Role of Dissociatively Adsorbed Water on the Formation of Shallow

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Article

Role of Dissociatively Adsorbed Water on the Formation of Shallow Trapped Electrons in TiO Photocatalysts 2

Anton Litke, Emiel J. M. Hensen, and Jan P Hofmann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01151 • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

1

Role of Dissociatively Adsorbed Water on the

2

Formation of Shallow Trapped Electrons in TiO2

3

Photocatalysts

4 5

Anton Litke, Emiel J.M. Hensen*, Jan P. Hofmann*

6 7

Laboratory of Inorganic Materials Chemistry, Department of Chemical Engineering and

8

Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The

9

Netherlands

10

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

12

The mismatch between short lifetimes of free charge carriers and slow kinetics of surface redox

13

reactions substantially limits the efficiency of most photocatalytic systems. Hence, the

14

knowledge of trapping and recombination of photogenerated electrons and holes at different time

15

scales is key for a rational optimization of photocatalytic materials. In this study, we used

16

subsecond time-resolved diffuse-reflectance FTIR spectroscopy to investigate how energy and

17

intensity of the incident irradiation affect the dynamics of photogenerated charge carriers in TiO2

18

P25 photocatalysts subjected to different pre-treatments and how shallow trapped electrons

19

(STE) are formed under these conditions. Intensity dependent measurements demonstrated that

20

electrons and holes generated by 325 nm and 409 nm irradiation undergo bimolecular and trap-

21

assisted recombination, respectively. Analysis of characteristic times of photogenerated electron

22

absorption rise and decay indicated that the apparent charge carrier dynamics at the time scale of

23

seconds to minutes relate to chemical trapping of photogenerated electrons and holes. The

24

presence of dissociatively adsorbed water on the oxide surface was required for efficient STE

25

formation. This suggests that STE form at s−min time scale upon surface-mediated self-trapping

26

of electrons.

27


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28 29

1. Introduction

30

is the solar light-driven production of chemical fuels from CO2 and water. Photocatalysis is one

31

of the main approaches explored to address this challenge. However, solar-to-chemical energy

32

conversion efficiency of most photocatalytic systems remains low due to severe charge carrier

33

recombination. Trapping of photogenerated electrons and holes plays an important role in this

34

process. Trap states can either act as recombination centers increasing electron-hole

35

recombination rates,1–3 or they can aid charge carrier separation, increasing their lifetimes and

36

enabling slow redox reactions.4,5 Thus, understanding of charge carrier trapping and

37

recombination at different time scales as well as the role of surface species in these processes is

38

key for optimization of photocatalytic materials.

39

One of the most desired and yet very challenging processes for sustainable energy conversion

Dynamics of photogenerated electrons and holes can be accessed by means of different

40

spectroscopic

techniques

such

as

electron

paramagnetic

resonance,

time-resolved

41

photoluminescence, microwave conductivity, and transient absorption spectroscopy.6–15 Often, a

42

combination of several methods is needed to obtain information about the charge carrier

43

lifetimes and the related interfacial chemical processes. In this regard, time-resolved mid-

44

infrared spectroscopy (trIR) is a versatile tool that can access both the majority charge carriers

45

through their characteristic absorption, and interfacial chemical reactions via the changes of

46

molecular vibrational bands.15–20 Temporal resolution of modern IR instruments ranges from ps–

47

ns15,16,20,21 to ms−s ranges,17,18 covering both fast transfer22 and trapping16,21 of photogenerated

48

charge carrier as well as slow redox reactions facilitated by them.17,23,24

49

Over the last decades, steady-state and time-resolved IR spectroscopies have been used to

50

study trapping and recombination of photogenerated electron in TiO2-based materials.15–18,20 This 3 ACS Paragon Plus Environment


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wide-bandgap n-type oxide semiconductor is used for photocatalytic degradation of pollutants,25

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self-cleaning surfaces26 and as the anode material in dye-sensitized solar cells.27,28

