Boosting molecular oxygen activation of SrTiO3 by engineering exposed facets for highly efficient photocatalytic oxidation

Article abstract of DOI:10.1039/c7ta08061a

Engineering exposed facets for improving the activation of molecular oxygen into O₂^- species and charge separation is a promising way to improve photocatalytic oxidation activity. Herein, SrTiO₃ nanocrystals with controllably exposed {001} and {110} facets are prepared as a proof-of-concept candidate to study the effect of exposed facets on the activation of molecular oxygen into O₂^- species, finally acting on the photocatalytic oxidation of gaseous HCHO. The photocatalytic test demonstrates that {110} facet exposed SrTiO₃ nanoparticles present a HCHO photocatalytic degradation rate about 6.8 times faster than that of the {001} facet exposed one. The experiments and theoretical calculations together confirm that as compared to the {001} facet exposed sample, the {110} facet exposed sample is favorable for activating molecular oxygen to produce O₂^- species under the irradiation of light, owing to the relatively higher conduction band position and lower surface adsorption energy for O₂ molecules as well as advantageous charge accumulation on adsorbed O₂ molecules. Finally, the new production of O₂^- species and the higher charge separation contribute to the superior HCHO photocatalytic oxidation activity of {110} facet exposed SrTiO₃. This interesting finding is probably useful for the design of highly efficient single crystal photocatalysts for photocatalytic oxidation reactions.

Full text of DOI:10.1039/c7ta08061a

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PAPER
Boosting molecular oxygen activation of SrTiO3 by
engineering exposed facets for highly efficient
photocatalytic oxidation†
Cite this: DOI: 10.1039/c7ta08061a
ab
Xiaoyong Wu,‡a Xiaoyang Wang,‡a Jun Lia and Gaoke Zhang
*
Engineering exposed facets for improving the activation of molecular oxygen into O2cꢀ species and charge
separation is a promising way to improve photocatalytic oxidation activity. Herein, SrTiO3 nanocrystals with
controllably exposed {001} and {110} facets are prepared as a proof-of-concept candidate to study the
effect of exposed facets on the activation of molecular oxygen into O2cꢀ species, finally acting on the
photocatalytic oxidation of gaseous HCHO. The photocatalytic test demonstrates that {110} facet
exposed SrTiO3 nanoparticles present a HCHO photocatalytic degradation rate about 6.8 times faster
than that of the {001} facet exposed one. The experiments and theoretical calculations together confirm
that as compared to the {001} facet exposed sample, the {110} facet exposed sample is favorable for
activating molecular oxygen to produce O2cꢀ species under the irradiation of light, owing to the
relatively higher conduction band position and lower surface adsorption energy for O2 molecules as well
as advantageous charge accumulation on adsorbed O2 molecules. Finally, the new production of O2cꢀ
species and the higher charge separation contribute to the superior HCHO photocatalytic oxidation
activity of {110} facet exposed SrTiO3. This interesting finding is probably useful for the design of highly
efficient single crystal photocatalysts for photocatalytic oxidation reactions.
Received 13th September 2017
Accepted 24th October 2017
DOI: 10.1039/c7ta08061a
rsc.li/materials-a
photocatalysts has rarely been studied for the photocatalytic
oxidation of HCHO.
Introduction
On the other hand, it is well acknowledged that charge
separation of photo-generated electron and hole pairs, the
relative position of the conduction band (CB) and valence band
(VB) of semiconductors and the adsorption ability between the
semiconductor surface and molecules (e.g. O2, H2O, CO2 or
organic molecules) are three key factors for the high efficiency
of photocatalysis.8–16 In order to fulll the high rate of charge
separation, the formation of a heterostructure between two
semiconductors has been popularly employed.17–20 For instance,
Ismail, et al.21 synthesized novel heterostructure Fe2O3/Bi2S3.
