Synthesis, Crystal Structure, UV–Vis Adsorption Properties

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Article Cite This: ACS Omega 2018, 3, 14021−14026

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Synthesis, Crystal Structure, UV−Vis Adsorption Properties, Photoelectric Behavior, and DFT Computational Study of AllInorganic and Lead-Free Copper Halide Salt K2Cu2Cl6 Huawei Zhou,*,† Xuejing Liu,‡ Guohang He,† Lin Fan,† Shaozhen Shi,† Jiazhen Wei,† Wenli Xu,† Cang Yuan,† Ning Chai,† Baoli Chen,† Yingtian Zhang,† Xianxi Zhang,† Jinsheng Zhao,† Xinting Wei,† Jie Yin,*,† and Dongxu Tian*,§

ACS Omega 2018.3:14021-14026. Downloaded from pubs.acs.org by 95.181.176.30 on 10/25/18. For personal use only.

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School of Chemistry and Chemical Engineering, College of Materials Science and Engineering, Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, Liaocheng University, Liaocheng 252000, China ‡ Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, People’s Republic of China § State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, People’s Republic of China S Supporting Information *

ABSTRACT: In this study, all-inorganic copper halide salt K2Cu2Cl6 singlecrystal and thin films were prepared. The single-crystal diffraction data belonged to the monoclinic K2Cu2Cl6 (space group = P2(1)/C, unit cell parameters of a = 4.0340 Å, b = 13.7987 Å, c = 8.7445 Å, α = 90.000, β = 97.123, and γ = 90.000). As far as we know, this is the first study of the copper halide salt K2Cu2Cl6 for optoelectronic applications. The band gap of K2Cu2Cl6 is calculated to be approximately 1.85 eV. A low-cost photodetector based on the K2Cu2Cl6 thin film was efficient under different monochromatic light from 330 to 390 nm with different chopping frequencies (1.33−30 Hz). Density functional theory (DFT) computational results indicate that the valence bands (VBs) and conduction bands (CBs) are shifted up in energy using the orbital-dependent correction to the DFT energy. Partial density of states reveals that the VBs and narrow CBs are derived from the hybrid orbitals of Cu2+ 3d and Cl− p, respectively.

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INTRODUCTION

RESULTS AND DISCUSSION Herein, all-inorganic copper halide salt K2Cu2Cl6 single-crystal and thin films were prepared by a mixture (as shown in Figure 1a) of 1 mM KCl and 1 mM CuCl2 in an aqueous solution. Figure 1b shows the synthesized dark-red single crystals. The single structure is solved by a direct method and expanded using difference Fourier techniques with the SHELXL-97 program. The single-crystal diffraction data are assigned to the monoclinic K2Cu2Cl6 (space group = P2(1)/C, unit cell parameters of a = 4.0340 Å, b = 13.7987 Å, c = 8.7445 Å, α = 90.000, β = 97.123, and γ = 90.000). The schematic diagram of the unit cell structure is shown in Figure 1c (ball-stick), 1d (polyhedron), which indicates that the Cu2Cl62− dimers are located at the corners and center of the unit cell in the b−c plane. The structural refinement and detailed crystal data for the K2Cu2Cl6 single crystals can be found in Tables 1 and 2 and Tables S1 and S2. To characterize the absorption property of K2Cu2Cl6, UV− vis absorption spectroscopy was carried out. Figure 2a shows

As one kind of metal halide salts (MHSs), lead halide salts (LHSs) have attracted widespread attention because of organic−inorganic LHSs.1,2 Recently, the power conversion efficiencies (PCEs) of solid thin-film solar cells based on organic−inorganic LHS materials are more than 22%,3 which is close to the PCEs of the commercially silicon solar cells. In addition, organic−inorganic LHSs present promising performance in field-effect transistors,4 light-emitting diodes,5 and so on. However, using organic−inorganic LHS involves a number of challenges. First, the toxicity of Pb in organic−inorganic LHSs cannot meet environmental friendliness.6 Second, the structure of organic−inorganic LHSs can be easily destroyed by moisture and heat, resulting in poor stability of the photoelectronic device.7 Thus, it is necessary to develop a high-performance, low-cost, corrosion-resistant, nonprecious all-inorganic and lead-free MHS. Recently, numerous researchers have developed alternative lead-free MHS materials,8 such as tin (Sn) halide salts,9 germanium (Ge) halide salts,10 bismuth (Bi) halide salts,11 manganese (Mn) halide salts,12,13 copper halide salts (Cu),14 and Sb(V) halide complexes15 for optoelectronic applications. © 2018 American Chemical Society

