Bismuth sulfide nanoflowers as high performance near-infrared laser detectors and visible-light-driven photocatalysts

Abstract

Flower-like bismuth sulfide nanostructures were synthesized and applied for both rigid and flexible near-infrared laser detectors. The rigid laser detector exhibits excellent photoresponse characteristics to an 808 nm laser beam, and its response time and decay time were found to be relatively fast as 2 s and 2.5 s, respectively. The flexible laser detector not only has high flexible, light-weight and adequate bendability, but also revealed good sensitivity to an 808 nm laser beam. The photocatalytic properties of bismuth sulfide nanoflowers were evaluated by the decomposition of RhB in aqueous solution under visible light irradiation. The results demonstrated that the photodegradation ratio of RhB was up to nearly 96% after 3 h visible light irradiation, indicating the bismuth sulfide samples were good candidates for visible light photocatalysts.

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1. Introduction

Photodetectors based on nanoscale semiconducting metal chalcogenides have been studied for decades and the performance is found to greatly depend on the morphology, microstructure, size, and so on.^1–10 For example, Deng and his co-workers reported a type of SnS nanoribbon based photodetector having highly sensitive and rapid photocurrent responses.¹¹ ZnS nanostructures-based UV-light photodetectors show good photocurrent reproducibility under 320 nm light illumination.¹² Shen's group showed high photoresponse characteristics for the Sb₂S₃ nanoflower rigid and flexible photodetector in the visible light region.¹³ Up to now, many approaches have been studied to improve the sensitivity of metal chalcogenide photodetectors.^14–19

Bismuth sulfide, a member of compound semiconductor of the A^V₂B^VI₃ family, has received intensive attention due to its wide application in optoelectronic devices, thermoelectric device, nonlinear optical device, optical modulators and sensors.^20–27 In particular, with a narrow direct bandgap of 1.3–1.7 eV,^22–24,28 which makes it a good candidate in fabricating high performance infra-red or visible photodetectors. In 2009, the rigid device based on the Bi₂S₃ core–shell microspheres reported by Qi's group, had their on/off switching ratio was 1.1 times to the AM 1.5 sunlight.²⁹ Xiao et al. built their rigid photodetector with hierarchical architectures, with the response and decay time of 0.5 s and 0.8 s.³⁰ At the same time, Li and his co-works reported the synthesis of rigid photodetector based on hierarchical Bi₂S₃ nanostructures.³¹ Moreover, several methods including hydro/solvothermal process,^32,33 self-assembly method,³⁴ template-directed,³⁵ and organometallic synthetic methods³⁶ have been reported for the synthesis of Bi₂S₃ nanostructure. Thus, developing lower temperature, convenience synthetic, benignancy to environment, simple and effective methods is still important in the current scientific field. However, to the best of our knowledge, near-infrared photoconductive laser detectors on Bi₂S₃ nanostructures have not been synthesized. In this work, by synthesizing Bi₂S₃ nanoflowers via a facile polyol refluxing process under the open-air condition, we successfully fabricated two types of high performance Bi₂S₃-based near-infrared 808 nm laser detectors-rigid and flexible laser detectors. The laser detectors have the features of high stability, fast response and recovery time. Finally, photocatalytic properties of Bi₂S₃ nanoflowers were also studied.

2. Experimental section

2.1 Preparation of materials

Bismuth sulfide nanoflowers were synthesized through a simple refluxing process in ethylene glycol solution. In a typical procedure, 0.97 g bismuth nitrate pentahydrate and 0.5 g thiourea were first added into 75 mL ethylene glycol in a round bottom flask. After stirring for 5 min, the reaction system was heated and kept at 197 °C for 30 min. After cooling to room temperature naturally, the brown-black precipitates were collected, washed with absolute ethanol and distilled water for several times, and then dried in a vacuum at 70 °C for 3 h.

