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Article

Carbon Quantum Dots Bridged TiO2/CdIn2S4 toward Photocatalytic Upgrading of Polycyclic Aromatic Hydrocarbons to Benzaldehyde

by
Jiangwei Zhang
1,2,†,
Fei Yu
1,†,
Xi Ke
3,
He Yu
1,
Peiyuan Guo
1,
Lei Du
2,
Menglong Zhang
1,* and
Dongxiang Luo
2,*
1
Institute of Hydrogen Eergy for Carbon Peaking and Carbon Neutralization, School of Semiconductor Science and Technology, South China Normal University, Foshan 528225, China
2
School of Chemistry and Chemical Engineering, Huangpu Hydrogen Innovation Center, Guangzhou University, Guangzhou 510006, China
3
Great Bay Area Branch of Aerospace Information Research Institute, Chinese Academy of Sciences, Guangzhou 510530, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2022, 27(21), 7292; https://doi.org/10.3390/molecules27217292
Submission received: 29 September 2022 / Revised: 22 October 2022 / Accepted: 23 October 2022 / Published: 27 October 2022
(This article belongs to the Special Issue New Trends in Photochemistry in Eurasia)
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Abstract

:
Conversion of hazardous compounds to value-added chemicals using clean energy possesses massive industrial interest. This applies especially to the hazardous compounds that are frequently released in daily life. In this work, a S-scheme photocatalyst is optimized by rational loading of carbon quantum dots (CQDs) during the synthetic process. As a bridge, the presence of CQDs between TiO2 and CdIn2S4 improves the electron extraction from TiO2 and supports the charge transport in S-scheme. Thanks to this, the TiO2/CQDs/CdIn2S4 presents outstanding photoactivity in converting the polycyclic aromatic hydrocarbons (PAHs) released by cigarette to value-added benzaldehyde. The optimized photocatalyst performs 87.79% conversion rate and 72.76% selectivity in 1 h reaction under a simulated solar source, as confirmed by FT-IR and GC-MS. A combination of experiments and theoretical calculations are conducted to demonstrate the role of CQDs in TiO2/CQDs/CdIn2S4 toward photocatalysis.

1. Introduction

In fossil industrial societies, environmental pollution is closely related to the rapid progress of modern industry [1,2,3]. Among the kinds of pollutants, more and more public attention has been paid to the ecological environment change caused by organic pollutants [4,5]. In domestic sewage, organic pollutants usually include benzene compounds (Polychlorinated biphenyls) [6], formaldehyde [7], tobacco tar (PAHs) [8]. When tobacco and cigarette paper are insufficiently burned, the tobacco tars with polycyclic aromatic hydrocarbons (PAHs) as major pollutants may be formed [9]. PAHs are aromatic hydrocarbons with two or more fused aromatic rings, which can be combined in two forms, the non-viscous ring type and the viscous ring type [10]. It is well known that PAHs in the environment will be particularly harmful to human health [11], causing a variety of hazards to the respiratory system and circulatory system. Currently, the general methods of dealing with PAHs in pollutants include biological degradation [12], adsorption process [10] and photocatalytic degradation [13]. Although biodegradation is environmentally friendly, it can be restricted by various factors such as temperature, pH and microbial adaptations, and adsorption treatment prefers transferring organic pollutants to the adsorbent rather than reducing the concentration of it [14]. In contrast, semiconductor photocatalysis is considered as an appealing method for breaking down toxic compounds into non-toxic carbon dioxide and water under mild conditions [15]. Semiconductor-based photocatalytic reactions are environment friendly, having a high surface area, low cost and high modification, etc. [16,17]. It is worth noting that there is still little research on tobacco tar via photocatalytic degradation.
In recently years, abundant types of photocatalysts have been playing an important role, including ZnO [18], SrTiO3 [19], SnO2 [20], Bi2S3 [21], and so on. Unfortunately, the narrow absorption spectra and unsatisfactory photocatalytic performance limit their application. Thus, exploring highly active at the range of visible-light-driven photocatalysts is still a constant pursuit of researchers.
Titanium oxide (TiO2) is a classical semiconductor with favorable thermal and chemical stability [22,23]. However, limited by the wide band gap (3.2 eV) [24,25] and rapid photo-generated electron-hole recombination, the solar energy utilization of TiO2 is typically inefficient, resulting in a low photocatalytic efficiency [26,27]. Therefore, researchers have developed a variety of strategies for enhancing the photocatalytic performance of TiO2, such as precious metal deposition, pigment sensitization and metal ion doping, etc. [28]. Comparing with the above strategies, the construction of heterojunctions has been demonstrated to be an effective way for tuning the absorption and manipulating the charge separation [29]. For instance, a TiO2/CdIn2S4 hierarchical nano hetero-structure photocatalyst demonstrated an ~5.5 times higher in hydrogen production from water compared to bare TiO2 under visible light [30].
CdIn2S4, as a ternary metal chalcogenide compound, is equipped with an appropriate band gap (2.03–2.26 eV) and photoredox potential, promoting the light capture efficiency at visible light range [31,32]. Considering these advantages, CdIn2S4 has been reported to contribute to photocatalytic degradation of pollutants [33], water splitting [31], and carbon dioxide (CO2) photoreduction [34]. However, due to the photoinduced hole self-oxidation of bare CdIn2S4 and the rapid recombination of photoexcited charges, the practical application suffers a huge impediment [35]. In recent years, combining semiconductor materials with CdIn2S4 has been one of the approaches to solving the transfer direction of photogenerated carriers [36]. Moreover, the charge transfer rate of the composite photocatalyst system is closely related to the electrical conductivity between the material interfaces, which directly affects its catalytic activity [37]. Therefore, the introduction of materials with good electrical conductivity can facilitate the charge transfer of photoexcited semiconductors and co-catalysts.
Carbon quantum dots (CQDs), as a novel carbon nano material, are universally employed in various fields because of their excellent properties [38]. In the field of photocatalysis, CQDs have been used to modify binary semiconductor nano materials for its efficient electronic transmission capability [39]. For instance, Liang et al. showed a novel Z-scheme g-C3N4/CQDs/CdIn2S4 heterojunction showing an improvement of ~1.5 times higher photocatalytic degradation than g-C3N4/CdIn2S4 composite [40]. Pei et al. synthesized the ternary NiS/CQDs/ZnIn2S4 nanocomposite by a simple hydrothermal method. Compared with NiS/ZnIn2S4 heterojunction, the hydrogen production efficiency was more than 1.75 times [37]. Looking at these facts, it can be anticipated that CQDs play a crucial role in the charge transfer between TiO2 and CdIn2S4.
In this study, an active photocatalyst was developed by hydrothermal method that involves TiO2/CdIn2S4 heterojunction being modified with CQDs as bridge to improve the electron transport efficiency. Meanwhile, we investigated the effect of different ratios of CQDs on the photocatalytic performance of the TiO2/CQDs/CdIn2S4 heterojunction to obtain the optimum fabrication parameters. The composition, structure, morphology and optical properties of the prepared heterojunction were analyzed thoroughly. PAHs, a typical hazardous organic chemical in products of incomplete combustion tobacco, was chosen as the typical pollutant in tobacco tar. The photocatalytic degradation mechanism of PAHs by prepared heterojunction, the electron transfer and decomposition products were analyzed in detail.