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Photogenerated electrons exist in titania in the form of free conduction band (CBE) and shallow

54

trapped electrons (STE).15–18,20 CBE species give rise to distinct exponential absorption features: () = –

55

(1)

56

where ACBE(ν) is the CBE absorbance at the frequency of the probing light ν, K – the

57

proportionality coefficient, and n – an exponent with values between 1.5 and 3.5 depending on

58

the electron scattering mode.29 In contrast to the featureless exponential CBE absorption, STE

59

give rise to broad bands which can be described by optical excitation of electrons from shallow

60

trap states into the conduction band: () =

61

( )/ ( )

(2)

62

where E is energy of IR irradiation in eV, K is the proportionality coefficient, Eop and Ethermal are

63

optical and thermal energy required to detrap an electron from the shallow state into the

64

conduction band continuum, respectively.18,30 Trapping of photogenerated electrons affects the

65

efficiency of photocatalytic systems,4,5,28 but the mechanism of STE formation in TiO2 and the

66

nature of the trap states remain unclear. Some works attributed these trap states to oxygen

67

vacancies,4 while other studies advocate polaronic self-trapping of photogenerated charge

68

carriers.16–19

69

In this work, we used sub-second time-resolved diffuse-reflectance Fourier transform infrared

70

spectroscopy (DRIFTS) to investigate CBE and STE formation in commercial TiO2 P25 (Eg =

71

3.2 eV), anatase (Eg = 3.2 eV) and rutile (Eg = 3.0 eV) titania. We used 409 nm (3.03 eV) and

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325 nm (3.81 eV) laser irradiation to study how excitation energy and intensity influence the 4 ACS Paragon Plus Environment

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dynamics of mid-IR photogenerated electron absorption. Above-bandgap excitation (i.e. 325 nm)

74

is expected to yield CBE which can then be trapped and form STE species, while sub-bandgap

75

excitation with a 409 nm laser can lead to direct population of shallow trap states (Scheme 1).

76

Formation of a prominent STE signal was observed in oxidized TiO2 P25 under both 325 nm and

77

409 nm excitation. In untreated titania, STE were formed under a broad range of 409 nm light

78

intensities and under low-intensity 325 nm irradiation (≤ 3.0 mW/cm2). Intensity-dependent

79

measurements revealed that for 325 nm and 409 nm excitation the steady-state concentration of

80

free photogenerated charge carriers is controlled by bimolecular and trap-assisted recombination,

81

respectively. The apparent electron absorption rise and decay kinetics at the s−min time scale, on

82

the other hand, relate to chemical trapping of free charge carriers and their concomitant

83

recombination. The presence of dissociatively adsorbed water on the oxide was found to be

84

crucial for the formation of STE. This suggests that STE form in a surface-mediated

85

physicochemical process involving hydroxyl groups rather than upon trapping in the bulk of

86

titania particles.

87 88 89

2. Experimental

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Industries), anatase (Sigma-Aldrich) and rutile (Sigma-Aldrich) were used as TiO2 sources. The

91

materials were characterized by bright field transmission electron microscopy (FEI Technai G2

92

type Sphera operated at 200 kV), powder XRD (Bruker Endeavour D2 Bragg-Brentano

93

diffractometer with Cu cathode and a 1D LYNXEYE detector), diffuse-reflectance UV-Vis

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spectrometry (Shimadzu UV-2401PC) and liquid N2 physisorption (Tristar II, Micromeritics).

2.1 Materials and characterization. Commercial mixed phase Aeroxide P25 (Evonik



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More details about instrument configuration and experimental procedures can be found in the

96

Supporting Information.

