Enhanced photoelectric performance of GQDs anchored WO3 with a ‘dot-on-nanoparticle’ structure

WO3/GQDs-H composites were synthesized by a hydrothermal method using WCl6 as the tungsten source. Various analyses were conducted to investigate the composition, structure, morphology and performance of the composites. WO3/GQDs-H composites formed a special ‘dot-on-nanoparticle’ structure by anchoring GQDs on the surface of WO3. The lattice spacings of 0.34 and 0.386 nm were attributed to the (002) facets of GQDs and WO3, respectively. Compared to blank WO3, an obvious shift to higher value in the binding energy of W6+ and W5+ and a decreased ID/IG value in the Raman spectra could be observed for WO3/GQDs-H composites. The photocurrent value of hydrothermal synthesized WO3/GQDs-H composites achieved 1.56 × 10–5 A cm−2, which was obviously prior to that of blank WO3 and mechanically mixed WO3/GQDs. The result indicated that the hydrothermal process promoted GQDs as a conductive route to transfer photoexcited electrons and improve the photoelectric performance of WO3/GQDs in comparison to the mechanical mixture process.


Introduction
As an n-type semiconductor with a tunable bandgap (2.4-2.8eV), tungsten oxide (WO 3 ) possessed rapid response to visible light, excellent gas sensitivity, and favorable electron transport ability. Therefore, it is widely used in the fields of photocatalysis, gas sensors, and photoelectronics [1][2][3][4][5]. For instance, Yang et al fabricated light-emitting devices based on n-typed WO 3 nanorod arrays [6]. Wang et al investigated the effects of crystallinity on the electron-transfer of sol-gel WO 3 films [7]. It is crucial for electron-transporting materials to retard the carrier recombination at transfer interfaces, but the photoelectric performance of pristine WO 3 was still limited by its low separation efficiency and high recombination rate of photogenerated electron-hole pairs. To solve these obstacles, various efforts have been made to improve the photoelectron transfer of WO 3 by introducing conductive materials. For example, Ibrahim et al employed a method of pulsed laser ablation in liquids to anchor WO 3 nanoparticles on reduced graphene oxide sheets [8]. Jun et al constructed reduced graphene oxide/tungsten trioxide heterojunction by coupling WO 3 preferential planes with graphene sheets [9]. The charge-transfer ability of WO 3 -graphene was promoted by the incorporation of WO 3 and graphene sheets, but the sheets at micrometer scale tend to aggregate in a stacking structure and precipitate in solvents, which significantly limits their photoelectric performances in nanometer-scale.
Nowadays, there are more and more in-depth studies on the development and structural-properties of carbon nanostructures [10][11][12][13][14]. As 2D graphene sheets were cut down to 0D pieces, nano-sized graphene quantum dots (GQDs) have captured considerable attention due to the quantum confinement effect, small size effect, and superior electron-transfer ability [15][16][17][18][19][20]. Several reports on GQDs based composites are available in the literature. Li et al provided an approach in designing modified MoS 2 /graphene quantum dots heterostructures [21]. Fei et al prepared graphene quantum dots modified Bi 2 WO 6 composites with a low recombination rate of photo-induced electrons [22]. Yuan et al employed a hydrothermal treatment to fabricate GQDs decorated graphitic carbon nitride nanorods [23]. GQDs can accelerate the photo-induced charge separation, shorten the charge-transfer path, and improve the conductivity and mobility of GQDs based composites. However, literatures on the construction of 'dot-on-nanoparticle' structure with GQDs and WO 3 were rarely discussed, and reports integrating interfacial charge-transfer and photocurrent responses were scarcely investigated.
Based on the above analysis and our previous work [24][25][26], a continuing effort was proposed to construct GQDs anchored WO 3 nanoflakes, which was aimed to obtain a special structure to break the shackles of WO 3 widely used in sensors and supply the foundation in the field of photoelectronics. A simple hydrothermal process was employed to prepare WO 3 /GQDs composites, and various analyses are conducted to determine the structure, morphology, and materials performance of the synthesized composite.

