Reduced Graphene Oxide nanocomposites and their enhanced photoelectric detection application

Sensors and Actuators B 245 (2017) 435–440

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Research Paper

CsPbBr3 /Reduced Graphene Oxide nanocomposites and their enhanced photoelectric detection application Xiaosheng Tang a,∗ , Zhiqiang Zu a , Zhigang Zang a,∗ , Zhiping Hu a , Wei Hu a , Zhiqiang Yao b , Weiwei Chen a , Shiqi Li a , Shuai Han a , Miao Zhou a a Key Laboratory of Optoelectronic Technology and Systems of the Education Ministry of China, College of Optoelectronic Engineering, Chongqing University, Chongqing, 400044, China b State Centre for International Cooperation on Designer Low-Carbon and Environmental Materials, ICDLCEM, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 450001, China

a r t i c l e

i n f o

Article history: Received 22 November 2016 Received in revised form 21 January 2017 Accepted 25 January 2017 Available online 30 January 2017 Keywords: CsPbBr3 /RGO nanocomposites Photodetectors

a b s t r a c t CsPbBr3 /Reduced Graphene Oxide (RGO) nanocomposites were prepared by a facile hot-injection method. The crystalline structure and morphologies of the as-synthesized CsPbBr3 /RGO nanocomposites were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The photoluminescence (PL) spectra indicated that CsPbBr3 /RGO nanocomposites displayed strong quenching phenomenon compared with the pure CsPbBr3 as the probably reason of the fast separation and transfer of the photogenerated hole-electron pair. Moreover, the PL decay results of the CsPbBr3 /RGO nanocomposites ( ave = 4.52 ns) was relatively shorter than that of the corresponding CsPbBr3 nanoparticles ( ave = 14.72 ns), demonstrated the charge transfer between the CsPbBr3 nanoparticles and graphene. Meanwhile, an enhanced strong photoresponse was demonstrated by the CsPbBr3 /RGO nanocomposites, which showed tremendous potential applications in the field of photoelectric detection. © 2017 Elsevier B.V. All rights reserved.

1. Introduction As the rapid development of nanotechnology and nanoscience, semiconductor nanocrystals (NCs) have been widely applied in the field including industry, life science, information technology and environmental protection. In general, NCs have many excellent characteristics such as easy fabrication proecess, strong light absorbance coefficient and size dependent band gap [1–5], which allowing their applications in the development of lightemitting diodes (LEDs) [6], solar cells [7], and biolabels [8]. Recently, as the progress of perovskite materials based solar cells, great attention has been paid to hybrid organic–inorganic lead halide perovskites materials such as MAPbX3 (X = Cl, Br, I) [9]. However, more and more research found that the instability of hybrid organic–inorganic would effect its further commercialization. And in the past two years, one interesting all inorganic cesium lead halide (CsPbX3 , X = Cl, Br, I) perovskite materials with high quantum efficiency of 90% were demonstrated by Kovalenko et al. [10]. In addition, Ramasamy et al further reported bandgap tunable CsPbX3 perovskite NCs by anion exchange method [11]. There-

∗ Corresponding authors. E-mail address: [email protected] (X. Tang). http://dx.doi.org/10.1016/j.snb.2017.01.168 0925-4005/© 2017 Elsevier B.V. All rights reserved.

