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S, N co-doped Graphene Quantum Dots Embedded in (BiO)2CO3: Incorporating Enzymatic-like Catalysis in Photocatalysis Penghui Ding, Jun Di, Xiaoliu Chen, Mengxia Ji, Kaizhi Gu, Sheng Yin, Gaopeng Liu, Fei Zhang, Jiexiang Xia, and Huaming Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01552 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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ACS Sustainable Chemistry & Engineering

S, N Co-doped Graphene Quantum Dots Embedded in (BiO)2CO3: Incorporating Enzymatic-like Catalysis in Photocatalysis Penghui Ding,a Jun Di,a Xiaoliu Chen,b Mengxia Ji,a Kaizhi Gu,c Sheng Yin,a Gaopeng Liu,a Fei Zhang,a Jiexiang Xia,*a Huaming Li*a

a

School of Chemistry and Chemical Engineering, Institute for Energy Research,

Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, P. R. China. b

School of the Environment, Jiangsu University, Zhenjiang, 212013, P. R. China

c

School of Chemistry and Molecular Engineering, East China University of Science

and Technology, Shanghai 200237, P. R. China

*Corresponding author: Tel.:+86-511-88791108; Fax: +86-511-88791108; E-mail address: [email protected]; [email protected]

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Abstract: In this study, S, N co-doped graphene quantum dots/(BiO)2CO3 hollow microspheres have been fabricated by a facile electrostatic self-assembly method. The nanosized S, N:GQDs, which can be obtained by a bottom-up approach, are a superior surface modification material for photocatalytic applications due to their better electron transfer and peroxidase mimetic properties. The excellent oxidation property of the synthesized nanocomposite is confirmed by degradation of different model pollutants, such as rhodamine B, tetracycline, and bisphenol A under light irradiation or dark situation. Based on several experiments, the essential roles of S, N:GQDs can be described as: (i) a photocarrier transport center strengthening photo-induced charge carriers (h+-e-) separation and (ii) an enzymatic-like catalysis center to accelerate H2O2 decomposition to produce ·OH since the surface accumulation of H2O2 is harmful for photocatalytic processes. The present work may pave the way for integrating enzymatic-like co-catalysis into a photocatalytic process to generate more reactive oxygen species, thus advancing the field of environmental remediation and synthetic chemistry.

KEYWORDs: S, N:GQDs; (BiO)2CO3; Rose-like hollow microspheres; H2O2 decomposition; Photocatalytic


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Introduction Due to excess human activities, a multitude of pollutants are being released into previously pure waters (such as lakes, seas, and groundwater) in both underdeveloped and developed places: from existing macroscopic contaminants such as plastics to emergent micropollutants such as tracer dyes, antibiotics, and endocrine-disrupting chemicals.1,2 To date, a vast array of water treatment technologies and plants have been developed or constructed to mitigate the long-standing problem facing humankind. However, most of these water purification treatments are time-consuming and high-cost.3 In this scenario, photocatalysis has attracted researchers' interest since the seminal work on titanium dioxide (TiO2)-based photoelectrochemical water splitting published by Fujishima and Honda because of its fast, low-price, and “green” nature.4-6 TiO2, characterized by inexpensiveness and chemical stability, has been commonly utilized as a reliable photocatalyst for pollutant removal,7 water splitting,8 CO2 reduction,9 H2O2 production,10 and organic transformation.11 However, the inherent large band gap (3.0–3.2 eV) greatly hinders its wide use, since ultraviolet (UV) light constitutes approximately 5% of the entire solar spectrum.12 Although the scientific community is making a concerted effort to engineer the pure white crystal by doping (nonmetal ion doping,13 metal ion doping,14 and codoping15) and heterostructure formation,16 its photoactivity still suffers from quick electron-hole recombination, low quantum efficiency (QE), and reduced oxidative activity.17 To fully address the problems, finding new catalysts meeting relevant standards might be a good choice. Of all the newly discovered photocatalysts, bismuth-containing materials have aroused the interest of researchers thanks to their narrow band gap, layered structure, and internal polar electric field.18,19 Bismuth subcarbonate ((BiO)2CO3), a typical Aurivillius phase oxide, is composed of alternate stacking of [Bi2O2]2+ slabs intercalated by CO32- layers. It exhibits relatively high photocatalytic performance towards NO removal,20 H2 evolution,21 and nitrogen fixation.22 Nevertheless, in order to additionally enhance its photocatalytic effect, further steps are still needed.



