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Novel GQD-PVP-CdS composite with enhanced visible-light-driven photocatalytic properties
Applied Surface Science 367 (2016) 518–527
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Novel GQD-PVP-CdS composite with enhanced visible-light-driven photocatalytic properties Tao Fan, Yinle Li, Jianfeng Shen ∗ , Mingxin Ye ∗ Institute of Special Materials and Technology, Fudan University, 220 Handan Road, Shanghai 200433, China
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Article history: Received 15 October 2015 Received in revised form 20 January 2016 Accepted 22 January 2016 Available online 26 January 2016 Keywords: Graphene quantum dots Cadmium sulfide Photocatalysis Heterogeneous photocatalysts
a b s t r a c t A facile one-step hydrothermal method to synthesize graphene quantum dots (GQDs)-polyvinyl pyrrolidone (PVP)-CdS nanocomposite was reported. The nanocomposite was thoroughly characterized with X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and ultraviolet–visible spectroscopy. The results confirmed the formation of GQD-PVP-CdS composite with a uniform size (5–10 nm) and a relatively low band gap (Eg = 2.23 eV). Moreover, the as-prepared composite exhibited enhanced photocatalytic activity toward the degradation of organic contaminants, with 92.3% of methyl orange (10 mg/L) removed after 3 hours of visible light illumination. This enhancement in photocatalytic activity was postulated to be attributed to the upconversion property of GQDs and a more efficient charge distribution between GQDs and CdS particles. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Since the first discovery of photocatalytic splitting of water on a titanium dioxide (TiO2 ) electrode in 1972, heterogeneous photocatalysis has attracted increasing attention in recent years [1–5]. Photocatalysis has been proven to be an efficient way to split water to produce hydrogen and oxygen, and also to remove hazardous contaminants from wastewater. Compared to other wastewater treatment techniques, photocatalysis possesses advantages in oxidation efficiency, catalytic rate and oxidation of pollutants in low levels [6,7]. Semiconductors are usually selected as photocatalysts due to their ability to absorb light and produce electron–hole pairs, which can further generate free radicals. Among the known photocatalytic semiconductors, TiO2 possesses extraordinary properties as a photosensitive catalyst, such as environment-friendly behavior, long-term chemical stability, good photostability and strong oxidation activity, which make TiO2 an ideal material for the treatment of organic pollutants in wastewater [8,9]. Thus, TiO2 has been widely investigated and is still a benchmark material for photocatalytic reactions to this day. However, as a wide-gap semiconductor, TiO2 can only absorb high-energy ultraviolet (UV) radiation of wavelengths under 388 nm. Since UV light only counts for less than 5% of the whole solar spectrum, the application of TiO2 in the field of photocatalysis has been greatly restricted [10,11]. To this end,
∗ Corresponding authors. E-mail addresses: [email protected] (J. Shen), [email protected] (M. Ye). http://dx.doi.org/10.1016/j.apsusc.2016.01.194 0169-4332/© 2016 Elsevier B.V. All rights reserved.
numerous efforts have been made to explore new photocatalysts with suitable band gap, so that the solar energy can be utilized more efficiently. On the other hand, more and more attention has been poured into cadmium sulfide (CdS). CdS possesses a relatively narrow band gap (2.35 eV), so it could be employed as an effective photocatalyst toward the utilization of visible light in solar radiation [12,13]. Various methods have been developed to synthesize CdS particles, such as sol–gel, precipitation, and template synthesis [14]. However, pristine CdS particles are not stable and tend to aggregate, which greatly decreases their specific surface area and increases the recombination of photoinduced electron–hole pairs. Adding polymeric materials is an efficient way to address these problems. By coating CdS particles with polymers, it is feasible to tune their particle sizes, extend their light absorption in the visible region and improve their stability against photocorrosion [7,15]. Polyvinyl pyrrolidone (PVP) is often chosen as a polymeric capping agent and plays an important role in the transfer of the photogenerated electrons and holes and prevents their recombination [16]. Moreover, compared to bare CdS, PVP-capped CdS nanoparticles can diffuse better in water and make more effective contact with dye molecules [15,17]. In recent years, GQDs, an emerging kind of zero-dimensional carbon-based nanomaterials, have been widely investigated [18–21]. Due to quantum confinements and edge effects, GQDs possess a size-dependent band gap and unique up-converted photoluminescence behavior. Compared to fluorescent semiconductor nanocrystals, GQDs hold such favorable characteristics as
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nontoxicity, biocompatibility and eco-hospitality, which make GQDs a promising candidate in chemical sensing, bioimaging, supercapacitors and photovoltaics [22–25]. In addition, the application of GQDs in the field of photocatalysis has also been extensively explored recently. A quintessential example should be cited that the design of complex photocatalysts (GQDs/TiO2 , GQDs/Cu2 O and GQDs/Fe2 O3 , etc.) to harness the full spectrum of solar radiation has been achieved by many research groups [26–29]. In these composites, GQDs play a crucial part in the enhancement of the absorption in the visible region and ultimately increase their photocatalytic performance. Herein, we demonstrate a novel GQD-PVP-CdS nanocomposite fabricated by a facile one-step hydrothermal route. GQDs and polymeric material are compounded with CdS nanoparticles simultaneously for the first time. GQDs and PVP are added to not only modify the surface properties of CdS particles, but also promote their visible light response and ultimately enhance their photocatalytic ability. The as-prepared nanocomposite exhibited uniform size, extended visible light absorption, narrower band gap and excellent photocatalytic activity. 2. Experimental 2.1. Reagents Citric acid, thiourea, cadmium diacetate dihydrate (Cd(OAc)2 ·2H2 O), PVP and sodium sulfide nonahydrate (Na2 S·9H2 O) were of analytical grade, supplied by Sinopharm chemical reagent Co., Ltd (Shanghai, China) and used without further purification. Deionized (DI) water was applied in all experiments.
