Photocatalytic Ability of Visible-Light-Responsive TiO2 Nanoparticles

Article pubs.acs.org/JPCC

Photocatalytic Ability of Visible-Light-Responsive TiO2 Nanoparticles Ivana Vukoje,† Tijana Kovač,‡ Jasna Džunuzović,§ Enis Džunuzović,∥ Davor Lončarević,§ S. Phillip Ahrenkiel,⊥ and Jovan M. Nedeljković*,† †

Institute of Nuclear Sciences Vinča, University of Belgrade, P.O. Box 522, 11001 Belgrade, Serbia Innovation Center, Faculty of Technology and Metallurgy and ∥Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, Belgrade 11120, Serbia § Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia ⊥ South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, South Dakota 57701, United States ‡

ABSTRACT: The synthetic procedures for preparation of free-standing and attached to polymer support surfacemodified TiO2 nanoparticles (NPs) with absorption extended into the visible spectral region due to charge transfer complex formation were developed. The one-step synthesis of colloids consisting of surface-modified TiO2 NPs is based on the reaction between titanium(IV) isopropoxide (TTIP) and lauryl galatte in nonprotic organic solvents (tetrahydrofuran, xylol, chloroform). The poly(GMA-co-EGDMA) copolymer decorated with surface-modified TiO2 NPs was obtained in two steps. First, copolymer was functionalized with dopamine and then treated with TTIP in organic solvent at slightly elevated temperature. Thorough microstructural and optical characterization of free-standing and attached to polymer support surface-modified TiO2 NPs was performed involving transmission electron microscopy as well as absorption and reflection spectroscopy. Infrared spectroscopy was used to understand coordination of ligands to surface Ti atoms. Photoredox chemistry of surface-modified TiO2 NPs attached to the polymer support was tested. Enhanced photooxidative ability of composite was demonstrated by degradation of organic dye crystal violet under visible light illumination, i.e., using photons with energy smaller than 2.75 eV. On the other hand, photocatalytic hydrogen production was used to demonstrate photoreduction ability of surfacemodified TiO2 NPs attached to the polymer support.

1. INTRODUCTION The properties of nanocrystals are strongly dependent on size,1−7 shape,8,9 and surface features.10−12 Preparation of nanocrystals with precise control of size and shape often requires quite specific conditions, and it is difficult to adjust the surface properties of the nanocrystal with requirements of a final application. With advances in nanoscience, a new class of composite materials was developed that physically integrates inorganic nanoparticles and organic molecules.13 These hybrid composites exhibit multifunctional properties that are responsible for diverse applications ranging from site-selective catalysis, biosensing, energy transduction, as well as advanced medical therapies. The TiO2 has been extensively used in many industrially relevant processes ranging from environmental applications to clean energy and from cosmetics to paint. Wide use of TiO2 is based on its exceptionally efficient photoactivity, high chemical stability, and low cost. The bulk TiO2 material appears in three major crystal phases: rutile (tetragonal), anatase (tetragonal), and brookite (rhombohedral). Rutile is a high-temperature stable phase with a band gap energy of 3.0 eV. Anatase, which is formed at a lower temperature, with a band gap energy of 3.2 eV, and refractive index of 2.3, is common in fine-grained © XXXX American Chemical Society

(nanoscale) natural and synthetic samples. Due to its large band gap, TiO2 absorbs less than 5% of the available solar light photons. Surface modification of TiO2 with organic and organometallic molecules is largely motivated by the application of these materials in photocatalysis14−16 and photovoltaics.17,18 Recently, a new approach for creating and utilizing bioinorganic composites of nanoscale TiO2 has been extensively investigated for biomedical applications.19 Sensitization of TiO2 crystals and nanoparticles (NPs) with appropriately chosen molecules can indeed induce a significant red shift of their absorption from the UV to the visible, thus making TiO2 functional over a more practical range of the solar spectrum. The key process is the charge injection from the adsorbate to the semiconductor, and two distinct mechanisms classified as Type I (straddling) and Type II (staggered) can be recognized.20,21 In Type I, the electrons from dye molecules are photoexcited to a dye excited state, and then an electron transfer to the semiconductor conduction band occurs. Type II is a direct mechanism in which electron injection occurs in one step from the dye Received: April 28, 2016 Revised: August 2, 2016

