Carbon quantum dot decorated hollow In₂S₃ microspheres with efficient visible-light-driven photocatalytic activities

Abstract

Carbon quantum dot (CQDs) decorated hollow In₂S₃ microspheres were firstly synthesized by a facile hydrothermal method. CQDs with an average size of 5 nm were attached on the surfaces of hollow In₂S₃ microspheres. The photocatalytic activities of the as-prepared samples were investigated by the photocatalytic degradation of methyl orange under visible light, and the 3 wt% CQDs/In₂S₃ sample presented the most efficient photocatalytic activity which was almost 3 times the pure In₂S₃ sample. On the basis of the active species trapping experiment and ESR analysis, holes and superoxide radicals were proved to be the main active species in the photocatalytic degradation process, and a possible reaction mechanism was proposed.

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1. Introduction

In recent years, semiconductor photocatalytic technology has drawn increasing attentions owing to its efficient applications in solving the environmental and energy issues all over the world.^1–3 Currently, the development of visible-light-driven photocatalysts has become a new trend, since the efficient utilization of the visible-light region which occupies the largest proportion of the incoming solar spectrum.^4,5 Meanwhile, metal sulfides have been extensively investigated and proven to be a group of highly efficient catalysts for photochemical reactions.^6,7 Typically, indium sulfide (In₂S₃), an III–VI group sulfide, has attracted special interest in photoelectron chemical and photocatalysis.^8–11 As is known, In₂S₃ (α-In₂S₃, β-In₂S₃, and γ-In₂S₃), especially β-In₂S₃, an n-type semiconductor with relatively narrow band gap of 2.0–2.3 eV and comparatively negative potential (approximate −0.9 eV vs. NHE) of conduction band, is considered as a promising candidate for photocatalytic applications.^12–14

Unfortunately, the pure In₂S₃ possesses poor photocatalytic activity and low quantum efficiency due to the rapid recombination of photogenerated electrons and holes. Up to now, many efforts have made to solve this drawback and various In₂S₃-based photocatalytic materials with enhanced photocatalytic activities have been reported. Juan et al.¹⁵ synthesized Bi₂S₃/In₂S₃ core/shell microspheres with admirable visible-light photocatalytic activities in the degradation of 2,4-dichlorophenol. Xin et al.¹⁶ synthesized In₂S₃/carbon nanofibers/Au composite, and this ternary synergetic system exhibited increased photocatalytic activity in the decomposition of Rhodamine B. Chao et al.¹⁴ constructed In₂S₃/g-C₃N₄ heterojunction which exhibited favorable photocatalytic activity in the degradation of Rhodamine B. However, some defects will emerge in the interfaces of these heterojunctions because of the large-size of materials.¹⁷ Thus, the photogenerated electrons and holes will be apt to recombine on the defects and the photocatalytic activities of these heterojunctions will be decreased. So as to overcome this limitation, carbon quantum dots materials could be used because of uniform distribution and the formation of sufficient construction.¹⁸

Carbon quantum dots (CQDs), a novel class of carbon nanomaterials, are attracting increasing interest in wide range of applications, including chemical sensing, bioimaging, biosensing, photodynamic therapy, and electrochemical.^19–24 Considering the excellent electron–hole pair's separation ability, photochemical stability, and up-conversion fluorescence emission, CQDs have been applied to decorate the photocatalysts to enhance photocatalytic performance, such as TiO₂,^25–27 C₃N₄,^28–30 Ag₃PO₄,³¹ Bi₂MO₆,³² NiFe,³³ and ZnO.³⁴ Nevertheless, the functions of CQDs in the composites are still controversial, and the mechanism of improvement photoactivity need to be further studied. To the best of our knowledge, there is no research on the CQDs decorated hollow In₂S₃ microspheres or their application in degradation of organic pollutants.

