ARTICLE pubs.acs.org/JPCC
Quenching of Photoactivity in Phthalocyanine Copper(II) -Titanate Nanotube Hybrid Systems W. Alves,† A. O. Ribeiro,† M. V. B. Pinheiro,‡ K. Krambrock,‡ F. El Haber,§ G. Froyer,§ O. Chauvet,§ R. A. Ando,|| F. L. Souza,† and W. A. Alves*,† †
Centro de Ci^encias Naturais e Humanas, Universidade Federal do ABC, CEP 09210-170, Santo Andre, SP, Brazil Departamento de Física, ICEx, UFMG, Avenido Ant^onio Carlos 6627, 31270-901 Belo Horizonte, MG, Brazil § Institut des Materiaux Jean Rouxel (IMN), Universite de Nantes, CNRS, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 03, France Instituto de Química, Universidade de S~ao Paulo, C.P. 26077, 05513-970 S~ao Paulo, SP, Brazil
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‡
ABSTRACT: Titanate nanotubes (TiNTs) were obtained by hydrothermal treatment of anatase powder in aqueous NaOH solution and then modified with 2,9,16,23-tertracarboxyl phthalocyanine copper(II) (CuPc). This hybrid organicinorganic nanoscopic system was characterized by X-ray diffraction, microscopy, and spectroscopy. Transmission electron microscopy (TEM) images of pure and modified TiNTs revealed multiwall structures with an average outer diameter of 9 nm and a length of several hundred nanometers. The tubular morphology of the TiNTs was covered with CuPc-film. The amount of CuPc adsorbed onto the TiNTs was quantified by electron paramagnetic resonance (EPR). Using the same technique and spin-trapping methodology, the photogeneration of reactive oxygen species (ROS) from the TiNTs was systematically investigated. A drastic quenching of photoactivity was observed in the CuPc/TiNT hybrid system. Electron transfer from excited CuPc states to the TiNT conduction band followed by electron recombination may be the cause of this quenching.
’ INTRODUCTION In recent years, inorganic semiconducting one-dimensional nanostructures, such as nanowires, nanorods, and nanotubes, have been actively investigated for renewable energy applications.1,2 In this context, titanate nanotubes (TiNTs) have received significant attention due to their large specific surface area (typically 200 to 300 m2 g1).3 This characteristic of TiNTs may enhance photocatalytic activity and lead to a higher potential for many applications, including sensors,4 photocatalysis devices,5 and heterogeneous catalysis.68 More than a decade after Kasuga9 described a novel hydrothermal method for titanate nanotube formation, the structure and properties of these materials are still not well understood. TiNTs obtained by different researchers can differ significantly in crystal structure and composition, which is attributed to the utilization of different precursors and reaction conditions. Therefore, the major inconsistencies are related to the crystal structure and photocatalytic activity. Recent results obtained by atomic pair distribution function (APDF) analysis suggest that the crystal structure of these nanotubes is similar to the H2Ti3O7type structure model.10 The photocatalytic activity of TiNTs is also unknown. Several papers indicate that TiNTs have potential applications in photocatalysis.11,12 However, the available results are limited and inconsistent. In addition, several works published in recent years have indicated that these nanotubes exhibit low or no r 2011 American Chemical Society
photocatalytic activity.1315 Recently, several reports have suggested that some dyes, such as polychelate porphyrin chromophores16 and phthalocyanine derivates,17 can be used to sensitize TiO2 and TiO2 nanotubes for organic detoxification and to enhance dye-sensitized solar cell efficiencies. Metallophthalocyanines have been studied because of their wide range of applications as catalysts, chemical sensors, optical and electronic devices, organic solar cells,18 and gas sensors.19 Particularly, copper phthalocyanine (CuPc) represent a class of largely investigated compounds that are assumed to improve the spatial separation and enhance the lifetime of photoproduced hole pairs in TiO2, increasing the efficacy of photodegradation of organic contaminations.2026 For example, modified amorphous titania (am-TiO2) with 2,9,16,23-tetracarboxyl phthalocyanine copper (CuPc) has been investigated for the detoxification of methyl orange (MO) under visible irradiation.27 The CuPc/am-TiO2 hybrid photocatalyst exhibits excellent photocatalytic activity for illumination with wavelengths around 550 nm. CuPc has been suggested to react with am-TiO2 by supplying electrons and generating CuPcþ• radicals and active oxygen species (O2•) that react with MO to induce photodegradation. Received: March 4, 2011 Revised: May 20, 2011 Published: May 21, 2011 12082
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In this work, the photophysical properties of pure and CuPcmodified TiNTs were investigated by electron paramagnetic resonance (EPR). With EPR spectroscopy, the following objectives were possible: (i) to determine the adsorption rate of CuPc on TiNTs and (ii) to demonstrate quenching of the photogeneration of reactive oxygen species (ROS) for the hybrid system. In addition, the structural and morphological properties of pure and CuPc-modified TiNTs were characterized by microscopic and spectroscopic techniques.
