Unraveling the Photocatalytic Activity of Multiwalled Hydrogen

ARTICLE pubs.acs.org/JPCC

Unraveling the Photocatalytic Activity of Multiwalled Hydrogen Trititanate and Mixed-Phase Anatase/Trititanate Nanotubes: A Combined Catalytic and EPR Study Stefan Ribbens,† Ignacio Caretti,‡,§ Evi Beyers,† Sepideh Zamani,‡ Evi Vinck,‡ Sabine Van Doorslaer,*,‡ and Pegie Cool*,† †

Laboratory of Adsorption and Catalysis, University of Antwerpen (UA), Universiteitsplein 1, B-2610 Wilrijk, Belgium Department of Physics, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium § Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Campus de Cantoblanco, 28049 Madrid, Spain ‡

bS Supporting Information ABSTRACT: A detailed study of the photocatalytic activity of hydrogen trititanate nanotubes (H-TNT), formed by a hydrothermal treatment, was carried out. H-TNT show a limited activity toward pyridinium chloride degradation under UV-light and even no activity under visible light. In contrast, H-TNT show activity toward the degradation of rhodamine 6G (R6G), both under UV and visible light. EPR spectroscopy is used to gain insight into this difference. UV-light excitation of H-TNT leads to the predominant formation of Ti3þ centers by trapping of electrons at Ti sites, whereas almost no reactive oxygen-based species are formed. Upon visible light excitation of these nanotubes, no light-induced EPR signals are observed. The activity toward R6G degradation thus stems from the excitation of R6G (under both UV and visible light) and the subsequent transfer of electrons into the conduction band of TiO2. After a short calcination process at 623 K, the H-TNT undergo a partial phase transformation into anatase, without affecting the shape and morphology of the nanotubes, and the photocatalytic activity increases to a great extent. The EPR analysis now reveals the formation of different types of species characterized by g values larger than ge, both upon UV and visible light excitation. These reactive species, such as O2- and O-, are known to play an important role in the photocatalytic process.

’ INTRODUCTION Heterogeneous titanium-dioxide-based photocatalysts are very promising materials for alternative water and air treatments since the photoinduced “advanced oxidation processes” can lead to a complete mineralization of most pollutants emitted by industrial and domestic activities.1,2 Therefore, a lot of synthesis procedures have already been developed to prepare a cheap, stable, mesoporous material with high surface area and excellent photocatalytic properties.3,4 However, in 1998, Kasuga et al.5 discovered the alkaline hydrothermal route for the synthesis of titaniumoxide nanotubes, which allows a complete conversion of an initial raw TiO2 powder to titanate nanotubes6 at relatively low hydrothermal temperatures. In the beginning, most studies focused on the understanding of the formation mechanism7,8 of these nanotubes, the improvement of the synthesis method,9,10 and the electrochemical properties.11,12 In this way, the potential of the titanate-phase nanotubes toward fuel cell technology, supercapacitors, and lithium batteries was investigated. Nowadays, more researchers focus on the photocatalytic properties of the nanotubes.9,13a Whereas nanoparticle-based photocatalysts are hard to remove out of solution, which can have adverse effects on algae, fish, and higher organisms,13b titanium-oxide nanotubes can be easily recovered from a solution by sedimentation. Despite the fact that titanate nanotubes are very promising as cheap r 2011 American Chemical Society

photocatalysts toward the photocatalytic degradation of dyes, and allow a facile scaling-up of production, a detailed study of the photoinduced activity of these Ti-based nanotubes is still awaited. In this work, the photocatalytic activity of trititanate and mixed-phase (trititanate/anatase) nanotubes was studied by evaluating the photocatalytic degradation of rhodamine 6G (R6G) and pyridinium chloride (PyCl) over long time intervals using optical absorption measurements and microvolume total organic carbon analysis (TOC). Furthermore, using X-band light-induced electron paramagnetic resonance (EPR), the origin of the created paramagnetic centers as well as their time evolution under illumination is unraveled. In this way, more details concerning charge separation, charge-trapping mechanisms, and the resulting photocatalytic activity are obtained.

’ EXPERIMENTAL SECTION Chemical Reagents. All the products were used as received without any modification or purification, unless stated otherwise. Ultrapure milli-Q water was used to prepare the different solutions. Received: December 17, 2010 Published: January 14, 2011 2302

dx.doi.org/10.1021/jp112005m | J. Phys. Chem. C 2011, 115, 2302–2313

The Journal of Physical Chemistry C Table 1. Molar Mass, Absorbance Maximum, and Structural Formule of Rhodamine 6G and Pyridinium Chloride

Sample Preparation. Trititanate nanotubes (TNTs) are rolled-up mesoporous materials, which can be prepared without using any template. Here, 4.5 g of TiO2 (Fluka) (anatase powder, 12 m2/g) was dispersed into 80 mL of 10 M NaOH solution under vigorous stirring during 1 h. The mixture was transferred into a Teflon-lined autoclave and hydrothermally treated at 423 K for 48 h. After hydrothermal treatment, the solid was recovered by centrifugation. The precipitate was washed 3 times with deionized water. In this way, sodium trititanate nanotubes (NaTNT) are obtained. To prepare hydrogen trititanate nanotubes (H-TNT), the wet cake was dispersed into 240 mL of 0.1 M HCl solution and stirred for 30 min. The solid was recovered again by centrifugation and dried for 2 days. Afterward, the dried powder was dispersed again in 100 mL of 0.1 M HCl for 5 min. The precipitate was separated by filtration and washed three times with water. Finally, the washed solid was dried at 373 K for several days. By calcination at 623 K for 6 h (heating rate 278 K min-1), trititanate nanotubes partially recrystallize into anatase nanotubes. These mixed-phase nanotubes will be henceforth referred to as H-TNT C623. Samples of H-TNT with adsorbed R6G (hereafter referred to as H-TNT/R6G) were prepared by dispersing 16 mg of catalyst in 50 mL of 4 10-5 M R6G solution. After 40 min, the nanotubes were recovered by filtration, washed with 25 mL of distilled water, and dried at room temperature for 2 days. Instrumentation. UV-vis Absorption Spectroscopy. UV-vis absorption scans of the aqueous pollutant solution were taken at fixed time intervals (30 min) on a Thermo-electron evolution 500 UV-vis double beam spectrometer. In this way, the photobleaching of the initial pollutant (oxidation toward intermediates) was evaluated. Using Lambert-Beer’s law, the measured absorbance can be converted to the corresponding concentration. In Table 1, the molar mass, the absorbance maximum, and the structural formula of both pollutants are plotted. Figure S1 (Supporting Information) shows the optical absorbance spectrum of R6G and PyCl in the 200-600 nm range. Total Organic Carbon Analysis (TOC). To evaluate the photomineralization (oxidation to CO2), microvolume TOC analysis was used. Measurements were performed on a Shimadzu TOC-VCPH equipped with a palladium normal sense catalyst and a Shimadzu designed gas injection kit. Samples are injected with a