Three-dimensional (3D) hierarchical photocatalysts possess considerable advantages over zero-dimensional (0D), one-dimensional (1D) or two-dimensional (2D) photocatalysts in many ways: (1) enhanced mass transport of target contaminants, (2) improved light-absorbing capacity due to surface scattering and reflecting effect (SSR effect) and (3) much higher specific surface area (SSA).23-26 Considering the merits above, constructing a rose-like (BiO)2CO3 hollow microspheres-based photochemical system might be an optional approach. Nonetheless, such a nanostructured porous photocatalyst cannot easily make complete contact with other coupled materials (such as MoS2, polyaniline, or graphene) due to its inner complex networks, thereby reducing the photocatalytic ability to remove pollutants.27-29 Nanosized quantum dots (QDs) are being extensively used in the field of photocatalysis, because of the ease of forming a maximal accessible area in the inner and outer structure of pure catalyst.30,31 Thus, it is highly desirable to combine 0D QDs with (BiO)2CO3 hollow microspheres. Apart from light-harvesting ability and target contaminants diffusion, efficient separation of the charge carriers should be one of the most critical factors for photocatalytic ability. Higher specific surface area (SSA) of a 3D flower-like photocatalyst means better adsorption ability towards contaminants, thus leading to more active sites. Unfortunately, it also indicates much more surface trapping sites for electron-hole recombination.32 Thus, it is of great importance to add surface materials to accelerate carriers separation. Another long-forgotten factor affecting photoactivity should also be dealt with: the effectiveness of transforming charge carriers to reactive oxygen species (ROS) or specific oxidant for pollutants removal. Several ROS can be recognized in a photocatalytic process, namely superoxide ion (.O2-), hydroxyl radical (·OH) and hydrogen peroxide (H2O2).33 Due to its relatively small oxidation potential (E0=1.78 V), H2O2 is not very powerful for photocatalytic oxidation.34 Besides that, excess H2O2 accumulated on the surface of photocatalyst might do harm to the catalyst itself.35 Therefore, in order to achieve high photocatalytic activity and good stability simultaneously, urgent attention should be concentrated on the transformation of the photoinduced H2O2. It is generally accepted that the non-selective ·OH is the most powerful ROS in the process of photocatalytic removal of pollutants.36 Thus, if a


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high concentration of ·OH could be achieved in a photochemical system, a good photocatalytic performance might be guaranteed. Considering the downside of photogenerated H2O2 and the upside of ·OH in photocatalytic oxidation reactions, the degradation efficiency could be enhanced due to synergistic effects originated from in-situ conversion of generated H2O2 to useful ·OH. However, semiconductor photocatalysts per se cannot produce specific ROS with ease.37 Consequently, adding materials possessing peroxidase-like properties are also very essential. Still, achieving the two goals (i.e. better electron transfer and H2O2 decomposition) at the same time remains challenging. Graphene quantum dots (GQDs), a subset of carbon dots (CDs) with diameters smaller than 10 nm, are being thoroughly explored due to their exotic physicochemical properties. Because of the conjugated π-system, GQDs usually display good electron transfer property, thus giving rise to better photoactivity when employed as a surface modification material.38 Recent reports have discussed the potential use of GQDs in photocatalytic environmental preservation or energy conversion, and the mechanisms behind these systems.39-41 Interestingly, as a kind of carbon-based enzyme, GQDs are also capable of generating ·OH from H2O2 decomposition via its unique peroxidase mimetic property.42,43 Compared with natural enzymes, such as horseradish peroxidase (HRP), GQDs are much more robust (operation over wide pH range) and easier to obtain or engineer through many established methods. Moreover, introduction of heteroatoms (such as sulfur, nitrogen or

phosphorus)

could

result

in

electron

delocalization

due

to

different

electronegativities between carbon atoms and doped atoms and the change of amount of relevant catalytic sites for enzymatic-like catalysis. Therefore, it may improve carriers separation and peroxidase-like property.44-47 Herein, we report a nanocatalyst composed of S, N:GQDs and rose-like (BiO)2CO3 hollow microspheres in order to tune the pathway of charge carriers and facilitate ·OH generation by enzymatic-like catalysis of H2O2 decomposition. Although previous work concerning GQDs/N-(BiO)2CO3 nanocomposite has been reported, it does not discuss the formation mechanisms and special features of GQDs



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in photocatalysis detailedly.40 Moreover, the pivotal roles of S, N:GQDs are evaluated by removing different representative persistent pollutants (such as organic dyes, antibiotics, and endocrine disrupter chemicals) under various conditions (with or without external light).We believe this work would be beneficial for enhancing the performance of other three-dimensional (3D) hierarchical photocatalysts, and highlighting the unique roles of S, N:GQDs in relevant photocatalytic applications.

Experimental section Reagents and Chemicals. Ammonium bismuth citrate (C12H22BiN3O14, 99%), urea (CH4N2O, 99%), citric acid monohydrate (C6H8O7·H2O, 99%), thiourea (CH4N2S, 99%), hydrogen peroxide (H2O2, 30% w/w aqueous), terephthalic acid (C8H6O4, 99%), sodium

hydroxide

(NaOH,

96%),

p-benzoquinone

(C6H4O2,

98%),

ethylenediaminetetraacetic acid disodium (C10H14N2Na2O8·2H2O, 97%), isopropanol (C3H8O, 99.7%), ethanol (C2H6O, 97%), and rhodamine B (C28H31ClN2O3, AR) were kindly supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Bisphenol A (C15H16O2, 99%) was purchased from Aladdin. Tetracycline hydrochloride (C22H24N2O8·HCl, USP) was purchased from Macklin. All chemicals were used as received without further purification. Deionized (DI) water was used in all experiments. Fabrication of photocatalyst. The synthesis of S, N:GQDs aqueous solution was directly from the work of Sun et al. with a modification.48 Typically, 1 mmol portion of citric acid and 3 mmol portion of thiourea were dissolved in 10 mL deionized water under stirring for 30 minutes. After that, the transparent solution was poured into a 25 ml Teflon-lined autoclave and heated at 160 °C for 4 hours. Finally, the as-obtain product experienced dialysis under stirring for 24 h to obtain S, N:GQDs aqueous solution. To obtain rose-like (BiO)2CO3 hollow microspheres (denoted as BOC), a hydrothermal method was employed. In a typical procedure, 2 mmol of ammonium bismuth citrate and 10 mmol of urea were dissolved in 70 mL deionized water under


