Atomic Layer Deposition-Confined Nonstoichiometric TiO2

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Letter 2

Atomic Layer Deposition-Confined Nonstoichiometric TiO Nanocrystals with Tunneling Effects for Solar Driven Hydrogen Evolution Peng Zhang, Takashi Tachikawa, Mamoru Fujitsuka, and Tetsuro Majima J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00227 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016

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The Journal of Physical Chemistry Letters

Atomic Layer Deposition-Confined Nonstoichiometric TiO2 Nanocrystals with Tunneling Effects for Solar Driven Hydrogen Evolution Peng Zhang,† Takashi Tachikawa,*, ‡,§ Mamoru Fujitsuka,† and Tetsuro Majima*,†

† The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 81, Ibaraki, Osaka 567-0047, Japan. ‡ Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan. § PRESTO, Japan, and Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan. * E-mail address: [email protected] (Takashi Tachikawa), [email protected] (Tetsuro Majima)

ACS Paragon Plus Environment

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ABSTRACT: Ti3+ self-doped TiO2 nanocrystals (TNCs) confined with controllable atomic layer deposition (ALD) amorphous layers were developed to provide a novel model of metal-insulatorsemiconductor (MIS) photocatalysts for hydrogen generation in the UV to near-IR region. Photoexcitation of optimized MIS nanostructures consisting of a metal co-catalyst (Pt), electron tunneling layer (ALD TiO2), and photoactive nonstoichiometric core (Ti3+-doped TNC) exhibited efficient hydrogen generation (52 µmol h–1∙g–1), good reusability (16 h), and long-term stability (> 7 d). The charge transfer dynamics were examined using transient absorption spectroscopy to clarify the relationship between the photocatalytic activity and the tunneling effect. Our strategies highlight defect engineering in fabricating MIS photocatalysts with improved charge separation and tailored solar energy conversion properties.

Table of Contents Graphic and Synopsis

KEYWORDS: Atomic layer deposition, Nonstoichimetric TiO2, Tunneling effect, Hydrogen evolution, Charge transfer dynamics.


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With the increasing global demand for energy solutions that alleviate the environmental issues associated with fossil fuels, efficient utilization of solar energy has attracted considerable attention over the past few decades. For instance, the production of hydrogen gas as a potential candidate in practical solar energy conversion has been studied extensively since 1972. 1 With promising physicochemical abilities, anatase TiO2 is one of the most preferable photocatalysts for hydrogen generation.2-4 At the same time, numerous studies have attempted to engineer the bandgap of TiO 2 in a way that achieves a visible-light response, which would greatly expand the application potential of this class of materials. So far, a variety of modifications have extended the optical response of TiO2 from the UV region to the edges of the visible region, through the use of nonmetals (N, S, C, F), metals (Au, Ag, Fe), semiconductors (CdS, CdSe), crystal defects (Ti 3+, oxygen vacancies (VO)), and dyes (Ru complex).5, 6 However, most of these are restricted by decreased thermal/crystal stability, increased charge recombination centers after extrinsic incorporation, and complicated synthetic procedures.7 Dopant-free engineering of self-doped TiO2 has also attracted a great deal of attention with regards to energy and environmental concerns. For instance, VO-Ti3+ states can be formed by visible-light excitation,8 electrochemical doping,9, 10 chemical reduction,11-13 or low-temperature vacuum treatment,14,

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and exhibit visible-light

photocatalytic activity with a color change from white to blue or black-grey. Despite the interest of nonstoichiometric TiO2-x materials, the stability of these materials is still under debate. 16 Thus, there is room for improvement in terms of efficiency and stability if the non-stoichiometric surface is completely protected.17 Atomic layer deposition (ALD) is gaining an increased attention in energy and environmental fields as a promising method of surface modification and protection. 18, 19 Several nanostructures fabricated using ALD have been developed for solar water splitting, where facile charge separation


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is achieved by stagger band alignment, 20,

