Na-Doping Effects on Thermoelectric Properties of Cu 2 − x Se Nanoplates

For this work, a β-phase Cu2−xSe nanowire and nanoplate, and a Na-doped Cu2−xSe nanoplate were successfully synthesized using solution syntheses. The morphologies of the Cu2−xSe nanowire and nanoplate could be easily controlled by changing the synthetic condition. The Na-doped Cu2−xSe nanoplate was prepared by a simple treatment of the Cu2−xSe nanoplate with a sodium hydroxide-ethylene glycol solution. The nanopowders were then consolidated to bulk materials using spark plasma sintering (SPS). The phase structure and the microstructure of all of the samples were checked using X-ray diffraction (XRD), high-resolution transmission electron microscope (HR-TEM), and scanning electron microscope (SEM) analyses. The thermoelectric transport properties, such as the electrical conductivity, Seebeck coefficient, carrier concentration, carrier mobility, and thermal conductivity, were measured at temperature ranges from 323 to 673 K. The results show that Na played two important roles: one is reducing the carrier concentration, thereby improving the Seebeck coefficient, the other is reducing the thermal conductivity. Overall, the maximum thermoelectric figure of merit (ZT) of 0.24 was achieved at 673 K in the Na-doped Cu2−xSe nanoplate.


Introduction
Thermoelectrics (TEs) is one of the most fascinating topics in the field of sustainable energy utilization due to its ability of direct conversion between electricity and heat based on either Seebeck or Peltier effects [1][2][3].The efficiency of thermoelectric conversion is mostly dependent on the materials' dimensionless figure of merit (ZT) that is defined as ZT = (S 2 σ/κ tot )T, where S is the Seebeck coefficient (or thermopower), σ is the electrical conductivity, κ tot is the thermal conductivity (κ tot = κ elec.+ κ latt., where κ elec.and κ latt.are the electronic and lattice contributions, respectively), and T is the temperature in Kelvin [4].High-performance TE materials require a large Seebeck coefficient, low thermal conductivity, and high electrical conductivity.It is difficult, however, to concurrently improve all of the parameters because these three parameters are interrelated by carrier concentration.Recently, several strategies and concepts indicated that higher ZT values could be obtained in nanomaterials by a tailoring of the band structure and the phonon scattering [5][6][7][8][9][10][11].Therefore, the pursuit of different TE materials using low-dimensional systems has recently become an active research field.
Following this idea, in the present study, Cu 2−x Se nanocrystals were successfully synthesized via an aqueous approach.The morphologies of Cu 2−x Se can be tunable from plate to wire by changing synthetic conditions.A sodium doped Cu 2−x Se nanoplate was prepared.After consolidating the nanopowders using spark plasma sintering (SPS), an evaluation of the thermoelectric properties was performed.

Synthesis of Cu 2−x Se Nanoplate and Nanowire
Cu 2−x Se was synthesized with different morphologies according to the method reported by Xu et al. with a slight modification [28].For the synthesis of the Cu 2−x Se nanoplate, 0.02 mol of Se powder, 1.25 mol of NaOH, and 8.00 mL N 2 H 4 •H 2 O were dissolved in 200 mL of distilled water.The solution was heated to 373 K and held there for 1 h to completely dissolve the selenium powder, thereby forming a clear deep red colored solution.Then, 15.39 mL of a 0.5 M Cu(NO 3 ) 2 aqueous solution was quickly added to the selenium precursor solution and kept for 30 min under this condition.The as-obtained product was washed with distilled water several times, and then dried under a vacuum at room temperature.For the Cu 2−x Se nanowire, 0.02 mol of Se powder, 1.25 mol of NaOH, and 8.00 mL of N 2 H 4 •H 2 O was dissolved in 50 mL of distilled water, and followed the same procedure used in the case of Cu 2−x Se nanoplates except heating the solution for 1 h at 373 K after injecting the Cu source.

Synthesis of Na-Doped Cu 2−x Se Nanoplate
Sodium was incorporated via the following step: the as-synthesized Cu 2−x Se nanoplate (2.6 g) were reacted with sodium hydroxide-ethylene glycol solution (3.1 M) under vigorous stirring at 413 K for 24 h.Finally, the products were collected by centrifugation after a number of washing procedures with acetone and ethanol.

