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Photoelectrochemical Determination of Shallow and Deep Trap States of Pt Decorated TiO Nanotube Arrays for Photocatalytic Applications 2

Maryam Zare, Abdollah MortezaAli, and Azizollah Shafiekhani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11987 • Publication Date (Web): 14 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016

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The Journal of Physical Chemistry

Photoelectrochemical Determination of Shallow and Deep Trap States of Pt Decorated

1

TiO2 Nanotube Arrays for Photocatalytic Applications

2

Maryam Zare a, Abdollah Mortezaali a,*, Azizollah Shafiekhani a,b

3 4

a b

Physics Department, Alzahra University, Vanak, Tehran 1993893973, Iran

5

School of Physics, Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5531, Tehran,

6

Iran

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1 ACS Paragon Plus Environment


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Abstract

27

A novel insight to the effect of Pt decoration on electrochemical and photoelectrochemical

28

behavior of TiO2 nanotube arrays (TNA) was developed in this study. TNA samples were

29

prepared via a two-step anodization of a titanium foil and decorated with Pt by a facile

30

photodeposition method. The formation of metallic Pt were confirmed by X-ray

31

photoelectron spectroscopy (XPS). Based on our calculations, the localized states and surface

32

states induced by Pt deposition into the bandgap of titania nanotubes, play a dominant role in

33

trapping/detrapping charge carriers and electron transfer to electrolyte. In Pt/TNAs the

34

appropriate electrical connection between Pt nanoparticles and TNA induces sufficiently

35

shallow traps in the vicinity of conduction band edge of TNA which creates a fast lane for

36

electrons toward semiconductor/electrolyte interface and decreases the density of deep trap

37

levels compared to the pristine TNA. However, there is an optimum amount for deposited Pt.

38

Higher amount of optimum Pt can impose the monoenergetic deep trap levels which act as

39

recombination centers.

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1.

56

Introduction

Photoelectrochemical (PEC) reaction is one of the best strategies for clean production of

57

hydrogen and oxygen from water splitting using radiative energy. This conversion process

58

has been studied extensively over the past a few decades.1-3 In recent years, TiO2 as a

59

transition metal oxide semiconductor has attracted much attention as photocatalyst because of

60

its unique electronic and ionic properties, biocompatibility, non-toxic and stability against

61

corrosion.4-8 Among all of nanosized geometries of this material, one- dimensional highly

62

ordered TiO2 nanotube

on Ti substrates is a suitable

63

photoelectrocatalyst electrode for hydrogen generation via water splitting because of its

64

superior electron mobility, quantum confinement effect and improved specific surface

65

area.9,10 Nevertheless, high recombination rate of photogenerated electron- hole (e-/h+) is one

66

of the main disadvantage of this material which limits its efficiency in photcatalytic

67

applications. Decorating TNA surface with noble metal co-catalysts is one of the most

68

applicable routs to overcome this problem.11-15 Most of researchers believed that the noble

69

metal nanoparticles effectively collect electrons via larger work function (i.e. lower Fermi

70

levels), so provide an interface to enhance e-/h+ separation.16 Another role of co-catalyst is to

71

lower the electrochemical overpotentials associated with the multielectron reactions, because

72

the metal co-catalyst can work as an electron-sink. The deposition of noble metal co-

73

catalysts such as gold and silver on TiO2 surface has also been used widely to enhance the

74

PEC activity of TiO2. Various methods were utilized by researchers to synthesis metal/TiO2

75

nanocomposite. Sol- gel,17,18 using an electrochemically active biofilm,19-21 chemical

76

reduction,22 electrodeposition23 and photodeposition24 are the recent methods. Although the

77

use of other noble metals such as Ag or Au are more reasonable, Platinum (Pt), with the

78

largest work function (5.7 eV) and lower Fermi level, can produce the highest Schottky

79

barrier at the noble metal/ TiO2 interface.25 It facilitates the electron capture from TiO2 and

80

arrays (TNA) formed



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reduces e-/h+ recombination. Moreover, proton reduction sequence on co-catalysts contains at

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least two steps: first, H+ reduction to atomic H and second, the catalytic activity for the

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combination of surface hydrogen atoms into molecular H2. Trasatti found that Pt has the

83

lowest activation energy for H2 evolution.16 Therefore, Pt was proposed to be the most

84

promising applicant of co-catalyst for e-/h+ separation and the active reaction site for H2

85

production. Although, the mechanism of collecting and transferring electrons through Pt/TiO2

86

has studied extensively by researchers,26-28 the nature of electron transport in Pt/TiO2

87

nanostructures and the effect of Pt concentration on TiO2 PEC properties are not already well

88

understood. To our knowledge, little analytic and comprehensive studies have been

89

performed on electron transport in Pt decorated TiO2 nanostructured electrodes in

90

electrochemical (EC) and photoelectrochemical (PEC) systems.

91

Deep and shallow trap states distributed in the bandgap of nanostructured metal oxide

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semiconductors play an essential role in the dynamics of the electron transport and

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performance of the PEC systems. Bisquert and co-workers reviewed electrochemical

94

techniques utilized for determination of the density of electronic states in these

95

nanostructured metal oxide electrodes.29 In recent communications, they also showed

96

voltammetry techniques applied to a semiconductor/ electrolyte junction could provide an

97

improved approach for density of states (DOS) measurement of a semiconductor in the

98

energy axis.30,31 Moreover, they indicated that the study of the sharp cathodic peak appeared

99

in capacitance plot could determine the distinct contribution of trapped electrons in

100

monoenergetic levels of surface states and deep exponential DOS. Gomez et al. have

101

demonstrated these sharp cathodic responses in TiO2 nanostructured electrodes are due to

102

deep trap states associated with grain boundaries.32 Two different hole transfer processes

103

from the valence band and surface states were also investigated by Bisquert group utilizing

104

low frequency capacitance measurements for potential application in solar fuels production.33

105


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In this work, we have fabricated vertical and highly ordered TiO2 nanotube arrays (TNA)

106

with top open surface by anodic oxidation in an organic electrolyte through a double step

107

process. Then, different content of Pt nanoparticles were deposited on TNA by a facile

108

photodeposition method (PD). In addition, the electron transport property of the Pt/TNAs,

109

was evaluated by using capacitive analysis in the Bisquert approach. This calculation

110

provides a new physical insight for further understanding the role of Pt nanoparticles in

111

trapping/detrapping charge carriers in the PEC systems for the first time. Finally, we

112

proposed a mechanism for photo carrier transfer in Pt/TNAs based on our results.

