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Nanospherical composite of WO3 wrapped NaTaO3: Improved photodegradation of tetracycline under visible light irradiation
Accepted Manuscript Title: Nanospherical Composite of WO3 Wrapped NaTaO3 : Improved Photodegradation of Tetracycline under Visible Light Irradiation Author: Lingnan Qu Junyu Lang Shuwei Wang Zhanli Chai Yiguo Su Xiaojing Wang PII: DOI: Reference:
S0169-4332(15)03100-1 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.095 APSUSC 32078
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-9-2015 7-12-2015 13-12-2015
Please cite this article as: L. Qu, J. Lang, S. Wang, Z. Chai, Y. Su, X. Wang, Nanospherical Composite of WO3 Wrapped NaTaO3 : Improved Photodegradation of Tetracycline under Visible Light Irradiation, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.12.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
Highlights
1. A novel ternary NaTaO3@WO3 photocatalyst was successfully fabricated.
nanosheets wrapped on the cube NaTaO3 nanoparticles.
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2. NaTaO3@WO3 composites exhibited spherical assemblies with fine WO3
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3. NaTaO3@WO3 showed the enhanced photocatalytic performance in the
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tetracycline hydrochloride degradation under visible light irradiation.
4. An adsorption-degradation photocatalytic mechanism promoted through a Z type
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heterojunction was proposed.
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Graphical Abstract (for review)
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*Manuscript
Nanospherical Composite of WO3 Wrapped NaTaO3: Improved Photodegradation of Tetracycline under Visible Light Irradiation Lingnan Qu, Junyu Lang, Shuwei Wang, Zhanli Chai, Yiguo Su and Xiaojing Wang *
City 010021, P. R. China Corresponding Author. Tel: 0471-4994406; Fax: 0471-4994406;
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E-mail: [email protected]
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School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot
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Abstract
In this paper, WO3 wrapped NaTaO3 nanospheres photocatalysts with different W/Ta
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molar ratios were successfully prepared via a facile hydrothermal method. The samples were characterized by X-ray diffraction, transmission and scan electron microscopy, X-ray photoelectron spectroscopy, FT-IR spectrum, UV−vis diffuse
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reflectance spectroscopy, and Barrett−Emmett−Teller technique. The photocatalytic activities for degrading tetracycline hydrochloride under visible light irradiation were
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examined. The results indicated that the as-prepared NaTaO3@WO3 photocatalysts showed the obvious enhancement in the tetracycline hydrochloride degradation ratio,
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compared to the pure NaTaO3 and WO3 under visible light irradiation. The optimum percentage of NaTaO3@WO3 composites with a 60.88% degradation rate was W:Ta=0.3:1 in mole, which was mainly attributed to the effective separation of the
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photo-generated electron and hole as well as the expanding of the absorption edge to the visible region due to the spherical heterojunction by wrapping WO3 on the surface of NaTaO3. The radicals trapping experiments demonstrated that there were multiple active species during the degrading process of TC. The possible mechanism of tetracycline hydrochloride degradation by NaTaO3@WO3 composite was also proposed. Keywords: NaTaO3@WO3; tetracycline hydrochloride; degradation; visible light irradiation; spherical heterojunction. 1. Introduction
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In recent years for the purpose of remedying infectious disease, the utilization of antibiotics in pharmaceutical therapies and agricultural husbandry has become more and more widespread [1-2]. These antibiotics are very difficult to completely metabolize and ultimately were released to ground water and surface soil. Thus,
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antibiotics residing in environment have become one of the most concerning issues due to their great threat to human health as a toxic and the suspected resistant
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bacterial strains even at low concentrations [3]. For example, as one of the most
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commonly used antibiotics in aquiculture and live stocking, tetracycline (TC) has been identified to bring out the microbial respiration disruption, nitrification,
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arthropathy, nephropathy, central nervous system alterations, spermatogenesis anomalies in human beings [4-6]. The methods developed in recent years to remove
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antibiotics from wastewater include membrane filtration, physical adsorption, biological remediation strategies, and advanced oxidation process [7-10]. Among
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which, the photocatalytic oxidation of pharmaceutical pollutants has appeared as new emerging technique with the advantages of low cost and environmental friendly
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[11–13]. However, since the chemical structure of drug compounds usually are various heterocyclic ring groups the incompletely degradation frequently occurs, leading to the formation of different ionic species such as imidazole ring,
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nitrobenzene, phenol, pyridine, which sometimes cause even more toxicity than the maternal. Thus, the deep degradation and complete mineralization of these antibiotics residing will be very important which require the highly photocatalytic redox ability. For a given semiconductor, a larger band gap usually gives rise to a stronger driving force for the photo-redox reaction, while a wider band gap would reduce light responsive range and seriously inhibit solar adsorption [14-15]. Besides, many visible-light-driven photocatalysts few show activities on the degradation of TC due
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to its narrow band gap. Efforts have been made by researchers to realize the photodegradation of TC under visible light such as C–N–S tridoped TiO2 [16], three-dimensional mesoporous multiwalled carbon nanotubes-Bi2WO6 [17], Fe-doped SrTiO3 [18], Au/Pt/g-C3N4 [19], Ag/Ag3PO4/AC [20], Mn-doped SrTiO3 [21],
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Sr–Bi2O3 [22], In2S3 [23], Ag2O [24], and so on. Although these photocatalysts have various merits in environmental procedures, to improve the low photon utilization, the
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incomplete degradation and poor removal ratio in a cost efficient way are still the
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most intriguing challenges in terms of the practical application.
It has been well-documented that tantalates are superior over other
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semiconductors such as TiO2 in photocatalytic oxidation due to its very positive electrochemical potential, which allows tantalates photocatalyst to be more efficient in
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photo-degrading tetracycline hydrochloride. So a variety of mixed metal oxides containing Ta5þ transition-metal ions have been studied recently [25–30]. In contrast,
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tungsten trioxide is a visible-light responsive photocatalyst with a narrow band gap that absorbs light in the region up to 480 nm. The valence band potential of WO3 is
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higher than the redox potential of TC (2.38 V vs NHE) as well as H2O/.OH (1.9 V vs NHE), indicating that WO3 possesses the ability to oxide TC by visible-light-driven photocatalytic degradation [31-33]. Additionally, WO3 is an inexpensive material with
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high stability in aqueous solution under acidic condition. However the basic disadvantage of WO3 as a photocatalyst is its low photonic efficiency [34-35]. In fact, few single composite photocatalysts can provide enhanced photo-oxidation power (which is necessary for complete decomposition of harmful antibiotics) and, at the same time, effective utilization of solar energy. The coupling strategy with appropriate semicoductors is frequently used to pursue highly separation of the electron–hole pairs and extend the range of useful excitation light to the visible spectra, beneficial
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for the synchronous promotion both degradation efficiency and solar light utilization [36]. Aiming at producing photocatalytic materials with better performance for the completely removal of TC under irradiation of visible light, a novel WO3 coupled
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NaTaO3 composite was prepared in the present study. Compared to pure WO3 and NaTaO3, the NaTaO3@WO3 samples exhibit a higher photocatalytic activity in
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photo-degrading tetracycline hydrochloride under visible light irradiation which was
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elucidated according to the reduced charge carrier recombination via forming the spherical heterojunction. Finally, a possible photocatalytic reaction mechanism for
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NaTaO3@WO3 was proposed based on the experimental results. 2. Experiment
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2.1. Materials
Sodium tungsten oxide, tantalic oxide, tetracycline hydrochloride and sodium
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hydroxide were purchased from Aladdin (Shanghai, China). All the reagents were analytical grade and used without further purification.
