Ionothermal synthesis of black Ti3+-doped single-crystal TiO2 as an active photocatalyst for pollutant degradation and H2 generation

Article abstract of DOI:10.1039/c4ta02873b

A black Ti³⁺-doped single-crystal TiO₂ (Ti³⁺/TiO₂) was one-pot synthesized by treating Ti metal in an ionic liquid containing LiAc and HAc under mild ionothermal conditions. The ionic liquid (1-methyl-imidazolium tetrafluoroborate) supplied an environment enriched with fluoride ions for dissolving titanium foil under ionothermal conditions, followed by reducing protons in acetic acid to form Ti³⁺ ions, leading to Ti³⁺-doped single-crystal TiO₂ as a black powder. EPR and XPS results indicated the high concentrations of both Ti³⁺-dopants and oxygen vacancies. The Ti³⁺ incorporated into the TiO₂ lattice could narrow the energy band gap of TiO₂via forming intermediate energy levels, leading to a visible photocatalyst. Meanwhile, the oxygen vacancies could inhibit the photoelectron-hole recombination. As expected, such a black Ti³⁺/TiO₂ exhibited high activity in the photocatalytic degradation of organic pollutants and water splitting for H₂ production under irradiation with visible light and/or simulated solar light.

Full text of DOI:10.1039/c4ta02873b

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Table of Contents Entry
3
+
Ionothermal synthesis of black Ti -doped single-crystal
TiO as an active photocatalyst for pollutant degradation
2
and H generation
2
1
*
1
1
1
1
2
3*
Guisheng Li , Zichao Lian , Xin Li , Wenchao Wang , Dieqing Zhang , Fenghui Tian , Hexing Li
1
. Chinese Education Ministry Key Laboratory of Resource Chemistry, Shanghai Normal
University, Shanghai 200234, China. Email: liguisheng@shnu.edu.cn. 2. Institute of
Computational Science and Engineering, Qingdao University, Qingdao, 266071, China. 3.
Shanghai University of Electric Power, Shanghai 200090, China. Email: Hexing-Li@shnu.edu.cn.
3
+
Black Ti /TiO single-crystals synthesized via an ionothermal process exhibited high
2
activity for photocatalytic degradation of pollutants and hydrogen evolution reaction.

Journal of Materials Chemistry A
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3
+
Ionothermal synthesis of black Ti ꢀdoped singleꢀcrystal TiO as an
2
active photocatalyst for pollutant degradation and H generation
2
1
*
1
1
1
1
1
2
Guisheng Li , Zichao Lian , Xin Li , Yuanyuan Xu , Wenchao Wang , Dieqing Zhang , Fenghui Tian ,
3*
Hexing Li
1
. Chinese Education Ministry Key Laboratory of Resource Chemistry, Shanghai Normal
University, Shanghai 200234, China. Email: liguisheng@shnu.edu.cn. 2. Institute of
Computational Science and Engineering, Qingdao University, Qingdao, 266071, China. 3.
Shanghai University of Electric Power, Shanghai 200090, China. Email: HexingꢀLi@shnu.edu.cn.
Abstract
3
+
3+
A black Ti ꢀdoped singleꢀcrystal TiO
an ionic liquid containing LiAc and HAc under mild ionothermal conditions. The ionic liquid
1ꢀmethylꢀimidazolium tetrafluoroborate) supplied an environment enriched with fluoride ions for
dissolving titanium foil at ionothermal condition, followed by reducing protons in acetic acid to
2 2
(Ti /TiO ) was oneꢀpot synthesized by treating metal Ti in
(
3
+
3+
form Ti ions, leading to Ti ꢀdoped singleꢀcrystal TiO in black powder. EPR and XPS results
2
3
+
3+
indicated the highꢀconcentrations of both Ti ꢀdopants and oxygen vacancies. The Ti
incorporated into TiO lattice could narrow the energy band gap of TiO via forming intermediate
energy levels, leading to the visible photocatalyst. Meanwhile, the oxygen vacancies could inhibit
2
2
3
+
the photoelectronꢀhole recombination. As expected, such a black Ti /TiO
2
exhibited high activity
production under
in photocatalytic degradation of organic pollutants and water splitting for H
2
irradiation with visible lights and/or simulated solar lights.
3
+
3+
Keywords: Black Ti ꢀdoped singleꢀcrystal TiO
2
(Ti /TiO
2
), ionothermal synthesis, visible
photocatalyst, pollutant degradation and H
2
generation
Introduction:
2
TiO photocatalyst has received more and more attentions in environmental cleaning and
green energy production owing to its high activity, nonꢀtoxicity, easy availability, and strong
1
2
stability for degradation of organic pollutants , H
2
production via water splitting and direct
3
4
photoelectric transformation
.
Since the report of Fujishima and Honda in 1972 , TiO
2
ꢀbased
1

