Transient Absorption Spectroscopy of Anatase and Rutile: The Impact

Article pubs.acs.org/JPCC

Transient Absorption Spectroscopy of Anatase and Rutile: The Impact of Morphology and Phase on Photocatalytic Activity Xiuli Wang,†,‡,⊥ Andreas Kafizas,*,‡,⊥ Xiaoe Li,‡ Savio J. A. Moniz,§ Philip J. T. Reardon,§ Junwang Tang,§ Ivan P. Parkin,∥ and James R. Durrant*,‡ †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, 457 Zhongshan Road, Dalian, China Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K. § Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K. ∥ Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K. ‡

S Supporting Information *

ABSTRACT: We employed transient absorption spectroscopy (TAS) to investigate the kinetic dependences of photocatalysis in anatase and rutile TiO2 films of varying morphology. In mesoporous films, anatase was ∼30 times more efficient than rutile in the photocatalytic degradation of an intelligent ink model system. Independent of phase, up to 100 lower levels of photocatalysis were found in dense films. Charge carrier lifetimes were probed by TAS on the microsecond to second time scale. For both rutile and anatase, recombination was independent of morphology. Rutile exhibited up to 10 times slower recombination kinetics than anatase. Efficient, irreversible hole scavenging by alcohols was present in mesoporous anatase alone, resulting in the generation of long-lived electrons (τ ≈ 0.7 s) which, upon the addition of the dye reduction target resazurin, enabled efficient electron transfer (τ ≈ 3 ms). Hole scavenging by alcohols on mesoporous rutile was substantially less efficient and more reversible than anatase, resulting in only a marginal increase in electron lifetime. The lower activity of rutile was not due to differences in recombination but rather to the deficiency of rutile holes to drive efficient and irreversible alcohol oxidation.

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surface area is needed to achieve high quantum yields.7 On the basis of similar arguments, mesoporous TiO2 films have been actively developed.8 However, it is the dense form of TiO2 that is found in commercial self-cleaning products, where efficiency has been compromised for material durability. Although substantial efforts have been made to control the morphology of TiO2 to improve photocatalysis, the role of morphology in photocatalysis is not clear from the aspect of kinetics. To understand this, in this study we will examine the difference in photocatalytic activities for mesoporous and dense thin films of both pure anatase and rutile phase. We will then compare these differences in activity with the lifetime and population of photogenerated charges using transient absorption spectroscopy (TAS). TAS is a form of laser flash spectroscopy that tracks transient changes in absorption after an excitation pulse.9−11 It has been shown that the dynamics specific to photogenerated electron or hole carriers could be studied in TiO2 by tracking transient absorbance changes at particular wavelengths.12 This has allowed many research groups to use TAS to understand the dynamics of charge transfer in many photocatalytic processes,

INTRODUCTION TiO2 is the most widely employed material for photocatalytic water splitting and degradation of organic pollutants. Its importance can be attributed to its unique physicochemical properties.1−3 For photocatalytic applications, anatase and rutile are the two most studied TiO2 polymorphs. Despite possessing the same chemical composition, the different arrangements of TiO6 octahedra in anatase (4 edge sharing partly distorted octahedra) and rutile (2 edge sharing nondistorted octahedra) result in different physicochemical properties. Aside from phase mixtures and composites, the anatase form is generally regarded as the more active phase in photocatalysis,1 especially for environmental applications,2 despite rutile often showing superior photocatalytic performance in specific cases, such as water oxidation.4,5 This is further supported by its commercial application, where anatase is used exclusively in self-cleaning windows (Pilkington Activ, SaintGobain Bioclean, PPG Sunclean) and tiles (TOTO Hydrotect).6 Understanding why anatase often shows a higher activity compared with rutile in most photocatalytic reactions remains a key challenge that will support strategies to enhance TiO2 photocatalysis. Nowadays, most studies of photocatalytic reactions are focused on particulate systems (as opposed to their dense thinfilm counterparts) due to the apparent reality that a high © XXXX American Chemical Society