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vigorous stirring for 30 minutes. After that, the mixed solution was transferred into a 100 ml Teflon-lined autoclave and heated at 180 °C for 7 hours. The as-obtained white catalyst was further washed by deionized water and absolute ethanol for three times, and dried at 60 °C overnight. To fabricate S, N:GQDs/(BiO)2CO3-X photocatalyst (X=1, 3 or 5) (denoted as S, N:GQDs/BOC-X), a facile electrostatic self-assembly method was employed. To a beaker containing 0.1 g of as-prepared (BiO)2CO3 were added X mL of as-obtained S, N:GQDs aqueous solution and 10 mL of deionized water in order. Then, the turbid solution was subjected to vigorous stirring for 24 hours. The final nanocatalyst was obtained by successive washing with deionized water and absolute ethanol for three times followed by drying at 60 °C overnight. Note that S, N:GQDs/(BiO)2CO3-3 catalyst is the main nanocomposite we will focus on according to relevant experiments. Characterizations. Zeta potential measurements were conducted with a Brookhaven Instruments Zeta Potential Analyzer. Power X-ray diffraction (PXRD) (Shimadzu, XRD-6000 diffractometer) was employed to ascertain the crystalline structure with a Cu Kα irradiation from 10° to 70° at a rate of 7° min-1. Fourier transform infrared (FT-IR) spectra (Nexus 470 FT-IR spectrophotometer) were carried out to determine the relevant functional groups from 500 cm-1 to 4000 cm-1. The surface chemical states of the as-synthesized materials were recorded by X-ray photoelectron (XPS) spectroscopy (VG MultiLab 2000). The microstructures and morphologies of nanocatalyst were elucidated by a field-emission scanning electron microscope (FE-SEM) (JEOL, JSM-7001F) and a high-resolution transmission electron microscope (HR-TEM) (JEOL, JEM-2100). The optical properties of the nanocomposite were determined by ultraviolet−visible (UV−vis) diffuse reflectance spectra (DRS) (Shimadzu, UV2450). Brunauer-Emmett-Teller (BET) (TriStar II 3020 Surface Area and Porosity Analyzer) measurements were conducted to decide the pore size and pore distribution. The photoluminescence (PL) spectra were acquired by a Varian Cary Eclipse spectrometer with an excitation wavelength of 360 nm. The Raman spectra were obtained by a Raman spectrometer (Renishaw Invia). Electron



spin resonance (ESR) signals were acquired by a ESR spectrometer (Bruker , JES-FA200) by employing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in different aquatic environment (aqueous environment for ·OH; alcohol environment for .O2-). The photoelectrochemical tests were performed on a typical three-electrode system (CHI 660B), which employed catalysts-coated-ITO as working electrode, silver-silver chloride electrode as reference electrode and a platinum (Pt) wire as counter electrode. Photocurrent and electrochemical impedance spectroscopy experiments were carried out in different aqueous environment: phosphate buffer solution (PBS, 0.1 M, pH 7.0) for photocurrent measurements; 0.1 M KCl solution containing 5 Mm Fe(CN)63/Fe(CN) 64- for electrochemical impedance spectroscopy (EIS) measurements. Photocatalytic performance. The photocatalytic activity was assessed by photocatalytic degradation of rhodamine B (RhB) (10 mg L-1), tetracycline (TC) (20 mg L-1), and bisphenol A (BPA) (10 mg L-1). To the model pollutants solution (RhB, TC and BPA, 100 mL) was added 10 mg, 20 mg, 90 mg catalyst, respectively. The light source was a 250 W high pressure Hg lamp. Prior to reaction, the mixture containing pollutants and catalyst was subjected to vigorous stirring for 30 minutes under dark condition so as to reach adsorption-desorption equilibrium. In the process of photoreaction, air was constantly supplied by an air pump. 3 ml of the mixture was withdrawn from the photoreaction system at specific intervals, and then centrifuged to obtain the supernatant for further study. The degradation rate of RhB and TC was determined by its characteristic absorption peak at 553 and 356 nm; and high-performance liquid chromatography (HPLC) was employed to determine the removal efficiency of BPA.49

Result and discussion Formation process and morphology information. While the stable nanocomposite can be obtained by simply mixing S, N:GQDs and BOC, the formation mechanism behind it remains elusive. To this end, zeta potential measurement was performed since it could reveal the surface charge density of material.50 The results of these


