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Bi2O3 composite photocatalyst based on biomass
Chemical Engineering Journal 304 (2016) 351–361
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Enhanced photocatalytic activity of a double conductive C/Fe3O4/Bi2O3 composite photocatalyst based on biomass Nailing Gao a, Ziyang Lu b,⇑, Xiaoxu Zhao b, Zhi Zhu a, Youshan Wang a, Dandan Wang a, Zhoufa Hua a, Chunxiang Li a,⇑, Pengwei Huo a, Minshan Song c a b c
School of Chemistry & Chemical Engineering, Jiangsu University, Jiangsu, Zhenjiang 212013, PR China School of the Environment and Safety Engineering, Jiangsu University, Jiangsu, Zhenjiang 212013, PR China School of Mathematics and Physics, Jiangsu University of Science and Technology, Zhenjiang 212003, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The C/Fe3O4/Bi2O3 has double
conductivity which enhanced the photocatalytic activity greatly. Using biomass as carbon source realized the rational utilization of waste. C/Fe3O4 was obtained by one-step method and showed good magnetic property. The C/Fe3O4 is conductive and it can receive and transport electrons.
a r t i c l e
i n f o
Article history: Received 18 March 2016 Received in revised form 8 June 2016 Accepted 10 June 2016 Available online 14 June 2016 Keywords: C/Fe3O4 Biomass carbon Bi2O3 Visible light photocatalysis Photodegradation mechanism
a b s t r a c t In order to realize the rational utilization of waste, C/Fe3O4 was obtained by using biomass as the carbon source and prepared via one-step method, afterwards, a double conductive C/Fe3O4/Bi2O3 composite photocatalyst was further synthesized via solvothermal method. Due to the good double conductivity, the electrons in Bi2O3 could be transported to C/Fe3O4, which greatly inhibited the recombination of electron-hole pairs, it is worth noting that the double conductivity effectively blocked the reverse transfer of electrons to Bi2O3. Consequently, the photodegradation rate of C/Fe3O4/Bi2O3 composite photocatalyst reached 91%, which was much higher than that of pure Bi2O3. In addition, the mechanism exploration experiment showed that h+ was the main activity specie, meanwhile, the photocatalytic electron transfer mechanism was also investigated. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Environmental pollution and energy crisis coexist in human society which are closely related to life. Accordingly, it is an important task to seek a suitable method to solve above problems. Photocatalysis has been regarded as one of most efficient strategies to solve the environmental pollution [1,2]. TiO2, as the most used ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Lu), [email protected] (C. Li). http://dx.doi.org/10.1016/j.cej.2016.06.063 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.
photocatalyst, possesses its unique performance [3,4], such as high efficiency and good stability [5]. Nevertheless, the large bandgap and the narrow range of light absorption greatly hinder the practical application of TiO2, therefore, finding a suitable visible light responsive material is very critical. Bi2O3, a kind of Bi-based material with a smaller band gap [6], is widely used in environment in virtue of its nontoxic and good visible photocatalytic performance [7], whereas the pragmatic photocatalytic efficiency of Bi2O3 is still at a low state because of the fast recombination of electron-hole pairs [8]. Some modification
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methods [9–14] are proved to be important strategies to ameliorate these shortcomings by constructing a composite structure to improve the separation of photogenerated electron-hole pairs [15]. However, the electrons transport between simple composite structures is insufficient and a part of electrons would be returned to the photocatalyst and caused the decrease of photocatalytic performance. Numerous researches have been down to overcome the shortcoming, previous literatures [16–20] have attested that the combination of carbon and photocatalyst can enhance the photocatalytic degradation rate by improving the electron transfer rate and blocking the recombination of electron-hole pairs, the degree of electrons transferred back to photocatalyst can be reduced [21]. Therefore, it is convinced that the photocatalytic performance of Bi2O3 can be improved by combining with carbon material. However, the carbon source used in mentioned above researches were organic compound which makes the cost higher, it will be more reasonable that if they had fully consideration of the economic efficiency and environment pollution. Using biomass as carbon source not only controls the waste of agricultural wastes [22] to relieve the environment pollution as well as energy crisis [23] but also increases the activity of photocatalyst. Up to now there are few reports about the application of biomass carbon on photocatalytic. Biomass is a kind of renewable feedstock [24] which is widely distributed in our country, the biomass resources, such as corn cobs, straw and rice husk are defined as agricultural waste. Hitherto many researches have been carried out to study the conductivity of carbon which derived from biomass [25–27], open up new avenues for the reuse of agricultural waste. Carbon, fabricated from biomass by calcination or hydrothermal method, can be used as supporter to let the metal particles grow on surface in dispersed state to increase the stability [28], meanwhile, it often acts as adsorbent to absorb pollutants toward to the photocatalyst, furthermore, the excited electrons will be transferred away by carbon rather than aggregated on the photocatalyst, therefore, the recombination probability of the holes and electrons on the photocatalyst will be impeded and the lifetime of photogenerated electrons will be prolong [29]. Nevertheless, the recycle of the composite photocatalyst is so hard that the cost becomes higher, in order to further realize the low carbon idea, Fe3O4, being introduced into the composite photocatalyst, is regarded as an ideal magnetic material, the combination of Fe3O4 and semiconductor to form a composite structure has been frequently investigated [29,30], the addition of Fe3O4 makes the photocatayst become more easily recovered from the solution [31], from another point of view, the Fe3O4 is conductive which can facilitate the separation of the electron-hole pairs effectively [32] and achieve the purpose of increasing the photocatalytic activity ultimately. In this work, we demonstrate a method to synthesis the C/Fe3O4/Bi2O3 composite photocatalyst, tetracycline is selected as the target pollutant, and corn cobs are used as biomass carbon raw materials, the obtained carbon is used for the matrix material. Herein, a series of factors such as different proportion of C/Fe3O4 and different matrix materials are put forward, aimed at studying the role of carbon in the reaction of photocatalytic. The main active species during the process of photocatalytic are evaluated through mechanism explore experiment, the photocatalytic electron transfer mechanism will be discussed in this paper.
95.0%), Fe(NO3)39H2O (98.0%), activated carbon, tetracycline were purchased from Sinopharm Chemical Reagent Co., Ltd. Corn cobs were obtained from Zhenjiang, Jiangsu Province, China. Tetracycline was analytical pure and used without further purification, distilled water was used in the whole experiments. 2.2. Sample preparation 2.2.1. Synthesis of C/Fe3O4 materials All reagents were of analytical grade and used without further purification. Corn cob powder (5 g) and Fe(NO3)39H2O (5 g) were dissolved in 100 mL ethanol, the mixed materials were stirred by a magnetic stirrer under room temperature for one hour, and then the suspension was filtered and dried in an oven at 80 °C overnight. Put the dried powder into tube furnace, calcined at 650 °C for 3 h under H2 atmosphere. The carbon was synthesized by the same way in addition to Fe(NO3)39H2O at the beginning. Put some Fe (NO3)39H2O powders into porcelain boat, calcined in tube furnace to form Fe3O4 by the same condition. 2.2.2. Synthesis of C/Fe3O4/Bi2O3 composite materials The as-prepared C/Fe3O4 (0.04 g) was dispersed into ethylene glycol (35 mL), ultrasonicated for 1 h, afterwards, Bi(NO3)35H2O (0.73 g, 1.5 mmol) was dissolved in the mixed solution, ultrasound for 1 h and then magnetic stirred for 2 h, the mixture was poured into a Teflon-lined stainless-steel autoclave, sealed and maintained at 160 °C for 15 h. After the reaction was completed, the resulting precipitates were collected by magnetic, washed several times with deionized water and ethanol, dried in an oven at 80 °C. Finally the product was calcined in tube furnace under air at 300 °C for 1 h. Samples synthesized with C/Fe3O4 content 2.5 wt.%, 5 wt.%, 10 wt. %, 15 wt.% and 20 wt.% were denoted as C/Fe3O4/Bi2O3-2.5, C/Fe3O4/Bi2O3-5, C/Fe3O4/Bi2O3-10, C/Fe3O4/Bi2O3-15 and C/Fe3O4/ Bi2O3-20, respectively. For comparison, the pure Bi2O3, C/Bi2O3 and Fe3O4/Bi2O3 were synthesized by the method mentioned above. The preparation process of C/Fe3O4/Bi2O3 is shown in Scheme 1. 2.3. Characterization The type and phase structure of the as-prepared product was detected using powder X-ray diffraction (XRD, Bruker D8 Advance diffractometer using Cu Ka = 1.5406 Å), Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectra were taken to observe and analyze the morphology, structure and chemical element composition of product. Transmission electron microscopy (TEM) was used to observe the inner microcosmic
2. Experimental section 2.1. Materials Bi(NO3)35H2O (98.0%), ethylene glycol (EG, 98.0%) were all supported by Aladdin Chemistry Co., Ltd. ethanol (C2H5OH,
Scheme 1. Illustration of the possible formation process of C/Fe3O4/Bi2O3 composite photocatalyst.