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high precision syringe of 250 μL (Hamilton 1725 gastight syringe with Chaney Adapter, RN type 2). Combustion of the injected samples to CO2 takes place at 953 K. The amount of CO2 is measured with an NDIR (nondispersive infrared) detector. UV-vis Diffuse Reflectance Spectroscopy (UV-DR). This technique was used to get information about the band gap energy and was recorded on a Thermo-electron evolution 500 UV-vis spectrometer equipped with a Thermo-electron RSA-UC40 Diffuse Reflectance cell. N2-Sorption. The surface area and porosity of the mesoporous nanomaterials were determined on a Quantachrome Quadrasorb SI automated gas adsorption system with AS-6 degasser. The samples were outgassed at 423 K for 16 h. Subsequent N2sorption was carried out at 77 K. The Brunauer-Emmet-Teller (BET) method was used to calculate the specific surface area. The volume adsorbed at a relative pressure P/P0 = 0.97 was used to determine the total pore volume. FT-Raman Spectroscopy. Samples were measured on a Nicolet Nexus 670 bench equipped with a InGaAs detector in a 180° reflective sampling configuration using a 1064 nm Nd:YAG laser and a laser power of 0.8 W. Thermal Gravimetric Analysis/Derivative Thermogravimetric Analysis (TGA/DTG). Thermogravimetric analysis gives information on the weight loss as a function of the temperature. Measurements were executed on a Mettler Toledo TGA/SDTA 851e. Samples were heated until 1073 K with a heating rate of 278 K/min. Electron Energy Loss Spectroscopy (EELS). Reference spectra of trititanate and amorphous TiO2 were recorded. All the reference signals have been background subtracted, deconvoluted, and normalized. The spectra for the unknown samples were then interpreted as a linear combination of the two references spectra, taking into account a background and multiple scattering.14 Electron Paramagnetic Resonance (EPR). X-band continuous wave (CW)-EPR experiments were performed on a Bruker ESP300E spectrometer (microwave (mw) frequency 9.45 GHz) equipped with a gas-flow cryogenic system (Oxford, Inc.), allowing operation from room temperature down to 2.5 K. The magnetic field was measured with a Bruker ER035 M NMR Gaussmeter. The spectra were recorded at 10 K, 100 K, and room temperature using a microwave power of 2 or 5 mW, a modulation amplitude of 0.1 or 0.5 mT, and a modulation frequency of 100 kHz (see details in figure captions). Light Sources. For the light-induced (LI)-EPR experiments, the H-TNT samples were irradiated with UV or visible light from a Kr-ion laser (Spectra Physics 2580) with wavelengths of 350.7 and 406.7 nm or with visible light from an Ar-ion laser (Spectra Physics 2020), with a wavelength of 514.5 nm (see details in figure captions). Laser powers of 5 mW were used. For the photocatalytic activity tests, the following light sources were used: (1) illumination with UV-light: polymeric light source (equipped with filters, peak at 365 nm), 100 W Hg-lamp (Sylvania Par 38; 21.7 mW/cm2 at 5 cm) and (2) irradiation in the visible region (395-800 nm): Philips Plusline double-ended linear halogen lamp with UV-block (0.16 mW/cm2 at 100 cm). Measurement of Photocatalytic Activity by UV-vis Analysis (Photobleaching). The photocatalytic activity was tested by photodegradation of R6G and PyCl in aqueous solution. For the photodegradation of R6G, 8 mg of catalyst was dispersed in 50 mL of 4 10-5 M R6G (pH of solution is 6.5). If PyCl was used as a synthetic pollutant, 32 mg of catalyst was added to 50 mL of 16 10-5 M PyCl (pH of solution is 5). The pH did 2303

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The Journal of Physical Chemistry C

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Table 2. Surface Area, Total Pore Volume, Morphology, Crystal Phase, and Bandgap Energy of H-TNT, H-TNT C623, and H-TNT C723 SBETa (m2/g)

Vpb (mL/g)

H-TNT

338

1.10

nanotube

H-TNT C623 H-TNT C723

318 104

1.10 0.11

nanotube nanotube þ nanoparticles

morphologyc

crystal phased

bandgape (ev)

crystallinityf(%)

trititanate

3.3

45

trititanate þ anatase anatase

3.2 3.2

79 -

a

SBET (specific surface area) deduced from N2-sorption; BET, 77 K. b Vp (total pore volume) deduced from N2-sorption; BET, 77 K. c HR-TEM. Crystal phase determined with FT-Raman spectroscopy. e Bandgap energy determined using UV-DR spectroscopy. f Crystallinity determined by EELS. d

not change when intermediate, mineral acids were converted toward CO2. These catalyst amounts and concentrations were applied in all experiments, unless stated otherwise. Different concentrations of pollutant solution were chosen to allow optimum TOC measurements. To maintain the same catalyst/pollutant ratio, the amount of catalyst was also adapted. To establish an adsorption-desorption equilibrium between the pollutant molecules and the catalyst surface, the catalyst was stirred in the pollutant solution for 40 min. Then, the solution was irradiated for 360 min with UV-light. During illumination, samples with a volume of 5 mL were taken out of the suspension at fixed time intervals (30 min) and analyzed by UV-vis spectroscopy. The determination of the decolorization efficiency (photobleaching) can be given by the following equation efficiency ¼

