High-performance self-driven broadband photoelectrochemical photodetector based on reduced graphene oxide/Bi2Te3 heterojunction

Abstract

Attributed to its excellent physicochemical properties, graphene (GR) has very active applications in the fields of catalysis, optoelectronic devices, and battery electrode materials. However, until now, regulating the type and density of carriers in GR is still crucial for its practical applications. Here, reduced graphene oxide (RGO)-Bi₂Te₃ heterojunctions doped with different contents were prepared by a simple one-step method. The Bi₂Te₃ materials containing different RGO were made into broadband (365-850 nm) photoelectrochemical-type detectors, and the effects of the doping amount of RGO on the optoelectronic behavior of the devices and the intrinsic operation mechanism of the devices were investigated in detail. The results show that the values of I_ph/I_dark, R_i, and D· of Bi₂Te₃/RGO heterojunction devices obtained with 1 mg of RGO doping are 412, 6.072 mA/W, and 2.406 × 10¹⁰ Jones, respectively. It is anticipated that this work will provide a research basis for future quantitative tuning of the performance of micro-nano devices by GR.

Headnote

ARTICLE INFO

Keywords:

Photoelectrochemical

Bi₂Te₃/RGO heterojunctions

Doping regulation

Self-driven

Broadband

ABSTRACT

Attributed to its excellent physicochemical properties, graphene (GR) has very active applications in the fields of catalysis, optoelectronic devices, and battery electrode materials. However, until now, regulating the type and density of carriers in GR is still crucial for its practical applications. Here, reduced graphene oxide (RGO)-Bi₂Te₃ heterojunctions doped with different contents were prepared by a simple one-step method. The Bi₂Te₃ materials containing different RGO were made into broadband (365-850 nm) photoelectrochemical-type detectors, and the effects of the doping amount of RGO on the optoelectronic behavior of the devices and the intrinsic operation mechanism of the devices were investigated in detail. The results show that the values of I_ph/I_dark, R_i, and D· of Bi₂Te₃/RGO heterojunction devices obtained with 1 mg of RGO doping are 412, 6.072 mA/W, and 2.406 × 10¹⁰ Jones, respectively. It is anticipated that this work will provide a research basis for future quantitative tuning of the performance of micro-nano devices by GR.

(ProQuest: ... denotes formulae omitted.)

1. Introduction

With the advent of graphene (GR), a two-dimensional (2D) material system different from nanomaterials and bulk-phase materials was discovered [1,2]. Owing to their unique layered structure and physicochemical properties, 2D materials have very promising applications in the fields of photoelectric catalysis, photoelectric detection, gas-sensitive components, electrode materials, and medical therapy [3-7]. As a typical 2D material, GR has become a very important candidate in the field of optoelectronic devices due to its excellent thermal and electrical conductivity, as the internal charge migration rate is much higher than that of a typical semiconductor (2000 cm²/V/s) [8-10]. The zero-bandgap structure of GR determines that its internal charge is free to flow just like any metal, and cannot be regulated by external energy excitation like a semiconductor with a bandgap, which greatly limits application in the field of optoelectronic devices [1,2]. Regulating the type and density of carriers in GR is crucial for its practical applications. In recent years, a lot of work has been done to open the band gap of GR [11]. The development of GR is mainly expanded in two aspects: on one hand, the construction of heterojunction graphene as a charge collector and transport channel [12, 131, and on the other hand, the modulation of electronic properties and band gap of GR through the doping of N, B, S and other elements [14-16]. Photonics and optoelectronic devices integrated with graphene and 2D materials are expected to be at the core of future industrial technology development [17].

