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A simple and low-cost approach to fabricate TiO2-NO2 hollow spheres with excellent simulated sunlight photocatalytic activity
Materials Chemistry and Physics xxx (2016) 1e9
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A simple and low-cost approach to fabricate TiO2-NO2 hollow spheres with excellent simulated sunlight photocatalytic activity Kexin Li, Shufen Liu, Shaoting Yi, Liushui Yan*, Huiqin Guo, Yuhua Dai, Xubiao Luo, Shenglian Luo* Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, NanChang Hangkong University, NanChang 330063, People’s Republic of 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
TiO2-NO2 HSs were successfully fabricated by a simple and low-cost method. Their morphology, composition, and optoelectronic property were well characterized. TiO2-NO2 HSs exhibit considerably high activity under simulated sunlight irradiation. High activity is attributed to hollow spherical structure and introduced -NO2 groups.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2016 Received in revised form 10 May 2016 Accepted 19 June 2016 Available online xxx
Nitryl surface functionalized TiO2 hollow spheres (TiO2-NO2 HSs) composite material was successfully fabricated by a simple sol-gel combined with solvothermal and concentrated HNO3 treatment strategy using carbon microspheres obtained from hydrothermal carbonization of waste Camellia oleifera shells as a template. The microstructure, surface property, compositional and structural information, and optical and electronic property of as-prepared TiO2-NO2 HSs were well characterized. Compare with conventional g-C3N4 and TiO2 photocatalysts, the as-prepared TiO2-NO2 HSs exhibited excellent simulated sunlight photocatalytic activity toward aqueous organic pollutants degradation and hydrogen evolution from water-splitting, which can be attributed to their unique hollow spherical microstructure and the introduction of -NO2 groups. © 2016 Elsevier B.V. All rights reserved.
Keywords: Sol-gel Microstructure Surface property Composite material
1. Introduction The efficient photocatalytic reaction directly carried out by sunlight irradiation is an ideal goal in the course of various new photocatalytic materials development [1e6]. In the sunlight spectrum, the percentages of visible and infrared light are
* Corresponding authors. E-mail addresses: [email protected] (L. Yan), [email protected] (S. Luo).
approximately 46% and 49%, respectively [7]. Therefore, the visiblelight response photocatalysts have been widely studied and a few near-infrared light driven photocatalysts have also been reported in recent years [8e11]. However, the activity study of photocatalytic materials in the sunlight irradiation is considered to have more practical significance. Currently, graphitic carbon nitride (g-C3N4) is considered to be one of the most market development prospect photocatalysts because of its low cost and visible-light activity characteristics [12e14]. However, g-C3N4 obtained by hightemperature polycondensation of melamine exhibits some
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K. Li et al. / Materials Chemistry and Physics xxx (2016) 1e9
serious drawbacks such as very low quantum efficiency, light corrosion due to the oxidation of lattice N3 by photogenerated holes, and hard changed bulk layered structure [15e17]. Therefore, the practical application of g-C3N4 is limited. Compare with g-C3N4, titania (TiO2) has a relatively higher quantum efficiency, more stable chemical structure, and more easily controlled morphology [18e20]. Therefore, the development of new TiO2-based photocatalysts with excellent photocatalytic activity is still the hot content concerned by many researchers [21e24]. Construction of hollow spherical microstructure and surface functionalization are the effective strategies for improving the photocatalytic activity of TiO2 [25,26]. On the one hand, compare with bulk TiO2, the separation and transportation of photoinduced charge carriers and the contact between photocatalyst and reactants are more convenient for the TiO2 hollow spheres. Therefore, the photocatalytic activity of TiO2 hollow spheres is obviously higher than that of bulk TiO2 in the same photocatalytic reaction conditions. On the other hand, the affinity between surface functionalized photocatalyst and reactants significantly increase owing to the electronic effects of surface functional groups. Therefore, the surface functionalized TiO2 exhibits a significantly higher photocatalytic activity than traditional TiO2 photocatalyst. In addition, the light harvesting capability of surface functionalized TiO2 is increased because the introduction of nonmetallic elements enhance the electronic transition efficiency from valence band (VB) to conduction band (CB). Currently, nonmetallic elements doped TiO2 has been concerned by many researchers [27e30]. However, research related to surface functionalized TiO2 is less. In this article, nitryl (-NO2) surface functionalized TiO2 hollow spheres (TiO2-NO2 HSs) was successfully fabricated by a simple solgel combined with solvothermal and concentrated HNO3 treatment strategy using carbon microspheres as a template. In the TiO2-NO2 HSs preparation process, the carbon microspheres template was obtained by hydrothermal carbonization of waste Camellia oleifera shells, and the -NO2 groups were introduced in the course of template removal by concentrated HNO3 treatment. Therefore, the above preparation process is a simple and low-cost method that is suitable for actual production. Finally, the as-prepared materials were well characterized and their photocatalytic activities were evaluated through the degradation of aqueous organic pollutants and hydrogen evolution from water-splitting under simulated sunlight irradiation. 2. Experimental 2.1. Chemicals and reagents Waste Camellia oleifera shells (abbreviated WCOSs) were collected from Jiangxi Green Sea Oil Co. Ltd., China. Titanium butoxide (Ti(OC4H9)4, 99.0%, abbreviated TBT) and p-chlorophenol (C6H5OCl, GC grade, abbreviated PCP) were purchased from Aladdin Chemistry Co. Ltd. Triethylamine ((C2H5)3N, AR grade, abbreviated TEA) was purchased from Shanghai Fine Chemical Technology Co. Ltd. Methyl orange (C14H14N3NaO3S, AR grade, abbreviated MO) was purchased from Shanghai Fine Chemical Technology Co. Ltd. All chemicals were used without further purification. Double distilled water was used in the catalyst preparation and subsequent catalytic tests. 2.2. Preparation 2.2.1. Preparation of carbon microspheres template directly from WCOSs In a typical synthesis, 2.0 g of WCOSs powder was uniformly dispersed into 60 mL of deionized water by using a 500 W
ultrasonic crasher for 1 h. The resulting suspension was stirred at room temperature for 6 h and then subjected to hydrothermal treatment at 230 C for 12 h at a heating rate of 1 C/min. The byproducts of hydrothermal carbonization were removed through Soxhlet and ultrasonicemicrowave synergistic extraction by using water as a solvent, and the final products were obtained after drying at 60 C for 24 h. 2.2.2. Preparation of TiO2-NO2 HSs by a simple sol-gel combined with solvothermal and concentrated HNO3 treatment strategy In a typical synthesis, 100 mg of carbon microspheres template was uniformly dispersed into 20 mL EtOH using a 500 W ultrasonic crasher for 30 min, and then 0.25 mL NH3$H2O was dropwise added. A TBT/EtOH solution was prepared by pouring 2 mL TBT into 5.0 mL EtOH with vigorous stirring for 5 min. Subsequently, the above TBT/EtOH solution was slowly added to the carbon microspheres template/EtOH/NH3$H2O suspension to form a translucent black sol, and the stirring was continued until the hydrogel was formed. The obtained hydrogel was subject to solvothermal treatment at 180 C for 24 h at a heat rate of 1 C/min. After drying at 60 C for 24 h to form a xerogel, the carbon microspheres template was removed by concentrated HNO3 treatment at 70 C for 4 h. The final product was obtained after washing with water for several times, and denoted as TiO2-NO2 HSs. 2.3. Characterizations Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2010 microscope at an accelerating voltage of 200 kV. Field emission scanning electron microscopy (FESEM) images were recorded using a Nova NanoSEM450 field emission scanning electron microscope. The chemical compositions and elemental mappings of the samples were determined by energydispersive X-ray spectrometer (EDX) equipped on FESEM. Nitrogen gas porosimetry measurements were performed on a Quantachrome NOVA 2000e surface area and porosity analyzer after the samples were outgassed under a vacuum at 70 C for 20 min and 150 C for 6 h. X-ray diffraction (XRD) patterns were obtained using a Panalytical X’Pert PRO diffractometer via Cu Ka radiation. Fourier transform infrared (FTIR) spectra were recorded on a Bruker VERTEX 70 FTIR apparatus. X-ray photoelectron spectra (XPS) was performed using a VG-ADES 400 instrument with an Mg Ka-ADES source at a residual gas pressure of less than 108 Pa. UVevisible/ diffuse reflectance spectroscopy (UVevis/DRS) was conducted using a Lambda 750S UV/VIS/NIR spectrometer. Photoluminescence (PL) measurements were carried out on a HITACHI F-7000 fluorescence spectrophotometer. 2.4. Photocatalytic tests 2.4.1. Photocatalytic degradation of aqueous MO and PCP A PLS-SXE300 Xe lamp (Beijing PerfectLight Co. Ltd., China) served as the simulated sunlight source, and the output wavelength l ¼ 320e2500 nm. 100 mg of solid photocatalyst and 100 mL of organic pollutant (MO or PCP) aqueous solution were poured into a 300 mL self-designed quartz reactor. The initial concentrations of MO and PCP are 10 mg L1 and 20 mg L1, respectively. The suspension was ultrasonicated for 10 min and stirred in the dark until adsorption-desorption equilibrium. Subsequently, the light source was switched on, and further stirring was applied. The temperature of the suspension was maintained at 35 ± 2 C by circulation of water through an external cooling jacket. At specific intervals of irradiation, fixed amounts of the reaction solution were extracted, centrifuged, and filtered. Changes in the MO concentrations were analyzed using an UNICO UV-2000 spectrophotometer at l ¼ 464.