A

DOI: 10.1021/acs.jpcc.6b04293 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

perform under visible light irradiation. The simplicity of the synthetic procedures for the preparation of free-standing surface-modified TiO2 NPs is the main advantage of the presented approach. However, preparation of hybrid composites is more sophisticated and can be described as “in-situ, ship-in-a-bottle” synthesis of surface-modified TiO2 NPs within porous media. It should be emphasized that this synthetic approach has general character and is not limited to use of a specific ligand (in this case gallic acid or its derivatives). Thorough microstructural and optical characterization of synthesized materials was performed involving transmission electron microscopy as well as absorption and reflection spectroscopies. Infrared spectroscopy was used to understand coordination of the catecholate type of ligands to surface Ti atoms. Photocatalytic performance of surface-modified TiO2 NPs attached to the polymer support was tested. Photooxidative ability of composite was demonstrated by degradation of organic dye crystal violet under visible light illumination, while hydrogen evolution was used to demonstrate photoreduction reactions. In addition, the photocatalytic performance of synthesized composites was compared to the performance of commercial Degussa P25 TiO2 powder.

ground state with energy located in the semiconductor band gap to the semiconductor conduction band. In last two decades, surface modification of commercial TiO2 (Degussa P25) with benzene derivatives (mainly catechol and salicylic acid) was extensively studied.22−37 However, colloidal TiO2 NPs are unique compared to the bulk because of their larger surface area and the existence of surface sites with distorted coordination. Due to a large curvature of TiO2 particles on the nanosize scale, formation of undercoordinated surface structure with square pyramidal geometry instead of octahedral takes place.38,39 Consequently, surface Ti atoms are very reactive, and their binding to electron-donating ligands simultaneously adjusts their coordination to octahedral geometry and changes the electronic properties of TiO2. In such hybrid structures, localized orbitals of surface-attached ligands are electronically coupled with the delocalized electron levels from the conduction band of a TiO2 semiconductor.40 As a consequence, absorption of light by the charge transfer complex (CTC) yields excitation of electrons from the chelating ligand directly into the conduction band of TiO2 nanocrystallites (Type II). Thus far, the CTC formation accompanied by a red shift of the absorption onset by up to 1.3 eV has been reported for colloidal TiO2 NPs surface modified with either catecholate or salicylate type of ligands38,41−46 and, recently, for dispersions consisting of TiO2 nanorods.47 However, for many applications it is advantageous to use TiO2 in powder form. A few recent papers have investigated CTC formation between either catecholate or salicylate types of ligands with different TiO2 in powder form: submicronic TiO2 spheres,48,49 mesoporous nanopowders,50 commercial Deggusa P25,50 and commercial sodium trititanate (Na2Ti3O7) nanotubes.51 Although the main purpose of extending the absorption spectrum of TiO2 into the red spectral region is use of less energetic photons to drive photoinduced reactions, there is a lack of information concerning photocatalytic performance of surface-modified TiO2 particles. Thus far, hydrogen evolution under visible light illumination of surfacemodified TiO2 NPs by catechol and its derivatives has been demonstrated52 as well as degradation of organic dye crystal violet using commercial Degussa P25 powder modified with catechol.50 Cross-linked macroporous poly(glycidyl methacrylatecoethylene glycol dimethacrylate) (poly(GMA-co-EGDMA)) resins, due to the possibility to control their properties (size, porosity, etc.) and due to the presence of reactive epoxy group, have found a variety of applications, such as sorbents and column packaging in different types of chromatograph,53 enzyme supports,54 in biotechnological and biomedical applications,55 for heavy and precious metal sorption,56 and the sorption of organic compounds.57 On the other hand, relatively little published data concerning poly(GMA-coEGDMA)-supported nanoparticles can be found in the literature. Recently, Au,58 Pd,59 and Ag NPs60,61 were attached to the surface of functionalized poly(GMA-co-EGDMA) in order to achieve their efficient separation, catalytic, and antibacterial properties, respectively. This study is a continuation of our efforts to improve light absorption of TiO2 by extending the absorption spectrum into the red and making TiO2 functional over a more practical range of solar spectrum. We focused on two main points in this investigation: first, in establishing a novel, one-step approach for the synthesis of visible-light absorbing TiO2 NPs and, second, to demonstrate their ability to photocatalytically