’ EXPERIMENTAL DETAILS Reagents. Titanium(IV) oxide (99.8% anatase), sodium hydroxide (NaOH, Synth), hydrochloric acid (HCl, Synth), and 2,9,16,23-tertracarboxyl phthalocyanine copper(II) were utilized for the synthesis of TiNTs. Ultrapure water (18 M Ω cm1) was used in all experiments. For EPR/spin-trapping experiments, R-phenyl-N-tert-butyl nitrone (PBN) spin-trap (Sigma-Aldrich) and isopropyl-myristate (IPM) solvents from VETEC were used, following the manufacturer’s instructions. Synthesis and Modification of TiNTs. The TiNTs were synthesized with the following hydrothermal treatment. Anatase powder (1.8 g) was suspended in 60 mL of 10 mol 3 L1 aqueous NaOH solution for 30 min. The white suspension was transferred to a 60 mL Teflon flask and maintained at 125 C for 24 h. The mixture was treated with distilled water and centrifuged at 8000 rpm for 15 min to separate the product from the solution. This procedure was repeated until the pH of the supernatant reached 10. Afterward, 600 mL of 0.1 mol L1 HCl aqueous solution was added and ultrasonically dispersed for 8 h. The materials were repeatedly washed with deionized water until the pH of the supernatant reached 4. Modification of the TiNTs with CuPc was performed at room temperature by mechanically mixing the following weight proportions of TiNTs and CuPc: 1:3, 1:5, 1:7, and 1:10 m/m. After the addition of 10 mL of ultrapure deionized water, the mixture was stirred for 1 h. The resultant product was maintained under agitation for 24 h in the dark. The resulting solid was centrifuged at 8000 rpm, repeatedly washed with deionized water, and monitored by Soret band absorbance. Structural, Morphological, and Physical Characterization. Transmission electron microscopy (TEM) images were obtained using a JEM-2100F (JEOL) transmission electron microscope at the LNLS-Campinas-Brazil. Diluted samples (approximately 10 μm thick) were placed on copper grids and analyzed at 200 kV of tension. Energy dispersive X-ray spectroscopy (EDS) data were collected using a Noran system (Thermo Electron Corporation) attached to the transmission electron microscope. X-ray diffraction (XRD) patterns were recorded using a Rigaku X-ray diffractometer with CuKR radiation (λ = 0.154 nm) in the 2θ range of 1.5 to 70, operating at 30 mA and 40 kV. A scan rate of 1 min1 was employed. X-ray photoelectron spectroscopy (XPS) data were recorded on an ESCALAB 250 spectrometer, and MgKR radiation was used as the X-ray source at the Institute des Materiaux Jean Rouxel (France). The C1s peak of adventitious carbon (284.5 eV) was used as a reference for estimating the binding energy.28 The binding energies were given with an accuracy of (0.2 eV. To deconvolute the various peak components, a Gaussian curve fitting procedure was made after linear background subtraction. Literature data22,2935 were used as a reference for the assignment of the peak components.
