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AC photocatalyst by microwave irradiation for the photocatalytic degradation of naphthalene
Journal of Alloys and Compounds 676 (2016) 489e498
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Synthesis of N and La co-doped TiO2/AC photocatalyst by microwave irradiation for the photocatalytic degradation of naphthalene Dandan Liu, Zhansheng Wu*, Fei Tian, Bang-Ce Ye, Yanbin Tong* School of Chemistry and Chemical Engineering, Shihezi University, Shihezi, 832003, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 December 2015 Received in revised form 13 March 2016 Accepted 17 March 2016 Available online 18 March 2016
La and N co-doped TiO2 nanoparticles supported on activated carbon (TiO2/AC) were synthesized through a microwave-assisted solegel method for the synergistic removal of naphthalene solution by photocatalytic degradation. Results showed that the La and N ions were incorporated into the TiO2 framework in both the anatase and rutile phases of TiO2 for single doped and co-doped samples, which narrowed the band gap of TiO2 from 2.82 to 2.20 eV. The PL spectra of the samples showed a decrease in the recombination centers when N and La were introduced in TiO2/AC. The 0.001La-N-TiO2/AC photocatalyst exhibited the highest degradation efficiency of 93.5% for naphthalene under visible light within 120 min. This result was attributed to a synergistic effect involving the efficient inhibition of the recombination of photogenerated electrons and holes, the increase in surface hydroxyl, surface area, volume pores, and the increase of uptake in the visible light region. In addition, the high apparent rate constant indicated that La and N co-doping result in the increase of photoactivity. This study demonstrated the co-doped TiO2/AC is a highly efficient photocatalyst for the removal of naphthalene. The results provided valuable information on the mechanism of naphthalene decomposition. © 2016 Elsevier B.V. All rights reserved.
Keywords: Photocatalytic degradation Microwave irradiation TiO2/activated carbon Naphthalene
1. Introduction Naphthalene is one of the 16 polycyclic aromatic hydrocarbons (PAHs) classified as priority pollutants by the Environmental Protection Agency of the United States, and has been identified as carcinogenic, mutagenic, and teratogenic substance [1]. Therefore, the effective treatment of naphthalene in aqueous solutions is extremely important for public health and environmental safety. Conventional physicochemical techniques such as adsorption, flocculation, reverse osmosis, and extraction have been developed to remove naphthalene from aqueous solutions [1,2]; however, these methods merely transfer the naphthalene from one phase to another while not transform it into harmless compounds. Classical biological treatment was proven to be inefficient for naphthalene decomposition because it was time consuming and difficult to control [3]. Photodegradation of naphthalene by UV irradiation from aqueous solution also exists many limitations in practical application [4]. Thus, advanced alternative technologies are required to effectively decompose naphthalene from aqueous solution.
* Corresponding authors. E-mail addresses: [email protected] (Z. Wu), [email protected] (Y. Tong). http://dx.doi.org/10.1016/j.jallcom.2016.03.124 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Photocatalytic degradation of TiO2 with superior photocatalytic activity, high chemical stability, and non-toxicity as photocatalyst has been considered as a promising approach and an environmentfriendly strategy for the elimination of organic pollutants [5]. However, the photocatalytic efficiency and practical application of pure TiO2 nanomaterials have been greatly restricted because of their relatively large band gap with low absorption characteristics under visible light, high photogenerated electrons and hole pairs recombination and poor adsorption performance [5,6]. Many attempts have been performed to eliminate the drawbacks of pure TiO2 materials. On the one hand, doped non-metal elements such as C, N, S, F, and P could be easily incorporated into the lattice of TiO2 nano-crystallites to induce a narrower band gap, which lead to enhance their visible light response [7e10]. On the other hand, doped rare earth elements such as La, Ce, Gd, and Sm could trap photogenerated electrons and hole (e/hþ) pairs, which improve their catalytic efficiency by enhancing the separation of photogenerated charge carriers [11e13]. Recently, co-doping TiO2 with non-metal and rare earth elements has attracted considerable attention [14e18]. Yu et al. [14,15] employed that introducing N and La into the TiO2 matrix narrows the band gap of titania and enhances the utilization efficiency of visible light, and thereby increases photodegradation activity of Rh
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B under both visible and UV light irradiation, as well N and La codoping could produce a synergistic photodegradation effect. Similarly, the photocatalyst of Ce and N co-doped TiO2 prepared through hydrothermal method was remarkably enhanced the visible-light-induced degradation of Acid Orange 7 compared with N-TiO2 and pure TiO2 [16]. Wang et al. [17] reported that Gd, C, N and P quaternary doped anatase-TiO2 nano-photocatalyst prepared by a modified sol-solvothermal process exhibited the highest photocatalytic performance of 4-chlorophenol under simulated sunlight irradiation among as-prepared undoped, single-doped and codoped samples. Similarly, the (Yb, N)-TiO2 synthesized by solegel method had shown much more higher photocatalytic activity for the degradation of methylene blue than that of the TiO2-P25, the NTiO2 and the Yb-TiO2 under the visible light [18]. So, co-doping TiO2 with non-metal and rare earth elements was reported as one of the most promising strategies for photocatalytic degradation of organic pollutants under visible-light irradiation. Furthermore, supporting TiO2 on porous activated carbon (AC) could increase its adsorption capacity for organic compounds [6,19e21]. Ragupathy et al. [6] reported that the removal efficiencies of the TiO2-loaded cashew nut shell AC on brilliant green and methylene blue dyes were 99.75% and 96.35% under sunlight radiation, respectively. Similar, Liu et al. [19] prepared ordered mesoporous TiO2/AC by the template technique with the aid of an ultrasonic method, which had higher photoactivity for acid red B degradation than pure TiO2. Very recently, the immobilization of TiO2 on powdered activated carbon is indicated as very effective for phenol solar photocatalytic degradation [20]. Luo et al. [21] synthesized TiO2/wood charcoal composites photocatalysts with efficient removal of hydrophobic bisphenol A by synergistic adsorption and photocatalytic degradation. In addition, microwave-assisted synthesis of TiO2 nanoparticles has attracted much interest in recent years [22e25]. Compared with conventional heating methods, the microwave-irradiated method is more environmentally friendly, rapid, and cost effective [23,24]. Recently, the Cu/N co-doped TiO2 catalyst prepared by microwave-assisted hydrothermal exhibited the highest photoactivity for hydrogen production [25]. However, few studies on the microwave-assisted synthesis of N and La co-doped TiO2/AC with high photocatalytic activity have been reported so far. Thus, developing techniques for the degradation of naphthalene by N and La co-doped TiO2/AC synthesized via microwave irradiation under the visible light is very interesting and significant. In the present work, the synthesis and structural characterization of the N- and La-doped AC/TiO2 photocatalysts prepared by solegel method combined with microwave irradiation were studied. The photocatalysts were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET analysis, UVevisible diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR). The photocatalytic activity for the degradation of naphthalene was measured under visible light. The synergistic effect of N and La in enhancing photocatalytic activity was discussed. The proposed mechanisms for the increase of visible light absorption and photocatalytic activity were also investigated. The present work may provide a valuable implication on the environmental fate of PAHs. 2. Materials and methods