97

2.2 IR spectroscopy. Time-resolved and steady state DRIFT spectra of TiO2 samples were

98

recorded in a low temperature reaction chamber (Harrick Scientific) connected to a home-built

99

gas delivery and vacuum system. The IR signal was coupled in and out through two polished

100

KBr windows while the sample was irradiated with 409 nm or 325 nm light through a third

101

fused-silica window. The cell exterior was thermostated at 293 K by cooling water during all

102

experiments. A Bruker Vertex 70v FTIR spectrometer equipped with a Praying Mantis diffuse-

103

reflectance accessory (Harrick Scientific) and a liquid-nitrogen cooled MCT detector was used

104

for the rapid scan and steady-state measurements. The sample compartment of the spectrometer

105

was purged by dry nitrogen during the experiments, the interferometer and IR optics were kept

106

under vacuum (ca. 3 mbar). The spectra were recorded at 4 cm–1 resolution in the spectral range

107

3950–600 cm–1. 100 scans were averaged per steady-state spectrum. A low-pass IR filter (cut-off

108

frequency

109

cm–1) was placed in front of the detector compartment to prevent aliasing and block UV/vis

110

excitation stray light. Dry KBr (Sigma Aldrich, IR grade) was used as the reference for the

111

survey DRIFT spectra. The sample in the dark state was used as the reference for difference

112

spectra and for the time-resolved measurements.

3950

113

Time-resolved experiments were conducted in rapid scan mode. Single-sided interferograms at

114

forward and backward mirror movement were acquired at 4 cm–1 spectral resolution and 40 kHz

115

scanner velocity. The backward and forward components of the interferograms were

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automatically split by Bruker OPUS 7.5 software. With these settings a single scan took ca. 125

117

ms and a delay between consecutive mirror movements was ca. 40 ms. Temporal profiles of the 6 ACS Paragon Plus Environment

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IR signal at particular wavenumbers were extracted from the 3D data blocks consisting of

119

individual transient spectra stacked against recording time with Bruker OPUS 7.5 and further

120

processed in Origin. When the signal-to-noise ratio of individual measurements was too low,

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several kinetic curves were averaged during data processing. O2 (≥ 99.95%), H2 ( ≥ 99.999%)

122

and Ar ( ≥ 99.999%) gasses were supplied by Linde and passed through moisture and/or oxygen

123

filters (Agilent technologies). The 325 nm and 409 nm emission lines of a continuous-wave He-

124

Cd (Kimmon Koha) and diode (GaN) lasers, respectively were used to irradiate the samples.

125

Intensity of the incident light was adjusted with reflective neutral-density filters. The area of the

126

circular laser beam at the fused silica window was about (1.0 ± 0.1) cm2 in all experiments.

127

Exposure of the sample during measurements was controlled with a mechanical shutter

128

(Thorlabs) and a software script. More details on data conversion and processing can be found in

129

the Supporting Information.

130

2.3 Sample treatment. In this work, we used TiO2 either as delivered (untreated sample) or

131

after a pre-treatment in O2 atmosphere at elevated temperatures (oxidized sample). For untreated

132

TiO2, the oxide was placed in the low temperature reaction chamber, evacuated to the lowest

133

stable pressure (p < 10–3 mbar) at 293 K and kept under dynamic vacuum for several hours prior

134

to the experiments. The oxidized TiO2 was prepared as follows: First, the oxide was evacuated in

135

the low temperature reaction chamber (Harrick Scientific). Then, the sample was heated to 523 K

136

under static vacuum, exposed to 36 mW/cm2 409 nm irradiation for 30 min. Over this time,

137

intensities of CHx bands decreased and bands of carboxylates became more prominent. After this,

138

100 mbar O2 was admitted to the cell and irradiation continued until no spectral changes were

139

observed anymore (i.e. bands of CO2 gas, hydrocarbons and organic oxygenates did not change

140

in intensity anymore). Then, the reaction chamber was evacuated, refilled with 50 mbar O2, and 7 ACS Paragon Plus Environment


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the sample was cooled to room temperature, irradiated with 409 nm light for 1 h and then kept in

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300 mbar O2 overnight in dark prior to spectroscopic experiments. This treatment was found to

143

increase hydration of the material and result in a substantial increase of the STE signal intensity

144

in comparison with untreated TiO2 P25.