Received: June 13, 2018 Accepted: September 11, 2018 Published: October 25, 2018 14021

DOI: 10.1021/acsomega.8b01337 ACS Omega 2018, 3, 14021−14026

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Figure 3b shows the corresponding crystal faces of each diffraction peak. Strong peaks at 12.08°, 12.94°, 16.52°, 26.70°, and 37.90° are assigned to the (011), (020), (021), (111), and (132) planes, respectively. The results indicate that the surface of the TiO2 electron-collecting layer induces the growth of K2Cu2Cl6 along (132). To measure the uniformity and thickness of each layer in fluorine-doped tin dioxide (FTO)/ TiO2/K2Cu2Cl6 device, we tested the scanning electron microscopy (SEM) image of the cross-sectional structure, which is shown in Figure 3c. The thickness of FTO is 348.54 nm. The thickness of the K2Cu2Cl6 film is from 18.45 to 142.66 nm. Moreover, the thickness of the dense-TiO2 film on the FTO conductive glass is 33.54 nm. Also, the voids between the TiO2 particles are very small, which will prevent short circuit. Under AM1.5, 100 mW/cm2 simulated illumination, we studied the photoresponse properties of the device (Figure 4a) by recording the photocurrent−time characteristics. An evident Ion/Ioff photoresponse is observed, as shown in Figure 4b. We estimated the Q generated in the initial 5 s illumination by integrating the I−t area, as shown in Figure 4c. Q is calculated to be 66.4 nC in the initial 5 s. To probe the photoresponse of the device under the monochromatic light, we recorded the Ion/Ioff photoresponse under different wavelengths with a flashlight frequency of 1.33 Hz, as shown in Figure 5. The Ion/Ioff photoresponse is not observed under 300 nm. When the wavelength is shifted to 330 nm, the I on /I off photoresponse is obvious. The Ion/I off photoresponse clearly decreases when the light wavelength is tuned to 390 nm. However, the device still has a photoelectric response at this wavelength. The Ion/Ioff photoresponse is irregular when the wavelength of light changed to 400 nm. Although the K2Cu2Cl6 thin film absorbs infrared light, we cannot measure the photoelectric response in the infrared wavelength range. The reason may be the unsuited structure or mismatched energy level. We will optimize the unsuited structure or mismatched energy level to make it infrared-light response in another study. We studied the Ion/Ioff photoresponse of the device at 330 nm with different flashlight frequencies, as shown in Figure 6. The experimental results show that the response of the device at 1.33 Hz is obvious. The photoelectric response device at 20 Hz is also obvious. The regularity of the device at 40 Hz becomes weak. To better explain the density of states (DOS), the DOS and band structure of K2Cu2Cl6 were calculated with spin-polarized density functional theory (DFT). The results in Figure 7 indicate that the band structure of K2Cu2Cl6 along the highly symmetric points has a band gap of about 0.26 eV. It is wellknown that the calculated band gaps based on the general Kohn−Sham theory are often to be underestimated compared to the experimental value 1.70 eV. The total magnetic moment is 3.775 μB. Additionally, we considered the K2Cu2Cl6 band gaps obtained with these U−J values of 2, 3, and 4 eV as 0.39, 0.45, and 0.47 eV, respectively. The total magnetic moment is increased to 4.00 μB with the U−J value of 4 eV. It is worthwhile that the valence bands (VBs) and conduction bands (CBs) are shifted up in energy using the orbitaldependent correction to the DFT energy. To further reveal the factors controlling the band gap trends, DOS and partial DOS (PDOS) were calculated. PDOS reveals that the VBs and narrow CBs are derived from the hybrid orbitals of Cu2+ 3d and Cl− p, respectively. Thus, the small gap of 0.26 eV is

Figure 1. (a) Mixture of 1 mM KCl and 1 mM CuCl2 in deionized water. (b) Picture of the K2Cu2Cl6 single crystal. (c) Illustration of the unit cell structure (ball-stick). (d) Schematic diagram of the cell structure (polyhedron).