in anhydrous alcohol (EtOH) in proportion of 1:1 (volume ratio) by stirring for 40 min and named solution A. Solution B was prepared by mixing 14 mL of glacial acetic acid and 7 mL of distilled water in 35 mL of absolute alcohol. Solution B was added dropwise to solution A and continuously stirred for 1 h to obtain pale yellow clear TiO2 sol. AC (10 g) was added into the TiO2 sol (100 g) to solidify, then the mixed powder of TiO2/AC was prepared under microwave irradiation at 700 W for 15 min. To prepare N-doped AC/TiO2, urea was dissolved in solution B, the dosage of N was 0.4 g, and the resulted sample was noted as N-TiO2/AC. while for La-N-codoped AC/TiO2, urea and lanthanum nitrate were dissolved in solution B. The N: Ti molar ratio is 6.09. The dosages of La were 0.0005, 0.001 and 0.01 g, and the resulted samples were noted as 0.0005 La-NTiO2/AC, 0.001La-N-TiO2/AC and 0.01La-N-TiO2/AC, respectively. The La: Ti molar ratios are 0.0002, 0.0004 and 0.004, respectively. 2.2. Characterization of photocatalyst The crystal phase composition of photocatalyst was identified by XRD using a Rigaku Giegerflex D/Max B diffractometer with CuKa radiation in the region 2ɵ ¼ 10e80 with an increment of 0.02 on the 2ɵ scale at room temperature. The surface morphology of the photocatalyst was examined using SEM (JSM-6490 LV, Japan) at accelerating voltages of 10 KV. TEM was performed on a Tecnai G2 F20 (USA) microscope at 100 kV. The porous structure of photocatalyst was characterized by N2 adsorption at 77 K using a surface area analyzer (Micromeritics, ASAP-2020, USA). A diffuse reflectance accessory attached to the UVeVisible spectrophotometer (Hitachi U4100, Japan) was used to record the reflectance spectra to determine the optical band gap of the photocatalysts. Photoluminescence (PL) spectra of the samples were measured with a fluorescence spectrophotometer (FLsp920, England) at room temperature using Xe lamp as an excitation light source with excitation wavelength of 280 nm. The XPS analysis was conducted using a 250XI ESCA system with Mg Ka X-ray source (1253.6 eV) under a vacuum pressure < 106 Pa, the binding energy 284.6 eV of C1s was used to charge correction reference. FTIR spectra were recorded with a Magna-IR 750 spectrometer in the range of 4000e400 cm1 using KBr pellet at resolution of 1 cm1. 2.3. Photocatalystic experiments The photocatalytic activity of prepared photocatalyst was evaluated in terms of the degradation of naphthalene, a 500 W Xe lamp (any irradiation below 420 nm removed by using a cut-off filter) was served as the source of visible light. An average irradiation intensity of 350 W/m2 was maintained throughout the experiments. The distance between the reactor and lamp housing is 8.5 cm. The temperature was maintained at 25 C during the photocatalytic reaction. The photocatalyst (20.0 mg) was added into 50 mL of naphthalene solution (30 mg/L). Prior to illumination, the mixture was stirred for 60 min in the dark until reaching the adsorption-desorption equilibrium, the degradation experiment of each sample at different time interval was carried out in turn, each sample was taken at regular interval after the reaction and was immediately centrifuged at 8000 rpm for 15 min to remove catalyst particles for analysis, the concentration of naphthalene in supernatant liquid were measured by a UV-752N spectrophotometer at lmax ¼ 218 nm. The removal efficiency ðhÞ of the photocatalyst was calculated as follows:
co ct 100% co
2.1. Preparation of photocatalyst
h¼
The AC and photocatalyst were prepared based on our previous studies [22]. 30 mL of tetrabutyl orthotitanate (TBOT) was dissolved
the apparent rate constants (k) of the samples are calculated as follows:
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3.2. SEM and TEM
lnðct =c0 Þ ¼ kt where Co and Ct are the concentrations of naphthalene at initial and different irradiation time, respectively.
3. Results and discussion 3.1. Phase analysis Fig. 1 shows the XRD patterns of the photocatalyst. The characteristic diffraction peaks of anatase (A) TiO2 at 25.2 , 36.1, 37.8 , 47.7, 54.4 , 62.7, and 67.4 were clearly observed, and the miller indexes of these diffraction peaks are calculated to be [101], [004], [112], [200], [105], [204] and [116], respectively, as well as rutile (R) TiO2 at 27.4 , 41.3 and 56.8 were clearly observed, and the miller indexes of which are [110], [111] and [211], respectively. It can be concluded that codoping with La and N elements favored the formation of anatase and suppressed the formation of rutile. No other peaks were observed in the doped TiO2 that could be related to segregated phases of the dopants, it could be due to the doping amounts of La and N were too low or the high dispersion of the doping element. The anatase peaks became broader with the decrease in intensity when the amount of La was increased in the La-N-TiO2. This indicates that La incorporation result in the decrease of crystallinity of the co-doped samples, which may be attributed to the inhibitive action of crystal growth is prompted by the increased dopants. The gradual weakening intensity of the peaks also showed that the interaction of Ti-O bond weakened with the increase in the amount of La in La-N-TiO2 [15]. The average crystallite sizes of the samples were calculated from the anatase (101) and rutile (110) peaks using the Scherrer equation [24] (Table 1). The particle sizes of La-N-TiO2/AC were smaller than those of TiO2/AC because the O-Ti-N and La-O-Ti linkages were formed on the surface and on the interstitial sizes of the samples, which inhibited the growth of crystal grains by restricting the aggregation and growth of TiO2 crystallite [26e28]. This may increase the surface area of the photocatalyst and result in high photocatalytic activity, which was confirmed by the BET result below (Table 1).
Fig. 1. XRD patterns of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC samples.
Fig. 2 presents the SEM images of AC and various photocatalyst powders. After doping, the nano-TiO2 particles were dispersed nonuniformly on the surface of AC in all samples. With the increase of dopants amount, more particles dispersed on TiO2/AC. In order to further investigate the morphology and microstructure of the samples, these TEM images showed that the synthesized samples are nearly spherical shape with high density (Fig. 3). The average particle sizes were measured by about 200 particles. The calculated average particle size for N-TiO2/AC was 17.20 ± 0.01 nm, which was smaller than that of TiO2/AC (20.70 ± 0.02 nm). The average particle sizes for 0.0005 La-N-TiO2/AC, 0.001 La-N-TiO2/AC, and 0.01 La-NTiO2/AC were about 20.20 ± 0.03, 15.80 ± 0.02, and 16.60 ± 0.01 nm respectively, which agree with the XRD results (Table 1). 3.3. BET Fig. 4 presents the N2 adsorption-desorption isotherms of AC, TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC samples. Referring to the IUPAC classification, the adsorption isotherms of the samples were of type IV with a hysteresis loop of type H2, indicating the presence of a mesoporous structure. In addition, the presence of either N or La species induces a shift of the hysteresis loop to lower pressures (Fig. 4), indicating a decrease in the average pore size (Table 1) [29]. N-TiO2/AC and La-N co-doped TiO2/AC samples had larger BET surface areas, pore volumes, and average pore diameters than TiO2/AC, which was mainly because of the smaller crystallite sizes of the doped TiO2/AC (Table 1); The fact that the crystal sizes of photocatalyst are greater than the average pore size of AC (Table 1), entrance of TiO2 into the pores is ruled out. Instead, TiO2 nanoparticles mostly deposited on the surface of AC and resulted in the decrease of surface area of photocatalysts. While little crystal size and fine disperse of TiO2 could resist the decrease of surface area and pore volume of photocatalysts to some extent. A larger surface area can offer more active adsorption sites and photocatalytic reaction centers, and this means the higher specific surface area will be beneficial to improve the photocatalytic activity [17,30]. 3.4. UVevis diffuse reflectance spectra (DRS) The UVevis DRS of the TiO2/AC, N-TiO2/AC, and La-N co-doped TiO2/AC samples are shown in Fig. 5 (inset). It was seen that La-N co-doped TiO2/AC exhibited higher absorption than N-TiO2/AC and TiO2/AC. The N-TiO2/AC and La-N co-doped TiO2/AC samples were red shifted and exhibited extended absorption in the visible-light region (>400 nm). The red shift could be attributed to the chargeetransfer transition between the La 3 d electrons or N 2p state and the TiO2 conduction or valance band; thus, the generation of electronehole pairs improved the visible light response [15]. The increased visible light absorption of the co-doped TiO2/AC samples was due to the introduction of new energy levels by doping La and N in the band gap of TiO2, resulting in additional energy states [14]. And the bulk electrons originating from La dopants can be localized onto N centers, making therefore available more charge density to be promoted into the CB upon visible-light photoexcitation [31]. Thus, the band gap of TiO2 was reduced and it became visible light. Sample 0.001La-N-TiO2/AC showed the strongest photo-absorption in the visible light region, which implies its potential for higher photocatalytic activity under visible light. Anatase TiO2 is an indirect band gap semiconductor, for which the Kubelka-Munk function between absorption coefficient (a) and the incident photon energy (hn) can be written as a ¼ Bi(hn-Eg)2/hn, where Bi is the absorption constant for indirect transitions [27], hv
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Table 1 Characteristics of AC, TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC samples. Samples
Anatase size (nm)
Rutile size (nm)
Ae (m2/g)
SBET (m2/g)
Vtot (cm3/g)
Average pore size (nm)
Band energy (eV)
Ratio of A and R %
AC N-TiO2/AC 0.0005La-N-TiO2/AC 0.001La-N-TiO2/AC 0.01La-N-TiO2/AC
e 17.20 20.10 15.80 16.80
e 18.60 20.30 16.30 18.10
956.85 68.57 45.40 57.32 52.80
1915.00 404.20 321.24 335.17 316.37
1.02 0.25 0.20 0.21 0.20
3.82 2.48 2.47 2.49 2.51
e 2.61 2.44 2.20 2.31
e 62/38 67/41 72/31 70/36
± ± ± ±
0.01 0.02 0.01 0.03
± ± ± ±
0.02 0.02 0.01 0.02
SBET, specific surface area obtained by BET equation; Ae, external surface area; Vt, total pore volume; Average pore size from pore size distribution determined by BET method.