145 146

Scheme 1. Excitation of a semiconductor with the bandgap Eg by above- (hν1) and sub-bandgap

147

(hν2) irradiation, free conduction band electrons (CBE), and shallow trapped electrons (STE)

148

along with the characteristic IR spectra of these species. For hν2 two possible situations are

149

shown: valence band-donor STE states (left) and acceptor states-valence band (right) transition.

150 151 152

3. Results and discussion

153

325 nm irradiation strongly depend on the hydration state of the oxide.31 In this work, the

154

material subjected to an oxidative treatment exhibited a strong STE signal, while photogenerated

Recently, we found that the dynamics of electrons generated in TiO2 P25 by above-bandgap


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electron absorption in untreated titania was dominated by CBE. In the present study, we

156

investigated the formation mechanism of STE by studying photogenerated electron absorption

157

dynamics in untreated and oxidized mixed-phase TiO2 P25 as well as in commercial anatase and

158

rutile under variable intensity of 325 nm (3.81 eV, above bandgap) and 409 nm (3.03 eV, below

159

bandgap) excitation at different temperatures.

160 161

Figure 1. Difference room temperature DRIFT spectra of untreated (a) and oxidized (b) TiO2

162

P25 in static vacuum under 325 nm irradiation of different intensity. Inset – zoomed in O−H

163

spectral region. Note: the spectra are vertically offset to match the baseline in the 3950–3700 cm–

164

1

region to zero absorbance.



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3.1 Above-bandgap 325 nm excitation of untreated and oxidized TiO2 3.1.1 Intensity-dependent steady-state difference spectra. Survey DRIFT spectra of

168

untreated and oxidized TiO2 P25 are shown in Figure S1 (Supporting Information). In oxidized

169

titania, the 3693 cm–1 band of dissociatively adsorbed water32–34 was more prominent than in the

170

untreated material and so was the broad band of molecular associated water. When these samples

171

were exposed to 325 nm light, a broad absorption feature with intensity increasing toward lower

172

wavenumbers emerged (Figure 1). The cut-off of the absorbance band at wavenumbers < 1000

173

cm–1 and high noise level this spectral region are due to the high absorbance of the TiO2 lattice.35

174

The behavior of untreated and oxidized TiO2 P25 was very similar under ca. 0.5−3.0 mW/cm2

175

325 nm irradiation except for the intensity of the photogenerated electron absorption feature,

176

which was higher for the oxidized material (Figure 1). In both samples, 325 nm irradiation of

177

intensities ≤ 3.0 mW/cm2 generated very broad absorption bands with an absorbance onset in the

178

3500−3000 cm−1 region and intensities almost linearly increasing toward lower wavenumbers.

179

Similar broad absorption bands have been previously ascribed to STE.17,18 When the intensity of

180

325 nm light exceeded 3.0 mW/cm2, the stretching (3400 cm–1) and bending (1622 cm–1)

181

vibrational bands of associated adsorbed water and the stretching vibrational bands of surface O–

182

H groups at 3695 cm–1, 3665 cm–1 and 3632 cm–1 decreased in intensity.32–34 Decrease of the

183

molecularly adsorbed water band intensity can be due to light-induced sample heating which has

184

been previously reported in literature for TiO2-based materials.36,37 The difference between the

185

photogenerated electron absorption bands developed in the untreated and oxidized samples

186

became apparent. The onset of the absorption feature formed in the untreated material under 6.0–

187

15.6 mW/cm2 325 nm occurred at lower wavenumber. The shift of the electron absorbance onset 10 ACS Paragon Plus Environment

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188

was partially due to bleaching of the associated water band which spans through the 3500−2500

189

cm–1 spectral region. Besides this, the band formed under 6.0–15.6 mW/cm2 325 nm appeared

190

narrower, compared with features observed under lower light intensities, and could be fit by the

191

exponential function (1) characteristic for CBE.29,38 Even though oxidized TiO2 P25 developed

192

more prominent bleaching of associated water bands than untreated material, the photogenerated