2. EXPERIMENTAL SECTION 2.1. One-Step Synthesis of Surface-Modified Colloidal TiO2 NPs. All chemicals used were of the highest purity available and used without further purification (Alfa Aesar, JT Baker). One-step synthesis of surface-modified TiO2 NPs was achieved in the reaction between titanium(IV) isopropoxide (TTIP) and gallates with a different length of the aliphatic part of the molecule (propyl gallate (PG), octyl gallate (OG), and lauryl gallate (LG)). Typically, LG and TTIP were separately dissolved in organic solvent (tetrahydrofuran (THF), xylol, etc.). Then 1 mL of solution containing TTIP was injected into 25 mL of solution containing LG under vigorous stirring at room temperature. The molar ratio between TTIP and LG was in the range from 5:1 to 1:1, while the concentration of TTIP was always 10.0 mM. Formation of surface-modified TiO2 NPs was indicated by immediate appearance of a red color. For comparison reasons the colloid consisting of 45 Å TiO2 NPs was prepared by dropwise addition of titanium(IV) chloride to cooled water, as described elsewhere.62 Briefly, the pH of the solution was between 0 and 1, depending on the TiCl4 concentration. Slow growth of the particles was achieved by using dialysis at 4 °C against water until pH 3.5 was reached. The concentration of the TiO2 colloids was determined from the concentration of the peroxide complex obtained after dissolving the colloid in concentrated H2SO4.63 Phase transfer and surface modification of 45 Å TiO2 NPs from water to organic solvents containing LG was as described elsewhere.64 2.2. Synthesis of Surface-Modified TiO2 NPs on Polymer Support. Macroporous copolymer, based on glycidyl methacrylate (GMA) and ethylene glycol dimethacrylate (EGDMA), was obtained by suspension copolymerization according to the procedure described elsewhere.65 Briefly, distilled water (237.6 g) and 2.4 g of polyvinylpyrrolidone (PVP) were placed into four-necked flask equipped with a mechanical stirrer, a water condenser, and a nitrogen inlet tube. The flask was placed in a water bath previously heated at 70 °C, and the mixture was stirred with a stirring rate of 200 rpm. After complete dissolution of PVP, GMA (24.2 g), EGDMA (10.3 g), AIBN (0.8 g), and 45.6 g of inert component (36.5 g of cyclohexanol and 9.1 g of tetradecanol) were added. B


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The Journal of Physical Chemistry C Polymerization was carried out at 70 °C for 2 h and at 80 °C for 4 h with a constant stirring rate. After completion of the reaction, the precipitated poly(GMA-co-EGDMA) particles were washed with water and ethanol, kept in ethanol overnight, and then dried in a vacuum oven at 40 °C. Size separation was performed by sieves; the fraction with a size of about 30 μm was used in all experiments. Functionalization of the poly(GMA-co-EGDMA) with dopamine was performed according to a similar procedure already described in the literature.54 Typically, 0.512 g of DOPA and 0.1 g of poly(GMA-co-EGDMA) were dispersed in 25 mL of tetrahydrofuran (THF). The mixture was vigorously stirred for 4 h at 50 °C. Finally, when the reaction was over, functionalized poly(GMA-co-EGDMA) particles were separated by centrifugation (4000 rpm for 10 min), washed several times with ethanol, and dried at 40 °C in a vacuum oven for 24 h. For the sake of clarity, functionalized poly(GMA-co-EGDMA) will be named further in the text G-DOPA. Coordination of TiO2 NPs to G-DOPA was achieved by hydrolysis of TTIP in organic solvent at room temperature. Typically, 25 mg of G-DOPA was dispersed in 19 mL of THF, and then a solution consisting of 0.1 mL of TTIP and 1 mL of THF was added drop by drop at room temperature. Appearance of red color characteristic of surface-modified TiO2 NPs was observed after approximately 30 min. The mixture was left overnight under vigorous stirring. Finally, synthesized composite was separated by centrifugation (4000 rpm for 10 min), washed several times with ethanol, and dried at 40 °C in a vacuum oven for 24 h. For the sake of clarity, the obtained composite will be named further in the text G-DOPA/ TiO2. 2.3. Characterization. Absorption spectra of TiO2 colloids were measured using a Thermo Evolution 600 spectrophotometer, while for reflection measurements of G-DOPA/TiO2 composites a spectrophotometer was equipped with a Labsphere RSA-PE-19 accessory. Formation kinetics of surfacemodified TiO2 NPs was measured using a Hi-Tech stopped flow accessory from T2g Scientific. The accessory (20 ms dead time) with a 1 cm path length thermostated quartz cuvette (25 μL) was connected to a spectrophotometer and used for reaction rate measurements. Infrared spectroscopy measurements were carried out using a Thermo Nicolet 6700 FTIR spectrometer at a spectral resolution of 8 cm−1 in the region of 4000−400 cm−1. Elemental analysis (C, H, and N) was carried out using a LECO Elemental Analyzer CHNS-628 Model. The content of the TiO2 in the G-DOPA/TiO2 composites was determined using inductively coupled plasma atomic-emission measurements (ICP-AES Spectroflame 17 instrument). The samples for ICP-AES measurements were prepared by complete dissolution of TiO2 in concentrated HCl. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100 LaB6 instrument operated at 200 kV. TEM images were acquired with a Gatan Orius CCD camera at 2× binning. X-ray diffraction (XRD) powder patterns were recorded using a Rigaku SmartLab instrument under Cu Kα1,2 radiation. The intensity of diffraction was measured with continuous scanning at 2°/min. The data were collected at 0.02° intervals. All XRD analyses were performed by built-in software. 2.4. Photocatalytic Performance of Surface-Modified TiO2 NPs Supported by the Poly(GMA-co-EGDMA). Photocatalytic ability of surface-modified TiO2 NPs supported