Owing to this exceptional heterostructure, the charge carriers
could be effectively separated, nally leading to much enhanced
photocatalytic activity of composites. In addition to the heter-
ostructure, engineering the crystal facet of a single semi-
conductor has also demonstrated great advantages for charge
separation and preferentially assembling photogenerated holes
or electrons on a typical facet due to various charge densities
and band structures in different facets of a single crystal.22–25 For
example, Zhang, et al.26 prepared various facet exposed TiO2 to
photocatalytic oxidation of ammonia. They found that {001}
facet dominantly exposed TiO2 exhibited superior photo-
catalytic performance over those of {101} and {010} exposed
ones, due to higher improved charge separation and more
assembled photogenerated holes in the {001} facet dominantly
exposed one. Li, et al.27 reported a {101} and a {010} facet
Formaldehyde (HCHO) is well-known as one of the most toxic
indoor air pollutants, which originates from building and
decoration materials.1,2 Long-term exposure to HCHO can cause
serious human diseases such as leukemia, respiratory tract
diseases, nasal tumors, etc. Thus, HCHO removal has become
an urgent issue to be addressed.3,4 Recently, photocatalysis has
attracted peculiar interest from researchers to remove HCHO
due to its low cost, environmental friendliness and high effi-
ciency. Some photocatalysts (e.g. TiO2, ZnO, Bi12TiO20, etc.) are
correspondingly developed to oxidize HCHO to nontoxic
compounds.5–7 However, SrTiO3 as one of the most popular
aHubei Key Laboratory of Mineral Resources Processing and Environment, Hubei
Provincial Collaborative Innovation Center for High Efficient Utilization of
Vanadium Resources, School of Resources and Environmental Engineering, Wuhan
University of Technology, 122 Luoshi Road, Wuhan 430070, China
bState Key Laboratory of Silicate Materials for Architectures, Wuhan University of
Technology, Wuhan 430070, China. E-mail: gkzhang@whut.edu.cn; Fax: +86-27-
87887445; Tel: +86-27-87651816
† Electronic supplementary information (ESI) available: The calculated fraction of
exposed facets, XPS, photocatalytic stability, XRD and FTIR of samples. See DOI:
10.1039/c7ta08061a
‡ X. Y. Wu and X. Y. Wang contributed equally to this paper. G. K. Zhang and X. Y.
Wu conceived the project and designed the experiments; X. Y. Wang performed
the experiments; X. Y. Wu wrote the paper. J. Li and G. K. Zhang helped to
discuss the results and the corresponding analysis.
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exposed BiVO4 single crystal. The photogenerated electrons and water/pentaerythritol) stirring in an ice bath. Aer 10 min stir-
holes could be effectively separated and preferentially assem- ring, 30 mL 3 M LiOH solution and 10 mL 0.24 M SrCl2 solution
bled in {101} and {010} facets, respectively, nally resulting in were introduced into the above mixed solution and was stirred
high efficiency of water splitting. Besides, the relative positions for another 30 min. Then, the nal solution was transferred into
of the CB and the VB in a various facet exposed single crystal not a 100 mL Teon-lined stainless steel autoclave and heated at
ꢁ
only connect with the spatial charge separation, owing to the 180 C for 48 h. Aer automatically cooling to room tempera-
driving of the potential difference, but also probably affect the ture, the sample was centrifuged and washed with water and
activation of molecular oxygen in the crystal surface. In other ethanol several times. Finally, the sample particles were dried at
words, a different facet exposed crystal28 can present different 60 ꢁC overnight in a vacuum oven. The samples prepared using
abilities for activation of adsorbed molecular oxygen owing to distilled water/ethanol, distilled water/ethylene glycol and
the various band potentials. It will further affect the production distilled water/pentaerythritol mixed solutions are denoted as
or concentration of active species (O2cꢀ), nally contributing to SrTiO3-001, SrTiO3-110-1, and SrTiO3-110-2, respectively. To
the photocatalytic activity. Additionally, as we all know, the investigate the effect of carbon content on the photocatalytic
adsorption ability of the semiconductor surface for molecules activity of samples, the sample SrTiO3-110-2 was also further
and the corresponding charge distribution, which are different calcined at 450 ꢁC in air to eliminate the carbon content in the
in various facets,29,30 are also essential for the activation of particle and denoted as SrTiO3-110-2-C.
molecular oxygen to produce O2cꢀ species. Therefore, the
research about engineering the crystal facets of a single semi-
conductor to investigate the effect of band potentials, surface
molecule adsorption abilities and charge distribution in various
exposed facets on the activation of molecular oxygen for pho-
tocatalytic performance is promising, but rarely studied. Typi-
cally, SrTiO3 as one of the most promising photocatalysts has
been widely studied for controlling crystal facets to enhance
photocatalytic activities.22,23,31,32 However, all studies were
mainly focused on the charge separation and charge carrier
assembly in various facets, and a detailed study about boosting
activation of molecular oxygen by controlling exposed facets for
photocatalytic oxidation activity as mentioned above has not
been reported up to now.