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i.e., effective heterojunctions composed of a

semiconductor and an ultrathin insulating layer.22, 23 It is a critical characteristic of metal-insulatorsemiconductor (MIS) that the electromotive efficiency and reactive stability are enhanced by the facilitated electron tunneling and corroding protection offered by the ultrathin insulator. Apart from native oxides such as SiO2 and Al2O3, TiO2 is an alternative candidate as an ultrathin insulator for efficient solar water splitting.24 However, the chemical and structural characteristics of MIS structures with TiO2 are currently undefined. Along with the consideration of favorable deposition of the insulator onto the bulk materials, applying ALD techniques to nanocrystal-based MIS systems remains a challenge. In this letter, we report the synthesis of Ti3+ self-doped TiO2 nanocrystals (TNCs) under vacuum treatment, which are simultaneously coated with amorphous TiO 2 layers by ALD. Under visiblelight excitation, core-shell architectures modified with Pt to form model MIS systems showed efficient and stable hydrogen generation, which are attributed to interfacial electron tunneling and surface protection. The dependence of the charge transfer dynamics on layer thickness was examined in order to prove a correlation with the photocatalytic activity. The proposed MIS model with improved visible-near infrared (NIR) response and long-term reusability will be important in the design and application of novel photocatalysts for solar driven environmental remediation and solar energy fields. As shown in Figure 1a, ALD was performed in a custom-built ALD chamber equipped with ports for the TiO2 precursor tetrakis(dimethylamido)titanium (TDMAT) and H2O under vacuum. The TNC powder was uniformly dispersed in the homemade sandwich holder. Ultrathin amorphous TiO2 layers were subsequently coated on TNCs using an improved exposure mode for particle coating under a controlled nitrogen flow. Ti3+ self-doped TNCs with different layer


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thicknesses were prepared via a self-limiting reaction at 160 °C. The obtained samples are labeled as AC-X, where X represents the number of ALD cycles. The crystal structures were first analyzed by transmission electron microscopy (TEM). As shown in Figure 1b, the as-prepared AC-200 maintains the square plate shape of the parent TNC, ~30 nm in size and ~10 nm in thickness after the ALD treatment (Figure S1). High-resolution TEM (HRTEM) images display a well-defined structure that is indicative of a {001} crystal facet with characteristic atomic planes of anatase (200) and (020) at an angle of 90° (Figure 1c). The corresponding fast Fourier transformation (FFT) pattern recorded from the crystal further confirms this single crystalline structure in the direction along the [001] zone axis (Figure 1d). The two parallel lines in Figure 1c indicate an ultrathin amorphous layer around ~1.5 nm in thickness. As seen in Figure 1e, the enlarged edge surface (the white circle in panel c) shows a disordered amorphous structure, which is closed to the anatase {001} facet with 0.24 nm space lattice. The results of other AC-X samples also exhibit good crystallinity in the core and an amorphous shell on the surface (Figure S2). The mean layer thickness on TNCs follows a linear correlation with the number of ALD cycles as shown in Figure 2c (left axis), indicating uniform and continuous growth.


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a

b

c

d

[001]

e disorder

50 nm

2 nm

0.24 nm

Figure 1. (a) Preparation procedures of Ti3+ self-doped TiO2 nanocrystals (TNC) by the exposure mode of ALD with a special holder. (b) Low magnification TEM image of AC-200. (c) HRTEM image of the marked particle in the panel b. The parallel lines indicate the amorphous region. (d) Corresponding FFT pattern of the particle shown in (c). (e) Enlarged image of the region enclosed by a circle in (c).