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structure. The Brunauer-Emmett-Teller (BET) specific surface area of the powders was analyzed by nitrogen adsorption. The photocurrent and electrochemical impedance spectroscopy (EIS) were measured on an electrochemical system (CHI-660B). UV–vis diffuse reflectance spectra (DRS) were recorded on a UV–vis spectrometer (Shimadzu UV-2550) by using BaSO4 as a reference. 2.4. Photocatalytic activity measurement 2.4.1. The tetracycline adsorption measurement In a typical adsorption procedure, 0.1 g sample was added into 100 mL tetracycline solution (20 mg mL1) in photocatalytic reaction bottle. Prior to irradiation, kept the mixed solution in the dark with continuous magnetic stirring, 8 mL suspension was collected by magnet and centrifuged to remove the solid adsorbents at each given time interval. The concentration of tetracycline was measured by UV spectra at a wavelength of 357 nm. 2.4.2. Photocatalytic activity measurement The photocatalytic activity of the sample was evaluated by the degradation of tetracycline under visible light irradiation (Xe lamp which was filtered out UV light by filter). In a typical run of photocatalytic reactions, 0.1 g sample was added to 100 mL tetracycline solution (20 mg mL1). Prior to light irradiation, keep the suspension in the dark under stirring for a certain time to reach the balance of adsorption-desorption. Afterwards, turned the light on and made the solution exposed to Xe lamp irradiation under stirring, 8 mL solution was collected by magnet and centrifuged to remove the solid adsorbents at each given time interval. The concentration of tetracycline was measured by UV spectra. All the photocatalytic experiments were carried out at room temperature. 3. Results and discussion 3.1. Characterization of catalysts 3.1.1. Structure and chemical composition Fig. 1 presents the typical XRD patterns of the as-prepared composite photocatalysts. From Fig. 1, all diffraction peaks of the sample before and after calcination can be assigned to the Bi (JCPDS No. 85-1329) [33] and Bi2O3 (JCPDS No. 74-1374), respectively. It is well known that EG was often used as the reducing agent to prepare metal nanoparticles through its intrinsic reducing property [34], the metallic Bi was generated by the solvothermal method, and it turned into Bi2O3 reveals the fact that calcination is absolutely necessary in the experiment. The calcination temperature
Fig. 1. XRD patterns of samples before and after calcination at 300 °C for 1 h.
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was chosen as 300 °C because the metallic Bi can react with O2 to form Bi2O3 when the ambient temperature is higher than melting point of bismuth [33]. The peaks of Fig. 2d located at 2h = 30.2°, 35.5°, 43.2°, 57.3° and 62.6° were well matched to Fe3O4 (JCPDS No. 88-0866), indicating that the C/Fe3O4 material was successfully synthesized by one-step method. The peaks in Fig. 2 have no obvious difference between other photocatalysts in addition to the peaks of Fe3O4, illustrating that no impurity emerged. The sharp peaks of composite photocatalysts make known that the samples have good crystallinity. As is known that Raman spectra has been proven to be an effective tool to analyze the structure of carbon material. The recorded spectrum of Fig. 3a shows two broad Raman bands of biomass carbon at 1366 cm1 and 1590 cm1, they can be assigned to D band and G band [35]. It is known that the relative intensity ratio between the D and G bands (ID/IG) reflects the degree of graphitization [36], lower ID/IG value means high graphitization, the ID/IG value is 0.84, suggesting that the C/Fe3O4 has a certain degree of graphitization [37,38], the ID/IG value of C/Fe3O4/Bi2O3-10 is 0.82, there was a slight change of graphitization compared with the C/Fe3O4. The graphitization of biomass carbon can make carbon material from disorder to order, and it will be beneficial to achieve better electronic conduction between composite adjacent Fe3O4 [39]. The conductive capacity of C, C/Fe3O4 and activated carbon were shown by the Electrochemical impedance spectroscopy (EIS) (Fig. S1, see support information), illustrating that the obtained C and C/Fe3O4 both have good conductive performance, the resistance of C is smaller than activated carbon indicating that the graphitization of carbon helps the transmission of electrons while the arc radius of C/Fe3O4 is much smaller than C, which means the higher efficiency of electron-transmission of C/Fe3O4. The peaks located at 315 cm1 and 415 cm1 of Fig. 3b stem from the vibration modes of Bi2O3 [40]. It can be found that the peaks of the composite photocatalyst (Fig. 3c) were composed of two types of peaks, one is the characteristic peaks of C/Fe3O4 and the other is Bi2O3, which validated that Bi2O3 have been successfully grown on the surface of C/Fe3O4. XPS was used to detect the elements and its chemical state on the surface of material. It is observed that the composite photocatalyst consists of C, O, Fe and Bi elements (Fig. 4a). The highresolution of Bi element peaks were shown in Fig. 4b, the Bi 4f peaks related to Bi3+ in Bi-O band in C/Fe3O4/Bi2O3-10 which were divided into Bi 4f5/2 and Bi 4f7/2 peaks, and the binding energy were 164.3 eV and 158.9 eV, respectively. There is no obvious difference between C/Fe3O4/Bi2O3-10 and pure Bi2O3, making it
Fig. 2. XRD patterns of different materials: (a) pure Bi2O3, (b) C/Fe3O4/Bi2O3-10, (c) C/Bi2O3, (d) C/Fe3O4.
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corresponded to C/Fe3O4/Bi2O3-10 and pure Bi2O3, respectively. The peaks at 284.9 eV could be assigned to CAC bond of the graphitized carbon [42], and the peak at 286.6 eV was CAO or CAOH group, they all belonged to the adventitious carbons on the surface of Bi2O3. The peak of O 1s in Fig. S2 was attributed to Bi-O band in Bi2O3, these values are in good agreement with the results in previous literatures [43,44].
Fig. 3. Raman spectra of as-prepared product: (a) C/Fe3O4, (b) pure Bi2O3, (c) C/Fe3O4/Bi2O3-10.
clear that no impurities emerged. It can be observed that no Fe element peaks (Fig. 4c) emerged in pure Bi2O3 relative to the composite product, revealing that Fe3O4 do exist in composite. The peaks located in 710.5 eV and 724.5 eV of binding energy were Fe 2p3/2 and Fe 2p1/2 (Fig. 4c), respectively, they all both consisted with Fe2+ (FeO) and Fe3+ (Fe2O3) peaks which are typical characteristics of the Fe3O4 [41]. The XPS spectrum of C 1s was shown in Fig. 4d, the peaks situated at 284.9 eV and 286.6 eV were
3.1.2. Morphology analysis and surface area analysis The surface morphology and internal structure of product were detected by SEM and TEM, the results were shown in Fig. 5. The surface of biomass carbon was smooth (Fig. 5a) and the microstructure was schistose (Fig. 5b), obviously, the smooth surface turned into rough (Fig. 5c) after Fe3O4 loaded on nanoparticles by one-step process, meanwhile, it can be seen from Fig. 5d that the Fe3O4 nanospheres have good dispersity on carbon plate which size is about 30 nm. Moreover, it can be observed from Fig. 5e that many Bi2O3 nanospheres which the diameter size is about 100 nm (the red circles of Fig. 5f have pointed out) covered on the surface of C/Fe3O4 via hydrothermal reaction. The EDS area scans (Fig. S3) further confirmed the presence of Bi, Fe, C and O elements and it is in keeping with the results of XRD and Raman. The BET surface areas were obtained by nitrogen-adsorption–desorption isotherms, pore size was investigated by using the Barrett–Joyner–Halenda model. Table 1 summarized that the composite photocatalyst belonged to mesoporous and the specific BET surface area of C/ Fe3O4/Bi2O3-10 was calculated to be 24.11 m2 g1, which is higher than that of pure Bi2O3 (10.82 m2 g1). The increasing of specific
Fig. 4. (a) XPS spectrum of C/Fe3O4/Bi2O3-10 and pure Bi2O3; (b–d) high-resolution XPS spectrum of Bi 4f, Fe 2p and C 1s, respectively.