C0 - C C0

ð1Þ

where C0 is the initial concentration of the dye after an adsorption-desorption equilibrium was established and C is the final concentration after illumination by UV-light for a certain time. Under the applied experimental conditions, photolysis of the pollutant is negligible. All photocatalytic reactions were performed at room temperature. The dissolved oxygen demand was measured by an oxygen electrode and determined to be 7.45 ( 0.04 mg/L for the rhodamine 6G solution and 7.52 ( 0.16 mg/L for the pyridine chloride solution. Measurement of Photocatalytic Activity by Microvolume (μV)-TOC Analysis (Photomineralization). Although evaluation of the photobleaching process (breaking of conjugated system of pollutant) can give interesting information on the activity of the catalyst, it is also important to evaluate the photomineralization (oxidation to CO2) since not all photobleached molecules will be fully photomineralized. In this way, detailed information on the photodegradation (photobleaching þ photomineralization) is obtainend. As mentioned above, a 5 mL sample was taken every 30 min for the UV-vis analysis. From this sample, 0.5 mL was used for μV-TOC-analysis. The remaining 4.5 mL of the sample was brought back to the illuminated test solution to prevent large changes in volume/catalyst ratio. Using this 0.5 mL volume, two separate measurements were performed in which (i) the total amount of carbon (TC) was measured and (ii) the total amount of inorganic carbon (IC) was determined. For both TC and IC measurements, a minimum of three sequential injections was performed. Samples for TC analysis were injected directly on the combustion oven, using the special designed gas injection kit. Samples for IC analysis were injected directly on the acid-containing vile. The TOC value can be calculated using the following equation TOC ¼ TC - IC

ð2Þ

’ RESULTS Pure trititanate (H-TNT) and mixed-phase anatase/trititanate nanotubes (H-TNT C623) are prepared following the abovedescribed synthesis procedure. As reported in an earlier study,9 calcination of H-TNT at 623 K allows partial recrystallization of trititanate to anatase without changing the nanotube morphology and surface area. Indeed, results of N2-sorption (Table 2) show that the total pore volume and specific surface area of H-TNT and H-TNT C623 are similar, whereas H-TNT starts to degrade and transform into anatase nanoparticles at 723 K. EELS results show that H-TNT C623 is much more crystalline due to calcination compared to noncalcined H-TNT (Table 2). Furthermore, the formation of anatase slightly lowers the bandgap energy of the initial material as shown in the UV-DR spectrum (Figure S2, Supporting Information). Due to the smaller bandgap of H-TNT C623, it can absorb light with a maximum wavelength of 410 nm (violet region of visible light), whereas H-TNT can absorb light with a maximum wavelength of 398 nm. Although these wavelengths can be absorbed, the absorbance is very limited. In the following sections, the photocatalytic activity of both catalysts is evaluated by studying the bleaching and mineralization of pollutants rhodamine 6G and pyridinium chloride over long irradiation times of the catalyst-pollutant mixtures. Both UV-light and visible light were used to activate the studied catalysts. Detailed EPR measurements are presented that investigate the formation of photoinduced paramagnetic centers in support of the information obtained by the photocatalytic tests. Note that a study of the degradation mechanism and identification of intermediate reaction products of the pollutants lies beyond the scope of this article and has already been thoroughly described in the literature.15-17 The outcome of the latter studies is used here in the interpretation of the obtained data. Photocatalytic Degradation of Pyridinium Chloride by H-TNT and H-TNT C623 under UV-Light Illumination.

Figure 1 shows the bleaching and mineralization of pyridinium chloride catalyzed by H-TNT and H-TNT C623 under UV-light illumination. Here, it can be seen that H-TNT C623 has a somewhat higher adsorption capacity toward PyCl compared to H-TNT (see concentrations at t = 0), although the surface areas of H-TNT and H-TNT C623 are quite similar. Despite the good interaction between H-TNT and PyCl, the bleaching of PyCl is rather limited. Indeed, after 6 h of intensive UV-irradiation, only 15% of the initial solution has been degraded into smaller compounds by H-TNT. Furthermore, no oxidation to CO2 (0%) takes place. In contrast to H-TNT, high photocatalytic efficiency can be observed for H-TNT C623. After 6 h, 99% of the initial PyCl molecules have been bleached. The results of the TOCanalysis show that, at this stage, the aqueous solution of PyCl contains only 11% of carbon, which implies that 89% of the initial PyCl molecules and bleached intermediates are completely 2304



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Figure 1. Results of the TOC-analysis and the analysis with UV-vis spectroscopy for 32 mg of H-TNT/H-TNT C623 in 50 mL of 16 10-5 M PyCl. The experimental procedure is explained in the text. nc = noncalcined.

converted to CO2. It is clear that H-TNT C623 has excellent photocatalytic properties, whereas the photocatalytic activity of H-TNT toward PyCl is rather limited. Photocatalytic Degradation of Rhodamine 6G by H-TNT and H-TNT C623 under UV-Light Illumination. To gain more insight into the photocatalytic activity of H-TNT and H-TNT C623 under UV-light, the photocatalytic degradation of another artifical pollutant was studied. In Figure 2, the results of bleaching and mineralization of rhodamine 6G photocatalyzed by H-TNT and H-TNT C623 are shown. Here, it can be seen that the bleaching and mineralization processes of R6G by H-TNT and H-TNT C623 are very different. The UV-vis absorption analysis of the photodegradation of R6G by H-TNT shows that, after 120 min of irradiation, 75% of the initial dye solution has been degraded into smaller compounds, while H-TNT C623 has bleached 84% of the dye. This indicates that the rate of bleaching of R6G by H-TNT C623 is slightly higher compared to H-TNT. After 6 h, the bleaching of R6G by both catalysts is almost complete (99%). In contrast, TOC-analysis shows that, during the first 90 min of UV-irradiation, the rate of photocatalytic mineralization by H-TNT (31%) is significantly higher compared to H-TNT C623 (13%). After 90 min of UV illumination, the photocatalyzed mineralization by H-TNT stops abruptly, whereas the oxidation of intermediate reaction products by H-TNT C623 is a continuous process, taking place in small steps. After 6 h of irradiation, H-TNT C623 has mineralized 48% of the initial solution, while no additional photomineralization could be observed for H-TNT after 90 min. Photocatalytic Degradation of Pyridinium Chloride by H-TNT and H-TNT C623 under Visible Light. Here, the photocatalytic properties of H-TNT and H-TNT C623 are studied under visible light illumination. For these experiments, only the photocatalytic bleaching and mineralization of PyCl is studied because dye molecules, such as R6G, may induce photocatalytic activity under visible light via a fully different mechanism.18-20

