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Photoresponse properties of ultrathin Bi2Se3 nanosheets synthesized by hydrothermal intercalation and exfoliation route
Applied Surface Science 316 (2014) 341–347
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Photoresponse properties of ultrathin Bi2 Se3 nanosheets synthesized by hydrothermal intercalation and exfoliation route Chen Zang, Xiang Qi ∗ , Long Ren, Guolin Hao, Yundan Liu, Jun Li, Jianxin Zhong ∗ Hunan Key Laboratory for Micro-Nano Energy Materials and Devices, People’s Republic of China Laboratory for Quantum Engineering and Micro-Nano Energy Technology and Faculty of Materials and Optoelectronic Physics, Xiangtan University, Hunan 411105, People’s Republic of China
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Article history: Received 3 March 2014 Received in revised form 8 July 2014 Accepted 11 July 2014 Available online 18 July 2014 Keywords: Topological insulator Bismuth selenide Nanosheets Responsivity
a b s t r a c t The photoresponse properties of Bi2 Se3 nanosheets prepared by a simple hydrothermal intercalation and exfoliation route are studied. Photoelectrochemical results indicate that the as-prepared Bi2 Se3 nanosheets devices have excellent sensitivity, high-speed and good reproducibility as a photodetector, which are superior to the bulk Bi2 Se3 . Especially, the response time, responsivity, and external quantum efficiency are found to be about 0.7 s, 20.48 mA/W, and 8.360 , respectively. It is proposed that the two-dimensional nanostructure of Bi2 Se3 can be effectively used in high performance nanoscale photodetectors. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Much attention has been focused on a special type of quantum materials of condensed matter physics in recent years, which called topological insulators (TI), their bulks are insulating but surfaces are conducting [1]. TI provide a gateway to investigate fundamental quantum behaviors of exotic quasi-particles [2]. These remarkable materials have fascinating potential applications, such as almost dissipationless surface transport and applications in spintronics and quantum computing [3]. This family of compounds like Bi2 Se3 , Bi2 Te3 , and Sb2 Te3 possess graphene-like two-dimensional materials with layers linked by weak van der Waals forces, they have been theoretically predicted and experimentally proved to be three-dimensional TIs due to their Dirac surface states [4–6]. Their applications such as optical recording system [7], spintronic devices [8] and thermoelectric devices [9–11] have been discovered. Due to its large surface-to-volume ratio and abundant surface states, Bi2 Se3 nanostructures is currently becoming one of the widely studied TI materials for its potential applications in solar selective, optoelectronic devices and thermoelectric coolers [12]. It is reported that Bi2 Se3 thin films behaved excellent photosensitive performances [13]. Unfortunately, the thickness of
∗ Corresponding author at: Faculty of Materials and Optoelectronic Physics, Xiangtan University, Hunan 411105, People’s Republic of China. Tel.: +86 731 58292195. E-mail addresses: [email protected], franck [email protected] (X. Qi), [email protected] (J. Zhong). http://dx.doi.org/10.1016/j.apsusc.2014.07.064 0169-4332/© 2014 Elsevier B.V. All rights reserved.
as-prepared film is still problematic and does not reach the requirement for nanoscale devices. Most recently, Zhang et al. illustrated that graphene-like materials had been behaved superior optical properties [14–17]. They found that TI nanosheets dominated veryhigh-modulation-depth (up to 95%) saturable absorber [18] and could work as a passive Q-switcher for solid-state laser operation [19]. In addition, Bi2 Se3 membrane composited with nanosheets was proposed to be a novel type of optical saturable absorber [20]. While, the sample of Bi2 Se3 nano-platelets was suggested to be a promising nonlinear optical material [21]. In this paper, ultrathin Bi2 Se3 nanosheets are prepared via a top-down intercalation and exfoliation route. The photoresponse properties of the as-prepared Bi2 Se3 nanosheets are investigated, which was comparing with the pristine Bi2 Se3 bulk and the heat-treated Bi2 Se3 nanosheets. 2. Experimental 2.1. Synthesis In a typical procedure, 0.23 g Bi2 Se3 bulk powder was added into 30 mL ethylene glycol solution with 0.2 g LiOH. After constant stirring, the solution was transferred into a Teflon-lined autoclave of 50 mL capacity. The autoclave was heated at 200 ◦ C for about 24 h to achieve the intercalation of Bi2 Se3 by dissolving the Lithium ions (Li+ ) in the solution, followed by cooling to room temperature. The dispersions in the solution was then carefully collected by filtration and washed with acetone to eliminate the excess ethylene glycol solution of lithium hydroxide. Colloidal suspensions of
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bismuth selenium can be readily prepared by exfoliating the lithiated powder in deionized water. In order to explain the effect of Lithium ions, the controlled experiments without LiOH in the ethylene glycol solution during the hydrothermal process were preformed.
and ultrasonicated for 1 h to yield a stable Bi2 Se3 suspension. The Na2 SO4 aqueous solution (0.5 M, 98.0%) was used as electrolyte and photoresponse measurements were carried out under 0.12 W/cm2 illumination via a Xe lamp with AM 1.5 G filter as simulated sunlight.