In this work, firstly, the CQDs decorated hollow In₂S₃ microspheres were synthesized by a facile one pot hydrothermal method. Then, multiple techniques were used to explore the micro-, nano-structures and chemical composition of the as-prepared materials. Clearly, CQDs with the average size of 5 nm were attached on the surfaces of hollow In₂S₃ microspheres. The photocatalytic activities of as-prepared samples were explored under visible light irradiation, and methyl orange (MO) was chosen to be the target pollutant. Consequently, CQDs/In₂S₃ samples exhibited significantly enhancement of photocatalytic activity for MO degradation compared to pure In₂S₃. Moreover, the 3 wt% CQDs/In₂S₃ sample displayed the highest photocatalytic degradation efficiency, which was about 3 times as much as that of pure In₂S₃. Furthermore, the possible photocatalytic mechanism for the enhanced photocatalytic activity was also investigated based on the active species trapping experiment and electron spin resonance (ESR) analysis.

2. Experimental

2.1. Materials

Citric acid, ethylenediamine, indium trichloride, L-cysteine, triethanolamine (TEOA), 1,4-benzoquinone (BQ), isopropanol (IPA), 5,5-dimethy l-1-pyrroline N-oxide (DMPO), and methyl orange (MO) were analytical grade agents purchased from Aladdin (China) and used as received without further purification.

2.2. Fabrication of photocatalysts

CQDs were prepared by a hydrothermal method according to the previous report.³⁵ In a typical fabrication, 1.0507 g citric acid and 335 μL ethylenediamine were dilute in 10 mL distilled water under low-speed stirring. Then the above solution was transport to a 30 mL Teflon-lined autoclave, and heated at 180 °C for 5 h. After that, the reactors were cooled to room temperature naturally. In order to obtain CQDs, the product was further treated using a semipermeable membrane and dried by freeze drying.

CQDs decorated hollow In₂S₃ microspheres were synthesized by a facial one pot hydrothermal method. In detail, 1 mmol indium trichloride was dissolved to 25 mL distilled water which containing a certain amount of CQDs. Then, 1.5 mmol L-cysteine was added, and the solution was stirring for 5 min. Afterward, the above solution was transport to a 30 mL Teflon-lined autoclave, and heated at 150 °C for 24 h. A dark red product was collected after centrifugation, and the product was washed with deionized water and ethanol three times. Finally, the product was dried in a vacuum drier at 60 °C for 24 h. The hollow In₂S₃ microspheres and CQDs decorated hollow In₂S₃ microspheres containing different mass ratios (1%, 3%, and 5%) were fabricated following the similar procedure by changing the content of CQDs. To make clarity, the CQDs decorated hollow In₂S₃ microspheres with expected CQDs contents of 0, 1, 3, 5 wt% are referred to as In₂S₃, 1 wt% CQDs/In₂S₃, 3 wt% CQDs/In₂S₃, and 5 wt% CQDs/In₂S₃, respectively.

2.3. Characterization

X-ray diffraction (XRD) patterns were obtained on a D/MAX-2500 diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 1.54178 Å), the scanning rate was 7.0° min⁻¹ and the scanning range (2θ) was 5.0–80°. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) patterns of as-prepared samples were measured by an S-4800 field emission scanning electron microscope (FESEM, Hitachi, Japan). The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were also used to characterize the morphologies of as-prepared samples, the TEM images were acquired on a transmission electron microscopy (Tecnai G2 F30 S-Twin, FEI) operated under the accelerating voltage of 200 kV, and HRTEM images were acquired on a transmission electron microscopy (JEM-2100F, Japan). X-ray photoelectron spectroscopy (XPS) was determined by a Thermo ESCALAB 250X (America) electron spectrometer with a 150 W Al Kα X-ray sources. Fourier transform infrared (FT-IR) spectra of the samples were acquired in the 400–4000 cm⁻¹ range on a Bruker Vertex 70 spectrometer using KBr as the dispersion medium. The UV-vis diffused reflectance spectra (DRS) of the samples were determined on an UV-2450 spectrophotometer (Shimadzu, Japan) using BaSO₄ as the reflectance standard. The photoluminescence (PL) spectra of samples were measured on a Quanta Master™ 40 fluorescence spectrofluorometer (Photon Technology International Inc.) at room temperature. Photocurrent and electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI 660B (Chenhua, China) electrochemical workstation with a standard three-electrode cell at room temperature. The electron spin resonance (ESR) analysis was conducted with a Bruker EPRA 300-10/12 spectrometer under visible light.