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for TEM tests were prepared by dropping the solution onto a copper grid with carbon film and dried in air. Scanning electron microscopy (SEM) and elemental mapping images were acquired with a Tescan MAIA3 scanning electron microscope. The Fourier transform infrared (FT-IR) spectra were acquired with a Nicolet Is10 spectrometer. Raman spectra were recorded on a Raman microspectrometer (spex/403). X-ray photoelectron spectrometry (XPS) analyses were carried out on a PHI 5000C ESCA system. The Brunaure–Emmett–Teller (BET) surface areas of the samples were determined by nitrogen adsorption–desorption isotherm measurement at room temperature (Quadrasorb SI, Quantachrome). Room temperature UV–vis absorption and UV–vis diffusive reflectance spectra were recorded using a Shimadzu UV-3600 spectrophotometer. 2.5. Photoelectrochemical measurements Electrochemical impedance spectroscopy (EIS) and transient photocurrent response under visible light irradiation were measured on an electrochemical workstation (Autolab PG 302N) in a standard three-electrode system. The working electrodes were prepared as follows: 0.5 g of the sample was grounded with 0.1 g polyvinylidene fluoride (PVDF) and 1 mL ethanol, then the mixture was stirred for 3 hours to make a slurry. The slurry was coated onto an indium tin oxide (ITO) glass electrode by the doctor blade technique, and the active area was controlled to be 1.0 cm2 . A Pt wire was employed as the counter electrode, while Ag/AgCl (saturated KCl) was applied as the reference electrode. Na2 SO4 aqueous solution (1 mol/L) was used as the electrolyte. 2.6. Photocatalytic activity measurements
2.2. Preparation of GQDs GQDs were prepared from citric acid through a modified hydrothermal method [30]. Briefly, 1.26 g (6 mmol) citric acid and 1.38 g (18 mmol) thiourea were dissolved in 30 mL DI water, and stirred for an hour to form a clear solution. Then the mixture was transferred into a 100 mL Teflon lined stainless autoclave and kept in an electric oven at 180 ◦ C for 6 hours. After cooling to room temperature, the solvent of suspension was removed with the aid of a rotary evaporator. The final product was collected by adding ethanol to the residue and centrifuged at 10,000 rpm for 5 min. 2.3. Fabrication of GQD-PVP-CdS composite 1.33 g (5 mmol) Cd(OAc)2 ·2H2 O and 1 g PVP were added into 25 mL GQDs aqueous solution and stirred for 2 hours to form a clear solution. 25 mL Na2 S solution (0.2 mol/L) was added drop by drop into the mixture solution under vigorous stirring. Then the mixture was transferred into a 100 mL Teflon lined stainless autoclave and kept at 120 ◦ C for 4 hours. The resulting solids were collected by centrifugation and washed with water for 3 times, and then dried in vacuum at 60 ◦ C overnight. PVP-CdS composite was prepared in the same way by replacing GQD solution with DI water, and pristine CdS particles were fabricated without either PVP or GQDs.
The photocatalytic activity of the samples was evaluated through the degradation of MO and phenol in aqueous solution under visible light ( > 400 nm) from a high-pressure mercury lamp with a UV 400 cut filter. The experiments were carried as follows: 50 mg of photocatalysts were added into 100 mL of MO or phenol solution (10 mg/L). Before irradiation, the solution was stirred in dark for 1 hour to achieve adsorption/desorption equilibrium between the catalysts and organic pollutant. Then the solution was exposed to visible light irradiation while keeping stirring. 3 mL aliquots were taken at time intervals of 15 min and analyzed on a UV–vis spectrophotometer to examine the concentrations of the organic contaminant. Before the spectroscopy measurement, the aliquots were centrifuged at 10,000 rpm for 5 min to remove the photocatalysts. 2.7. Active species trapping experiments To detect the active species during photocatalytic process, several kinds of sacrificial agents were used as scavengers. 2-propanol (IPA), ammonium oxalate (AO) and 1,4-benzoquinone (BQ) were chosen as the hydroxyl radical (• OH) scavenger, hole (h+ ) scavenger and superoxide radical (• O2 − ) scavenger, respectively. The experiments were carried out similarly as the former photocatalytic activity measurement with the addition of different quenchers [31,32].
2.4. Characterization 3. Results and discussion To examine the crystal phase of the products, powder X-ray diffraction (XRD) patterns were recorded with an X-ray diffractometer (Bruker D8 Advance) at a scanning rate of 5◦ /min in the 2 range of 10–90◦ . Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained with a Tecnai G2 F20 S-Twin transmission electron microscope. The samples
3.1. XRD patterns The typical X-ray diffraction (XRD) patterns of the as-prepared CdS, PVP-CdS and GQD-PVP-CdS samples are shown in Fig. 1. It is clear to see that all these diffraction peaks could be indexed to cubic
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3.2. TEM analysis To characterize the morphology of the as-prepared GQDs and GQD-PVP-CdS composite, transmission electron microscopy (TEM) was employed. Fig. 2(a) shows the TEM image of GQDs. It is clearly indicated that the as-prepared GQDs are similar in size with diameters mainly distributed in the range of 2–4 nm. The high-resolution transmission electron microscopy (HRTEM) image displayed in Fig. 2(c) indicates that the GQDs are highly crystalized with an interplanar spacing of 0.194 nm, which corresponds well to the (104) crystal plane of graphitic carbon (JCPDS 26-1076) [34]. Furthermore, the typical TEM image of GQD-PVP-CdS composite is shown in Fig. 2(b), demonstrating that the product mainly consists of uniform particles with sizes ranging from 5 to 10 nm. HRTEM was also applied and the result is exhibited in Fig. 2(d). Interplanar spacings of 0.194, 0.293 and 0.332 nm were clearly observed, which are in agreement with the (104) facet of GQDs and the (200) and (111) facets of cubic CdS, respectively [12,33]. Fig. 1. Typical XRD patterns of CdS, PVP-CdS and GQD-PVP-CdS samples.
3.3. SEM analysis and elemental mapping CdS (JCPDS card no. 65-2887). All three samples showed three main peaks at 2 ≈ 27.8◦ , 43.5◦ and 52.4◦ , which matched well with (111), (220) and (311) crystal planes of cubic CdS, respectively [17,33]. In addition, a careful study of the patterns revealed the asymmetry of the (111) peak, indicating a relatively weak peak at 2 ≈ 30.3◦ , which corresponds to the (200) crystal face [17]. By comparing all three patterns, we found that the addition of PVP and GQDs had no distinct effect on the crystal structure of CdS particles.
Fig. 3(a) shows the SEM image of the as-prepared GQD-PVP-CdS composite, suggesting that CdS nanoparticles tended to aggregate. The elemental mapping images are displayed in Fig. 3(b–f), indicating the presence of Cd, S, C, N and O as major chemical components. The existence of Cd and S suggests the formation of CdS, while C, N and O elements are attributed to PVP and GQDs. These images confirmed that GQDs and PVP were eventually loaded on the surface of CdS particles.
Fig. 2. TEM images of (a) GQDs and (b) GQD-PVP-CdS; HRTEM images of (c) GQDs and (d) GQD-PVP-CdS.
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Fig. 3. (a) SEM image of GQD-PVP-CdS; elemental mapping of (b) Cd, (c) S, (d) C, (e) N and (f) O elements in GQD-PVP-CdS composite.
indicating that there exist many hydroxyl and amino groups on the surface of GQDs. The vibrational absorption band of C O is observed at 1707 cm−1 , while the bands at 1578 and 1401 cm−1 are attributed to the bending vibrations of C C and C N, respectively. The weak peaks at 1110 and 617 cm−1 correspond to the stretching vibrations of C S and C S, respectively, which means the as-prepared products are S,N co-doped GQDs [30]. The spectrum of PVP is close to that in previous reports [35,36]. The bands at 2960, 1703 and 1292 cm−1 could be assigned to the vibrations of C H stretching, C O stretching and CH2 deformation, severally. All of the peaks above can be spotted in the FT-IR spectrum of the final product GQD-PVP-CdS composite, with slight changes in peak positions and intensities. The spectrum of pure CdS shows a major absorption peak at 596 cm−1 , which is assigned to the Cd S stretching frequency [37,38]. However, this peak is not visible in the spectrum of GQD-PVP-CdS, probably due to its relatively low intensity.
Fig. 4. FT-IR spectra of GQDs, PVP and GQD-PVP-CdS samples.
