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Lotus–leaf–derived activated carbon supported nano–CdS as energy–efficient photocatalysts under visible irradiation Hai-Bo Huang, Yu Wang, Wen-Bin Jiao, Feng-Ying Cai, Min Shen, Shungui Zhou, Hai-Lei Cao, Jian Lü, and Rong Cao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01021 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 28, 2018

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ACS Sustainable Chemistry & Engineering

Lotus–Leaf–Derived

Activated

Carbon

Supported

Nano–CdS

as

Energy–Efficient

Photocatalysts under Visible Irradiation Hai–Bo Huang†,‡, Yu Wang†, Wen–Bin Jiao†, Feng–Ying Cai†, Min Shen‡,§, Shun–Gui Zhou†, Hai–Lei Cao†,*, Jian Lü†,‡,* and Rong Cao‡,* †

Fujian Provincial Key Laboratory of Soil Environmental Health and Regulation, College of

Resources and Environment, Fujian Agriculture and Forestry University, No. 15 Shang Xia Dian Road, Fuzhou 350002, P.R. China; ‡State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, No. 155 Yang Qiao Xi Road, Fuzhou 350002, P.R. China; §The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 313 Ferst Drive, Atlanta, Georgia 30332, USA.

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

ABSTRACT: Lotus–leaf–derived activated carbon materials (denoted as LAC–T) were fabricated at different temperatures (T = 600, 700 and 800 ºC) that resulted in carbonaceous materials with various microstructures and porosity. BET surface area of LAC–T increased from 1184 to 1807 m2·g−1 with activation temperature increasing from 600 to 800 ºC. These microporous carbonaceous materials were subsequently advanced as ideal platforms for cadmium sulfide (CdS) composite photocatalysts, through the deposition of nano–CdS precursors on LAC–T supports (CdS@LAC–T). It was revealed that the CdS@LAC–T nanocomposites displayed enhanced photocatalytic efficiency, in comparison with the nano–CdS, toward the degradation of various organic dyes under visible light. More specifically, CdS@LAC–800, prepared from a carbonaceous support with the highest BET, gave the best photocatalytic efficiency. Estimated band gap energy for CdS@LAC–800 (2.01 eV) was considerably lower than that of nano–CdS (2.22 eV), which was among the lowest band gap energies observed for CdS photocatalysts. Band gap narrowing observed for nanocomposites indicated noticeable interface interaction between nano–CdS and the carbonaceous supports, and excellent light harvesting ability. Furthermore, the improved photocatalytic activity shown by the best performing CdS@LAC–800 was achieved thanks to the effective production of catalytically active species (h+, O2•−, OH• and H2O2), which were demonstrated by means of extensive mechanism study. Overall, the highly ordered and porous carbonaceous support accounted for the outstanding photocatalytic efficiency of CdS@LAC–800 by boosting synergistically the substrate accessibility, the solar energy harvesting efficiency, and the electron–hole separation in this photocatalytic system. KEYWORDS: Biomass, Activated carbon, Cadmium sulfide, Composites, Photocatalysis

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INTRODUCTION Biomass–derived black carbon (BC, known also as biochar) production, which is typically achieved from the carbonization of agricultural and garden wastes at elevated temperatures,1 has been regarded as a key strategy for biomass carbon sequestration and recycle, and thus to provide a source of renewable energy.2–4 Current investigation is primarily focused on the structure and physiochemical property of BCs as a function of pyrolysis conditions.5–7 The prominent property of BCs, i.e. high specific surface area and porosity, hierarchical porous structures, abundant surface functional groups and mineral compositions, endows them with superior adsorption capacity as efficient and eco–friendly remediation materials.8–10 Furthermore, the BCs are intrinsically adaptable to couple with nanoparticles (NPs) of metals and metal oxides, by which their capability in contaminants treatment is largely enhanced due to the optimization of pore structure and functionality, as well as the implantation of potential active sites.11–13 More recently, BCs have been advanced as ideal catalyst carriers thanks to their particularly excellent heat and acid/alkali resistance that are favorable for catalytic reactions.14 Unsurprisingly, many BCs supported material catalysts have shown enhanced catalytic efficiency toward the degradation of organic pollutants that are of current environmental concerns.15–18 Of special research interest, the advance of practically applicable photocatalysts for effective use of solar energy has become a viable platform in environmental remediation,19–21 in view of the unprecedented demands on sustainable resources and renewable energy supplies.22–24 To this end, a typical and important semiconductor,

