Nitrogen-doped graphene quantum dots decorated ZnxCd1-xS semiconductor with tunable photoelectric properties

Journal of Alloys and Compounds 812 (2020) 152096

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Nitrogen-doped graphene quantum dots decorated ZnxCd1-xS semiconductor with tunable photoelectric properties Zicong Jiang, Yun Lei*, Zheng Zhang, Jiaxin Hu, Yuanyuan Lin, Zhong Ouyang School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 June 2019 Received in revised form 25 August 2019 Accepted 29 August 2019 Available online 30 August 2019

In this study, ZnxCd1-xS/N-GQDs composites with different Zn/Cd ratios were synthesized via a facile solvothermal process. A series of techniques were introduced to investigate the effect of Zn/Cd ratios and N-GQDs on the performances of ZnxCd1-xS/N-GQDs. The results show that the crystal structure and photoelectric performance of the composites can be tuned via adjusting the Zn/Cd ratios. ZnxCd1-xS/NGQDs composites transform from a hexagonal structure to a cubic structure with the increase of Zn content, and the diffraction peaks are located between hexagonal phase CdS and cubic phase ZnS. Meanwhile, the photocurrent responses and electrochemical impedance present good tunability via adjusting the Zn/Cd ratios. Zn0.9Cd0.1S/N-GQDs composites with the higher photocurrent value of 3.79 mA cm
2 and lower interfacial impedance are superior to ZnxCd1-xS/N-GQDs composites with other Zn/Cd ratios. Moreover, the introduction of N-GQDs obviously enhances the photoelectric properties of the semiconductor. © 2019 Elsevier B.V. All rights reserved.

Keywords: ZnxCd1-xS N-GQDs Composites Photoelectric properties

1. Introduction As an important IIeVI semiconductor, ternary alloy ZnCdS possesses the performances of both CdS and ZnS and exhibits better ability in durability [1e3]. What's more, ternary alloy ZnCdS may offer more unique performances than the corresponding binary semiconductors. Their band gap, absorption of visible light and level of the conduction band, can be effectively tuned via adjusting the constituent components [4e8]. Consequently, numerous scientific studies have concentrated on ZnxCd1
xS and ZnxCd1
xSbased materials. Ray et al. synthesized Zn1-xCdxS compound and investigated the effect of cadmium concentration on the charge transport performances of the alloy compound [5]. Hu et al. reported that the band gap of the ZnCdS semiconductor could be continuously tuned via altering the Zn/Cd molar ratios [9]. Ye et al. demonstrated that the photocatalysis performances of RGOeZnCdS composites could be improved and controlled via adjusting the interaction between RGO and ZnCdS [10]. Moradlou et al. investigated the effect of crystalline phase on the photoresponsivity of graphene oxide/ZnxCd1-xS nanocomposites via a facile hydrothermal route [11]. Based on the others and our works [12e15], ZnxCd1-

* Corresponding author. E-mail address: [email protected] (Y. Lei). https://doi.org/10.1016/j.jallcom.2019.152096 0925-8388/© 2019 Elsevier B.V. All rights reserved.

xS

composites have already been investigated, but far less information is available regarding novel ZnxCd1-xS decorated with GQDs, as the GQDs have been considered a candidate for novel materials. Graphene quantum dots (GQDs), as the graphene on a nanoscale, have drawn much attention in biosensors, photodetectors, optoelectronic devices, drug delivery, energy and environmental cleaning because of the quantum confinement and edge effects [16e22]. Recently, some studies indicate that the introduction of heteroatom into carbon lattice can effectively tune the intrinsic performances of GQDs such as local chemical features, surface, optical and electronic performances [23e29]. Among various doping elements, Nitrogen possesses comparable atomic radius to carbon and greater electronegativity than carbon, which makes it easier to incorporate nitrogen into carbon network via substitution doping. Doping N into graphene quantum dots (N-GQDs) leads to polarization and affects the fluorescence emission [30e32]. NGQDs can also serve as an electron sink and reservoir for accelerating the photogenerated electron migration [33]. In addition, some researchers introduced N-GQDs into semiconductors for accelerating the separation of photo-generated carriers to enhance the materials property, such as N-GQDs/TiO2 [34], WSe2/N-GQD [35], N-GQD@V2O5 [36], N-GQD@npg-C3N4 [37], BiOX/N-GQDs [26], and so on. However, there are limited reports on N-GQDs decorated ZnCdS semiconductor. Therefore, the research of ZnxCd1-xS