113

2.

Materials and Methods

114

2.1

115

Preparation of TiO2 Nanotube Arrays and Pt/TNAs

One-Dimensional TiO2 nanotube arrays were fabricated through electrochemical anodization

116

of Ti substrates in a two electrode vertical cell with Ti sheet (purity > 99%, 0.7 mm

117

thickness) as a cathode and Ti foil (purity > 99%, 0.033 mm thickness) as an anode.

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Electrolyte was consisted of 0.3 wt% NH4F and 2 vol% deionized (DI) water in ethylene

119

glycol. The potentiostatic anodization was performed under 50 VDC in a two- step process.

120

In the first step, a sacrificial layer was formed on the Ti substrate. Then, it was removed by

121

ultrasonication for prevention of debris formation. The second step was carried out on the

122

former dimpled substrate at the same potential. The as-anodized samples were annealed in the

123

barometric air at 450ºC for 3 hours with heating rate of 2 ºC/min.

124

TNA electrodes were decorated with Pt nanoparticles by a straightforward photodeposition

125

method. As a typical synthesize, an aqueous solution containing 2g/lit Chloroplatinic acid

126

(H2PtCl6. 6 H2O, Aldrich) named as “M” and 20wt% methanol solution in DI water were

127

prepared, separately. Then, M solution was mixed with methanol solution in 1/1 volume ratio

128

to obtain M1. M2, M3 and M4 were prepared by mixing M1 and methanol solution in 1/1, ½

129

and ¼ volume ratios, respectively. Pt deposition was performed when TNA samples were

130



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transferred to a glass plate and rinsed in appropriate Pt solution (M#) and illuminated from

131

top side by a high pressure Mercury lamp (125 W) with illumination density of 13.4 Wm-2.

132

The overall pH of the solutions (M#) was 3. Each deposition process takes 10 minutes. The

133

color of the solutions was changed to dark gray during the reaction which indicated reduction

134

of Pt anion (PtCl62−) to metallic Pt.

135

2.3

Characterization and PEC Measurements

136

The surface and cross sectional morphologies of as-prepared electrodes were characterized by

137

field emission scanning electron microscope (FESEM) (Hitachi S-4160). The crystalline

138

structures were investigated by X-ray diffraction (XRD, Rigaku, Japan) using Cu Kα

139

radiation (λ=1.54056 Å). The surface chemical state of the Pt/TNA nanostructures was

140

examined by X-ray photoelectron spectroscope (XPS) using a monochromated Al Ka

141

radiation source (1486.6 eV). The UV-Vis diffuse reflection spectroscopy (UV-Vis DRS)

142

was measured using an Avantes spectrometer equipped with an integrating sphere.

143

Photoluminescence (PL) spectra were measured by means of fluorescence spectrophotometer

144

(Cary Eclipse). The excitation energy was set on 3.94 eV for PL measurement.

145

All photoelectrochemical experiments were performed on a SAMA500 (SAMA Research

146

Center, Iran) electro-analyzer with 500 ml of 0.5 M KOH as electrolyte solution. It was

147

carried out in a 3 electrode cell where the prepared electrode with an area of 1 cm2, a

148

platinum rod and an Ag/AgCl electrode were employed as working (WE), counter and

149

reference electrodes, respectively. The WE was illuminated by a Xe light source (EIKI,

150

Japan) with illumination intensity of 11 kW/m2 at the WE location through a quartz window.

151

All of PEC measurements repeated several times with different aging. The results show no

152

changes which indicating reasonable stability of pure TNA and Pt/TNA samples.

153 154 155


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3.

Results and Discussions

156

3.1. Characterization of Pt/TNA Layers

157

Table S1 shows the content of Pt in initial PD solutions. The surface and cross sectional

158

morphology of pristine TNA electrode is shown in Figure 1a. The homogeneous 1-D highly

159

ordered nanotube arrays grown successfully on the Ti substrate are demonstrated in this

160

figure. Open-mouth tubes are achieved using this improved 2-step formation procedure and

161

no collapsing or bundling of walls in the form of debris are observed on the top layer. The

162

internal diameter and wall thickness of tubes are 90, 40 nm, respectively. As well, the

163

thickness of TNA layer is ca. 5 µm from cross sectional image.

164

FESEM shows Pt nanoparticles are randomly distributed on the top surface and inner walls of

165

nanotubes. It is obvious from Figure 1b that in some positions Pt nanoclusters are formed on

166

Pt1/TNA, however Pt nanoparticles in Figure 1c are not clearly observed probably because of

167

very small size of particles. Image processing estimates the size of Pt clusters in Pt1/TNA can

168

be up to 160 nm.

169

170 FIGURE 1. FESEM images of a) pristine TNA layer, b and c) Pt1/TNA and Pt/4TNA layers.

171

The crystal structures of the prepared samples were studied by XRD. Figure S1 (a and b)

172

shows the XRD patterns of as-anodized and annealed TNA electrodes. It is demonstrated that

173

as-anodized arrays of nanotubes has amorphous crystallographic phase while walls of

174



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nanotubes transform to anatase due to annealing at 450ºC in air For 3 h and no rutile fraction

175

was found in the samples.

