A Mixed Titanium Oxides Strategy for Enhanced Photocatalytic

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A Mixed Titanium Oxides Strategy for Enhanced Photocatalytic Hydrogen Evolution Jiayu Chu, Yanchun Sun, Xijiang Han, Bin Zhang, Yunchen Du, Bo Song, and Ping Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04787 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Applied Materials & Interfaces

A Mixed Titanium Oxides Strategy for Enhanced Photocatalytic Hydrogen Evolution Jiayu Chu,†,‖ Yanchun Sun,‡,‖Xijiang Han,*,† Bin Zhang,† Yunchen Du, †

†

Bo Song*,§ and Ping Xu*,†

MIIT Key Laboratory of Critical Materials Technology for New

Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China ‡

Heilongjiang

River

Fisheries

Research

Institute

of

Chinese

Academy of Fishery Sciences, Laboratory of Quality & Safety Risk Assessment for Aquatic Products (Harbin), Ministry of Agriculture, Harbin 150070, China. §Academy

of Fundamental and Interdisciplinary Sciences, Department

of Physics, Harbin Institute of Technology, Harbin 150001, China ‖

These authors contributed equally.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (P.X.); [email protected] (X.H.); [email protected] (B.S.)

ACS Paragon Plus Environment

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KEYWORDS: photocatalysis, water splitting, titanium oxide, mixed phase strategy, hydrogen evolution

ABSTRACT: Titanium dioxide is a promising photocatalyst material for water splitting, but is limited by its low utilization of solar energy and rapid recombination of electron-hole pairs. Herein, a mixed

titanium

oxides

strategy,

utilizing

TiO2/Ti2O3

heterostructures consisting of in situ grown TiO2 nanotubes with mixed

anatase

and

rutile

phases

on

bulk

Ti2O3

materials,

is

demonstrated for efficient and recyclable hydrogen evolution from photocatalytic water splitting. Taking advantange of the formed heterostructures

and

the

created

porous

structures,

the

photogenerated electrons from the conduction band of anatase TiO2 can be first delivered to rutile TiO2 and then transferred to Ti2O3. Meanwhile, the presence of Ti2O3 in TiO2/Ti2O3 heterostructures can substantially

promote

the

charge

mobility

and

suppress

the

recombination of photogenerated electron-hole pairs. Hence, with tuned

bandgap

structure

that

enables

rapid

electron-hole

separation, increased charge carrier density and enhanced light absorption, the TiO2/Ti2O3 heterostructures provide an enhanced photocatalytic hydrogen evolution rate as high as 1440 μmol g-1 h-1 under full-sunlight irradiation and without any other cocatalyst. This mixed titanium oxides strategy may open up new avenues for


2

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designing

and

constructing

highly

efficient

TiO2-based

photocatalytic materials for various applications.


3


Page 4 of 32

1. Introduction With increasing concerns regarding the present energy crisis and environmental pollution, photocatalytic hydrogen (H2) evolution has been the focus of copious research activities to substantially reduce

the

decades,

fossil

various

consumption.1-3

energy materials

(e.g.

During

sulfides,4-5

the

past

few

nitrides,6-7

and

oxides8-10) have been developed for photocatalytic H2 evolution.1113

Amongst

these

materials,

TiO2

is

recognized

as

an

ideal

photocatalyst due to its low cost, nontoxicity, high chemical stability and environmental benignity.14-18 However, due to the large bandgap and rapid recombination of photogenerated carriers, solar energy utilization on pure TiO2 is extremely low, leading to very low photocatalytic activity.

19-22

Hence, considerable efforts

have been devoted to improving the photocatalytic efficiency of TiO2, including dye sensitization,

23-26doping

with metal and non-

metal elements,27-30 and constructing heterojunctions with other semiconductor materials.31-34 Notably, Ti3+ self-doped TiO2, with reduced bandgap as compared to pure TiO2, has shown enhanced photocatalytic performance in the visible light region, which is usually obtained by generating oxygen

vacancies

borohydride,

metal

from

a

reducing

reduction reagents

process (Al

and

calcination under a reducing gas atmosphere.33,

35-39

using Zn,

sodium

etc.),

or

In this regard,


4

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a uniformly hydrogenated blue H-TiO2-x prepared by a simple lowtemperature solvothermal method (using Li-dissolved ethanediamine as

a

solvent)