Please cite this article in press as: K. Li, et al., A simple and low-cost approach to fabricate TiO2-NO2 hollow spheres with excellent simulated sunlight photocatalytic activity, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.06.072
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Changes in the PCP concentrations were analyzed using an Agilent 1100 series high-performance liquid chromatography (HPLC): C18 column, UV detector (l ¼ 277 nm), and acetonitrile/water (60/40 v/ v) was used as a mobile phase at a flow rate of 1.0 mL/min. 2.4.2. Hydrogen evolution from water-splitting by using TEA as an electron donor 100 mg of solid photocatalyst loaded with 3 wt% of Pt co-catalyst and 100 ml of H2O containing 10 vol% TEA were poured into a 300 mL quartz reactor. The above suspension was ultrasonicated for 10 min and stirred in dark for 1 h. Subsequently, the light source was switched on, and further stirring was applied. The temperature of the suspension was maintained at 35 ± 2 C by circulation of water through an external cooling jacket. After l ¼ 320e2500 nm simulated sunlight irradiation for 3 h, the generated hydrogen was in situ analyzed with a GC 7890-II TCD gas chromatograph (TECHCOMP) using an MS-5 A column, which was connected to the gas circulating line with argon carrier. 3. Results and discussion 3.1. Characterizations 3.1.1. Microstructure and surface property The microstructure of as-prepared materials was characterized by TEM and FESEM observations (Fig. 1). As shown in Fig. 1a and b, the solid and hollow spherical microstructures can be clearly observed for the carbon microspheres template and TiO2-NO2 HSs, indicating that the self-assembly process between carbon microspheres template and TBT is successfully carried out in the course of catalyst preparation. From Fig. 1c and d it can be observed that both TiO2-NO2 and g-C3N4 show a bulk structure and the additional mesoporous structure exist in the TiO2-NO2. As shown in Fig. 1e and f, TiO2-NO2 HSs before concentrated HNO3 treatment exhibit a rough surface, clear carbon core, and high carbon content. Conversely, TiO2-NO2 HSs after concentrated HNO3 treatment show a smooth surface, almost disappeared carbon core, and significantly
3
low carbon content (Fig. 1g and h). The above result indicates that Ti(OH)4 impurities and carbon microspheres template were successfully removed by concentrated HNO3 treatment. The surface property of as-prepared materials was characterized by nitrogen gas porosity measurement (Fig. 2). The type II isotherm of g-C3N4 shown in Fig. 2a corresponds to its non-porous characteristic. The type IV isotherms with H2-type hysteresis loops for the TiO2, TiO2-NO2, and TiO2-NO2 HSs samples correspond to their mesoporous structures. The different shapes of hysteresis loops of TiO2 and TiO2-NO2 samples imply their different pore geometries. Specifically, some accumulated mesoporous structures fabricated by the accumulation of Ti(OH)4 impurities exist in the TiO2 sample. For the TiO2-NO2, the accumulated mesoporous structures by Ti(OH)4 impurities were destroyed and removed after concentrated HNO3 treatment, and the rest of mesoporous structures were constructed by the connection of threedimensional TiO2 network. As shown in Fig. 2b, the relatively wide pore-size distribution peak in the range of 3e12 nm for the TiO2 sample corresponds to its accumulated mesoporous structure, and the relatively narrow pore-size distribution peak in the range of 3e6 nm for the TiO2-NO2 and TiO2-NO2 HSs samples correspond to their mesoporous structure constructed by the connection of three-dimensional TiO2 network. The BET (BrunauereEmmetteTeller) specific surface areas of TiO2, TiO2-NO2, and TiO2-NO2 HSs are larger than that of g-C3N4 owing to the existence of mesoporous structures. The BET specific surface area of TiO2-NO2 HSs is larger than that of TiO2-NO2 because of the construction of hollow spherical microstructure.
3.1.2. Compositional and structural information The composition and structure of as-prepared materials are characterized by XRD, FTIR spectra, and XPS. The phase structures of as-prepared materials are analyzed by XRD (Fig. 3). For the gC3N4, the strong (002) peak at 27.4 corresponds to an interlayer distance of d ¼ 0.33 nm, whereas the (100) peak at 12.8 represents in-plane structural packing motif with a period of 0.675 nm. As for the TiO2, TiO2-NO2, and TiO2-NO2 HSs samples, the
Fig. 1. TEM images of carbon microspheres template (a), TiO2-NO2 HSs (b), TiO2-NO2 (c), g-C3N4 (d); FESEM images, EDX elemental mappings, and EDX spectra of TiO2-NO2 HSs before (e and f) and after (g and h) concentrated HNO3 treatment.
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Fig. 3. XRD patterns of as-prepared materials.
Fig. 2. Nitrogen gas sorption isotherms (a) and pore-size distribution curves (b) of asprepared materials.
characteristic diffractions at 25.3 (101), 37.6 (004), 48.1 (200), 53.8 (105), 55.0 (211), and 62.7 (204) represent their pure anatase crystal phase (JCPDS No. 21e1272). Compare with TiO2, the relatively lower crystallinity of TiO2-NO2 can be attributed to the TiO2 framework was destroyed somewhat by the oxidation of concentrated HNO3. The obviously higher crystallinity of TiO2-NO2 HSs than that of TiO2-NO2 indicates that the dispersion effect of carbon microspheres template facilitates the growth of anatase crystal phase. FTIR spectra of as-prepared materials are shown in Fig. 4. For the TiO2, the broad peak in the range of 420e1000 cm1 is attributed to the stretching vibration of Ti-O-Ti covalent bonds, and the peaks at 1629 and 3412 cm1 are assigned to the H-O-H bending and O-H stretching vibration of adsorbed water and surface -OH groups. As for the TiO2-NO2, a new peak assigned to -NO2 groups appears at the position of 1384.6 cm1 implying that the surface -OH groups within TiO2 have been substituted with -NO2 groups during the concentrated HNO3 treatment. Compare with TiO2-NO2, the obviously lower intensity of -NO2 peak for the TiO2-NO2 HSs indicate that the surface -OH groups and carbon microspheres template react with concentrated HNO3 are mutual competitive reaction.