cadmium

sulfide

(CdS),

has

received

enormous

research

attention

in

visible–light–driven photocatalysis.25–27 The suitable band gap of CdS ensures successful activation under visible irradiation28 that is superior for applications in photocatalysis.29–31 However, the

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development of CdS photocatalysts has been largely plagued because of photocorrosion from which toxic Cd2+ ions are released to increas environmental risks. In this context, BCs can be ideal material supports for composite CdS photocatalysts to achieve favorable transport and contact of organic substrates due to the possible interplay between the supports and catalytically active sites, and more importantly, to avoid catalyst deactivation, particle aggregation and photocorrosionin aqueous systems.32–34 Besides, BCs are able to photogenerate reactive oxygen species (ROS), for example hydroxyl radicals (•OH) and singlet oxygen (1O2), that may facilitate photocatalytic reactions.35 In the light of the advantages of BCs, specific research focusing on how morphological and structural changes of the carbonaceous materials determine the property of attached nanoparticles (NPs). This study seeks to bridge the gap in knowledge of relationship between structural modification of BCs and their synergic effects with NPs, aiming at functional optimization of photocatalysis under visible irradiation. For this purpose, we have developed for the first time a family of facile and low–cost nanocomposites by loading cadmium CdS NPs (nano–CdS) on programmed BCs originated from lotus leaves (Nelumbo). The lotus–leaf–derived carbons (LCs) are expected to retain the hierarchical and porous surface morphology after the high temperature carbonization.36,37 In addition, the activation of LCs into activated carbons (LACs) is known to create well–developed microstructures and porosity, as well as surface functionality. In this work, lotus–leaf–derived activated carbon materials (LAC–T) have been well programmed through chemical activation at various temperatures (T = 600, 700 and 800 ºC). The microporous carbonaceous materials are further advanced as viable supports for composite cadmium sulfide (CdS) photocatalysts due to their remarkable Brunauer–Emmett–Teller (BET) surface areas (1184 to 1807 m2·g−1) derived from high temperature activation (600 to 800 ºC). The resultant nanocomposites

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(denoted as CdS@LAC–T) have shown generally improved photocatalytic capacity toward the degradation of various dye molecules under visible irradiation, in comparison with nano–CdS. Specifically, the CdS@LAC–800, prepared from a carbonaceous support with the highest BET, exhibits a significantly decreased band gap energy (ca. 2.01 eV versus 2.22 eV for nano–CdS) and is among the narrowest band gap for CdS photocatalysts. Further study confirms that CdS@LAC–800 possesses excellent photocatalytic efficiency that is 50 times higher than that for nano–CdS, toward rhodamine B (RhB) degradation. Moreover, CdS@LAC–800 is applicable to other organic dyes with inspiringly high photocatalytic efficiency. Mechanism study indicates that the enhanced photocatalytic activity shown by CdS@LAC–800 is originated from the production of various active species i.e. h+, OH•, O2•− and H2O2, as well as the highly ordered and porous carbonaceous support that accounts for the synergistic enhancement with respect to the accessibility of organic substrates, the efficiency of solar energy harvesting, and the transportation of electrons.