2.2. Characterization
3. Results and discussion
The morphologies and microstructure of the samples were studied by scanning electron microscope (SEM, JEOL JSM-6610LV), and transmission electron microscopy (TEM, JEOL JEM-2100F). The crystallographic information of the obtained samples was analyzed by X-ray diffraction (XRD, Rigaku D/Max 2500) using the Cu K␣ radiation in the 2 range of 10–80◦ . Atomic force microscopic (AFM, SPI-3800N) studies were carried out in air at ambient condition. Raman spectra were collected for bulk samples and nanosheets of Bi2 Se3 by employing Renishaw in-Via micro Raman spectrometer, excited at room temperature with laser light (532 nm). N2 adsorption–desorption measurements were conducted at 77 K on a Nove 2200e analyzer. The specific surface area was obtained by Brunauer–Emmett–Teller (BET) method.
Fig. 1(a) shows the digital photos of three samples in aqueous dispersion, they are the raw bulk Bi2 Se3 , the exfoliated nanosheets and the Bi2 Se3 bulk after a hydrothermal process in the ethylene glycol solution without LiOH, respectively. It is clear that the aqueous dispersion of exfoliated Bi2 Se3 is remarkably different from the former pellucid solution with a large number of Bi2 Se3 bulk in the bottom, which presents a turbid and uniform dispersion state, implying the smaller size of the exfoliated sample. To gain insight into the transformation of the bulk Bi2 Se3 before and after the hydrothermal intercalation, the morphologies of bulk and exfoliated Bi2 Se3 samples are further characterized by SEM. It is obviously shown in Fig. 1(b) that the layered surface of the huge bulk is smooth and flat. On the contrary, for the exfoliated sample shown in Fig. 1(c), large-scale isolated nanosheets with large lateral dimensions are widespread. We propose the formation of nanosheets via hydrothermal exfoliation route from bulk crystals follows the following process [22], as shown schematically in Figure 1(d). Here, Li+ on the process is confirmed to be one indispensable factor by checking the experiment of preparing Bi2 Se3 bulk in the ethylene glycol solution but without lithium hydroxide (LiOH). Without the addition of Li, the solution is still clear (right one in Fig. 1(a)) and the Bi2 Se3 bulk after hydrothermal process also has original smooth and flat layered surface (inset of Fig. 1(b)), same
2.3. Photoelectrochemical measurements Photoelectrochemical test systems were composed of a CHI 660D electrochemistry workstation, an illumination source, and a homemade three-electrode cell. The indium–tin oxide (ITO) conductor glass coated with as-prepared Bi2 Se3 suspension was used as working electrode with Pt foil and saturated calomel electrode as counter and reference electrodes, respectively. Typically, a total of 10 mg as-prepared samples was mixed with 2 mL de-ionized water
Fig. 1. (a) Digital photos of the change in the solution containing Bi2 Se3 bulk before (I) and after reaction in the ethylene glycol solution with LiOH (II) or without (III); SEM image of (b) Bi2 Se3 bulk and (c) exfoliated Bi2 Se3 nanosheets (the image inserted in (b) is the sample after reaction with the absent of LiOH); (d) Schematic representation of the formation of Bi2 Se3 nanosheets.
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as that of pristine Bi2 Se3 bulk. For the Bi2 Se3 consist with weakly van der Waals-bonded Se Bi Se Bi Se slabs [12], the Li cations dissolve in the ethylene solvent and insert into interlayer of Bi2 Se3 , then form Liy Bi2 Se3 during the reduction process as given in Eq. (1). During this Li-intercalated process, the ethylene glycol serves as both reductant and solvent. After exposing to water, the lithium ions in Liy Bi2 Se3 dissolve rapidly, resulting in the production of LiOH) and hydrogen gas, as given in Eq. (2). −y Bi2 Se3 + yLi+ + ye− → Li+ y (Bi2 Se3 )
Liy Bi2 Se3 + yH2 O → Bi2 Se3 (nano) + yLiOH +
(1) y H2 ↑ 2
(2)
Due to the rapid expansion in the layers and forming suspensions of Bi2 Se3 nanosheets, the original quintuple layers are homogeneously exfoliated. It is noted that the powder of the Bi2 Se3 nanostructures could be easily be collected by filtration. The phases of the products obtained above are determined by X-ray diffraction experiments, as shown in Fig. 2. The XRD patterns of exfoliated Bi2 Se3 and bulk Bi2 Se3 match the rhombohedral phase of Bi2 Se3 (JCPDS no. 89-2008), no other impurities are observed. For the Li intercalated Bi2 Se3 , additional diffraction peaks at 19◦ , 21◦ , 28◦ , 31◦ , 39◦ and 59◦ , may be originated from Bi2 Se3 being intercalated Li to form Liy Bi2 Se3 , which is similar with previous results [23,24]. Interestingly, the additional peaks are disappeared after exfoliated process, indicating that the Li ion in the Li intercalated Bi2 Se3 are completely excluded to form Bi2 Se3 nanosheets. To further investigate the microstructures of the nanosheets, TEM and HRTEM measurements are carried out. Fig. 3(a) displays a typical low-magnification TEM image of an individual Bi2 Se3 sheet, which has a very thin layer with a smooth surface, in good agreement with the SEM observation. Corresponding high-resolution TEM lattice fringes and the SAED spot pattern are shown in Fig. 3(b) to identify the single crystalline quality of the nanosheet. It is clear
