Simultaneously Broadened Visible Light Absorption and Boosted

May 22, 2019 - ... decreased by about 30%, implying adsorption of CFC by CN–Pt. When irradiating the mixture of CFC and CN–Pt with a white LED for...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Simultaneously Broadened Visible Light Absorption and Boosted Intersystem Crossing in Platinum-Doped Graphite Carbon Nitride for Enhanced Photosensitization Chaobi Li,†,‡ Ying Wang,∥ Chenghui Li,∥ Shuxia Xu,*,‡ Xiandeng Hou,∥ and Peng Wu*,†,∥ †

State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu 610065, China College of Environment and Ecology, Chengdu University of Technology, Chengdu 610059, China ∥ Analytical & Testing Center, Sichuan University, Chengdu 610064, China ‡

Downloaded by BUFFALO STATE at 23:10:05:989 on May 30, 2019 from https://pubs.acs.org/doi/10.1021/acsami.9b02767.

S Supporting Information *

ABSTRACT: Herein, taking graphite carbon nitride (g-C3N4) as the example, we demonstrated that the two limiting factors that determine the photosensitization performance, namely, light absorption and intersystem crossing (ISC), could be simultaneously enhanced through Pt2+ doping. Specifically, as a πconjugated two-dimensional semiconductor, g-C3N4 is capable of absorbing light shorter than 460 nm (2.7 eV). Upon Pt2+ doping that allows metal-to-ligand charge transfer (MLCT) from Pt2+ to the substrate g-C3N4, the light absorption of g-C3N4 was greatly expanded up to 1000 nm. Meanwhile, the large atomic number of Pt2+ ensures promotion of ISC to activate the triplet state of gC3N4 via heavy atom effect (HAE), which was confirmed via both photosensitization performance and photophysical characterizations. Further, the enhanced light absorption and photosensitization of Pt2+-doped g-C3N4 were harvested for antibiotics removal, a type of environment contaminants that gained global attention because of their worldwide abuse. Compared with its undoped counterpart, Pt2+-doped g-C3N4 featured significantly improved antibiotics removal in the presence of low-power white LED irradiation, which is promising for photosensitized environmental remediation. KEYWORDS: photosensitization, carbon nitride, Pt2+ doping, metal-to-ligand charge transfer, heavy atom effect, antibiotics

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INTRODUCTION Photosensitization (Scheme 1A) refers to the light-induced reaction mediated by a photosensitizer (PS).1,2 During such a process, PS is first excited to the excited singlet state upon light absorption, followed by either decaying back to the ground state to emit prompt fluorescence, or undergoing intersystem crossing (ISC) to activate the triplet state. The triplet-state PS may interact with substrates and oxygen in its immediate environment, leading to the generation of reactive oxygen species (ROSs) through either type I or II pathway.3,4 The generated ROSs are responsible for the applications of photosensitization in environmental remediation,5,6 photodynamic therapy,3,7 as well as photooxidation catalysis for organic synthesis.8−10 Therefore, light absorption and ISC are the two limiting factors that determine the photosensitization performance (Scheme 1A). Graphitic carbon nitride (g-C3N4), featuring as a πconjugated two-dimensional material and a visible light absorption semiconductor,11−13 has also been explored for photosensitization applications.14−17 However, the ROS generation efficiency of g-C3N4 is largely lower than that of the previously reported PSs. Besides, the light absorption of gC3N4 is still restricted by its bandgap (2.7 eV, λ < 460 nm) and © XXXX American Chemical Society

most of the visible-near-infrared (vis−NIR) photons in the solar spectrum thus cannot be utilized. To increase the ROS generation efficiency, a simple oxidization treatment was reported to incorporate carbonyl groups into the structure of g-C3N4.18 However, the π-conjugated structure of g-C3N4 may be destroyed during the oxidization treatment, and the light absorption of the oxidized g-C3N4 was barely changed. Metal ion doping has proven to be an efficient route for alternating the electronic structure, as well as the light absorption of gC3N4,19−21 but the ROS generation efficiency was seldom investigated. Herein, we proposed the integration of metal-to-ligand charge transfer (MLCT) and heavy atom effect (HAE) for improving the photosensitization efficiency of g-C3N4. MLCT, which relies on electronic charge transfer from the molecular orbit of metal to the ligand, is an efficient way of light harvesting at longer wavelengths.22 The combination of MLCT and g-C3N4 allowed the broadening of the light absorption up to 1000 nm for efficient visible-near-infrared photocatalysis.23 Received: February 13, 2019 Accepted: May 20, 2019