193

electron absorption band formed under 325 nm irradiation was broader and its signal almost

194

linearly increased toward lower wavenumbers. This band, and the bands formed under lower

195

light intensities could be fit with a complex function consisting of a sum of function (1) and a

196

number of components described by function (2) (for detailed fitting see Figure S2, Supporting

197

Information). Such a combined function has been previously used in literature to fit similar broad

198

absorption bands due to the contribution of CBE and STE species.18 Besides the different

199

structures of the photogenerated electron absorption bands formed in untreated and oxidized

200

TiO2 P25 under higher UV light intensities, these samples showed different rearrangement of

201

surface hydroxyls. In the untreated sample, the 3632 cm–1 band was affected the most while in

202

the oxidized titania the bands at 3695 and 3660 cm–1 showed a more prominent intensity

203

decrease. These surface hydroxyls form upon dissociative adsorption of water molecules.32–34

204

Reversible bleaching of these bands under 325 nm irradiation suggests either partial removal of

205

these hydroxyls or dissociation of the O−H bonds. Addition of oxygen quenched intensities of

206

the low-wavenumber absorption features developed under 325 nm excitation in both oxidized

207

and untreated TiO2 P25, thereby confirming their assignment to photogenerated electrons.

208

3.1.2 Intensity-dependent time-resolved measurements: above-bandgap excitation.

209

Temporal profiles of the photogenerated electrons absorbance rise and decay were found to be

210

wavenumber-independent in the spectral ranges free from molecular vibrational bands which 11 ACS Paragon Plus Environment


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211

agrees well with results reported in litearure.13,15,38,39 In this work, temporal changes of the

212

photogenerated electron signal were monitored at 1280 cm−1, because there are no molecular

213

bands, while the signal-to-noise ratio was found to be the highest. Experimental profiles of the

214

electron absorption generated in the oxidized TiO2 P25 under 325 nm irradiation of high and low

215

intensity at 1280 cm−1 are shown in Figure 2. From this figure one can see that the increase of the

216

light intensity had a more prominent effect on the electron absorption signal rise than on its

217

decay.

218

219 220

Figure 2. Experimental kinetic curves of photogenerated electron absorption rise (a) and decay

221

(b) at 1280 cm−1 normalized to OD = 1.0. Oxidized sample, 293 K, static vacuum, 325 nm 12 ACS Paragon Plus Environment

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222

excitation. The dashed blue line shows the time when the exposure to UV light started (a) and

223

ended (b).

224

The dependencies of the electron absorption rise and decay times from the 325 nm light

225

intensity are shown in Figure 3. Both the untreated and oxidized TiO2 P25 demonstrated a linear

226

correlation between characteristic times of the electron absorption rise and decay and the square

227

root of the UV light intensity. Besides this, the decay was significantly slower than the rise.

228

Intensity-dependent plots obtained for untreated TiO2 P25 exhibited two distinct linear regions

229

intersecting at ca. 3.0 mW/cm2. Up until this value, the electron absorption rise and decay rates

230

increased with increasing 325 nm light intensity but showed much weaker intensity

231

independence at higher intensities (Figure 3 a and b). On the other hand, the oxidized sample

232

showed no such behavior and the electron absorption rise and decay rates decreased with

233

increasing UV light intensity throughout the entire range (Figure 3 c and d). Besides this, the

234

electron absorption dynamics were slower in the oxidized material than in untreated TiO2 P25.



Page 14 of 33

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236 237

Figure 3. Dependence of the half-life rise (left) and decay (right) times of photogenerated

238

electron absorption in the untreated (a, b) and oxidized (c, d) TiO2 P25 from the square root of

239

325 nm light intensity measured at 293 K under static vacuum.

240 241

The substantial difference between the electron absorption rise and decay rates (Figure 3)

242

suggests that accumulation and recombination of photogenerated charge carriers in TiO2 P25 at

243

the s−min time scale did not reflect the dynamics of free photogenerated charge carriers directly.

