- Nano Express
- Open Access
S, N Co-Doped Graphene Quantum Dot/TiO2 Composites for Efficient Photocatalytic Hydrogen Generation
© The Author(s). 2017
- Received: 27 February 2017
- Accepted: 23 April 2017
- Published: 12 June 2017
S, N co-doped graphene quantum dots (S,N-GQDs) coupled with P25 (TiO2) (S,N-GQD/P25) have been prepared via simply hydrothermal method. The as-prepared S,N-GQD/P25 composites exhibited excellent photocatalytic hydrogen generation activities, with a significantly extended light absorption range and superior durability without loading any noble metal cocatalyst. The photocatalytic activity of this composite under visible light (λ = 400–800 nm) was greatly improved compared with that of pure P25. This remarkable improvement in photocatalytic activity of the S,N-GQD/P25 composites can be attributed to that S,N-GQDs play a key role to enhance visible light absorption and facilitate the separation and transfer of photogenerated electrons and holes. Generally, this work could provide new insights into the facile fabrication of photocatalytic composites as high performance photocatalysts.
- Graphene quantum dots
- Elemental doping
- Hydrogen evolution
Hydrogen energy is a new green pollution-free energy with many advantages including high calorific value, easy storage and transportation, no pollution, etc. Given that water and sunlight are two of the most abundant and easily accessible sources in the real world, transferring the solar energy into H2 from aqueous solution has become a hot research topic in the field of photocatalysis and hydrogen energy. Compared with CdS, SiC and many other semiconductors those have been widely used for photocatalytic H2 evolution [1–6], TiO2 has several advantages, such as low cost, non-toxicity, good photochemical stability and long service life, which benefits its industrial applications . However, the large bandgap (3.2 eV) of TiO2 and fast recombination of photogenerated electrons and holes restrict its solar energy conversion efficiency . Massive strategies have been taken to solve this problem, such as doping with metal elements [9, 10], depositing with noble metal  sensitizing with organic dyes [12, 13] and so on. Recently, a great deal of interest has been attracted on TiO2-based composites with combining metal-free carbon materials, such as graphene and carbon nanotubes (CNTs), which could efficiently enhance photocatalytic activity due to the superior charge transport properties to reduce the recombination rate of photogenerated electron-holes. For example, Du et al.  has reported a photocatalysis based on graphene/TiO2 core–shell nanoparticles, and the enhanced photocatalytic activity was associated with the large extended photoresponsive range and high electron–hole separation efficiency due to the synergetic interactions among TiO2 and graphene material. However, graphene is intrinsically a semimetal with a zero bandgap, which considerably impedes its application in photocatalysis . Besides, graphene as well as CNTs absorb a wide range of light, therefore may block other photocataysis from light irradiation . Above drawbacks limit the photocatalytic performance of graphene- and CNTs-based composite photocataysis.
Graphene quantum dots (GQDs), as a new rising carbon nanomaterial, consist of few layers of graphene with a lateral dimension less than 10 nm and process unique properties derived from graphene . Compared with traditional semiconductor quantum dots, such as ZnO , CdSe  and so on, GQDs exhibit higher water solubility, better chemical stability, low toxicity, excellent biocompatibility and photoelectrical properties. Therefore they have attracted a wide range of interests in sensing [20, 21], solar cells [22–24], bio-imaging [25, 26] and photocatalysis [27–30]. Recently, Qu et al.  has prepared GQD/TiO2 nanotube (GQD/TiO2 NT) composites by a simple hydrothermal method at low temperature. The photocatalytic activity of prepared GQD/TiO2 NT composites on the degradation of methyl orange (MO) was significantly enhanced compared with that of pure TiO2 nanotubes. Sudhagar et al.  has prepared GQDs/TiO2 hollow nanowires (HNW) architecture electrode for enhancing the light harvesting efficiency and the catalytic activity for water oxidation, without the need of sacrificial agents and demonstrated the underlying mechanism of photocarrier (e-/h+) transfer characteristics at GQDs/metal oxide interface during operation. Though there have been several reports suggesting the potential of GQDs as visible-light-driven photocatalysts, the lack of emission under long wavelength excitation and broad absorption in the visible region (λ > 400 nm) of GQDs still call for optimized methods . Recently, nitrogen and sulfur co-doped graphene quantum dots (S,N-GQDs) are studied due to their broad photoabsorption in wide spectral range, high carrier transport mobility and excellent chemical stability. Qu et al  has demonstrated that S,N-GQDs processed much better absorption of visible light than pure GQDs and multicolor emission under visible light excitation. These results indicate that elemental doping of GQDs could produce promising catalysts for solar photocatalysis. Further researches should focus on the modification of GQDs to regulate the bandgap, broaden the photo absorption region, and improve photo-quantum efficiency. But major challenges remain in developing low-cost, stable, and highly active GQD-based photocatalysts.