Bismuth telluride (Bi₂Te³) has a very promising application in the field of thermoelectric devices owing to high electrical conductivity and low thermal conductivity [18,19]. As a typical narrow bandgap material, Bi₂Te₃ has a theoretical bandgap value of 0.15 eV and is promising for a wide range of applications in the mid-infrared detection field [20]. However, compared with high mobility materials such as GR, black phosphorus (BP), and Bi₂O₂Se, because of the thermal effect of the device prompted by light radiation [10,21-23], the internal dark current will disappear with the light radiation and produce a significant hysteresis, which is very harmful to the sensitivity of the device [24-26]. To expand the application of Bi₂Te₃ materials in the field of optoelectronic devices, a lot of research work has been done to construct Bi₂Te₃/GR heterojunction devices to enhance photodetection performance. Currently, two methods have been reported for the preparation of Bi₂Te₃/GR heterojunctions, one of which is the vapor deposition technique, which usually involves material transfer and is a relatively complex and time-consuming process. The other is the synthesis of Bi₂Te₃/GR nano-heterojunctions in a high-temperature and high-pressure liquid-phase reaction. However, a serious challenge for such heterojunctions is the preparation of large-scale devices. Shivananju prepared a Bi₂Te₃/GR heterojunction by a two-step chemical vapor deposition method and integrated an ultra-broad band (400 nm-10.768 µm) optical fiber Bragg grating device [17]. Zhang et al. prepared Bi₂Te₃/GaAs heterojunctions by molecular beam epitaxial growth technique and integrated GR/Bi>. Te₃/GaAs array devices by transferring and vaporizing electrodes. The GR/Bi₂Te₃/GaAs array device was found to have a response rate of 405 nm to 4.5 pm in a wide wavelength range and a response rate of microsecond at room temperature [27]. Islam et al. used hydrothermally synthesized Bi, Tes nanowires modified with GR to obtain highly sensitive infrared photoelectric detectors [28]. Similarly, Yoo et al. used Bi₂ Te₃-modified GR to obtain a broad-band detector with an optical gain of up to 3 x 10⁴ covering 400-2200 nm [29]. Wang et al. synthesized Bi₂Te₃/GR heterojunctions by a one-step hydrothermal method and used them as photoelectrochemical (PEC)-type electrode materials to detect the photoresponsive behavior under simulated solar irradiation [30]. Although the reported constructed GR/Bi₂Te₃ heterojunctions devices all showed different degrees of performance enhancement [31]. Unfortunately, none of the studies addressed the effect of GR doping amount on the optoelectronic properties of Bi₂Te₃ materials. Therefore, it is necessary to give a quantitative standard for graphene doping so that subsequent optimization and enhancement of micro and nanodevices can be carried out.

In our work, Bi₂Te₃/RGO heterojunctions were synthesized by a simple one-step hydrothermal method, and the effects of RGO doping on the morphology and optoelectronic properties of two-dimensional Bi₂Te₃ nanosheets were investigated in detail. Further, the role played by RGO in Bi₂Te₃/RGO devices was investigated.

2. Materials and methods

The one-step hydrothermal reaction method was used to prepare Bi₂Te₃/RGO heterojunctions, which were synthesized as follows: 0.4650 g of Bi₂O₃ and 0.4788 g of TeO₂ and 0.8 g of polyvinylpyrrolidone (PVP) were added to a 50 mL beaker containing 32 mL of ethylene glycol (EG) with continuous stirring. Then, 4 mL of aqueous sodium hydroxide (4 M) was added to the mixed solution. Five different groups of graphene oxide (GO) samples, 0.5 mg, 1 mg, 2 mg, 3 mg, and 4 mg, were weighed and added to the prepared Bi₂Te₃ precursor solution, stirred for 1 h, and sonicated for 30 min, then loaded into a 50 mL stainless steel reactor and set at 200 °C for 4.5 h. After the reaction was cooled to room temperature, the mixed solution was placed in a 50 mL centrifuge tube, washed with acetone, deionized water, and anhydrous ethanol more than three times, and finally dried in a vacuum chamber at 40 °C for 24 h.

Here, the samples with pure Bi₂Te₃ and added graphene oxide 0.5 mg, 1 mg, 2 mg, 3 mg, and 4 mg are noted as BGO.5, BG1, BG2, BG3, and BG4, respectively. The pure Bi₂Te₃ sample obtained without the addition of graphene is noted as B.