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composites based on N-GQDs will be investigated by a series of experiments. Herein, a facile solvothermal way was used to prepare ZnxCd1xS/N-GQDs composites by changing the Zn/Cd ratios. The synthesized composites were characterized via various techniques including atomic force microscopy, UVevisible spectra, X-ray photoelectron spectroscopy, FTIR, zeta potential, photoluminescence spectra, XRD, TEM, SEM, photocurrent responses, linear sweep voltammetry and electrochemical impedance spectroscopy. The roles of Zn/Cd ratios and N-GQDs in the improvement of composite performance were studied in detail. The photoelectric performances of ZnxCd1-xS/N-GQDs composites present good tunability at different Zn/Cd ratios. Moreover, the introduction of NGQDs effectively enhanced the photoelectric properties of the semiconductor. 2. Experimental 2.1. Chemical All of the chemicals were analytical grade and used directly. 2.2. Preparation of N-GQDs Graphite oxide (GO) was previously prepared using the modified Hummers method. Concretely, 20 mg GO was dispersed into the 40 ml hydrogen peroxide, and the mixture was refluxed at 90  C for 12 h. After that, the resulting product was transferred to a rotary evaporation for removing the hydrogen peroxide residue. The final solution was diluted to 80 ml using the distilled water and adjusted the pH to 9 with ammonia. Finally, the solution was transferred into an autoclave and then reacted at 180  C for 12 h. The reaction product was filtered by 0.1 mm membrane. This filtrate was dialyzed for three days with frequently changing distilled water. 2.3. Preparation of ZnxCd1-xS/N-GQDs composites A facile solvothermal method was used to prepare ZnxCd1-xS/NGQDs (x ¼ 0.1, 0.3, 0.5, 0.7, 0.9) composites by using the precursors of zinc acetate, cadmium acetate, thiourea and N-GQDs. To synthesize Zn0.9Cd0.1S/N-GQDs, 5 mmol thiourea, 4.5 mmol zinc acetate and 0.5 mmol cadmium acetate were dissolved into 80 ml ethylene glycol. Then, N-GQDs were dispersed into the mixture.

The mixture was sonicated for 30 min, loaded into the autoclaves of 100 ml capacity and reacted at 140  C for 4 h. After the solvothermal reaction, the as-prepared composites were collected by centrifuged at 6000 rpm for 3 min and washed by ethanol for five times. The composites were dried in vacuum at 60  C. The compared ZnxCd1xS/N-GQDs (x ¼ 0.1, 0.3, 0.5, 0.7) composites were prepared by the same experiment process with different molar ratios of Zn/Cd precursors. 2.4. Characterization Atomic force microscopy (AFM) images were obtained using a Bruker Multimode 8 at the scanning range of 5 mm  5 mm. Transmission electron microscope was performed on an FEI Tecnai G2 F20. X-ray diffraction (XRD) was conducted on a diffractometer (X'Pert3 Powder) with Cu Ka radiation (l ¼ 1.5406 nm). Scanning electron microscopy (SEM) image was recorded on Sigma HD. Photoluminescence spectra (PL) were obtained by a Cary Eclipse fluorescence spectrometer. FTIR spectra were collected by a Nicolet IS-10 FTIR spectrometer. Zeta potential tests were performed on a Malvern zetasizer nano-ZS90. The UVevisible spectra were performed using a UV-5500 spectrophotometer. XPS analysis was collected by a Thermo ESCALAB 250XI. Photocurrent responses, linear sweep voltammetry and the electrochemical impedance were conducted on a three-electrode system in electrochemical work station (CHI650e). 3. Results and discussion The thickness of prepared N-GQDs was collected by AFM. As shown in Fig. 1a, N-GQDs are dispersed homogeneously, as the oxygenic groups on the N-GQDs surface are beneficial to improve the dispersion of N-GQDs. To further demonstrate the thickness of N-GQDs, the height statistical chart (Fig. 1b) was collected. It can be clearly observed that the height of N-GQDs is mainly distributed between 1.2 and 1.8 nm, with the average height of 1.5 nm. These results reveal that the prepared N-GQDs have the average height of 3 layer graphene structure. In order to analyse the size distribution, crystal structure, functional group and optical performance of N-GQDs, a series of characterization was performed. In Fig. 2a, the size distribution of N-GQDs is uniform, and particle size is about 4e5 nm. As can be seen from the high-resolution electron microscopy, the lattice

