Article pubs.acs.org/JPCC
Bio-Template Mediated In Situ Phosphate Transfer to Hierarchically Porous TiO2 with Localized Phosphate Distribution and Enhanced Photoactivities Tao He,*,† Yonggen Weng,† Peng Yu,† Chuanlin Liu,‡ Haiqin Lu,§ Yuanping Sun,∥ Shangzhou Zhang,⊥ Xin Yang,† and Guoqun Liu# †
College of Chemistry and Chemical Engineering, ‡College of Life Science, ∥College of Optoelectronic Information Science and Technology, and ⊥School of Environment and Materials Engineering, Yantai University, China § Medical School, Southeast University, China # School of Materials and Chemical Engineering, Zhongyuan University of Technology, China S Supporting Information *
ABSTRACT: A localized phosphate distribution (LPD) was introduced for the first time into a porous TiO2 nanostructure by using a biotemplate synthetic strategy, that is, Staphylococcus aureus (S. aureus)-assisted in situ phosphate transfer. The resulting novel nanostructures have shown remarkable enhancement of photoactivities for both selective dye degradation and photoelectrochemical water reduction. Mechanistic understanding reveals that improved separation, directional transport, and less limited interface transfer of the photogenerated electron and hole may be achieved simultaneously within the LPD-modified TiO2 nanostructures because of the existence of the confined negative surface electrostatic field (NSEF) and the spatially oriented upward band bending (UBB). On the contrary, a homogeneous phosphate distribution (HPD) will greatly increase electron interface transfer resistance, which will cause the increase of recombination in bulk. The most important inspiration we can obtain herein is that a comprehensive consideration of the influence of nanostructure on all of the critical aspects of the carrier’s dynamics is needed during the rational design and construction of the advanced nanostructured photocatalyst systems. Considering the available resources for the synthesis and strong covalent interaction of phosphate with many other transition metal cations, the authors think that the novel strategy for a simultaneous optimization of the dynamic processes of the charge pairs by introducing LPD is promising for several applications including photocatalysis, photoelectrochemical hydrogen production, and solar cell.
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INTRODUCTION
It has been reported that the photocatalytic activity of TiO2 nanoparticles can be enhanced to some extent when an appropriate number of phosphate ions are adsorbed to their surface.10−15 This enhancement has been recently attributed to the improved separation of the charge pairs due to the existence of NSEF.14,15 However, the photoactivities decrease greatly when too many phosphate anions are adsorbed,10−13 and the main reason to be revealed in this work is the increase of interfacial electron transfer resistance. This implies that the influence of phosphate modification on the dynamic process of the charge pairs is actually like a double-edged sword. Thus, we hypothesize that we can design and construct a novel nanostructure of phosphate-modified TiO2 to maximize the benefit on separation and to minimize the disadvantage of increased interface electron transfer resistance at the same time.
Currently, nanostructured semiconductors are attracting multidisciplinary research areas due to their promising potential applications in photovoltaic cells, photocatalysis, and environmental remediation.1−9 It has been demonstrated that the rational design and construction of novel nanostructures can efficiently optimize the dynamics of photogenerated electron and hole, and consequently can result in improved performance. For example, by constructing heterojunction nanostructures,1,2 faceting nanocrystallites,3−5 improving interparticle connections,6 employing one-dimensional nanostructured semiconductors,7,8 loading noble metal nanoparticles,9 and so on, the separation,1−5 transport,6−8 or interface charge transfer behaviors9 can be improved, respectively. To date, these optimizations are usually focused on a certain aspect of the dynamics. Since the photoactivity is obviously coinfluenced by all the critical aspects of the dynamics, it is highly necessary to take into account all of them during the preparation of semiconductor nanostructures. © 2014 American Chemical Society
Received: October 30, 2013 Revised: January 18, 2014 Published: February 18, 2014 4607
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Figure 1. Schematic diagrams of (a) a homogeneous distribution of negatively charged phosphate groups (gray circle with horizontal white line through middle) in the surface of a TiO2 particle (gray sphere); (b) an upward band bending within n-type anatase TiO2 (gray square) due to adsorbing anionic phosphate groups; (c) a localized phosphate distribution (phosphate groups are localized only within each of the isolated pores (dark sphere) of a porous TiO2 (gray square)).
effective way to improve photoactivity by constructing LPDmodified nanostructures, and the way is meaningful for materials/device design and fabrication in photovoltaic cell, photocatalysis, and environmental remediation.
