3D Hierarchical Bi2S3 Nanostructures by Polyvinylpyrrolidone (PVP) and Chloride Ion-Assisted Synthesis and Their Photodetecting Properties
© Ding et al. 2015
Received: 12 April 2015
Accepted: 27 June 2015
Published: 9 July 2015
A solvothermal method has been employed to synthesize bismuth sulfide (Bi2S3) with three-dimensional (3D) hierarchical architectures. The influences of different types of surfactants and Cl− species on the size and morphology were investigated. A possible formation mechanism was also proposed on the basis of time-dependent experiments. The photoresponse properties show that the conductivity of Bi2S3 micro-flowers is significantly enhanced and the photocurrent is approximately two orders of magnitude larger than the dark current. The response and decay times are estimated to be 142 and 151 ms, respectively. It is expected that hierarchical architectures Bi2S3 may provide a new pathway to develop advanced nanomaterial for high-speed and high-sensitivity photoelectrical switches and photodetecting devices.
Bismuth sulfide (Bi2S3) that is an important member of group V–VI binary semiconductors, has drawn increasing attention in solar cells , photodetectors [2–10], gas sensors , Schottky diode , lithium-ion battery , X-ray computed tomography imaging (CT) , and thermoelectric devices . In recent years, various morphologies of Bi2S3 micro-/nanostructures, including one-dimensional (1D) nanoribbons/nanowires [16–19] and nanorods , two-dimensional (2D) nanosheets , and three-dimensional (3D) hierarchically complex architectures [13, 16], have been fabricated. Among them, 3D hierarchically porous and hollow nanostructures have showed enhanced properties for applications in lithium-ion batteries , photocatalysts , and gas sensors , because of their large surface area and facile electron (ion) transport. 3D hierarchical Bi2S3 nanoarchitectures are usually produced through solution-based synthesis [4, 13, 24, 25], and chemical vapor deposition routes  and their excellent physical and chemical properties have been revealed. In 2009, Li et al. built their photodetectors with Bi2S3 core-shell mircospheres. The light current increased by 1.1 times upon exposure to the simulated sunlight. The signal-to-noise ratio (SNR) of the photodetectors was probably too low . The devices based on the Bi2S3 hierarchical architectures reported by Xiao et al. had their response time and decay time of 0.5 and 0.8 s, respectively . That cannot meet the demand of high-speed photodetectors. Besides, the methods they employed were too complex, compared with hydrothermal or solvothermal approaches. At the same year, Li et al. reported a photodetector based on the Bi2S3 hierarchical architectures, with a fast response time of ~50 ms via a hydrothermal method . However, their light current was about 30 nA with illumination of 100 mW cm2 (AM 1.5), which may be too low for high-performance photodetectors. Therefore, fast and high response photodetectors based on Bi2S3 are still a challenge for practical applications. To improve the crystal quality and morphology of Bi2S3, nanostructures may be a good route to enhance their photodetecting properties. It has been reported that chloride ion (Cl−) could monitor the crystal growth of Cu2O , silver nanocubs , and silver nanoparticles , because Cl− could retard the nucleation and growth and reduce the surface energy by binding strongly to seeds. Thus it is reasonable to conjecture that the Cl− could affect the crystallinity and growth of 3D hierarchical Bi2S3 considering the similar crystalline structure and growth behavior of Bi2S3 and Cu2O. However, there is no report to prepare Bi2S3 nanostructures by introducing Cl− to monitor the morphology of Bi2S3 during solvothermal process. Moreover, the influence of surfactants on the morphologies of 3D hierarchical Bi2S3 has not been investigated systematically. For example, Jiang et al. reported that they had synthesized flower-like Bi2S3 by an ionic liquid-assisted templating route . In 2010, the flower-like Bi2S3 had been synthesized via a hydrothermal method by Tang et al . And the similar Bi2S3 had also been obtained by Wang et al. via the same hydrothermal method and assembled into the dye-sensitized solar cells with a good performance . Chen et al. reported that they had synthesized ultrathin Bi2S3 nanosheets via an organometallic synthetic route .
In this paper, we report a facile synthetic route to synthesize 3D hierarchical flower-like Bi2S3 consisted of nanowires via a solvothermal approach. Three types of surfactants including polyvinylpyrrolidone (PVP), sodium dodecyl sulfate (SDS), and centrimonium bromide (CTAB) were employed in the synthesis of Bi2S3 nanostructures, and PVP shows more manifest effects on the morphologies than the other two surfactants. The potassium chloride was added first in the solution to investigate the influence of chloride ions on 3D hierarchical Bi2S3 during solvothermal process. Our results demonstrate that Cl− plays a critical role on monitoring the shapes of Bi2S3 nanostructures. A possible formation mechanism of 3D Bi2S3 hierarchical nanostructures is proposed. Furthermore, a photodetector has been constructed based on as-prepared 3D Bi2S3 hierarchical nanostructures. The results show that the photocurrent is enhanced by two orders of magnitude compared with the dark current and the response time and decay time are estimated to be 142 and 151 ms, respectively, indicating promising applications of the as-prepared 3D hierarchical Bi2S3 for photodetecting and photoelectric switches.