Table 1. Crystal Data and Structure Refinement for K2Cu2Cl6 compound

K2, Cu2, Cl6

chemical composition crystal system space group, Z temperature/K a, Å b, Å c, Å α, deg β, deg γ, deg volume, Å3 ρ, Mg/m3 μ, mm−1 total reflections, R(int) data/restraints/parameters final R1, wR2 [I > 2γ(I)]

K, Cu, Cl monoclinic P2(1)/C 298(2) K 4.0340(3) 13.7987(13) 8.7445(7) 90 97.1230(10) 90 483.00(7) 4, 2.874 6.844 2298/846, 0.0715 846/0/47 R1 = 0.0428, wR2 = 0.1026

the UV−vis absorption curve of K2Cu2Cl6 single crystals. Strong absorption from 300 to 568 nm is observed. The absorbance is decreased quickly from 568 to 628 nm, which indicates the existence of the band gap. It is very interesting that the absorbance is relatively strong from 700 to 1100 nm. The reason may be the existence of the Jahn−Teller effect or double spin chains in the K2Cu2Cl6 crystal structure. To obtain the band gap of K2Cu2Cl6, diffuse reflection spectroscopy of K2Cu2Cl6 single crystals was carried out as shown in Figure 2b. According to the previous literature,16 K2Cu2Cl6 can be regarded as direct allowed transition. Thus, Eg for the thin film is determined to be approximately 1.85 eV from the intercept of the linear part of the [F(R)hv]2 plot on the X-axis. The K2Cu2Cl6 thin films (Figure 3a) were formed by a spinning−coating mixture of 1 mM KCl and 1 mM CuCl2 in an aqueous solution. To determine the structure of the film, we measured the X-ray diffraction (XRD) of the film. Figure 3b shows the XRD results of the K2Cu2Cl6 thin film on TiO2. The diffraction peaks of K2Cu2Cl6 films at TiO2 are basically corresponding to the diffraction peaks obtained by singlecrystal diffraction data. 14022

DOI: 10.1021/acsomega.8b01337 ACS Omega 2018, 3, 14021−14026

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Table 2. Bond Lengths [Å] and Angles [deg] for K2Cu2Cl6 bond Cu(1)−Cl(3) Cu(1)−Cl(2) Cu(1)−Cl(1) Cu(1)−Cl(1)#1 Cu(1)−K(1)#2 Cl(1)−Cu(1)#1 Cl(1)−K(1)#3 Cl(1)−K(1)#2 Cl(3)−K(1)#4 Cl(3)−K(1) Cl(3)−K(1)#5 Cl(3)−K(1)#6 Cl(2)−K(1)#2 Cl(2)−K(1) Cl(2)−K(1)#7 K(1)−Cl(3)#8 K(1)−Cl(2)#2 K(1)−Cl(2)#7 K(1)−Cl(1)#9 K(1)−Cl(3)#10 K(1)−Cl(3)#11 K(1)−Cl(1)#2 K(1)−Cu(1)#2 K(1)−K(1)#11 K(1)−K(1)#6 Cl(3)−Cu(1)−Cl(2) Cl(3)−Cu(1)−Cl(1) Cl(2)−Cu(1)−Cl(1) Cl(3)−Cu(1)−Cl(1)#1 Cl(2)−Cu(1)−Cl(1)#1 Cl(1)−Cu(1)−Cl(1)#1 Cl(3)−Cu(1)−K(1)#2 Cl(2)−Cu(1)−K(1)#2 Cl(1)−Cu(1)−K(1)#2 Cl(1)#1−Cu(1)−K(1)#2 Cu(1)−Cl(1)−Cu(1)#1 Cu(1)−Cl(1)−K(1)#3 Cu(1)#1−Cl(1)−K(1)#3 Cu(1)−Cl(1)−K(1)#2 Cu(1)#1−Cl(1)−K(1)#2 K(1)#3−Cl(1)−K(1)#2 Cu(1)−Cl(3)−K(1)#4 Cu(1)−Cl(3)−K(1) K(1)#4−Cl(3)−K(1) Cu(1)−Cl(3)−K(1)#5 K(1)#4−Cl(3)−K(1)#5 K(1)−Cl(3)−K(1)#5 Cu(1)−Cl(3)−K(1)#6 K(1)#4−Cl(3)−K(1)#6 K(1)−Cl(3)−K(1)#6 K(1)#5−Cl(3)−K(1)#6 Cu(1)−Cl(2)−K(1)#2 Cu(1)−Cl(2)−K(1) K(1)#2−Cl(2)−K(1) Cu(1)−Cl(2)−K(1)#7 K(1)#2−Cl(2)−K(1)#7 K(1)−Cl(2)−K(1)#7 Cl(3)#8−K(1)−Cl(3) Cl(3)#8−K(1)−Cl(2)#2 Cl(3)−K(1)−Cl(2)#2 Cl(3)#8−K(1)−Cl(2)