The morphology and elemental composition of the material are observed by a field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectroscopy (EDS). The finer microstructure of the samples is obtained by transmission electron microscopy (TEM). The phase composition, chemical environment, and surface composition of the hydrothermally synthesized Bi₂Te₃/RGO heterojunctions were analyzed by the X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence spectrometer (PL) and high-resolution Raman spectroscopy (Raman). UV-vis-IR absorption spectra of RGO, Bi₂Te₃ and RGO/Bi₂Te₃ (BG1) heterojunctions recorded by UV-vis spectrometer.

The Fluorine doped indium tin oxide glass (FTO) coated with BisTez/ RGO and the FTO with Pt electrodes is encapsulated at 150 °C using a 30 µm-thick thermoplastic film to form an electrochemical photodetector. The encapsulated devices were injected into the polysulfide electrolyte for subsequent performance measurements. Similarly, pure Bi₂Te₃ devices were prepared and encapsulated by the above method. The photovoltaic properties of the devices were obtained by recording with a Keithley 2400 digital source meter. The electrochemical impedance spectra (EIS) were recorded by an electrochemical workstation (CHI660E). The detailed synthesis of the materials and the preparation process of the device as well as the schematic diagram of the device operation are shown in

3. Results and discussion

3.1. Morphology and structure

The morphology of the hydrothermally synthesized Bi₂Te₃/RGO heterojunctions was characterized in detail by SEM. depicts the SEM images of the heterojunction samples prepared by mixing different ratios of Bi₂Te₃ with RGO. a illustrates the SEM image of simple Bi₂Te₃ without the addition of RGO, where the stacked Bi₂Te₃ hexagonal nanosheets with a size of about 500 nm can be observed. b-f shows the SEM images of Bi₂Te₃ nanosheets doped with different contents of RGO. The RGO in the heterojunction sample is seen as a lamellar undulating sheet with folds (marked in red in ). In addition, the addition of graphene did not change the morphology of the Bi₂Te₃ hexagonal nanosheets. As the addition of GO precursors increased, more RGO doping in Bi₂Te₃ nanosheets was observed in the obtained hydrothermal samples. The SEM images in b-f also exhibit that the GO is reduced to RGO and densely clustered with Bi₂Te₃ in contact with each other, which is conducive to the interconnection of electrons and the application of detectors. g-j represents the SEM and elements mappings of the Bi₂Te₃/RGO heterojunctions. The elemental mapping shows that Bi and Te elements are uniformly distributed on the surface of the sample, while the C elements come from RGO. From the point of view of the C elemental distribution, the RGO has been homogeneously mixed with the Bi₂Te₃ nanosheets to form Bi₂Te₃/RGO heterojunctions. To further observe the fine structure of Bi₂Te₃/RGO, the results of the prepared samples were investigated by TEM as shown in k-1. The thin complete and regular Bi₂Te₃ nano-hexagonal sheets can be seen in the low magnification TEM images, while the thin folded irregular sheet samples are the RGO. The white arrow in the indicates the RGO. The presence of Bi₂Te₃ nanosheets and reduced graphene were also observed in the low magnification TEM images, confirming the synthesis of Bi₂Te₃ heterojunctions.

To obtain detailed structural information on Bi₂Te₃, RGO, and Bi₂Te₃/RGO, HRTEM was applied to record the lattice fringes of the three materials [32,33]. shows the TEM image of Bi₂Te₃ nanosheets, Which are hexagonal in shape and about 500 nm in size, which is consistent with the SEM results. While RGO is an irregular film ( ) with a size of more than 1 pm. shows a layer of RGO stacked between two Bi₂Te₃ nanosheets. shows the HRTEM of Bi₂Te₃ hexagonal nanosheets, where the lattice fringes spacing was measured to be 0.22 nm, corresponding to the (1120) crystal plane of Bi₂Te₃ [34,35]. The HRTEM image of RGO partially shows ordered lattice fringes (Fig.S1e), while some amorphous structures are present at the edges of RGO, and the spacing of the lattice fringes is 0.25 nm, which is consistent with literature reports [36]. Fig. S1f shows the HRTEM of the RGO/-Bi₂Te₃ heterojunction, where it is observed that the RGO is in the middle sandwich and the lattice fringes spacing is 0.25 nm, whereas the lattice fringes spacing of Bi₂Te₃ is 0.22 nm. Additionally, the presence of an amorphous layer is observed on the outer surface of Bi₂Te₃, suggesting that the surface of Bi₂Te₃ in the RGO/Bi₂Te₃ heterojunctions is oxidized.