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Fig. 5. SEM images and TEM images of the obtained materials: (a, d) C, (b, e) C/Fe3O4, (c, f) C/Fe3O4/Bi2O3-10.
surface area helps to increase the photocatalytic reaction sites and let more pollutants adsorbed on the surface of photocatalyst. 3.1.3. Photoabsorption and energy band structures The optical absorption property of semiconductor plays an essential role in photocatalytic performance. The photophysical properties of photocatalysts were investigated by UV–vis diffuse reflanctance spectroscopy. Obviously, all the products exhibit strong absorbance ability owe to the remarkable strong and broad absorption in the range of 400–700 nm in visible light region. The UV–vis diffuse reflectance spectra indicate that the composites can be excited by visible light easily, the maximal absorbance wavelengths of pure Bi2O3 is approximately 470 nm, it can be seen from Fig. S5 that C and C/Fe3O4 all had a widely absorption both in visible light region and ultraviolet light region which played a critical role in the enhanced visible light absorption of the C/Fe3O4/Bi2O3 composite photocatalyst, and the composite photocatalysts Table 1 BET surface area, pore volume and mean pore diameters of as-synthesized pure Bi2O3 and C/Fe3O4/Bi2O3-10. Sample
Contents of BET surface Pore volume Mean pore C/Fe3O4 (wt.%) areas (m3 g1) (cm3 g1) diameters (nm)
Pure Bi2O3 0 C/Fe3O4/Bi2O3-10 10
10.82 24.11
0.0421 0.0309
15.59 5.65
exhibited a better absorption than pure Bi2O3 and shown red shift in visible light region, the absorption enhancement is beneficial for forming more electron-hole pairs [45]. The energy of the band gap energy (Eg ) was calculated by the Eq. (1) [33]:
ahm ¼ Aðhm Eg Þn=2
ð1Þ
where a, h, m, and A are the adsorption coefficient, Plank’s constant, light frequency, and proportionality a constant, respectively. The value of n depends on the characteristic of the transition in semiconductor including direct (n = 1) or indirect (n = 4) [46], herein we suppose that n = 1 [47], it can be calculated from the inset that the Eg of pure Bi2O3 is 2.6 eV. In order to accurately understand the position of the conduction band and valence band of Bi2O3, the Eqs. (2) and (3) were carried out to calculate the exact location of conduction and valence band of the composite photocatalyst [48].
ECB ¼ v Ee 0:5Eg
ð2Þ
EVB ¼ ECB þ Eg
ð3Þ
where v refers to electro-negativity which is calculated by ionization energy and electron affinity energy of the semiconductor [49]. The final calculated value of v is 5.96 eV, Ee is the energy of free electrons on the Hydrogen scale (4.5 eV) [50]. Eg is band gap of composite photocatalyst. According to the above equations, the conduction band and valence band is 0.16 eV and 2.76 eV,
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3.3. Photocatalytic performances
Fig. 6. DRS spectrum of as-prepared samples: (a) C/Fe3O4/Bi2O3-2.5, (b) C/Fe3O4/ Bi2O3-10, (c) C/Fe3O4/Bi2O3-20, (d) pure Bi2O3; the inset shows the plots of (Ahm)2 as a function of photon energy hm, respectively.
respectively. The band gap of as-synthesized pure Bi2O3 is 2.6 eV, and it is in good agreement with the inset of Fig. 6. The result was consistent with the hypothesis above, the as-prepared Bi2O3 is direct semiconductor (n = 1) [51].
3.2. Adsorption property The absorption curves of different photocatalysts were shown in Fig. 7A, the solution system reached the adsorption-desorption balance within 40 min, the adsorption value has a little change during the remaining part of adsorption time (40–60 min) but that can be ignored. The adsorption capacity of C/Fe3O4/Bi2O3-10 reached to 9.5 mg g1 much higher than pure Bi2O3, and the C/Bi2O3 and Fe3O4/Bi2O3 followed. Fig. 7B shows the absorption curves of different mass proportion of C/Fe3O4, the solution system reached the balance of adsorption-desorption within 40 min, and the adsorption capacity increased with the change of mass proportion, but the gap is not big (from 8.56 mg g1 to 9.62 mg g1). The XRD patterns of different materials after absorption were shown in Fig. S6, it can be seen that the crystal forms of composite photocatalysts have not changed after several times of adsorption, illustrating that the as-prepared composite photocatalysts have a stable property.
The photocatalytic activity was carried out by the degradation of tetracycline. As presented in Fig. 8A, the degradation rate of C/Fe3O4/Bi2O3-10 reached to 91% in 90 min exhibited the best photocatalytic activity while the pure Bi2O3 reached to 37% within the same time due to the effective separation of photo-generated electron–hole pair which was investigated by Photoluminescence (PL), the intensity of peaks of pure Bi2O3 is higher than C/Fe3O4/Bi2O3-10 (Fig. 8B), illustrating that the recombination probability of C/Fe3O4/ Bi2O3-10 is smaller than pure Bi2O3. The diameter of the arc radius on the EIS Nyquist plot [52] of C/Fe3O4/Bi2O3-10 (Fig. S4) is smaller than that of pure Bi2O3, indicating the charge transfer speed on the interface is fast which impelled the separation of electron–hole pairs more effective, resulting in good photocatalytic performance. The degradation rate of C/Bi2O3 and Fe3O4/Bi2O3 is higher than pure Bi2O3 because it is easy for carbon and Fe3O4 to accept the electrons from Bi2O3 to react with O2 which couldn’t happened on pure Bi2O3. Moreover more electrons were transported away by the C/Fe3O4/Bi2O3 composite photocatalyst which has two ways of electron transport contributes to the higher degradation rate compared with C/Bi2O3 and Fe3O4/Bi2O3. Besides, the different mass proportion of C/Fe3O4 exhibits small influence on the crystalline and purity (Fig. 8D), but affects the photodegradation efficiency greatly. As shown in Fig. 8C, the most suitable point of mass proportion was 10 wt.% in the degradation of tetracycline. The different mass proportion of C/Fe3O4 plays the pivotal factor role in photocatalytic, when the content of C/Fe3O4 is 20 wt.%, the absorption of composite photocatalyst is the best, but the black C/Fe3O4 has the properties of light absorption which will reduce the visible light utilization of bismuth oxide, and the content of Bi2O3 which was the main photocatalytic active material became less relatively in the composite photocatalyst, resulting in the low degradation efficiency. On the contrary, when the content of C/Fe3O4 is 2.5 wt.%, the C/Fe3O4 content is so low that it is unfavorable for the transmission of electrons, thereby the degree of recombination of electron-hole pairs increased and inhibited the photocatalytic efficiency directly. In the cycle experiment (Fig. 9), the degradation rate was reduced less than 5% after 5 cylices indicating that the composite photocatalyst possessed favorable stability for reuse. The removal of total organic carbon (TOC) was selected as a mineralization index to characterize the tetracycline degradation, the time independence of the TOC data in the tetracycline solution was shown in Fig. S7. It can be observed from Fig. S7 that 65% of the
Fig. 7. The absorption curves of samples: (A) different composite materials and pure Bi2O3; (B) different composite photocatalysts: (a) C/Fe3O4/Bi2O3-2.5, (b) C/Fe3O4/Bi2O3-5, (c) C/Fe3O4/Bi2O3-10, (d) C/Fe3O4/Bi2O3-15, (e) C/Fe3O4/Bi2O3-20.
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Fig. 8. (A) Photocatalytic degradation of tetracycline over the samples under visible light; (B) Photoluminescence (PL) spectra of pure Bi2O3, C/Bi2O3 and C/Fe3O4/Bi2O3-10, excited at the wavelength of 270 nm; (C) photo-degradation of tetracycline curves and (D) XRD patterns of different composite photocatalysts: (a) C/Fe3O4/Bi2O3-2.5, (b) C/Fe3O4/Bi2O3-5, (c) C/Fe3O4/Bi2O3-10, (d) C/Fe3O4/Bi2O3-15, (e) C/Fe3O4/Bi2O3-20.