Fig. 2. SEM images of AC, TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
is the photon energy, Eg the band gap energy. Plots of (ahn)1/2 versus hn from the spectral data are presented in Fig. 5, which suggested that the absorption edge of them were caused by direct transitions, the band gap energies were deduced by extrapolating a straight line to the abscissa axis [32]. The results showed that N and La can reduce the band gap of energy compared with the undoped TiO2/AC (Table 1), indicating the internal electronehole recombination rate of TiO2 reduce and eventually improving the photocatalytic efficiency [33]. Such decrease of the band gap can
be explained that La doping could lead to lattice deformation and form vacancy, thus probably resulting in an impurity state in TiO2 band gap [15]. In a previous report, the band narrowing for the codoped samples mainly resulted from N-doping, which not only reduced the band gap by mixing N 2p states with O 2p states on the top of the valance band (VB), but also created an N-induced midgap level [14]. In addition, an increase in band energy was observed at a higher concentration of La (0.01), which may have resulted from the deposition of the metal on the photocatalyst that
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Fig. 3. TEM images of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
covered the surface of TiO2 and reduced the effective surface area for adsorbing light [26]. 3.5. PL spectra Fig. 6 shows the PL spectra of various photocatalyst samples. The PL spectra exhibited similar curve shapes for all samples, indicating that the doping dose affected the intensity but did not lead to a new PL phenomenon. The spectra also indicated that the incorporation of La reduced the intensity of the PL spectra because of the reduced recombination centers and the decline in the recombination of electrons and holes. The major emissive peaks were obtained in the wavelength range of 430e530 nm. The maximum peak at around 440 nm was assigned to a combined peak originating from the band edge emission and defect emission; previous studies have suggested that the PL signals obtained from TiO2 mainly originated from the surface states as a result of the oxygen vacancies and defects present on the surface of TiO2 [34]. The intensity of the PL spectra decreased in the order of TiO2/AC > N-TiO2/AC > 0.0005LaN-TiO2/AC > 0.01La-N-TiO2/AC > 0.001La-N-TiO2/AC. As PL emission represents the recombination of free carriers; thus, a lower PL intensity may indicate a lower electronehole recombination rate and
higher separation efficiency [17,35]. Therefore, the 0.001La-N-TiO2/ AC sample exhibited the best photocatalytic activity, which is discussed in the photocatalytic section below. When the concentration of La is too high, the space charge region becomes very narrow and the penetration depth of light into TiO2 greatly exceeds the space charge layer. The excess La on the surface contributed to the possible introduction of new trap sites or recombination centers on the surface of the samples, making the recombination of the photogenerated electronehole pairs in TiO2 easier. 3.6. XPS analysis As shown in Fig. 7, the Ti 2p XPS spectra of all samples were mainly composed of the main peaks of Ti 2p1/2 at 464.7 eV and Ti 2p3/2 at 458.8 eV. Compared with TiO2/AC, the Ti 2p3/2 of N-TiO2/AC and La-N co-doped TiO2/AC samples showed a red shift to lower energy, which was attributed to the Ti ions influenced by N and La incorporation [27]. In addition, these shifting of the peaks to the higher binding energy depict a decrease of the Ti in the electron density, which indicate the existence of only Ti4þ on the surface of the photocatalyst, and confirm the absence of the Ti3þ [16]. The O 1s XPS spectra of N-TiO2/AC and La-N co-1doped TiO2/AC
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Fig. 4. Nitrogen adsorption-desorption BET isotherms and pore size distribution curves (inset) of AC, TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
Fig. 5. UVevisible DRS of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
samples were very similar. The peaks at around 530 eV of the samples were attributed to the Ti-O bond in TiO2 and the surface hydroxyl groups of TiO2. The increase of the peak at 531.6 eV
Fig. 6. PL spectra of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
showed that the surface hydroxyl groups were increased (from 18.92% for TiO2/AC to 25.89% for 0.001La-N-TiO2/AC as calculated from XPS), the surface hydroxyl groups play an important role in photocatalysis by reacting with photogenerated holes to produce
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Fig. 7. XPS spectra for (a) TiO2/AC, (b) N-TiO2/AC, (c) 0.0005La-N-TiO2/AC, (d) 0.001La-N-TiO2/AC and (e)0.01La-N-TiO2/AC.
hydroxyl radicals ($OH), which are responsible for the photodegradation of contaminants [18]. The broad peak in 397e400 eV range can be allotted to the existence of interstitial N present in the form of (Ti-O-N) in the structure [24], which indicated that some O atoms were substituted. According to the most of previous reports, the peak around 397 eV is attributed to the b-N assigned to the N3 anions that replace oxygen atoms in the TiO2 lattice to form N-Ti-N [17]. These results show that N is doped in the lattice of TiO2. Moreover, the similar peaks indicated that the incorporation of La ions did not induce an obvious change in N chemical state. The splitting of the main peaks of La 3d5/2 and La 3d3/2 were at around 834.2 and 851.0 eV, respectively. Compared with the bonding energy of pure La2O3 (834.9 eV), the slight shift of La 3d5/2 (834.2 eV) to lower binding energy indicates the formation of La-OTi in the samples [36].
