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Novel design of porous hollow hydroxyapatite microspheres decorated by reduced graphene oxides with superior photocatalytic performance for tetracycline removal
Solid State Sciences 99 (2020) 106067
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Novel design of porous hollow hydroxyapatite microspheres decorated by reduced graphene oxides with superior photocatalytic performance for tetracycline removal Rongjiang Zou a, 1, Tianhong Xu b, 1, Xiaofang Lei b, Qiang Wu b, *, Song Xue a, ** a
Department of Cardiovascular Surgery, Ren Ji Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai, 200127, PR China Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai, 200090, PR China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Reduced graphene oxides (RGO) Hydroxyapatite (HAp) Tetracycline (TC) Photocatalysis
Photocatalysis is a well-established and green technology with cost-effective, high-performance and environment benign for tetracycline (TC) removal in wastewater. Recently, HAp-based materials have been proved to be cheap and green photocatalysts for wastewater pollutant treatment. However, the photocatalytic removal efficiency of these existing HAp-based materials is still not desirable. Herein, a series of new porous hollow hydroxyapatite microspheres decorated with small amounts of reduced graphene oxides (0.5, 1.5 and 3 wt%) were firstly and successfully fabricated by a facile hydrothermal method. The as-prepared RGO/HAp composites were charac terized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer Emmett-Teller (BET) and photoelectrochemical measurements. Furthermore, their photocatalytic applications were investi gated by using TC as a model contaminant. It turned out that the as-prepared RGO (1.5 wt%)/HAp composite exhibits an outstanding photocatalytic activity for TC degradation (60 mg/L) under a 300 W xenon lamp with full spectrum irradiation (92.1%, 30 min). Note that its adsorption efficiency for TC in dark period was only 7.9% after 30 min. Thus, the total removal efficiency for TC was nearly 100%. Furthermore, the as-prepared RGO (1.5 wt%)/HAp exhibited remarkable stability and repeatability, demonstrating its promising potential as an efficient photocatalyst. A plausible photocatalytic reaction mechanism was proposed on the basis of electron spin resonance (ESR) and trapping experiments. This fundamental research will provide a promising strategy for developing highly efficient and compatible photocatalysts with wide applications.
1. Introduction It is well-known that water pollution is an important topic of global environmental issues [1–4]. Note that photocatalysis is a well-established and efficient strategy in removing a variety of pollut ants from wastewater [5–10]. The development of highly efficient, sta ble and low-cost photocatalyst has been a major research objective for decades. Recently, hydroxyapatite (Ca10(PO4)6(OH)2, HAp) has excel lent biocompatibility, bioactivity, environmental friendliness and low cost, and has been extensively used as biomaterials [11–14]. In addition, HAp-based materials possess prospective applications as green photo catalysts in wastewater pollutant treatment. For example, Chang et al.
[15] reported that HAp supported N-CQDs composites exhibited a pro nounced photocatalytic performance on the degradation of methylene blue (MB) under visible-light irradiation. Rgolshan et al. [16] proved that Fe3O4@HAp nanocomposite possesses very high photocatalytic �rquez et al. [17] efficiency toward acid red 73 under UV irradiation. Ma reported that HAp-titania multicomponent materials were employed as efficient UV light photocatalysts to degrade some pharmaceutical persistent pollutants such as fluoxetine and diclofenac. Nevertheless, it should be pointed out that the photocatalytic removal efficiency of HAp-based materials is still far from satisfactory. Therefore, designing and preparing unique-structured HAp-based photocatalytic materials that with high efficiency and stability in wastewater pollutant treatment
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Wu), [email protected] (S. Xue). 1 These authors contributed equally to this paper. https://doi.org/10.1016/j.solidstatesciences.2019.106067 Received 15 September 2019; Received in revised form 25 October 2019; Accepted 9 November 2019 Available online 13 November 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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prepared based on different chemical synthetic route [28–30]. Most research focus on either nano-HAp supporting on RGO surface or special structured-HAp decorating with RGO. Evidence shows that HAp-RGO composites tend to display better properties and possess potential ap plications as comparison to pure HAp. However, the ability to control unique morphology of HAp-RGO for potential application remains a great challenge. In the present study, for the first time, a series of porous hollow HAp microspheres decorated with small amounts of RGO were successfully synthesized via a facile synthetic route. It should be pointed out that porous hollow HAp microspheres tend to offer high surface areas and enhanced active sites for adsorption as well as photocatalysis during applications. As a result, porous hollow HAp microspheres decorated
Fig. 1. XRD patterns of (a) RGO, (b) HAp, (c) RGO (0.5 wt%)/HAp, (d) RGO (1.5 wt%)/HAp, and (e) RGO (3 wt%)/HAp.
is a hot research field. On the other hand, recent research reports have proved that reduced graphene oxides (RGO) can serve as high-performance reinforcements to HAp, due to its two-dimensional (2D) structure [18,19], good chemical stability [20], outstanding optical property [21–24], excellent electrical conductivity [25], and high specific surface areas [26,27]. The coupling of RGO with HAp may endow the composite material with expected properties and other additional desirable characteristics, which is helpful for adsorption as well as photocatalysis during applications. Besides, RGO is partially negatively charged because of some rich oxy gen functional groups on its surface. Notably, the negatively RGO can promote the adsorption of some kinds of antibiotics, and thus tend to facilitate the removal efficiency of target pollutant. To date, many HAp-RGO composites with various morphologies have been successfully
Fig. 3. Raman spectra of (a) HAp, (b) RGO, and (c) RGO (1.5 wt%)/HAp.
Fig. 2. FE-SEM images of (a) RGO, (b) HAp, (c) RGO (1.5 wt%)/HAp, and (d) Elemental mapping images of RGO (1.5 wt%)/HAp. 2
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Fig. 4. (a) XPS spectrum of RGO (1.5 wt%)/HAp, and (b) High resolution spectra of Ca2p, (c) P2p, (d) O1s, and (e) C1s.
with small amounts of RGO would hold promise as highly functionalized materials for potential application in wastewater pollutant treatment. In order to verify the excellent performance of the above-mentioned RGO/ HAp composites, tetracycline (TC) was adopted as a model antibiotic for photocatalytic degradation. The experimental results show that porous hollow HAp microspheres decorated with small amounts of RGO can act as efficient photocatalyst for TC degradation. These findings may pro vide new prospects for designing novel composites with wide applications.
2. Experimental 2.1. Materials Sodium polystyrene sulfonate (PSS) was purchased from MacLean Chemical Reagent. Calcium chloride (CaCl2), sodium carbonate (Na2CO3), ethanol (C2H5OH), sodium hydrogen phosphate (Na2HPO4), sodium hydroxide (NaOH), graphite powder, potassium peroxodisulfate (K2S2O8) and phosphorus pentoxide (P2O5) were purchased from Sino pharm Chemical Reagent (Shanghai, China). Concentrated sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 36–38%) and hydrochloric acid (HCl) were purchased from JunHui Chemical Reagent (Shanghai, China). All materials were used as received without further purification. 3
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2.2. Synthesis of reduced graphene oxide Graphene oxide was prepared from oxidation of graphite flakes via a modified Hummers’ method. Firstly, a mixture of natural graphite powder (3 g), phosphorus pentoxide (3 g) and potassium persulfate (3 g) was slowly dispersed in 150 mL of concentrated sulfuric acid (98%). The solution was then subjected to a magnetic stirring at 353 K for 5 h. After cooling naturally, the corresponding sample was fully cleaned by deionized water and dried in an oven at 353 K overnight. Next, both the above sample and 100 mL concentrated sulfuric acid were transferred to a round bottom flask and stirred for 1 h in an ice bath, followed by was the slow addition of 6 g potassium permanganate over 30 min with subsequent vigorous stirring for 2 h. Further, the resulting solution was moved to a pre-heated oil bath at 308 K under a stirring treatment for 3 h 300 mL deionized water was carefully added and stirred for another 2 h. Subsequently, 15 mL of 30% hydrogen peroxide was added drop-wise until the color of the solution became bright yellow. Later 10% hydro chloric acid solution was used to remove the excess of residual salts. After several times of washing by deionized water and centrifugation procedures, target RGO powder was collected after being dried in an oven at 333 K overnight.