Fig. 9. Cycling runs of photocatalytic activities of C/Fe3O4/Bi2O3-10 composite photocatalyst under visible light irradiation for degradation of tetracycline.
TOC was eliminated after 90 min under visible light irradiation, indicating that the tetracycline was mineralized by C/Fe3O4/ Bi2O3-10 composite photocatalyst during the photocatalytic degradation process under visible light irradiation. The photocatalytic mechanism exploration experiment was carried out to investigate the main active species during photocatalytic
Fig. 10. Photocatalytic degradation efficiency with different scavengers.
degradation process via dissolving different trapping agents in solution. The added trapping agent benzoquinone (BQ) is used to cap+ ture the superoxide radical ( O 2 ), while TEOA is for h and TBA is for OH. The results were shown in Fig. 10, the degradation rate decreased obviously after adding the capture agent compared with no scavenger, elucidating that superoxide radicals ( O 2 ), hydroxyl radicals ( OH) and photogenerated h+ are the activity species in pho-
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rich groups (negative oxygen ions) occurred nucleophilic reaction with carbonyl, generating lactone structure product B (m/ z = 361), and it turned into product C (m/z = 318) by loss of Nmethyl groups. Besides, on the other way of degradation, the tetracycline was fragmented into the product D (m/z = 345) by the leaving groups of ACH3, AN(CH3)2 and ACO(NH2), and it turned into product E (m/z = 329) by dislodging of AOH, the product 5 decomposed into product F (m/z = 274) through the smash of ring ‘‘b” [55], and the product I (m/z = 246) was formed by the loss of AOH and ACOH groups. Product G (m/z = 429) was stemmed from the tetracycline (m/z = 445) via loss of hydroxyl group during the photocatalytic reaction, afterwards, the product G transferred to product H (m/z = 415). Finally, with the change of time, the tetracycline is gradually broken down into small molecules. 3.4. Photocatalytic mechanism The photocatalytic mechanism was investigated in previous study [56]. In Fig. 13, when a photon with energy matches the band gap energy (Eg ) of C/Fe3O4/Bi2O3 composite photocatalyst, a phototo-generated electron (e ) in the valence band is excited into the conduction band of Bi2O3, and leave a positive hole (h+) in the valence band (Eq. (4)). According to the results which were calculated as mentioned above, the conductive band of Bi2O3 is 0.16 eV (vs NHE), it is more positive than E0 (O2 = O 2 = 0.28 eV vs NHE), thus there is no O 2 production from electrons on the surface of Bi2O3. The double conductive C/Fe3O4/Bi2O3 composite photocatalyst has two ways of electron transmission. On the step 1, the phototo-generated electrons (e ) of Bi2O3 transferred to Fe3O4 further transferred to carbon (Eq. (5)), the electrons which transferred on the surface of carbon reacted with O2 to generate O 2 (Eq. (6)), while on the step 2, the electrons firstly transferred to carbon and further to Fe3O4 (Eq. (7)), it has been reported that Fe3+ ions exist in Fe3O4, it can act as a photo-excited electron-trapping site to prevent the fast recombination of photo-induced charge carriers and prolong their lifetime [57–59], the electrons were captured by Fe3+ ion and it results in the formation of Fe2+ ion (Eq. (8)), then the Fe2+ ion would react with the dissolved O2 to generate Fe3+ ions and O 2 radicals (Eq. (9)). The valence band data of Bi2O3 (EVB = 2.76 eV) is positive than OH /OH (2.38 eV), the hole (h+) of Bi2O3 can reacted with OH to produce OH (Eq. (10)). According + to Fig. 10, the O 2 , OH and h are the main activity specie in the process of photocatalytic, they can react with tetracycline and degrade it into small molecules under visible light irradiation (Eqs. (11)–(13)), specific reaction formula is as follows: þ
Fig. 11. m/z of degrading tetracycline over C/Fe3O4/Bi2O3 composite photocatalyst: (A) the initial tetracycline solution; (B) degradation of tetracycline after 30 min; (C) degradation of tetracycline after 90 min.
Bi2 O3 þ Vis ! e ðBi2 O3 Þ þ h ðBi2 O3 Þ e ðBi2 O3 Þ ! e ðFe3 O4 Þ ! e ðcarbonÞ
ð4Þ step1
O2 þ e ðcarbonÞ ! O2 ðcarbonÞ todegradation tetracycline. From the picture it can be concluded that the degradation rate decreased heavily when TEOA is added, suggesting that h+ is the main activity specie in solution during photocatalytic, besides, the BQ could capture the electrons to form O2 radicals, which is beneficial to promote the degradation of tetracycline, the OH also plays a necessary part of photocatalytic. As is widely known that mass spectrometry is used to detect the photocatalytic intermediate product, the possible intermediate products of photodegradation of tetracycline were tested in this experiment. The results were shown in Fig. 11, the prominent anion with m/z of 445 was the deprotonated tetracycline molecular ion [53], during the process of degradation, the tetracycline was fragmented into small molecules (Fig. 12), it could form the intermediate product A (m/z = 362, via breakage of naphthalene ring ‘‘a” [54]), under alkaline conditions, the formation of hydroxyl electron
e ðBi2 O3 Þ ! e ðcarbonÞ ! e ðFe3 O4 Þ
ð5Þ ð6Þ
step2
ð7Þ
Fe3þ þ e ! Fe2þ
ð8Þ
Fe2þ þ O2 ! Fe3þ þ O2 ðFe3 O4 Þ
ð9Þ
þ
h ðBi2 O3 Þ þ OH ! OH
ð10Þ
O2 ðFe3 O4 or carbonÞ þ tetracycline ! Molecules
ð11Þ
OH þ tetracycline ! Molecules
ð12Þ
þ
h ðBi2 O3 Þ þ tetracycline ! Molecules
ð13Þ
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OH
O
OH
d
c
b
OH
O
O
NH2
a OH
HO
OH
O
OH
OH
CH3
m/z=445
N(CH3) 2
OH
O
OH
O
OH
OH
O
OH
O
OH
O
O
NH2
H CH2 CH3
HO
G, m/z=429
OH
N(CH3)2
HO CH3
D, m/z=345
HO
N(CH3) 2
A, m/z=362
O
OH
OH
O
O OH
O
H
O
OH
OH OH
O
OH
O
OH
O
O
NH2 CH2
H, m/z=415
N(CH3) 2
CH3
B, m/z=361
O
OH O
OH
O
HO
OH
O
H
O
E, m/z=329
OH
OH
O
HO CH3
OH
NH(CH3)
O
OH
H
CH3 CH3
HO
F, m/z=274 HO
C, m/z=318
I, m/z=246
products (CO2, H2O and other molecules) Fig. 12. The possible intermediate products of photodegradation of tetracycline.
Fig. 13. Scheme for electron-hole separation and transport of double conductive C/Fe3O4/Bi2O3 composite photocatalyst under the visible-light irradiation.
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4. Conclusions In this thesis, the double conductive C/Fe3O4/Bi2O3 composite photocatalyst was successfully synthesized, a series of characterization of composite photocatalyst were carried out. The results demonstrated that the as-synthesized C/Fe3O4/Bi2O3 composite photocatalyst exhibited strong photoabsorption in visible light region and the light absorption range is larger than pure Bi2O3. The existence of C/Fe3O4, part of double conductive C/Fe3O4/Bi2O3 composite photocatalyst, not only offer the magnetic properties but also serve as the center of electron transfer and storage which helps to transfer electrons from Bi2O3 and reduce the probability of electron return to Bi2O3. The degradation experiment showed that the photocatalytic degradation rate can reached to 91% in 90 min when the mass proportion of C/Fe3O4 was 10 wt.%. In addition, the photocatalytic mechanism study revealed that h+ was the main active specie in the process of degradation. And the recycle experiment showed that the composite photocatalyst has good reuse performance. It is proved that the introduction of C/Fe3O4 can improve the photocatalytic performance of Bi2O3 photocatalyst. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21306068 and 21546013), the Natural Science Foundation of Jiangsu Province (Nos. K20130487 and BK20140532), the Postdoctoral Science Foundation of Jiangsu Province (No. 1501102B), the Innovation Programs Foundation of Jiangsu Province (Nos. SJZZ_0136 and SJLX15_0504).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2016.06.063.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Enhanced photocatalytic activity of a double conductive C/Fe3O4/Bi2O3 composite photocatalyst based on biomass Nailing Gao a, Ziyang Lu b,⇑, Xiaoxu Zhao b, Zhi Zhu a, Youshan Wang a, Dandan Wang a, Zhoufa Hua a, Chunxiang Li a,⇑, Pengwei Huo a, Minshan Song c a b c
School of Chemistry & Chemical Engineering, Jiangsu University, Jiangsu, Zhenjiang 212013, PR China School of the Environment and Safety Engineering, Jiangsu University, Jiangsu, Zhenjiang 212013, PR China School of Mathematics and Physics, Jiangsu University of Science and Technology, Zhenjiang 212003, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The C/Fe3O4/Bi2O3 has double
conductivity which enhanced the photocatalytic activity greatly. Using biomass as carbon source realized the rational utilization of waste. C/Fe3O4 was obtained by one-step method and showed good magnetic property. The C/Fe3O4 is conductive and it can receive and transport electrons.