3.4. FT-IR spectra FT-IR spectra were used to identify the hyperfine chemical structure of the products. As displayed in Fig. 4, the spectrum of GQDs shows a broad absorption band at 3000–3500 cm−1 , which is assigned to the stretching vibrations of O H and N H bonds,
3.5. Raman spectra Raman spectroscopic measurements were carried out to further elucidate the structure of the prepared samples. As shown in Fig. 5, pure CdS exhibited two main Raman peaks at 307 cm−1 and 600 cm−1 , which correspond to 1LO modes and 2LO modes of CdS, respectively [39,40]. After the formation of PVP-CdS, additional Raman peaks at 754 cm−1 and 935 cm−1 were found, which are ascribed to PVP [41]. Moreover, after the loading of GQDs, the
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composite showed extra peaks at 1357 cm−1 and 1560 cm−1 , which are attributed to the D band and G band of GQDs, respectively [30,42]. However, compared to pure GQDs, these two peaks both showed a certain shift, which could be caused by the charge transfer between GQDs and CdS [43,44]. 3.6. XPS spectra
Fig. 5. Raman spectra of GQDs, CdS, PVP, PVP-CdS and GQD-PVP-CdS samples.
X-ray photoelectron spectroscopy (XPS) measurements were carried out to further prove the formation of GQD-PVP-CdS composite. Fig. 6 displays several regions of the XPS spectra of the GQD-PVP-CdS sample. The full scan XPS spectrum is shown in Fig. 6(a). The peaks at 285 eV and 533 eV are assigned to carbon (C 1s) and oxygen (O 1s), respectively, which could originate from GQDs and PVP. In addition, the XPS spectrum also confirms the existence of sulfur and cadmium species from the appearance of an S 2s peak at 225 eV, an S 2p peak at 162 eV and a characteristic Cd 3d5/2 peak at 406 eV, a Cd 3d3/2 peak at 413 eV, which suggests the formation of CdS [45–48]. No obvious N 1s peak was discovered, probably due to its relatively low content and being too close to the Cd 3d5/2 peak. Fig. 6(b) shows the high resolution scan of the C 1s region, indicating that carbon exists in three different chemical environments. The deconvoluted peak located at 284.6 eV corresponds to the sp2 C in graphene, namely C C and C C bonds. The
Fig. 6. (a) XPS survey spectra of GQD-PVP-CdS composite; peak deconvolution of (b) C 1s region and (c) S 2p region of GQD-PVP-CdS composite.
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the elemental composition of the region less than 10 nm from the surface, the top CdS layer could block the signal of the S C and S O bonds. Thus, the S C and S O peaks at 164 eV and 168 eV were not observed [2,33]. 3.7. Specific surface area analysis
Fig. 7. Nitrogen adsorption–desorption isotherms of CdS, PVP-CdS and GQD-PVPCdS samples.
sp3 C in C OH bonds contributes to the peak at 285.5 eV, and the peak at 288 eV is attributed to the C O bonds from carbonyls and carboxylates [49]. Besides, the high resolution scan of S 2p region is displayed in Fig. 6(c), the two peaks at 161.2 and 162.2 eV are both ascribed to the S in CdS. Since XPS technique can only detect
Full nitrogen adsorption–desorption isotherms are shown in Fig. 7. All three samples exhibited adsorption– desorption isotherms of type IV according to the Brunauer– Deming–Deming–Teller (BDDT) classification, indicating the presence of mesopores (2–50 nm) within CdS nanocrystals [50]. In addition, the isotherms showed high adsorption at a high relative pressure (P/P0 ) approaching 1.0, and the shape of all three hysteresis loops was of type H3. These results suggest the presence of large macropores formed by the aggregation of primary CdS nanocrystals [51]. The BET specific surface area of pure CdS sample is 36.8 m2 /g, while that of PVP-CdS is 56.8 m2 /g. However, after the addition of GQDs, the specific surface area of the sample decreases to 37.0 m2 /g, which can be attributed to the blockage of partial mesopores by the GQDs on the surface of CdS particles [52]. 3.8. DRS spectra The optical properties of the as-prepared samples were investigated by UV-vis diffuse reflection spectroscopy, and the obtained
Fig. 8. (a) UV–visible DRS spectra of different samples, (b) energy dependence of (ah)2 .
Fig. 9. (a) Transient photocurrent response and (b) EIS Nyquist plots of CdS, PVP-CdS and GQD-PVP-CdS under visible light irradiation.
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Fig. 10. (a) Absorption spectral changes over irradiation time for the MO aqueous solution (10 mg/L) in the presence of GQD-PVP-CdS composite under visible light illumination; (b) Normalized changes in the MO concentration (C/C0 ) and (c) linear plots of −ln(C/C0 ) for the photodegradation of MO under visible light in the presence of different photocatalysts.
DRS results are shown in Fig. 8. From the DRS data, we can see that the addition of PVP and GQDs has influence on the optical properties of CdS particles. Compared to bare CdS and PVP-CdS, GQD-PVPCdS showed higher absorption intensity in the visible light region. This enhancement could be attributed to the black body effect of the additional GQDs, which was beneficial for the composite to absorb visible light, thus improving its photoactivity [3,53]. According to the Kubelka–Munk theory, the optical absorption band gap Eg of a semiconductor can be calculated by the following equation: (ah)n = A(h − Eg ), where a stands for the absorption coefficient, hv means the incident photon energy, n is the index depending on the type of the semiconductor and A represents the proportionality constant relative to the materials.[3,31] In the case of CdS, which is a direct gap semiconductor, n equals 2 [54]. Therefore, as shown in Fig. 8(b), the Eg can be determined by extrapolation of the linear portion of the (ah)2 curve versus the photon energy h to (ah)2 = 0. It can be concluded that both CdS and PVP-CdS have close band gap values of 2.35 and 2.36 eV, respectively, while
GQD-PVP-CdS composite exhibits a slight red-shift, with a band gap value of 2.23 eV. 3.9. Photoelectrochemical measurements One of the important roles of GQDs and PVP is to facilitate the migration and separation of the photogenerated electrons, and we have carried out transient photocurrent response and EIS tests of the prepared samples to investigate their charge transfer capability. Fig. 9(a) shows the transient photocurrent response results of the as-prepared samples. For all three samples, the value of photocurrent density rose rapidly under visible light illumination and decreased to almost zero when the light was off. It can also be seen that GQD-PVP-CdS nanocomposite exhibited the highest photocurrent density, with a value of nearly 0.3 mA cm−2 , while CdS and PVP-CdS generated relatively lower photocurrent density of 0.04 and 0.19 mA cm−2 , respectively. Furthermore, the typical Nyquist plot of the EIS measurements is displayed in Fig. 9(b). It can be