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measurements (S, N:GQDs, BOC, S, N:GQDs/BOC-3) are shown in Table S1. The results indicate that S, N:GQDs are negatively charged due to its rich surface organic functional groups, and that BOC is positively charged. Due to the opposite surface charge of S, N:GQDs and BOC, the S, N:GQDs/BOC-3 material could be facilely formed by electrostatic self-assembly or Coulomb force. It is anticipated that S, N:GQDs will tightly deposit on the inner and outer structure of BOC, since the Brownian motion (reinforced by long-time, vigorous stirring) of particles in water is random.51 Moreover, the negatively-charged S, N:GQDs/BOC-3 materials could adsorb more cationic organic dyes (such as RhB and MB), which might be advantageous to enhance photocatalytic ability towards specific contaminants.52 Field-emission scanning electron microscope (FE-SEM) images demonstrate that the average lateral size of the S, N:GQDs/BOC-3 material is roughly 1 µm, and that its thickness is approximately 0.6 µm (Fig. 1a). Moreover, the rose-like microspheres consists of many thin nanosheets with a thickness of about 25 nm (inset of Fig. 1a). A feature revealed by transmission electron microscope (TEM) is that the composite possesses a hierarchical hollow structure due to the strong contrast between the dark edge and the comparatively bright center (Fig. 1b). Compared to S, N:GQDs/BOC-3 material, the modification process does not change the main morphologies of pure BOC, which retains its inherent 3D hierarchical structure (Fig. S2). Fig. 1c shows that many nanosized black dots are on the inner and outer surface of BOC, illustrating the successful incorporation of S, N:GQDs due to electrostatic self-assembly coupled with strong Brownian effect (also shown in Fig. S3). The presence of the S, N:GQDs could be further confirmed by high-resolution transmission electron microscope (HR-TEM). As seen in Fig. 1d, clear lattice spacing of 0.35 nm and 0.271 nm corresponds to the (002) facet of S, N:GQDs and (110) facet of (BiO)2CO3, respectively.53,54 The elemental mapping (Fig. S4) further demonstrates the existence of S, N:GQDs. Compositional and structural information. X-ray diffraction (XRD), X-ray photoelectron (XPS), Fourier transform infrared (FT-IR) and Raman spectroscopy are carried out to investigate the crystalline phase, chemical state, and functional group of



the nanocatalyst. Fig. 2 exhibits a series of sharp and intense peaks, indicating the highly crystalline state of the catalyst. These peaks correspond precisely to the different facets of tetragonal (BiO)2CO3 (JCPDS 41−1488). Moreover, the slightly more intense peak at 26.0° of the S, N:GQDs/BOC-X samples compared with pure BOC can be easily discerned probably due to the superposition effect of the (002) facet (~26.0°) of S, N:GQDs and the (004) facet of pure BOC, demonstrating the existence of S, N:GQDs.12 The composition and surface chemical state of the S, N:GQDs/BOC-3 material is further elaborated by XPS. Survey spectrum indicates the presence of relevant elements of the samples; moreover, the S, N:GQDs/BOC-3 material possesses distinct sulfur (S) and nitrogen (N) peaks. It suggests the successful introduction of S, N:GQDs into the pure BOC matrix (Fig. 3a). As can be seen from Fig. 3b, the peaks at roughly 164.3 eV and 159.0 eV of pure BOC can be assigned to the Bi 4f5/2 and Bi 4f7/2 of Bi3+; and a slightly red shift of Bi 4f peak could be found.55 The O 1s peak of the composite can be deconvoluted into three peaks at 531.3 eV, 530.5 eV and 529.7 eV, corresponding to the O-H, O=C and O-Bi bonds, respectively (Fig. 3c). Likewise, all peaks of oxygen-related bonds of S, N:GQDs/BOC-3 material are shifted to higher binding energy. The red shifts of Bi 4f and oxygen-related peaks might be caused by surface interaction between the abundant organic functional groups on S, N: GQDs and the dangling bonds of BOC.40 However, the C 1s peaks are negatively shifted possibly due to the enhanced electron density of C atoms (Fig. 3d). The XPS peak of the S 2p centered at 164.3 eV is ascribed to S 2p3/2 of thiophene (Fig. 3e).56 The high-resolution N 1s spectra further prove the existence of N element (Fig. 3f). The results above will suffice to confirm the successful introduction of S, N:GQDs. The functional groups of the catalyst are evaluated by FT-IR spectra (Fig. 4). The broad and medium band at 3419 cm-1 can be assigned to the stretching vibrations of O-H or N-H presumably due to existence of S, N:GQDs. The weak absorption bands at 1560 cm-1 can be ascribed to the existence of protonated imine (=NH+).57 Moreover, the stretching modes at 1067 cm-1 and 666 cm-1 can be attributed to different sulfur-based functional groups (C=S and C-S, respectively).48 These results are


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indicative of strong coupling of S, N:GQDs in the composite. The characteristic absorption band at 548 cm-1 is formed in all samples, evidencing the existence of Bi-O. In addition, several internal vibrations of “free” carbonate ion can be found: the out-of-plane bending mode (846 cm-1); the corresponding anti-symmetric vibration (1469 and 1389 cm-1). It further illustrates the existence of BOC. Apart from that, several strong peaks (160 cm-1 and 361 cm-1) could be attributed to the "lattice vibrations" of BOC from the Raman spectra (Fig. S7). Compared with bare BOC, the nanocomposite displays two distinct peaks at roughly 1384 cm-1 and 1577 cm-1, which exemplifies the disordered (D) band and graphitic (G) band of graphene. The ratio of ID/IG equals 0.89, demonstrating the highly graphitic nature of the S, N:GQDs. Optical properties. Optical properties of the samples across the UV−vis region are rated by diffuse reflectance spectra (DRS) in Fig. 5. The pure BOC exhibits relatively strong absorption from 200 nm 370 nm, which arises from its intrinsic band-to-band transition (Fig. 5a). Due to the indirect nature of BOC, the band gap (Eg) of the BOC can be derived from the Tauc plot of (αhν)0.5 vs photon energy (hν). It can be seen from the plot that pure BOC have a large band gap of 3.17 eV (Fig. 5b). In addition, the absorption intensity in the region of 370 nm-800 nm is dramatically increased due to the modification of S, N:GQDs. Thus, it may induce more carriers production because of increased light-harvesting ability, thereby generating more ROS participating in photocatalytic reaction. Photocatalytic tests and stability/reusability. The photocatalytic ability and stability of the catalyst before and after experiments are assessed by consecutive experiments. The RhB adsorption ability of different catalysts is shown in Fig. S8. The results indicate that the adsorption performance of samples might be enhanced due to π–π interactions between S, N:GQDs and organic molecule, which is of benefit to degradation tests. As shown in Fig. 6a, the modified BOC catalysts all display better photocatalytic removal of tracer dyes (RhB) than that of bare BOC, emphasizing the important roles of S, N:GQDs in environmental remediation. Notably, the S, N:GQDs/BOC-3 sample is superior to other modified or unmodified samples due to the introduction of proper amount of cocatalyst (S, N:GQDs), since too much guest