2. Experimental

2.1. Chemicals

Thioacetamide (C2H5NS, Macklin®99% purity), indium nitrate hydrate, (InH3O9·xH2O, Aladdin®99% purity), cadmium nitrate tetrahydrate (CdN2O6·4H2O, Aladdin®99% purity), citric acid (C6H8O7, Macklin®98% purity), urea (CH4N2O, Aladdin®99.5% purity), acenaphthylene (ANY) (C12H8, Aladdin®97% purity), titanium oxide, rutile (60 nm) (TiO2, Macklin®99.8% purity) were used in this study.

2.2. Synthesis of CQDs Colloidal Solution

In a typical synthesis, 4 g of citric acid (AC) and 4 g of urea were added to 80 mL of distilled water and the prepared solution was stirred for 15 min at 600 rpm. Then, the mixture solution was transferred into a Teflon-lined cylinder in a stainless-steel autoclave for hydrothermal synthesis of CQDs at 120 ℃ for 12 h. The brownish-green final CQD colloidal solution was collected after cooling at room temperature naturally [41].

2.3. Preparation of the TiO2/CdIn2S4 Photocatalyst

The hydrothermal method was used to synthesize the TiO2/CdIn2S4 photocatalysts. The 80 mg TiO2 (60 nm), 0.308 g cadmium nitrate (Cd(NO3)2·4H2O), 0.600 g indium nitrate (In(NO3)3·3H2O) and 0.302 g thioacetamide (C2H5NS) were dissolved in 70 mL of distilled water. Then, the solution was taken to be stirred for about 4 h with the Teflon magneton. After that, the mixed solution was sent to a 100 mL Teflon-lined autoclave with hydrothermal method at 120 °C for 12 h. After cooling to room temperature, the as-prepared photocatalyst was washed with distilled water and ethanol three times and dried at 70 °C for 4 h.

2.4. Preparation of the TiO2/CQDs/CdIn2S4 Photocatalysts

The TiO2/CQDs/CdIn2S4 photocatalysts were prepared by the hydrothermal method. In detail, 0.154 g cadmium nitrate (Cd(NO3)2·4H2O), 0.300 g indium nitrate (In(NO3)3·3H2O) and 0.151 g thioacetamide (C2H5NS) were dissolved in 30 mL of distilled water (as solution A). Meanwhile, the CQDs dispersion divided into different gradients from 250uL to 2500 uL was dripped in 40 mL distilled water before 40 mg TiO2 was added and stirred for 1 h (as solution B). Then, solution A was slowly added into solution B to obtain solution C. After being stirred for 10 h, solution C was subjected to a 100 mL Teflon-lined autoclave and maintained at 120 °C for 12 h. The obtained final product was washed with distilled water and ethanol three times and dried at 70 °C for 4 h [30]. The scheme for the synthesis is presented in Scheme 1.