exhibited

splitting.35

photocatalytic

water

nanoparticles

obtained

temperature

displayed

exceptional

by

much

H2

Black

hydrogen higher

generation

rate

from

hydrogenated

treatment

photocatalytic

under

TiO2 high

activity

in

generating H2 from water-methanol solution.1 However, it should be noted that although a certain concentration of doped Ti3+ could enhance

the

photocatalytic

activity,

the

issues

of

rapid

recombination of electron–hole pairs remain to be solved. In addition, the reduction methodology for obtaining Ti3+ species is relatively

complicated

because

of

the

requirements

of

high

temperature, high pressure and additional reducing agents. The mixed-phase (anatase and rutile) TiO2 has been widely applied in photocatalysis, and has a better photocatalytic performance than single-phase TiO2 due to the formed heterojunction that can promote the seperation of photogenerated carriers.40-41 So far, there have been many proposed synthetic methods for fabricating the mixed-phase TiO2 nanocrystals.

40, 42-43

In general, as a common

method, the mixed-phase TiO2 was prepared by treating the asprepared amorphous TiO2 at a high temperature atmosphere (>650℃) 43-44

However, repugnant agglomerations of TiO2 particles will be

formed under the high temperature, and this process will reduce


5


Page 6 of 32

the surface areas and decrease the photocatalytic activity. Hence, hydrothermal or solvothermal method are adopted to lower the reaction

temperature.41,

45

However,

the

hydrogen

production

efficiency is still a bit low. Moreover, combining TiO2 with other titanium oxides (TixOy) with proper band structure alignments may be a promising strategy for boosting the photocatalytic activity, but has been rarely reported. With this regard, it was reported that through a photo-assisted sol-gel method, the as-prepared binary TiO2-Ti2O3 photocatalyst displayed remarkable photocatalytic activity in the decomposition of organic dye pollutants.46 Herein, we report the first example of

efficient

photocatalytic

hydrogen

evolution

on

TiO2/Ti2O3

heterostructures consisting of in situ grown TiO2 nanotubes with mixed anatase and rutile phases on bulk Ti2O3 materials. With tuned bandgap structure and higher carrier density, the as-prepared TiO2/Ti2O3 heterostructures can provide a photocatalytic hydrogen evolution rate as high as 1440 μmol g-1 h-1 without any other cocatalyst, which is the best amongst the reported pure titanium oxide materials under full-sunlight irradiation and even superior to

some

TiO2-based

composite

materials.

Such

an

enhanced

photocatalytic activity based only on oxide materials strongly implies that this mixed titanium oxides strategy is promising for constructing highly efficient photocatalysts.


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2. Experimental Section Preparation of TiO2/Ti2O3 heterostructures. In a typical synthesis, 0.15 g of commercial Ti2O3 bulk powders was dispersed in 20 mL of 10 M NaOH solution to get the uniform suspension by the method of ultrasonication for 30 min. Then, the suspension was transferred into a Teflon-lined autoclave and kept in an oven at 140 °C for 48 h. Next, the synthesized wet powder was re-dispersed in a 0.05 M HCl aqueous solution and stirred for 2 h and then washed thoroughly with deionized water/ethanol and dried in a vacuum drier at 60 °C for overnight. The powder was then sealed in a glass ampule under a vacuum of 5 × 10-4 Pa. After annealing at 400 °C for 2 h with a heating rate at 5 °C min-1, the grey-blue TiO2/Ti2O3 heterostructures were obtained. Characterization. Scanning electron microscopy (SEM) measurements were conducted on a HELIOS NanoLab 600i (FEI), and transmission electron microscopy (TEM) images were obtained on a TECNAI F20 (FEI

Instruments).

Powder

X-ray

diffraction

(XRD)

data

were

recorded using a Cu Kα radiation source at 45.0 kV and 50.0 mA on a

Rigaku

D/MAXRC

X-ray

diffractometer.

X-ray

photoelectron

spectroscopy (XPS) was performed using a PHI-5400 ESCA system with Al

Kα

radiation

as

the

source.