Characterization of Materials
The synthesized samples were characterized using powder X-ray diffraction (PXRD), scanning electron microscope (SEM), high-resolution transmission electron microscope (HR-TEM), and inductively coupled plasma-optical emission spectroscope (ICP-OES) analyses.The PXRD patterns were obtained with a Rigaku powder X-ray Diffractometer (Rigaku Co., Shibuya-Ku, Tokyo, Japan) using Cu-K α radiation (λ = 1.5418Å) at 40 kV and 30 mA with the 2θ range from 20 • to 80 • .The diffraction data were collected at a scanning rate of 2 • /min.The SEM images were investigated using a JEOL JSM-6700F (JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 10 kV.The TEM images, selected area electron diffraction (SAED) pattern and energy-dispersive X-ray spectroscopy mapping (EDS-mapping) were recorded on a JEOL JEM-2100F instrument (JEOL Ltd., Tokyo, Japan).The chemical compositions were collected by ICP-OES using an OPTIMA 8300 (PerkinElmer, Inc., Waltham, MA, USA).

Characterization of the Thermoelectric Properties
In order to measure thermoelectric properties, pellets with a diameter of ~10 mm and a thickness of ~2 mm were fabricated using the spark plasma sintering (SPS-211lx, Fuji Electronic Industrial Co., Ltd., Osaka, Japan).Typically, ~2 g of the powder was loaded in a graphite die with an inner diameter of 10 mm.The powder was heated to the 450 • C at a heating rate of 100 • C/min under the conditions of vacuum and at a uniaxial pressure of ~50 MPa, and the holding time was 5 min at the sintering temperature.The electrical conductivity and the Seebeck coefficient were measured under a low-pressure helium atmosphere using ZEM-3 equipment (Ulvac-Riko, ULVAC Inc., Yokohama, Kanagawa, Japan).Thermal conductivity was determined by combining thermal diffusivity (D), specific heat (C p ) and sample density (ρ) according to κ tot = D × C p × ρ.The thermal diffusivity (D) and specific heat (C p ) of the samples were measured using a NETZSCH LFA 457 MicroFlash™ instrument (NETZSCH, Selb, Germany) under a vacuum atmosphere.The ρ was measured using the sample's geometry and mass.Generally, the uncertainties of the electrical conductivity, Seebeck coefficient, and thermal conductivity are estimated to be approximately 7%, 5% and 7%, respectively.The uncertainty of ZT is to be about 20%.The Hall resistivity was measured using a physical property measurement system (PPMS Dynacool-14T, Quantum Design, San Diego, CA, USA).

Results and Discussion
The XRD patterns of the as-synthesized samples of undoped Cu 2−x Se nanowire and nanoplate and Na-doped Cu 2−x Se nanoplate are shown in Figure 1.All of the reflection peaks can be indexed to the face-centered-cubic Cu 1.8 Se (JCPDS 71-0044), and no impurities were detected within the detection limit of the X-ray.The peaks for Cu 2−x Se nanowire sample are quite broad mainly due to the finite size of our products.The lattice constants can be refined as a = 5.674 (1), 5.737 (1) and 5.738 (1) Å for nanowire, nanoplate and Na-doped nanoplate Cu 2−x Se samples, respectively, which match the literature value listed on JCPDS Card No. 71-0044 well.It can be concluded that the lattice constant of the as-synthesized nanopowders is unchanged upon Na doping.This implies that Cu + (0.077 nm) ions are not substituted by much larger Na + (0.133 nm) ions in this experimental condition.A similar result was observed when Na was doped in a system of Cu 9 S 5 [29].On the other hand, Na-doped Cu 2−x Se shows a peak shift in comparison with the pattern of as-synthesized Na-doped Cu 2−x Se nanoplates after the SPS process (refer to the inset 002 peaks).The lattice constants for Na-doped nanoplate Cu 2−x Se samples after SPS is 5.766 (9) Å.This small lattice expansion (~5%) may be the result of Na incorporation into the structure of Cu 2−x Se by electrochemical reactions at the interfaces during the SPS process.Rather than the shift peak position, the SPS process did not significantly modify the crystal structure of nanoplate and Na-doped nanoplate sample.However, the structural changes occur for nanowire samples from cubic to orthorhombic Cu 2−x Se after SPS treatment, although they were sintered under the same conditions (i.e., 450 Ltd., Osaka, Japan).Typically, ~2 g of the powder was loaded in a graphite die with an inner diameter of 10 mm.The powder was heated to the 450 °C at a heating rate of 100 °C/min under the conditions of vacuum and at a uniaxial pressure of ~50 MPa, and the holding time was 5 min at the sintering temperature.The electrical conductivity and the Seebeck coefficient were measured under a lowpressure helium atmosphere using ZEM-3 equipment (Ulvac-Riko, ULVAC Inc., Yokohama, Kanagawa, Japan).Thermal conductivity was determined by combining thermal diffusivity (D), specific heat (Cp) and sample density (ρ) according to κtot = D × Cp × ρ.The thermal diffusivity (D) and specific heat (Cp) of the samples were measured using a NETZSCH LFA 457 MicroFlash™ instrument (NETZSCH, Selb, Germany) under a vacuum atmosphere.The ρ was measured using the sample's geometry and mass.Generally, the uncertainties of the electrical conductivity, Seebeck coefficient, and thermal conductivity are estimated to be approximately 7%, 5% and 7%, respectively.The uncertainty of ZT is to be about 20%.The Hall resistivity was measured using a physical property measurement system (PPMS Dynacool-14T, Quantum Design, San Diego, CA, USA).

