CuInSe2

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Catalyst-Assisted Solution−Liquid−Solid Synthesis of CdS/CuInSe2 and CuInTe2/CuInSe2 Nanorod Heterostructures Guanwei Jia†,‡ and Jiang Du*,†,§ †

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Henan Province Industrial Technology Research Institute of Resources and Materials, Zhengzhou University, Zhengzhou 450001, People’s Republic of China ‡ School of Physics and Electronics, Henan University, Kaifeng 475004, People’s Republic of China § Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Axial nanowire heterostructures composed of cadmium sulfide (CdS)/copper indium diselenide (CuInSe2) and copper indium telluride (CuInTe2)/copper indium diselenide (CuInSe2) were synthesized by a solution−liquid−solid (SLS) method with the catalyzer of bismuth nanocrystals. Electron microscopy and diffraction studies show CuInTe2 and CuInSe2 segments growing along the [112] direction with a clear epitaxial interface between them. In CdS/CuInSe2 nanorod heterostructures, CuInSe2 and CdS segments grow along the [112] and [111] direction, respectively, with an obvious epitaxial interface between them. Energy-dispersive X-ray spectrometry demonstrates the alloy-free composition modulation in two nanorod heterostructures. In CuInTe2/CuInSe2 nanorod heterostructures, Te and Se are localized in CuInTe2 and CuInSe2 segments, respectively. Cu/In/Se and Cd/S are localized in the CuInSe2 and CdS sections of the CdS/CuInSe2 nanorod heterostructures. This research confirms that the SLS mechanism provides a general alternate technique to prepare multicomponent axial 1D heterostructures that have been difficult to generate by using either catalyst-free solution-phase synthesis or vapor−liquid−solid growth.

1. INTRODUCTION

various quantum dot geometries based on p−i−n and p−n junctions.21,22 To obtain the desired heteronanostructures, several mechanisms have been explored, for instance, vapor-phase methods,4−15 solution-phase methods,18,19 template-directed synthesis,23,24 and so on. Vapor-phase methods include pulsedlaser deposition,4 catalyst-assisted metal−organic chemical vapor deposition,4,21 metal−organic vapor-phase epitaxy,6 and chemical beam epitaxy.5 However, these methods require unordinary equipment, extreme conditions, and the products cannot be in mass production-scale, which obstructs their practical applications. On the contrary, solution-phase methods employed in synthesis of 1D heterostructures circumvent above problems. Especially, the solution−liquid−solid (SLS) method,25−30 on the basis of low-melting-point metal nano-

Recent research indicates that one-dimensional (1D) heterostructure nanomaterials have attracted much great interest because of their applicability as versatile building blocks in nanophotonic and nanoelectronic devices.1−3 Various 1D heterostructure nanomaterials,4−15 such as Si/SiGe,4 Si/Ge,5 GaP/GaAs,6−8 InP/InAs,9 GaAsP/GaP,10,11 InAs/InAsP,12−14 and GaAs/GaP,15 have been reported, including axial or segmented,4,6,10 coaxial or core/shell,6,8,16,17 and hierarchical/ branched heterostructures.18,19 1D heterostructure nanomaterials possess the great potential to innovate nanomaterial research by offering means to define multiple functionalities within a single nanostructure. A large number of device functionalities indicating the unique properties of 1D heterostructures have already been demonstrated, including heterostructure field-effect transistors,12,13 infrared photodetectors,14 nanoscale photonics, and electronics,15,20 and © XXXX American Chemical Society

Received: October 11, 2018

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DOI: 10.1021/acs.inorgchem.8b02870 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

