Surface Modification of TiO2 with Au Nanoclusters for Efficient Water

Article pubs.acs.org/JPCC

Surface Modification of TiO2 with Au Nanoclusters for Efficient Water Treatment and Hydrogen Generation under Visible Light M. G. Méndez-Medrano,†,‡ E. Kowalska,§ A. Lehoux,§ A. Herissan,† B. Ohtani,§ S. Rau,∥ C. Colbeau-Justin,† J. L. Rodríguez-López,*,‡ and H. Remita*,†,⊥ †

Laboratoire de Chimie Physique, UMR 8000 CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France Advanced Materials Department, Instituto Potosino de Investigación Científica y Tecnológica, A.C., 78216 San Luis Potosí, SLP México § Institute for Catalysis, Hokkaido University, North 21, West 10, 001-0021 Sapporo, Japan ∥ Institute for Inorganic Chemistry 1, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany ⊥ CNRS, Laboratoire de Chimie Physique, UMR 8000, 91405 Orsay, France ‡

S Supporting Information *

ABSTRACT: Small gold nanoparticles (Au-NPs) were used to modify the surface of titanium dioxide as visible-light absorbers and thermal redox active centers. Au-NPs were synthesized on commercial TiO2 (P25) by reduction with tetrakis(hydroxymethyl)phosphonium chloride. The Au/P25 composites were characterized by different techniques including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS). Time-resolved microwave conductivity (TRMC) was used to study the chargecarrier dynamics. The photocatalytic activity of Au/TiO2 was evaluated for the degradations of phenol, 2-propanol, and acetic acid and for H2 production from aqueous methanol solution. The modification of TiO2-P25 with Au-NPs with preferential localization on the anatase phase led to an increase in its photocatalytic activity under UV and visible-light. TRMC signals showed the injection of electrons from Au-NPs into the conduction band of TiO2 under visible-light excitation, as a result of the activation of the localized surface plasmon resonance (LSPR) of the Au-NPs. The action spectra (AS) correlated with the absorption spectra, confirming that the decomposition of acetic acid occurs by a photocatalytic mechanism. The modified TiO2P25 was also found to provide promising results for hydrogen generation under visible-light. The stability of these plasmonic photocatalysts was also investigated, and the results showed that they can be reused several times without appreciable loss of activity.

1. INTRODUCTION The photocatalytic process requires semiconductor (SC) materials to absorb photons from incident irradiation, to generate electrons and holes capable of achieving the reduction and oxidation of the chemical reagents, making SCs useful for a wide range of applications such as air purification, CO2 reduction, effective degradation of organic pollutants in water or air, and hydrogen (H2) production through photocatalytic water-splitting (PWS) processes.1−5 Among PWS processes, there are many methods for producing hydrogen from water, including solar, thermal, combined photovoltaic/electrolytic, artificial photosynthetic, and photocatalytic approaches.6 However, the main drawbacks of the currently used photocatalysts such as TiO2 are high rates of recombination between electrons and holes, resulting in low quantum yields,7,8 and ineffective (or limited) responses under solar light, which is considered to be a green energy source because of its natural availability, low cost, and abundance. One way to enhance the efficiency of photocatalysts and activate them under visible-light is based on the recently © 2016 American Chemical Society

developed plasmonic photocatalysis systems, which involve the surface modification of semiconductors (such as titanium dioxide, which is actually the most frequently used semiconductor for photocatalytic processes) with plasmonic metal nanoparticles (MNPs).9,10 In this context, gold nanoparticles (Au-NPs) have attracted much attention because of their localized surface plasmon resonance (LSPR), that is, the oscillation of metal free electrons in constructive interference with the electric field of the incident light. These plasmonic properties can be used to induce photocatalytic activity in the semiconductor material under visible-light.2,10−16 Indeed, Au-NPs can enhance the visible-light absorption of TiO2, and this effect has been attributed to the interaction of the LSPR of the Au-NPs and the optical band of the semiconductor. Furthermore, an electric field is created by the LSPR. This electric field can power the Received: July 8, 2016 Revised: October 10, 2016 Published: October 10, 2016 25010

