Doping Zn2+ in CuS Nanoflowers into Chemically Homogeneous Zn0

Jun 14, 2016 - Doping Zn2+ in CuS nanoflower into chemically homogeneous superlattice crystal structure is proposed to convert p-type CuS semiconducto...
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Doping Zn2+ in CuS Nanoflowers into Chemically Homogeneous Zn0.49Cu0.50S1.01 Superlattice Crystal Structure as High-Efficiency n‑Type Photoelectric Semiconductors Peipei Wang,†,‡ Yuanhao Gao,*,† Pinjiang Li,† Xiaofei Zhang,‡ Helin Niu,§ and Zhi Zheng† †

Key Laboratory of Micro-Nano Materials for Energy Storage and Conversion of Henan Province and Institute of Surface Micro and Nano Materials, Xuchang University, Xuchang 461000, China ‡ College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450000, China § Department of Chemistry, Anhui University, Hefei 230039, China S Supporting Information *

ABSTRACT: Doping Zn2+ in CuS nanoflower into chemically homogeneous superlattice crystal structure is proposed to convert p-type CuS semiconductor to an n-type CuS semiconductor for significantly enhanced photoelectric response performance. In this study, the chemically homogeneous Zn-doped CuS nanoflowers (Zn0.06Cu0.94S, Zn0.26Cu0.73S1.01, Zn0.36Cu0.62S1.02, Zn0.49Cu0.50S1.01, Zn0.58Cu0.40S1.02) are synthesized by reacting appropriate amounts of CuCl and Zn(Ac)2·2H2O with sulfur powders in ethanol solvothermal process. By tuning the Zn/Cu atomic ratios to ∼1:1, the chemically homogeneous Zn-doped CuS nanoflowers could be converted to the perfect Zn0.49Cu0.50S1.01 superlattice structure, corresponding to the periodic Cu−S−Zn atom arrangements in the entire crystal lattice, which can induce an effective built-in electric field with n-type semiconductor characteristics to significantly improve the photoelectric response performance, such as the lifetime of photogenerated charge carriers up to 6 × 10−8−6 × 10−4 s with the transient photovoltage (TPV) response intensity to ∼44 mV. This study reveals that the Zn2+ doping in CuS nanoflowers is a key factor in determining the superlattice structure, semiconductor type, and the dynamic behaviors of charge carriers. KEYWORDS: copper sulfide, nanoflowers, superlattice, photoelectric property, photoluminescence, transient photovoltage

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INTRODUCTION

Copper sulfide (CuS), as an important p-type semiconductor, has extensive prospects for application as photocatalysts, supercapacitors, optical sensors, and photovoltaic devices, as well use in microwave absorption, because of its excellent optical, electronic, and other physical properties.14−18 Various topological structures of CuS nanomaterials with their relative properties have been reported, such as nanorods, nanotubes, nanoplates, nanowire arrays, and nanoflowers.14−18 Compared to one-dimensional (1D) or two-dimensional (2D) CuS structures, the three-dimensional (3D) CuS nanoflower has received more attention, because of its high specific surface area for effective light absorption, as well as interconnected networks for effective electronic transportation in the 3D scaffolds.19,20 However, the low quantum yield of photogenerated charge carriers still hindered its practical applications.9 Therefore, to improve CuS photoelectric properties by fabricating 3D nanoflowers, one should pay close attention to increasing the overall efficiency of generation, separation, and transfer of the photogenerated charges.

Studies on high-efficient photoelectric semiconductor materials for solving solar energy supply problems are an attractive process. Many semiconductor materials, such as metal oxides, sulfides, nitrides, and their mixed solid solutions could act as sunlight absorbers responding to almost the entire solar spectrum.1−5 However, they cannot meet the practical needs, because of the short lifetime of their photogenerated electron− hole pairs. To date, some efforts that have been aimed at the heterostructure semiconductor materials have achieved many outstanding results, such as Zn-doped In2O3−SnO2 nanowires,6 CdS/CdS:SnS2 microwires,7 ZnO/BiOI heterostructures,8 and BiVO4/Bi2S3 mesoporous discoids.9 In this case, the photoelectric response performances are significantly enhanced because the heterointerface induce the staggered alignment of band edges, and further improve spatial separation of photogenerated charge carriers.9−13 However, these semiconductor heterostructures are often inconvenient to synthesize, which made them difficult to use, for practical applications. Therefore, it is very important to develop an new strategy toward the hybrid nanostructures with perfect photoelectric properties for practical applications. © 2016 American Chemical Society