Powder X-ray diffraction (XRD) was performed in order to identify the crystal structures of the TNCs coated with amorphous layers (Figure S3). It was found that all diffraction peaks with similar line widths were readily indexed to the tetragonal phase of anatase TiO 2 without any impurities present. The (101) and (004) peaks on AC-300 broaden slightly due to the thicker amorphous TiO2 on the surfaces.25, 26 Raman spectroscopy was employed to obtain additional structural information (Figure 2a). After ALD treatment in right panel of Figure 2a, the Eg peak


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shifts to a higher frequency (from 144 cm–1 to 150 cm–1) and broadens (from 11.6 cm–1 to 12.6 cm– 1

) when compared with those of untreated TNC. These results are attributed to the presence of V O

and lattice disorder in non-stoichiometric TiO2-x.27, 28 The decreased intensity is likely due to the poor crystallinity of the shells. The optical absorption ability was investigated using steady-state diffuse reflectance spectroscopy. Compared with the intrinsic absorption of TNC, Ti 3+ self-doping during the ALD process under vacuum has a significant effect on the optical characteristics of the TNCs. As shown in Figure 2b, the observed absorption tail from visible to NIR was enhanced as the number of ALD cycles is increased, which can be visually observed as a color change from white to deep blue (inset). Notably, onsets of the absorption edge were red shifted from 3.12 eV to 3.02 eV (Figures 3a and b), which were estimated from the Tauc plots.27, 29 This can potentially be attributed to the existence of VO; a higher concentration of VO on the ALD-treated samples was also verified from the pronounced green emissions in the photoluminescence spectrum (Figure S4).30 Furthermore, X-ray photoelectron spectroscopy (XPS) analyses (Figure S5) reveals that the Ti3+/Ti4+ ratios increased from TNC (5.8%) to AC-300 (10%) (right axis in Figure 2c).31 These results support Ti3+ self-doping with VO. To characterize the band structure of these systems, valance band XPS (VB XPS) was carried out (Figure 3c). The pristine TNC shows a VB maximum (VBM) energy of 2.62 eV below the Fermi level, as determined by linear extrapolation of the leading edges of the VB spectra to the base line. The VBM energies (2.50–2.47 eV) of TNCs after ALD treatment (AC-100–300) are shifted to the Fermi level, accompanied by slight VB tailing at 1.91–1.83 eV. Combined with the optical band gap (Figure 3b), the conduction band minima (CBM) of TNCs and AC-100–300 should be located at approximately −0.50 eV and −0.56 (0.01) eV, respectively. The formation of


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VO-Ti3+ color centers and the accompanying structural disorders result in a narrowing of the band gap, leading to a visible-light response of the ALD-treated samples.11, 27, 32

a

Eg

Intensity (a.u.)

144 cm

B1g

A1g

150 cm

-1

-1

Eg

12.6 cm

AC-300

12.4 cm

AC-250

12.1 cm

AC-150

11.8 cm -1 11.7 cm -1 11.6 cm

-1

TNC

120

c

2.0 0.4

1.5 0.2

1.0 0.0 400

0.5 0.0 300

600

AC-300 AC-250 AC-200 AC-150 AC-100 TNC

800

160

180 AC-300

500

600

700

Wavelength (nm)

800

10

AC-250

2.5

9 2.0 AC-200

8

1.5 AC-100 AC-150

7

1.0

Wavelength (nm)

400

140

Raman shift (cm-1)

3.0

0.6

Thickness (nm)

Kubelka-Munk function

b

-1

AC-200 AC-100

100 200 300 400 500 600 700 800 Raman shift (cm-1) 2.5

-1

-1

0.5 100

Ti3+/Ti4+ (%)

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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150

200

250

300

6

ALD (cycles)

Figure 2. (a) Raman spectra of pristine and ALD-treated TNCs. The enlarged spectra are shown with the full width at half maximum (FWHM) values in the right panel. (b) Steady-state diffuse reflectance spectra of pristine and ALD-treated TNCs. Insets show the enlarged spectra and photographs of the samples. (c) The dependence of layer thickness (left axis) and Ti 3+/Ti4+ ratio (right axis) on the ALD cycles.