Herein, SrTiO3 nanoparticles with controllably exposed {001}
and {110} facets were chosen as a proof-of-concept candidate to
study the inuence of exposed facets on the activation of
molecular oxygen into O2cꢀ active species, nally contributing
to the HCHO photocatalytic oxidation activity. In order to
largely eliminate the effect of preparation method and specic
surface area of SrTiO3 particles, a one-pot solvothermal method
was used to synthesize SrTiO3, and the crystal facets were
engineered by employing various solvents. More importantly, in
addition to various characterizations and trapping experiments,
the density functional theory (DFT) calculation was also
Characterization
The crystallinity of the samples was tested by X-ray diffraction
(XRD) (D/MAX-RB diffractometer) with CuKa radiation. The
valence band position and atomic state of samples were evalu-
ated by X-ray photoelectron spectroscopy (XPS, VG Multilab
2000). The binding energy of samples in XPS was calibrated by C
1s with 284.6 eV as the reference. The structure and morphology
of the specimens were checked by transmission electron
microscopy (TEM), high-resolution transmission electron
microscopy (HRTEM) and selected area electron diffraction
(SAED) (JEOL-2010). The light absorption properties of products
were measured using a Lambda 750S UV-Vis-NIR spectropho-
tometer. The Brunauer–Emmett–Teller (BET) specic surface
area was measured by a multipoint BET method (AUTOSORB-1).
The O2cꢀ radicals produced by samples were characterized
using a electron paramagnetic resonance (EPR, A300-10/12)
spectrometer in methanol solution containing 50 mM, 0.2 mL
of 5,5-dimethyl-1-pyrroline N oxide (DMPO) under the irradia-
tion of a Xe lamp.
Photocatalytic test
combined to explore the detailed mechanisms about the acti- The photocatalytic oxidation of gaseous HCHO was employed to
vation of molecular oxygen on a various facet exposed SrTiO3 investigate the photocatalytic activity of the samples. In a typical
crystal from the viewpoint of potentials of the CB, surface O2 procedure, rstly, 0.1 g sample powders were homogeneously
adsorption abilities and the corresponding charge distribution. dispersed in a glass dish with a diameter of 7 cm; then, the
sample dish was sited in a sealed stainless reactor with
a volume of 600 mL; secondly, 2 mm of liquid HCHO was
injected into the reactor and then irradiated using an IR lamp to
be evaporated completely. Aer achieving a stable gaseous
Experimental
Photocatalyst preparation
Ethanol ($99.7%), ethylene glycol ($99.0%), pentaerythritol HCHO concentration in the reactor, the 300 W Xe lamp was
($98.0%), SrCl2$6H2O ($99.0%), LiOH$H2O ($95.0%), and turned on to irradiate the sample and the temperature of the
TiCl4($98.0%) were bought from Guoyao chemical reagent co. reactor was xed at 25 ꢁC using a water recycling system. Finally,
Ltd. All reagents were of analytical grade and used without the concentrations of HCHO gas and CO2 were evaluated using
further purication. SrTiO3 particles were prepared by a facile a photoacoustic eld gas monitor (Innova1412, Denmark).
solvothermal method.33 Typically, to begin with, 0.265 mL TiCl4
The apparent quantum yield (AQY) of O2 reduction and
was added into 25 mL of mixed solution (distilled water (15.45 HCHO degradation was calculated based on the following eqn
MU cm)/ethanol, distilled water/ethylene glycol or distilled (1) and (2), respectively:34,35
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nO ;tNAhc
2
AQYð%Þ ¼
ꢂ 100
(1)
(2)
PSlt
4nHCHO;tNAhc
PSlt
AQYð%Þ ¼
ꢂ 100
where nO ,t and nHCHO,t (mol) indicates the number of oxygen
2
and HCHO molecules evolved in the irradiation time, NA is
Avogadro's constant, h (m2 kg sꢀ1) is Planck's constant, c (m sꢀ1
)
is the light speed, P (W mꢀ2) is the power density of the incident
light (365 nm), S (m2) is the area of sample irradiated, l indi-
cates the wavelength of irradiated monochromatic light and t (s)
is the irradiation time.34
Trapping experiment
In order to better understand the photocatalytic oxidation of
HCHO by the sample, trapping experiments were carried out by
using potassium iodide (KI), tert-butyl alcohol (TBA) and 1,4-
benzoquinone as scavenging agents for h+, cOH and O2cꢀ
species under the irradiation of a Xe lamp.
