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Where Do Photo-generated Holes Go in Anatase:Rutile TiO? A Transient Absorption Spectroscopy Study of Charge Transfer and Lifetime Andreas Kafizas, Xiuli Wang, Stephanie R. Pendlebury, Piers R.F. Barnes, Min Ling, Carlos Sotelo-Vazquez, Raul Quesada-Cabrera, Can Li, Ivan P. Parkin, and James R. Durrant J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11567 • Publication Date (Web): 18 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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The Journal of Physical Chemistry

Where Do Photo-Generated Holes Go in Anatase:Rutile TiO2? A Transient Absorption Spectroscopy Study of Charge Transfer and Lifetime Andreas Kafizas, ‡a* Xiuli Wang, ‡ab Stephanie R. Pendlebury,a Piers Barnes,a Min Ling,c Carlos Sotelo-Vazquez,c Raul Quesada-Cabrera,c Can Li,b Ivan P. Parkinc and James R. Durranta* a

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

2AZ, UK b

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Dalian National

Laboratory for Clean Energy, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian,116023, China c

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

UK KEYWORDS: anatase rutile composites, transient absorption spectroscopy, hole transfer, recombination kinetics, trapped charges

ABSTRACT: Anatase:rutile TiO2 junctions are often shown to be more photocatalytically active than anatase or rutile alone, but the underlying cause of this improvement is not fully understood.

ACS Paragon Plus Environment

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Herein, we employ transient absorption spectroscopy to study hole transfer across the anatase:rutile heterojunction in films as a function of phase composition. By exploiting the different signatures in the photo-induced absorption of trapped charges in anatase and rutile, we were able to separately track the yield and lifetime of holes in anatase and rutile sites within phase composites. Photo-generated holes transfer from rutile to anatase on sub-microsecond timescales. This hole transfer can significantly increase the anatase hole yield, with a 20: 80 anatase:rutile composite showing a five-fold increase in anatase holes observed from the microsecond. Hole transfer does not result in an increase in charge-carrier lifetime, where an intermediate recombination dynamic between that of pure anatase (t1/2 ≈ 0.5 ms) and rutile (t1/2 ≈ 20 ms) is found in the anatase:rutile junction (t1/2 ≈ 4 ms). Irrespective of what the formal band energy alignment may be, we demonstrate the importance of trap-state energetics for determining the direction of photo-generated charge separation across heterojunctions and how transient absorption spectroscopy, a method that can specifically track the migration of trapped charges, is a useful tool for understanding this behavior.

INTRODUCTION Inorganic heterojunctions have been reported to enhance photo-induced charge separation in a range of materials and devices, including solar cells and photocatalysts for splitting water.1,2 Developing methods that can track charge transfer kinetics and recombination within such heterojunctions is crucial to guide future materials development. Herein, we study the most applied inorganic heterojunction in photocatalysis, the anatase: rutile junction formed from the two polymorphs of titanium dioxide (TiO2). We focus in particular upon the direction and efficiency of photo-induced hole transfer between these two phases and its impact on photocatalytic function.


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TiO2 has been extensively studied as it has multi-functional applications in self-cleaning,3,4 water-splitting,5,6 gas-sensing7 and solar-cell devices.8,9 Its self-cleaning function is exploited commercially by coating windows (Pilkington Activ, Saint-Gobain Bioclean, PPG Sunclean) and tiles (TOTO Hydrotect). Over the past decade, the number of studies using TiO2 as a photocatalyst for pollutant degradation and water-splitting has grown rapidly, now amassing more than 2000 publications per annum. TiO2 is found naturally in three semiconducting polymorphs – anatase, rutile and brookite. The anatase phase is often regarded the most photocatalytically active phase.10,11 However, forming an anatase:rutile junction can synergistically enhance photocatalysis and is fast becoming the method of choice for improving the photocatalytic activity of TiO2.12–20 The photocatalytic process begins with a photo-excitation, forming electron-hole pairs. At the material surface, these electrons and holes can react with surface species, where electrons can reduce oxygen to superoxide radicals and holes can oxidize water to hydroxyl radicals.3 Such radicals are highly reactive towards organic species (i.e. pollutants/dirt), breaking them down to CO2, H2O and mineral acids. It has been speculated that the synergy observed in anatase:rutile heterojunctions is due to an increased separation of photo-generated electron- hole pairs across phase boundaries, inhibiting electron-hole recombination and thus prolonging their lifetime to conduct photocatalysis.21 For example, Evonik P25 (formerly Degussa), a commercially available anatase:rutile composite,22 is so active it is considered the benchmark photocatalyst.23 Over the past decade, many groups have studied the photocatalytic synergy in anatase:rutile heterojunctions,24–35 with no consensus being reached on the direction of charge transfer. Obtaining a better understanding of this synergy is crucial to finding the role of each phase in photocatalysis. Thus far, the time-resolved methods used to investigate this system have focused