reported methods59 and dissolved in THF to make a 1 M solution. 5.0 g of Ganex V-216 and 15 g of DPE were combined in a 100 mL three-neck flask. The flask was attached to a Schlenk line, placed under vacuum, heated to 70 °C, and allowed to degas for 1.5 h while stirring. In parallel, 0.5 mL of Bi[N(SiMe3)2]3/THF was mixed with 2.0 mL of 1 M NaN−(SiMe3)2 in THF and placed into a syringe. After the Ganex V-216 and DPE solution was degassed, the flask was refilled with N2 and the temperature was increased to 180 °C. At 180 °C, the Bi precursor solution was quickly injected into the hot solution and reacted for 30 min. After 30 min, the mixture was cooled to 60 °C and 15 mL of room-temperature toluene was added. Bi nanocrystals were isolated by centrifuging the reaction product for 3 min at 5000 rpm. The supernatant was collected, and the precipitate was discarded. Thirty milliliters of MeOH was added to the supernatant and centrifuged for 5 min at 10 000 rpm. The supernatant was discarded. The precipitate was dispersed in 10 mL of toluene. This process was repeated several times, and the final precipitate was dispersed and stored in toluene at a concentration of 5 mg/mL. The Bi nanocrystals were spherical with an average diameter of 12 nm (see Figure S1). 2.3. Bi/CdS/CuInSe2 Nanorod Heterostructures Synthesis. First, Bi/CuInSe2 nanorods were synthesized by the SLS method.60,61 A 0.5 M stock solution of Se in TOP (TOP/Se) was made by dissolving 0.79 g of Se powder in 20 mL of TOP. This solution was made in a glovebox under an inert atmosphere, stirred overnight to ensure that the Se was completely dissolved, and stored in the same glovebox. Copper acetate (30.6 mg, 0.25 mmol), indium acetate (70.3 mg, 0.25 mmol), 0.25 mL of OA, and 4 mL of TOP were combined in a 25 mL three-neck flask. The flask was attached to a Schlenk line and degassed while heating to 100 °C. After reaching 100 °C, the solution was held under vacuum for 15 min and stirred vigorously. The flask was then filled with N2 and cooled to room temperature under N2 flow. The Cu and In precursor solution (1 mL) was then combined with 0.25 mL of the 0.5 M TOP/Se stock solution and placed in a syringe as the Cu, In, and Se precursor solution. TOP (8.5 mL) was added in a separate 100 mL three-neck flask. The flask was attached to the Schlenk line and degassed under vacuum at room temperature for 5 min. Then, the TOP was heated to 360 °C under N2 flow. To initiate the reaction, 0.25 mL of a 20 mg/ mL solution of Bi nanocrystals in toluene was swiftly injected into the hot TOP, followed immediately by the Cu, In, and Se precursor solution. The reaction proceeded for 5 min followed by removal of the heating mantle, allowing the products to cool to ∼60 °C. To separate and clean the products, this mixture was centrifuged at 4000 rpm for 5 min. The supernatant was discarded, and the precipitate was redispersed in 30 mL of toluene. This cleaning procedure was repeated three times, and the final product was redispersed in 1 mL of toluene. CdO (0.0048 g, 0.037 mmol) and TDPA (0.0214 g, 0.074 mmol) with 5 g of TOPO were loaded into a 100 mL reaction flask and then heated under nitrogen flow. The mixture turned clear at around 300 °C in TOPO solution. After the solution was kept at the dissolving temperature for 30−60 min, the flask was cooled to 270 °C. To initiate the reaction, 0.5 mL of a 10 mg/mL solution of Bi/CuInSe2 nanorods in toluene was swiftly injected into the hot Cd/TDPA TOPO solution, followed immediately by 0.15 mL of 8 wt % S/TOP solution was injected into the reaction system. The reaction proceeded for 5 min followed by removal of the heating mantle, allowing the products to cool to ∼60 °C. 10 mL of toluene was then injected into the flask, after which this solution was removed from the Schlenk line. To separate and clean the products, this mixture was centrifuged at 4000 rpm for 5 min. The supernatant was discarded, and the precipitate was redispersed in 30 mL of toluene. This cleaning procedure was repeated three times, and the final product was redispersed in 1 mL of toluene. 2.4. Bi/CuInTe2/CuInSe2 Nanorod Heterostructures Synthesis. The preparation of TOP/Te stock solution was similar to the TOP/Se. 1.27 g of Te powder was dissolved in 20 mL of TOP to