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clearly seen that the arc of the spectrum became smaller when PVP was introduced into CdS nanoparticles. After the addition of GQDs, the arc became even smaller, indicating a smaller electron transfer resistance and a higher charge transfer rate [55,56]. 3.10. Photocatalytic performance It is well known that semiconductor particles are capable of producing electron–hole pairs under illumination with photon energy larger than their band gap. When the particle size is small enough, those electrons and holes are able to transfer to the surface of the crystal, which makes their reaction with H2 O and molecular oxygen possible. When they get in contact with H2 O and O2 , hydroxyl radicals and superoxide radicals are generated, which are highly reactive and oxidative [57,58]. They are able to degrade various kinds of organic pollutants into less harmful products such as CO2 , H2 O, NO3 − , etc. Since most organic pollutants show absorption in ultraviolet–visual (UV–vis) region, it is feasible to detect their concentrations using a UV–vis spectrophotometer. With the goal of investigating the visible light photocatalytic performance of the as-prepared samples toward the degradation of organic pollutants, methyl orange (MO) was selected as a model contaminant. Prior to visible light irradiation, the reaction solution was magnetically stirred in dark for 1 hour to ensure adsorption/desorption equilibrium. Under dark conditions, no significant degradation of the MO solution was observed after 1 hour in the presence of GQD-PVP-CdS composite. Fig. 10(a) shows the evolutions of absorption spectra of MO solution exposed to visible light ( > 400 nm) over time in the presence of the as-prepared GQDPVP-CdS composite. The MO spectrum revealed a major absorption band at 464 nm. It is clear that under visible light irradiation, the absorption peaks dropped gradually, indicating that MO underwent a photocatalytic decomposition process. Fig. 10(b) compares the photocatalytic performance of CdS, PVP-CdS and GQD-PVPCdS samples, and it is obvious that GQD-PVP-CdS exhibits the best photocatalytic properties. According to the Lambert–Beer law, the degradation efficiency, de %, can be calculated by the following equation: de = (C0 − Cf )/C0 * 100% = (A0 − Af )/A0 * 100%, where C0 and Cf represent the initial and final MO concentrations, while A0 and Af represent the initial and final absorbance, respectively [12,59]. After 3 hours of visible light illumination, the degradation efficiencies of MO under the presence of CdS and PVP-CdS are 56.0% and 59.5%, respectively. In contrast, GQD-PVP-CdS composite leads to a remarkable increase with a degradation efficiency of 92.3%. The photocatalytic degradation reaction of MO is usually assumed to follow pseudo-first-order expression: −ln(C0 /C) = kt, where C/C0 represents the normalized MO concentration and k stands for the apparent reaction rate (min−1 ). By plotting −ln(C0 /C) as a function of irradiation time through linear regression, the observed rate constant k, which is an important index to evaluate photoactivity, can be obtained from the slopes of the simulated straight lines [6,60]. Fig. 10(c) displays the relationship between −ln(C0 /C) and the irradiation time (t) for the photocatalytic degradation of MO under visible light. The reaction rate constant for the pristine CdS particles is 4.38 × 10−3 min−1 , and for PVP-capped CdS particles, it slightly increased to 4.71 × 10−3 min−1 . After the addition of GQDs, the photoactivity of the particles exhibited a major improvement, with a much higher rate constant of 1.47 × 10−2 min−1 . Furthermore, we have investigated the photostability of CdS and GQD-PVP-CdS samples by repeated photocatalytic experiments. As shown in Fig. 11, in the first three cycles of photocatalytic tests, CdS showed degradation efficiencies of 56.0%, 41.9% and 32.7%, while GQD-PVP-CdS nanocomposite exhibited 92.3%, 91.7% and 89.6%, respectively. This result shows that GQD-PVP-CdS nanocomposite possesses not only much higher photocatalytic activity but also
Fig. 11. Recycled photodegradation of MO under visible light irradiation of pure CdS and GQD-PVP-CdS composite.
Fig. 12. Normalized changes in the MO concentration (C/C0 ) under visible light in the presence of GQD-PVP-CdS composite with the addition of different scavengers.
better stability than those of CdS, suggesting better protection against photocorrosion [46,61]. This enhancement may be attributed to the synergetic effect between GQDs and CdS, since the photogenerated photons and electron could transfer from CdS to GQDs, thus protecting CdS from photocorrosion and improving its photostability [62]. Another series of tests were carried out to probe the active species during photocatalytic degradation process. As stated in the experimental section, IPA, BQ and AO were selected as the scavengers for • OH, • O2 − and h+ , respectively. The results are shown in Fig. 12. With the addition of IPA as a scavenger for • OH radicals, the degradation rate only slightly decreased, indicating • OH did not play an important part during the photocatalytic process. However, when BQ was added to the system, the degradation of MO was significantly prohibited, and only less than 20% of MO was degraded after 3 hours of visible light illumination. This dramatic decrease in photodegradation efficiency suggests that the • O2 − pathway had a crucial role in the process of MO oxidation. To detect the influence h+ had on the photocatalytic process, we chose AO as h+ scavenger. The results show that after the addition of AO, the degradation efficiency dropped to 45%, indicating h+ did play a part in the removal of MO, though not as important as • O2 − .
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4. Conclusion In summary, a novel GQD-PVP-CdS composite photocatalyst with efficient visible light activity was fabricated by a facile onestep method successfully. Significantly, compared to pristine CdS and PVP-CdS, the as-prepared GQD-PVP-CdS displayed enhanced photocatalytic ability and high photodegradation efficiency toward MO under visible light illumination. This improvement may be caused by the synergetic effect between GQDs and CdS particles, since GQDs possess upconversion properties and GQD-PVP-CdS exhibited a relatively higher dispersion of electric charge in the system. Since ultraviolet light only accounts for no more than 5% of the whole solar energy, it is desirable to develop visible light sensitive photocatalysts. Therefore, we expect that our findings may provide a new strategy to synthesize photocatalysts that are sensitive to visible light and help research in current environmental pollution, energy issues and other related fields. Fig. 13. Normalized changes in the concentration (C/C0 ) for the photodegradation of phenol under visible light in the presence of different photocatalysts.
Fig. 14. Schematic presentation of the photodegradation process of MO.
Moreover, in order to rule out the photosensitization role of dyes during the photocatalytic reaction, we have selected phenol as a colorless contaminant to test the photcatalytic behavior of the asprepared samples. As shown in Fig. 13, after 3 hours of visible light illumination, CdS, PVP-CdS and GQD-PVP-CdS exhibited degradation efficiencies of 26.1%, 47.2% and 59.0% toward the degradation of phenol solution (10 mg/L), respectively. The samples showed less photoactivity toward phenol degradation than those of MO degradation, however, it can be clearly seen that the introduction of GQDs and PVP greatly enhanced the photocatalytic behavior of CdS. This improvement in photoactivity could be attributed to the synergetic effect of GQDs and CdS particles. The proposed photodegradation mechanism is displayed in Fig. 14. Firstly, GQDs possess unique upconversion properties, which can absorb visible light and then emit photon with shorter wavelength [30,63]. The black body effect of GQDs also can help with the absorption of visible light. As a result, GQD-PVP-CdS composite has an expanded light absorption and an enhanced photoactivity in visible light region compared to CdS and PVP-CdS. Secondly, under visible light illumination, the photo-induced electrons are able to transfer between GQDs and CdS particles easily. Through this electron transfer process, GQDs may protect CdS from photocorrosion and enhance the photostability of GQD-PVP-CdS particles [14,62]. Furthermore, GQDs can also play the role as an electron reservoir to trap electrons emitted from CdS particles, thus preventing the photoinduced electron–hole pairs from recombination [27,64,65]. As a result of all these advantages, the as-prepared GQD-PVP-CdS composite shows greater photocatalytic performance.