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only on the behavior of photo-generated electrons. However, such studies have not reached a clear consensus of the direction of electron transfer, complicated in part by the difficulty in distinguishing rutile and anatase electrons spectroscopically.28–32 Using a tool that can identify the movement of photo-generated holes would shed a unique light in understanding this system and guide the future development of more efficient photocatalysts. In the present work, we use transient absorption spectroscopy (TAS), a method that can specifically track the migration of trapped photo-generated charges, to study the direction of hole transfer in the anatase:rutile system. We observe transfer of holes from rutile to anatase on submicrosecond timescales. This hole transfer can significantly increase the anatase hole yield, where a 20: 80 anatase:rutile junction showed an ~ five-fold increase relative to pure anatase, but does not substantially inhibit the rate of recombination. Irrespective of the formal band energy alignment, this paper highlights the importance of trap-state energetics for determining the direction of charge separation in heterojunctions and how transient absorption spectroscopy is a useful tool for monitoring this behavior.

EXPERIMENTAL SECTION Preparation of nanocrystalline thin-films. Dense anatase thin-films grown by atmospheric pressure chemical vapor deposition (APCVD, Figure S1 and Table S1) were thermally converted into dense rutile and a range of intimately mixed anatase:rutile heterojunctions. Films were calcined at 600, 700, 800, 950 or 1050 °C for 2 hours. Conditions that formed anatase:rutile heterojunctions are labeled “TiO2-T”, in which T is the calcination temperature (°C). The pure anatase films prepared by APCVD at 500 °C are labeled “anatase” and the pure rutile films prepared by thermal conversion at 1050 °C are labeled “rutile”.


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Transient absorption spectroscopy. Charge carrier dynamics of TiO2 films were measured using transient absorption spectroscopy (TAS) from the µs – s timescale at room temperature (~ 22 °C). The TAS apparatus has been described in detail elsewhere.36 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 excites the sample (third harmonic of an Nd: YAG laser; 355 nm, 6 ns pulse width). Relatively low laser intensities were employed (~ 300 µJ cm-2 pulse-1), with a laser repetition rate of 1 Hz. All experiments were performed in an inert Ar gas environment, with the exception of chemical scavenger studies, which employed pure methanol or aqueous silver nitrate (2 mM) purged with Ar gas. Each TAS trace is the result of averaging between 50–500 scans. TAS was conducted both with and without probe light, so that contributions from laser scatter and / or sample photoluminescence could be subtracted.

RESULTS Physical characterization. Phase-fraction. Our study is focused on dense, nanocrystalline films of TiO2 deposited by APCVD. Crystal structures (Figure S2) and phase fractions (Equation S1) were analyzed by XRD. The as grown films were purely composed of anatase (I41/ amd). When these films were annealed at 950 or 1050 °C for two hours, only the rutile (P42/ mnm) phase was observed. Samples annealed at 600, 700 and 800 °C showed varying amounts of the two phases. The anatase to rutile phase transformation typically occurs above 600 °C, however, the transition to rutile is not rapid and can take several hours even at temperatures as high as 950 °C.37 All patterns were fit to a Le Bail refined model, with phase fractions listed in Table 1 and


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lattice parameters listed in Table S2. The degree of preferred growth was also quantified by comparing the percentage change in the intensity of each peak relative to the single crystal (Figure S3). Phase composition was also assessed using Raman spectroscopy (Figures S4 & S5).