by the poly(GMA-co-EGDMA) was studied using degradation of organic dye crystal violet (CV) as well as hydrogen evolution. The photocatalytic oxidation reactions of CV were induced using an Osram Vitalux lamp at 300 W that simulates solar radiation. Also, the photocatalytic ability of red-shifted TiO2 NPs supported by poly(GMA-co-EGDMA) was tested under visible light illumination. For that purpose, a low-energy band-pass 450 nm cutoff filter was used to eliminate photons with energy higher than 2.75 eV. The acidity of the solutions was not adjusted, and pH values were in the range of 6.7−7.0. Initial concentrations of organic day (CV) as well as its decrease during photodegradation reactions were determined by measuring absorption at the peak position (λmax = 590 nm, ε590 = 1.0 × 105 M−1 cm−1).66 For hydrogen evolution experiments, the suspension (50 mg of G-DOPA/TiO2, 2.5 mL of ethanol, and 250 mL of water) was placed in a photoreactor and purged with Ar. The Ar flow was adjusted to be 15 mL/min and used to displace the photocatalytically generated hydrogen from the photoreactor to the GC measuring system. The 100 W mercury-vapor lamp (7825-30 Ace Glass, Inc.) was used for illumination, while hydrogen production was quantified using a PerkinElmer F33 gas chromatograph equipped with a thermal conductivity detector and a 5 Å molecular sieve column. For comparison, photocatalytic hydrogen generation experiments were also performed under identical experimental conditions with commercial TiO2 Degussa P25 powder.

3. RESULTS AND DISCUSSION Immediate appearance of red color upon reactions between TTIP and various derivatives of gallic acid in nonprotic organic solvents indicated formation of surface-modified TiO2 NPs. The kinetics of CTC formation are on the second time scale, and a typical kinetic curve obtained in the reaction between TTIP and LG in chloroform is shown as an inset to Figure 1A. The stability of colloids consisting of surface-modified TiO2 NPs in organic solvents was tested as a function of the length of hydrocarbon tail of various gallic acid derivatives, different ratios between TTIP and modifier molecules, and polarity of solvents. Some general features concerning the stability of synthesized dispersions can be made: (a) organic sols are unstable if gallates with short hydrocarbon tails are used (PG and OG), (b) the stability of dispersions is better when solvents of lower polarities are used (chloroform or xylole in comparison to THF), and (c) the stability of dispersions is better when the ratio of precursor concentrations is equimolar in comparison to excess TTIP. The dispersions of surfacemodified TiO2 NPs obtained in one-step reactions between TTIP and LG are stable for at least 1 week. The absorption spectrum of surface-modified TiO2 NPs, prepared by one-step reaction between TTIP and LG in chloroform, is shown in Figure 1A, curve a. The absorption onset of surface-modified TiO2 NPs (675 nm) is significantly red shifted in comparison to the absorption threshold of unmodified titania with anatase crystal structure (390 nm). The decrease of the effective band gap energy of TiO2 NPs for about 1.35 eV can be assigned to the formation of CTC between electron-donating ligand and coordinately unsaturated Ti atoms at the surface. The extent of red shift is comparable to the extent obtained by modifying the 45 Å TiO2 NPs dispersed in water with gallic acid and pyrogallol42 or after their phase transfer to organic solvents with gallic acid esters.64 Kubelka− Munk transformation of UV−vis diffuse reflection data of C


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Figure 2. High-resolution TEM image of surface-modified TiO2 NPs with LG and corresponding SAED pattern (inset).

45 Å TiO2 NPs prepared by acid hydrolysis of TiCl4, which is followed with improved photocatalytic performance.67 The coordination of ligands to the TiO2 surface was investigated by using FTIR spectroscopy. Since the infrared spectrum of dried TiO2 has only the characteristic broad band in the 3700−2000 cm−1 region68 and peak at 1630 cm−1 originated from adsorbed water (see Figure 3, curve a), it is

Figure 1. (A) Absorption spectrum of colloid consisting of surfacemodified TiO2 NPs with LG (a), and Kubelka−Munk transformation of UV−vis diffuse reflection data of surface-modified TiO2 powder with PG (b). (Inset) Kinetic curve obtained in the reaction between 10.0 m M TTIP and 5.0 mM LG in xylole. (B) Absorption spectra of water colloid consisting of 45 Å TiO2 NPs (a), and surface-modified 45 Å TiO2 NPs with LG dispersed in chloroform (b).