It is of significant importance to mention that the band gap energies of amorphous TiO 2 layers with different thicknesses are expected to be subject to quantum confinement effects. As the number of ALD cycles decreases from 300 to 100 cycles (corresponding to 2.7 to 1.0 nm thickness), the band gap energy of the TiO2 layers is expected to increase up to 4 eV, which is approximately 1 eV greater than that of pristine TNCs.33, 34 This suggests that an ultrathin TiO2 layer can act as


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an insulator between TNCs and subsequently modified Pt nanoparticles, as a co-catalyst for hydrogen evolution.

a 3.0

c

(F(R) h)1/2

TNC

AC-300 AC-250 AC-200 AC-150 AC-100 TNC

2.5 2.0 1.5

2.62 eV

AC-100

1.0

10

0.0

b 1.0

2.5

3.0

3.5

Photon energy (eV)

4.0

AC-300 AC-250 AC-200 AC-150 AC-100 TNC

Intensity (a.u.)

0.5

(F(R) h)1/2

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


8

6

4

2

0

1.91 eV

2.50 eV

AC-200

10

8

6

4

2

0

1.90 eV

2.47 eV

AC-300

0.5 8 05 3. 6 48 01 3. 3.0

7 06

3.

10

3 06

8

6

4

3.

6 11 3.

0.0 3.00 3.05 3.10 3.15 3.20 3.25

Photon energy (eV)

2.47 eV 10

8

6

4

Binding Energy (eV)

2

0

1.83 eV 2

0

Figure 3. (a) Tauc plots for pristine and ALD-treated TNCs. (b) The enlarged spectra from (a). (c) XPS valence band spectra of pristine and ALD-treated TNCs.

The ALD-treated TNCs were surface-modified with Pt nanoparticles via thermal deposition to form MIS nanostructures (Figure S6). The photocatalytic hydrogen generation was then evaluated during visible-light irradiation of the aqueous suspensions of the powder samples with a sacrificial electron donor (ED; formic acid) (Figure 4a). It is noteworthy that the activities of the Pt-modified AC-100–300 are significantly higher than those of the Pt-modified TNCs and Pt free AC-100–300 (Figure S7). In particular, Pt-modified AC-200 shows the best performance of approximately 52


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µmol h–1∙g–1. Furthermore, the action spectrum of apparent quantum efficiency (AQE) obtained for Pt-modified AC-200 is well matched with its absorption spectrum, demonstrating that the Ti 3+ self-doped MIS structure responds in the UV to NIR region (Figure 4b). There is the heterogeneous electron-tunneling process happened in the MIS structure, indicating the diffused electrons in nonstoichiometric TiO2 were tunneled through the ultrathin amorphous layer to the noble metal for H2 generation (Figure 4c). These samples show good long-term stability (up to 16 h) without any considerable reduction in activity (Figure 4d). The active color centers are stable even after being exposed to air for one week, whereas the activity of the samples without ALD treatment (i.e., TNC annealed in vacuum) significantly decreased over this same time period, implying the importance of surface protection (Figure S8).

b Kubelka-Munk function

a AC-300 AC-250 AC-200 AC-150 AC-100 TNC

2.0

0.4

1.5

0.3

1.0

0.2

0.5

TNC

0.0 0

10

20

30

40

50

60

-1

500

-1

750

Rate of hydrogen generation (mol g h )

2H+ H2 Pt tunneling e– diffusion

Ti3+ self-doped TNC amorphous TiO2

0.0 1250

1st

2nd

3rd

4th

H2 evolution (mol g-1)

hv

1000

Wavelength (nm)

d

c

0.1

AC-200

AQE (%)

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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200

100

0 0

4

8 Time (h)

12

16

Figure 4. (a) Hydrogen evolution from the Pt-modified samples under visible light ( > 420 nm). (b) Diffuse reflectance spectra and action spectra of AQE. (c) The MIS structure and reaction processes. (d) Repeated runs of H2 evolution from the AC-200 sample under visible light


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irradiation ( > 420 nm).