The morphology transformation and interaction between -NO2 groups and TiO2 matrix can be confirmed by high-resolution XPS analysis (Fig. 5). For the TiO2, the determined binding energies of Ti 2p3/2 and Ti 2p1/2 are 458.6 and 464.4 eV, which is in good agreement with anatase TiO2 (Fig. 5a). A slight shift of Ti 2p peaks was observed for the TiO2-NO2 indicating that the surface -OH groups within TiO2 were substituted with -NO2 groups after concentrated HNO3 treatment. Compare with TiO2-NO2, the obvious shift of Ti 2p peaks to higher binding energy region for TiO2-NO2 HSs indicate that the formation of hollow spherical microstructure changes the binding state of surface atoms. The XPS result of O 1s is consistent with that of Ti 2p. As shown in Fig. 5b, TiO2 shows two O 1s peaks with the binding energies of 529.9 and 531.4 eV, which attribute to the lattice-oxygen (Ti-O) and hydroxyl-oxygen (O-H) in anatase TiO2. A slight shift of O 1s peaks for the TiO2-NO2 is owing to the existence of -NO2 groups after concentrated HNO3 treatment. Compare with TiO2-NO2, the obvious shift of O 1s peaks to higher binding energy region for the TiO2-NO2 HSs can be attributed to the changes of binding state of surface atoms after the formation of hollow spherical microstructure. As shown in Fig. 5c, the C 1s peaks centered at 285.0 and 288.8 eV are typically assigned to carbon impurities, which originate from the organic species in the preparation process. Compare with TiO2, the decreased intensity of C 1s peaks for TiO2-NO2 indicates that the carbon impurities originated from organic species is partly removed in the course of concentrated HNO3 treatment. Compare with TiO2-NO2, the increased intensity of C 1s peaks for TiO2-NO2 HSs can be attributed to the small amount of residual carbon microspheres template. As shown in Fig. 5d, the N 1s peak centered at 400.2 eV is assigned to nitrogen impurities, which originate from ammonia in the preparation process. As for the TiO2-NO2, a new N 1s peak appeared at 407.0 eV corresponds to the -NO2 groups. This result further confirms that the surface -OH groups within TiO2 are substituted with -NO2 groups during the concentrated HNO3 treatment. Compare with TiO2-NO2, the obviously lower intensity of -NO2 XPS peak for the TiO2-NO2 HSs indicates that the surface -OH groups and carbon microspheres template react with concentrated HNO3 are mutual competitive reaction. 3.1.3. Optical and electronic properties The light harvesting capabilities of as-prepared materials were studied by UVevis/DRS. As shown in Fig. 6, g-C3N4 shows a typical
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5
Absorbance (a.u.)
1.6
g-C3N4
1.2
TiO2 TiO2-NO2 TiO2-NO2 HSs
0.8
0.4
0.0 200
400
600
800
Wavelength (nm) Fig. 6. UVevis/DRS of as-prepared materials.
Fig. 4. FTIR spectra of as-prepared materials.
semiconductor absorption within the region of 200e450 nm, which originates from the electron transition from VB populated by N 2p orbit to CB formed by C 2p orbit. TiO2 shows a light response within the range of 200e400 nm, which can be attributed to the electron transition from the VB (O 2p) to the CB (Ti 3d). Compare with TiO2, the enhanced light harvesting capability for the TiO2-
(b)
Ti 2p3/2
Ti 2p
O 1s
Ti 2p1/2
Intensity (a.u.)
Ti-O
TiO2
TiO2-NO2
-OH
Intensity (a.u.)
(a)
NO2 can be attributed to the increased electronic transition efficiency. Specifically, after the introduction of -NO2 groups, the electronic transition from N 2p to Ti 3d are more efficient than the electronic transition from O 2p to Ti 3d under the same irradiation condition (Scheme 1). Compare with TiO2-NO2, the enhanced light harvesting capability of TiO2-NO2 HSs can be attributed to the fabrication of hollow spherical microstructure. Specifically, hollow spherical microstructure is more conductive to capture photons than bulk structure, therefore, the electronic transition efficiency of TiO2-NO2 HSs were further increased.
TiO2
-NO2
TiO2-NO2
TiO2-NO2 HSs
TiO2-NO2 HSs
452
456
460
464
468
524
528
532
536
(c)
(d)
N 1s TiO2
TiO2
TiO2-NO2
284
Intensity (a.u.)
Intensity (a.u.)
C 1s
280
540
Binding energy (eV)
Binding energy (eV)
-NO2
TiO2-NO2
TiO2-NO2 HSs
TiO2-NO2 HSs
292
396
288
Binding energy (eV)
400
404
408
Binding energy (eV)
Fig. 5. High-resolution XPS of as-prepared materials in the Ti 2p (a), O 1s (b), C 1s (c), and N 1s (d) binding energy regions.
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Scheme 1. The fabrication route and band gap structure of as-prepared TiO2-NO2 HSs and the separation and transportation of photoinduced charge carriers in the TiO2-NO2 HSs under simulated sunlight irradiation.
The photocatalytic quantum efficiencies of as-prepared materials were studied by PL measurements. As shown in Fig. 7, g-C3N4 exhibits a broad high fluorescence emission peak in the range of 350e700 nm. This finding suggests that photoinduced eehþ pairs are generated and recombined within the g-C3N4. Compare with gC3N4, the obviously decreased PL intensity of TiO2 indicates the efficient separation and transportation of photoinduced charge carriers. Compare with TiO2, the increased PL intensity of TiO2-NO2 can be attributed to the increased electronic transitions efficiency from VB to CB simultaneously accelerate the recombination of photoinduced eehþ pairs (Scheme 1). For the TiO2-NO2 HSs, the decreased PL intensity compared with TiO2-NO2 suggest that the efficient separation and transportation of photoinduced charge carriers are realized after the construction of hollow spherical microstructure.
1000 g-C3N4
Intensity (a.u.)
TiO2 TiO2-NO2
750
TiO2-NO2 HSs 500
250
0 400
500
600
Wavelength (nm) Fig. 7. PL spectra of as-prepared materials.