bond lengths [Å] and angles [deg]

bond

bond lengths [Å] and angles [deg]

2.2510(13) 2.2806(13) 2.3120(13) 2.3314(13) 3.9421(13) 2.3314(13) 3.2935(17) 3.6906(15) 3.0859(16) 3.1112(16) 3.4060(17) 3.4606(16) 3.1797(16) 3.2045(17) 3.2116(16) 3.0859(16) 3.1797(16) 3.2116(16) 3.2935(17) 3.4060(17) 3.4606(16) 3.6906(15) 3.9421(13) 4.0340(3) 4.0340(3) 93.35(4) 175.85(5) 90.73(5) 91.89(5) 172.99(5) 83.99(5) 116.74(4) 53.75(4) 66.51(4) 127.24(4) 96.01(5) 164.76(5) 98.91(4) 78.42(4) 135.40(5) 88.71(3) 107.03(5) 102.97(5) 149.92(5) 105.97(5) 76.66(4) 97.15(4) 101.09(5) 96.51(4) 75.53(3) 152.93(4) 90.90(4) 99.54(5) 111.31(4) 153.13(5) 78.27(4) 107.30(4) 135.97(4) 125.37(5) 90.60(4) 147.75(5)

Cl(3)−K(1)−Cl(2) Cl(2)#2−K(1)−Cl(2) Cl(3)#8−K(1)−Cl(2)#7 Cl(3)−K(1)−Cl(2)#7 Cl(2)#2−K(1)−Cl(2)#7 Cl(2)−K(1)−Cl(2)#7 Cl(3)#8−K(1)−Cl(1)#9 Cl(3)−K(1)−Cl(1)#9 Cl(2)#2−K(1)−Cl(1)#9 Cl(2)−K(1)−Cl(1)#9 Cl(2)#7−K(1)−Cl(1)#9 Cl(3)#8−K(1)−Cl(3)#10 Cl(3)−K(1)−Cl(3)#10 Cl(2)#2−K(1)−Cl(3)#10 Cl(2)−K(1)−Cl(3)#10 Cl(2)#7−K(1)−Cl(3)#10 Cl(1)#9−K(1)−Cl(3)#10 Cl(3)#8−K(1)−Cl(3)#11 Cl(3)−K(1)−Cl(3)#11 Cl(2)#2−K(1)−Cl(3)#11 Cl(2)−K(1)−Cl(3)#11 Cl(2)#7−K(1)−Cl(3)#11 Cl(1)#9−K(1)−Cl(3)#11 Cl(3)#10−K(1)−Cl(3)#11 Cl(3)#8−K(1)−Cl(1)#2 Cl(3)−K(1)−Cl(1)#2 Cl(2)#2−K(1)−Cl(1)#2 Cl(2)−K(1)−Cl(1)#2 Cl(2)#7−K(1)−Cl(1)#2 Cl(1)#9−K(1)−Cl(1)#2 Cl(3)#10−K(1)−Cl(1)#2 Cl(3)#11−K(1)−Cl(1)#2 Cl(3)#8−K(1)−Cu(1)#2 Cl(3)−K(1)−Cu(1)#2 Cl(2)#2−K(1)−Cu(1)#2 Cl(2)−K(1)−Cu(1)#2 Cl(2)#7−K(1)−Cu(1)#2 Cl(1)#9−K(1)−Cu(1)#2 Cl(3)#10−K(1)−Cu(1)#2 Cl(3)#11−K(1)−Cu(1)#2 Cl(1)#2−K(1)−Cu(1)#2 Cl(3)#8−K(1)−K(1)#11 Cl(3)−K(1)−K(1)#11 Cl(2)#2−K(1)−K(1)#11 Cl(2)−K(1)−K(1)#11 Cl(2)#7−K(1)−K(1)#11 Cl(1)#9−K(1)−K(1)#11 Cl(3)#10−K(1)−K(1)#11 Cl(3)#11−K(1)−K(1)#11 Cl(1)#2−K(1)−K(1)#11 Cu(1)#2−K(1)−K(1)#11 Cl(3)#8−K(1)−K(1)#6 Cl(3)−K(1)−K(1)#6 Cl(2)#2−K(1)−K(1)#6 Cl(2)−K(1)−K(1)#6 Cl(2)#7−K(1)−K(1)#6 Cl(1)#9−K(1)−K(1)#6 Cl(3)#10−K(1)−K(1)#6 Cl(3)#11−K(1)−K(1)#6 Cl(1)#2−K(1)−K(1)#6 Cu(1)#2−K(1)−K(1)#6