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that there is a lattice fringe with a lattice space of 0.30 nm, which is consistent with that of (0 1 5) plane for rhombohedral phase of Bi2 Se3 . In order to further confirm the thickness of as-prepared sample, AFM topography images of Bi2 Se3 nanosheets are investigated. As shown in Fig. 4, the surface of the nanosheets is very clean and flat with a uniform thickness across the lateral dimensions. The height profiles corresponding to the dashed line-cut are shown in the right frame. From the height profile, it is clearly seen that the large nanosheets have uniform thicknesses of 3.05 nm and 2.08 nm. The nitrogen gas adsorption–desorption isotherms is performed to have further detailed insight on the high surface area of the nanosheets. As shown in Fig. 5, the as-prepared Bi2 Se3 nanosheets possess a relatively high specific surface area (∼12.5 m2 /g), which is much larger than that of the pristine Bi2 Se3 bulk (∼2.3 m2 /g). Fig. 6 are the Raman spectra of a bulk crystal and Bi2 Se3 nanosheets, in which the inset image displays the vibrational normal modes (only Raman active) of a quintuple layer Bi2 Se3 . Identified in Raman spectra, there are three characteristic peaks (72, 131 and 173 cm−1 ) within the scanned frequency range in bulk crystalline Bi2 Se3 . The peak at 131 cm−1 has to be assigned to the Eg 2 mode, whereas the 72 cm−1 and 173 cm−1 correspond to A1g 1 and A1g 2 mode, respectively [25,26]. These three modes are also observed in the as-prepared Bi2 Se3 nanosheets. It is worth noting that the A1g 1 peak for the nanosheets shifts (∼3 cm−1 ) to lower wave number, possibly due to the out-of-plane vibration is less restrained in the nanosheets than in the bulk [25]. It was also found the Eg 2 mode considerably broadened with the decrease of their thickness, mainly resulted from an enhanced electronphonon coupling in the few QL regime [25]. The Raman spectra indicate that the exfoliated Bi2 Se3 nanosheets with pure and good crystalline quality are successfully prepared via a top-down exfoliation route from the pristine Bi2 Se3 bulk. The applications of the few-layer Bi2 Se3
Fig. 2. XRD patterns of bulk Bi2 Se3 , Li intercalated Bi2 Se3 and exfoliated Bi2 Se3 nanosheets, respectively.
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Fig. 3. TEM characterizations of exfoliated Bi2 Se3 nanosheets. (a) TEM image of one typical Bi2 Se3 nanosheet. HRTEM image (b) and SAED pattern (the inset image) acquired from the same Bi2 Se3 nanosheets.
nanosheets are greatly widened and promoted in many fields, such as in photovoltaic and photoelectrochemical devices, for their high surface-to-volume ratio and tuning capability via thickness variation [27]. We further successively expose the as-prepared Bi2 Se3 nanosheets and the bulk Bi2 Se3 to illuminated and dark conditions, in order to record the I–V feature and instantaneous photoresponse properties. Compared with bulk Bi2 Se3 electrode, the nanosheets electrode shows larger photocurrent during the irradiation of simulated sunlight, as shown in Fig. 7(a). In addition, with the increase of applied potential, the photocurrent of the Bi2 Se3 nanosheets electrode increases much rapidly implying the generation of a photo-induced current, which is mainly attributed to fast the separation of the photogenerated electron–hole pairs under an electric field [28]. To gain insight into the reproducibility of data with time, the illumination source is turned on and off at a given interval, as shown in Fig. 7(b). It is noteworthy that there is a fast and uniform photocurrent responding to each switch-on and switch-off event in the Bi2 Se3 nanosheets electrode, confirming the excellent reproducibility behavior of the device under continuous cycling. In contrast, for bulk Bi2 Se3 , it exhibits a very faint photocurrent density, which is much lower than that of nanosheets electrode
under the same bias. The improvement of the photoresponse properties in Bi2 Se3 nanosheets electrode is proposed to be attributed to their high surface area and ultrathin two-dimensional feature, which is benefited to the absorption of photo and separation of photo-generated electron–hole pairs under an electric field. The parameter of response time is an important symbol to evaluate the performances of photo-sensor device. Generally, the growth time depends on the time period of charges entering the electrode, while the decay time relies on the time period of electron–hole recombination. Fig. 7(c) and (d) depict the photocurrent growth and decay time for bulk Bi2 Se3 and as-prepared Bi2 Se3 nanosheets devices under simulated sunlight illumination, where the experimental points are fitted using the following equations (experimental data: open-line circle and fit data: solid line). The dynamic response of electrodes to the light source can be described by I(t) = I + A exp(−t/) for growth and decay [29], where is the time constant and t indicates the time when light source switches on or off, I represents the initial current, and A is scaling constants. The time constants for growth and decay are estimated from the fitting data I(t) = I + A exp(−t/) for both of them. From the fitting data of Fig. 7(c), it is shown that the time constants for decay and growth of nanosheets electrode are
Fig. 4. Typical (a) 2D and (b) 3D AFM images and height profiles (corresponding to the dashed lines in the image) of Bi2 Se3 nanosheets.