In addition, XRD, XPS, and Raman spectroscopy were performed to characterize the material in detail to better understand the composition of the Bi₂Te₃/RGO heterojunctions and the chemical environment of the elements. The XRD of Bi₂Te₃/RGO samples obtained with different additions of RGO can be observed in Fig. 3a, from which it can be seen that the gradual increase of RGO content has almost no effect on the diffraction peaks of Bi₂Te₃ samples. Comparing with the diffraction peaks of the standard Bi₂Te₃ (JCPDS#15-0863), it was found that all the diffraction peaks of Bi₂Te₃ in the prepared series of Bi₂Te₃/RGO heterojunctions corresponded well with the standard peaks. It is important to emphasize that no diffraction peak is found at 20 = 10.7° for the (001) crystal plane of GO, which proves that all the added GO is reduced to RGO during the hydrothermal process [37,38]. Fig. 3b shows a local magnification of the XRD spectrum of Bi₂Te₃ and Bi₂Te₃/RGO. It can be observed that with the increase of the RGO amount, the gradual accentuation of the two diffraction peaks located near 20 = 54°, corresponds to the (004) crystal plane of RGO [39]. To further verify the presence of RGO in Bi₂Te₃/RGO, Raman was utilized to observe and record the results of the samples as shown in Fig. 3c. The Raman spectra of pure Bi₂Te₃ nanosheets can be understood to be located at 89-96 cm^-1 1, 113 cm^-1, and 135 cm^-1 attributed to E²_g, A¹_w, and A²_1g vibrations on the sample surface, respectively [28,29]. Similarly, the prototype of the Raman peak belonging to Bi₂Te₃ was also observed in the Bi₂Te₃/RGO heterojunctions. More importantly, two significant peaks belonging to RGO are found near 1500 cm^-1, namely the D band (1345 cm^-1) and the G band (1580 cm^-1) [40]. Further, the intensity ratio Ip/Ig of the band of Bi₂Te₃/RGO is 0.91, implying that the oxidation group of the initial GO is reduced in the hydrothermal reaction resulting in the product RGO slightly exhibiting the properties of the 2D material with sp² hybridization [38]. Fig. 3d reveals the XPS survey of Bi₂Te₃ (B) nanosheets and Bi₂Te₃/RGO (BG1) heterojunctions. The XPS peaks typical of Bi and Te as well as С elements found in heterojunctions on RGO/Bi₂Te₃ are observed to be present on the XPS Survey. The chemical environment of typical C elements belonging to RGO can be seen by XPS fine spectroscopy (Fig. 3e), in which the peak at the binding energy of 284.5 eV corresponds to the sp² hybridization of C elements. The peak near 285.8 eV corresponds to the sp³ hybridization of C elements. The peak appearing near 287.5 eV corresponds to C-OH, indicating that the RGO surface contains hydroxyl functional groups [41,42]. The fine spectrum of elemental Bi in the RGO/Bi₂Te₃ heterojunction contains two typical splitting peaks belonging to Bi 4f (Fig. 35), corresponding to the binding energies at 157.8 eV and 163.0 eV, respectively, which show an increase in the spectral peaks of the binding energies by more than 0.6 eV when compared to pure Bi₂Te₃ Bi 4f [43-45]. The higher binding energy is attributed to the strong interaction between Bi₂Te₃ and RGO, which puts the Bi element in Bi₂Te₃ in a high valence state where it loses electrons. Miraculously, two peaks in the oxidation state of Bi-O at 158.6 eV and 164.0 eV were found in RGO/Bi₂Te₃, which are not found in unoxidized pure Bi₂Te₃ [46], and it is obvious that the chemical environment of the elements in Bi₂Te₃ is very much altered by the addition of GO. Similarly, the fine spectrum of elemental Te in the RGO/Bi₂Te₃ heterojunction contains two peaks belonging to Te 3d at 572.3 eV and 582.4 eV [44,45], respectively (Fig. 3g). Two peaks in the oxidation state of Te-O at 575.9 eV and 586.3 eV [46], respectively, were also found in the RGO/Bi₂Te₃ heterojunctions, which again verifies that the interaction between the two RGO/Bi₂Te₃ influences the chemical environment of the Te element of the Bi₂Te₃.