The charge transfer kinetics of the MIS system were directly investigated using time-resolved diffuse reflectance spectroscopy. Here, we focus on the dynamics of the photogenerated electrons observed in the NIR region because the transient absorption of (trapped) holes in the visible region largely overlaps with the broad absorption band of the (trapped) electrons. 35 The TiO2 core particles are selectively and fully excited by 330 nm radiation, because the ALD TiO 2 layers are very thin and the light penetration depth is 100 nm (1/ε,36 ε = 105 cm−1 for anatase37). The electron transfer rate constant (ket) was evaluated using eq. 138 ket = 1/τPt − 1/τ0

(1)

where τPt and τ0 are the decay times with and without Pt, respectively. As shown in Figure 5a, the lifetimes of trapped electrons monitored at 900 nm shorten after Pt modification. The decay kinetics are multi-exponential and fitted by three exponentials to parameterize the distribution of lifetimes (Table S1). Using the shorter τ1 and τ2 values, which are dominant for electron transfer to Pt, we calculate the ket values as 5.4 × 109–1.4 × 1011 s–1 for TNC, 1.2 × 1010–2.0 × 1011 s–1 for AC-100, 1.1 × 1010–1.4 × 1011 s–1 for AC-200, and 2.3 × 1010–2.0 × 1011 s–1 for AC-300. Surprisingly, the obtained ket values for all samples are the same range of magnitude, indicating that the ultrathin TiO2 layer does not act as an electron blockade. There is a negligible potential barrier necessary for driving electron tunneling through the ultrathin insulator layer from the semiconductor to the noble metal. In other words, electron diffusion in the TiO2 core is the rate-determining step when the tunneling step in the ultrathin layer is very fast. Indeed, a simple calculation using the electron diffusion coefficient (10–6 m2∙s–1)39 reveals that the mean times required for electron diffusion from the center of TNC to the Pt


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nanoparticles on the basal and lateral surfaces are several to tens of picoseconds (4–38 ps).35 In the MIS system, the Pt nanoparticles are isolated from the anatase TNCs by amorphous TiO 2 layers, which have larger bandgap energies than TNCs (Figure 5b). On the basis of electron transfer theory,40, 41 the electron tunneling rate across the MIS interface (ketMIS) can be described using eq. 2 ketMIS = ketMS·e-βd

(2)

where ketMS is the interfacial electron transfer rate between Pt and TNC at a fixed distance, which is assumed to be 1013–1014 s–1,40 β is the decay constant for tunneling, and d is the thickness of the insulator. It was recently confirmed that the β correlates well with (ΔEIS)1/2 for several metal oxides,41 where ΔEIS is the barrier height (Figure 5b). As summarized in Table 1, ketMIS increases as d increases due to the reduced ΔEIS, which would partially explain the comparable ket values among the samples.

a Normalized %Abs at 900 nm

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b 1.0 0.8 0.6 0.4 0.2 without Pt 0.0 1.0 0.8 0.6 0.4 0.2 with Pt 0.0 0 10 20 30 40

TNC AC-100 AC-200 AC-300

CB Fermi level

TNC (30 nm)

VB 100

1000

DEIS

Pt

(3.5 nm)

e– electron tunneling

h+

ED

ALD TiO2 (13 nm)

Delay time (ps) Figure 5. (a) Effects of Pt modification on the normalized transient absorption traces at 900 nm. (b) Schematic illustration of energy diagram and electron transfer processes. e − is the electron, h+ is the hole, and ED is the electron donor.


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Table 1. Parameters Related to Electron Tunneling for ALD-treated TNCs . a Thickness of ALD TiO2 layers.

b

Samples

d (nm)a

EgI (eV)b

∆EIS (eV)c

β (nm–1) d

ketMIS/ketMS

AC-100

1.0

4.0

0.42

3.1

0.044

AC-200

1.4

3.6

0.18

2.1

0.051

AC-300 2.7 3.3 0.04 0.9 0.087 The bandgap energies of amorphous TiO2 with d. The data were adapted from ref 34 reporting

the TiO2 quantum dots. c

The ∆EIS values were calculated from (EgI − EgS)/2, where EgS is the bandgap energy of anatase

TiO2 (3.2 eV). d

(∆EIS)1/2/β = 6.5 meV. See ref 41 for details.