Indeed, R6G can absorb visible light (Figure S1, Supporting Information), which leads to excitation of electrons from π to π*. Because of the strong interaction between the positively charged rhodamine molecules and negatively charged trititanate surface, the electron diffusion distance is short allowing the adsorbed photoexcited dye molecules to inject electrons into the conduction band of the catalyst (Figure 3). This may lead to the formation of radicals, which in turn results in the degradation of R6G. Therefore, only the results of the photocatalytic degradation of PyCl by H-TNT and H-TNT C623 under visible light are studied in detail (see Table 3 and Figure S4, Supporting Information). In Table 3, it can be seen that no bleaching of PyCl takes place by H-TNT under visible light irradiation, whereas it bleaches 14% of the initial PyCl solution under UV-light illumination. In contrast to H-TNT, the mixed-phase H-TNT C623 can clearly be photocatalytically activated under visible light. However, the bleaching of PyCl by H-TNT C623 under visible light (15%) is ∼6.5 times smaller compared to its performance under UV-light (99%). Furthermore, despite the noticeable bleaching of PyCl by H-TNT C623, no photomineralization is observed under visible light irradiation, while 89% of the initial PyCl solution is mineralized under UV-light illumination. This indicates that photoinduced degradation reactions take place under visible light to convert the initial PyCl molecules into smaller intermediates but that the number of degradation reactions is too limited to obtain a large conversion into CO2. EPR Study of Light-Induced Paramagnetic Species. To gain insight into the paramagnetic species that are formed upon irradiation with UV- and visible light and hence the mechanism of the photocatalysis, LI-EPR experiments were undertaken. Light-Induced EPR Signals in Hydrogen Tritantate Nanotubes upon Irradiation with UV-Light (350.7 nm). EPR experiments were performed at 10 and 100 K, where the recombination rate of the photogenerated electron-hole pairs is low. Figure 4A shows the EPR spectra of H-TNT and H-TNT C623 2305



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Figure 2. Results of the TOC-analysis and the analysis with UV-vis spectroscopy for 8 mg of H-TNT/H-TNT C623 in 50 mL of 4 10-5 M R6G. The experimental procedure is explained in the text. nc = noncalcined.

Figure 3. Energy level diagram of H-TNT and R6G.

Table 3. Percentage of Photodegradation of PyCl by H-TNT and H-TNT C623 under UV-Light and Visible Light for 32 mg of H-TNT/H-TNT C623 in 50 mL of 16 10-5 M PyCl UV-analysis (%) TOC-analysis (%) H-TNT (UV-irradiation)

15

H-TNT (visible light irradiation)

0

0

0

H-TNT C623 (UV-irradiation)

99

89

H-TNT C623 (visible light irradiation)

15

0

after irradiation at 10 K with 350.7 nm UV-light during 1 h. The spectra reveal the formation of several light-induced paramagnetic species, which are clearly different for H-TNT and H-TNT C623. The EPR spectrum of H-TNT (Figure 4A) is dominated by a contribution with principal g values of g1 = 1.953, g2 =1.938, and g3 =1.89 and broad linewidths (indicated as species I in Table 4, simulation in Figure S5, Supporting Information). This signal is typical for surface Ti3þ centers21a-21e that are formed when an electron is trapped at a Ti site Ti

4þ

þe

-

f Ti

ð3Þ

3þ 3þ

Similar EPR parameters have been reported for Ti centers in colloidal anatase24c and in partially reduced TiO2 nanoparticles.22,23

Figure 4. X-band LI-EPR signals of (a), (c) H-TNT and (b), (d) H-TNT C623, upon irradiation with (A) UV-light (350.6 nm) and (B) visible light (406.7 nm). The experiments were performed at 10 K under air with modulation amplitude of 0.5 mT (A) or 0.1 mT (B) and a mw power of 5 mW.

The EPR spectrum of H-TNT C623 (Figure 4B) also contains contributions due to electrons trapped at Ti sites (species II and III, Table 4, simulation in Figure S5, Supporting Information), although these are only minority species. Species II (Table 4) can again be assigned to surface Ti3þ centers. Species III (Table 4) is 2306



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Table 4. EPR Parameters of the Ti3þ Centers Formed upon Illumination of H-TNT and H-TNT C623 with UV Laser Light (λ = 350.6 nm) as Determined from Spectral Simulationa

a

g1

g2

g3

species I

1.953(3)

1.938(3)

1.89(1)

species II

1.942(4)

1.942(4)

1.8(5)

H-TNT C623

species III

1.982(2)

1.982(2)

1.954(6)

H-TNT C623

occurrence H-TNT

The number in brackets represents the error on the last digit.

more typical for excess electrons located in the d orbitals of lattice Ti atoms.24 The g values are similar to those reported for Ti3þ in an anatase phase.24d,24e This result supports that calcination of the tubes leads to anatase formation without affecting their surface area and nanotubular shape.9 In addition, both H-TNT and H-TNT C623 show several overlapping LI-EPR signals at g > ge after UV irradiation (Figure 4A). These signals dominate the spectra of H-TNT C623 but are only minor in H-TNT. Although the relative intensity of the g > ge versus the g < ge signals varies slightly upon batch, the overall difference between H-TNT and H-TNT C623 was reproduced in all batches. For H-TNT C623, the UV-light-induced signals resemble those observed in anatase TiO2 powder calcined at 523 K24e and in anatase TiO2 nanoparticles.25 This also corroborates the transformation to the anatase phase upon calcination of the tubes. Kumar et al.24e interpreted these LI-EPR signals of TiO2 anatase powder at g > ge in terms of a single contribution, with g values of [2.016, 2.012, and 2.002], and assigned it to a hole trapped at a subsurface oxygen center (Ti4þO-Ti4þOH-). Although species with similar g values have often been observed in titania materials,24e,26 their origin is still debated. These g values are also typical for O3- centers24e,27 and have even been assigned to •OH radicals.26 Furthermore, Kumar et al.24e did not show spectral simulations, and inspection of the EPR spectrum of the 523 K calcined anatase in their paper clearly shows spectral features at g values higher than 2.016. Berger et al.24d interpreted the g > ge part of the CW-EPR spectrum of TiO2 nanoparticles illuminated under UV-light in terms of three species: O2-[I]: g1 = 2.0248, g2 = 2.0096, g3 = 2.0033; O2-[II]: g1 = 2.0184, g2 = 2.0096, g3 = 2.0033; O-: g1 = 2.0121, g2 = 2.0121, g3 = 2.0046. They were able to distinguish different components because the CW-EPR measurements were carried out at 140 K, whereas Kumar et al.24e recorded the spectra at 4.2 K. Similarly, the line width of the EPR components in H-TNT C623 reduces considerably at higher temperatures (Figure S6A, Supporting Information), facilitating the interpretation of the spectrum. The spectral features found in UVilluminated H-TNT C623 could not be fully reproduced using the three species observed in TiO2 nanoparticles, and additional components were included to fit the data (Table S1, Supporting Information). Due to the high degree of overlap, it was not possible to unambiguously determine all components in the spectrum. However, it is clear that a variety of species are formed. Most of the EPR parameters are typical for adsorbed O2- or O-/O3sites (see Supporting Information for an elaborate discussion). It has been previously shown that UV irradiation of TiO2 materials under air generally results in the formation of O2- radicals, according to several possible mechanisms,24b such as Ti3þ þ O2 f Ti4þ 3 3 3 O2 -

ð4Þ

Ti4þ þ O2 þ e - f Ti4þ 3 3 3 O2 -

ð5Þ

This was also confirmed by a recent study on the influence of the vacuum annealing temperature of hydrogen trititanate tubes on the formation of O2- defects under air upon UV illumination.28 It was found that at lower annealing temperature (470 K) the formation of O2- is higher than the formation of O- sites. These studies show that the presence of O2centers in the tubes is very likely and supports the EPR assignments. Similar results are found for H-TNT. The weak signals at g > ge can be assigned to a combination of adsorbed O2- or O-/O3sites (Table S3, Supporting Information). Dynamics of the LI-EPR Species under UV-Light. Figure 5 shows the dynamics of the light-induced EPR species under UV-light, recorded at positions (i), (ii), and (iii) (see arrows in Figure 4). Position (i) agrees with surface Ti3þ sites, whereas positions (ii) and (iii) fall within the g > ge region. In H-TNT, the Ti3þ sites build up slowly, whereas the signals at g > ge are formed very fast and quickly level off. When the laser is switched off, an immediate increase in the intensity of the Ti3þ EPR signal is observed, followed by a gradual increase in time. On the contrary, the signals at g > e show an initial small decay, followed by a stabilization after the laser is switched off. In H-TNT C623, the EPR intensity at positions (i) and (ii) builds up fast upon UV irradiation, followed by a slower, continuous increase. After the laser is switched off, signals (i) and (ii) decrease slowly. This was also observed for the Ti3þ sites (position (iii)), contrary to the kinetics of this species in H-TNT. Note that the EPR signals at positions (i)-(iv) remain relatively constant up to one hour after switching off the laser. This indicates that the charge separation is maintained, as was also observed previously for titania nanoparticles.29 The charge separation is still maintained at 100 K (not shown) but is lost at room temperature, where the depletion of the trapped sites and electron-hole recombination rates becomes too fast. Light-Induced EPR Signals upon Irradiation with Visible Light. To verify the visible LI-EPR signals, the tubes were irradiated with laser light of 514.5 and 406.7 nm. As expected, no light-induced EPR signals are observed at 514.5 nm (not shown). As mentioned previously, H-TNT absorbs light at a maximum wavelength of 398 nm, whereas the band edge maximum of HTNT C623 lies around 410 nm (Figure S2, Supporting Information). In parallel, no LI-EPR signals are observed upon illumination at 406.7 nm of H-TNT (Figure 4c), whereas weak but clear signals can be observed in the EPR spectra of H-TNT C623 (Figure 4d). Figure S8 (Supporting Information) compares the EPR spectra of H-TNT C623 illuminated at 406.7 nm at 100 K (A) and 10 K (B). These EPR spectra differ but can be explained in terms of the same four species with varying relative ratio (Table 5). Species III can be ascribed to Ti3þ centers, which are formed when an electron is trapped by a Ti4þ site22,24e,30,31 (eq 3). This species was also observed upon UV irradiation of H-TNT C623 (Table 4). Species with principal g values similar to species IV have often been observed in TiO2 materials.24e,26 As mentioned above, their origin is still debated, as they were ascribed to a hole trapped by subsurface oxygen (Ti4þO-Ti4þOH-24d,26c) and even to •OH radicals.26a The g values are, however, also typical for O3radicals.26a,25 The principal g values of O3- centers depend only slightly on the matrix (Table S2, Supporting Information). 2307



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Figure 5. UV-light-induced formation of paramagnetic species in (a) H-TNT and (b) H-TNT C623, at 10 K. The EPR signal intensities were measured under in situ illumination at positions indicated with (i)-(iv) during 64 min of UV irradiation (350.6 nm) and during 1 h after switching off the laser.

Table 5. EPR Parameters of the Species Formed upon Illumination of H-TNT C623 with Visible Laser Light (λ = 406.7 nm) at 100 and 10 K as Determined from Spectral Simulation of Figure S8 (Supporting Information)a relative contribution to spectrumb

principal g values

a

g1

g2

g3

100 K

10 K

species III

1.982(2)

1.982(2)

1.954(6)

33%

12%

species IV

2.017(1)

2.013(1)

2.002(2)

36%

29%

species V species VI

2.025(1) 2.0048(1)

2.011(2) 2.0048(1)

2.002(2) 2.0048(1)

12% 19%

50% 9%

The number in brackets represents the error on the last digit. b Full spectral intensity = 100%. The experimental error is ∼5%.

In contrast, a large range of principal g values have been reported for O- radicals.27a,32 O3- species are formed according to hVB þ þ O2 - f O -

ð6Þ

O - þ O2 f O 3 -

ð7Þ

As the EPR experiments were conducted under air, the assignment of species IV to O3- is quite probable. The EPR characteristics of species V have been ascribed to O2-,24b,30,33a-33d which are formed according to eqs 4 and 5. Note that species with similar g values have also been ascribed to surface-trapped holes (Ti4þO2-Ti4þO-) in solid anatase and rutile calcined at 1023 K.24e However, since these signals were not found in solid anatase and rutile calcined at temperatures below 700 K, and since all the experiments were performed under air, the assignment to O2- is most probable. A signal similar to species VI has been observed in vacuumannealed titanate tubes34 and in some TiO2 materials.35a-35d Note that this signal was also observed upon illumination of H-TNT C623 at room temperature (with 406.7 nm). Although many radical-type species can in principle lead to an isotropic EPR signal near g = ge, the g = 2.0048 signal in titania materials is usually assigned to an electron trapped in an oxygen vacancy (VO), forming the so-called color center Fþ V O þ e - f Fþ