Bi(NO3)•5H2O, thiourea (TU), polyvinylpyrrolidone (PVP), and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co., Ltd., (Shanghai) without further purification.
In a typical procedure, 0.6 g Bi(NO3)•5H2O, 0.3 g TU, and 0.1 g PVP were added successively into 40 mL EG. The resulting mixture was sonicated to obtain a clear, yellow solution, which was then transferred into a 100-mL Teflon-lined autoclave and heated at 60 °C for 24 h. Finally, the sample was collected and washed with distilled water and ethanol for three times, and then dried at 60 °C for 12 h in a vacuum oven. This final sample was designated as V-Bi2S3.
To investigate the influence of surfactants on the morphologies of the Bi2S3 micro-structures, another two surfactants sodium dodecyl sulfate (SDS) and centrimonium bromide (CTAB) were selected. Moreover, potassium chloride with different amounts was added to investigate the effects of chloride ions on the final morphologies. Each control experiment was performed in the same conditions except the change of the surfactants and chloride concentrations.
The morphologies, structures, and compositions were characterized by field emission scanning electron microscopy (FE-SEM, FEI Nova NanoSEM 450) and transmission electron microscopy (TEM; FEI Tecnai G20). X-ray powder diffraction (XRD) characterization was performed on Shimadzu XRD-7000s diffractometer equipped with Cu Kα radiation (λ = 0.15418 nm). X-ray photoelectron spectra (XPS) were characterized with Kratos AXIS Ultra DLD-600W X-ray photo electron spectroscopy.
The photodetectors were fabricated by a simple drop-casting method. Typically, 10 mg V-Bi2S3 was first suspended in 2 mL ethanol by sonication. The Au interdigital electrodes (1.5*1.0 cm, the electrode gap size is 1 μm) on Al2O3 substrates were cleaned by distilled water, ethanol, and acetone successively for 15 min, respectively. And then 10 μL of suspension was dropped on the Au electrodes. Finally, the devices were put in an oven at 30 °C for 12 h. Electrical property measurements and photo-sensing tests were conducted in ambient condition by a semiconductor characterization system (Keithley 2420) and a solar simulator (Newport 91160-1000) in the dark and under simulated AM 1 and 1.5 illumination.
Results and Discussion
where D is the average crystalline size, k is a constant whose value is typically 0.9 of non-spherical crystals, B is the full width at half maximum (FWHM) of the diffraction peak (in radians) that has the maximum intensity in the diffraction pattern, λ is the wavelength of incident X-ray beam (0.154184 nm), and θ is diffraction angle or Bragg angle. From this formula, the average crystalline size of Bi2S3 was calculated 12.27 nm. The full spectrum of XPS shows four distinct peaks corresponding to bismuth, carbon, sulfur, and oxygen, respectively (Fig. 1b). The peaks for O can be attributed to the absorbed oxygen species on the sample surface, which is commonly observed for samples exposed to the atmosphere and more pronounced for ultrafine powders with high surface areas. The C is from the absorbed carbon species during XPS measurement. The fine-spectrum of Bi is shown in Fig. 1b, two peaks located at ca. 163.65 and 158.3 eV (Fig. 1b) are assigned to the Bi 4f5/2 and Bi 4f7/2, respectively. Two peaks between the Bi4f7/2 and Bi 4f5/2 can be ascribed to the S 2p3/2 and S 2p1/2 that located 160.95 and 162.4 eV [33, 34]. The binding energy of located at 225.3 eV can be attributed to the S2− (2s) Fig. 1c. The reason for the asymmetric S2s peak is that there is a combination of both S8 which is expected at 228 eV, and SOx species , where x < 3 (that differs from metal sulfite salts), are typically at ~230 eV. Metal sulfites are typically found at ~230 eV. S8 is a byproduct of the reaction that is difficult to remove during purification.
The Morphology and Proposed Formation Mechanism
The Photoresponse Properties of the V-Bi2S3
In summary, a facile solvothermal procedure has been developed for large-scale production of 3D micro-structures consisted of ultra-long Bi2S3 nanowires with a diameter of 12 nm and axial dimension of up to 1 μm. The influences of surfactant, KCl, and time on the final morphologies of Bi2S3 nanostructures have been investigated, and the growth mechanism is proposed. The capping effect of the PVP and chloride ions and the specific amount of Cl− species seem to be the most pivotal factors in guiding the formation of Bi2S3 micro-flowers and microspheres. A high efficient photodetector was constructed based on Bi2S3 micro-flowers. The photoresponse properties show that the conductivity of Bi2S3 micro-flowers is significantly enhanced and the photocurrent is approximately two orders of magnitude larger than the dark current. The response and decay times are estimated to be 142 and 151 ms, respectively, suggesting promising applications in photodetectors.
This work was supported by the National Basic Research Program of China (Grant No. 2012CB619302, 2010CB923204), The Science and Technology Bureau of Wuhan City (No. 2014010101010006), Natural Science Foundation of Hubei Province (Grant Nos. 2011CDA81), Science Foundation from Hubei Provincial Department of Education (Grant Nos. D20131001), the National Natural Science Foundation of China (Grant No. 10990103, 51002058, 61274010) and the Science and technology project of Zhejiang Province (2012C33057).
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