2.5. Characterization

The crystalline phase and crystalline size of the samples were analyzed by using an X-ray powder diffraction (XRD) technique (Rigaku Corporation, Japan) with Cu Kα1 radiation (λ = 1.541866 Å), scanning angle from 10° to 80° (2θ). The morphology and structure of the TiO2/CQDs/CdIn2S4/photocatalysts were measured by transmission electron microscope (TEM, Japan-JEOL-JEM 2100F) and scanning election microscopy (SEM, American-FEI-Quanta FEG 250). Transmission electron microscope was employed for the elemental mapping (Japan-JEOL-JEM 2100 F). X-ray photoelectron spectroscopy (XPS) was measured by Thermo SCIENTIFIC ESCALAB 250Xi (ThermoFisher Nexsa, America, X-ray source-Al Kα ray (hν = 1486.8 eV) the work function: 4.97 eV). The FT-IR spectra in the range from 400 to 4000 cm−1 was analyzed by a thermogravimetric spectrum instrument (America-Agilent-7890). Electron paramagnetic resonance (EPR) measurements was obtained using a CW-EPR Bruker spectrometer (Germany Bruker-A200). Spectrum measurement system was applied to collect. The UV-vis absorption spectroscopy of the samples (DH-2000-BAL) and the steady-state photoluminescence (PL) spectra of the photocatalysts were recorded by fluorescence spectrometer (QE65000-FL). The gas chromatography-mass spectrometer (GC-MS) was used to analyze the photocatalytic degradation of PAHs. In this experiment, the temperature is programmed as follows: maintain the initial 45 °C for 1.5 min; 10 °C/min to 200 °C for 1 min; 2.5 °C/min to 250 °C for 1 min; 4 °C/min to 280 °C for 3 min; 5 °C/min to 300 °C and hold for 10 min.

2.6. Photocatalytic Experiment

The photocatalytic activities of TiO2/CQDs/CdIn2S4 (different ratio of CQDs), TiO2/CdIn2S4 and bare CdIn2S4 were studied by comparative absorption spectra, and the optimal proportion of CQDs in the photocatalyst was determined. The 50 mg of the as-prepared photocatalysts was added in the substrate solution. The 300 W Xe lamp was used as the light source to provide UV or visible light (long-pass filter (>400 nm)). To evaluate the photocatalytic performance for cigarette tar degradation, acenaphthylene (ANY) was employed for the photocatalytic experiments, due to its predominant content in cigarette tar. The substrate was dissolved in toluene solvent at 1 mg/L, and the amount of the photocatalyst was 0.5 g/L. Before the illumination, the mixture was stirred with Teflon magneton for 30 min under dark conditions to ensure uniform mixing of photocatalyst and polycyclic aromatic hydrocarbon molecules.
To calculate density functional theory (DFT) within the generalized gradient approximation (GGA) using the PBE [42] formulation, Vienna Ab Initio Package (VASP) was exploited [43,44]. In this study, we performed the projected augmented wave (PAW) potentials for describing the ionic cores and to account for valence electrons, working on 450 eV kinetic energy cutoffs [45,46]. With Gaussian smearing, partial occupancies of the Kohn–Sham orbitals were allowed at a width of 0.05 eV. The on-site corrections (DFT+U) were subjected to the 3D electron of Ti atoms (Ueff = 4.5 eV). Electronic energy was identified as self-consistent when the change in energy was less than 10−5 eV. The geometry optimization was known as convergent when the force change was less than 0.02 eV/Å. Grimme’s DFT-D3 approach was employed to illustrate the dispersion interactions.
We determined a = b = 3.858 Å and c = 9.652 Å for anatase TiO2 unit cells when employing a 10 × 10 × 4 Monkhorst-Pack k-point grid for Brillouin zone sampling. For Brillouin zone sampling, the equilibrium lattice constant of cubic CdIn2S4 is a = 10.920 Å, in the presence of a 2 × 2 × 2 Monkhorst-Pack k-point grid. Hexagonal graphene unit cells isolated by a vacuum layer of 15 Å depth were optimized for equilibrium lattice constants, being regarded as a = 2.468 Å, when employing a 15 × 15 × 15 Monkhorst-Pack k-point grid for Brillouin zone sampling. After that, two heterojunction surface models were constructed. The first was a heterojunction surface model based on TiO2/CdIn2S4 (101). The part of CdIn2S4 has a p (1 × 2) periodicity in the X and Y directions and one stoichiometric layer in the Z direction; the TiO2 (101) part has a p (1 × 6) periodicity in the X and Y directions and two stoichiometric layers in the Z direction; the TiO2/CdIn2S4 (101) slab was added with a vacuum layer in the Z direction at the depth of 15 Å in order to separate the surface slab from its periodic duplicates. The second heterojunction model was built by adding a graphene monolayer with a (4 × 5√3) periodicity between the two parts of model 1. During structural optimizations, the Γ point in the Brillouin zone was used for k-point sampling, and the bottom stoichiometric layer of the TiO2 (101) part was fixed while the rest were allowed to fully relax.