700
3.2. Photocatalytic tests 3.2.1. Photocatalytic degradation of aqueous MO and PCP The photocatalytic degradation tests were conducted in an aqueous solution containing oxygen molecules from dissolved air. As shown in Fig. 8a and b, the results of direct photodegradation experiment indicate that changes of MO and PCP concentrations in the reaction system are negligible under simulated sunlight irradiation for 60 min. The adsorption test results show that the adsorption-desorption equilibrium had been reached prior to irradiation. Compare with g-C3N4, the relatively higher photocatalytic activity of TiO2 for the aqueous MO and PCP degradation can be attributed to the efficient separation and transportation of photoinduced charge carriers in TiO2. The photocatalytic degradation activity of P25 commercial product is higher than that of as-prepared TiO2 owing to the mature manufacturing process of P25 commercial product. As shown in Fig. 8a, TiO2-NO2 shows the highest adsorption capacity and photocatalytic activity among all the tested materials for the aqueous MO adsorption and degradation because of the strong electrostatic interaction between -NO2 groups and MO molecules. Compare with TiO2-NO2, the relatively poorer adsorption capacity and photocatalytic activity of TiO2-NO2 HSs toward aqueous MO adsorption and degradation corresponds to their relatively small amount of -NO2 groups. As shown in Fig. 8b, all the tested materials show a similar adsorption capacity toward PCP, and the photocatalytic activities of asprepared materials for the aqueous PCP degradation followed the order of TiO2-NO2 HSs > TiO2-NO2 > P25 > TiO2 > g-C3N4. Compare with TiO2, the increased photocatalytic activity of TiO2NO2 toward aqueous PCP degradation can be attributed to the increased electronic transition efficiency of TiO2-NO2. Compare with TiO2-NO2, the increased photocatalytic activity of TiO2-NO2 HSs toward aqueous PCP degradation can be attributed to the fabrication of hollow spherical microstructure with large BET specific surface area, perfect anatase crystal phase, increased
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hydrogen evolution activities of the tested materials follow the order of TiO2-NO2 HSs > TiO2-NO2 > P25 > TiO2 > g-C3N4, which is similar as aqueous PCP degradation. This result further indicates that the large BET specific surface area, perfect anatase crystal phase, increased electronic transition efficiency, and decelerated recombination of photoinduced eehþ pairs play an important role to improve the photocatalytic activity of TiO2-NO2 HSs. 3.2.3. Recyclability of TiO2-NO2 HSs composite material The above PCP degradation and hydrogen evolution reactions were repeated for five times to evaluate the stability of as-prepared TiO2-NO2 HSs composite material in the current photocatalytic degradation and hydrogen evolution systems. After the first catalytic run, the catalyst was recovered by centrifugation, and then it was washed by water at room temperature. The recovered catalyst was used for the subsequent catalytic runs under the same experimental conditions. As shown in Fig. 9, the tested catalyst exhibits considerably high stability, and it can maintain a similar level of reactivity after five catalytic cycles. A gradually decreased photocatalytic activity can be attributed to the loss of photocatalyst in the recovery process. 3.2.4. Discussion to photocatalytic reaction mechanisms in degradation and hydrogen evolution systems Based on the above photocatalytic activity results, the photocatalytic reaction mechanisms in degradation and hydrogen
Fig. 8. Degradation of aqueous organic pollutants (a and b) and hydrogen evolution from water-splitting (c) over as-prepared photocatalysts under simulated sunlight irradiation.
electronic transition efficiency, and decelerated recombination of photoinduced eehþ pairs. 3.2.2. Hydrogen evolution from water-splitting by using TEA as an electron donor The photocatalytic activities of as-prepared materials were further evaluated by hydrogen evolution from water-splitting under simulated sunlight irradiation. As shown in Fig. 8c, the
Fig. 9. Recycling experiments of aqueous PCP degradation (a) and hydrogen evolution from water-splitting (b) over TiO2-NO2 HSs photocatalyst.
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(OH) scavenger, hole (hþ) scavenger, and superoxide radical (O 2) scavenger, respectively. As shown in Fig. 10a, the degradation rate of PCP is inhibited somewhat compared with scavenger-free photocatalytic system after adding t-BuOH (1 mmol L1). However, the degradation rate of PCP is decelerated obviously in the presence of EDTA (1 mmol L1). Finally, the degradation of PCP is inhibited completely after adding BQ (1 mmol L1). The above results indiþ cate that O 2 is the principal active species, h is the secondary active species, and OH is the unimportant active species in the course of photocatalytic degradation. In the active species masking experiments, the dissolved oxygen was consumed because of the addition of BQ in the photocatalytic degradation system. Therefore, the redox reaction between oxygen and organic pollutant is inhibited completely. In addition, the electron transfer effect of photocatalyst is weakened because the photogenerated hþ is consumed by the added EDTA. Therefore, the redox reaction between the oxygen and organic pollutant is also inhibited significantly. The unobvious OH masking effect of t-BuOH can be understood that t-BuOH or organic pollutant reacts with dissolved oxygen are competitive. To further explore the reaction mechanism of photocatalytic hydrogen evolution, the initial hydrogen evolution system, that is 100 mL of H2O containing 10 vol% TEA, was replaced by 100 ml of pure H2O, pure TEA, or H2O containing 10 vol% CH3OH. As shown in Fig. 10b, in the single-component (pure H2O or TEA) hydrogen evolution system, the redox reaction only carries out within the H2O or TEA molecule due to the lack of reducing agent (TEA) or oxidizing agent (H2O). Therefore, the hydrogen yield is low. For the H2O/CH3OH hydrogen evolution system, the redox reaction between H2O and CH3OH hardly carries out due to the poor electrondonating ability of CH3OH. Therefore, the hydrogen yield is significantly decreased compared with H2O/TEA photocatalytic hydrogen evolution system.
Fig. 10. Influence of various scavengers on the photocatalytic degradation of aqurous PCP (a) and the hydrogen yield in different photocatalytic hydrogen evolution systems (b).
evolution systems are discussed below. Firstly, there are two questions must be clear that both photocatalytic degradation and hydrogen evolution reactions belong to the redox processes and photocatalyst act as an electron transfer medium in the redox process (Scheme 1). Specifically, oxygen act as an oxidizing agent and organic pollutant act as a reducing agent in the photocatalytic degradation system, moreover, water act as an oxidizing agent and electron donor act as a reducing agent in the photocatalytic hydrogen evolution system. The redox reaction can be successfully carried out owing to the simultaneous presence of oxidizing and reducing agent in the photocatalytic reaction system. Therefore, the effect of photocatalytic reaction is mainly influenced by the properties of the photocatalyst. The unique hollow spherical microstructure, large BET specific surface area, perfect anatase crystal phase, increased electronic transition efficiency, and decelerated recombination of photoinduced eehþ pairs play positive roles to enhance the activity of the photocatalyst. Therefore, the asprepared TiO2-NO2 HSs photocatalyst exhibits excellent photocatalytic activity in both degradation and hydrogen evolution systems. In order to clarify what kind of active species played a key role during the photocatalytic degradation, the active species masking experiments were carried out by using TiO2-NO2 HSs as a representative photocatalyst and PCP as a target pollutant. In this study, tert-butyl alcohol (t-BuOH), ethylenediaminetetraacetic acid (EDTA), and 1,4-benzoquinone (BQ) are used as the hydroxyl radical
4. Conclusions Nitryl surface functionalized TiO2 hollow spheres (TiO2-NO2 HSs) were successfully fabricated by a simple and low-cost method. The as-prepared TiO2-NO2 HSs exhibit considerably high photocatalytic activity toward representative aqueous organic pollutants degradation and hydrogen evolution from water-splitting under simulated sunlight irradiation. The high photocatalytic activity is attributed to their unique hollow spherical microstructure and the introduction of -NO2 groups. The as-prepared TiO2-NO2 HSs are expected to be widely used as a photocatalyst in many areas such as wastewater treatment, air purification, and hydrogen energy development. Acknowledgements This work was financially supported by National Natural Science Foundation of China (51568049, 51208248, 21366024, 21165013, 51468043, 51238002, 51272099); National Science Fund for Excellent Young Scholars (51422807); Science and Technology Major Bidding Project of Jiangxi Province, China (No. Gan Ke Fa Ji Zi [2010] 217); Youth Science Foundation of Jiangxi Province, China (20114BAB213015, 20114BAB213016); Natural Science Foundation of Jiangxi Provincial Department of Education, China (GJJ14515, GJJ12456). References [1] M. Zhu, C. Zhai, L. Qiu, C. Lu, A.S. Paton, Y. Du, M.C. Goh, ACS Sustain. Chem. Eng. 3 (2015) 3123e3129. [2] S. Tonda, S. Kumar, S. Kandula, V. Shanker, J. Mater. Chem. A 2 (2014)
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A simple and low-cost approach to fabricate TiO2-NO2 hollow spheres with excellent simulated sunlight photocatalytic activity Kexin Li, Shufen Liu, Shaoting Yi, Liushui Yan*, Huiqin Guo, Yuhua Dai, Xubiao Luo, Shenglian Luo* Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, NanChang Hangkong University, NanChang 330063, People’s Republic of 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
TiO2-NO2 HSs were successfully fabricated by a simple and low-cost method. Their morphology, composition, and optoelectronic property were well characterized. TiO2-NO2 HSs exhibit considerably high activity under simulated sunlight irradiation. High activity is attributed to hollow spherical structure and introduced -NO2 groups.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2016 Received in revised form 10 May 2016 Accepted 19 June 2016 Available online xxx
Nitryl surface functionalized TiO2 hollow spheres (TiO2-NO2 HSs) composite material was successfully fabricated by a simple sol-gel combined with solvothermal and concentrated HNO3 treatment strategy using carbon microspheres obtained from hydrothermal carbonization of waste Camellia oleifera shells as a template. The microstructure, surface property, compositional and structural information, and optical and electronic property of as-prepared TiO2-NO2 HSs were well characterized. Compare with conventional g-C3N4 and TiO2 photocatalysts, the as-prepared TiO2-NO2 HSs exhibited excellent simulated sunlight photocatalytic activity toward aqueous organic pollutants degradation and hydrogen evolution from water-splitting, which can be attributed to their unique hollow spherical microstructure and the introduction of -NO2 groups. © 2016 Elsevier B.V. All rights reserved.