2.2 Material characterization

The morphology and microstructure of the bismuth sulfide nanoflowers were characterized by field emission scanning electron microscopy (FESEM, JEOL JSM-6700F, 5 kV) and transmission electron microscopy (TEM, JEM-2010). The X-ray diffraction patterns (XRD) were obtained from a X-ray diffractometer (X' Pert PRO, PANalytical B.V., the Netherlands) with radiation of a Cu target (Ka, λ = 0.15406 nm). The UV-vis absorption spectra were recorded on a UV-vis spectrophotometer (Shimadzu UV-2550) and were converted from reflection to absorbance by the standard Kubelka–Munk theory. Photoresponse properties were measured under 808 nm laser conditions beam (output wavelength: 808 nm ± 5 nm, minimum diameter of beam at the focus point is around 1.0 mm, output power: 300 mW) with an Autolab (model AUT84315).

2.3 Fabrication of rigid and flexible laser detectors

To fabricate the rigid laser detector, suitable amounts of the as-obtained Bi₂S₃ products were dispersed in ethanol solution containing small quantity of ethylene cellulose and terpineol to form uniform paste. Then the paste was coated onto the electric surface of FTO substrate. The electrode of another FTO substrate was pressed on the surface of the Bi₂S₃ nanoflowers coated on one of the FTO substrate. To fabricate the flexible laser detector, two parallel silver wires with an interval of 0.8 mm were deposited on the flexible polyethylene terephthalate (PET) substrate with the help of silver paste. A suitable amount of the as-synthesized Bi₂S₃ nanoflowers paster was coated onto the PET substrate with Ag electrodes at room temperature. Finally the flexible devices were dried at 70 °C for 3 h in a vacuum oven to improve the mechanical strength and electrical contact.

3. Result and discussion

Fig. 1a shows the XRD patterns of the as-synthesized samples. All the diffraction peaks can be readily indexed to a pure orthorhombic Bi₂S₃ crystal phase (JCPDS: 17-0320) with lattice constants a = 1.1149, b = 1.1304, and c = 0.3981. No peaks of any other phases are detected in this pattern, indicating the high purities of the samples. The UV-vis absorption spectrum of the as-synthesized Bi₂S₃ nanoflowers is shown in Fig. 1b, which demonstrates continuous broad absorption in visible light and near-infrared area. A plot of (αhν)² against hν based on the direct transition is shown in Fig. 1c, corresponding to the bandgap energy of 1.4 eV, which is consistent with the previously reported data.²⁸

Fig. 1 (a) XRD patterns, (b) UV-vis absorption spectrum and (c) (αhν)² vs. hν plot of the as-synthesized Bi₂S₃ nanoflowers.

The morphology and microstructure of the as-synthesized Bi₂S₃ nanoflowers from the facile refluxing process were characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Fig. 2a shows a low-magnification SEM image of the Bi₂S₃ products, which clearly exhibits that the products actually consist of many nanoflowers. The higher magnification SEM images (Fig. 2b and its inset) show that the Bi₂S₃ nanoflowers with smooth surface are constructed by many uniform nanorods interconnected to each other. TEM images in Fig. 2c and d clearly display that the as-obtained samples are composed of nanorods with the length of several micrometers and form a diverging microstructure. The corresponding selected-area electron diffraction (SAED) pattern (inset of Fig. 2d) reflected the excellent single-crystalline in nature of the Bi₂S₃ nanoflowers.

Fig. 2 (a and b) SEM images, (c and d) TEM images, the inset is the corresponding SAED pattern.

The photoelectric measurement system for the Bi₂S₃ nanoflowers based rigid laser detector is similar to that in our previous reports.¹³ The corresponding device structure was depicted in Fig. 3a inset. Both the current–voltage characteristics and the reproducible switching of the device were recorded upon 808 nm near-infrared laser light illumination with an Autolab. Fig. 3a shows the typical I–V curves of the rigid photodetector exposed to 808 nm near-infrared laser light and under dark condition with the bias from −1 V to 1 V, respectively. Obviously, the current intensity increased from 0.07 mA at dark to 0.31 mA upon laser light illumination with a power density of 200 mW cm⁻² at the bias of 1 V. Compared with the dark condition, the photocurrent increases by more than four times. The light-on and light-off states at an applied voltage of 1 V for a rigid photodetector were shown in Fig. 3b. It shows a typical time dependent photocurrent response under 808 nm near-infrared laser light illumination. From the curves, it is noted that all the photocurrent are stable and the laser detector has a fast time response.