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2.2. Catalyst preparation
The fabrication of NaTaO3 nanoparticles: 0.2209g tantalic oxide, 5.0g sodium hydroxide, and 15 mL deionized water were fully mixed under stirring. Then the
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mixture was transferred into a Teflon-lined stainless steel vessel with a capacity of 25 mL, and the hydrothermal process was carried out at 200 oC for 72 h. After cooling to room temperature naturally, the white precipitate was collected by centrifugation, thoroughly washed several times with deionized water and ethanol, and then dried at 60 oC in air for 12 h to yield pure NaTaO3. Fabrication of WO3：2 mL nitric acid was first dropped slowly into 8 mL of deionized water solution and stirred for 10 min. 0.1484g Na2WO4·2H2O was
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dissolved homogeneously in 3 mL deionized water to form a transparent solution. The above two solutions were mixed and stirred for 30 min. Then, a precipitate appeared with its color gradually turning from white to light yellow. The suspension was transferred into a Teflon-lined stainless steel of 25 mL capacity. The hydrothermal
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route was carried out at different temperature and reactive time. The centrifugal sedimentation was washed with the deionized water and ethanol for several times, and
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dried in vacuum at 60 °C for 12 h. The final yellow products were collected and
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characterized with XRD (SI: Fig. S1 and S2).
Fabrication of WO3 wrapped NaTaO3 composites: NaTaO3 was first dissolved
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into 15 mL of sodium tungstate and sodium hydroxide solution under vigorous stirring with a different molar fractions of W:Ta (in mol) (X=5:1, 3:1, 1:1, 0.5:1, 0.3:1,
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0.2:1, respectively). Then HCl solution was added the mixture under continuous stirring for 0.5 hours, and migrated into 25 mL Teflon-lined stainless steel vessels.
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The autoclave was sealed and maintained at 180 oC for 12 hours. Subsequently, the obtained WO3 wrapped NaTaO3 composites were denoted as 5W/Ta, 3W/Ta, W/Ta,
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0.5W/Ta, 0.3W/Ta, and 0.2W/Ta, respectively, in terms of the initial addition ratio of W:Ta in mol.
2.3. Characterization
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X-ray power diffraction (XRD) was used to characterize the phase structures and particle sizes of all as-prepared samples, which was performed on a Panalytical Empyream diffractometer with a copper target. A Perkin Elmer IR spectrometer was used to measure the surface structure of the samples. The specific surface areas of the samples were determined from the nitrogen absorption data at liquid nitrogen temperature using Barrett-Emmett-Teller (BET) technique on a Micromeritics ASAP 2000 Surface Area and Porosity Analyzer. Particle morphologies of the samples were
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characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) on a HITACHI S-4800 apparatus and a Tecnai G2 F20 S-TWIN apparatus with an acceleration voltage of 20.0 kV, respectively. The energy dispersive X-ray energy spectroscopy (EDS) was performed at an acceleration voltage of 20 kV
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on a HITACHI S-4800 apparatus. X-ray photoelectron spectroscopy (XPS) measurement were performed on a Thermo ESCALAB 250 with Al Kα (1486.6 eV)
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line at 150 W. To compensate for surface charges effects, the binding energies were
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calibrated using the C1s peak at 284.60 eV as the reference. A diffusive reflectance UV–Vis spectrophotometer (UVIKON XL/XS) was used to measure the diffuse
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reflectance spectra of the samples. BaSO4 was taken as the reference sample, and the spectra were recorded in the range of 190–800 nm. The band gap of the samples was
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estimated from the onset of the absorption using the formula Eg (eV)= 1240/λg(nm). 2.4. Photocatalytic activity estimating
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Photocatalytic experiments were tested in a SGY-II photochemical reactor containing 25 mg catalytic samples and 100 mL of 20 mg/L tetracycline hydrochloride. A 500 W
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Xe lamp with a UV-cutoff filter (>420 nm) was placed inside the reactor. Water was circulated through the annulus to avoid heating during the reaction. The suspension was stirred continuously for 120 min in the dark to establish the equilibrium of
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tetracycline hydrochloride absorption/desorption on the samples before light irradiation. At certain time intervals, 4 mL solution was centrifuged to test the changes of the absorption peak with a UVIKON XL/XS spectrometer and was used to determine the concentration of TC. The photocatalytic degradation rate (Dr) was calculated by the following equation: Dr
C0 Ct *100% C0
(1)
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where C0 is the initial concentration of tetracycline solution and Ct is the concentration at irradiation time (t). To investigate the reactive species, various scavengers were introduced into the TC solution prior to the addition of the photocatalyst. The experiment process was
were referred to those adopted in previous studies [37].
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3. Results and discussion
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similar to the above photodegradation experiment. The dosages of these scavengers
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Figure 1 shows the XRD patterns of WO3, NaTaO3 and NaTaO3@WO3 composites with the different molar radio of W: Ta= 5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively.
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It presents that there existed the respective crystal phases of the cubic phase of NaTaO3 (JCPDS NO. 00-002-0873) and the monoclinic WO3 (JCPDS 072-1465). In
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addition, no any other characteristic peaks can be detected and the characteristic peaks of WO3 became more intensive with the increased contents of WO3 in NaTaO3@WO3
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composites, which testified that complex metal oxide or other new phases are not
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formed during the preparation process.
Fig. 1 XRD patterns of NaTaO3@WO3 composites with the molar radio of W: Ta= 5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively. Vertical bars below the patterns represent the standard diffraction data from JCPDS file for NaTaO3 (JCPDS NO. 00-002-0873) and WO3 (JCPDS 072-1465).