cocatalyst might induce adverse shielding effect, impairing host BOC’s ability to absorb light and covering the active sites for photocatalysis by S, N:GQDs agglomeration. Fig. 6b displays the photocatalytic kinetics fit of RhB decomposition based on Langmuir−Hinshelwood model (pseudo-first-order model), which assumes that the concentration of pollutants is relatively low, and that adsorption-desorption equilibrium must be reached quickly. Similarly, the degradation efficiency of RhB is much improved after adding S, N:GQDs. Specifically, the degradation rate of the S, N:GQDs/BOC-3 is approximately 2 times better than that of unmodified catalyst. The photoreactivity of the as-obtained samples is further evaluated by decomposition of TC. TC is a kind of persistent antibiotics contaminant, which is ubiquitous in waters and lands due to excess use for medical treatment. The accumulated amount of TC might result in bacteria resistance, microorganism selection failure and even health problems of human beings. Therefore, it is of great importance to reduce the amount of TC under ambient environment. Herein, the S, N:GQDs/BOC materials are tentatively utilized to remove TC; and we can also prove the importance of S, N:GQDs from another angle. From Fig. 6c, the removal efficiency is comparatively low when employing pure BOC as a catalyst. The degradation rate of TC reaches 50% within 60 minutes when using bare BOC; however, the degradation efficiency is dramatically enhanced after adding S, N:GQDs. Identically, the S, N:GQDs/BOC-3 sample displays the best photocatalytic oxidation ability towards TC (Fig. S9). More than 80% of TC could be eliminated in a relatively short time span of 120 minutes. It is much better than previously reported BOC-based catalyst towards TC degradation, demonstrating the important roles of S, N:GQDs in a photocatalytic process.21,58 To test the non-selective oxidation ability of the catalysts, we further carry out BPA degradation experiments. BPA, a common kind of endocrine-disrupting chemicals (EDCs) or metabolism-disrupting chemicals (MDCs), is threatening people’s health due to mounting amount of it on this planet. It can alter the energy homeostasis and affect several organs of human being after exposure to it, thus leading to a series of diseases, such as obesity and diabetes.59 Then BPA is subjected


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to photocatalytic reaction in order to address the serious problem. The results also indicate the superior catalytic activity compared with pure catalyst (Fig. 6d). The results above suggest that introducing S, N:GQDs might be an effective and easy way to increase the photocatalytic property of BOC. To evaluate the samples’ ability to be used in everyday life or industries, several experiments regarding repeated tests and physical characterizations of the used catalyst are conducted. According to Fig. 7a, the degradation rate shows no obvious change even after 4 successive cycles for RhB elimination, demonstrating its excellent ability in avoiding photocorrosion (Fig. 7a). Moreover, the stability or reusability of the catalyst is also verified by the nearly same crystalline structure before and after reaction, further indicating the good stability of the catalyst (Fig. 7b). The SEM and TEM images of the S, N:GQDs/BOC-3 catalyst after 4 consecutive cycles are shown in Fig. S10. The morphologies and microstructures of the used samples are nearly the same compared with the fresh catalyst, except that few broken nanosheets are present in the picture probably due to the successive washing process for reuse. Physical and chemical characteristics of S, N:GQDs. To investigate the generation, mobility and recombination kinetics of photoinduced electron in modified BOC, photo-luminescence spectroscopy (PL), electrochemical impedance spectroscopy (EIS), and photocurrent–time experiments are carried out. Fig. 8a shows that all samples possess a strong emission peak at ca. 470 nm as well as few weak shoulder peaks under excitation at 360 nm, displaying a Stokes shift of 0.75 eV. This phenomenon could be found in other reports.20,60 What’s more, the S, N:GQDs/BOC-3 material displays an obvious quenching (decreased emission peak intensity) compared with BOC. This phenomenon usually suggests an increasing probability of hole-electron pairs separation, which is good for photocatalytic applications. The charge carriers separation behavior is further evaluated by photoelectrochemical analysis. Fig. 8b displays amperometric transient photocurrent response of S, N:GQDs/BOC-3 and BOC samples. All samples show a sharp current increase when the light source is switched on due to the intrinsic photoresponse of a semiconductor. After turning on the light source, the photocurrent of S,



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N:GQDs/BOC-3 is strikingly higher than that of bare BOC, i.e. 27 times higher than that of pure BOC. Moreover, the photocurrent can be easily reproduced in successive turn-on/turn-off tests, evidencing the enhanced charge carriers separation and long-term stability of the catalysts. The results of photocurrent–time (I–t) test are further confirmed by electrochemical impedance spectroscopy (EIS). According to Fig. 8c, the semicircle arc of S, N:GQDs/BOC-3 in the Nyquist plot is, as expected, smaller than that of pure BOC, indicating strong inhibition of interface carriers recombination or enhanced carriers density. Based on the results above, S, N:GQDs might serve as an electron sink or acceptor, which will facilitate spatial separation of photo-induced charge carriers and may be favorable for enhanced ROS generation. Besides excellent physical characteristics of S, N:GQDs, viz charge carriers transfer property, the unique peroxidase like activity of S, N:GQDs should also be given enough attention. The measured value of valence band (VB) of pure BOC is 2.13 eV from extrapolation to the x-axis of valence-band XPS spectra (Fig. S11). Considering that the typical difference between exact value of VB and measured value from valence-band XPS spectra to be roughly 0.63 eV, the valence band maximum (VBM) is determined to be ca. 2.76 eV.25 Notably, the measured value of VBM of the hybrid catalyst is also 2.13 eV (Fig. S11). The results suggest that the introduction of S, N:GQDs does not alter the electronic properties of BOC matrix, and that S, N:GQDs may only serve as a surface decoration material. The conduction band minimum (CBM) could be easily estimated to be –0.41 eV by subtracting band gap (Eg) from VBM (EVB - Eg). Given the CBM of BOC is more negative than the reduction potentials of O2/.O2- (−0.046 V vs NHE) and O2/H2O2 (0.68 V vs NHE), H2O2 can be in theory generated either by a multistep one-electron (Eqs (1)-(3)) or a one-step two-electron reduction reaction (Eq. (4)) by pure BOC and S, N:GQDs/ BOC-3 catalysts.33,61

O2 + e- → .O2-

(1)

O2- + H+ → .O2H

(2)

O2H + H+ + e- → H2O2

(3)

. .


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O2 + 2H+ + 2e- → H2O2

(4)

To confirm the hypothesis, we measure the concentration of the H2O2 in the reaction system over the entire degradation time. Compared with pure BOC, the S, N:GQDs/BOC-3 composite obviously generates much less H2O2 (Fig. 9a). Although pure BOC can produce ·OH by one-step one-electron reduction of H2O2 ((Eq. (5)), the efficiency of this reaction is rather low without proper catalyst, such as Fe 2+.62 On the contrary, the concentration of the H2O2 is relatively low in S, N:GQDs/BOC-3 sample, which indicates the adverse accumulation of H2O2 is effectively prohibited. This result may demonstrate the unique H2O2 decomposition property of S, N:GQDs.

H2O2 + e- →·OH + OH-

(5)

Due to complicated scenarios (such as many redox reactions and charge carriers recombination/ separation processes) in a photochemical system, we further conduct degradation tests of RhB with proper addition of H2O2 under dark condition in order to clarify the enzymatic-like catalysis of S, N:GQDs. Similar to natural enzymes, the nanocarbon peroxidase, such as GQDs and carbon quantum dots (CQDs), should be also operated in a suitable substrate concentration. Therefore, we firstly conduct concentration-activity experiments; and the results confirm that introducing 3 mL of H2O2 into the reaction system might be the best for the catalytic activity (Fig. S12a). Fig. 9b reveals that there is no notable change (15%) can be achieved in S, N:GQDs/BOC-3, evidencing superior peroxidase mimetic property of S, N:GQDs possibly arising from its richer ketonic carbonyl groups than pure BOC (enhanced by 5% according to relevant XPS integration).63 Moreover, we employ fluorescence spectroscopy to further ascertain and quantify the active species involved in this process. The S, N:GQDs/BOC-3 material could effectively decompose H2O2 to obtain ·OH, as evidenced by much stronger fluorescence at 425 nm under excitation at 315 nm compared with other two



samples (Fig. S13). Fig. 9c clearly indicates the enzymatic-like catalysis of S, N:GQDs due to the fact that the fluorescence difference (Δ PL intensity) of S, N:GQDs is nearly one order of magnitude higher than that of pure BOC and absence of catalyst. Considering the relatively efficient degradation property of S, N:GQDs/BOC-3 with or without light, a question naturally arises: Can we incorporate the enzymatic-like catalysis for photocatalytic applications so as to save electrical energy? To answer the question above, we devise a photochemical system where light illumination is given for 30 minutes followed by turning off the light source and adding 3 mL of H2O2 simultaneously. According to Fig. 9d, the S, N:GQDs/BOC-3 sample displays best RhB removal ability under light irradiation because of increased light-harvesting ability, enhanced charge carriers transport and H2O2 decomposition ability. After experiencing light illumination for 30 minutes, the reaction system is subjected to cease of irradiation and 3 mL of H2O2 addition. Under dark condition, the degradation ability of S, N:GQDs/BOC-3 is still far better than that of control materials. Considering the light equipment is switched off, the elimination ability of S, N:GQDs/BOC-3 is solely attributed to the remarkable peroxidase mimics function of S, N:GQDs. Furthermore, we employ several methods to appraise the catalyst’s stability under such switch-on/switch-off condition. As shown in Fig. S14, the XRD patterns of used S, N:GQDs/BOC-3 catalyst is as same as the fresh one. However, the degradation efficiency is reduced by 7% possibly because of the poisoning effect of successive addition of H2O2. Based on the results above, the embedding of S, N:GQDs would improve the electrical property of pure BOC and induce pronounced enzymatic-like catalysis of H2O2 decomposition. Mechanism analysis. To determine which active species are essential for photocatalytic degradation (photocatalysis), we successively add BQ, IPA and EDTA-2Na into the reaction system under light illumination. After introducing BQ (scavenger for .O2-) or EDTA-2Na (scavenger for h+), the degradation efficiency is greatly inhibited (Fig. S15). This illustrates .O2- and direct hole oxidation are important in the photocatalytic process. Moreover, adding IPA (scavenger for ·OH) will also undermine the degradation ability of S, N:GQDs/BOC-3 material. These