3.7. FTIR analysis In Fig. 8, the broad absorption peak of the samples at around 3422 cm1 was attributed to the stretching vibration of O-H bonds [16]. The absorption peak at 1630 cm1 corresponded to the OH bending mode of H2O adsorbed on the surface of the TiO2, which may play a crucial role in photocatalytic activity [24]. From the peak at 3422 cm1, it was seen that as the amount of La increased, the amount of surface-adsorbed hydroxyl groups also increased. The absorption peak at 1370 cm1 was due to the scissoring vibration of CH3 species [35]. In comparison with TiO2/AC, the N-TiO2/AC and La-N co-doped TiO2/AC samples displayed additional peaks at around 1080 cm1, which can be assigned to the vibration of the NTi bond [9]. Additionally, the La-N co-doped TiO2/AC samples displayed an absorption peak of 518 cm1 characteristic of the LaeO
Fig. 8. FTIR spectra of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
bond, which showed the successful incorporation of the La-dopant with the TiO2 [37]. The peaks in the 1000 to 400 cm1 region are related to the bend vibration of Ti-O-Ti bonds [35]. 3.8. Photocatalytic activity The photocatalytic activities of various photocatalysts were assessed by the degradation of 30 mg/L of naphthalene solution under visible light irradiation (Fig. 9). For comparison, under visible light irradiation over a period of 120 min, no self-degradation of
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naphthalene was observed in the absence of photocatalyst, indicating that the direct photolysis of naphthalene could not undergo under visible light irradiation. The N-TiO2/AC was more efficient for the degradation of naphthalene as compared with undoped TiO2/ AC. This is could be that the nitrogen doping more efficiently respond to the visible light by reducing band gap as shown in DRS. The photoactivity of La-N co-doped TiO2/AC was further enhanced by the introduction of La in the photocatalyst. In addition, the degradation of naphthalene by the samples appeared to follow apparent first-order kinetics and kinetic curves (Fig. 10); 0.001 LaN-doped TiO2/AC achieved an excellent photocatalytic performance with ka of 0.0239 min1, which was 3.92 times that of TiO2/AC (ka ¼ 0.0061 min1). The apparent rate constants of La-N-doped TiO2/AC samples were higher than that of La-TiO2/AC and N-TiO2/ AC (Table 2), indicating that the La and N co-doping resulted in the further increase of TiO2/AC photoactivity. In addition, the larger red shift and lower PL intensity was found in La-N-TiO2/AC sample as confirmed by the UVevis DRS and PL results; thus, the visible light absorption ability of La-N-TiO2/AC was enhanced. La, N codoped materials show both an effect on the absorption edge and a localized absorption in the visible region [15]. Moreover, the La-induced increase in the activity of the samples was attributed to the decrease in particle size and increase in the surface area of photocrystal. High surface area offering more sites to adsorb naphthalene resulted in enhancing the photoactivity of photocatalyst. Moreover, La-N-doped TiO2/AC may enhance the formation of labile oxygen vacancies [16], which also was confirmed from the XPS and FTIR analyses results. In addition, the number of hydroxyl groups on the surface of N and La co-doped TiO2/AC was increased compared with N-TiO2/AC. Among the samples, the degradation efficiency of 0.001La-NTiO2/AC for naphthalene is 93.5% within 120 min under visible light radiation, namely, the photodecomposition is 85.5%, and the pure adsorption is 8.0% (Fig. 9). The increase in photocatalytic activity of 0.001La-N-TiO2/AC by increasing the concentration of La from 0.0005 to 0.001 g may be due to reduced band gap. Another explanation for the increase of photocatalytic activity could be attributed to the fact that the doping of TiO2 with La introduces new trapping sites. This affects the lifespan of charge carriers by splitting the arrival time of photogenerated electrons and holes needed to reach the surface of the photocatalyst, thereby reducing
Fig. 10. Kinetic curves for the photodegradation of naphthalene under visible light irradiation over the samples.
Table 2 First order kinetic constants and relative coefficients for photocatalytic degradation of naphthalene over the samples. Samples
ka (min1)
R2
TiO2/AC N-TiO2/AC La-TiO2/AC 0.0005La- N-TiO2/AC 0.001La- N-TiO2/AC 0.01La- N-TiO2/AC
0.0061 0.0109 0.0122 0.0144 0.0239 0.0165
0.9786 0.9947 0.9939 0.9788 0.9984 0.9904
the electronehole recombination. At a higher dopant concentration (0.01La), multiple trapping of charge carriers occurs, increasing the possibility of electronehole recombination [26] and reducing the number of charge carriers that will reach the surface to initiate the degradation of naphthalene [38]. Thus, a decrease in degrading efficiency at higher La doping concentration was observed. It was seen from Table 3, the La-N-TiO2/AC is efficient to degrade naphthalene among the materials reported, it is valuable to develop the photocatalyst. 3.9. Photostability over La-N-TiO2/AC under visible light To investigate the reusability of La-N co-doped TiO2/AC photocatalysts, the photocatalytic degradation of recycled 0.001La-NTiO2/AC was carried out under visible light irradiation. After each cycle, the photocatalyst was collected through filtration and dried in the oven at 110 C for 2 h before reuse. As shown in Fig. 11, the photocatalytic activity of the recovered La-N-TiO2/AC photocatalyst showed no noticeable change and still reached 92.2% after five recycling runs. This result indicated that 0.001La-N-TiO2/AC photocatalyst is highly effective and stable under visible light irradiation. 3.10. Photodegradation mechanism of naphthalene over La-N-TiO2/ AC under visible light
Fig. 9. Photocatalytic degradation of naphthalene over phtocatalysts under visible light irradiation (Blank: without a photocatalyst; C0 ¼ 30 mg/L, 50 mL of naphthalene solution, 20 mg of photocatalyst).
It was concluded that La and N work together to improve the photocatalytic activity of TiO2/AC under visible light, it can be found that after N doping, the band gap was narrowed from 2.82 to 2.61 eV, while after N and La co-doping, the band gap was narrowed from 2.82 to 2.20 eV (Fig. 5, Table 1). In addition, it is reported that a dopant level can form above the valence band for the substitution
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Table 3 Comparison of different kinds of materials. Materials
Degradation
Time (h)
Degradation efficiency
Reference
N-TiO2/Bi2O3 Ce-N-TiO2 Yb-N-TiO2 Mo-TiO2 TiO2/Wood Charcoal La-N-TiO2/AC
2,4-Dichlorophenol (50 mg/L) Phenol (20 mg/L) Methylene blue (10 mg/L) Acid red 88 Bisphenol A (20 mg/L) Naphthalene
6 2 5 3 18 2
90.0% 93.8% 93.6% 77.0% 80.0% 93.5%
[39] [40] [18] [26] [21] This work
was reduced (Fig. 5, Table 1), which can enhance the visible light utilization and degradation efficiency. In addition, AC has been proved to be an efficient support in promoting the photocatalytic process by providing a synergistic effect by creating a common interface between both the AC and TiO2 phase. An enhanced adsorption of the naphthalene molecules onto the AC followed closely by a transfer through an interface to the TiO2 has given rise to a complete photo-degradation process [6]. Some hot-spots could be formed on the surface of the AC particles, and the naphthalene molecules around the “hot spots” could be decomposed in the presence of O2 dissolved in water [42]. Thus, the photocatalytic activity of La-N-TiO2/AC was significantly increased.
Fig. 11. Repetitive photocatalytic degradation of naphthalene over 0.001La-N-TiO2/AC under visible light.
nitrogen and lanthanum ion doping can provide a dopant energy level (La3þ impurity level) below the conduction band of TiO2 to generate a red shift [33]. Therefore, there are two impurity levels in the band gap of La-N-TiO2/AC. The possible mechanisms are proposed (Fig. 12). As visible light irradiate La-N-TiO2/AC, TiO2 is active to produce the electrons and holes that react with H2O and O2, respectively, to form $OH and superoxide radical anion ($O 2 ). The $O 2 can also yield $OH through a series of chemical reactions. These generated $OH can oxidize naphthalene in aqueous solution and may result in the formation of CO2 and H2O. The excited electrons can either shift from the N 2p states above the VB to the conduction band (CB) of TiO2 to be trapped by the surface absorbed O2, or they can transfer from the VB of TiO2 and the N 2p to the La 3 d states. Such electron transfer enhances the separation of photoinduced holes and electrons, further prolongs the lifetime of photoinduced pairs [41], explaining why the intensity of PL decreased. When N and La are doped in the TiO2 lattice, the band gap energy of TiO2
4. Conclusions A high-performance La-N-TiO2/AC photocatalyst was successfully synthesized via an efficient and fast microwave-assisted method. La-N co-doping effectively extended the absorption spectra to the visible light region, resulting in high photocatalytic activity under visible light irradiation. The results showed that La3þ was mostly dispersed onto TiO2/AC, and a number of La atoms modified the surface of TiO2 in the form of Ti-O-La. The 0.001La-NTiO2/AC photocatalyst exhibited the highest degradation efficiency of 93.5% for naphthalene within 120 min under visible light irradiation, which could be attributed to the high adsorption capacity (small particle size, large surface and pore volume). In addition, the introduction of La to the TiO2 promoted the formation of oxygen vacancies and Ti4þ onto the surface of photocatalyst, which facilitated the adsorption of naphthalene molecules, and the La-Ncodoping can promote the separation of the photogenerated electrons and holes to accelerate the transmission of photocurrent carrier. Hence, the La-N codoing TiO2/AC photocatalyst could be considered a promising material for the application for the removal of PAHs in aqueous solution.
Fig. 12. Proposed mechanism for the visible light photocatalytic degradation of naphthalene on the surface of the La-N-TiO2/AC photocatalyst.