The morphologies of pure WO3 and NaTaO3 were analyzed by field emission
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scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The prepared NaTaO3 were cubic shape with an average size of 70-90 nm (Fig.2a). WO3 particles exhibited square-like sheet feature with an area about 200*300 nm and a thickness of 10-20 nm as showed in Fig.2b and Fig.S3. In order to investigate their
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photon absorbance ability, UV-vis spectrum was determined. For the pure NaTaO3 sample, the absorption spectrum was cut off at about 303 nm which corresponded to
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the intrinsic electron excitation from the valence band of O 2p orbitals to the
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conduction band of Ta 5d of the semiconductor (Fig.2c). The band gap of NaTaO3 was obtained to be 4.09 eV by extrapolating the linear portion of diffuse reflectance
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absorption spectrum on the x-axis (insert of Fig.2c). Obviously, pure NaTaO3 are inactive in the visible-light range. Fig.2c also shows the absorption spectrum of pure
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WO3, importantly, the excellent visible absorption was found for WO3 located in the regions of 456 nm consistent with the reported literatures, which was assigned to the
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excitation of O 2p to W 6s with a band gap of 2.72 eV (see Fig. S4). To characterize the specific surface area and porosity of the as-prepared samples, N2 desorption
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analysis was carried out. As shown in Fig. 2d, the isotherm curves of WO3 and NaTaO3 are characterized by type II with an H3 hysteresis loop according to Brunauer-Deming-Deming-Teller (BDDT) classification [38]. It is worth noting that
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the BET surface area of WO3 nanocrystal is only about 2cm3g-1 whereas it is 15cm3g-1 for NaTaO3 (Insert of Fig. 2d). The enlarged surface area may predict significantly enhanced adsorption performance. Thus it could be concluded that the materials formed via compositing WO3 and NaTaO3 may have the merits of two discrete components, excellent visible light response, pollution adsorption abilities, and the higher redox potential, which are significantly necessary for degrading through a photo-oxidation mechanism.
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Fig.2 (a) SEM image of pure NaTaO3, (b) SEM image of pure WO3, Insert is TEM, (c) UV-vis diffuse reflectance spectra, Insert is Tauc plot of pure NaTaO3 and WO3. (d) Nitrogen adsorption/desorption isotherms of pure WO3 and NaTaO3, Insert is the BET surface area.
A series of tightly spherical heterojunction structure of NaTaO3@WO3 were
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fabricated via simply hydrothermal method in the present work. Sodium tungstate was added into NaOH solution mixed with cube NaTaO3 nanoparticles. Then WO3 nanosheets were formed and meanwhile uniformly wrapped the surface of NaTaO3 to
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yield sphere-like composites NaTaO3@WO3 as showed in Fig.3a and 3b. The insert of Fig.3a is HRTEM image of NaTaO3@WO3 composite with W/Ta=1:1. The fringe intervals of 0.376 nm in the outer surface of the composite particle was corresponding to the interplanar spacing of (0 2 0) of monoclinic phase WO3, while that of 0.389 nm corresponding to the interplanar spacing of the cubic phase NaTaO3 (1 0 0) in the interior region of the composite particle. It indicates that WO3 are firmly attached onto the surface of NaTaO3. The result was further confirmed by EDS element
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analysis. Fig. 3c showed that NaTaO3@WO3 consisted of Ta, W, O and Na elements. The atomic ratio of W/O was calculated to be 0.304, very close to the theoretical value of WO3. However, atomic percent of Ta element was found to be small. As we known, the EDS data mainly represents the elements distribution on the surface. Thus
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it suggested that the tungsten trioxide were highly uniformly dispersed on the surface of the composite. Fig. 3d shows the FT-IR spectra of WO3, NaTaO3, and
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NaTaO3@WO3 composites. The absorption bands around 1632 cm-1 is due to the
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H–O–H bending vibrations of surface hydroxyls and crystal water molecules. The obviously characteristic peak of WO3 at 715 cm-1 appeared in composite samples. The
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characteristic peaks of NaTaO3 at 600-700 cm-1 were covered by WO3 peaks at the same regions, confirming the existence of both WO3 and NaTaO3 in composites
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[39,40].
Fig. 3 (a) SEM image of NaTaO3@WO3 (W/Ta=1:1) composite, Insert is HRTEM, (b) Schematic of a spherical heterojunction formed by WO3 coated on the surface of NaTaO3 particles, (c) EDS analysis of NaTaO3@WO3 sample, (d) FT-IR spectra of WO3, NaTaO3, and NaTaO3@WO3
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composites with the initial molar ratio W: Ta of 5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively.
XPS survey spectrum in Fig. 4a illustrates that the composite sample consists of W, Ta, and O elements. Through analyzing the component, it was found the W contents are obviously higher than Ta （W/Ta=1.81）, indicating surface existence of
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WO3. The high resolution spectrum of O 1s for NaTaO3@WO3 and pure WO3 samples were shown in Fig. 4b. It can be observed that the O 1s spectrum is broad and
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complicated due to the presence of more than one peak arising from the overlapping.
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In order to differentiate these oxygen species, the spectra was resolved with Gaussian-Lorenz model functions and the curve fitted into two peaks at 530.2, and
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530.8 eV both in NaTaO3@WO3 and in pure WO3, which were attributed to lattice oxygen atoms and surface adsorbed oxygen, respectively [41]. The result indicates the
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consistently surface component of WO3 in the pure WO3 and NaTaO3@WO3 samples. In order to certify the chemical states of W elements, the high-resolution XPS peaks
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of W elements are showed in Fig. 4c. It was clearly seen that there were two individual peaks at 37.7 and 35.6 eV for pure WO3, corresponding to W 4f5/2 and 4f7/2
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of W6+ [42]. However, for composite NaTaO3@WO3, the two strong peaks at binding energies around 37.8 and 35.6 eV were fitted into two sets of peaks (Figure 4c), indicating binding energy shift in the composite photocatalyst due to the
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heterojunction was formed between the WO3 and NaTaO3. As for Ta 4f, individual peaks at 28.2 and 26.3 eV, which were attributed to Ta 4f5/2 and Ta 4f7/2 of Ta5+, respectively [43].
UV–visible diffuse reflectance spectra of WO3, NaTaO3, and NaTaO3@WO3 composites with the different W:Ta ratios are shown in Fig. 5. The optical absorption near the band edge follows the formula (αhυ)2 = A(hυ- Eg), where α, h, υ, Eg, and A are the absorption coefficient, Planck constant, light frequency, band gap, and a
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constant, respectively. According to the spectra, pure NaTaO3 presents an intrinsic band gap transition at around 303 nm, while the absorbance peaks of NaTaO3@WO3 composites shifted to long wave regions with the increased component of WO3 phase. By coupling the NaTaO3 nanoparticles with WO3, the band gap energy (Eg) ranged
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from 4.09 eV of pure NaTaO3 to 2.82 eV of NaTaO3@WO3 composite (molar ratio 5:1) as estimation by extrapolating the line, which were listed in Table 1.