particle catalyzers (e.g., Au, Bi, In, and Sn nanoparticles), has relatively mild reaction conditions by using simple operation and inexpensive equipment. By the conventional SLS approach, a variety of crystallized semiconductor nanowires have been successfully synthesized. 31−46 Moreover, branched,47 core−shell and doped48,49 nanowires can be synthesized via the SLS mechanism. Axial nanorod heterostructures composed of CdS and CdSe50 were prepared by the SLS method with the assistance of bismuth nanocrystals.51 However, few studies of the ternary nanorod heterostructures have been reported by the SLS method because of weak stoichiometric control of ternary compounds. Here, we report the fabrication of CdS/CuInSe2 heterostructure nanorods via the SLS method. In detail, CuInSe2 has been proved to be a stable and effective light-harvesting material in thin-film solar cells owing to its high absorption coefficients, optimal band-gaps, and high photostability. CuInSe2 is related to Cu(InxGa1−x)Se2 (CIGS), which holds the solar cell efficiency record of all thin film semiconductors (over 20%).52 Meanwhile, cadmium sulfide (CdS), a typical ptype semiconductor with an intermediate bandgap (2.42 eV) is generally employed as buffer layers between absorber layers (CuInSe2) and window layers (ZnO and ITO), forming a p−n junction. CdS/CuInSe2 heterostructure nanorods have potential properties that can be used as high efficiency single nanorod photovoltaics because of its excellent electrical and optical properties for both CuInSe2 and CdS. Furthermore, this is a general method to fabricate binary or ternary heterostructure nanorods. Further research on the synthesis of ternary axial CuInTe2/ CuInSe2 nanorod heterostructures via a Bicatalyzed SLS growth has been achieved. CuInTe2, an important I−III−VI2 semiconductor with a direct bandgap of 1.02 eV, is a promising material for application in thermoelectric,53 photoluminescence,54 and photovoltaic devices.55−57 CuInTe2 exhibits a larger Bohr radius and a stronger quantum confinement effect than CuInSe2 and CuInS2, owing to its covalent property of tellurium.56 CuInTe2 photovoltaics have been made with power conversion efficiency of up to 5.1%.58 In this paper, diffraction and electron microscopy studies show that both CuInTe2 and CuInSe2 segments exhibit the chalcopyrite (tetragonal) structure, along with the [112] growth direction. Importantly, the heterostructures between CuInTe2 and CuInSe2 portions exhibit a clear epitaxial interface and alloyfree localization of Te and Se within each section. The heterostructures of the two important solar materials will broaden the potential application of photovoltaic nanodevices.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were used as received. Tetrahydrofuran (THF, anhydrous, 99.9%, inhibitor-free), sodium bis(trimethylsilyl)-amide (Na[N(SiMe3)2], 1.0 M in THF), bismuth chloride (BiCl3, 98%), diphenyl ether (DPE, 99%), copper(I) acetate (97%), indium(III) acetate (99.99%), cadmium oxide (CdO, 99.99%), elemental selenium (Se, 99.99%), elemental tellurium (Te, 99.99%), elemental sulfur (S, 99.99%), trioctylphosphine (TOP, 90%), oleic acid (OA, 99%), tri-n-octylphosphine oxide (TOPO, 99%), and n-tetradecylphosphonic acid (TDPA, 98%) were obtained from Aldrich; methanol (MeOH), toluene, and hexane were from Fisher Scientific. Polyvinylpyrrolidone−hexadecane copolymer (Ganex V-216, MW = 7300 g/mol, product ID 72289D) was from ISP Technologies, Inc. 2.2. Bismuth Nanocrystal Synthesis. Tris[bis(trimethylsilyl)amino]bismuth (Bi[N(SiMe3)2]3) was synthesized by previously B


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Inorganic Chemistry prepare a 0.5 M stock solution TOP/Te. The color of TOP/Te is bright yellow. Indium acetate (35.1 mg, 0.125 mmol), copper acetate (15.3 mg, 0.125 mmol), 0.125 mL of OA, and 2 mL of TOP were combined in a 25 mL three-neck flask. The flask was attached to a Schlenk line and degassed while heating to 100 °C. After reaching 100 °C, the solution was held under vacuum for 15 min and stirred vigorously. The flask was then filled with N2 and cooled to room temperature under N2 flow. The Cu and In precursor solution (1 mL) was then combined with 0.25 mL of the 0.5 M TOP/Se stock solution and placed in a syringe as the Cu, In, and Se precursor solution. The concentration of Se was 0.1 M in the mixed solution. The Cu and In precursor solution (0.5 mL) was then combined with 0.125 mL of the 0.5 M TOP/Te stock solution and 0.625 mL TOP. Then the above mixed solution was taken 0.625 mL and placed in a syringe as the Cu, In, and Te precursor solution. The mole ratio of Se/Te in precursor solution was 4:1. TOP (8.5 mL) was added in a separate 100 mL three-neck flask. The flask was attached to the Schlenk line and degassed under vacuum at room temperature for 5 min. Then, the TOP was heated to 360 °C under N2 flow. To initiate the reaction, 0.25 mL of a 20 mg/mL solution of Bi nanocrystals in toluene was swiftly injected into the hot TOP, followed immediately by the Cu, In, and Se precursor solution. After the reaction proceeded for 5 min and the temperature reached 370 °C, the Cu, In, and Te precursor solution was rapidly injected into the reaction system and the reaction proceeded for 10 s followed by removal of the heating mantle rapidly. 20 mL TOP was swiftly injected into the flask and the air quickly blew on the flask, allowing the products to rapidly cool to ∼100 °C. To separate and clean the products, this mixture was centrifuged at 4000 rpm for 5 min. The supernatant was discarded, and the precipitate was redispersed in 30 mL of toluene. This cleaning procedure was repeated three times, and the final product was redispersed in 10 mL of toluene.