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Novel GQD-PVP-CdS composite with enhanced visible-light-driven photocatalytic properties Tao Fan, Yinle Li, Jianfeng Shen ∗ , Mingxin Ye ∗ Institute of Special Materials and Technology, Fudan University, 220 Handan Road, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 15 October 2015 Received in revised form 20 January 2016 Accepted 22 January 2016 Available online 26 January 2016 Keywords: Graphene quantum dots Cadmium sulfide Photocatalysis Heterogeneous photocatalysts
a b s t r a c t A facile one-step hydrothermal method to synthesize graphene quantum dots (GQDs)-polyvinyl pyrrolidone (PVP)-CdS nanocomposite was reported. The nanocomposite was thoroughly characterized with X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and ultraviolet–visible spectroscopy. The results confirmed the formation of GQD-PVP-CdS composite with a uniform size (5–10 nm) and a relatively low band gap (Eg = 2.23 eV). Moreover, the as-prepared composite exhibited enhanced photocatalytic activity toward the degradation of organic contaminants, with 92.3% of methyl orange (10 mg/L) removed after 3 hours of visible light illumination. This enhancement in photocatalytic activity was postulated to be attributed to the upconversion property of GQDs and a more efficient charge distribution between GQDs and CdS particles. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Since the first discovery of photocatalytic splitting of water on a titanium dioxide (TiO2 ) electrode in 1972, heterogeneous photocatalysis has attracted increasing attention in recent years [1–5]. Photocatalysis has been proven to be an efficient way to split water to produce hydrogen and oxygen, and also to remove hazardous contaminants from wastewater. Compared to other wastewater treatment techniques, photocatalysis possesses advantages in oxidation efficiency, catalytic rate and oxidation of pollutants in low levels [6,7]. Semiconductors are usually selected as photocatalysts due to their ability to absorb light and produce electron–hole pairs, which can further generate free radicals. Among the known photocatalytic semiconductors, TiO2 possesses extraordinary properties as a photosensitive catalyst, such as environment-friendly behavior, long-term chemical stability, good photostability and strong oxidation activity, which make TiO2 an ideal material for the treatment of organic pollutants in wastewater [8,9]. Thus, TiO2 has been widely investigated and is still a benchmark material for photocatalytic reactions to this day. However, as a wide-gap semiconductor, TiO2 can only absorb high-energy ultraviolet (UV) radiation of wavelengths under 388 nm. Since UV light only counts for less than 5% of the whole solar spectrum, the application of TiO2 in the field of photocatalysis has been greatly restricted [10,11]. To this end,
∗ Corresponding authors. E-mail addresses: [email protected] (J. Shen), [email protected] (M. Ye). http://dx.doi.org/10.1016/j.apsusc.2016.01.194 0169-4332/© 2016 Elsevier B.V. All rights reserved.
numerous efforts have been made to explore new photocatalysts with suitable band gap, so that the solar energy can be utilized more efficiently. On the other hand, more and more attention has been poured into cadmium sulfide (CdS). CdS possesses a relatively narrow band gap (2.35 eV), so it could be employed as an effective photocatalyst toward the utilization of visible light in solar radiation [12,13]. Various methods have been developed to synthesize CdS particles, such as sol–gel, precipitation, and template synthesis [14]. However, pristine CdS particles are not stable and tend to aggregate, which greatly decreases their specific surface area and increases the recombination of photoinduced electron–hole pairs. Adding polymeric materials is an efficient way to address these problems. By coating CdS particles with polymers, it is feasible to tune their particle sizes, extend their light absorption in the visible region and improve their stability against photocorrosion [7,15]. Polyvinyl pyrrolidone (PVP) is often chosen as a polymeric capping agent and plays an important role in the transfer of the photogenerated electrons and holes and prevents their recombination [16]. Moreover, compared to bare CdS, PVP-capped CdS nanoparticles can diffuse better in water and make more effective contact with dye molecules [15,17]. In recent years, GQDs, an emerging kind of zero-dimensional carbon-based nanomaterials, have been widely investigated [18–21]. Due to quantum confinements and edge effects, GQDs possess a size-dependent band gap and unique up-converted photoluminescence behavior. Compared to fluorescent semiconductor nanocrystals, GQDs hold such favorable characteristics as
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nontoxicity, biocompatibility and eco-hospitality, which make GQDs a promising candidate in chemical sensing, bioimaging, supercapacitors and photovoltaics [22–25]. In addition, the application of GQDs in the field of photocatalysis has also been extensively explored recently. A quintessential example should be cited that the design of complex photocatalysts (GQDs/TiO2 , GQDs/Cu2 O and GQDs/Fe2 O3 , etc.) to harness the full spectrum of solar radiation has been achieved by many research groups [26–29]. In these composites, GQDs play a crucial part in the enhancement of the absorption in the visible region and ultimately increase their photocatalytic performance. Herein, we demonstrate a novel GQD-PVP-CdS nanocomposite fabricated by a facile one-step hydrothermal route. GQDs and polymeric material are compounded with CdS nanoparticles simultaneously for the first time. GQDs and PVP are added to not only modify the surface properties of CdS particles, but also promote their visible light response and ultimately enhance their photocatalytic ability. The as-prepared nanocomposite exhibited uniform size, extended visible light absorption, narrower band gap and excellent photocatalytic activity. 2. Experimental 2.1. Reagents Citric acid, thiourea, cadmium diacetate dihydrate (Cd(OAc)2 ·2H2 O), PVP and sodium sulfide nonahydrate (Na2 S·9H2 O) were of analytical grade, supplied by Sinopharm chemical reagent Co., Ltd (Shanghai, China) and used without further purification. Deionized (DI) water was applied in all experiments.