Results and Discussion
The XRD patterns of the as-synthesized samples of undoped Cu2−xSe nanowire and nanoplate and Na-doped Cu2−xSe nanoplate are shown in Figure 1.All of the reflection peaks can be indexed to the face-centered-cubic Cu1.8Se (JCPDS 71-0044), and no impurities were detected within the detection limit of the X-ray.The peaks for Cu2−xSe nanowire sample are quite broad mainly due to the finite size of our products.The lattice constants can be refined as a = 5.674 (1), 5.737 (1) and 5.738 (1) Å for nanowire, nanoplate and Na-doped nanoplate Cu2−xSe samples, respectively, which match the literature value listed on JCPDS Card No. 71-0044 well.It can be concluded that the lattice constant of the as-synthesized nanopowders is unchanged upon Na doping.This implies that Cu + (0.077 nm) ions are not substituted by much larger Na + (0.133 nm) ions in this experimental condition.A similar result was observed when Na was doped in a system of Cu9S5 [29].On the other hand, Na-doped Cu2−xSe shows a peak shift in comparison with the pattern of as-synthesized Na-doped Cu2−xSe nanoplates after the SPS process (refer to the inset 002 peaks).The lattice constants for Na-doped nanoplate Cu2−xSe samples after SPS is 5.766 (9) Å.This small lattice expansion (~5%) may be the result of Na incorporation into the structure of Cu2−xSe by electrochemical reactions at the interfaces during the SPS process.Rather than the shift peak position, the SPS process did not significantly modify the crystal structure of nanoplate and Na-doped nanoplate sample.However, the structural changes occur for nanowire samples from cubic to orthorhombic Cu2−xSe after SPS treatment, although they were sintered under the same conditions (i.e., 450 °C, 50 MPa, and 5 min) by the SPS technique.According to the Hall coefficient measurements, the carrier concentrations of nanoplate and Na-doped nanoplate are estimated to be 5.71 × 10 22 /cm 3 and 2.80 × 10 22 /cm 3 , respectively.Na doping reduce the carrier concentration of Cu 2−x Se.This means that Na acts as an electron donor to the nanoplate.The generated electrons by Na recombine with holes, resulting in a decrease of the hole concentration by the electron-hole pairs [30].This is consistent with the observed decreased electrical conductivity in Na-doped Cu 2−x Se nanoplates shown in the next section.As expected, Na-doped nanoplate showed a lower carrier mobility than an undoped nanoplate due to the higher carrier concentration.The carrier mobility of nanoplate and Na-doped nanoplate were estimated to be 0.96 and 1.32 cm 2 /Vs, respectively.
In our synthetic process, Cu 2−x Se with different shapes were achieved under different conditions (see Experimental Section for details).Figure 2 shows the morphologies of the as-obtained Cu 2−x Se observed by SEM.It was found that the morphology of products depends on the concentration of reactants and the reducing reagent.It is not very clearly known at present why reaction concentration can change the morphologies significantly.One possible reason may be that the concentration affects the growth rate of crystals [31].Figure 2a,b display the SEM images of the Cu 2−x Se nanoplate with a hexagonal morphology.In general, the lateral dimensions of the nanoplates are 1-3 µm.The thickness of the nanoplate was about ~50 nm, shown in a vertical SEM image of a single Cu 2−x Se nanoplate (Figure 2b). Figure 2c,d show SEM images of Cu 2−x Se nanowire with lengths larger than ~30 µm and diameters of ~200 nm.
According to the Hall coefficient measurements, the carrier concentrations of nanoplate and Nadoped nanoplate are estimated to be 5.71 × 10 22 /cm 3 and 2.80 × 10 22 /cm 3 , respectively.Na doping reduce the carrier concentration of Cu2−xSe.This means that Na acts as an electron donor to the nanoplate.The generated electrons by Na recombine with holes, resulting in a decrease of the hole concentration by the electron-hole pairs [30].This is consistent with the observed decreased electrical conductivity in Na-doped Cu2−xSe nanoplates shown in the next section.As expected, Na-doped nanoplate showed a lower carrier mobility than an undoped nanoplate due to the higher carrier concentration.The carrier mobility of nanoplate and Na-doped nanoplate were estimated to be 0.96 and 1.32 cm 2 /Vs, respectively.
In our synthetic process, Cu2−xSe with different shapes were achieved under different conditions (see Experimental Section for details).Figure 2 shows the morphologies of the as-obtained Cu2−xSe observed by SEM.It was found that the morphology of products depends on the concentration of reactants and the reducing reagent.It is not very clearly known at present why reaction concentration can change the morphologies significantly.One possible reason may be that the concentration affects the growth rate of crystals [31].Figure 2a,b display the SEM images of the Cu2−xSe nanoplate with a hexagonal morphology.In general, the lateral dimensions of the nanoplates are 1-3 μm.The thickness of the nanoplate was about ~50 nm, shown in a vertical SEM image of a single Cu2−xSe nanoplate (Figure 2b). Figure 2c,d        The SPS pressed Na-doped Cu2−xSe bulk sample was also characterized by X-ray photoelectron spectroscopy (XPS).The XPS spectra provide useful information for understanding the chemical bonding states of the constituent atoms.The binding energies obtained in the XPS analysis were corrected