Ultraviolet

photoelectron

spectroscopy (UPS) was carried out with the He I line (hν=21.22 eV) as the excitation source, and the Fermi level position was


7


Page 8 of 32

calibrated based on the gold reference with a work function of 5.0 eV. UV-Vis diffuse reflectance spectra of the samples were recorded on

a

Hitachi

resonance

UH4150

(EPR)

spectrometer.

spectra

were

The

electron

characterized

on

paramagnetic a

JEOL-FA200

spectrometer. Raman spectra were collected using a 532 nm laser on a micro-Raman spectroscopy system (Renishaw in Via). Nitrogen adsorption/desorption isotherms were collected after heating the materials under vacuum for 2 h at 120 °C on a QUADRASORB SI-KR/MP (Quantachrome, USA). The temperature distribution of the reaction system was recorded by a Fluke TiS65 IR camera. An SDTQ600 TGA (TA Instruments) with the temperature range of room temperature to 1000 °C at a heating rate of 10 °C min−1 was employed to conduct the thermogravimetric (TG) analysis. Photocatalytic Hydrogen Production Test. A vacuum-closed gascirculation system, whose top window was sealed by the silicone rubber septum, was employed to test the amount of photocatalytic hydrogen evolution. A 300 W Xe lamp (PLS-SXE300/300UV) with fullsunlight

filter

intensity

was

was

~250

fixed mW

10

cm-2).

cm

away

For

the

from

the

reactor

photocatalytic

(the

hydrogen

production test, 50 mg of catalyst were dispersed into 100 mL aqueous solution with 20 mL methanol, where no noble metal cocatalyst (H2PtCl6) was added. The reaction system was pumped to vacuum for 45 min before irradiation. A gas chromatograph (Techcomp


8

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GC7900) with a thermal conductivity detector (TCD) and N2 as the carrier gas was employed to measure the amount of H2. Apparent

quantum

yield

(AQY)

for

photocatalytic

hydrogen

production is calculated by equation (1): 𝑁𝑒

𝐴𝑄𝑌 = 𝑁𝑝 × 100% = 2𝑀𝑁𝐴ℎ𝑐 𝑆𝑃𝑡𝜆

(1)

× 100%

where Ne is the amount of reaction electrons, Np is the amount of incident photons, M is the amount of H2 molecules, NA is Avogadro’s constant, h is the Planck constant, c is the speed of light, S is the irradiation area, P is the intensity of the irradiation light, t is the reaction time, and λ is the wavelength of the incident light (350 nm). Mott-Schottky measurement. Mott-Schottky analysis was applied to calculate and compare the density of charge carriers (Nd) of the photocatalysts, providing further insight into the process of photocatalytic hydrogen evolution. The carrier density can be calculated from the following equation (2): 2

𝑁d＝d(1

47

𝑒0𝜀𝜀0 𝐶2) d𝑉

(2) where e0 is the electron charge, ε is the dielectric constant of the semiconductor, ε0 is the permittivity of vacuum, C is the


9


Page 10 of 32

capacitance, and V is the applied bias at the electrode. Here, we take ε=55 for TiO2.48 Photoelectrochemical test. The photoelectrochemical measurements were carried out using a standard three-electrode cell on a CHI 660D (CH Instruments, Inc.) electrochemical workstation, with a Ag/AgCl reference electrode, a platinum foil as a counter electrode and the as-prepared photocatalyst film electrodes on FTO glass as the working electrode. A spray coating method was adopted to prepare the photoanodes. Therein, a paste containing 30 mg of catalyst and 2 mL of EtOH were rolled by a glass-rod on an FTO substrate, which was calcined for 120 min at 400 °C under a N2 atmosphere with a heating rate of 5 °C min-1. KOH (0.1 M) was purged with N2 and used as the electrolyte. Electrochemical impedance spectroscopy was measured at the potential of 0.6 V vs Ag/AgCl. 3. Results and Discussion

Scheme 1. Schematic illustration of the in situ fabrication of TiO2/Ti2O3 heterostructures for photocatalytic hydrogen evolution through hydrothermal treatment, acid washing and calcination processes from Ti2O3 bulk powders.


10

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TiO2/Ti2O3 heterostructures have been fabricated through an in situ conversion of Ti2O3 surface into TiO2 nanotubes, including hydrothermal treatment in alkaline, acid washing and calcination processes

(Scheme

Information).

