Article pubs.acs.org/JPCC
Microsecond Transient Absorption Spectra of Suspended Semiconducting Metal Oxide Nanoparticles Herme G. Baldoví, Belén Ferrer, Mercedes Á lvaro, and Hermenegildo García* Instituto Universitario de Tecnologia Química, CSIC-UPV, and Departamento de Química, Univ. Politécnica de Valencia, Avd. de los Naranjos, 46022 Valencia, Spain S Supporting Information *
ABSTRACT: The time-resolved absorption spectra of opaque semiconductor powders have been typically studied by diffuse reflectance techniques. Herein, we show that time-resolved transmission spectroscopy of suspended nanoparticles is a suitable way to detect electrons and holes generated photochemically in these solids. Ultrasound dispersion of nanoparticulate metal oxide semiconductors, including TiO2, ZnO, NiO, and α-Fe2O3, in acetonitrile or water leads to the formation of indefinitely persistent suspensions in which metal oxide nanoparticles are dispersed in the liquid phase. Dynamic laser scattering reveals the presence of these nanoparticles of dynamic dimensions ranging from about 30 to 200 nm. Time-resolved transmission absorption spectroscopy of these suspensions allows detecting transients decaying in the microsecond time scale that are characterized by a broad absorption spanning the whole visible range and almost coincident temporal profiles in every wavelength that have been assigned to the state of charge separation. This assignment was supported by quenching experiments and by steady-state irradiation of the suspensions containing methyl viologen (electron acceptor) and tetramethyl-p-phenylenediamine (hole acceptor) as probe molecules that lead to the generation of the visually observable corresponding radical cations.
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INTRODUCTION Photocatalysis is continuously gaining increasing interest because in addition to conventional applications for environmental remediation and pollutant degradation,1−7 it can also be applied to the conversion of solar light into electrical or chemical energy.8−17 Most of the photocatalysts are based on semiconducting solids in which light absorption leads, as a general phenomenon, to charge separation with the creation of electrons in the conduction band and positive holes in the valence band.18 While most of the studies in this area report on the photocatalytic activity of semiconductors for different applications, there is a paucity in the understanding of the reasons for the overall photocatalytic activity. This lack of rationalization derives from the complexity of photocatalysis whose mechanism encompasses several elementary steps including light absorption and charge separation, charge migration through the particle to the surface, and electron and hole transfer to adsorbed substrates. Direct time-resolved spectroscopic techniques capable of monitoring the generation and decay of electrons and holes are powerful tools that can allow monitoring of the charge-separated state and measurement of their decay kinetics.19 In solution molecular photochemistry, a considerable degree of rationalization and understanding on the photophysical and photochemical events has been gained by applying transient absorption spectroscopy to the detection of electronic excited © 2014 American Chemical Society
states and reactive intermediates generated photochemically.20,21 In contrast to the situation of molecular photochemistry applied to optically transparent solutions of the organic probe, similar studies for semiconductors have been less abundant.22−26 This is in part due to the fact that semiconductors are opaque powders that are not suitable for transmission mode. Diffuse reflectance (DR) techniques in which the light reflected by a thick surface is compared before and after the laser pulse has been applied to the detection of transients for these solid samples with more difficulty than transmission mode due to the weak signals generally observed for powders and the lower sensitivity of DR compared to transmission.27,28 To overcome this limitation, several transient absorption measurements have been carried out using micrometric films of these semiconductors on a transparent substrate.23 In the solid state, in the absence of any liquid phase in contact with the semiconductor, time-resolved studies of the most important metal oxide semiconductor TiO2 has led to the conclusion that most of the photogenerated charges collapse in the subnanosecond time scale29,30a fact that indicates that most of the electrons and holes must decay by recombination without the occurrence of chemical reactions. Chemical Received: February 21, 2014 Revised: April 9, 2014 Published: April 14, 2014 9275
dx.doi.org/10.1021/jp5018345 | J. Phys. Chem. C 2014, 118, 9275−9282
The Journal of Physical Chemistry C
Article
circuit was measured by a potentiostat (AMEL model 7050), which transferred the data through the A/D converter to the PC controlling the monochromator.
transformations, such as those carried out by TiO2 photocatalysis, typically require much longer-lived electrons and holes in order to have the opportunity to react and to be quenched by the substrates. It could be that the conditions needed for DR transient spectroscopy of dry powders are not relevant for photocatalytic reactions taking place in suspension. For this reason, it is of interest to perform transient absorption spectroscopy of semiconductors suspended in liquid phase, but this type of measurement has been rarely made. In the present study, we exemplifly that transmission absorption spectroscopy can be applied to detect transients on suspensions of semiconducting metal oxide nanoparticles (NPs).
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RESULTS AND DISCUSSION In the present article, we report transient absorption spectroscopic studies by transmission technique of persistent suspensions of a series of metal oxide semiconductors that have been widely used as photocatalysts. The series includes Fe2O3, NiO, ZnO, and TiO2 in its two phases anatase and rutile. The series of semiconductors under study and their main textural properties are collected in Table 1. All of them are commercial samples constituted by agglomerated NPs that have been widely used as photocatalysts.31
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EXPERIMENTAL SECTION All the chemicals employed have been purchased from Aldrich. The powders (1.5 mg) were suspended in acetonitrile or water (5 mL) by sonication (250 W/30 min), and the resulting transparent suspensions, after separation of any solid residue, were characterized by optical absorption spectroscopy and laser scattering. UV−vis absorption spectra of suspended NPs have been recorded with a Jasco V-650 spectrophotometer. Laser scattering measurements were performed in a Malvern Instrument, and the size distribution of the samples was obtained using a Zetasizer program. Laser flash photolysis (LFP) experiments of the suspensions in transmission mode were carried out using the third (355 nm) harmonic of a Surelite Nd:YAG laser for excitation (20 mJ, pulse ≤10 ns). The signal from the monochromator/photomultiplier detection system was captured by a Tektronix 2440 digitizer and transferred to a PC computer that controlled the experiment and provided suitable processing and data storage capabilities. LFP experiments in DR mode were performed by using a Qswitched Nd:YAG laser (Quantel Brilliant, 355 nm, 20 mJ/ pulse, 5 ns fwhm) coupled to a mLFP-122 Luzchem miniaturized detection equipment. This transient absorption spectrometer includes a 300 W ceramic xenon lamp, 125 mm monochromator, Tektronix TDS-2001C digitizer, compact photomultiplier and power supply, cell holder and fiber-optic connectors, computer interfaces, and a software package developed in the LabVIEW environment from National Instruments. The LFP equipment supplies 5 V trigger pulses with programmable frequency and delay. The rise time of the detector/digitizer is ∼3 ns up to 300 MHz (2.5 GHz sampling). The monitoring beam is provided by a ceramic xenon lamp and delivered through fiber-optic cables. The laser pulse is probed by a fiber that synchronizes the LFP system with the digitizer operating in the pretrigger mode. Transient spectra of suspensions were recorded using 10 mm × 10 mm quartz cells of 4 mL capacity and were bubbled for 30 min with argon or the corresponding gas quencher (O2 or N2O) before data acquisition. The absorbance of the samples was 0.3 OD at the laser excitation wavelength (355 nm). Transient spectra of powdered solid samples were recorded using 3 mm × 7 mm quartz cells. For the photocurrent measurements the samples were placed in a one-compartment cell and were deoxygenated by N2 bubbling prior to each experiment that was also run under a N2 atmosphere. Photocurrent measurements were performed using a three-electrode standard configuration with a platinum wire as counter electrode, saturated Ag/AgCl electrode as reference, and a steel sponge as working electrode. The samples were excited with a 150 W xenon lamp through a Czerny Turner monochromator. The current output at short
Table 1. Appearance and Textural Properties of the Set of Commercial Semiconductors under Study color Fe2O3 NiO ZnO TiO2 rutile TiO2 anatase
red-brown dark greenblack white white white
av particle size (nm)
surface area BET (m2/g)