catalyzers with the same size (Figure 1a), thus, the heterostructure components could thus be clear distinguished by TEM images even without elemental analysis (Figure 1b). The diameters of the CuInSe2 nanorods range distributes from 40 to 60 nm according to the size of the Bi seed, and the lengths vary from 200 nm up to 1 μm (Figure 1c). There is an epitaxial growth at the heterojunction. From the junction, CdS nanorod diameters gradually reduced to around 20 nm (Figure 1d). Figure 2 shows a high-resolution TEM (HR-TEM) image of CdS/CuInSe2 heterojunction with a clear lattice fringes,

Figure 2. HR-TEM image of CdS/CuInSe2 nanorod heterostructures.

confirming two component material’s high crystallinity and the epitaxial growth. The growth directions of CuInSe2 and CdS segments are parallel to the [112] and [111], respectively. The approximate 0.34 nm lattice spacing agrees well with interplanar distance of (002) direction parallel in the hexagonal wurtzite phase of CdS, which also indicates that [100] is the main growth direction for the CdS part. The X-ray diffraction (XRD) pattern of Bi/CdS/CuInSe2 nanorod heterostructures is shown in Figure 3. The peaks appear at 27.2°, 38.0°, and 39.6°, which correspond well with the (012), (104), and (110) diffractions of the hexagonal bismuth (JCPDS card no. 98-000-0118). Unfortunately, the diffraction peaks of CuInSe2 and CdS are almost complete overlap. Therefore, XRD data cannot confirm the composition of the Bi/CuInTe2/CuInSe2 nanorod heterostructures.

3. RESULTS AND DISCUSSION Representative transmission electron microscopy (TEM) images of CdS/CuInSe2 nanorod heterostructures are shown in Figure 1a−d. The CuInSe2 sections possess larger diameters and stronger contrast than CdS sections grown from the Bi

Figure 3. XRD pattern of Bi/CdS/CuInSe2 nanorod heterostructures. The blue pattern corresponds to hexagonal bismuth (JCPDS card no. 98-000-0118). The green pattern corresponds to wurtzite CdS (JCPDS card no. 01-080-0019). The red pattern corresponds to chalcopyrite CuInSe2 (JCPDS no. 01-089-5649).