According to the literature,24, 42 a thin insulator (< 2 nm) functions as a tunnel barrier because the insulator lowers the barrier height. In other words, the Fermi-level depinning occurs after equilibration. However, as the insulating layer becomes thicker (> 2 nm), the tunnel effect is almost negligible. In the present MIS system, the electron transfer rates are not dependent on the thicknesses of the insulators in the range from 1.0 to 2.7 nm, but the hydrogen generation rates significantly decrease as the thickness increases over 1.5 nm (Figure 4a). One possible explanation for this inconsistency is that electron transfer from the ED to the holes in TNC is blocked by the thicker insulator (thickness > 2 nm). From these findings, it can be inferred that efficient electron tunneling from the excited TiO2 core to Pt through the amorphous TiO2 layer is attributed to a larger density of states in the Pt as compared to those available for the molecular redox species. 43 In conclusion, a novel ALD method was presented to prepare the Ti 3+ self-doped TNCs with an


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electron tunneling layer in one step. The nonstoichiometric core-shell MIS nanostructures with optical absorption extending into NIR region exhibited efficient photocatalytic activity and excellent durability. From the point of view of charge transfer dynamics, precise control of the thickness and electronic energy level of the insulator is crucial for optimizing the interfacial electron tunneling between the TiO2 core and modified Pt or molecular redox species in solution. Continued developments of these MIS photocatalysts will be essential in motivating interests in the field of solar energy conversion.

EXPERIMENTALAND METHODS Preparation of TiO2 Nanocrystals (TNC). The detailed synthetic procedure and grown mechanism of TiO2 nanocrystal have been described according to the literature. 44 Briefly, 0.16 g of TiF4 (Sigma-Aldrich) was dissolved in 40 mL of tert-butyl alcohol (Tokyo Chemical Industry Co., Ltd.) under continuous stirring at room temperature. Then the transparent solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and heated at 160 °C for 72 h. After the autoclave was cooled to room temperature, the precipitated powders were centrifuged, washed thoroughly with absolute ethanol, and dried at 100 °C. Finally, the as-synthesized powders were annealed at 500 °C in air for 30 min to remove surface residues. Atomic Layer Deposition (ALD). ALD was performed in a custom-built ALD reactor (Cambrige Nanotech,

Savannah

100)

equipped

with

the

ports

for

TiO 2

precursor

of

tetrakis(dimethylamido)titanium (TDMAT) and H2O. The nanocrystal powders were dispersed in the homemade sandwich holder. The ultrathin TiO2 layers were coated on nanocrystals using a deposition technique with an exposure mode under a controlled nitrogen flow (20 sccm). All deposited processes briefly consist of a 0.1 s pulse of TDMAT, a 20 s purge, a 0.015 s pulse of