176

The XPS survey spectrum of the Pt1/TNA sample is indicated in Figure 2. As expected, XPS

177

analysis revealed the existence of Ti, O and Pt elements on the sample. A trace amount of

178

carbon

adsorbed

179

pollutions/contaminations (such as CO/CO2) in air. It is known that the chemical

180

compositions of the platinum particles via photodeposition method strongly depend on the pH

181

of the precursor solution.34-36 According to Han et al.37 proposed mechanism, in the acidic

182

solutions, the metallic Pt (Pt0) nanoparticles are formed on TiO2 surface while PtO and PtO2

183

could be formed when precursor solution was basic. So, we expect that metallic Pt

184

nanoparticles was achieved in our experiment with pH=3. Figure 2b shows the Gaussian

185

fitted core-level Pt(4f) of the Pt/TNA sample. Deconvoluting Pt(4f) core level spectra

186

exhibits three characteristic peaks of 4f7/2 at binding energies (B.E.) of 67.96, 69.08 ,71.51 eV

187

and one 4f5/2 peak at binding energy of 74.60 eV. The first three peaks can be assigned to

188

metallic Pt with an oxidation state of zero (Pt0) and the last one (4f5/2) is corresponded to Pt2+

189

or Pt4+ according to National Institute of Standards and Technology (NIST) X-ray

190

Photoelectron Spectroscopy Database. The binding energies of individual peaks along with

191

other important parameters are provided in Table 1. Although the mixed valence states of

192

platinum is present in Pt deposited TNA samples, comparing the area surrounded by each

193

peak demonstrates that the main oxidation state is attributed to Pt0, as predicted. Moreover,

194

the last peak with the lowest B.E. (named as E in Figure 2b is attributed to Pt(5p) that is too

195

small in compared with others. The presence of PtO and PtO2 (peak C in Figure 2b) on the

196

surface of TiO2 samples probability is related to oxygen chemisorption at the step and kink

197

sites present on the Pt surface.21

198

was

also

observed

in

the

survey

as

a

result


of

surface

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199 FIGURE 2. a) XPS survey spectra and b) High resolution XPS spectra of Pt1/TNA.

200 201 202

Table 1. XPS peak positions and other important parameters of Pt/TNA.

Name B.E. (eV) FWHM

Area

Oxidation state Peak Rel %

A

71.50

2.09

1062.5

Pt0

4f7/2

47.67

B

68.03

1.41

557.3

Pt0

4f7/2

25.00

C

74.62

2.81

384.9

Pt4+ and Pt2+

4f5/2

17.27

D

69.25

1.23

185.6

Pt0

4f7/2

8.32

E

65.55

2.53

38.94

Pt0

5p1/2

1.75 203

UV–Vis absorbance spectra were calculated from diffuse reflectance spectra (DRS) by Beer-

204

Lambert low as illustrated in Figure 3a. This Figure demonstrates that the fundamental

205

absorbance edge in all our samples is near 392 nm. Calculating bandgap from Tauc formula

206

for direct bandgap materials (Figure 3b) shows that the corresponding bandgap of pure TNA

207

is about 3.1 eV as expected for anatase. Furthermore, visible light (>400nm) absorption is

208

observed in samples is probably attributed to trapping visible light inside nanotubes.

209

Comparing samples with different content of Pt (Figure 3a), proved that Pt4/TNA exhibits

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higher absorbance in the visible region compared with pristine TNA. We believe, it is due to

211

electron transition between valence band and new electronic surface states induced by surface

212

decoration of TNA with Pt nanoparticles. However, Pt1/TNA does not exhibit such an

213

enhancement because of shielding effect of large nanoclusters of Pt, as illustrated in FESEM

214

images (Figure 1b). Indeed, less incident light can reach TiO2 surface in Pt1/TNA.

215

FIGURE 3. a) UV–Vis DRS absorbance spectra of pristine TiO2 nanotube layer, Pt1/TNAs and Pt4/TNAs. b)

216

Analysis of optical bandgap of pristine TiO2.

217


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Photoluminescence is a consequence of radiative recombination of photoinduced electron-

218

hole. Figure 4 shows a comparison of the PL spectra of pure TNA and Pt/TNAs. In pure

219

TNA, the broadest emission is close to fundamental absorption of TNA. Another peak also

220

existed in the wavelength region of below the bandgap. Energies of these two peaks are close

221

to bandgap of anatase TiO2 and represent the strongest emissions occur at energies near to

222

bandgap transmission. These UV emissions are attributed to direct exciton transitions which

223

mean the excited electrons recombine radiatively with holes in the valence band (VB) or in

224

traps near the VB. Significant increase in PL intensity near 600 and 900 nm are due to second

225

or higher orders of excitation energy. Other emissions in the wavelength range of 400-900

226

nm (mostly visible emission) with relative low intensity than formers are ascribed to defects

227

and oxygen vacancies which cause deep trap states into bandgap.38 Indeed, recombination of

228

an electron from the conduction band (CB) with a hole in a deep trap or recombination of a

229

hole from the VB with a deeply trapped electron in TNA is responsible for these emissions.

230

231 FIGURE 4. Photoluminescent spectra of different TNA and Pt/TNA electrodes. The emission spectra were

232

obtained with 3.94 eV excitation. Peaks at 630 and 945 nm are second and third modes of excitation,

233

respectively.