Fig. 5. Nitrogen adsorption/desorption isotherms of RGO, HAp and RGO (1.5 wt%)/HAp.
2.3. Controllable synthesis of RGO/HAp composites A series of RGO/HAp composites were prepared via a hydrothermal method. Firstly, a hard sacrificial CaCO3 template was prepared by a PSS polymer-template method [31]. Briefly, CaCl2 solution (30 mL, 0.2 M) was added into PSS solution (300 mL, 10 g/L) under magnetic stirring, followed by was the drop-wise addition of Na2CO3 (30 mL, 0.2 M) into the prepared mixture. Afterwards, under a stirring treatment for 1 h, the resulting solution was centrifuged and fully washed by deionized water and ethanol, respectively. Target CaCO3 template was collected after vacuum drying at 333 K overnight. In a next step, 0.2 g of the obtained CaCO3 template was added into Na2HPO4 solution (100 mL, 0.2 M) under a stirring treatment. The pH value was adjusted to 11.0 by using NaOH solution. Later the above suspension was transferred into a 200 mL Teflon-lined autoclave, followed by was the addition of 0.5, 1.5 and 3 wt% RGO into the autoclave, respectively. After a hydrothermal treatment at 393 K for 1 h and cooling naturally, the corresponding sample was collected by centrifugation, washed by deionized water and ethanol for several times, respectively. Finally, the resulting sample was dried under vacuum at 333 K overnight to obtain RGO/HAp composites. In addition, porous hollow HAp microspheres (without decorating of RGO) were also prepared under the similar condition for comparison. 2.4. Characterization X-ray diffraction (XRD) measurements were accomplished on a BRUKER-D8 diffractometer with a Cu-Kα radiation. Field emission scanning electron microscopy (FE-SEM) measurements were character ized by a Hitachi SU-1500 instrument. Ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy was performed on a Shimadzu UV-2550 spectrophotometer. X-ray photoelectron spectroscopy (XPS) were taken on an ESCALAB250Xi photoelectron spectrometer. Raman spectrum was collected by an NRS-1000 (JASCO) FT-Raman spectrometer. BrunauerEmmett-Teller (BET) surface area was measured using a Gemini VII 2390 instrument (Micromeritics instrument Ltd). Total organic carbon (TOC) was measured on a TOC analyzer (Shimadzu, Japan). Electron Spin-Resonance spectroscopy (ESR) was recorded by a Bruker EPR A300-10/12 spectrometer. A high-performance liquid chromatographymass spectroscopy (HPLC-MS) was conducted to identify the photo catalytic degradation intermediates of TC. In addition, photocurrent (PC) and electrochemical impedance spectra (EIS) tests were carried out in a three-electrode system using a CHI660E electrochemical station (CH Instruments, Inc., Shanghai).
Fig. 6. (a) UV–Vis absorbance of RGO, HAp, and RGO (1.5 wt%)/HAp. (b) The plots of transformed Kubelka-Munk function vs. energy of light.
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Fig. 7. Photocatalytic degradation of TC over (a) Different samples, (b) Initial pH, (c) Pseudo-first order reaction kinetics, (d) Values of reaction constants, and (e) TOC removal efficiency in the presence of RGO (1.5 wt%)/HAp under 300 W xenon lamp with full spectrum irradiation.
2.5. Photocatalytic experiment for TC
Table 1 Comparison of the photocatalytic degradation TC of RGO (1.5 wt%)/HAp with other photocatalysts in the reported literatures. Photocatalyst
mCat (g/ L)
t (min)
Removal efficiency (%)
Ref.
Ca2þ@CdSe/rGO Cu2O/Bi2O3/rGO RGO/CdIn2S4/CN RGO/g-C3N4/ BiVO4 CdS/CoFe2O4/rGO RGO/HAp
0.5 0.5 1.0 1.0
60 180 180 150
85.6% 75.0% 74.0% 72.5%
[37] [38] [39] [40]
1.0 1.0
60 30
56.3% nearly 100%
[41] this work
Photocatalytic degradation of TC was performed in a home-made glass reactor at room temperature. A 300 W Xe arc lamp (Perfectlight Co., PLS-SXE 300) with full spectrum irradiation was used as light source. In a typical run, 100 mg of given RGO/HAp photocatalyst was firstly dispersed in a 100 mL TC aqueous solution (60 mg/L) at pH value 5.0. Followed by was the magnetically stirring treatment in darkness for 30 min to ensure an adsorption-desorption equilibrium. After that, the lamp was turned on. At given time intervals, 4 mL of suspension was withdrawn and centrifuged, and then the supernatant was analyzed to determine its residual concentration by an UV–vis spectrophotometer at the maximal TC adsorption of wavelength of 357 nm. The photocatalytic degradation efficiency (D %) of TC was calculated as follows: D% ¼ 5
C0 Ct � 100% C0
(1)
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Fig. 8. (a) Photocatalytic recycling runs for RGO (1.5 wt%)/HAp, and (b) XRD spectra of RGO (1.5 wt%)/HAp photocatalyst before and after TC degradation. Fig. 9. (a) Transient photocurrent (PC) response, and (b) Electrochemical impedance spectroscopy (EIS) analysis of the as-prepared samples.
where C0 and Ct represent the initial TC concentration and instantaneous TC concentration after a certain irradiation time (t). Note that porous hollow HAp microspheres and pure RGO were also adopted for com parison. Furthermore, the recycle experiment was performed to test the durability and recyclability of the obtained photocatalyst.
To further confirm the morphology of the as-prepared RGO/HAp composites, FE-SEM was conducted and the representative image of RGO (1.5 wt%)/HAp composite is shown in Fig. 2. For comparison, the typical morphologies of the as-prepared RGO and HAp are also included in Fig. 2. Fig. 2a displays a thin layer of RGO with the characteristic wrinkles and ripples structure. Note that the as-prepared HAp possesses a uniform spherical morphology with a hollow urchin-like structure (inset of Fig. 2b) and its size distribution is ca. 1 μm. Evidence from Fig. 2b shows some broken microspheres with hollow interiors, providing good proof that HAp microspheres have hollow structures [32]. A good feature for such hollow HAp microspheres is offering high surface areas and more active sites, which is helpful for both adsorption and photocatalytic degradation during applications. As for RGO (1.5 wt %)/HAp composite, it is evident from the FE-SEM image that some hollow HAp microspheres were partially wrapped with thin layer of RGO (Fig. 2c). In addition, the corresponding elemental mapping images (Fig. 2d) demonstrate the existence of all elements (C, O, Ca and P) in the composite together, further confirming the successful synthesis of RGO/HAp composite. As it well-known that Raman spectroscopy is an important and informative technique for structural analysis of both HAp and carbo naceous materials. Fig. 3 displays the typical Raman spectra of the asprepared RGO, porous hollow HAp microspheres and RGO (1.5 wt
3. Results and discussion 3.1. Characterization of RGO/HAp composites The phase composition and crystallinity of various RGO/HAp com posites were firstly investigated by XRD patterns. For comparison, the XRD patterns of the as-prepared RGO and HAp were also measured. Obviously, the bulk RGO (Fig. 1a) gives a typical broad diffraction peak centered at 25� , corresponds to (002) plane and confirms short range order in the stacked graphene sheets, suggesting the formation of RGO. As for HAp (Fig. 1b), all the diffraction peaks match very well the typical hexagonal structured HAp (JCPDS No. 09-0432). There are no peaks of any other phases or impurities involved. Moreover, the as-prepared different RGO/HAp composites exhibit similar XRD patterns (Fig. 1c–e) to those of porous hollow HAp microspheres. Note that the disappearance of RGO peak in the composites was due to trace amount of RGO without being measured. No peaks of any other phases or im purities are involved in the composites. The results indicate the suc cessful synthesis of RGO/HAp composites in the present study. 6
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Fig. 10. (a) Photocatalytic activities of RGO (1.5 wt%)/HAp towards TC degradation in the presence of different scavengers under 300 W xenon lamp full spectrum irradiation, (b) ESR signals of DMPO-⋅O2 for RGO (1.5 wt%)/HAp, (c) ESR signals of DMPO-⋅OH for RGO (1.5 wt%)/HAp, and (d) ESR signals of TEMPO-hþ for RGO (1.5 wt%)/HAp in the dark and under 300 W xenon lamp full spectrum irradiation.