DOI: 10.1021/acs.jpcc.6b06854 J. Phys. Chem. C 2016, 120, 25010−25022

Article

The Journal of Physical Chemistry C

propanol, and acetic acid oxidations) and photoreduction for hydrogen generation by PWS using methanol dehydrogenation. Surface modification of TiO2 with small ( 450 nm) or UV irradiation, respectively. The photocatalytic tests were conducted under oxygen bubbling at a fixed rate flow. Aliquots of 0.5 mL were sampled from the reactor at different time intervals. The powder was separated by centrifugation, and then the resultant transparent solution was analyzed by highperformance liquid chromatography (HPLC; Agilent 1260 infinity quaternary liquid chromatograph) equipped with a UV detector set at 260 nm for phenol analysis. The column was an adsorbosphere C18 in reverse phase (5 μm, I = 150 mm, i.d. = 4.6 mm, Alltech) combined with an All-Guard cartridge system (7.5 × 4.6 mm, Alltech) for elution at a flow rate of 1 mL min−1, and the isocratic mobile phase was composed of 80% H2O and 20% acetonitrile (ACN). Star software was used for data acquisition. 2.4.2. 2-Propanol Degradation under Visible-Light Irradiation. The photocatalytic decomposition of 2-propanol was carried out with 50 mg of photocatalyst suspended in 5 mL of an aqueous solution of 2-propanol (5% v/v). The photocatalyst was irradiated using a xenon lamp and a cutoff filter (Y48, Asahi Techno Glass) with wavelengths above 450 nm (see the lamp spectra in Figure S6) and under magnetic stirring. The vials with aerated liquid suspensions were covered with a septum to avoid the evaporation of acetone. The reactor was immersed in a water bath to maintain the reaction temperature at 25 °C. Samples were taken at different irradiation times up to 3 h: Every hour, a volume of the liquid phase was removed, and the suspension was filtered using syringeless filters [Whatman Mini-UniPrep, poly(vinylidene difluoride) (PVDF)]. Then, 0.2 mL of the filtered solution was analyzed using a Shimadzu GC14B gas chromatograph equipped with a flame ionization detector to follow the amount of acetone generated according to the reaction16

samples are labeled as Aux/P25, where x is the weightpercentage loading of Au. 2.3. Material Characterization. The samples were characterized by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) using an FEI Tecnai F30 microscope equipped with a tungsten field-emission gun operated at 300 keV. The samples were dispersed in 2-propanol under sonication, and then, a few drops of the suspension were deposited on a holeycarbon-coated copper grid (Quantifoil Micro Tools GmbH, Großlöbichau, Germany). X-ray photoelectron spectroscopy (XPS) characterization was conducted on a JEOL JPS-9010 MC spectrometer with a hemispherical electron energy analyzer using Mg Kα radiation and with a PHI 5000 VersaProbe II microprobe (Physical Electronics) using monochromatic Al Kα X-rays at an energy of 1486.6 eV. The samples were mounted on carbon films. After overnight degassing in the preparation chamber, the samples were inserted into the analysis chamber at a pressure lower than 10−7 Torr. High-resolution scans were performed for five elements, and the number of scans differed depending on the element content in the sample; specifically, 50 scans were recorded for Ti and O, 100 scans for C, and 300−500 scans for Au. Diffuse reflectance spectroscopy (DRS) scans were recorded on a UV−vis−NIR spectrophotometer (model Cary 5000 Series from Agilent Technologies) equipped with an integrating sphere for diffuse and total reflection measurements and using a KBr reference sample. The structures of the modified TiO2 samples were characterized by X-ray diffraction (XRD) with a DX8 Bruker Advance diffractometer using Cu Kα radiation (λ = 1.5406 Å) over the 2θ range of 10−80° in step mode, with steps of 0.02° and 2 s per step. The time-resolved microwave conductivity (TRMC) method was used to study the dynamics of photogenerated charge carriers under UV and visible irradiation. The TRMC technique uses a pulsed laser source with an optical parametric oscillator (OPO; EKSPLA, NT342B), tunable in the range of 200−2000 nm. The full width at half-maximum of one pulse was 8 ns, the repetition frequency of the pulses was 10 Hz, and microwaves were generated with a Gunn diode (30 GHz). The principle of this technique has been described in previous works.19,20,33,34 In brief, TRMC involves the measurement of the microwave power reflected by a semiconductor sample when it is irradiated by a nanosecond pulsed laser. The signal generated by the diode detector is transformed into voltage for input to the oscilloscope. As follows: ΔP(t ) = AΔσ(t ) = Aμi Δni(t ) P