Keywords: Sol-gel Microstructure Surface property Composite material
1. Introduction The efficient photocatalytic reaction directly carried out by sunlight irradiation is an ideal goal in the course of various new photocatalytic materials development [1e6]. In the sunlight spectrum, the percentages of visible and infrared light are
* Corresponding authors. E-mail addresses: [email protected] (L. Yan), [email protected] (S. Luo).
approximately 46% and 49%, respectively [7]. Therefore, the visiblelight response photocatalysts have been widely studied and a few near-infrared light driven photocatalysts have also been reported in recent years [8e11]. However, the activity study of photocatalytic materials in the sunlight irradiation is considered to have more practical significance. Currently, graphitic carbon nitride (g-C3N4) is considered to be one of the most market development prospect photocatalysts because of its low cost and visible-light activity characteristics [12e14]. However, g-C3N4 obtained by hightemperature polycondensation of melamine exhibits some
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Please cite this article in press as: K. Li, et al., A simple and low-cost approach to fabricate TiO2-NO2 hollow spheres with excellent simulated sunlight photocatalytic activity, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.06.072
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K. Li et al. / Materials Chemistry and Physics xxx (2016) 1e9
serious drawbacks such as very low quantum efficiency, light corrosion due to the oxidation of lattice N3 by photogenerated holes, and hard changed bulk layered structure [15e17]. Therefore, the practical application of g-C3N4 is limited. Compare with g-C3N4, titania (TiO2) has a relatively higher quantum efficiency, more stable chemical structure, and more easily controlled morphology [18e20]. Therefore, the development of new TiO2-based photocatalysts with excellent photocatalytic activity is still the hot content concerned by many researchers [21e24]. Construction of hollow spherical microstructure and surface functionalization are the effective strategies for improving the photocatalytic activity of TiO2 [25,26]. On the one hand, compare with bulk TiO2, the separation and transportation of photoinduced charge carriers and the contact between photocatalyst and reactants are more convenient for the TiO2 hollow spheres. Therefore, the photocatalytic activity of TiO2 hollow spheres is obviously higher than that of bulk TiO2 in the same photocatalytic reaction conditions. On the other hand, the affinity between surface functionalized photocatalyst and reactants significantly increase owing to the electronic effects of surface functional groups. Therefore, the surface functionalized TiO2 exhibits a significantly higher photocatalytic activity than traditional TiO2 photocatalyst. In addition, the light harvesting capability of surface functionalized TiO2 is increased because the introduction of nonmetallic elements enhance the electronic transition efficiency from valence band (VB) to conduction band (CB). Currently, nonmetallic elements doped TiO2 has been concerned by many researchers [27e30]. However, research related to surface functionalized TiO2 is less. In this article, nitryl (-NO2) surface functionalized TiO2 hollow spheres (TiO2-NO2 HSs) was successfully fabricated by a simple solgel combined with solvothermal and concentrated HNO3 treatment strategy using carbon microspheres as a template. In the TiO2-NO2 HSs preparation process, the carbon microspheres template was obtained by hydrothermal carbonization of waste Camellia oleifera shells, and the -NO2 groups were introduced in the course of template removal by concentrated HNO3 treatment. Therefore, the above preparation process is a simple and low-cost method that is suitable for actual production. Finally, the as-prepared materials were well characterized and their photocatalytic activities were evaluated through the degradation of aqueous organic pollutants and hydrogen evolution from water-splitting under simulated sunlight irradiation. 2. Experimental 2.1. Chemicals and reagents Waste Camellia oleifera shells (abbreviated WCOSs) were collected from Jiangxi Green Sea Oil Co. Ltd., China. Titanium butoxide (Ti(OC4H9)4, 99.0%, abbreviated TBT) and p-chlorophenol (C6H5OCl, GC grade, abbreviated PCP) were purchased from Aladdin Chemistry Co. Ltd. Triethylamine ((C2H5)3N, AR grade, abbreviated TEA) was purchased from Shanghai Fine Chemical Technology Co. Ltd. Methyl orange (C14H14N3NaO3S, AR grade, abbreviated MO) was purchased from Shanghai Fine Chemical Technology Co. Ltd. All chemicals were used without further purification. Double distilled water was used in the catalyst preparation and subsequent catalytic tests. 2.2. Preparation 2.2.1. Preparation of carbon microspheres template directly from WCOSs In a typical synthesis, 2.0 g of WCOSs powder was uniformly dispersed into 60 mL of deionized water by using a 500 W
ultrasonic crasher for 1 h. The resulting suspension was stirred at room temperature for 6 h and then subjected to hydrothermal treatment at 230 C for 12 h at a heating rate of 1 C/min. The byproducts of hydrothermal carbonization were removed through Soxhlet and ultrasonicemicrowave synergistic extraction by using water as a solvent, and the final products were obtained after drying at 60 C for 24 h. 2.2.2. Preparation of TiO2-NO2 HSs by a simple sol-gel combined with solvothermal and concentrated HNO3 treatment strategy In a typical synthesis, 100 mg of carbon microspheres template was uniformly dispersed into 20 mL EtOH using a 500 W ultrasonic crasher for 30 min, and then 0.25 mL NH3$H2O was dropwise added. A TBT/EtOH solution was prepared by pouring 2 mL TBT into 5.0 mL EtOH with vigorous stirring for 5 min. Subsequently, the above TBT/EtOH solution was slowly added to the carbon microspheres template/EtOH/NH3$H2O suspension to form a translucent black sol, and the stirring was continued until the hydrogel was formed. The obtained hydrogel was subject to solvothermal treatment at 180 C for 24 h at a heat rate of 1 C/min. After drying at 60 C for 24 h to form a xerogel, the carbon microspheres template was removed by concentrated HNO3 treatment at 70 C for 4 h. The final product was obtained after washing with water for several times, and denoted as TiO2-NO2 HSs. 2.3. Characterizations Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2010 microscope at an accelerating voltage of 200 kV. Field emission scanning electron microscopy (FESEM) images were recorded using a Nova NanoSEM450 field emission scanning electron microscope. The chemical compositions and elemental mappings of the samples were determined by energydispersive X-ray spectrometer (EDX) equipped on FESEM. Nitrogen gas porosimetry measurements were performed on a Quantachrome NOVA 2000e surface area and porosity analyzer after the samples were outgassed under a vacuum at 70 C for 20 min and 150 C for 6 h. X-ray diffraction (XRD) patterns were obtained using a Panalytical X’Pert PRO diffractometer via Cu Ka radiation. Fourier transform infrared (FTIR) spectra were recorded on a Bruker VERTEX 70 FTIR apparatus. X-ray photoelectron spectra (XPS) was performed using a VG-ADES 400 instrument with an Mg Ka-ADES source at a residual gas pressure of less than 108 Pa. UVevisible/ diffuse reflectance spectroscopy (UVevis/DRS) was conducted using a Lambda 750S UV/VIS/NIR spectrometer. Photoluminescence (PL) measurements were carried out on a HITACHI F-7000 fluorescence spectrophotometer. 2.4. Photocatalytic tests 2.4.1. Photocatalytic degradation of aqueous MO and PCP A PLS-SXE300 Xe lamp (Beijing PerfectLight Co. Ltd., China) served as the simulated sunlight source, and the output wavelength l ¼ 320e2500 nm. 100 mg of solid photocatalyst and 100 mL of organic pollutant (MO or PCP) aqueous solution were poured into a 300 mL self-designed quartz reactor. The initial concentrations of MO and PCP are 10 mg L1 and 20 mg L1, respectively. The suspension was ultrasonicated for 10 min and stirred in the dark until adsorption-desorption equilibrium. Subsequently, the light source was switched on, and further stirring was applied. The temperature of the suspension was maintained at 35 ± 2 C by circulation of water through an external cooling jacket. At specific intervals of irradiation, fixed amounts of the reaction solution were extracted, centrifuged, and filtered. Changes in the MO concentrations were analyzed using an UNICO UV-2000 spectrophotometer at l ¼ 464.