3. Result and Discussions

3.1. Characterizations of Structure and Morphology

The morphology and microstructure of the as-prepared photocatalysts were initially checked by scanning electron microscope (SEM). As shown in Figure 1A, TiO2 presented a spherical shape with uniform particle size of ca. 65 nm. After coupling TiO2 with CdIn2S4 through a hydrothermal method, the stacked nanospheres with wrinkles were monitored (Figure 1B), which is consistent with the previous reports [32,47]. The morphology can be attributed to that the anisotropy of CdIn2S4 drives a directional growth, leading to the presence of nanosheets and subsequent self-assembly. While, the decoration of carbon quantum dots (CQDs) on TiO2/CdIn2S4 resulted in no obvious morphological changes as observed by SEM, which can be explained by the small size of the CQDs (Figure 1C) [48].
To confirm the phase and crystal structures of the as-prepared samples, powder X-ray diffraction (PXRD) (Figure 1D and Figure S1) analysis was performed. On the one hand, the main diffraction peaks located at 2θ = 23.15°, 27.24°, 33.00°, 40.73°, 43.31° and 47.41° can be indexed to cubic crystal structure of CdIn2S4 (JCPDS#27-0060), corresponding to the indices of (220), (311), (400), (422), (511) and (440) planes, respectively [49,50]. On the other hand, the diffraction peaks in TiO2/CQDs/CdIn2S4 composite exhibit a standard phase of tetragonal TiO2 (JCPDS#21-1272) with the lattice (101), (004), (200), (105) and (211) [30]. In addition, CQDs (top right) showed a broad peak centered at 2θ = 22.57° with low intensity owing to the small size, which is consistent with the previous reports [48,51,52,53]. Thus, the (220) plane of CdIn2S4 at 2θ = 23.2° in TiO2/CQDs/CdIn2S4 did not alter significantly compared with TiO2/CdIn2S4 due to the low intensity of CQDs characteristic peak. The crystal size of CQDs is evaluated based on the PXRD patterns using Debye–Scherrer equation [53,54,55,56], which is capable of calculating the size of nanocrystals based on the X-ray diffraction features, regardless of the aggregation of these nanocrystals. Meanwhile, the full width at half maximum (FWHM) of as-prepared CQDs is measured to be 546.01×10−4 rad, suggesting an ultra-small size of ca. 2.5615 nm (Table 1), confirming that the synthesized samples were quantum dots with the unique property of quantum dots, whose size ranges from 1 to 10 nm. As seen in Figure 1D, in comparison to TiO2/CdIn2S4, TiO2/CQDs/CdIn2S4 presents no significant change, which is ascribed to the weak XRD signal of CQDs.
Transmission electron microscopy (TEM) was employed to further demonstrate the microscopic structure of TiO2/CQDs/CdIn2S4. The uniformly distributed lattice fringes in Figure 1E,F clearly present the intimate interfacial contact between TiO2 and CdIn2S4. The lattice spacings of 0.357 nm and 0.327 nm are exhibited in Figure 1F,G, which correspond to (101) planes of the tetragonal phase of TiO2 and (311) planes of the cubic phase of CdIn2S4 [33,57], respectively; this is in full accord with the PXRD results. Based on the crystal parameters of TiO2 and CdIn2S4, the lattice mismatch can be obtained as ∆d/dTiO2 = 8%. The small lattice mismatch typically indicates rapid transfer of photogenerated carriers between interfaces of TiO2/CdIn2S4 due to the lower interfacial trap state [58]. Next, energy dispersive spectra (EDS) mapping was employed to investigate the elemental composition and distribution of TiO2/CQDs/CdIn2S4 photocatalysts. As shown in Figure 1H, carbon can be clearly detected to be evenly distributed in the whole TiO2/CdIn2S4 region. In addition, Cd, In, S, Ti and O elements were also confirmed from the sample, which can correspond to the elemental composition of TiO2/CQDs/CdIn2S4 photocatalysts. For further confirming the content difference of CQDs between TiO2/CQDs/CdIn2S4 and TiO2/CdIn2S4, Figures S5 and S6 and Table S1 show that C element was not found in TiO2/CdIn2S4 photocatalysts.