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Fig. 5. N2 adsorption–desorption isotherms of the Bi2 Se3 nanosheets and the Bi2 Se3 bulk (inset).
estimated to be 0.70 s and 1.48 s, respectively. In contrast, the growth and decay time are determined to be 5.32 s and 9.54 s from Fig. 7(d) for the device using the bulk Bi2 Se3 , indicating that the Bi2 Se3 nanosheets device behaves faster response and recovery times under the light irradiation. Other important merit figures of a photodetector are current responsivity (R ) and external quantum efficiency (EQE). Thereinto, current responsivity is defined for the photocurrent generated per unit power of the incident light on the effective area of a photodetector, and the EQE is expressed as the number of detected electrons. The higher values of R and EQE means high sensitivity. R and EQE can be calculated by R = I /(P S) and EQE = hcR /(e)
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Fig. 6. Raman spectra of exfoliated Bi2 Se3 and bulk Bi2 Se3 , respectively. The inset shows the vibrational modes of Bi2 Se3 .
[30,31], where, I represents the magnitude of current change between light illumination on and off, P is the light intensity, S indicates the effective illuminated area, h is the Planck’s constant, c is the velocity of light, e is the electronic charge, and represents the wavelength of the incident light. We defined the parameter hc/(e) as 0 . According to the experimental results, all of the calculated parameters are listed in Table 1. Current responsivities of the bulk and nanosheets devices are determined to be 2.45 mA/W and 20.48 mA/W, respectively. Correspondingly, the EQE is 0 and 8.360 , respectively. It is clearly expressed that the Bi2 Se3 nanosheet is an attractive candidate for photodetector.
Fig. 7. (a) Current–voltage characteristics of pristine Bi2 Se3 bulk electrode and exfoliated Bi2 Se3 nanosheets electrode. (b) Photocurrent response of electrodes with the applied potential of 0.6 V. Time responses of photocurrent growth and decay (inset images) for (c) exfoliated Bi2 Se3 nanosheets electrode and (d) pristine Bi2 Se3 bulk electrode. (The open circles are the experimental points and the solid lines are the fit data according to the exponential equations).
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Fig. 8. a The photoresponse of heat-treated Bi2 Se3 nanosheets electrode under continuous cycling and (b) its time response of photocurrent growth and decay (inset image). The open circles are the experimental points and the solid lines are the fit data to the exponential equations.
Table 1 Table showing the time constants of growth and decay, R and EQE for pristine Bi2 Se3 bulk, exfoliated Bi2 Se3 nanosheets, and heat-treated Bi2 Se3 nanosheets, respectively. Samples
Pristine Bi2 Se3 bulk Exfoliated Bi2 Se3 nanosheets Heat-treated Bi2 Se3 nanosheets
Time (s) Growth
Decay
5.32 0.70 0.45
9.54 1.48 1.32
R (A/W)
EQE (ratio)
2.45 20.48 16.19
0 8.360 6.610
Acknowledgments This work was supported by the Grants from National Natural Science Foundation of China (nos. 51002129, 51172191 and 11204261), Open Fund based on innovation platform of Hunan Colleges and Universities (no. 13K045), Provincial Natural Science Foundation of Hunan (no. 14JJ3079) and Program for Changjiang Scholars and Innovative Research Team in University (IRT13093). References
Generally, the crystalline of materials can be reformed through an annealing process. Datta et al. had reported that the photoresponse performance of Bi2 Se3 thin films prepared by the chemical deposition method could be improved via a low temperature annealing treatment [13]. The influence of the annealing treatment on the properties of Bi2 Se3 nanosheets was also studied in the present work. As shown in Fig. 8, the photocurrent response of the Bi2 Se3 nanosheets after annealing at 150 ◦ C for 1 h also behaves excellent photo-sensitivity. The photocurrent value drops to minimum value instantaneously when the incident light is turned off and returned to the original value just as the light is turned on again. The changes of both “on” and “off” currents are still nearly vertical, indicating that charge transport in the electrode material proceeds very quickly, which is similar with the phenomenon in Bi2 Se3 nanosheets. The lower photocurrent present in the heattreated Bi2 Se3 nanosheets may due to some powders flake off from the conductor glass substrate. It is interesting noted that the growth and decay time of the nanosheets after annealing are estimated to be 0.45 s and 1.32 s, respectively, which are much close with that of as-prepared ones. The effect of annealing on the photo-response performances of Bi2 Se3 nanosheets is negligible, which is proposed to be resulted from the high-crystalline in Bi2 Se3 nanosheets prepared via top-down intercalation and exfoliation route.
4. Conclusion In summary, ultrathin Bi2 Se3 nanosheets are successfully prepared via simple top-down Li ions intercalation and exfoliation route. These nanosheets have high purity with flat and smooth surfaces, and high crystallinity. Compared with the bulk Bi2 Se3 , the as-prepared Bi2 Se3 nanosheets possess a dramatically enhanced photoresponse with very fast response time, superior current responsivity and external quantum efficiency. It is determined that the response time, responsivity, and external quantum efficiency of Bi2 Se3 nanosheets are 0.7 s, 20.48 mA/W, and 8.360 , respectively. It is proposed that the two-dimensional nanostructure of Bi2 Se3 can be effectively used in high performance nanoscale photodetectors.