2.4. Photocatalytic evaluation

Photocatalytic activities of In₂S₃ and CQDs/In₂S₃ samples were investigated by the degradation of MO under visible light irradiation. A 250 W xenon lamp with a 420 nm cutoff filter was used as the source of visible light in the photochemical reactor. Typically, 50 mg as-prepared photocatalyst were added into 50 mL 10 mg L⁻¹ pollutants aqueous solution. In order to achieve the adsorption–desorption equilibrium between photocatalysts and MO molecules, the suspension was kept stirring in the dark for 30 min. The content of pollutant in reactors was sampled (about 5 mL) every 20 minutes, and the samples were treated by centrifugation (10 000 rpm, 10 min) to remove the photocatalysts. Finally, the concentration of residual dye in the sample was determined at the wavelength of 464 nm over a TU-1810 UV-vis spectrophotometer.

2.5. Active species trapping and ESR experiment

In order to determine the key active species which are responsible for the photocatalytic reaction, the active species trapping experiment was carried out. In detail, TEOA (0.5 mmol),^36,37 BQ (0.5 mmol),^38,39 and IPA (0.5 mmol)^40,41 were employed as the scavengers for h⁺, ˙O₂⁻ and ˙OH in the similar photocatalytic evaluation with 3 wt% CQDs/In₂S₃, respectively. The ESR technique was also used to confirm the result of active species trapping experiment. As we all know, ˙OH and ˙O₂⁻ radicals can be easily trapped by the DMPO and the intermediate of DMPO-˙OH and DMPO-˙O₂⁻ adducts can be detected by ESR analysis. Before the test, 10.0 mg samples were dissolved in 1 mL deionized water or 1 mL methanol, and then 30 μL DMPO was added, respectively. The ESR analysis was carried out under visible light, which was similar to the photocatalytic evaluation.

3. Results and discussion

The morphologies and microstructures of pure In₂S₃ and 3 wt% CQDs/In₂S₃ samples are characterized by SEM and TEM techniques. The SEM image acquired at low-magnification, Fig. 1a, shows that the pure In₂S₃ sample possesses spherical-like morphology with the diameter from 2 to 5 μm. The high-magnification SEM images of pure In₂S₃, Fig. 1b and c, reveal that the In₂S₃ microspheres are built from small nanosheets, and there are a number of porous randomly distributed on its surfaces. These porous could play a significant role in improving surface area, shorting charge-carrier transport paths, and enhancing light harvesting. Fig. 1d–f show the low and high magnification SEM images of 3 wt% CQDs/In₂S₃ sample, respectively. No significant change appears compared to pure In₂S₃. That is to say, the introduction of CQDs does not affect the morphology of In₂S₃.

Fig. 1 SEM images of the pure In₂S₃ sample (a–c) and 3 wt% CQDs/In₂S₃ sample (d–f).

TEM image of pure In₂S₃ is shown in Fig. 2a, some nanosheets can be found from the edge of spherical-like structure. Meanwhile, the obvious contrast between the relatively bright center and the dark edge confirms their hollow structure with the diameter of approximate 420 nm. The lattice spacing shown in Fig. 2c is 0.325 nm, which corresponding well to the (109) crystal plane of In₂S₃. As shown in Fig. 2b, 3 wt% CQDs/In₂S₃ sample has the similar spherical-like structure with hollow nature. Fig. 2d shows the HRTEM image of 3 wt% CQDs/In₂S₃ sample, the lattice spacing of CQD is determined to be 0.212 nm, which is corresponding to the (100) crystal plane for graphitic carbon.^42–44 At the same time, the lattice spacing around 0.325 nm can be also found, which agree well with the crystallographic (109) spacing of In₂S₃. The above results further demonstrate that the CQDs and In₂S₃ have been coupled together successfully.

Fig. 2 TEM and HRTEM images of the pure In₂S₃ sample (a, c) and 3 wt% CQDs/In₂S₃ sample (b, d).