Figure 1. TEM images of as-prepared TiNTs: (A) pure and (B) modified with CuPc.
Raman spectra were obtained using a Renishaw InVia Raman spectrometer with excitation at 632.8 nm from a HeNe laser, an objective with 50 magnification (Olympus BTH2 microscope), a numerical aperture of 0.75, and a spot size of 0.9 μm. EPR measurements were performed using a custom-built X-band spectrometer (9.38 GHz) and a commercial cylindrical cavity (Bruker). The microwave Klystron source (Varian) provided approximately 100 mW, which could be attenuated to 60 dB. The magnetic field was produced by an electromagnet (Varian) and an automated current source (Heinzinger). For detection, standard lock-in techniques (EG&G) and a 100 kHz field modulation were employed. The microwave frequency was stabilized to one part in 105 by an automatic frequency control (AFC) using a secondary lock-in at 8 kHz. EPR measurements were performed as a function of microwave power and variable temperatures (from 6 K to room temperature) using a He-flow cryostat and an ITC (Oxford) temperature controller. The magnetic field calibration was carried out using a diphenylpicryl-hydrazyl (DPPH) standard with g = 2.0037. The Cu2þ EPR signal from CuPc was compared with the EPR signal of Cu2þ of a CuSO4 3 5H2O standard to quantify the adsorption of CuPc. The photogeneration of ROS assisted by the TiNTs and TiNT/CuPc was evaluated under UVA (366 nm) and LED (600 nm) illumination by EPR using the spin-trapping technique. Each sample consisted of an isopropyl myristate (IPM) suspension of two compounds: the spin-trap PBN (R-phenyl-Ntert-butyl nitrone) and TiNTs at concentrations of 200 mmol L1 and 2.5 mg/mL, respectively. IPM was chosen as a solvent that is nonpolar and has a high viscosity, which suppresses decantation of TiNTs during the experiment. Two reference suspensions, which contained the same concentration of PBN with either 2.0 mg/mL of pure anatase powder (used for the TiNT synthesis) or CuPc (2.25 mg/mL), were prepared for comparison. All samples were prepared under ambient laboratory light. A fixed volume of 100 μL for these solutions was placed inside borosilicate EPR tubes with a 3 mm inner diameter (Wilmad). Illumination was performed outside the EPR cavity with an UVA lamp (System Eickhorst) at 366 nm (fwhm = ∼4 nm, 5 W) for 130 min and a 600 nm light-emitting diode (LED; fwhm = 20 nm; 50 mW) for up to 60 min under agitation.
’ RESULTS AND DISCUSSION Morphology and X-ray Diffraction. Figure 1A,B shows TEM images of the as-prepared TiNTs and TiNTs modified with CuPc (TiNT/CuPc), respectively. Figure 1A shows that the TiNTs have a tubular morphology and are multiwalled with four shells (see insert in Figure 1A). The average outer (inner) diameter was 12083
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The Journal of Physical Chemistry C 9 nm (3 nm), and the lengths were several hundred nanometers with a uniform diameter distribution. TiNTs obtained after adsorption of CuPc exhibit tubular morphology with diameter slightly higher than pure TiNTs as shown in insert Figure 1B. It can be seen that TiNTs has most of its surface covered with CuPc-film, allowing the tubes to laterally self-assemble through metallo-phthalocyanines adsorbed on the surface of two distinct nanotube if one of them is close enough to the CuPc moiety adsorbed onto an adjacent nanotube. This is in agreement with previously reported work from Ouyang et al.,21 which shows that CuPc molecules could hardly enter the channel of the TiO2 nanotubes with diameter smaller than 45 nm during the deposition process. Figure 2 shows XRD patterns of pure TiNTs (curve A), pure CuPc (curve B), and TiNT/CuPc (curve C). The XRD pattern
Figure 2. X-ray patterns of (A) as-prepared TiNTs, (B) pure CuPc, and (C) TiNTs modified with CuPc.