EXPERIMENTAL Fabrication of LAC Materials. The LAC materials were fabricated via the carbonization of lotus leaves, and activated with KOH under various temperatures. In a typical procedure, the lotus leaves were washed with ultrapure water, dried overnight in oven (50 °C), and grounded to gain fine powder samples with particle sizes below 100 mesh. Pre–treated lotus leave powders were carbonized at 600 ºC for 4 h in a tubular furnace under N2 atmosphere (5 ºC min–1). The as–prepared LACs were activated for 1 hour under nitrogen atmospheric environment at 600, 700 and 800 ºC, respectively, with three equivalents of KOH (named as LAC–T; T = 600, 700 and 800). LAC–T samples were collected to wash with diluted hydrochloric acid (HCl, 0.1 M) to remove excess KOH

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and ash contents in the materials until pH of the filtrates reached approximately 7.0. Further washing with ultrapure water was performed before drying overnight in oven (50 °C).38,39 Preparation of Nano–CdS. The nano–CdS was prepared from a modified procedure reported in the literature.40 Cd(CH3COO)2·2H2O (533 mg, 2.0 mmol) was dissolved in ethanediamine (60 mL) under magnetic stirring, and thioacetamide (105 mg, 1.4 mmol) was then added to allow stirring continuously for 1 h. The obtained solution was sealed in a solvothermal autoclave and kept in oven (180 °C) for 5 h. After slow cooling the reaction system to ambient temperature, the orange powder sample was collected collected to wash with ultrapure water and absolute ethanol, respectively, and collected by using centrifuge and drying overnight in oven (50 °C). Fabrication of CdS@LAC–T Nanocomposites. LAC–T (200 mg) and nano–CdS (100 mg) were added in ultrapure water (30 mL) with magnetic stirring for 1 hour and and dried overnight in oven (105 °C). Then the above mixture was calcinated at 300 ºC for 1 h in a tubular furnace under N2 atmosphere (5 ºC min–1), and washed with ultrapure water and dried overnight (50 °C) to afford CdS@LAC–T nanocomposites. Photocatalytic Reaction. A portion of nanocomposite (10 mg) and dye solution (100 mL, 40 mg L–1) was added in a Pyrex glass vessel (200 mL) with simultaneous shaking. The above mixture was left for 30 min in dark then exposed to visible light. Photocatalytic experiments were monitored by UV–vis measurements of the characteristic absorbency of dye molecules after certain time of intervals. Dye residues absorbed on photocatalysts were extracted by using ethanol for three times into a constant volume as used for initial dye solutions and the concentration of dye residues was determined as detailed above. Degradation efficiency was estimated by the following equation: D = C / C0 × 100%

(1)

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where D, C0 and C represented for degradation efficiency, initial, and tested characteristic absorbency of organic dyes, respectively. Rate constant (κ) was estimated by the following equation: ln (C0 / C) =κt

(2)

Photocatalyst Stability Test. Stability of the photocatalyst (CdS@LAC–800) was studied by repeated cycles of photocatalytic reactions at set conditions. Between two consecutive runs of a cycling reaction, the photocatalyst was recyled by using centrifuge and washed with ultrapure water and ethanol for several times until UV absorbance of the filtrated was nearly invisible, before drying at 50 °C in oven. To further examine the stability of CdS@LAC–800, the nanocomposite was separated at the end of a cycling reaction, washed and dried as stated above and used for further XRD measurements.

RESULTS AND DISCUSSION Characterizations. Surface morphology of lotus–leaf–derived carbon (LC) and activated carbon (LAC) materials was investigated by means of the scanning electron microscopy (SEM) technique. SEM images of the LC material exhibit considerably rough and hierarchical surface that is derived from the pristine lotus leaves (Figure 1a).39 Upon chemical activation, uniform surfaces with patches of nanosized particles are observed for the LAC materials (Figure 1b, 1c and 1d). Moreover, higher activation temperature seems to favor the formation of highly ordered surface structures (Figure 1c and 1d). In order to evaluate the porosity of LAC–T (T = 600, 700 and 800), N2 adsorption–desorption isotherms were recorded, which revealed classic type–I adsorption behaviors (Figure S1a). Specific surface area of the LAC materials according to Brunauer–Emmett–Teller (BET) analyses were calculated to be in the range of 1184 (LAC–600), 1508 (LAC–700) and 1807

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(LAC–800) m2·g−1 (Figure 1e), respectively. The results clearly indicate that activation temperature enhances, by a large margin, the surface area of LAC–T materials by improving their microstructures and porosity.