244

This can be understood by considering a simple kinetic model describing generation and

245

recombination of free electrons and holes in a semiconductor under above-bandgap excitation:40


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&'

∗ ! + *

246

!

+ ℎ$ →

247

∗ !

+ * + ℎ →

+

!

+ ℎ

(3)

+ ℎ! ,- .

(4)

248

When process (4) is the primary pathway of charge carrier recombination, the concentrations of

249

electrons and holes in the system under irradiation is determined by the charge carriers

250

generation (i.e. light intensity I) and recombination rates, while relaxation of the system to the

251

ground state in the dark (I = 0) is described solely by the latter process: /[12 ]

252

=

/4

/[5 6 ] /4

= 78 − .: [* ][ℎ ]

(5)

253

where α is the effective absorption coefficient, I is the incident light flux in photons/s, and k4 is

254

the rate constant of electron-hole recombination. For such a system, steady state concentrations

255

of photogenerated charge carriers ([e–]0, [h+]0) under irradiation, and characteristic times of their

256

accumulation and recombination (i.e. signal rise and decay half-life times) are:

257

[* ]; = [ℎ ]; = 523 K. On the other hand, 409 nm light produced a

445

rather strong STE signal (ca. 0.05 a.u.) up to 573 K but no STE absorbance was observed at 623

446

K. At this temperature, most of the molecularly and dissociatively adsorbed water desorbed from

447

the oxide as was evident from the intensity losses at 3693 cm–1, 3632 cm–1, 3400 cm–1 and 1624

448

cm–1.32–34 When such a dehydrated sample was cooled to 293 K, no STE signal was produced by

449

409 nm light for several hours even though molecularly adsorbed water re-appeared on titania

450

upon cooling down. Only when the bands of surface hydroxyls formed upon dissociative water 25 ACS Paragon Plus Environment


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451

adsorption water (i.e. 3693 cm–1 and 3632 cm–1) emerged in the survey DRIFT spectra, the STE

452

signal emerged under sub-bandgap excitation again. This correlation between the STE formation

453

and the presence of 3693 and 3632 cm–1 hydroxyl bands on TiO2 P25 further confirms that these

454

surface species are involved in trapping of photogenerated electron in titania. The absence of the

455

STE signal in dehydrated TiO2 P25 suggests that bulk electron trapping (i.e. by the crystal

456

defects) did not have any significant contribution to the apparent STE absorbance.

457

Formation of CBE and STE in TiO2-based materials has been previously explored by different

458

techniques.16–19,36–39,46 For instance, Yates et al. used electron paramagnetic resonance (EPR) to

459

study trapped electrons and holes in TiO2 under above-bandgap irradiation at 90 K.36,46

460

Comparison of the trapped electron and hole signals suggests that trapping of photogenerated

461

holes is much more favorable than trapping of electrons. Hence, the major part of photogenerated

462

electrons is present in the form of free conduction band electrons at 90 K. This agrees with our

463

observation that electron absorbance formed under 325 nm at 163 K was dominated by the CBE

464

component (Figure 6a). Interestingly, EPR signal of trapped electrons was not observed at

465

temperatures higher than 90 K,46 while a rather strong STE absorbance formed in oxidized TiO2

466

P25 under 409 nm irradiation at 163 K (Figure 6b). This can be due to involvement of different

467

processes in STE formation under sub-bandgap irradiation of titania at T ≤ 163 K or because of

468

different nature of trapped electrons observed by EPR and IR spectroscopies. The fact that

469

almost no STE signal was observed when water adsorption on TiO2 P25 was perturbed by high

470

intensity 409 nm irradiation suggests that interaction between adsorbed and the oxide surface is

471

crucial for photogenerated electron trapping in TiO2. This agrees with the fact that no STE signal

472

was observed when hydroxyls formed upon dissociative water adsorption (i.e. bands at 3695 and

473

3632 cm−1) were removed from TiO2 at elevated temperatures (see previous sub-section). 26 ACS Paragon Plus Environment

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474

The strong apparent STE signal formed in TiO2 P25 (Figure 4) and anatase (Figure S4a,

475

Supporting Information) under 409 nm irradiation may suggest that shallow trapped states can be

476

directly populated by sub-bandgap excitation (Scheme 1). However, this conclusion contradicts

477

finding of Antila et al.16 who observed no electron absorbance at delay times up to 1 ns for sub-

478

bandgap excitation. Antila et al. proposed that STE states are not pre-existing features of the

479

material (i.e. crystal defects) but form upon polaronic self-trapping of photogenerated electrons.