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Photoresponse properties of ultrathin Bi2 Se3 nanosheets synthesized by hydrothermal intercalation and exfoliation route Chen Zang, Xiang Qi ∗ , Long Ren, Guolin Hao, Yundan Liu, Jun Li, Jianxin Zhong ∗ Hunan Key Laboratory for Micro-Nano Energy Materials and Devices, People’s Republic of China Laboratory for Quantum Engineering and Micro-Nano Energy Technology and Faculty of Materials and Optoelectronic Physics, Xiangtan University, Hunan 411105, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 3 March 2014 Received in revised form 8 July 2014 Accepted 11 July 2014 Available online 18 July 2014 Keywords: Topological insulator Bismuth selenide Nanosheets Responsivity
a b s t r a c t The photoresponse properties of Bi2 Se3 nanosheets prepared by a simple hydrothermal intercalation and exfoliation route are studied. Photoelectrochemical results indicate that the as-prepared Bi2 Se3 nanosheets devices have excellent sensitivity, high-speed and good reproducibility as a photodetector, which are superior to the bulk Bi2 Se3 . Especially, the response time, responsivity, and external quantum efficiency are found to be about 0.7 s, 20.48 mA/W, and 8.360 , respectively. It is proposed that the two-dimensional nanostructure of Bi2 Se3 can be effectively used in high performance nanoscale photodetectors. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Much attention has been focused on a special type of quantum materials of condensed matter physics in recent years, which called topological insulators (TI), their bulks are insulating but surfaces are conducting [1]. TI provide a gateway to investigate fundamental quantum behaviors of exotic quasi-particles [2]. These remarkable materials have fascinating potential applications, such as almost dissipationless surface transport and applications in spintronics and quantum computing [3]. This family of compounds like Bi2 Se3 , Bi2 Te3 , and Sb2 Te3 possess graphene-like two-dimensional materials with layers linked by weak van der Waals forces, they have been theoretically predicted and experimentally proved to be three-dimensional TIs due to their Dirac surface states [4–6]. Their applications such as optical recording system [7], spintronic devices [8] and thermoelectric devices [9–11] have been discovered. Due to its large surface-to-volume ratio and abundant surface states, Bi2 Se3 nanostructures is currently becoming one of the widely studied TI materials for its potential applications in solar selective, optoelectronic devices and thermoelectric coolers [12]. It is reported that Bi2 Se3 thin films behaved excellent photosensitive performances [13]. Unfortunately, the thickness of
∗ Corresponding author at: Faculty of Materials and Optoelectronic Physics, Xiangtan University, Hunan 411105, People’s Republic of China. Tel.: +86 731 58292195. E-mail addresses: [email protected], franck [email protected] (X. Qi), [email protected] (J. Zhong). http://dx.doi.org/10.1016/j.apsusc.2014.07.064 0169-4332/© 2014 Elsevier B.V. All rights reserved.
as-prepared film is still problematic and does not reach the requirement for nanoscale devices. Most recently, Zhang et al. illustrated that graphene-like materials had been behaved superior optical properties [14–17]. They found that TI nanosheets dominated veryhigh-modulation-depth (up to 95%) saturable absorber [18] and could work as a passive Q-switcher for solid-state laser operation [19]. In addition, Bi2 Se3 membrane composited with nanosheets was proposed to be a novel type of optical saturable absorber [20]. While, the sample of Bi2 Se3 nano-platelets was suggested to be a promising nonlinear optical material [21]. In this paper, ultrathin Bi2 Se3 nanosheets are prepared via a top-down intercalation and exfoliation route. The photoresponse properties of the as-prepared Bi2 Se3 nanosheets are investigated, which was comparing with the pristine Bi2 Se3 bulk and the heat-treated Bi2 Se3 nanosheets. 2. Experimental 2.1. Synthesis In a typical procedure, 0.23 g Bi2 Se3 bulk powder was added into 30 mL ethylene glycol solution with 0.2 g LiOH. After constant stirring, the solution was transferred into a Teflon-lined autoclave of 50 mL capacity. The autoclave was heated at 200 ◦ C for about 24 h to achieve the intercalation of Bi2 Se3 by dissolving the Lithium ions (Li+ ) in the solution, followed by cooling to room temperature. The dispersions in the solution was then carefully collected by filtration and washed with acetone to eliminate the excess ethylene glycol solution of lithium hydroxide. Colloidal suspensions of
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bismuth selenium can be readily prepared by exfoliating the lithiated powder in deionized water. In order to explain the effect of Lithium ions, the controlled experiments without LiOH in the ethylene glycol solution during the hydrothermal process were preformed.
and ultrasonicated for 1 h to yield a stable Bi2 Se3 suspension. The Na2 SO4 aqueous solution (0.5 M, 98.0%) was used as electrolyte and photoresponse measurements were carried out under 0.12 W/cm2 illumination via a Xe lamp with AM 1.5 G filter as simulated sunlight.
