Article pubs.acs.org/JPCC
Influence of Formate Adsorption and Protons on Shallow Trap Infrared Absorption (STIRA) of Anatase TiO2 During Photocatalysis David M. Savory and A. James McQuillan* Department of Chemistry, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand S Supporting Information *
ABSTRACT: ATR-IR spectroscopy has been used to probe the spectral changes resulting from UV irradiation of thin particulate films of anatase TiO2 under aqueous anoxic conditions. Irradiation under circumneutral solutions removed adsorbed impurities and produced a transient broad IR absorption peaking at ∼880 cm−1 that has been attributed to excitations associated with shallow electron traps. This shallow trap IR absorption (STIRA) was long-lived under acidic conditions with peak absorbance rising with decreasing pH. Formate was used as a hole scavenger to probe the impact of hole removal on the electron trapping processes. The adsorption of formate to anatase TiO2 was investigated in the absence of UV irradiation with the spectra showing a mixture of innerand outer-sphere adsorbed species. UV irradiation of anatase films in the presence of formate at low concentrations enhanced STIRA intensity substantially. The STIRA scaled with TiO2 mass was more pronounced from larger crystallite anatase and was sensitive to [H+], suggesting that formation of the shallow trap state is accompanied by proton intercalation.
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INTRODUCTION Titanium dioxide (TiO2) is a well-known semiconducting metal oxide that has been investigated on many fronts. The large amount of research directed toward TiO2 can be attributed to its key role in early studies of photocatalytic water splitting,1 its use as a substrate in dye-sensitized solar cells,2 and the relative ease with which it can photocatalytically degrade many chemicals with input of ultraviolet (UV) light. It is additionally favored for its low cost, high availability, nontoxicity, and photostability that makes it a prime candidate for many realworld photocatalysis applications. It is therefore no surprise that TiO2 has been utilized in a variety of practical research areas such as environmental remediation,3,4 self-cleaning surfaces,5 advanced oxidation,6 and photocatalytic processes.7,8 Such areas of emerging and experimental science have progressed our understanding of TiO2 properties, aided by thriving activity in the fields of materials science, spectroscopy and computational chemistry. TiO2 occurs naturally in three phases, anatase, rutile and brookite, with anatase and rutile being the more commonly investigated and synthesized. TiO2 has been produced in morphologies ranging from nanoparticles, well-defined nanocrystals, inverse opals, and doped materials to highly crystalline slabs. Differences in morphologies and phase aside, pristine TiO2 has a fundamental band gap absorption in the UV with onset at 380−420 nm depending on phase. Absorption of UV photons produces electrons (e−) and holes (h+) in the conduction band (CB) and valence band (VB), respectively, which can migrate to available surface sites where they may participate in and drive chemical (photocatalytic) reactions. However, the nature of photocatalytic reactions at hydrated © 2013 American Chemical Society
interfaces remains poorly understood. More specifically, how electrons and holes interact with chemical species at or near the surface of nanoparticles in an aqueous environment and the resulting impacts on electronic behavior. There have been many investigations probing the kinetic and mechanistic aspects of nanoparticulate TiO2-photocatalysed reactions. However, most have employed experimental techniques that provide information on the bulk TiO2 behavior with little specificity toward the nanoparticle surfaces, thus insight into interfacial reactions is limited. The comprehensive review by Fujishima et al.9 provides an excellent précis of the surface-related studies pertinent to TiO2. Spectroscopic methods able to probe TiO2 systems during photocatalytic conditions can provide molecular insight; indeed many spectroscopic techniques have been utilized to explore these systems, such as electron paramagnetic resonance,10 infrared,11 X-ray,12 and UV−visible spectroscopies.13 However, there are few techniques and experimental designs that allow surfacesensitive in situ probing of TiO2 systems in an aqueous environment during chemical changes. Attenuated total-internal reflection infrared (ATR-IR) spectroscopy is an example of such a technique and one that has been used for the investigation of thin oxide films in both dry14 and aqueous15−17 environments. Conventional vibrational spectroscopy is expected to give limited insight into electronic processes occurring at TiO2 surfaces due to (a) the short timeframes of electronic processes Received: May 1, 2013 Revised: October 14, 2013 Published: October 15, 2013 23645