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for TEM tests were prepared by dropping the solution onto a copper grid with carbon film and dried in air. Scanning electron microscopy (SEM) and elemental mapping images were acquired with a Tescan MAIA3 scanning electron microscope. The Fourier transform infrared (FT-IR) spectra were acquired with a Nicolet Is10 spectrometer. Raman spectra were recorded on a Raman microspectrometer (spex/403). X-ray photoelectron spectrometry (XPS) analyses were carried out on a PHI 5000C ESCA system. The Brunaure–Emmett–Teller (BET) surface areas of the samples were determined by nitrogen adsorption–desorption isotherm measurement at room temperature (Quadrasorb SI, Quantachrome). Room temperature UV–vis absorption and UV–vis diffusive reflectance spectra were recorded using a Shimadzu UV-3600 spectrophotometer. 2.5. Photoelectrochemical measurements Electrochemical impedance spectroscopy (EIS) and transient photocurrent response under visible light irradiation were measured on an electrochemical workstation (Autolab PG 302N) in a standard three-electrode system. The working electrodes were prepared as follows: 0.5 g of the sample was grounded with 0.1 g polyvinylidene fluoride (PVDF) and 1 mL ethanol, then the mixture was stirred for 3 hours to make a slurry. The slurry was coated onto an indium tin oxide (ITO) glass electrode by the doctor blade technique, and the active area was controlled to be 1.0 cm2 . A Pt wire was employed as the counter electrode, while Ag/AgCl (saturated KCl) was applied as the reference electrode. Na2 SO4 aqueous solution (1 mol/L) was used as the electrolyte. 2.6. Photocatalytic activity measurements
2.2. Preparation of GQDs GQDs were prepared from citric acid through a modified hydrothermal method [30]. Briefly, 1.26 g (6 mmol) citric acid and 1.38 g (18 mmol) thiourea were dissolved in 30 mL DI water, and stirred for an hour to form a clear solution. Then the mixture was transferred into a 100 mL Teflon lined stainless autoclave and kept in an electric oven at 180 ◦ C for 6 hours. After cooling to room temperature, the solvent of suspension was removed with the aid of a rotary evaporator. The final product was collected by adding ethanol to the residue and centrifuged at 10,000 rpm for 5 min. 2.3. Fabrication of GQD-PVP-CdS composite 1.33 g (5 mmol) Cd(OAc)2 ·2H2 O and 1 g PVP were added into 25 mL GQDs aqueous solution and stirred for 2 hours to form a clear solution. 25 mL Na2 S solution (0.2 mol/L) was added drop by drop into the mixture solution under vigorous stirring. Then the mixture was transferred into a 100 mL Teflon lined stainless autoclave and kept at 120 ◦ C for 4 hours. The resulting solids were collected by centrifugation and washed with water for 3 times, and then dried in vacuum at 60 ◦ C overnight. PVP-CdS composite was prepared in the same way by replacing GQD solution with DI water, and pristine CdS particles were fabricated without either PVP or GQDs.
The photocatalytic activity of the samples was evaluated through the degradation of MO and phenol in aqueous solution under visible light ( > 400 nm) from a high-pressure mercury lamp with a UV 400 cut filter. The experiments were carried as follows: 50 mg of photocatalysts were added into 100 mL of MO or phenol solution (10 mg/L). Before irradiation, the solution was stirred in dark for 1 hour to achieve adsorption/desorption equilibrium between the catalysts and organic pollutant. Then the solution was exposed to visible light irradiation while keeping stirring. 3 mL aliquots were taken at time intervals of 15 min and analyzed on a UV–vis spectrophotometer to examine the concentrations of the organic contaminant. Before the spectroscopy measurement, the aliquots were centrifuged at 10,000 rpm for 5 min to remove the photocatalysts. 2.7. Active species trapping experiments To detect the active species during photocatalytic process, several kinds of sacrificial agents were used as scavengers. 2-propanol (IPA), ammonium oxalate (AO) and 1,4-benzoquinone (BQ) were chosen as the hydroxyl radical (• OH) scavenger, hole (h+ ) scavenger and superoxide radical (• O2 − ) scavenger, respectively. The experiments were carried out similarly as the former photocatalytic activity measurement with the addition of different quenchers [31,32].
2.4. Characterization 3. Results and discussion To examine the crystal phase of the products, powder X-ray diffraction (XRD) patterns were recorded with an X-ray diffractometer (Bruker D8 Advance) at a scanning rate of 5◦ /min in the 2 range of 10–90◦ . Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained with a Tecnai G2 F20 S-Twin transmission electron microscope. The samples
3.1. XRD patterns The typical X-ray diffraction (XRD) patterns of the as-prepared CdS, PVP-CdS and GQD-PVP-CdS samples are shown in Fig. 1. It is clear to see that all these diffraction peaks could be indexed to cubic
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3.2. TEM analysis To characterize the morphology of the as-prepared GQDs and GQD-PVP-CdS composite, transmission electron microscopy (TEM) was employed. Fig. 2(a) shows the TEM image of GQDs. It is clearly indicated that the as-prepared GQDs are similar in size with diameters mainly distributed in the range of 2–4 nm. The high-resolution transmission electron microscopy (HRTEM) image displayed in Fig. 2(c) indicates that the GQDs are highly crystalized with an interplanar spacing of 0.194 nm, which corresponds well to the (104) crystal plane of graphitic carbon (JCPDS 26-1076) [34]. Furthermore, the typical TEM image of GQD-PVP-CdS composite is shown in Fig. 2(b), demonstrating that the product mainly consists of uniform particles with sizes ranging from 5 to 10 nm. HRTEM was also applied and the result is exhibited in Fig. 2(d). Interplanar spacings of 0.194, 0.293 and 0.332 nm were clearly observed, which are in agreement with the (104) facet of GQDs and the (200) and (111) facets of cubic CdS, respectively [12,33]. Fig. 1. Typical XRD patterns of CdS, PVP-CdS and GQD-PVP-CdS samples.
3.3. SEM analysis and elemental mapping CdS (JCPDS card no. 65-2887). All three samples showed three main peaks at 2 ≈ 27.8◦ , 43.5◦ and 52.4◦ , which matched well with (111), (220) and (311) crystal planes of cubic CdS, respectively [17,33]. In addition, a careful study of the patterns revealed the asymmetry of the (111) peak, indicating a relatively weak peak at 2 ≈ 30.3◦ , which corresponds to the (200) crystal face [17]. By comparing all three patterns, we found that the addition of PVP and GQDs had no distinct effect on the crystal structure of CdS particles.
Fig. 3(a) shows the SEM image of the as-prepared GQD-PVP-CdS composite, suggesting that CdS nanoparticles tended to aggregate. The elemental mapping images are displayed in Fig. 3(b–f), indicating the presence of Cd, S, C, N and O as major chemical components. The existence of Cd and S suggests the formation of CdS, while C, N and O elements are attributed to PVP and GQDs. These images confirmed that GQDs and PVP were eventually loaded on the surface of CdS particles.
Fig. 2. TEM images of (a) GQDs and (b) GQD-PVP-CdS; HRTEM images of (c) GQDs and (d) GQD-PVP-CdS.
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Fig. 3. (a) SEM image of GQD-PVP-CdS; elemental mapping of (b) Cd, (c) S, (d) C, (e) N and (f) O elements in GQD-PVP-CdS composite.
indicating that there exist many hydroxyl and amino groups on the surface of GQDs. The vibrational absorption band of C O is observed at 1707 cm−1 , while the bands at 1578 and 1401 cm−1 are attributed to the bending vibrations of C C and C N, respectively. The weak peaks at 1110 and 617 cm−1 correspond to the stretching vibrations of C S and C S, respectively, which means the as-prepared products are S,N co-doped GQDs [30]. The spectrum of PVP is close to that in previous reports [35,36]. The bands at 2960, 1703 and 1292 cm−1 could be assigned to the vibrations of C H stretching, C O stretching and CH2 deformation, severally. All of the peaks above can be spotted in the FT-IR spectrum of the final product GQD-PVP-CdS composite, with slight changes in peak positions and intensities. The spectrum of pure CdS shows a major absorption peak at 596 cm−1 , which is assigned to the Cd S stretching frequency [37,38]. However, this peak is not visible in the spectrum of GQD-PVP-CdS, probably due to its relatively low intensity.
Fig. 4. FT-IR spectra of GQDs, PVP and GQD-PVP-CdS samples.