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of the TiNTs (curve A) is similar to those observed in the literature,36,37 which suggests a crystalline structure of lepidocrocitetype TiNTs with the general formula Na0,54Ti1,86500,135O4 (0: vacancy).15,3841 The X-ray diffractograms are typical of layered materials, which suggest that the nanotubes are multiwalled,3842 which is in agreement with the TEM images. The layered structures give rise to a characteristic (200) reflection in the XRD pattern at small 2θ values of 9.0 (d200 = 0.98 nm),43 which corresponds to the distance between the TiNT wall layers. The common peak at 2θ = 48.4 (d020 = 0.188 nm) can be found in different bulk titanates and is probably linked to Ti separation in the layers of edge-sharing TiO6 octrahedra.44 The other peaks at 2θ ∼ 24.5 and 28.0 correspond to the titanate phase. All observed peaks are in agreement with reported values.45 The XRD pattern for samples of TiNT/CuPc (Figure 2, curve C) shows peaks at 2θ ∼5.5 and 27.06 that are attributed to the CuPc structure with its configuration in the R-form.46 The superposition of the TiNTs and CuPc diffraction peaks at ∼27 does not allow inferring about structural changes in the modified TiNT/CuPc. Moreover, the common peaks at 2θ = 9.0 and 48 may indicate that the structure of TiNTs remained unchanged and that the CuPc molecules are adsorbed at the surface of the TiNTs as shown in insert Figure 1(B). The presence of CuPc on the TiNTs was also confirmed by optical absorption spectroscopy. The spectrum of the nanocomposites shows absorption bands at 448, 528, and 644 nm (data not shown). Raman and X-ray Photoelectron Spectroscopy. The Raman spectrum for the TiNTs is shown in Figure 3A and is characteristic of TiNTs, as discussed in detail by Viana et al.47 The Raman band at approximately 909 cm1 has been assigned to a 4-fold coordinated TiO vibration by Kasuga et al.48 or to a short symmetric TiO stretching mode by Menzel et al.49 The Raman bands at 190 and 667 cm1 have been attributed to TiO vibrations of anatase.48,50 Although the Raman band at 270 cm1 has been recognized as an intrinsic mode related to new titanate phases (NaOTi),50,51 analysis of the Raman scattering data
Figure 3. Raman spectra of (A) as-prepared TiNTs and (B) pure CuPc and TiNTs modified with CuPc. 12084
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Figure 4. XPS spectra for O1s and N1s in CuPc, TiNTs and TiNT/CuPc.
(Figure 3A) in terms of symmetry properties reveals that the intense Raman bands at 270, 450, and 667 cm1 can be assigned to the Ag modes characteristics of TiO2,38 which correspond to pure framework TiOTi vibrations. The peak at 146 cm1 can be attributed to the Eg mode in some nonreacted anatase-phase TiO2 particles among the TiNTs, while the peak at 820 cm1 is assigned to TiO lattice vibrations within TiO6 octahedral host layers.38 Raman measurements by Gao et al. support the suggestion that the protonic titanate is similar to Na0,54Ti1,86500,135O4 (0: vacancy) with an orthorhombic structure representing an acceptable structural basis for the TiNTs.38 Indeed, the TiNTs obtained in this work possess Naþ intercalated in the layered structure, which has been demonstrated by EDS (1.04 keV) and XPS spectra. The Ti/ O ratio obtained from EDS analysis was 0.430, which is consistent with a structure similar to that of lepidocrocite-type sodium titanates.52 The Raman spectra of pure CuPc and TiNT/CuPc