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Table 1 Estimated Eg of NaTaO3,WO3, and NaTaO3@WO3 with the initial W/Ta molar ratios of
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5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively. P-NaTaO3
0.2W/Ta
0.3W/Ta
0.5W/Ta
W/Ta
3W/Ta
5W/Ta
P-WO3
Absorption edge(nm)
303
437
425
427
436
437
440
456
Estimated Eg(eV)
4.09
2.84
2.92
2.90
2.83
2.82
2.72
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Sample
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2.84
Fig. 4 XPS spectrum: (a) survey spectrum of NaTaO3@WO3 (W/Ta=1:1) and pure WO3, (b) O
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1s orbital,（c）W 4f orbital, and （d）Ta 4f orbital of NaTaO3@WO3 (W/Ta=1:1).
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Fig.5 UV–vis diffuse reflectance spectra of NaTaO3@WO3 with the W/Ta molar ratios of 5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively.
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Fig. 6 shows the photocatalytic activities of as-prepared NaTaO3@WO3 composites with the different W:Ta ratios for the degradation of TC aqueous solution
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under visible light irradiation (λ>420 nm) together with pure WO3 and NaTaO3. For comparison, direct photolysis of TC solution was also studied without catalyst. It was
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observed the self-degradation of TC is negligible under visible light irradiation. NaTaO3 showed weak adsorption-degradation (only 20%) in visible light irradiation due to its large band-gap. As for WO3, the fast recombination of electron-hole pairs made it showed the poor photocatalytic efficiency (about 40%). However, when WO3
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and NaTaO3 were constructed together, the well photocatalytic performance was found superior than that of the single members. In addition, the photocatalytic activity of NaTaO3@WO3 nanocomposite was related to the content of WO3 loaded on NaTaO3. The NaTaO3@WO3 (molar ratio 0.3:1) presented the best photocatalytic activity, corresponding to 60.88% of degradation TC after 7 hours under visible light irradiation. Therefore, the effects of compositing NaTaO3 and WO3 as heterojunction photocatalysts can be clearly drawn, not only increasing the absorption of visible light but also improving the photocatalytic activity.
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irradiation.
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Fig.6 The photocatalytic degradation of TC with different photocatalysts under visible light
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In order to investigate the active species in TC degradation, the radical-trapping experiments were performed by using different scavengers, benzoquinone (BQ,
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superoxide anion •O2− radical scavenger), tertbutanol (TBA, •OH radical scavenger),
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carbon tetrachloride (CCl4, electron scavenger) and ammonium oxalate (AO, as hole scavenger) [44-47]. As shown in Fig. 7, it could be found that the presence of AO and
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BQ, the photodegradation was greatly restrained compared to the reaction without radical scavengers. Therefore, it could be concluded that h+ and •O2− were the main active species of 0.3W/Ta in aqueous solution under visible light irradiation. And the degradation of TC was slightly lower than in the reaction without radical scavengers
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when added TBA into the reaction system. It indicated that .OH played only a minor role in TC degradation. In contrast, the photocatalytic degradation of TC increased obviously with the addition of CCl4. This increasing suggested that the scavenger of ehad less of an opportunity to recombine electron–hole pairs, facilitating the production of more holes.
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Fig.7 Photocatalytic degradation of TC over NaTaO3@WO3 in different inhibitor under visible
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light irradiation.
The valence band (VB) and conductor band (CB) potentials of NaTaO3 and WO3
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at the point of zero charge were calculated by the following formulas [48]: (2)
EVB = ECB+ Eg
(3)
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ECB = χ- Ee -0.5 Eg
in which EVB or ECB are the band edge potentials of VB or CB, respectively. χ is the
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absolute electronegativity which is the geometric mean of the electronegativity of its constituent atoms. Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV vs NHE), and Eg is the band gap of semiconductor. Thus, the valence band (EVB) and
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the conduction band (ECB) position of NaTaO3 were estimated to be 2.79 and -1.36 eV vs NHE. Furthermore, the EVB and ECB values of WO3 were calculated to be 3.29 and 0.69 eV, respectively. Obviously, the band edge potentials of VB and CB in NaTaO3 are much more negative than that in WO3, as illustrated in Figure 8. As a result, there is a greater tendency the generated holes transferred from WO3 into NaTaO3 whereas the photo-excited electrons remain to stay in the conductor band of WO3. In this process, a significant factor responsible for the increase in the efficiency is the
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preventing recombination of electrons and holes through a Z type of NaTaO3@WO3 composite. According to the above investigations, the photocatalytic mechanism of NaTaO3@WO3 was proposed in Fig. 8. Hence, the highly uniform NaTaO3@WO3 composite exhibits a significantly enhanced performance in visible light, which could
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be ascribed to the extended light absorption, well dyes adsorption abilities, and
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quickly separation of photogenerated electron.
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4. Conclusion
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Fig.8 Photocatalytic degradation schematic illustration of TC over NaTaO3@WO3 composite.
In this study, we have successfully fabricated a series of NaTaO3@WO3 photocatalysts with different molar ratios of W to Ta by hydrothermal method. It was
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found that the as-prepared NaTaO3@WO3 composites exhibited spherical assemblies with fine WO3 nanosheets wrapped on the cube NaTaO3 nanoparticles. As established in the experiment of UV-vis and BET surface area, WO3 could adsorb visible light with a narrowed band gap while NaTaO3 has a merit of adsorption pollution due to its larger surface area. The individual NaTaO3 and WO3 showed negligible efficiencies in decomposing tetracycline hydrochloride, whereas NaTaO3@WO3 demonstrated notably higher photocatalytic activities in adsorption-degradation tetracycline hydrochloride over a wide composition range. The optimal photocatalytic activity
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occurred when the NaTaO3@WO3 molar ratio is 0.3:1, prepared at 180 0C for 12 hours. An adsorption-degradation photocatalytic mechanism promoted through a Z type heterojunction was proposed based on the active species trapping experiments and the estimation of band potentials. It is indicated a uniform sphere composite
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formed by WO3 and NaTaO3 exhibited the improved the photo-quantum efficiency due to the synergistic effects of well visible light response, quick separation of the
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hole-electrons pairs, and excellent adsorption of pollution. The present work may
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provide deep insight into the solar light driving degradation of TC, and also offer new opportunities for their industrial application in the elimination of pharmaceutical
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pollutants from wastewater. Acknowledgements
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This work is financially supported by National Natural Science Foundation of China (Grants 21267014 and 21567017), the Project of Scientific and Technological
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Innovation Team of Inner Mongolia University (12110614).