Fig. 1 XRD patterns of the SrTiO3 samples with various exposed
facets.
peaks were observed, indicating that the pure SrTiO3 particles
had been successfully obtained by a solvothermal method using
various alcohols as solvents. To further obtain the formation on
the exposed facets of the samples, TEM, HRTEM and SAED
measurements were conducted as shown in Fig. 2 and the
corresponding SEM images of samples are also shown in
Fig. S1.‡ From Fig. 2a, d and g, it can be obviously seen that the
sample SrTiO3-001 prepared using ethanol mixed solution
presents a perfect cubic morphology; two interplanar spacings
with the same distance of 0.27 nm and an angle of 90ꢁ were
observed in the HRTEM image, which are assigned to the 110
Photoelectrochemical measurement
Photoelectrochemical measurements were conducted on a CHI-
660E station. A Pt wire and a saturated Ag/AgCl electrode were
respectively utilized as the counter and reference electrode. The
working electrodes prepared by coating sample suspension on
uorine doped stannic oxide (FTO) glass and 0.1 M Na2SO4
solution was used as the electrolyte. Finally, the photocurrent
and electrochemical impedance spectra were obtained under
the irradiation of a 300 W Xe lamp at room temperature.
DFT calculation
The electronic band structure of the samples was calculated by
the density functional theory (DFT) method with the assistance
of generalized gradient approximation (GGA) with the Perdew–
Burke–Ernzerhof (PBE) functional, encoding via the Vienna ab
initio simulation package (VASP). Firstly, the SrTiO3 (110) and
(001) surfaces were modeled with slabs cut from the crystal
along the [110] and [001] orientation, respectively. Two-layer
˚
slabs of a 2 ꢂ 2 surface unit cell with a vacuum of 15 A were
selected. The unit cell parameters were optimized, until the
residual force of components was less than 0.05 eV Aꢀ1 and the
˚
iterative energy variation was no more than 10ꢀ5 eV, to improve
the accuracy of calculation.26 Besides, the adsorption energy
(Eads) between the SrTiO3 surface and O2 molecules was also
calculated. The Eads was determined by using the equation Eads
¼ EO -surface ꢀ (Esurface + EO ), where EO -surface is the total energy
2
2
2
of the surface with O2, Esurface is the total energy of the surface
without O2, and EO is the energy of the O2.33
2
Results and discussion
Fig. 1 shows the XRD patterns of the samples SrTiO3-001,
SrTiO3-110-1 and SrTiO3-110-2. It is apparent that all of the
diffraction peaks in the three samples are well indexed to the
Fig. 2 TEM images, HRTEM images, SAED patterns of samples SrTiO3-
001 (a, d, and g), SrTiO3-110-1 (b, e, and h) and SrTiO3-110-2 (c, f, and
cubic SrTiO3 crystal (JCPDS No. 35-0734) and no other impurity i), respectively.