for specimen charging by a setting C 1s to 284.6 eV.Representative XPS spectra of Na, Cu, and Se are shown in Figure 4.As shown in Figure 4b, one strong peak located at 1071.7 eV, indicative of Na 1s, suggested the existence of Na + species in Na-doped Cu2−xSe nanoplates.As shown in Figure 4c,d, the resulted binding energies of Se 3d and Cu 2p3/2 are given as 54.0 and 933.4 eV, respectively.All of the observed binding energy values for Cu 2p and Se 3d are nearly in agreement with the reported data in the literature [30].In the literature, the binding energy of Cu 2p3/2 has been reported to be ~933 eV regardless of its oxidation state (Cu 0 , Cu 1+ and Cu 2+ ) [31,32].Generally, Cu 2+ is best distinguished from other Cu oxidation states by observing low-intensity satellite peaks in the region of 940-950 eV.According to Figure 4c, the satellite peaks indicate that the presence of Cu 2+ ion in bulk Na-doped Cu2−xSe nanoplates.A significant broadening of the Se 3d spectra is observed, as shown in Figure 4d.Similar features were previously found in InSe compounds upon Na deposition [33].The SPS pressed Na-doped Cu 2−x Se bulk sample was also characterized by X-ray photoelectron spectroscopy (XPS).The XPS spectra provide useful information for understanding the chemical bonding states of the constituent atoms.The binding energies obtained in the XPS analysis were corrected for specimen charging by a setting C 1s to 284.6 eV.Representative XPS spectra of Na, Cu, and Se are shown in Figure 4.As shown in Figure 4b, one strong peak located at 1071.7 eV, indicative of Na 1s, suggested the existence of Na + species in Na-doped Cu 2−x Se nanoplates.As shown in Figure 4c,d, the resulted binding energies of Se 3d and Cu 2p 3/2 are given as 54.0 and 933.4 eV, respectively.All of the observed binding energy values for Cu 2p and Se 3d are nearly in agreement with the reported data in the literature [30].In the literature, the binding energy of Cu 2p 3/2 has been reported to be ~933 eV regardless of its oxidation state (Cu 0 , Cu 1+ and Cu 2+ ) [31,32].Generally, Cu 2+ is best distinguished from other Cu oxidation states by observing low-intensity satellite peaks in the region of 940-950 eV.According to Figure 4c, the satellite peaks indicate that the presence of Cu 2+ ion in bulk Na-doped Cu 2−x Se nanoplates.A significant broadening of the Se 3d spectra is observed, as shown in Figure 4d.Similar features were previously found in InSe compounds upon Na deposition [33].Figure 5 shows the electrical and thermal properties of SPSed bulk pellets of nanowire, nanoplate, and Na-doped Cu2−xSe nanoplate, and compared to that of bulk Cu1.75Se sample [32].For all of the samples, with the increase of the temperature, the electrical conductivity decreases, indicating a degenerated semiconductor behavior, shown in Figure 5a.The electrical conductivity of SPSed bulk pellets of nanowire is relatively lower than those of the other two samples, which is mainly due to the presence of point defects and dislocations by Cu vacancies in the pellet along with lower symmetry.Compared to the bulk, the undoped Cu2−xSe nanoplates exhibited a similar electrical conductivity.It is observed that a Na doped Cu2−xSe nanoplate sample shows lower electrical conductivity than a Cu2−xSe nanoplate sample.The reason for this may be the reduction of the carrier concentration by Na-doping.
Figure 5b shows the temperature dependences of the Seebeck coefficients.All samples possess a positive Seebeck coefficient in the measured temperature range.This means that the Cu2−xSe samples exhibit p-type conduction and the majority carriers are holes, which is consistent with the Hall measurements.It can be seen that the Seebeck coefficients increase from 10 to 70 μV/K with increasing temperature.It is observed that Na doped Cu2−xSe nanoplate sample shows a slightly higher Seebeck coefficient than that of the undoped Cu2−xSe nanoplate sample.This result can be attributed to the lower carrier concentration by Na doping.
Based on the results of the electrical conductivity and the Seebeck coefficient, the power factor (PF) of all of the samples can be calculated according to the equation PF = σS 2 .As shown in Figure 5c, the power factor increases as temperature increases due to the increased values of Seebeck coefficients.Because of a high electrical conductivity, the maximum power factor of the undoped Cu2−xSe nanoplate is 6.5 μW/cmK 2 (at ~673 K).A Na-doped Cu2−xSe nanoplate has a slightly lower PF when compared to the Cu2−xSe nanoplate.The peak PF values of 3.9 μW/cmK 2 (at ~673 K) was observed for the Na-doped Cu2−xSe nanoplate.This lower PF is mainly owing to the lower electrical conductivity of Na-doped Cu2−xSe nanoplate samples.Figure 5 shows the electrical and thermal properties of SPSed bulk pellets of nanowire, nanoplate, and Na-doped Cu 2−x Se nanoplate, and compared to that of bulk Cu 1.75 Se sample [32].For all of the samples, with the increase of the temperature, the electrical conductivity decreases, indicating a degenerated semiconductor behavior, shown in Figure 5a.The electrical conductivity of SPSed bulk pellets of nanowire is relatively lower than those of the other two samples, which is mainly due to the presence of point defects and dislocations by Cu vacancies in the pellet along with lower symmetry.Compared to the bulk, the undoped Cu 2−x Se nanoplates exhibited a similar electrical conductivity.It is observed that a Na doped Cu 2−x Se nanoplate sample shows lower electrical conductivity than a Cu 2−x Se nanoplate sample.The reason for this may be the reduction of the carrier concentration by Na-doping.
