Single‐precursor phase‐controlled synthesis of copper selenide nanocrystals and their conversion to amorphous hollow nanostructures

The crystal phases are essential to the physicochemical properties and functionalities of materials. Copper selenide has emerged as an important and appealing semiconductor, which can exist in a variety of polymorphic phases. However, the richness of polymorphs also makes it a challenge to the direct preparation of copper selenide nanocrystals with tunable phases. Herein, two polymorphs, that is, quasi‐tetragonal Cu2−xSe nanocubes and metastable wurtzite Cu2Se nanodisks, are successfully synthesized by using a single precursor, copper(I) selenocyanate (CuSeCN), as the Cu and Se sources. The key to phase modulation is the optimal choice of the ligand in the synthesis. The as‐prepared nanocrystals possess different morphologies and compositions, giving rise to distinct optical properties and electrical conductivities. Interestingly, the copper selenide nanocrystals can provide a platform for the rational construction of two types of amorphous hollow Au─Cu─Se nanostructures by reaction with Au(I) precursor, in which their final shapes are well kept as that of the original nanocrystal templates. This work provides an easy strategy for the phase‐controlled synthesis of copper selenide nanocrystals and enables the design of new materials for broad applications.


| INTRODUCTION
The crystal phases greatly affect the properties of materials from optical absorption to electrical conductivity and reactivity. For instance, the conductivity of 1T MoS 2 is 10 7 times higher than that of its 2H counterpart. 1 2H-Pd displays better electrocatalytic performance toward ethanol oxidation than that of thermodynamically stable face-centered cubic (fcc)-Pd. 2 As an appealing semiconductor, copper selenide can exist as a variety of crystal phases with variable stoichiometries (including monoclinic and tetragonal Cu 2 Se, cubic Cu 7 Se 4 , tetragonal Cu 3 Se 2 , and hexagonal CuSe) and nonstoichiometries (Cu 2−x Se, x = 0−0.25). [3][4][5][6][7] According to Cu─Se phase diagram, Cu 2 Se can exist as two main phases, that is, α-phase and β-phase Cu 2 Se. 8 The structure of αphase Cu 2 Se is complicated since it contains at least three types of crystals (monoclinic, tetragonal, and cubic structures), while the β-phase Cu 2−x Se only has an fcc structure. 9 In addition, the α-phase Cu 2 Se is stable at room temperature, whereas it tends to turn into β-phase at temperatures higher than 400 K and the phase transition is reversible. 10 The richness of polymorphs renders them with distinct properties and facilitates their extensive applications in batteries, optoelectronics, thermoelectrics, catalysis, and biological therapy. [11][12][13][14][15][16] Until now, the wet-chemistry method has been widely used for the preparation of copper selenide nanocrystals, and it is found that the morphology, composition, and crystal structure of the product are highly dependent on the Se precursor and reaction temperature. In particular, conventional Se precursors, such as 1-octadecene (ODE-Se), 17 oleylamine (OM-Se), 18 tri-n-octylphosphine (TOP-Se), 19 and diphenyl diselenide (Ph 2 Se 2 ), 20 tend to produce cubic phase Cu 2−x Se nanoparticles. A complex molecular Se precursor, imidazoline-2-selenone, 6 has been developed to control synthesized Cu 2−x Se nanoplates. Moreover, the ligand also plays an important role in determining the morphology and crystal phase of the copper chalcogenide nanocrystals. For instance, roxbyite Cu 1.75 S nanoplates and chalcocite Cu 2 S nanoparticles have been successfully prepared when trioctylphosphine (TOP) and tributylphosphine (TBP) are used as the ligands, respectively. 21 It is worth noting that conventional synthesis usually needs to be performed at high temperatures to decompose the precursors, which would favor the formation of products with thermodynamically stable phases. 22 Recently, the Macdonald group reported the direct synthesis of metastable wurtzite Cu 2−x Se at moderate reaction temperature by using a new homemade precursor, didodecyl diselenide. 23 However, it still remains a great challenge to control the synthesis of copper selenide nanocrystals with unconventional morphology and metastable phase by using the easy-available precursors.