A

DOI: 10.1021/acsami.9b02767 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Illustration of the Energy Diagram of Photosensitization for ROS Generation (A) and Pt2+Doping for Promoting the Photosensitization Efficiency of g-C3N4 (B)a

Figure 1. UV−vis diffused reflectance spectra of g-C3N4 and CN−Pt. The inset shows the Pt2+ contents measured by ICP-OES and also the photos of the corresponding samples.

and the color of the sample changed from yellowish to brown, allowing the use of white light for photosensitization and photocatalysis. The actual Pt contents were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), with Pt contents ranging from 0.93% to 4.13% (Figure 1). Control investigation on postmixing of H2PtCl6 with g-C3N4 did not show such spectra and color change (Figure S2). Therefore, premixing the precursors in the stage of monomer is the key for successful Pt2+ doping, which may permit complete coordination of Pt2+ with the N sites before the formation of g-C3N4. Upon doping of Pt2+ into the matrix of g-C3N4, CN acts as a polymeric ligand with the N coordination sites for Pt2+ to be located in. On the basis of the coordination number of Pt2+ (4), it is presumable that the dopants (Pt2+) locate at the interlayers of CN, acting as a bridge between the adjacent sheet of CN (Figure 2A). Such linkage can be evidenced from X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterizations. As shown in Figure 2B and Figure S3A, the XRD patterns of CN and CN−Pt shows two main diffraction peaks around 13° and 27.4°, which can be indexed to the (001) interplanar structural packing and (002) interlayer stacking, respectively.31 Compared with g-C3N4, the peak intensity decreased gradually and the peak became broader along with Pt2+ doping (Figure S3B), clearly demonstrating that the layered structure of g-C3N4 has been perturbed upon Pt2+ doping.32,33 In other words, more N-conjugated aromatic pores were filled with Pt2+ by increasing the Pt2+ doping concentrations. In addition, doping of Pt2+ also resulted in increased interlayer distance of g-C3N4 from 0.324 to 0.33 nm, as derived from the (002) interlayer stacking peaks, which is consistent with the changes of the peak intensity. TEM further confirmed that the sheet structure of CN−Pt had changed little as compared with CN (after ultrasonic liquid exploitation, Figure 2C versus Figure 2D), but the sheet of CN−Pt is obviously thicker than that of CN, indicating that Pt2+ locates at the interlayers of CN. The perturbation of the chemical structure of CN by Pt2+ was further confirmed with Fourier transform infrared (FT-IR) spectroscopy. As displayed in Figure S4, the stretching (810 cm−1) and bending (1200−1700 cm−1) modes of the tri-striazine units in CN and CN−Pt were both identified, but the

a

For simplification, here, only a single layer of C3N4 is presented. The actual structure of C3N4 and the location of the Pt2+ dopants were provided in the following characterization section.

On the other hand, metal ions with large atomic number are expected to inducing heavy atom effect (HAE), which results in significantly increased ISC rate constant via promoting spin−orbit coupling (SOC) between the electronic states of different spin multiplicity. The promoted ISC will result in activation of the triplet state of PSs and subsequent boosted ROS generation.24,25 Therefore, here our concept is to integrate MLCT and HAE in metal ion-doped g-C3N4, namely, MLCT for broadening the light absorption and HAE for improving the ROS generation efficiency (Scheme 1B). Specifically, on the basis of the intriguing effects of Pt2+ doping into various materials,26−28 as well as the appealing roles of Pt2+ in inducing both HAE and MLCT,23,29 we proposed the use of Pt2+ doping to make g-C3N4 an efficient visible-light-absorbing photosensitizer. The photosensitization performance of the Pt2+-doped g-C3N4(CN−Pt) was probed via photooxidation of an antibiotics, cefaclor (CFC).