Fig. 11. Photocatalytic removal mechanism of TC over the as-prepared RGO/HAp composite.
%)/HAp composite. As for pure RGO, the characteristics peaks of RGO at 1332 cm 1 and 1584 cm 1 are assignable to sp3 D bands and sp2 G bands, respectively. Note that the intensity ratio of ID/IG is a key parameter to estimate the structural defects in carbonaceous materials. Moreover, in case of porous hollow HAp microspheres, Raman spectra shows a very weak peak at 952 cm 1, which is assigned to the symmetric stretching ν1(PO34 ) corresponding to free tetrahedral phosphate ions, as
confirmed in other literatures [11,33]. As expected, further Raman spectroscopy analysis demonstrated the coexistence of HAp and RGO in RGO (1.5 wt%)/HAp composite. This is a good proof that RGO/HAp composite was formed in the present study, as evidenced by the above XRD and FE-SEM results. The intensity of the ID/IG ratio of RGO (1.5 wt %)/HAp composite (ID/IG ¼ 1.26) is much higher than RGO (ID/IG ¼ 0.95), indicating a large defects density in its structure. 7
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Furthermore, a slight peak shift with lower frequency of RGO (1.5 wt %)/HAp composite in Raman spectra indicates the interaction between HAp and RGO. Next, XPS analysis was employed to identify the surface composition of the as-prepared RGO (1.5 wt%)/HAp composite, and the results are shown in Fig. 4. It can be seen the photoelectron lines at binding energy (EB) values at ca. 285 eV, 532 eV, 348 eV, and 133 eV further confirm the presence of C, O, Ca, and P elements, respectively. In addition, highresolution XPS spectra of the Ca2p, P2p, C1s, and O1s are demon strated in Fig. 4b–e. As shown in Fig. 4b, the peak at 348 eV and 351.5 eV are belong to the Ca 2p3/2 and Ca 2p1/2. The peak at 133.9 eV in Fig. 4c is assigned to the presence of phosphate group (P2p) in HAp. Moreover, the O1s exhibit an intense peak at 532.1 eV (Fig. 4d), asso ciated with the anionic oxygen in phosphate radical (PO34 ) and hy droxyl (OH) in the HAp and residual oxygen functional groups presented in RGO sheets, as reported in other literature [34]. As it can be seen from Fig. 4e, the C1s spectra is positioned at 284.7 eV, corresponding to – C and C–C) and the other carbon components on the benzene ring (C– – O) groups [35]. peak is assigned to carboxylate (O–C– Fig. 5 presents N2 adsorption-desorption isotherms of the asprepared RGO (1.5 wt%)/HAp composite. The isotherm shows a type IV loop at a relative pressure range from 0.4 to 1. The BET specific surface area was calculated to be 46.8 m2/g for RGO (1.5 wt%)/HAp composite. As for porous hollow HAp microspheres and pure RGO, the BET specific surface area was 44.2 and 16.6 m2/g, respectively. It should be noted that the increased BET specific surface area is not derived from HAp but mainly derived from RGO. Hopefully, high specific surface area and synergistic effect of RGO and porous hollow HAp microspheres can endow RGO (1.5 wt%)/HAp composite with strong adsorption ability, and thus favors the removal efficiency of target pollutant in wastewater. UV–Vis diffuse reflectance spectrum was further conducted to characterize the absorption capability of the as-prepared RGO (1.5 wt %)/HAp composite, and the result is shown in Fig. 6. For comparison, the UV–Vis diffuse reflectance spectra of the as-prepared RGO and porous hollow HAp microspheres are also included in Fig. 6. As for RGO, due to its well-performing light harvesting feature, it manifested a typical light absorption capacity. In contrast, an obvious absorption peak in the UV-light range (<400 nm) was observed for porous hollow HAp microspheres. In case of the as-prepared RGO (1.5 wt%)/HAp composite, the presence of RGO led to an increase of absorption in the visible-light range, due to the characteristic light absorption capacity of carbon materials. In addition, the bandgaps of HAp and RGO (1.5 wt %)/HAp composite obtained from the Tauc plot (Fig. 6b) [36], were determined to be 5.00 eV and 4.90 eV, respectively. It can be inferred that the as-prepared RGO (1.5 wt%)/HAp composite show better pho tocatalytic activity in the whole light range, as compared with porous hollow HAp microspheres.
a 300 W xenon lamp with full spectrum irradiation, which was in the order of RGO (1.5 wt%)/HAp > RGO (0.5 wt%)/HAp > HAp > RGO (3 wt%)/HAp > RGO. As for porous hollow HAp microspheres, after adsorbing 20.8% of TC (Fig. S1) for 30 min, about 68.1% of TC was degraded under a 300 W xenon lamp full spectrum irradiation within 30 min, and reaching 88.9% removal efficiency in total. In case of RGO, after adsorbing 17.7% of TC (Fig. S1) for 30 min, about 12.8% of TC was degraded under the similar experimental conditions. While the total removal efficiency of TC over RGO was about 30.5%. Among the three tested RGO/HAp composites, RGO (1.5 wt%)/HAp exhibits the best photocatalytic performance (ca. 92.1%) towards TC degradation within 30 min under a 300 W xenon lamp with full spectrum irradiation. Note that its adsorption efficiency for TC in dark period was 7.9% (Fig. S1) after 30 min. Thus, the total removal efficiency for TC was ca. 100% for RGO (1.5 wt%)/HAp. It should be pointed out that in all cases, the dosage amount of photocatalyst is 0.1 g. Undoubtedly, the as-prepared RGO (1.5 wt%)/HAp showed much higher photocatalytic efficiency in TC degradation, taking advantage over the corresponding porous hollow HAp microspheres and RGO samples. Table 1 shows the comparison of the photocatalytic degradation TC of RGO (1.5 wt%)/HAp with other photocatalysts in the reported literatures. It can be clearly seen that GO (1.5 wt%)/HAp has the highest photocatalytic ability to degrade TC. It is well-known that the initial pH value has an important influence on the photodegradation of TC. A series of experiments (Fig. 7b) shows the removal efficiency of TC at different pH (3–9) over RGO (1.5 wt %)/HAp photocatalysts under a 300 W xenon lamp with full spectrum irradiation. It was shown that the photocatalytic degradation efficiency of TC was slightly influenced by pH value over RGO (1.5 wt%)/HAp, and it follows the order of pH (5) > pH (7) > pH (9) > pH (3). Therefore, in the current study, the optimal initial pH value is pH 5. Fig. 7c indicates that all the photocatalytic degradation data can fit well with the pseudo-first-order correlation (ln (C0/Ct) ¼ κt), confirming that the photocatalytic degradation TC reaction kinetics over the asprepared RGO/HAp photocatalysts is followed pseudo-first-order. In addition, the largest apparent rate constant of RGO (1.5 wt%)/HAp in Fig. 7d was 0.1816 min 1, which was about 31.9 and 14.4 times higher than that of RGO and porous hollow HAp microspheres, respectively. The above experimental results demonstrate that an optimum deco rating amount (1.5 wt%) of RGO onto porous hollow HAp microspheres can act as an effective photocatalyst for TC degradation. In order to evaluate the photocatalytic degradation ability of the asprepared RGO (1.5 wt%)/HAp for TC, the mineralization activity was investigated based on TOC analysis. As shown in Fig. 7e, a mineraliza tion efficiency of 85% can be obtained over RGO (1.5 wt%)/HAp for TC after 30 min irradiation, indicating the as-prepared RGO (1.5 wt%)/HAp photocatalyst possessed higher mineralization ability for TC degrada tion. It is worth noting that 85% mineralization efficiency was lower than that of degradation efficiency (almost 100%), suggesting that complete mineralization could not be achieved, and some organic in termediates might generate in the photocatalytic process. The reaction intermediates in the photocatalytic process was further analyzed by HPLC-MS (Fig. S2). As observed, there was a characteristic anion peak at m/z 445 belonging to the deprotonated TC. From Fig. S3, it can be seen that the ions with m/z ¼ 317 and m/z ¼ 216 were detected with the increasing reaction time. The above results confirming that TC molecule is degraded into CO2, H2O and some other intermediates. As it well-known that the stability and recycling performance of a photocatalyst is important for practical applications. As indicated in Fig. 8a, there is no obvious change in the photocatalytic activity of RGO (1.5 wt%)/HAp composite even after three successive photocatalytic reaction, indicating its good recyclability in TC degradation during re cycles. Thus, it can be inferred that the as-prepared RGO/HAp composite can be used as a stable and high-performance photocatalyst. The structural stability of RGO (1.5 wt%)/HAp composite before and after five TC photocatalytic degradation cycles were characterized by XRD patterns. As observed in Fig. 8b, the XRD patterns of three recycled