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Acknowledgments This work was supported financially by funding from the National Natural Science Foundation of China (51262025), International scientific and technological cooperation project of Xinjiang Bingtuan (2013BC002) and Graduate Research Innovation Project in Xinjiang (XJGRI2014053).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of N and La co-doped TiO2/AC photocatalyst by microwave irradiation for the photocatalytic degradation of naphthalene Dandan Liu, Zhansheng Wu*, Fei Tian, Bang-Ce Ye, Yanbin Tong* School of Chemistry and Chemical Engineering, Shihezi University, Shihezi, 832003, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 December 2015 Received in revised form 13 March 2016 Accepted 17 March 2016 Available online 18 March 2016
La and N co-doped TiO2 nanoparticles supported on activated carbon (TiO2/AC) were synthesized through a microwave-assisted solegel method for the synergistic removal of naphthalene solution by photocatalytic degradation. Results showed that the La and N ions were incorporated into the TiO2 framework in both the anatase and rutile phases of TiO2 for single doped and co-doped samples, which narrowed the band gap of TiO2 from 2.82 to 2.20 eV. The PL spectra of the samples showed a decrease in the recombination centers when N and La were introduced in TiO2/AC. The 0.001La-N-TiO2/AC photocatalyst exhibited the highest degradation efficiency of 93.5% for naphthalene under visible light within 120 min. This result was attributed to a synergistic effect involving the efficient inhibition of the recombination of photogenerated electrons and holes, the increase in surface hydroxyl, surface area, volume pores, and the increase of uptake in the visible light region. In addition, the high apparent rate constant indicated that La and N co-doping result in the increase of photoactivity. This study demonstrated the co-doped TiO2/AC is a highly efficient photocatalyst for the removal of naphthalene. The results provided valuable information on the mechanism of naphthalene decomposition. © 2016 Elsevier B.V. All rights reserved.
Keywords: Photocatalytic degradation Microwave irradiation TiO2/activated carbon Naphthalene
1. Introduction Naphthalene is one of the 16 polycyclic aromatic hydrocarbons (PAHs) classified as priority pollutants by the Environmental Protection Agency of the United States, and has been identified as carcinogenic, mutagenic, and teratogenic substance [1]. Therefore, the effective treatment of naphthalene in aqueous solutions is extremely important for public health and environmental safety. Conventional physicochemical techniques such as adsorption, flocculation, reverse osmosis, and extraction have been developed to remove naphthalene from aqueous solutions [1,2]; however, these methods merely transfer the naphthalene from one phase to another while not transform it into harmless compounds. Classical biological treatment was proven to be inefficient for naphthalene decomposition because it was time consuming and difficult to control [3]. Photodegradation of naphthalene by UV irradiation from aqueous solution also exists many limitations in practical application [4]. Thus, advanced alternative technologies are required to effectively decompose naphthalene from aqueous solution.
* Corresponding authors. E-mail addresses: [email protected] (Z. Wu), [email protected] (Y. Tong). http://dx.doi.org/10.1016/j.jallcom.2016.03.124 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Photocatalytic degradation of TiO2 with superior photocatalytic activity, high chemical stability, and non-toxicity as photocatalyst has been considered as a promising approach and an environmentfriendly strategy for the elimination of organic pollutants [5]. However, the photocatalytic efficiency and practical application of pure TiO2 nanomaterials have been greatly restricted because of their relatively large band gap with low absorption characteristics under visible light, high photogenerated electrons and hole pairs recombination and poor adsorption performance [5,6]. Many attempts have been performed to eliminate the drawbacks of pure TiO2 materials. On the one hand, doped non-metal elements such as C, N, S, F, and P could be easily incorporated into the lattice of TiO2 nano-crystallites to induce a narrower band gap, which lead to enhance their visible light response [7e10]. On the other hand, doped rare earth elements such as La, Ce, Gd, and Sm could trap photogenerated electrons and hole (e/hþ) pairs, which improve their catalytic efficiency by enhancing the separation of photogenerated charge carriers [11e13]. Recently, co-doping TiO2 with non-metal and rare earth elements has attracted considerable attention [14e18]. Yu et al. [14,15] employed that introducing N and La into the TiO2 matrix narrows the band gap of titania and enhances the utilization efficiency of visible light, and thereby increases photodegradation activity of Rh
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B under both visible and UV light irradiation, as well N and La codoping could produce a synergistic photodegradation effect. Similarly, the photocatalyst of Ce and N co-doped TiO2 prepared through hydrothermal method was remarkably enhanced the visible-light-induced degradation of Acid Orange 7 compared with N-TiO2 and pure TiO2 [16]. Wang et al. [17] reported that Gd, C, N and P quaternary doped anatase-TiO2 nano-photocatalyst prepared by a modified sol-solvothermal process exhibited the highest photocatalytic performance of 4-chlorophenol under simulated sunlight irradiation among as-prepared undoped, single-doped and codoped samples. Similarly, the (Yb, N)-TiO2 synthesized by solegel method had shown much more higher photocatalytic activity for the degradation of methylene blue than that of the TiO2-P25, the NTiO2 and the Yb-TiO2 under the visible light [18]. So, co-doping TiO2 with non-metal and rare earth elements was reported as one of the most promising strategies for photocatalytic degradation of organic pollutants under visible-light irradiation. Furthermore, supporting TiO2 on porous activated carbon (AC) could increase its adsorption capacity for organic compounds [6,19e21]. Ragupathy et al. [6] reported that the removal efficiencies of the TiO2-loaded cashew nut shell AC on brilliant green and methylene blue dyes were 99.75% and 96.35% under sunlight radiation, respectively. Similar, Liu et al. [19] prepared ordered mesoporous TiO2/AC by the template technique with the aid of an ultrasonic method, which had higher photoactivity for acid red B degradation than pure TiO2. Very recently, the immobilization of TiO2 on powdered activated carbon is indicated as very effective for phenol solar photocatalytic degradation [20]. Luo et al. [21] synthesized TiO2/wood charcoal composites photocatalysts with efficient removal of hydrophobic bisphenol A by synergistic adsorption and photocatalytic degradation. In addition, microwave-assisted synthesis of TiO2 nanoparticles has attracted much interest in recent years [22e25]. Compared with conventional heating methods, the microwave-irradiated method is more environmentally friendly, rapid, and cost effective [23,24]. Recently, the Cu/N co-doped TiO2 catalyst prepared by microwave-assisted hydrothermal exhibited the highest photoactivity for hydrogen production [25]. However, few studies on the microwave-assisted synthesis of N and La co-doped TiO2/AC with high photocatalytic activity have been reported so far. Thus, developing techniques for the degradation of naphthalene by N and La co-doped TiO2/AC synthesized via microwave irradiation under the visible light is very interesting and significant. In the present work, the synthesis and structural characterization of the N- and La-doped AC/TiO2 photocatalysts prepared by solegel method combined with microwave irradiation were studied. The photocatalysts were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET analysis, UVevisible diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR). The photocatalytic activity for the degradation of naphthalene was measured under visible light. The synergistic effect of N and La in enhancing photocatalytic activity was discussed. The proposed mechanisms for the increase of visible light absorption and photocatalytic activity were also investigated. The present work may provide a valuable implication on the environmental fate of PAHs. 2. Materials and methods