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RESULTS AND DISCUSSIONS Confirmation of the Extended Absorption of g-C3N4 by Pt2+ Doping. To dope Pt2+ into the g-C3N4 (CN) pores, the dicyandiamide (monomer for g-C3N4) was mixed with H2PtCl6·6H2O (Pt2+ precursor) prior to the thermal polymerization for forming g-C3N4 (see Figure S1).22 The number of Pt2+ in the CN−Pt samples was altered by varying the concentrations of H2PtCl6·6H2O in the synthetic system. As viewed from the UV−vis diffused reflectance spectra in Figure 1, doping of Pt2+ into the g-C3N4 matrix resulted in significantly broadened absorption spectra of g-C3N4. Such broadened absorption spectra could be ascribed to MLCT, which possess lower photoexcitation energy than the HOMO− LUMO transition (here the bandgap transition of g-C3N4).30 Upon increasing the concentrations of Pt2+, the absorption spectra were expanded to ∼1000 nm (460 nm for g-C3N4), B

DOI: 10.1021/acsami.9b02767 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Verification of the Enhanced Photosensitization of gC3N4 by Pt2+ Doping. To verify the photosensitization performance of CN−Pt, antibiotic CFC was taken as the model pollutant here. As clearly demonstrated in Figure 3A,

Figure 3. Photosensitized oxidation of CFC (2.0 mg/L) by CN−Pt (0.1 mg/mL): (A) UV−vis absorption spectra of CFC in the presence various conditions and (B) HPLC chromatograms of CFC before and after photosensitized oxidation.

CFC has a characteristic absorbance band peaked at ∼264 nm. Upon mixing of CFC with CN−Pt, the absorbance of the 264 nm-band decreased by about 30%, implying adsorption of CFC by CN−Pt. When irradiating the mixture of CFC and CN−Pt with a white LED for 45 min, the absorption band of CFC almost disappeared, indicating photodegradation of CFC. Without CN−Pt, no such photodegradation was observed. The photosensitization performance of CN−Pt was further subjected to high-performance liquid chromatography spectrometry−mass spectrometry (HPLC-MS) analysis. As shown in Figure 3B, after photosensitized oxidation, the characteristic peak of CFC (4.63 min) disappeared, giving rise to two new peaks at 2.70 and 2.46 min. According to MS analysis, they were identified as the molecular fragments of CFC, namely, 7amino-3-chloro-3-cephem-4-carboxylic acid (7-ACCA) and αamino-benzeneacetaldehyde (Figure S8). Therefore, both absorption and HPLC-MS results clearly demonstrate the photosensitized oxidation ability of CN−Pt. Photosensitization occurs first by photon absorption to trigger the transition from the ground state to the excited state (Scheme 1A). To demonstrate the effect of Pt2+ doping in promoting the photosensitization performance of CN, the photoinduced CFC removal rates mediated by CN and CN− Pt in the presence of various light irradiation were compared. Here, the light irradiation was supplied with miniaturized LEDs of different lighting wavelengths (3 V, 3 W). As shown in Figure 4, both CN and CN−Pt could induce CFC removal in the dark, probably because of CFC adsorption (see Supporting Information for adsorption details). In the presence of LED irradiation (from violet to red), CN−Pt exhibited better visible light response (violet ≈ white > blue > green > orange > red > dark) over CN (violet > white > blue > green ≈ orange ≈ red ≈ dark). However, it can not only ascribe the increased CFC removal to the extended absorption, since the increased absorption cannot correlate with the increased photosensitization. Therefore, Pt2+ doping substantially promoted the photosensitization performance of CN. More importantly, CN−Pt enabled remarkably improved (up to 2-fold) photosensitized CFC degradation performance over CN with a white LED irradiation (Figures 4 and S11),