Figure 1. The change in anatase:rutile phase fraction (%) with a 2 hr annealing stage (° C), starting from phase-pure anatase on quartz. The red dotted line represents a guide to the eye.

Table 1. Physical properties of the TiO2 films studied herein, with anatase grown by CVD (500 °C) and post-annealed at various temperatures to form anatase:rutile junctions (600 – 950 °C) and phase-pure rutile (1050 °C). Sample

phase composition (%), [A: R]

bandgap (eV)

Crystal thickness size (nm), (nm) [A: R]

absorption (%)

XRD

Raman

Tauc

355 nm

anatase

100: 0

100: 0

100: 0

3.20

130: 0

390

68

TiO2-600

95: 5

100: 0

78: 22

3.16

118: 17

330

66

TiO2-700

60: 40

70: 30

43: 57

3.09

135: 79

470

89

TiO2-800

12: 88

17: 84

22: 78

3.05

35: 100

390

93


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TiO2-950

0: 100

0: 100

10: 90

3.02

nm: 123

320

93

rutile

0: 100

0: 100

0: 100

3.00

0: 175

400

95

[nm = not measurable]

A varying degree of absorption was observed at 355 nm – the excitation wavelength used for our transient absorption studies (Figure S6). All samples retained their high transparency in the visible region. Bandgaps were determined from a Tauc plot (Figure S7).38 A progressive redshift in the band-edge was due to a gradual transition from anatase to rutile. These bandgaps were used to estimate the anatase:rutile phase fraction by a rule of weighted averages (Equation S2). Trends were similar to those determined by XRD and Raman methods, with the exception of sample TiO2-950. In TiO2-950 a 90 % conversion of anatase to rutile was determined from a Tauc analysis, however, no presence of anatase was observed by either XRD or Raman spectroscopy. This we attribute to either a low anatase crystal size or phase fraction, making it hard to detect. However, our TAS studies later show that sample TiO2-950 behaved similarly to all other anatase:rutile junctions studied herein, and therefore should contain some amount of anatase . The phase fractions determined by XRD, Raman and UV-visible spectroscopy are plotted in Figure 1. The most rapid conversion to rutile was observed between ~ 650 to 800 °C. The activation energy for the conversion of anatase to rutile was determined using an Arrhenius and Avrami model39 (Figure S8), yielding activation energies between 100 and 130 kJ.mol-1 – similar to previous studies of nanocrystalline anatase.40 Surface topography and phase-junction contact. Anatase, rutile and TiO2-700 films were investigated by SEM (Figure S9). The thin and compact nature of these films was demonstrated by cross-sectional imaging. Top-down imaging showed that the anatase film was composed of


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tetragonal domains ~ 100-200 nm in width, and the rutile film was composed of larger and more rounded domains ~ 200-400 nm in width. TiO2-700 was composed of a mixture of intimately mixed tetragonal and larger rounded domains. The average crystal size was determined from the Scherrer line-broadening of XRD patterns.41 The average rutile crystal diameter increased almost monotonically with anneal temperature (Table 1). In anatase and rutile, the average crystal diameters were found to be 130 and 175 nm respectively. This indicated that the domains observed by SEM imaging were most probably composed of single or twinned crystals. The degree of contact between anatase and rutile sites in TiO2-700 was investigated by highresolution transmission electron microscopy (HRTEM) (Figure 2). Throughout the material, a highly intimate crystalline contact was observed between anatase and rutile sites, with no presence of any amorphous interlayer at the phase boundaries.