Fig. 1. AFM image (a) and its height statistical chart (b) of N-GQDs.

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Fig. 2. (a) TEM image of N-GQDs. (b) HRTEM image of N-GQDs with indicated lattice parameters. (c) XRD pattern of N-GQDs. (d) FTIR spectrum of N-GQDs. (e) PL spectra of N-GQDs. (f) UVevisible spectrum of N-GQDs.

stripes of 0.19 nm are shown clearly, corresponding to the (104) plane of graphene, which indicates the high crystallinity of the synthesized N-GQDs. The XRD pattern of N-GQDs shows a broad diffraction peak at 22 , which corresponds to the (002) plane of graphene [30]. In FTIR (Fig. 2d), the peaks at 3060-3700 cm
1 and 1384 cm
1 are assigned to O-H stretching vibration. The peak at 1640 cm
1 is corresponded to the vibrational bands of C]O in the carboxylic group. The peaks at 920-1234 cm
1 and 1619 cm
1 are attributed to C-N and eCOeNH-, respectively [16,18,38]. The above results of FTIR analysis demonstrate that part oxygenic groups remained in the GQDs and that the N element has been doped into

the GQDs. The optical performances of N-GQDs were investigated by PL and UVevisible spectra. As shown in Fig. 2e, N-GQDs have an emission peak at 390 nm which is derived to the surface oxygencontaining functional groups of N-GQDs. During the excitation wavelength changed from 240 to 340 nm, the PL emission shows excitation dependent PL behavior, which indicates that both the size and surface state of N-GQDs are uniform [39]. The UVevis absorption spectrum of N-GQDs has a weak shoulder in the range 292e323 nm as the result of n /p* transition of C]O bonds. The morphology of Zn0.9Cd0.1S/N-GQDs was observed by SEM and TEM (Fig. 3). It can be observed from Fig. 3a that the particles of

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Fig. 5. The FTIR spectra of ZnxCd1-xS/N-GQDs with the x value of 0.1, 0.3, 0.5, 0.7 and 0.9.

Fig. 3. SEM (a) and TEM (b,c) images of Zn0.9Cd0.1S/N-GQDs composites.

Zn0.9Cd0.1S are present in nanospheres with a relatively uniform diameter of 50 nm. No information of N-GQDs can be obtained from the SEM due to low content, small size and little thickness of NGQDs. Fig. 3b&c present the TEM images of Zn0.9Cd0.1S/N-GQDs. The lattice spacing of 0.32 nm is indexed to the (111) plane of Zn0.9Cd0.1S. In addition, the layer can be clearly observed, indicating the well-crystallized Zn0.9Cd0.1S nanoparticles. The lattice spacing of 0.19, 0.24, and 0.34 nm are agreed well with the (104), (100), and (002) planes of N-GQDs [17,28]. XRD patterns (Fig. 4) were used to investigate the structure changes of ZnxCd1-xS/N-GQDs with the x value of 0.1, 0.3, 0.5, 0.7 and 0.9. For Zn0.1Cd0.9S/N-GQDs, it can be found that six mainly diffraction peaks appear at 25.05 , 26.67, 28.39 , 44.02 , 48.29 , 52.19 indexed to the (100), (002), (101), (110), (103) and (112) planes of hexagonal structure. Obviously, ZnxCd1-xS/N-GQDs