Copper selenide nanocrystals usually exhibit p-doping characteristics, and their electrical conductivity is related to the concentration of the copper vacancies in the lattice. Meanwhile, the copper vacancies in copper selenide nanocrystals also cause the appearance of the localized surface plasmon resonance (LSPR) in the near-infrared (NIR) band. 24 Owing to the high mobility and redox properties of the Cu + in these nanocrystals, the concentration of copper vacancies can be well-tuned by a redox reaction in solution; thus, the electrical conductivity and LSPR band can be reversibly adjusted by selective oxidation/reduction. Furthermore, copper selenide provides a platform for the construction of novel nanomaterials via cation exchange reaction, even for hollow and concave structures. [25][26][27] The advantage of the cation exchange is that the original morphology of nanocrystals could be kept due to the good maintenance of Se sublattice, 28 so uncommon phases can be produced by using copper selenides with tunable phases as the templates. For example, the metastable wurtzite Cu 2 Se can serve as an intermediate in the conversion of CdSe to metastable wurtzite ZnSe nanoparticles. 29 Notably, it reveals that the kinetics of the cation exchange is related to the concentration of copper vacancy in the templates, that is, the higher density of copper vacancies can accelerate the reaction rate as vacancy diffusion is a main driving force. Although copper selenide nanocrystals have been widely applied as templates for the preparation of nanomaterials with unconventional phases and specific morphologies, the rational construction of complex nanostructures, particularly hollow nanostructures, is rarely explored.
Herein, we report the phase-controlled synthesis of two types of copper selenide nanocrystals by thermal decomposition of a single precursor, copper(I) selenocyanate (CuSeCN). Quasi-tetragonal Cu 2−x Se nanocubes (NCs) and metastable wurtzite Cu 2 Se nanodisks (NDs) have been prepared when 1-dodecanethiol (DDT) and TOP are employed as the ligands, respectively. These nanocrystals exhibit distinct optical absorption and electrical conductivity, which should be associated with their compositions and crystal phases. Impressively, amorphous hollow Au─Cu─Se nanostructures have been successfully produced by the introduction of Au(I) precursor into these nanocrystal dispersions. The final shapes of the products are well kept as that of the original templates, so they are named amorphous hollow Au─Cu─Se NCs and NDs, respectively. The underlying mechanism for the formation of hollow nanostructures could attribute to the Kirkendall effect, that is, the imbalance between the in-diffusion of Au + and outdiffusion of Cu + across an interface. Meanwhile, the redox reaction between these two species would also induce the deposition of metallic gold on the nanostructures. The ultimate amorphous structure should result from the incompatibility of the lattice between the template and the product, and the entrance of a larger radius Au + leading to the collapse of the original metastable Se sublattice.

| Synthesis of copper(I) selenocyanate (CuSeCN)
The synthesis of the CuSeCN was according to the previous report with slight modifications. 30 Briefly, CuSO 4 ·5H 2 O (4992.0 mg, 20.0 mmol) was dissolved in 100 mL distilled water, followed by the addition of Na 2 S 2 O 3 ·5H 2 O (4960.0 mg, 20.0 mmol) under stirring to obtain a green solution. The mixed solution was transferred to an ice-water bath and cooled for 5 min. The KSeCN aqueous solution (1928.3 mg, 13.4 mmol KSeCN dissolved in 5 mL distilled water) was dropped into the above solution via a syringe pump (0.25 mL/min) under magnetic stirring at 300 r/min. After a reaction for 20 min, the brown precipitate was filtered and washed sequentially with water, ethanol, diethyl ether, and ethanol. Finally, the product was dried in a vacuum oven at 60°C for 10 h and then ground for further use.

| Phase-controlled synthesis of copper selenide nanocrystals
All the preparations were performed using a standard Schlenk line technique with nitrogen (N 2 ) as the inert gas. For the standard synthesis of quasi-tetragonal Cu 2−x Se NCs, 8 mL of OM was loaded into 50 mL three-necked flask, and the flask was degassed at 120°C for 20 min under vigorous magnetic stirring. After purging with N 2 , the temperature of the solution was raised to 170°C, and then the precursor dispersion was quickly injected into the flask under magnetic stirring at 800 r/min. The precursor dispersion was prepared by dissolution of 0.2 mmol of CuSeCN in 2 mL of OM by sonication and followed by the addition of 100 μL of DDT to obtain a slightly gray dispersion. The temperature of the solution rapidly decreased to 160°C after injection and held the reaction at this temperature for 30 min. The solution was then gradually cooled to room temperature. The product was collected by centrifugation at 6000 r/min for 2 min and washed with toluene two times. Finally, the product was dispersed in 10 mL of toluene for further use.