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photocatalysts have been widely investigated in designing new photocatalysts, exploring
fundamental theory and putting into practical applications. However, the application of pure TiO
2
photocatalyst is still quite limited since it can be activated by only UV lights due to the big energy
band gap (anatase with a bandꢀgap of 3.2 eV and rutile with a bandꢀgap of 3.0 eV), taking into
account that the sunlight contain large proportion of visible lights and very little fraction of UV
5
, 6
7
8
lights. In the past two decades, cationꢀdoping , metal hybrid , semiconductorꢀdoping , and
5
, 6
nonmetalꢀdoping ect.
have been widely used to modify the electronic structure and
consequently the light absorption range of TiO for achieving visible photocatalysts in which there
2
9
, 10
were using many methods to synthesis visibleꢀlightꢀdrivenꢀmaterials , such as ionothermal
11
12, 13
method which could prepare TiO
2
and other materials
. In addition, Also changing the
1
4
15
16
morphology , preparing TiO ꢀbased or doped composite and enhancing (001) facet exposure
2
17ꢀ20
rate
of TiO
2
would be increasing the photocatalytic activity.
Recently, both the experimental results and theoretical prediction demonstrate that the
3
+
3+
21
Ti ꢀdoped TiO
2 2
with great number of oxygen vacancies (Ti /TiO ) usually displays black color
2
2
and strong spectral response in visible area. Up to now, a variety of methods have been
3
+
developed for synthesizing black Ti /TiO
2
, such as hydrogen reduction of TiO
high temperature induced molten aluminum reduction of
TiO etc. Besides the harsh conditions and tedious experimental steps, these reported black
2
at high
2
3,24
calcination temperature and pressure,
25
2
3
+
Ti /TiO
2
samples usually exhibited low photocatalytic activity under visible light irradiation due
3
+
3+
to the lowꢀcontent of Ti ꢀdopants and the poor stability of Ti ꢀdopants against oxidation. Herein,
3
+
a facile oneꢀstep approach was developed for synthesizing black Ti ꢀdoped anatase TiO
2
3
+
singleꢀcrystal (Ti /TiO
2
) by treating Ti foil in 1ꢀmethylꢀimidazolium tetrafluoroborate ionic liquid
containing acetic acid (HAc), NꢀN dimethylformamide (DMF), and lithium acetate (LiAc) under
o
3+
2
00 C ionothermal conditions. The asꢀformed black Ti /TiO
2
photocatalyst contained
highꢀcontents of Ti ꢀdopants and oxygen vacancies. The Ti ꢀdopants in the TiO lattice could
narrow energy band gap of TiO semiconductor via forming intermediate energy levels, leading to
3
+
3+
2
2
the visible photocatalyst. Meanwhile, the oxygen vacancies could diminish the photoelectronꢀhole
recombination and thus, could enhance the quantum efficiency of photocatalysis. As expected, this
3
+
black Ti /TiO
2
exhibited high activity in photocatalytic degradation of organic pollutants and
waterꢀsplitting for H
2
generation under irradiation with visible lights and/or simulated solar lights.
2

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Experiments
Catalyst preparation
All chemicals were analytical grade and used as received without any further purification. In a
.
typical run of synthesis, 12 mL DMF (as solvent), 0.6 g LiAc 2H O and 18 mL glacial HAc (as
2
buffer solution) were mixed in a 100ꢀmL pyrex beaker and magnetically stirred for 15 min to get a
clear solution. The mixture was then transferred to an autoclave (50 mL). Meanwhile, a piece of
Ti foil (99.5 %, 20 mm*33 mm*0.3 mm) was ultrasonically cleaned in a mixed solution of
deionized water, acetone, and ethanol with volume ratios of 1:1:1, followed by adding into the
autoclave together with 2 mL ionic liquid (1ꢀmethylꢀimidazolium tetrafluoroborate). The mixture
o
was kept at 200 C for 24 h and then, the black product was washed with water and ethanol for
o
three times, followed by drying at 80 C for 12 h.
Characterizations
Xꢀray diffraction (XRD) patterns were collected on a Rigacu Dmaxꢀ3C with Cu Kα radiation.
Selected area electron diffraction (SAED) images and transmission electronic microscopy (TEM)
morphologies were recorded on a JEMꢀ2010. Field emission scanning electron microscopy
(
FESEM) images were performed on a JEOL JSMꢀ6380LV. Raman spectra and UVꢀvis diffuse
reflectance spectra (UVꢀvis DRS) were conducted on Dilor Super LabRam II and MCꢀ2530
instruments, respectively. For expelling the local heating effect, same laser source (wavelength =
633 nm) was utilized in the Raman characterization for all samples. The photoluminescence
spectroscopy (PLS) was collected on Varian CaryꢀEclipse 500 excited with 280 nm lights. The
photocurrent responses in the light onꢀoff process were determined in a homemade three electrode
2 4
quartz cell containing 0.5 mol/L Na SO aqueous solution under visible lights (> 420 nm)
irradiation at an applied potential of 0.5 V vs. SCE with electrochemical workstation (CHI 660D,
Chen Hua Instrument Co., Ltd.). Xꢀray photoelectron spectroscopy (XPS) analysis was performed
on a PerkinꢀElmer PHI 5000C ESCA system. All binding energies were calibrated by using the
contaminant carbon (C1s =284.6 eV) as a reference. Thermogravimetric analysis (TG) were carried
out on a DTGꢀ60H thermogravimetric analyzer with a heating speed of 10 °C/min under air
atmosphere. The electron paramagnetic resonance (EPR) spectra were recorded at 100 K using a
3