surface-modified TiO2 nanopowder with PG, obtained from unstable organo-sol, is shown in Figure 1A, curve b. A steep rise of absorption occurring in the visible spectral region bellow 675 nm corresponds to the absorption onset of surface-modified TiO2 NPs with LG. The identical optical behavior of surfacemodified TiO2 NPs with various derivatives of gallic acid is easy to understand. The headgroup that participates in the CTC formation is the same for all surfactant molecules, while tails with different lengths do not participate in the CTC formation and only have an influence on the stability of synthesized dispersions. For comparison reasons, phase transfer of the 45 Å TiO2 NPs from water into chloroform containing LG was performed. The phase transfer is quantitative and followed with immediate coloration of the organic phase. The absorption spectrum of organo-sol (Figure 1B, curve b) is identical to the absorption spectrum of surface-modified TiO2 NPs obtained in the reaction between TTIP and LG in chloroform (Figure 1A, curve a), and it is significantly shifted compared to bare TiO2 NPs dispersed in water (Figure 1B, curve a). This result indicates that different pathways of surface modification of TiO2 NPs can induce the same/similar red shift of absorption onset. High-resolution TEM imaging of free-standing surfacemodified TiO2 NPs prepared in one-step reaction between TTIP and LG in xylol revealed nearly spherical TiO2 NPs with a low level of crystallinity and average size of about 3 nm. Neither the selected area electron diffraction (SAED) analysis (see inset to Figure 2) nor the XRD measurements (pattern is not shown) indicated the presence of any crystalline phase. The formation of crystalline particles in nanosize domain usually takes place when reaction is performed at elevated temperature. However, the appearance of crystalline phase can occur after months of aging, like, for example, in the case of the colloidal

Figure 3. FTIR spectra of 45-Å TiO2 NPs (a) as well as LG: (b) free, (c) adsorbed on 45 Å TiO2 NPs, and (d) upon reaction with TTIP in xylol.

possible to measure spectra of modified colloids and to determine the characteristic bands of modifiers. The FTIR spectra of LG, free and adsorbed on 45 Å TiO2 NPs, as well as surface-modified TiO2 NPs prepared in one-step reaction between TTIP and LG in xylol are shown in Figure 3 (curves b, c, and d, respectively). Since the way of binding between surface Ti atoms and gallic acid derivatives with different length of the tail is the same, the FTIR spectra of free and adsorbed PG and OG are not shown. The main bands in the FTIR spectrum of free LG and their assignments33,69 are as follows: stretching vibrations of −OH groups at 3450 and 3348 cm−1, asymmetric and symmetric C− H stretching in the methyl group at 2953 and 2869 cm−1, asymmetric and symmetric C−H stretching vibrations in methylene groups at 2933 and 2850 cm−1, stretching vibrations of the ester carbonyl at 1669 cm−1, stretching vibrations of the D


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The Journal of Physical Chemistry C aromatic ring at 1610, 1533, and 1410 cm−1, aliphatic C−H bending and stretching vibrations of the aromatic ring at 1467 cm−1, the C−O stretching vibrations of the phenolic group at 1379 cm−1, in-plane −OH bending vibrations (shoulder at 1331 cm−1) overlapped with the band at 1305 cm−1 from the C−O stretching vibrations of the phenolic group, (CO)−O stretching vibration from the ester group at 1258 cm−1, in-plane bending vibrations of the phenolic group at 1196 cm−1, and O− C−C stretching vibration from the ester group at 1030 cm−1. The FTIR spectra of surface-modified TiO2 nanopowders, obtained after adsorption of LG onto 45 Å TiO2 NPs (Figure 3, curve c) or in one-step reaction of TTIP with LG in xylol (Figure 3, curve d), have the same features indicating the same coordination of LG to the surface Ti atoms. The bands are assigned to the stretching vibrations of −OH groups (3450 and 3348 cm−1), and the band that belongs to the bending vibrations of phenolic group (1196 cm−1 ) completely disappeared. On the other hand, the intensity of the band at 1304 cm−1, assigned to the C−O stretching vibrations of phenolic group, decreased, but this band did not completely disappear. These results indicate that LG is coordinated to the surface Ti atoms through the adjacent −OH groups forming binuclear (bridging) complex, while the third phenolic −OH group remains unbound/free. The proposed surface structure is in agreement with the literature data.42,64 Except free-standing ones, surface-modified TiO2 NPs were also synthesized on epoxy support, previously functionalized with DOPA. The poly(GMA-co-EGDMA) copolymer is frequently used as a starting material for various kinds of functionalization due to the presence of the reactive epoxy groups and macroporous structure. The specific surface area (36 m2/g) and average pore size (130 nm) of the poly(GMAco-EGDMA) copolymer have been determined in our previous study.61 In order to follow each synthetic step during the preparation of G-DOPA/TiO2 composites, the FTIR measurements of DOPA, poly(GMA-co-EGDMA), G-DOPA, and GDOPA/TiO2 were performed (Figure 4, curves a, b, c and d, respectively). After dopamine modification of poly(GMA-co-EGDMA) copolymer, complete disappearance of the peaks that belong to the epoxy ring (asymmetric vibrations centered at 845 and 910 cm−1) can be observed (compare FTIR spectra b and c in