1

The

and

Experimental

morphology

Section

evolution

of

in

Supporting

the

TiO2/Ti2O3

heterostructures has been studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Commercial Ti2O3 is typical bulk powders with smooth surface (Figure 1a), and the surface

is

greatly

roughened

after

being

transformed

into

TiO2/Ti2O3 heterostructures (Figure 1b), due to the reaction of bulk Ti2O3 with NaOH during the hydrothermal process at the surface, and then with HCl during the acid treatment and final calcination at 400

oC.49

TiO2/Ti2O3

A close observation reveals that the surface of the

heterostructures

is

actually

composed

of

uniform

nanotube-like structures (Figure 1c). TEM images confirm that the surface of bulk Ti2O3 has been transformed into TiO2 nanotubes that are about 10-20 nm in diameter, and from the interface between the TiO2 and Ti2O3, TiO2 layer is about 100-200 nm in thickness (Figure 1d and 1e). Moreover, the lattice fringes of 0.352 and 0.324 nm in the high resolution TEM (HRTEM) image of the surface nanotubes can be assigned to the (101) plane of the anatase TiO2 phase and the (110) plane of the rutile TiO2 phase, respectively (Figure 1f). The TiO2 nanotubes was further analyzed by high-angle annular dark


11


Page 12 of 32

field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive spectroscopy (EDS) mapping shows that Ti and O elements

are

uniformly

distributed

(Figure

S1

in

Supporting

Information). Ti2O3 is too thick to see the clear lattice fringes, and it is also invisible in the HRTEM image of the interface in the

TiO2/Ti2O3

Information).

heterostructures This

indicates

(Figure that

the

S2

in

Supporting

prepared

TiO2/Ti2O3

heterostructures are composed of mixed anatase and rutile TiO2 nanotubes supported on the Ti2O3 particles. Notably, the nanotube structures at the

surface

would

be readily formed after the

hydrothermal process, which could be stably maintained during the acid washing and subsequent calcination processes (Figure S3 in Supporting

Information).

N2

adsorption-desorption

isotherms

(Figure S4 in Supporting Information) clearly display that the TiO2/Ti2O3

heterostructures

have

a

distinctly

higher

specific

surface area (15.9 m2 g-1), in comparison to an almost negligible value for the bulk Ti2O3 powders. Moreover, pore size distribution analysis reveals an average pore size of about 50 nm in the TiO2/Ti2O3 heterostructures. This is resulting from the nanoscale surface modification by in situ generation of TiO2 nanotubes, and a higher specific surface area and created porous structure may provide more catalytically active sites and promote the charge carrier transport.


12

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Figure 1. SEM images of (a) the bulk Ti2O3 and (b, c) TiO2/Ti2O3 heterostructures, TEM (d, e) image of the as-prepared TiO2/Ti2O3 heterostructures, and (f) HRTEM image of the TiO2 nanotubes at the surface of the TiO2/Ti2O3 heterostructures.

In

order

to

better

understand

the

crystal

structure

and

composition of the TiO2/Ti2O3 heterostructures, X-ray diffraction (XRD), electron paramagnetic resonance (EPR) spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were applied. The XRD pattern of Ti2O3 in Figure 2a reveals that bulk Ti2O3 powders are predominantly in the rhombohedral phase (JCPDS No. 43-1033). As for the TiO2/Ti2O3 heterostructures, four wellresolved diffraction peaks at 2θ= 23.8, 33.0, 34.7 and 53.7° can be assigned to the (012), (104), (110), and (116) planes of Ti2O3, respectively. Additionally, the diffraction peaks at 2θ= 27.4, 36.0 and 54.3° belong to the (110), (101) and (211) planes of rutile TiO2 phase (JCPDS No. 21-1276), while the peaks at 2θ= 25.2 and 48.0° strongly indicate the co-existence of anatase TiO2 phase


13


Page 14 of 32

(JCPDS No. 21-1272), which agrees well with the HRTEM results. Another

strong

evidence

for

the

formation

of

TiO2/Ti2O3

heterostructures lies in the low-temperature EPR spectrum (Figure 2b), where the signal at g = 1.97 can be attributed to the paramagnetic Ti3+ centers.50-52

However, Raman and XPS studies can

only provide the information of TiO2 at the surface of the TiO2/Ti2O3 heterostructures. Raman bands at

146.2, 196.3, 391.9, 509.3 and

632.4 cm-1 can be only ascribed to TiO2, and the Raman modes of the bulk Ti2O3 cannot be detected in the TiO2/Ti2O3 heterostructures (Figure

2c).53-55

Also,

Ti

2p

XPS

spectrum

of

the

TiO2/Ti2O3

heterostructures only shows the binding energies at 458.7 and 464.3 eV, corresponding to Ti4+ 2p3/2 and Ti4+ 2p1/2, respectively (Figure 2d),51,