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1
Bruker EMXꢀ8/2.7 EPR spectrometer. Solid state HNMR was recorded on a Bruker Avance400.
Photoelectrochemical measurements were carried out in a conventional threeꢀelectrode,
3
+
singleꢀcompartment quartz cell on an electrochemical station (CHI 660D). The Ti /TiO
2
2
electrodes with an active area of ca. 1 cm were served as working electrode. For the working
electrodes, 20 mg of the photocatalyst was ground with 0.01 g polyethylene glycol (PEG,
molecular weight 20000) and 0.5 mL ethanol to make a slurry. Afterward, the slurry was coated
onto a 3 cm*1 cm Fꢀdoped SnO
2
ꢀcoated glass (FTO glass) electrode by the doctor blade method,
2
o
with an active area of ca.1 cm . The electrodes were calcined at 200 C for 3 h in a ceramic plate
heater. All the electrodes used had a similar thickness (1.0ꢀ1.5mm). The counter electrode and the
reference electrode were platinum sheet and saturated calomel electrode (SCE), respectively. A
bias voltage of 0.5 V was utilized for driven the photoꢀgenerated electrons transfer from working
electrode to platinum electrode. A 300 W Xe lamp with an ultraviolet filter (λ > 420 nm) was used
as visible light source and positioned 10 cm away from the photoꢀelectrochemical cell. A 0.5
2 4
mol/L Na SO aqueous solution was used as the electrolyte. The transient photocurrent curves
were measured under an applied bias of 0.5 V.
Activity test
1
. Photocatalytic degradation. Briefly, 50 mg photocatalyst was dispersed in 50 mL Rhodamine B
RhB) aqueous solution (10 mg/L) and stirred in dark for 1 h for reaching the
adsorption/desorption equilibrium. During photocatalysis, 300 Xenon lamp
PLSꢀSXE300CUV, UV percent = 5.2 %) was used to simulate solar lights. For achieving visible
(
a
W
(
lights induced photocatalysis, all the lights with wavelength shorter than 420 nm were cutꢀoff by a
filter to get visibleꢀlights. After reaction for 4 h under 1000 r/min stirring, 2.0 mL suspension was
sampled into plastic tube and centrifuged to remove the photocatalyst particles. The concentration
of remained RhB was determined by UVꢀvisible spectrophotometer analysis at its characteristic
wavelength of 554 nm, from which the photocatalytic degradation yield was obtained by
comparing with the initial RhB concentration. For comparison, the photocatalytic degradation of
RhB was also conducted by using a 300 W Xenon lamp without removing UV lights with
2
wavelength shorter than 420 nm. Meanwhile, 8 LED lamps (light intensity = 3.5 mW/cm ,
UVECꢀ4 Ⅱ devices purchased from Shenzhen Lamplic Science Co., Ltd) with single wavelength
of 420 nm were used instead of a 300 W Xenon lamp for driving photocatalytic degradation of
4

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RhB. In addition, photocatalytic degradation of aniline in aqueous solution (10 mg/L) was also
carried out by using visible lights with wavelength longer than 420 nm from a 300 W Xenon lamp.
The concentration of remained aniline was determined by UVꢀvisible spectrophotometer analysis
at its characteristic wavelength of 230 nm, from which the photocatalytic degradation yield was
obtained by comparing with the initial aniline concentration.
2
2
. Photocatalytic H generation. Hydrogen production by photocatalytic waterꢀsplitting was
carried out at room temperature in a three flatꢀbottomed flask reaction cell. Typically, 50 mg
photocatalyst doped with 1 wt% Pt through in situ photoꢀreduction of H PtCl was dispersed in 80
2
6
mL aqueous solution containing 20 mL methanol as the sacrificial reagent under ultrasonication.
A 300 W Xenon lamp was used as simulated solar lights, positioned 1 cm away from the reactor.
The evolved H amount was determined using a gas chromatograph (Shanghai Ke Chuang,
2
GCꢀ9800, TCD, N
2
carrier and 5 A molecular sieve columns).
Results and discussion:
3
+
As shown in Fig. 1a, the asꢀreceived Ti /TiO sample was present in black powders. TGA
2
o
analysis (Fig. S1) displayed no significant weight loss until 450 C, indicating the absence of
organic species. The XRD pattern in Fig. 1b demonstrated the singleꢀcrystal anatase TiO
2
(JCDPS
21ꢀ1272) in about 20ꢀ30 µm regular particles (see the SEM image in Fig. 1c). HRTEM (Fig. 1d)
2
shows that the asꢀprepared TiO nanocrystals were covered with disordered layers. Both HRTEM
and the SAED images (Fig. 1d) further confirmed the singleꢀcrystal anatase phase. Based on the
SAED, it can be defined that the asꢀprepared samples had the exposed facets which were {001}
and {101} combined with Fig 1c.In addition, the SAED in Fig. 1(d) shows the (002) and (011)
atomic planes with a lattice spacing of 0.189 nm and 0.352 nm respectively. The XRD patterns
3
+
(
Fig. S2) further confirmed that the asꢀprepared black Ti /TiO
2
remained anatase phase until the
o
o
calcination temperature reaching 600 C and then totally transferred into rutile phase at 700 C,
showing excellent thermal stability against phase change.
5