Received: April 12, 2016 Accepted: June 3, 2016 Published: June 14, 2016 15820

DOI: 10.1021/acsami.6b04378 ACS Appl. Mater. Interfaces 2016, 8, 15820−15827

Research Article

ACS Applied Materials & Interfaces

sample (20 mg) in ethanol (1 mL) by spreading it on an indium tin oxide (ITO) conducting glass and then dried at 150 °C for 1 h under vacuum. A 300 W Xe arc lamp (Beijing Trusttech, Model PLS-SXE 300) was utilized as the irradiation source. A 0.1 M Na2SO4 aqueous solution was used as the electrolyte. Photoluminescence (PL) spectra were registered on a fluorescence spectrophotometer (Hitachi, Model F-4500) at room temperature. Transient photovoltage (TPV) responses were carried out on a self-made instrument.21 The samples were excited by a laser radiation pulse (pulse width of 5 ns, wavelength of 355 nm) from a third-harmonic Nd/YAG laser (New Wave Research, Inc., Model Polaris II). The TPV signal was recorded using a digital phosphor oscilloscope (500 MHz, Tektronix, Model TDS 5054). The photocatalytic activity of the obtained Zn-doped CuS samples was evaluated by the decomposition of Rhodamine B (RhB) at room temperature in air, using the XPA-7 photochemical reactor (Xujiang Electromechanical Plant, Nianjing, China). The Zn-doped CuS sample (20 mg) was dispersed in 50 mL of RhB aqueous solution (C0 = 5 mg/L) and stirred for 30 min in darkness to obtain an adsorption−desorption equilibrium. A high-pressure xenon lamp (500 W) was used as the light source for photocatalytic reaction. A spectrophotometer (PerkinElmer, Model Lambda 35 UV-vis) was used to monitor the reaction at different intervals.

In this work, we address this issue by fabricating the chemically homogeneous Zn-doped CuS nanoflowers. By reacting appropriate amounts of CuCl and Zn(Ac)2·2H2O with sulfur powders in ethanol in a facile solvothermal process, the chemically homogeneous Zn-doped CuS nanoflowers (Zn 0 . 0 6 Cu 0 . 9 4 S, Zn 0 . 2 6 Cu 0 . 7 3 S 1 . 0 1 , Zn 0 . 3 6 Cu 0 . 6 2 S 1 . 0 2 , Zn0.49Cu0.50S1.01, and Zn0.58Cu0.40S1.02) were obtained. It was found that doping Zn2+ in CuS nanocrystals are favorable for significantly enhanced photoelectric response performance. It is mainly due to the following natural characteristics: (i) the Zn2+ can act as “shallow electron traps” to increase the lifetime of photogenerated electrons and holes in Zndoped CuS nanocrystals, because the energy level of the lowest unoccupied molecular orbital (LUMO) of Zn2+ is higher than that of Cu2+; and (ii) the heterocontacts in Zn-doped CuS nanocrystals can resist the charge recombination. Importantly, by tuning the Zn/Cu atomic ratios to ∼1:1, the chemically homogeneous Zn-doped CuS nanoflowers could be converted to the Zn0.49Cu0.50S1.01 superlattice crystal structure consisting of the periodic alternate of Cu−S−Zn atom arrangements. We found that the chemically homogeneous Zn0.49Cu0.50S1.01 superlattice structure can induce an effective self-built electric field with n-type semiconductor characteristics to significantly promote the separation and transfer of photogenerated charges, which were systematically investigated by transient photocurrent (TPC) response and transient photovoltage (TPV) technique. To exploit the potential applications of the fabricated nanostructures, the photocatalytic activity of samples is also discussed.