62.91(4) 68.69(4) 81.78(4) 135.22(5) 78.27(4) 72.70(4) 62.07(4) 73.90(4) 144.18(5) 126.35(4) 134.98(5) 76.66(4) 89.54(4) 77.44(4) 135.02(5) 128.58(4) 70.49(4) 88.95(4) 75.53(3) 137.70(5) 69.43(4) 84.08(4) 70.22(4) 140.46(4) 69.35(4) 141.08(5) 56.22(3) 113.68(4) 63.15(3) 119.91(4) 65.62(3) 142.30(4) 98.37(4) 123.76(4) 35.34(2) 78.62(3) 47.15(3) 154.97(4) 90.66(3) 128.17(4) 35.07(2) 55.24(3) 123.84(3) 128.78(3) 92.85(3) 50.51(3) 85.62(3) 131.90(3) 48.31(3) 94.51(3) 95.829(18) 124.76(3) 56.16(3) 51.22(3) 87.15(3) 129.49(3) 94.38(3) 48.10(3) 131.69(3) 85.49(3) 84.171(18)

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Figure 5. Photocurrent density−time characteristics of the FTO/ TiO2/K2Cu2Cl6/carbon electrode device under different wavelengths (300, 330, 390, and 400 nm) with a flashlight frequency of 1.33 Hz.

Figure 2. (a) UV−vis absorption spectrum of the K2Cu2Cl6 single crystal. (b) Diffuse reflection spectrum of the K2Cu2Cl6 single crystal. (c) Kubelka−Munk spectrum of the K2Cu2Cl6 single crystal.

Figure 3. (a) Picture of K2Cu2Cl6 thin films on the TiO2 layer. (b) Calculated powder XRD profiles from the K2Cu2Cl6 single crystal and K2Cu2Cl6 thin films on the TiO2 layer. (c) Amplified cross-sectional SEM image of the FTO/TiO2/K2Cu2Cl6 device.

Figure 6. Photocurrent density−time characteristics of the FTO/ TiO2/K2Cu2Cl6/carbon electrode device under different flashlight frequencies (1.33, 20, and 40 Hz) with a 330 nm light beam.

limited to the contribution of the hybrid orbitals of Cu2+ 3d and Cl− p.

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CONCLUSIONS In summary, all-inorganic copper halide salt K2Cu2Cl6 singlecrystal and thin films were prepared. The single-crystal diffraction data belonged to the monoclinic K2Cu2Cl6 (space group = P2(1)/C). The band gap of K2Cu2Cl6 is determined to be approximately 1.85 eV. A low-cost and efficient photodetector based on the K2Cu2Cl6 thin film under different monochromatic light from 330 to 390 nm with different chopping frequencies (1.33−30 Hz) is beneficial for future industrial production. The performance is related to narrow CBs derived from the hybrid orbitals of Cu2+ 3d and Cl p.