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(
101)
(
b)
(
004)
(200)
(
105)
(
211) (204)
(
215)
(
103) (112)
(116) (220)
(
213)
20
30
40
50
60
70
80
2
Theta (Degree)
Figure 1. (a) Digital picture, (b) XRD pattern, (c) SEM image, and (d) HRTEM image of the
3
+
asꢀprepared black Ti /TiO
2
. The inset is the SAED image. Scale bar in (a): 10 mm.
1
As shown in Fig. 2, the H NMR spectra revealed that, besides a weaker signal at 4.7 ppm
26, 27
3+
indicative of the H at bridging sites observed in pure while TiO2
, the black Ti /TiO2
displayed three additional strong signals at ꢀ0.18 ppm, 0.97 ppm and 1.90 ppm, corresponding to
2
6
the terminal and internal hydroxyl groups of anatase TiO
2
. These groups were associated with
3
+
28
the H located in the disordered surface layer of the black Ti /TiO , which could be completely
2
o
removed after being calcined in air at 700 C.
(a)
(b)
-4
-2
0
2
4
6
8
10
-4
-2
0
2
4
6
-1
8
10
-1
Chemical shift(cm )
Chemical shift (cm )
1
3+
3+
Figure 2. H NMR spectra of black Ti /TiO
2
(a) and white TiO
2
obtained by calcining Ti /TiO
2
o
at 700 C (b).
3
+
Scheme 1 briefly illustrates the formation of black Ti /TiO
2
by the reaction between the Ti
6

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foil and the ionic liquids (ILs) under ionothermal conditions. During the reaction, the fluoride
enriched ionic liquid (IL) could dissolve Ti foil gradually. The SEM images and XRD patterns
(
Fig. S3) revealed that very little ionic liquid (0.5 mL) could dissolve only the outer surface of Ti
foil, leading to a thin anatase TiO film on the Ti foil. The Ti foil could be severely dissolved into
bumps in the presence of 1.0 mL IL, corresponding to the formation of black Ti /TiO
the solution or on the surface of Ti bumps. Further increase of IL to 2.0 mL resulted in complete
2
3
+
2
either in
3
+
2
dissolve of Ti foil, leading to the pure black Ti /TiO in regular particles with an average size
about 20~30 µm in Fig. 1c. During the dissolving process of Ti foil under ionothermal conditions,
+
H
2
gas could be produced via the reduction of H from acetic acid by metal Ti, which could be
confirmed by the following controlled experiment. After ionothermal treatment of Ti foil for 24 h,
the autoclave was opened in a selfꢀmade glove box (volume = 60 L) filled with high purity N 0.5
2
.
mL gas was sampled and analyzed by GC (GC9800, Shanghai Ke Chuang Chromatograph
Instruments Co. Ltd, China, TCD, with nitrogen as a carrier gas and 5 A molecular sieve columns),
which clearly displayed a typical H
2
peak as observed by introducing H
2
into autoclave before
ionothermal process (see Fig. S4), indicating the H
ionothermal conditions.
2
generation during Ti foil dissolving under
3
+
2
Scheme 1. The formation process of the black Ti /TiO under ionothermal conditions.
3
+
Based on the above results, it is reasonable that the black Ti /TiO formation process may
2
+
include the following 4 steps. 1) Ti foil reacted with the H ions resulted from ionization of HAc
4
+
3+
4+
to form Ti /Ti ions and hydrogen gas in which the concentration of Ti ions could be much
7