Figure 1. SEM images of LC and LAC (a–d; scale bar: 1 µm, inset 100 nm) and (e) physiochemical parameters of LC, LAC–T and CdS@LAC–T nanocomposites. The as–prepared microporous carbonaceous materials were thus applied to develop a series of novel nanocomposites (CdS@LAC–T) based on nano–CdS precursors and LAC–T supports. Compositions of CdS@LAC–T nanocomposites were first demonstrated by elemental analysis and ICP measurements, which suggested consistent CdS contents varying from ca. 31% to ca. 34% (Figure 1e). XRD patterns of the nano–CdS and CdS@LAC–T nanocomposites exhibit similar characteristics that were indexed to mixed–phased cubic and hexagonal CdS (JCPDS 10–0454 and 41–1049; Figure 2a).41,42 It was found that the crystalline of nano–CdS in nanocomposites generally increased when the LAC support was carbonized at a higher activation temperature. SEM and TEM

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images of CdS@LAC–T nanocomposites further confirmed the coexistence of CdS and LAC domains (Figure S2, S3 and S4). Moreover, rational decreases of BET surface areas were observed for CdS@LAC–T nanocomposites by 32% to 38% (Figure 1e, 2b and S1a), which was inconsistent with the CdS loadings determined by ICP results (Figure 1e). Pore size distribution (PSD) analyses revealed that major pores with mean diameters of ca. 1.6 nm existed in the CdS@LAC–T nanocomposites (Figure 1e), which suggested the structural feature of dominant micoporosity (Figure S1b).

Figure 2. (a) XRD, (b) N2 adsorption/desorption isotherms, and (c) DRS spectra (Inset: K–M plot) of nano–CdS and CdS@LAC–T nanocomposites; and (d) XPS survey spectra of nanocomposites. Surface chemical compositions, valence state of elements, as well as surface functionality of CdS@LAC–T nanocomposites were characterized by XPS analyses (Figure 2d). Taking the CdS@LAC–800 as a typical example, the doublet peaks at 412.3 eV (Cd 3d3/2) and 405.6 (Cd 3d5/2) in high–resolution XPS spectrum were characteristics of Cd(II) species (Figure 2d). Peaks at 163.2 eV (S 2p1/2) and 162.0 (S 2p3/2) were from the spin–orbit doublet of S2–(Figure 2d). The peak at 9



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284.5 eV was the characteristics of sp2–carbon species (i.e. C=C), and peaks at 285.4, 287.5, and 288.9 eV were tentatively ascribed to oxygenated carbon species (C–O, C=O and COOR). The peak at a higher binding energy (291.7 eV) was the characteristics of π–π* (Figure 2d). These results demonstrated the surface of LAC supports was functionalized.43,44 Furthermore, an approximately 1:1 atomic ratio of Cd and S was obtained, which suggested the possible form as CdS. Therefore, the cominational characterization by using ICP, XRD, TEM and XPS verified that nano–CdS was successful loaded on various LAC supports.

Figure 3. (a) Indexed XRD (inset: TEM image), (b) and (c) HRTEM image, (d) EDX elemental analysis, and (e) surface scanning of the CdS@LAC–800 nanocomposite. It is known that mix–phased CdS generally shows high catalytic efficiency

41,42

and the

formation of nanocomposites facilitates the catalytic activity, especially in the case of porous material supports. Thus the UV–vis DRS and TGA analyses of CdS@LAC–T were performed to evaluate their possible applicability as viable photocatalysts. Indeed, the UV–vis DRS of