Figure 2. HRTEM image of sample TiO2-700, with the boundaries drawn for anatase (101) [black-dotted lines] and rutile (110) particles [red-dotted lines]. The Fourier transform (top-right)


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identified these phases as anatase (101) [inner circle] and rutile (110) [outer circle], indicating an intimate junction between the two phases, with no presence of any amorphous material at the interface. Transient absorption spectroscopy. TAS is a form of laser flash spectroscopy that can be used to monitor the generation, recombination, trapping, charge transfer etc of photo-generated charges in semiconductors. The dynamics specific to photo-generated electrons or holes can be studied by tracking transient changes in absorbance at particular wavelengths.4 The technique has primarily been used to study charge transfer processes in solar cells (organic-organic or inorganic-organic)42–44 but has also been used to study charge transfers in heterojunction photocatalysts (inorganic-inorganic)45 as well as the kinetics of photocatalysis.46–48


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Figure 3. Transient absorption kinetics for nanocrystalline (a) anatase and (b) rutile thin-films in an inert argon gas atmosphere, measured from 10 µs after a laser pulse (355 nm, 6 ns pulsewidth, 300 µJ.cm-2.pulse-1). Charge carrier recombination kinetics and spectral features of anatase and rutile. In the study herein, we focus on the long lived (micro- to millisecond) charge carriers whose yields and lifetimes are thought to be particularly critical to photocatalytic function49 (for example oxygen reduction by TiO2 electrons has been reported to proceed on millisecond timescales).46 The transient change in absorption was measured for phase pure anatase and rutile samples in argon (i.e. an inert environment) following bandgap excitation at 355 nm. Typical decay kinetics are


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shown in Figure 3, with spectra at selected times shown in Figure S10. Our anatase sample showed similar transient decays to those observed previously in mesoporous anatase photoanodes,50 following dispersive power law dynamics assigned previously to trap/de-trap limited bimolecular recombination.48 Our rutile sample showed similar transient decays to those observed previously.49,51 The rate of decay of photo-generated charge carriers in rutile was significantly slower than in anatase, decaying linearly on a log timescale, which indicates recombination is due to an electron tunneling process. Comparing the time constants “t50%” at 550 nm (i.e. the time to reach half of the initial absorption seen at10 µs), we find the decay in anatase (t50% = 0.5 ms) is 40 times faster than in rutile (t50% = 20 ms). These kinetics of electronhole recombination were largely independent of morphology (Figure S11) and have previously been shown to be relatively independent of excitation density (from 2 – 300 µJ.cm-2.pulse-1).49


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Figure 4. Transient absorption spectra of all samples in an inert argon gas atmosphere at (a) 10 µs and (b) 100 ms after a laser pulse (355 nm, 6 ns pulse-width, 300 µJ.cm-2.pulse-1). In anatase, a maximum absorption was observed at 460 nm, which decreased steadily with increasing probe wavelength (Figure S10a). In rutile, a maximum absorption was found at around 550 nm, which decreased more sharply with increasing probe wavelength (Figure S10b). The maximum absorption in rutile (~ 0.45 m∆ O.D) was more than double that of anatase (~ 0.20 m∆ O.D), which is partly due to the increased absorption of the laser light excitation in our rutile sample (95 %) compared with our anatase sample (65 %). Understanding transient spectral signals using chemical scavengers. We have previously shown that TAS in the presence of chemical scavengers can be used to identify photo-generated electrons and holes in TiO2.49 Now we extend this analysis to build a deconvolution model for separating electron and hole signals in anatase and rutile. Two scavengers were employed: (i) a methanol hole scavenger52 or (ii) a silver nitrate electron scavenger53. In general, when a hole scavenger is used during TAS, signals due to photogenerated holes can be quenched, allowing one to discern what signals are caused by photogenerated electrons (and vice-versa for electron scavengers). In our case, mesoporous films of


