Introduction
In recent years, various morphologies of nanostructures, including one dimensional (1D) nanoribbons/nanowires [ 1, 2] and nanorods [ 3], two dimensional (2D) nanosheets [ 4], and three dimensional (3D) hierarchically complex architectures [ 5- 7] have been fabricated. Among them, 1D nanostructured material is believed to play an important role in the next-generation building blocks for electronic devices [ 8], solar cells [ 9- 11], photocatalysis [ 12], lithium-ion batteries [ 13] and piezoelectric nanogenerators [ 14] for their high surface-to-volume ratio and excellent electron transport property. As an important member of bismuth chalcogenides (Bi2S3, Bi2Se3, and Bi2Te3), bismuth sulfide (Bi2S3) has drawn increasing attention in solar cells [ 15], photo-detectors [ 16- 18], gas sensors [ 19], Schottky diode [ 20], lithium-ion battery [ 21], X-ray computed tomography imaging (CT) [ 22], and thermoelectric devices [ 23]. Actually, Bi2S3 has also been considered as one of the most promising materials for photoactive materials due to its low band gap, high absorption coefficients and reasonable energy conversion efficiency. In view of the high surface area, high crystal quality for fast electron separation and transport, Bi2S3 nanostructure has been used as an efficient photoactive material more frequently [ 15]. It is well known that photoresponse properties are largely determined by morphology, and many approaches have been proposed to synthesize various Bi2S3 nanostructures, such as microwave irradiation [ 24], chemical vapor deposition (CVD) method [ 25- 27], anodized alumina membrane method [ 28], sonochemical approach [ 29], solvothermal process [ 30], electrochemical deposition [ 31] and biomolecule-assisted route [ 32]. Compared with the approaches mentioned above, hydrothermal process [ 16] is a more facile, low-cost and easy route to obtain Bi2S3 nanostructures with high crystallinity.
Herein, in this paper, Bi2S3 nanorods were successfully prepared via a one-pot and surfactant-free hydrothermal method. Typically, Bi(NO3)3, thiourea (TU) and ethylenediaminetetraacetic acid disodium salt (EDTA) were used as starting materials to obtain the Bi2S3 nanorods structures. The current-voltage (I-V) characteristic of optical switches based on Bi2S3 nanorods have been improved significantly, which exhibited ca. 2 orders of magnitude larger than the dark current. This result suggests an enhanced conductivity and high sensitivity of Bi2S3 nanorods based photodetectors. Furthermore, the response and decay time of the photodetector was estimated to be ~371.66 and 386 ms, respectively. These results indicate that the Bi2S3 nanorod is a promising candidate material for photoelectrical witches and light sensitive devices.
Experiment
Materials synthesis
Bi(NO3)•5H2O, TU, EDTA, ammonium hydroxide(NH3•H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai) without further purification.
In a typical procedure, 1.22 g Bi(NO3)•5H2O, 0.57 g TU and 0.66 g EDTA were added successively into 60 mL distilled water, followed with ammonium hydroxide to adjust pH to 8. A clear solution was obtained with assistant of sonication, which was then transferred into a 100 mL teflon-lined autoclave and heated at 130°C for 9 h. Finally, the product was collected and washed with distilled water and ethanol for three times, and dried at 60°C for 12h in an vacuum oven.
The morphologies, structures and composition were characterized by field emission scanning electron microscopy (FE-SEM, FEI Nova NanoSEM 450), transmission electron microscopy (TEM, FEI Tecnai G20). X-ray powder diffraction (XRD) characterization were 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 photoelectron spectroscopy.
Device fabrication
The photodetectors were fabricated by a simple drop-casting method. Typically, 10 mg Bi2S3 nanorodswas first suspended in 2 mL ethanol by sonication. The Au interdigital electrodes 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 light 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
Crystal structure
The XRD pattern of the as prepared Bi2S3 nanorods can be found in Fig. 1(a). Clearly, all the diffraction peaks can be indexed as the orthorhombic phase of Bi2S3 (JCPDS No. 17-0320) with cell constants a = 3.981, b = 11.147 and c = 11.305. The Bi-EDTA precursors were entirely decomposed after the hydrothermal reaction without any characteristic peaks. In another investigation, the average crystalline size was derived from the Scherrer formula as shown below:
where D is the average crystalline size, k is a constant whose value is typically 0.9 for 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 to be 28.26 nm.
The chemical composition of the Bi2S3 was further analyzed by XPS, as shown in Fig. 1(b). The main peaks could be indexed to Bi 4f, S 2s, O 1s and C 1s regions, which implied that no other metallic or inorganic containments were involved in the outcomes. The peaks of O could be attributed to the O2, CO2 and H2O absorbing on the surface of the sample, which is common for powders exposed to the atmosphere and more pronounced for ultrafine powders with high surface areas. Figure 1(c) presents high resolution XPS spectra of Bi 4f in Bi2S3 nanorods. The Bi 4f spectrum exhibits spin-orbit splitting into 4f5/2 (163.65 eV) and 4f7/2 (158.3 eV) components, and both contained the same chemical information. As illustrated in Fig. 1(d), the peak measured in the S energy region detected at 225.3 eV can be attributed to the S 2s transition.