Preparation of graphene quantum dots
GQDs were prepared by an uncomplicated and simplified process where the nitric acid was considered as a shearing agent [27]. In brief, 50 mg of graphite oxide prepared by the modified Hummers' method was dispersed in 50 ml of concentrated nitric acid solution, and the mixture was ultrasonicated for 4 h. Subsequently, the brownish-yellow solution was poured into an autoclave and heat-treated at 100°C for 12 h. The prepared product was rinsed with ethanol for several times. When the product was dried entirely under natural conditions, it was put in a tube furnace and calcined at 400°C for 4 h under the protection of nitrogen. The resulting black powders were GQDs.

Preparation of WO 3 /GQDs composites
WO 3 /GQDs composites were synthesized by a hydrothermal method (WO 3 /GQDs-H). Briefly, 1.0 g of WCl 6 was dispersed in 60 ml of deionized water, and then a certain amount of GQDs powders were added to the mixed solution. The solution was mixed for 1 h and autoclaved at 180°C for 24 h. After the resulting product was washed several times with ethanol and deionized water, it was transferred into a tube furnace and annealed at 400°C for 2 h under N 2 atmosphere. For contrast, blank WO 3 was prepared by the same way in the absence of GQDs. The prepared WO 3 and GQDs were mechanically mixed, and the mixture was denoted as WO 3 /GQDs-M.

Characterization
The structure and morphology were characterized via a D8/Adwance x-ray diffractometer (XRD), a Zeiss Ultra Plus Feild emission scanning electron microscope (FESEM) and a JEM-2100F transmission emission microscopy (TEM), respectively. The thickness of GQDs was characterized via a Multimode 8 atomic force microscope (AFM). The Raman spectra of samples were measured on a RENISHAW Raman microscope. The detailed chemical components of samples were characterized by an ESCALAB 250XI XPS. UV-vis absorption spectra were recorded on a UV5500 spectrophotometer. Photocurrent-time, linear sweep voltammetry and interfacial impedance curves were recorded using a standard three-electrode electrochemical workstation (CHI650E) with a saturated calomel reference electrode, a Pt counter electrode, and a working electrode coated with WO 3 or WO 3 /GQDs.