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RESULTS AND DISCUSSION Synthesis of Zn-Doped CuS Nanoflowers. In our experiments, Zn-doped CuS nanoflowers were synthesized by reacting CuCl, Zn(Ac)2·2H2O, and sulfur powders without any shape-directing surfactants. The most arresting feature in this reaction design is the use of CuCl. It is well-known that the Cu+ center usually has a 4-coordinated planar tetragonal geometry, thus resulting in nanocrystalline growth as unfolded nanopetals. If Cu+ is replaced by Cu2+, such as CuCl2, it is difficult to obtain flowerlike nanostructures under the present conditions without any shape-directing surfactants. In the present experiments, excess sulfur powders were used to ensure a coprecipitation of Cu+ and Zn2+, which, consequently, transformed structurally into target Zn-doped CuS nanoflowers in the ethanol solvothermal oxidation process. The composition of the resulting Zn-doped CuS nanoflowers was controlled by using the different amounts of Zn(Ac)2·2H2O. The atomic ratios in Zn-doped CuS series were validated with energy-dispersive Xray spectroscopy (EDS), and the obtained samples were Zn0.06Cu0.94S, Zn0.26Cu0.73S1.01, Zn0.36Cu0.62S1.02, Zn0.49Cu0.50S1.01, and Zn0.58Cu0.40S1.02, respectively (see Figure S1 in the Supporting Information). Just taking the obtained Zn0.49Cu0.50S1.01 sample as an example, the EDS data indicate that the atomic ratio (at. %) of Zn, Cu, and S in the Zn0.49Cu0.50S1.01 nanocrystals is 24.7:25.1:50.2, which is very close to the marked 0.49:0.50:1.01 ratio (see Figure S1e). Structure Investigation of Zn-Doped CuS Nanoflowers. The morphology of the obtained samples was characterized by SEM. Figure 1 shows the typical SEM images of the pure CuS and Zn0.49Cu0.50S1.01 nanoflowers. It appears that the flower-like architecture with the diameter of ∼1 μm consist of nanopetals with a thickness of ∼30 nm. For all of the Zn-doped CuS nanoflowers, their morphologies are just negligibly different (see Figure S2 in the Supporting Information). The crystallographic structures of Zn-doped CuS nanoflowers are first investigated by XRD analysis. Figure 2a shows the typical diffraction pattern of pure CuS nanoflowers with an intense peak at 2θ = 32.92° oriented along the (006) crystal plane, and the other peaks are indexed to the (100), (101), (102), (103), (110), (108), and (116) planes in good crystalline quality, which is consistent with the hexagonal

EXPERIMENTAL MATERIALS AND METHODS

All reagents were analytical grade and used without further purification. In a typical procedure, 0.25 mmol of CuCl, 0.25 mmol of Zn(Ac)2·2H2O, 0.75 mmol of sulfur powders and 15 mL absolute ethanol were first added into a Teflon-lined stainless steel autoclave with a capacity of 20 mL. After magnetic stirring for 30 min, the autoclave was sealed and maintained at 180 °C for 24 h, and then cooled naturally to room temperature. Finally, the black powder of Zn0.49Cu0.50S1.01 samples were obtained via the centrifugation of precipitates, washed several times with distilled water and absolute ethanol, and then dried at 60 °C under vacuum for 3 h. Other Zndoped CuS samples (Zn0.06Cu0.94S, Zn0.26Cu0.73S1.01, Zn0.36Cu0.62S1.02, Zn0.58Cu0.40S1.02) were prepared under the same conditions, with appropriate amounts of CuCl (0.45, 0.35, 0.30, 0.20 mmol) and Zn(Ac)2·2H2O (0.05, 0.15, 0.20, 0.30 mmol). The pure CuS sample was also synthesized under the same conditions without the addition of Zn(Ac)2·2H2O. Measurements. The morphology of the samples was characterized by scanning electron microscopy (SEM, Model JSM-4800), coupled with energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer (Cu Ka, λ = 1.5418 Å). Raman spectra were obtained on a confocal laser micro-Raman spectrometer (LABRAM-1B) with the radiation of 532 nm. The selected-area electron diffraction (SAED) patterns were obtained using a high-resolution transmission electron microscopy (HRTEM) system (JEOL, Model JEM 2100) operated at 200 kV. Ultraviolet−visible light (UV-vis) absorption spectra were recorded on an ultraviolet−visible light−near-infrared (UV-vis-NIR) spectrophotometer (Shimadzu, Model UV-3600). Transient photocurrent (TPC) responses were measured with a photoelectrochemical analyzer (CH Instruments, Model CHI660E) in a standard three-electrode system, with the as-obtained sample as the working electrode, a Pt foil as the counter electrode, and a saturated calomel electrode as the reference electrode.8 The working electrode was fabricated using the as-obtained 15821

DOI: 10.1021/acsami.6b04378 ACS Appl. Mater. Interfaces 2016, 8, 15820−15827

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Figure 1. SEM images of (a) pure CuS and (b) Zn0.49Cu0.50S1.01 nanoflowers.