3.2. Band Structure of TiO2/CQDs/CdIn2S4 Photocatalysts

Figure 2A illustrates the UV-Vis diffuse reflectance spectrum (DRS) of bare TiO2, CdIn2S4, TiO2/CdIn2S4 and TiO2/CQDs/CdIn2S4 photocatalysts. The tetragonal TiO2 exhibited a typical absorption edges at 387 nm, situated in the absorption range of ultraviolet light [59]. Meanwhile, the absorption edge of CdIn2S4 can be observed at 610 nm corresponding to the band gap energy of 2.03 eV [49]. Furthermore, the loading of CdIn2S4 on TiO2 lead to a light absorption range of the composite extended to the visible region comparing to pristine TiO2 [30,60]. After the introduction of CQDs, TiO2/CQDs/CdIn2S4 presented a slightly enhanced absorption capability in range from 600 to 900 nm, which can be attributed to the narrow band gap energy of CQDs [48]. The valence state of the samples was evaluated by X-ray photoelectron spectroscopy (XPS), from which the characteristic binding energies of Cd (3d), In (3d), S (2p) and Ti (2p) were in good agreement with the rational valence state of the composite (Figure 2B–E). As shown in Figure 2F, the XPS profile of O 1s in the initial state presented three peaks located at 529.67, 521.97 and 533.02 eV, corresponding to the lattice oxygen, adsorbed oxygen [61] and surface hydroxyl, respectively [62,63,64]. Under UV illumination, we observed a dramatically reduced O 1s (533.50 eV) peak corresponding to the surface hydroxyl (Figure 2F). Considering the reductivity of hydroxyl species and that the valance of TiO2 was composed by O orbitals, this result indicates that the O in TiO2 featured highly active oxidation sites in TiO2/CQDs/CdIn2S4, possibly as a S-scheme structure (Figure S6) [60], where the hydroxyl species were oxidized by the photo-generated holes in the valence band of TiO2. Based on the valence plots depicted in the Figure S2, the valence band (EVB) of the CdIn2S4 and TiO2 were calculated to be 0.56 eV and 2.23 eV [65]. Meanwhile, due to the band gap energy (Eg) of the CdIn2S4 (Eg = 2.02 eV) and TiO2 (Eg = 3.20 eV) shown in Figure 2A, the band structure of the S-scheme of TiO2/CQDs/CdIn2S4 and type-II of TiO2/CdIn2S4 were constructed in Figure 2G. Notably, compared with the type-II, the photogenerated electrons in CB of TiO2 and the holes produced in the VB of CdIn2S4 are inclined to recombine on the CQDs bridge during the photocatalytic reaction of S-scheme under the Coulombic attraction between electrons and holes [66]. Since more holes are accumulated in the VB of TiO2, the oxidation ability of the S-scheme photocatalyst becomes stronger.

3.3. Influence of S Defects Formation

Figure 3A presents the cubic spinel structure CdIn2S4 indexing to the space group FD-3m (No.227), where the purple, magenta and yellow balls represent the Cd, In and S atoms, respectively [67]. The In atom is connected to six S atoms, forming the In-S octahedral structure. The blue and red balls represent the Ti and O atoms, which constitutes the tetragonal TiO2. The CQDs serve as a bridge to conduct electricity between TiO2 and CdIn2S4 (Figure 3B). To further investigate the roles of CQDs in TiO2/CQDs/CdIn2S4, the density of state (DOS) was conducted using first-principle calculations. As depicted in Figure 3C, the shallow trap states that primarily consisted of S (3p), O (2p) and In (5p) orbitals were projected, which are consistent with the pristine component (Figure S3). A peak corresponding to S 3p orbital at the Fermi level (E-Ef = 0 eV) indicates the presence of S defects [68]. The generation of surface sulfur vacancies typically leads to the accumulation of charges on adjacent Cd and In atoms, which may serve as highly active sites for intermediate chemisorption during photocatalytic degradation. Additionally, with respect to TiO2/CdIn2S4 (Figure 3C), after the introduction of CQDs (Figure 3D), the DOS of valence band orbital of O atom shows a clear downward trend due to the increased electron extraction from O, suggesting an enhancement in the oxidation capacity of TiO2 [69], which is consistent with our in-situ XPS result. Furthermore, in consideration of the critical role of radicals in the photocatalytic process, electron paramagnetic resonance (EPR) was applied to monitor the influence of CQDs on radical generation under illumination. The EPR results in Figure S4 confirmed that the samples with/without are both capable of producing superoxide and hydroxyl radicals under illumination, and the radical concentration generated from the sample with CQDs is slightly higher than that of the control group.

3.4. Influence of CQDs Content on Oxidation Performance

The as-prepared photocatalysts were exploited to degrade PAHs from cigarette tar to produce valuable products. First, 10 mg of the as-prepared photocatalysts was added in PAHs substrate, then the reaction was conducted under 100 mW cm−2 simulated solar source with AM 1.5 G filter. A combination of gas chromatography-mass spectroscopy (GC-MS), Fourier-transform infrared spectroscopy (FT-IR) and UV-vis DRS were employed to evaluate the degradation rate and products. Before the photocatalytic tests, the PAHs substrate and photocatalysts were initially mixed and stirred under dark conditions for 30 min to achieve adsorption equilibrium, during which we monitored no obvious change in UV-vis DRS.
In terms of the photocatalytic performance, the ratio of CQDs in TiO2/CQDs/CdIn2S4 presented a strong impact on the photocatalytic degradation rate over PAHs (Figure 4A). Namely, the photocatalytic activity increased with the increased ratio of CQDs, it and reached the zenith at 20 to 30 mg/L, which was then followed with a deceased activity. This may be due to the overloading of CQDs leading to a reduced charge transfer efficiency between TiO2 and CdIn2S4, which suppresses the S-scheme features of the photocatalysts. To further optimize the ratio of CQDs, we tested the range of CQDs between 20 to 25 mg/L, in which the 23 mg/L presented a degradation rate up to 78.5% (Figure 4B). As illustrated in Figure 4C, the color of the substrate changed from the initial brown to a transparent colorless solvent in 1 h photocatalytic process. In addition to the loss of color, a blue shift of the absorption maximum (from 430 to 385 nm) was also monitored during the photocatalytic reaction (Figure 4D). As it is known that the molecular structure can be reflected by the absorption spectra, to be specific, a decreased number of conjugated carbon-carbon double bonds typically result in an absorption spectrum with a shorter wavelength [70]. In our case, the blue shift suggests that carbon–carbon double bonds in PAHs is possibly destroyed during the photocatalytic degradation. The comparison of the degradation performances among the serials of samples were exhibited in Figure 4E, particularly, the loading of rational content of CQDs between TiO2 and CdIn2S4 dramatically improved the photocatalytic activity, comparing to the control group (from 45.81 to 78.50%), suggesting the photoactivity can be efficiently promoted by adding CQDs. Furthermore, the PAHs degradation performance was compared as shown in Table 2. Besides, the stability of TiO2/CQDs/CdIn2S4 was checked by repeating the experiments 3 times using the recycled samples (Figure 4F) Comparing to the initial cycle, ca. 7.2% photoactivity loss was observed in the second cycle. Meanwhile, for further confirming the stability of photocatalysts in PAHs degradation, as shown in Figure S7, 97.6% samples were retained in the two TiO2/CQDs/CdIn2S4 sample groups after 5 h photocatalytic degradation. While in the third cycle, the photoactivity loss was less than 2%, relative to the second cycle. As confirmed by PXRD (Figure 4G,H), the characteristic patterns of TiO2/CQDs/CdIn2S4 basically remain the same after 3 reaction cycles. As depicted in Figure S8, the hot filtration test was conducted to confirm the compatibility of TiO2/CQDs/CdIn2S4 photocatalysts [71]. After 10 min of reaction under simulated sunlight (degradation of 65.2% PAHs), the photocatalyst was removed and filtered. Then the filtrate was illuminated for another 50 min under the same condition with no significant increase in degradation, confirming that no leaching of the photocatalyst components emerged [72]. Meanwhile, the ICP-MS analysis was performed to reveal the metal leaching of the pure filtrate [73]. The contents of Cd, In and Ti were 0.00055, 0.000554 and 0.05477 ppm, respectively, indicating the TiO2/CQDs/CdIn2S4 photocatalyst was stable and that the photocatalytic activity derives from the whole as-prepared photocatalyst, rather than from the constituents of possible leaching.

3.5. Photocatalytic Performance for PAHs

The obtained products were initially analyzed by FT-IR spectroscopy, as shown in Figure 5A. After degradation, the most obvious changes in FT-TR spectra are two newly appeared signals located at ca. 1260 and 1730 cm−1 with high intensity, which can be assigned to the C-C key stretching vibration and aldehyde group, respectively [79,81]. In addition, increased intensity of FT-IR signals were also observed at 2860 and 2930 cm−1, which can be indexed to the cyclohexane bands [82]. These results are in good agreement with the benzaldehyde (m/z = 106) and dimethylcyclohexane (m/z = 112) products recorded by GC-MS (Table 3) [83]. It should be noted that the conversion rates based on GC-MS are slightly higher than that of UV-vis DRS, since some side products might not be reflected by absorption spectra. In contrast to the conversion rate of 58.42% presented by the control group without CQDs, the degradation of PAHs using the optimal TiO2/CQDs/CdIn2S4, (Figure 5B) GC-MS monitored that 87.79% of PAHs was decomposed to benzaldehyde (63.88%), dimethylcyclohexane (21.18%) and side products (2.73%) of very low proportion after 1 h of reaction.

4. Conclusions

In summary, TiO2/CdIn2S4 with S-Scheme band structure was modified by the introduction of CQDs during the synthetic process. Based on the in-situ XPS tests and theoretical calculations, we found that the presence of CQDs between TiO2 and CdIn2S4 can efficiently promote the oxidation capability of the photocatalyst, which benefits the photocatalytic degradation of PAHs. Thanks to this, the TiO2/CQDs/CdIn2S4 presents outstanding photoactivity in converting the polycyclic aromatic hydrocarbons (PAHs) released by cigarettes to value-added benzaldehyde. The optimized photocatalyst performs 87.79% conversion rate and 72.76% selectivity in 1 h reaction under simulated solar source, as confirmed by FT-IR and GC-MS, which was more efficient than the control group (58.42% conversion rate).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules27217292/s1, Figure S1: XRD of (a) bare CdIn2S4 and (b) bare TiO2; Figure S2: The valance band edges of (a) CdIn2S4 and (b) TiO2; Figure S3: (a,b) the band structure of bare CdIn2S4 bulk and bare TiO2 bulk, (c,d) the density of states for bare CdIn2S4 bulk and bare TiO2 bulk; Figure S4: EPR spectrum of (a) TiO2/CdIn2S4 and (b) TiO2/CQDs/CdIn2S4; Figure S5: The elemental composition of (a) TiO2/CQDs/CdIn2S4 and (b) TiO2/CdIn2S4 analyzed by TEM-EDS; Figure S6: (a) HAADF TEM image of TiO2/CdIn2S4 heterostructure, (b–g) respective elemental mapping of Cd, In, S, Ti and O; Figure S7: Stability of PAHs degradation of TiO2/CQDs/CdIn2S4; Figure S8: Hot filtration test of TiO2/CQDs/CdIn2S4; Table S1: Relative atomic content of TiO2/CQDs/CdIn2S4 and TiO2/CdIn2S4; References [65,84].

Author Contributions

Conceptualization, J.Z. and F.Y.; methodology, J.Z. and M.Z.; software, X.K. and L.D.; validation, J.Z., H.Y. and M.Z.; formal analysis, J.Z. and F.Y.; investigation, J.Z. and P.G.; resources, M.Z. and D.L.; data curation, J.Z.; writing—original draft preparation, J.Z. and F.Y.; writing—review and editing, J.Z. and M.Z.; visualization, J.Z.; supervision, M.Z.; project administration, M.Z. and D.L.; funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful to the Guangdong Basic and Applied Basic Research Foundation (Grant NO. 2020B1515020032; 2020B1515120022), Guangzhou Basic and Applied Basic Research Foundation (Grant NO. 202201010440), the National Natural Science Foundation of China (Grant No. 62074060; 22102058) and the Science and Technology Program of Guangzhou (Grant No. 2019050001). They also acknowledge eceshi (www.eceshi.com, accessed on 3 March 2022) for the EPR analyses.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data are available in the main text or the electronic supplementary information (ESI).

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

Sample Availability

Samples of the compounds of TiO2/CQDs/CdIn2S4 and TiO2/CdIn2S4 photocatalysts are available from the authors.

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Molecules 27 07292 sch001 550
Scheme 1. Synthesis scheme for TiO2/CQDs/CdIn2S4 photocatalysts.
Scheme 1. Synthesis scheme for TiO2/CQDs/CdIn2S4 photocatalysts.
Molecules 27 07292 sch001
Molecules 27 07292 g001 550
Figure 1. SEM images of (A) as-prepared TiO2; (B) TiO2/CdIn2S4; (C) TiO2/CQDs/CdIn2S4; (D) PXRD patterns of TiO2/CdIn2S4 and TiO2/CQDs/CdIn2S4; (E–G) TEM images of TiO2/CQDs/CdIn2S4 at different magnifications and locations; (H) TEM-EDS elemental mapping of TiO2/CQDs/CdIn2S4.
Figure 1. SEM images of (A) as-prepared TiO2; (B) TiO2/CdIn2S4; (C) TiO2/CQDs/CdIn2S4; (D) PXRD patterns of TiO2/CdIn2S4 and TiO2/CQDs/CdIn2S4; (E–G) TEM images of TiO2/CQDs/CdIn2S4 at different magnifications and locations; (H) TEM-EDS elemental mapping of TiO2/CQDs/CdIn2S4.
Molecules 27 07292 g001
Molecules 27 07292 g002 550
Figure 2. (A) UV-vis DRS of CdIn2S4, TiO2/CdIn2S4 and TiO2/CQDs/CdIn2S4; (B–E) XPS spectra of the main elements in TiO2/CQDs/CdIn2S4; (F) In-situ XPS spectra of O1s orbital in TiO2/CQDs/CdIn2S4, the data was collected with/without 365 nm irradiation; (G) The band position and charge transfer of S-scheme photocatalytic reaction process of TiO2/CQDs/CdIn2S4 and the type-II photocatalytic reaction process of TiO2/CdIn2S4 under illumination.
Figure 2. (A) UV-vis DRS of CdIn2S4, TiO2/CdIn2S4 and TiO2/CQDs/CdIn2S4; (B–E) XPS spectra of the main elements in TiO2/CQDs/CdIn2S4; (F) In-situ XPS spectra of O1s orbital in TiO2/CQDs/CdIn2S4, the data was collected with/without 365 nm irradiation; (G) The band position and charge transfer of S-scheme photocatalytic reaction process of TiO2/CQDs/CdIn2S4 and the type-II photocatalytic reaction process of TiO2/CdIn2S4 under illumination.
Molecules 27 07292 g002
Molecules 27 07292 g003 550
Figure 3. The crystal structures of (A) TiO2/CdIn2S4 and (B) TiO2/CQDs/CdIn2S4; density of states of (C) TiO2/CdIn2S4 and (D) TiO2/CQDs/CdIn2S4.
Figure 3. The crystal structures of (A) TiO2/CdIn2S4 and (B) TiO2/CQDs/CdIn2S4; density of states of (C) TiO2/CdIn2S4 and (D) TiO2/CQDs/CdIn2S4.
Molecules 27 07292 g003
Molecules 27 07292 g004 550
Figure 4. (A,B) The PAHs degradation rate by TiO2/CQDs/CdIn2S4 with different CQDs content; (C) UV-vis DRS of PAHs with different photocatalytic degradation time; (D) digital photograph of PAHs with different photocatalytic degradation time; (E) UV-vis DRS of PAHs degraded by different photocatalysts; (F) repeated reactions of PAHs degradation over TiO2/CQDs/CdIn2S4; (G,H) PXRD patterns of TiO2/CQDs/CdIn2S4 before and after 3 reaction cycles. Reaction conditions: 100 mW cm−2 illumination for 1 h with 50 mg photocatalyst under room temperature.