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ꢀ
and (110) planes, respectively; in addition, the SAED pattern Fig. 3a, it is apparent that the blank experiments display
displayed a parallel spot pattern, implying that the as- negligible HCHO concentration change, when the tests were
synthesized sample was a single crystal and two closest spots conducted under light irradiation without a SrTiO3 photo-
with the angle of 90ꢁ are attributed to the 020 and 200 planes, catalyst as well as with a SrTiO3 photocatalyst without light
respectively. Thus, combining HRTEM and SAED, it can be irradiation, respectively, indicating that the photolysis of HCHO
learned that the cubic SrTiO3-001 is totally covered by six {001} hardly occurred and the physical adsorption of HCHO by the
facets. While as observed from Fig. 2b, e, h, c, f and i, the sample could be eliminated. However, as SrTiO3 particles were
samples SrTiO3-110-1 and SrTiO3-110-2, respectively prepared introduced into the reactor, all three samples exhibited good
by ethylene glycol and pentaerythritol mixed solutions, present HCHO concentration decrement with the increment of light
a similar octagonal shape but with difference degrees of edge irradiation time, implying that this decrement trend was really
truncations. The interplanar spacings of 0.27 and 0.39 nm are from the photocatalysis. More importantly, the sample SrTiO3-
ascribed to the 011 and 100 planes with the angle of 90ꢁ, 110-2 showed the best HCHO degradation ability followed by
respectively. Meanwhile, the two closest spots in SAED patterns the samples SrTiO3-110-1 and SrTiO3-001. The corresponding
belonged to the 022 and 200 planes, respectively. So the exposed CO2 concentration variation is illustrated in Fig. 3b. It can be
facets for samples SrTiO3-110-1 and SrTiO3-110-2 were domi- clearly observed that the three samples presented the same
nantly {110}.33,36 According to these results and combining SEM increment tendency in CO2 concentration with the HCHO
images, we calculated the fraction of exposed facets for each decrement, meaning that the HCHO was actually decomposed
sample as shown in Table S1.‡ Samples SrTiO3-110-1 and into CO2 and H2O by SrTiO3. As shown in Fig. 3c, the photo-
SrTiO3-110-2 exposed the dominant {110} facet with the ratios of catalytic degradation of HCHO could follow the pseudo-rst
65.4 and 87.5%, respectively. Therefore, SrTiO3 with con- order reaction kinetics and for the sample SrTiO3-110-2, 95%
trollably exposed {001} and {110} facets has been successfully of HCHO was degraded in the rst 40 min and then reached the
obtained by a solvothermal method using various solvents. photocatalytic equilibrium, so only a preliminary 40 min reac-
Meanwhile, the specic surface area of samples SrTiO3-001, tion was used to calculate the reaction rate k. Under these
SrTiO3-110-1 and SrTiO3-110-2 was determined to be 5.34, 4.89 conditions, the decomposition rate constant of HCHO by the
and 5.23 m2 gꢀ1, respectively. These similar surface areas for the sample SrTiO3-110-2 is nearly 4.1 and 6.8 times faster than that
three samples are benecial for us to investigate the effect of by the samples SrTiO3-110-1 and SrTiO3-001, respectively. The
exposed facets on the photocatalytic activity of materials aer results in Fig. 3 demonstrate that the exposed facets played
eliminating the inuence of surface area. In addition, the a signicant role for the HCHO photocatalytic oxidation and the
composition and surface atomic states of samples were ob- {110} facet is much more active than the {001} facet. The
tained by XPS as supplemented in Fig. S2.‡ All of the three detailed mechanism for this variation would be discussed in the
samples showed similar results, which were in good agreement following part. Furthermore, in order to visually display HCHO
with the features of SrTiO3 crystals and further conrmed the degradation, the detailed concentration change of HCHO and
formation of pure SrTiO3 materials.37
CO2 with respect to light irradiation time by the sample SrTiO3-
In this work, the photocatalytic oxidation of gaseous HCHO 110-2 is shown in Fig. 3d. During 60 min light irradiation, the
was employed to check the photocatalytic activity of SrTiO3 concentration of HCHO was decreased from ca. 650 to 30 ppm,
samples. Fig. 3 presents the HCHO photocatalytic activity of i.e. 95% degradation ability, while the concentration of CO2 was
samples with various exposed facets and relevant kinetics. In increased from ca. 600 to 1150 ppm. The concentration decre-
ment of HCHO is a little bit lower than the concentration
increment of CO2 rather than 1 : 1, the abnormality of which is
mainly due to desorption of some HCHO adsorbed on the
reactor surface during the photocatalytic test and then oxidizing
to CO2 as well as the measurement deviation of the device. In
addition, Fig. S3‡ also added the HCHO degradation activity of
samples SrTiO3-110-2 and SrTiO3-110-2-C to investigate the
effect of carbon content on the photocatalytic oxidation
performance and no big difference was found for these two
samples, indicating the inuence of carbon content on HCHO
degradation could be eliminated. On the other hand, the HCHO
degradation performance was also measured under the irradi-
ation of monochromic light (365 nm, 80 mW cmꢀ2) as pre-
sented in Fig. S4.‡ The calculated AQY for HCHO conversion by
samples SrTiO3-110-2 and SrTiO3-001 were about ca. 0.89 and
0.52% during 2 h light irradiation, respectively, and the AQY for
O2 reduction by sample SrTiO3-110-2 was about 2.41% and that
Fig. 3 HCHO photocatalytic oxidation (a), the corresponding CO2
yield (b) and HCHO decomposition kinetics (c) of the three samples as
well as the concentration changes of HCHO and CO2 as a function of
irradiation time for the sample SrTiO3-110-2 (d).