To further certify the successful fabrication of In₂S₃ and CQDs decoration hollow In₂S₃ microspheres, the as-prepared samples are examined by powder XRD characterizations. As shown in Fig. 3a, the diffraction peaks of the pure In₂S₃ sample appears at 27.40°, 33.45°, 43.80° and 47.90°, which are attributed to the (109), (0012), (309), and (2212) diffraction planes of β-In₂S₃ phase (JCPDS card no. 73-1366), respectively. Moreover, the characteristic peak of carbon at 26° is too weak to be observed, perhaps due to the small amount of carbon and its relatively low diffraction intensity in the hybrid materials.^26,33,45 The EDX spectra of In₂S₃ and 3 wt% CQDs/In₂S₃ samples were shown in Fig. S1,† the presence of C, S, and In elements in Fig. S1b† is a direct evidence for successful synthesis of the hybrid material.

Fig. 3 XRD patterns of as-prepared samples.

The surface valence state and the chemical composition of as-prepared samples are investigated by XPS. The survey scan XPS spectrum, Fig. 4a, shows that the 3 wt% CQDs/In₂S₃ sample contain indium, sulfur, oxygen and carbon elements. Note that the O 1s signal may be from the absorbed CO₂ and H₂O on the surface of the samples. In Fig. 4b, In 3d peaks of pure In₂S₃ sample are centered at approximate 444.8 eV and 452.3 eV, which attribute to the binding energies of In 3d5/2 and In 3d3/2. Moreover the phenomena of spin orbit separation between In 3d5/2 and In 3d3/2 (7.5 eV) suggests the existence of In³⁺ in the In₂S₃ sample.¹⁵ The In 3d peaks of 3 wt% CQDs/In₂S₃ sample appear a slight shift compared to that of pure In₂S₃, which indicates that the surface In chemical environment in the 3 wt% CQDs/In₂S₃ sample has changed and there exist interactions between CQDs and In₂S₃. The high-resolution S 2p peaks of 3 wt% CQDs/In₂S₃ sample, shown in Fig. 4c, are located at 161.7 eV and 162.9 eV, ascribing at the binding energy of S 2p3/2 and S 2p1/2.⁴⁶ The high-resolution XPS spectra of C 1s analysis of In₂S₃ and 3 wt% CQDs/In₂S₃ sample were shown in Fig. S2† and 4d. Fig. 4d revealed three different types of carbon atoms: graphitic or aliphatic (C=C or C–C, 284.75 eV), oxygenated (C=O, 286.15 eV), and nitrous (C–NH–C, 287.7 eV).¹⁹ The result of XPS analysis reveals the coexistence of In₂S₃ and CQDs in the 3 wt% CQDs/In₂S₃ sample.

Fig. 4 XPS spectra of In₂S₃ and 3 wt% CQDs/In₂S₃ sample: survey of the samples (a); In 3d (b); S 2p (c); C 1s (d).

FT-IR spectra are used to further ensure the presence of CQDs in CQDs/In₂S₃ samples. In Fig. 5, the absorb peaks at 1373 cm⁻¹ and 1615 cm⁻¹ are attributed to In–S vibration mode. The absorb peak at 1049 cm⁻¹ is ascribed to epoxy group.¹⁹ The stretching vibrations of C–OH at 3430 cm⁻¹ and C–H at 2923 cm⁻¹ can be also observed in CQDs and CQDs/In₂S₃ samples.⁴⁷ The result of FT-IR analysis indicates that CQDs and In₂S₃ have been coupled together successfully.

Fig. 5 FT-IR of as-prepared materials with different contents of CQDs.