a r t i c l e
i n f o
Article history: Received 18 March 2016 Received in revised form 8 June 2016 Accepted 10 June 2016 Available online 14 June 2016 Keywords: C/Fe3O4 Biomass carbon Bi2O3 Visible light photocatalysis Photodegradation mechanism
a b s t r a c t In order to realize the rational utilization of waste, C/Fe3O4 was obtained by using biomass as the carbon source and prepared via one-step method, afterwards, a double conductive C/Fe3O4/Bi2O3 composite photocatalyst was further synthesized via solvothermal method. Due to the good double conductivity, the electrons in Bi2O3 could be transported to C/Fe3O4, which greatly inhibited the recombination of electron-hole pairs, it is worth noting that the double conductivity effectively blocked the reverse transfer of electrons to Bi2O3. Consequently, the photodegradation rate of C/Fe3O4/Bi2O3 composite photocatalyst reached 91%, which was much higher than that of pure Bi2O3. In addition, the mechanism exploration experiment showed that h+ was the main activity specie, meanwhile, the photocatalytic electron transfer mechanism was also investigated. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Environmental pollution and energy crisis coexist in human society which are closely related to life. Accordingly, it is an important task to seek a suitable method to solve above problems. Photocatalysis has been regarded as one of most efficient strategies to solve the environmental pollution [1,2]. TiO2, as the most used ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Lu), [email protected] (C. Li). http://dx.doi.org/10.1016/j.cej.2016.06.063 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.
photocatalyst, possesses its unique performance [3,4], such as high efficiency and good stability [5]. Nevertheless, the large bandgap and the narrow range of light absorption greatly hinder the practical application of TiO2, therefore, finding a suitable visible light responsive material is very critical. Bi2O3, a kind of Bi-based material with a smaller band gap [6], is widely used in environment in virtue of its nontoxic and good visible photocatalytic performance [7], whereas the pragmatic photocatalytic efficiency of Bi2O3 is still at a low state because of the fast recombination of electron-hole pairs [8]. Some modification
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methods [9–14] are proved to be important strategies to ameliorate these shortcomings by constructing a composite structure to improve the separation of photogenerated electron-hole pairs [15]. However, the electrons transport between simple composite structures is insufficient and a part of electrons would be returned to the photocatalyst and caused the decrease of photocatalytic performance. Numerous researches have been down to overcome the shortcoming, previous literatures [16–20] have attested that the combination of carbon and photocatalyst can enhance the photocatalytic degradation rate by improving the electron transfer rate and blocking the recombination of electron-hole pairs, the degree of electrons transferred back to photocatalyst can be reduced [21]. Therefore, it is convinced that the photocatalytic performance of Bi2O3 can be improved by combining with carbon material. However, the carbon source used in mentioned above researches were organic compound which makes the cost higher, it will be more reasonable that if they had fully consideration of the economic efficiency and environment pollution. Using biomass as carbon source not only controls the waste of agricultural wastes [22] to relieve the environment pollution as well as energy crisis [23] but also increases the activity of photocatalyst. Up to now there are few reports about the application of biomass carbon on photocatalytic. Biomass is a kind of renewable feedstock [24] which is widely distributed in our country, the biomass resources, such as corn cobs, straw and rice husk are defined as agricultural waste. Hitherto many researches have been carried out to study the conductivity of carbon which derived from biomass [25–27], open up new avenues for the reuse of agricultural waste. Carbon, fabricated from biomass by calcination or hydrothermal method, can be used as supporter to let the metal particles grow on surface in dispersed state to increase the stability [28], meanwhile, it often acts as adsorbent to absorb pollutants toward to the photocatalyst, furthermore, the excited electrons will be transferred away by carbon rather than aggregated on the photocatalyst, therefore, the recombination probability of the holes and electrons on the photocatalyst will be impeded and the lifetime of photogenerated electrons will be prolong [29]. Nevertheless, the recycle of the composite photocatalyst is so hard that the cost becomes higher, in order to further realize the low carbon idea, Fe3O4, being introduced into the composite photocatalyst, is regarded as an ideal magnetic material, the combination of Fe3O4 and semiconductor to form a composite structure has been frequently investigated [29,30], the addition of Fe3O4 makes the photocatayst become more easily recovered from the solution [31], from another point of view, the Fe3O4 is conductive which can facilitate the separation of the electron-hole pairs effectively [32] and achieve the purpose of increasing the photocatalytic activity ultimately. In this work, we demonstrate a method to synthesis the C/Fe3O4/Bi2O3 composite photocatalyst, tetracycline is selected as the target pollutant, and corn cobs are used as biomass carbon raw materials, the obtained carbon is used for the matrix material. Herein, a series of factors such as different proportion of C/Fe3O4 and different matrix materials are put forward, aimed at studying the role of carbon in the reaction of photocatalytic. The main active species during the process of photocatalytic are evaluated through mechanism explore experiment, the photocatalytic electron transfer mechanism will be discussed in this paper.
95.0%), Fe(NO3)39H2O (98.0%), activated carbon, tetracycline were purchased from Sinopharm Chemical Reagent Co., Ltd. Corn cobs were obtained from Zhenjiang, Jiangsu Province, China. Tetracycline was analytical pure and used without further purification, distilled water was used in the whole experiments. 2.2. Sample preparation 2.2.1. Synthesis of C/Fe3O4 materials All reagents were of analytical grade and used without further purification. Corn cob powder (5 g) and Fe(NO3)39H2O (5 g) were dissolved in 100 mL ethanol, the mixed materials were stirred by a magnetic stirrer under room temperature for one hour, and then the suspension was filtered and dried in an oven at 80 °C overnight. Put the dried powder into tube furnace, calcined at 650 °C for 3 h under H2 atmosphere. The carbon was synthesized by the same way in addition to Fe(NO3)39H2O at the beginning. Put some Fe (NO3)39H2O powders into porcelain boat, calcined in tube furnace to form Fe3O4 by the same condition. 2.2.2. Synthesis of C/Fe3O4/Bi2O3 composite materials The as-prepared C/Fe3O4 (0.04 g) was dispersed into ethylene glycol (35 mL), ultrasonicated for 1 h, afterwards, Bi(NO3)35H2O (0.73 g, 1.5 mmol) was dissolved in the mixed solution, ultrasound for 1 h and then magnetic stirred for 2 h, the mixture was poured into a Teflon-lined stainless-steel autoclave, sealed and maintained at 160 °C for 15 h. After the reaction was completed, the resulting precipitates were collected by magnetic, washed several times with deionized water and ethanol, dried in an oven at 80 °C. Finally the product was calcined in tube furnace under air at 300 °C for 1 h. Samples synthesized with C/Fe3O4 content 2.5 wt.%, 5 wt.%, 10 wt. %, 15 wt.% and 20 wt.% were denoted as C/Fe3O4/Bi2O3-2.5, C/Fe3O4/Bi2O3-5, C/Fe3O4/Bi2O3-10, C/Fe3O4/Bi2O3-15 and C/Fe3O4/ Bi2O3-20, respectively. For comparison, the pure Bi2O3, C/Bi2O3 and Fe3O4/Bi2O3 were synthesized by the method mentioned above. The preparation process of C/Fe3O4/Bi2O3 is shown in Scheme 1. 2.3. Characterization The type and phase structure of the as-prepared product was detected using powder X-ray diffraction (XRD, Bruker D8 Advance diffractometer using Cu Ka = 1.5406 Å), Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectra were taken to observe and analyze the morphology, structure and chemical element composition of product. Transmission electron microscopy (TEM) was used to observe the inner microcosmic
2. Experimental section 2.1. Materials Bi(NO3)35H2O (98.0%), ethylene glycol (EG, 98.0%) were all supported by Aladdin Chemistry Co., Ltd. ethanol (C2H5OH,
Scheme 1. Illustration of the possible formation process of C/Fe3O4/Bi2O3 composite photocatalyst.