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CdS@LAC–T nanocomposites displayed red–shifted absorption bands, in comparison with that of nano–CdS (Figure 2c). The red shifts of absorption bands indicate the generally better light harvesting capacity of CdS@LAC–T nanocomposites, which is favorable for photocatalysis under visible irradiation.45,46 The band gap energy of CdS@LAC–T nanocomposites was approximately 2.12, 1.94 and 2.01 eV, respectively, which were considerably lower than that for nano–CdS (ca. 2.22 eV) and bulk cadmium sulfide (ca. 2.4 eV),47 and were among the lowest band gap energies observed for CdS composite photocatalysts.46,48,49 Previous studies have indicated that the degree of band gap narrowing partly reflects the strength of interactions or the contact of interfaces between CdS and material supports.47,50 Thus, the larger band gap narrowing observed for CdS@LAC–800 may suggest stronger interaction or more contact interfaces between CdS and LAC supports in these nanocomposites. Inspired by the above observations, our special focus is given to the CdS@LAC–800, showing remarkable material crystalline, structural porosity, as well as energy harvesting efficiency (Figure 2a, 2b and 2c), which is expected to display outstanding photocatalytic activity. Therefore, the detailed structural and morphological information of CdS@LAC–800 was studied by XRD and HRTEM (Figure 3a, 3c and 3d). The diffraction characteristics in XRD pattern of CdS@LAC–800 matched well with the mix–phased nano–CdS (Figure 3a) and the TEM image unambiguously confirmed different morphologies of the two CdS phases (Figure 3a, inset). HRTEM image and selected area electron diffraction (SAED) identified crystal spacing of 0.356 nm and 0.336 nm, which were assigned to the (1 0 0) and (1 1 1) crystal planes of hexagonal and cubic CdS, respectively (Figure 3b and 3c). Moreover, EDX analysis and surface scanning suggested the uniform distribution of nano–CdS on LAC supports (Figure 3d and 3e).

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Photocatalytic Properties. The photocatalytic performance of nano–CdS and CdS@LAC–T nanocomposites were studied by RhB degradation in aqueous phases (Figure 4a). The CdS@LAC–T nanocomposites generally exhibit enhanced photocatalytic efficiency than nano–CdS, which is reasonable since the band gap energy of nanocomposites were considerably lower than that for nano–CdS determined by UV–vis DRS analyses. Meanwhile, photocatalytic efficiency of the nanocomposites follows an ascending order of CdS@LAC–600 < CdS@LAC–700 < CdS@LAC–800. Notably, the CdS@LAC–700, possessing the lowest band gap energy, displays slightly lower photocatalytic efficiency than the CdS@LAC–800 which is known to have the highest BET surface area albeit. We then tested the photocatalytic performance of a commercial activated carbon supported nano–CdS (CdS@CAC) that shows a low BET of 429.3 m2·g−1 (Figure 1e) and a rationally low photocatalytic efficiency (Figure 4a). These results indicate the photocatalytic process is likely dominated by dual factors involving the accessibility of organic substrates and light harvesting efficiency of the photocatalysts. To quantitatively clarify the reaction kinetics of RhB degradation in this system, the pseudo–first order model was applied to estimate rate constant (κ) of visible–light–driven catalytic reactions (Figure S5a).48,49 CdS@LAC–800 was found to possess the highest degradation efficiency (κ = 9.76×10–2 min–1) that was ca. 50 and ca. 37 times of nano–CdS (κ = 1.94×10–3 min–1) and CdS@CAC (κ = 2.66×10–3 min–1), respectively. To be specific, degradation efficiency of CdS@LAC–800 reached ca. 95.9% at 60 minutes. By contrast, the nano–CdS (ca. 6.0%) and CdS@CAC (ca. 21.9%) exhibited fairly low RhB degradation efficiency under the same conditions. Further study on the photodegradation of MO and MB were performed by using the best–performing CdS@LAC–800 photocatalysts and nano–CdS, as a reference. As shown in Figure