(1)

2(CH3)2 CHOH + O2 → 2(CH3)2 CO + 2H 2O

The difference between the incident and reflected microwave power gives the microwave power absorbed by the sample (ΔP), which is directly proportional to the variation of the conductance Δσ(t) induced by the laser, where Δni(t) is the number of excess charge carriers i at time t and μi is the mobility of the charge carrier. The sensitivity factor A is independent of time but depends on the microwave frequency and the conductivity of the sample. The main types of data provided by TRMC are the maximum value of the signal (Imax), which reflects the number of excess charge carriers created by the pulsed laser, weighted by the mobility of the charge carriers and by the influence of charge-carrier decay processes during the excitation, and the

(2)

2.4.3. Acetic Acid Degradation: Action Spectrum. The photocatalytic decomposition of acetic acid was carried out using 30 mg of photocatalyst suspended in 3 mL of an aqueous solution of acetic acid (5% v/v), inside quartz cells with a volume of ca. 12 mL. The cells were sealed with a septum to avoid leakage of the generated CO2. The suspensions were stirred in the dark for 10 min (to attain adsorption equilibrium). The reactions were carried out using a 300-W xenon lamp (Hamamatsu Photonics C2578-02) equipped with a diffraction-grating-type illuminator (Jasco CRM-FD; see the spectra in Figure S6), allowing the selection of the irradiation wavelength in the range between 350 and 680 nm, in steps of 25012

DOI: 10.1021/acs.jpcc.6b06854 J. Phys. Chem. C 2016, 120, 25010−25022

Article

The Journal of Physical Chemistry C

3. RESULTS AND DISCUSSION 3.1. Characterization of the Photocatalysts. TEM observations showed small gold clusters well dispersed on the TiO2-P25 surface for all of the modified titania samples. In all cases, the Au-NPs were homogeneous in size and had diameters smaller than 4 nm, with a mean size of about 2−3 nm. With an increase of the Au loading, a slightly larger particle size was observed (see Table 1 and Figure S1).

30 nm. The samples were irradiated with a monochromatic light with a full-width at half-maximum (fwhm) of 15 nm irrespective of the selected wavelength, and the intensity of irradiation [(1.24−3.9) × 10−8 einstein s−1]. measured with a Hioki 3664 optical power meter, was maintained at ca. 3.5 mW. The samples were irradiated for 90 min under stirring. Every 30 min, a 0.2 mL gas sample was removed by syringe from the quartz cell and analyzed with a gas chromatograph (Shimadzu GC-14B) equipped with a flame ionization detector (FID) and a methanizer (Shimadzu MTN-1), to enhance the sensitivity by converting of CO2 into methane, to measure the amount of CO2 produced during the reaction. The reaction of acetic acid degradation is as follows16

Table 1. Characteristics of the Au/P25 Photocatalysts Prepared by a Chemical Method with THPC photocatalyst

theoretical weight (wt %)

sample color

average Au-NP sizea (nm)

std dev

(3)