are shown in Figure 3B. Comparison of the spectra shows no significant differences between pure CuPc and CuPc adsorbed on TiNTs. The only minor observed difference is a shift in the band at 1343 cm1 for pure CuPc to 1339 cm1 in the TiNT/CuPc. Considering that this vibrational mode is related to the CC ring stretching/CH bending mode from phthalocyanine, there is most likely a significant interaction between the CuPc and TiNTs, which is probably due to COOH groups from the CuPc compound interacting with the titanate surface. Notably, Raman spectra analysis of the samples with different TiNT/CuPc proportions resulted in the same conclusion, that is, the spectra are very similar to the pure CuPc spectrum, and the only difference is the 4 cm1 shift at 1343 cm1. The samples were also analyzed by XPS to examine the chemical bonds of the elements under investigation. The binding energy in the XPS analysis was calibrated using a C1s standard 12085
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The hyperfine parameters for 63Cu (I = 3/2 and a natural abundance of 69.17%) and 65Cu (I = 3/2 and a natural abundance of 30.16%) and for 14N (I = 1 and a natural abundance of 99.632%) were determined at low temperatures in frozen solutions of H2SO4: A (Cu) = 19.65 20.7 mT, A^ (Cu) = 11.0 mT, A (N) = 1.32 mT, and A^ (N) = 1.6 mT.57 The anisotropic g-factors and hyperfine constants in the polycrystalline samples account for the large linewidths (∼15 mT) and asymmetric lineshapes in the CuPc EPR powder spectra. The hyperfine satellite lines are not well resolved in the polycrystalline samples. However, the line width of the former is larger by about 20% in comparison with the linewidths of the normalized EPR lines for both the CuPc and TiNT/CuPc. This result indicates that the CuPc molecules interact with the TiNTs by a partial transfer of spin density from the CuPc to the TiNT structure. The EPR spectra of four different samples (CuPc and four TiNT/CuPc samples with nominal starting concentration ratios (M0CuPc/M0TiNT) of 3:1, 5:1, 7:1, and 10:1) were measured at 30 K and 30 mW of microwave power to quantify the amount of CuPc attached to the TiNTs. The EPR spectra of the Cu2þ signals were integrated twice and compared to the CuSO4 3 5H2O standard. The mass ratio of MCuPc/MTiNT/CuPc was obtained from these results. Figure 6 shows that with increasing CuPc amounts in the mixture the mass of CuPc attached to the TiNTs increases sublinearly and follows a single exponential saturation. The photophysical properties of the pure and modified TiNTs were investigated by a series of spin-trapping experiments to investigate the photogenerated radicals in air-saturated IPM suspensions at room temperature (300 K) and under illumination. PBN (200 mM) was used as the spin-trap. The samples were illuminated with either an UVA lamp (366 nm and 5 W) or an orange LED (600 nm and 50 mW). In the first series of experiments using the UVA lamp, four samples were studied: a TiO2 (anatase) powder suspension (2 mg/mL), a CuPc solution (2.25 mg/mL), a TiNT suspension (2.5 mg/mL), and a TiNT/ CuPc suspension (2.5 mg/mL) with illumination times of 0 (dark), 1, 3, 5, and 10 min. For the second series of experiments (orange LED), the following samples were investigated: a CuPc solution, a TiNT suspension, and a TiNT/CuPc suspension (all samples at 2.5 mg/mL concentration). The illumination times in this series were extended to 0 (dark), 5, 10, 20, 30, 40, and 60 min because the LED was much less intense than the UVA lamp. )