234



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It is clearly seen from Figure 4 that the intensity of PL decreases when a little Pt

235

nanoparticles is loaded on TNA and increases when the amount of Pt nanoparticles decorated

236

on TNA is enhanced. Indeed, the reduction of PL is an evidence of superior connection

237

between Pt and TNA which creates a Schottky junction in the interface of noble metal and

238

semiconductor. The energetic difference at Pt/TNA interface move photogenerated electrons

239

to Pt nanoparticles. Therefore separation of e-/h+ occurs efficiently and recombination

240

reduces. Another reason for quenching of PL is the creation of the surface electronic states as

241

a result of the deposition of noble metal particles on the surface of TiO2 electrodes.39 These

242

surface states act as trapping states which trap electrons or holes preventing recombination.

243

The energies of these electronic states depend on the nature of metal and the average size of

244

metal particles as well as on the properties of semiconductor. On the other hand, increasing

245

the average size of nanoparticles corresponds to the increase of the depth of energy levels

246

formed by Pt nanoparticles relative to the edge of conduction band (EC).39 It causes their

247

electric interaction with the conduction band becomes weak in order that trapping life time of

248

electron and hole for these states becomes almost equal. Therefore, large Pt nanoparticles

249

could act as recombination centers which raise the radiative e-/h+ recombination and PL

250

intensity.

251 252

3.2. EC and PEC Measurements In continuation, for better understanding of electron transfer processes at the interface

253

between an electrolyte and TNA or Pt loaded TNA electrodes we will investigate the

254

electrochemical and photoelectrochemical response of the prepared electrodes. Then, we will

255

calculate the density of traps in both type of samples.

256

3.2.1. The Transient Photocurrent Densities of Electrodes

257

The transient photocurrent densities of Pt/TNA electrodes and pure TNA versus time at zero

258

potential vs. Ag/AgCl reference (i.e. 0.9646 V vs. RHE at pH of 13) under cycles of light-off

259


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and light-on states in an aqueous solution of 0.5 M KOH (pH=13) are compared in Figure 5.

260

The photoresponse current in all electrodes is negligible in dark shows that no reaction

261

happens in dark. It is worthy to note that rapid anodic photocurrents are created in all

262

electrodes when imposing illumination. The transient photocurrent density is decreasing with

263

increasing Pt content of the electrodes and in pure TNA is higher than Pt/TNAs. Indeed,

264

decorating TNA with Pt nanoparticles causes an anodic current leakage from TiO2 anode

265

toward electrolyte, so that the net anodic current in external circuit is decreased when TNA is

266

decorated with Pt.

267

268 FIGURE 5. Photocurrent density versus time for pure TNA and Pt/TNA electrodes at zero potential vs.

269

Ag/AgCl reference.

270

3.2.2. EC Measurements and Trap States Study in Dark Conditions

271

For investigation of the trap states in pure and Pt/TNA electrodes we present cyclic

272

voltammetry (CV) experiment in two dark and illuminated pretreatment conditions as

273

described by Bisquert and co-workers.30 In first experiment, a positive potential (1.66 vs.

274

RHE) were applied to electrodes in dark conditions each for constant time of 60 s. Then a

275

cyclic voltammetry scan starts from positive to negative potentials with constant scan rate of

276

0.1 V/s for pure TNA and Pt/TNA electrodes. In second test, the same constant positive

277



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potential (1.66 vs. RHE) along with one sun illumination were applied to electrodes for 60 s.

278

Next, CVs were recorded at different scan rates from plus to minus voltages and vice versa.

279

Both CV experiments were performed at dark conditions in KOH 0.5 M (pH=13) vs.

280

Ag/AgCl reference electrode, then potentials converted to RHE. All CV experiments have

281

been started from voltage of pretreatment conditions (1.66 eV vs. RHE) and swept toward

282

negative values until cathodic current saturation happened. Figure 6a shows the CV plots of

283

dark pretreatment for pure TNA. Where the cathodic current density reach saturation, the

284

position of conduction band edge (Ec) has been estimated according to literatures.40,31 In pure

285

TNA, by sweeping potentials toward negative values, we can observe two distinctive

286

voltammetric features: An increase in cathodic current density with an exponential trend

287

along with a capacitive peak positioned at around 0 V vs. RHE. In reverse direction, the

288

broad and intense anodic peak detected around -0.5 V vs. RHE relates to oxidation of H2,41

289

and no peak can be seen around 0 V vs. RHE. Bisquert and co-workers,30,31 interpret these

290

features in terms of relaxation of electrons in trap states. According to their description, the

291

exponential feature starts from conduction band edge (Ec) shows the density of localized

292

states (DOS) located in the band gap. Among these traps, shallow ones are in equilibrium

293

with extended states (i.e. exchange electrons with conduction band) in spite of deep ones

294

which gather electrons avoiding them to detrap. We refer the separation level between the

295

shallow and deep states as the demarcation level (Ed). For the electrons at the demarcation

296

level the response time of trapping and detrapping is equal. Acutely, the capacitive peak

297

originates from deep traps which themselves are involved of two distinctive source: an

298

exponential DOS tail and a monoenergetic surface state. Indeed, capacitive peak is detected

299

because the velocity of transfer of charges is slow relative to catching them by traps. In

300

anodic direction, the absence of corresponding peak is an evident of extremely slow

301

depopulating of deep trap states of electrons by holes. Indeed, we suppose that anodic current

302


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density is not eliminated up to the end of the experiment (1.66 V vs. RHE). In nanoparticular

303

TiO2, the density functional theory (DFT) study has shown that under-coordinated surface Ti

304

ions at the (100) facets42 cause an exponential tail of inherent trap states and Ti interstitials or

305

oxygen vacancies 43-45 are the origin of a monoenergetic level of surface states.