Figure 1. TEM images of CdS/CuInSe2 nanorod heterostructures. C


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Inorganic Chemistry

sequential Bi seeding SLS strategy. Uniform Bi/CdS NWs are synthesized by the SLS method (Figure 6a,b). The length of CdS NWs is several micrometers (Figure 6a), with an approximate diameter around 35 nm (Figure 6b). Then, Bi− CdS NWs are used as seeds to induce the subsequent heterostructures growth. Dispersing Bi−CdS NWs in toluene and injecting them to the TOP at 360 °C under N2 atmosphere, followed swiftly by the injection of copper acetate, indium acetate, and Se precursors solution in hot TOP solution. Representative TEM images of products with different additive amount of precursor are shown in Figure 6c−f. From the different contrast of two segments in Figure 6c,e, it seems that the Bi/CuInSe2/CdS heterostructure NWs have been synthesized. The diameters of upper segments range from 40 to 60 nm, and the lengths vary from 200 to 500 nm depending on the additive amount of precursors (Figure 6d,f). The composition of the product obtained by using Bi−CdS NWs as seeds to induce the subsequent heterostructure growth is confirmed by the element maps from EDS (Figure 7a−f). Cu element is only distributed at the upper segment of NWs. However, except for the distribution at the upper segment, some In elements are diffused into the lower segment of NWs. The Se element is distributed throughout the whole NWs. More interesting, the Cd element is also diffused from lower to upper, while S is only distributed at the lower segment. Thus, EDS results clearly displayed the detailed chemical components of two different parts, which indicated that it seems impossible to get idealized Bi/CuInSe2/CdS heterostructure due to the different reactivities of precursors and the different diffusion speeds of components. Furthermore, the XRD pattern and EDS data of this sample (Figure S2 and Table S1) both confirmed that CdS and Cu9In4 were synthesized. Representative TEM images of the CuInSe2 nanorod and Bi/CuInTe2/CuInSe2 nanorod heterostructures are shown in Figure 8a,b, respectively. There is an epitaxial growth at the heterostructure and the CuInTe2 sections always have a stronger contrast and larger diameters than CuInSe2 sections (Figure 8b). The heterostructure could be clearly distinguished by TEM even without elemental analysis. The diameters of CuInSe2 nanorods distribute from 40 to 60 nm according to the size of the Bi seed, and the lengths vary from 500 nm up to few microns. The CuInTe2 nanorod diameters and lengths are more uniform because of the shorter reaction time than CuInSe2. The lengths vary from 80 to 120 nm, and the diameters range from 50 to 80 nm. The structure of the Bi/CuInTe2/CuInSe2 nanorod heterostructures is confirmed using XRD (Figure 9). The peaks appear at 27.2°, 38.0°, and 39.6°, which correspond well with the (012), (104), and (110) diffractions of the hexagonal bismuth (JCPDS card no. 98-000-0118). The peaks with 2θ values of 24.9°, 41.3°, and 48.8° are assigned to the (112), (220)/(204), and (116)/(312) diffractions of tetragonal (chalcopyrite) CuInTe2 (JCPDS card no. 03-065-2747). The peaks at 26.6°, 44.1°, and 52.3° correspond well with the (112), (220)/(204), and (116)/(312) diffractions of tetragonal (chalcopyrite) CuInSe2 (JCPDS card no. 01-089-5649). XRD data confirms the co-existence of three components. The CuInTe2 reaction time affects the final product a lot. When the CuInTe2 reaction time is extended from 10 s to 5 min, CuInSexTe(2−x) nanowires were obtained instead of Bi/ CuInTe2/CuInSe2 nanorod heterostructures. In addition, the injection order of CuInSe2 and CuInTe2 precursors is also a key factor of the obtained CuInTe2/CuInSe2 heterostructure.

Energy-dispersive X-ray (EDX) spectrometry line scan further confirms the content change of the specific atomic composition in the heterostructures. The resulting scanning line of the heterostructures obtained in scanning TEM (STEM) mode are illustrated in Figure 4. Cu, In, and Se are

Figure 4. STEM dark field image and EDX spectrometry line scan of a Bi/CdS/CuInSe2 nanorod heterostructure.

only distributed at the left segment of the heterostructure. The ratio of Cu/In/Se is nearly 1:1:2 which is consistent with the ratio of reactants added. Meanwhile, Cd and S are distributed at the right segment. The Cd/S ratio is approximately 1:1. The elemental line scan convincingly confirms that CuInSe2 and CdS segments of the heterostructure, respectively. The composition of the CdS/CuInSe2 nanorod heterostructures is ascertained by the element maps from EDX spectroscopy (Figure 5a−f). According to Figure 5a, Cd and S

Figure 5. STEM and EDX elemental mapping of Cu, In, Se, Cd, and S for Bi/CdS/CuInSe2 nanorod heterostructures.

are only distributed at the right segment (Figure 5b,c), while Cu, In, and Se distributed at the left segment (Figure 5d−f). Thus, the EDX elemental mapping of a nanowire unambiguously confirms the elements distribution of the Bi/CdS/ CuInSe2 nanorod heterostructure. Next, we have attempted inversion growth to synthesize the Bi/CuInSe2/CdS heterostructure NWs by utilizing the D


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Inorganic Chemistry

Figure 6. TEM images of (a,b) Bi−CdS NWs, and the products obtained by using Bi−CdS NWs as seeds to induce the subsequent heterostructures growth, (c,d) 0.25 mL Cu, In, and Se precursor solution, (e,f) 0.5 mL Cu, In, and Se precursor solution (EDS data was collected from red dots ① to ⑤ and shown in Supporting Information Table S1).

Figure 7. STEM and EDX elemental mapping of Cu, In, Se, Cd, and S for the product obtained by using Bi−CdS NWs as seeds to induce the subsequent heterostructures growth. The dosage of Cu, In, and Se precursor solution is 0.5 mL.