(285 eV). The doublet spectral line of Ti2p in both samples is characterized by a binding energy of 459.8 ( 0.2 eV (Ti2p3/2) and 465.5 ( 0.2 eV (Ti2p1/2) with a separation energy of 5.7 eV. The spectral lines of both Ti2p3/2 and Ti2p1/2 are very close to the reported values.53,54 Figure 4 shows the XPS spectra for the O1s and N1s signal of the CuPc, TiNTs, and TiNT/CuPc samples. The O1s photoemission signal for the pure TiNTs splits into two peaks. The first peak at 531.0 eV is attributed to oxygen in the titanate phase, and the second peak at 532.6 eV is 1.6 eV higher and assigned to OH groups. This splitting of 1.6 eV is not due to NaOH but is consistent with the existence of NaþOTi bonds.55 The O1s photoemission signal in the CuPc (Figure 4) sample splits into two peaks. The first peak is at 532.8 eV (COH bond due to the carboxyl group), and the second peak is at 534.4 eV (CdO bond). After the intercalation of TiNTs with CuPc, the O1s binding energy is shifted from 532.8 to 531.5 eV (Figure 4). This decrease in binding energy can be attributed to the electronic coupling between the CuPc and TiNTs, which is possibly due to the chemical bonding through ester formation between COOH groups of CuPc that interact with OH groups on the TiNT surface. The carboxylic groups promote electronic coupling between the donor levels of the CuPc and the acceptor levels of the titanate.56 Analysis of the N1s photoemission signal in the CuPc and TiNT/CuPc (Figure 4), indicate the existence of at least two chemically different nitrogen atoms, namely four pyrrolic and four aza-bridging nitrogens atoms in the copper-phthalocyanine. In both spectra, the weak peak at ∼400.8 eV in the N1s photoemission signal was also observed, which can be assigned to the satellite of normal N1s photopeaks. This peak does not arise from a simple photoionization but from the photoionization process accompanied by the simultaneous excitation of a valence electron to the upper vacant energy level.22,2933 The binding energy of Cu2p3/2 for Cu2þ in the CuPc molecule is 935.8 eV. EPR Results and Photophysical Measurements. Figure 5 shows the X-band EPR spectra of 17.7 mg pure CuPc and 3.1 mg TiNT/CuPc measured at 30 K and low microwave power (30 mW) after mass normalization to 1 mg. Under these experimental conditions, no saturation of the EPR signal was observed. The broad EPR line belongs to Cu2þ in the typical R-Cu phthalocyanine, which is surrounded by four nitrogen atoms.57 The spin Hamiltonian parameters of this paramagnetic center are well-known: g = 2.133 and g^ = 2.045 at room temperature and g = 2.200 and g^ = 2.062 in a H2SO4 frozen solution (77 K).57
Figure 6. Mass ratio of CuPc to the total sample mass (MCuPc/ MTiNT/CuPc = 0.21(1), 0.47(2), 0.50(2), 0.52(3)) as a function of the initial mass ratio of CuPc to TiNTs to (M0CuPc/M0TiNT = 3:1, 5:1, 7:1, 10:1) in the mixture. MCuPc/MTiNT/CuPc was determined after double integration of the Cu2þ EPR line and comparison with Cu2þ spectra of the CuSO4 3 5H2O concentration standard.
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Figure 5. EPR spectra of Cu2þ in TiNT/CuPc and CuPc powders, measured at 30 K and 30 mW of microwave power.
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Figure 7. EPR spectra of PBN spin adducts photogenerated after illuminating IPM suspensions of CuPc (2.25 mg/mL), TiNTs (2.5 mg/mL), TiNT/CuPc (2.5 mg/mL), and TiO2 anatase powder (2.0 mg/mL). The latter was used in the TiNT synthesis. All spectra were measured at 300 K and 2 dB of attenuation. The illumination lasted (A) 10 min with a UVA lamp (366 nm and 5 W).