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4b, the CdS@LAC–800 displays excellent photocatalytic ability toward MO degradation with an efficiency of ca. 97.8% within 60 min, which is 17 times of the value for nano–CdS (ca. 5.9%). Similarly, the CdS@LAC–800 exhibits photocatalytic efficiency of ca. 96.3% at 150 min that is 7 times higher than that of nano–CdS (ca. 13.6%) toward the degradation of MB (Figure S5b). These results suggested that CdS@LAC–800 might be applied as a highly efficient visible–light–driven photocatalyst toward degradation of organic pollutants in aqueous systems. The excellent photocatalytic activity of CdS@LAC–800 might be attributable to its high BET (1245 m2·g−1), low band gap energy (ca. 2.01 eV), as well as the uniform deposition of nano–CdS precursors on the LAC support. It is noteworthy that dye adsorption on the nanocomposites contributes largely in the initial dye removal from solutions in dark. However, the photocatalytic degradation of dye molecules for nanocomposites is rather unaffected as long as the dye concentration reaches a lower level than that of the solution. We have demonstrated that the degradation of dye molecules (RhB, Figure 4a; MB, Figure 4b; MO, Figure 4b) absorbed on CdS@LAC–800 is highly efficient within a certain time (2 h), by which the overall photocatalytic degradation efficiency is fairly estimated in this current system. In other words, dye adsorption facilitates synergistically the substrate accessibility and photocatalytic efficiency owing to the presence of micorposous carbonaceous supports in the nanocomposite photocatalysts, and therefore contributes largely to the excellent photocatalytic performance of CdS@LAC–800.

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Figure 4. (a) RhB degradation efficiency of various photocatalysts under visible irradiation (hollow stars represent dye residue concentrations in solid photocatalysts); (b) MO and MB degradation efficiency of nano–CdS and CdS@LAC–800; (c) RhB degradation efficiency of CdS@LAC–800 at various pH; (d) solution Cd2+ levels in catalytic systems of nano–CdS and CdS@LAC–800; (e) recycling reactions by using CdS@LAC–800; and (f) XRD of as–prepared and recycled CdS@LAC–800 samples. Stability and Reusability of CdS@LAC–800 Nanocomposites. In general, stability and reusability

of the photocatalysts are

key indices for further applications. Therefore,

thermogravimetric analyses (TGA) were first recorded to assess the thermal stability of CdS@LAC–T nanocomposites. The TG plots indicate that these materials are able to maintain good thermal stability up to 300 ºC and structural decomposition occurred afterwards (Figure S6). It is speculated that the CdS@LAC–T nanocomposites will exhibit rational material stability for photocatalysis. On the other hand, the reaction system with CdS@LAC–800 nanocomposite revealed extraordinarily low and steady Cd2+ levels during photocatalysis (below 0.4 ppm within 120 minutes; Figure 4d). By contrast, increasingly high Cd2+ concentrations (up to 10.9 ppm; Figure 4d) were observed in the photocatalysis system using nano–CdS. In addition, acid–base tolerance is another key factor that evaluates the reliability of photocatalysts in water treatment. For this purpose, the 14


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photocatalytic RhB degradation efficiency of CdS@LAC–800 was investigated at various pH conditions (3.0 to 11.0). The photocatalytic efficiency of CdS@LAC–800 remained nearly unchanged (Figure 4c), which indicated the CdS@LAC–800 was stable over a wide pH range. These results further confirmed the formation of CdS@LAC nanocomposite photocatalysts favored greatly the stabilization of nano–CdS, by which Cd2+ release resulted from photocorrosion of CdS was largely prevented. Moreover, recycling reactions of RhB degradation were performed to verify the photostability and reusability of CdS@LAC–800. It was demonstrated that CdS@LAC–800 could preserve over 75% of its initial catalytic ability at the end of four consecutive runs, as indicated in Figure 4e. The rational decrease of photocatalytic ability might be a result of local aggregation and deactivation of nano–CdS, as well as the loss of photocatalysts from the wash–and–dry process. XRD patterns recorded on as–prepared and recycled CdS@LAC–800 samples matched well (Figure 4f), suggesting significant structural and crystalline stability of nanocomposites. Mechanism for Dye Degradation over CdS@LAC–800 Nanocomposites. Mott–Schottky plots for CdS@LAC–800 were measured at frequencies of 500, 1000 and 1500 Hz. As shown in Figure 5a, the positive slope in plot of C–2 versus potential was typical for n–type semiconductor.50 The intersection point was independent on frequency and flat band potential (Vfb) of CdS@LAC–800 estimated form the intersection was approximately –1.32 V vs. Ag/AgCl (–1.12 V vs. NHE), which was more negative than the redox potential of Cd2+/Cd0 (E(Cd2+/Cd0) = –0.40 V vs. NHE).51 The conduction band (CB) of CdS@LAC–800 was estimated to be–1.12V vs. NHE,52 and the valence band (VB) of CdS@LAC–800 was calculated to be 0.89 V vs. NHE with a band gap energy (Eg) of ca. 2.01 eV (Figure 2c, inset).