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structure. The Brunauer-Emmett-Teller (BET) specific surface area of the powders was analyzed by nitrogen adsorption. The photocurrent and electrochemical impedance spectroscopy (EIS) were measured on an electrochemical system (CHI-660B). UV–vis diffuse reflectance spectra (DRS) were recorded on a UV–vis spectrometer (Shimadzu UV-2550) by using BaSO4 as a reference. 2.4. Photocatalytic activity measurement 2.4.1. The tetracycline adsorption measurement In a typical adsorption procedure, 0.1 g sample was added into 100 mL tetracycline solution (20 mg mL1) in photocatalytic reaction bottle. Prior to irradiation, kept the mixed solution in the dark with continuous magnetic stirring, 8 mL suspension was collected by magnet and centrifuged to remove the solid adsorbents at each given time interval. The concentration of tetracycline was measured by UV spectra at a wavelength of 357 nm. 2.4.2. Photocatalytic activity measurement The photocatalytic activity of the sample was evaluated by the degradation of tetracycline under visible light irradiation (Xe lamp which was filtered out UV light by filter). In a typical run of photocatalytic reactions, 0.1 g sample was added to 100 mL tetracycline solution (20 mg mL1). Prior to light irradiation, keep the suspension in the dark under stirring for a certain time to reach the balance of adsorption-desorption. Afterwards, turned the light on and made the solution exposed to Xe lamp irradiation under stirring, 8 mL solution was collected by magnet and centrifuged to remove the solid adsorbents at each given time interval. The concentration of tetracycline was measured by UV spectra. All the photocatalytic experiments were carried out at room temperature. 3. Results and discussion 3.1. Characterization of catalysts 3.1.1. Structure and chemical composition Fig. 1 presents the typical XRD patterns of the as-prepared composite photocatalysts. From Fig. 1, all diffraction peaks of the sample before and after calcination can be assigned to the Bi (JCPDS No. 85-1329) [33] and Bi2O3 (JCPDS No. 74-1374), respectively. It is well known that EG was often used as the reducing agent to prepare metal nanoparticles through its intrinsic reducing property [34], the metallic Bi was generated by the solvothermal method, and it turned into Bi2O3 reveals the fact that calcination is absolutely necessary in the experiment. The calcination temperature
Fig. 1. XRD patterns of samples before and after calcination at 300 °C for 1 h.
353
was chosen as 300 °C because the metallic Bi can react with O2 to form Bi2O3 when the ambient temperature is higher than melting point of bismuth [33]. The peaks of Fig. 2d located at 2h = 30.2°, 35.5°, 43.2°, 57.3° and 62.6° were well matched to Fe3O4 (JCPDS No. 88-0866), indicating that the C/Fe3O4 material was successfully synthesized by one-step method. The peaks in Fig. 2 have no obvious difference between other photocatalysts in addition to the peaks of Fe3O4, illustrating that no impurity emerged. The sharp peaks of composite photocatalysts make known that the samples have good crystallinity. As is known that Raman spectra has been proven to be an effective tool to analyze the structure of carbon material. The recorded spectrum of Fig. 3a shows two broad Raman bands of biomass carbon at 1366 cm1 and 1590 cm1, they can be assigned to D band and G band [35]. It is known that the relative intensity ratio between the D and G bands (ID/IG) reflects the degree of graphitization [36], lower ID/IG value means high graphitization, the ID/IG value is 0.84, suggesting that the C/Fe3O4 has a certain degree of graphitization [37,38], the ID/IG value of C/Fe3O4/Bi2O3-10 is 0.82, there was a slight change of graphitization compared with the C/Fe3O4. The graphitization of biomass carbon can make carbon material from disorder to order, and it will be beneficial to achieve better electronic conduction between composite adjacent Fe3O4 [39]. The conductive capacity of C, C/Fe3O4 and activated carbon were shown by the Electrochemical impedance spectroscopy (EIS) (Fig. S1, see support information), illustrating that the obtained C and C/Fe3O4 both have good conductive performance, the resistance of C is smaller than activated carbon indicating that the graphitization of carbon helps the transmission of electrons while the arc radius of C/Fe3O4 is much smaller than C, which means the higher efficiency of electron-transmission of C/Fe3O4. The peaks located at 315 cm1 and 415 cm1 of Fig. 3b stem from the vibration modes of Bi2O3 [40]. It can be found that the peaks of the composite photocatalyst (Fig. 3c) were composed of two types of peaks, one is the characteristic peaks of C/Fe3O4 and the other is Bi2O3, which validated that Bi2O3 have been successfully grown on the surface of C/Fe3O4. XPS was used to detect the elements and its chemical state on the surface of material. It is observed that the composite photocatalyst consists of C, O, Fe and Bi elements (Fig. 4a). The highresolution of Bi element peaks were shown in Fig. 4b, the Bi 4f peaks related to Bi3+ in Bi-O band in C/Fe3O4/Bi2O3-10 which were divided into Bi 4f5/2 and Bi 4f7/2 peaks, and the binding energy were 164.3 eV and 158.9 eV, respectively. There is no obvious difference between C/Fe3O4/Bi2O3-10 and pure Bi2O3, making it
Fig. 2. XRD patterns of different materials: (a) pure Bi2O3, (b) C/Fe3O4/Bi2O3-10, (c) C/Bi2O3, (d) C/Fe3O4.
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corresponded to C/Fe3O4/Bi2O3-10 and pure Bi2O3, respectively. The peaks at 284.9 eV could be assigned to CAC bond of the graphitized carbon [42], and the peak at 286.6 eV was CAO or CAOH group, they all belonged to the adventitious carbons on the surface of Bi2O3. The peak of O 1s in Fig. S2 was attributed to Bi-O band in Bi2O3, these values are in good agreement with the results in previous literatures [43,44].
Fig. 3. Raman spectra of as-prepared product: (a) C/Fe3O4, (b) pure Bi2O3, (c) C/Fe3O4/Bi2O3-10.
clear that no impurities emerged. It can be observed that no Fe element peaks (Fig. 4c) emerged in pure Bi2O3 relative to the composite product, revealing that Fe3O4 do exist in composite. The peaks located in 710.5 eV and 724.5 eV of binding energy were Fe 2p3/2 and Fe 2p1/2 (Fig. 4c), respectively, they all both consisted with Fe2+ (FeO) and Fe3+ (Fe2O3) peaks which are typical characteristics of the Fe3O4 [41]. The XPS spectrum of C 1s was shown in Fig. 4d, the peaks situated at 284.9 eV and 286.6 eV were
3.1.2. Morphology analysis and surface area analysis The surface morphology and internal structure of product were detected by SEM and TEM, the results were shown in Fig. 5. The surface of biomass carbon was smooth (Fig. 5a) and the microstructure was schistose (Fig. 5b), obviously, the smooth surface turned into rough (Fig. 5c) after Fe3O4 loaded on nanoparticles by one-step process, meanwhile, it can be seen from Fig. 5d that the Fe3O4 nanospheres have good dispersity on carbon plate which size is about 30 nm. Moreover, it can be observed from Fig. 5e that many Bi2O3 nanospheres which the diameter size is about 100 nm (the red circles of Fig. 5f have pointed out) covered on the surface of C/Fe3O4 via hydrothermal reaction. The EDS area scans (Fig. S3) further confirmed the presence of Bi, Fe, C and O elements and it is in keeping with the results of XRD and Raman. The BET surface areas were obtained by nitrogen-adsorption–desorption isotherms, pore size was investigated by using the Barrett–Joyner–Halenda model. Table 1 summarized that the composite photocatalyst belonged to mesoporous and the specific BET surface area of C/ Fe3O4/Bi2O3-10 was calculated to be 24.11 m2 g1, which is higher than that of pure Bi2O3 (10.82 m2 g1). The increasing of specific
Fig. 4. (a) XPS spectrum of C/Fe3O4/Bi2O3-10 and pure Bi2O3; (b–d) high-resolution XPS spectrum of Bi 4f, Fe 2p and C 1s, respectively.