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S0169-4332(15)03100-1 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.095 APSUSC 32078
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-9-2015 7-12-2015 13-12-2015
Please cite this article as: L. Qu, J. Lang, S. Wang, Z. Chai, Y. Su, X. Wang, Nanospherical Composite of WO3 Wrapped NaTaO3 : Improved Photodegradation of Tetracycline under Visible Light Irradiation, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.12.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
Highlights
1. A novel ternary NaTaO3@WO3 photocatalyst was successfully fabricated.
nanosheets wrapped on the cube NaTaO3 nanoparticles.
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2. NaTaO3@WO3 composites exhibited spherical assemblies with fine WO3
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3. NaTaO3@WO3 showed the enhanced photocatalytic performance in the
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tetracycline hydrochloride degradation under visible light irradiation.
4. An adsorption-degradation photocatalytic mechanism promoted through a Z type
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heterojunction was proposed.
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Graphical Abstract (for review)
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*Manuscript
Nanospherical Composite of WO3 Wrapped NaTaO3: Improved Photodegradation of Tetracycline under Visible Light Irradiation Lingnan Qu, Junyu Lang, Shuwei Wang, Zhanli Chai, Yiguo Su and Xiaojing Wang *
City 010021, P. R. China Corresponding Author. Tel: 0471-4994406; Fax: 0471-4994406;
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E-mail: [email protected]
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School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot
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Abstract
In this paper, WO3 wrapped NaTaO3 nanospheres photocatalysts with different W/Ta
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molar ratios were successfully prepared via a facile hydrothermal method. The samples were characterized by X-ray diffraction, transmission and scan electron microscopy, X-ray photoelectron spectroscopy, FT-IR spectrum, UV−vis diffuse
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reflectance spectroscopy, and Barrett−Emmett−Teller technique. The photocatalytic activities for degrading tetracycline hydrochloride under visible light irradiation were
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examined. The results indicated that the as-prepared NaTaO3@WO3 photocatalysts showed the obvious enhancement in the tetracycline hydrochloride degradation ratio,
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compared to the pure NaTaO3 and WO3 under visible light irradiation. The optimum percentage of NaTaO3@WO3 composites with a 60.88% degradation rate was W:Ta=0.3:1 in mole, which was mainly attributed to the effective separation of the
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photo-generated electron and hole as well as the expanding of the absorption edge to the visible region due to the spherical heterojunction by wrapping WO3 on the surface of NaTaO3. The radicals trapping experiments demonstrated that there were multiple active species during the degrading process of TC. The possible mechanism of tetracycline hydrochloride degradation by NaTaO3@WO3 composite was also proposed. Keywords: NaTaO3@WO3; tetracycline hydrochloride; degradation; visible light irradiation; spherical heterojunction. 1. Introduction
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In recent years for the purpose of remedying infectious disease, the utilization of antibiotics in pharmaceutical therapies and agricultural husbandry has become more and more widespread [1-2]. These antibiotics are very difficult to completely metabolize and ultimately were released to ground water and surface soil. Thus,
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antibiotics residing in environment have become one of the most concerning issues due to their great threat to human health as a toxic and the suspected resistant
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bacterial strains even at low concentrations [3]. For example, as one of the most
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commonly used antibiotics in aquiculture and live stocking, tetracycline (TC) has been identified to bring out the microbial respiration disruption, nitrification,
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arthropathy, nephropathy, central nervous system alterations, spermatogenesis anomalies in human beings [4-6]. The methods developed in recent years to remove
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antibiotics from wastewater include membrane filtration, physical adsorption, biological remediation strategies, and advanced oxidation process [7-10]. Among
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which, the photocatalytic oxidation of pharmaceutical pollutants has appeared as new emerging technique with the advantages of low cost and environmental friendly
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[11–13]. However, since the chemical structure of drug compounds usually are various heterocyclic ring groups the incompletely degradation frequently occurs, leading to the formation of different ionic species such as imidazole ring,
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nitrobenzene, phenol, pyridine, which sometimes cause even more toxicity than the maternal. Thus, the deep degradation and complete mineralization of these antibiotics residing will be very important which require the highly photocatalytic redox ability. For a given semiconductor, a larger band gap usually gives rise to a stronger driving force for the photo-redox reaction, while a wider band gap would reduce light responsive range and seriously inhibit solar adsorption [14-15]. Besides, many visible-light-driven photocatalysts few show activities on the degradation of TC due
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to its narrow band gap. Efforts have been made by researchers to realize the photodegradation of TC under visible light such as C–N–S tridoped TiO2 [16], three-dimensional mesoporous multiwalled carbon nanotubes-Bi2WO6 [17], Fe-doped SrTiO3 [18], Au/Pt/g-C3N4 [19], Ag/Ag3PO4/AC [20], Mn-doped SrTiO3 [21],
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Sr–Bi2O3 [22], In2S3 [23], Ag2O [24], and so on. Although these photocatalysts have various merits in environmental procedures, to improve the low photon utilization, the
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incomplete degradation and poor removal ratio in a cost efficient way are still the
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most intriguing challenges in terms of the practical application.
It has been well-documented that tantalates are superior over other
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semiconductors such as TiO2 in photocatalytic oxidation due to its very positive electrochemical potential, which allows tantalates photocatalyst to be more efficient in
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photo-degrading tetracycline hydrochloride. So a variety of mixed metal oxides containing Ta5þ transition-metal ions have been studied recently [25–30]. In contrast,
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tungsten trioxide is a visible-light responsive photocatalyst with a narrow band gap that absorbs light in the region up to 480 nm. The valence band potential of WO3 is
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higher than the redox potential of TC (2.38 V vs NHE) as well as H2O/.OH (1.9 V vs NHE), indicating that WO3 possesses the ability to oxide TC by visible-light-driven photocatalytic degradation [31-33]. Additionally, WO3 is an inexpensive material with
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high stability in aqueous solution under acidic condition. However the basic disadvantage of WO3 as a photocatalyst is its low photonic efficiency [34-35]. In fact, few single composite photocatalysts can provide enhanced photo-oxidation power (which is necessary for complete decomposition of harmful antibiotics) and, at the same time, effective utilization of solar energy. The coupling strategy with appropriate semicoductors is frequently used to pursue highly separation of the electron–hole pairs and extend the range of useful excitation light to the visible spectra, beneficial
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for the synchronous promotion both degradation efficiency and solar light utilization [36]. Aiming at producing photocatalytic materials with better performance for the completely removal of TC under irradiation of visible light, a novel WO3 coupled
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NaTaO3 composite was prepared in the present study. Compared to pure WO3 and NaTaO3, the NaTaO3@WO3 samples exhibit a higher photocatalytic activity in
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photo-degrading tetracycline hydrochloride under visible light irradiation which was
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elucidated according to the reduced charge carrier recombination via forming the spherical heterojunction. Finally, a possible photocatalytic reaction mechanism for
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NaTaO3@WO3 was proposed based on the experimental results. 2. Experiment
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2.1. Materials
Sodium tungsten oxide, tantalic oxide, tetracycline hydrochloride and sodium
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hydroxide were purchased from Aladdin (Shanghai, China). All the reagents were analytical grade and used without further purification.
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2.2. Catalyst preparation
The fabrication of NaTaO3 nanoparticles: 0.2209g tantalic oxide, 5.0g sodium hydroxide, and 15 mL deionized water were fully mixed under stirring. Then the
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mixture was transferred into a Teflon-lined stainless steel vessel with a capacity of 25 mL, and the hydrothermal process was carried out at 200 oC for 72 h. After cooling to room temperature naturally, the white precipitate was collected by centrifugation, thoroughly washed several times with deionized water and ethanol, and then dried at 60 oC in air for 12 h to yield pure NaTaO3. Fabrication of WO3：2 mL nitric acid was first dropped slowly into 8 mL of deionized water solution and stirred for 10 min. 0.1484g Na2WO4·2H2O was
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dissolved homogeneously in 3 mL deionized water to form a transparent solution. The above two solutions were mixed and stirred for 30 min. Then, a precipitate appeared with its color gradually turning from white to light yellow. The suspension was transferred into a Teflon-lined stainless steel of 25 mL capacity. The hydrothermal
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route was carried out at different temperature and reactive time. The centrifugal sedimentation was washed with the deionized water and ethanol for several times, and
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dried in vacuum at 60 °C for 12 h. The final yellow products were collected and
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characterized with XRD (SI: Fig. S1 and S2).
Fabrication of WO3 wrapped NaTaO3 composites: NaTaO3 was first dissolved
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into 15 mL of sodium tungstate and sodium hydroxide solution under vigorous stirring with a different molar fractions of W:Ta (in mol) (X=5:1, 3:1, 1:1, 0.5:1, 0.3:1,
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0.2:1, respectively). Then HCl solution was added the mixture under continuous stirring for 0.5 hours, and migrated into 25 mL Teflon-lined stainless steel vessels.
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The autoclave was sealed and maintained at 180 oC for 12 hours. Subsequently, the obtained WO3 wrapped NaTaO3 composites were denoted as 5W/Ta, 3W/Ta, W/Ta,
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0.5W/Ta, 0.3W/Ta, and 0.2W/Ta, respectively, in terms of the initial addition ratio of W:Ta in mol.
2.3. Characterization
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X-ray power diffraction (XRD) was used to characterize the phase structures and particle sizes of all as-prepared samples, which was performed on a Panalytical Empyream diffractometer with a copper target. A Perkin Elmer IR spectrometer was used to measure the surface structure of the samples. The specific surface areas of the samples were determined from the nitrogen absorption data at liquid nitrogen temperature using Barrett-Emmett-Teller (BET) technique on a Micromeritics ASAP 2000 Surface Area and Porosity Analyzer. Particle morphologies of the samples were
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characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) on a HITACHI S-4800 apparatus and a Tecnai G2 F20 S-TWIN apparatus with an acceleration voltage of 20.0 kV, respectively. The energy dispersive X-ray energy spectroscopy (EDS) was performed at an acceleration voltage of 20 kV
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on a HITACHI S-4800 apparatus. X-ray photoelectron spectroscopy (XPS) measurement were performed on a Thermo ESCALAB 250 with Al Kα (1486.6 eV)
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line at 150 W. To compensate for surface charges effects, the binding energies were
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calibrated using the C1s peak at 284.60 eV as the reference. A diffusive reflectance UV–Vis spectrophotometer (UVIKON XL/XS) was used to measure the diffuse
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reflectance spectra of the samples. BaSO4 was taken as the reference sample, and the spectra were recorded in the range of 190–800 nm. The band gap of the samples was
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estimated from the onset of the absorption using the formula Eg (eV)= 1240/λg(nm). 2.4. Photocatalytic activity estimating
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Photocatalytic experiments were tested in a SGY-II photochemical reactor containing 25 mg catalytic samples and 100 mL of 20 mg/L tetracycline hydrochloride. A 500 W
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Xe lamp with a UV-cutoff filter (>420 nm) was placed inside the reactor. Water was circulated through the annulus to avoid heating during the reaction. The suspension was stirred continuously for 120 min in the dark to establish the equilibrium of
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tetracycline hydrochloride absorption/desorption on the samples before light irradiation. At certain time intervals, 4 mL solution was centrifuged to test the changes of the absorption peak with a UVIKON XL/XS spectrometer and was used to determine the concentration of TC. The photocatalytic degradation rate (Dr) was calculated by the following equation: Dr
C0 Ct *100% C0
(1)
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where C0 is the initial concentration of tetracycline solution and Ct is the concentration at irradiation time (t). To investigate the reactive species, various scavengers were introduced into the TC solution prior to the addition of the photocatalyst. The experiment process was
were referred to those adopted in previous studies [37].
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3. Results and discussion
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similar to the above photodegradation experiment. The dosages of these scavengers
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Figure 1 shows the XRD patterns of WO3, NaTaO3 and NaTaO3@WO3 composites with the different molar radio of W: Ta= 5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively.
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It presents that there existed the respective crystal phases of the cubic phase of NaTaO3 (JCPDS NO. 00-002-0873) and the monoclinic WO3 (JCPDS 072-1465). In
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addition, no any other characteristic peaks can be detected and the characteristic peaks of WO3 became more intensive with the increased contents of WO3 in NaTaO3@WO3
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composites, which testified that complex metal oxide or other new phases are not
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formed during the preparation process.
Fig. 1 XRD patterns of NaTaO3@WO3 composites with the molar radio of W: Ta= 5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively. Vertical bars below the patterns represent the standard diffraction data from JCPDS file for NaTaO3 (JCPDS NO. 00-002-0873) and WO3 (JCPDS 072-1465).
The morphologies of pure WO3 and NaTaO3 were analyzed by field emission
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scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The prepared NaTaO3 were cubic shape with an average size of 70-90 nm (Fig.2a). WO3 particles exhibited square-like sheet feature with an area about 200*300 nm and a thickness of 10-20 nm as showed in Fig.2b and Fig.S3. In order to investigate their
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photon absorbance ability, UV-vis spectrum was determined. For the pure NaTaO3 sample, the absorption spectrum was cut off at about 303 nm which corresponded to
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the intrinsic electron excitation from the valence band of O 2p orbitals to the
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conduction band of Ta 5d of the semiconductor (Fig.2c). The band gap of NaTaO3 was obtained to be 4.09 eV by extrapolating the linear portion of diffuse reflectance
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absorption spectrum on the x-axis (insert of Fig.2c). Obviously, pure NaTaO3 are inactive in the visible-light range. Fig.2c also shows the absorption spectrum of pure
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WO3, importantly, the excellent visible absorption was found for WO3 located in the regions of 456 nm consistent with the reported literatures, which was assigned to the
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excitation of O 2p to W 6s with a band gap of 2.72 eV (see Fig. S4). To characterize the specific surface area and porosity of the as-prepared samples, N2 desorption
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analysis was carried out. As shown in Fig. 2d, the isotherm curves of WO3 and NaTaO3 are characterized by type II with an H3 hysteresis loop according to Brunauer-Deming-Deming-Teller (BDDT) classification [38]. It is worth noting that
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the BET surface area of WO3 nanocrystal is only about 2cm3g-1 whereas it is 15cm3g-1 for NaTaO3 (Insert of Fig. 2d). The enlarged surface area may predict significantly enhanced adsorption performance. Thus it could be concluded that the materials formed via compositing WO3 and NaTaO3 may have the merits of two discrete components, excellent visible light response, pollution adsorption abilities, and the higher redox potential, which are significantly necessary for degrading through a photo-oxidation mechanism.
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Fig.2 (a) SEM image of pure NaTaO3, (b) SEM image of pure WO3, Insert is TEM, (c) UV-vis diffuse reflectance spectra, Insert is Tauc plot of pure NaTaO3 and WO3. (d) Nitrogen adsorption/desorption isotherms of pure WO3 and NaTaO3, Insert is the BET surface area.
A series of tightly spherical heterojunction structure of NaTaO3@WO3 were
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fabricated via simply hydrothermal method in the present work. Sodium tungstate was added into NaOH solution mixed with cube NaTaO3 nanoparticles. Then WO3 nanosheets were formed and meanwhile uniformly wrapped the surface of NaTaO3 to
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yield sphere-like composites NaTaO3@WO3 as showed in Fig.3a and 3b. The insert of Fig.3a is HRTEM image of NaTaO3@WO3 composite with W/Ta=1:1. The fringe intervals of 0.376 nm in the outer surface of the composite particle was corresponding to the interplanar spacing of (0 2 0) of monoclinic phase WO3, while that of 0.389 nm corresponding to the interplanar spacing of the cubic phase NaTaO3 (1 0 0) in the interior region of the composite particle. It indicates that WO3 are firmly attached onto the surface of NaTaO3. The result was further confirmed by EDS element
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analysis. Fig. 3c showed that NaTaO3@WO3 consisted of Ta, W, O and Na elements. The atomic ratio of W/O was calculated to be 0.304, very close to the theoretical value of WO3. However, atomic percent of Ta element was found to be small. As we known, the EDS data mainly represents the elements distribution on the surface. Thus
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it suggested that the tungsten trioxide were highly uniformly dispersed on the surface of the composite. Fig. 3d shows the FT-IR spectra of WO3, NaTaO3, and
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NaTaO3@WO3 composites. The absorption bands around 1632 cm-1 is due to the
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H–O–H bending vibrations of surface hydroxyls and crystal water molecules. The obviously characteristic peak of WO3 at 715 cm-1 appeared in composite samples. The
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characteristic peaks of NaTaO3 at 600-700 cm-1 were covered by WO3 peaks at the same regions, confirming the existence of both WO3 and NaTaO3 in composites
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[39,40].
Fig. 3 (a) SEM image of NaTaO3@WO3 (W/Ta=1:1) composite, Insert is HRTEM, (b) Schematic of a spherical heterojunction formed by WO3 coated on the surface of NaTaO3 particles, (c) EDS analysis of NaTaO3@WO3 sample, (d) FT-IR spectra of WO3, NaTaO3, and NaTaO3@WO3
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composites with the initial molar ratio W: Ta of 5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively.
XPS survey spectrum in Fig. 4a illustrates that the composite sample consists of W, Ta, and O elements. Through analyzing the component, it was found the W contents are obviously higher than Ta （W/Ta=1.81）, indicating surface existence of
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WO3. The high resolution spectrum of O 1s for NaTaO3@WO3 and pure WO3 samples were shown in Fig. 4b. It can be observed that the O 1s spectrum is broad and
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complicated due to the presence of more than one peak arising from the overlapping.
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In order to differentiate these oxygen species, the spectra was resolved with Gaussian-Lorenz model functions and the curve fitted into two peaks at 530.2, and
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530.8 eV both in NaTaO3@WO3 and in pure WO3, which were attributed to lattice oxygen atoms and surface adsorbed oxygen, respectively [41]. The result indicates the
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consistently surface component of WO3 in the pure WO3 and NaTaO3@WO3 samples. In order to certify the chemical states of W elements, the high-resolution XPS peaks
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of W elements are showed in Fig. 4c. It was clearly seen that there were two individual peaks at 37.7 and 35.6 eV for pure WO3, corresponding to W 4f5/2 and 4f7/2
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of W6+ [42]. However, for composite NaTaO3@WO3, the two strong peaks at binding energies around 37.8 and 35.6 eV were fitted into two sets of peaks (Figure 4c), indicating binding energy shift in the composite photocatalyst due to the
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heterojunction was formed between the WO3 and NaTaO3. As for Ta 4f, individual peaks at 28.2 and 26.3 eV, which were attributed to Ta 4f5/2 and Ta 4f7/2 of Ta5+, respectively [43].
UV–visible diffuse reflectance spectra of WO3, NaTaO3, and NaTaO3@WO3 composites with the different W:Ta ratios are shown in Fig. 5. The optical absorption near the band edge follows the formula (αhυ)2 = A(hυ- Eg), where α, h, υ, Eg, and A are the absorption coefficient, Planck constant, light frequency, band gap, and a
Page 13 of 23
constant, respectively. According to the spectra, pure NaTaO3 presents an intrinsic band gap transition at around 303 nm, while the absorbance peaks of NaTaO3@WO3 composites shifted to long wave regions with the increased component of WO3 phase. By coupling the NaTaO3 nanoparticles with WO3, the band gap energy (Eg) ranged
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from 4.09 eV of pure NaTaO3 to 2.82 eV of NaTaO3@WO3 composite (molar ratio 5:1) as estimation by extrapolating the line, which were listed in Table 1.
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Table 1 Estimated Eg of NaTaO3,WO3, and NaTaO3@WO3 with the initial W/Ta molar ratios of
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5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively. P-NaTaO3
0.2W/Ta
0.3W/Ta
0.5W/Ta
W/Ta
3W/Ta
5W/Ta
P-WO3
Absorption edge(nm)
303
437
425
427
436
437
440
456
Estimated Eg(eV)
4.09
2.84
2.92
2.90
2.83
2.82
2.72
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Sample
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2.84
Fig. 4 XPS spectrum: (a) survey spectrum of NaTaO3@WO3 (W/Ta=1:1) and pure WO3, (b) O
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cr
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1s orbital,（c）W 4f orbital, and （d）Ta 4f orbital of NaTaO3@WO3 (W/Ta=1:1).
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Fig.5 UV–vis diffuse reflectance spectra of NaTaO3@WO3 with the W/Ta molar ratios of 5:1, 3:1, 1:1, 0.5:1, 0.3:1, 0.2:1, respectively.
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Fig. 6 shows the photocatalytic activities of as-prepared NaTaO3@WO3 composites with the different W:Ta ratios for the degradation of TC aqueous solution
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under visible light irradiation (λ>420 nm) together with pure WO3 and NaTaO3. For comparison, direct photolysis of TC solution was also studied without catalyst. It was
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observed the self-degradation of TC is negligible under visible light irradiation. NaTaO3 showed weak adsorption-degradation (only 20%) in visible light irradiation due to its large band-gap. As for WO3, the fast recombination of electron-hole pairs made it showed the poor photocatalytic efficiency (about 40%). However, when WO3
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and NaTaO3 were constructed together, the well photocatalytic performance was found superior than that of the single members. In addition, the photocatalytic activity of NaTaO3@WO3 nanocomposite was related to the content of WO3 loaded on NaTaO3. The NaTaO3@WO3 (molar ratio 0.3:1) presented the best photocatalytic activity, corresponding to 60.88% of degradation TC after 7 hours under visible light irradiation. Therefore, the effects of compositing NaTaO3 and WO3 as heterojunction photocatalysts can be clearly drawn, not only increasing the absorption of visible light but also improving the photocatalytic activity.
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irradiation.
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Fig.6 The photocatalytic degradation of TC with different photocatalysts under visible light
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In order to investigate the active species in TC degradation, the radical-trapping experiments were performed by using different scavengers, benzoquinone (BQ,
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superoxide anion •O2− radical scavenger), tertbutanol (TBA, •OH radical scavenger),
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carbon tetrachloride (CCl4, electron scavenger) and ammonium oxalate (AO, as hole scavenger) [44-47]. As shown in Fig. 7, it could be found that the presence of AO and
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BQ, the photodegradation was greatly restrained compared to the reaction without radical scavengers. Therefore, it could be concluded that h+ and •O2− were the main active species of 0.3W/Ta in aqueous solution under visible light irradiation. And the degradation of TC was slightly lower than in the reaction without radical scavengers
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when added TBA into the reaction system. It indicated that .OH played only a minor role in TC degradation. In contrast, the photocatalytic degradation of TC increased obviously with the addition of CCl4. This increasing suggested that the scavenger of ehad less of an opportunity to recombine electron–hole pairs, facilitating the production of more holes.
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Fig.7 Photocatalytic degradation of TC over NaTaO3@WO3 in different inhibitor under visible
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light irradiation.
The valence band (VB) and conductor band (CB) potentials of NaTaO3 and WO3
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at the point of zero charge were calculated by the following formulas [48]: (2)
EVB = ECB+ Eg
(3)
ed
ECB = χ- Ee -0.5 Eg
in which EVB or ECB are the band edge potentials of VB or CB, respectively. χ is the
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absolute electronegativity which is the geometric mean of the electronegativity of its constituent atoms. Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV vs NHE), and Eg is the band gap of semiconductor. Thus, the valence band (EVB) and
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the conduction band (ECB) position of NaTaO3 were estimated to be 2.79 and -1.36 eV vs NHE. Furthermore, the EVB and ECB values of WO3 were calculated to be 3.29 and 0.69 eV, respectively. Obviously, the band edge potentials of VB and CB in NaTaO3 are much more negative than that in WO3, as illustrated in Figure 8. As a result, there is a greater tendency the generated holes transferred from WO3 into NaTaO3 whereas the photo-excited electrons remain to stay in the conductor band of WO3. In this process, a significant factor responsible for the increase in the efficiency is the
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preventing recombination of electrons and holes through a Z type of NaTaO3@WO3 composite. According to the above investigations, the photocatalytic mechanism of NaTaO3@WO3 was proposed in Fig. 8. Hence, the highly uniform NaTaO3@WO3 composite exhibits a significantly enhanced performance in visible light, which could
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be ascribed to the extended light absorption, well dyes adsorption abilities, and
M
an
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cr
quickly separation of photogenerated electron.
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4. Conclusion
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Fig.8 Photocatalytic degradation schematic illustration of TC over NaTaO3@WO3 composite.
In this study, we have successfully fabricated a series of NaTaO3@WO3 photocatalysts with different molar ratios of W to Ta by hydrothermal method. It was
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found that the as-prepared NaTaO3@WO3 composites exhibited spherical assemblies with fine WO3 nanosheets wrapped on the cube NaTaO3 nanoparticles. As established in the experiment of UV-vis and BET surface area, WO3 could adsorb visible light with a narrowed band gap while NaTaO3 has a merit of adsorption pollution due to its larger surface area. The individual NaTaO3 and WO3 showed negligible efficiencies in decomposing tetracycline hydrochloride, whereas NaTaO3@WO3 demonstrated notably higher photocatalytic activities in adsorption-degradation tetracycline hydrochloride over a wide composition range. The optimal photocatalytic activity
Page 18 of 23
occurred when the NaTaO3@WO3 molar ratio is 0.3:1, prepared at 180 0C for 12 hours. An adsorption-degradation photocatalytic mechanism promoted through a Z type heterojunction was proposed based on the active species trapping experiments and the estimation of band potentials. It is indicated a uniform sphere composite
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formed by WO3 and NaTaO3 exhibited the improved the photo-quantum efficiency due to the synergistic effects of well visible light response, quick separation of the
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hole-electrons pairs, and excellent adsorption of pollution. The present work may
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provide deep insight into the solar light driving degradation of TC, and also offer new opportunities for their industrial application in the elimination of pharmaceutical
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pollutants from wastewater. Acknowledgements
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This work is financially supported by National Natural Science Foundation of China (Grants 21267014 and 21567017), the Project of Scientific and Technological
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Innovation Team of Inner Mongolia University (12110614).
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