Figure 4). On the other hand, the appearance of new peaks that originate from DOPA (the FTIR of free DOPA is given in Figure 4, curve a) such as stretching vibrations of −OH groups at 3348 cm−1, stretching vibrations of aromatic ring ν(C−C)/ ν(CC) at 1619, 1598, 1513, and 1468 cm−1, stretching vibrations of the phenolic group ν(C−OH) at 1278, 1254, and 1238 cm−1, bending vibrations of the phenolic group δ(C− OH) at 1361, 1185, 1164, and 1149 cm−1, and aromatic δ(C− H) bending vibrations at 1093 and 1040 cm−1 can be observed. These results indicate that DOPA is quantitatively attached onto poly(GMA-co-EGDMA) support over amino groups, which is in agreement with already published literature data.60,61,70 However, the bands originating from N−H stretching vibrations (over 3000 cm−1) and N−H bending vibrations (at 1618 cm−1) indicate that, except chemically bound DOPA, a certain amount of DOPA is physically adsorbed on the surface of poly(GMA-co-EGDMA). After completion of the reaction between DOPA, either chemisorbed or physically adsorbed onto poly(GMA-coEGDMA) support, and TTIP, indicated with the appearance of red color characteristic of surface-modified TiO2 NPs, the disappearance of the prominent stretching vibrations of −OH groups at 3348 cm−1 in the FTIR spectrum can be noticed (see Figure 4, curve d). This result indicates that the binding of DOPA and synthesized TiO2 NPs is the result of the formation of the CTC via two adjacent phenolic groups. The proposed binding is in accordance with literature data reported for CTC formation between TiO2 NPs and catechol41 as well as gallic acid42 and gallic acid esters.64 The elemental composition of poly(GMA-co-EGDMA), GDOPA, and G-DOPA/TiO2 was determined by elemental analysis and ICP-AES measurements and the obtained results are presented in Table 1. Elemental composition analysis for Table 1. Elemental Composition of Poly(GMA-coEGDMA), G-DOPA, and G-DOPA/TiO2 Determined by the Elemental Analysis (C, H, and N), ICP-AES Measurements (Ti), and Difference (O) content (%) sample

C

H

N

O

Ti

poly(GMA-co-GDMA) G-DOPA G-DOPA/TiO2

58.9 52.7 22.1

8.6 7.4 5.3

0.0 4.9 2.1

32.5 35.0 45.3

0.0 0.0 25.2

poly(GMA-co-GDMA) shows a C/O weight ratio of 1.81, which is in agreement with the calculated value from the initial concentrations of GMA and EGDMA (1.79). Also, despite the fact that FTIR measurements indicated that a certain amount of DOPA is physically adsorbed on the surface of epoxy support, the experimentally determined O/N ratio of 7.1 is the same as the value expected for G-DOPA obtained by quantitative opening of the epoxy ring of poly(GMA-co-GDMA) copolymer by DOPA. The percentage of Ti determined by ICP-AES indicates that the inorganic part of G-DOPA/TiO2 composite is about 42 wt %. On the basis of the FTIR measurements and elemental composition data, the proposed mechanism for insitu synthesis of surface-modified TiO2 NPs attached to polymer support is presented in Scheme 1. TEM measurements of G-DOPA/TiO2 composites at low and high magnification revealed that the polymer support is decorated with a large number of well-separated TiO2 NPs with a size distribution similar to the size distribution of free-

Figure 4. FTIR spectra of (a) DOPA, (b) poly(GMA-co-EGDMA), (c) G-DOPA, and (d) G-DOPA/TiO2. E


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The Journal of Physical Chemistry C Scheme 1. Proposed Reaction Mechanism for the in-Situ Formation of Surface-Modified TiO2 NPs Attached to Polymer Support

Figure 6. Kubelka−Munk transformations of diffuse reflection data for G-DOPA/TiO2 composite; (inset) photograph of G-DOPA/TiO2 composite.