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Fig. 5. SEM images and TEM images of the obtained materials: (a, d) C, (b, e) C/Fe3O4, (c, f) C/Fe3O4/Bi2O3-10.
surface area helps to increase the photocatalytic reaction sites and let more pollutants adsorbed on the surface of photocatalyst. 3.1.3. Photoabsorption and energy band structures The optical absorption property of semiconductor plays an essential role in photocatalytic performance. The photophysical properties of photocatalysts were investigated by UV–vis diffuse reflanctance spectroscopy. Obviously, all the products exhibit strong absorbance ability owe to the remarkable strong and broad absorption in the range of 400–700 nm in visible light region. The UV–vis diffuse reflectance spectra indicate that the composites can be excited by visible light easily, the maximal absorbance wavelengths of pure Bi2O3 is approximately 470 nm, it can be seen from Fig. S5 that C and C/Fe3O4 all had a widely absorption both in visible light region and ultraviolet light region which played a critical role in the enhanced visible light absorption of the C/Fe3O4/Bi2O3 composite photocatalyst, and the composite photocatalysts Table 1 BET surface area, pore volume and mean pore diameters of as-synthesized pure Bi2O3 and C/Fe3O4/Bi2O3-10. Sample
Contents of BET surface Pore volume Mean pore C/Fe3O4 (wt.%) areas (m3 g1) (cm3 g1) diameters (nm)
Pure Bi2O3 0 C/Fe3O4/Bi2O3-10 10
10.82 24.11
0.0421 0.0309
15.59 5.65
exhibited a better absorption than pure Bi2O3 and shown red shift in visible light region, the absorption enhancement is beneficial for forming more electron-hole pairs [45]. The energy of the band gap energy (Eg ) was calculated by the Eq. (1) [33]:
ahm ¼ Aðhm Eg Þn=2
ð1Þ
where a, h, m, and A are the adsorption coefficient, Plank’s constant, light frequency, and proportionality a constant, respectively. The value of n depends on the characteristic of the transition in semiconductor including direct (n = 1) or indirect (n = 4) [46], herein we suppose that n = 1 [47], it can be calculated from the inset that the Eg of pure Bi2O3 is 2.6 eV. In order to accurately understand the position of the conduction band and valence band of Bi2O3, the Eqs. (2) and (3) were carried out to calculate the exact location of conduction and valence band of the composite photocatalyst [48].
ECB ¼ v Ee 0:5Eg
ð2Þ
EVB ¼ ECB þ Eg
ð3Þ
where v refers to electro-negativity which is calculated by ionization energy and electron affinity energy of the semiconductor [49]. The final calculated value of v is 5.96 eV, Ee is the energy of free electrons on the Hydrogen scale (4.5 eV) [50]. Eg is band gap of composite photocatalyst. According to the above equations, the conduction band and valence band is 0.16 eV and 2.76 eV,
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3.3. Photocatalytic performances
Fig. 6. DRS spectrum of as-prepared samples: (a) C/Fe3O4/Bi2O3-2.5, (b) C/Fe3O4/ Bi2O3-10, (c) C/Fe3O4/Bi2O3-20, (d) pure Bi2O3; the inset shows the plots of (Ahm)2 as a function of photon energy hm, respectively.
respectively. The band gap of as-synthesized pure Bi2O3 is 2.6 eV, and it is in good agreement with the inset of Fig. 6. The result was consistent with the hypothesis above, the as-prepared Bi2O3 is direct semiconductor (n = 1) [51].
3.2. Adsorption property The absorption curves of different photocatalysts were shown in Fig. 7A, the solution system reached the adsorption-desorption balance within 40 min, the adsorption value has a little change during the remaining part of adsorption time (40–60 min) but that can be ignored. The adsorption capacity of C/Fe3O4/Bi2O3-10 reached to 9.5 mg g1 much higher than pure Bi2O3, and the C/Bi2O3 and Fe3O4/Bi2O3 followed. Fig. 7B shows the absorption curves of different mass proportion of C/Fe3O4, the solution system reached the balance of adsorption-desorption within 40 min, and the adsorption capacity increased with the change of mass proportion, but the gap is not big (from 8.56 mg g1 to 9.62 mg g1). The XRD patterns of different materials after absorption were shown in Fig. S6, it can be seen that the crystal forms of composite photocatalysts have not changed after several times of adsorption, illustrating that the as-prepared composite photocatalysts have a stable property.
The photocatalytic activity was carried out by the degradation of tetracycline. As presented in Fig. 8A, the degradation rate of C/Fe3O4/Bi2O3-10 reached to 91% in 90 min exhibited the best photocatalytic activity while the pure Bi2O3 reached to 37% within the same time due to the effective separation of photo-generated electron–hole pair which was investigated by Photoluminescence (PL), the intensity of peaks of pure Bi2O3 is higher than C/Fe3O4/Bi2O3-10 (Fig. 8B), illustrating that the recombination probability of C/Fe3O4/ Bi2O3-10 is smaller than pure Bi2O3. The diameter of the arc radius on the EIS Nyquist plot [52] of C/Fe3O4/Bi2O3-10 (Fig. S4) is smaller than that of pure Bi2O3, indicating the charge transfer speed on the interface is fast which impelled the separation of electron–hole pairs more effective, resulting in good photocatalytic performance. The degradation rate of C/Bi2O3 and Fe3O4/Bi2O3 is higher than pure Bi2O3 because it is easy for carbon and Fe3O4 to accept the electrons from Bi2O3 to react with O2 which couldn’t happened on pure Bi2O3. Moreover more electrons were transported away by the C/Fe3O4/Bi2O3 composite photocatalyst which has two ways of electron transport contributes to the higher degradation rate compared with C/Bi2O3 and Fe3O4/Bi2O3. Besides, the different mass proportion of C/Fe3O4 exhibits small influence on the crystalline and purity (Fig. 8D), but affects the photodegradation efficiency greatly. As shown in Fig. 8C, the most suitable point of mass proportion was 10 wt.% in the degradation of tetracycline. The different mass proportion of C/Fe3O4 plays the pivotal factor role in photocatalytic, when the content of C/Fe3O4 is 20 wt.%, the absorption of composite photocatalyst is the best, but the black C/Fe3O4 has the properties of light absorption which will reduce the visible light utilization of bismuth oxide, and the content of Bi2O3 which was the main photocatalytic active material became less relatively in the composite photocatalyst, resulting in the low degradation efficiency. On the contrary, when the content of C/Fe3O4 is 2.5 wt.%, the C/Fe3O4 content is so low that it is unfavorable for the transmission of electrons, thereby the degree of recombination of electron-hole pairs increased and inhibited the photocatalytic efficiency directly. In the cycle experiment (Fig. 9), the degradation rate was reduced less than 5% after 5 cylices indicating that the composite photocatalyst possessed favorable stability for reuse. The removal of total organic carbon (TOC) was selected as a mineralization index to characterize the tetracycline degradation, the time independence of the TOC data in the tetracycline solution was shown in Fig. S7. It can be observed from Fig. S7 that 65% of the
Fig. 7. The absorption curves of samples: (A) different composite materials and pure Bi2O3; (B) different composite photocatalysts: (a) C/Fe3O4/Bi2O3-2.5, (b) C/Fe3O4/Bi2O3-5, (c) C/Fe3O4/Bi2O3-10, (d) C/Fe3O4/Bi2O3-15, (e) C/Fe3O4/Bi2O3-20.
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Fig. 8. (A) Photocatalytic degradation of tetracycline over the samples under visible light; (B) Photoluminescence (PL) spectra of pure Bi2O3, C/Bi2O3 and C/Fe3O4/Bi2O3-10, excited at the wavelength of 270 nm; (C) photo-degradation of tetracycline curves and (D) XRD patterns of different composite photocatalysts: (a) C/Fe3O4/Bi2O3-2.5, (b) C/Fe3O4/Bi2O3-5, (c) C/Fe3O4/Bi2O3-10, (d) C/Fe3O4/Bi2O3-15, (e) C/Fe3O4/Bi2O3-20.