Received: February 25, 2015 Revised: April 9, 2015

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DOI: 10.1021/acs.jpcc.5b01858 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Scheme 1. Two-Step Reduction of Resazurin (Royal Blue) to Resorufin (Pink) and Then Dihydro-resorufin (Colorless)

such as the reduction of O2,13 the oxidation of NO,14 and the oxidation of water.15 To date, such studies have largely been limited to anatase, where no direct comparison of the charge carrier dynamics in anatase and rutile TiO2 has been made. This study begins with a comparison of the oxidation of methanol, a widely used hole scavenger in photocatalysis. Although there have been TAS studies of the hole scavenging effect of alcohols with anatase,16 to our knowledge, there has been no such study with rutile. This study is then extended to intelligent ink, previously shown to be an effective model system for quantifying photocatalytic activity.17,18 The ink consists of three components dissolved in water: (i) a polymer thickener, (ii) glycerol, and (iii) a resazurin redox dye. Intelligent ink is made rapid acting through the use of a hole scavenger, glycerol. Such alcohols are well-known for their hole scavenging potency and are regularly used to increase the efficiency of photocatalysis, including water splitting.2 In removing holes, electron−hole recombination is retarded, and photogenerated electrons have more time to take part in reduction processes of the dye, as summarized in Scheme 1. The focus of this study is to understand how the phase and morphology of TiO2 impacts the kinetic competition between electron−hole recombination and their reaction with organics in solutionenabling better materials design for future optimization of the photocatalytic function.

using the Barret−Joyner−Halenda (BJH) method.24 Mesoporous film thickness was measured using an Alpha 200 Profilometer. Photocatalytic Activity. The photocatalytic test involved spray-coating intelligent ink25 evenly over the surface of the film.18 The intelligent ink (royal blue in color) consisted of hydroxyl ethyl cellulose polymer (0.45 g), glycerol (3.0 g), and the redox dye resazurin (40 mg) dissolved to 30 mL in distilled water. Samples were spray-coated with the ink using an aerosol spray gun (SIP Emerald Spray Gun, Halfords, Plc.) at an airpressure feed of 3.5 bar until a blue coating was formed. UV− visible absorption spectra were measured from 450 to 700 nm using a PerkinElmer Lambda 25 device. A soft UVA light source was used as the excitation source for the photocatalytic reaction (365 nm, Vilber Lourmat 2 × 8W, Figure S1, Supporting Information), which was shone from above (i.e., lamp−ink−sample−substrate). The lamp flux was measured using a UVX-Radiometer equipped with a UVX-36 sensor (λ = 365 ± 15 nm). Previous studies have shown that the photocatalytic reduction of intelligent ink on anatase TiO2 is first order, where the rate constant for the reduction of resazurin to resorufin is almost 4 times larger than the reduction of resorufin to dihydro-resorufin.26 As such, the rate of the first reduction (Scheme 1) can be determined by assessing the decrease in absorbance at 608 nm (λmax, resazurin) at early times (i.e., times in which the rate of decrease is linear). The thickness of the ink coating could be derived using the Beer−Lambert law given the dye was 5.8 mM in concentration and possesses an extinction coefficient of 43 500 M−1 cm−1 at 608 nm. The rate of decrease at 608 nm could then be converted into a molecular rate. Thus, with knowledge of the light flux from the 365 nm excitation source (450 μW·cm−2 ≈ 1.1 × 1015 photons·cm−2·s−1) coupled with the material’s absorbance at 365 nm the quantum yield (QY) was derived

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EXPERIMENTAL SECTION Sample Preparation. Mesoporous TiO2 Film. A colloidal anatase paste was prepared from the aqueous hydrolysis of titanium isopropoxide, as described in our previous publications.19 A colloidal rutile paste was prepared from a commercial powder (Shanghai ST-Nano Science and Technology Co.).20 Mesoporous films were formed by doctor-blading each colloidal paste onto glass microscope slides, cleaned with acetone prior to use. The pastes were allowed to air-dry for 30 min before calcination at 450 °C for 30 min. Dense TiO2 Film. Dense anatase thin films were grown on quartz substrates by chemical vapor deposition from the reaction of TiCl4 and ethyl acetate vapors carried by N2 gas at 500 °C. Dense rutile films were made by simply annealing the dense anatase films at 1050 °C in air for 5 h to ensure the complete conversion to its thermodynamic phase.21 Physical Characterization. Surface microstructures were investigated by scanning electron microscopy (SEM) on a JEOL-6301F field emission instrument. X-ray diffraction (XRD) patterns were measured with a modified Bruker-Axs D8 diffractometer with parallel beam optics equipped with a PSD LinxEye silicon strip detector. Patterns were fit to a Rietveld refined model using GSAS-EXPGUI software.22 UV− visible transmittance and reflectance spectra were recorded from 250−1100 nm using a Helios double beam instrument equipped with an integrating sphere. Specific surface area measurements were measured using the Brunauer−Emmett− Teller (BET) method23 in N2 using a TriStar 3000 Micromeritics device. Pore diameter distributions were determined

QY (%) =

molecules degraded (cm−2·s−1) photons absorbed (cm−2· s−1)

× 100 (1)

Transient Absorption Spectroscopy. Charge carrier dynamics of TiO2 films were measured using transient absorption spectroscopy (TAS) from the microsecond to second time scale at room temperature. The TAS apparatus has been described in detail elsewhere.27 In brief, a 75 W Xe lamp is used as a probe beam with a monochromator placed before the sample. The change in transmitted light is measured by a Si PIN photodiode after a UV laser excitation pulse is applied on the sample using the third harmonic of a Nd:YAG laser (355 nm, 6 ns pulse width). Reasonably low laser intensities were used (∼30−500 μJ cm−2 pulse−1) with a laser repetition rate of 1 Hz. Each TAS trace is the result of averaging between 50 and 500 scans.

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RESULTS Materials Characterization. X-ray diffraction showed that the mesoporous and dense films of anatase (I41/ amd) and

B


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Figure 1. SEM images of (a) mesoporous anatase, (b) mesoporous rutile, (c) dense anatase, and (d) dense rutile films.

Figure 2. (a) Changing UV−visible absorption spectra during the photocatalysis of intelligent ink on mesoporous rutile under excitation at 365 nm and (b) the change in normalized absorbance at 608 nm (λmax resazurin) with time (s, log time scale) during the photocatalysis of intelligent ink for mesoporous anatase [open circles], mesoporous rutile [black circles], dense anatase [open squares], dense rutile [black squares], and a blank piece of glass [gray circles].

was investigated by SEM (Figure 1). Mesoporous films were rough and porous, with anatase possessing primarily spherical particles ∼20 nm in width and rutile possessing more fluffy particles that ranged from 100 to 500 nm in width. Dense films were more compact and flat, with anatase possessing tetragonal domains 100−200 nm wide and rutile possessing larger and more rounded domains 200−600 nm wide. Photocatalytic Activity. The photocatalytic activity of mesoporous anatase and rutile films was examined using the intelligent ink test as a model system.25 Figure 2a depicts the changes observed by UV−visible spectroscopy at selected times during the reaction on mesoporous rutile. At 0 s, a spectrum characteristic of the sole presence of resazurin is observed.30 At the early stages of the reaction, predominantly resazurin (λmax = 608 nm) is reduced to resorufin (λmax = 575 nm). By 450 s most of the resazurin was consumed, and a maximum concentration of resorufin was reached. By 1000 s the resazurin dye was completely reduced, and a spectrum characteristic of the sole presence of resorufin is observed.26 Further irradiation

rutile (P42/ mnm) were phase-pure (Figure S2, Supporting Information). Scherrer line broadening studies28 showed that the average width of crystallites were ∼40 nm in the mesoporous anatase film, ∼ 40 nm in the dense anatase film, ∼ 110 nm in the mesoporous rutile film and ∼80 nm in the dense rutile film. Dense films showed strong preferred growth effects, akin to epitaxially grown films by CVD.29 No preferred growth was observed in mesoporous films, typical of particulate growth. The specific surface areas of mesoporous anatase and rutile (Figure S4, Supporting Information) as determined by the Brunauer−Emmett−Teller (BET) method were measured to be 100 m2.g−1 and 72 m2.g−1 respectively. The specific surface areas of our dense films were lower than the detection limits of the device ( dense rutile

(2)

Transient Absorption Spectroscopy. Employing Scavengers to Identify Photogenerated Electron and Hole Spectra. Insight into photogenerated charge carrier events, including phenomena occurring at the material−reactant interface, can be acquired using transient absorption spectroscopy (TAS).12 In both anatase and rutile, TAS spectra measured in N2 were composed of broad slopes that showed stronger absorbance at shorter wavelengths (Figure 3). When in N2 (an inert atmosphere), the photogenerated electrons and holes in TiO2 recombine nongeminately15 (i.e., no surface reactions can occur). To identify the regions where photogenerated electrons and holes were present in TAS spectra, chemical scavengers were D


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Figure 5. Transient absorption decays recorded in either an inert N2 environment (black lines) or a N2-purged pure methanol solution (red lines) for mesoporous anatase following a 355 nm laser excitation pulse (6 ns pulse width, 1 Hz, 250 μJ·cm−2·pulse−1). Samples were probed at (a) 900 nm or (b) 460 nm.

Figure 6. Transient absorption decays recorded in either an inert N2 environment (black lines) or a N2-purged pure methanol solution (red lines) for mesoporous rutile following a 355 nm laser excitation pulse (6 ns pulse width, 1 Hz, 250 μJ·cm−2·pulse−1). Samples were probed at (a) 900 nm or (b) 550 nm.

nongeminate recombination of charges is relatively independent of topography in TiO2 (i.e., particle size, density, morphology, etc.) but dependent on crystal structure. Decays in anatase showed a modest retardation with a decrease in excitation density (Figure S5a, Supporting Information), whereas decays in rutile were almost independent of excitation intensity (Figure S5b, Supporting Information). At the lower excitation densities studied, carrier lifetimes in anatase were more similar to rutile (Figure S6, Supporting Information). Decay Kinetics in Methanol and Other Alcohols. The hole scavenging effect of methanol was studied in both mesoporous and dense films. Interestingly, methanol (and other alcohols such as isopropanol and glycerol) showed no noticeable effect on the transient absorption decay dynamics in dense films (Figure S7, Supporting Information). However, substantial changes were observed in mesoporous films (Figures 5 and 6). This clearly shows how nanostructuring TiO2 enables charge carriers to reach the surface and thus produces a more active photocatalyst. Figure 5 shows the effect of methanol on the photogenerated carrier dynamics in mesoporous anatase. The presence of methanol both increases the initial absorption of mesoporous anatase at 900 nm almost 3-fold and increases the half-lifetime of the decay from ∼100 μs to ∼0.7 s signifying a reaction with methanol on the premicrosecond time scale (Figure 5a). Correspondingly, the initial absorption at 460 nm decreases to one-third of the original signal, with the residual signal in the presence of methanol decaying with the same dynamics as

employed. For holes, the well-known electron scavenger AgNO3 was used.32 Similar to previous studies of anatase,33,34 the large rise in absorbance at shorter wavelengths was attributed to the presence of photogenerated holes. The change in spectral signature for both anatase and rutile shows how photogenerated electrons are quenched by AgNO3 on the premicrosecond time scale. In the measured wavelength range, hole absorption increased at shorter wavelengths. For electrons, the well-known hole scavenger methanol was used.15 In anatase, photogenerated holes are completely quenched by methanol on the premicrosecond time scale, whereas in rutile they are only partly quenched (evidenced below). Similar to previous studies,35,36 a rise in absorbance at longer wavelengths was attributed to the presence of photogenerated electrons (at around 900 nm for both anatase and rutile). It is apparent that the electron signal in rutile is relatively small compared with anatase, most probably due to a lower extinction coefficient. Charge Recombination in the Absence of Electron or Hole Scavengers. Figure 4 shows the transient absorption decays at 900 nm of both mesoporous and dense TiO2 in N2. At this probe wavelength, the photoinduced absorption comes primarily from TiO2 electrons,12 as demonstrated in Figure 3. For anatase, the decays show power law dynamics, and for rutile, the signal decays linearly on a semilog time scale, consistent with previous reports.37,38 Photogenerated carriers are longer lived in rutile than in anatase. Irrespective of dense or mesoporous morphology, analogous transient absorption decay kinetics were found in each phase. This shows how the E


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Figure 7. Transient absorption spectroscopy study of the various photocatalytic processes in intelligent ink. The normalized transient decays were recorded in air, water, glycerol, resazurin dye (4 mM), and intelligent ink for (a) mesoporous anatase and (b) mesoporous rutile after back-excitation with a 355 nm laser (6 ns pulse width, 1 Hz, 250 μJ·cm−2·pulse−1) probed at 900 nm [electron signal].

Table 1. Summary of the Charge Transfer/Decay Times (t1/2) for Mesoporous Anatase and Rutile in a Range of Media Studied by Transient Absorption Spectroscopy on the Microsecond to Second Time Scale

mesoporous anatase mesoporous rutile

medium

N2

AgNO3(aq) (2 mM) in N2

methanol in N2

intelligent ink in air

processes

e− + h+

e− + Ag+, h+ + H2O/other

h+ + methanol, e− + methanol radicals

h+ + glycerol, e− + resazurin

initial time

from 10 μs

from 10 μs

from 10 μs

from 300 μs

80 μs 80 μs 1 ms 1 ms

completely quenched 0.7 s completely quenched 0.9 s

0.7 s completely quenched 2 ms ∼2/3 quenched

10 ms completely quenched transfer not observed ∼2/3 quenched

e− h+ e− h+

(900 (460 (900 (550

nm) nm) nm) nm)

studies that showed electron transfer to water and O2 is slow (up to 100 μs).39 The addition of glycerol resulted in long-lived electrons (τ ≈ 0.2 s) similar to those observed with the addition of methanol (Figure 5). In the presence of resazurin dye, but no glycerol, the electron signal decayed in a similar fashion to its natural decay in air, indicating electron−hole recombination was faster than dye reduction in the absence of a hole scavenger. However, in the presence of both glycerol and resazurin, the electron signal showed different decay kinetics, assigned to the transfer of electrons from mesoporous anatase to the resazurin dye (τ ≈ 3 ms). This electron transfer is 100 times faster than the electron decay in glycerol alone and consistent with the high quantum yield observed in our photocatalysis studies (QY = 62%, Figure 2). In contrast, all transient decays in mesoporous rutile showed similar kinetics, with only a modest retardation of the transient decay at long times in the presence of glycerol. This relative independence of the transient decay kinetics in the presence of the intelligent ink clearly indicates poor hole scavenging by glycerol and poor electron transfer from rutile to resazurin, which was also consistent with the low quantum yield observed in our photocatalysis studies (QY = 1.9%).

those at 900 nm and therefore assigned to the tail of electron absorption extending to this wavelength. The effect of methanol on photogenerated carrier dynamics in mesoporous rutile was also studied (Figure 6). Decay kinetics were probed at 900 and 550 nm to monitor changes in photogenerated electrons and holes, respectively (see Figure 3). The initial absorption at 550 nm was reduced by ∼2/3 in methanol compared with N2 alone. However, unlike anatase, the signal at 550 nm decayed to zero by 1 ms. As the kinetics at 550 nm did not track the kinetics at 900 nm, they each represent different processes. The decay at 550 nm is attributed to the hole scavenging of methanol that started on the premicrosecond time scale but ended by 1 ms. The decay at 900 nm is attributed to a partially enhanced electron signal. The decay dynamics at 900 nm was not significantly prolonged by the addition of methanol until after 1 msthe same time in which all holes at 550 nm had been scavenged. Although the addition of methanol results in a scavenging of approximately 2/3 of rutile holes on the premicrosecond time scale, it does not significantly increase the lifetime of photogenerated rutile electrons until 1 ms. Similar decay kinetics were observed in isopropanol and glycerol (Figure S8, Supporting Information). Analysis of Photocatalytic Function. In order to directly correlate our transient absorption data with photocatalytic activity, our transient absorption studies were extended to our intelligent ink study. In breaking the reaction down, the different photocatalytic processes that occur within intelligent ink were studied by TAS (Figures 7 and Figure S9, Supporting Information). In mesoporous anatase, the electron decay kinetics in air, water, and N2 were indistinguishable, consistent with previous

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DISCUSSION Charge transfer and decay dynamics of the photogenerated electrons and holes in mesoporous anatase and rutile were studied by transient absorption spectroscopy (TAS) in dense and mesoporous anatase and rutile TiO2. The results were correlated with the photocatalytic activity of reducing an intelligent ink, which shows that anatase is more efficient than rutile and mesoporous films more efficient than dense. The F


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Scheme 2. Summary of the Time Scales of the Processes in the Photocatalysis of Intelligent Ink with (A) Mesoporous Anatase and (B) Mesoporous Rutilea

a

t1/2 half-times were measured from 10 μs at ∼300 μJ·cm−1·pulse−1. Dotted lines represent the slowest transfer process in each system.

generating long-lived electrons. In mesoporous anatase, an almost 3-fold increase in electron population was observed from 10 μs with a dramatic increase in the decay half-time from ∼100 μs to ∼0.7 s. However, in mesoporous rutile, a mere 25% increase in electron population was observed from 10 μs with only a marginal increase in decay half-time from the millisecond time scale onward. This strongly suggests that methanol oxidation by rutile holes is at least partially reversible, such that it is relatively ineffective at preventing electron losses. This inability of methanol (and other alcohols, see Supporting Information) to suppress electron−hole recombination in rutile compared with anatase is consistent with rutile’s lower photocatalytic activity as assayed in our intelligent ink study. Our observation of less effective hole scavenging on rutile is in line with recent studies of anatase and rutile single-crystal surfaces, where anatase was consistently more active than rutile in the photo-oxidation of methanol.42 The photocatalytic oxidiation of methanol by anatase has been well studied.42 It is suggested that the C−H bond of methanol is first cleaved by a surface hole forming the ●CH2OH radical. Contrary to anatase, the methanol must first dissociate on the rutile surface to form absorbed methoxy groups before C−H bond cleavage can occur, 43 which is perhaps why slower/more reversible scavenging kinetics are observed. This difference in hole scavenging behavior was in contrast to the reactions of mesoporous anatase and rutile with the electron scavenger AgNO3, where efficient and highly irreversible electron scavenging was observed in both materials (Figure 3 and Figure S10, Supporting Information). This was a strong indication that the differences in hole scavenging were not due to differences in porosity/surface area and also showed how photogenerated electrons could reach the surface of both mesoporous materials in high yield. Correlation with Photocatalytic Activity. We now turn to correlating the observation charge carrier dynamics with the efficiency of photocatalytic function, considering the organic photodegradation of intelligent ink. In mesoporous anatase, rapid and highly irreversible hole scavenging by alcohol prolongs the electron lifetime to ∼0.7 s and thus strongly suppresses electron−hole recombination. These long-lived electrons reduce the dye, resazurin, with a time constant of 3 ms, ∼100 times faster than competing decay pathways, leading to highly efficient photocatalysis reaction (QY = 62%), as illustrated in Scheme 2a. In contrast, the low photocatalytic activity of mesoporous rutile (QY = 1.9%) results from less

results of these kinetic studies are summarized in Table 1 and discussed in detail below. Charge Recombination in the Absence of Electron or Hole Scavengers. Our transient absorption data show that electron−hole charge carrier lifetimes are longer in rutile than anatase, assigned to a slower nongeminate recombination. These lifetimes are also invariant in mesoporous and dense microstructures. As such, the higher photocatalytic activity observed for anatase and mesoporous films could not be assigned to a slower nongeminate recombination loss. In anatase, the carrier decay dynamics showed power law, intensity-dependent recombination, characteristic of bimolecular recombination in the presence of shallow trap states.40 In contrast, rutile showed linear decay dynamics on the log time scale and intensity-independent recombination. The origin of this distinct behavior for rutile is unclear but is most probably associated with a tunneling recombination process between deeply trapped electrons and holes, where the presence of deep trap states has been reported.13,41 We note that recombination in titania is primarily nonradiative, so differences in direct versus indirect bandgaps reported between anatase and rutile are unlikely to impact these charge carrier dynamics. Hole Scavenging by Methanol. Methanol (and other alcohols such as isopropanol and glycerol) did not show any effect on the TAS signals in dense samples, indicating that photogenerated holes in these dense films are mostly unable to access the film surface (Figure S7). This correlates with the poor photocatalytic activity observed for these dense films. This highlights the importance of photocatalyst surface area in achieving a high hole scavenging efficiency−demonstrating how nanostructure in TiO2 is essential if bulk recombination is to be reduced. The ability to generate long-lived electrons by methanol hole scavenging was far more pronounced in mesoporous anatase than mesoporous rutile (Figures 5 and 6). Holes in anatase were completely scavenged by methanol on the premicrosecond time scale, consistent with previous fs-TAS studies that show 80% of holes were scavenged in 1 ns.16 In the absence of any reduction targets, this results in the efficient generation of long-lived electrons (∼0.7 s). In contrast, hole scavenging by methanol in mesoporous rutile appears to be much less effective. The addition of methanol results in somewhat less efficient early time hole scavengingwith ∼2/3 of rutile holes being scavenged before 10 μs. However, more significantly, this hole scavenging by methanol is much less effective at G



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efficient and more reversible hole scavenging. As a result, hole scavenging results in only a modest increase in electron lifetime, such that these electrons are less able to reduce resazurin, as illustrated in Scheme 2b. For both mesoporous anatase and rutile, direct electron transfer to our dye reduction target was not observed in the absence of a hole scavenger due to the kinetics of this dye reduction being slower than bulk electron/hole recombination. We note that this inability to achieve dye reduction in the absence of a hole scavenger does not result from the inability of photogenerated electrons to access the TiO2 surface, as efficient electron scavenging was observed in both materials with AgNO3 (Figure S10, Supporting Information). Rather it appears to result from the slow kinetics of electron transfer from the TiO2 surface to the dye. We thus attribute the highly efficient and irreversible hole scavenging of alcohols by mesoporous anatase as the primary reason for its enhanced photocatalytic activity. The lower photocatalytic activity of dense rather than mesoporous TiO2 films (Figure 2) also appears to result from inefficient hole scavenging, attributed to bulk electron−hole recombination competing with hole transfer to the TiO2 surface. The key impact of the efficiency of hole scavenging on photocatalytic function may be understood in terms of the energetics of the TiO2 valence and conduction bands. TiO2 conduction band electrons are not strongly reducing, such that reduction of organic materials (i.e., resazurin dye or water) is kinetically slow. In contrast, TiO2 valence band holes are highly oxidizing, enabling very rapid oxidation of suitable organic targets such as alcohols.

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ASSOCIATED CONTENT

S Supporting Information *

Lamp excitation source emission spectra, XRD patterns, UV− visible absorption spectra, BET analysis, SEM images, and additional transient absorption spectra provided. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b01858.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: a.kafi[email protected]. *E-mail: [email protected]. Author Contributions ⊥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS AK wishes to thank the Ramsay Memorial Fellowships Trust for funding his fellowship. JRD wishes to thank the ERC project Intersolar (291482).

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ABBREVIATIONS TAS, transient absorption spectroscopy; CVD, chemical vapor deposition; QY, quantum yield; SEM, scanning electron microscopy; BET, Brunauer−Emmett−Teller; BJH, Barret− Joyner−Halenda

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REFERENCES

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CONCLUSION

Mesoporous anatase was ∼30 times more efficient than mesoporous rutile in the photocatalysis of the “intelligent ink” model system. Independent of phase, significantly lower levels of intelligent ink photocatalysis were observed in dense films compared with mesoporous structures. This was attributed to a high ratio of photogenerated charges residing in the material bulk and not taking part in photocatalysis at the material surfacethus demonstrating the need for nanostructuring TiO2 to achieve a higher efficiency of photocatalysis. In the presence of alcohols, photogenerated holes on mesoporous anatase were scavenged faster and more irreversibly than mesoporous rutile. As such, the addition of methanol is substantially more effective at generating long-lived electrons on anatase compared to rutile. In the presence of intelligent ink, efficient electron transfer from anatase to the redox dye resazurin was observed on the millisecond time scale. It is concluded that the poorer photocatalytic activity of rutile compared to anatase results not from differences in nongeminate recombination, nor the efficiency of electron access to the TiO2 surface, but rather from the poorer ability of rutile holes to drive the efficient and irreversible oxidation of alcohol scavengers. In the case of mesoporous rutile, hole scavenging was reversible on a time scale faster than the photocatalysis of the dye, resulting in a low efficiency. This is of high significance to those studying colloidal TiO2 photocatalysts, where alcohol hole scavengers are often employed to drive photocatalytic processes. H


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Transient Absorption Spectroscopy of Anatase and Rutile: The Impact

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