fore, all-inorganic lead halide perovskites were recongized as one of probably substitue for hybrid organic–inorganic perovskite materials as the simialr optical and electronic properties, especially its high stabilty. Most recently, all-inorganic perovskites materials based optoelectronic devices of solar cells [12], quantum dots based light emitting diodes (QLEDs) [13] and lasers [14] were intense studied. In particularly, CsPbX3 perovskite NCs have gained a close attention in photoelectric detection application [11], attributed to the fast separation of photo-induced carriers in CsPbX3 perovskite NCs such as CsPb(Br/I)3 nanorods [15], CsPbBr3−x Ix nanoparticles [16] and CsPbBr3 nanosheets [17]. As one type of two-dimensional (2-D) nanostructured sp2 carbon material, graphene nanosheet showed perfect mechanical and electronic properties. Thus, the graphene nanosheet showed promising applications such as gas sensors, light-emitting diodes and solar cells [18–21]. Nowadays, a large number of studies focused on graphene based nanocomposites, such as PbS/RGO [22], TiO2 /RGO [23] and AgInZnS/RGO nanocomposites [24]. And the RGO based nanocomposites were proved as one effective way to load semiconductor nanoparticles onto graphene surface with no further aggregation and stacking of graphene sheets. On the other side, it would induce huge improvement of optical and electrical properties [25,26]. However, till now, there were still no reports about the nanocompistes of perovskites/RGO. Therefore, it is one

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interesting and challenging study for combining the two excellent materials of graphene and all-inorganic perovskites materials for optoelectronic application. In this study, we demonstrated a facile method for synthesizing CsPbBr3 /RGO nanocomposites. The PL of the as-prepared CsPbBr3 /RGO nanocomposites showed obviously quenching compared to the pure CsPbBr3 nanoparticles as the reason of the fast separation and transfer of photo-induced carriers between CsPbBr3 nanoparticles and graphene. Moreover, the CsPbBr3 /RGO nanocomposites showed shorter lifetime (4.52 ns) than pure CsPbBr3 nanoparticles (14.72 ns), which further testified the qunenching phenomenon. Finally, the CsPbBr3 /RGO nanocomposites based photodetector demonstrated the enhanced strong photoresponse of short decay-time (0.414s) and rise-time (0.417s). 2. Experimental 2.1. Chemicals Cs2 CO3 (99%, Adamas), octadecene (ODE, 90%, Acros), oleic acid (OA, 90%, Aldrich), PbBr2 (99.99%, Xi’an Polymer Light Technology Core), PbI2 (99.99%, Xi’an Polymer Light Technology Core), oleylamine (OLA, 70%, Aldrich), oleylamine (99.9%, Acros), graphite powder (99.85%, Chengdu Kelong Chemical Reagent Factory), NaNO3 (99%, Chengdu Kelong Chemical Reagent Factory), concentrated H2 SO4 (98%, Chengdu Kelong Chemical Reagent Factory), KMnO4 (99%, Chengdu Kelong Chemical Reagent Factory), H2 O2 (20%, Chengdu Kelong Chemical Reagent Factory), HCl (37%, Chengdu Kelong Chemical Reagent Factory), Zn powder (98%, Chengdu Kelong Chemical Reagent Factory). All chemicals were used as received without further purification. 2.2. Synthesis of CsPbBr3 nanoparticles Caesium stock solution was obtained by dissolving 100 mg CsCO3 powder in 600 ␮L oleic acid (OA) and 4 mL octadecene (ODE) in a 100 mL three-necked flask, then heated to 120 ◦ C under N2 for standby. 69 mg PbBr2 and 5 mL ODE were mixed in a 100 mL three-necked flask, which was placed in a heating jacket with the environment of nitrogen and heated to 120 ◦ C with magnetic stirring for 1 h, and then heated to 150 ◦ C. Subsequently, 500 ␮L OA and 500 ␮L oleylamine (OLA) were injected into the three-necked flask. Following, 400 ␮L Caesium stock solution was extracted and injected into the precursor solution quickly which containing PbBr2 solution at 150 ◦ C for 3–5 s. Finally, the CsPbBr3 nanoparticles were obtained. The CsPbBr3 nanoparticles was purified by using toluene to resolve the unreacted OA and OLA, CsPbBr3 nanoparticles were obtained after several purification process, and then the CsPbBr3 nanoparticles were dispersed in toluene for testing. 2.3. Synthesis of RGO The RGO was prepared by oxidation of natural flake graphite powder using a modified Hummers method [27]. In a typical synthesis, 1 g graphite powder and 1 g NaNO3 was mixed, and then put into 46 mL concentrated H2 SO4 (98%) with an ice bath. After magnetically stirring for one hour, 6 g KMnO4 was gradually added into the mixture under stirring. The mixture was then transferred to a water bath of 35 ◦ C for one day. Successively, 40 mL ultrapure water was slowly added into the mixture, during which the temperature of the mixture rose to approximately 80 ◦ C. Finally, 100 mL H2 O followed by 20 mL 20% H2 O2 solution was added into the mixture, and the mixture was further stirred for 30 min. The final product, GO, was collected by centrifugation. Then fifty milligrams oxidized graphite was dispersed in 50 mL H2 O under ultrasonication with a