480

Taking into account these findings and our experimental results, we surmise that STE form in a

481

surface-mediated process which involves surface hydroxyls originating from dissociative water

482

adsorption (i.e. 3693 and 3630 cm−1 bands). Involvement of a rather slow chemical process can

483

then explain the absence of the STE signal at time scales up to 1 ns.16 The strong correlation

484

between the presence of surface hydroxyls and STE formation is in line with a recent theoretical

485

study which showed that an excess of electrons in anatase TiO2 can trigger water dissociation on

486

(101) surface and form polarons with the resulting surface hydroxyls.42 This surface-mediated

487

trapping of photogenerated electrons can play an important role in photocatalytic processes by

488

slowing down charge carrier recombination and improves their accessibility to the reactants

489

when photocatalytic reactions are carried out either on a hydrated oxide or on TiO2 particles

490

suspended in aqueous media.

491 492

4. Conclusions

493

P25 under 325 nm and 409 nm irradiation were investigated in detail in a broad range of

494

temperatures. Intensity-dependent measurements evidenced that the steady-state concentrations

495

of free charge carriers were governed by a bimolecular and trap-assisted recombination in the

496

case of above- and sub-bandgap excitation, respectively. STE formation in TiO2 P25 was

The s−min dynamics of photogenerated electron absorption in untreated and oxidized TiO2



Page 28 of 33

497

observed in a broad range of temperatures under 325 nm and 409 nm irradiation both in mixed-

498

phase TiO2 P25 and anatase titania but not in the rutile phase. At 163 K, the STE signal

499

substantially decreased when high intensity 409 nm irradiation perturbed hydrogen-bonding

500

between the oxide and adsorbed water and decreased the intensity of surface hydroxyls formed

501

upon dissociative water adsorption. In keeping with this, almost no STE signal was detected at

502

room- or elevated temperatures when 3695 and 3632 cm−1 hydroxyl bands were removed from

503

titania. Characteristic rise and decay times of the STE signal suggest that formation of these

504

species is coupled with a rather slow interfacial chemical process. The prominent correlation

505

between the STE absorbance, crystal phase composition and the presence of certain hydroxyls in

506

the material showed that shallow trapping of photogenerated electrons occurs is a surface-

507

mediated process involving surface hydroxyls originating from dissociative water adsorption on

508

anatase TiO2.

509 510 511

Associated content

512

detailed data processing are available in the Supporting Information

513

Author Information

514

Corresponding authors:

515

* J.P. Hofmann e-mail: [email protected]; phone: +31 (0) 40 247 3466

516

* E.J.M. Hensen e-mail: [email protected]; phone: +31 (0) 40 247 5178

TEM images, survey and difference DRIFT spectra, XRD patterns, UV-vis spectra, and


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517


Acknowledgements

518

The authors thank J.J.G. van Velzen and M.W.G.M. Verhoeven (both Eindhoven University of

519

Technology) for their assistance in setting up the equipment. Development of the time-resolved

520

FTIR setup was supported by the Dutch National Research School Combination Catalysis

521

Controlled by Chemical Design (NRSC-C). This work was supported by NanoNextNL, a micro

522

and nanotechnology consortium of the Government of The Netherlands and 130 partners: project

523

NNNL.02B.08CO2Fix-1.

524 525

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Figure for Table of Contents

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Role of Dissociatively Adsorbed Water on the Formation of Shallow

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