range of the solar spectrum. The spectral features and extent of the red shift of G-DOPA/TiO2 composite are the same/similar to the free-standing surface-modified TiO2 NPs prepared in one-step reaction between TTIP and LG in organic solvents and/or surface-modified TiO2 nanopowders obtained from unstable organo-sols upon the reaction of TTIP with short tail gallic acid esters (PG, OG). The obtained results are in agreement with literature data concerning CTC formation between colloidal TiO2 NPs,41,42,52,64 colloidal TiO2 nanorods,47 TiO2 nanopowders prepared by the sol−gel technique,50 submicrometer TiO2 spheres obtained using nanometer in size TiO2 colloids as a precursor in the aerosol-assisted processing,48,49 commercially available TiO2 powders (Degussa P25,50 Ishihara Co., Ltd.52), and commercial sodium trititanate (Na2Ti3O7) nanotubes51 with catecholate type of ligands. The photooxidative power of G-DOPA/TiO2 composite was tested using the degradation reaction of the organic dye crystal violet. The CV was chosen for this purpose because direct photolysis of dye solution, in the absence of the photocatalyst, induces negligible degradation of dye.71,72 Having in mind that the main purpose of extending the absorption spectrum of TiO2 into the red spectral region is use of less energetic photons to drive photoinduced reactions, the photocatalytic performance of G-DOPA/TiO2 composite was tested under visible light illumination. The low-energy band-pass 450 nm cutoff filter was used to eliminate photons with energy higher than 2.75 eV. The absorption spectra of CV as a function of time, prior to (in dark) and under visible light illumination, are presented in Figure 7. Also, a comparison of kinetic data obtained using a light source that simulates solar radiation with kinetic data obtained by excluding photons with energy higher than 2.75 eV is presented as the inset to Figure 7. The preliminary results clearly indicate that G-DOPA/TiO2 composite is able to photocatalytically perform under visible light illumination. Of course, the kinetics of photocatalytic degradation of CV is faster under illumination of light that mimics the solar spectrum (compare kinetic curves given in inset to Figure 7). The photodegradation reactions of organic pollutants occur after their adsorption on the surface of photocatalysts. It is important to note that the adsorption behavior of this material contributes simultaneously to accelerate the photocatalytic action and to participate for the total removal of dyes. Due to

standing ones (compare Figure 5 with Figure 2). The formation of nanometer in size TiO2 NPs and their homogeneous

Figure 5. TEM images of G-DOPA/TiO2 composite at low (A) and high magnification (B).

distribution through the entire polymer is related to distribution of the aminated GMA chain segment separated from each other by EGDMA blocks. SAED analysis and XRD measurements (results are not shown) did not indicate the presence of any crystalline phase. Kubelka−Munk transformation of diffuse reflection data for G-DOPA/TiO2 composite is shown in Figure 6. The absorption spectrum of G-DOPA/TiO2 composite is shifted into the red, making TiO2 functional over a more practical F


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hydrogen production rates of G-DOPA/TiO2 composite suspended in aqueous solution containing ethanol are shown in Figure 8. The rate of steady state photocatalytic hydrogen

Figure 7. Absorption spectra of CV as a function of time during adsorption onto G-DOPA/TiO2 composite (in dark) and upon illumination with visible light (photons with energy higher than 2.75 eV were eliminated using a cutoff filter). Initial concentration of CV was 1.5 × 10−5 M, while the concentration of G-DOPA/TiO2 was 2 mg/mL. (Inset) Photocatalytic degradation kinetic curves of CV under illumination that simulates solar radiation and under visible light illumination.

Figure 8. Rates of photocatalytic generation of hydrogen as a function of illumination time over G-DOPA/TiO2 composite (a) and TiO2 Degussa P25 powder (b) (50 mg of photocatalyst; 2.5 mL of ethanol; 250 mL of water; medium-pressure Hg lamp (100W)).