3.4. FT-IR spectra FT-IR spectra were used to identify the hyperfine chemical structure of the products. As displayed in Fig. 4, the spectrum of GQDs shows a broad absorption band at 3000–3500 cm−1 , which is assigned to the stretching vibrations of O H and N H bonds,
3.5. Raman spectra Raman spectroscopic measurements were carried out to further elucidate the structure of the prepared samples. As shown in Fig. 5, pure CdS exhibited two main Raman peaks at 307 cm−1 and 600 cm−1 , which correspond to 1LO modes and 2LO modes of CdS, respectively [39,40]. After the formation of PVP-CdS, additional Raman peaks at 754 cm−1 and 935 cm−1 were found, which are ascribed to PVP [41]. Moreover, after the loading of GQDs, the
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composite showed extra peaks at 1357 cm−1 and 1560 cm−1 , which are attributed to the D band and G band of GQDs, respectively [30,42]. However, compared to pure GQDs, these two peaks both showed a certain shift, which could be caused by the charge transfer between GQDs and CdS [43,44]. 3.6. XPS spectra
Fig. 5. Raman spectra of GQDs, CdS, PVP, PVP-CdS and GQD-PVP-CdS samples.
X-ray photoelectron spectroscopy (XPS) measurements were carried out to further prove the formation of GQD-PVP-CdS composite. Fig. 6 displays several regions of the XPS spectra of the GQD-PVP-CdS sample. The full scan XPS spectrum is shown in Fig. 6(a). The peaks at 285 eV and 533 eV are assigned to carbon (C 1s) and oxygen (O 1s), respectively, which could originate from GQDs and PVP. In addition, the XPS spectrum also confirms the existence of sulfur and cadmium species from the appearance of an S 2s peak at 225 eV, an S 2p peak at 162 eV and a characteristic Cd 3d5/2 peak at 406 eV, a Cd 3d3/2 peak at 413 eV, which suggests the formation of CdS [45–48]. No obvious N 1s peak was discovered, probably due to its relatively low content and being too close to the Cd 3d5/2 peak. Fig. 6(b) shows the high resolution scan of the C 1s region, indicating that carbon exists in three different chemical environments. The deconvoluted peak located at 284.6 eV corresponds to the sp2 C in graphene, namely C C and C C bonds. The
Fig. 6. (a) XPS survey spectra of GQD-PVP-CdS composite; peak deconvolution of (b) C 1s region and (c) S 2p region of GQD-PVP-CdS composite.
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the elemental composition of the region less than 10 nm from the surface, the top CdS layer could block the signal of the S C and S O bonds. Thus, the S C and S O peaks at 164 eV and 168 eV were not observed [2,33]. 3.7. Specific surface area analysis
Fig. 7. Nitrogen adsorption–desorption isotherms of CdS, PVP-CdS and GQD-PVPCdS samples.
sp3 C in C OH bonds contributes to the peak at 285.5 eV, and the peak at 288 eV is attributed to the C O bonds from carbonyls and carboxylates [49]. Besides, the high resolution scan of S 2p region is displayed in Fig. 6(c), the two peaks at 161.2 and 162.2 eV are both ascribed to the S in CdS. Since XPS technique can only detect
Full nitrogen adsorption–desorption isotherms are shown in Fig. 7. All three samples exhibited adsorption– desorption isotherms of type IV according to the Brunauer– Deming–Deming–Teller (BDDT) classification, indicating the presence of mesopores (2–50 nm) within CdS nanocrystals [50]. In addition, the isotherms showed high adsorption at a high relative pressure (P/P0 ) approaching 1.0, and the shape of all three hysteresis loops was of type H3. These results suggest the presence of large macropores formed by the aggregation of primary CdS nanocrystals [51]. The BET specific surface area of pure CdS sample is 36.8 m2 /g, while that of PVP-CdS is 56.8 m2 /g. However, after the addition of GQDs, the specific surface area of the sample decreases to 37.0 m2 /g, which can be attributed to the blockage of partial mesopores by the GQDs on the surface of CdS particles [52]. 3.8. DRS spectra The optical properties of the as-prepared samples were investigated by UV-vis diffuse reflection spectroscopy, and the obtained
Fig. 8. (a) UV–visible DRS spectra of different samples, (b) energy dependence of (ah)2 .
Fig. 9. (a) Transient photocurrent response and (b) EIS Nyquist plots of CdS, PVP-CdS and GQD-PVP-CdS under visible light irradiation.
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Fig. 10. (a) Absorption spectral changes over irradiation time for the MO aqueous solution (10 mg/L) in the presence of GQD-PVP-CdS composite under visible light illumination; (b) Normalized changes in the MO concentration (C/C0 ) and (c) linear plots of −ln(C/C0 ) for the photodegradation of MO under visible light in the presence of different photocatalysts.
DRS results are shown in Fig. 8. From the DRS data, we can see that the addition of PVP and GQDs has influence on the optical properties of CdS particles. Compared to bare CdS and PVP-CdS, GQD-PVPCdS showed higher absorption intensity in the visible light region. This enhancement could be attributed to the black body effect of the additional GQDs, which was beneficial for the composite to absorb visible light, thus improving its photoactivity [3,53]. According to the Kubelka–Munk theory, the optical absorption band gap Eg of a semiconductor can be calculated by the following equation: (ah)n = A(h − Eg ), where a stands for the absorption coefficient, hv means the incident photon energy, n is the index depending on the type of the semiconductor and A represents the proportionality constant relative to the materials.[3,31] In the case of CdS, which is a direct gap semiconductor, n equals 2 [54]. Therefore, as shown in Fig. 8(b), the Eg can be determined by extrapolation of the linear portion of the (ah)2 curve versus the photon energy h to (ah)2 = 0. It can be concluded that both CdS and PVP-CdS have close band gap values of 2.35 and 2.36 eV, respectively, while
GQD-PVP-CdS composite exhibits a slight red-shift, with a band gap value of 2.23 eV. 3.9. Photoelectrochemical measurements One of the important roles of GQDs and PVP is to facilitate the migration and separation of the photogenerated electrons, and we have carried out transient photocurrent response and EIS tests of the prepared samples to investigate their charge transfer capability. Fig. 9(a) shows the transient photocurrent response results of the as-prepared samples. For all three samples, the value of photocurrent density rose rapidly under visible light illumination and decreased to almost zero when the light was off. It can also be seen that GQD-PVP-CdS nanocomposite exhibited the highest photocurrent density, with a value of nearly 0.3 mA cm−2 , while CdS and PVP-CdS generated relatively lower photocurrent density of 0.04 and 0.19 mA cm−2 , respectively. Furthermore, the typical Nyquist plot of the EIS measurements is displayed in Fig. 9(b). It can be