3.2. Photocatalytic performance for TC over RGO/HAp composites To investigate the photocatalytic efficiencies of the as-prepared RGO/HAp composites, TC was used as a model antibiotic for photo catalytic degradation. Fig. 7a illustrates the photocatalytic degradation of TC over different samples under a 300 W xenon lamp with full spec trum irradiation. For comparison, the photocatalytic degradation of TC over the as-prepared porous hollow HAp microspheres and RGO were also tested. In addition, the photocatalytic degradation of TC over RGO (1.5 wt%)/HAp under a 300 W xenon lamp equipped with an optical cutoff filter (λ > 420 nm) that offered a visible-light irradiation was performed and compared. As observed, blank test demonstrates little photolysis and the corresponding efficiency is only ca. 9.4%. Note that the RGO (1.5 wt%)/HAp composite exhibited much higher photo catalytic removal efficiency under a 300 W xenon lamp with full spec trum irradiation than that of the sample performed under a visible-light irradiation. It can be seen that the as-prepared photocatalysts demon strated different photocatalytic degradation rates for TC removal under 8
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RGO (1.5 wt%)/HAp composite had almost no change compared with fresh RGO (1.5 wt%)/HAp composite. Therefore, it can be concluded that the as-prepared RGO (1.5 wt%)/HAp composite is reusable and stable in the photocatalytic process, and has a promising potential for practical application. 3.3. Photocatalytic mechanistic aspects
HAp þ hν→e þ hþ
(2)
e þ RGO→RGO ðe Þ
(3)
O2 þ e ðRGOÞ→⋅O2
(4)
� ⋅O2 hþ þ TC→degraded products
(5)
As demonstrated in Fig. 11, porous hollow HAp microspheres and RGO can act as an electron donor and an electron acceptor, respectively. Under a 300 W xenon lamp full spectrum irradiation, the photo generated electrons (e ) in valence band (VB) of porous hollow HAp microspheres can be excited to conduction band (CB), leaving behind the promotion of positive holes (hþ). Due to the good conductivity and many defects of RGO in the composite, the excited electrons (e ) in the CB of porous hollow HAp microspheres can be transferred to the surface of RGO. Note that the CB and VB potential of HAp are 2.20 V and 2.80 V (vs. NHE). HAp has a more negative CB potential than the O2/ �O2 ( 0.33 V vs. NHE), thus the photogenerated electrons (e ) accu mulating at RGO could interact with O2 to form ⋅O2 , resulting in partial degradation of TC molecules. Besides, the separated positive holes (hþ) in the VB of porous hollow HAp microspheres can directly participate in the oxidation of TC molecules.
Photocurrent response (PC) is an important technique for analyzing charge transfer and separation in a semiconductor which could be significantly affects its photocatalytic performance. Usually, a higher photocurrent density means a higher ability in separating the photogenerated electrons and holes. Fig. 9a shows the photocurrent response of the as-prepared RGO, HAp, and RGO (1.5 wt%)/HAp com posite. As clearly observed, RGO (1.5 wt%)/HAp composite demon strates the highest photocurrent intensity, much higher than those of the as-prepared RGO and HAp. The highest photocurrent was due to the efficient transfer and separation of photo-generated electrons and holes in RGO (1.5 wt%)/HAp composite. The introduction of RGO plays an important role in charge transfer and separation, which is beneficial for photocatalytic process. Moreover, electrochemical impedance spectra (EIS) test is another effective way to analyze the transfer and separation of the photogenerated electrons and holes. Fig. 9b displays the EIS Nyquist plots of the as-prepared RGO, HAp, and RGO (1.5 wt%)/HAp composite under the identical experimental conditions. Obviously, the arc radius of RGO (1.5 wt%)/HAp composite in the EIS Nyquist plot was the smallest among these samples, indicating its highest charge transfer ability. The above point was well consistent with the result of photocurrent response. Next, trapping experiments were conducted to elucidate the reasonable photocatalytic mechanism. 2-Propanol (IPA), disodium ethylene diamine tetraacetic acid (EDTA), and 1, 4-benzoquinone (BQ) were used as the scavengers to detect the main active species during the photocatalytic process. As can be seen from Fig. 10a, with the addition of IPA into the reaction system, there was a slight decrease in the degra dation efficiency, suggesting that ⋅OH is not a main active species in the present study. In contrast, the degradation efficiency is remarkably reduced in the presence of BQ, this indicating that ⋅O2 plays an important role in TC photocatalytic degradation process. Similarly, an obvious loss could be found when EDTA was used for the scavenger in the photocatalytic degradation system, verifying that hþ is another crucial pathway for TC removal. Thus, it can be inferred that both ⋅O2 and hþ are the main active species that responsible for the photocatalytic degradation of TC. The above scavenger additional experiments provide good proof for the photocatalytic mechanistic analysis. In order to further confirm the active species generated in the pho tocatalytic process, ESR analysis were performed and the corresponding results are shown in Fig. 10b–d. Note that ⋅OH and ⋅O2 radicals can be captured by DMPO, while hþ radicals can be captured by the TEMPO. As indicated in Fig. 10b, there appeared obvious characteristic peaks of DMPO-⋅O2 , however no signals were found in the dark. The results demonstrate that ⋅O2 plays an important role in the photocatalytic process. Note that very weak characteristic peak of DMPO-⋅OH can be observed From Fig. 10c, suggesting that ⋅OH is not a main active species in the photocatalytic reaction. Furthermore, the characteristic peaks of TEMPO-hþ were also clearly detected in Fig. 10d, and the signals were so strong in the dark, while it slightly decreased under the xenon lamp irradiation. The results indicate that hþ also act as main active species in the photocatalytic reaction. The ESR results confirm that both ⋅O2 and hþ are the main active species in the photocatalytic process, which was well consistent with the above trapping experimental result. Based on the above analysis, a possible photocatalytic mechanism of RGO/HAp composite photocatalyst is proposed. The possible photo catalytic experiment equations are expressed as follows:
4. Conclusion In summary, a facile hydrothermal route was adopted and optimized for small amounts of RGO decorating on the porous hollow HAp mi crospheres for the first time. Their subsequent applications were inves tigated by photocatalytic degradation of TC. It turned out that the asprepared RGO (1.5 wt%)/HAp composite displays an excellent photo catalytic activity for TC degradation under a 300 W xenon lamp with full spectrum irradiation, which was much faster than those over porous hollow HAp microspheres and RGO. High specific surface areas, desir able absorption capability, suppression of charge recombination, and synergistic effects of RGO and HAp should be responsible for the high photocatalytic performance. This research will offer a promising plat form for developing highly efficient and stable photocatalyst that with potential application prospects. Declaration of competing interest We declare that we do not have any commercial or associative in terest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21107069), and the Science and Technology Commission of Shanghai Municipality (14DZ2261000). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solidstatesciences.2019.106067. References [1] L. Alamonole, S. Bailonruiz, T. Lunapineda, O. Peralesperez, F.R. Roman, Photocatalytic activity of quantum dot-magnetite nanocomposites to degrade organic dyes in the aqueous phase, J. Mater. Chem. A 1 (2013) 5509–5516. [2] H. Wang, Y. Sun, Y. Wu, W. Tu, S. Wu, X. Yuan, G. Zeng, Z.J. Xu, S. Li, J.W. Chew, Electrical promotion of spatially photoinduced charge separation via interfacialbuilt-in quasi-alloying effect in hierarchical Zn2In2S5/Ti3C2(O,OH)x hybrids toward efficient photocatalytic hydrogen evolution and environmental remediation, Appl. Catal. B Environ. 245 (2019) 290–301.