Figure 9. XRD pattern of Bi/CuInTe2/CuInSe2 nanorod heterostructures.

fast Fourier transform (FFT) patterns (Figure 10b) clearly exhibit pairs of diffraction peaks along the different lattice directions and indicate that the CuInTe2/CuInSe2 interface is formed without any alloying, and the growth directions of both CuInTe2 and CuInSe2 segments are parallel to the [112] direction. Representative HR-TEM image of the Bi/CuInTe2 interface and corresponding FFT patterns are shown in Figure 10c,d, respectively. The lattice spacing of the dark-contrast domains is 3.9 Å, which matches best with the (003) plane of the hexagonal bismuth crystal structure. This, evidenced by the EDS data in Figure 11, confirmed that the dark-contrast domains are mainly bismuth seed, which has been observed in our prior studies of the CuInTe2 NWs synthesis, and the growth direction of CuInTe2 is still parallel to the [112] direction.62 The composition of the Bi/CuInTe2/CuInSe2 nanorod heterostructure is ascertained by the element maps from EDX spectroscopy (Figure 11a−f). The Bi seed is limited to the tip of the wire. Cu and In are distributed throughout the whole nanorod, while Te is only distributed at the upper segment of nanorod, and Se is only distributed at the lower segment of the nanorod. Thus, the EDX elemental mapping of a nanorod unambiguously confirms the element distribution of the Bi/CuInTe2/CuInSe2 heterostructure.

Figure 8. TEM images of (a) CuInSe2 nanorod and (b) Bi/CuInTe2/ CuInSe2 nanorod heterostructures.

If we reverse the injection order of CuInSe2 and CuInTe2 precursors, the product is only Bi−CuInTe2. These data indicate that CuInTe2 has higher reactivity than CuInSe2. Therefore, in this SLS system, it seems impossible to make the Bi/CuInSe2/CuInTe2 heterostructure due to the different reactivities of CuInTe2 and CuInSe2. Figure 10a shows a HR-TEM image of the CuInTe2/ CuInSe2 junction with clear crystal lattice, confirming the high crystallinity and epitaxial growth of the two components. The E


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Inorganic Chemistry

Figure 12. EDX spectrometry line scan of a Bi/CuInTe2/CuInSe2 nanorod heterostructure showing the change of composition as a function of the position of detection. Insets: STEM dark field image of a Bi/CuInTe2/CuInSe2 nanorod heterostructure.

materials science research but also promising in the further development in nanodevices and nanosolar cells. We also learnt from the “failed” experiments of Bi/CuInSe2/ CdS and Bi/CuInSe2/CuInTe2 synthesis attempts which indicate that the growth of axial heterojunctions by the SLS method is not common. Parameters such as precursor activity, growth order, reaction time, and temperature highly affect the morphology and component of the final products. Therefore, further research of the SLS mechanism is interesting and significative.

Figure 10. (a) HR-TEM image of the CuInTe2/CuInSe2 junction, (b) FFT taken from the whole image (a). (c) HR-TEM image of the Bi/CuInTe2 junction, (d) FFT taken from the whole image (c).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02870.

Figure 11. STEM and EDX elemental mapping of Cu, In, Se, Te, and Bi for the Bi/CuInTe2/CuInSe2 nanorod heterostructure.

TEM image of Bi seeds, XRD data of Bi/CuInSe2/CdS synthesis attempts and EDS data collected from red dots ① to ⑤ in Figure 6b,d,f (PDF)

The EDX spectrometry line scan further confirms the content change of the specific atomic composition of the heterojunction. The elemental line scan profiles are illustrated in Figure 12. Bi is only distributed the tip of the nanorod. Cu and In are distributed throughout the whole structure and the content of In is larger than that of Cu, which indicates the Bi/ CuInTe2/CuInSe2 nanorod heterostructure made by this SLS process always tends to be Cu deficient. The elemental line scan convincingly confirms that Te and Se only locally appear in the CuInTe2 and CuInSe2 segments, respectively. At the junction, the content of Te decreases swiftly and the signal of Se increases rapidly, which further ascertains the heterostructure of the Bi/CuInTe2/CuInSe2 nanorod.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiang Du: 0000-0001-8949-6230 Author Contributions

All authors have given approval to the final version of the manuscript. Funding

This work was supported by the National Key Research and Development Program of China (grant no. 2016YFB0301101), the Robert A. Welch Foundation (no. F-1464) and the National Science Foundation Industry/University Cooperative Research Center on Next Generation Photovoltaics (no. IIP1134849).

4. CONCLUSIONS In conclusion, we successfully demonstrated the SLS mechanism for obtaining axial Bi/CdS/CuInSe2 and Bi/ CuInTe2/CuInSe2 heterostructures. This general mechanism possesses a great potential for the synthesis of binary, ternary, even the quaternary nanorod with built-in p−p, p-n, or n−n heterojunctions. This research is not only significant in the

Notes

The authors declare no competing financial interest. F


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Inorganic Chemistry

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