3.2. Performance of devices

GR has attracted much attention owing to the ultra-fast response speed and broadband optical response while possessing zero-bandgap properties that severely hinder the application in the field of photodetection [47,48]. In our work, to broaden the application of graphene in the field of detection, Bi₂Te₃/RGO heterojunctions are skillfully constructed to assemble into self-driven broadband photodetectors. Some details and key parameters for the preparation of PEC-type RGO/Bi₂Te₃ devices as well as the device package schematic (Fig. S2 and Fig. 1) are detailed in the Supporting Information. Switching ratio (I_ph/I_dark), responsivity (R_i), and detectivity (D·) are very important evaluation metrics for photodetectors [49-51]. To better compare the performance of different Bi;Te3/RGO devices, the quantization of J, R_i, and D· is calculated as follows [52-54]:

... Eq (1)

... Eq (2)

... Eq (3)

Where I_ph and I_dark are the photocurrent and dark current of the device, J is the photocurrent density, 5 is the effective irradiated area of the device, P, is the laser power, h is Planck's constant, c is the speed of light under vacuum, e is the elementary charge, and λ is the wavelength.

Fig. 4 depicts the switching behavior of Bi₂Te₃ and different components of Bi₂Te₃/RGO heterojunction with different wavelengths of laser irradiation. To evaluate the switching performance of the prepared devices, the devices were placed in a specific dark room environment and given different laser wavelength irradiation The switching experiments were performed in 20 s cycles, where the devices were exposed to light for 10 s and in a dark room environment for 10 s. Fig. 4a exhibits the switching behavior of Bi₂Te₃ and Bi₂Te₃/RGO heterojunctions under 365 nm laser irradiation (30 mW/cm²), which reveals that the photocurrent of the Bi; Te3/RGO (denoted as BG1) device obtained by adding 1 mg of RGO is as high as 90.58 pA under the identical conditions with 0 V bias. Compared to pure Bi₂Te₃ and other components of Bi₂Te₃/RGO devices, BG1 devices exhibit excellent switching ratios under 365 nm laser irradiation (I_ph/I_dark, 412). It is worth emphasizing that when the addition of RGO is less than 2 mg, the photocurrent of the device increases with the addition of RGO. While the amount of RGO added >2 mg, the photocurrent of the device shows a gradual decay with the increase of RGO content. In addition, the switching behavior of all devices except BG4 was found to be better than that of the pure Bi₂Te₃ devices (denoted as B), and the switching ratio of BG1 devices was 21.68 times higher than that of B devices. This phenomenon suggests that the presence of a small amount of RGO (<2 mg) has an enhancing effect on the photoresponse performance of Bi₂Te₃ devices under UV irradiation. Similarly, the switching behaviors of the above devices were investigated under visible blue light (470 nm, 30 mW/cm²) laser irradiation as shown in Fig. 4b. An interesting phenomenon is observed, the photocurrent values of BGO.5 and BG1 devices are 32.72 pA and 32.95 pA with almost no difference, while the switching ratios of the devices are 24 and 195, respectively. The photocurrent of the device shows a trend of increasing and then decreasing with increasing RGO content under the irradiation of 470 nm visible light. The excellent switching ratio of BG1 devices implies that the addition of a moderate amount of RGO has the potential to improve the photocurrent and suppress the dark current of the Bi>Tez device. In addition, the switching behavior of the devices under visible irradiation at 530 nm and 625 nm was also investigated as shown in Fig. 4c,d. The switching performance found that the BG1 device still has large photocurrent values and high switching ratios under 530 nm and 625 nm laser irradiation. Similarly, the switching behavior in the infrared (850 nm) was investigated for the prepared devices, as shown in Fig. 4e. The BGO.5 device has the highest photocurrent value (2.80 pA), while the switching ratio of the device (6.18) is much lower than that of the BG1 device (13.49). Fig. 4f depicts the photocurrents of various devices at different wavelengths of laser irradiation. Compared to other devices, BG1 devices are found to exhibit superior switching ratios in both UV-vis-IR bands. In conclusion, the BG1 device obtained by adding 1 mg RGO to the prepared self-driven PEC devices possesses excellent optoelectronic properties. Simultaneously, the devices showed a consistent pattern that the photocurrent of the devices showed an increasing trend followed by a decreasing trend with increasing RGO content in the system under all laser irradiations.