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clearly seen that the arc of the spectrum became smaller when PVP was introduced into CdS nanoparticles. After the addition of GQDs, the arc became even smaller, indicating a smaller electron transfer resistance and a higher charge transfer rate [55,56]. 3.10. Photocatalytic performance It is well known that semiconductor particles are capable of producing electron–hole pairs under illumination with photon energy larger than their band gap. When the particle size is small enough, those electrons and holes are able to transfer to the surface of the crystal, which makes their reaction with H2 O and molecular oxygen possible. When they get in contact with H2 O and O2 , hydroxyl radicals and superoxide radicals are generated, which are highly reactive and oxidative [57,58]. They are able to degrade various kinds of organic pollutants into less harmful products such as CO2 , H2 O, NO3 − , etc. Since most organic pollutants show absorption in ultraviolet–visual (UV–vis) region, it is feasible to detect their concentrations using a UV–vis spectrophotometer. With the goal of investigating the visible light photocatalytic performance of the as-prepared samples toward the degradation of organic pollutants, methyl orange (MO) was selected as a model contaminant. Prior to visible light irradiation, the reaction solution was magnetically stirred in dark for 1 hour to ensure adsorption/desorption equilibrium. Under dark conditions, no significant degradation of the MO solution was observed after 1 hour in the presence of GQD-PVP-CdS composite. Fig. 10(a) shows the evolutions of absorption spectra of MO solution exposed to visible light ( > 400 nm) over time in the presence of the as-prepared GQDPVP-CdS composite. The MO spectrum revealed a major absorption band at 464 nm. It is clear that under visible light irradiation, the absorption peaks dropped gradually, indicating that MO underwent a photocatalytic decomposition process. Fig. 10(b) compares the photocatalytic performance of CdS, PVP-CdS and GQD-PVPCdS samples, and it is obvious that GQD-PVP-CdS exhibits the best photocatalytic properties. According to the Lambert–Beer law, the degradation efficiency, de %, can be calculated by the following equation: de = (C0 − Cf )/C0 * 100% = (A0 − Af )/A0 * 100%, where C0 and Cf represent the initial and final MO concentrations, while A0 and Af represent the initial and final absorbance, respectively [12,59]. After 3 hours of visible light illumination, the degradation efficiencies of MO under the presence of CdS and PVP-CdS are 56.0% and 59.5%, respectively. In contrast, GQD-PVP-CdS composite leads to a remarkable increase with a degradation efficiency of 92.3%. The photocatalytic degradation reaction of MO is usually assumed to follow pseudo-first-order expression: −ln(C0 /C) = kt, where C/C0 represents the normalized MO concentration and k stands for the apparent reaction rate (min−1 ). By plotting −ln(C0 /C) as a function of irradiation time through linear regression, the observed rate constant k, which is an important index to evaluate photoactivity, can be obtained from the slopes of the simulated straight lines [6,60]. Fig. 10(c) displays the relationship between −ln(C0 /C) and the irradiation time (t) for the photocatalytic degradation of MO under visible light. The reaction rate constant for the pristine CdS particles is 4.38 × 10−3 min−1 , and for PVP-capped CdS particles, it slightly increased to 4.71 × 10−3 min−1 . After the addition of GQDs, the photoactivity of the particles exhibited a major improvement, with a much higher rate constant of 1.47 × 10−2 min−1 . Furthermore, we have investigated the photostability of CdS and GQD-PVP-CdS samples by repeated photocatalytic experiments. As shown in Fig. 11, in the first three cycles of photocatalytic tests, CdS showed degradation efficiencies of 56.0%, 41.9% and 32.7%, while GQD-PVP-CdS nanocomposite exhibited 92.3%, 91.7% and 89.6%, respectively. This result shows that GQD-PVP-CdS nanocomposite possesses not only much higher photocatalytic activity but also
Fig. 11. Recycled photodegradation of MO under visible light irradiation of pure CdS and GQD-PVP-CdS composite.
Fig. 12. Normalized changes in the MO concentration (C/C0 ) under visible light in the presence of GQD-PVP-CdS composite with the addition of different scavengers.
better stability than those of CdS, suggesting better protection against photocorrosion [46,61]. This enhancement may be attributed to the synergetic effect between GQDs and CdS, since the photogenerated photons and electron could transfer from CdS to GQDs, thus protecting CdS from photocorrosion and improving its photostability [62]. Another series of tests were carried out to probe the active species during photocatalytic degradation process. As stated in the experimental section, IPA, BQ and AO were selected as the scavengers for • OH, • O2 − and h+ , respectively. The results are shown in Fig. 12. With the addition of IPA as a scavenger for • OH radicals, the degradation rate only slightly decreased, indicating • OH did not play an important part during the photocatalytic process. However, when BQ was added to the system, the degradation of MO was significantly prohibited, and only less than 20% of MO was degraded after 3 hours of visible light illumination. This dramatic decrease in photodegradation efficiency suggests that the • O2 − pathway had a crucial role in the process of MO oxidation. To detect the influence h+ had on the photocatalytic process, we chose AO as h+ scavenger. The results show that after the addition of AO, the degradation efficiency dropped to 45%, indicating h+ did play a part in the removal of MO, though not as important as • O2 − .
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4. Conclusion In summary, a novel GQD-PVP-CdS composite photocatalyst with efficient visible light activity was fabricated by a facile onestep method successfully. Significantly, compared to pristine CdS and PVP-CdS, the as-prepared GQD-PVP-CdS displayed enhanced photocatalytic ability and high photodegradation efficiency toward MO under visible light illumination. This improvement may be caused by the synergetic effect between GQDs and CdS particles, since GQDs possess upconversion properties and GQD-PVP-CdS exhibited a relatively higher dispersion of electric charge in the system. Since ultraviolet light only accounts for no more than 5% of the whole solar energy, it is desirable to develop visible light sensitive photocatalysts. Therefore, we expect that our findings may provide a new strategy to synthesize photocatalysts that are sensitive to visible light and help research in current environmental pollution, energy issues and other related fields. Fig. 13. Normalized changes in the concentration (C/C0 ) for the photodegradation of phenol under visible light in the presence of different photocatalysts.
Fig. 14. Schematic presentation of the photodegradation process of MO.
Moreover, in order to rule out the photosensitization role of dyes during the photocatalytic reaction, we have selected phenol as a colorless contaminant to test the photcatalytic behavior of the asprepared samples. As shown in Fig. 13, after 3 hours of visible light illumination, CdS, PVP-CdS and GQD-PVP-CdS exhibited degradation efficiencies of 26.1%, 47.2% and 59.0% toward the degradation of phenol solution (10 mg/L), respectively. The samples showed less photoactivity toward phenol degradation than those of MO degradation, however, it can be clearly seen that the introduction of GQDs and PVP greatly enhanced the photocatalytic behavior of CdS. This improvement in photoactivity could be attributed to the synergetic effect of GQDs and CdS particles. The proposed photodegradation mechanism is displayed in Fig. 14. Firstly, GQDs possess unique upconversion properties, which can absorb visible light and then emit photon with shorter wavelength [30,63]. The black body effect of GQDs also can help with the absorption of visible light. As a result, GQD-PVP-CdS composite has an expanded light absorption and an enhanced photoactivity in visible light region compared to CdS and PVP-CdS. Secondly, under visible light illumination, the photo-induced electrons are able to transfer between GQDs and CdS particles easily. Through this electron transfer process, GQDs may protect CdS from photocorrosion and enhance the photostability of GQD-PVP-CdS particles [14,62]. Furthermore, GQDs can also play the role as an electron reservoir to trap electrons emitted from CdS particles, thus preventing the photoinduced electron–hole pairs from recombination [27,64,65]. As a result of all these advantages, the as-prepared GQD-PVP-CdS composite shows greater photocatalytic performance.
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