in anhydrous alcohol (EtOH) in proportion of 1:1 (volume ratio) by stirring for 40 min and named solution A. Solution B was prepared by mixing 14 mL of glacial acetic acid and 7 mL of distilled water in 35 mL of absolute alcohol. Solution B was added dropwise to solution A and continuously stirred for 1 h to obtain pale yellow clear TiO2 sol. AC (10 g) was added into the TiO2 sol (100 g) to solidify, then the mixed powder of TiO2/AC was prepared under microwave irradiation at 700 W for 15 min. To prepare N-doped AC/TiO2, urea was dissolved in solution B, the dosage of N was 0.4 g, and the resulted sample was noted as N-TiO2/AC. while for La-N-codoped AC/TiO2, urea and lanthanum nitrate were dissolved in solution B. The N: Ti molar ratio is 6.09. The dosages of La were 0.0005, 0.001 and 0.01 g, and the resulted samples were noted as 0.0005 La-NTiO2/AC, 0.001La-N-TiO2/AC and 0.01La-N-TiO2/AC, respectively. The La: Ti molar ratios are 0.0002, 0.0004 and 0.004, respectively. 2.2. Characterization of photocatalyst The crystal phase composition of photocatalyst was identified by XRD using a Rigaku Giegerflex D/Max B diffractometer with CuKa radiation in the region 2ɵ ¼ 10e80 with an increment of 0.02 on the 2ɵ scale at room temperature. The surface morphology of the photocatalyst was examined using SEM (JSM-6490 LV, Japan) at accelerating voltages of 10 KV. TEM was performed on a Tecnai G2 F20 (USA) microscope at 100 kV. The porous structure of photocatalyst was characterized by N2 adsorption at 77 K using a surface area analyzer (Micromeritics, ASAP-2020, USA). A diffuse reflectance accessory attached to the UVeVisible spectrophotometer (Hitachi U4100, Japan) was used to record the reflectance spectra to determine the optical band gap of the photocatalysts. Photoluminescence (PL) spectra of the samples were measured with a fluorescence spectrophotometer (FLsp920, England) at room temperature using Xe lamp as an excitation light source with excitation wavelength of 280 nm. The XPS analysis was conducted using a 250XI ESCA system with Mg Ka X-ray source (1253.6 eV) under a vacuum pressure < 106 Pa, the binding energy 284.6 eV of C1s was used to charge correction reference. FTIR spectra were recorded with a Magna-IR 750 spectrometer in the range of 4000e400 cm1 using KBr pellet at resolution of 1 cm1. 2.3. Photocatalystic experiments The photocatalytic activity of prepared photocatalyst was evaluated in terms of the degradation of naphthalene, a 500 W Xe lamp (any irradiation below 420 nm removed by using a cut-off filter) was served as the source of visible light. An average irradiation intensity of 350 W/m2 was maintained throughout the experiments. The distance between the reactor and lamp housing is 8.5 cm. The temperature was maintained at 25 C during the photocatalytic reaction. The photocatalyst (20.0 mg) was added into 50 mL of naphthalene solution (30 mg/L). Prior to illumination, the mixture was stirred for 60 min in the dark until reaching the adsorption-desorption equilibrium, the degradation experiment of each sample at different time interval was carried out in turn, each sample was taken at regular interval after the reaction and was immediately centrifuged at 8000 rpm for 15 min to remove catalyst particles for analysis, the concentration of naphthalene in supernatant liquid were measured by a UV-752N spectrophotometer at lmax ¼ 218 nm. The removal efficiency ðhÞ of the photocatalyst was calculated as follows:
co ct 100% co
2.1. Preparation of photocatalyst
h¼
The AC and photocatalyst were prepared based on our previous studies [22]. 30 mL of tetrabutyl orthotitanate (TBOT) was dissolved
the apparent rate constants (k) of the samples are calculated as follows:
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3.2. SEM and TEM
lnðct =c0 Þ ¼ kt where Co and Ct are the concentrations of naphthalene at initial and different irradiation time, respectively.
3. Results and discussion 3.1. Phase analysis Fig. 1 shows the XRD patterns of the photocatalyst. The characteristic diffraction peaks of anatase (A) TiO2 at 25.2 , 36.1, 37.8 , 47.7, 54.4 , 62.7, and 67.4 were clearly observed, and the miller indexes of these diffraction peaks are calculated to be [101], [004], [112], [200], [105], [204] and [116], respectively, as well as rutile (R) TiO2 at 27.4 , 41.3 and 56.8 were clearly observed, and the miller indexes of which are [110], [111] and [211], respectively. It can be concluded that codoping with La and N elements favored the formation of anatase and suppressed the formation of rutile. No other peaks were observed in the doped TiO2 that could be related to segregated phases of the dopants, it could be due to the doping amounts of La and N were too low or the high dispersion of the doping element. The anatase peaks became broader with the decrease in intensity when the amount of La was increased in the La-N-TiO2. This indicates that La incorporation result in the decrease of crystallinity of the co-doped samples, which may be attributed to the inhibitive action of crystal growth is prompted by the increased dopants. The gradual weakening intensity of the peaks also showed that the interaction of Ti-O bond weakened with the increase in the amount of La in La-N-TiO2 [15]. The average crystallite sizes of the samples were calculated from the anatase (101) and rutile (110) peaks using the Scherrer equation [24] (Table 1). The particle sizes of La-N-TiO2/AC were smaller than those of TiO2/AC because the O-Ti-N and La-O-Ti linkages were formed on the surface and on the interstitial sizes of the samples, which inhibited the growth of crystal grains by restricting the aggregation and growth of TiO2 crystallite [26e28]. This may increase the surface area of the photocatalyst and result in high photocatalytic activity, which was confirmed by the BET result below (Table 1).
Fig. 1. XRD patterns of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC samples.
Fig. 2 presents the SEM images of AC and various photocatalyst powders. After doping, the nano-TiO2 particles were dispersed nonuniformly on the surface of AC in all samples. With the increase of dopants amount, more particles dispersed on TiO2/AC. In order to further investigate the morphology and microstructure of the samples, these TEM images showed that the synthesized samples are nearly spherical shape with high density (Fig. 3). The average particle sizes were measured by about 200 particles. The calculated average particle size for N-TiO2/AC was 17.20 ± 0.01 nm, which was smaller than that of TiO2/AC (20.70 ± 0.02 nm). The average particle sizes for 0.0005 La-N-TiO2/AC, 0.001 La-N-TiO2/AC, and 0.01 La-NTiO2/AC were about 20.20 ± 0.03, 15.80 ± 0.02, and 16.60 ± 0.01 nm respectively, which agree with the XRD results (Table 1). 3.3. BET Fig. 4 presents the N2 adsorption-desorption isotherms of AC, TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC samples. Referring to the IUPAC classification, the adsorption isotherms of the samples were of type IV with a hysteresis loop of type H2, indicating the presence of a mesoporous structure. In addition, the presence of either N or La species induces a shift of the hysteresis loop to lower pressures (Fig. 4), indicating a decrease in the average pore size (Table 1) [29]. N-TiO2/AC and La-N co-doped TiO2/AC samples had larger BET surface areas, pore volumes, and average pore diameters than TiO2/AC, which was mainly because of the smaller crystallite sizes of the doped TiO2/AC (Table 1); The fact that the crystal sizes of photocatalyst are greater than the average pore size of AC (Table 1), entrance of TiO2 into the pores is ruled out. Instead, TiO2 nanoparticles mostly deposited on the surface of AC and resulted in the decrease of surface area of photocatalysts. While little crystal size and fine disperse of TiO2 could resist the decrease of surface area and pore volume of photocatalysts to some extent. A larger surface area can offer more active adsorption sites and photocatalytic reaction centers, and this means the higher specific surface area will be beneficial to improve the photocatalytic activity [17,30]. 3.4. UVevis diffuse reflectance spectra (DRS) The UVevis DRS of the TiO2/AC, N-TiO2/AC, and La-N co-doped TiO2/AC samples are shown in Fig. 5 (inset). It was seen that La-N co-doped TiO2/AC exhibited higher absorption than N-TiO2/AC and TiO2/AC. The N-TiO2/AC and La-N co-doped TiO2/AC samples were red shifted and exhibited extended absorption in the visible-light region (>400 nm). The red shift could be attributed to the chargeetransfer transition between the La 3 d electrons or N 2p state and the TiO2 conduction or valance band; thus, the generation of electronehole pairs improved the visible light response [15]. The increased visible light absorption of the co-doped TiO2/AC samples was due to the introduction of new energy levels by doping La and N in the band gap of TiO2, resulting in additional energy states [14]. And the bulk electrons originating from La dopants can be localized onto N centers, making therefore available more charge density to be promoted into the CB upon visible-light photoexcitation [31]. Thus, the band gap of TiO2 was reduced and it became visible light. Sample 0.001La-N-TiO2/AC showed the strongest photo-absorption in the visible light region, which implies its potential for higher photocatalytic activity under visible light. Anatase TiO2 is an indirect band gap semiconductor, for which the Kubelka-Munk function between absorption coefficient (a) and the incident photon energy (hn) can be written as a ¼ Bi(hn-Eg)2/hn, where Bi is the absorption constant for indirect transitions [27], hv
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Table 1 Characteristics of AC, TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC samples. Samples
Anatase size (nm)
Rutile size (nm)
Ae (m2/g)
SBET (m2/g)
Vtot (cm3/g)
Average pore size (nm)
Band energy (eV)
Ratio of A and R %
AC N-TiO2/AC 0.0005La-N-TiO2/AC 0.001La-N-TiO2/AC 0.01La-N-TiO2/AC
e 17.20 20.10 15.80 16.80
e 18.60 20.30 16.30 18.10
956.85 68.57 45.40 57.32 52.80
1915.00 404.20 321.24 335.17 316.37
1.02 0.25 0.20 0.21 0.20
3.82 2.48 2.47 2.49 2.51
e 2.61 2.44 2.20 2.31
e 62/38 67/41 72/31 70/36
± ± ± ±
0.01 0.02 0.01 0.03
± ± ± ±
0.02 0.02 0.01 0.02
SBET, specific surface area obtained by BET equation; Ae, external surface area; Vt, total pore volume; Average pore size from pore size distribution determined by BET method.