Fig. 4. XRD patterns of ZnxCd1-xS/N-GQDs with the x value of 0.1, 0.3, 0.5, 0.7 and 0.9, and the standard patterns of hexagonal phase CdS (PDF No. 80-0006) and cubic phase ZnS (PDF No. 80-0020) for comparison.

composites transform from a hexagonal structure to a cubic structure with the increase of Zn content, which is in good agreement with the results reported earlier [11,40,41]. Transformation to cubic structure is causal, as cubic ZnS is the thermodynamically stable phase at low temperatures [42]. Meanwhile, the XRD pattern of Zn0.9Cd0.1S/N-GQDs composites is in good agreement with PDF No 80-0020 of cubic ZnS. In addition, a successive shift towards larger angles with the x increasing indicates that the small size Zn2þ ion (0.74 Å) incorporates in the CdS lattice or enters its interstitial sites, with the radii of Cd2þion being 0.97 Å. The phenomenon suggests the formation of the alloyed ZnxCd1-xS instead of the independent ZnS and CdS. No obvious peaks of N-GQDs can be observed because of the low content and high dispersibility of NGQDs. Fig. 5 showed the FTIR spectra of ZnxCd1-xS/N-GQDs (x ¼ 0.1, 0.3, 0.5, 0.7, 0.9) composites. Compared with N-GQDs, the peak at 1384 cm
1 is decreased, indicating the O-H group can be removed partly in the solvothermal process. There are still remain the O-H, C]O, C-N, and eCOeNH- groups which are originated to the NGQDs. The remained oxygenic groups are beneficial to the stability of the N-GQDs anchored onto the ZnxCd1-xS surface [10]. As increasing of Zn content, the peak intensity of oxygenic groups become stronger, which clearly confirms that N-GQDs are strongly

Fig. 6. Zeta potentials of Zn0.9Cd0.1S, N-GQDs, and Zn0.9Cd0.1S/N-GQDs.

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Fig. 7. Full scan XPS spectrum of the Zn0.9Cd0.1S/N-GQDs composites (a), high-resolution C 1s (b), N 1s (c), O 1s (d), Zn 2p (e), Cd 3d (f) and S 2p (g).

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Fig. 8. (a) UVevis absorption spectra of ZnxCd1-xS/N-GQDs with the x value of 0.1, 0.3, 0.5, 0.7 and 0.9. (b) The comparison of Zn0.9Cd0.1S/N-GQDs, N-GQDs and pure Zn0.9Cd0.1S.

attached to the surface of ZnxCd1-xS molecules. To confirm the FTIR results, the zeta potentials were carried out to analyse the surface charge of Zn0.9Cd0.1S, N-GQDs, and Zn0.9Cd0.1S/N-GQDs (Fig. 6). N-GQDs show a
14.1 mV zeta potential, resulting from the oxygenic groups on the surface of N-GQDs, which supports the FTIR result of N-GQDs. The pure Zn0.9Cd0.1S shows a high positive zeta potential of 11.3 mV. It is reasonable to suppose that the N-GQDs still maintain negative zeta potential in the solvothermal process due to the remained oxygenic groups on the N-GQDs surface, which is beneficial for the anchoring of NGQDs to ZnxCd1-xS surface via electrostatic attraction. The physical combination explain why the Zn0.9Cd0.1S/N-GQDs decrease to the low positive zeta potential of 5.2 mV and why no more chemical bond appear to the FTIR of ZnxCd1-xS/N-GQDs. The surface chemical composition of the Zn0.9Cd0.1S/N-GQDs composites was detected by XPS and shown in Fig. 7. A full XPS spectrum (Fig. 7a) contains the binding energies corresponding to Zn 2p, O 1s, Cd 3d, N 1s, C 1s and S2p, which is originated from Zn0.9Cd0.1S and N-GQDs. High-resolution spectra of C 1s region are resolved into four main peaks indicating the carbon present in four different chemical environment (Fig. 7b). The binding energy peaks at 284.6 eV, 285.5 eV, 286.2 eV and 288.6 eV correspond to sp2 C peak of C]C or C-C bonds [17,43], sp3 C peak of C]N or C-N bonds [18,21], C-O bond or C]O bond [16,44], respectively. The