dx.doi.org/10.1021/jp404321f | J. Phys. Chem. C 2013, 117, 23645−23656
The Journal of Physical Chemistry C
Article
(femto to milliseconds) cf. the time of IR spectrum acquisition (seconds to minutes), and (b) lack of specific interaction of electrons and holes with infrared radiation. However, conventional IR spectroscopic observation of free/trapped electrons and holes is not novel. Work in the 1960s by Kukimoto et al. probed doped ZnS samples under UV irradiation and observed a broad IR absorption peaking at ∼0.12 eV, which was attributed to electron excitation from a hydrogenic donor center to the conduction band.18,19 Later work by Boccuzzi et al. showed exposure of ZnO to gaseous H2 produced a similar broad IR absorption that was attributed to excitation of electron levels populated as a consequence of H atom spillover.20,21 Quenching by O2 was consistent with an electronic origin. Guglielminotti and Bond22 observed similar IR spectra from reducing Ru/TiO2 with H2 as did Benvenutti et al. who investigated Pt-doped oxide samples.23 More recently, Szczepankiewicz et al. presented broad IR continua arising from photoexcited TiO2 that were attributed to IR absorption by mobile CB electrons, although a separate absorption at ∼3600 cm −1 was assigned to trapped electrons.24,25 Yamakata et al. probed platinized TiO2 particles using time-resolved IR spectroscopy and similarly observed a broad absorption that increased to lower wavenumber.26 CB electron scattering was considered and direct excitation of electrons in a shallow trap ΔE below the CB edge was alternatively suggested. However, the anticipated cutoff in absorption corresponding to the shallow trap ΔE was below the ∼1100 cm−1 experimental limits of their transmission experiment. In 2004, Warren and McQuillan presented ATR-IR spectra showing a broad absorption arising from anhydrous thin film P25 TiO2 under UV irradiation.14 With an extended spectral window, a sharp cutoff in the absorption was detected at 880 cm−1 allowing an unequivocal assignment of the absorption to a shallow electron trap level ∼0.1 eV below the CB edge. In subsequent studies,27−37 similar shallow trap IR absorptions (STIRA) have been observed and have prompted discussion of the chemical nature of the trap state. Panayotov and Yates30 doped TiO2 with atomic hydrogen and suggested hydrogen diffusion into the TiO2 lattice produced shallow electron donor Ti−O(H)−Ti groups that ionize resulting in population of electron traps. Savory et al. presented preliminary work on the pH sensitivity of the STIRA observed under aqueous conditions and supported a shallow electron trap being responsible for the absorption (Figure 1) though the mechanism of optical absorption remained unclear. Prior to this report most of the reported STIRA data was from TiO2 samples under vacuum conditions. Under aqueous conditions, H+ is able to intercalate into the TiO2 lattice,38,39 a process that can have significant impact on electron mobility.40−45 Furthermore, under band gap excitation H+ and photogenerated electrons may produce chemical structures equivalent to those arising from atomic hydrogen exposure, possibly giving (H+)(e−) pairs known to form in ionic oxides.46−50 The role of protons is therefore of great interest as proton intercalation can produce trapping levels near the CB edge.51−55 In order to further explore this behavior under aqueous conditions, a simple system is advantageous so formate ion has been chosen as a hole trapping species. Formate is chemically simple, extensively investigated, and interacts weakly with TiO2 surfaces in the experimental conditions used. Additionally, the expected products of formate degradation,56 CO2 and H+, are
Figure 1. Schematic of TiO2 band gap excitation producing holes and electrons, hole scavenging by solution species, trapping of electrons, and subsequent absorption of IR photons by a shallow trap state. Arrows are not to scale.
unlikely to chemically modify the TiO2 surface. Indeed formate is the practical choice as a starting point for the investigation of carboxylic acid interactions with TiO2 and consequences during photoexcitation. Here, we present ATR-IR spectroscopic investigations of UV-irradiated anatase TiO2 under aqueous conditions, as well as anatase exposed to formate ion, and discuss the adsorption and electronic processes occurring.
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MATERIALS AND METHODS All chemicals were used as received. Sodium hydroxide (AR grade, BDH) and hydrochloric acid (∼36%, AR grade, Fisher Scientific) were used for adjustment of solution pH; sodium formate (>99%, Ajax Finechem) and sodium chloride (AR grade, BDH) were used where discussed. Given concentrations of formate ion at different pH include formate contained in formic acid. Water used throughout was obtained from a MilliQ RG (Millipore, resistivity 18 MΩ cm) system fed by a distilled water source. All solutions used were sparged with argon (Industrial grade, >99.9%, BOC). Titanium dioxide samples were obtained courtesy of Bunsho Ohtani (Hokkaido University, Japan) and are part of a series of Japanese Catalysis Society TiO2 samples. The sample used in photocatalytic experiments was JRC-TiO-13, JRC-TiO-2 was used in hole scavenger free experiments and JRC-TiO-8 used for adsorption measurements only. Table 1 lists sample properties reported by Ohtani et al.57 and scanning electron microscopic images are available in the Supporting Information (Figure 1, Supporting Information) Table 1. Material Property Data from This Work1 and That Published by Ohtani et al.2−4 a sample
crystallite size1 (XRD)/ nm
primary particle2 size/nm
secondary3 particle size/ nm
BET surface area4/m2 g−1
JRC-TiO-2 JRC-TiO-8 JRC-TiO-13
43.6 6.7 22.6
45b 4 30
510 240 1002
18 338 59
a
Primary particle size is from XRD analysis of dry powder samples and Scherrer equation analysis. Secondary particle size was obtained from aqueous suspensions using a laser diffraction particle analyser and represents an average hydrated particle size. bRevised datum provided by Bunsho Ohtani. 23646
dx.doi.org/10.1021/jp404321f | J. Phys. Chem. C 2013, 117, 23645−23656
The Journal of Physical Chemistry C
Article
ATR-IR spectroscopic experiments were carried out using a Digilab FTS4000 FTIR spectrometer, equipped with a DuraSamplIR 3-reflection diamond-coated ZnSe prism mounted in a stainless steel housing (ASI SensIR Technologies, U.S.A.). The inaccessible spectral range ∼2500−1900 cm−1 with this accessory is due to diamond phonon absorptions. The diamond layer is unaffected by UV irradiation and insulates the TiO2 from the consequences of UV excitation of the underlying ZnSe.58 Between experiments the diamond optical surface was cleaned by polishing with aqueous γ-Al2O3 (0.015 μm diameter, Alfa-Aesar) slurry on a Buehler Microcloth. The spectrometer was purged with dry air, consequently temporal fluctuations in gaseous CO2 absorption (∼2350 cm−1, doublets at ∼3600 and ∼3700 cm−1) are evident in some spectra. Normal spectra were obtained at a resolution of 4 cm−1 and 64 coadded scans (75 s acquisition times). For kinetic measurements, a temporal resolution of 2−4 s was used corresponding to 2−4 coadded scans per spectrum. Room temperature was regulated at 23 ± 1 °C. Spectra were collected and processed using Resolutions Pro (v4.0, Digilab) and spectra have not been corrected for the wavelength dependence of penetration depth. Figures were prepared in OriginPro (v8.5, OriginLab) and further processed in Adobe Illustrator (CS5). The light-emitting diode (LED) used had a diameter of 5 mm, peak output at 370 nm (∼16 nm fwhm), power of 2−4 mW, and threshold voltage of ∼4.0 V (model 370−5R15-M, VioLED International Inc., Taiwan). This was powered using a 12 V dc supply connected to a custom built constant-current source. All UV irradiation data was obtained using an LED current of 20 mA. The UV light intensity at the TiO2 surface was ∼1 mW cm−2. Thin TiO2 films were prepared using the following method: dry TiO2 powder was suspended in pure water to give a concentration of 1 mg mL−1 and sonicated for 15 min (Bandelin Sonorex) to disperse the suspension thoroughly. Subsequently a 10 μL droplet was deposited on the cleaned diamond surface and left to dry in ambient for ∼30 min giving films of thickness ∼1 μm (Figure 1, Supporting Information). The dried film was immersed in a 10 μL droplet of water to allow gentle rehydration as the flow cell was assembled, followed by pure water (argon sparged) flow. The film was then subjected to a UV cleaning procedure, alkaline wash, and subsequently adjusted back to circumneutral and/or acidic conditions with HCl as later described. The studied films were stable in solutions between pH 2 and 11 and spectral changes indicative of TiO2 losses were not observed at the conclusion of the experiments, nor were the films visibly altered. Aqueous flow experiments were performed using a custom built ∼1 mL volume flow cell (Figure 2) consisting of an 25 mm diameter rubber O-ring, 6 mm thick glass block with pressfitted plastic flow cell inserts, and shrouding plastic allowing for secure clamping of the flow cell system and reproducible positioning of the LED. Solutions were flowed through Tygon Lab tubing (R-3603, Masterflex) using a peristaltic pump (Masterflex C/L, Cole-Parmer) at a flow rate of ∼2.0 mL min−1. Solutions that were argon-sparged in a reservoir and flowed into the ATR-IR accessory via Tygon tubing showed residual oxygen electrode (YSI Pro 2030) levels typically less than 0.5 mg L−1 (1.6 × 10−5 mol L−1) with the oxygen concentration being determined by the permeability and length of the tubing. Intrinsic variations in film structure (effective density and porosity, distribution of crystal surfaces exposed, surface site availability, etc.) between experiments significantly
Figure 2. Schematic of custom flow cell assembly on the ATR-IR accessory.
impact adsorption kinetics and photocatalytic behavior. Thus, a single TiO2 film was used when behavior variations with changes in conditions were assessed.
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RESULTS AND DISCUSSION a. IR Spectral Changes from UV Irradiation of Anatase TiO2 Films in a Circumneutral Aqueous Environment. In contrast to many studies of UV illuminated TiO2 under dry/ vacuum conditions there have been few IR spectroscopic investigations under aqueous conditions, owing to the difficulties associated with IR absorption by water in regions of interest. The ATR-IR technique can circumvent this obstacle and allows for probing of thin films under aqueous photocatalytic conditions. Prior to discussing the photocatalytic activity of TiO2 in the presence of formate ion it is sensible to address the activity of TiO2 under aqueous conditions when subjected to UV irradiation in the absence of exogenous photooxidisable species. Indeed this is necessary to benchmark any intrinsic response that may overlay later observations. There have been few comments in the photocatalysis literature on the impact of UV irradiation on TiO2 films in aqueous conditions. Here we present a general example of an anatase TiO2 film, prepared from aqueous suspension of the as-received powder, subjected to UV irradiation and alkaline washing procedures. Experiments were carried out with flowing argon-saturated solutions. Thus with relative absence of oxygen, minimal scavenging of CB electrons is expected and oxidation of water is the expected dominant fate of VB holes. Figure 3 presents the spectral changes occurring during initial TiO2 film cleaning procedures. The film was first subjected to circumneutral pure water flow (measured pH of 6−8) for 15 min, followed by UV irradiation for approximately one hour, alkaline washing at pH ∼10.5 with NaOH for 20−30 min, and subsequent adjustment to acidic conditions (typically pH 3) with HCl. Spectrum (A) shows the presence of the shallow trap IR absorption (STIRA) of interest during UV irradiation under circumneutral conditions. In this circumstance the STIRA is transient, generally disappearing before acquisition of a second spectrum (lifetime