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anatase (Figure S12) and rutile (Figure S13) were examined as the quantum yield in nanocrystalline films was too low to show differences in transient absorption response. Photo-generated electrons and holes showed absorption maxima at 800 and 460 nm in anatase and 850 and 550 nm in rutile respectively. Of critical importance to this study, rutile showed only a weak negative transient absorption at the hole maximum of anatase (Figure 3). We note these absorption difference spectra include contributions from the loss (‘bleaching’) of ground state absorption and appearance of absorption due to photo-generated charges. Using the observed transient difference spectra for rutile and anatase observed in our films as spectral fingerprints, this allows us to track the populations of anatase and rutile holes as a function of time and phase composition. We also note that on the timescales studied herein (post microsecond), the observed holes are expected to have undergone trapping/ relaxation processes, and therefore to exhibit spectra distinct from those of untrapped valence band holes studied on ultrafast timescales.54 As sufficient differences are present in the hole spectra of anatase and rutile, holes residing in anatase or rutile states could be distinguished in our anatase:rutile composites. Charge carrier dynamics and spectral features of anatase:rutile heterojunctions. The transient change in absorption was measured for all anatase:rutile junctions in argon, 10 µs after photoexcitation (Figure 4a). All junctions showed an increased level of absorbance at 460 nm relative to pure anatase – an absorption attributed solely to the presence of holes in anatase. The spectral shapes from all junctions were similar to that of pure anatase, indicative of a significant presence of holes on anatase sites. This was a particularly intriguing result for TiO2-950 as this sample was composed of very little anatase (< 10 %). TiO2-800 showed the highest photo-induced absorbance at 460 nm (0.92 m∆ O.D.), which was nearly 5 times more than pure anatase even


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though it contained only ~ 20 % anatase. Considering that rutile absorbs the 355 nm laser excitation pulse more strongly than anatase (Table 1) and that TiO2-800 was composed of ~ 80% rutile, most of the incident light would have been absorbed by the rutile fraction – forming photo-generated electrons and holes primarily in rutile. As such, the ~ five-fold increase in absorption at 460 nm (attributed to anatase holes) could only be explained by hole transfer from the rutile fraction into neighboring anatase sites, which occurred on sub-µs timescales. In all anatase:rutile junction materials, these hole transfers inhibited sub-µs electron-hole recombination, resulting in a consistently higher anatase hole yield on the microsecond timescale.

Figure 5. Normalized transient absorption kinetics in an inert argon gas atmosphere comparing the decays at 460 nm in anatase and TiO2-800 and 550 nm in rutile and TiO2-800 after a laser pulse (355 nm, 6 ns pulse-width, 300 µJ.cm-2.pulse-1). By 100 ms, the spectral features of samples TiO2-800 and -950 had changed significantly (Figure 4b). At these long timescales the spectra are more similar to the pure rutile sample. In contrast to the relative populations observed on earlier timescales, the absorption due to longlived holes in anatase (460 nm) is significantly lower than those found in rutile (550 nm). This


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stronger resemblance to the spectrum of rutile is attributed to a portion of rutile holes that are not transferred to anatase sites in the junction materials. These rutile holes were more clearly discerned at longer time delays (~ 100 ms), when the anatase holes have largely recombined due to their shorter lifetime (Figure S14). In Figure 5, the hole decay dynamics (due to electron/hole recombination) of pure anatase and rutile are overlaid with the corresponding hole decay dynamics of the TiO2-800 anatase:rutile heterojunction. It is apparent that this heterojunction exhibits intermediate hole decay dynamics between the two phase pure materials, where anatase (t1/2 ≈ 0.5 ms) < anatase:rutile (t1/2 ≈ 4 ms) < rutile (t1/2 ≈ 20 ms). In this heterojunction, the photo-induced absorption results primarily from anatase holes, as discussed above. Our observation of intermediate decay dynamics in the heterojunction strongly indicates that this recombination results from anatase holes recombining with rutile electrons across the anatase:rutile heterojunction. It is thus apparent that micro-millisecond recombination in anatase:rutile junctions is significantly faster than in pure rutile, but slower than pure anatase. This is significance to the photocatalysis community, where it has often been speculated that the higher photocatalytic activity of anatase:rutile heterojunctions is due to an increase in charge carrier lifetime, not observed herein.