To further investigate the optoelectronic performance of the BG1 device, the switching behavior of the device under different light intensities was researched. Fig. 5a shows the switching behavior of the BG1 device under different intensities of 365 nm laser irradiation. The photocurrent of the BG1 device is seen to increase with laser intensity, which is consistent with what is reported in the literature [27,50]. To better visualize the photocurrent of the device versus the light intensity, the data of photocurrent versus light intensity were fitted to the results shown in Fig. 5b. The photocurrent density of the BG1 device was found to increase with the increase of light intensity, which conforms to a linear relationship. The photocurrent value of the BG1 device was found to become larger with increasing light intensity, which conforms to a linear relationship. The responsivity of the BG1 device gradually decreases with increasing light intensity, which has similar experimental results as that reported previously. Moreover, the responsivity of the BG1 device reaches 7.15 mW/A at 10 mW/cm² of laser irradiation (Fig. 5b). Similarly, under laser irradiation at a wavelength of 470 nm, the photocurrent of the BG1 device at O V bias still increases with increasing light intensity, while the overall response of the device decreases with increasing light intensity as shown in Fig. 5c,d. Owing to the excellent photovoltaic performance of BG1, it was compared with the pure Bi₂Te₃ device to confirm the effect of adding the appropriate amount of GO on the photovoltaic performance of the devices. As shown in Fig. 5e, the photocurrent of the BG1 device is much higher than that of the B device at different wavelengths of light radiation. Meanwhile, the optical responsivity and detectivity of BG1 devices were found to be better than those of B devices under light radiation at various wavelengths (Fig. 5f, g). It is worth emphasizing that under 365 nm UV irradiation, the responsivity and detectivity of the BG1 device are 6.072 mA/W and 2.406 x 1019 Jones, respectively, which are 118 % and 338 % higher than those of the B device.

Stability is one of the important evaluation parameters for PEC-type devices because it involves nano-heterojunctions, which are very susceptible to agglomeration and oxidation, and we directly measured the performance of RGO/Bi₂Te₃ heterojunction materials after 12 months of placement in an atmospheric environment by integrating the devices through the same encapsulation-injection electrolyte process. It was surprising to find that the average value of the photocurrent of the heterojunction material placed for 12 months was about 9.75 pA, which is still 10.76 % (90.58 pA) of the photocurrent of the fresh device (Fig. 53). The average dark current of the device is 0.20 pA, and the ratio of photocurrent to dark current (switching ratio) remains surprisingly high at 48.75. Experiments confirm that heterojunction-integrated PEC-type devices placed in atmospheric environments for 12 months can maintain a relatively good performance. To visualize the performance of the RGO/ Bi₂Te₃ heterojunction devices, the device performance of the currently reported PEC-type devices in the literature is recorded in Table 1. The comparison reveals that the prepared RGO/Bi₂Te₃ heterojunction PEC-type devices exhibit excellent performance.