In order to understand the role which CQD plays in the adsorption–desorption equilibrium, the photocatalytic reaction system was placed in the dark continuous stirring for 50 min, and the content of pollutant in reactors was sampled every 5 minutes. As shown in the Fig. S3,† CQDs play an important role in aiding dye bandaging to the photocatalyst. Moreover, the adsorption–desorption equilibrium between photocatalyst and MO molecules will be almost achieved after 30 min. The photocatalytic performances of the pure In₂S₃ and CQDs/In₂S₃ samples with different contents of CQDs are explored by the degradation of MO under visible light irradiation. As shown in Fig. 6a, the blank test without catalyst reveals that the photolysis of MO molecules is so slowly that can be ignored. Meanwhile, the CQDs/In₂S₃ samples express a better photocatalytic behavior than that of the pure In₂S₃, and the 3 wt% CQDs/In₂S₃ sample expresses the most outstanding photocatalytic behavior which is almost 3 times as much as that of the pure In₂S₃. Fig. 6b reveals the UV-vis spectral absorbance changes of MO solution during the photocatalytic degradation experimentation over 3 wt% CQDs/In₂S₃ sample. Obviously, the main absorption peak of MO molecules, decreasing rapidly over the degradation time, are located at 464 nm. In Fig. 6c, the zero-order reaction kinetic model is applied to fit the relationship between the measured dye concentration (C_t) and irradiation time (t):

C_t = −kt + C₀ (1)

where C₀ and k indicate the initial dye concentration and the rate constant.⁴⁸ Clearly, the degradation of MO is very consistent with the zero order kinetic model, and the k values are shown in Fig. 6d.

Fig. 6 (a) The photocatalytic activities of as-prepared samples for MO degradation under visible light; (b) the UV-vis spectral absorption changes of MO solution photodegraded over 3 wt% CQDs/In₂S₃ sample under visible light irradiation; (c) the zero-order reaction kinetics for MO degradation; (d) the apparent rate constants for MO degradation.

It is expected that the introduction of CQDs would have a large effect on the optical properties of In₂S₃. Thus the optical properties of as-prepared samples were explored by UV-vis absorption and up-converted PL spectroscopy. As shown in Fig. 7a, the CQDs/In₂S₃ samples presents outstanding optical absorption and a clear red shift of the optical absorption can be observed with the increasing of CQDs. Furthermore, much broader and stronger absorption tail of CQDs/In₂S₃ samples are also clearly observed, and the absorbance is gradually increasing with the increased dosage of CQDs. Moreover, the band gap energy (E_g) of In₂S₃ sample, shown in Fig. 7b, is calculated by the following equation:

(αhν) = A(hν − E_g)^0.5 (2)

where α, h, ν, A and E_g indicate absorption coefficient, Planck constant, light frequency, proportionality and band gap energy, respectively.^14,49 As shown in Fig. 7c, the band gap of the pure In₂S₃ is approximate 2.0 eV, which is similar to previous study.¹² Up-converted photoluminescence (PL) spectra of the as-prepared CQDs was studied using different excitation wavelengths. As shown in Fig. 7c, the emission wavelengths of pure CQDs are located at the range of 400–650 nm when excited by long wavelength light at 600, 650, 700, 750, 800, 850, 900, 950 and 1000 nm. That is to say, CQDs can absorb light with longer wavelength (>620 nm) and then emit visible light (400–650 nm) which in turn excites In₂S₃ to form more electron/hole pairs. To further confirm the up-conversion effect in CQDs decorated In₂S₃ microspheres, the photocatalytic behavior of In₂S₃ and 3 wt% CQDs/In₂S₃ samples were explored for the degradation of MO dye under infrared light irradiation. As shown in Fig. S4,† we can see that CQDs/In₂S₃ still exhibit photocatalytic activities under infrared light, while In₂S₃ do not show any obvious activities.

Fig. 7 UV-vis absorption spectra of as-prepared samples (a); plots of the (αhν)² versus (hν) for In₂S₃ sample (b); up-converted PL spectra of CQDs with different excitation wavelengths (c).