56-57

but no characteristic Ti3+ peak at 456.6 eV.50,

58

Besides,

O 1s XPS spectrum also indicates the presence of only TiO2 at the surface of TiO2/Ti2O3 heterostructures (Figure S5 in Supporting Information).1,

36

We believe the failure to monitor Ti3+ species by

Raman and XPS is due to limited detection depth of these two techniques, and actually this phenomenon had been witnessed in previous works.50 Taking all these together, it is safe to say that TiO2/Ti2O3 heterostructures consisting of TiO2 nanotubes with a mixed anatase and rutile phases supported on Ti2O3 particles have been successfully fabricated by our method. Thermogravimetric (TG) analysis was used to measure the relative amounts of TiO2 and Ti2O3 in the TiO2/Ti2O3 heterostructures, and the weight percentages of


14

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TiO2 and Ti2O3 were calculated to be 58.2% and 41.8%, respectively (Figure S6 in Supporting Information). Interestingly, by raising the hydrothermal temperature to 200 °C and subject to identical acid

washing

and

calcination

processes,

only

TiO2 nanobelts

composed of anatase and rutile phase can be obtained (Figure S7 and S8 in Supporting Information), withtout the presence of Ti2O3 component as evidencd by the EPR spectrum (Figure 2b).

Figure 2. (a) XRD (b) EPR spectra nanobelts, (c) heterostructures, heterostructures.

As

the

connected

patterns of the bulk Ti2O3 and TiO2/Ti2O3 heterostructures, of the bulk Ti2O3, TiO2/Ti2O3 heterostructures and TiO2 Raman spectra of the bulk Ti2O3 and TiO2/Ti2O3 and (d) high resolution Ti 2p XPS spectrum of the TiO2/Ti2O3

catalytic with

properties

their

optical

of

photocatalysts

absorption

are

features,

strongly

the

UV-vis


15


Page 16 of 32

diffuse reflectance spectra of Ti2O3, TiO2/Ti2O3 heterostructures and TiO2 nanobelts are compared in Figure 3. The composition variation can also be reflected by the color of the samples, which turned from black (Ti2O3), to grey (TiO2/Ti2O3 heterostructures), and

to

white

(TiO2 nanobelts).

heterostructures

show

stronger

As

predicted,

absorption

in

the the

TiO2/Ti2O3

ultraviolet

region than the bulk Ti2O3 and better visible light absorption as compared

to

the

TiO2

nanobelts,

which

may

enhance

the

photocatalytic activity. Accordingly, the bandgap of the bulk Ti2O3 is

calculated

to

be

about

1.30

eV

(Figure

S9

in

Supporting

Information), and the TiO2/Ti2O3 heterostructures is about 2.95 eV, narrower

than

substantially

that enhance

of

TiO2

the

nanobelts

photocatalytic

(3.15

eV),

activity

which due

to

may the

broadened light harvesting.

Figure 3. Ultraviolet-Visible absorption spectra of the bulk Ti2O3, TiO2/Ti2O3 heterostructures and TiO2 nanobelts. Insets show the optical images of the corresponding samples.


16

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With

three

typical

samples,

the

photoelectrochemical

performances have been compared. The separation of photogenerated charge carriers can be enhanced by constructing the TiO2/Ti2O3 heterostructures

(Figure

4a).

The

TiO2/Ti2O3

heterostructures

possess much higher photocurrent intensity than the TiO2 nanobelts and

bulk

Ti2O3,

mainly

because

of

the

formed

heterojunctions

between the Ti2O3 and TiO2 that enable more efficient charge carrier transport charge

and

lower

trapping

recombination.

and

Moreover,

separation

was

the

behavior

verified

by

of the

photoluminescence (PL) emission spectra (Figure S10 in Supporting Information). It is found that the PL peak intensity of TiO2/Ti2O3 heterostructures is lower than that of TiO2 nanobelts due to the precence of Ti2O3, thus suppressing the electron-hole recombination and

enhancing

the

photocatalytic

activity.

According

to

the

Mott−Schottky analysis (Figure 4b), the density of charge carrier (Nd) of the photocatalysts is calculated and the charge-transfer process can

be further

probed.