2.2. Characterization
3. Results and discussion
The morphologies and microstructure of the samples were studied by scanning electron microscope (SEM, JEOL JSM-6610LV), and transmission electron microscopy (TEM, JEOL JEM-2100F). The crystallographic information of the obtained samples was analyzed by X-ray diffraction (XRD, Rigaku D/Max 2500) using the Cu K␣ radiation in the 2 range of 10–80◦ . Atomic force microscopic (AFM, SPI-3800N) studies were carried out in air at ambient condition. Raman spectra were collected for bulk samples and nanosheets of Bi2 Se3 by employing Renishaw in-Via micro Raman spectrometer, excited at room temperature with laser light (532 nm). N2 adsorption–desorption measurements were conducted at 77 K on a Nove 2200e analyzer. The specific surface area was obtained by Brunauer–Emmett–Teller (BET) method.
Fig. 1(a) shows the digital photos of three samples in aqueous dispersion, they are the raw bulk Bi2 Se3 , the exfoliated nanosheets and the Bi2 Se3 bulk after a hydrothermal process in the ethylene glycol solution without LiOH, respectively. It is clear that the aqueous dispersion of exfoliated Bi2 Se3 is remarkably different from the former pellucid solution with a large number of Bi2 Se3 bulk in the bottom, which presents a turbid and uniform dispersion state, implying the smaller size of the exfoliated sample. To gain insight into the transformation of the bulk Bi2 Se3 before and after the hydrothermal intercalation, the morphologies of bulk and exfoliated Bi2 Se3 samples are further characterized by SEM. It is obviously shown in Fig. 1(b) that the layered surface of the huge bulk is smooth and flat. On the contrary, for the exfoliated sample shown in Fig. 1(c), large-scale isolated nanosheets with large lateral dimensions are widespread. We propose the formation of nanosheets via hydrothermal exfoliation route from bulk crystals follows the following process [22], as shown schematically in Figure 1(d). Here, Li+ on the process is confirmed to be one indispensable factor by checking the experiment of preparing Bi2 Se3 bulk in the ethylene glycol solution but without lithium hydroxide (LiOH). Without the addition of Li, the solution is still clear (right one in Fig. 1(a)) and the Bi2 Se3 bulk after hydrothermal process also has original smooth and flat layered surface (inset of Fig. 1(b)), same
2.3. Photoelectrochemical measurements Photoelectrochemical test systems were composed of a CHI 660D electrochemistry workstation, an illumination source, and a homemade three-electrode cell. The indium–tin oxide (ITO) conductor glass coated with as-prepared Bi2 Se3 suspension was used as working electrode with Pt foil and saturated calomel electrode as counter and reference electrodes, respectively. Typically, a total of 10 mg as-prepared samples was mixed with 2 mL de-ionized water
Fig. 1. (a) Digital photos of the change in the solution containing Bi2 Se3 bulk before (I) and after reaction in the ethylene glycol solution with LiOH (II) or without (III); SEM image of (b) Bi2 Se3 bulk and (c) exfoliated Bi2 Se3 nanosheets (the image inserted in (b) is the sample after reaction with the absent of LiOH); (d) Schematic representation of the formation of Bi2 Se3 nanosheets.
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as that of pristine Bi2 Se3 bulk. For the Bi2 Se3 consist with weakly van der Waals-bonded Se Bi Se Bi Se slabs [12], the Li cations dissolve in the ethylene solvent and insert into interlayer of Bi2 Se3 , then form Liy Bi2 Se3 during the reduction process as given in Eq. (1). During this Li-intercalated process, the ethylene glycol serves as both reductant and solvent. After exposing to water, the lithium ions in Liy Bi2 Se3 dissolve rapidly, resulting in the production of LiOH) and hydrogen gas, as given in Eq. (2). −y Bi2 Se3 + yLi+ + ye− → Li+ y (Bi2 Se3 )
Liy Bi2 Se3 + yH2 O → Bi2 Se3 (nano) + yLiOH +
(1) y H2 ↑ 2
(2)
Due to the rapid expansion in the layers and forming suspensions of Bi2 Se3 nanosheets, the original quintuple layers are homogeneously exfoliated. It is noted that the powder of the Bi2 Se3 nanostructures could be easily be collected by filtration. The phases of the products obtained above are determined by X-ray diffraction experiments, as shown in Fig. 2. The XRD patterns of exfoliated Bi2 Se3 and bulk Bi2 Se3 match the rhombohedral phase of Bi2 Se3 (JCPDS no. 89-2008), no other impurities are observed. For the Li intercalated Bi2 Se3 , additional diffraction peaks at 19◦ , 21◦ , 28◦ , 31◦ , 39◦ and 59◦ , may be originated from Bi2 Se3 being intercalated Li to form Liy Bi2 Se3 , which is similar with previous results [23,24]. Interestingly, the additional peaks are disappeared after exfoliated process, indicating that the Li ion in the Li intercalated Bi2 Se3 are completely excluded to form Bi2 Se3 nanosheets. To further investigate the microstructures of the nanosheets, TEM and HRTEM measurements are carried out. Fig. 3(a) displays a typical low-magnification TEM image of an individual Bi2 Se3 sheet, which has a very thin layer with a smooth surface, in good agreement with the SEM observation. Corresponding high-resolution TEM lattice fringes and the SAED spot pattern are shown in Fig. 3(b) to identify the single crystalline quality of the nanosheet. It is clear