Au0.5%/P25

0.5

2.3

0.4

The apparent quantum efficiency was calculated as the rate of CO2 evolution from the decomposition of acetic acid versus the flux of incident photons, assuming that four photons were required. 2.4.4. Photocatalytic Production of Hydrogen (H2). The production of H2 from an aqueous methanol solution was performed in a closed Pyrex glass reactor with an argon atmosphere and under vigorous stirring. For these experiments, 2 mg of each photocatalyst was suspended in 2 mL of a degassed aqueous solution with 25 vol % of methanol, used as a sacrificial agent. As mentioned by Ortega Méndez et al.,18 methanol is advantageous because it does not contain carbon− carbon links, thus reducing the risk of the formation of more carbon-based subproducts (the main intermediates generated in the production of H2 from methanol are formaldehyde, formic acid, and CO2) and, thereby, the fouling of the photocatalyst.18,36 The photocatalyst was dispersed in the solution by sonication. Two types of light-emitting diodes (LEDs; Innotas Elektronik) with peak emission at wavelengths of 400 and 470 nm were used as irradiation sources (see spectra in Figure S7). The amount of hydrogen produced was determined by gas chromatography (GC) on a Bruker Scion gas chromatograph/ mass spectrometer, with a thermal conductivity detector (column, molecular sieve 5 A, 75 m × 0.53 mm i.d.; oven temperature, 70 °C; flow rate, 22.5 mL min−1; detector temperature, 200 °C; carrier gas, argon). 2.4.5. Stability under Visible-Light. The stability of the material was studied during cycling for the Au0.5%/P25 sample in the photodecomposition of phenol, used as a model pollutant in water (50 ppm). The photodegradation was carried out in a Pyrex tube reactor containing 15 mL of the phenol solution and 15 mg of the photocatalyst. Before irradiation, the photocatalyst was dispersed in the solution by sonication, and then the suspension was stirred (magnetic stirring) for 30 min in the dark to ensure equilibrium between adsorption and desorption. Then, the solution was irradiated using a xenon lamp (Oriel 300 W) with a cutoff filter (AM32603-1, LOT-Oriel) for experiments carried out under visiblelight (λ > 450 nm). The photocatalytic tests were conducted under oxygen bubbling at a fixed rate flow. Aliquots of 1 mL were sampled from the reactor after 8 h of irradiation. The powder was separated by centrifugation and dried at 60 °C overnight. The collected sample was weighed and then reused at the same ratio of 1 mg/1 mL, and the resultant transparent supernatant (obtained after centrifugation) after each cycle was analyzed by HPLC.

Au1%/P25 Au2%/P25

1 2

light purple purple dark purple

2.4 2.8

0.4 0.4

CH3COOH + 2O2 → 2CO2 + 2H 2O

a

TEM size measured manually with DigitalMicrograph software.

In the HRTEM images (Figure 1), the anatase and rutile phases of TiO2 were identified based on the interplanar distances determined using DigitalMicrograph software. The HRTEM images show that the Au-NPs tended to be located on the anatase phase after thermal treatment at 500 °C. This is in agreement with literature results: Indeed, Tsukamoto et al.37 and Wen et al.38 reported that gold nanoparticles have different localizations on P25 (anatase or rutile) depending on the treatment temperature. The increase of the calcination temperature (from 473 to 873 K, increasing in 100 K steps) had an effect on the location and size of the Au-NPs, giving larger sizes and a preference for an interphase location. The interplanar distance (d) deduced by fast Fourier transform (see Figure 1d) was found to be equal to d = 0.204 nm, which corresponds to the interplanar distance for (200) planes of Au, according to JCPDS file no. 01-1174. The Brunauer−Emmett− Teller (BET) surface areas were measured in previous works, and a slight decrease in the BET surface area (about 47.0 m2· g−1) was found after Au deposition.39,40 The dependence of the Au particle size on the support has been attributed to differences in metal/support interactions.41 As reported by Murdoch et al., Au-NPs supported on anatase are smaller than Au-NPs supported on rutile.24 The higher Fermi level of anatase can result in stronger electronic interactions with Au-NPs, which can inhibit Au agglomeration, leading to smaller Au-NPs.24 Figure S2 shows the XRD patterns of the as-prepared Au/ P25 samples: The characteristic diffraction peaks of the anatase and rutile phases of TiO2-P25, according to JCPDS file nos. 211272 and 21-1276, respectively, can be observed for all of the samples.42 For the Au/TiO2 samples, the patterns also show peaks attributable to Au diffraction planes (200), (220), and (311), according to JCPDS file no. 01-1174. As expected, the intensity of the observed peaks for Au planes increase with increasing amount of gold. To analyze the chemical composition of the modified TiO2P25 samples and to identify the chemical state of the Au-NPs, XPS analysis was performed, supporting the metallic nature of Au-NPs, as shown in Figure S3a. The doublet at 82.8 eV (fwhm = 0.74 eV) corresponds to Au−, associated with the interfacial interaction between Ti3+ and Au nanoparticles.43 The main 4f doublet at 83.2 eV (fwhm = 0.67 eV) corresponds to metallic 25013

