambient Facile Synthesis of Gram-Scale Copper Selenide Nanostructures from Commercial Copper and Selenium Powder.

TLDR: The research provides a novel ambient approach for preparation of Cu(2-x)Se nanocrystallines on a large scale for various applications.

Abstract: Grams of copper selenides (Cu2–xSe) were prepared from commercial copper and selenium powders in the presence of thiol ligands by a one-pot reaction at room temperature. The resultant copper selenides are a mixture of nanoparticles and their assembled nanosheets, and the thickness of nanosheets assembled is strongly dependent on the ratio of thiol ligand to selenium powder. The resultant Cu2–xSe nanostructures were treated with hydrazine solution to remove the surface ligands and then explored as a potential thermoelectric candidate in comparison with commercial copper selenide powders. The research provides a novel ambient approach for preparation of Cu2–xSe nanocrystallines on a large scale for various applications.

Figures

Figure 4. SEM images of Cu2-xSe nanostructures prepared from different molar ratios of Cu to 2- mercaptoethanol: (a) 1/5, (b) 1/7, (c) 1/10, (d) 1/20, (e) 1/30, (f) 1/50.

Figure 1. (a-c) FESEM and TEM images of as-prepared Cu2-xSe nanostructures. The inset in (c) is the SAED pattern of a typical single layer of Cu2-xSe. (d) HRTEM image of the selected area in (c), with the inset of FFT pattern from the area selected with the dashed square in (d).

Figure 5. Temperature dependence of the thermoelectric properties of synthetic Cu2-xSe and commercial Cu2Se samples: (a) electrical conductivity; (b) Seebeck coefficient; (c) power factor; (d) specific heat capacity (Cp); (e) thermal conductivity; (f) figure-of merit calculated using Equation (2).

Figure 3. XPS spectra of (a) Cu 2p and (b) Se 3d of as-prepared Cu2-xSe; (c) Cu 2p and (d) Se 3d of Cu2-xSe powder treated with N2H4.

Figure 6. XRD patterns of (a) as-prepared Cu2-xSe nanopowder (Original), Cu2-xSe pellet after sintering at 430℃ under 65 MPa (SPS), and Cu2-xSe pellet after thermoelectric measurements (Measured); (b) commercial Cu2Se powder, Cu2Se pellet sintered under the same conditions as in (a), and Cu2Se pellet after thermoelectric measurements. FESEM images of cross-sections of (c) Cu2-xSe and (d) Cu2Se pellets after sintering at 430℃ under 65 MPa by the spark plasma sintering technique and thermoelectric measurements.

Figure 2. (a) XRD patterns and (b) TGA curves of as-prepared Cu2-xSe nanostructures before and after treatment with N2H4. The vertical lines in (a) mark the line positions of the Cu2Se standard (JCPDS 88-2043).

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University of Wollongong
Research Online
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Ambient facile synthesis of gram-scale copper
selenide nanostructures from commercial copper
and selenium powder
Xinqi Chen
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Zhen Li
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S X. Dou
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Ambient facile synthesis of gram-scale copper
selenide nanostructures from commercial copper and
selenium powder
Xinqi Chen,
†, §
Zhen Li,
‡,§*
and Shi Xue Dou
§
†
Institute of Nanoscience and Nanotechnology, Department of Physics, Central China Normal
University, Wuhan, 430079, China.
‡
School of Radiation Medicine and Radiation Protection, Collaborative Innovation Center of
Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, 199 Ren Ai
Road, Suzhou Industrial Park, Suzhou 215123, China.
§
Institute for Superconducting and Electronic Materials, Squires Way, Innovation Campus of the
University of Wollongong, Wollongong, NSW 2500, Australia.
1

KEYWORDS Copper selenide, thermoelectric properties, semiconductor
ABSTRACT Grams of copper selenides (Cu
2-x
Se) were prepared from commercial copper and
selenium powders in the presence of thiol ligands by a one-pot reaction at room temperature. The
resultant copper selenides are a mixture of nanoparticles and their assembled nanosheets, and the
thickness of nanosheets assembled is strongly dependent on the ratio of thiol ligand to selenium
powder. The resultant Cu
2-x
Se (0 ≤ x ≤ 0.25) nanostructures were treated with hydrazine solution
to remove the surface ligands and then explored as a potential thermoelectric candidate in
comparison with commercial copper selenide powders. The research provides a novel ambient
approach for preparation of Cu
2-x
Se nano-crystallines in a large scale for various applications.
2

Introduction
Transition metal chalcogenides have attracted considerable attention due to the wealth of
physical and chemical properties that can be tuned through the careful manipulation of synthesis
conditions. Copper selenides are typical transition metal chalcogenides with diverse applications
ranging from energy conversion/storage to the biomedical regime,
1-4
e.g., lithium-ion or sodium-
ion batteries,
5, 6
electrocatalysis,
7
quantum-dot-sensitized solar cells,
8, 9
and photothermal
therapy.
10
Some of these applications require large amounts of samples for testing and measuring
their performance, e.g., thermoelectric measurement requires gram-scale powders for sintering a
small pellet,
11, 12
photovoltaic test needs tens of individual cells in a batch.
13-17
In the thermoelectric regime, introduction of nanoscale structures into conventional
thermoelectric bulk materials has been proven to be an effective way for improvement of
thermoelectric performance. For example, non-doped β-phase Cu
2
Se and Cu
2-x
Se fabricated by
high-temperature solid state reaction have a figure of merit, ZT, of around 1.5 at 1000 K,
18
which
can be enhanced to 1.7−1.8 at 700℃ by the introduction of nanoscale precipitates through fast
quenching of their liquids in the latest report.
19
Although various strategies, such as
hydrothermal or solvothermal approaches,
20-30
sonochemistry,
31, 32
electrochemical-deposition,
33-
35
the microwave-assisted route,
36
and the cation exchange method,
37
have been developed to
prepare nanoscale copper selenides, most methods are not able to produce large-scale samples
for thermoelectric, photovoltaic, or other applications. For example, the yield of copper selenide
nanomaterials prepared from hydrothermal or solvothermal approaches are limited by the
volume, temperature, and pressure of Teflon-lined autoclaves, which make difficulties for scale-
up preparation. Jiang et al prepared Cu
2-x
Se nanostructures from CuO and Se powder in a
mixture of ethylenediamine and hydrazine hydrate at room temperature. The use of expensive
3

Frequently asked questions

Q1. What was used to characterize the chemical composition and crystal structure of the samples?

Energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma − atomic emission spectroscopy (ICP-AES), and Raman spectroscopy with a 10 mW He/Ne laser at 632.8 nm (Jobin Yvon HR800) were used to characterize the chemical composition and crystal structure of the samples. 

Q2. What contributions have the authors mentioned in the paper "Ambient facile synthesis of gram-scale copper selenide nanostructures from commercial copper and selenium powder" ?

The resultant copper selenides are a mixture of nanoparticles and their assembled nanosheets, and the thickness of nanosheets assembled is strongly dependent on the ratio of thiol ligand to selenium powder. The research provides a novel ambient approach for preparation of Cu2-xSe nanocrystallines on a large scale for various applications. The resultant Cu2-xSe ( 0 ≤ x ≤ 0. 25 ) nanostructures were treated with hydrazine solution to remove the surface ligands and then explored as a potential thermoelectric candidate in comparison with commercial copper selenide powders. 

Q3. What is the advantage of the method?

An advantage of their method is that it is capable of producing grams of Cu2-xSe nanostructures in a one pot reaction for investigation of their properties and applications. 

Q4. What is the effect of the hydrazine treatment on Cu2Se?

The hydrazine treatment destroyed the sheet-like structure, and resulted in pristine Cu2-xSe nanoparticles, which were sintered into a pellet. 

Q5. What is the effect of increased carrier concentration on the electrical conductivity of Cu2Se?

Increased carrier concentration enhances the the electrical conductivity according to Equations (3-4), however, the Seebeck coefficient of the Cu2−xSe compounds was found to decrease with increasing carrier concentration. 

Q6. How much weight loss was achieved after hydrazine treatment?

The weight loss was reduced from 13% to only 3% after hydrazine treatment, which means that most of the surface ligands were removed. 

Q7. What was the process of sintering the copper selenide nanopowders?

The hydrazine-treated copper selenide nanopowders were loaded into a graphite die with a diameter of 20 mm and sintered into a pellet at 430 ℃ under 65 MPa for 10 min by the spark plasma sintering (SPS) technique. 

Q8. What is the simplest method for synthesis of copper selenide?

In this work, large amounts of Cu2-xSe nanostructures were synthesized through the ambient reaction of commercial copper powders with selenium powders in the presence of 2- mercaptoethanol and traces of NaOH. 

Q9. What is the role of -SH group in the removal of surface organic material?

Previous reports on the preparation of high crystalline metal chalcogenides, such as CuSe made from copper and selenium in pure 2-mercaptoethanol,49 and Bi2S3 nanorods prepared from bismuth(III) monosalicylate in the presence of thioglycolic acid, demonstrates that the key role of -SH group is to provide S2- or dissolve selenium to form highly reactive selenothiolate. 

Q10. What is the effect of the addition of more 2-mercaptoethanol on the nanostructures?

These results demonstrate that more 2- mercaptoethanol promotes the growth of nanoparticles along the [111] direction, which is the automatically smooth surface with the lowest energy in the fcc structure. 

Q11. Why is the Seebeck coefficient lower than that of the commercial sample?

Due to its excellent electrical conductivity, the Seebeck coefficient of Cu2-xSe is lower than that of the commercial sample (i.e., 35 μV/K at room temperature and 80 μV/K at 480 ℃) [Figure 5(b)]. 

Q12. What is the effect of hydrazine on the structure of the sheet?

The sheet structure was destroyed after treatment with hydrazine solution due to the removal of ligands (Figure S2), which indicates that nanoparticles were assembled with the assistance of ligands. 

Q13. How many nanoparticles were formed after the addition of selenium powder?

5 min after the addition of selenium powder into the reaction solution, small nanoparticles were formed and assembled into thin nanosheets with assistance of thiol ligands [Figure S5(b)]. 

Q14. What is the average Cu/Se ratio in the commercial samples?

18 The average Cu/Se ratios in pellets of their Cu2-xSe and the commercial samples after SPS and measurement were determined to be 1.93 and 2.02, respectively, by ICP-AES, which shows a higher Cu deficiency in the Cu2-xSe sample. 

Q15. What is the Seebeck coefficient of Cu2Se?

𝑍𝑍𝑍𝑍 = 𝑆𝑆 𝟐𝟐𝜎𝜎𝜎𝜎 𝜅𝜅(2)where S, σ, κ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the absolute temperature, respectively. 

Q16. What is the thermal conductivity of the Cu2Se sample?

The overall thermalconductivity of the Cu2-xSe and commercial samples calculated from Equation (1) demonstrates that their Cu2-xSe sample has lower thermal conductivity than that of the commercial Cu2Se sample [Figure 5(e)]. 

Q17. What is the difference between the commercial and the synthetic Cu2Se samples?

The calculated ZT values of the Cu2-xSe and Cu2Se samples are plotted in Figure 5(f), where a slight decrease in ZT at 140℃ can be observed due to the presence of a sharp endothermal peak at this temperature [Figure 5(d)].