Fig. 3 (a) I–V and (b) I–T curves of the Bi₂S₃ nanoflowers based rigid photodetector, (c) I–V and (d) I–T curves of the Bi₂S₃ nanoflowers based flexible photodetector illuminated with 808 nm laser light.

In recent years, flexible optoelectric devices have attracted extensive attention due to their potential application in binary switches, optical communication, imaging techniques and optoelectronic circuits.^37–42 The flexible laser detector was also fabricated by using the Bi₂S₃ nanoflowers as the active materials. Fig. 3c inset shows the optical image of the device on flexible polyethylene terephthalate (PET) substrate and the corresponding data are shown in Fig. 3c and d. The photocurrents were measured for the same illumination power density 300 mW cm⁻². Fig. 3c illustrates that the I–V curves taken under 808 nm light illumination and dark condition at a bias voltage from −5 V to 5 V, respectively. It also clearly shows that the photocurrent increases as the light intensity is elevated. The excellent photoresponse behavior was further proved by the I–T curves of the flexible device (Fig. 3d). A low dark current of 87 nA and high photocurrent of 335 nA were recorded, increased about four times. From the I–T curves measured with an external of 5 V, we find that the flexible laser detector showed stable and substantial response with a repetitive switching of near-infrared laser light illumination.

As one of the parameter for laser detector performance, a fast time response is very important. Fig. 4a and b show the enlarged view of a single on/off cycle of the Bi₂S₃ nanoflowers based rigid device to the 808 nm wavelength laser light, in which the response and the recovery time were measured to be 2 s and 2.5 s, respectively, indicating fast photoresponse sensitivity to near-infrared laser light. Fig. 4c and d show the photoresponse characteristics of the flexible device. It clearly demonstrates the response and the recovery time were estimated to be 2.2 s and 3 s, respectively. It would be innovative to fabricate flexible laser detector on transparent PET substrate. Nevertheless, comparing to the rigid laser detector, there are still some challenges in the fabrication and performance enhancements of the flexible device. For the previously reported device, including Li³¹ and Ding' work⁴³ as shown in Table 1, most of the devices focused on full spectrum (AM 1.5G, one sun). Our work first looked outward to single wavelength of laser. The flexible laser detector is a significative exploration, it shows high flexible, light-weight and adequate bendability comparing to the rigid device, and we believe the properties can be further improve in the near future.

Fig. 4 (a and b) Enlarged view of a single on/off cycle of the rigid laser detector, (c and d) enlarged view of a single on/off cycle of the Bi₂S₃ nanoflowers based flexible laser detector.

Table 1 Comparison of the important performance parameters between this work and other Bi₂S₃ detectors

Detector style	Light source	Response/decay time
Micro-flower (rigid)^43	AM 1.5G	142 ms/151 ms
Hierarchical nanostructure (rigid)^31	AM 1.5G	5 ms/240 ms
Nanoflowers (rigid)^this work	808 nm laser beam	2 s/2.5 s
Nanoflowers (flexible)^this work	808 nm laser beam	2.2 s/3 s

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The photoresponse mechanism of the Bi₂S₃ nanoflowers laser detectors with two types of substrates can be expressed as the follows: in the dark condition, oxygen molecules absorbed on the Bi₂S₃ nanoflowers surface capture free the concentration of holes increase, the low conductivity depletion layer formed: O₂ (g) + e⁻ → O₂⁻ (ad). Once a beam of laser at a wavelength below 878 nm (corresponding to the bandgap energy of 1.4 eV mentioned above), illuminates on the materials' surface, electron–hole pairs are generated: hν → e⁻ + h⁺. Then the holes move to the surface of the samples to combine the O₂⁻ (ad): h⁺ + O₂⁻ (ad) → O₂ (g). At the same time, the conductivity layer dramatically increases with the increasing of photo-generated electrons.