Sonics & Materials Vibracell VCX 150 ultrasonic processor. A homogeneous brown GO solution was obtained, and no particles were observed in naked eyes. After the pH value of the solution was adjusted to 7 by the addition of 37% HCl solution, 100 mg Zn powder was added into the GO solution. The mixture was immediately sonicated with the ultrasonic processor for another 1 min. The top part of the brown solution turned into transparent, and black precipitates appeared at the bottom after the sonication. The excessive Zn powder was dissolved by adding 5 mL 37% HCl. RGO was collected by vacuum filtration. It was rinsed with excessive water and successively desiccated with the freezedrier for two days. 2.4. Synthesis of CsPbBr3 /RGO nanocomposites Caesium stock solution was obtained by dissolving 100 mg CsCO3 powder in 600 ␮L oleic acid (OA) and 4 mL octadecene (ODE) in a 100 mL three-necked flask, then heated to 120 ◦ C under N2 for standby. 69 mg PbBr2 , 5 mL ODE and RGO (10 mg, 20 mg, 30 mg, 40 mg, 50 mg) were mixed in a 100 mL three-necked flask, which was placed in heating jacket with the environment of nitrogen and heated to 120 ◦ C with magnetic stirring for 1 h, and then heated to 150 ◦ C. Subsequently, 500 ␮L OA and 500 ␮L oleylamine (OLA) were injected into the three-necked flask. Following, 400 ␮L Caesium stock solution was extracted and injected into the precursor solution quickly which containing PbBr2 solution at 150 ◦ C for 3–5 s. Finally, the CsPbBr3 /RGO nanocomposites were obtained. The CsPbBr3 /RGO nanocomposites was purified by using toluene to resolve the unreacted OA and OLA, CsPbBr3 /RGO nanocomposites were obtained after several purification process, and then the CsPbBr3 /RGO nanocomposites were dispersed in toluene for testing. 2.5. Design of the photodetectors The gold interdigital electrode based on SiO2 substrate with 50 ␮m space of adjacent fingers was prepared by the method of photolithography, which the thickness of the gold electrode was 10 nm. And the photodetector device was prepared by dropping CsPbBr3 /RGO nanocomposites on a gold interdigital electrode. 2.6. Characterization The crystal phases of all samples were characterized by XRD with CuKa radiation (XRD-6100, SHIMADZU, Japan). TEM was recorded using a ZEISS LIBRA 200FE microscope. SEM images were observed by JSM-7800F, JAPAN. Absorption was adopted by a Scan UV–vis spectrophotometer (UV–vis: UV-2100, Shimadzu, Japan). Spectra were recorded at room temperature ranging from 200 to 800 nm. Photoluminescence spectroscopy was measured by a fluorescence spectrophotometer (PL: Agilent Cary Eclipse, Australia), which includes a Xe lamp as an excitation source with optical filters). The photoreponse were detected by Keithley 4200-SCS, America. 3. Result and discussion The X-ray diffraction (XRD) pattern of CsPbBr3 /RGO (black line) nanocomposites and CsPbBr3 nanoparticles (red line) are shown in Fig. 1. It could be seen that the as-synthesized CsPbBr3 nanoparticles showed cubic structure (JCPDS No. 54-0752) due to the combined effect of the high synthesis temperature and the surface energy [28]. By comparsion, one additional peak for the CsPbBr3 /RGO nanocomposites (black line) sample located at about 25◦ , which could be corresponded to the (002) reflection of graphene [29], which indicated that the CsPbBr3 nanoparticles have been coated on the graphene nanosheets.