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that there is a lattice fringe with a lattice space of 0.30 nm, which is consistent with that of (0 1 5) plane for rhombohedral phase of Bi2 Se3 . In order to further confirm the thickness of as-prepared sample, AFM topography images of Bi2 Se3 nanosheets are investigated. As shown in Fig. 4, the surface of the nanosheets is very clean and flat with a uniform thickness across the lateral dimensions. The height profiles corresponding to the dashed line-cut are shown in the right frame. From the height profile, it is clearly seen that the large nanosheets have uniform thicknesses of 3.05 nm and 2.08 nm. The nitrogen gas adsorption–desorption isotherms is performed to have further detailed insight on the high surface area of the nanosheets. As shown in Fig. 5, the as-prepared Bi2 Se3 nanosheets possess a relatively high specific surface area (∼12.5 m2 /g), which is much larger than that of the pristine Bi2 Se3 bulk (∼2.3 m2 /g). Fig. 6 are the Raman spectra of a bulk crystal and Bi2 Se3 nanosheets, in which the inset image displays the vibrational normal modes (only Raman active) of a quintuple layer Bi2 Se3 . Identified in Raman spectra, there are three characteristic peaks (72, 131 and 173 cm−1 ) within the scanned frequency range in bulk crystalline Bi2 Se3 . The peak at 131 cm−1 has to be assigned to the Eg 2 mode, whereas the 72 cm−1 and 173 cm−1 correspond to A1g 1 and A1g 2 mode, respectively [25,26]. These three modes are also observed in the as-prepared Bi2 Se3 nanosheets. It is worth noting that the A1g 1 peak for the nanosheets shifts (∼3 cm−1 ) to lower wave number, possibly due to the out-of-plane vibration is less restrained in the nanosheets than in the bulk [25]. It was also found the Eg 2 mode considerably broadened with the decrease of their thickness, mainly resulted from an enhanced electronphonon coupling in the few QL regime [25]. The Raman spectra indicate that the exfoliated Bi2 Se3 nanosheets with pure and good crystalline quality are successfully prepared via a top-down exfoliation route from the pristine Bi2 Se3 bulk. The applications of the few-layer Bi2 Se3
Fig. 2. XRD patterns of bulk Bi2 Se3 , Li intercalated Bi2 Se3 and exfoliated Bi2 Se3 nanosheets, respectively.
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Fig. 3. TEM characterizations of exfoliated Bi2 Se3 nanosheets. (a) TEM image of one typical Bi2 Se3 nanosheet. HRTEM image (b) and SAED pattern (the inset image) acquired from the same Bi2 Se3 nanosheets.
nanosheets are greatly widened and promoted in many fields, such as in photovoltaic and photoelectrochemical devices, for their high surface-to-volume ratio and tuning capability via thickness variation [27]. We further successively expose the as-prepared Bi2 Se3 nanosheets and the bulk Bi2 Se3 to illuminated and dark conditions, in order to record the I–V feature and instantaneous photoresponse properties. Compared with bulk Bi2 Se3 electrode, the nanosheets electrode shows larger photocurrent during the irradiation of simulated sunlight, as shown in Fig. 7(a). In addition, with the increase of applied potential, the photocurrent of the Bi2 Se3 nanosheets electrode increases much rapidly implying the generation of a photo-induced current, which is mainly attributed to fast the separation of the photogenerated electron–hole pairs under an electric field [28]. To gain insight into the reproducibility of data with time, the illumination source is turned on and off at a given interval, as shown in Fig. 7(b). It is noteworthy that there is a fast and uniform photocurrent responding to each switch-on and switch-off event in the Bi2 Se3 nanosheets electrode, confirming the excellent reproducibility behavior of the device under continuous cycling. In contrast, for bulk Bi2 Se3 , it exhibits a very faint photocurrent density, which is much lower than that of nanosheets electrode
under the same bias. The improvement of the photoresponse properties in Bi2 Se3 nanosheets electrode is proposed to be attributed to their high surface area and ultrathin two-dimensional feature, which is benefited to the absorption of photo and separation of photo-generated electron–hole pairs under an electric field. The parameter of response time is an important symbol to evaluate the performances of photo-sensor device. Generally, the growth time depends on the time period of charges entering the electrode, while the decay time relies on the time period of electron–hole recombination. Fig. 7(c) and (d) depict the photocurrent growth and decay time for bulk Bi2 Se3 and as-prepared Bi2 Se3 nanosheets devices under simulated sunlight illumination, where the experimental points are fitted using the following equations (experimental data: open-line circle and fit data: solid line). The dynamic response of electrodes to the light source can be described by I(t) = I + A exp(−t/) for growth and decay [29], where is the time constant and t indicates the time when light source switches on or off, I represents the initial current, and A is scaling constants. The time constants for growth and decay are estimated from the fitting data I(t) = I + A exp(−t/) for both of them. From the fitting data of Fig. 7(c), it is shown that the time constants for decay and growth of nanosheets electrode are
Fig. 4. Typical (a) 2D and (b) 3D AFM images and height profiles (corresponding to the dashed lines in the image) of Bi2 Se3 nanosheets.