Herein, we propose that an LPD-modified porous TiO2 may be such one of the desired nanostructures. The conventional phosphate-modification methods, typically such as sol−gel and impregnation, can only make homogeneous phosphate distribution on TiO2 surface.10−15 For n-type anatase TiO2, the HPD will cause uniform NSEF14,15 and UBB will point outward16 (Figure 1a,b). Both the NSEF and UBB will uplift the barrier of electron interface transfer through whatever place around the surface (Figure 1a,b). Alternatively, if a porous TiO2 nanostructure where phosphate groups are localized only within isolated pores can be constructed, NSEF will form within each of the pores alone and UBB will be spatially oriented pointing toward the pores (Figure 1b,c). Under this situation, holes will move directionally toward the pores and electrons will drift away from them (in other words, moving directionally toward the pure TiO2 domains). Therefore, an LPD-modified porous TiO2 not only improves the separation and directional transport but also avoids great increase of interface transfer resistance of electrons. In order to achieve such a goal, we have developed a biotemplate process to prepare the LDP-modified porous TiO2 nanostructure. Staphylococcus aureus (S. aureus), a gram-positive bacterium, is selected as the biotemplate. S. aureus is very stable and abundant, with phosphate residues on its wall.17,18 And it has a strong ability to induce a fast interparticle aggregation in a dialyzed TiO2 colloidal solution. The aggregation of dialyzed TiO2 colloidal particles and the elimination of S. aureus bacteria give disordered meso- and macropores in the final products. Although biotemplate routes have been extensively applied for the preparation of inorganic nanostructures,19−21 the novelty of this synthetic process lies in that an in situ transfer of phosphate at the biotemplate/inorganic interface causes a fixed-point introduction of anionic phosphate to the macropore domains. As being demonstrated in this article, the photoactivities of the novel TiO2 nanostructure for both the selective photocatalytic oxidation of cationic organics and the photoelectrochemical hydrogen production are remarkably improved. We have investigated herein the formation mechanism of the LPD-modified nanostructure. The existences of LPD and confined NSEF have been proven by combining selective dye adsorption experiments, FE-SEM and selected-area element analysis with energy dispersive X-ray microanalyzer. FT-IR, solid state 31P MAS NMR spectra, and Ti K-edge XANES are used to characterize the covalent connection of P−O−Ti. UV− visible diffusion reflectance spectra, photoluminescence (PL) spectra, and a series of photoelectrochemical characterizations are applied to reveal the influence of LPD on the dynamic processes of the charge pairs. This article provides a novel and
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EXPERIMENTAL SECTION Preparation of Porous TiO2 with Localized Phosphate Distribution. TiO2 colloidal solution was prepared by using dialysis (see Supporting Information, SI1, for details). The procedure for S. aureus culturing and quantification is carried out according to the methods described in ref 22 and also see SI2 for details. The synthetic process is briefly described as follows (see SI3 for more details): The dialyzed colloidal solution is mixed with S. aureus dispersion to induce a fast interparticle aggregation; the aggregates are separated by centrifugation and then transformed to the final products (0.6 g TiO2) after having been processed at high temperature. During the synthesis, 0, 1, 4, 12, and 48 × 1011 S. aureus bacteria are used, respectively, and the corresponding products are named as S0, S1, S2, S3, and S4, respectively. Characterization. The nature of the prepared samples (especially