production was found to be 450 μmol h−1 g−1. It should be noted that hydrogen production rates presented in Figure 8 are normalized to the mass of G-DOPA/TiO2 composite, not to the mass of TiO2 (about 40 wt %). Also, the photoreduction ability of composite was demonstrated without metal catalyst that facilitates efficient separation and transfer of photogenerated charges. Although the value of steady state photocatalytic hydrogen production of G-DOPA/TiO2 composite is comparable to the reported literature values obtained for commercial photocatalyst,75−77 knowing that the small variations in experimental conditions can considerably change photocatalytic data, additional measurements with the most studied commercial TiO2 photocatalyst (Dugussa P25) were performed. Comparison of photocatalytic hydrogen production rates, obtained under identical experimental conditions, between G-DOPA/ TiO2 composite (curve a) and Degussa P25 TiO2 powder (curve b) are presented in Figure 8. It is clear that the steady state photocatalytic hydrogen production rate of synthesized visible-light-responsive TiO2 samples is about two times larger compared to commercial TiO2 photocatalyst without taking into consideration the fact that the inorganic component of composite is 40 wt %. On the basis of presented results and considering the limited number of recently published data for photocatalytic generation of hydrogen over surface-modified commercial TiO2 powder with catechol and its derivatives,52 most likely the top and bottom of the valence and conduction band, i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the CTC, are located above the valence and conduction bands of the pristine TiO2, respectively. Upon excitation, generated electrons and holes are separated. The photogenerated holes are localized in the organic moiety of the surface complex and react with the hole scavenger, ethanol, generating radicals with sufficient reducing power capable of transferring electrons to the conduction band of TiO2. On the other hand, photogenerated electrons, localized in the inorganic part of the surface complex (conduction band), participate in the reduction of protons to form hydrogen. Although our results clearly indicate that

the macroporous structure of the poly(GMA-co-EGDMA) (specific surface area 36 m2/g and average pore size 130 nm61) and small size of attached surface-modified TiO2 NPs, the GDOPA/TiO2 composite has a large adsorption capacity. The adsorption capacity could be expressed as AC = (C0 − Ce) × V /W

(1)

where AC is the adsorption capacity, C0 (mg/L) and Ce (mg/ L) are the concentration of CV before and after adsorption experiments, V (L) is the solution volume, and W (g) is the dosage of adsorbent. On the basis of the results of the dark adsorption of CV on the G-DOPA/TiO2 composite of about 20% (see inset to Figure 7), the adsorption capacity of CV by the G-DOPA/TiO2 composite was found to be reasonably high (0.92 mg/g). This value is slightly higher compared to the literature data concerning adsorption of organic dyes onto commercial Degussa P25 photocatalyst.72 The energy conversion efficiency from solar to hydrogen by TiO2 photocatalytic water splitting is still low, mainly due to the inability of TiO2 to utilize visible light, as well as due to fast recombination of photogenerated electron/hole pairs and fast backward reaction. While the photocatalytic mechanism with pristine TiO2 is well understood,14−16 there is a lack of information concerning photocatalytic performance of surfacemodified TiO2 particles. Thus far, only Higashimoto et al.52 made an attempt to find relationships between the electronic structures of the CTCs and their photocatalytic activities combining electrochemical measurements and density functional theory (DFT) calculations. The photoredox property of red-shifted G-DOPA/TiO2 composite was tested in the presence of ethanol (1.0 vol %). Ethanol serves as a sacrificial electron donor that can protect the surface modifier (DOPA) from oxidation. Also, ethanol is known as a current doubling agent.73 Thus, the photogenerated holes are transferred to adsorbed ethanol, which is oxidized to the ethanol radical instead of oxidizing DOPA, and the large negative potential (−1.25 V, NHE74) of ethanol radical facilitates electron injection to the conduction band of TiO2. Photocatalytic G


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surface-modified TiO2 NPs attached to the polymer support can perform under visible light illumination, the level of enhancement of redox chemistry by taking advantage of electron promotion from the ground state of CTC, i.e., donor levels of small colorless organic ligands into the conduction band of TiO2, still needs to be addressed.

4. CONCLUSION We presented a procedure for preparation of colloidal and/or supported surface-modified TiO2 NPs. The method is based on the hydrolysis of titanium(IV) isopropoxide induced by gallates or dopamine in nonprotic organic solvents and provides TiO2 NPs with absorption extended in the visible spectral region. The main advantage of the presented approach is simplicity. Also, to the best of our knowledge, the methodology for preparation of decorated polymers with surface-modified TiO2 NPs has never been reported in the literature. The size distribution, composition, optical properties, and way of binding of ligand molecules to surface Ti atoms was thoroughly investigated. However, special attention was paid to the photocatalytic ability of synthesized composite. The degradation of organic dye crystal violet indicated that red-shifted TiO2 attached to polymer support can photocatalytically perform under illumination with photons whose energy is smaller than 2.75 eV. Also, photocatalytic hydrogen production was achieved at a higher rate compared to the most studied commercial TiO2 photocatalyst (Degussa P25). It is clear that presented results are preliminary, but it is our belief that development of new materials whose optical properties can be tuned opens up the possibility to extend not only fundamental but, also applied aspects of this type of research.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +381118066428. Fax: +381113408607. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express our gratitude to Dr. Milovan Stoiljković for performing ICP-AES measurements. Financial support for this study was granted by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project 45020).

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REFERENCES

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Photocatalytic Ability of Visible-Light-Responsive TiO2 Nanoparticles

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