DOI: 10.1021/acs.jpcc.6b06854 J. Phys. Chem. C 2016, 120, 25010−25022

Article

The Journal of Physical Chemistry C

Figure 1. (a−c) HRTEM images of (a) Au0.5%/P25, (b) Au1%/P25, and (c) Au2%/P25, showing localization of the Au-NPs on the surface of TiO2P25. (d) HRTEM image of Au-NPs. Insets in a−d: Fast Fourier transform (FFT) images of Au NPs with indexed planes. (e) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) panoramic view of the Au0.5%/P25 sample.

gold, shifted by −0.8 eV, which indicates the Au−TiO2 interaction.19,39 The doublet at 83.7 eV (fwhm = 0.64 eV) corresponds to Au+, which can be related to the outer surface of the gold nanoparticles.43,44 It should be noted that XPS signals of Au increase with Au loading. In Figure S3b, the Ti 2p peaks are characteristic of Ti4+ in the TiO2 lattice with two main doublets at 458.6 eV (fwhm =1.19 and 2.01 eV).19,43,44 The doublet at 458.0 eV (fwhm =1.03 and 1.86 eV) corresponds to Ti3+, associated with defects in the TiO2 surface, which interact preferentially with the anionic gold found in the Au 4f region.43 In Figure S3c, the peak at 529.7 eV corresponds to oxygen in the TiO 2 lattice, and the peak at 530.8 eV denotes hydroxylation of the titanium oxide surface.19,43,44 The other peaks at 531.8 and 533.57 eV correspond to adventitious contamination by OC and COH, respectively. For C element, Figure S3d shows peaks at 288.6 and 286.9 eV corresponding to adventitious contamination by OC and COH, respectively. The main peak at 284.9 eV corresponds to CC bonds from adventitious contamination. The optical properties of the modified surface of TiO2-P25 were studied by DRS. In Figure 2, the spectrum of TiO2-P25 shows an absorption edge at about 400 nm due to the presence of rutile.26 The photoabsorption properties of the Au/P25 materials are higher than those of pure TiO2-P25, because AuNPs induce a shift of the absorbance toward visible-light as a

result of the interaction between the metal and the semiconductor, that is, the so-called Schottky barrier,12 and because of the plasmon of gold. This plasmon band is sensitive to size, shape, and environment15,45 and can be shifted depending on the stabilizer and substrate. Because of the coupling between the metal nanoparticles and the TiO2 support having a high reflective index, the plasmon band in the case of modified titania is usually red-shifted, as already reported for Au/ TiO2.16,38,46,47 A weak LSPR band from 500 to 650 nm with maximum values at 548, 554, and 560 nm for 0.5 wt % Au, 1 wt % Au, and 2 wt % Au , respectively, due to the plasmon of small nanoclusters,37,38 can be observed in Figure 2. These absorptions result in a pink-purple color of the modified TiO2 -P25 samples. Note that the LSPR maximum absorption peak shifts toward longer wavelengths with increasing loading, which is consistent with the fact that the size of the Au-NPs increases, as previously observed in the TEM images. The electronic properties of the samples were studied by the TRMC technique at different excitation wavelengths considering both the UV and visible regions. The samples were excited at different wavelengths (365, 400, 450, 470, 500, and 560 nm) to study the charge-carrier dynamics under UV and visible-light irradiation, with particular attention paid to excitation energies close to the plasmon of gold. The corresponding laser energies were 1.4, 0.7, 6.1, 5.7, 5.3, and 3.7 mJ·cm−2, respectively. The corresponding TRMC signals are shown in Figure 3. The surface modification with Au-NPs shows a strong influence on the charge-carrier dynamics in TiO2-P25. At 365 nm, the intensity of the signal, Imax, for the modified TiO2-P25 samples was lower than that for pure TiO2-P25, especially for the cases with lower loadings of gold, namely, 0.5 and 1 wt %. These results indicate that there was a lower amount of mobile electrons (lower Imax value) in the conduction band of the modified semiconductor and that the lifetime of the photogenerated electrons in the samples was decreased. Three phenomena linked to metal deposition and corresponding to the loss of charge carriers during the pulse might be responsible for this Imax reduction: (i) a shield effect by NPs, (ii) a surface recombination center induced by the synthesis method, and (iii) fast electron scavenging by the metal (