Fig. 9. Cycling runs of photocatalytic activities of C/Fe3O4/Bi2O3-10 composite photocatalyst under visible light irradiation for degradation of tetracycline.
TOC was eliminated after 90 min under visible light irradiation, indicating that the tetracycline was mineralized by C/Fe3O4/ Bi2O3-10 composite photocatalyst during the photocatalytic degradation process under visible light irradiation. The photocatalytic mechanism exploration experiment was carried out to investigate the main active species during photocatalytic
Fig. 10. Photocatalytic degradation efficiency with different scavengers.
degradation process via dissolving different trapping agents in solution. The added trapping agent benzoquinone (BQ) is used to cap+ ture the superoxide radical ( O 2 ), while TEOA is for h and TBA is for OH. The results were shown in Fig. 10, the degradation rate decreased obviously after adding the capture agent compared with no scavenger, elucidating that superoxide radicals ( O 2 ), hydroxyl radicals ( OH) and photogenerated h+ are the activity species in pho-
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rich groups (negative oxygen ions) occurred nucleophilic reaction with carbonyl, generating lactone structure product B (m/ z = 361), and it turned into product C (m/z = 318) by loss of Nmethyl groups. Besides, on the other way of degradation, the tetracycline was fragmented into the product D (m/z = 345) by the leaving groups of ACH3, AN(CH3)2 and ACO(NH2), and it turned into product E (m/z = 329) by dislodging of AOH, the product 5 decomposed into product F (m/z = 274) through the smash of ring ‘‘b” [55], and the product I (m/z = 246) was formed by the loss of AOH and ACOH groups. Product G (m/z = 429) was stemmed from the tetracycline (m/z = 445) via loss of hydroxyl group during the photocatalytic reaction, afterwards, the product G transferred to product H (m/z = 415). Finally, with the change of time, the tetracycline is gradually broken down into small molecules. 3.4. Photocatalytic mechanism The photocatalytic mechanism was investigated in previous study [56]. In Fig. 13, when a photon with energy matches the band gap energy (Eg ) of C/Fe3O4/Bi2O3 composite photocatalyst, a phototo-generated electron (e ) in the valence band is excited into the conduction band of Bi2O3, and leave a positive hole (h+) in the valence band (Eq. (4)). According to the results which were calculated as mentioned above, the conductive band of Bi2O3 is 0.16 eV (vs NHE), it is more positive than E0 (O2 = O 2 = 0.28 eV vs NHE), thus there is no O 2 production from electrons on the surface of Bi2O3. The double conductive C/Fe3O4/Bi2O3 composite photocatalyst has two ways of electron transmission. On the step 1, the phototo-generated electrons (e ) of Bi2O3 transferred to Fe3O4 further transferred to carbon (Eq. (5)), the electrons which transferred on the surface of carbon reacted with O2 to generate O 2 (Eq. (6)), while on the step 2, the electrons firstly transferred to carbon and further to Fe3O4 (Eq. (7)), it has been reported that Fe3+ ions exist in Fe3O4, it can act as a photo-excited electron-trapping site to prevent the fast recombination of photo-induced charge carriers and prolong their lifetime [57–59], the electrons were captured by Fe3+ ion and it results in the formation of Fe2+ ion (Eq. (8)), then the Fe2+ ion would react with the dissolved O2 to generate Fe3+ ions and O 2 radicals (Eq. (9)). The valence band data of Bi2O3 (EVB = 2.76 eV) is positive than OH /OH (2.38 eV), the hole (h+) of Bi2O3 can reacted with OH to produce OH (Eq. (10)). According + to Fig. 10, the O 2 , OH and h are the main activity specie in the process of photocatalytic, they can react with tetracycline and degrade it into small molecules under visible light irradiation (Eqs. (11)–(13)), specific reaction formula is as follows: þ
Fig. 11. m/z of degrading tetracycline over C/Fe3O4/Bi2O3 composite photocatalyst: (A) the initial tetracycline solution; (B) degradation of tetracycline after 30 min; (C) degradation of tetracycline after 90 min.
Bi2 O3 þ Vis ! e ðBi2 O3 Þ þ h ðBi2 O3 Þ e ðBi2 O3 Þ ! e ðFe3 O4 Þ ! e ðcarbonÞ
ð4Þ step1
O2 þ e ðcarbonÞ ! O2 ðcarbonÞ todegradation tetracycline. From the picture it can be concluded that the degradation rate decreased heavily when TEOA is added, suggesting that h+ is the main activity specie in solution during photocatalytic, besides, the BQ could capture the electrons to form O2 radicals, which is beneficial to promote the degradation of tetracycline, the OH also plays a necessary part of photocatalytic. As is widely known that mass spectrometry is used to detect the photocatalytic intermediate product, the possible intermediate products of photodegradation of tetracycline were tested in this experiment. The results were shown in Fig. 11, the prominent anion with m/z of 445 was the deprotonated tetracycline molecular ion [53], during the process of degradation, the tetracycline was fragmented into small molecules (Fig. 12), it could form the intermediate product A (m/z = 362, via breakage of naphthalene ring ‘‘a” [54]), under alkaline conditions, the formation of hydroxyl electron
e ðBi2 O3 Þ ! e ðcarbonÞ ! e ðFe3 O4 Þ
ð5Þ ð6Þ
step2
ð7Þ
Fe3þ þ e ! Fe2þ
ð8Þ
Fe2þ þ O2 ! Fe3þ þ O2 ðFe3 O4 Þ
ð9Þ
þ
h ðBi2 O3 Þ þ OH ! OH
ð10Þ
O2 ðFe3 O4 or carbonÞ þ tetracycline ! Molecules
ð11Þ
OH þ tetracycline ! Molecules
ð12Þ
þ
h ðBi2 O3 Þ þ tetracycline ! Molecules
ð13Þ
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OH
O
OH
d
c
b
OH
O
O
NH2
a OH
HO
OH
O
OH
OH
CH3
m/z=445
N(CH3) 2
OH
O
OH
O
OH
OH
O
OH
O
OH
O
O
NH2
H CH2 CH3
HO
G, m/z=429
OH
N(CH3)2
HO CH3
D, m/z=345
HO
N(CH3) 2
A, m/z=362
O
OH
OH
O
O OH
O
H
O
OH
OH OH
O
OH
O
OH
O
O
NH2 CH2
H, m/z=415
N(CH3) 2
CH3
B, m/z=361
O
OH O
OH
O
HO
OH
O
H
O
E, m/z=329
OH
OH
O
HO CH3
OH
NH(CH3)
O
OH
H
CH3 CH3
HO
F, m/z=274 HO
C, m/z=318
I, m/z=246
products (CO2, H2O and other molecules) Fig. 12. The possible intermediate products of photodegradation of tetracycline.
Fig. 13. Scheme for electron-hole separation and transport of double conductive C/Fe3O4/Bi2O3 composite photocatalyst under the visible-light irradiation.
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4. Conclusions In this thesis, the double conductive C/Fe3O4/Bi2O3 composite photocatalyst was successfully synthesized, a series of characterization of composite photocatalyst were carried out. The results demonstrated that the as-synthesized C/Fe3O4/Bi2O3 composite photocatalyst exhibited strong photoabsorption in visible light region and the light absorption range is larger than pure Bi2O3. The existence of C/Fe3O4, part of double conductive C/Fe3O4/Bi2O3 composite photocatalyst, not only offer the magnetic properties but also serve as the center of electron transfer and storage which helps to transfer electrons from Bi2O3 and reduce the probability of electron return to Bi2O3. The degradation experiment showed that the photocatalytic degradation rate can reached to 91% in 90 min when the mass proportion of C/Fe3O4 was 10 wt.%. In addition, the photocatalytic mechanism study revealed that h+ was the main active specie in the process of degradation. And the recycle experiment showed that the composite photocatalyst has good reuse performance. It is proved that the introduction of C/Fe3O4 can improve the photocatalytic performance of Bi2O3 photocatalyst. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21306068 and 21546013), the Natural Science Foundation of Jiangsu Province (Nos. K20130487 and BK20140532), the Postdoctoral Science Foundation of Jiangsu Province (No. 1501102B), the Innovation Programs Foundation of Jiangsu Province (Nos. SJZZ_0136 and SJLX15_0504).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2016.06.063.
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