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Figure 5. (a) Mott–Schottky plots for CdS@LAC–800 (Inset: energy diagram of the CB and VB levels); (b) transient photocurrent plots for nano–CdS and CdS@LAC–800; (c) photoluminescence (excitation: 430 nm) and (d) time–resolved fluorescence (excitation: 375 nm) spectra of nano–CdS and CdS@LAC–800; (e) photocatalytic efficiency of CdS@LAC–800 with exposure to various scavengers; (f) DMPO–O2•− spin–trapping ESR spectra; and (g) time–dependent fluorescence spectra of TAOH using CdS@LAC–800 (*fluorescent characteristics of sodium terephthalate), inset: fluorescent intensity in the presence of h+/O2•− scavenger versus the original one at 60 min; (h) proposed photodegradation mechanism for the nanocomposites under visible irradiation. In most cases, separation of photogenerated electron–hole pairs are considered as the starting stage of a visible–light–driven catalysis. In this context, electrochemical analyses were performed to reveal the generation and transportation of photongenerated carriers in this system. The difference between nano–CdS and CdS@LAC–800 in photocurrent generation was compared according to the transient photocurrent plots, as shown in Figure 5b. Upon irradiation, a photocurrent generated and reached stable rapidly, and the photocurrent remained stable within the time intervals of 20 seconds. The photocurrent decreased quickly to the initial levels when light was switched off. Of special notice, the photocurrent of CdS@LAC–800 nanocomposite was significantly increased, comparing with the nano–CdS. It was obvious that the photogexcited electrons and holes were efficiently separated and a long lifetime existed in photogenerated charge carriers. Furthermore, a significant photoluminescence quenching was observed for CdS@LAC–800 compared with that of nano–CdS (Figure 5c), indicating an efficient transfer of electrons in the excited–state from nano–CdS to LAC 16


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support. Time–resolved fluorescence spectroscopy was performed to detect the fluorescence lifetime of nano–CdS and CdS@LAC–800 (Figure 5d). In comparison with the fluorescence spectrum of nano–CdS, a faster fluorescence decay was observed for CdS@LAC–800 which was ascribed to efficient electron transportation in the nanocomposite photocatalyst. These results demonstrated that the separation of photogenerated carriers (h+, e-) was efficiently enhanced in the nanocomposite systems. To further investigate the mechanism for dye degradation, scavenger experiments were applied to detect photogenerated active species.53,54 Noticeable decreases on the photocatalytic efficiency of CdS@LAC–800 were observed upon addition of ammonium oxalate (AO), tert–butyl alcohol (TBA) and benzoquione (BQ), which were used as scavengers for h+, OH• and O2•− radicals, respectively.55 Specifically, BQ exhibited better quenching effects toward the photocatalytic reactions, which was rather superior to AO and TBA. Specifically, O2•− accounted for ca. 47.6% of the total degradation efficiency, whereas h+ (ca. 10.7%) and OH• (ca. 13.9%) contributed relatively less to the overall photodegradation of dye molecules (Figure 5e). These results suggested O2•− radicals were likely dominant active species, whereas h+ and OH• radicals played minor roles. Moreover, OH• and O2•− radicals were detected by performing ESR spin–trapping in the presence of DMPO.56–58 As expected, CdS@LAC–800 generated strong characteristic of DMPO–O2•− species, while the ESR signal of DMPO–OH• remains silent over repeated attempts, which supported our hypothesis of dominant O2•− species present (Figure 5f). Nonetheless, OH• was successfully detected by means of fluorescence measurements in the presence of terephthalic acid (TA), by which OH• and TA reacted to produce highly fluorescent 2–hydroxyterephthalic acid (TAOH). Moreover, the intensity of fluorescence characteristics (emission: 425 nm, excitation: 315 nm) was found to increase