Figure 2. Structural characterization of CN and CN−Pt: (A) presumable structures of CN and CN−Pt, (B) XRD patterns of CN and CN−Pt, (C) TEM image of CN, (D) TEM image of CN−Pt, (E) XPS survey spectra of CN and CN−Pt, and (F) XPS Pt 4f spectrum of CN−Pt.

intensities are decreased after Pt2+ doping. Particularly, the N− H stretching vibration (3000−3500 cm−1, possibly due to hydrogenation of the sp2-N in g-C3N434) was shifted toward higher frequency after Pt2+ doping, possibly because of the formation of Pt−N bond. The valence state of Pt in CN−Pt was identified with X-ray photoelectron spectroscopy (XPS). As shown in Figures 2E and S5, both CN and CN−Pt exhibited the C, N, and O signals. The O signal is probably arising as a result of the small number of absorbed oxygen molecules. Besides, two tiny peaks of Pt 4d and Pt 4f were observed in CN−Pt but not in CN (Figure 2E). Such Pt species were identified as Pt(II) according to the peak at 73.3 eV (Pt 4f7/2) and 76.5 eV (Pt 4f5/2) (Figure 2F), which resulted from the reduction of Pt(VI) (H2PtCl6) by the released NH3 during the formation of C3N4. Before and after Pt2+ doping, the C 1s XPS spectra changed little (Figure S6), but the C−N−H peak (sp2) in N 1s XPS spectrum of CN−Pt is shifted toward higher energy (Figure S7). The reduced electron density of N atoms is another evidence of the filling the lone-pair electrons of the sp2-bonded N atoms into the empty orbitals of Pt2+ (coordination, Scheme 1B). C

DOI: 10.1021/acsami.9b02767 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Confirmation of the Enhanced Photosensitization through ROS Identification. Since photosensitization can generate ROS through the reaction with molecular oxygen via either type I or II pathway (Scheme 1A), the enhanced photosensitization can thus be revealed through ROS identification, which has been frequently explored previously.18,35,36 Therefore, To further identify the role of Pt2+ doping in promoting the photosensitization performance of CN, the ROSs generated from such process were evaluated. As showed in Figure 5A, the highest CFC removal rate was obtained in oxygen-enriched solution and the lowest in oxygen-depleted solution. Furthermore, the specific ROS responsible for the CFC degradation were identified with ROS-specific scavengers, namely tryptophan (Trp) for 1O2, isopropanol (IPA) for •OH, and benzoquinone (BQ) for O2•−.18,36−38 All these scavengers showed inhibitory effect for CFC removal (Figures 5A and S12), with contribution order of 1 O2 > •OH > O2•−. Obviously, CN−Pt experienced a dominant type-II photosensitization process upon white light irradiation. Moreover, it was also observed from the holescavenging study (triethanolamine, TEOA)36 that hole (h+) generated from a typical semiconductor exciton process also took part in the photodegradation, but its contribution was largely lower than ROSs. The ROSs generated from photosensitization of CN and CN−Pt were further identified with electron paramagnetic

Figure 4. Comparison of the photosensitized CFC (2.0 mg/L) removal performance of CN and CN−Pt (0.1 mg/mL) in the presence of various LEDs (3 V, 3 W). Power irradiance: violet LED, 16.8 mW/cm2; blue LED, 122 mW/cm2; green LED, 43.9 mW/cm2; orange LED, 5.0 mW/cm2; red LED, 10.1 mW/cm2; white LED, 25.7 mW/cm2.

indicating that CN−Pt is a good candidate white-light absorption material for photosensitized oxidation.