deconvolution peaks of the N 1s spectra are fitted into three components, centered at 399.5 eV, 404.4 eV and 405.7 eV, which can be indexed to N-C, eNH2 and eNO2, respectively (Fig. 7c). These results indicate that N was successfully doped into the graphene structure. High-resolution spectra of O 2s (Fig. 7d) clearly show three peaks at 530.6 eV, 531.4 eV, and 532.3 eV which represent OC, O]C and O-H respectively. For the reason of spin-orbit coupling, high-resolution Zn 2p XPS spectra (Fig. 7e) are divided to Zn 2p1/2 and Zn 2p3/2 located at 1044.4 eV and 1021.4 eV, respectively. XPS spectra of Cd 3d (Fig. 7f) are divided to Cd 3d3/2 and Cd 3d5/2 that are centered at 411.2 eV and 404.5 eV, respectively. The binding energies of Zn 2p and Cd 3d are similar to the literature data for Zn2þ and Cd2þ in which Zn 2p3/2 and Cd 3d5/2 can be ascribed to the Zn-S bond and Cd-S bond, respectively [2,7,11]. Besides, as shown in Fig. 7g, the peaks at 161.4 eV and 162.6 eV in the high-resolution spectra of S 2p are both ascribed to the S2- in Zn0.9Cd0.1S. The UVevisible absorption spectra for all ZnxCd1-xS/N-GQDs composites with different Zn/Cd ratios were shown in Fig. 8. The Zn0.1Cd0.9S/N-GQDs composites show a strong absorption peak in 483 nm, corresponding to the band edge absorption peak of Zn0.1Cd0.9S. Specifically, the specific absorption peaks have an obvious blue-shift from 483 nm (2.56 eV) to 430 nm (2.88 eV) with the increase of Zn content. The blue-shift is caused by the increase of the band gap energy when the Zn ions are incorporated in CdS crystals [9]. The obvious shift of band edge absorption indicates that the band gap of ZnxCd1-xS/N-GQDs composites can be tuned by adjusting the Zn/Cd ratios. As shown in Fig. 8b, it is obvious that Zn0.9Cd0.1S/N-GQDs show an absorption peak at 322 nm after introduction of N-GQDs, corresponding to the n /p* transition of N-GQDs. The peak shows a slight red-shift to the pure N-GQDs, which could be the result of interaction of N-GQDs and Zn0.9Cd0.1S. Fig. 9 shows the photocurrent responses of the ZnxCd1-xS/NGQDs and pure Zn0.9Cd0.1S composites. As shown in Fig. 9a, the photocurrent value of all composites increases rapidly exposed on the light and decreases to zero when the light is off. The photocurrent density of ZnxCd1-xS/N-GQDs is altered by varying the Zn/ Cd ratios. It is clear that the photocurrent first decreased and then increased with the increase of Zn content. Obviously, Zn0.9Cd0.1S/NGQDs exhibit a higher photocurrent density of 3.79 mA cm
2. Compared with the pure Zn0.9Cd0.1S, notably, the photocurrent curve has an obvious anodic photocurrent spike which is caused by the separation of the electron-hole pairs at the initial time of irradiation for the Zn0.9Cd0.1S/N-GQDs [45]. As shown in Fig. 9b, photocurrent intensity of the Zn0.9Cd0.1S/N-GQDs composites exhibits a great enhancement relative to pure Zn0.9Cd0.1S, which indicates that photoinduced electrons are separated more efficiently in the presence of N-GQDs. The photocurrent intensity was also investigated by linear sweep voltammetry (LSV). It can be observed in Fig. 9c that the anodic current response increases with increase in positive bias voltage, and that the current intensity show a great improvement when introduction N-GQDs to the Zn0.9Cd0.1S. The results of LSV favor the fact that N-GQDs is beneficial to the separation of electron-hole, which corresponds to the photocurrent responses. In order to demonstrate the stability of Zn0.9Cd0.1S/NGQDs composites, the photocurrent curve as a function of time was performed and presented in Fig. 9d. The photocurrent density of Zn0.9Cd0.1S/N-GQDs composites remains 90% after 900s, which exhibits a good stability. To further study the influence of the Zn/Cd ratios and N-GQDs on electrochemical performance of the composites, electrochemical impedance and equivalent circuit were used to study the interfacial electron transfer. As shown in Fig. 10a, the CPE and Rs are the associated doublelayer capacitance and electrolyte resistance, respectively. Furthermore, Rw denotes Warburg impedance which is related to the diffusion of ion OH
to the fabricated layer. Rct is