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Figure 6. The separated absorption of (a) holes on anatase sites and (b) holes on rutile sites acquired by a deconvolution of transient absorption spectra. Labels in the key represent the anatase: rutile phase fraction for each sample. DISCUSSION The direction of charge transfer in anatase:rutile heterojunctions. Our transient absorption spectroscopy of these anatase:rutile heterojunctions has revealed that rutile holes are transferred to anatase sites on sub-µs timescales. In deconvoluting TAS spectra, we were able to quantify the degree of hole transfer. An example is shown for TiO2-800 in Figures S15 and S16. A summary of the changing hole absorption with time is presented in Figure 6.


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The change in anatase hole yield (Figure 6a) followed the change in transient absorption kinetics at 460 nm (Figure S15a), providing strong evidence for hole transfer from rutile to anatase sites. In junctions TiO2-600 and -700, which were composed of ~ 10 and ~ 40 % rutile respectively, no rutile holes were observed over the entire analysis period (Figure 6b). This is attributed to a complete hole transfer from rutile to anatase on the pre-measurement (sub-µs) timescale. In TiO2-800 and -950, which were composed of ~ 80 and ~ 95 % rutile respectively, we observed an incomplete transfer of rutile holes to anatase. The initial rutile hole yields in TiO2-800 and -950 were substantially lower than for pure phase rutile, and did not scale with a decrease in rutile phase fraction. This shows that most of the holes generated in rutile were transferred to anatase. The reason that some holes remained in rutile may be due to the presence of larger rutile crystals from 800 °C (> 100 nm in diameter) coupled with the low diffusion length of rutile holes, speculated to be ~ 10 nm.55 If we account for the light absorbed by the rutile fraction, we can estimate the proportion of long-lived holes transferred. This equates to ~ 70 % and ~ 60 % hole transfer efficiencies in TiO2-800 and in TiO2-950, respectively. If we also consider the extinction coefficient of holes in anatase (ε ~ 2900 M-1.cm-1, λ = 460 nm),50 then we can estimate the overall quantum yield of hole transfer from rutile to anatase sites (including losses due to ultrafast recombination processes not directly monitored in this study). In TiO2-800 an absorption of 0.9 m∆O.D. is observed from 10 µs, which equates to roughly 1 x 1014 holes cm-2 on anatase sites. If we consider the laser power (~ 300 µJ.cm-2.pulse-1, λ = 355 nm, 5.4 x 1014 photons cm-2), this equates to an overall quantum yield of ~ 20%. Over the past decade, several techniques have been used to investigate the charge separation function of anatase:rutile heterojunctions, however, there is still no consensus on the direction of charge transfer. Using low temperature electron paramagnetic resonance (EPR) at 10 K, Hurum


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et. al. observed electron transfer from rutile to anatase sites in Evonik P25 TiO2.24 Conversely, Komaguchi et. al. observed electron transfer from anatase to rutile in Evonik P25 using EPR at 77 K.25 Indirect evidence for electron transfer from rutile to anatase sites was studied by Ag labeling studies of bi-layered anatase:rutile materials, where Ag was deposited through the photo-chemical

reduction

of

AgNO3.26,27

Conversely,

transient

photo-voltage

(TPV)

spectroscopy studies indicated electrons transfer from anatase to rutile sites.28,29 Using time-resolved microwave conductivity (TRMC), Carneiro et. al. inferred an increase in microwave reflectance (i.e. free charge carrier conductivity) was due to hole transfer from anatase to rutile sites in nanoparticulate anatase:rutile heterojunctions, but provided no direct evidence.12 This is because the TRMC signals in TiO2 are primarily due to electron movement as their mobility is far greater than holes.56 Carneiro et. al. observed that the lifetime and conductivity of charges increased as the rutile phase fraction increased, where in all cases the measured conductivity was almost zero by 10 µs. Although they did not compare their results with that of pure rutile, it was previously shown by Colbeau-Justin et. al. that free charge carrier conductivity decreases several orders of magnitude faster in rutile (t50% ~ 15 ns) than in anatase (t50% > 200 ns).31 What is important to note is that TRMC cannot distinguish between the trapping of free-charge carriers or electron-hole recombination events, which both amount to a loss in conductivity. Our work shows that the electron-hole recombination lifetime, as measured by TAS, was one order of magnitude longer in rutile (t50% ~ 20 ms) than in anatase (t50% ~ 0.5 ms). As such, it appears likely that TRMC studies of TiO2 are strongly influenced by the trapping of free charges as well as electron-hole recombination. These differences can be related to the electron trap-state energies of anatase and rutile, where rutile possesses a mono-energetic trap


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state below the conduction band57 whereas anatase possesses an exponential distribution of trap states.58 Recently, Shen et. al. provided strong evidence for electron transfer from anatase to rutile sites on sub-microsecond timescales in anatase:rutile heterojunctions from TAS studies in the infrared.32 The direction of this observed electron transfer is consistent with the study herein, which has focused on the complimentary hole transfer process from rutile to anatase. Charge separation in anatase:rutile junctions. It has previously been reported that photogenerated electron and holes in anatase54 and rutile59 TiO2 can undergo significant ultrafast bimolecular recombination on picosecond timescales. Of note, these ultrafast transient absorption spectra are typically more broad, and do not show the well defined, and clearly distinguishable, transient absorption maxima we observe herein for anatase and rutile trapped holes.49 This difference in transient spectra is most probably related to the more localized/trapped nature of the holes observed on the micro- to millisecond timescales studied herein. Of note, the slower timescales monitored herein (from the microsecond - second) typically correspond to the timescales of most interfacial photocatalytic processes, and is therefore more likely to be of relevance to its photocatalytic function. In any case, our observation of increased yields of long lived anatase holes in our anatase:rutile heterojunction films strongly indicates hole transfer from rutile to anatase competes with such ultrafast electron/hole recombination, and therefore must proceed on the picosecond-nanosecond timescale. Modeling the density of states in anatase:rutile heterojunctions. A recent XPS/ modeling study has suggest that the bend energy alignment in such heterojunctions should result in electron transfer to anatase and hole transfer to rutile.34 However, our TAS studies showed that holes transferred to anatase, not rutile. In order to address this apparent contradiction further, we


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modeled the band bending at the anatase:rutile interface, as detailed in the supporting information (Figure S17). Two type II junctions were studied: a configuration in which the CBM of anatase is more negative than rutile (Figure S17a) and vice versa (Figure S17b). The n-type dopant level was included for both anatase and rutile, with, following literature data, lower doping densities in anatase (ranging from ~ 4 x 1016 – 3 x 1019 in anatase60 and ~ 2 x 1019 – 1 x 1020 cm-3 in rutile61,62). In the first configuration (Figure S17a), electron transfer from anatase to rutile and hole transfer from rutile to anatase is favorable, independent of the doping density assumed. As almost no band-bending is present in the rutile layer (due to the high charge carrier concentration), hole transfer would be diffusion limited (an effect consistent with our TAS data, vide-supra). Electron transfer from anatase to rutile would also be partially inhibited by the formation of a non-rectifying junction (Ea ~ 40 mV). In the reverse configuration (Figure S17b), the higher band bending resulting from the higher doping density of rutile reduces the band energy offset by up to 0.2 eV for physically reasonable levels of doping density. However, this reduction in band offset appears to be too small to explain the difference between the band energy alignment34 and what was concluded from our TAS studies. Critical to this study, one should note that such modeling studies do not account for intra-bandgap states (i.e. trap-states). Several studies have reported exponential tails of shallow traps states extending into the bandgap in TiO2,58,63,64 with these states playing a key role in the charge carrier dynamics at long timescales. It is possible that differences in the energetic distributions of these trap-states can facilitate the movement of trapped charges in the opposing direction to the formal band offsets; although a detailed analysis of this issue is beyond the scope of this study. Overall implications on TiO2 photocatalyst design. Anatase:rutile junctions often show enhanced levels of photocatalytic activity compared with anatase or rutile alone.65 In a typical


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experiment assessment of H2 generation from water splitting, methanol is most frequently used as a hole scavenger. Previous studies have shown how anatase is more active than rutile in oxidizing methanol.66 We have recently shown that this is due to the relatively slow and reversible hole transfer kinetics from rutile to methanol, indicating that rutile holes are less photocatalytically reactive than anatase holes in this process.49 It has also been reported that hole radicals formed on anatase are mobile and can diffuse several microns, whereas those formed on rutile are static.67 As such, if holes in rutile can be transferred to anatase sites, the benefits to photocatalytic function are clear. Our TAS studies demonstrated that holes were transferred from rutile to anatase sites in our anatase:rutile heterojunctions, and that these anatase holes show an increase in yield and lifetime compared with anatase holes in phase pure anatase films. It is this increased yield of long lived anatase holes which appears to be the origin of the enhanced photocatalytic activity of anatase:rutile heterojunctions compared to phase pure materials. Careful photocatalyst design is therefore essential if the fruits of an increased anatase hole yield are to be exploited, where anatase sites are exposed to the reactant-material interface whilst retaining a high degree of contact with rutile. This has already been realized in the field of water oxidation, where high surface area rutile nanorods coated with anatase particles have achieved record conversion efficiencies and photocurrents for any TiO2-based material.68

CONCLUSIONS Anatase, rutile and anatase:rutile composites were examined by transient absorption spectroscopy (TAS), which can be used specifically to track the migration of trapped charges.


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Signals specific to holes in anatase and rutile allowed the direction and degree of hole transfer in anatase:rutile junctions to be discerned. Photo-generated holes transfer from rutile to anatase sites on sub-µs timescales. In samples of high rutile content an incomplete hole transfer was observed, which was attributed to the limited diffusion length of rutile holes. An almost five time increase in anatase hole yield was observed in a 20: 80 anatase:rutile junction. Electron-hole recombination in anatase:rutile junctions showed intermediate decay dynamics (t1/2 ≈ 4 ms) between that of pure anatase (t1/2 ≈ 0.5 ms) and rutile (t1/2 ≈ 20 ms), dispelling the notion that the synergic enhancement in photocatalytic activity is due to an increase in charge carrier lifetime alone. Irrespective of what the formal band energy alignment may be, we demonstrate the importance of monitoring trapped charges, where differences in the energetic distributions of these trapped charges may facilitate movement in the opposite direction to the formal band offset. To the best of our knowledge, this is the first time in which hole transfer has been examined in the anatase:rutile system, shedding a unique light on the behavior of this heterojunction and the impacts on photocatalysis, water splitting and photocatalyst design.

ASSOCIATED CONTENT Supporting Information. Experimental details of the chemical vapor deposition of the nanocrystalline anatase thin-films, preparation of mesoporous films, physical characterization, X-ray diffraction, lattice parameters, texturing, Raman spectroscopy, UV-visible spectroscopy, Tauc plots, activation energy of anatase-to-rutile phase transition, scanning electron microscopy, transient absorption spectra, kinetics and chemical scavenging.


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AUTHOR INFORMATION Corresponding Author * e-mail: [email protected] & [email protected] Author Contributions ‡These authors contributed equally. Funding Sources Ramsay Memorial Fellowships Trust European Research Council (291482) ACKNOWLEDGMENT AK wishes to thank the Ramsay Memorial Fellowships Trust for funding his fellowship. We also thank the ERC project Intersolar (291482) for funding. ABBREVIATIONS CVD, chemical vapor deposition; APCVD, atmospheric pressure chemical vapor deposition; EPR, electron paramagnetic resonance; HRTEM, high resolution transmission electron microscopy; TAS, transient absorption spectroscopy; TPV, transient photo-voltage; TRMC, time-resolved microwave conductivity; SEM, scanning electron microscopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction. REFERENCES (1)

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Insert Table of Contents Graphic and Synopsis Here: Transient absorption spectroscopy can monitor the generation, migration and recombination of trapped charges. We show how this technique can be used to study the direction of hole transfer in the most applied inorganic heterojunction in photocatalysis, the anatase: rutile junction. This article highlights the importance of monitoring trap state behavior, where differences in the energetic distributions of these trapped charges may facilitate movement in the opposite direction to the formal band offset.


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A Transient Absorption Spectroscopy Study of Cha - ACS Publications

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