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3
+
4+
3+
higher than that of Ti ions. 2) Some of the formed Ti ions will be further reduced to Ti ions
29
4+
3+
by the metal Ti . 3) The Ti and Ti ions underwent ionothermal hydrolysis owing to the
presence of trace H O resulted from the dehydration of acetic acid. The LCꢀMS patterns (Fig. S5)
clearly displayed the presence of acetic anhydride in the ionic liquid obtained after ionothermal
2
4
+
treatment of Ti foil for 24 h. During ionothermal hydrolysis process, Ti transformed into anatase
3
+
TiO
2
and Ti ions may corporate into TiO
2 2
lattice. 4) The in-situ generated H could be strongly
adsorbed onto the TiO
2
surface and dissociated into H atom to form HꢀTiO2ꢀx disordered layer.
Some important reactions involved the whole process are listed as follows.
ꢀ
O
O
H+
1
H
3
C
OH
OH
H
3
C
O:
O
O
O
O
H O
2
2
H
3
C
HO
CH
3
H
3
C
O
CH
2
H+
Ti
Ti
Ti3+ Ti4+ H2
Ti3+
3
4
Ti4+
Ti4+
TiO2
H O
TiO2
5
6
2
diffusion
H2
HꢀTiO2
3
+
The EPR spectra (Fig. S6) of the asꢀprepared black Ti /TiO
2
displayed strong signals in the
g range from 1.99 to 1.94 which could be remained after being calcined at the temperature up to
o
3+
30
o
5
00 C, confirming the presence of Ti species. After being calcined at 700 C, nearly all the
3
+
Ti species were oxidized into pure TiO
2
. The XPS spectra in Ti 2p level (Fig. S7) also
3
+
4+
demonstrated the coꢀexistence Ti (457.5 eV 2P3/2 and 463.7 eV 2P ) and Ti (459.1 eV 2P3/2
1/2
3
1
3+
o
and 464.7 eV 2P1/2
transformed into pure TiO
molar ratio could be calculated based on the XPS spectra (Table S1). The fresh black Ti /TiO
)
in the black Ti /TiO
2
when it was calcined below 500 C and completely
o
3+
4+
2
after being calcined at 700 C. A semiꢀquantitative analysis of Ti /Ti
3
+
2
3
+
4+
displayed a Ti /Ti molar ratio of 0.162, which quickly decreased after calcinations at elevated
3
+
o
o
3+
4+
temperatures due to the oxidation of Ti . From 300 C to 500 C, the Ti /Ti molar ratio
o
3+
4+
decreased from 0.128 to 0.101. After being calcined at 700 C, the Ti /Ti ratio decreased to
3
+
4+
about 0, indicating all of the Ti ions were transferred to Ti ions owing to the oxidation effect of
O
2
. In addition, the Raman spectra also confirmed the presence of oxygen vacancies. As well
known, the pure anatase TiO
2
displays 6 typical peaks around 144, 197, 639, 399, 519, and 513
8

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ꢀ1
32,33
cm , corresponding to the Eg(1), Eg(2), Eg(3), B1g(1), B1g(2) and A1g modes . As shown in Fig. 3,
3
+
ꢀ1
the fresh black Ti /TiO
2
exhibited a principal peak around 157.0 cm characteristic of oxygen
o
vacancies. Calcining at 300 C had no significant influence on the Raman spectra. Increase the
o
ꢀ1
calcination temperature to 500 C resulted in a negative shift of the principal peak from 157.0 cm
ꢀ1
o
to 151.3 cm . Further increasing the calcining temperature to 700 C led to the formation of
anatase TiO to rutile TiO as indicated in both Fig. 3a and aforementioned XRD pattern (Fig. S2),
together with the continued negative shift to about 147.1 cm , implying a great decrease in
2
2
ꢀ1
34
oxygen vacancies . These results clearly demonstrated that the oxygen vacancies produced due to
4
+
3+
35
the replacement of Ti ions with Ti ions in TiO
2
lattice (see the following equation) and the
number of oxygen vacancies decreased gradually during calcinations at elevated temperatures due
3
+
to the oxidation of Ti into TiO2.
4
+
2ꢀ
4+
ꢀ
4+
3+
4
2 2
Ti + O → 4Ti + 2e /□ + 0.5 O → 2Ti + 2Ti + □ + 0.5 O
(a)
(b)
144
144
151
D
C
640
398
515
D
C
B
A
198
156
1
57
B
A
200
400
600
800
1000
100
120
140
160
180
200
ꢀ
1
ꢀ1
Raman shift (cm )
Raman shift (cm )
ꢀ
1
Figure 3. Raman spectra (a) and the corresponding enlarged Raman spectra from 100 to 200 cm
3
+
(
b) of the asꢀprepared black Ti /TiO
2
before (A) and after being calcined at elevated temperatures
o
o
o
300 C (B), 500 C (C), 700 C (D).
3
+
The UVꢀVis DRS spectra in Fig. 4 revealed that the black Ti /TiO displayed an extended
2
absorption band up to ca. 800 nm. And the calcined samples had the broad peak at ca. 680 nm
which is consistent with the color change of the samples. The broad peak at 680 nm might be due
to forming oxygen vacancy energy level in the bandgap. In the illumination, the photoꢀgenerated
3+
electron was graded transition. Calcining the black Ti /TiO
2
samples at elevated temperatures
2
1
3+
resulted in the gradual blueꢀshift of the light absorbance . The black Ti /TiO
2
and calcining at
9

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different temperature samples all had the decreasing optical bandgap with the colorꢀdependence.
3
+
Thus, the visible light absorbance of the black Ti /TiO
of defect states in TiO band gap owing to the presence of Ti ꢀdopants and oxygen vacancies in
accordance with the XPS analysis.
2
could be mainly ascribed to the existence
3
+
36
2
1.8
1.5
1.2
0.9
0.6
0.3
0.0
black TiO2
o
C
300
o
5
00 C
o
7
00 C
200
300
400
500
600
700
800
Wavelength (nm)
Figure 4. The UVꢀvis diffuses reflectance spectra (DRS) and the digital photos of the asꢀprepared
3
+
2
Ti /TiO before and after being calcined at elevated temperatures.
Based on 1*1*2 and 2*2*1 anatase TiO super cells, the electronic structure of the black
2
3
+
37 38
Ti /TiO
2
could be calculated by using CASTEP model
. To calculate the concentration of
oxygen vacancy, one O atom is removed from each super cell. During the calculation, a suitable
planeꢀwave basis set with an energy cutoff of 408 eV was selected for the ultraꢀsoft
pseudopotential with PerdewꢀBurkeꢀErnzerhof exchange correlations. Brillouinꢀzone integration is
computed with k points in a Monkhorstꢀ Pack (10, 10, 5) grid. As known, the H disorderꢀadsorbed
3
+
2
on the surface of black Ti /TiO could form a midꢀgap state. It can form a continuum extending
2
4
to or overlapping with the conduction band edge, named band tail states . It is found that the
Fermi level is much closer to the conduction band tail states at high oxygen vacancy concentration
(
see Fig. S8). Thus, it is reasonable that the high concentration of oxygen vacancy may break the
selection rule for indirect transitions, leading to an enhanced absorption for photon energy below
39
the direct band gap, in good accordance with the UVꢀvis DRS spectra (see Fig. 4). Based on
above calculation, a scheme of the structure and density of states (DOS) is illustrated in Fig. 5.
The gap states form a miniꢀband below the conduction band edge. It could be seen that the
conduction band edge will turn broad at enhanced oxygen vacancy concentration. Meanwhile, the
band of defectꢀstates resulted from oxygen vacancy is very close to the conduction band edge,
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allowing photoꢀgenerated electrons to easily exchange between two bands. Under visibleꢀlight
irradiation, the electron from the valence band (VB) can easily transfer to the oxygen vacancy
level existing in the band gap. Therefore, the electronic transfer from both valence band and
oxygen vacancy level localized states to tailed conduction band (CB).
E (eV)
Ec
Visible light
eꢀ
ꢀ
e
hυ
O vacancies
eꢀ
Ev
Figure 5. The scheme of the structure and density of states (DOS) for pure TiO
2
(left) and black
3
+
Ti /TiO
2
(right), together with conduction and valence levels of bulk semiconductor (Ec and Ev).
3
+
As shown in Fig. 6a, all the Ti /TiO
2
samples display PLS peaks at around 560 nm
regardless of the calcination temperature. The intensity increased with the enhanced calcination
40
temperature, indicating the increased photoelectronꢀhole recombination rate, since the reduced
3
+
concentration of Ti ꢀdopants caused the decrease in oxygen vacancies, which could capture
photoelectrons and thu, inhibit the photoelectronꢀhole recombination. Meanwhile, as shown in Fig.
3
+
6
b, the black Ti /TiO
2
exhibited strong photocurrent response. Calcinations of the black
at elevated temperatures in air resulted in significant decrease in photocurrent response,
which could be attributed to both the decrease in absorbance for visible lights (see Fig. 4) and the
3
+
Ti /TiO
2
41
increase of photoelectronꢀhole recombination rate (see Fig. 6a) due to the reduced concentration
3
+
of Ti ꢀdopants and oxygen vacancies.
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DOI: 10.1039/C4TA02873B
4
5
0
3
+
3+
(a)
black Ti /TiO2
black Ti /TiO2
(b)
4
o
o
3
5
7
00 C
300 C
o
35
30
o
00 C
500 C
o
o
00 C
700 C
25
20
15
10
5
0
5
20
540
560
580
600
0
20
40
60
80
100
Time (s)
Wavelength (nm)
Figure 6. (a) PLS spectra (excited by 280 nm) and (b) the visible lights (λ > 420 nm) induced
3+
2
photocurrent of the black Ti /TiO before and after being calcined at elevated temperatures.
3
+
As shown in Fig. 7a, the asꢀprepared black Ti /TiO
2
exhibited relatively high activity
during visible light driven photocatalytic degradation of RhB. The photocatalytic activity
o
o
decreased gradually after being calcined at elevated temperature from 300 C to 700 C, which
3
+
could mainly be attributed to the decrease in both the Ti content and the oxygen vacancy number
o
due to the oxidation in air at high temperature. The white TiO obtained at 700 C displays no
2
3
+
significant activity, obviously due to the complete transformation of Ti into TiO
2
and the total
disappearance of oxygen vacancies, taking into account that either the pure anatase or the pure
39
rutile TiO could not be activated by visibleꢀlight due to their big energy band gaps . As shown in
2
3
+
2
Fig. 7b, the asꢀprepared black Ti /TiO also exhibited strong durability and could be used for 4
times without any deactivation. For excluding the selfꢀsensitization effect of RhB at 554 nm, the 8
2
LED lamps with wavelength of 420 nm and light intensity of 3.5 mW/cm were employed as a
singleꢀwavelength light source for driving the photocatalytic reaction. As shown in Fig. 7c, 40 %
3
+
2
RhB removal rate was obtained after reaction for 4 h over the fresh black Ti /TiO , which was
similar to that obtained under Xenon light irradiation with wavelength longer than 420 nm.
Meanwhile, Fig. 7d also demonstrated the efficient degradation of colorless aniline under visible
light irradiation (λ > 420 nm, Xenon lamp). It was also noted that only a about 19 % removal rate
was obtained via choosing phenol as another colorless organic pollutants. For better comparison,
commercial P25 was also utilized as standard. P25 exhibited an 79.7 % removal rate of RhB under
under Xenon light irradiation with wavelength longer than 420 nm, however its activity was
greatly decreased to very low (about 5 % removal rate) under 420 nm LED irradiation while
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keeping other conditions unchanged. 79.7 % removal rate of RhB for P25 is attributed to its strong
4
2
RhBꢀsensitization effect, which was evidenced by our earlier reported work. To the case of 420
nm LED irradiation, the RhBꢀsensitization effect was avoided with the formation of a negative
activity for P25. All these results indicated that the present visibleꢀlight induced photocatalytic
3
+
degradation was attributed to the intrinsic properties of the black Ti /TiO
2
rather than the
photosensitizing effect.
It was also found that the RhB degradation yield greatly enhanced up to 91% even after
reaction for 3 h by using a 300 W Xenon lamp irradiation without removing UV lights (see Fig.
S9), implying that the UV lights in the Xenon lamp could more effectively activate the black
3
+
3+
2 2
Ti /TiO , which could also be observed for the black Ti /TiO after being calcined at elevated
3
+
temperatures. Moreover, it was found that the concentrations of both the Ti ꢀdopants and oxygen
vacancies had very little influence on the photocatalytic activity induced by UV lights since no
3
+
much change in RhB degradation yield was observed even after calcination of the black Ti /TiO
2
o
3+
up to 500 C. From those results, we could guess that the Ti ꢀdopants and oxygen vacancies
promoted photocatalytic activity mainly via generating intermediate energy levels to narrow the
energy band gap rather than via capturing photoelectrons to reduce photoelectronꢀhole
3
+
o
recombination. To our surprise, the black Ti /TiO
2
after being calcined at 700 C exhibited
slightly higher activity than the fresh sample, which could be ascribed to the formation
43
2
rutile/anatase TiO heterojunctions, allowing a fast photoꢀgenerated electron transfer rate.
1
.0
(a)
(b)
4
0
0
0
0.9
0.8
2
0.7
0.6
0.5
o
C
o
C
o
0
2
4
6
8
10
12
14
16
As-prepared
300
500
700 C
Time (h)
1
3

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1
0
0
0
0
.0 (c)
1.0 (d)
RhB
Aniline
.8
.6
.4
0.8
0.6
0.4
0.2
.2
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
3
+
Figure 7. (a) Photocatalytic activities of the black Ti /TiO before and after being calcined at
2
3
+
2
elevated temperature and (b) Recycling test of the fresh black Ti /TiO for RhB degradation in
aqueous solution under visibleꢀlight (λ > 420 nm) irradiation. Reaction conditions: 50 mg
photocatalyst, 50 mL RhB aqueous solution (10 mg/L), room temperature, 4 h. (c) The
3
+
photocatalytic degradation of RhB with the fresh black Ti /TiO
2
under 8 LED lamps (each light
2
intensity = 3.5 mW/cm ) illumination. Other reactions are the same as those in Figure 7(a). (d)
3
+
The photocatalytic degradation of aniline (10 mg/L) with the fresh black Ti /TiO
2
under
visibleꢀlight illumination (λ > 420 nm). Other reactions are the same as those in Figure 7(a).
To clarify the mechanism of the photocatalytic degradation of aqueous RhB in the presence
3+
of the asꢀprepared black Ti /TiO , we introduced various reactive species scavengers, including
2
sodium oxalate (10 mg), isopropanol (1.0 mL), AgNO
3
(10 mg), and nitrogen gas, to the
+
photocatalytic reaction system for investigating the effect of holes (hvb ), hydroxyl radicals (·OH),
2
−
photoꢀgenerated electrons (e‑) and oxygen anion molecules (·O ) on the removal efficiency of
RhB under visibleꢀlight irradiation. As shown in Figure 8, it was found that the addition of AgNO
3
,
as the electron scavenger, had only a weak influence on the photocatalytic degradation of RhB.
This indicates that the photogenerated electrons are not the key for degrading RhB. As known, the
.
ꢀ
formation of superoxide anions ( O ), an important active radical for photocatalytic reactions,
2
resulted from the reaction between the dissolved oxygen in the reaction system and
photogenerated electrons. Upon purging with nitrogen gas, oxygen molecules could be expelled
from the reaction system, inhibiting the formation of superoxide anions. The removal rate of RhB
.
ꢀ
was also decreased slightly. This indicates that O
2
is not the key factor affecting the
3
+
photocatalytic performance of the black Ti /TiO
2
. In addition, when adding isopropanol (IPA) to
.
minimize the formation of OH radicals, it was found that the addition of isopropanol greatly
3
+
2
reduced the photodegradation rate of RhB from 43 % to about 15 % in the black Ti /TiO .
.
The OH radicals were further detected by a terephthalic acid photoluminescence probing. The
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fluorescence intensity was found to increase steadily with irradiation time (Fig. S10a), indicating
3
+
that hydroxyl radicals can be generated in the black Ti /TiO
2
system under visible light (λ> 420
nm) irradiation. The ESR spinꢀtrap technique (with DMPO) was also further chosen to probe the
3
+
hydroxyl radicals generated on the surface of the black Ti /TiO
2
. As shown in Fig. S10b, four
characteristic peaks of DMPOꢀ•OH were obviously observed in the suspension of black
3
+
44
2
Ti /TiO . This is consisted with the description of by using the terephthalic acid to probe
hydroxyl radicals in PL experiment.
Moreover, a significant 72 % decrease in the photocatalytic activity was attributed to the
+
addition of sodium oxalate (Na
2
C O
2 4
) for trapping photogenerated holes (hvb ). These results
3+
imply that the degradation of RhB by the black Ti /TiO
2
photocatalyst seemed to be ascribed to a
+
.
dual mechanism involving both surface hvb and OH radicals. To the case of the sample, the black
3
+
2
Ti /TiO , the absorptive intensity of RhB at a wavelength of 554 nm gradually decreases and the
absorption band shifts to shorter wavelengths at 540 nm as shown in Figure S11. Furthermore, no
new absorption bands appear in either the visible or the UV region. This suggests the complete
3+
photocatalytic decomposition of RhB during the reaction on black Ti /TiO
2
sample.
Therefore, it can be concluded that the photoꢀoxidation mechanism occurring on the surface
3
+
2
of the black Ti /TiO may involve the direct reaction of the organic chemical (dye) with surface
+
.
+
hvb and indirect reaction with OH radicals, or a dual mechanism involving both surface h
vb
.
and OH radicals (equation 7).
+
.
h
vb / OH + dye (RhB) → CO
2
+ H
2
O
(7)
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50
40
30
20
10
0
Sodium oxalate AgNO3
Bubbling N2 Isopropanol Initial sample
2 2 4 3 2
Figure 8. Effect of Effect of sodium oxalate (Na C O ), AgNO , N and isopropanol on the
3
+
photocatalytic performance of black Ti /TiO for degrading RhB (10 mg/L) in 4 hours under
2
visible light.
Besides the high activity in photocatalytic degradation of organic pollutants, such a black
3
+
Ti /TiO could also be employed in photocatalytic waterꢀsplitting for H evolution under
2
2
simulated solar lights irradiation (a 300 W Xenon lamp without removing UV lights) via choosing
3
+
methanol as the sacrificial reagent. As shown in Table 1, the asꢀprepared black Ti /TiO
2
2
2
5
45
displayed similar activity to both the Sꢀdoped TiO
and the Iꢀdoped TiO
2
, which were widely
2
recognized as highly active visible photocatalysts for H evolution via water splitting. Meanwhile,
2
its H
2
evolution activity (0.26 mmol/h*m ) was about two times of that of P25, suggesting the
3
+
asꢀpreapred black Ti /TiO
2
had excellent intrinsic activity.
a
Table 1. H
2
evolution rates over different photocatalysts irradiated with simulated solar lights
2
Photocatalyst
H evolution rate (mmol/h⋅m )
2
3
+
Black Ti /TiO
2
0.26
Sꢀdoped TiO
2
0.29
0.25
0.13
Iꢀdoped TiO2
P25
a
Reaction conditions: 50 mg photocatalyst doped with 1 wt% Pt, 80 mL aqueous solution
containing 20 mL methanol, a 300 W Xenon lamp located at 10 cm away from the reaction system,
room temperature.
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Conclusions
This work developed a facile strategy to prepare black Ti ꢀdoped singeꢀcrystal anatase TiO
3
+
2
3
+
(
Ti /TiO
2
) in oneꢀpot by treating Ti in an ionic liquid containing LiAc and HAc under mild
3
+
3+
ionothermal conditions. This Ti /TiO contained highꢀcontents of Ti ꢀdopants incorporated into
2
TiO
2
lattice and oxygen vacancies, which facilitated the activation by visible lights and also
reduced the photoelectronꢀhole recombination. It exhibited high activity in both photocatalytic
2
degradation of organic pollutants and H generation under irradiation with visible lights and/or
simulated solar lights.
Supporting Information
3
+
4+
Ti /Ti and oxygen vacancy molar percentages, O2s XPS spectra of black TiO , FESEM
2
images of the asꢀprepared samples via choosing various amount of ionic liquids
2
(1ꢀmethylꢀimidazolium tetrafluoroborate), TGA curve of the black TiO sample under an air flow,
the detection of hydrogen produced in the ionothermal treatment process of Ti foil by the gas
chromatograph measurement, XRD patterns for asꢀprepared samples with different calcining
temperature, EPR spectra, Ti 2p XPS spectra of various samples, calculated density of electronic
2
states for anatase TiO , the photocatalytic degradation of RhB with the samples calcined at various
temperature in air under Xenon lamp illumination. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgements
This work was supported by National Natural Science Foundation of China (21207090,
21477079, 21261140333, 21237003), Doctoral program of Higher Education (20123127120009),
Shanghai Government (11SG42, 11ZR1426300, 14ZR1430900, IRT1269), and by a scheme
administrated by Shanghai Normal University (SK201104, DXL122, and S30406).
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