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results show that .O2-, h+ and ·OH determine RhB elimination under light irradiation. ESR spectra are employed to further reveal the degradation mechanism under light irradiation. As shown in Fig. 10a, the pure BOC shows characteristic signals of DMPO-.O2-; however the intensity of DMPO-.O2- of S, N:GQDs/BOC-3 is greatly enhanced, signaling the good electron transfer property for the one-electron reduction of molecular oxygen (Eq. (1), Fig. 10c). The characteristic 1:2:2:1 quartet pattern of DMPO-·OH can be observed in pure BOC catalyst according to Fig. 10b.64 Similarly, the intensity of nanocomposite is also higher after introducing S, N:GQDs (Fig. 10d). Considering the VB edge and CB edge of pure BOC is 2.76 eV and –0.41 eV based on previous analysis, the holes on the VB and electrons on the CB can thermodynamically oxidize OH- and reduce molecular oxygen to generate ·OH (·OH/OH-, 2.38 V vs NHE) and .O2- (O2/.O2-, -0.046 V vs NHE) , respectively. According to the results of photocatalysis and enzymatic-like catalysis, the different carriers formation and transformation paths in persistent pollutants degradation of bare BOC and S, N:GQDs/BOC-3 under different situations are presented in Fig. 11. Under light illumination, the pure BOC cannot produce sufficient specific ROS (such as ·OH and .O2-) to drive photocatalytic oxidation reactions due to fast carriers recombination (Fig. 11a). Even worse, it may generate excess H2O2, which not only is inefficient for contaminants degradation, but also may affect the catalyst itself. However, after S, N:GQDs modification, the mobility of photoinduced electrons is greatly enhanced due to the excellent electron transfer of S, N:GQDs, thus finally leading to more powerful oxidants for photochemical reaction (Fig. 11b). Interestingly, loading of S, N:GQDs opens another door to produce ·OH by enzymatic-like catalysis (without electron) of H2O2 decomposition. The importance of S, N:GQDs is further emphasized in removing contaminant under dark condition because of its special peroxidase mimetic property, thereby opening up the possibility for all-day-working photocatalysis (Fig. 11c,d).

Conclusion



In conclusion, S, N:GQDs/(BiO)2CO3 hollow microspheres photocatalyst was constructed by a simple electrostatic self-assembly method. The photocatalytic property for removing persistent pollutants (RhB, TC and BPA) is greatly enhanced. The increased photooxidation ability can be ascribed to effective and increased ROS production by electron–hole pairs through introduction of S, N:GQDs. The embedded S, N:GQDs can successfully suppress carriers recombination and improve the transformation of H2O2 to non-selective ·OH reactive radical by enzymatic-like catalysis, which altogether is beneficial for a photocatalytic process. This study may offer some insights about incorporating enzymatic-like catalysis into a hierarchical photocatalyst-based system, which eventually is favorable for environmental and energy applications under different situations.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional experimental data including zeta potentials of the samples in deionized water, comparison with other GQDs-based photocatalysts, the formation mechanism of pure BOC, SEM and TEM images, N2 adsorption−desorption experiments, Raman spectra, degradation ability, stability tests, valence-band XPS spectra, relevant enzymatic-like co-catalysis experiments of pure BOC and S, N:GQDs/BOC-3 nanocomposite, various characterizations of S, N:GQDs.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21476098, 21471069, 21576123, and 21676128).


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TOC (For Table of Contents Use Only)

Synopsis (BiO)2CO3 hollow microspheres functionalized with S, N:GQDs exhibits better photocatalytic ability because of enhanced carriers separation and enzymatic-like catalysis.


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Fig. 1. (a) SEM image, (b-c) TEM images and (d) HRTEM images of S, N:GQDs/BOC-3 sample.



: S, N: GQD (002) : BOC (004) (011)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10

S, N: GQDs/BOC-5 S, N: GQDs/BOC-3 S, N: GQDs/BOC-1 BOC

(110)

/

(002)

20

30

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40

50

2-Theta (degree)

60

70

Fig. 2. XRD patterns of pure BOC and S, N:GQDs/BOC composites with different S, N: GQDs loading.


Bi4f

Survey

Bi5d

C1s S2s

Intensity (a.u.)

O KLL

Bi4p3

S, N: GQDs/BOC-3

N1s

(a)

BOC

1200

1000

800

600

400

200

0

Binding Energy (eV)

(b) Bi 4f S, N: GQDs/BOC-3

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O1s Bi4d3 Bi4d5

Page 29 of 43

BOC

170

165

160

Binding Energy (eV)


155


(c) O 1s

O=C O-Bi

O-H

Intensity (a.u.)

S, N: GQDs/BOC-3

BOC

535

530

525

Binding Energy (eV)

S, N: GQDs/BOC-3

(d) C 1s

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 43

BOC

290

288

286

Binding Energy (eV)


284

282

Page 31 of 43

Intensity (a.u.)