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Changes in the PCP concentrations were analyzed using an Agilent 1100 series high-performance liquid chromatography (HPLC): C18 column, UV detector (l ¼ 277 nm), and acetonitrile/water (60/40 v/ v) was used as a mobile phase at a flow rate of 1.0 mL/min. 2.4.2. Hydrogen evolution from water-splitting by using TEA as an electron donor 100 mg of solid photocatalyst loaded with 3 wt% of Pt co-catalyst and 100 ml of H2O containing 10 vol% TEA were poured into a 300 mL quartz reactor. The above suspension was ultrasonicated for 10 min and stirred in dark for 1 h. Subsequently, the light source was switched on, and further stirring was applied. The temperature of the suspension was maintained at 35 ± 2 C by circulation of water through an external cooling jacket. After l ¼ 320e2500 nm simulated sunlight irradiation for 3 h, the generated hydrogen was in situ analyzed with a GC 7890-II TCD gas chromatograph (TECHCOMP) using an MS-5 A column, which was connected to the gas circulating line with argon carrier. 3. Results and discussion 3.1. Characterizations 3.1.1. Microstructure and surface property The microstructure of as-prepared materials was characterized by TEM and FESEM observations (Fig. 1). As shown in Fig. 1a and b, the solid and hollow spherical microstructures can be clearly observed for the carbon microspheres template and TiO2-NO2 HSs, indicating that the self-assembly process between carbon microspheres template and TBT is successfully carried out in the course of catalyst preparation. From Fig. 1c and d it can be observed that both TiO2-NO2 and g-C3N4 show a bulk structure and the additional mesoporous structure exist in the TiO2-NO2. As shown in Fig. 1e and f, TiO2-NO2 HSs before concentrated HNO3 treatment exhibit a rough surface, clear carbon core, and high carbon content. Conversely, TiO2-NO2 HSs after concentrated HNO3 treatment show a smooth surface, almost disappeared carbon core, and significantly
3
low carbon content (Fig. 1g and h). The above result indicates that Ti(OH)4 impurities and carbon microspheres template were successfully removed by concentrated HNO3 treatment. The surface property of as-prepared materials was characterized by nitrogen gas porosity measurement (Fig. 2). The type II isotherm of g-C3N4 shown in Fig. 2a corresponds to its non-porous characteristic. The type IV isotherms with H2-type hysteresis loops for the TiO2, TiO2-NO2, and TiO2-NO2 HSs samples correspond to their mesoporous structures. The different shapes of hysteresis loops of TiO2 and TiO2-NO2 samples imply their different pore geometries. Specifically, some accumulated mesoporous structures fabricated by the accumulation of Ti(OH)4 impurities exist in the TiO2 sample. For the TiO2-NO2, the accumulated mesoporous structures by Ti(OH)4 impurities were destroyed and removed after concentrated HNO3 treatment, and the rest of mesoporous structures were constructed by the connection of threedimensional TiO2 network. As shown in Fig. 2b, the relatively wide pore-size distribution peak in the range of 3e12 nm for the TiO2 sample corresponds to its accumulated mesoporous structure, and the relatively narrow pore-size distribution peak in the range of 3e6 nm for the TiO2-NO2 and TiO2-NO2 HSs samples correspond to their mesoporous structure constructed by the connection of three-dimensional TiO2 network. The BET (BrunauereEmmetteTeller) specific surface areas of TiO2, TiO2-NO2, and TiO2-NO2 HSs are larger than that of g-C3N4 owing to the existence of mesoporous structures. The BET specific surface area of TiO2-NO2 HSs is larger than that of TiO2-NO2 because of the construction of hollow spherical microstructure.
3.1.2. Compositional and structural information The composition and structure of as-prepared materials are characterized by XRD, FTIR spectra, and XPS. The phase structures of as-prepared materials are analyzed by XRD (Fig. 3). For the gC3N4, the strong (002) peak at 27.4 corresponds to an interlayer distance of d ¼ 0.33 nm, whereas the (100) peak at 12.8 represents in-plane structural packing motif with a period of 0.675 nm. As for the TiO2, TiO2-NO2, and TiO2-NO2 HSs samples, the
Fig. 1. TEM images of carbon microspheres template (a), TiO2-NO2 HSs (b), TiO2-NO2 (c), g-C3N4 (d); FESEM images, EDX elemental mappings, and EDX spectra of TiO2-NO2 HSs before (e and f) and after (g and h) concentrated HNO3 treatment.
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Fig. 3. XRD patterns of as-prepared materials.
Fig. 2. Nitrogen gas sorption isotherms (a) and pore-size distribution curves (b) of asprepared materials.
characteristic diffractions at 25.3 (101), 37.6 (004), 48.1 (200), 53.8 (105), 55.0 (211), and 62.7 (204) represent their pure anatase crystal phase (JCPDS No. 21e1272). Compare with TiO2, the relatively lower crystallinity of TiO2-NO2 can be attributed to the TiO2 framework was destroyed somewhat by the oxidation of concentrated HNO3. The obviously higher crystallinity of TiO2-NO2 HSs than that of TiO2-NO2 indicates that the dispersion effect of carbon microspheres template facilitates the growth of anatase crystal phase. FTIR spectra of as-prepared materials are shown in Fig. 4. For the TiO2, the broad peak in the range of 420e1000 cm1 is attributed to the stretching vibration of Ti-O-Ti covalent bonds, and the peaks at 1629 and 3412 cm1 are assigned to the H-O-H bending and O-H stretching vibration of adsorbed water and surface -OH groups. As for the TiO2-NO2, a new peak assigned to -NO2 groups appears at the position of 1384.6 cm1 implying that the surface -OH groups within TiO2 have been substituted with -NO2 groups during the concentrated HNO3 treatment. Compare with TiO2-NO2, the obviously lower intensity of -NO2 peak for the TiO2-NO2 HSs indicate that the surface -OH groups and carbon microspheres template react with concentrated HNO3 are mutual competitive reaction.