Morphology of Bi2S3 sample
The SEM morphologies of the obtained Bi2S3 nanorods are shown in Figs. 2(a) and 2(b). It can be clearly seen that nanorods are dispersed in the field of version. The length and the diameter of the nanrodsareca. 0.5-1 μm and 70-100 nm, respectively. TEM and HRTEM were employed to get further information about the morphology of the obtained Bi2S3 nanorods. Figure 2(c) is a TEM image of a single Bi2S3 nanorod radiating from the center; it can be found that the surface of the individual nanorod is smooth. Figure 2(d) shows the HRTEM image of the individual nanorod. The single nanorod exhibits a lattice spacing of 0.79 nm, correspond to the (110) plane of orthorhombic Bi2S3. The TEM results are consistent with those reported for the Bi2S3 nanostructures previously, which are scribed to the highest surface energy of the (001) plane as well as the strongest bond energy of the c axis [ 33].
Photoresponse properties of Bi2S3 nanorods
As we all know, photoelectrical switches and photodetectors are widely used in imaging techniques, optical communications [ 34]. Generally, conventional photodetectors are usually in film or bulk configurations, which mean higher power consumption compared with the photodetectors constructed by micro- or nano-scale materials. Therefore, Bi2S3 nanorods were chosen as a representative micro- and nano- materials to evaluate the potential application in photodetectors. The photodetectors constructed with Au interdigital electrode and Bi2S3 nanorods were shown in the inset of Fig. 3(a). Evidently, the I-V curves were presented as linear whether the photoconductors were exposed to light or in dark, as shown in Fig. 3(a). Therefore, we can conclude that the contact between Bi2S3 and Au were ohmic-type. A distinct light current was observed when the photodetectors were exposed in light, demonstrating these Bi2S3 nanorods have an excellent photovoltaic response upon light illumination. It is evidently shown that the photocurrent is increased with the increase of bias voltage. According to the formula of photocurrent,
where U is the bias voltage, L is the length of photodetector, A is the sectional area, Δσ is the photoconductivity. As can be seen from the formula, Iph is proportional to the bias voltage. And our results also proved this point, as shown in Fig. 3(b). Based on the formula of the transit time of electrons,
where L is the length of photoconductor, μn is the mobility of electrons and U is the bias voltage. We can conclude that electrons have a higher mobility, their transit time is short than τ (carrier lifetime) from the formula above. So the separation of photo-generated electrons and holes is more efficient in a high bias. In addition, the light current significantly increases approximately 2 orders of magnitude compared with the dark state as shown in Figs. 3(a) and 3(b), indicating excellent optical sensitivity. Good photoresponse property should result from the increased charge carrier concentration via direct electron-hole pair generation under light illumination and the enhanced conductivity of Bi2S3. Figure 3(c) depicts the photoresponse as a function of time with the light regularly chopped at a bias of 5 V to reveal the stability and response capability. The photocurrent quickly reaches to a maximum value (the steady-state), and then rapidly returned to its initial ones (the normal state) once the light was turned off, revealing the Bi2S3 nanorods respond quickly to the light. Such on-off cycles were repeated several times without any distinguishable degradation, showing its excellent stability and reproducible behavior. The response time is generally defined as the time required to recover from 10% to 90% of the maximum photocurrent. The recovery time has similar definition. The response and recovery time at a bias of 5 V for our photodetector are calculated to be 371.66 and 386 ms, respectively. The above results indicate that the photodetectors based on Bi2S3 nanorods have a good stability and responds quickly to light, suggesting promising applications of Bi2S3 micro-flower in photoelectrical switches and photodetectors. More importantly, the Bi2S3 nanorods as well as the relevant photosensitive devices presented in this paper were very easy to fabricate and no complex equipment and procedures are needed. Thus, this method offers probability for low-cost and large-scale applications in the integration circuits.
Fig.3 Photoresponsive sensitivity of the Bi2S3 nanorods as a representative system was studied. (a) I-V characteristic of device in the dark and under simulated A M 1.5 illumination; (b) logarithmic plot of (a); (c) time dependence of current of Bi2S3 micro-flower at a bias of 5 V in the dark and under simulated A M 1 illumination; (d) enlarged portion of the 510-513 s and 661-664 s |
Conclusions
In summary, we have successfully synthesized Bi2S3 nanorodsvia a facile, low-cost, and low power consumption while effective one-pot solution-based method. And we also investigated the photosensitivity properties based on Bi2S3 nanorods in an ambient environment. It is worth noting that the light current significantly increases by 2 times compared with the dark state. The devices based on Bi2S3 nanorods show an excellent stability and fast response to the light as well. These findings indicate these nanorods may have great potential applications in high speed and high-sensitivity photoelectrical switches and light sensitive devices.