Photocurrent and electrochemical impedance spectroscopy (EIS) were used to estimate the photogenerated charge carriers separation and transfer ability of the as-prepared samples. Photocurrent tests of the pure In₂S₃ and 3 wt% CQDs/In₂S₃ samples are carried out in an on-and-off cycle mode under visible light, and the result is shown in Fig. 8a. Obviously, the 3 wt% CQDs/In₂S₃ electrode exhibits a higher photocurrent than that of the pure In₂S₃ sample. The significant improvement of photocurrent response reveals that the photogenerated electrons and holes separate efficiently, which can prolong the lifetime of electrons and holes, and consequently improve the photocatalytic activity. The EIS Nyquist plots of the working electrodes were acquired in the dark or under illumination condition, and the result was shown in Fig. 8b. The semicircle part at the high frequency region corresponds to the charge-transfer process of electrolyte/electrode interface and a smaller radius implies more efficient charge transfer process.^50,51 Clearly, when measured in the dark, the radius of the Nyquist semicircle in the high frequency region of the 3 wt% CQDs/In₂S₃ electrode is smaller than that of the pure In₂S₃ electrode, which implies that the 3 wt% CQDs/In₂S₃ material has lower electron transfer resistance. Moreover, compared with the plot obtained in the dark, a much smaller radius was acquired under irradiation for both electrodes, illustrating the increased electron conductivity of the electrode under light.²⁸ The result of EIS demonstrating that the introduction of CQDs can efficiently reduce the electron transfer resistance of In₂S₃, and then improve the efficiency of electron hole separation.

Fig. 8 Photocurrent response (a) and Nyquist plots (b) of pure In₂S₃ and 3 wt% CQDs/In₂S₃ samples.

In order to reveal the photocatalytic mechanism, firstly, the active species trapping experiment is carried out over the 3 wt% CQDs/In₂S₃ sample. In detail, IPA, TEOA, and BQ are used as the scavengers of ˙OH, h⁺ and ˙O₂⁻ radicals, respectively. As shown in Fig. 9, when IPA is added, the photocatalytic degradation rate (80%) is almost unchanged compared with that of no scavenger. However, when BQ is added, the photocatalytic degradation rate (3%) significantly decreases, which indicate that the ˙O₂⁻ is the predominant active species. Meanwhile, after the addition of TEOA, the photocatalytic degradation rate is also inhibited seriously (7%), which implies that the h⁺ is also playing an important role in photocatalytic degradation. Therefore, ˙O₂⁻ and h⁺ radicals are the major active species in the photocatalytic degradation.

Fig. 9 Effects of a series of scavengers on the photocatalytic efficiency of 3 wt% CQDs/In₂S₃ sample.

The ESR spin-trap tests with DMPO is performed over 3 wt% CQDs/In₂S₃ sample to further confirm the result of the active species trapping experiment. As shown in Fig. 10a, no ESR signals can be detected when the reaction is carried out in the dark. However, the characteristic peaks of the DMPO-˙O₂⁻ adducts can be observed under visible light irradiation. The intensity of the ESR signal is related to the concentration of reactive oxygen species generated in each catalyst suspension. Thus, the weak characteristic peaks of DMPO-˙OH adducts, shown in Fig. 10b, may be attributed to further reduction of ˙O₂⁻.³⁶

Fig. 10 ESR spectra of (a) DMPO-˙O²⁻ adducts in methanol solution and (b) DMPO-˙OH adducts in aqueous solution recorded with 3 wt% CQDs/In₂S₃ sample under visible light irradiation.

In order to fully understand the photocatalytic mechanism, the energy band positions of conduction band (CB) and valence band (VB) of In₂S₃ microspheres are calculated with the following equation:

E_CB = X − E^e − 0.5E_g (3)

E_VB = E_CB + E_g (4)

where E_CB and E_VB are the CB and VB edge potential of the pure In₂S₃ sample, X is the absolute electronegativity of the pure In₂S₃ sample (4.7 eV),¹⁴ E^e is the energy of free electrons on the hydrogen scale (4.5 eV), and E_g is the bandgap energy of the semiconductor which is determined by UV-vis absorption spectra (2.0 eV). Thus, the E_CB of In₂S₃ sample has been calculated to −0.8 eV, which is more negative than the potential of O₂/˙O₂⁻ (−0.33 eV vs. NHE).⁵² That is to say, the photogenerated electrons in the CB of 3 wt% CQDs/In₂S₃ sample is enough to reduce O₂ to ˙O₂⁻. However, the E_VB of the pure In₂S₃ sample, estimated to 1.2 eV, is lower than the potential of ˙OH, H⁺/H₂O (2.72 eV vs. NHE).⁵³ Thus, the E_VB of 3 wt% CQDs/In₂S₃ sample cannot yield ˙OH. This is consistent with the results of active species trapping experiment and ESR analysis.