Figure 4. (A,B) The PAHs degradation rate by TiO2/CQDs/CdIn2S4 with different CQDs content; (C) UV-vis DRS of PAHs with different photocatalytic degradation time; (D) digital photograph of PAHs with different photocatalytic degradation time; (E) UV-vis DRS of PAHs degraded by different photocatalysts; (F) repeated reactions of PAHs degradation over TiO2/CQDs/CdIn2S4; (G,H) PXRD patterns of TiO2/CQDs/CdIn2S4 before and after 3 reaction cycles. Reaction conditions: 100 mW cm−2 illumination for 1 h with 50 mg photocatalyst under room temperature.
Molecules 27 07292 g004
Molecules 27 07292 g005 550
Figure 5. (A) FT-TR spectra of PAHs before and after the photocatalytic reactions for 1 h; (B) the performance for PAHs photocatalytic degradation over TiO2/CQDs/CdIn2S4 and TiO2/CdIn2S4.
Figure 5. (A) FT-TR spectra of PAHs before and after the photocatalytic reactions for 1 h; (B) the performance for PAHs photocatalytic degradation over TiO2/CQDs/CdIn2S4 and TiO2/CdIn2S4.
Molecules 27 07292 g005
Table
Table 1. The crystal sizes of the photocatalysts and CQDs calculated by Debye–Scherrer equation.
Table 1. The crystal sizes of the photocatalysts and CQDs calculated by Debye–Scherrer equation.
Sampleβ (rad)2θ (°)D311 (nm)
TiO2/CQDs/CdIn2S486.873 × 10−427.18116.275
TiO2/CdIn2S475.534 × 10−427.22018.681
CQDs546.01 × 10−422.5702.5615
Table
Table 2. Review of photocatalytic degradation performance for PAHs by varies photocatalysts.
Table 2. Review of photocatalytic degradation performance for PAHs by varies photocatalysts.
PhotocatalystLight SourceWeight of PhotocatalystAmount of PAHsIrradiation
Time (min)		Degradation
Efficiency (%)		Reference
TiO2TiO2/CQDs/CdIn2S4Xe lamp (300 W) with a cutoff filter (>400 nm)50 mg50 mL aqueous solution,
1 mg/L		6087.79This work
Co3O4/Bi2O2CO3500 W xenon lamp25 mg50 mL aqueous solution,
10 mg/L		15091.02[74]
ZnO/g-C3N4Visible lightNot mentionedNot mentioned24084.50[75]
ZnO15 W UVC lamp with emission at 254 nmNot mentioned3.5 mL of anthracene5560.00[76]
CeVO4 nanoparticlesXe lamp (300 W) with a 420 nm cutoff filter50 mg200 mL Nap solution,
30 mg/L		60~60.00[77]
ZnFe2O4-CSdaylight20 mg5 mL aqueous solution,
2−10 mgL−1		72095.00[78]
TiO2-Graphene300 W high pressure mercury lamp (λ > 320 nm)25 mg250 mL aqueous solution,
0.5–5.0 μg/mL		120~80.00[79]
ZnO NPsUV lamp (368 nm, 40 W)10 mg50 mL aqueous solution,
23 mg/L		23090.00[80]
Table
Table 3. GC-MS data of the main products from photocatalytic degradation of PAHs over TiO2/CQDs/CdIn2S4.
Table 3. GC-MS data of the main products from photocatalytic degradation of PAHs over TiO2/CQDs/CdIn2S4.
PeakRetention Timem/zEmpirical FormulaProduct
24.472112C8H16dimethylcyclohexane
34.565112C8H16dimethylcyclohexane
44.743112C8H16dimethylcyclohexane
54.859112C8H16dimethylcyclohexane
64.938154C12H8acenaphthene
75.447106C7H6Obenzaldehyde
85.588106C7H6Obenzaldehyde
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Zhang, J.; Yu, F.; Ke, X.; Yu, H.; Guo, P.; Du, L.; Zhang, M.; Luo, D. Carbon Quantum Dots Bridged TiO2/CdIn2S4 toward Photocatalytic Upgrading of Polycyclic Aromatic Hydrocarbons to Benzaldehyde. Molecules 2022, 27, 7292. https://doi.org/10.3390/molecules27217292

AMA Style

Zhang J, Yu F, Ke X, Yu H, Guo P, Du L, Zhang M, Luo D. Carbon Quantum Dots Bridged TiO2/CdIn2S4 toward Photocatalytic Upgrading of Polycyclic Aromatic Hydrocarbons to Benzaldehyde. Molecules. 2022; 27(21):7292. https://doi.org/10.3390/molecules27217292

Chicago/Turabian Style

Zhang, Jiangwei, Fei Yu, Xi Ke, He Yu, Peiyuan Guo, Lei Du, Menglong Zhang, and Dongxiang Luo. 2022. "Carbon Quantum Dots Bridged TiO2/CdIn2S4 toward Photocatalytic Upgrading of Polycyclic Aromatic Hydrocarbons to Benzaldehyde" Molecules 27, no. 21: 7292. https://doi.org/10.3390/molecules27217292

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