Figure 5. Identification of the ROS generated from photosensitization of CN and CN−Pt: (A) photodegradation of CFC (2.0 mg/L) by CN−Pt (0.1 mg/mL) under different conditions, (B) EPR spectra of CN and CN−Pt with TEMP (specific for 1O2), (C) EPR spectra of CN and CN−Pt with DMPO (specific for O2•− and •OH), (D) fluorescence spectra of terephthalate incubated with CN and CN−Pt under white LED irradiation, and (E) photocurrent response (i−t curve) of CN/ITO and CN−Pt/ITO electrodes in the presence of CFC. The photoelectrochemical tests were performed in 0.10 M PBS at a working potential of 0.0 V and white LED irradiation.39 D

DOI: 10.1021/acsami.9b02767 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 6. Photophysical characterization of g-C3N4 and CN−Pt: (A) Fluorescence spectra of pristine g-C3N4 and CN−Pt, (B) fluorescence lifetime of g-C3N4 (458 nm) and CN−Pt (477 nm), and (C) phosphorescence lifetime of g-C3N4 (508 nm) and CN−Pt (527 nm).

resonance (EPR) and terephthalate fluorescence. 2,2,6,6tetramethylpiperidine (TEMP) and 5,5-dimethyl-1-pyrrolineN-oxide (DMPO) were employed as the 1O2 and O2•−/•OH trapping agents, respectively.40,41 As illustrated in Figure 5B, the EPR spectra of both CN and CN−Pt display a typical 1:1:1 triplet signal, which are in accordance with those of 2,2,6,6tetramethylpiperidine-noxyl (TEMPO), suggesting the generation of 1O2.42 The quantum yields of singlet oxygen (ΦΔ) of g-C3N4 and CN−Pt were evaluated through comparing the EPR integrated area with Rose Bengal (RB) as the standard photosensitizer (ΦΔ = 0.75).43 As shown in Figure S14, singlet oxygen QY of g-C3N4 and CN−Pt were roughly estimated as ∼0.108 and ∼0.216, respectively. Meanwhile, an appreciable amounts of •OH were detected from CN and CN−Pt via the characterized 1:2:2:1 quadruplets of DMPO in water (Figure 5C),44 which can be further confirmed with terephthalic acid as the fluorescent probe (Figures 5D and S13).45,46 Obviously, the amount of each ROS generated from light-irradiated CN− Pt is all largely higher (∼2 fold) than that of CN, confirming that Pt2+-doping could promote the photosensitization of CN. However, no noticeable signal for O2•− was detected, probably because of the low generated amounts. For the hole (h+), CN− Pt also exhibited higher generate rate over CN (Figure 5E). Spectroscopic Confirmation of HAE. To further convince the Pt2+ doping-induced heavy atom effect, further photophysical characterizations were carried out. Generally, the apparent signature of heavy atom effect is simultaneous fluorescence quenching and phosphorescence enhancing.24 Besides, since heavy atom effect can accelerate ISC, the phosphorescence lifetime will also be shortened.47−50 As can be seen from Figure 6A, upon Pt2+ doping, the fluorescence of CN is dramatically quenched, accompanied also by significant reduction of the fluorescence lifetime from 3.19 to 1.02 ns (Figure 6B). For solid samples, it is difficult to collect phosphorescence since oxygen cannot be removed efficiently (Figure S5). However, according to the relationship between phosphorescence and singlet oxygen (Scheme 1A), it is reasonable to conclude that Pt2+ doping does result in phosphorescence enhancing (Figure 5B). Next, we carried out phosphorescence lifetime measurements for both g-C3N4 and CN−Pt. In a previous work,18 a ΔEST value of ∼0.25 eV was observed for pristine g-C3N4. Therefore, the phosphorescence lifetime was collected at the wavelengths about 50 nm longer than those of fluorescence. As shown in Figure 5C,

appreciable phosphorescence lifetime shortening of g-C3N4 upon Pt2+ doping was observed. Therefore, the heavy atom effect of Pt2+ in CN−Pt can be confirmed, which promotes ISC for activation of the triplet state and enhances photosensitization (Scheme 1A). Employing the Enhanced Photosensitization of CN− Pt for Improved Antibiotics Removal. To maximize the photodegradation effects for pollutants, a series of factors were optimized, including the concentration of CN−Pt, illumination time, and the media pH. As shown in Figure S15A, the optimal dosage of CN−Pt suspension used in this work is 0.1 mg/mL. The degradation efficiency significantly increased with the illumination time and the equilibrium was reached within 45 min (Figure S15B). The optimal pH of this system was 7.0 (Figure S15C). Under the optimized conditions, the degradation efficiency for 2 mg/L CFC under 45 min white light LED (3 V, 3W) irradiation is higher than 95%. The photosensitized performance of CN−Pt for antibiotic removal was further compared with two commercialized materials, namely, titanium dioxide (TiO2)51 and graphene oxide (GO),36 which feature similar adsorption and photosensitization ability as CN−Pt. As depicted in Table 1, the Table 1. Comparison of the Adsorption Capacity and Degradation Efficiency of CFC by GO, TiO2, and CN−Pta materials

adsorption capacity (mg/g)

degradation efficiency (%)