The morphology transformation and interaction between -NO2 groups and TiO2 matrix can be confirmed by high-resolution XPS analysis (Fig. 5). For the TiO2, the determined binding energies of Ti 2p3/2 and Ti 2p1/2 are 458.6 and 464.4 eV, which is in good agreement with anatase TiO2 (Fig. 5a). A slight shift of Ti 2p peaks was observed for the TiO2-NO2 indicating that the surface -OH groups within TiO2 were substituted with -NO2 groups after concentrated HNO3 treatment. Compare with TiO2-NO2, the obvious shift of Ti 2p peaks to higher binding energy region for TiO2-NO2 HSs indicate that the formation of hollow spherical microstructure changes the binding state of surface atoms. The XPS result of O 1s is consistent with that of Ti 2p. As shown in Fig. 5b, TiO2 shows two O 1s peaks with the binding energies of 529.9 and 531.4 eV, which attribute to the lattice-oxygen (Ti-O) and hydroxyl-oxygen (O-H) in anatase TiO2. A slight shift of O 1s peaks for the TiO2-NO2 is owing to the existence of -NO2 groups after concentrated HNO3 treatment. Compare with TiO2-NO2, the obvious shift of O 1s peaks to higher binding energy region for the TiO2-NO2 HSs can be attributed to the changes of binding state of surface atoms after the formation of hollow spherical microstructure. As shown in Fig. 5c, the C 1s peaks centered at 285.0 and 288.8 eV are typically assigned to carbon impurities, which originate from the organic species in the preparation process. Compare with TiO2, the decreased intensity of C 1s peaks for TiO2-NO2 indicates that the carbon impurities originated from organic species is partly removed in the course of concentrated HNO3 treatment. Compare with TiO2-NO2, the increased intensity of C 1s peaks for TiO2-NO2 HSs can be attributed to the small amount of residual carbon microspheres template. As shown in Fig. 5d, the N 1s peak centered at 400.2 eV is assigned to nitrogen impurities, which originate from ammonia in the preparation process. As for the TiO2-NO2, a new N 1s peak appeared at 407.0 eV corresponds to the -NO2 groups. This result further confirms that the surface -OH groups within TiO2 are substituted with -NO2 groups during the concentrated HNO3 treatment. Compare with TiO2-NO2, the obviously lower intensity of -NO2 XPS peak for the TiO2-NO2 HSs indicates that the surface -OH groups and carbon microspheres template react with concentrated HNO3 are mutual competitive reaction. 3.1.3. Optical and electronic properties The light harvesting capabilities of as-prepared materials were studied by UVevis/DRS. As shown in Fig. 6, g-C3N4 shows a typical
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5
Absorbance (a.u.)
1.6
g-C3N4
1.2
TiO2 TiO2-NO2 TiO2-NO2 HSs
0.8
0.4
0.0 200
400
600
800
Wavelength (nm) Fig. 6. UVevis/DRS of as-prepared materials.
Fig. 4. FTIR spectra of as-prepared materials.
semiconductor absorption within the region of 200e450 nm, which originates from the electron transition from VB populated by N 2p orbit to CB formed by C 2p orbit. TiO2 shows a light response within the range of 200e400 nm, which can be attributed to the electron transition from the VB (O 2p) to the CB (Ti 3d). Compare with TiO2, the enhanced light harvesting capability for the TiO2-
(b)
Ti 2p3/2
Ti 2p
O 1s
Ti 2p1/2
Intensity (a.u.)
Ti-O
TiO2
TiO2-NO2
-OH
Intensity (a.u.)
(a)
NO2 can be attributed to the increased electronic transition efficiency. Specifically, after the introduction of -NO2 groups, the electronic transition from N 2p to Ti 3d are more efficient than the electronic transition from O 2p to Ti 3d under the same irradiation condition (Scheme 1). Compare with TiO2-NO2, the enhanced light harvesting capability of TiO2-NO2 HSs can be attributed to the fabrication of hollow spherical microstructure. Specifically, hollow spherical microstructure is more conductive to capture photons than bulk structure, therefore, the electronic transition efficiency of TiO2-NO2 HSs were further increased.
TiO2
-NO2
TiO2-NO2
TiO2-NO2 HSs
TiO2-NO2 HSs
452
456
460
464
468
524
528
532
536
(c)
(d)
N 1s TiO2
TiO2
TiO2-NO2
284
Intensity (a.u.)
Intensity (a.u.)
C 1s
280
540
Binding energy (eV)
Binding energy (eV)
-NO2
TiO2-NO2
TiO2-NO2 HSs
TiO2-NO2 HSs
292
396
288
Binding energy (eV)
400
404
408
Binding energy (eV)
Fig. 5. High-resolution XPS of as-prepared materials in the Ti 2p (a), O 1s (b), C 1s (c), and N 1s (d) binding energy regions.
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Scheme 1. The fabrication route and band gap structure of as-prepared TiO2-NO2 HSs and the separation and transportation of photoinduced charge carriers in the TiO2-NO2 HSs under simulated sunlight irradiation.
The photocatalytic quantum efficiencies of as-prepared materials were studied by PL measurements. As shown in Fig. 7, g-C3N4 exhibits a broad high fluorescence emission peak in the range of 350e700 nm. This finding suggests that photoinduced eehþ pairs are generated and recombined within the g-C3N4. Compare with gC3N4, the obviously decreased PL intensity of TiO2 indicates the efficient separation and transportation of photoinduced charge carriers. Compare with TiO2, the increased PL intensity of TiO2-NO2 can be attributed to the increased electronic transitions efficiency from VB to CB simultaneously accelerate the recombination of photoinduced eehþ pairs (Scheme 1). For the TiO2-NO2 HSs, the decreased PL intensity compared with TiO2-NO2 suggest that the efficient separation and transportation of photoinduced charge carriers are realized after the construction of hollow spherical microstructure.
1000 g-C3N4
Intensity (a.u.)
TiO2 TiO2-NO2
750
TiO2-NO2 HSs 500
250
0 400
500
600
Wavelength (nm) Fig. 7. PL spectra of as-prepared materials.