306

307

308 FIGURE 6. a) Cyclic voltammetry plot with pretreatment in dark for pure TNA electrode and b) Total

309

capacitance of traps and deep trap capacitance obtained from CV plot of pure TNA at scan rate of 0.1 V/s for

310

initial voltage of 1.66 V vs. RHE

311



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Chemical capacitance calculations from dividing current density by scan rates in relative

312

potential range are shown in Figure 6b. The exponential curve represents the total density of

313

localized states (NL). For determination of NL, we fitted the decay part of capacitance curve

314

with an exponential equation of the form a exp (-bV) as illustrated by the dashed line in

315

Figure 6b. This exponential equation is relative to shallow trap capacitive activities with

316

=

exp

and =

, where kB is the Boltzmann constant and T is a

317

parameter with temperature unit that determine the depth of distribution and α represents the

318

relative temperature (T) to temperature at equilibrium (T0).31 The found values of α and NL

319

are given in Table 2. It is worthy to note that total traps capacitance is the sum of shallow and

320

deep traps capacitances. Therefore, we obtained the contribution of deep traps capacitance

321

with the subtraction of exponential fitting curve to the total capacitance (Figure 6b). Based on

322

this method, the amount of accumulated charge in deep traps (Q) is obtained by integrating

323

on the resultant deep traps capacitance. According to these descriptions, the charge stored in

324

the deep traps of pure TNA electrode in dark pretreatment conditions is equal to ca. 10.53

325

mC/cm2 at scan rate of 0.1 V/s.

326

Table 2. Important parameters for pure TNA and Pt/TNA electrodes with pretreatment under both dark and

327

illuminated conditions for initial voltage of 1.66 V vs. RHE

328

Dark condition (scan rate =0.1 V/s)

Illuminated condition (scan rate =0.1 V/s)

Pure TNA

Pt1/TNA

Pt2/TNA

Pt3/TNA

Pt4/TNA

Pure TNA

Pt1/TNA

Pt2/TNA

Pt3/TNA

Pt4/TNA

α

0.170

0.304

0.251

0.226

0.172

0.135

0.294

0.276

0.212

0.153

NL×10-21 (cm-3)

0.338

93.5

17.3

9.09

2.04

0.390

98.4

40.1

9.88

2.80

Q (mCcm-2)

10.53

12.48

4.47

3.22

1.57

29.75

22.18

8.67

7.11

9.02 329

For comparison between pristine TNA and Pt/TNAs the CV curves of all electrodes at similar

330

conditions (scan rate of 0.1V/s) were investigated in Figure 7. It can be found that the

331


Page 17 of 54

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position of cathodic peak associated to deep trap states and the demarcation level (Ed) are

332

slightly moved toward more positive values which indicate deep traps in Pt/TNAs became

333

deeper relative to TNA. Also, the corresponding peak current densities are also improved in

334

different Pt/TNAs when the amount of Pt enhanced. It implies that the velocity of trapping

335

has been grown by deposition of Pt nanoparticles on TNA, so that the more Pt, the faster

336

trapping of electron carriers. The density of localized states and accumulated electron carriers

337

are given in Table 2 as well. It was well documented that the Pt nanoparticles facilitate the

338

transfer of electrons from the titania.46 Our results could illustrate this point obviously. It

339

should be noted that presence of Pt on the surface of nanotubes could increase the total

340

number of localized states (one order of magnitude), however the accumulated charge

341

decreased. We suppose Pt deposition induces a plenty of shallow traps in the vicinity of

342

conduction band edge which create a fast lane for electron transfer to electrolyte in addition

343

to reducing the density of deep traps. A more quantitative estimation of deep trap states

344

densities will give later on. Of course, intense, broad and symmetric cathodic and anodic

345

peaks in Figure 7a are evident for rapid trapping/detrapping kinetics involving deep localized

346

states. Consequently, the detrapping in opposite direction is more rapid in Pt/TNAs than pure

347

TNA electrode. It is concluded that with Pt increment, besides that the density of localized

348

states was elevated, trapping and detrapping both occur rapidly. Moreover, the accumulation

349

of charges is reversible and the occupation probability in both anodic and cathodic directions

350

is equal.

351



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Page 18 of 54

352

353 FIGURE 7. a, b) Cyclic voltammetry and capacitance plots for pure TNA and Pt/TNA electrodes with

354

pretreatment under dark conditions for initial voltage of 1.66 V vs. RHE at scan rate of 0.1 V/s.

355 356

3.2.3. PEC Measurements and Trap States Study in Illuminated Conditions

357

In the second pretreatment procedure, we applied one sun illumination accompanied by

358

positive voltage of 1.66 V vs. RHE to electrodes before scanning starts. Figure 8 displays CV

359

curves of Pure TNA in second pretreatment route in comparison with first at the same scan

360

rate (0.1 V/s). It is obvious from Figure 8 that cathodic peak is present in both dark and under

361


Page 19 of 54

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illumination indicating deep traps are attributed to intrinsic surface states associated with the

362

morphology of the semiconductor.30 This fact that the peak intensity is lower under dark

363

compared to under illumination reveals that in pretreatment under dark, some traps are not

364

fully filled whith holes (i.e. partialy filled with electrons).Consequently, these partially filled

365

traps can not catch electron carriers and do not contribute in cathodic current density.

366

However, when TNA exposed to one sun illumination through pretreatment, the trapped

367

electrons migrate to conduction band and more trapping states are holes full filled. During

368

pretreatment under illumination, bubbles were observed at working and counter electrodes

369

indicating traps saturation was happened and transfer of holes from semiconductor/electrolyte

370

interface and electrons from external circuit to electrolyte. The maximum charge stored in the

371

deep traps for pure TNA electrode in illumination condition at scan rate of 0.1 V/s is equal to

372

ca. 29.75 mC/cm2 which has about 3 times increase relative to dark condition at the same

373

scan rate.