of sample SrTiO3-001 was ignored since no signicant O2cꢀ
species was produced. Besides, the stability of the sample
SrTiO3-110-2 is shown in Fig. S5.‡ It is explicit that the sample
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SrTiO3-110-2 still presented high HCHO photocatalytic oxida-
Fig. 4b and c show that the photocatalytic activity of the
tion ability even aer being recycled for ve runs, indicating samples SrTiO3-110-1 and SrTiO3-110-2 with partially exposed
SrTiO3 with the {110} exposed facet not only is effective for the {110} facets was dramatically weakened not only upon addition
HCHO degradation but also has high photocatalytic stability. of KI but also in the presence of 1,4-benzoquinone, implying
Meanwhile, XRD patterns and FTIR spectra of the sample that when SrTiO3 exposed dominantly the {110} facet with
SrTiO3-110-2 for a fresh one without photocatalysis and a used partial {001} facet, both h+ and O2cꢀwere the main active species
one aer ve runs are also shown in Fig. S6.‡ All peaks in XRD in the photocatalytic reaction. More importantly, the effect of
patterns and FTIR spectra did not present an obvious differ- O2cꢀ on the HCHO photocatalytic decomposition by the sample
ence, indicating that the sample is very stable aer several runs SrTiO3-110-2 is more than that of h+, which could be ascribed to
of photocatalytic reaction.
the fact that the sample SrTiO3-110-2 has a higher fraction of
The above results have veried that the {110} exposed facet of {110} exposed facets than the sample SrTiO3-110-1 and {110}
SrTiO3 is much more effective for HCHO photocatalytic oxida- facet exposure apparently increased the production of O2cꢀ
tion than its {001} exposed facet. To further study the detailed species.
photocatalytic mechanisms on different exposed facets, trap-
In order to further conrm that the {110} facet exposure
ping experiments, EPR, DRS, photocurrent, electrochemical induced the production of O2cꢀ species, HCHO photocatalytic
impedance spectra and DFT calculation were combined to oxidation by SrTiO3 with exposed {001} or {110} facets was
address this issue as follows.
carried out in an Ar atmosphere and in air, respectively, and is
As we all know, h+, O2cꢀ and cOH species are the main active shown in Fig. 4d. Since the O2cꢀ species is produced by the
radicals for the photocatalytic oxidation. Thus, KI, 1,4-benzo- reaction of electron and O2 molecules, the production of O2cꢀ
quinone and TBA were employed as scavengers to investigate species would be hindered in an Ar atmosphere without O2. It is
the effect of h+, O2cꢀ and cOH on the HCHO photocatalytic apparent in Fig. 4d that the HCHO photocatalytic degradation
oxidation process by various facet exposed SrTiO3 as shown in by the sample SrTiO3-001 exhibited negligible change in air and
Fig. 4. In Fig. 4a, it is clear that in the presence of 1,4-benzo- an Ar atmosphere, but for sample SrTiO3-110-2, the HCHO
quinone and TBA, the photocatalytic activity of the sample photocatalytic degradation was signicantly reduced in an Ar
SrTiO3-001 decreased a little bit compared with the pure one but atmosphere as compared to that in air. This result veries that
signicantly reduced in the presence of KI, indicating that the O2cꢀ species played an essential role for the HCHO degradation
h+ was the main active species in the photocatalytic reaction by by the {110} facet exposed sample. EPR analysis was combined
SrTiO3 with the {001} exposed facet.