Figure 3. Raman spectra of pure CuS (spectrum a), Zn0.06Cu0.94S (spectrum b), Zn0.26Cu0.73S1.01 (spectrum c), Zn0.36Cu0.62S1.02 (spectrum d), Zn0.49Cu0.50S1.01 (spectrum e), and Zn0.58Cu0.40S1.02 (spectrum f).

of all the obtained samples, for comparison. For the pure CuS samples (Figure 3a), the observed Cu−S vibration modes show a strong signal at 474 cm−1, consistent with those reported earlier.22,23 For all the Zn-doped CuS samples, the Raman intensity is reduced, while the full width at half maximum (fwhm) is increased (see Figures 3b−f), indicating the smaller polarizability of sulfur atoms in the these Zn-doped CuS samples. This result is consistent with the strong covalent nature from S-atom π-donation to the metal center. It is reasonable to expect that the Zn2+(3d10) occupy the Cu2+(3d9) site as the Cu−S−Zn combinations in the CuS lattice, resulting in some degree of electron delocalization of the S atoms to their neighboring Zn2+, and thus decreasing the polarizability of S atoms in the these Zn-doped CuS samples. Also, note that the Raman frequency peak of Zn-doped CuS samples obviously shifts downward to reach a minimum at 467 cm−1 and then return to the raw value of pure CuS, and again shifts upward to 477 cm−1 with the increasing Zn atomic ratios. This frequency shift is closely related with the induced strain/stress in the Zndoped CuS nanocrystals.24,25 If the Raman shift is shifted downward in frequency, the induced strain is present in the nanocrystal; the converse is observed for induced stress.24,25 In our cases, the Zn0.36Cu0.62S1.02 samples display the maximum downward shift in Raman frequency (Figure 3d), indicating the presence of the greatest strain. For the Zn0.58Cu0.40S1.02 samples, the Raman peak position is shifted upward to 477 cm−1 (Figure 3f), revealing induced stress in the Zn0.58Cu0.40S1.02 crystals. Zn doping in the CuS host lattice could undoubtedly vary the force constants, because of the bond strength of Zn−S−Cu being different from that of the pure Cu−S bond. Thus, the varying bond strengths of Zn−S−Cu, depending on the Zn atomic ratios in the lattice, induce strain or stress in the entire crystals. Interestingly, for the Zn0.49Cu0.50S1.01 samples, Zn/Cu atomic ratios of ∼1:1 represent an appropriate amount for the even arrangement of Cu and Zn atoms to obtain an even permutation of the Zn−S−Cu bonds in the entire lattice; therefore, the new force constants in the entire lattice are close to the raw values of CuS. As a result, the intensity of the Raman frequency peak is obviously reduced, whereas the peak position is almost consistent with the pure CuS sample (Figure 3e). We believe that increasing the number of Zn atoms combined into the Cu−S−Zn atom permutations give rise to induced stress in the Zn0.58Cu0.40S1.02 nanoflowers.

Figure 2. XRD patterns of pure CuS (spectrum a), Zn0.06Cu0.94S (spectrum b), Zn0.26Cu0.73S1.01 (spectrum c), Zn0.36Cu0.62S1.02 (spectrum d), Zn0.49Cu0.50S1.01 (spectrum e), and Zn0.58Cu0.40S1.02 (spectrum f).

CuS standard spectrum, with lattice constants of a = 3.792 Å and c = 16.344 Å (JCPDS File No. 06-464). For all the Zn-doped CuS samples, the XRD pattern is consistent with the features of pure CuS nanoflowers (Figures 2b−f). However, the diffraction peak of the (006) crystal plane is weakened as the atomic ratios of Zn in these Zn-doped CuS samples increased. At a relatively low Zn-dopant content (such as Zn0.06Cu0.94S), the (006) plane corresponds to a stronger diffraction peak (Figure 2b), while at a relatively high Zndopant content (such as Zn0.58Cu0.40S1.02), there is a weak diffraction peak for the (006) plane (Figure 2f). This means that the growth of the (006) plane is suppressed as the Zndopant content increases. Furthermore, there is no XRD peak related to other Zn compounds in all the Zn-doped CuS samples, which indicate that the crystals obtained are a chemically homogeneous solid solution. Interestingly, no obvious peak shift in all diffraction peak positions is observed for any of the Zn-doped CuS samples, suggesting the feasibility of incorporation of Zn2+ into the host sites of CuS lattice, because both Zn2+ and Cu2+ have similar ionic radius (0.74 and 0.73 Å). We believed that the Zn atoms have entered the sites of Cu atoms in the CuS lattice and further influenced the growth of (006) crystal plane. More-detailed structural information about the Zn-doped CuS nanoflowers can be obtained from Raman spectra. From XRD data, the obtained samples all have a hexagonal crystal structure, so only S atoms could vibrate in Raman-active mode. Therefore, Raman spectra can provide direct information about the polarizability changes of S atoms once Zn atoms occupy the Cu sites in the CuS lattice. Figure 3 shows the Raman spectra 15822

DOI: 10.1021/acsami.6b04378 ACS Appl. Mater. Interfaces 2016, 8, 15820−15827

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cation exchange between the Cu2+ and Zn2+ cations can easily occur, because of their similar ionic radius. Crystallographically, the crystal systems of the CuS and all the Zn-doped CuS products are the same hexagonal phase, which is beneficial for the cation exchange behavior. Thus, the formation of Zn0.49Cu0.50S1.01 superlattice structure is only related to the strain−stress equilibrium in the entire lattice for stable even arrangement of Cu and Zn atoms. This means that the strain− stress equilibrium established by tuning the Zn/Cu atomic ratios in lattice is the key to the formation of the present chemically homogeneous superlattice. The perfect Zn0.49Cu0.50S1.01 superlattice structure with the periodic Cu− S−Zn atom arrangements meet the strain−stress equilibrium in the entire lattice, as observed from Raman spectroscopy. In fact, with the greater number of Zn atoms combined into the Cu− S−Zn atom permutations, such as the Zn0.58Cu0.40S1.02 samples, the induced stress arises in crystals as observed from Raman spectroscopy; thus, the quality of the superlattice structure deteriorates. As shown in Figure 4f, the diffuse electron diffraction rings imply an incompetent commensurability. Photoelectric Properties of Zn-Doped CuS Nanoflowers. The detailed structural translation from CuS phase to the Zn0.49Cu0.50S1.01 superlattice structure has an important effect on their optical absorption properties. As seen in Figure 5a, the pure CuS nanoflowers display broad absorption that is

It is well-known that the nanoscale Cu−S compounds have characteristics that are sensitive to electron beam radiation. This makes the measurement of HRTEM images of these chemically homogeneous Zn-doped CuS nanocrystals difficult. However, the detailed structural difference in these Zn-doped CuS nanocrystals is reflected in their SAED pattern (Figure 4).

Figure 4. Selected area electron diffraction (SAED) patterns of (a) pure CuS, (b) Zn0.06Cu0.94S, (c) Zn0.26Cu0.73S1.01, (d) Zn0.36Cu0.62S1.02, (e) Zn0.49Cu0.50S1.01, and (f) Zn0.58Cu0.40S1.02.

Note that, to obtain the SAED pattern, the manipulation must been achieved quicxkly, within 10 s, at an accelerating voltage of 200 kV. For the pure CuS sample, as shown in Figure 4a, the SAED pattern displays a set of diffraction spots indexed as (100), (101), and (201), corresponding to the hexagonal CuS structure. For Zn-doped CuS samples, such as Zn0.06Cu0.94S, Zn0.26Cu0.73S1.01, and Zn0.36Cu0.62S1.02, the corresponding SAED pattern is shown in Figures 4b, 4c, and 4d, respectively. These paterns are comprised of primary diffraction spots with some satellite spots adjacent to primary spots, and the trend is toward a continuous line with the increasing Zn atomic ratios, which is the natural feature during the formation of a superlattice structure tuned by adjusting the Zn contents in the entire lattice.26−29 Typically, for the Zn0.49Cu0.50S1.01 samples, the appearance of the characteristic superlattice is obvious (Figure 4e). The diffraction spots clearly are arrayed into a continuous line, while the clear subcell spots imply the good commensurability of the superlattice structure.28 It is well-known that the commensurability of the superlattice is due to dopants throughout the entire lattice toward a periodic modulated structure.28 Obviously, the present Zn0.49Cu0.50S1.01 superlattice is a periodic modulated structure, tuned by the Zn/Cu atom ratio in the lattice. We believe that the Zn0.49Cu0.50S1.01 superlattice structure is related to the even arrangement of Cu and Zn atoms in nanocrystals corresponding to the periodic Cu−S−Zn atom arrangements in the entire lattice configuration. From a structural perspective, the Zn0.49Cu0.50S1.01 can be represented formally as 1/2[ZnS]·1/2[CuS]. Half of the Cu atoms are substituted by Zn atoms in the entire lattice. Thus, the even arrangement of Cu and Zn atoms in the entire lattice configuration, sharing their neighboring S atoms, cause the periodic alternate combination and permutation of the Cu−S− Zn atomic layer, similar to our previously reported S2-doped Cu7.2(S)2(S2)2.1 superlattice structure with the alternate combination of Cu−S and Cu−S2 atomic layer.30 The structural translation from the CuS phase to the Zn0.49Cu0.50S1.01 superlattice structure is related to the ion exchange between the Cu2+ and Zn2+ ions. Kinetically, the