Fig. 2. SEM images of AC, TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
is the photon energy, Eg the band gap energy. Plots of (ahn)1/2 versus hn from the spectral data are presented in Fig. 5, which suggested that the absorption edge of them were caused by direct transitions, the band gap energies were deduced by extrapolating a straight line to the abscissa axis [32]. The results showed that N and La can reduce the band gap of energy compared with the undoped TiO2/AC (Table 1), indicating the internal electronehole recombination rate of TiO2 reduce and eventually improving the photocatalytic efficiency [33]. Such decrease of the band gap can
be explained that La doping could lead to lattice deformation and form vacancy, thus probably resulting in an impurity state in TiO2 band gap [15]. In a previous report, the band narrowing for the codoped samples mainly resulted from N-doping, which not only reduced the band gap by mixing N 2p states with O 2p states on the top of the valance band (VB), but also created an N-induced midgap level [14]. In addition, an increase in band energy was observed at a higher concentration of La (0.01), which may have resulted from the deposition of the metal on the photocatalyst that
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Fig. 3. TEM images of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
covered the surface of TiO2 and reduced the effective surface area for adsorbing light [26]. 3.5. PL spectra Fig. 6 shows the PL spectra of various photocatalyst samples. The PL spectra exhibited similar curve shapes for all samples, indicating that the doping dose affected the intensity but did not lead to a new PL phenomenon. The spectra also indicated that the incorporation of La reduced the intensity of the PL spectra because of the reduced recombination centers and the decline in the recombination of electrons and holes. The major emissive peaks were obtained in the wavelength range of 430e530 nm. The maximum peak at around 440 nm was assigned to a combined peak originating from the band edge emission and defect emission; previous studies have suggested that the PL signals obtained from TiO2 mainly originated from the surface states as a result of the oxygen vacancies and defects present on the surface of TiO2 [34]. The intensity of the PL spectra decreased in the order of TiO2/AC > N-TiO2/AC > 0.0005LaN-TiO2/AC > 0.01La-N-TiO2/AC > 0.001La-N-TiO2/AC. As PL emission represents the recombination of free carriers; thus, a lower PL intensity may indicate a lower electronehole recombination rate and
higher separation efficiency [17,35]. Therefore, the 0.001La-N-TiO2/ AC sample exhibited the best photocatalytic activity, which is discussed in the photocatalytic section below. When the concentration of La is too high, the space charge region becomes very narrow and the penetration depth of light into TiO2 greatly exceeds the space charge layer. The excess La on the surface contributed to the possible introduction of new trap sites or recombination centers on the surface of the samples, making the recombination of the photogenerated electronehole pairs in TiO2 easier. 3.6. XPS analysis As shown in Fig. 7, the Ti 2p XPS spectra of all samples were mainly composed of the main peaks of Ti 2p1/2 at 464.7 eV and Ti 2p3/2 at 458.8 eV. Compared with TiO2/AC, the Ti 2p3/2 of N-TiO2/AC and La-N co-doped TiO2/AC samples showed a red shift to lower energy, which was attributed to the Ti ions influenced by N and La incorporation [27]. In addition, these shifting of the peaks to the higher binding energy depict a decrease of the Ti in the electron density, which indicate the existence of only Ti4þ on the surface of the photocatalyst, and confirm the absence of the Ti3þ [16]. The O 1s XPS spectra of N-TiO2/AC and La-N co-1doped TiO2/AC
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Fig. 4. Nitrogen adsorption-desorption BET isotherms and pore size distribution curves (inset) of AC, TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
Fig. 5. UVevisible DRS of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
samples were very similar. The peaks at around 530 eV of the samples were attributed to the Ti-O bond in TiO2 and the surface hydroxyl groups of TiO2. The increase of the peak at 531.6 eV
Fig. 6. PL spectra of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
showed that the surface hydroxyl groups were increased (from 18.92% for TiO2/AC to 25.89% for 0.001La-N-TiO2/AC as calculated from XPS), the surface hydroxyl groups play an important role in photocatalysis by reacting with photogenerated holes to produce
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Fig. 7. XPS spectra for (a) TiO2/AC, (b) N-TiO2/AC, (c) 0.0005La-N-TiO2/AC, (d) 0.001La-N-TiO2/AC and (e)0.01La-N-TiO2/AC.
hydroxyl radicals ($OH), which are responsible for the photodegradation of contaminants [18]. The broad peak in 397e400 eV range can be allotted to the existence of interstitial N present in the form of (Ti-O-N) in the structure [24], which indicated that some O atoms were substituted. According to the most of previous reports, the peak around 397 eV is attributed to the b-N assigned to the N3 anions that replace oxygen atoms in the TiO2 lattice to form N-Ti-N [17]. These results show that N is doped in the lattice of TiO2. Moreover, the similar peaks indicated that the incorporation of La ions did not induce an obvious change in N chemical state. The splitting of the main peaks of La 3d5/2 and La 3d3/2 were at around 834.2 and 851.0 eV, respectively. Compared with the bonding energy of pure La2O3 (834.9 eV), the slight shift of La 3d5/2 (834.2 eV) to lower binding energy indicates the formation of La-OTi in the samples [36].
3.7. FTIR analysis In Fig. 8, the broad absorption peak of the samples at around 3422 cm1 was attributed to the stretching vibration of O-H bonds [16]. The absorption peak at 1630 cm1 corresponded to the OH bending mode of H2O adsorbed on the surface of the TiO2, which may play a crucial role in photocatalytic activity [24]. From the peak at 3422 cm1, it was seen that as the amount of La increased, the amount of surface-adsorbed hydroxyl groups also increased. The absorption peak at 1370 cm1 was due to the scissoring vibration of CH3 species [35]. In comparison with TiO2/AC, the N-TiO2/AC and La-N co-doped TiO2/AC samples displayed additional peaks at around 1080 cm1, which can be assigned to the vibration of the NTi bond [9]. Additionally, the La-N co-doped TiO2/AC samples displayed an absorption peak of 518 cm1 characteristic of the LaeO
Fig. 8. FTIR spectra of TiO2/AC, N-TiO2/AC and La-N co-doped TiO2/AC.