The carrier densities

of the

TiO2/Ti2O3 heterostructures, TiO2 nanobelts and the bulk Ti2O3 are 1.14×1018, 5.67×1017 and 6.14×1016 cm-3, respectively. The increase in the carrier density in the TiO2/Ti2O3 heterostructures may also play a vital role in improving the efficiency of hydrogen evolution from photocatalytic water splitting. Linear sweep voltammograms (LSV) of the photocatalysts were conducted in the potential range of 0.4 to 1.2 V (vs Ag/AgCl) at a scan rate of 10 mV s-1 (Figure


17


4c).

The

TiO2/Ti2O3

heterostructures

Page 18 of 32

possess

higher

current

densities as compared to that of TiO2 nanobelts. Similarly, the current density obtained under light conditions is significantly higher than that under dark conditions, further demonstrating the enhanced

separation

Electrochemical

of

impedance

photoinduced spectroscopy

charge (EIS)

was

carriers. used

to

investigate the charge transfer resistance of the samples.59-60 The Nyquist plots for TiO2/Ti2O3 heterostructures and TiO2 nanobelts were recorded at the potential of 0.6 V (vs Ag/AgCl) under dark and light illumination (Figure 4d), where the diameter of the semicircles of the TiO2/Ti2O3 heterostructures is smaller than that of

TiO2

nanobelts

under

either

light

or

dark

condition,

demonstrating that introduction of Ti2O3 can substantially promote the

charge

mobility

and

suppress

the

recombination

of

photogenerated electron-hole pairs.


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Figure 4. (a) Photocurrent responses of the bulk Ti2O3, TiO2 nanobelts and TiO2/Ti2O3 heterostructures, (b) Mott-Schottky plots of the bulk Ti2O3, TiO2 nanobelts and TiO2/Ti2O3 heterostructures, (c) Linear sweep voltammograms of TiO2 nanobelts and TiO2/Ti2O3 heterostructures under dark and light conditions, and (d) Nyquist plots of electrochemical impedance spectroscopy of TiO2 nanobelts and TiO2/Ti2O3 heterostructures under dark and light conditions.

Notably, the

Ti2O3 has no photocatalytic

activity

under

our

experimental conditions, possibly due to the small surface area and

rapid

electron-hole

recombination

caused

by

a

relatively

narrow bandgap.60-61 As shown in Figure 5a, the hydrogen evolution rate of the TiO2/Ti2O3 heterostructures can reach up to 1440 μmol g-1 h-1 under full-sunlight irradiation, which is higher than that of TiO2 nanobelts (508 μmol g-1 h-1) and most previously reported TiO2-based photocatalysts (Table S1 in Supporting Information).54, 62

For comparison, a control sample of physically mixed TiO2 (P25)

and Ti2O3 with same percentage as in the TiO2/Ti2O3 heterostructures was prepared, which provides much lower hydrogen evolution rate


19


Page 20 of 32

(620 μmol g-1 h-1) than the TiO2/Ti2O3 heterostructures, but slightly higher than the TiO2 nanobelts, and addition of Pt co-catalyst can further increase the hydrogen evolution rate of the TiO2/Ti2O3 heterostructures to 2060 μmol g-1 h-1 (Figure S11 in Supporting Information). This convinces the superiority of constructing the heterojunctions between TiO2 and Ti2O3, and also the indispensable role of Ti2O3. Moreover, the apparent quantum yield (AQY) under different light wavelengths were measured to further characterize the hydrogen evolution efficiency of photocatalysts (Figure S12 in Supporting Information). A high AQY value of 72.64% was obtained at 350 nm for TiO2/Ti2O3 heterostructures, which is higher than that of TiO2 nanobelts (10.65%), clealy illustrating that presence of Ti2O3 increases the photocatalytic hydrogen production rate of the

TiO2/Ti2O3

(TiO2/Ti2O3

heterostructures.

heterostructures)

and

Very

low

0.02%

AQY (TiO2

values

of

nanobelts)

0.1% are

obtained at 420 nm due to the poor absorption in visible light region. In addition, both photocatalysts have no photocatalytic activity at 500 nm. The TiO2/Ti2O3 heterostructures can also provide stable

photocatalytic

hydrogen

evolution,

with

no

obvious

degradation after five cycles (Figure 5b). Moreover, there are no obvious changes in the morphology as well as the composition and crystallinity of the TiO2/Ti2O3 heterostructures after five cycles (Figure S13-S15 in Supporting Information), revealing the high


20

Page 21 of 32 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


stability

of

the

heterostructures

during

the

photocatalytic

hydrogen evolution process.

Figure 5. (a) Photocatalytic hydrogen evolution activity and (b) recycled hydrogen evolution for 25 h of the bulk Ti2O3, TiO2 nanobelts and TiO2/Ti2O3 heterostructures.

Ti2O3, as a narrow-bandgap semiconductor,61 has been demonstrated nearly 100% internal solar–thermal conversion efficiency,63 and one may consider whether the photothermal effect also plays a role in the photocatalysis. Infrared imaging technology was employed to explore

the

temperature

distributions

of

the

photocatalytic

reaction systems (Figure 6a). It can be seen that the temperature of the reaction cell is ~13°C using TiO2/Ti2O3 heterostructures as the photocatalyst. While, the temperature can be as high as ~28°C with the presence of only bulk Ti2O3 (Figure S16 in Supporting Information). This indicates that the photothermal effect of Ti2O3 is not high enough to affect the photocatalytic hydrogen evolution process of TiO2/Ti2O3 heterostructures,64-65 and again interprets the


21


importance

of

the

formed

heterojunctions

Page 22 of 32

in

the

TiO2/Ti2O3

heterostructures in realizing the highly efficient photocatalytic water splitting. Based on above results, a tentative mechanism for the high hydrogen evolution activity of the TiO2/Ti2O3 heterostructures is proposed according to the bandgap structures (Figure 6b). The in situ growth of TiO2 nanotubes on the bulk Ti2O3 provides close contact between TiO2 and the bulk Ti2O3, wherein TiO2 is composed of mixed anatase and rutile phases, thus facilitating the electron transfer and suppressing the charge recombination.42,

66

Although

the TiO2 nanotubes layer is about 100-200 nm in thickness, the presence of the porous feature allows for efficient electron transfer as well as luquid diffusion. Taking advantange of the formed heterostructures and the created porous structures, the photogenerated electrons from the conduction band of anatase TiO2 can be first delivered to rutile TiO2 and then transferred to Ti2O3.67-69 This electron transfer pathway is also strongly supported by the measured work function of bulk Ti2O3 (5.28 eV) and TiO2 nanobelts (4.27 (Figure

S17

in

eV) by ultraviolet photoelectron spectroscopy Supporting

Information).

Hence,

this

highly

efficient photocatalytic hydrogen evolution activity for TiO2/Ti2O3 heterostructures can be mainly ascribed to the synergistic effect of the three components in the heterostructures.


22

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Figure 6. (a) IR photograph of the reaction system using TiO2/Ti2O3 heterostructures as the photocatalyst. (b) Schematic illustration of the possible photocatalytic charge transfer and hydrogen evolution processes in the TiO2/Ti2O3 heterostructures.

Conclusions In conclusion, we have successfully constructed the TiO2/Ti2O3 heterostructures

through

an

in

situ

surface

growth

of

TiO2

nanotubes with mixed anatase and rutile phases on bulk Ti2O3 for highly

efficient

hydrogen

evolution

from photocatalytic water

splitting. Although Ti2O3 has no photocatalytic activity itself, this

mixed

titanium

oxides

strategy

in

the

TiO2/Ti2O3

heterostructures enables a high photocatalytic hydrogen evolution activity of 1440 μmol g-1 h-1 without any conventional noble metal co-catalyst.

Such

an

improved

photocatalytic

activity

can

be

ascribed to the synergetic effect of the tuned bandgap alignments,


23


enahnced

light

absorption,

and

higher

charge

Page 24 of 32

carrier

density

generated by the heterojunctions between the Ti2O3 and TiO2. We believe this high photocatalytic hydrogen evolution activity based only on titanium oxides may open new avenues for the development of efficient photocatalytic water splitting systems.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental details and additional characterizations (PDF) AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank the financial support from the National Natural Science Foundation of China (21471039, 21571043, 21671047, 21871065), and Natural Science Foundation of Heilongjiang Province (B2015001). REFERENCES (1) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746-750. (2) Liu, L.; Chen, X. Titanium Dioxide Nanomaterials: SelfStructural Modifications. Chem. Rev. 2014, 114, 9890-9918. (3) Wang, M.; Wang, B.; Huang, F.; Lin, Z. Enabling PIEZOpotential in PIEZOelectric Semiconductors for Enhanced


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TOC GRAPHICS


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A Mixed Titanium Oxides Strategy for Enhanced Photocatalytic

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