In this paper, we reported a hydrothermal method for simultaneously synthesizing and doping GQDs with S and N. We further prepared the S,N-GQD/TiO2 (P25) composites by a facile hydrothermal route. This composite showed an excellent photocatalytic performance in H2 production from methanol aqueous solution under UV-vis irradiation without the assistance of any noble metal cocatalysts. The photocatalytic activities of S,N-GQD/TiO2 with different S,N-GQD loading amounts were also investigated. Finally, the mechanism for the improvement of photocatalytic performance was discussed based on experimental results.
Synthesis of the S,N-GQDs
The detailed synthesis process of S,N-GQDs has been reported elsewhere . Typically, 1.26 g (6 mmol) citric acid and 1.38 g (18 mmol) thiourea were dissolved in 30 mL DMF and stirred for several minutes to obtain a clear solution. Then the solution was transferred in a 50 mL Teflon lined stainless steel autoclave. The sealed autoclave was heated up to constant 180 °C for 8 h and cooled down to room temperature. The final product was collected precipitate by adding ethanol into the solution and then centrifuged at 10,000 rpm for 15 min.
Synthesis of the S,N-GQD/P25 Composites
The S,N-GQD/P25 composites were obtained by a hydrothermal method. Typically, 0.5 g P25 and 5 mL S,N-GQD (2 mg mL−1) were added into 20 mL distilled water. The mixture was kept stirring for 4 h at room temperature to obtain a homogeneous suspension. After that, the suspension was transferred into a 40 mL Teflon-sealed autoclave and maintained at 150 °C for 6 h. Then the S,N-GQD/P25 composites were collected precipitate by centrifugation at 4000 rpm for 5 min. And finally the solid was dried in vacuum oven at 50 °C overnight. To investigate the effect of the S,N-GQD content on the photocatalytic H2 evolution rate, the S,N-GQD/P25 composites with different contents of S,N-GQD (0, 1, 2, 3, 5, 8 and10 wt%) were prepared.
Transmission electron microscopic (TEM) and high resolution TEM (HRTEM) images were obtained by a JEOL JEM-2100 F microscope operating at 200 kV; X-ray diffraction (XRD) pattern were recorded on a Rigaku D/max-2500 diffractometer with a nickel filtrated Cu Kα radiation operated at 40 kV and 300 mA; Fourier transform infrared (FTIR) spectra were performed using Nicolet 6700 (Thermo Fisher); Raman spectra were carried out by NEXUS670 (Thermo Nicolet Corporation); UV–vis absorption spectra were measured using a UV-vis spectrophotometer Lambda 950 (Perkin Elmer, USA).
Photocatalytic Hydrogen Generation
Fifty milligrams of photocatalyst powders were dispersed in a 100 mL aqueous solution which contains 10 mL methanol as the sacrificial agent. The UV-light and visible-light irradiations were generated from a 300 W Xe lamp without and with a 400 nm filter, respectively. The amount of generated H2 was determined with an online gas chromatograph.
The transient photocurrent responses were measured in an electrochemical workstation with a conventional three-electrode system: a Pt plate as the counter electrode, a saturated calomel electrode as the reference electrode, and the as-prepared sample was coated on the ITO substrate as the working electrode. Specifically, the working electrode was prepared by coating the slurry made of 0.05 g photocatalyst, 0.2 g polyethylene glycol (PEG20000), and 1.0 mL water onto ITO glass electrodes by the doctor blade method, with subsequent calcining at 450 °C for 30 min. The active surface area of the working electrode that exposed to the electrolyte was about 2 cm2 and the thickness of the coated layer was about 8 mm. The electrolyte was 0.5 M Na2SO4 aqueous solution. The light source was a 300 W Xe lamp.
In conclusion, we successfully prepared the S,N-GQD/P25 composites in aqueous solution. The composites were studied by TEM, HRTEM, FTIR, Raman and XRD analyses. Our results demonstrated that S,N-GQDs decorated on P25 can obvious broaden the visible light absorption of P25 and enhanced the activity on photocatalytic H2 production under UV–vis light irradiation. Especially, the 3 wt% S,N-GQD/P25 showed the best photocatalytic ability, which is about 3.6 times higher than that of the pure P25. Furthermore, the S,N-GQD/P25 composites also exhibited efficient photocatalytic H2 production activity under visible light, which won an advantage over P25. Overall, the S,N-GQD/P25 composites showed improved utilization of solar light for hydrogen production and energy conversion.
We gratefully acknowledge the financial support by Natural Science Foundation of China [No. 51572046, 51603037], The Shanghai Natural Science Foundation [15ZR1401200, 16ZR1401500], the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning,Program of Shanghai Academic Research Leader [16XD1400100], the Program of Introducing Talents of Discipline to Universities [No.111-2-04], the Fundamental Research Funds for the Central Universities [2232014A3-06] the Shanghai ChenGuang Program [15CG33], and the Shanghai Sailing Program [16YF1400400].
HX, CH, and YL carried out the experiments and wrote the manuscript. QZ and HW prepared the samples and performed the characterizations. All authors checked and revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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