374

375 FIGURE 8. Cyclic voltammetry plot for pure TNA electrode with pretreatment under dark and illuminated

376

conditions for initial voltage of 1.66 V vs. RHE at scan rate of 0.1 V/s.

377



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Page 20 of 54

Similar consequences were achieved in all Pt/TNA electrodes with pretreatment in light. For

378

better comparison, Table 2 offers the accumulated charges and other important parameters in

379

Pt/TNAs under both pretreatment conditions.

380 381 382

3.2.4. Deep Trap States Study In order to investigate in greater sense the deep trap changes induced by Pt loading on TiO2

383

nanotubes the dependence of the current density, corresponding capacitance and associated

384

accumulated charges as a function of the scan rate were measured and calculated in pure

385

TNA and Pt/TNAs. As represented in Figure 9a , while increasing scan rate a slight shift in

386

peak position (Vpeak) and also a noticeable increase in peak current intensity happened, except

387

for the latest scan rate (i.e. 0.2 V/s) which its peak intensity are approximately equal to the

388

one earlier. Indeed, at scan rate of 0.2 V/s the sweep was too fast to give time to the trap

389

states to fill so, the traps capacitance declined. These results confirm that deep trap levels

390

have a dynamic nature in order that the demarcation level of energy (Ed) is not pinned.

391

Certainly, when trapping/detrapping scan rate increases the number of trapes which are not in

392

equilibrium with Ec enhances as well. Accordingly, Ed shifts up with growing scan rates. As

393

demonstrated by Bisquert and co-workers,40 if deep traps are originated from a

394

monoenergetic level only, the peak value of capacitance should be independent of scan rates.

395

In our work, the peak capacitance changes with variation of scan rate (Figure 9b) concluding

396

that the deep traps are originated from not only the monoenergetic level, but also the

397

exponential band tail. Our results about variation of peak current density with scan rate about

398

pure TNA are in agreement with recent report of Zhang et al47 which fabricated array of TiO2

399

nanotubes and examined it in Na2CO3 0.1 M at pH 10. Figure 9c displays the dependence of

400

cathodic peak intensities and accumulated charges to scan rates. Assuming the surface area

401

occupied by each nanotube to be 10-10 cm2 (ca. 10000 nm2 from image processing), thus the

402


Page 21 of 54

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surface density of nanotubes should be 1010 cm-2. Considering total accumulated charge in

403

deep traps to be ca. 29.75 mC/cm2 (at scan rate of 0.1 V/s) we can concluded an average

404

value of 29.75 × 10-10 mC or ca. 18.6× 106 electrons exist per single nanotube. The estimated

405

number of stored electrons in deep trap states of a nanotube in Zhang et al study was 5×

406

104.47 It shows that the geometrical properties of TNA in addition to type and pH of

407

electrolyte and the pretreatment conditions are seminal factors here. The associated current

408

densities for Pt/TNAs were given in Figure S2. For Pt1/TNA the value of electrons per

409

nanotube decrease to 13.8× 106 and for other Pt/TNA electrodes the value is lower. On the

410

other hand, the presence of Pt nanoparticles on the surface of nanotubes could reduce the

411

number of trapped electrons in deep trap states and assist the transfer of charges to

412

electrolyte.

413

414



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Page 22 of 54

415

416 FIGURE 9. a,b) Cyclic voltammetry and capacitance plot for pure TNA electrode, c) cathodic peak intensity

417

and accumulated charge in deep traps at various scan rates with pretreatment under illuminated conditions and

418

initial voltage of 1.66 V vs. RHE.

419

According to model developed by Bisquert and co-workers,31 variation of Cpeak and Vpeak

420

with scan rate allows us to distinguish the contribution of deep traps of the exponential DOS

421

and the monoenergetic level (Nt) in total chemical capacitance as follows:

422

exp(

!"

where ν( =

/$% &) = ν/ν(

)*

(1)

and τ0 is the trapping lifetime of free electrons at equilibrium.31 22 ACS Paragon Plus Environment

423 424

Page 23 of 54

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The peak capacitance intensity can also be expressed as a function of the relative scan rate,

425

i.e.

426

-

!"

.

=

!

0

/( + 2 3

(2)

0*

where e is the Neper number and NL0 is

427 428

( = exp(6(78( 79 )/$% &).

(3)

429

EF0 is the electron Fermi level at equilibrium. It should be noted that the first term on the right

430

side of eq.6 represents the capacitance from deep traps of the exponential DOS and is

431

associated to scan rate, but the second term is independent of scan rate showing the

432

capacitance of the monoenergetic level.31

433

According to the above description, first we plotted Vpeak as a function of scan rate (ν). Then,

434

we fitted our resulting points with logarithmic equation of the form V

!"

?

= =>( ) @

435

(Figure 10a). From fitting parameters a and b we could conclude a = ν( A( and = ν( .31

436

After finding ν( , we plotted Cpeak as a function of ν/ν( and fitted the subsequent dots with

437

.

0

/B + B 3 to extract B = NL0 and B = Nt (Figure 10b).31 Also,

438

we used NL from fitting of capacitance curve at scan rate of 0.01 V/s as represented in Figure

439

9b for pure TNA. The plots for all Pt/TNAs were shown in Figure S2. Table 3 has been

440

summarized the resulting parameters.

441

equation -

!"

=

!

0*



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

Page 24 of 54

442

443 FIGURE 10. a) Voltage of the deep trap capacitance peak versus scan rate. (b) Deep trap capacitance peak

444

versus normalized scan rate. Both plots were extracted from Figure 9b for pure TNA electrode with pretreatment

445

in illuminated condition for initial voltage of 1.66 V vs. RHE.