(e) S 2p

S, N: GQDs/BOC-3

168

166

164

162

Binding Energy (eV)

(f)

N 1s

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S, N: GQDs/BOC-3

404

402

400

398

396

Binding Energy (eV) Fig. 3. XPS spectra of pure BOC and S, N:GQDs/BOC-3 samples: (a) survey of the sample (b) Bi 4f, (c) O 1s, (d) C 1s, (e) S 2p and (f) N 1s.



=NH+

O-H or N-H

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C=S

Page 32 of 43

C-S

S, N: GQDs/BOC-5

S, N: GQDs/BOC-3

S, N: GQDs/BOC-1

BOC

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1) Fig. 4. FT-IR spectra of BOC and S, N:GQDs/BOC materials.


Page 33 of 43

Absorbance (a.u.)

(a)


200

300

400

500

600

700

800

Wavelength (nm)

3

(b)

(h)0.5/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 BOC

1 3.17 eV

0 2

3

4

5

6

h Fig. 5. UV-Vis diffuse reflectance spectra (DRS) of pure BOC and S, N:GQDs/BOC materials and the corresponding Tauc plot of the pure BOC.



(a)

1.0

C/C0

0.8 0.6 BOC S, N: GQDs/BOC-1 S, N: GQDs/BOC-3 S, N: GQDs/BOC-5 without catalyst

0.4 0.2 0

30

60

90

120

Time (min)

(b) 1.6

-ln(C/C0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 43


1.2 0.8 0.4 0.0 0

30

60

90

Time (min)


120

Page 35 of 43

1.0

(c)

C/C0

0.8 0.6 BOC S, N: GQDs/BOC-1 S, N: GQDs/BOC-3 S, N: GQDs/BOC-5 without catalyst

0.4 0.2 0

30

60

90

120

Time (min)

1.0

(d)

0.8

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4 BOC S, N: GQDs/BOC-3 without catalyst

0.2 0

1

2

3

4

5

6

Time (h) Fig. 6. Photocatalytic performance curves of degradating (a) RhB, (b) kinetic fit of RhB degradation, (c) TC and (d) BPA.


Page 36 of 43

(a)

80

60

40

20

0 1

2

3

4

Cycle times

(b)

Before photocatalysis After photocatalysis

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

degradation efficiency (%)


10

20

30

40

50

60

70

2-Theta (degree) Fig. 7. (a) Cycling experiments of RhB degradation by S, N:GQDs/BOC-3 samples and (b) Comparison between the XRD patterns of S, N:GQDs/BOC-3 samples after 4 consecutive cycles of photocatalytic degradation.


Page 37 of 43

(a) Intensity (a.u)

BOC S, N: GQDs/BOC-3

390

420

450

480

510

Wavelength (nm)

1.0

S, N: GQDs/BOC-3 BOC

(b)

0.6 0.4

Light on

Current (A)

0.8

Light off

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2 0.0 50

100

150

200

Time (s)


250


300

(c)

S, N: GQDs/BOC-3 BOC

240

-Z'' (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 43

180 120 60 0 54

108

162

216

270

Z' (Ohm) Fig. 8. (a) steady-state photoluminescence (PL) spectra, (b) transient photocurrent responses (c) electrochemical impedance spectra of pure BOC and S, N:GQDs/BOC-3 materials.


Page 39 of 43

(a) BOC without catalyst S, N: GQDs/BOC-3 dark control

[H2O2] (μM)

6

4

2

0 0

30

60

90

120

90

120

Time (min)

(b) 1.0 0.8

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BOC without catalyst S, N: GQDs/BOC-3

0.6 0.4 0.2 0

30

60

Time (min)



PL intensity (a.u.)

(c)

BOC

S, N: GQDs/BOC-3

without catalyst

(d) 1.0 0.8

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 43

0.6 0.4

light on

light off

0.2

BOC without catalyst S, N: GQDs/BOC-3

H2O2 addition

0.0 0

30

60

90

120

150

180

Time (min) Fig. 9. (a) H2O2 production over BOC and S, N:GQDs/BOC-3 materials, (b) RhB degradation under dark condition (addition of 3 ml H2O2), (c) Relative PL intensity from fluorescence spectra of different samples, and (d) All-day-active photocatalytic performance of S, N:GQDs/BOC-3 samples.


Page 41 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) BOC

superoxide radical Light on

Dark

318

320

322

324

326

328

B/mT

(b) BOC

hydroxyl radical Light on

Dark

317.8

317.9

318.0

318.1

318.2

B/mT


318.3

318.4


(c) S, N: GQDs/BOC-3

Page 42 of 43

superoxide radical Light on

Dark

318

320

322

324

326

328

B/mT (d) S, N: GQDs/BOC-3

hydroxyl radical Light on

Dark

317.8

317.9

318.0

318.1

318.2

318.3

318.4

B/mT Fig. 10. ESR spectra of the (a and c) DMPO-.O2- in methanol dispersion and (b and d) DMPO-·OH in aqueous dispersion in the presence of (a and b) pure BOC and (c and d) S, N: GQDs/BOC-3.


Page 43 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 11. The proposed transfer and transformation routes of reactive radicals under light irradiation (a, b) or dark condition (c, d) over pure BOC (a, c) and S, N: GQDs/BOC-3 (b,d) materials.


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