Results and discussion
AFM image of GQDs was shown in figure 1(a). GQDs prepared by nitric acid shearing presented a thickness of 1.8∼3 nm corresponding to 3∼6 graphene layers. The statistical distribution of thickness was demonstrated in figure 1(b). In order to obtain further morphology of WO 3 /GQDs-H composites, the SEM, EDS with elemental mapping images of WO 3 /GQDs-H composites were shown in figures 2(a)-(e). As shown in figure 2(a), WO 3 /GQDs-H composites exhibited the nanolamellae-like structure. GQDs weren't observed in this image due to the possible reason that the size of GQDs was too small to be observed. Only W, O and C were observed in EDS image (figure 2(b)) for WO 3 /GQDs-H composites, and no other hetero elements existed. It could be seen in element mapping images of W, O, and C (figures 2(c)-(e)) that GQDs were evenly distributed in WO 3 . TEM images of WO 3 /GQDs-H composites prepared via a one-step hydrothermal method were shown in figures 3(a) and (b). It could be seen from the figures that WO 3 /GQDs-H composites were constructed by the nanolamellae-like structure as shown in SEM results. In the enlarged HRTEM images (figures 3(c) and (d)) of WO 3 /GQDs-H composites, three interplanar spacings of 0.386, 0.377 and 0.365 nm were indexed to the (002), (020) and (200) planes of WO 3 , respectively. Moreover, the interplanar distance of 0.34 nm was ascribed to the (002) plane of GQDs which were anchored on the surface of WO 3 nanoplates to form 'dot-on-nanoparticle' structure. The SAED pattern of WO 3 /GQDs-H composites was shown in figure 3(e). The (200) and (1120) facets were attributed to WO 3 and GQDs, respectively. The highly crystalline monoclinic phase of WO 3 can still be maintained with adding GQDs.
The XRD patterns of pure WO 3 and hydrothermally synthesized WO 3 /GQDs-H composites were shown in figure 4(a). Three strong diffraction peaks of WO 3 /GQDs-H appeared at 23.00°, 23.48°and 24.26°, matching (002), (020), and (200) crystal plane of WO 3 , respectively [9]. Other diffraction peaks were also assigned to JCPDS 72-0677, and no impurities were observed in the patterns of WO 3 /GQDs. According to the Bragg's Law: 2dsinθ=nλ, where d, θ, λ and n represented interplanar spacing, glancing angle, wavelength of x-rays, and diffraction order, respectively. Three kinds of interplanar spacings corresponding to three strong diffraction peaks could be calculated as 0.38636 nm, 0.37860 nm, and 0.36657 nm, respectively. The theoretical calculation results were also roughly consistent with the interplanar spacing observed by figures 3(c) and (d). The diffraction peaks of WO 3 were similar to those of WO 3 /GQDs-H, indicating that GQDs anchored on WO 3 nanolamellaelike structure could not change the crystalline phase of WO 3 . Moreover, the XRD pattern of GQDs was provided in figure 4(b). A broad diffraction peak attributed to GQDs appeared at around 2θ of 25.6°, and the corresponding lattice spacing calculated according to Bragg's Law is 0.34768 nm. The broad peak of GQDs was not observed from WO 3 /GQDs-H, which was ascribed to the possible reason that the amount of GQDs was too low to be detected.
Furthermore, the structure of the samples was characterized by Raman spectroscopy. As shown in figure 5, the Raman spectrum of GQDs (curve a) exhibited the D and G peaks at 1351 and 1600 cm −1 , respectively. The intensity ratio of D and G peaks (I D /I G ) achieved 0.95, which was used to demonstrate the structural disorder of graphite materials. Meanwhile, the Raman spectrum of hydrothermally synthesized WO 3 /GQDs-H (curve b) presented the D and G peaks at 1351 and 1591 cm −1 , respectively, and the value of I D /I G decreased to 0.90. The results demonstrated that the hydrothermal process lowered the structural disorder and promoted to form a To explore the elemental composition and binding states of WO 3 and WO 3 /GQDs-H, XPS was employed and displayed in figure 6. The XPS survey spectra of WO 3 ( figure 6(a)) and hydrothermal synthesized WO 3 /GQDs-H ( figure 6(d)) demonstrated the existence of W&O and W&O&C atoms, respectively. The element C was associated with GQDs in the composites, which was consistent with the results of Raman analysis. The high-resolution spectrum of W 4f for WO 3 ( figure 6(b)) was deconvoluted into four peaks at the binding energy of 33.59, 34.94, 35.87, and 37.08 eV. Therein, two strong peaks of 34.94 and 37.08 eV corresponded to the binding energy of W 4f 7/2 and W 4f 5/2 of W 6+ state, while two weak peaks located at 33.59 and 35.87 eV were    assigned to W 5+ state [29]. After GQDs were anchored on WO 3 , that of W 4f for WO 3 /GQDs-H (figure 6(e)) was split into two pairs of peaks including 35.24 & 37.39 eV and 34.08 & 36.11 eV, which were indexed to W 6+ and W 5+ states, respectively. Part of the reduced WO 3 might originate from the formation of surface defects, which was reported in other literatures [30,31]. By comparing the high-resolution W 4f spectra of WO 3 and WO 3 /GQDs, the binding energy of W 6+ and W 5+ for WO 3 /GQDs-H shifted to higher values, probably owing to the interaction between WO 3 and WO 3 /GQDs during the hydrothermal synthesis of the composites. Two peaks at 529.8 and 530.3 eV appeared in the high-resolution O 1s spectrum of pure WO 3 ( figure 6(c)), corresponding to the binding energy of lattice oxygen and the oxygen in WO 3 , respectively [28]. While three peaks at 530.0, 530.5, and 531.2 eV in the high-resolution O 1s spectrum of WO 3 /GQDs-H, matching with the lattice oxygen, the lattice oxygen or O=C attributed to GQDs and chemisorbed oxygen species, respectively. Figure 6(g) exhibited three peaks at 284.2, 285.8, and 288.14 eV, which corresponded to the binding energy of C-C, C-O or C-OH, and C=O [32].