bond, which showed the successful incorporation of the La-dopant with the TiO2 [37]. The peaks in the 1000 to 400 cm1 region are related to the bend vibration of Ti-O-Ti bonds [35]. 3.8. Photocatalytic activity The photocatalytic activities of various photocatalysts were assessed by the degradation of 30 mg/L of naphthalene solution under visible light irradiation (Fig. 9). For comparison, under visible light irradiation over a period of 120 min, no self-degradation of
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naphthalene was observed in the absence of photocatalyst, indicating that the direct photolysis of naphthalene could not undergo under visible light irradiation. The N-TiO2/AC was more efficient for the degradation of naphthalene as compared with undoped TiO2/ AC. This is could be that the nitrogen doping more efficiently respond to the visible light by reducing band gap as shown in DRS. The photoactivity of La-N co-doped TiO2/AC was further enhanced by the introduction of La in the photocatalyst. In addition, the degradation of naphthalene by the samples appeared to follow apparent first-order kinetics and kinetic curves (Fig. 10); 0.001 LaN-doped TiO2/AC achieved an excellent photocatalytic performance with ka of 0.0239 min1, which was 3.92 times that of TiO2/AC (ka ¼ 0.0061 min1). The apparent rate constants of La-N-doped TiO2/AC samples were higher than that of La-TiO2/AC and N-TiO2/ AC (Table 2), indicating that the La and N co-doping resulted in the further increase of TiO2/AC photoactivity. In addition, the larger red shift and lower PL intensity was found in La-N-TiO2/AC sample as confirmed by the UVevis DRS and PL results; thus, the visible light absorption ability of La-N-TiO2/AC was enhanced. La, N codoped materials show both an effect on the absorption edge and a localized absorption in the visible region [15]. Moreover, the La-induced increase in the activity of the samples was attributed to the decrease in particle size and increase in the surface area of photocrystal. High surface area offering more sites to adsorb naphthalene resulted in enhancing the photoactivity of photocatalyst. Moreover, La-N-doped TiO2/AC may enhance the formation of labile oxygen vacancies [16], which also was confirmed from the XPS and FTIR analyses results. In addition, the number of hydroxyl groups on the surface of N and La co-doped TiO2/AC was increased compared with N-TiO2/AC. Among the samples, the degradation efficiency of 0.001La-NTiO2/AC for naphthalene is 93.5% within 120 min under visible light radiation, namely, the photodecomposition is 85.5%, and the pure adsorption is 8.0% (Fig. 9). The increase in photocatalytic activity of 0.001La-N-TiO2/AC by increasing the concentration of La from 0.0005 to 0.001 g may be due to reduced band gap. Another explanation for the increase of photocatalytic activity could be attributed to the fact that the doping of TiO2 with La introduces new trapping sites. This affects the lifespan of charge carriers by splitting the arrival time of photogenerated electrons and holes needed to reach the surface of the photocatalyst, thereby reducing
Fig. 10. Kinetic curves for the photodegradation of naphthalene under visible light irradiation over the samples.
Table 2 First order kinetic constants and relative coefficients for photocatalytic degradation of naphthalene over the samples. Samples
ka (min1)
R2
TiO2/AC N-TiO2/AC La-TiO2/AC 0.0005La- N-TiO2/AC 0.001La- N-TiO2/AC 0.01La- N-TiO2/AC
0.0061 0.0109 0.0122 0.0144 0.0239 0.0165
0.9786 0.9947 0.9939 0.9788 0.9984 0.9904
the electronehole recombination. At a higher dopant concentration (0.01La), multiple trapping of charge carriers occurs, increasing the possibility of electronehole recombination [26] and reducing the number of charge carriers that will reach the surface to initiate the degradation of naphthalene [38]. Thus, a decrease in degrading efficiency at higher La doping concentration was observed. It was seen from Table 3, the La-N-TiO2/AC is efficient to degrade naphthalene among the materials reported, it is valuable to develop the photocatalyst. 3.9. Photostability over La-N-TiO2/AC under visible light To investigate the reusability of La-N co-doped TiO2/AC photocatalysts, the photocatalytic degradation of recycled 0.001La-NTiO2/AC was carried out under visible light irradiation. After each cycle, the photocatalyst was collected through filtration and dried in the oven at 110 C for 2 h before reuse. As shown in Fig. 11, the photocatalytic activity of the recovered La-N-TiO2/AC photocatalyst showed no noticeable change and still reached 92.2% after five recycling runs. This result indicated that 0.001La-N-TiO2/AC photocatalyst is highly effective and stable under visible light irradiation. 3.10. Photodegradation mechanism of naphthalene over La-N-TiO2/ AC under visible light
Fig. 9. Photocatalytic degradation of naphthalene over phtocatalysts under visible light irradiation (Blank: without a photocatalyst; C0 ¼ 30 mg/L, 50 mL of naphthalene solution, 20 mg of photocatalyst).
It was concluded that La and N work together to improve the photocatalytic activity of TiO2/AC under visible light, it can be found that after N doping, the band gap was narrowed from 2.82 to 2.61 eV, while after N and La co-doping, the band gap was narrowed from 2.82 to 2.20 eV (Fig. 5, Table 1). In addition, it is reported that a dopant level can form above the valence band for the substitution
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Table 3 Comparison of different kinds of materials. Materials
Degradation
Time (h)
Degradation efficiency
Reference
N-TiO2/Bi2O3 Ce-N-TiO2 Yb-N-TiO2 Mo-TiO2 TiO2/Wood Charcoal La-N-TiO2/AC
2,4-Dichlorophenol (50 mg/L) Phenol (20 mg/L) Methylene blue (10 mg/L) Acid red 88 Bisphenol A (20 mg/L) Naphthalene
6 2 5 3 18 2
90.0% 93.8% 93.6% 77.0% 80.0% 93.5%
[39] [40] [18] [26] [21] This work
was reduced (Fig. 5, Table 1), which can enhance the visible light utilization and degradation efficiency. In addition, AC has been proved to be an efficient support in promoting the photocatalytic process by providing a synergistic effect by creating a common interface between both the AC and TiO2 phase. An enhanced adsorption of the naphthalene molecules onto the AC followed closely by a transfer through an interface to the TiO2 has given rise to a complete photo-degradation process [6]. Some hot-spots could be formed on the surface of the AC particles, and the naphthalene molecules around the “hot spots” could be decomposed in the presence of O2 dissolved in water [42]. Thus, the photocatalytic activity of La-N-TiO2/AC was significantly increased.
Fig. 11. Repetitive photocatalytic degradation of naphthalene over 0.001La-N-TiO2/AC under visible light.
nitrogen and lanthanum ion doping can provide a dopant energy level (La3þ impurity level) below the conduction band of TiO2 to generate a red shift [33]. Therefore, there are two impurity levels in the band gap of La-N-TiO2/AC. The possible mechanisms are proposed (Fig. 12). As visible light irradiate La-N-TiO2/AC, TiO2 is active to produce the electrons and holes that react with H2O and O2, respectively, to form $OH and superoxide radical anion ($O 2 ). The $O 2 can also yield $OH through a series of chemical reactions. These generated $OH can oxidize naphthalene in aqueous solution and may result in the formation of CO2 and H2O. The excited electrons can either shift from the N 2p states above the VB to the conduction band (CB) of TiO2 to be trapped by the surface absorbed O2, or they can transfer from the VB of TiO2 and the N 2p to the La 3 d states. Such electron transfer enhances the separation of photoinduced holes and electrons, further prolongs the lifetime of photoinduced pairs [41], explaining why the intensity of PL decreased. When N and La are doped in the TiO2 lattice, the band gap energy of TiO2
4. Conclusions A high-performance La-N-TiO2/AC photocatalyst was successfully synthesized via an efficient and fast microwave-assisted method. La-N co-doping effectively extended the absorption spectra to the visible light region, resulting in high photocatalytic activity under visible light irradiation. The results showed that La3þ was mostly dispersed onto TiO2/AC, and a number of La atoms modified the surface of TiO2 in the form of Ti-O-La. The 0.001La-NTiO2/AC photocatalyst exhibited the highest degradation efficiency of 93.5% for naphthalene within 120 min under visible light irradiation, which could be attributed to the high adsorption capacity (small particle size, large surface and pore volume). In addition, the introduction of La to the TiO2 promoted the formation of oxygen vacancies and Ti4þ onto the surface of photocatalyst, which facilitated the adsorption of naphthalene molecules, and the La-Ncodoping can promote the separation of the photogenerated electrons and holes to accelerate the transmission of photocurrent carrier. Hence, the La-N codoing TiO2/AC photocatalyst could be considered a promising material for the application for the removal of PAHs in aqueous solution.
Fig. 12. Proposed mechanism for the visible light photocatalytic degradation of naphthalene on the surface of the La-N-TiO2/AC photocatalyst.
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Acknowledgments This work was supported financially by funding from the National Natural Science Foundation of China (51262025), International scientific and technological cooperation project of Xinjiang Bingtuan (2013BC002) and Graduate Research Innovation Project in Xinjiang (XJGRI2014053).
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