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Fig. 9. (a) Transient photocurrent responses curves of ZnxCd1-xS/N-GQDs with the x value of 0.1, 0.3, 0.5, 0.7 and 0.9. (b) The comparison of Zn0.9Cd0.1S/N-GQDs and pure Zn0.9Cd0.1S. (c) Linear sweep voltammetry Zn0.9Cd0.1S/N-GQDs and pure Zn0.9Cd0.1S. (d) The stability test of Zn0.9Cd0.1S/N-GQDs composites.

the resistance of deposited layer, which can be measured by the diameter of the semicircle at high-frequency district [46,47]. Small diameter indicates the small impedance. The Rct value first increased and then decreased with the increase of Zn content. The Zn0.9Cd0.1S/N-GQDs present the smallest semicircle, indicating that the Zn0.9Cd0.1S/N-GQDs own smallest electron transfer resistance. When the electron is motivated, low electron transfer resistance makes the electron transport more quickly, resulting in low recombination efficiency of electron-hole. Compared with the pure sample, the Zn0.9Cd0.1S/N-GQDs has a smaller charge-transfer resistance, indicating that N-GQDs make the result of a much lower interfacial electron transport resistance. To illustrate the tunable photoelectric properties of Zn0.9Cd0.1S/ N-GQDs composites, the mechanism was proposed and observed in Fig. 11, On the basis of the above results, the band gap becomes larger with increasing Zn content in the ZnxCd1-xS composites, resulting that the conduction band edge potential becomes more negative, which indicates the photoinduced electrons have stronger power for transferring to the lower conduction band [48]. The conduction band of N-GQDs lies between the ZnxCd1-xS conduction band and the FTO substrate. N-GQDs serve as the electron acceptor and conductive way provider for electron delivery quickly. The Zn0.9Cd0.1S composites present the most powerful conduction band, so that the photoinduced electrons transfer from the conduction band of Zn0.9Cd0.1S to the conduction band of N-GQDs, then

transfer to the external circuit via the FTO substrate. The existence of N-GQDs improves the separation efficiency of photoinduced electron effectively. In a word, the enhanced and tunable photoelectric properties of ZnxCd1-xS/N-GQDs are originated from the tunable band gap and the existence of N-GQDs. 4. Conclusion In summary, ZnxCd1-xS/N-GQDs composites with difference Zn/ Cd ratios were prepared by the facile solvothermal process. The experiment of investigating the effect of Zn/Cd ratios on the performance has yielded promising results. The results show that the composites of ZnxCd1-xS/N-GQDs belong to the alloy materials instead of the independent ZnS and CdS. Moreover, crystal structure and photoelectric performance of the composites can be tuned by adjusting the Zn/Cd ratios. The photocurrent responses and electrochemical impedance of Zn0.9Cd0.1S/N-GQDs are superior to ZnxCd1-xS/N-GQDs composites with other Zn/Cd ratios. Moreover, N-GQDs act as an important part in enhancing the semiconductor performance of the composites. The present of N-GQDs accelerates the electron of transport and reduces the electron transfer resistance, resulting in the reduction of electron-hole recombination and the increase of electron transferred to the external circuit. The present work leads to a promising application of N-GQDs in ZnxCd1xS/N-GQDs composites.

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Fig. 10. (a) EIS Nyquist plots of ZnxCd1-xS/N-GQDs with the x value of 0.1, 0.3, 0.5, 0.7 and 0.9. (b) The comparison of Zn0.9Cd0.1S/N-GQDs and pure Zn0.9Cd0.1S.

Fig. 11. Schematic illustration of the tunable photoelectric properties in ZnxCd1-xS/NGQDs composites.

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