GO TiO2 CN−Pt

15.04 16.36 15.86

68.99 82.17 99.22

a

Concentration of the photosensitizers: 0.1 mg/mL. Lighting conditions: white LED (3 V, 3 W).

adsorption capacity for CFC of TiO2 and GO is similar to that of CN−Pt, but the photosensitization activity was in the order of CN−Pt > TiO2 > GO, which endows CN−Pt further added degradation efficiency for antibiotic contaminants. In practical use of CN−Pt for antibiotics removal, the stability and reusability are economically important. For investigating the stability, five cycling tests to degrade CFC were carried out. As shown in Figure S16, CN−Pt showed a slight activity decline after five cycles. Upon further investigation, the decreased degradation performance could be ascribed to the loss of photosensitizer (CN−Pt) during the E

DOI: 10.1021/acsami.9b02767 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

of the photosensitizer, the two limiting factors that determine the photosensitization performance. Considering that a large number of new two-dimensional polymeric semiconductor materials are discovering, the present approach will be extremely useful in promoting the photosensitization performances of these new materials.

removal-elution run (Figure S16), thus ruling out the issue of photosensitization activity loss. Besides, introduction of Pt2+ did not add appreciable toxicity to g-C3N4, as revealed from the cytotoxicity of g-C3N4 and CN−Pt (Figure S17). After demonstration of the successful CFC degradation, we next investigated the degradation performance of CN−Pt for other antibiotics, that is, the universality. Recently, the worldwide abuse of antibiotics has resulted in global environment and health concerns as well as antibiotic-resistant bacteria. A series of antibiotic contaminants (Figure S18), including tetracycline hydrochloride (TC-HCl), acetylspiramycin (ASPM), amoxicillin (AMX), ciprofloxacin (CIP), sulfadiazine (SDZ), sulfamethoxazole (SMZ), tinidazole (TNZ), cefixime, chloramphenicol (CM), and metronidazole (MTZ), were subjected to CN−Pt adsorption and photosensitized degradation. As displayed in Figure 7, all these

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METHODS

Synthesis of CN and CN−Pt. The synthesis of g-C3N4 was based on a protocol in literature.19 Briefly, dicyandiamide (500 mg) was put into a crucible (15 mL) with a cover in a tube furnace (OTF-1200X, Hefei Kejing, China), heated at a rate of 5 °C min−1 to reach a temperature of 550 °C, and then, maintained at this temperature for about 4 h in a nitrogen-flowing atmosphere. After it was cooled down, the yellowish product was collected and washed with distilled water to remove residual ammonium. The sample was then dried and stored under dark condition for further characterizations. For the synthesis of CN−Pt (Figure S1), mixture of dicyandiamide (500 mg) mixed and deionized water (5 mL) was heated and stirred at 90 °C with an aqueous solution containing H2PtCl6·6H2O (0.01 M, 1−5 mL). The mixed solution was continually heated at 90 °C for total removal of the residual water. Then, the resulting reddish mixture was subjected to the above synthetic procedure to obtain the brown product. To remove extra Pt2+, a thorough washing procedure with distilled water was carried out. Adsorption Procedures. The CFC solutions for adsorption experiments were prepared via serial dilution of a CFC stock solution (1 mg/mL) to desired concentrations. And the CN−Pt suspensions were prepared by diluting the stoke solution (1 mg/mL) to the desired concentration with ultrapure water. The adsorption studies were performed with batch equilibration method. Specifically, 1 mL of CN−Pt (0.2 mg/mL) suspension was added with 0.1 mL of CFC solution of known concentration and 0.9 mL of phosphate buffered saline solution (PBS, pH 7.0) in a 2 mL centrifugal tube. The adsorption was carried out for 45 min at room temperature. Then, the suspension was centrifugated at 10 000 rpm for 5 min. The concentrations of CFC in the supernatant were analyzed by a UV− visible spectrophotometer at 264 nm. The amount of CFC adsorbed on CN−Pt was calculated from the expression