Figure 5b shows the temperature dependences of the Seebeck coefficients.All samples possess a positive Seebeck coefficient in the measured temperature range.This means that the Cu 2−x Se samples exhibit p-type conduction and the majority carriers are holes, which is consistent with the Hall measurements.It can be seen that the Seebeck coefficients increase from 10 to 70 µV/K with increasing temperature.It is observed that Na doped Cu 2−x Se nanoplate sample shows a slightly higher Seebeck coefficient than that of the undoped Cu 2−x Se nanoplate sample.This result can be attributed to the lower carrier concentration by Na doping.
Based on the results of the electrical conductivity and the Seebeck coefficient, the power factor (PF) of all of the samples can be calculated according to the equation PF = σS 2 .As shown in Figure 5c, the power factor increases as temperature increases due to the increased values of Seebeck coefficients.Because of a high electrical conductivity, the maximum power factor of the undoped Cu 2−x Se nanoplate is 6.5 µW/cmK 2 (at ~673 K).A Na-doped Cu 2−x Se nanoplate has a slightly lower PF when compared to the Cu 2−x Se nanoplate.The peak PF values of 3.9 µW/cmK 2 (at ~673 K) was observed for the Na-doped Cu 2−x Se nanoplate.This lower PF is mainly owing to the lower electrical conductivity of Na-doped Cu 2−x Se nanoplate samples.As shown in Figure 5d, the thermal conductivity of all of the samples decreased gradually with increasing temperature.Owing to the α-β phase transition in Cu2−xSe compounds, an anomalous behavior of thermal conductivity is observed near 373 K [35].The thermal conductivities of the Nadoped Cu2−xSe samples are lower than that of the undoped Cu2−xSe nanoplate sample.The total thermal conductivity (κtot) is the sum of two contributions, one from the charge carriers (κelec.),and the other from the lattice vibrations (κlatt.),(κtot = κelec.+ κlatt.).Here κelec. is estimated by the Wiedemann-Franz relationship, κelec.= L0σT, where L0 is the Lorenz number, σ is the electrical conductivity and T the absolute temperature.The value of Lorenz number (L0 = 1.5 × 10 −8 WΩ/K 2 ) [35] for Cu2−xSe is used to estimate κelec..The κlatt.values are calculated by subtracting κelec.from κtot, present in Figure 5d.The κlatt.values of undoped and Na-doped Cu2−xSe at 300 K are ~3.03W/m•K and ~0.83 W/m•K, respectively.Regardless of the exact mechanism, Na appears to play a role in lowering the lattice thermal conductivity.
According to the above measured results of electrical conductivity, the Seebeck coefficient and the thermal conductivity, the dimensionless figure of merit ZT can be calculated using the formula ZT = S 2 σT/κ, as shown in Figure 6.It can be seen that the ZT values of all of the samples were increased with the increasing temperature.Benefiting from the carrier concentration optimization and low thermal conductivity, the Na-doped Cu2−xSe shows the highest ZT value among all of the samples over the entire temperature range.The maximum ZT value of 0.24 was obtained at 673 K for a Nadoped Cu2−xSe sample, which is greater than those of undoped Cu2−xSe nanowire and nanoplate.It indicates that Na doping can indeed increase the figure of merit of the Cu2−xSe nanoplates.As shown in Figure 5d, the thermal conductivity of all of the samples decreased gradually with increasing temperature.Owing to the α-β phase transition in Cu 2−x Se compounds, an anomalous behavior of thermal conductivity is observed near 373 K [35].The thermal conductivities of the Na-doped Cu 2−x Se samples are lower than that of the undoped Cu 2−x Se nanoplate sample.The total thermal conductivity (κ tot ) is the sum of two contributions, one from the charge carriers (κ elec.), and the other from the lattice vibrations (κ latt.), (κ tot = κ elec.+ κ latt.).Here κ elec. is estimated by the Wiedemann-Franz relationship, κ elec.= L 0 σT, where L 0 is the Lorenz number, σ is the electrical conductivity and T the absolute temperature.The value of Lorenz number (L 0 = 1.5 × 10 −8 WΩ/K 2 ) [35] for Cu 2−x Se is used to estimate κ elec. .The κ latt.values are calculated by subtracting κ elec.from κ tot , present in Figure 5d.The κ latt.values of undoped and Na-doped Cu 2−x Se at 300 K are ~3.03W/m•K and ~0.83 W/m•K, respectively.Regardless of the exact mechanism, Na appears to play a role in lowering the lattice thermal conductivity.
According to the above measured results of electrical conductivity, the Seebeck coefficient and the thermal conductivity, the dimensionless figure of merit ZT can be calculated using the formula ZT = S 2 σT/κ, as shown in Figure 6.It can be seen that the ZT values of all of the samples were increased with the increasing temperature.Benefiting from the carrier concentration optimization and low thermal conductivity, the Na-doped Cu 2−x Se shows the highest ZT value among all of the samples over the entire temperature range.The maximum ZT value of 0.24 was obtained at 673 K for a Na-doped Cu 2−x Se sample, which is greater than those of undoped Cu 2−x Se nanowire and nanoplate.It indicates that Na doping can indeed increase the figure of merit of the Cu 2−x Se nanoplates.