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Fig. 5. N2 adsorption–desorption isotherms of the Bi2 Se3 nanosheets and the Bi2 Se3 bulk (inset).
estimated to be 0.70 s and 1.48 s, respectively. In contrast, the growth and decay time are determined to be 5.32 s and 9.54 s from Fig. 7(d) for the device using the bulk Bi2 Se3 , indicating that the Bi2 Se3 nanosheets device behaves faster response and recovery times under the light irradiation. Other important merit figures of a photodetector are current responsivity (R ) and external quantum efficiency (EQE). Thereinto, current responsivity is defined for the photocurrent generated per unit power of the incident light on the effective area of a photodetector, and the EQE is expressed as the number of detected electrons. The higher values of R and EQE means high sensitivity. R and EQE can be calculated by R = I /(P S) and EQE = hcR /(e)
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Fig. 6. Raman spectra of exfoliated Bi2 Se3 and bulk Bi2 Se3 , respectively. The inset shows the vibrational modes of Bi2 Se3 .
[30,31], where, I represents the magnitude of current change between light illumination on and off, P is the light intensity, S indicates the effective illuminated area, h is the Planck’s constant, c is the velocity of light, e is the electronic charge, and represents the wavelength of the incident light. We defined the parameter hc/(e) as 0 . According to the experimental results, all of the calculated parameters are listed in Table 1. Current responsivities of the bulk and nanosheets devices are determined to be 2.45 mA/W and 20.48 mA/W, respectively. Correspondingly, the EQE is 0 and 8.360 , respectively. It is clearly expressed that the Bi2 Se3 nanosheet is an attractive candidate for photodetector.
Fig. 7. (a) Current–voltage characteristics of pristine Bi2 Se3 bulk electrode and exfoliated Bi2 Se3 nanosheets electrode. (b) Photocurrent response of electrodes with the applied potential of 0.6 V. Time responses of photocurrent growth and decay (inset images) for (c) exfoliated Bi2 Se3 nanosheets electrode and (d) pristine Bi2 Se3 bulk electrode. (The open circles are the experimental points and the solid lines are the fit data according to the exponential equations).
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Fig. 8. a The photoresponse of heat-treated Bi2 Se3 nanosheets electrode under continuous cycling and (b) its time response of photocurrent growth and decay (inset image). The open circles are the experimental points and the solid lines are the fit data to the exponential equations.
Table 1 Table showing the time constants of growth and decay, R and EQE for pristine Bi2 Se3 bulk, exfoliated Bi2 Se3 nanosheets, and heat-treated Bi2 Se3 nanosheets, respectively. Samples
Pristine Bi2 Se3 bulk Exfoliated Bi2 Se3 nanosheets Heat-treated Bi2 Se3 nanosheets
Time (s) Growth
Decay
5.32 0.70 0.45
9.54 1.48 1.32
R (A/W)
EQE (ratio)
2.45 20.48 16.19
0 8.360 6.610
Acknowledgments This work was supported by the Grants from National Natural Science Foundation of China (nos. 51002129, 51172191 and 11204261), Open Fund based on innovation platform of Hunan Colleges and Universities (no. 13K045), Provincial Natural Science Foundation of Hunan (no. 14JJ3079) and Program for Changjiang Scholars and Innovative Research Team in University (IRT13093). References
Generally, the crystalline of materials can be reformed through an annealing process. Datta et al. had reported that the photoresponse performance of Bi2 Se3 thin films prepared by the chemical deposition method could be improved via a low temperature annealing treatment [13]. The influence of the annealing treatment on the properties of Bi2 Se3 nanosheets was also studied in the present work. As shown in Fig. 8, the photocurrent response of the Bi2 Se3 nanosheets after annealing at 150 ◦ C for 1 h also behaves excellent photo-sensitivity. The photocurrent value drops to minimum value instantaneously when the incident light is turned off and returned to the original value just as the light is turned on again. The changes of both “on” and “off” currents are still nearly vertical, indicating that charge transport in the electrode material proceeds very quickly, which is similar with the phenomenon in Bi2 Se3 nanosheets. The lower photocurrent present in the heattreated Bi2 Se3 nanosheets may due to some powders flake off from the conductor glass substrate. It is interesting noted that the growth and decay time of the nanosheets after annealing are estimated to be 0.45 s and 1.32 s, respectively, which are much close with that of as-prepared ones. The effect of annealing on the photo-response performances of Bi2 Se3 nanosheets is negligible, which is proposed to be resulted from the high-crystalline in Bi2 Se3 nanosheets prepared via top-down intercalation and exfoliation route.
4. Conclusion In summary, ultrathin Bi2 Se3 nanosheets are successfully prepared via simple top-down Li ions intercalation and exfoliation route. These nanosheets have high purity with flat and smooth surfaces, and high crystallinity. Compared with the bulk Bi2 Se3 , the as-prepared Bi2 Se3 nanosheets possess a dramatically enhanced photoresponse with very fast response time, superior current responsivity and external quantum efficiency. It is determined that the response time, responsivity, and external quantum efficiency of Bi2 Se3 nanosheets are 0.7 s, 20.48 mA/W, and 8.360 , respectively. It is proposed that the two-dimensional nanostructure of Bi2 Se3 can be effectively used in high performance nanoscale photodetectors.
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