To characterize the photo-response performances of WO 3 and WO 3 /GQDs-H, the UV-vis spectra, photocurrent-time curves and linear sweep voltammetry were performed and displayed in figures 7-9, respectively. GQDs ( figure 7(a)) exhibited a shoulder peak in the range of 260-290 nm corresponding to the π→π * transition of C=C [33]. The characteristic absorption peak of WO 3 ( figure 7(b)) appeared at 346 nm, while that of WO 3 /GQDs-H ( figure 7(c)) presented a red-shift from 346 nm to 381 nm. Photocurrent responses curves of WO 3 , WO 3 /GQDs-H, and WO 3 /GQDs-M were collected in a standard three-electrode system with a continuous 60s 'on/off' procedure as illustrated in figure 8. The photocurrent value of WO 3 /GQDs-H was  boosted at 1.56×10 −5 A cm −2 , which is about 1.6 times as high as that of pure WO 3 (0.98×10 −5 A cm −2 ), while the value of WO 3 /GQDs-M merely reached 1.05×10 −5 A cm −2 , which presented almost no significant improvement compared to that of pure WO 3 . The results indicated that the hydrothermal synthesis played a vital role in improving the photoelectric properties of WO 3 /GQDs in comparison to the mechanical mixture. Figure 9 showed the LSV curves of WO 3 and WO 3 /GQDs-H composites under optical radiation. When the positive potential continues to increase, WO 3 /GQDs-H composites had a more obvious advantage in generating a larger photocurrent density under the optical radiation. This result was also consistent with that of photocurrent responses. The possible reason for this result was that the introduction of GQDs promoted the  separation of electron-hole pairs in WO 3 , in the meantime, GQDs could also generate photo-generated electrons.
On the basis of photocurrent-time analyses, a possible mechanism was presented in figure 10 [34 -38]. When the light source was irradiated on the surface of FTO glass coated with WO 3 /GQDs, photo-generated electrons were transported from valence band (VB) to the conduction band (CB) and subsequently conducted to the FTO substrate. As shown in figure 10(a), GQDs served as a conductive route for photoexcited electrons, promoting the charge transfer rate of WO 3 and weakening the combination of electron-hole pairs in the hydrothermally synthesized composites. While nano-sized GQDs tended to aggregate instead of anchoring on the surface of WO 3 in the mechanical mixture system, as illustrated in figure 10(b), only a small part of electrons transported from WO 3 to GQDs since aggregated GQDs were not beneficial to anchor on the surface of WO 3 , leading to a blocked route transported to the FTO substrate.
Electrochemical impedance spectra measurements of WO 3 (a), WO 3 /GQDs-H (b), and WO 3 /GQDs-M (c) were shown in figure 11 used a Nyquist diagram. The charge-transfer resistance of blank WO 3 was significantly larger than that of WO 3 /GQDs composites synthesized by the hydrothermal method and mechanical mixture. The result showed that GQDs promoted a valid path to transport charges in the electrode-electrolyte interface [39]. The interfacial conductivity of WO 3 /GQDs-H was prior to that of WO 3 /GQDs-M since the smaller charge-transfer impedance and the faster charge-transfer rate occurred on the composites by hydrothermal decorating with GQDs.

Conclusion
In summary, a simple hydrothermal method was employed to combine WO 3 with GQDs and form a special 'dot-on-nanoparticle' structure. Compared to blank WO 3 , the Raman spectrum of WO 3 /GQDs-H presented a decreased I D /I G value corresponding to the higher order degree constructed by the hydrothermal process. Meanwhile, the binding energy of W 6+ and W 5+ for WO 3 /GQDs shifted to higher values, probably owing to the interaction between WO 3 and WO 3 /GQDs-H during the hydrothermal synthesis of the composites. The photocurrent value and charge-transfer resistance of hydrothermal synthesized WO 3 /GQDs-H were prior to those of WO 3 /GQDs-M and blank WO 3. The result showed that GQDs promoted a valid path to transport charges in hydrothermally synthesized WO 3 /GQDs as compared to mechanically mixed composites. According to the existing results in this work, it is hoped that the WO 3 /GQDs composites can be helpful in the field of photoelectronics, photocatalysis, etc.

Acknowledgments
The work was supported by National Natural Science Foundation of China No. 51204129.