Conclusions
In conclusion, surfactant-free Cu2−xSe nanowires and nanoplates have been successfully synthesized by an aqueous approach, and a Na-doped Cu2−xSe nanoplate was prepared by heat treatment of Cu2−xSe nanoplates in a sodium hydroxide-ethylene glycol solution at 413 K for 24 h.The decreasing carrier concentrations and electrical conductivities upon Na-doping indicate that sodium cations act as an n-type dopant, which is a key for increasing Seebeck coefficient and decreasing electronic thermal conductivity.The ZT value of 0.24 was obtained at 673 K from Na-doped Cu2−xSe nanoplate, which is much higher than that of undoped Cu2−xSe nanoplate (ZT ~ 0.09 at 673 K).This study provides a strategy for improvement of the thermoelectric performances.

Conclusions
In conclusion, surfactant-free Cu 2−x Se nanowires and nanoplates have been successfully synthesized by an aqueous approach, and a Na-doped Cu 2−x Se nanoplate was prepared by heat treatment of Cu 2−x Se nanoplates in a sodium hydroxide-ethylene glycol solution at 413 K for 24 h.The decreasing carrier concentrations and electrical conductivities upon Na-doping indicate that sodium cations act as an n-type dopant, which is a key for increasing Seebeck coefficient and decreasing electronic thermal conductivity.The ZT value of 0.24 was obtained at 673 K from Na-doped Cu 2−x Se nanoplate, which much higher than that of undoped Cu 2−x Se nanoplate (ZT ~0.09 at 673 K).This study provides a strategy for improvement of the thermoelectric performances.

Figure 1 .
Figure 1.Powder X-ray diffraction (XRD) patterns of as-synthesized samples of nanowire, nanoplate and Na-doped nanoplate Cu2−xSe compared with those of the standard cards (JCPDS 71-0044).The inset shows the comparison of peak position for as-synthesized (dashed line) and after SPSed (solid line) samples of Na-doped Cu2−xSe nanoplate.(SPS: spark plasma sintering) show SEM images of Cu2−xSe nanowire with lengths larger than ~30 μm and diameters of ~200 nm.

Figure
Figure 3a shows the SEM images of Na-doped Cu2−xSe nanoplates.The size and shape of Nadoped nanoplate are similar to those of undoped Cu2−xSe nanoplate.The TEM image of a typical Nadoped Cu2−xSe nanoplate is shown in Figure 3b, where a hexagonal shaped nanoplate can be clearly identified.The inset of Figure 3b is a selected-area electron diffraction pattern taken along the [111] direction of the nanoplate, showing the single crystallinity of the nanoplate.To check the chemical composition of the Cu2−xSe samples, energy-dispersive spectroscopy (EDS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) are performed.The result of elemental-mapping using the TEM-EDS analysis is shown in Figure 3c.The distributions of the elements Cu, Se, and Na are homogeneous in Na-doped Cu2−xSe nanoplate.About ~16% of Na was detected in Na-doped Cu2−xSe nanoplate.Similar results were observed by inductively coupled plasma optical emission

Figure
Figure 3a shows the SEM images of Na-doped Cu 2−x Se nanoplates.The size and shape of Na-doped nanoplate are similar to those of undoped Cu 2−x Se nanoplate.The TEM image of a typical Na-doped Cu 2−x Se nanoplate is shown in Figure 3b, where a hexagonal shaped nanoplate can be clearly identified.The inset of Figure 3b is a selected-area electron diffraction pattern taken along the [111] direction of the nanoplate, showing the single crystallinity of the nanoplate.To check the chemical composition of the Cu 2−x Se samples, energy-dispersive spectroscopy (EDS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) are performed.The result of elemental-mapping

Figure 5 .
Figure 5. Temperature dependent thermoelectric properties of nanowire, nanoplate and Na-doped nanoplate Cu2−xSe samples: (a) electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) thermal conductivity along with the lattice contribution to the thermal conductivities.Solid dots are experimental data in this work, while open dots are bulk Cu1.75Se samples from reference [34].

Figure 5 .
Figure 5. Temperature dependent thermoelectric properties of nanowire, nanoplate and Na-doped nanoplate Cu 2−x Se samples: (a) electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) thermal conductivity along with the lattice contribution to the thermal conductivities.Solid dots are experimental data in this work, while open dots are bulk Cu 1.75 Se samples from reference [34].

Figure 6 .
Figure 6.Temperature-dependent dimensionless figure of merit (ZT) for nanowire, nanoplate and Na-doped Cu 2−x Se nanoplate samples along with the data for bulk Cu 1.75 Se samples from reference [34].

Table 1 .
Atomic ratio of Cu, Se, and Na for Cu2−xSe calculated from ICP-AES.

Table 1 .
Atomic ratio of Cu, Se, and Na for Cu 2−x Se calculated from ICP-AES.