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water, and a 20 s purge in one cycle. The desired thickness was obtained via controlling the number of the ALD cycles. During the treatment, the TDMAT source was held at 75 °C and introduced to the ALD chamber at 160 °C. Finally, as-prepared samples were collected at room temperature. As a control sample, the nanocrystals were treated with same procedures, but without introducing TDMAT, referred to as TNC-vacuum. Characterizations. The crystal structures of the samples were examined using X-ray diffraction (XRD; Rigaku, Smartlab; operated at 40 kV and 200 mA, Cu Kα source). The morphologies were investigated using field-emission scanning electron microscopy (FESEM) equipped with EDX analyzer (JEOL, JSM-6330FT) and transmission electron microscopy (TEM) equipped with EDX analyzer (JEOL, JEM 3000F operated at 300 kV or JEM-2100 operated at 200 kV). Scanning TEM (STEM) and energy dispersive spectroscopy (EDS) mapping were performed using a Cs-corrected JEM-ARM200F microscope operated at 200 kV. The steady-state UV-Vis absorption and diffuse reflectance spectra were measured by UV-Vis-NIR spectrophotometers (Shimadzu, UV-3100 or Jasco, V-570) at room temperature. The PL spectra were measured using a Nikon Ti-E inverted wide-field fluorescence microscope. The 405-nm CW laser (Coherent) was used to excite the samples in epi-illumination geometry. A suitable dichroic mirror (Di02-R405, Semrock) and a longpass filter (BLP01-458R, Semrock) were used to improve the signal-to-noise ratio. Only the emission that passed through a slit entered the imaging spectrograph (MS3504i, SOL instruments) equipped with a CCD camera (DU416A-LDC-DD, Andor). All experimental data were obtained at room temperature. Photocatalytic H2 Generation Test. Prior to the tests, Pt nanoparticles (1 wt%) were deposited on the TiO2 surface by evaporating water from the aqueous suspensions of TNCs and H2PtCl6 at 100 °C. The catalyst powder (2 mg) was suspended by shaking in 2 mL aqueous solution


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containing formic acid (50 vol%) and the 10 mL quartz cell was then sealed with a rubber septum and purged with Ar gas for 20 min before initiating the irradiation. The suspensions were irradiated with visible light (Asahi Spectra, HAL-C100;  420 nm, 200 mW cm−2), with constant magnetic stirring at room temperature. After the reaction, 0.1 mL of gas was collected from the headspace of the reactor and analyzed using a Shimadzu GC-8A gas chromatograph equipped with an MS5A column and a thermal conductivity detector. The apparent quantum efficiency (AQE) for hydrogen evolution at each centered wavelength of the monochromatic light with width of 5 nm (Asahi Spectra, HAL-320; 0.7 mW cm−2) was calculated via the following equation: AQE = (2  number of hydrogen molecules / number of incident photons)  100%. Time-Resolved Diffuse Reflectance Measurements. The femtosecond diffuse reflectance transient absorption spectra were measured by the pump and probe method using a regeneratively amplified titanium sapphire laser (Spectra-Physics, Spitfire Pro F, 1 kHz) pumped by a Nd:YLF laser (Spectra-Physics, Empower 15). The seed pulse was generated by a titanium sapphire laser (Spectra-Physics, Mai Tai VFSJW; FWHM 80 fs). The fourth harmonic generation (330 nm, 3 μJ pulse−1) of the optical parametric amplifier (Spectra-Physics, OPA-800CF-1) was used as the excitation pulse. A white light continuum pulse, which was generated by focusing the residual of the fundamental light on a sapphire crystal after the computer controlled optical delay, was divided into two parts and used as the probe and the reference lights, of which the latter was used to compensate the laser fluctuation. Both probe and reference lights were directed to the sample powder coated on the glass substrate, and the reflected lights were detected by a linear InGaAs array detector equipped with the polychromator (Solar, MS3504). The pump pulse was chopped by the mechanical chopper synchronized to one-half of the laser repetition rate, resulting in a pair of spectra with and without the pump, from which the absorption change (% Abs) induced by the


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pump pulse was estimated. All measurements were carried out at room temperature.

Conﬂict of Interest: The authors declare no competing ﬁnancial interest.

Supporting Information Available: Additional table, figures and data. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Authors *E-mail: [email protected]; [email protected]

Acknowledgment We are thankful for the help of the Comprehensive Analysis Center of SANKEN, Osaka University. This work has been partly supported by a Grant-in-Aid for Scientific Research (Project 25220806, 25288035, and 15H03771) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. This work was performed under the Cooperative Research Program of "Net-work Joint Research Center for Materials and Devices".

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Atomic Layer Deposition-Confined Nonstoichiometric TiO2

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