for S0, S1, S2, and S3) is similar to that of hard ceramic. Powder-like samples with an average particle size of about 1−10 μm (as determined by optical microscope observing) are obtained by grinding in agate mortar. These powder-like samples obtained are used for the following experiments. XRD patterns are obtained with a Shimadzu-6100 diffractometer with a Cu Kα radiation. Scanning electron micrographs (SEM) are obtained with HITACHI S-4800 FE-SEM, selectedarea element analyses are carried out with EX-350 energy dispersive X-ray microanalyzer (HORIBAEX-350). Nitrogen adsorption−desorption isotherms are obtained with a Quantachrome Instruments NOVA 3000e at 77 K. FT-IR spectra are obtained with a Nicolet 5DX FT-IR instrument by using a KBr pellet technique. Size distributions of the TiO2 colloidal particles in the dialysate are measured based on dynamic light scattering (DLS) with PSS Nicomp 380 ZLS. UV−visible diffusion reflectance spectra are obtained with a Hitachi U-4100 UV/vis spectrophotometer equipped with an integrating sphere. Element analyses are carried out with an inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS Intrepid II XSP Thermo Electron). The dissolving TiO2 is carried out according to ref 11 (also see SI4 for details). The solid-state 31P MAS NMR spectra are obtained on a Bruker Avance III 400 spectrometer (Bruker BioSpin, Rheinstetten, Germany) with a resonance frequency of 161.9 MHz for 31P (see SI5 for details). Ti K-edge XANES measurements are 4608
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Figure 2. SEM images of (a) S1, (b) S2, (c) S3, and (d) S4.
Figure 3. (a) N2 adsorption−desorption isotherms of S0 and S4, and also see Figure s3 for the isotherms of S1−S3 and Table s2 for specific surface area, pore diameter, and pore volume of S0−S4; (b) XRD spectra of S0−S4, with inset showing the crystallite sizes.
electrochemical analyzer (CHI660D) with a standard threeelectrode mode and 1 M KNO3 (pH = 7.0) solution as the supporting electrolyte. The working electrodes are prepared according to the doctor blade method which is modified in order to mold the powder-like samples into continuous film on FTO glass (SI9), and the thickness of the final TiO2 film is controlled with adhesive tape to about 30 μm based on SEM characterization. A Pt sheet serves as the counter-electrode and an Ag/AgCl (saturated KCl) electrode (218; Shanghai Leici Inc.) is used as the reference electrode. A LED lamp (LHFC084−10, 365 nm, 0.6 W/cm2, Shenzhen Lamplic Science Co., Ltd.) connected to a controller (UVEC-4II, Shenzhen Lamplic Science Co., Ltd.) is used as the light source. Photoelectrochemical water reduction is carried out with the same 365 nm LED lamp as the light source. A solution of 10 vol% ethanol in 1 M KNO3 is used as both sacrificial agent and
carried out at the beamline 4B7A of Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (see SI6 for details). For photoluminescence (PL) measurements, the samples are excited by a 325 nm He−Cd laser (Melles Griot Inc., 45-MRM801-230) and the emission is detected by a monochromator (Acton Research, Spectropro-2500i) with PMT detector (Hamamatsu, PD471). The spectra are recorded in air at room temperature. Adsorption and Photocatalytic Dye Degradation. Five anionic and five cationic organic dyes are used for adsorption experiments, respectively (see SI7 for details). Photocatalytic oxidations of different dye solutions including each of or both of the cationic dyes and the anionic dyes are carried out. A detailed experimental procedure is described in SI8. Photoelectrochemical Measurement and H2 Production. Photoelectrochemical tests are carried out by using a CHI 4609
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supporting electrolyte and is bubbled with N2 to remove dissolved O2 before the reaction.
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RESULTS AND DISCUSSION Porous TiO2 Nanostructure. SEM images show that macropores with uniform size of about 500 nm are highly dispersed within the monolith-like products which are formed through aggregation of primary nanoparticles, and the number density of the macropore increases from S1 to S4 (Figure 2a− d). Sample S4 is full of the macropores by means of being constructed with only a single layer of aggregated nanoparticles (Figure s1). In addition to the macropores, the aggregation of primary nanoparticles gives disordered mesopores (Figure s2). A further detailed SEM characterization reveals that the macropore and mesopore are wholly interconnected (Figure s2). The porosity of the samples is further characterized by nitrogen sorption measurements (Figures 3a and s3). The isotherm of S0 exhibits a hysteresis loop at a relative pressure range of 0.7−0.9, indicating the presence of mesopores (type IV).23 Besides the hysteresis loop, the isotherms of S1−S4 show a quite steep step at relative pressures very close to unity, which is an indicative of capillary condensation and evaporation of N2 in the macropore network (Figures 3a and s3).24 This kind of isotherms has been reported for several hierarchically porous materials and reveals that the entire hierarchical pore system is interconnected within the monolith, which is consistent with SEM characterization.24−26 The interconnected hierarchical porosity is very attractive, guaranteeing a less limited mass diffusion throughout the whole nanostructure. XRD patterns (Figure 3b) indicate that S0−S4 are pure anatase TiO2 (PDF 21−1272). The calculated crystallite sizes based on Scherrer equation27 decrease sequentially from S0 (24.4 nm) to S4 (6.7 nm). Localized Phosphate Distribution and Confined Negative Surface Electrostatic Field. Energy dispersive spectrometer (EDS) spectra reveal the presence of elements Ti, O, P, K, and a trace amount of Na in the porous TiO2. P, K, and Na are from S. aureus. ICP-AES analysis results indicate that the molar ratios of Ti/P in S1−S4 are 816.4, 187.8, 60.6, and 14.4, respectively, and the molar ratio of K/P in S4 is about 0.4. Selected-area EDS characterizations applied to S1 indicate that phosphorus can only be detected in the macropores (typically shown in Figure 4, and also see SI, Figure s4, for details). Moreover, the molar ratio of Ti/P in the domain of macropores is 119.8 and is much lesser than the average value of 816.4 given by ICP-AES analysis. The above characterizations confirm that a fixed-point introduction of phosphate to the walls of macropores has been successfully achieved during the preparation. FT-IR spectra of the porous TiO2 (Figure 5a) show broad absorptions in the range of 1140−970 cm−1, which is typical for covalently bonded phosphates in TiO2 surface.10−15 The characteristic absorption of adsorbed water molecules at about 1630 cm−1 appears only in the spectra of S4, which is consistent with previous report12 that phosphate-modified TiO2 can improve adsorption ability to water molecules. The 31P MAS NMR spectra of S4 give a broad signal with chemical shift between 10 and 25 ppm (Figure 5b), which is typical for a wide distribution of phosphorus in different microenvironments and has been assigned to a mixture of tetrahedral P environments with a connectivity of 3 and 4 (the number of Ti atoms bonded to one PO4 unit).28,29
Figure 4. EDS spectra acquired at selected areas marked with gray squares in the inset SEM images of S1 (see SI, Figure s4, for details).
Herein, adsorption experiments by using both cationic and anionic dyes as probe molecules are carried out to explore the surface charge properties of the samples. The adsorption abilities of S0 and S4 are comparatively shown in Figure 6a. For S0, all five cationic dyes can be adsorbed to some extent and three anionic dyes (cresol red, methyl orange, and indigo carmine) exhibit no or weak adsorption, indicating that S0 is probably negatively charged in these dye solutions. The pH(PZC) (point of zero charge) of TiO2 nanoparticles is known to be around 5.5.30 While the pH values of most dye solutions used for the adsorption experiments are close to neutral (higher than the pH(pzc) of TiO2), which causes the TiO2 surface negatively charged. However, it should be pointed out that the pH-induced surface charging of TiO2 in the diluted dye solutions is weak, which can be confirmed by the fact that S0 show relatively weaker adsorption to the five cationic dyes when compared with S4 (see Results and Discussion in the following text). In addition, among the five anionic dyes, XOS has four carboxyl groups and congo red has two amine groups in their molecule structure. Both groups have strong coordination ability to transition metal cations, which explains much stronger adsorption of XOS and congo red in S0 and XOS in S4 (Figure 6a). Compared with S0, S4 presents much more pronounced adsorption preference to the five cationic dyes (Figure 6a). Moreover, the amount of adsorbed MB increases from S0 to S4 (Figure 6b). It has been proposed that phosphate modification can introduce NSEF.14,15 Chemically adsorbing anionic phosphate groups at TiO2 surface may result in much stronger surface charging (compared with S0). However, the specific surface area is an important factor to materials’ adsorption behavior and the total specific surface area increases successively from S0 to S4, so further experiment works need to be done to demonstrate the cause of the adsorption preference. By using the method introduced in ref 31, 0.2 g S4 powder is dispersed in 200 mL of 0.1 M ammonia solution at room temperature overnight, ICP-AES analysis indicates that about 50 mol % phosphate ions are eliminated in S4. N2 4610
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Figure 5. (a) FT-IR spectra of S4 and S0; (b) 31P MAS NMR spectra of S4. Peaks marked with * indicate the spinning sidebands.
Figure 6. (a) Column plots comparatively illustrate the adsorption preferences of S4 and S0 (0.1 g) to various dyes. Ce is the equilibrium concentration, the dashed line indicates the initial concentration C0 (2.5 × 10−5 mol·L−1) of the dye solutions (160.0 mL) and the values given in the columns are the measured pH of the dye solutions; (b) plots of C/C0 along with time exhibit adsorption preferences of S0−S4 (0.1 g) to MB (2.5 × 10−5 mol·L−1, 160.0 mL); (c, d) SEM images of S1 monoliths where MB or Ag+ are preadsorbed, selected-area EDS analysis are carried out in the places marked with squares and molar ratio of P/Ti, S/Ti, and Ag/Ti are given (see Figure s5 for the characterizations in details). Note that sulfur is in the cationic part of the MB molecule (C16H18ClN3S). “(−)” and “(+)” refer to anionic and cationic dye, respectively. XOS is xylenol orange sodium.
adsorption preference to cationic dyes is also improved successively. The localized phosphate distributions make us suspect that the NSEF may come from the macropores. In order to confirm the distribution of the NSEF, selected-area EDS analyses are applied to the porous monoliths which have been preadsorbed, respectively, with MB and Ag+, and the analysis reveals that the adsorbed MB and Ag+ can only be detected within the macropores (Figure 6c,d and also see Figure s5 for characterizations in detail). The results above definitely prove a localized distribution of phosphate and confined NSEF within the macropores. Formation Mechanism. Spherical S. aureus with uniform size of about 800 nm are highly dispersed in DI water (Figure
adsorption−desorption measurements confirm that the specific surface area does not decrease after the phosphate groups are removed. While dye adsorption experiments reveal that the elimination of phosphate groups results in nearly a halfreduction of the adsorbed amount of MB (Figure s6). On the contrary, S0 does not show any decrease of the adsorption to MB molecules after subjecting to the same ammonia treatment (Figure s6). The experiments above prove that it is the phosphate-modified negatively charged surface (i.e., NSEF) rather than pure TiO2 surface that is responsible for the improved adsorption preference of the LPD-modified TiO2 samples. The successive increase of the number of the bacteria used during preparation of S1−S4 results in a successive increase of the phosphate-modified surface area. As a result, the 4611
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Figure 7. Schematic diagram illustrating the formation mechanism of the LPD-modified porous TiO2 nanostructure.