to further conrm the production of O2cꢀ species as exhibited in
Fig. 4e. The sample SrTiO3-001 presented no obvious signal for
the DMPO–O2cꢀ adduct under the irradiation of visible light,
but the sample SrTiO3-110-2 showed strong feature peaks of the
DMPO–O2cꢀ adduct, indicating that the O2cꢀ species had been
largely produced in the {110} facet exposed sample but not in
the {001} facet exposed one.38,39
Based on the above discussion, it can be well conrmed that
the {110} exposed facet of SrTiO3 boosted the activation of
molecular oxygen into O2cꢀ species, which resulted in a high
degradation rate of HCHO, while the {001} facet exposed one was
infeasible. For this fascinating phenomenon, the possible mech-
anism was further studied from the points of band positions of
the VB and the CB and the adsorption ability of the particle
surface with various facets for O2 molecules. Fig. 5 shows the light
absorption properties and valence band XPS spectra of the
samples as well as the corresponding calculated band structure. It
is explicit in Fig. 5a that the sample SrTiO3-001 exhibited the
highest visible light absorption background in the range of 400–
600 nm, followed by the samples SrTiO3-110-1 and SrTiO3-110-2.
These abnormal visible light response properties should be owing
to the light reectance of the particle surface instead of absorp-
tion since SrTiO3 only can be excited by UV light.40 The variation of
the light reectance for the three samples was probably due to the
different exposed facets. More essentially, compared to the
sample SrTiO3-001 with the band gap of 3.27 eV, the absorption
edges of the samples SrTiO3-110-1 and SrTiO3-110-2 were blue-
shied to a short wavelength and the band gaps were enlarged
to 3.31 eV as displayed in Fig. 5b, indicating that exposed facets
Fig. 4 HCHO photocatalytic oxidation activity of the samples SrTiO3-
001 (a), SrTiO3-110-1 (b) and SrTiO3-110-2 (c) in the presence of
different scavengers and in various atmospheres (d) as well as EPR of
samples in the methanol solution under the irradiation of a Xe lamp (e). actually affect the band gaps of SrTiO3 to some extent.
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adsorption energy. Table 1 shows the calculated adsorption
energies between various surfaces and O2 molecules by the DFT
method. It can be learned from Table 1 that compared with the
adsorption energies between Ti atoms on the SrTiO3 surface
and O2 molecules, the adsorption energies between Sr atoms on
the SrTiO3 surface and O2 molecules were much less, implying
that the O2 molecules are preferably adsorbed on the Sr atoms.
The corresponding adsorption geometry is presented in Fig. 6.
More importantly, when O2 molecules are adsorbed by Sr atoms
on the {110} surface, the adsorption energy is ꢀ1.68 eV, which is
much more negative than that on the {001} surface with the
adsorption energy of 0.4 eV. Thus, O2 molecules are preferably
adsorbed by Sr atoms on the {110} facet exposed surface.
Besides, the charge distributions of samples with O2 adsorption
in various facets were also calculated as presented in Fig. 7. It is
apparent that as O2 is adsorbed on the {001} facet, the charge
distribution between the O2 molecules and {001} surface
displays an inverted funnel-like shape, meaning that the elec-
trons preferably accumulate in the {001} facet rather than
transferring to O2 molecules; while O2 is adsorbed on the {110}
facet, the result is opposite and the electrons are in favor of
accumulating on O2 molecules. Therefore, based on Fig. 6 and
Fig. 5 DRS (the inset is the enlarged area from 320–400 nm) (a), the
plots of (Ahn)2 versus photon energy (b), valence band XPS spectra (c)
and TDOS (d) of samples.
To determine the relative band positions of the VB and the 7, it can be learned that molecular oxygen is easier to activate to
CB, the valence band position of XPS spectra for the three produce O2cꢀ species on the {110} facet of SrTiO3 than on the
samples have been estimated as presented in Fig. 5c. The three {001} facet during the photocatalytic oxidation of HCHO.
SrTiO3 samples exhibited similar prole features with the same
Besides the active species, the separation ability of photo-
VB positions of ca. 2.93 eV. So based on the equation ECB ¼ EVB generated electron–hole pairs is another key factor for the high
ꢀ Eg,41,42 the CB positions of samples SrTiO3-110-2, SrTiO3-110-1 photocatalytic oxidation efficiency of the samples. From this
and SrTiO3-001 were ca. ꢀ0.38 eV, ꢀ0.38 eV and ