Transient Absorption Spectroscopy of Anatase and

Rutile: the Impact of Morphology and Phase on

Photocatalytic Activity

Xiuli Wang,ab‡ Andreas Kafizas,b‡* Xiaoe Li,b Savio J. A. Moniz,c Philip J. T. Reardon,c Junwang

Tangc Ivan P. Parkind and James R. Durrantb*

aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, 457 Zhongshan Road,

Dalian, China

bDepartment of Chemistry, Imperial College London, South Kensington Campus, London, SW7

2AZ, UK

cDepartment of Chemical Engineering, University College London, Torrington Place, London

WC1E 7JE, UK

dDepartment of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ,

UK

‡These authors contributed equally.

ABSTRACT

1

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 was

found in dense films. Charge carrier lifetimes were probed by TAS on the μs – s timescale. 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 from the deficiency of rutile holes to drive efficient and irreversible alcohol oxidation.

KEYWORDS: Transient absorption spectroscopy, photo-generated carrier lifetime,

photocatalysis, morphology, intelligent ink

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 non-distorted

2

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 applications2, 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, Saint-Gobain 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 thin film counterparts) due to the apparent reality that a high surface area

is needed to achieve high quantum yields.7 Based on 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 photo-generated 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 photo-generated 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

3

charge transfer in many photocatalytic processes, such as the reduction of O2,13 the oxidation of

NO14 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 photo-generated

electrons have more time to take part in reduction processes of the dye, as summarized in

Scheme 1.

Scheme 1. Two-step reduction of resazurin (royal blue) to resorufin (pink) and then dihydro-

resorufin (colorless).

The focus of this study is to understand how the phase and morphology of TiO2 impacts on 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.

4

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 oC 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 oC. Dense rutile films were

made by simply annealing the dense anatase films at 1050 oC in air for 5 hours to ensure the

complete conversion to its thermodynamic phase.21

Physical characterization

Surface microstructures were investigated by scanning electron microscopy (SEM) on a JOEL-

6301F field emission instrument. X-ray diffraction (XRD) patterns were measured with a

modified Bruker-Axs D8 diffractometer with parallel beam optics equipped with 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

5

using a TriStar 3000 Micromeritics device. Pore diameter distributions were determined 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 air-pressure feed of 3.5 bar until a blue coating was formed. UV-

visible absorption spectra were measured from 450 – 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 x 8W, Figure S1), 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 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-1cm-1 at 608 nm. The rate of

decrease at 608 nm could then be converted into a molecular rate. Thus with knowledge of the

6

light flux from the 365 nm excitation source (450 μW.cm-2 ≈ 1.1 x 1015 photons.cm-2.s-1) coupled

with the material’s absorbance at 365 nm the quantum yield (QY) was derived:

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 μs – s timescale 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

– 500 scans.

RESULTS

Materials characterization

X-ray diffraction showed that the mesoporous and dense films of anatase (I41/ amd) and rutile

(P42/ mnm) were phase-pure (Figure S2). 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.

7

The specific surface areas of mesoporous anatase and rutile (Figure S4) 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 (< 1 m2.g-1). Barret-Joyner-Halenda (BJH) analysis showed that the average pore

diameters in mesoporous anatase (~ 25– 30 nm) and rutile (~ 24 – 31 nm) were similar. Anatase

was more porous, possessing an average pore volume of 0.82 cm3.g-1 compared with 0.20 cm3.g-1

for rutile. Film topography 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 – 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.

8

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

(d) dense rutile films.

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

9

s the resazurin dye was completely reduced and a spectrum characteristic of the sole presence of

resorufin is observed.26 Further irradiation caused the resorufin spectrum to decrease in intensity

as it was reduced to its colorless counterpart dihydro-resorufin.

By plotting the change in absorbance at 608 nm against irradiation time (Figure 2b) one can

track the kinetics of dye reduction from resazurin to resorufin.26 Almost no change was observed

in the blank glass sample, proving dye reduction was due solely to semiconductor mediated

photocatalysis. The photocatalytic conversion of resazurin to resorufin was two orders of

magnitude faster in mesoporous anatase than dense anatase, two orders of magnitude faster than

mesoporous rutile and more than three orders of magnitude faster than dense rutile.

Figure 2. (a) The 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 timescale) 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 [grey circles].

The quantum yield (QY) for resazurin photo-degradation was determined, taking into account

the differences in film optical densities at the excitation wavelength (365 nm) derived from true

absorbance spectra (Figure S3). Mesoporous anatase and rutile films showed QYs of 62 % and

10

1.9 % respectively. As such, the reaction was ~ 30 times more efficient in mesoporous anatase.

This dramatic difference could not be attributed to the marginal difference in sample surface area

(anatase = 100 m2.g-1 versus rutile = 72 m2.g-1). The dense anatase film showed a QY of 1.2 %

and the dense rutile film showed a QY of 0.09 %. This decrease in efficiency for each phase

relative to mesoporous form was unsurprising as dense films typically possess surface areas < 1

m2.g-1.31 Even though dense samples possessed a far lower surface area, dense anatase possessed

a QY similar to that of mesoporous rutile. Such comparisons show that both phase and surface

area are important if one wishes to produce an efficient TiO2 photocatalyst where in this study:

mesoporous anatase > dense anatase ≈ mesoporous rutile > dense rutile (2)

Transient absorption spectroscopy

Employing scavengers to identify photo-generated electron and hole spectra

Insight into photo-generated 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

photo-generated electrons and holes in TiO2 recombine non-geminately15 (i.e. no surface

reactions can occur).

11

Figure 3. Transient absorption spectra recorded in N2, a methanol hole scavenger solution or a 2

mM AgNO3(aq) electron scavenger solution for (a) mesoporous anatase and (b) mesoporous rutile

20 µs / 20 ms after 355 nm laser excitation (6 ns pulse width, 1 Hz, 250 μJ.cm-2.pulse-1). Dotted

arrows indicate which y axis to refer to.

To identify the regions where photo-generated electrons and holes were present in TAS spectra,

chemical scavengers were 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 photo-generated holes. The change in spectral

signature for both anatase and rutile shows how photo-generated electrons are quenched by

AgNO3 on the pre-μs timescale. In the measured wavelength range, hole absorption increased at

shorter wavelengths. For electrons, the well known hole scavenger methanol was used.15 In

anatase, photo-generated holes are completely quenched by methanol on the pre-μs timescale,

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 photo-

generated electrons (at around 900 nm for both anatase and rutile). It is apparent that the electron

12

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 photo-induced 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 semi-log timescale, consistent with previous reports.37,38

Photo-generated 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 non-geminate 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), whereas decays in rutile were almost independent of excitation intensity (Figure

S5b). At the lower excitation densities studied, carrier lifetimes in anatase were more similar to

rutile (Figure S6).

13

Figure 4. Transient absorption decays probed at 900 nm and recorded in an inert N2 atmosphere

for (a) mesoporous and (b) dense TiO2 (normalized to 1 at 10 µs; 355 nm laser excitation, 6 ns

pulse width, 1 Hz, 250 and 300 μJ.cm-2.pulse-1 for mesoporous and dense TiO2 respectively).

Data are shown for anatase (black) and rutile (red) TiO2. Non-normalized decays are shown

inset.

The 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).

However, substantial changes were observed in mesoporous films (Figures 5 & 6). This clearly

shows how nano-structuring TiO2 enables charge carriers to reach the surface and thus produces

a more active photocatalyst.

Figure 5 shows the effect of methanol on the photo-generated carrier dynamics in mesoporous

anatase. The presence of methanol both increases the initial absorption of mesoporous anatase at

900 nm almost threefold and increases the half-lifetime of the decay from ~ 100 µs to ~ 0.7 s

signifying a reaction with methanol on the pre-μs timescale (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 those at 900 nm, and therefore

assigned to the tail of electron absorption extending to this wavelength.

14

Figure 5. Transient absorption decays recorded in either an inert N2 environment (black lines) or

an 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.

The effect of methanol on photo-generated carrier dynamics in mesoporous rutile was also

studied (Figure 6). Decay kinetics were probed at 900 and 550 nm to monitor changes in photo-

generated 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 pre-μs timescale 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 pre-µs timescale, it does not significantly increase the

15

lifetime of photo-generated rutile electrons until 1 ms. Similar decay kinetics were observed in

isopropanol and glycerol (Figure S8).

Figure 6. Transient absorption decays recorded in either an inert N2 environment (black lines) or

an 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.

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 S9).

16

Figure 7. A 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].

In mesoporous anatase, the electron decay kinetics in air, water and N2 were indistinguishable,

consistent with previous studies that showed electron transfer to water and O2 are slow (up to

100 μs).39 The addition of glycerol resulted in similarly long lived electrons (τ ≈ 0.2 s) 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

17

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 %)

DISCUSSION

Charge transfer and decay dynamics of the photo-generated 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 results of these kinetic studies are summarized

in Table 1, and discussed in detail below.

Medium N2AgNO3(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

mesoporous anatase

e- (900 nm) 80 μs completely quenched 0.7 s 10 ms

h+ (460 nm) 80 μs 0.7 s completely quenched completely quenched

mesoporous rutile

e- (900 nm) 1 ms completely quenched 2 ms transfer not observed

h+ (550 nm) 1 ms 0.9 s ~ 2/3 quenched ~ 2/3 quenchedTable 1. A 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 μs – s timescale

Charge recombination in the absence of electron or hole scavengers

18

Our transient absorption data shows that electron-hole charge carrier lifetimes are longer in

rutile than anatase, assigned to a slower non-geminate 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 non-geminate

recombination loss.

In anatase, the carrier decay dynamics showed power law, intensity dependent recombination,

characteristic of bi-molecular recombination in the presence of shallow trap states.40 In contrast,

rutile showed linear decay dynamics on log timescale 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 non-

radiative, so differences in direct versus indirect bandgaps reported between anatase and rutile

are unlikely to impact upon 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 photo-generated 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 nano-structure 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 & 6). Holes in anatase

were completely scavenged by methanol on the pre-μs timescale, consistent with previous fs-

19

TAS studies that showing 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 generating long lived electrons. In mesoporous

anatase, an almost threefold 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 ms timescale onwards. 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.43 It is suggested 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,44 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

20

observed in both materials (Figures 3 and S10). This was a strong indication that the differences

in hole scavenging was not due to differences in porosity/ surface area and also showed how

photo-generated 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 photo-degradation 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 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.

Scheme 2. Summary of the timescales of the processes in the photocatalysis of intelligent ink

with (a) mesoporous anatase and (b) mesoporous rutile; 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.

21

For both mesoporous anatase and rutile, direct electron transfer to our dye reduction target was

not observed in the absence of hole scavenger due to the kinetics of this dye reduction being

slower than bulk electron/ hole recombination. We note this inability to achieve dye reduction in

the absence of hole scavenger does not result from the inability of photo-generated electrons to

access the TiO2 surface, as efficient electron scavenging was observed in both materials with

AgNO3 (Figure S10). 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.

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 photo-generated charges residing in the material

22

bulk and not taking part in photocatalysis at the material surface – thus demonstrating the need

for nano-structuring TiO2 to achieve a higher efficiency of photocatalysis.

In the presence of alcohols, photo-generated 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 timescale. It is concluded that the poorer photocatalytic activity

of rutile compared to anatase results not from differences in non-geminate 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.

Supporting Information. Lamp excitation source emission spectra, XRD patterns, UV-visible

absorption spectra, BET analysis, SEM images and additional transient absorption spectra

provided. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION

Corresponding Authors

23

*a.kafizas@imperial.ac.uk and j.durrant@imperial.ac.uk

Funding Sources

ERC Intersolar (291482)

Ramsay Memorial Fellowships Trust

ACKNOWLEDGMENTS

AK wishes to thank the Ramsay Memorial Fellowships Trust for funding his fellowship. JRD

wishes to thank the ERC project Intersolar (291482).

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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