Figure 4. (a) Schematics of fabrication of the FTO/TiO2/K2Cu2Cl6/ carbon electrode device. (b) Photocurrent density−time characteristics of the FTO/TiO2/K2Cu2Cl6/carbon electrode device under the illumination of AM1.5 (1 sun; 100 mW/cm2) using a solar simulator. (c) The charge quantity (Q) generated by light is estimated from the integration of the area under the measured photocurrent density decay curve in initial 5 s.

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EXPERIMENTAL SECTION Fabrication and Characterizations of FTO/TiO2/ K2Cu2Cl6/Carbon Electrode Devices. The devices of the FTO/TiO2/K2Cu2Cl6/carbon electrode were fabricated and characterized according to the previous literature.16 The TiO2 14024

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Figure 7. Calculated band structure and DOS and PDOS projected on the constituted atoms of K2Cu2Cl6 using Perdew−Burke−Ernzerhof (PBE) functionals with spin polarization. Note that the band gaps are often underestimated in PBE calculations.

Notes

layer was covered by the K2Cu2Cl6 layer by spin-coating the aqueous solution of 170 mg CuCl2 and 75 mg KCl (3000 rpm for 30 s). The active area of the FTO/TiO2/K2Cu2Cl6/carbon electrode devices was 1 cm2. The photoresponse of the devices was measured under the bias voltage of 0.00 V. Growth of K2Cu2Cl6 Single Crystals. Typically, 170 mg of CuCl2 and 75 mg of KCl were dissolved in 4 mL of deionized water. The above solution was heated to 80 °C via an oil bath. Then the solution was transported to an oven and heated to 80 °C. The dark-red single crystal will be observed by slow evaporation (from 80 to 40 °C for 20 h). Single-Crystal Characterization. The characterization of the single crystal was according to the previous literature.16 Computational Methods. The DOS and band structure of K2Cu2Cl6 were calculated with spin-polarized DFT implemented with the Vienna ab initio simulation package.17,18 The interaction between the electron and the ion is described by the projector-augmented wave method.19 The PBE20 functional is applied to describe the electron exchange− correlation and the kinetic energy cutoff is 520 eV. The experimental lattice parameters for K2Cu2Cl6 are used with a = 4.034 Å, b = 13.7987 Å, c = 8.7445 Å, and β = 97.123°. The atomic positions were optimized until the force on each atom was less than 0.01 eV/Å. The Monkhorst−Pack k point used in the structural optimization and calculations of DOS were 6 × 2 × 3 and 10 × 4 × 6, respectively. The band structure of K2Cu2Cl6 was calculated along the highly symmetrical point in the Brillouin zone.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Shandong Province Natural Science Foundation (grant nos. ZR2016BQ20, BS2015NJ013, ZR2014BQ010, and ZR2016BQ21), Colleges and Universities in Shandong Province Science and Technology projects (grant nos. J16LC05 and J17KA097), the Science and Technology Innovation Foundation for the University or College Students (grant no. 26312160502), the National Science and Technology Innovation Foundation for the University or College Students (grant no. 201510447021), the Research Fund for the Doctoral Program of Liaocheng University (grant no. 31805), the National Natural Science Foundation of China (grant nos. 21503104, 21601078, and 21401095), and the National Basic Research Program of China (grant no. 2011CBA00701).

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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/acsomega.8b01337.

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REFERENCES

Atomic coordinates (×104) and equivalent isotropic displacement parameters (A2 × 103) for K2Cu2Cl6 (PDF) Crystallographic data of K2Cu2Cl6 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Z.). *E-mail: [email protected] (J.Y.). *E-mail: [email protected] (D.T.). ORCID

Huawei Zhou: 0000-0002-9428-5400 14025

DOI: 10.1021/acsomega.8b01337 ACS Omega 2018, 3, 14021−14026

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DOI: 10.1021/acsomega.8b01337 ACS Omega 2018, 3, 14021−14026