Figure 8. (a) Plots of ln(C/C0) along with reaction time during photocatalytic oxidation of MB; (b) Linear sweep voltametry curves measured in a mixed solution (30 mL of 1.0 M KNO3 and 3 mL of absolute ethanol) under 365 nm UV light irradiation (0.6 W/cm−2), with an inset showing the onset potentials of the photocurrents.
TAs include wall teichoic acid (WTA) and lipoteichoic acid (LTA), and take up 60% of the total cell wall mass.17,18 The negatively charged WTA is located on the outer surface of the bacteria and is a great long biopolymer whose main chain is composed of glycerol- or ribitol-phosphate repeats, with n equal to 20−40 (Figure 7).17,18 On the other hand, S. aureus has a much thicker peptidoglycan (PGN) layers constructing the cell wall with much higher chemical and mechanical stability.18,34 Other phosphate-containing biomacromolecules such as phospholipids and DNA are inside of the bacteria, separated by the thick PGN layers and far away from TiO2 aggregates (please see Figure 7). It is reasonable to think that the long WTA chains are the first to be exposed to and then directly contacted with TiO2 colloidal particles and finally buried within their aggregates (please see Figure 7). Therefore, TAs are probably the main source of phosphate. In addition, the fast aggregation and moderate pH 5−6 of the S. aureus/TiO2 mixture reduce the probability of the hydrolysis of TAs; otherwise, phosphate will be released into solution to cause a homogeneous distribution. Phosphorus is not detected in the supernatant of precipitates upon ICP-AES analysis, which indicates that TAs are chemically stable under this condition. To further confirm this, FT-IR spectroscopy, a reliable technique for characterizations of bacterial composition,35,36 is applied to track the synthetic process, definitely proving that no premature hydrolysis of TAs occurs (Figure s9 and Table s3). Therefore, spherical S. aureus serve as a template of the macropores, and phosphate anions formed during thermal
s7a). The bacterial surface is abundant with anionic phosphaterich teichoic acids (TAs) which create a “continuum of negative charge” around the bacteria (Figure 7).17,18 Potentiometric titration curve reveals that the action of S. aureus dispersion is similar to adding acid into a buffer (Figure s7c). At a final pH of 3.5 under the measurement conditions, one S. aureus is able to consume 1.6 × 1010 H3+O ions (Figure s7c). On the other hand, hydrolysis of titanium tetrabutoxide in acidic conditions (pH is 0.3) gives a stable TiO2 sol solution. Dialysis provides a method of effectively eliminating excess H3+O without causing precipitation or introducing impurities. Consequently, a transparent and light-blue TiO2 colloid solution is obtained when the pH increases slowly to 3.5. Consistent with former reports, we find that the acidity is crucial to the stability of the aqueous colloidal systems.32,33 DLS measurements reveal that the average size of the colloidal particles increases from 18.8 to 92.0 nm as the pH of the solution slowly increases from 0.7 to 3.0 along dialysis (Figure s8). The pH is known to influence the surface charge density of metal oxide colloidal particles, which further determines the thermodynamic stability.32,33 Therefore, when the dialyzed colloidal solution is added into S. aureus dispersion, the H3+O ions are immediately and massively consumed. As a result, the TiO2 colloidal particles lose their stability and interparticle aggregation will occur. The negatively charged surface of S. aureus probably acts as nucleation site, which consequently leads to the formation of large TiO2 aggregation within which the bacteria are highly dispersed (Figure s7b). 4612
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Figure 9. (a) UV−visible reflectance spectra (the inset is an enlarged part) and (b) photoluminescence (PL) spectra of the electrode films of S0−S4.
Figure 10. Mott−Schottky plots of S0−S4 (a−e) collected at a frequency of 1 kHz in 1 M KNO3.
Influence of the Phosphate Distribution on the Photocatalytic Activity. Absorption Behavior. UV−visible diffusion reflectance spectra show that the absorption abilities to UV light (