446 447 448 449 450 451 452


Page 25 of 54

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


Table 3. Obtained parameters from the fitting of the experimental data at various scan rates. α, NL and Q have

453

been found from fitting of capacitance at scan rate of 0.01 V/s.

454

Illuminated condition Pure TNA

Pt1/TNA

Pt4/TNA

α

0.196

0.495

0.225

NL×10-21 (cm-3)

1.17

24800

20.3

DE (V/s)

0.04424

0.702

0.7844

FE (s)

1.525

0.08112

0.09162

NL0 ×10-19

(cm-3)

1.545

1.085

35.07

Nt ×10-19

(cm-3)

3.077

6.454

0.9266

25.27

22.60

6.39

Q (mCcm-2)

455

Table 3 illustrates the density of trap states due to monoenergetic level (Nt) is significantly

456

lower than total trap states (NL) (i.e. 2, 4 and 6 orders of magnitude in pure TNA, Pt4/TNA

457

and Pt1/TNA, respectively). Moreover, Nt decreases when TNA has been decorated by small

458

amount of Pt nanoparticles. In contrary, Nt increases when large amount of Pt is decorated on

459

TNA surface. Considering PL as a result of radiative e-/h+ recombination, we suppose the

460

reduction of PL in Pt4/TNA relates to decline of monoenergetic deep trap states acting as

461

recombination centers and conversely the growing PL in Pt1/TNA is corresponded to

462

enhancement of Nt. In comparison to the pure anatase nanotubes, the trapping lifetime of free

463

electrons at equilibrium is also decreased by increasing Pt content.

464 465

3.2.5. Mechanism Let us now describe in more details the influence of the charge trap states on the current-

466

voltage and capacitance characteristics. First of all, illumination with energy more than

467

bandgap of semiconductor generates electron/hole pairs. Applying reverse bias coupled with

468

illumination, drives holes to the interface of n- type semiconductor/electrolyte and electrons

469



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

Page 26 of 54

to the back of electrode (external circuit). On the other hand, positive voltage causes valence

470

band and localized surface states inside the band gap fully filled with holes and in this way

471

suppress e-/h+ recombination. After 60 s, the lamp was turned off and sweeping potentials

472

from initial positive voltage toward negative values was begun. By reducing deterrent force,

473

the holes in traps start to recombine with electrons. Therefore, the subsequent current density

474

is almost zero at earlier stages of experiment (i.e. in positive voltages vs. Ag/AgCl reference).

475

Gradually, when the potential reaches the negative value (vs. Ag/AgCl reference) the density

476

of electrons in semiconductor/electrolyte interface increases and electrons start to fill trap

477

states. During charging, electrons collected in deep traps (closer to valence band) could not

478

be released. Indeed, the velocity of transfer of charges is slower than velocity of trapping. So,

479

the chemical capacitance associated with deep traps shows a peak. However electrons which

480

put in shallow traps can equilibrate with conduction band (i.e. they can detrap easily). The

481

density of trapping states varies exponentially from ECB with an exponential DOS tail. A

482

demarcation level which determines the boundary of deep and shallow traps is where the

483

response time for trapping and detrapping becomes equal. Deep traps could associate with an

484

exponential DOS tail or a monoenergetic surface state or both of them. The capacitance of

485

deep traps attributed to monoenergetic surface state is independent of trapping velocity,

486

however, the capacitance of deep traps associated with exponential DOS tail is exclusively

487

dependent of the scan rate. In reverse bias, detrapping kinetics is slow so that anodic peak

488

corresponding to capacitive peak is not appearing. With introducing minor Pt nanoparticles

489

on the surfaces of TiO2 nanotubes both the capacitance of monoenergetic deep surface state

490

and the density of total deep trap states are decreased while the capacitance of total density of

491

localized states is increased. Consequently, the density of shallow traps in the vicinity of

492

conduction band edge is effectively increased. Accordingly, electrons after charging deep

493

traps tend to transfer via new fast lane to electrolyte (Scheme 1).

494


Page 27 of 54

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Increment of Pt content (e.g similar content as Pt1/TNA) increases the capacity of

495

monoenergetic surface state but the density of total deep trap states are decreased compared

496

to the pure TNA.

497

498

499 SCHEME 1. a) Energy band diagram for Pt–TiO2 junction in equilibrium. Evac, EF, G and H represents vacuum

500

level, Fermi level, work function of Pt and electron affinity of TiO2 conduction band, respectively (all values in

501

eV), b) Proposed mechanism of electron transfer in both pure TNA and Pt/TNA electrodes in forward bias

502

direction.

503 504



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Page 28 of 54

We demonstrate the formation of highly ordered pure anatase nanotube arrays on the Ti

505 506 507

substrate with top- open and ultra-flat surface. Then decorated TNAs with Pt by a facile

508

photodeposition method. X-Ray photoelectron spectroscopy (XPS) confirmed the formation

509

of metallic Pt nanoparticles on TNA. The effect of Pt decoration on physical and

510

electrochemical properties of TNA is highlighted in the present study. Based on our results,

511

in Pt/TNAs the appropriate electrical connection between Pt nanoparticles and TNA induces

512

sufficiently shallow traps in the vicinity of conduction band edge which creates a fast lane for

513

electrons to transfer to electrolyte and decrease the density of deep trap levels and

514

corresponding stored charges. The density of deep and shallow trap levels can be tuned by the

515

content of Pt deposited on the TNA surface. Generally, in pure TNA the electron population

516

on deep trap states manifesting itself by a sharp cathodic peak when potential sweeping

517

toward negative values, while the electrons trapped in shallow traps are in equilibrium with

518

conduction band edge and are able to rapidly detrap and transfer to electrolyte. Moreover, the

519

accumulated charges are detrapped in reverse direction in order that the occupation

520

probability in both anodic and cathodic directions is equal in the presence of Pt nanoparticles.