Figure 5. UV-vis-IR absorption spectra of pure CuS (spectrum a), Zn 0.06 Cu 0.94 S (spectrum b), Zn 0.26 Cu 0.73 S 1.01 (spectrum c), Zn0.36Cu0.62S1.02 (spectrum d), Zn0.49Cu0.50S1.01 (spectrum e), and Zn0.58Cu0.40S1.02 (spectrum f).

responsive in the visible light and near-infrared (NIR) regions. The absorbance arrives a minimum at ∼600 nm, and then again run up over longer wavelengths, which are attributed to freecarrier intraband absorbance.31 For the three Zn-doped CuS nanoflow ers, Zn 0 . 0 6 Cu 0 . 9 4 S, Zn 0 . 2 6 Cu 0 . 7 3 S 1 . 0 1 , and Zn0.36Cu0.62S1.02, with the induced strains in the nanocrystals, their UV-vis-IR spectra in Figures 5b, 5c, and 5d exhibit the similar feature of pure CuS nanoflowers. However, with the increasing induced strains (increasing Zn/Cu atomic ratios), the corresponding absorption edges are all red-shifted, relative to their counterparts. The phenomenon has also been observed in Zn-doped CuO polycrystals25 and Cu-doped ZnS polycrystals.32 It has been well-known that the induced strains in nanocrystals can create forbidden energy levels in the lower states of conduction band, resulting in the narrowing band gap energy.24,25 Comparatively, for the Zn0.58Cu0.40S1.02 nanoflowers with the induced stress in the nanocrystals, the corresponding 15823

DOI: 10.1021/acsami.6b04378 ACS Appl. Mater. Interfaces 2016, 8, 15820−15827

Research Article

ACS Applied Materials & Interfaces absorption edge is obviously blue-shifted (see Figure 5f). Interestingly, the perfect Zn0.49Cu0.50S1.01 superlattice nanoflowers exhibit smooth absorption in, seemingly, the same absorbance in the entire UV-vis-IR regions with only a weak absorption peak at 345 nm (Figure 5e). This is the first time novel optical absorption properties were observed in a transition-metal sulfide, which may be a typical feature of the chemically homogeneous superlattice structure, and can be expected to generate more electron−hole pairs in the entire UV-vis-IR regions. The photoelectric properties of Zn-doped CuS nanoflowers were investigated by the photocurrent transient response measurement. Figure 6 shows the prompt photocurrent

Figure 7. Room-temperature photoluminescence (PL) spectra of pure CuS (spectrum a), Zn0.06Cu0.94S (spectrum b), Zn0.26Cu0.73S1.01 (spectrum c), Zn0.36Cu0.62S1.02 (spectrum d), Zn0.49Cu0.50S1.01 (spectrum e), and Zn0.58Cu0.40S1.02 (spectrum f), with an excitation wavelength of 365 nm.

nanocrystals to contribute to the defect-related emission. Interestingly, the perfect Zn0.49Cu0.50S1.01 superlattice nanoflowers (Figure 7e) exhibit significantly stronger emission at 405 nm, with its shouder being observed at 429 nm, which is different from other Zn-doped CuS nanoflowers. This phenomenon cannot be rashly attributed to the native trapstate emissions. It is most likely that the perfect superlattice structure induces an effective built-in electric field in the nanocrystals to improve the charge generation, separation, and transfer, thus giving rise to the strong luminescences. In fact, for the Zn0.58Cu0.40S1.02 sample, because of the greater number of Zn atoms combined into the Cu−S−Zn atom permutations, the built-in electric field is harmed, resulting in its decreasing luminescences at 405 and 429 nm, as shown in Figure 7f. Also, an additional emission peak at 460 nm was observed in the PL spectra of the Zn0.58Cu0.40S1.02 sample, in agreement with earlier results in Cu-doped ZnS nanoparticles related to the zinc vacancy.34 To further understand the dynamic behaviors of of charge generation, separation, transfer, and recombination in the Zndoped CuS systems, the TPV response was measured. As shown in the inset of Figure 8a, the pure CuS nanoflowers display a negative TPV signal that increases abruptly to the maximum TPV (tmax) at 3 × 10−5 s upon irradiation. The negative transient signal is consistent with the characteristic of CuS as a p-type semiconductor. Afterward, the TPV signal sharply decreases with response times of 3 × 10−5−1 × 10−3 s, as a function of photogenerated charge recombination. This short lifetime (3 × 10−5−1 × 10−3 s) and weak photovoltage intensity (2.4 mV) suggest a low charge separation efficiency in the pure CuS nanoflowers. Under the same conditions, the two samples of Zn0.06Cu0.94S and Zn0.26Cu0.73S1.01 have stronger TPV response in negative signal as p-type characteristic with maximum TPV (tmax) at ∼4 × 10−8 s (Figure 8a). With the increasing Zn/Cu atomic ratios, The TPV intensity is obviously enhanced, implying the higher generation efficiency of photogenerated charge carriers. Furthermore, for the two samples, the lifetime (4 × 10−8−4 × 10−4 s) of excess charge carriers is much longer, indicating that the Zn2+ doping could act as “shallow electron traps” to resist the charge recombination, thus the separation of photogenerated

Figure 6. Transient photocurrent responses of (a) pure CuS, (b) Zn 0.06 Cu 0.94 S, (c) Zn 0.26 Cu 0.73 S 1.01 , (d) Zn 0.36 Cu 0.62 S 1.02 , (e) Zn0.49Cu0.50S1.01, and (f) Zn0.58Cu0.40S1.02 with 300 W of illumination from a xenon arc lamp.

responses under several switch-on and switch-off irradiation cycles. As shown in Figure 6a, the pure CuS sample electrode exhibits very low photocurrent density (∼0.08 μA/cm2), whereas the photocurrent density of all the Zn-doped CuS samples (Figures 6b−e) is obviously larger than that of the pure CuS sample. The enhanced photocurrent response indicates the effective charge separation and transfer in such a hybrid structure. Among all of the Zn-doped CuS samples, the perfect Zn0.49Cu0.50S1.01 superlattice structure electrode exhibits the highest photocurrent density (∼1.5 μA/cm2), which is ∼20 times greater than that of the pure CuS electrode (∼0.08 μA/ cm2). The room-temperature PL spectra of the pure CuS nanoflowers and those Zn-doped CuS nanoflowers are shown in Figure 7. The pure CuS nanoflowers display two peaks, at 405 and 429 nm, respectively (Figure 7a). For those Zn-doped CuS nanoflowers (Figures 7b−f), the two peak positions do not shift with the increasing Zn2+ dopants. Thus, the two peaks at 405 and 429 nm should originate from native defect states, not from Zn2+ impurity states. It has been well-known that the emission peak of 405 nm derives from the recombination between the valence band and the sulfur-vacancy-related donor,33 while the peak at 429 nm could be attributed to the trap-state emissions of CuS, related to copper vacancy in origin.34 Particularly, the peak intensities at 405 and 429 nm for these doped samples obviously increase as the amount of Zn2+ dopants increases. This mainly is due to the effect of native defect states. With more Zn2+ ions doped into the CuS nanocrystals, more native defect states will be available in the 15824

DOI: 10.1021/acsami.6b04378 ACS Appl. Mater. Interfaces 2016, 8, 15820−15827

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ACS Applied Materials & Interfaces

comparison, the TPV response of Zn0.58Cu0.40S1.02 sample is also shown in Figure 8b. The decreasing TPV response intensity indicates that the built-in electric field in the Zn0.58Cu0.40S1.02 superlattice structure has weakened, because of more Zn in the atomic configuration in lower symmetry, resulting in disorder of the heterocontacts. To demonstrate the applications of photoelectric response in these Zn-doped CuS nanoflowers, the photocatalytic degradation of RhB was investigated (see Figure S3 in the Supporting Information). As shown in Figure 9, the perfect Zn0.49Cu0.50S1.01

Figure 9. RhB concentration in the solution with the different Zndoped CuS catalysts (20 mg) of pure CuS (trace a), Zn0.06Cu0.94S (trace b), Zn0.26Cu0.73S1.01 (trace c), Zn0.36Cu0.62S1.02 (trace d), Zn0.49Cu0.50S1.01 (trace e), and Zn0.58Cu0.40S1.02 (trace f), relative to the exposure time under illumination with a 500 W high-pressure xenon lamp. The starting RhB concentration (C0) is 5 mg/L. Figure 8. Transient photovoltage (TPV) spectroscopy of (a) Zn 0.06 Cu 0.94 S, Zn 0.26 Cu 0.73 S 1.01 and Zn 0.36 Cu 0.62 S 1.02 , and (b) Zn0.49Cu0.50S1.01 and Zn0.58Cu0.40S1.02. Inset in panel (a) is the TPV spectroscopy of the pure CuS sample.

superlattice nanoflowers exhibit the highest photocatalytic activity, leading to the total decomposition of RhB after irradiation for 30 min (Figure 9e). Meanwhile, other Zn-doped CuS nanoflowers appear to have slightly lower photocatalytic activity, but those are still obviously greater than that of the pure CuS nanoflowers (Figure 9a). The photocatalytic activity of Zn-doped CuS nanoflowers is consistent with the result of the TPV response measurements above. In those Zn-doped CuS systems, the heterocontacts could induce the spontaneous separation and transfer of electrons and holes, improving the yields and lifetimes of photogenerated charge carriers, thereby enhancing the photocatalytic performance. In this case, the perfect Zn0.49Cu0.50S1.01 superlattice structure with periodic heterocontacts could induce an effective built-in electric field to significantly improve the dynamic behaviors of photogenerated charge carriers, thus giving rise to the optimal photocatalytic activity.

electron−hole pairs is improved. Interestingly, for the Zn0.36Cu0.62S1.02 nanoflowers (Figure 8a), the TPV response shows a positive signal with tmax at 3 × 10−8 s and a negative signal with tmax at 4 × 10−8 s. The signals provide indirect evidence that a self-built electric field has been established in the Zn0.36Cu0.62S1.02 crystals. We believe that appropriate amounts of Zn2+ doping could induce the internal electric field in heterocontacts to improve the spatial charge separation and transfer. In fact, for the perfect Zn0.49Cu0.50S1.01 superlattice structure, the efficiency of self-built electric field has obviously improved (Figure 8b). As shown in Figure 8b, the TPV response shows only a strong positive signal as an n-type semiconductor characteristic, with response times of 6 × 10−8− 6 × 10−4 s. Clearly, the TPV response intensity (∼44 mV) and the lifetime of the excess charge carriers significantly increase, suggesting much higher charge generation, separation, and transfer efficiencies in the Zn0.49Cu0.50S1.01 superlattice. Especially, the strong positive signal indicates generous positive charges at the top electrode, which means that an efficient builtin electric field dominates the charge separation and transfer process.8,35 In this case, the even alternate permutation of the Cu−S−Zn atom layer in the Zn0.49Cu0.50S1.01 superlattice structure could induce an effective internal electric field in the periodic heterocontacts of Cu−S−Zn atom layers. Therefore, the photogenerated holes could drift to the surface region of Zn0.49Cu0.50S1.01 crystals under the self-built electric field. For

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CONCLUSIONS Zn-doped CuS nanoflowers (Zn0.06Cu0.94S, Zn0.26Cu0.73S1.01, Zn0.36Cu0.62S1.02, Zn0.49Cu0.50S1.01, Zn0.58Cu0.40S1.02) were successfully synthesized by reacting CuCl, Zn(Ac)2·2H2O, and sulfur powders in a simple ethanol solvothermal process without any shape-directing surfactants. The use of CuCl facilitates the growth of nanocrystals as unfolded nanopetals in no shape-directing surfactant conditions. By tuning the Zn/Cu atomic ratios to ∼1:1 in the periodic Cu−S−Zn atom arrangements in strain−stress equilibrium in the entire lattice, the perfect Zn0.49Cu0.50S1.01 superlattice structure is obtained. The perfect Zn0.49Cu0.50S1.01 superlattice structure coud induce 15825

DOI: 10.1021/acsami.6b04378 ACS Appl. Mater. Interfaces 2016, 8, 15820−15827

Research Article

ACS Applied Materials & Interfaces an effective built-in electric field in the periodic heterocontacts of Cu−S−Zn atom layers, improving the charge generation, separation, and transfer efficiency. Among all the Zn-doped CuS samples, the perfect Zn0.49Cu0.50S1.01 superlattice nanoflower demonstrates the significantly enhanced photoelectric response performance. Moreover, the detailed structural translation in these Zn-doped CuS nanoflowers could cause the remarkable conversion between p-type characteristics and n-type characteristics. This work will encourage more study on conventional heterogeneous nanostructures for potential photoelectric and photocatalytic applications.

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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/acsami.6b04378. EDS spectra for Zn-doped CuS nanoflowers; SEM images for the Zn 0.06 Cu 0.94 S, Zn 0.26 Cu 0.73 S 1.01 , Zn0.36Cu0.62S1.02, and Zn0.58Cu0.40S1.02 nanoflowers; timedependent absorption spectrum of a solution of Rhodamine B (RhB) solution with the different Zndoped CuS catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-371-4369251. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21271152 (for Y.G.), 21273192 (for Z.Z.), 21471001 (for H.N.)), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 144200510014), and the Program for Science and Technology Innovation Talents in Universities of Henan Province (Grant No. 2011HASTIT029 for Y.G.). We thank Dr. Xuanjun Zhang, Prof. Zhonglin Lu, and Prof. Yupeng Tian for helpful discussions.

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