The response time of the device to the switching behavior of light is also an important parameter for evaluating photodetectors. The time required for the photocurrent of the device to rise to 63 % of the Ip is denoted as τ_rise, While the time required for the photocurrent of the device to fall to 37 % of the I is denoted as τ_decay. The τ_rise of the BG1 device is 29 ms, while the rise time τ_rise of the B device is 39.3 ms, implying that BG1 shows excellent response speed to the light on behavior under 365 nm laser irradiation (Fig. 6a). The fall time τ_decay Of the BG1 device is 68.8 ms, while the fall time τ_decay Of the B device is 39.32 ms (Fig. 6b). Similarly, the rise time of the BG1 device is 41.30 ms, while the rise time of the B device is 47.94 ms under 470 nm laser irradiation (Fig. 6 ). From Fig. 6d, it is observed that the response delay time of the BG1 device is 79.7 ms while the response delay time of the B device is 84.45 ms. It is worth mentioning that the rise response times of BG1 and B devices under 850 nm laser irradiation were 33.33 ms and 171.89 ms (Fig. 6e). And the delay times of BG1 and B devices are 46.34 ms and 99.99 ms (Fig. 6f). There is a very clear rule that as the wavelength of laser irradiation grows, the rise and delay time of the device is increased. The response times of BG1 and B devices at different wavelengths of light radiation indicate that the addition of an appropriate amount of graphene can accelerate the response rate of the devices. The excellent optical-response behavior of BG1 is attributed to the fast charge transfer between Bi₂Te₃ and RGO. The surface of 2D Bi₂Te₃ is topologically insulated with excellent electrical conductivity, while graphene possesses high electrical conductivity and excellent carrier mobility.

To understand the intrinsic mechanism of the devices, the prepared devices were analyzed using various characterization methods. The EIS spectra were performed to analyze the equivalent resistance values of the various devices prepared, and the results are illustrated in Fig. 7a. From the EIS spectrum, it can be found that the equivalent resistance of the device shows a gradual increase with the increase of RGO in the device. Such results imply that the optical response performance of the device with the smallest resistance value is not necessarily optimal and is only used as a reference criterion. To analyze the optimal ratio of RGO that effectively inhibits the complexation of photogenerated electrons at the interface of Bi₂Te₃ nanosheets and electrolytes by contacting Bi₂Te₃ with RGO to form heterojunctions. Various BG heterojunction detectors were exposed to a 365 nm laser for 10 s and then darkened to allow the voltage to decay to 0 V. The voltage decay results of the devices with time (V-t) are shown in Fig. 7b. The open-circuit voltage of the BGO.5 device reaches a maximum of 166 mV, while the open-circuit voltage of the BG1 device is 162 mV, with almost no significant difference. The open-circuit voltage of BG4 is the smallest at 110 mV, and it can be seen from the data that the open-circuit voltage of the device increases with the increase of the reduced graphene ratio and then decreases. It is worth emphasizing that the open-circuit voltage and carrier lifetime performance tests of the BG1 device show that the device has good optical response performance. Fig. 7c shows the PL spectra of Bi₂Te₃ with different amounts of RGO. It was observed that the luminescence intensity of the PL spectra tended to decrease and then increase with the increase of RGO content with identical experimental conditions, which indicates that the addition of an appropriate amount of RGO can effectively reduce the number of photogenerated carriers and hole complexes inside Bi₂Te₃. The UV-vis-IR absorption spectra of RGO, Bi₂Te₃, and RGO/Bi₂Te₃ (BG1) heterojunctions are shown in Fig. 7d. The absorbance of RGO decreases with increasing wavelength. The absorption of the RGO/Bi₂Te₃ heterojunction with 1 mg of RGO significantly increases from 350 nm to 850 nm compared to the pure Bi₂Te₃ nanosheets, suggesting that the junction photoelectrons from RGO in addition to those from Bi₂Te₃, which contributes to a significant increase in the current density of the RGO/Bi₂Te₃ heterojunction device. From the absorption spectrum, it can be observed that the addition of RGO can effectively enhance the absorption of light by the heterojunction, which is the primary factor that the RGO/Bi₂Te₃ heterojunction device exhibits relatively excellent performance. To reveal the photogenerated electron transport mechanism of Bi₂Te₃/RGO PEC-type devices, the mechanism of charge transport in Bi₂Te₃ and RGO was deeply analyzed from the energy band engineering perspective of the materials, as shown in Fig. 7e. When RGO nanosheets were irradiated by light, the ground-state e inside RGO absorbs energy to become excited state e and subsequently enters the conduction band of Bi₂Te₃ through the RGO surface [29,30]. Simultaneously, the ground state e^- inside the Bi₂Te₃ absorbs energy to become excited state e^-, thus achieving rapid enrichment of excited state e on the Bi; Tes conduction band [27,28,40]. The topological insulating states on the surface of the RGO and Bi₂Te₃ provide a fast transport channel for photogenerated electrons [62-64]. In contrast, the h· produced by Bi₂Te₃ is enriched in the valence band of RGO. Finally, the e^- and h⁺ entering the external circuit reacts with the polysulfide electrolyte through the Pt deposited on the FTO to achieve a complete circuit cycle (S²_n + 2(n-1) e^- = nS²- and nS² + 2(n-1) h⁺ = S^2-_n), thus ensuring that the Bi₂Te₃/RGO device can proceed stably and efficiently [65]. When external photons irradiate the surface of the Bi₂Te₃/RGO, the generated electrons enter the external circuit under the built-in electric field to form a photocurrent. The faster photo-response rate is due to the fast charge transfer between the Bis Tes and RGO surfaces and the unique charge transfer medium of GR. In addition, the heterostructure of the appropriate amount of RGO and Bi₂Te₃ exhibits a synergistic effect, driving the device to exhibit good optical response behavior over a wide wavelength range of 365-850 nm [66]. The main reasons why RGO doping can improve the photodetection performance of Bi₂Te₃/RGO heterojunction are found in the above analysis: 1) RGO, a 2D material with a large specific surface area has excellent electrical conductivity, which can provide a fast charge transport channel for the efficiently separated carriers in the device. 2) The generation of RGO/Bi₂Te₃ effective heterojunction can partially inhibit the composite of carriers and holes generated inside RGO, which can achieve the effective separation of part of the photogenerated charge, thus enhancing the photodetection performance of the device. 3) When excessive GO is doped into the precursor of Bi₂Te₃, the Bi₂Te₃ formed while synthesizing RGO will be oxidized to different degrees. The oxidation of Bi₂Te₃ implies the existence of defective states, and a potential barrier layer will appear inside and on the surface of the material, thus affecting the separation and transmission of photogenerated charges on the surface of Bi₂Te₃.