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Contents lists available at ScienceDirect
Solid State Sciences journal homepage: http://www.elsevier.com/locate/ssscie
Novel design of porous hollow hydroxyapatite microspheres decorated by reduced graphene oxides with superior photocatalytic performance for tetracycline removal Rongjiang Zou a, 1, Tianhong Xu b, 1, Xiaofang Lei b, Qiang Wu b, *, Song Xue a, ** a
Department of Cardiovascular Surgery, Ren Ji Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai, 200127, PR China Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai, 200090, PR China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Reduced graphene oxides (RGO) Hydroxyapatite (HAp) Tetracycline (TC) Photocatalysis
Photocatalysis is a well-established and green technology with cost-effective, high-performance and environment benign for tetracycline (TC) removal in wastewater. Recently, HAp-based materials have been proved to be cheap and green photocatalysts for wastewater pollutant treatment. However, the photocatalytic removal efficiency of these existing HAp-based materials is still not desirable. Herein, a series of new porous hollow hydroxyapatite microspheres decorated with small amounts of reduced graphene oxides (0.5, 1.5 and 3 wt%) were firstly and successfully fabricated by a facile hydrothermal method. The as-prepared RGO/HAp composites were charac terized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer Emmett-Teller (BET) and photoelectrochemical measurements. Furthermore, their photocatalytic applications were investi gated by using TC as a model contaminant. It turned out that the as-prepared RGO (1.5 wt%)/HAp composite exhibits an outstanding photocatalytic activity for TC degradation (60 mg/L) under a 300 W xenon lamp with full spectrum irradiation (92.1%, 30 min). Note that its adsorption efficiency for TC in dark period was only 7.9% after 30 min. Thus, the total removal efficiency for TC was nearly 100%. Furthermore, the as-prepared RGO (1.5 wt%)/HAp exhibited remarkable stability and repeatability, demonstrating its promising potential as an efficient photocatalyst. A plausible photocatalytic reaction mechanism was proposed on the basis of electron spin resonance (ESR) and trapping experiments. This fundamental research will provide a promising strategy for developing highly efficient and compatible photocatalysts with wide applications.
1. Introduction It is well-known that water pollution is an important topic of global environmental issues [1–4]. Note that photocatalysis is a well-established and efficient strategy in removing a variety of pollut ants from wastewater [5–10]. The development of highly efficient, sta ble and low-cost photocatalyst has been a major research objective for decades. Recently, hydroxyapatite (Ca10(PO4)6(OH)2, HAp) has excel lent biocompatibility, bioactivity, environmental friendliness and low cost, and has been extensively used as biomaterials [11–14]. In addition, HAp-based materials possess prospective applications as green photo catalysts in wastewater pollutant treatment. For example, Chang et al.
[15] reported that HAp supported N-CQDs composites exhibited a pro nounced photocatalytic performance on the degradation of methylene blue (MB) under visible-light irradiation. Rgolshan et al. [16] proved that Fe3O4@HAp nanocomposite possesses very high photocatalytic �rquez et al. [17] efficiency toward acid red 73 under UV irradiation. Ma reported that HAp-titania multicomponent materials were employed as efficient UV light photocatalysts to degrade some pharmaceutical persistent pollutants such as fluoxetine and diclofenac. Nevertheless, it should be pointed out that the photocatalytic removal efficiency of HAp-based materials is still far from satisfactory. Therefore, designing and preparing unique-structured HAp-based photocatalytic materials that with high efficiency and stability in wastewater pollutant treatment
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Wu), [email protected] (S. Xue). 1 These authors contributed equally to this paper. https://doi.org/10.1016/j.solidstatesciences.2019.106067 Received 15 September 2019; Received in revised form 25 October 2019; Accepted 9 November 2019 Available online 13 November 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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prepared based on different chemical synthetic route [28–30]. Most research focus on either nano-HAp supporting on RGO surface or special structured-HAp decorating with RGO. Evidence shows that HAp-RGO composites tend to display better properties and possess potential ap plications as comparison to pure HAp. However, the ability to control unique morphology of HAp-RGO for potential application remains a great challenge. In the present study, for the first time, a series of porous hollow HAp microspheres decorated with small amounts of RGO were successfully synthesized via a facile synthetic route. It should be pointed out that porous hollow HAp microspheres tend to offer high surface areas and enhanced active sites for adsorption as well as photocatalysis during applications. As a result, porous hollow HAp microspheres decorated
Fig. 1. XRD patterns of (a) RGO, (b) HAp, (c) RGO (0.5 wt%)/HAp, (d) RGO (1.5 wt%)/HAp, and (e) RGO (3 wt%)/HAp.
is a hot research field. On the other hand, recent research reports have proved that reduced graphene oxides (RGO) can serve as high-performance reinforcements to HAp, due to its two-dimensional (2D) structure [18,19], good chemical stability [20], outstanding optical property [21–24], excellent electrical conductivity [25], and high specific surface areas [26,27]. The coupling of RGO with HAp may endow the composite material with expected properties and other additional desirable characteristics, which is helpful for adsorption as well as photocatalysis during applications. Besides, RGO is partially negatively charged because of some rich oxy gen functional groups on its surface. Notably, the negatively RGO can promote the adsorption of some kinds of antibiotics, and thus tend to facilitate the removal efficiency of target pollutant. To date, many HAp-RGO composites with various morphologies have been successfully
Fig. 3. Raman spectra of (a) HAp, (b) RGO, and (c) RGO (1.5 wt%)/HAp.
Fig. 2. FE-SEM images of (a) RGO, (b) HAp, (c) RGO (1.5 wt%)/HAp, and (d) Elemental mapping images of RGO (1.5 wt%)/HAp. 2
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Fig. 4. (a) XPS spectrum of RGO (1.5 wt%)/HAp, and (b) High resolution spectra of Ca2p, (c) P2p, (d) O1s, and (e) C1s.
with small amounts of RGO would hold promise as highly functionalized materials for potential application in wastewater pollutant treatment. In order to verify the excellent performance of the above-mentioned RGO/ HAp composites, tetracycline (TC) was adopted as a model antibiotic for photocatalytic degradation. The experimental results show that porous hollow HAp microspheres decorated with small amounts of RGO can act as efficient photocatalyst for TC degradation. These findings may pro vide new prospects for designing novel composites with wide applications.
2. Experimental 2.1. Materials Sodium polystyrene sulfonate (PSS) was purchased from MacLean Chemical Reagent. Calcium chloride (CaCl2), sodium carbonate (Na2CO3), ethanol (C2H5OH), sodium hydrogen phosphate (Na2HPO4), sodium hydroxide (NaOH), graphite powder, potassium peroxodisulfate (K2S2O8) and phosphorus pentoxide (P2O5) were purchased from Sino pharm Chemical Reagent (Shanghai, China). Concentrated sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 36–38%) and hydrochloric acid (HCl) were purchased from JunHui Chemical Reagent (Shanghai, China). All materials were used as received without further purification. 3
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2.2. Synthesis of reduced graphene oxide Graphene oxide was prepared from oxidation of graphite flakes via a modified Hummers’ method. Firstly, a mixture of natural graphite powder (3 g), phosphorus pentoxide (3 g) and potassium persulfate (3 g) was slowly dispersed in 150 mL of concentrated sulfuric acid (98%). The solution was then subjected to a magnetic stirring at 353 K for 5 h. After cooling naturally, the corresponding sample was fully cleaned by deionized water and dried in an oven at 353 K overnight. Next, both the above sample and 100 mL concentrated sulfuric acid were transferred to a round bottom flask and stirred for 1 h in an ice bath, followed by was the slow addition of 6 g potassium permanganate over 30 min with subsequent vigorous stirring for 2 h. Further, the resulting solution was moved to a pre-heated oil bath at 308 K under a stirring treatment for 3 h 300 mL deionized water was carefully added and stirred for another 2 h. Subsequently, 15 mL of 30% hydrogen peroxide was added drop-wise until the color of the solution became bright yellow. Later 10% hydro chloric acid solution was used to remove the excess of residual salts. After several times of washing by deionized water and centrifugation procedures, target RGO powder was collected after being dried in an oven at 333 K overnight.