521

The photoluminescence reduces with adequate amount of Pt due to the reduction of deep trap

522

states which act as e-/h+ recombination centers. However, high amount of Pt induces more

523

deep traps and consequently increases the PL intensity by growing e-/h+ radiative

524

recombination.

525

AUTHOR INFORMATION

526

Corresponding Author:

527

*Tel.: +98 2185692640, Email: [email protected]

528

Notes

529

The authors declare no competing financial interest.

530

Acknowledgment

531

4.

Conclusion


Page 29 of 54

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M. Zare acknowledges the support by the Institute for Research in Fundamental Sciences

532

(IPM).

533

Supporting Information Description

534

Experimental details of Pt decorated TNAs are collected in Table S1. Figure S1 shows XRD

535

patterns of as-anodized and annealed TNA layers. Figures S3 and S4 include cyclic

536

voltammetry plot for Pt/TNAs in addition to other relevant details.

537

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538

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FIGURE.1- FESEM images of a) pristine TNA layer, b and c) Pt1/TNA and Pt/4TNA layers. 570x254mm (96 x 96 DPI)

ACS Paragon Plus Environment


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FIGURE 2. a) XPS survey spectra and b) High resolution XPS spectra of Pt1/TNA. 108x146mm (96 x 96 DPI)


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FIGURE 2. a) XPS survey spectra and b) High resolution XPS spectra of Pt1/TNA. 130x151mm (96 x 96 DPI)



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FIGURE 3. a) UV–Vis DRS absorbance spectra of pristine TiO2 nanotube layer, Pt1/TNAs and Pt4/TNAs. b) Analysis of optical bandgap of pristine TiO2. 177x123mm (150 x 150 DPI)


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FIGURE 3. a) UV–Vis DRS absorbance spectra of pristine TiO2 nanotube layer, Pt1/TNAs and Pt4/TNAs. b) Analysis of optical bandgap of pristine TiO2. 177x123mm (150 x 150 DPI)



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FIGURE 4. Photoluminescent spectra of different TNA and Pt/TNA electrodes. The emission spectra were obtained with 3.94 eV excitation. Peaks at 630 and 945 nm are second and third modes of excitation, respectively. 177x123mm (150 x 150 DPI)


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FIGURE 5. Photocurrent density versus time for pure TNA and Pt/TNA electrodes at zero potential vs. Ag/AgCl reference. 177x123mm (150 x 150 DPI)



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FIGURE 6. a) Cyclic voltammetry plot with pretreatment in dark for pure TNA electrode and b) Total capacitance of traps and deep trap capacitance obtained from CV plot of pure TNA at scan rate of 0.1 V/s for initial voltage of 1.66 V vs. RHE 177x123mm (150 x 150 DPI)


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FIGURE 6. a) Cyclic voltammetry plot with pretreatment in dark for pure TNA electrode and b) Total capacitance of traps and deep trap capacitance obtained from CV plot of pure TNA at scan rate of 0.1 V/s for initial voltage of 1.66 V vs. RHE 177x135mm (150 x 150 DPI)



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FIGURE 7. a, b) Cyclic voltammetry and capacitance plots for pure TNA and Pt/TNA electrodes with pretreatment under dark conditions for initial voltage of 1.66 V vs. RHE at scan rate of 0.1 V/s. 177x138mm (150 x 150 DPI)


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FIGURE 7. a, b) Cyclic voltammetry and capacitance plots for pure TNA and Pt/TNA electrodes with pretreatment under dark conditions for initial voltage of 1.66 V vs. RHE at scan rate of 0.1 V/s. 177x138mm (150 x 150 DPI)



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FIGURE 8. Cyclic voltammetry plot for pure TNA electrode with pretreatment under dark and illuminated conditions for initial voltage of 1.66 V vs. RHE at scan rate of 0.1 V/s. 177x138mm (150 x 150 DPI)


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FIGURE 9. a,b) Cyclic voltammetry and capacitance plot for pure TNA electrode, c) cathodic peak intensity and accumulated charge in deep traps at various scan rates with pretreatment under illuminated conditions and initial voltage of 1.66 V vs. RHE. 177x138mm (150 x 150 DPI)



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FIGURE 10. a) Voltage of the deep trap capacitance peak versus scan rate. (b) Deep trap capacitance peak versus normalized scan rate. Both plots were extracted from Figure 9b for pure TNA electrode with pretreatment in illuminated condition for initial voltage of 1.66 V vs. RHE. 177x123mm (150 x 150 DPI)


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FIGURE 10. a) Voltage of the deep trap capacitance peak versus scan rate. (b) Deep trap capacitance peak versus normalized scan rate. Both plots were extracted from Figure 9b for pure TNA electrode with pretreatment in illuminated condition for initial voltage of 1.66 V vs. RHE. 177x123mm (150 x 150 DPI)



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SCHEME 1. a) Energy band diagram for Pt–TiO2 junction in equilibrium. (Evac, EF, ߶ and ߯ represents vacuum level, Fermi level, work function of Pt and electron affinity of TiO2 conduction band, respectively (all values in eV), b) Proposed mechanism of electron transfer in both pure TNA and Pt/TNA electrodes in forward bias direction. 171x150mm (300 x 300 DPI)


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SCHEME 1. a) Energy band diagram for Pt–TiO2 junction in equilibrium. (Evac, EF, ߶ and ߯ represents vacuum level, Fermi level, work function of Pt and electron affinity of TiO2 conduction band, respectively (all values in eV), b) Proposed mechanism of electron transfer in both pure TNA and Pt/TNA electrodes in forward bias direction. 410x313mm (96 x 96 DPI)



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Table of Contents Image 133x102mm (96 x 96 DPI)


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