Figure 7. Removal efficiency of CN−Pt for a broad band of antibiotics in the absence and presence of white LED irradiation. CN−Pt concentration, 0.1 mg/mL; antibiotic pollutant concentration, 2 mg/L.

antibiotic contaminants could be adsorbed by CN−Pt (dark), implying the adsorption capacity of CN−Pt. Moreover, in the presence of white LED irradiation, further added removal for all these antibiotic contaminants were received, which should be ascribed to photosensitized degradation. In real-world sample matrices, the antibiotics removal efficiencies of CN−Pt for CFC, TC-HCl, and AMX (2.0 mg/L) were still higher than 95% (Table S6), indicating that such removal was not prone to be influenced by sample matrices. Therefore, CN−Pt could be explored for removal of a broad band of antibiotic contaminants.

qe =

(C0 − Ce)V m

where qe was the amount adsorbed at equilibrium (mg/g), C0 and Ce were the initial and equilibrium concentrations of CFC in solution (mg/L), respectively, m was the dosage of sorbent (mg), and V was the volume of the solution (mL). Study on the Photosensitization Property of CN−Pt. The photosensitization experiments were evaluated by photodegradation of the cefaclor (CFC) aqueous solution irradiated by a white LED (3 V, 3 W). Typically, 0.2 mg/mL CN−Pt suspension (1 mL) was well dispersed in 0.9 mL of PBS solution (pH 7.0) in the 24 well-plate, followed by addition of 0.1 mL of CFC solution (40 mg/L). Subsequently, the mixed suspension was stirred for several minutes to obtain relatively uniformly dispersion of CN−Pt and CFC in the dark. The above solution was irradiated with a white LED (3 V, 3 W). At given time intervals, the supernatant was separated by centrifugation at 10 000 rpm for 5 min. Afterward, 1.5 mL of supernatant was sampled and analyzed with a UV−visible spectrophotometer through the absorbance at 264 nm. The CFC was used as a process indicator in all the experiments unless otherwise specified.

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CONCLUSIONS In summary, we have demonstrated Pt2+ doping could endow dual roles for improving the photosensitization performance of g-C3N4, namely, MLCT-induced light absorption broadening and HAE-promoted ISC for ROS generation boosting. Through optimization of the Pt2+ doping amounts, the light absorption edge of g-C3N4 could be extended from ∼460 to ∼1000 nm, which permitted the use of low power white LED for excitation. Meanwhile, the HAE brought by Pt2+ doping resulted in boosted ISC rate, leading to ∼2-fold higher ROS generation efficiency. The enhanced light absorption and photosensitization of Pt2+-doped g-C3N4 were harvested for improved antibiotics removal, demonstrating promising performance in photosensitized environmental remediation. Overall, the present study provided an interesting approach of Pt2+ doping for increasing both of the light absorption and ISC

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b02767. Available experimental results for characterization of prepared CN and CN−Pt, adsorption studies, ROS F

DOI: 10.1021/acsami.9b02767 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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identification, parameters for CFC removal, and cytotoxicity of CN−Pt of CN (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiandeng Hou: 0000-0003-2488-2063 Peng Wu: 0000-0002-9128-9027 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.W. received funding from the National Natural Science Foundation of China (No. 21522505) and the Youth Science Foundation of Sichuan Province (Grant 2016JQ0019). We also would like to thank Dr. Yunfei Tian and Dr. Hanjiao Chen of Analytical & Testing Center, Sichuan University, for their help in XPS and EPR data collection, respectively.

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DOI: 10.1021/acsami.9b02767 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX