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In-situ growth of layered double hydroxide films on biomedical magnesium alloy by transforming metal oxyhydroxide
NANO: Brief Reports and Reviews Vol. 14, No. 10 (2019) 1950133 (11 pages) © World Scienti¯c Publishing Company DOI: 10.1142/S1793292019501339
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One-Step Synthesis of Si@Ti 3þ Self-Doped TiO2 Heterostructure with Enhanced Photocatalytic Performance Xingquan Zhang*,§, Yousong Liu†, Hongjia Ji*, Jun Wang†, Jin Chen†, Jichuan Huo* and Hongtao Song‡ *State Key Laboratory of Environment-friendly Energy Materials Southwest University of Science and Technology Mianyang 621010, P. R. China †Institute
of Chemical Materials China Academy of Engineering Physics Mianyang 621900, P. R. China ‡Institute
of Nuclear Physics and Chemistry China Academy of Engineering Physics Mianyang 621900, P. R. China §
[email protected]
Received 6 June 2019 Accepted 3 September 2019 Published 27 September 2019
Heterostructure construction and doping provide two powerful routes for manipulating charge carrier separation, electrical transport, optical response, and interface kinetics for photocatalysis. However, the literature reported synthetic methods until now were very time-consuming. In this work, a facile one-pot hydrothermal reaction route was designed to synthesize Si nanospheres @ Ti 3þ self-doped TiO2 nanosheet heterostructure with tunable Ti 3þ doping levels at di®erent hydrothermal temperatures. It was found that the precursor Si nanospheres not only serve as supporter of TiO2 nanosheets, but also are the inducement of Ti 3þ doping. Due to the synergistic e®ect of Ti 3þ doping and Si/TiO2 heterojunctions, the optimal sample exhibited 6.74-fold enhancement for rhodamine B (RhB) photodegradation under Xe lamp irradiation compared with the pristine TiO2. The construction of Si/TiO2 heterojunction, as well as the introduced Ti 3þ doping and associated oxygen vacancies, extended the optical absorption of TiO2 into visible region and enhanced the separation e±ciency of photogenerated electron-hole pairs, which ¯nally resulted in improved photocatalytic performance of the Si@Ti 3þ self-doped TiO2 core–shell nanospheres. The designed hydrothermal route opens up an opportunity to the synthesis of doped hetero-photocatalysts in an e±cient way. Keywords: Si/TiO2 heterostructure; Ti 3þ self-doped TiO2; oxygen vacancy; photocatalysis.
§
Corresponding author. 1950133-1
X. Zhang et al.
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1. Introduction The increasing energy exhausting and environmental contamination caused by industrialization and population growth have driven considerable interests on photocatalysis, including pollutant degradation and arti¯cial photosynthesis.1,2 Metal oxide semiconductors, in particular, have been widely studied as promising materials for photocatalysis due to their advanced photocatalytic properties and relatively abundant resources.3,4 Among the numerous semiconducting materials, TiO2 is one of the most famous photocatalysts due to its low cost, nontoxicity and excellent stability. However, the two obstacles, wide bandgap ( 3:0 eV for rutile and 3.2 eV for anatase) and rapid recombination of the photo-generated electron and hole pairs with the short hole di®usion length (10 nm for the rutile single crystal), hamper its industrial-scale application in photocatalytic process.5,6 Therefore, proposing reasonable approaches to narrow the bandgap and restrain the recombination of photo-generated electron-hole pairs for high-e±cient photocatalysts is essential. Among the various strategies to improve photocatalytic activity of TiO2, doping with nonmetal elements, metal elements and self-doping are considered as simple and feasible methods.7–14 In particular, the reduced TiO2 with Ti 3þ self-doping has attracted enormous attention owing to the interband level of Ti 3þ and increased electrical conductivity arising from its high donor density. It was ascertained that the formation of Ti 3þ self-doping is responsible for improving the separation e±ciency of photogenerated electron-hole pairs and extending the optical absorption of wide-bandgap semiconductors into the visible region, thereby, to a large extent improving the photocatalytic performance.15 Several synthetic approaches including hydrogen treatment, thermal treatment under oxygen depleted conditions, high energy bombardment, doping with other elements and treatment with reducing aqueous solutions have been developed to prepare Ti 3þ self-doped-TiO2, but the multiple steps or harsh conditions limited these methods for practical applications.16–19 Thus, a facile method is greatly desired to prepare defective TiO2 with Ti 3þ self-doping. A smart way to reduce photo-generated electronhole recombination rate is to construct a heterojunction structure including two semiconductors with energy-band matching. The heterojunctions
can accelerate the separation of electron and hole, prolong charge life and increase the e±ciency of photocatalysis. A number of semiconductors, such as Bi2MoO6, CdTe, and MoS2, are able to form heterojunctions with TiO2 and improve the photocatalytic activity to certain degrees.20–25 However, traditional TiO2 visible-light heterojunction photocatalysts are either unstable or have a low conversion e±ciency. So, fabricating high e±ciency and stable TiO2-based heterojunction photocatalysts is still a research focus in recent years. Of the various narrow-band semiconductors, silicon has considerable application in optoelectronic devices with its wide light absorption range, high light absorption e±ciency and high electron mobility.26–29 Recently, photocatalytic activity of silicon materials have been reported. Qu et al. prepared porous silicon nanowires and platinum nanoparticle loaded porous silicon nanowires by metal assisted wet-chemical etching of highly-doped silicon wafers and investigated their performance for the degradation of organic dyes and toxic pollutants under visible irradiation.28 Sampath et al. attained a novel composite material of TiO2 and porous silicon using atomic layer deposition and found that it exhibits much higher photocatalytic activity for the degradation of methylene blue.29 Chen et al. found that Si/TiO2 photocatalysts possess broad-band absorption with extended edge up to 700 nm, facilitating high photocatalytic activity for the urgent treatment of water pollutants.30 Therefore, combining of TiO2 with Si could be a potential pathway not only to extend the light absorption of TiO2 to visible range, but also to enhance the transfer of charge carriers by forming heterojunction at the interface. In this work, Si@Ti 3þ self-doped TiO2 (Si@Ti 3þ TiO2) core–shell nanospheres were successfully prepared via a facile one-pot hydrothermal method. Si nanospheres obtained from the magnesiothermic reduction of SiO2 st€ober nanospheres were used as the Si source and supporter for TiO2 nanosheets. Moreover, the produced H2 from Si and HF will capture partial O atoms from TiO2 nanosheets to generate Ti 3þ -doping at high temperature in the hydrothermal process. The morphology, structure, and properties of the samples were investigated and the mechanism of enhanced photocatalytic activity for the Si@Ti 3þ -TiO2 core–shell nanospheres was also discussed in detail.
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One-Step Synthesis of Si@Ti3+ Self-Doped Heterostructure with Enhanced Photocatalytic Performance
2. Materials and Methods
respectively. Photoluminescence (PL) spectra were obtained with an Edinburgh Instrument FLS 920 spectrometer.
2.1. Synthesis of the Si@Ti -TiO2 core–shell nanospheres
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3+
All chemical reagents were purchased from Aldrich and used without further puri¯cation. Si nanospheres were prepared via the magnesiothermic ober nanospheres, as reported reduction of SiO2 st€ 31 earlier. 0.5 g SiO2 nanospheres and 0.42 g Mg powder were ground for 5 min and then transferred into a crucible to calcine at 750 C for 5 h under N2 atmosphere. After cooling to room temperature, the powder was then added to 50 mL 32 wt.% HCl solution with stirring for 24 h. Si nanospheres were then cleaned several times by centrifugation and water dispersion and ¯nally dried into powders at 60 C in a vacuum oven for 12 h. The Si@Ti 3þ TiO2 core–shell nanospheres were fabricated via a facile hydrothermal reaction with tetrabutyl titanate and Si nanospheres at di®erent hydrothermal temperatures. In a typical process, 5 mL of Ti(OBu)4 and 0.3 g Si nanospheres were added in a 20 mL Te°on pot, respectively and 0.6 mL of hydro°uoric acid was added dropwise under stirring. After stirring for 15 min at room temperature, the Te°on pot was sealed and kept at 150 C, 180 C and 220 C for 24 h, respectively (noted as Si@Ti 3þ -TiO2-150, Si@Ti 3þ -TiO2-180 and Si@Ti 3þ -TiO2-220, respectively). Finally, the as-prepared Si@Ti 3þ -TiO2 core–shell nanospheres were obtained after centrifuging and washing with ethanol, and then dried in a vacuum oven for 12 h. Pure TiO2 was prepared in a same hydrothermal reaction without Si nanospheres.
2.2. Material characterization The crystal structure of all the samples were examined by X-ray di®raction analysis (XRD, Bruker D8 ADVANCE with Cu-K radiation, ¼ 1:5418 A). The morphology and particle size were determined by a transmission electron microscopy (TEM, JEOL JSM-2010) with an accelerating voltage of 200 kV. UV-Vis absorption spectra were obtained by using a UV-Vis spectrometer (Shimadzu UV-3600). Electron paramagnetic resonance spectra (EPR) were recorded at 110 K on a Bruker EMX-10/12 EPR spectrometer. The microwave power employed was 1 mW, and the modulation frequency and modulation amplitude were 100 kHz and 0.35 mT,
2.3. Photocatalytic measurements The photocatalytic activity was measured as follows: 0.100 g of the as-prepared Si, TiO2 and Si@Ti 3þ -TiO2 samples were added to a 250 mL pyrex glass vessel which contained 200 mL rhodamine B (RhB) solution (7.5 mg/L). The light source was a 300 W Xe arc lamp (CHF-XM500W, Beijing TrustTech Co. Ltd.) with an illumination intensity of 100 mW/cm2. Prior to irradiation, RhB solution suspended with photocatalysts were stirred in the dark for 30 min to ensure that the surface of photocatalysts reaches adsorption–desorption equilibrium. 3 mL of the suspension was withdrawn throughout the experiment after every 10 min. The samples were analyzed by a UV-Vis spectrophotometer after removing the catalyst powders by centrifugation.
3. Results and Discussion The formation mechanism of the Si@Ti 3þ -TiO2 core–shell nanospheres is illustrated in Fig. 1. Pure TiO2 nanosheets can be obtained by a hydrothermal method.32 For synthesis of the Si@Ti 3þ -TiO2 core– shell nanospheres, extra Si nanospheres prepared ober through magnesiothermic reduction of SiO2 st€ nanospheres are added as supporter for the TiO2 nanosheets to grown on. Under suitable hydrothermal conditions, Si/TiO2 crystal nuleus form gradually and grow into Si@TiO2 nanospheres with the extension of reaction time. Meanwhile, oxygen atom of TiO2 reacts with hydrogen generated from the reaction of Si powder and hydro°uoric acid, resulting in the oxygen loss from TiO2 and the formation of oxygen vacancy (Ov ), and then the remaining electrons will be trapped at Ti 4þ interstitials to form Ti 3þ . It is worth noting that H2 capturing oxygen from TiO2 occurs in the whole hydrothermal process, hence introducing Ti 3þ species and Ov not merely on the surface but also in the bulk of TiO2. The morphologies of the obtained Si@Ti 3þ TiO2 nanospheres were examined thoroughly by TEM. As shown in Fig. 2(a), the pure TiO2 shows plentiful square sheet-like structure with a thickness of about 10 nm and an average edge length of approximately 40 nm. It is clearly observed [Figs. 2(b)
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Fig. 1.
Schematic diagram for the formation of TiO2 nanosheets and Si@Ti 3þ -TiO2 core–shell nanospheres.
and 2(c)] that the as-prepared samples comprise Si@Ti 3þ -TiO2 nanospheres with Si spheres encapsulated by TiO2 nanosheets around the interfaces. The heterojunction structures of the Si@Ti 3þ -TiO2 nanospheres are further demonstrated by the
Fig. 2.
HRTEM image, as shown in Fig. 2(d). The Si@Ti 3þ -TiO2 sample shows two di®erent lattice fringes: one with d ¼ 0:31 nm matches the (111) plane of Si; the other with d ¼ 0:235 nm corresponding to the (001) plane of TiO2. Therefore, the
TEM images of (a) TiO2 nanosheets, TEM (b, c) and HRTEM images (d) of Si@Ti 3þ -TiO2-180 nanospheres. 1950133-4
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One-Step Synthesis of Si@Ti3+ Self-Doped Heterostructure with Enhanced Photocatalytic Performance
Fig. 3. XRD patterns for the pure TiO2, Si and Si@Ti 3þ -TiO2 nanospheres prepared at di®erent hydrothermal temperature with magni¯cation patterns showing in insets for 2 ranges of about 24–32.
above results further prove that plentiful Si/TiO2 heterojunctions were successfully constructed in the as-prepared Si@Ti 3þ -TiO2 samples via the hydrothermal method. The XRD patterns of the pure TiO2 and Si@Ti 3þ -TiO2 nanospheres are shown in Fig. 3. The analysis of those patterns revealed that the Si@Ti 3þ -TiO2 samples obtained by various hydrothermal temperatures basically remain in the crystal structures of TiO2. It is clearly observed that both the pure TiO2 nanosheets and the Si@Ti 3þ -TiO2 nanospheres grow along the preferred orientation of (101) and (200) with considerable intensities. For these samples, the characteristic di®raction peaks are around 2 values of 25.53, 38.08, 48.21, 55.05, 55.28, 62.78, 68.99, 74.98 and 82.88, which can be assigned to the (101), (004), (200), (105), (211), (204), (220), (215) and (303) crystal planes, respectively, indicating a typical anatase TiO2 structure. All observations are in good agreement with the reported anatase TiO2 values.10,33 Nevertheless, the XRD patterns do not reveal the existence of di®raction peak of Si other than a weak characteristic di®raction peak assigned to (111) plane of Si (JCPDS Card 27-1402), which could be attributed to the limited amount and relatively low di®raction intensity. It can also be found that the di®raction peak position of the Si@Ti 3þ -TiO2 nanospheres shifted to lower angle () in comparison with the
pure TiO2, which may be caused by the fact that the A) was larger than ionic radius of the Ti 3þ (0.67 4þ that of Ti ion (0.61 A) in TiO2, it is expected that partial Ti 4þ ion was reduced to form Ti 3þ , and accordingly, occupied some of the titanium lattice sites, leading to an increment in the lattice parameters and cell volume. Photoabsorption is a crucial factor a®ecting the photocatalytic performance. Hence, UV-Vis absorption spectra were obtained to represent the light absorption properties of the Si@Ti 3þ -TiO2 samples [Fig. 4(a)]. For comparison, the absorption spectra of pure TiO2 and Si are also provided. Compared to the Si and TiO2, it can be seen that the as-prepared Si@Ti 3þ -TiO2 nanospheres exhibit highly enhanced absorption in the ultraviolet region and nearly the entire visible range. It is worth nothing that the Si@Ti 3þ -TiO2-150 sample shows improved but inclined downward trend in the visible-light region, which is mainly resulted from the characteristic absorption of Si. With the increase of hydrothermal temperature, the Si@Ti 3þ -TiO2-180 and Si@Ti 3þ TiO2-220 samples exhibit upward trend owing to the presence of Ti 3þ ions/Ov in TiO2.34 Consistently with the absorption results, the color of the powders su®ers a matching variation from white to bluish from pure TiO2 to the Si@Ti 3þ -TiO2 samples. In order to con¯rm the existence of Ti 3þ species and explore the e®ect of their concentrations on the
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(a)
(b)
Fig. 4. (a) UV-Vis di®use re°ectance spectra of pure TiO2, Si and Si@Ti 3þ -TiO2 nanospheres prepared at di®erent hydrothermal temperature. (b) EPR spectra of pure TiO2 and the as-prepared Si@Ti 3þ -TiO2 samples.
photocatalytic performance, EPR analysis was carried out. Generally speaking, EPR is used to detect the paramagnetic centers, regardless inner or outer portion of the materials, ranging from transition metal ions to defects and radicals which are singly anionic ionized.35 As far as TiO2, the singly ionized Ov is deemed to the main paramagnetic species. In comparison to pure TiO2, a strong EPR signal is observed at g = 1.98 in Si@Ti 3þ -TiO2 nanospheres [Fig. 4(b)], which could be assigned to Ti 3þ ,36 thus con¯rming the existence of Ti 3þ in the synthesized samples. Widely accepted, the surface Ti 3þ species were unstable in air or water. However, the bluish colors of our prepared Si@Ti 3þ -TiO2 samples remain unchanged for several months, revealing that Ti 3þ was present not only on the surface but also in the bulk of TiO2. What is more, the Ti 3þ doping concentration increases with the hydrothermal temperature from 150 C to 220 C, which is consistent with the results of the light absorption properties. The photocatalytic activity of the Si@Ti 3þ -TiO2 nanospheres was studied by degradation of RhB in aqueous solution. Figure 5(a) shows the variation of RhB concentration (C/C0 ) over Si, TiO2 and Si@Ti 3þ -TiO2 nanospheres photocatalysts against reaction time under light irradiation. The concentration of RhB at di®erent times was determined by the absorption value at 553 nm. First, it can be observed that the C/C0 Values of RhB with Si@Ti 3þ -TiO2 samples prior to light irradiation were all lower than that of Si and TiO2 samples, indicating their improved RhB adsorption alilities
attributed to the Ti 3þ and Ov species. When the Xe arc lamp was turned on, nearly 100% of the RhB molecules were degraded by the sample Si@Ti 3þ TiO2-180 in 20 min, 95.8% by Si@Ti 3þ -TiO2-150 nanospheres and 93.7% by Si@Ti 3þ -TiO2-220 nanospheres while only 40% and 5% by pure TiO2 nanosheets and Si nanospheres, which indicated an enhanced photocatalytic performance of the Si@Ti 3þ -TiO2 nanospheres. Figure 5(b) shows the UV-Vis spectra of the RhB aqueous solutions treated by the Si@Ti 3þ -TiO2-180 photocatalysts at di®erent irradiation times. It can be found that all the absorption peaks of RhB disappeared when the irradiation time reach 20 min, suggesting the complete decomposition of RhB by the preprared Si@Ti 3þ -TiO2-180 sample. Figure 5(c) exhibits the kinetic study of photocatalytic degradation of RhB solution over the as-synthetized samples. The linear relationship of lnðC0 /C) versus irradiation time suggests that degradation of RhB is a ¯rst-order reaction. And the calculated rate constants for Si, TiO2, Si@Ti 3þ -TiO2-150, Si@Ti 3þ -TiO2-180 and Si@Ti 3þ -TiO2-220 nanospheres are 0.0008, 0.0271, 0.1142, 0.1827 and 0.0974 min 1 , respectively. It can be seen that all the Si@Ti 3þ -TiO2 nanospheres exhibit improved photocatalytic performance compared with pure TiO2, which show 4.22-, 6.74- and 3.61-fold higher activity as compared to TiO2 nanosheets, respectively. It is well known that the separation rate of photoexcited electron-hole pairs is an important factor for the photocatalytic activity of the photocatalyst. As shown in Fig. 5(d), PL spectroscopy
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One-Step Synthesis of Si@Ti3+ Self-Doped Heterostructure with Enhanced Photocatalytic Performance
(a)
(b)
(c)
(d)
Fig. 5. The variation of RhB concentration (C/C0 ) over di®erent photocatalysts with time under light irradiation (a) and UV-Vis spectral changes of the RhB in aqueous solution over Si@Ti 3þ -TiO2-180 composite photocatalysts at di®erent irradiation times, (c) Variation in the normalized lnðC/C0 ) of the RhB concentration as a function of irradiation time and (d) PL spectra of pure TiO2, Si and Si@Ti 3þ self-doped TiO2-180 samples under 320 nm excitation.
was employed for further investigation of the photocatalytic activities of the Si@Ti 3þ -TiO2 nanospheres. The spectrum of the Si@Ti 3þ -TiO2-180 nanospheres consist of a strong peak at 540 nm and a weak, broad peak from 400 to 520 nm, which are identical for the Si nanospheres and TiO2 nanosheets, respectively. It can be seen clearly that the peak intensities in PL intensity of the Si@Ti 3þ TiO2-180 nanospheres are much lower in contrast to
that of the Si nanospheres and TiO2 nanosheets. As the PL emission resulted from the recombination of photo-induced charge carriers and information regarding the e±ciency of charge carrier trapping, and their recombination kinetics can be drawn from the PL spectra, it can be inferred that the Si@Ti 3þ TiO2-180 nanospheres have a higher e±cient separation rate of photogenerated charge carriers than that of TiO2 nanosheets. The improved separation
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X. Zhang et al. Table 1. Comparison of the performance of Si@Ti 3þ -TiO2 photocatalytic decomposition system with that of some noblemetal-containing systems.
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Catalyst 20-BS/3D-TiO2 3% Cu–TiO2 GO/WO3 QDs/TiO2 TiO2 -B ZnFe2 O4 /TiO2 /CDs Si@Ti 3þ -TiO2
Pollutant
k (min 1 Þ
Ref.
RhB RhB RhB RhB RhB RhB
0.00772 0.076 0.06 0.058 0.1362 0.1827
37 38 39 40 41 This work
of the electron-hole pairs of Si@Ti 3þ -TiO2 samples can be ascribed to several aspects: (1) the appropriate proportion of Si nanospheres and Ti 3þ selfdoped TiO2 sheets helped form suitable Si@Ti 3þ TiO2 heterojunctions and sped up the separation of charge carriers; (2) the existence of Ti 3þ and associated Ov accelerated the electron transfer rate and suppressed the electronhole pair recombination. From the above results, it can be clearly seen that the Si@Ti 3þ -TiO2-180 samples exhibit the highest photocatalytic activity among all the as-prepared samples, which may be attributed to the moderate concentration of Ti 3þ and Ov in these samples. Moreover, the photocatalytic performance of the optimized Si@Ti 3þ -TiO2-180 was comparable to other TiO2 nanosheets of reported works (Table 1). It is known that Ti 3þ and Ov improve the photocatalytic activity of TiO2 mainly from the following two aspects: (i) it extends the photoresponse of
TiO2 from UV to visible light region, which gives rise to visible-light photo catalytic activity; (ii) it provides signi¯cant reactive agents for many adsorbates and results in the reduction of electronhole pairs recombination rate.42 In other words, the light absorption abilities and photocatalytic activities of the Si@Ti 3þ -TiO2 samples increase with the Ti 3þ and Ov doping concentrations. However, excessive Ti 3þ and Ov will act an opposite part and generally produced the combination centers for electrons and holes to reduce the concentration of photo-generated carriers as well as the sample's photocatalytic activity. Therefore, the optimal temperature to prepare Si@Ti 3þ -TiO2-180 nanospheres in the hydrothermal process is 180 C to generate a suitable quantity of Ti 3þ and reveal an optimum photocatalytic performance. It is known that RhB shows some self-sensitization e®ect due to its excellent visible light absorption ability, phenol was adopted to test the photocatalytic performance of the Si@Ti 3þ -TiO2 samples. For photocatalysis, 200 mL of 20 mg/L phenol containing 100 mg catalyst was exposed to Xe lamp as visible light source. The phenol concentration was monitored by high performance liquid chromatography (HPLC) at predetermined time intervals (6 min). The experimental results of photocatalytic degradation of phenol are shown in Fig. 6(a). The blank test con¯rmed that the phenol degradation was negligible without catalysts. For pure TiO2 nanosheets and Si nanospheres, the
(a)
(b)
Fig. 6. (a) Photocatalytic degradation of phenol over di®erent photocatalysts with time under light irradiation. (b) Photodegradation of RhB over Si@Ti 3þ -TiO2-180 with di®erent scavengers. 1950133-8
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One-Step Synthesis of Si@Ti3+ Self-Doped Heterostructure with Enhanced Photocatalytic Performance
degradation e±ciencies of phenol are only 33% after 75 min of photoreaction. However, it was obvious that nearly 100% of the RhB molecules were degraded by the sample Si@Ti 3þ -TiO2-180 in 20 min, 90% by Si@Ti 3þ -TiO2-150 nanospheres and 84% by Si@Ti 3þ -TiO2-220 nanospheres which indicated an enhanced photocatalytic performance of the Si@Ti 3þ -TiO2 nanospheres. To investigate the mechanism of the photodegradation process, scavengers for electrons (e ) and holes (h þ ) were employed to determine the speci¯c reactive species.43 Si@Ti 3þ -TiO2-180 was selected as the model photocatalysts to deeply understand the degradation behavior of RhB. In the photocatalytic process, the main oxidative species can be detected by adding benzoquinone (BQ, a scavenger of O 2 ), Ammonium oxalate (AO, a scavenger of holes), Tert-butyl alcohol (TBA, a scavenger of OH) and AgNO3 (TBA, a scavenger of e ). As shown in Fig. 6(b), di®erent scavengers have di®erent e®ects on the degradation of RhB over the Si@Ti 3þ -TiO2-180 samples. After BQ and AO were added into the reaction system, the rate for degradation of RhB over Si@Ti 3þ -TiO2-180 was remarkably decreased and only about 19% and 39% of RhB were degraded, respectively. In the presence of TBA and AgNO3, the photodegradation rate was slightly decreased. This evidence clearly indicated that the in°uences of BQ and AO on the photocatalytic activity were more obvious. In other words, O 2 and holes contributed the photodegradation of RhB. The above results con¯rmed that the synergistic e®ect of Ti 3þ self-doped TiO2 and Si e®ectively improved the photocatalytic performance. The schematics of the energy band of Si@Ti 3þ -TiO2 and the possible photocatalytic mechanism are illustrated in Fig. 7. According to the conduction band (CB) and valence band (VB) edge potentials of Si and TiO2 (0.41 eV and 0.71 eV, 0.29 eV and 2.91 eV, respectively), in the Si@Ti 3þ -TiO2 heterojunctions, Si could easily absorb the visible light because of its bandgap (1.12 eV). Once irradiated under Xe arc lamp, the electrons are able to jump from the VB to the CB of Si and then transfer from the CB of Si to that of TiO2. Such electron transition between heterojunctions can reduce the recombination of charge carriers and prolong the charge lifetime. In addition, the Ti 3þ forms a local state at the bottom of the TiO2 CB with visiblelight response capacity. Under Xe arc lamp
Fig. 7. Schematic of the energy band of TiO2 and Si as well as charge migration and separation on Si@Ti 3þ -TiO2 heterojunctions under Xe arc lamp irradiation.
irradiation, the electrons in the VB can jump to the CB of TiO2. Then these excited-state electrons transfer to the TiO2 nanoparticles surface to generate the main active groups of superoxide anion radicals (O 2 ) through reacting with dissolved oxygen, which further oxidize the RhB. In addition, since the VB edge potential of TiO2 is more positive than that of Si, the holes at the VB of TiO2 can transfer to the VB of Si, which could oxidize the organic molecules and simultaneously react with H2O to produce OH radical. These active species attack RhB at the aromatic chromophore leading to the degradation of the RhB structure rather than to de-ethylation. The pollutants are ¯nally degraded to CO2, H2O and other micromolecules.
4. Conclusions In summary, a facile and e®ective method for synthesizing the Si@Ti 3þ -TiO2 core–shell structured photocatalyst was designed and developed. The composite photocatalyst exhibited much enhanced photocatalytic activity than pure Si and TiO2 samples for the degradation of RhB under Xe arc lamp irradiation, which was attributed to the effective separation of electron–hole pairs and extended light absorption range to visible light region due to the formation of heterojunction between Si nanospheres and TiO2 nanosheets as well as the Ti 3þ doping and associated Ov . Therefore, such a Si@Ti 3þ -TiO2 core–shell structured photocatalyst is
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promising for water puri¯cation application and environmental remediation.
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Acknowledgments This work was supported by the National Natural Science Foundation of China [Grant No. 11702268]; the Scienti¯c Research Fund of Education Department of Sichuan Province, China [Grant No. 16ZB0135]; the Open Project of State Key Laboratory of Environment-friendly Energy Materials, China [Grant No. 15zxfk11]; Longshan academic talent research supporting program of Southwest University of Science and Technology, China [Grant Nos. 18LZX560 and 18LZXT10].
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16. F. Zuo, L. Wang, T. Wu, Z. Y. Zhang, D. Borchardt and P. Y. Feng, J. Am. Chem. Soc. 132, 11856 (2010). 17. C. Y. Wu, Z. G. Gao, S. M. Gao, Q. Y. Wang, Z. Y. Wang, B. B. Huang and Y. Dai, NANO 11, 1650088 (2016). 18. B. Bharti, S. Kumar, H. N. Lee and R. Kumar, Sci. Rep. 6, 32355 (2016). 19. J. L. Tang, Y. S. Liu, Y. J. Hu, G. Q. Lv, C. T. Yang and G. C. Yang, Chem. Eur. J. 24, 4390 (2018). 20. J. W. Yin, Z. P. Xing, J. Y. Kuang, Z. Z. Li, M. Li, J. J. Jiang, S. Y. Tan, Q. Zhu and W. Zhou, J. Alloys Compd. 750, 659 (2018). 21. X. F. Liu, Z. P. Xing, Y. Zhang, Z. Z. Li, X. Y. Wu, S. Y. Tan, X. J. Yu, Q. Zhu and W. Zhou, Appl. Catal. B - Environ. 201, 119 (2017). 22. B. Pal, B. L. Vijiayan, S. G. Krishnan, M. Harilal, W. J. Basirun, A. Lowe, M. M. Yuso® and K. R. Jose, J. Alloys Compd. 740, 703 (2017). 23. L. V. Bora and R. K. Mewada, Renew. Sust. Energ. Rev. 76, 1393 (2017). 24. X. F. Gao, H. B. Li, W. T. Sun, Q. Chen, F. Q. Tang and L. M. Peng, J. Phys. Chem. C 113, 7531 (2009). 25. C. J. Chen, X. Xin, J. N. Zhang, G. Li, Y. F. Zhang, H. B. Lu, J. Z. Gao, Z. B. Yang, C. L. Wang and Z. He, NANO 13, 1850129 (2018). 26. L. M. Zhang, C. Liu, A. B. Wong, J. Resasco and P. D. Yang, Nano Res. 8, 281 (2015). 27. X. L. Yang, H. Zhang, Y. H. Zhong, Y. H. Zhu, H. L. Jiang, J. H. Shen, J. F. Huang and C. Z. Li, J. Mater. Chem. A 2, 9040 (2014). 28. Y. Q. Qu, X. Zhong, Y. J. Li, L. Liao, Y. Huang and X. F. Duan, J. Mater. Chem. 20, 3590 (2010). 29. S. Sampath, P. Maydannik, T. Ivanova, M. Shestakova, T. Homola, A. Bryukvin, M. Sillanpaa, R. Nagumothu and V. Alagan, Superlattice. Microst. 97, 155 (2016). 30. C. Y. Chen and A. H. Chen, J. Phys. D Appl. Phys. 45, 365304 (2012). 31. M. Ibisate, D. Golmayo and C. López, Adv. Mater. 21, 2899 (2009). 32. X. G. Han, Q. Kuang, M. S. Jin, Z. X. Xie and L. S. Zheng, J. Am. Chem. Soc. 131, 3152 (2009). 33. Y. S. Liu, G. B. Ji, M. A. Dastageer, L. Zhu, J. Y. Wang, B. Zhang, X. F. Chang and M. A. Gondal, RSC Adv. 4, 56961 (2014). 34. Z. Wang, C. Y. Yang, T. Q. Lin, H. Yin, P. Chen, D. Y. Wan, F. F. Xu, F. Q. Huang, J. H. Lin, X. M. Xie and M. H. Jiang, Energy Environ. Sci. 6, 3007 (2013). 35. S. V. Doorslaer and D. M. Murphy, EPR Spectroscopy in Catalysis (Springer, Heidelberg, 2012). 36. S. Dohshi, M. Anpo, S. Okuda and T. Kojima, Top. Catal. 35, 327 (2005).
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41. Y. D. Lu, Y. P. Feng, F. L. Wang, X. G. Zou, Z. F. Chen, P. Chen, H. J. Liu, Y. H. Su, Q. X. Zhang and G. G. Liu, J. Photochem. Photobiol. A 353, 10 (2018). 42. L. B. Xiong, J. L. Li, B. Yang and Y. Yu, J. Nanomater. 83, 1524 (2012). 43. Y. Hu, D. Z. Li, H. B. Wang, G. P. Zeng, X. H. Li and Y. Shao, J. Mol. Catal. A - Chem. 408, 172 (2015).
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One-Step Synthesis of Si@Ti 3þ Self-Doped TiO2 Heterostructure with Enhanced Photocatalytic Performance Xingquan Zhang*,§, Yousong Liu†, Hongjia Ji*, Jun Wang†, Jin Chen†, Jichuan Huo* and Hongtao Song‡ *State Key Laboratory of Environment-friendly Energy Materials Southwest University of Science and Technology Mianyang 621010, P. R. China †Institute
of Chemical Materials China Academy of Engineering Physics Mianyang 621900, P. R. China ‡Institute
of Nuclear Physics and Chemistry China Academy of Engineering Physics Mianyang 621900, P. R. China §
[email protected]
Received 6 June 2019 Accepted 3 September 2019 Published 27 September 2019
Heterostructure construction and doping provide two powerful routes for manipulating charge carrier separation, electrical transport, optical response, and interface kinetics for photocatalysis. However, the literature reported synthetic methods until now were very time-consuming. In this work, a facile one-pot hydrothermal reaction route was designed to synthesize Si nanospheres @ Ti 3þ self-doped TiO2 nanosheet heterostructure with tunable Ti 3þ doping levels at di®erent hydrothermal temperatures. It was found that the precursor Si nanospheres not only serve as supporter of TiO2 nanosheets, but also are the inducement of Ti 3þ doping. Due to the synergistic e®ect of Ti 3þ doping and Si/TiO2 heterojunctions, the optimal sample exhibited 6.74-fold enhancement for rhodamine B (RhB) photodegradation under Xe lamp irradiation compared with the pristine TiO2. The construction of Si/TiO2 heterojunction, as well as the introduced Ti 3þ doping and associated oxygen vacancies, extended the optical absorption of TiO2 into visible region and enhanced the separation e±ciency of photogenerated electron-hole pairs, which ¯nally resulted in improved photocatalytic performance of the Si@Ti 3þ self-doped TiO2 core–shell nanospheres. The designed hydrothermal route opens up an opportunity to the synthesis of doped hetero-photocatalysts in an e±cient way. Keywords: Si/TiO2 heterostructure; Ti 3þ self-doped TiO2; oxygen vacancy; photocatalysis.
§
Corresponding author. 1950133-1
X. Zhang et al.
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1. Introduction The increasing energy exhausting and environmental contamination caused by industrialization and population growth have driven considerable interests on photocatalysis, including pollutant degradation and arti¯cial photosynthesis.1,2 Metal oxide semiconductors, in particular, have been widely studied as promising materials for photocatalysis due to their advanced photocatalytic properties and relatively abundant resources.3,4 Among the numerous semiconducting materials, TiO2 is one of the most famous photocatalysts due to its low cost, nontoxicity and excellent stability. However, the two obstacles, wide bandgap ( 3:0 eV for rutile and 3.2 eV for anatase) and rapid recombination of the photo-generated electron and hole pairs with the short hole di®usion length (10 nm for the rutile single crystal), hamper its industrial-scale application in photocatalytic process.5,6 Therefore, proposing reasonable approaches to narrow the bandgap and restrain the recombination of photo-generated electron-hole pairs for high-e±cient photocatalysts is essential. Among the various strategies to improve photocatalytic activity of TiO2, doping with nonmetal elements, metal elements and self-doping are considered as simple and feasible methods.7–14 In particular, the reduced TiO2 with Ti 3þ self-doping has attracted enormous attention owing to the interband level of Ti 3þ and increased electrical conductivity arising from its high donor density. It was ascertained that the formation of Ti 3þ self-doping is responsible for improving the separation e±ciency of photogenerated electron-hole pairs and extending the optical absorption of wide-bandgap semiconductors into the visible region, thereby, to a large extent improving the photocatalytic performance.15 Several synthetic approaches including hydrogen treatment, thermal treatment under oxygen depleted conditions, high energy bombardment, doping with other elements and treatment with reducing aqueous solutions have been developed to prepare Ti 3þ self-doped-TiO2, but the multiple steps or harsh conditions limited these methods for practical applications.16–19 Thus, a facile method is greatly desired to prepare defective TiO2 with Ti 3þ self-doping. A smart way to reduce photo-generated electronhole recombination rate is to construct a heterojunction structure including two semiconductors with energy-band matching. The heterojunctions
can accelerate the separation of electron and hole, prolong charge life and increase the e±ciency of photocatalysis. A number of semiconductors, such as Bi2MoO6, CdTe, and MoS2, are able to form heterojunctions with TiO2 and improve the photocatalytic activity to certain degrees.20–25 However, traditional TiO2 visible-light heterojunction photocatalysts are either unstable or have a low conversion e±ciency. So, fabricating high e±ciency and stable TiO2-based heterojunction photocatalysts is still a research focus in recent years. Of the various narrow-band semiconductors, silicon has considerable application in optoelectronic devices with its wide light absorption range, high light absorption e±ciency and high electron mobility.26–29 Recently, photocatalytic activity of silicon materials have been reported. Qu et al. prepared porous silicon nanowires and platinum nanoparticle loaded porous silicon nanowires by metal assisted wet-chemical etching of highly-doped silicon wafers and investigated their performance for the degradation of organic dyes and toxic pollutants under visible irradiation.28 Sampath et al. attained a novel composite material of TiO2 and porous silicon using atomic layer deposition and found that it exhibits much higher photocatalytic activity for the degradation of methylene blue.29 Chen et al. found that Si/TiO2 photocatalysts possess broad-band absorption with extended edge up to 700 nm, facilitating high photocatalytic activity for the urgent treatment of water pollutants.30 Therefore, combining of TiO2 with Si could be a potential pathway not only to extend the light absorption of TiO2 to visible range, but also to enhance the transfer of charge carriers by forming heterojunction at the interface. In this work, Si@Ti 3þ self-doped TiO2 (Si@Ti 3þ TiO2) core–shell nanospheres were successfully prepared via a facile one-pot hydrothermal method. Si nanospheres obtained from the magnesiothermic reduction of SiO2 st€ober nanospheres were used as the Si source and supporter for TiO2 nanosheets. Moreover, the produced H2 from Si and HF will capture partial O atoms from TiO2 nanosheets to generate Ti 3þ -doping at high temperature in the hydrothermal process. The morphology, structure, and properties of the samples were investigated and the mechanism of enhanced photocatalytic activity for the Si@Ti 3þ -TiO2 core–shell nanospheres was also discussed in detail.
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One-Step Synthesis of Si@Ti3+ Self-Doped Heterostructure with Enhanced Photocatalytic Performance
2. Materials and Methods
respectively. Photoluminescence (PL) spectra were obtained with an Edinburgh Instrument FLS 920 spectrometer.
2.1. Synthesis of the Si@Ti -TiO2 core–shell nanospheres
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3+
All chemical reagents were purchased from Aldrich and used without further puri¯cation. Si nanospheres were prepared via the magnesiothermic ober nanospheres, as reported reduction of SiO2 st€ 31 earlier. 0.5 g SiO2 nanospheres and 0.42 g Mg powder were ground for 5 min and then transferred into a crucible to calcine at 750 C for 5 h under N2 atmosphere. After cooling to room temperature, the powder was then added to 50 mL 32 wt.% HCl solution with stirring for 24 h. Si nanospheres were then cleaned several times by centrifugation and water dispersion and ¯nally dried into powders at 60 C in a vacuum oven for 12 h. The Si@Ti 3þ TiO2 core–shell nanospheres were fabricated via a facile hydrothermal reaction with tetrabutyl titanate and Si nanospheres at di®erent hydrothermal temperatures. In a typical process, 5 mL of Ti(OBu)4 and 0.3 g Si nanospheres were added in a 20 mL Te°on pot, respectively and 0.6 mL of hydro°uoric acid was added dropwise under stirring. After stirring for 15 min at room temperature, the Te°on pot was sealed and kept at 150 C, 180 C and 220 C for 24 h, respectively (noted as Si@Ti 3þ -TiO2-150, Si@Ti 3þ -TiO2-180 and Si@Ti 3þ -TiO2-220, respectively). Finally, the as-prepared Si@Ti 3þ -TiO2 core–shell nanospheres were obtained after centrifuging and washing with ethanol, and then dried in a vacuum oven for 12 h. Pure TiO2 was prepared in a same hydrothermal reaction without Si nanospheres.
2.2. Material characterization The crystal structure of all the samples were examined by X-ray di®raction analysis (XRD, Bruker D8 ADVANCE with Cu-K radiation, ¼ 1:5418 A). The morphology and particle size were determined by a transmission electron microscopy (TEM, JEOL JSM-2010) with an accelerating voltage of 200 kV. UV-Vis absorption spectra were obtained by using a UV-Vis spectrometer (Shimadzu UV-3600). Electron paramagnetic resonance spectra (EPR) were recorded at 110 K on a Bruker EMX-10/12 EPR spectrometer. The microwave power employed was 1 mW, and the modulation frequency and modulation amplitude were 100 kHz and 0.35 mT,
2.3. Photocatalytic measurements The photocatalytic activity was measured as follows: 0.100 g of the as-prepared Si, TiO2 and Si@Ti 3þ -TiO2 samples were added to a 250 mL pyrex glass vessel which contained 200 mL rhodamine B (RhB) solution (7.5 mg/L). The light source was a 300 W Xe arc lamp (CHF-XM500W, Beijing TrustTech Co. Ltd.) with an illumination intensity of 100 mW/cm2. Prior to irradiation, RhB solution suspended with photocatalysts were stirred in the dark for 30 min to ensure that the surface of photocatalysts reaches adsorption–desorption equilibrium. 3 mL of the suspension was withdrawn throughout the experiment after every 10 min. The samples were analyzed by a UV-Vis spectrophotometer after removing the catalyst powders by centrifugation.
3. Results and Discussion The formation mechanism of the Si@Ti 3þ -TiO2 core–shell nanospheres is illustrated in Fig. 1. Pure TiO2 nanosheets can be obtained by a hydrothermal method.32 For synthesis of the Si@Ti 3þ -TiO2 core– shell nanospheres, extra Si nanospheres prepared ober through magnesiothermic reduction of SiO2 st€ nanospheres are added as supporter for the TiO2 nanosheets to grown on. Under suitable hydrothermal conditions, Si/TiO2 crystal nuleus form gradually and grow into Si@TiO2 nanospheres with the extension of reaction time. Meanwhile, oxygen atom of TiO2 reacts with hydrogen generated from the reaction of Si powder and hydro°uoric acid, resulting in the oxygen loss from TiO2 and the formation of oxygen vacancy (Ov ), and then the remaining electrons will be trapped at Ti 4þ interstitials to form Ti 3þ . It is worth noting that H2 capturing oxygen from TiO2 occurs in the whole hydrothermal process, hence introducing Ti 3þ species and Ov not merely on the surface but also in the bulk of TiO2. The morphologies of the obtained Si@Ti 3þ TiO2 nanospheres were examined thoroughly by TEM. As shown in Fig. 2(a), the pure TiO2 shows plentiful square sheet-like structure with a thickness of about 10 nm and an average edge length of approximately 40 nm. It is clearly observed [Figs. 2(b)
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X. Zhang et al.
Fig. 1.
Schematic diagram for the formation of TiO2 nanosheets and Si@Ti 3þ -TiO2 core–shell nanospheres.
and 2(c)] that the as-prepared samples comprise Si@Ti 3þ -TiO2 nanospheres with Si spheres encapsulated by TiO2 nanosheets around the interfaces. The heterojunction structures of the Si@Ti 3þ -TiO2 nanospheres are further demonstrated by the
Fig. 2.
HRTEM image, as shown in Fig. 2(d). The Si@Ti 3þ -TiO2 sample shows two di®erent lattice fringes: one with d ¼ 0:31 nm matches the (111) plane of Si; the other with d ¼ 0:235 nm corresponding to the (001) plane of TiO2. Therefore, the
TEM images of (a) TiO2 nanosheets, TEM (b, c) and HRTEM images (d) of Si@Ti 3þ -TiO2-180 nanospheres. 1950133-4
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One-Step Synthesis of Si@Ti3+ Self-Doped Heterostructure with Enhanced Photocatalytic Performance
Fig. 3. XRD patterns for the pure TiO2, Si and Si@Ti 3þ -TiO2 nanospheres prepared at di®erent hydrothermal temperature with magni¯cation patterns showing in insets for 2 ranges of about 24–32.
above results further prove that plentiful Si/TiO2 heterojunctions were successfully constructed in the as-prepared Si@Ti 3þ -TiO2 samples via the hydrothermal method. The XRD patterns of the pure TiO2 and Si@Ti 3þ -TiO2 nanospheres are shown in Fig. 3. The analysis of those patterns revealed that the Si@Ti 3þ -TiO2 samples obtained by various hydrothermal temperatures basically remain in the crystal structures of TiO2. It is clearly observed that both the pure TiO2 nanosheets and the Si@Ti 3þ -TiO2 nanospheres grow along the preferred orientation of (101) and (200) with considerable intensities. For these samples, the characteristic di®raction peaks are around 2 values of 25.53, 38.08, 48.21, 55.05, 55.28, 62.78, 68.99, 74.98 and 82.88, which can be assigned to the (101), (004), (200), (105), (211), (204), (220), (215) and (303) crystal planes, respectively, indicating a typical anatase TiO2 structure. All observations are in good agreement with the reported anatase TiO2 values.10,33 Nevertheless, the XRD patterns do not reveal the existence of di®raction peak of Si other than a weak characteristic di®raction peak assigned to (111) plane of Si (JCPDS Card 27-1402), which could be attributed to the limited amount and relatively low di®raction intensity. It can also be found that the di®raction peak position of the Si@Ti 3þ -TiO2 nanospheres shifted to lower angle () in comparison with the
pure TiO2, which may be caused by the fact that the A) was larger than ionic radius of the Ti 3þ (0.67 4þ that of Ti ion (0.61 A) in TiO2, it is expected that partial Ti 4þ ion was reduced to form Ti 3þ , and accordingly, occupied some of the titanium lattice sites, leading to an increment in the lattice parameters and cell volume. Photoabsorption is a crucial factor a®ecting the photocatalytic performance. Hence, UV-Vis absorption spectra were obtained to represent the light absorption properties of the Si@Ti 3þ -TiO2 samples [Fig. 4(a)]. For comparison, the absorption spectra of pure TiO2 and Si are also provided. Compared to the Si and TiO2, it can be seen that the as-prepared Si@Ti 3þ -TiO2 nanospheres exhibit highly enhanced absorption in the ultraviolet region and nearly the entire visible range. It is worth nothing that the Si@Ti 3þ -TiO2-150 sample shows improved but inclined downward trend in the visible-light region, which is mainly resulted from the characteristic absorption of Si. With the increase of hydrothermal temperature, the Si@Ti 3þ -TiO2-180 and Si@Ti 3þ TiO2-220 samples exhibit upward trend owing to the presence of Ti 3þ ions/Ov in TiO2.34 Consistently with the absorption results, the color of the powders su®ers a matching variation from white to bluish from pure TiO2 to the Si@Ti 3þ -TiO2 samples. In order to con¯rm the existence of Ti 3þ species and explore the e®ect of their concentrations on the
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X. Zhang et al.
(a)
(b)
Fig. 4. (a) UV-Vis di®use re°ectance spectra of pure TiO2, Si and Si@Ti 3þ -TiO2 nanospheres prepared at di®erent hydrothermal temperature. (b) EPR spectra of pure TiO2 and the as-prepared Si@Ti 3þ -TiO2 samples.
photocatalytic performance, EPR analysis was carried out. Generally speaking, EPR is used to detect the paramagnetic centers, regardless inner or outer portion of the materials, ranging from transition metal ions to defects and radicals which are singly anionic ionized.35 As far as TiO2, the singly ionized Ov is deemed to the main paramagnetic species. In comparison to pure TiO2, a strong EPR signal is observed at g = 1.98 in Si@Ti 3þ -TiO2 nanospheres [Fig. 4(b)], which could be assigned to Ti 3þ ,36 thus con¯rming the existence of Ti 3þ in the synthesized samples. Widely accepted, the surface Ti 3þ species were unstable in air or water. However, the bluish colors of our prepared Si@Ti 3þ -TiO2 samples remain unchanged for several months, revealing that Ti 3þ was present not only on the surface but also in the bulk of TiO2. What is more, the Ti 3þ doping concentration increases with the hydrothermal temperature from 150 C to 220 C, which is consistent with the results of the light absorption properties. The photocatalytic activity of the Si@Ti 3þ -TiO2 nanospheres was studied by degradation of RhB in aqueous solution. Figure 5(a) shows the variation of RhB concentration (C/C0 ) over Si, TiO2 and Si@Ti 3þ -TiO2 nanospheres photocatalysts against reaction time under light irradiation. The concentration of RhB at di®erent times was determined by the absorption value at 553 nm. First, it can be observed that the C/C0 Values of RhB with Si@Ti 3þ -TiO2 samples prior to light irradiation were all lower than that of Si and TiO2 samples, indicating their improved RhB adsorption alilities
attributed to the Ti 3þ and Ov species. When the Xe arc lamp was turned on, nearly 100% of the RhB molecules were degraded by the sample Si@Ti 3þ TiO2-180 in 20 min, 95.8% by Si@Ti 3þ -TiO2-150 nanospheres and 93.7% by Si@Ti 3þ -TiO2-220 nanospheres while only 40% and 5% by pure TiO2 nanosheets and Si nanospheres, which indicated an enhanced photocatalytic performance of the Si@Ti 3þ -TiO2 nanospheres. Figure 5(b) shows the UV-Vis spectra of the RhB aqueous solutions treated by the Si@Ti 3þ -TiO2-180 photocatalysts at di®erent irradiation times. It can be found that all the absorption peaks of RhB disappeared when the irradiation time reach 20 min, suggesting the complete decomposition of RhB by the preprared Si@Ti 3þ -TiO2-180 sample. Figure 5(c) exhibits the kinetic study of photocatalytic degradation of RhB solution over the as-synthetized samples. The linear relationship of lnðC0 /C) versus irradiation time suggests that degradation of RhB is a ¯rst-order reaction. And the calculated rate constants for Si, TiO2, Si@Ti 3þ -TiO2-150, Si@Ti 3þ -TiO2-180 and Si@Ti 3þ -TiO2-220 nanospheres are 0.0008, 0.0271, 0.1142, 0.1827 and 0.0974 min 1 , respectively. It can be seen that all the Si@Ti 3þ -TiO2 nanospheres exhibit improved photocatalytic performance compared with pure TiO2, which show 4.22-, 6.74- and 3.61-fold higher activity as compared to TiO2 nanosheets, respectively. It is well known that the separation rate of photoexcited electron-hole pairs is an important factor for the photocatalytic activity of the photocatalyst. As shown in Fig. 5(d), PL spectroscopy
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One-Step Synthesis of Si@Ti3+ Self-Doped Heterostructure with Enhanced Photocatalytic Performance
(a)
(b)
(c)
(d)
Fig. 5. The variation of RhB concentration (C/C0 ) over di®erent photocatalysts with time under light irradiation (a) and UV-Vis spectral changes of the RhB in aqueous solution over Si@Ti 3þ -TiO2-180 composite photocatalysts at di®erent irradiation times, (c) Variation in the normalized lnðC/C0 ) of the RhB concentration as a function of irradiation time and (d) PL spectra of pure TiO2, Si and Si@Ti 3þ self-doped TiO2-180 samples under 320 nm excitation.
was employed for further investigation of the photocatalytic activities of the Si@Ti 3þ -TiO2 nanospheres. The spectrum of the Si@Ti 3þ -TiO2-180 nanospheres consist of a strong peak at 540 nm and a weak, broad peak from 400 to 520 nm, which are identical for the Si nanospheres and TiO2 nanosheets, respectively. It can be seen clearly that the peak intensities in PL intensity of the Si@Ti 3þ TiO2-180 nanospheres are much lower in contrast to
that of the Si nanospheres and TiO2 nanosheets. As the PL emission resulted from the recombination of photo-induced charge carriers and information regarding the e±ciency of charge carrier trapping, and their recombination kinetics can be drawn from the PL spectra, it can be inferred that the Si@Ti 3þ TiO2-180 nanospheres have a higher e±cient separation rate of photogenerated charge carriers than that of TiO2 nanosheets. The improved separation
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X. Zhang et al. Table 1. Comparison of the performance of Si@Ti 3þ -TiO2 photocatalytic decomposition system with that of some noblemetal-containing systems.
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Catalyst 20-BS/3D-TiO2 3% Cu–TiO2 GO/WO3 QDs/TiO2 TiO2 -B ZnFe2 O4 /TiO2 /CDs Si@Ti 3þ -TiO2
Pollutant
k (min 1 Þ
Ref.
RhB RhB RhB RhB RhB RhB
0.00772 0.076 0.06 0.058 0.1362 0.1827
37 38 39 40 41 This work
of the electron-hole pairs of Si@Ti 3þ -TiO2 samples can be ascribed to several aspects: (1) the appropriate proportion of Si nanospheres and Ti 3þ selfdoped TiO2 sheets helped form suitable Si@Ti 3þ TiO2 heterojunctions and sped up the separation of charge carriers; (2) the existence of Ti 3þ and associated Ov accelerated the electron transfer rate and suppressed the electronhole pair recombination. From the above results, it can be clearly seen that the Si@Ti 3þ -TiO2-180 samples exhibit the highest photocatalytic activity among all the as-prepared samples, which may be attributed to the moderate concentration of Ti 3þ and Ov in these samples. Moreover, the photocatalytic performance of the optimized Si@Ti 3þ -TiO2-180 was comparable to other TiO2 nanosheets of reported works (Table 1). It is known that Ti 3þ and Ov improve the photocatalytic activity of TiO2 mainly from the following two aspects: (i) it extends the photoresponse of
TiO2 from UV to visible light region, which gives rise to visible-light photo catalytic activity; (ii) it provides signi¯cant reactive agents for many adsorbates and results in the reduction of electronhole pairs recombination rate.42 In other words, the light absorption abilities and photocatalytic activities of the Si@Ti 3þ -TiO2 samples increase with the Ti 3þ and Ov doping concentrations. However, excessive Ti 3þ and Ov will act an opposite part and generally produced the combination centers for electrons and holes to reduce the concentration of photo-generated carriers as well as the sample's photocatalytic activity. Therefore, the optimal temperature to prepare Si@Ti 3þ -TiO2-180 nanospheres in the hydrothermal process is 180 C to generate a suitable quantity of Ti 3þ and reveal an optimum photocatalytic performance. It is known that RhB shows some self-sensitization e®ect due to its excellent visible light absorption ability, phenol was adopted to test the photocatalytic performance of the Si@Ti 3þ -TiO2 samples. For photocatalysis, 200 mL of 20 mg/L phenol containing 100 mg catalyst was exposed to Xe lamp as visible light source. The phenol concentration was monitored by high performance liquid chromatography (HPLC) at predetermined time intervals (6 min). The experimental results of photocatalytic degradation of phenol are shown in Fig. 6(a). The blank test con¯rmed that the phenol degradation was negligible without catalysts. For pure TiO2 nanosheets and Si nanospheres, the
(a)
(b)
Fig. 6. (a) Photocatalytic degradation of phenol over di®erent photocatalysts with time under light irradiation. (b) Photodegradation of RhB over Si@Ti 3þ -TiO2-180 with di®erent scavengers. 1950133-8
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One-Step Synthesis of Si@Ti3+ Self-Doped Heterostructure with Enhanced Photocatalytic Performance
degradation e±ciencies of phenol are only 33% after 75 min of photoreaction. However, it was obvious that nearly 100% of the RhB molecules were degraded by the sample Si@Ti 3þ -TiO2-180 in 20 min, 90% by Si@Ti 3þ -TiO2-150 nanospheres and 84% by Si@Ti 3þ -TiO2-220 nanospheres which indicated an enhanced photocatalytic performance of the Si@Ti 3þ -TiO2 nanospheres. To investigate the mechanism of the photodegradation process, scavengers for electrons (e ) and holes (h þ ) were employed to determine the speci¯c reactive species.43 Si@Ti 3þ -TiO2-180 was selected as the model photocatalysts to deeply understand the degradation behavior of RhB. In the photocatalytic process, the main oxidative species can be detected by adding benzoquinone (BQ, a scavenger of O 2 ), Ammonium oxalate (AO, a scavenger of holes), Tert-butyl alcohol (TBA, a scavenger of OH) and AgNO3 (TBA, a scavenger of e ). As shown in Fig. 6(b), di®erent scavengers have di®erent e®ects on the degradation of RhB over the Si@Ti 3þ -TiO2-180 samples. After BQ and AO were added into the reaction system, the rate for degradation of RhB over Si@Ti 3þ -TiO2-180 was remarkably decreased and only about 19% and 39% of RhB were degraded, respectively. In the presence of TBA and AgNO3, the photodegradation rate was slightly decreased. This evidence clearly indicated that the in°uences of BQ and AO on the photocatalytic activity were more obvious. In other words, O 2 and holes contributed the photodegradation of RhB. The above results con¯rmed that the synergistic e®ect of Ti 3þ self-doped TiO2 and Si e®ectively improved the photocatalytic performance. The schematics of the energy band of Si@Ti 3þ -TiO2 and the possible photocatalytic mechanism are illustrated in Fig. 7. According to the conduction band (CB) and valence band (VB) edge potentials of Si and TiO2 (0.41 eV and 0.71 eV, 0.29 eV and 2.91 eV, respectively), in the Si@Ti 3þ -TiO2 heterojunctions, Si could easily absorb the visible light because of its bandgap (1.12 eV). Once irradiated under Xe arc lamp, the electrons are able to jump from the VB to the CB of Si and then transfer from the CB of Si to that of TiO2. Such electron transition between heterojunctions can reduce the recombination of charge carriers and prolong the charge lifetime. In addition, the Ti 3þ forms a local state at the bottom of the TiO2 CB with visiblelight response capacity. Under Xe arc lamp
Fig. 7. Schematic of the energy band of TiO2 and Si as well as charge migration and separation on Si@Ti 3þ -TiO2 heterojunctions under Xe arc lamp irradiation.
irradiation, the electrons in the VB can jump to the CB of TiO2. Then these excited-state electrons transfer to the TiO2 nanoparticles surface to generate the main active groups of superoxide anion radicals (O 2 ) through reacting with dissolved oxygen, which further oxidize the RhB. In addition, since the VB edge potential of TiO2 is more positive than that of Si, the holes at the VB of TiO2 can transfer to the VB of Si, which could oxidize the organic molecules and simultaneously react with H2O to produce OH radical. These active species attack RhB at the aromatic chromophore leading to the degradation of the RhB structure rather than to de-ethylation. The pollutants are ¯nally degraded to CO2, H2O and other micromolecules.
4. Conclusions In summary, a facile and e®ective method for synthesizing the Si@Ti 3þ -TiO2 core–shell structured photocatalyst was designed and developed. The composite photocatalyst exhibited much enhanced photocatalytic activity than pure Si and TiO2 samples for the degradation of RhB under Xe arc lamp irradiation, which was attributed to the effective separation of electron–hole pairs and extended light absorption range to visible light region due to the formation of heterojunction between Si nanospheres and TiO2 nanosheets as well as the Ti 3þ doping and associated Ov . Therefore, such a Si@Ti 3þ -TiO2 core–shell structured photocatalyst is
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promising for water puri¯cation application and environmental remediation.
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Acknowledgments This work was supported by the National Natural Science Foundation of China [Grant No. 11702268]; the Scienti¯c Research Fund of Education Department of Sichuan Province, China [Grant No. 16ZB0135]; the Open Project of State Key Laboratory of Environment-friendly Energy Materials, China [Grant No. 15zxfk11]; Longshan academic talent research supporting program of Southwest University of Science and Technology, China [Grant Nos. 18LZX560 and 18LZXT10].
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