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Fig. 1. XRD pattern of CsPbBr3 nanoparticles and CsPbBr3 /RGO nanocomposites.

The morphology information of the CsPbBr3 /RGO nanocomposites, CsPbBr3 nanoparticles and RGO were tested by Transmission Electron Microscope (TEM) respectively. It could be observed that the RGO nanosheet are layer and curled shape (Fig. 2(A)). According to Fig. 2(B), the nanoparticles of CsPbBr3 were cubic shape and with the size of about 10 nm. Moreover, the high resolution TEM image of Fig. 2(C) showed the clear crystal lattice, and the spacing

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between two neighboring planes is 0.58 nm, which could correspond to the (110) planes of the cubic structure. Additionally, from the TEM image in Fig. 2(D), a large number of CsPbBr3 nanoparticles were sucessfully coated on the surface of graphene nanosheets. However, the size distribution of the CsPbBr3 nanoparticles on the RGO is relatively larger (about 25 nm), which may attributed to the process of Ostwald Ripening phenomenon [30]. As shown in Fig. 3(A), in comparison with the pure CsPbBr3 nanoparticles, the PL spectrum of CsPbBr3 /RGO nanocomposites is obviously weak, which probably could ascribe to the effective separation and transfer of the photo-excited charge carriers from the nanocomposites of CsPbBr3 /RGO [31]. And the peak of CsPbBr3 /RGO nanocomposites showed slightly blue shift for 15 nm compared to CsPbBr3 nanoparticles, which may originated from the interaction between the CsPbBr3 nanoparticles and graphene sheets, and this should be attributed to the Burstein Moss effect [32,33]. Moreover, different ratio of RGO nanosheets were used to synthesize the CsPbBr3 /RGO nanocomposites. The corresponding PL results were showed in Fig. 3(B), which demonstrated that the peaks of the as-prepared CsPbBr3 /RGO nanocomposites would slightly blue shift as the increase amout of the added RGO nanosheets. This is because that as the increasing ratio of graphene in the CsPbBr3 /RGO nanocomposites, more ␲-electrons would enter the lattice of CsPbBr3 nanoparticles, and the potential barrier between graphene and CsPbBr3 nanoparticles would increase, which would further lead to Burstein Moss shift. Therefore, the optical bandgap would become larger gradually, finally induce the blue shift

Fig. 2. (A) TEM image of RGO. (B) TEM image of CsPbBr3 nanoparticles (insert: size distribution of CsPbBr3 nanoparticles nanoparticles). (C) HRTEM image of CsPbBr3 nanoparticles. (D) TEM image of CsPbBr3 /RGO nanocomposites (insert: size distribution of CsPbBr3 nanoparticles nanoparticles coated on the RGO).

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Fig. 3. (A) The photoluminescence spectra (excited by light with ␭ = 365 nm) of CsPbBr3 nanoparticles (red line) and RGO/CsPbBr3 nanocomposite (blue line). (B) The photoluminescence spectra of CsPbBr3 /RGO nanocomposite with different ratio of RGO. (C) The absorption spectra of RGO (black line), CsPbBr3 /RGO nanocomposites (blue line) and CsPbBr3 nanoparticles (red line). (D) PL decay lifetime of CsPbBr3 /RGO nanocomposites (blue line) and pure CsPbBr3 nanoparticles (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

of photolunmincesce [34,35]. As shown in Fig. 3(C), the absorption spectrum peak of the CsPbBr3 /RGO nanocomposites is 495 nm and that of the pure CsPbBr3 nanoparticles is 475 nm, which have slightly red shift compared with CsPbBr3 nanoparticles. The reason should be the presence of graphene in CsPbBr3 nanoparticles, which would reduce the reflection of light [36,37]. For CsPbBr3 nanoparticles, the carbon species may modify the surface of CsPbBr3 nanoparticles and absorb the visible light, which can lead to the absorption edge of CsPbBr3 /RGO nanocomposites slightly shifted to the visible light range. And the size change of CsPbBr3 nanoparticles may be another reason for the red shift of the absorption peaks [38]. In addition, the presence of RGO nanosheets would lead to continuous absorption band in the range of 400–800 nm, making it as potential materials in solar cells and photodetectors. The time-resolved fluorescence spectroscopy (Fig. 3(D)) was employed to test the emission lifetime of CsPbBr3/RGO nanocomposites and pure CsPbBr3 nanoparticles. A two exponential decay model fit the decay curves was obtained I(t) = A1 e−t/1 + A2 e−t/2 , in which A is the amplitude, ␶ means the lifetime, and the wavelength of excitation is 400 nm,  ave represents the average lifetime, I(t) refers to the time-dependent fluorescence intensity: ave =

 i



Ai i2 /

Ai i

i

The emission lifetime of CsPbBr3 /RGO nanocomposites ( ave = 4.52 ns) was relatively shorter than that of the corresponding CsPbBr3 nanoparticles ( ave = 14.72 ns). The difference in the average emission lifetime between CsPbBr3 /RGO and the

CsPbBr3 counterpart indicates the emergence of a nonradiative pathway from the significant electronic interaction between CsPbBr3 nanoparticles and graphene [39]. Fig. 4(A) shows the I–V features of the photodetector, which were measured by changing the bias voltage from −20 V to 20 V, in which the Schottky barrier between the RGO and CsPbBr3 nanoparticles might be the reason for the non-linear lines [40]. As shown in Fig. 4(B), the photocurrent–time response of the photodetector was measured in the dark and illumination by using laser of 532 nm as the function of light intensity at the fixed applied bias of 8 V. It could be obviously found that the ratio of the current (with and without irradiation) could achieve as large as 170%, which is about two times higher than the simliar detector prepared by CsPbBr3 nanoparticles (102%). For pure CsPbBr3 nanoparticles, the carriers could not transfer to gold interdigital electrode effectively as the small size of 10 nm. However, for the CsPbBr3 /RGO nanocomposites system, the electron-hole pairs could be separated and transferd to the RGO effectively, thereby lead to enhanced high photocurrent [41,42]. In order to study the relationship between the photoresponse and different ratio of RGO nanosheets, five CsPbBr3 /RGO nanocomposites based photodetectors were prepared with different ratio of the RGO nanosheets (10 mg, 20 mg, 30 mg, 40 mg, 50 mg). It could be observed that the RGO nanosheets would changed the intensity of photocurrent remarkably. Fig. 4(B) showed the best perfomance of CsPbBr3 /RGO nanocomposites based photodetectors is the dosage of 20 mg RGO nanosheets, it suggesting this is the best separation for photoexcited electron-hole pairs of the

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Fig. 4. (A) I–V characteristic of CsPbBr3 /RGO nanocomposites with and without the irradiation. (B) The photoelectric response of CsPbBr3 nanoparticles and CsPbBr3 -RGO nanocomposites under different ratio of RGO with and without irradiation. (C) Schematic profiles of a single photocurrent response for CsPbBr3 nanoparticles and CsPbBr3 /RGO nanocomposites with and without the irradiation.

as-prepared CsPbBr3 /RGO nanocomposites. However, its photocurrent would decrease with the increase amount of RGO nanosheets when the content of RGO is more than 20 mg. This could attribute to the stacking of graphene sheets, and the corresponding restriction of the transportation of electrons [43,44]. According to Fig. 4(C), the decay time of CsPbBr3 /RGO nanocomposites based photodectctor was 0.414 s and the rise time was 0.417 s respectively, which were shorter than theCsPbBr3 nanoparticles based devices (about 0.83s). As the unique structure of CsPbBr3 /RGO nanocomposites, the photogenerated electrons could separated and transferred effectively, this the major reason for its relatively outstanding performance of the corresponding photodetector [45].

4. Conclusions In conclusion, we have adopted a simple method for the synthesis of CsPbBr3 /RGO nanocomposites and the CsPbBr3 nanoparticles were coated on the surface of RGO sheets. CsPbBr3 /RGO nanocomposites based photodetectors were further prepared on gold interdigital electrode, and it showed relatively strong photoresponse, quick rise time (0.417s) and decay time (0.414s), which displayed a promising application in photoelectric detectors.

Acknowledgements This work is supported by National Natural Science Foundation of China (61520106012, 61674023), the Fundamental Research Funds for the Central Universities, (106112015CDJZR125511, 106112015CDJXY120001, 106112016CDJCR121222), initial funding of Hundred Young Talents Plan at Chongqing University (0210001104430), The Chongqing Research Programm of Basic Research and Frontier Technology (cstc2015jcyjA1055,

cstc2015jcyjA90007), the Project-sponsored by SRF for ROCS, SEM (0210002409003).

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Biographies Xiaosheng Tang received Ph.D. degree from Department of Materials Science and Engineering, National University of Singapore, Singapore, in 2013. Now, he has been a Professor at college of optoelectronic engineering in Chongqing University, China. His current research interests include I-III-VI, perovskite quantum dots based optoelectronic applications such as light emitting diode, solar cell, photodetector, laser. Zhiqiang Zu received the B.S. degree in technique and instrumentation of measurements from Yantai University, Yantai, Shandong, China, in 2012. Currently, he is pursuing the M.E. degree in optical engineering at Chongqing University, Chongqing, China. Zhigang Zang received Ph.D. degree in electronics and materials science Kyushu University, Fukuoka, Japan, in 2011. Now, he has been a professor at college of optoelectronic engineering in Chongqing University, China. His current research interests include perovskite quantum dots based optoelectronic applications such as light emitting diode, solar cell, photodetector. Zhiping Hu received the B.S. degree in physics from Shangqiu Normal University, Shangqiu, Henan, China. She received the M.E. degree in optical engineering from Henan University, Kaifeng, Henan. Currently, he is pursuing the Ph.D. degree in optical engineering at Chongqing University, Chongqing, China Wei Hu received Ph.D. degree from department of physics and engineering, Sun YatSen University, Guangzhou, China, in 2015. Now, he has been a researcher at college of optoelectronic engineering in Chongqing University, China.His current research interests include super-capacitor and resistive switching random access memory. Zhiqiang Yao received Ph.D. degree from Department of Physics and Materials Science, City University of Hong Kong, Hong Kong in 2008. Now, he has been a professor at college of materials science and engineering in Zhengzhou University, China. His current research interests include thin film semiconductor materials, development of the surface and coatings technology,Special inorganic coating and film. Weiwei Chen received the B.S. degree in optical information science and technology from Henan University of Science and Technology, Luoyang, Shandong, China, in 2014. Currently, he is pursuing the Ph.D. degree in optical engineering at Chongqing University, Chongqing, China. Shiqi Li received the B.S. degree in technique and instrumentation of measurements from Jilin University, Changchun, Jilin, China. Currently, he is pursuing the M.E. degree in optical engineering at Chongqing University, Chongqing, China. Shuai Han received the B.E. degree in electronic information engineering from Liaocheng University, Liaocheng, Shandong, China. Currently, he is pursuing the M.E. degree in optical engineering at Chongqing University, Chongqing, China. Miao Zhou received Ph.D. degree in Physics from Department of Physics, National University of Singapore, Singapore, in 2012. Now, he has been a Professor at college of optoelectronic engineering in Chongqing University, China. His current research interests include the mass theoretical study of condensed matter physics, information materials and devices, catalytic chemistry.