ð8Þ

It has been shown that the synthesis of H-tubes favors the formation of oxygen vacancies.9 In most cases, however, the electrons in oxygen vacancies in TiO2 materials lead to the reduction of the nearby Ti4þ (formation of a vacancy-Ti3þ site).36 Unlike MgO, where formation of color centers is standard, these centers are not observed in solid TiO2 and have only been reported in some TiO2 nanoparticles and nanotubes.34,35 It should be noted that the EPR signal cannot be saturated at low temperatures and high microwave powers, in contrast to Fþ centers in other materials. Influence of Rhodamine Adsorption. As mentioned above, R6G induces photocatalytic activity under visible light, in both H-TNT and H-TNT C623. This likely proceeds by the transfer of photogenerated electrons in R6G to the conduction band of TiO2, leading to the formation of dye cationic radicals.37,38 These electrons then can further react with dioxygen adsorbed on the surface of TiO2 and generate a series of reactive oxygen species, which initiate the degradation of the organic dye. EPR experiments were performed to trace the radicals that are formed in these reactions. The EPR spectra of H-TNT R6G without illumination (not shown), recorded at 10 K, reveal a broad isotropic EPR line with giso ∼ 2.0048 and ΔHpp ∼ 2 mT (indicated as species VII, Table 6). These g values are identical with those of species VI, but the line width is much broader (line width species VI = 0.5 mT), which indicates that signals VI and 2308



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Table 6. EPR Parameters of the Species Formed upon Illumination of H-TNT R6G at 10 K and 100 K as Determined by Spectral Simulationa species I

0

VII a

g1

g2

1.961(3)

1.945(4)

g3 1.90(1)

2.0048(5) 2.0048(5) 2.0048(5)

rel. contr.

occurence

λex/nm

90%

HTNT R6G 350.6

10%

HTNT R6G 350.6

100%

HTNT R6G 514.5

The number in brackets represents the error on the last digit.

VII correspond to different species. Illumination with visible light of 514.5 nm increases the intensity of this EPR signal (Figure 6B). A very weak EPR signal corresponding to species VII is also observed in R6G powder and increases upon illumination with visible light of 514.5 nm (not shown). Therefore, this EPR signal can be assigned to R6G radicals. Similar EPR signals were observed in carotenoid-modified titania nanoparticles and were assigned to carotenoid cationic radicals.39 The intensity of the latter signals was also found to increase upon illumination in the UV or visible region. The intensity of species VII is much higher for H-TNT R6G compared to R6G (more than 50). As mentioned in the previous section, no EPR signals are observed upon illumination of bare H-TNT at 514.5 nm (Figure 6A). This shows that adsorption of R6G onto H-TNT and subsequent illumination with visible light result in the transfer of electrons from R6G to H-TNT with the formation of R6G radicals. The electrons injected in H-TNT do not lead to additional EPR signals and hence are probably conduction-band electrons that are not detectable by EPR. As R6G absorbs light in the UV region (Figure S1, Supporting Information), the influence of R6G adsorption on the UV-lightinduced EPR signals of H-TNT needs to be verified as well. Upon UV irradiation (350 nm) of H-TNT R6G, the intensity of species VII, corresponding to R6G radicals, increases (Figure 6D). Simultaneously, an additional signal corresponding to species I (Ti3þ centers, Figure 6D) appears, which is also observed upon UV illumination of bare H-TNT (Figure 6C). There is no marked difference in the amount of Ti3þ centers generated in bare H-TNT and in H-TNT R6G. This again indicates that adsorption of R6G to H-TNT and subsequent illumination with UV-light result in the transfer of electrons from R6G to the conduction band of H-TNT. Similar results (i.e., the formation of species VII) are obtained at 100 K, although in this case species I is not detected. The EPR signal of species I is indeed difficult or not detectable at temperatures g100 K. TGA/DTG Experiments. To facilitate the interpretation of some of the EPR spectra, TGA/DTG experiments were undertaken (Figure 7). The weight loss between 298 and 373 K can be attributed to the removal of physically adsorbed water and can be observed for all the samples. The removal of structural water present in the interlayers of the nanotube structure can be assigned to a weight loss between 373 and 474 K. This signal can be clearly observed for H-TNT but is less pronounced for H-TNT C623. Furthermore, no signal can be seen in this temperature range for H-TNT C623 O2. This can be explained by the fact that a part of the structural water is removed due to recrystallization to anatase. Because strong recrystallization toward anatase takes place under oxygen flow, no removal of structural water can be observed in TGA for H-TNT C623 O2.

Figure 6. X-band LI-EPR signals of (a,c) H-TNT and (b,d) H-TNT R6G upon irradiation with (a,b) visible light (514.5 nm) and (c,d) UVlight (350.6 nm). The experiments were performed at 10 K under air with a modulation amplitude of 0.5 mT and a mw power of (a,b) 5 mW and (c,d) 2 mW.

’ DISCUSSION Photocatalytic Activity of H-TNT and H-TNT C623 under UV-Light. The photocatalytic activity of trititanate nanotubes

(H-TNT) and anatase-containing nanotubes (H-TNT C623) was extensively studied by evaluating the degradation of two different synthetic pollutants (R6G and PyCl) in terms of bleaching (optical absorption spectroscopy) and mineralization (TOCanalysis) upon light irradiation. The degradation (bleaching þ mineralization) was analyzed over a long period of time under UV-illumination and visible-light illumination. Furthermore, the photoinduced species were characterized using EPR to obtain detailed information on the differences in photocatalytic activity of both H-TNT and H-TNT C623. To explain the differences in the photocatalytic activity of both catalysts, the interaction between the catalyst and the pollutant needs to be studied. Indeed, clear differences in adsorption capacity are observed (Figure 1 and Figure 2). Besides the surface area, the adsorption capacity is also determined by the net charge on the surface of the catalyst and the crystallinity of the catalyst. Because the surface area and morphology of both catalysts are quasi similar (Table 2), the other factors will have a major influence. When studying the adsorption of PyCl (Figure 1), it became clear that the adsorption capacity of H-TNT C623 is higher compared to H-TNT. In terms of the surface charges on both catalysts, trititanate is (slightly) negatively charged in the PyCl solution (pItrititanate = 3.5-4.5 < pHPyCl = 5),40 whereas anatase is positively charged (pIanatase = 5.8-6 > pHPyCl = 5).41 Therefore, an electrostatic repulsion will occur between PyCl and anatase, while a small electrostatic attraction is present between the cationic PyCl molecules and the weakly negatively charged trititanate phase. Therefore, it is expected that H-TNT C623 has a lower adsorption capacity compared to H-TNT due to the presence of anatase. However, opposite results are observed. Due to calcination, the crystallinity of H-TNT increases with 58% (H-TNT, 45% crystalline; and H-TNT C623, 79% crystalline; determined by EELS). Therefore, the adsorption capacity of H-TNT C623 toward PyCl is higher compared to H-TNT, although the introduction of anatase reduces the 2309



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Figure 7. TGA/DTG measurements of H-TNT nc, H-TNT C623, and H-TNT C623 O2.

interaction with PyCl molecules. Indeed, crystalline TiO2 has a higher adsorption capacity compared to amorphous TiO2 because the amorphous phase shows only a short-range order and a lot of structural defects (large amount of under- or overcoordinated structural Ti units and dangling bonds), whereas the anatase phase has a well-formed lattice with a small amount of structural defects and high amount of hydroxyl groups. H-TNT and H-TNT C623 show a comparable adsorption capacity toward R6G. Because the aqueous R6G solution has a pH around 6.5, the trititanate phase (pItrititanate = 3.5-4.5) and anatase phase (pIanatase = 5.8-6) will both be negatively charged. However, anatase will only be slightly charged compared to the trititanate phase because the pH of the dye solution approaches the isoelectric point of anatase. Therefore, it is expected that H-TNT C623 has a smaller adsorption capacity toward R6G compared to H-TNT. Again, the weak adsorption of R6G by anatase in H-TNT C623 is compensated by the increased crystallinity of H-TNT C623. This results in almost equal adsorption capacity for H-TNT and H-TNT C623 toward R6G. Although there is a good interaction between the pollutants and H-TNT, the photocatalytic degradation of PyCl by H-TNT is rather limited, whereas H-TNT shows high photocatalytic activity toward the R6G solution. To explain the large differences in photocatalytic activity of H-TNT toward both pollutants, the possibility of deactivation of H-TNT by the PyCl solution (influence of acid (pHPyCl = 5) and the influence of Cl-)1 has to be studied. Therefore, the photodegradation of an acidified R6G solution (pH of initial dye solution lowered until pH = 3 and pH = 5 by addition of HCl) by H-TNT was examined (Figure 8). As the pH decreases, a decreased adsorption capacity and photocatalytic activity can be observed. Indeed, as the pH of the solution approaches the isoelectric point of trititanate, the trititanate surface becomes neutral, which weakens the interaction between H-TNT and R6G. However, bleaching of acidified R6G can still be observed, even if the pH is lowered from pH = 6.5 to pH = 3. This proves that the acidic environment or the presence of chloride anions in the PyCl solution does not deactivate H-TNT. If PyCl does not have a negative effect on the photocatalytic activity of H-TNT, it may be suggested that R6G has a positive effect on the photocatalytic activity of H-TNT.

Indeed, the absorption spectrum of R6G (Figure S1, Supporting Information) shows that R6G molecules absorb light in the visible region but also in the UV-region. This implies that the irradiated UV-light (365 nm) can excite the R6G molecule. Subsequently, the R6G molecule can then inject electrons in the conduction band of H-TNT resulting in the formation of R6G radicals. Indeed, Figure 3 shows that the ELUMO of R6G (∼-1.10 V vs NHE, normal hydrogen electrode)42 is negative enough to inject electrons into the conduction band of titania (∼-0.3 V vs NHE).43 These electrons can further react with dioxygen adsorbed on the surface of TiO2 and generate a series of reactive oxygen species, which may initiate the degradation of the organic dye. To trace the radicals formed in these reactions, EPR experiments were performed. Upon excitation of H-TNT with UVlight (350 nm), predominantly Ti3þ centers (species I) are formed, which are indicative for electron trapping by Ti4þ sites, according to reaction scheme 3. When the tubes are loaded with R6G, an EPR signal, typical for R6G radicals, appears (Figure 7 d, species VII). Species VII was also observed in R6G after UV illumination, but in much lower amounts. Upon UV illumination of H-TNT R6G, the intensity of species VII increases. These results suggest electron transfer from the excited R6G dye to H-TNT, which explains the high photocatalytic activity of H-TNT toward R6G compared to PyCl. Furthermore, this can explain the observed inhibition of mineralization of R6G by H-TNT on the moment that the dye solution has been completely photobleached. At this stage, the photocatalytic activity of H-TNT is reduced because the intermediates are not able to inject electrons in the conduction band and contribute to the formation of radicals. Whereas the photocatalytic activity of H-TNT is limited, H-TNT C623 shows high photocatalytic activity toward both PyCl and R6G due to the presence of the highly photocatalytically active anatase phase. To gain insight in the photocatalysis of pure, undoped H-TNT and H-TNT C623, the different lightinduced paramagnetic centers were studied using EPR spectroscopy. Illumination of H-TNT with UV-light (350.7 nm) produces predominantly surface Ti3þ centers (species I), which are formed by the trapping of electrons at Ti4þ sites (reaction scheme 3). The EPR spectrum of H-TNT C623 also contains contributions due to electrons trapped at Ti sites (species II and III), 2310



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Figure 8. Results of the adsorption and photocatalytic activity analysis with UV-vis spectroscopy for 16 mg of catalyst in 50 mL of 4 10-5 M R6G. The photocatalytic activity was induced by UV-light.

but they are no longer the dominant species. The g values of the Ti3þcenters in H-TNT are similar to those reported for colloidal anatase24c and partially reduced TiO2 nanoparticles44,45 but differ from those of the Ti3þ centers in H-TNT C623. This is related to the high amount of structural water in H-TNT, as revealed by the TGA/DTG experiments. Two types of Ti3þ centers are induced by UV-light in H-TNT C623, of which one is characteristic of Ti3þ in anatase (species III), consistent with the formation of anatase during the calcination process. In comparison, the EPR spectrum of the well-studied Degussa P25 mixed phase TiO2 consists of contributions of two Ti3þ centers typical for its anatase and a rutile phase.24d The Ti3þ centers exhibit a different dynamic behavior for H-TNT compared to H-TNT C623. The sites build up slowly upon UV illumination (Figure 5). For H-TNT, an immediate increase in the intensity of the Ti3þ EPR signal is observed when the laser is switched off, followed by a gradual increase in time (Figure 5). In contrast, the Ti3þ EPR signal in H-TNT C623 decreases slowly after switching off the laser. This indicates that UV illumination of H-TNT is accompanied by the excitation of localized electrons to the conduction band and the transfer of electrons to EPR-inactive sites. When the laser is switched off, a fraction of these electrons is released and captured at Ti4þ sites, which explains the sudden increase in intensity of species I. A similar light-induced behavior was observed for trapped holes in MgO nanoparticles, which was ascribed to the release of localized holes from (unspecified) EPR-inactive sites.29 The storage of electrons in EPR-inactive sites and subsequent release to Ti4þ ions does not occur in H-TNT C623. The light-induced charge separation pattern of H-TNT C623 parallels that of TiO2 anatase nanoparticles.29 This again confirms the conversion into anatase after calcination of H-TNT. The photocatalytic process in TiO2 materials is believed to proceed by the generation of reactive oxygen species, such as O2-, •O2H, and •OH,26b,46 which are formed when the lightinduced electrons and holes react with dioxygen adsorbed on the TiO2 surface. Molecular oxygen is known to adsorb most frequently on or at the vicinity of defect sites such as oxygen vacancies.35d,47 As surface OH groups facilitate the adsorption and activation of molecular oxygen,48,49 they also play a role in the catalysis. Calcination at 623 K does not remove all the surface OH groups9,35d and leads to recrystallization toward anatase, which is an intrinsically active TiO2 phase toward oxygen

adsorption.50 Apart from the Ti3þ centers, the EPR spectra of both H-TNT and H-TNT C623 reveal several overlapping lightinduced EPR signals at g > ge, after illumination in the UV region. These signals dominate the spectra of H-TNT C623 but are only minor in H-TNT. The g > ge signals can be assigned to a combination of adsorbed O2- or O-/O3- sites and possibly Fþ centers. Hence, the current EPR results show that more reactive oxygen species (signals at g > ge) are formed in H-TNT C623, which confirms the higher photocatalytic activity of H-TNT C623 compared to H-TNT. Photocatalytic Activity of H-TNT and H-TNT C623 under Visible Light. As already discussed, the photocatalytic activity of H-TNT toward PyCl is very limited under UV-light compared to H-TNT C623. Using visible light, no photocatalytic activity of H-TNT is observed, whereas H-TNT C623 displays some photocatalytic activity. To understand the differences in visible light-induced photocatalysis, EPR experiments were undertaken. Upon visible light irradiation at 406.7 nm, no EPR signals are observed for H-TNT, which can be related to its high band gap energy (band edge maximum around 398 nm). In contrast, H-TNT C623, which absorbs light up to 410 nm, shows several light-induced signals at g > ge and minor contributions of species III (Ti3þ in anatase phase). Visible light illumination of pure anatase does not lead to the observation of EPR signals.24d However, it has been shown for Degussa P25 slurries that illumination with visible light leads to the observation of Ti3þ signals in both the anatase and rutile phase.24d This showed that for P25 the rutile phase plays an active role, whereby charges produced on rutile are stabilized through electron transfer to the lower-energy anatase lattice trapping sites. A similar procedure may explain the observation of a small contribution of species III in the H-TNT C623 case after illumination with light with 406.7 nm. The g > ge EPR signals involve reactive oxygen species and possibly also Fþ centers. As mentioned above, the reactive oxygen species are known to be involved in the photocatalysis. Furthermore, the visible light activity of TiO2 materials has also been related to the presence of Fþ centers.33b,51,52 Serpone and Kuznetsov53,54 observed that TiO2 species with absorption features reported in the visible spectral region usually underwent a heat treatment or photostimulation leading to reduction of TiO2. They therefore suggest that the red-shift of the absorption spectrum does not result from a bandgap narrowing but is caused 2311


The Journal of Physical Chemistry C by the formation of color centers due to the reduction of TiO2. The observation of the small red-shift in the absorption spectrum of the titania tubes after calcination, in combination with the observation of the EPR signal of Fþ centers, may suggest that the red-shift is due to a similar mechanism. However, given the observed photocatalytic activity of H-TNT C623 after illumination with violet light, this would then imply that the F center must be involved in the photocatalysis.

’ CONCLUSION In this study, it is shown that pure trititanate nanotubes have a limited photocatalytic activity under UV-light toward the degradation of PyCl. EPR studies reveal the predominant formation of Ti3þ centers upon UV excitation of H-TNT, which are formed via the trapping of electrons at Ti sites (eq 3). The EPR parameters of these centers are typical for TiO2 powders in a gel phase, which is related to the high amount of structural water in the untreated tubes as proven by TGA/DTG experiments. Relatively few reactive EPR species with g values at g > ge are formed. These reactive species are known to play an important role in photocatalysis, and hence the EPR results are in agreement with the low photocatalytic activity. It is proven that the photocatalytic activity of H-TNT strongly depends on the pollutant. If there is a good interaction between the pollutant and H-TNT, and if the pollutant can be excited by UV/vis-light, as is the case for R6G, electrons can be injected into the conduction band of H-TNT, which leads to an increased photocatalytic activity of H-TNT. A short calcination step at relatively low temperature results in the formation of mixed-phase anatase/trititanate nanotubes9 and increases the photocatalytic activity to a great extent. EPR studies show that only a minor amount of Ti3þ sites are formed after calcination. The predominant EPR signals have g values at g > ge and can be assigned to reactive species, which are known to play an important role in the photocatalysis. The photocatalytic activity is even maintained under visible light. Therefore, calcined H-TNT can be considered as an interesting candidate for commercial applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional experimental details, figures, and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: þ32 (0)3 265.24.61. Fax: þ32 (0)3 265.24.70. E-mail: [email protected]. be. Phone: þ32 (0)3 265.23.55. Fax: þ32 (0)3 265.23.74.

’ ACKNOWLEDGMENT This work has been performed in the frame of the FWO projects (Fund for Scientific Research-Flanders; G.0237.09 and G.0312.05) and the GOA project (Special Fund for Research of the University of Antwerp). Sepideh Zamani thanks the University of Antwerp for PhD funding via a BOF-NOI grant. Furthermore, Liang Zhang, Jo Verbeeck, and Gustaaf Van Tendeloo are gratefully acknowledged for the EELS measurements. Financial support from the Hercules Foundation, Flanders (contract AUHA013), is gratefully acknowledged.

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Unraveling the Photocatalytic Activity of Multiwalled Hydrogen

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