Figure 7 shows some of the PBN spin adduct spectra measured under the same experimental conditions for the longest illumination times with the UVA lamp (Figure 7A, 10 min) and the orange LED (Figure 7B, 60 min). A 14N (I = 1 and 99.632% natural abundance) hyperfine triplet due to the nitroxide groups in the PBN spin adducts is observed in the spectra, which occurs even at very low signal-to-noise ratios. The Cu2þ line of the CuPc is not resolved in the spectra because this signal is broad (∼15 mT) and very weak at room temperature. A primary concern with the photoinduced reactive oxygen species (ROS) in the presence of a photosensitizer is whether the main produced species are the superoxide (O2) or the strongly reactive singlet oxygen molecule (1O2). The first product is due to a charge transfer from the photosensitizer to the oxygen molecule (Type I mechanism), and the second results from resonant energy transfer that excites the oxygen molecule from the triplet ground state (3O2) (Type II mechanism) to the singlet state 1O2. However, the Type I mechanism is expected to be dominant under UVA illumination due to the semiconductor nature of the TiO2 and TiNTs. Light above the band gap creates electronhole pairs, and the electrons are readily transferred to the oxygen molecule, resulting in reduction to an anionic superoxide radical.58 The same process may also occur with light of energy below the band gap for n-type semiconductor nanoparticles, which is the case for pure n-type TiNTs (band gap of 3.45 eV59). From the measured EPR spectra, the Hamiltonian spin parameters of the hyperfine triplets in all samples are determined as g = 2.000 ( 0.002 and AHF (14N) = 1.54 ( 0.02 mT. This hyperfine splitting is due to PBNOH• spin adducts, which can be expected after self-dismutase of superoxide radicals.60 Both OH• and O2•- PBN adducts have been identified by superhyperfine splitting in 17O-enriched systems, that is, AHF (14N) = 1.48 mT for PBNO2•- and AHF (14N) = 1.53 mT for PBNOH•.61 The primary radicals produced for the TiO2 nanoparticle powders and TiNTs are superoxide O2•- radicals (mechanism Type I), which are dismuted into hydroxyl OH• radicals. For the TiO2
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anatase powder, the UVA illumination (∼3.39 eV) is above the band gap. Therefore, the concentration of radicals and spin adducts is enhanced. The expected mechanism for pure CuPc is Type II, that is, singlet oxygen is produced as the primary species for illumination in the visible spectral range.62 CuPc is not as efficient as phtalocyanines with closed shell metals, such as Al3þ and Zn2þ, due to shortened triplet lifetimes.62 However, free-radical derived PBN adducts were formed in the illumination experiments. The PBN adducts were also identified as PBNOH• spin adducts with AHF (14N) = 1.54 ( 0.02 mT. One possible explication is based on a complex mechanism of successive reductions of singlet oxygen ending in OH• radicals. Recently, a reduction of singlet oxygen into superoxide in the presence of an electron donor was observed for molecular photosensitizers, such as tetrazole-modified fullerenes in organic solvents, followed by dismutase into hydroxyl radicals to yield PBNOH• spin adducts.63 Therefore, even if the primary mechanism for the ROS production is Type II for CuPc, the final concentration of OH• radicals and PBNOH• spin adducts should be proportional to the concentration of the primary produced ROS.63 Both pure CuPc and TiNTs generate detectable radicals that most likely result from two distinct mechanisms under UVA and orange illumination, but no PBN spin adducts were formed at detectable concentrations for either wavelength in the TiNT/ CuPc hybrid structure, even at lower concentrations for the TiO2 powder with UVA light. This result was demonstrated by the EPR spectra of the TiNT/CuPc (Figure 7). Phthalocyanines are molecules with highest occupied molecular orbitallowest unoccupied molecular orbital (HOMO LUMO) energies in the visible spectral range. The energy is about 1.8 eV for CuPc.64 Excitation can occur under illumination, and these molecules can transfer part of the excitation energy to the triplet ground state of oxygen molecules to form singlet oxygen, which can be reduced to form superoxide radicals. This proposed mechanism is shown in Figure 8A and is well-known to occur in photosensitizer molecules with excited triplet states with relatively long lifetimes. On the other hand, TiNTs are semiconductors with a band gap located near 3.45 eV.59 Electron hole pairs are created with light above the band gap, and the electrons can be transferred to oxygen molecules forming superoxide radicals at the semiconductor surface. TiNTs are n-type semiconductors, and therefore, the formation of ROS by transfer of conduction band electrons to oxygen molecules may occur even when electrons are excited under the band gap to ionize donors. This process is characteristic of a Type I mechanism for inorganic semiconductor nanoparticles such as TiO2 and a schematic is shown in Figure 8B. As previously discussed, both mechanisms form PBNOH spin adducts that are detectable by EPR. However, the TiNTs modified with CuPc exhibited no photocatalytic activity toward the generation of reactive oxygen species at either 366 or 600 nm. A similar quenching in photocatalytic activity has recently been reported.65 This result suggests that the photocatalytic activity is suppressed, once the recombination of photogenerated electronholes pairs at oxygen vacancies takes place. However, recent ab initio calculations of phthalocyanines (Pc) on inorganic semiconductor substrates have demonstrated that the Pc HOMO levels of these molecules introduce donor levels in the semiconductor band gap, while the LUMO levels are resonant in the conduction band when the semiconductor has a high work function, as is the case for TiO2.66 12087
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Figure 8. Mechanism of photogenerated electron transfer in TiNT/CuPc samples. (A) mechanism type II for pure CuPc; (B) mechanism type I for a semiconductor nanoparticle/nanotube; and (C) quenching of the photoactivity for the TiNT/CuPc hybrid system due to the recombination of carries in deep levels of the TiNTs.
In addition, the HOMOLUMO gap is not significantly altered in the hybrid system for weakly bound molecules as compared to free molecules. These calculations also confirm that effective Pc semiconductor coupling can be achieved through the careful choice of Pc substrate and semiconductor doping. Therefore, Pc semiconductor surface charge transfer is possible in some cases. A high work function is essential, which is present in TiO2 nanoparticles and potentially TiNTs. Recent investigations development by Wang et al.21,67,68 provided the occurrence of the photoinduced charge transfer between organic and inorganic materials, since the large contact area between TiO2 and CuPc facilitated the photoinduced charge transfer from CuPc to TiO2, which together with the directional alignment of TiO2 nanowires was favorable to the following charge separation and transport processes.67 Therefore, the transfer of electrons from the excited states of CuPc to the TiNT conduction band is favorable, since the LUMO level of CuPc is higher than the conduction band of titanate. The electrons in the conduction band of the TiNTs may recombine at centers like oxygen vacancies of the TiNTs and not react with adsorbed molecular oxygen to generate ROS,65 which quenches photoactivity in the hybrid system. This mechanism is shown in Figure 8C, where the excitons generated in the charge generation material (CuPc) upon exposure to light are dissociated into free holes and electrons at the interface between the electron donor (CuPc) and the electron acceptor (TiNT).21 Then the arrow from CB band of TiNT to HOMO level of CuPc represents the electron/hole pair recombination.
’ CONCLUSIONS Multiwall titanate nanotubes (TiNTs) with diameters of about 9 nm and lengths of several tens of nanometers were successfully obtained by hydrothermal treatment of anatase powder in aqueous NaOH solution and then modified with 2,9,16,23tertracarboxyl phthalocyanine copper(II) (CuPc). Structural analysis revealed that the resulting TiNTs are lepidocrocite-type sodium titanates and that the CuPc adsorbs on the walls of the TiNTs. The photocatalytic activity of TiNT/CuPc was investigated by the EPR technique using spin-trapping methodology. Although the CuPc and TiNTs demonstrated photoactivity in the UVA and visible spectral ranges due to Type II and I mechanisms, respectively, the photoactivity of the CuPc-modified TiNTs was quenched. This quenching is the result of electron transfer from the CuP to the TiNTs, which is possibly due to the presence of oxygen vacancies in the TiNTs that function as recombination centers at the semiconductor surface. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ55 11 4996 0193. Fax: þ55 11 4996 3166. E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support from the Brazilian agencies of FAPESP (Grant 08/53576-9), FAPEMIG, and CNPq is gratefully 12088
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The Journal of Physical Chemistry C acknowledged. This work was also supported by INCT in Bioanalytics (FAPESP, Grant 08/57805-2, and CNPq, Grant 573672/2008-3). We are thankful to LME-LNLS (Project TEMMSC 9005 and 11338).
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dx.doi.org/10.1021/jp202101r |J. Phys. Chem. C 2011, 115, 12082–12089