4. Conclusion

Bi₂Te₃/RGO heterojunctions were successfully prepared by a one-step solvothermal method, and the effects of different RGO additions on the performance of PEC-type photodetectors were investigated. The BG1 device obtained by adding 1 mg of RGO to the Bi₂Te₃ sample demonstrated an I_ph/I_drak as high as 412 when irradiated by a 365 nm laser. The responsivity and the detectivity of the BG1 device were 6.072 mA/W, and 2.406 x 109 Jones respectively, which are enhanced by 118 % and 338 % relative to the responsivity and detectivity of the pure BisTe3 devices. Optical response results indicate that the incorporation of a moderate amount of graphene is important for the improvement of Bi₂Te₃ device performance, and the fast and excellent response performance is attributed to the synergistic provision of a fast charge transport channel by the Bi₂Te₃ and RGO topological insulating states.

Author contributions

The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

Data availability

Data will be made available on request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (Grant No. 2019YFA0705201), and the National Natural Science Foundation of China (Grant No. U2032129).

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Received 8 October 2023; Accepted 6 December 2023

Available online 20 February 2024

Footnote

* Corresponding author.

*· Corresponding author.

*·· Corresponding author.

*··· Corresponding author.

*···· Corresponding author.

E-mail addresses: [email protected] (D. Wang), hewenmseGhit.edu.cn (W. He), [email protected] (X. Fang), [email protected] (L. Zhao), jinzhong wang@ hit.edu.cn (J. Wang).

1 Chenchen Zhao and Yangyang Liu contributed to this work equally.

References

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Appendix A. Supplementary data

Supplementary data to this article can be found online at https ://doi.org/10.1016/j.nanoms.2023.12.008.