Finally, the photocatalytic activities of the Bi₂S₃ nanoflowers were studied by measuring the photodegradation of rhodamine B (RhB) in aqueous solution under visible light irradiation at room temperature. During the measurement, 0.1 g of Bi₂S₃ nanoflowers were suspended in 100 mL RhB aqueous solution with an initial concentration of 4 mg L⁻¹ in a pyrex reactor. The suspension was stirred in the dark condition for a certain time to reach an adsorption–desorption equilibrium, then the light was turn on. A 500 W Xe-lamp equipped with a cutoff filter (λ > 420 nm) and a water filter was used as the visible light source. At given time intervals, appropriate reaction suspension was collected and filtrated. Fig. 5a shows the UV-vis absorption spectra of the RhB solution in the presence of Bi₂S₃ nanoflowers. Clearly, during the dark condition, Bi₂S₃ nanoflowers architectures can adsorb about 80% RhB within 30 min, an adsorption–desorption equilibrium in the solution was reached. When the light was turned on, the main peaks decreased continuously with increased irradiation time.

Fig. 5 (a) UV-vis spectra of RhB aqueous solution in the presence of Bi₂S₃ nanoflowers, (b) concentration changes of RhB over no catalyst, commerce P25, and Bi₂S₃ nanoflowers under visible light irradiation. Catalyst 0.1 g; 500 W Xe-lamp (λ > 420 nm); RhB solution: 100 mL, 4 mg L⁻¹.

Furthermore, we also find that the absorption spectrum shows a weak blue shift, indicating that the cleavage of the whole conjugated chromophore structure is the main pathway. The plot for the concentration changes of RhB determined from its characteristic absorption peak is shown in Fig. 5b. For comparison, the photolysis, the RhB degradation over commerce P25 and Bi₂S₃ nanoflowers under visible light irradiation were measured at the same conditions and the corresponding results are also shown. The Bi₂S₃ nanoflowers have a significantly stronger adsorption and photocatalytic degradation ability than commerce P25, and RhB dye is considerably stable under visible light irradiation. It can be clearly seen that after 0.5 h of adsorption equilibrium and 3 h of irradiation, the degradation ratio of RhB for Bi₂S₃ nanoflowers is about 96%, while that of commerce P25 is about 70%. Due to the good adsorption and degradation effect of Bi₂S₃ nanoflowers exhibited in the RhB solution, we can conclude that the obtained product might be used in the waste-water treatment in industry.

4. Conclusions

In summary, high performance near-infrared laser detectors on both rigid FTO substrates and flexible PET substrate using Bi₂S₃ nanoflowers were fabricated. The two types of laser detectors exhibit excellent photoelectric characteristics to 808 nm laser light, such as high stability, excellent reproducible properties and fast response and recovery speed. Especially, the rigid laser detector has fast response time of 2 s and decay time of 2.5 s. The flexible laser detector was fabricated on PET substrate which cause both the response time and decay time to increase to 2.2 s and 3 s, respectively. In addition, under visible light irradiation, the Bi₂S₃ nanoflowers show much stronger adsorption and photodegradation abilities for RhB than commercial P25 powders. The results open a new vista for the potential application of the Bi₂S₃ nanoflowers in the field of visible-light-driven photocatalysts.

Acknowledgements

This work was supported by the National Natural Science Foundation (21001046, 51002059), the 973 Program of China (2011CB933300, 2011CBA00703), the Important Scientific Research Foundation of Henan Province Education Department (14A510002), the Research Fuuds of Anyang Institute of Technology (YJJ2015017).

Notes and references

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