On the basic of the above experimental results, the possible mechanism of photocatalytic degradation is put forward. As shown in Scheme 1, the CQDs play an important role in the improvement of the photocatalytic activity. The up-convention PL property of CQDs endows the CQDs decorated hollow In₂S₃ microspheres with a broadband spectrum photocatalytic activity. When the CQDs decorated hollow In₂S₃ microspheres are irradiated by shorter wavelength light (400–620 nm), the electrons on VB of In₂S₃ are excited to the CB of In₂S₃, leaving h⁺ on VB of In₂S₃. The h⁺ can directly oxidize MO,⁵⁴ and the photoelectrons on the CB of In₂S₃ are apt to transform to CQDs due to the excellent electrical conductivity and electron reservoir property of CQDs.³¹ This is propitious to the separation of e⁻ and h⁺ in the process of photocatalytic degradation.¹⁷ When the CQDs decorated hollow In₂S₃ microspheres are irradiated by NIR light (620–1000 nm), CQDs can absorb these light, and then emit shorter wavelength light (400 to 650 nm), which in turn excites In₂S₃ to form more electron/hole pairs. The electrons accumulated on CQDs can convert O₂ molecular to ˙O₂⁻ radicals, and the superoxide radical can break down the MO molecules into small molecules (e.g. CO₂ and H₂O).⁵⁵ In addition, excessive loading of black-colour CQDs (>5 wt%) will shield the active sites on the surface of catalyst and decrease the intensity of light through the depth of the reaction solution.⁵⁶ Thus, it is also crucial to control the ratios of CQDs in the CQDs decorated hollow In₂S₃ microspheres to gain the best photocatalytic activity.

Scheme 1 Schematic model for the important roles of CQDs for the high photocatalytic activity in the CQDs/In₂S₃ sample.

The stability of photocatalyst is extremely important for its assessment and application. Thus, the circulating experiment over 3 wt% CQDs/In₂S₃ sample is carried out to research the photostability of the as-prepared photocatalysts. After each run, the photocatalyst is collected, washed (by distilled water and absolute ethanol for three times), and dried (in vacuum at 80 °C for 12 h). As illustrated in Fig. 11, no obvious change can be found after five runs of photocatalytic degradation, which indicates that the as-prepared CQDs/In₂S₃ samples possessing excellent photostability. The outstanding photostability of the CQDs decorated hollow In₂S₃ microspheres may be attributed to the insoluble CQDs layer on the surface of In₂S₃, which can effectively protect In₂S₃ from dissolution in aqueous solution,³¹ thus the structural stability of CQDs/In₂S₃ samples can be greatly enhanced during the photocatalytic process.

Fig. 11 The repeated photocatalytic experiments of 3 wt% CQDs/In₂S₃ sample for degradation of MO under visible light irradiation.

4. Conclusion

In summary, CQDs decorated hollow In₂S₃ microspheres were firstly synthesized by a facile hydrothermal process. Obviously, CQDs decorated hollow In₂S₃ microspheres presented enhanced photocatalytic activity and stability compared with pure In₂S₃ sample. The significant improvement in photocatalytic performance was attributed to the up-conversion luminescence and photoinduced electron transfer properties of CQDs. Moreover, h⁺ and ˙O₂⁻ radicals were the main active species in the photocatalytic process, and the probable photocatalytic degradation mechanism was also proposed.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (21276116, 21477050, 21301076 and 21303074), Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011), and Chinese-German Cooperation Research Project (GZ1091). Program for High-Level Innovative and Entrepreneurial Talents in Jiangsu Province, Program for New Century Excellent Talents in University (NCET-13-0835), Henry Fok Education Foundation (141068) and Six Talents Peak Project in Jiangsu Province (XCL-025).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01348a

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