700
3.2. Photocatalytic tests 3.2.1. Photocatalytic degradation of aqueous MO and PCP The photocatalytic degradation tests were conducted in an aqueous solution containing oxygen molecules from dissolved air. As shown in Fig. 8a and b, the results of direct photodegradation experiment indicate that changes of MO and PCP concentrations in the reaction system are negligible under simulated sunlight irradiation for 60 min. The adsorption test results show that the adsorption-desorption equilibrium had been reached prior to irradiation. Compare with g-C3N4, the relatively higher photocatalytic activity of TiO2 for the aqueous MO and PCP degradation can be attributed to the efficient separation and transportation of photoinduced charge carriers in TiO2. The photocatalytic degradation activity of P25 commercial product is higher than that of as-prepared TiO2 owing to the mature manufacturing process of P25 commercial product. As shown in Fig. 8a, TiO2-NO2 shows the highest adsorption capacity and photocatalytic activity among all the tested materials for the aqueous MO adsorption and degradation because of the strong electrostatic interaction between -NO2 groups and MO molecules. Compare with TiO2-NO2, the relatively poorer adsorption capacity and photocatalytic activity of TiO2-NO2 HSs toward aqueous MO adsorption and degradation corresponds to their relatively small amount of -NO2 groups. As shown in Fig. 8b, all the tested materials show a similar adsorption capacity toward PCP, and the photocatalytic activities of asprepared materials for the aqueous PCP degradation followed the order of TiO2-NO2 HSs > TiO2-NO2 > P25 > TiO2 > g-C3N4. Compare with TiO2, the increased photocatalytic activity of TiO2NO2 toward aqueous PCP degradation can be attributed to the increased electronic transition efficiency of TiO2-NO2. Compare with TiO2-NO2, the increased photocatalytic activity of TiO2-NO2 HSs toward aqueous PCP degradation can be attributed to the fabrication of hollow spherical microstructure with large BET specific surface area, perfect anatase crystal phase, increased
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K. Li et al. / Materials Chemistry and Physics xxx (2016) 1e9
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hydrogen evolution activities of the tested materials follow the order of TiO2-NO2 HSs > TiO2-NO2 > P25 > TiO2 > g-C3N4, which is similar as aqueous PCP degradation. This result further indicates that the large BET specific surface area, perfect anatase crystal phase, increased electronic transition efficiency, and decelerated recombination of photoinduced eehþ pairs play an important role to improve the photocatalytic activity of TiO2-NO2 HSs. 3.2.3. Recyclability of TiO2-NO2 HSs composite material The above PCP degradation and hydrogen evolution reactions were repeated for five times to evaluate the stability of as-prepared TiO2-NO2 HSs composite material in the current photocatalytic degradation and hydrogen evolution systems. After the first catalytic run, the catalyst was recovered by centrifugation, and then it was washed by water at room temperature. The recovered catalyst was used for the subsequent catalytic runs under the same experimental conditions. As shown in Fig. 9, the tested catalyst exhibits considerably high stability, and it can maintain a similar level of reactivity after five catalytic cycles. A gradually decreased photocatalytic activity can be attributed to the loss of photocatalyst in the recovery process. 3.2.4. Discussion to photocatalytic reaction mechanisms in degradation and hydrogen evolution systems Based on the above photocatalytic activity results, the photocatalytic reaction mechanisms in degradation and hydrogen
Fig. 8. Degradation of aqueous organic pollutants (a and b) and hydrogen evolution from water-splitting (c) over as-prepared photocatalysts under simulated sunlight irradiation.
electronic transition efficiency, and decelerated recombination of photoinduced eehþ pairs. 3.2.2. Hydrogen evolution from water-splitting by using TEA as an electron donor The photocatalytic activities of as-prepared materials were further evaluated by hydrogen evolution from water-splitting under simulated sunlight irradiation. As shown in Fig. 8c, the
Fig. 9. Recycling experiments of aqueous PCP degradation (a) and hydrogen evolution from water-splitting (b) over TiO2-NO2 HSs photocatalyst.
Please cite this article in press as: K. Li, et al., A simple and low-cost approach to fabricate TiO2-NO2 hollow spheres with excellent simulated sunlight photocatalytic activity, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.06.072
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(OH) scavenger, hole (hþ) scavenger, and superoxide radical (O 2) scavenger, respectively. As shown in Fig. 10a, the degradation rate of PCP is inhibited somewhat compared with scavenger-free photocatalytic system after adding t-BuOH (1 mmol L1). However, the degradation rate of PCP is decelerated obviously in the presence of EDTA (1 mmol L1). Finally, the degradation of PCP is inhibited completely after adding BQ (1 mmol L1). The above results indiþ cate that O 2 is the principal active species, h is the secondary active species, and OH is the unimportant active species in the course of photocatalytic degradation. In the active species masking experiments, the dissolved oxygen was consumed because of the addition of BQ in the photocatalytic degradation system. Therefore, the redox reaction between oxygen and organic pollutant is inhibited completely. In addition, the electron transfer effect of photocatalyst is weakened because the photogenerated hþ is consumed by the added EDTA. Therefore, the redox reaction between the oxygen and organic pollutant is also inhibited significantly. The unobvious OH masking effect of t-BuOH can be understood that t-BuOH or organic pollutant reacts with dissolved oxygen are competitive. To further explore the reaction mechanism of photocatalytic hydrogen evolution, the initial hydrogen evolution system, that is 100 mL of H2O containing 10 vol% TEA, was replaced by 100 ml of pure H2O, pure TEA, or H2O containing 10 vol% CH3OH. As shown in Fig. 10b, in the single-component (pure H2O or TEA) hydrogen evolution system, the redox reaction only carries out within the H2O or TEA molecule due to the lack of reducing agent (TEA) or oxidizing agent (H2O). Therefore, the hydrogen yield is low. For the H2O/CH3OH hydrogen evolution system, the redox reaction between H2O and CH3OH hardly carries out due to the poor electrondonating ability of CH3OH. Therefore, the hydrogen yield is significantly decreased compared with H2O/TEA photocatalytic hydrogen evolution system.
Fig. 10. Influence of various scavengers on the photocatalytic degradation of aqurous PCP (a) and the hydrogen yield in different photocatalytic hydrogen evolution systems (b).
evolution systems are discussed below. Firstly, there are two questions must be clear that both photocatalytic degradation and hydrogen evolution reactions belong to the redox processes and photocatalyst act as an electron transfer medium in the redox process (Scheme 1). Specifically, oxygen act as an oxidizing agent and organic pollutant act as a reducing agent in the photocatalytic degradation system, moreover, water act as an oxidizing agent and electron donor act as a reducing agent in the photocatalytic hydrogen evolution system. The redox reaction can be successfully carried out owing to the simultaneous presence of oxidizing and reducing agent in the photocatalytic reaction system. Therefore, the effect of photocatalytic reaction is mainly influenced by the properties of the photocatalyst. The unique hollow spherical microstructure, large BET specific surface area, perfect anatase crystal phase, increased electronic transition efficiency, and decelerated recombination of photoinduced eehþ pairs play positive roles to enhance the activity of the photocatalyst. Therefore, the asprepared TiO2-NO2 HSs photocatalyst exhibits excellent photocatalytic activity in both degradation and hydrogen evolution systems. In order to clarify what kind of active species played a key role during the photocatalytic degradation, the active species masking experiments were carried out by using TiO2-NO2 HSs as a representative photocatalyst and PCP as a target pollutant. In this study, tert-butyl alcohol (t-BuOH), ethylenediaminetetraacetic acid (EDTA), and 1,4-benzoquinone (BQ) are used as the hydroxyl radical
4. Conclusions Nitryl surface functionalized TiO2 hollow spheres (TiO2-NO2 HSs) were successfully fabricated by a simple and low-cost method. The as-prepared TiO2-NO2 HSs exhibit considerably high photocatalytic activity toward representative aqueous organic pollutants degradation and hydrogen evolution from water-splitting under simulated sunlight irradiation. The high photocatalytic activity is attributed to their unique hollow spherical microstructure and the introduction of -NO2 groups. The as-prepared TiO2-NO2 HSs are expected to be widely used as a photocatalyst in many areas such as wastewater treatment, air purification, and hydrogen energy development. Acknowledgements This work was financially supported by National Natural Science Foundation of China (51568049, 51208248, 21366024, 21165013, 51468043, 51238002, 51272099); National Science Fund for Excellent Young Scholars (51422807); Science and Technology Major Bidding Project of Jiangxi Province, China (No. Gan Ke Fa Ji Zi [2010] 217); Youth Science Foundation of Jiangxi Province, China (20114BAB213015, 20114BAB213016); Natural Science Foundation of Jiangxi Provincial Department of Education, China (GJJ14515, GJJ12456). References [1] M. Zhu, C. Zhai, L. Qiu, C. Lu, A.S. Paton, Y. Du, M.C. Goh, ACS Sustain. Chem. Eng. 3 (2015) 3123e3129. [2] S. Tonda, S. Kumar, S. Kandula, V. Shanker, J. Mater. Chem. A 2 (2014)
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Please cite this article in press as: K. Li, et al., A simple and low-cost approach to fabricate TiO2-NO2 hollow spheres with excellent simulated sunlight photocatalytic activity, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.06.072