Fig. 5. Nitrogen adsorption/desorption isotherms of RGO, HAp and RGO (1.5 wt%)/HAp.
2.3. Controllable synthesis of RGO/HAp composites A series of RGO/HAp composites were prepared via a hydrothermal method. Firstly, a hard sacrificial CaCO3 template was prepared by a PSS polymer-template method [31]. Briefly, CaCl2 solution (30 mL, 0.2 M) was added into PSS solution (300 mL, 10 g/L) under magnetic stirring, followed by was the drop-wise addition of Na2CO3 (30 mL, 0.2 M) into the prepared mixture. Afterwards, under a stirring treatment for 1 h, the resulting solution was centrifuged and fully washed by deionized water and ethanol, respectively. Target CaCO3 template was collected after vacuum drying at 333 K overnight. In a next step, 0.2 g of the obtained CaCO3 template was added into Na2HPO4 solution (100 mL, 0.2 M) under a stirring treatment. The pH value was adjusted to 11.0 by using NaOH solution. Later the above suspension was transferred into a 200 mL Teflon-lined autoclave, followed by was the addition of 0.5, 1.5 and 3 wt% RGO into the autoclave, respectively. After a hydrothermal treatment at 393 K for 1 h and cooling naturally, the corresponding sample was collected by centrifugation, washed by deionized water and ethanol for several times, respectively. Finally, the resulting sample was dried under vacuum at 333 K overnight to obtain RGO/HAp composites. In addition, porous hollow HAp microspheres (without decorating of RGO) were also prepared under the similar condition for comparison. 2.4. Characterization X-ray diffraction (XRD) measurements were accomplished on a BRUKER-D8 diffractometer with a Cu-Kα radiation. Field emission scanning electron microscopy (FE-SEM) measurements were character ized by a Hitachi SU-1500 instrument. Ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy was performed on a Shimadzu UV-2550 spectrophotometer. X-ray photoelectron spectroscopy (XPS) were taken on an ESCALAB250Xi photoelectron spectrometer. Raman spectrum was collected by an NRS-1000 (JASCO) FT-Raman spectrometer. BrunauerEmmett-Teller (BET) surface area was measured using a Gemini VII 2390 instrument (Micromeritics instrument Ltd). Total organic carbon (TOC) was measured on a TOC analyzer (Shimadzu, Japan). Electron Spin-Resonance spectroscopy (ESR) was recorded by a Bruker EPR A300-10/12 spectrometer. A high-performance liquid chromatographymass spectroscopy (HPLC-MS) was conducted to identify the photo catalytic degradation intermediates of TC. In addition, photocurrent (PC) and electrochemical impedance spectra (EIS) tests were carried out in a three-electrode system using a CHI660E electrochemical station (CH Instruments, Inc., Shanghai).
Fig. 6. (a) UV–Vis absorbance of RGO, HAp, and RGO (1.5 wt%)/HAp. (b) The plots of transformed Kubelka-Munk function vs. energy of light.
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Fig. 7. Photocatalytic degradation of TC over (a) Different samples, (b) Initial pH, (c) Pseudo-first order reaction kinetics, (d) Values of reaction constants, and (e) TOC removal efficiency in the presence of RGO (1.5 wt%)/HAp under 300 W xenon lamp with full spectrum irradiation.
2.5. Photocatalytic experiment for TC
Table 1 Comparison of the photocatalytic degradation TC of RGO (1.5 wt%)/HAp with other photocatalysts in the reported literatures. Photocatalyst
mCat (g/ L)
t (min)
Removal efficiency (%)
Ref.
Ca2þ@CdSe/rGO Cu2O/Bi2O3/rGO RGO/CdIn2S4/CN RGO/g-C3N4/ BiVO4 CdS/CoFe2O4/rGO RGO/HAp
0.5 0.5 1.0 1.0
60 180 180 150
85.6% 75.0% 74.0% 72.5%
[37] [38] [39] [40]
1.0 1.0
60 30
56.3% nearly 100%
[41] this work
Photocatalytic degradation of TC was performed in a home-made glass reactor at room temperature. A 300 W Xe arc lamp (Perfectlight Co., PLS-SXE 300) with full spectrum irradiation was used as light source. In a typical run, 100 mg of given RGO/HAp photocatalyst was firstly dispersed in a 100 mL TC aqueous solution (60 mg/L) at pH value 5.0. Followed by was the magnetically stirring treatment in darkness for 30 min to ensure an adsorption-desorption equilibrium. After that, the lamp was turned on. At given time intervals, 4 mL of suspension was withdrawn and centrifuged, and then the supernatant was analyzed to determine its residual concentration by an UV–vis spectrophotometer at the maximal TC adsorption of wavelength of 357 nm. The photocatalytic degradation efficiency (D %) of TC was calculated as follows: D% ¼ 5
C0 Ct � 100% C0
(1)
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Fig. 8. (a) Photocatalytic recycling runs for RGO (1.5 wt%)/HAp, and (b) XRD spectra of RGO (1.5 wt%)/HAp photocatalyst before and after TC degradation. Fig. 9. (a) Transient photocurrent (PC) response, and (b) Electrochemical impedance spectroscopy (EIS) analysis of the as-prepared samples.
where C0 and Ct represent the initial TC concentration and instantaneous TC concentration after a certain irradiation time (t). Note that porous hollow HAp microspheres and pure RGO were also adopted for com parison. Furthermore, the recycle experiment was performed to test the durability and recyclability of the obtained photocatalyst.
To further confirm the morphology of the as-prepared RGO/HAp composites, FE-SEM was conducted and the representative image of RGO (1.5 wt%)/HAp composite is shown in Fig. 2. For comparison, the typical morphologies of the as-prepared RGO and HAp are also included in Fig. 2. Fig. 2a displays a thin layer of RGO with the characteristic wrinkles and ripples structure. Note that the as-prepared HAp possesses a uniform spherical morphology with a hollow urchin-like structure (inset of Fig. 2b) and its size distribution is ca. 1 μm. Evidence from Fig. 2b shows some broken microspheres with hollow interiors, providing good proof that HAp microspheres have hollow structures [32]. A good feature for such hollow HAp microspheres is offering high surface areas and more active sites, which is helpful for both adsorption and photocatalytic degradation during applications. As for RGO (1.5 wt %)/HAp composite, it is evident from the FE-SEM image that some hollow HAp microspheres were partially wrapped with thin layer of RGO (Fig. 2c). In addition, the corresponding elemental mapping images (Fig. 2d) demonstrate the existence of all elements (C, O, Ca and P) in the composite together, further confirming the successful synthesis of RGO/HAp composite. As it well-known that Raman spectroscopy is an important and informative technique for structural analysis of both HAp and carbo naceous materials. Fig. 3 displays the typical Raman spectra of the asprepared RGO, porous hollow HAp microspheres and RGO (1.5 wt
3. Results and discussion 3.1. Characterization of RGO/HAp composites The phase composition and crystallinity of various RGO/HAp com posites were firstly investigated by XRD patterns. For comparison, the XRD patterns of the as-prepared RGO and HAp were also measured. Obviously, the bulk RGO (Fig. 1a) gives a typical broad diffraction peak centered at 25� , corresponds to (002) plane and confirms short range order in the stacked graphene sheets, suggesting the formation of RGO. As for HAp (Fig. 1b), all the diffraction peaks match very well the typical hexagonal structured HAp (JCPDS No. 09-0432). There are no peaks of any other phases or impurities involved. Moreover, the as-prepared different RGO/HAp composites exhibit similar XRD patterns (Fig. 1c–e) to those of porous hollow HAp microspheres. Note that the disappearance of RGO peak in the composites was due to trace amount of RGO without being measured. No peaks of any other phases or im purities are involved in the composites. The results indicate the suc cessful synthesis of RGO/HAp composites in the present study. 6
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Fig. 10. (a) Photocatalytic activities of RGO (1.5 wt%)/HAp towards TC degradation in the presence of different scavengers under 300 W xenon lamp full spectrum irradiation, (b) ESR signals of DMPO-⋅O2 for RGO (1.5 wt%)/HAp, (c) ESR signals of DMPO-⋅OH for RGO (1.5 wt%)/HAp, and (d) ESR signals of TEMPO-hþ for RGO (1.5 wt%)/HAp in the dark and under 300 W xenon lamp full spectrum irradiation.
Fig. 11. Photocatalytic removal mechanism of TC over the as-prepared RGO/HAp composite.
%)/HAp composite. As for pure RGO, the characteristics peaks of RGO at 1332 cm 1 and 1584 cm 1 are assignable to sp3 D bands and sp2 G bands, respectively. Note that the intensity ratio of ID/IG is a key parameter to estimate the structural defects in carbonaceous materials. Moreover, in case of porous hollow HAp microspheres, Raman spectra shows a very weak peak at 952 cm 1, which is assigned to the symmetric stretching ν1(PO34 ) corresponding to free tetrahedral phosphate ions, as
confirmed in other literatures [11,33]. As expected, further Raman spectroscopy analysis demonstrated the coexistence of HAp and RGO in RGO (1.5 wt%)/HAp composite. This is a good proof that RGO/HAp composite was formed in the present study, as evidenced by the above XRD and FE-SEM results. The intensity of the ID/IG ratio of RGO (1.5 wt %)/HAp composite (ID/IG ¼ 1.26) is much higher than RGO (ID/IG ¼ 0.95), indicating a large defects density in its structure. 7
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Furthermore, a slight peak shift with lower frequency of RGO (1.5 wt %)/HAp composite in Raman spectra indicates the interaction between HAp and RGO. Next, XPS analysis was employed to identify the surface composition of the as-prepared RGO (1.5 wt%)/HAp composite, and the results are shown in Fig. 4. It can be seen the photoelectron lines at binding energy (EB) values at ca. 285 eV, 532 eV, 348 eV, and 133 eV further confirm the presence of C, O, Ca, and P elements, respectively. In addition, highresolution XPS spectra of the Ca2p, P2p, C1s, and O1s are demon strated in Fig. 4b–e. As shown in Fig. 4b, the peak at 348 eV and 351.5 eV are belong to the Ca 2p3/2 and Ca 2p1/2. The peak at 133.9 eV in Fig. 4c is assigned to the presence of phosphate group (P2p) in HAp. Moreover, the O1s exhibit an intense peak at 532.1 eV (Fig. 4d), asso ciated with the anionic oxygen in phosphate radical (PO34 ) and hy droxyl (OH) in the HAp and residual oxygen functional groups presented in RGO sheets, as reported in other literature [34]. As it can be seen from Fig. 4e, the C1s spectra is positioned at 284.7 eV, corresponding to – C and C–C) and the other carbon components on the benzene ring (C– – O) groups [35]. peak is assigned to carboxylate (O–C– Fig. 5 presents N2 adsorption-desorption isotherms of the asprepared RGO (1.5 wt%)/HAp composite. The isotherm shows a type IV loop at a relative pressure range from 0.4 to 1. The BET specific surface area was calculated to be 46.8 m2/g for RGO (1.5 wt%)/HAp composite. As for porous hollow HAp microspheres and pure RGO, the BET specific surface area was 44.2 and 16.6 m2/g, respectively. It should be noted that the increased BET specific surface area is not derived from HAp but mainly derived from RGO. Hopefully, high specific surface area and synergistic effect of RGO and porous hollow HAp microspheres can endow RGO (1.5 wt%)/HAp composite with strong adsorption ability, and thus favors the removal efficiency of target pollutant in wastewater. UV–Vis diffuse reflectance spectrum was further conducted to characterize the absorption capability of the as-prepared RGO (1.5 wt %)/HAp composite, and the result is shown in Fig. 6. For comparison, the UV–Vis diffuse reflectance spectra of the as-prepared RGO and porous hollow HAp microspheres are also included in Fig. 6. As for RGO, due to its well-performing light harvesting feature, it manifested a typical light absorption capacity. In contrast, an obvious absorption peak in the UV-light range (<400 nm) was observed for porous hollow HAp microspheres. In case of the as-prepared RGO (1.5 wt%)/HAp composite, the presence of RGO led to an increase of absorption in the visible-light range, due to the characteristic light absorption capacity of carbon materials. In addition, the bandgaps of HAp and RGO (1.5 wt %)/HAp composite obtained from the Tauc plot (Fig. 6b) [36], were determined to be 5.00 eV and 4.90 eV, respectively. It can be inferred that the as-prepared RGO (1.5 wt%)/HAp composite show better pho tocatalytic activity in the whole light range, as compared with porous hollow HAp microspheres.
a 300 W xenon lamp with full spectrum irradiation, which was in the order of RGO (1.5 wt%)/HAp > RGO (0.5 wt%)/HAp > HAp > RGO (3 wt%)/HAp > RGO. As for porous hollow HAp microspheres, after adsorbing 20.8% of TC (Fig. S1) for 30 min, about 68.1% of TC was degraded under a 300 W xenon lamp full spectrum irradiation within 30 min, and reaching 88.9% removal efficiency in total. In case of RGO, after adsorbing 17.7% of TC (Fig. S1) for 30 min, about 12.8% of TC was degraded under the similar experimental conditions. While the total removal efficiency of TC over RGO was about 30.5%. Among the three tested RGO/HAp composites, RGO (1.5 wt%)/HAp exhibits the best photocatalytic performance (ca. 92.1%) towards TC degradation within 30 min under a 300 W xenon lamp with full spectrum irradiation. Note that its adsorption efficiency for TC in dark period was 7.9% (Fig. S1) after 30 min. Thus, the total removal efficiency for TC was ca. 100% for RGO (1.5 wt%)/HAp. It should be pointed out that in all cases, the dosage amount of photocatalyst is 0.1 g. Undoubtedly, the as-prepared RGO (1.5 wt%)/HAp showed much higher photocatalytic efficiency in TC degradation, taking advantage over the corresponding porous hollow HAp microspheres and RGO samples. Table 1 shows the comparison of the photocatalytic degradation TC of RGO (1.5 wt%)/HAp with other photocatalysts in the reported literatures. It can be clearly seen that GO (1.5 wt%)/HAp has the highest photocatalytic ability to degrade TC. It is well-known that the initial pH value has an important influence on the photodegradation of TC. A series of experiments (Fig. 7b) shows the removal efficiency of TC at different pH (3–9) over RGO (1.5 wt %)/HAp photocatalysts under a 300 W xenon lamp with full spectrum irradiation. It was shown that the photocatalytic degradation efficiency of TC was slightly influenced by pH value over RGO (1.5 wt%)/HAp, and it follows the order of pH (5) > pH (7) > pH (9) > pH (3). Therefore, in the current study, the optimal initial pH value is pH 5. Fig. 7c indicates that all the photocatalytic degradation data can fit well with the pseudo-first-order correlation (ln (C0/Ct) ¼ κt), confirming that the photocatalytic degradation TC reaction kinetics over the asprepared RGO/HAp photocatalysts is followed pseudo-first-order. In addition, the largest apparent rate constant of RGO (1.5 wt%)/HAp in Fig. 7d was 0.1816 min 1, which was about 31.9 and 14.4 times higher than that of RGO and porous hollow HAp microspheres, respectively. The above experimental results demonstrate that an optimum deco rating amount (1.5 wt%) of RGO onto porous hollow HAp microspheres can act as an effective photocatalyst for TC degradation. In order to evaluate the photocatalytic degradation ability of the asprepared RGO (1.5 wt%)/HAp for TC, the mineralization activity was investigated based on TOC analysis. As shown in Fig. 7e, a mineraliza tion efficiency of 85% can be obtained over RGO (1.5 wt%)/HAp for TC after 30 min irradiation, indicating the as-prepared RGO (1.5 wt%)/HAp photocatalyst possessed higher mineralization ability for TC degrada tion. It is worth noting that 85% mineralization efficiency was lower than that of degradation efficiency (almost 100%), suggesting that complete mineralization could not be achieved, and some organic in termediates might generate in the photocatalytic process. The reaction intermediates in the photocatalytic process was further analyzed by HPLC-MS (Fig. S2). As observed, there was a characteristic anion peak at m/z 445 belonging to the deprotonated TC. From Fig. S3, it can be seen that the ions with m/z ¼ 317 and m/z ¼ 216 were detected with the increasing reaction time. The above results confirming that TC molecule is degraded into CO2, H2O and some other intermediates. As it well-known that the stability and recycling performance of a photocatalyst is important for practical applications. As indicated in Fig. 8a, there is no obvious change in the photocatalytic activity of RGO (1.5 wt%)/HAp composite even after three successive photocatalytic reaction, indicating its good recyclability in TC degradation during re cycles. Thus, it can be inferred that the as-prepared RGO/HAp composite can be used as a stable and high-performance photocatalyst. The structural stability of RGO (1.5 wt%)/HAp composite before and after five TC photocatalytic degradation cycles were characterized by XRD patterns. As observed in Fig. 8b, the XRD patterns of three recycled
3.2. Photocatalytic performance for TC over RGO/HAp composites To investigate the photocatalytic efficiencies of the as-prepared RGO/HAp composites, TC was used as a model antibiotic for photo catalytic degradation. Fig. 7a illustrates the photocatalytic degradation of TC over different samples under a 300 W xenon lamp with full spec trum irradiation. For comparison, the photocatalytic degradation of TC over the as-prepared porous hollow HAp microspheres and RGO were also tested. In addition, the photocatalytic degradation of TC over RGO (1.5 wt%)/HAp under a 300 W xenon lamp equipped with an optical cutoff filter (λ > 420 nm) that offered a visible-light irradiation was performed and compared. As observed, blank test demonstrates little photolysis and the corresponding efficiency is only ca. 9.4%. Note that the RGO (1.5 wt%)/HAp composite exhibited much higher photo catalytic removal efficiency under a 300 W xenon lamp with full spec trum irradiation than that of the sample performed under a visible-light irradiation. It can be seen that the as-prepared photocatalysts demon strated different photocatalytic degradation rates for TC removal under 8
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RGO (1.5 wt%)/HAp composite had almost no change compared with fresh RGO (1.5 wt%)/HAp composite. Therefore, it can be concluded that the as-prepared RGO (1.5 wt%)/HAp composite is reusable and stable in the photocatalytic process, and has a promising potential for practical application. 3.3. Photocatalytic mechanistic aspects
HAp þ hν→e þ hþ
(2)
e þ RGO→RGO ðe Þ
(3)
O2 þ e ðRGOÞ→⋅O2
(4)
� ⋅O2 hþ þ TC→degraded products
(5)
As demonstrated in Fig. 11, porous hollow HAp microspheres and RGO can act as an electron donor and an electron acceptor, respectively. Under a 300 W xenon lamp full spectrum irradiation, the photo generated electrons (e ) in valence band (VB) of porous hollow HAp microspheres can be excited to conduction band (CB), leaving behind the promotion of positive holes (hþ). Due to the good conductivity and many defects of RGO in the composite, the excited electrons (e ) in the CB of porous hollow HAp microspheres can be transferred to the surface of RGO. Note that the CB and VB potential of HAp are 2.20 V and 2.80 V (vs. NHE). HAp has a more negative CB potential than the O2/ �O2 ( 0.33 V vs. NHE), thus the photogenerated electrons (e ) accu mulating at RGO could interact with O2 to form ⋅O2 , resulting in partial degradation of TC molecules. Besides, the separated positive holes (hþ) in the VB of porous hollow HAp microspheres can directly participate in the oxidation of TC molecules.
Photocurrent response (PC) is an important technique for analyzing charge transfer and separation in a semiconductor which could be significantly affects its photocatalytic performance. Usually, a higher photocurrent density means a higher ability in separating the photogenerated electrons and holes. Fig. 9a shows the photocurrent response of the as-prepared RGO, HAp, and RGO (1.5 wt%)/HAp com posite. As clearly observed, RGO (1.5 wt%)/HAp composite demon strates the highest photocurrent intensity, much higher than those of the as-prepared RGO and HAp. The highest photocurrent was due to the efficient transfer and separation of photo-generated electrons and holes in RGO (1.5 wt%)/HAp composite. The introduction of RGO plays an important role in charge transfer and separation, which is beneficial for photocatalytic process. Moreover, electrochemical impedance spectra (EIS) test is another effective way to analyze the transfer and separation of the photogenerated electrons and holes. Fig. 9b displays the EIS Nyquist plots of the as-prepared RGO, HAp, and RGO (1.5 wt%)/HAp composite under the identical experimental conditions. Obviously, the arc radius of RGO (1.5 wt%)/HAp composite in the EIS Nyquist plot was the smallest among these samples, indicating its highest charge transfer ability. The above point was well consistent with the result of photocurrent response. Next, trapping experiments were conducted to elucidate the reasonable photocatalytic mechanism. 2-Propanol (IPA), disodium ethylene diamine tetraacetic acid (EDTA), and 1, 4-benzoquinone (BQ) were used as the scavengers to detect the main active species during the photocatalytic process. As can be seen from Fig. 10a, with the addition of IPA into the reaction system, there was a slight decrease in the degra dation efficiency, suggesting that ⋅OH is not a main active species in the present study. In contrast, the degradation efficiency is remarkably reduced in the presence of BQ, this indicating that ⋅O2 plays an important role in TC photocatalytic degradation process. Similarly, an obvious loss could be found when EDTA was used for the scavenger in the photocatalytic degradation system, verifying that hþ is another crucial pathway for TC removal. Thus, it can be inferred that both ⋅O2 and hþ are the main active species that responsible for the photocatalytic degradation of TC. The above scavenger additional experiments provide good proof for the photocatalytic mechanistic analysis. In order to further confirm the active species generated in the pho tocatalytic process, ESR analysis were performed and the corresponding results are shown in Fig. 10b–d. Note that ⋅OH and ⋅O2 radicals can be captured by DMPO, while hþ radicals can be captured by the TEMPO. As indicated in Fig. 10b, there appeared obvious characteristic peaks of DMPO-⋅O2 , however no signals were found in the dark. The results demonstrate that ⋅O2 plays an important role in the photocatalytic process. Note that very weak characteristic peak of DMPO-⋅OH can be observed From Fig. 10c, suggesting that ⋅OH is not a main active species in the photocatalytic reaction. Furthermore, the characteristic peaks of TEMPO-hþ were also clearly detected in Fig. 10d, and the signals were so strong in the dark, while it slightly decreased under the xenon lamp irradiation. The results indicate that hþ also act as main active species in the photocatalytic reaction. The ESR results confirm that both ⋅O2 and hþ are the main active species in the photocatalytic process, which was well consistent with the above trapping experimental result. Based on the above analysis, a possible photocatalytic mechanism of RGO/HAp composite photocatalyst is proposed. The possible photo catalytic experiment equations are expressed as follows:
4. Conclusion In summary, a facile hydrothermal route was adopted and optimized for small amounts of RGO decorating on the porous hollow HAp mi crospheres for the first time. Their subsequent applications were inves tigated by photocatalytic degradation of TC. It turned out that the asprepared RGO (1.5 wt%)/HAp composite displays an excellent photo catalytic activity for TC degradation under a 300 W xenon lamp with full spectrum irradiation, which was much faster than those over porous hollow HAp microspheres and RGO. High specific surface areas, desir able absorption capability, suppression of charge recombination, and synergistic effects of RGO and HAp should be responsible for the high photocatalytic performance. This research will offer a promising plat form for developing highly efficient and stable photocatalyst that with potential application prospects. Declaration of competing interest We declare that we do not have any commercial or associative in terest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21107069), and the Science and Technology Commission of Shanghai Municipality (14DZ2261000). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solidstatesciences.2019.106067. References [1] L. Alamonole, S. Bailonruiz, T. Lunapineda, O. Peralesperez, F.R. Roman, Photocatalytic activity of quantum dot-magnetite nanocomposites to degrade organic dyes in the aqueous phase, J. Mater. Chem. A 1 (2013) 5509–5516. [2] H. Wang, Y. Sun, Y. Wu, W. Tu, S. Wu, X. Yuan, G. Zeng, Z.J. Xu, S. Li, J.W. Chew, Electrical promotion of spatially photoinduced charge separation via interfacialbuilt-in quasi-alloying effect in hierarchical Zn2In2S5/Ti3C2(O,OH)x hybrids toward efficient photocatalytic hydrogen evolution and environmental remediation, Appl. Catal. B Environ. 245 (2019) 290–301.
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