Lots of controlled experiments were performed to clarify the influence of the using amounts of the DDT and CuSeCN, the solvent, and temperature on the phase and morphology of the product. To check the effect of the ligand, different dosages of the DDT (0, 25, 50, 100, and 200 μL) were used in the preparation of precursor dispersion, while all other synthetic conditions were kept the same. The standard synthesis was also performed at other temperatures (140°C and 180°C) or using different solvents (6/2 mL of OM/ODE, 6/2 mL of OM/OA) to study their impact on the product.
For the synthesis of metastable wurtzite Cu 2 Se NDs, 8 mL of OM was loaded into a 50 mL three-necked flask, and the flask was degassed at 120°C for 20 min under vigorous magnetic stirring. After purging with N 2 , the temperature of the solution was raised to 250°C, and then the precursor dispersion, prepared by dissolution of 0.2 mmol of CuSeCN in the mixture of 2 mL of OM and 250 μL of TOP, was quickly injected into the flask under magnetic stirring at 800 r/min. After injection, the reaction temperature was rapidly decreased to 240°C, and the reaction was held at this temperature for 30 min. The solution was then gradually cooled to room temperature. The product was collected by centrifugation at 6000 r/min for 2 min and washed with toluene twice. Finally, the product was dispersed in 10 mL of toluene for further use.

| Cation exchange of as-prepared copper selenide nanocrystals with Cd 2+ and Zn 2+
The cation exchange of Cu 2 − x Se NCs with Cd 2+ was carried out according to the previous report with slight modifications. 31 Briefly, 1 mmol of Cd(NO 3 ) 2 ·4H 2 O, 2 mL of methanol, 3 mL of Cu 2−x Se NCs stock solution, and 5 mL of toluene were added into a 50 mL three-neck flask. The mixture was heated to 50°C under magnetic stirring at 400 r/min. Subsequently, 1 mL of TBP was quickly injected into the flask and the reaction was held for 2 h. After the mixture was cooled to room temperature, the products were collected by centrifugation at 6000 r/min for 2 min and washed with toluene two times. The final products were dispersed into 10 mL of toluene.
The cation exchange of Cu 2 Se NDs with Cd 2+ was performed according to the previous report. 32 Briefly, 1 mmol of CdO was added into a 50 mL three-neck flask containing 2 mL OA and 4 mL ODE. The mixture was degassed at 120°C for 15 min and then heated to 220°C under N 2 for 15 min to obtain an optically clear solution.
After the temperature of the mixture was reduced to 160°C, the ND dispersion was quickly injected into the flask and kept at 160°C for 15 min. The ND dispersion was prepared by the centrifugation of 3 mL Cu 2 Se NDs stock solution and then the addition of 1 mL of TOP to disperse them by sonication. After the mixture was cooled to room temperature, the products were collected by centrifugation at 6000 r/min for 2 min and washed with toluene two times. The final products were dispersed into 10 mL of toluene.
The cation exchange of Cu 2 Se NDs with Zn 2+ was carried out according to the previous report with slight modifications. 31 The procedure is the same as that for cation exchange of Cu 2−x Se NCs with Cd 2+ , except that 1 mmol of Cd(NO 3 ) 2 ·4H 2 O is replaced by 1 mmol of ZnCl 2 , and the Cu 2 Se NDs substitutes for Cu 2−x Se NCs.

| Phase-transfer of copper selenide nanocrystals
The phase-transfer process was performed based on our recent report. 6 Generally, 4 μL of (NH4) 2 S and 4 mL of NMF mixed with 5 mL of Cu 2−x Se NCs or Cu 2 Se NDs dispersion in toluene. The nanocrystals were transferred from apolar toluene to polar NMF by ultrasonication for 30 min. The nanocrystals in the polar phase were precipitated by the toluene and then centrifuged for 5 min at 13,000 r/min. The collected product was washed with NMF once and redispersed into NMF for further studies.

| Self-assembly and transfer of copper selenide nanocrystal films
The preparation of copper selenide films was adapted from the previous report. 33 The phase-transferred nanocrystals dispersed in NMF were added into a polytetrafluoroethylene vial (diameter = 6 cm), and then a mixed solution of methanol and hexane (volume ratio = 1:1) was added dropwise to the surface of NMF. In this process, the nanocrystals gradually aggregated and got self-assembly into the film at the air/NMF interface. The film was then transferred to SiO 2 /Si substrate with prepatterned gold electrode arrays by stamping the substrate with the self-assembly film. The residual NMF on the film was slowly rinsed with ethanol three times and followed by annealing in a vacuum at 80°C for 2 h.

| Synthesis of amorphous hollow Au─Cu─Se nanostructures
The preparation of gold(I) precursor solution was according to the literature. 34 Briefly, 8 mg of HAuCl 4 · 3H 2 O, 15 mg DDAB, and 50 mg of DDA were dissolved in 4 mL of toluene under sonication for 15 min to get a slightly yellow solution at room temperature. Next, the above Au(I) precursor solution was mixed with 6 mL of nanocrystal solution (Cu 2−x Se NCs or Cu 2 Se NDs) in a 50 mL three-necked flask at room temperature with the magnetic stirring at 300 r/min. After 30 min, the product was collected by centrifugation at 6000 r/min for 2 min, followed by washing with toluene twice, and then redispersed in 10 mL of toluene. To get the amorphous Au─Cu─Se NCs with Au deposition, the amount of HAuCl 4 ·3H 2 O was increased to 80 mg, while the other synthetic procedures kept the same.

| Characterization
Transmission electron microscopy (TEM) measurements were carried out on JEOL JEM-1011 operating at 100 kV accelerating voltage and corresponding samples were prepared by dropping a diluted nanoparticles solution in toluene on a copper grid. High-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were obtained by an FEI Titan3 G2 60-300 operated at 300 kV, using carbon-coated Molybdenum grids. Scanning electron microscope (SEM) images were recorded on a field-emission scanning electron microscope (FESEM, JEOL JSM-7800F). Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 diffractometer equipped with a Cu Kα radiation (λ = 1.5406 Å) and corresponding samples were prepared by dropping concentrated Cu 2−x Se NCs solution on the silicon substrate. The UV-vis-NIR extinction spectra were measured on a Shimadzu UV-3600 spectrophotometer. Raman spectra were measured through a fully automated LabRAM Aramis Raman system, equipped with a 532 nm excitation source. X-ray photoelectron spectroscopy (XPS) patterns of samples were collected on a PHI-5000 Versa Probe with Al Kα as the excitation source. The binding energy was corrected by C 1s at 284.8 eV of adventitious carbon. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis of samples was performed on an Agilent ICP-OES 730 to quantify the element content. The current-voltage measurement was performed using a semiconductor parameter analyzer (4200A-SCS; Keithley) based on the double-probe method. Conductivity was calculated by the following equation: where κ is the conductivity; I and U represent the measured current and voltage, respectively. L and A defined as the length and area of resistance, respectively. Figure 1 illustrates the schematic diagram for the phasecontrolled synthesis of two polymorphs of copper selenides by the hot-injection method. In our synthesis, homemade CuSeCN was chosen as the single precursor for the copper and selenide sources simultaneously. The precursor mixture composed of CuSeCN and ligands in OM was injected quickly into the hot OM solvent. The key to the selective preparation is the optimal choice of the ligand, that is, quasi-tetragonal Cu 2−x Se NCs and metastable wurtzite Cu 2 Se NDs are successfully prepared by using the DDT and TOP as ligands, respectively. Figure 2A,B shows the TEM and SEM images of the as-prepared Cu 2−x Se NCs when DDT as the ligand, confirming its well-defined cubic shape with a narrow size distribution of 36.0 ± 5.0 nm (inset of Figure 2A). The molar ratio of Cu/Se in the NCs is~1.7, measured by an ICP-OES. The XRD pattern of the NCs cannot match well with standard cubic Cu 2−x Se, cubic Cu 7 Se 4 , hexagonal CuSe, and monoclinic Cu 2 Se phases ( Figure 2C), whereas it is consistent with that of previously reported copper selenide NCs. 35 In that study, NCs were prepared by the introduction of a large amount of impurity aluminum ions as modulators. The authors attributed the as-prepared product to umangite tetragonal Cu 3 Se 2 (JPCDS:01-071-0045), in spite of the XRD pattern not exactly matching the standard pattern. HRTEM of a representative Cu 2−x Se NC confirms its high crystalline and corresponding fast Fourier transformation (FFT) reveals one set of diffraction spots ( Figure 2D). The measured lattice parameters from the selected area electron diffraction (SAED) are a = 0.68 nm and c = 1.42 nm (c ≈ 2a) (Supporting Information: Figure S1A), so we ascribe our copper selenide NCs to quasi-tetragonal structure for convenience. The EDS of a representative NC indicates a homogeneous distribution of Cu and Se elements throughout the sample ( Figure 2E) and the measured atomic ratio of Cu to Se is about 1.8:1 (Supporting Information: Figure S1B), which is close to the value of ICP-OES. However, the Cu/ Se atomic ratio in our work is slightly higher than that of reports by other groups (1.5:1) 35,36 ; thus, the product is named as Cu 2−x Se instead of Cu 3 Se 2 . To check the unique structure of the Cu 2−x Se NCs, they are adopted as the templates for the cation exchange with Cd 2+ , and rhombus-like CdSe nanoparticles with wurtzite phase are obtained (Supporting Information: Figure S2A,B). There is remnant copper in the CdSe nanoparticles, as verified by the quantitative EDS (Supporting Information: Table S1). The distortion of the nanoparticles and the presence of many dislocations indicate the incompatibility of crystal structures between the original Cu 2−x Se NCs and the final products. The as-obtained Cu 2−x Se NCs show a broad NIR absorption peaked at 1300 nm, which can be attributed to its LSPR band ( Figure 2F). In addition, there is no obvious shift of the LSPR peak when the Cu 2−x Se NCs dispersion is exposed to air for 1 week.
It is found that the using amount of DDT has an impact on the shape and crystal phase of the as-obtained product when the CuSeCN dosage is fixed (Supporting Information: Figure S3). Only cubic phase Cu 2−x Se nanoparticles can be produced without the addition of DDT. When the DDT dosage was increased to 25 μL, small nanoparticles with a size of~13 nm are obtained, F I G U R E 1 Schematic diagram for the phase-controlled synthesis of copper selenides and their conversion to amorphous hollow Au─Cu─Se nanostructures.
while the product still processes cubic structure. As the dosages of DDT enlarge to 50 and 100 μL, monodispersed quasi-tetragonal Cu 2−x Se NCs with the size of~18 and 44 nm are produced, respectively. Whereas further increasing the volume of DDT to 200 μL would lead to the formation of cubic phase Cu 2−x Se again. Based on the above result, the size of Cu 2−x Se NCs could be easily tuned from 24 to 42 nm by increasing the dosage of CuSeCN, while keeping the DDT volume the same (Supporting Information: Figure S4A-D). In addition, it is found that the temperature has an influence on the final morphology of the product, whereas all crystal structures keep the same quasi-tetragonal phase (Supporting Information: Figure S4E). Cu 2 − x Se NCs can be got at 140°C and 160°C, while only nonuniform spherical nanoparticles are produced when the reaction temperature increases to 180°C (Supporting Information: Figure S5A-D). Furthermore, it reveals that the introduction of OA would also promote the formation of quasi-tetragonal Cu 2−x Se spherical nanoparticles (Supporting Information: Figure S5E-H).
Interestingly, the copper selenide NDs are obtained by adopting TOP as ligand, while the synthetic procedures are kept the same except that the reaction temperature is raised to 240°C. The product has a nearhexagonal shape ( Figure 3A) with a lateral size of 25.2 ± 2.0 nm (Supporting Information: Figure S6A). The packed NDs standing on the TEM grid indicate their two-dimensional morphology and the thickness is 9.2 ± 1.0 nm (Supporting Information: Figure S6B). XRD pattern of the NDs matches well with the previously reported metastable wurtzite Cu 2 Se ( Figure 3C). 23,37 HRTEM image of a single Cu 2 Se ND displays its highly crystalline structure and the measured lattice spacing of 0.20 nm, which could be ascribed to (1120) planes of the wurtzite Cu 2 Se ( Figure 3B). The corresponding FFT image reveals one set of spots with hexagonal symmetry, and two marked dots (yellow circle and green square) with a lattice spacing of 0.35 and 0.20 nm can be assigned to the (1010) and (1120) planes of wurtzite Cu 2 Se, respectively. The measured lattice spacing from the standing Cu 2 Se NDs is 0.34 nm, which matches well with (0002) planes of the wurtzite Cu 2 Se, so the basic plane of the Cu 2 Se ND is perpendicular to <0002> direction ( Figure 3D). To verify its metastable phase, cation exchange of Cu 2 Se (F) UV-vis-NIR extinction spectra of the fresh-prepared Cu 2−x Se NCs and the sample exposed to air for 1 week. EDS, energy-dispersive X-ray spectroscopy; FFT, fast Fourier transformation; HAADF, high-angle annular dark-field; HRTEM, high-resolution TEM; NC, nanocube; NIR, near-infrared; SEM, scanning electron microscope; TEM, transmission electron microscopy; XRD, X-ray diffraction.
NDs with Cd 2+ and Zn 2+ was performed (Supporting Information: Figure S7). The as-obtained CdSe and ZnSe nanoparticles have a wurtzite structure and well maintain the original disk shape. Furthermore, the absence of an obvious Cu element suggests that the cation exchange reaction is complete. Based on the above result, the Cu 2 Se ND is believed to be the metastable wurtzite phase if we consider the characteristics of cation exchange in the maintenance of the Se sublattice. EDS elemental mapping of a representative Cu 2 Se ND shows that the Cu and Se uniformly distribute through the ND (Figure 3E), and the corresponding atomic ratio of Cu to Se is 2.02:1 (Supporting Information: Figure S6C). UV-vis-NIR extinction spectra of the fresh Cu 2 Se NDs dispersion display a broad LSPR band peaked at 1950 nm ( Figure 3F). An obvious blue shift and the intensity enhancement of the LSPR band could be found when the Cu 2 Se NDs dispersion is exposed to air for 1 week. This phenomenon results from the increase of concentration of Cu vacancies in Cu 2 Se NDs because the Cu + is easily oxidized to Cu 2+ in the air; thus, the concentration of free carriers also increased to affect the LSPR. 18,38 XPS measurements were performed to reveal the composition and chemical state of the aforementioned nanocrystals. The XPS survey spectra illustrate the presence of Cu, Se, and O elements in both nanocrystals, where the weak O 1 s peak may be attributed to adsorbed oxygen or a thin copper oxide layer ( Figure 4A,E). The high-resolution XPS spectrum of Cu 2p from Cu 2−x Se NCs ( Figure 4B) shows two sharp peaks located at 932.2 and 952.1 eV, ascribing to Cu 2p 3/2 and Cu 2p 1/2 , respectively. Each peak is deconvoluted into two peaks, indicating the presence of Cu(I) and Cu(II) in Cu 2−x Se NCs. 5,14 In addition, the appearance of two satellite peaks located at 963.5 and 944.1 eV also imply the presence of Cu(II). Similar results are also found in the Cu 2 Se NDs ( Figure 4F), and two main peaks in the spectrum of Cu 2p can be deconvoluted into signals attributed to Cu(I) and Cu(II). Noted that the proportion of Cu(II) to Cu(I) is much lower than that of Cu 2−x Se NCs, and the signal of Cu(II) should come from the remnant copper oxide by oxidation of the Cu 2 Se NDs during the sample preparation, which is also confirmed by the absence of distinguishable satellite peaks from the Cu(II). The XPS spectra of Se 3d in Cu 2−x Se NCs ( Figure 4C) and Cu 2 Se NDs ( Figure 4G) are located at about 53.7 and 53.9 eV, (F) UV-vis-NIR extinction spectra of the fresh-prepared Cu 2 Se NDs and the sample exposed to air for 1 week. EDS, energydispersive X-ray spectroscopy; HAADF, high-angle annular dark-field; HRTEM, high-resolution TEM; ND, nanodisk; NIR, near-infrared; TEM, transmission electron microscopy; XRD, X-ray diffraction which are the typical binding energy of lattice Se in selenide. 21 The Raman spectrum of Cu 2−x Se NCs exhibits a feature signal at 262.3 cm −1 , which can be assigned to a Se−Se vibrational mode ( Figure 4D). 39 Additional peak located at 358.5 cm −1 may attribute to CuO with the Cu vacancy. 40,41 Similarly, the Cu 2 Se NDs also shows two peaks located at 261.6 and 358.6 cm −1 ( Figure 4H), respectively, which are slightly shifted compared with that of the Cu 2−x Se NCs.
The aforementioned results demonstrate the successful synthesis of quasi-tetragonal Cu 2−x Se NCs and metastable hexagonal Cu 2 Se NDs by selecting DDT and TOP as ligands, respectively. As we know, Cu + is Lewis acid, while DDT and TOP are the typical Lewis bases. 42 According to the hard soft acid base (HSAB) theory, the above ligand can form a strong complex with Cu + (Cu-DDT and Cu-TOP), which would reduce the reactivity of the precursor. In the synthesis, the decomposition of the SeCN − produces the selenide ions, which would react with the Cu + to produce the copper selenide nuclei. The formation of the complex reduces the release rate of the Cu + ; thus, the nucleation and growth rate of the copper selenide nanocrystals also slows down, which would regulate the crystal phase of the final product. In fact, the formation of the cubic phase Cu 2−x Se nanocrystals without the addition of a ligand is much more easily taking place than that of the reaction with the ligand. It is worth noting that the TOP shows much stronger binding capability with Cu + than DDT, which is also confirmed by the experimental fact of its higher reaction temperature. Moreover, the composition of the copper selenides also affected by the kinetics of the reaction. The Cu/Se ratio of Cu 2−x Se NCs is less than that of the thermodynamically stable cubic phase Cu 2−x Se nanocrystals by the formation of the Cu-DDT complex. However, the formation of the Cu 2 Se NDs in the presence of the TOP should attribute to the fast extraction capability of Se from the copper-vacancy nanocrystals at higher temperature reactions. 39 Of course, we cannot exclude the effect of the ligand on tuning the morphology of the product by its absorption behavior on different facets.
Copper selenide nanocrystals have great potential for electronic devices due to their low-toxic, low-cost, and high electrical conductivity. 43 Figure 5A shows SEM image of the fabricated Cu 2−x Se NCs film on patterned gold electrodes. The magnified image clearly illustrates a continuous film with a nearly single layer of NCs connecting together. The standard two-point probe method was performed to measure the conductivity of the film under ambient conditions. The typical linear I-V curve suggests an Ohmic contact between the Au electrode and the Cu 2−x Se NCs film ( Figure 5B). Specifically, the current can reach to ±6.3 mA at the applied voltage of ±3 V, and the calculated conductivity is about 61 Ω/cm. In comparison, the symmetric S-shape curve is found in the fresh-fabricated Cu 2 Se ND film, indicating the formation of Schottky junction between the gold electrode and NDs ( Figure 5C). 19 The film exhibits semiconductor behavior with a maximum current of ±10 nA at ±1 V. After exposure to air for 48 h, the Cu 2 Se NDs film shows an Ohmic behavior rather than semiconducting behavior with a significant increase in current at the same applied voltage ( Figure 5D). This phenomenon can be explained by the oxidation of Cu + to Cu 2+ in the Cu 2 Se NDs at ambient conditions, which exhibits a great solid-state ionic conductivity of Cu 2+ within the nanocrystal. 6 The electrical behavior transformation from Schottky to Ohmic demonstrates that the electrical property of Cu 2−x Se NDs film can be well regulated through selective oxidation.
Copper selenide nanocrystals are widely adopted as platforms for the preparation of various novel nanocrystals via cation exchange reaction. 44 Here, we investigate the reaction between the copper selenide nanocrystals and the Au(I) precursor solution in the presence of DDAB and DDA. Surprisingly, hollow nanostructures with similar shapes as the templates can be synthesized. Figure 6A shows a TEM image of the product template by Cu 2−x Se NCs, the cubic shape nanoparticles have obvious cavities in the center and some holes at the facets. The corresponding SAED of the product only shows some diffusive rings, implying the amorphous structure of the product. It is also confirmed by the XRD pattern, which only shows a broad peak at around 31°. The high-angle annular darkfield (HAADF) and corresponding EDS elemental mapping of the hollow NCs display the presence of Au, Cu, and Se elements ( Figure 6B) with an atomic percentage of 12.4%, 40.7%, and 46.9%, respectively (Supporting Information: Table S2), and these elements are distributed throughout the NCs with the higher intensity at the edge sites than that of the interior area. The amorphous hollow Au─Cu─Se NCs only show featureless UV-vis-NIR adsorption, and the original LSPR band of the Cu 2−x Se NCs disappears after the reaction. It is worth noting that the composition and the morphology of the final product are closely related to the dosage of the Au(I) precursor. When the concentration of Au(I) precursor increased to 10 times, amorphous Au─Cu─Se NCs with gold deposition could be obtained (Figure 1). The product still maintains a cubic shape, whereas it shows a darker contrast than the aforementioned hollow NCs under TEM characterization (Supporting Information: Figure S8A). XRD illustrates a broad diffraction peak at around 31.7°and another two strong peaks located at 38.2°and 44.4°, in which these strong signals can be assigned to (111) and F I G U R E 5 SEM images of the (A) Cu 2−x Se NCs film and (C) Cu 2 Se NCs film on prepatterned Au electrodes (inset: high-magnification images taken from the white dashed squares). I-V curves of (B) the Cu 2−x Se NCs film (D) the fresh-prepared Cu 2 Se NDs film and the film exposed to air for 48 h. NC, nanocube; ND, nanodisk; SEM, scanning electron microscope; XPS, X-ray photoelectron spectroscopy.
(200) planes of fcc Au, respectively (Supporting Information: Figure S8B), while the broad diffraction peak reflects the amorphous structure of the Au─Cu─Se NCs, in addition to the gold. The HAADF and EDS elemental mapping reveal that the product is still composed of Au, Cu, and Se elements (Supporting Information: Figure S8C-F), and the atomic ratio of the Au increases to 24.7%, about two times than that of the amorphous hollow Au─Cu─Se NCs (Supporting Information: Table S2). The XPS spectra of the amorphous hollow NCs confirm the presence of Au, Cu, and Se. Especially, the Au 4f 5/2 and 4f 7/2 peaks can be separately deconvoluted into two peaks, where the peaks located at 84.7 and 88.5 eV can be ascribed to Au + , while the others-centered at 83.7 and 87.4 eV indicates the presence of Au(0) (Supporting Information: Figure S9D). 45,46 Likewise, hollow NDs can be obtained when the Cu 2 Se NDs are used as the template, as the boundaries of the product are darker than the central area in the TEM image ( Figure 6E). Sometimes, some small nanoparticles (Au) anchored on the hollow NDs can be found (Supporting Information: Figure S10). The standing hollow NDs indicate their thickness is close to that of the original Cu 2−x Se NDs. HAADF image and EDS elemental mapping of the hollow NDs also show that the Au, Cu, Se elements are uniformly distributed throughout the NDs ( Figure 6F) and corresponding atomic percent of Au, Cu, and Se are 29.3%, 33.4%, and 37.3%, respectively (Supporting Information: Table S2). The XRD pattern ( Figure 6G) suggests that the amorphous structures of hollow Au─Cu─Se NDs, and the sharp peaks ascribed to fcc gold may come from the gold particles anchored on the hollow NDs. Figure 6H shows the UV-vis-NIR adsorption spectrum of hollow Au─Cu─Se NDs, and the LSPR absorption from Cu 2 Se NDs also disappears after conversion.
Copper chalcogenide nanocrystals provide versatile platforms for the construction of nanomaterials with various compositions, morphology, and even complex architectures. 47 However, the spontaneous formation of hollow structures templated by copper selenide nanocrystals is rarely reported. Considering the above results, we suggest that the formation of hollow structures should result from the well-known Kirkendall effect, 48,49 that is the imbalance between the faster outward diffusion of Cu + than the inward diffusion of Au + across the interface in our synthesis. It is worth noting that Au + can be reduced by the solvated Cu + due to the difference of their redox potentials; 50 thus, metallic gold can be detected in the samples as the increase of Au(I) concentration, which is consistent with our finding. Meanwhile, the oxidized Cu 2+ would dissolve into a solution, which is also proven by the blue supernatant after the separation of the product by centrifugation. To further check the influence of the crystal phase on the product, the thermodynamically stable cubic Cu 2−x Se nanoparticles, prepared without the addition of the DDT, were chosen to perform the same reaction (Supporting Information: Figure S11). On the contrary, there are no hollow structures in the product, and only gold nanoparticles with size of 15.2 nm anchored on the templated nanoparticles are found. However, the crystal structure of reacted nanoparticles cannot be well ascribed, which may also result from the interfacial cation diffusion. Furthermore, the formation of amorphous structures is believed to come from the incompatibility of the lattice between the original template and the product. The radius of Au + is larger than that of Cu + , and the large lattice mismatch leads to stronger lattice stress during the exchange process. 26 Therefore, the inward diffusion of Au + would lead to the collapse of the Se sublattice for the unconventional quasi-tetragonal phase and the metastable wurtzite phase but not for the thermodynamically stable phase.

| CONCLUSION
In summary, we have developed a facile singleprecursor strategy for the direct synthesis of two types of copper selenide nanocrystals with controlled crystal phases and shapes, that is, quasi-tetragonal Cu 2−x Se NCs and metastable wurtzite Cu 2 Se NDs. The phase tuning is achieved by the optimal selection of the ligand. Moreover, the different compositions of these nanocrystals affect their optical absorption and electrical transport properties. The conductivity of Cu 2−x Se NCs film can be achieved as high as 61 Ω −1 cm −1 , while the Cu 2 Se NDs film displays a change from semiconductor behavior to Ohmic behavior upon exposure to air. The transformation results from the oxidation of cuprous ions in the lattice, which also induces the blue shift of the LSPR band for Cu 2 Se NDs. Interestingly, two novel amorphous hollow Au─Cu─Se nanostructures are successfully prepared by treating these nanocrystals with the Au(I) precursor. The spontaneous formation of hollow structures is believed to come from the wellknown Kirkendall effect, and the ultimate amorphous structures should result from the incompatibility of the lattice between the original template and the product. We believe that the presented strategy could be extended for the phase-controlled synthesis of other nanocrystals and enables the design of new materials for broad applications.