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proportionally with the production of OH• (Figure 5g).59,60 This was however a solid evidence demonstrating the presence of OH• in this photocatalytic system. Moreover, the presence of either of h+ or O2•− scavenger resulted in significant yet comparable decrease (over 50% in comparison with the original one) of fluorescent intensity at a time of 60 min, indicating both h+ and O2•− radicals took part in an effective generation of OH•. It is rationalized that electrons first transfer from the VB to CB of nano–CdS during a visible–light–driven photocatalysis. Next, the carriers (h+, e-)

participate in the formation of

various active species (OH•, O2•−, H2O2 etc.) by which holes in the VB can oxidize dye molecules by capturing electrons.61 Meanwhile, dye molecules are degraded by OH• that is initiated from a complex process via the reduction of H2O2 generated by dominant O2•− oxidants. In particular, the presence of micorposous carbonaceous supports effectively increases the accumulation of dye molecules on interfaces of nano–CdS and LAC, and the surface–adsorbed dye molecules are concequently photodegraded via accepting active species from the CdS@LAC–800 nanocomposite. The overall photocatalytic degradation pathway is proposed and illustrated in Scheme 1.

CONCLUSION A series of lotus–leaf–derived activated carbons (LACs) with microporous surface structures and porosity have been prepared and advanced as material supports for nano–CdS photocatalysts. It is demonstrated that the photocatlytic capacity of CdS@LAC–T nanocomposites are superior to the nano–CdS precursors due to the introduction of microporous LAC supports. Surprisingly, CdS@LAC–800 displays outstanding visible–light–driven photocatalytic efficiency, which is over 50 times of nano–CdS precursors, toward various organic dye degradation. Mechanism study

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suggests that the narrowed bad gap energy of CdS@LAC–800 is responsible for the efficient light harvesting efficiency of the photocatalyst. Moreover, the formation and transportation of photogenerated carriers (h+,e-) and catalytically free radicals (OH• and O2•−) contribute largely to the exceptional photocatalytic capacity of CdS@LAC–800. This study provides a viable platform to optimize the performance of photocatalysts by means of a facile and inexpensive pathway. Further application of the CdS@LAC–T nanocomposites into other photocatalytic systems is currently underway.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at xxx. Materials and methods; Gas adsorption/desorption isotherms; SEM, TEM, and HRTEM images of nano–CdS, CdS@LAC–600 and CdS@LAC–700; Plots for rate constant calculations; TGA patterns (PDF)

AUTHOR INFORMATION Corresponding Authors *E–mail: [email protected] (R. C.). *E–mail: [email protected] (J. L.). *E–mail: [email protected] (H. L.C.). ORCID Jian Lü: 0000–0002–0015–8380

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Rong Cao: 0000–0003–2384–791X Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful for financial support from the National Key Research and Development Program of China (Grant No. 2017YFD0800900), the NSFC (Grant Nos. 91622114, 21520102001, 21521061 and 21331006), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB20000000), the State Key Laboratory of Structural Chemistry (Grant No. 20170032), the New Century Excellent Talents in Fujian Province University, and the International Science and Technology Cooperation and Exchange Project of Fujian Agriculture and Forestry University (Grant No. KXGH17010).

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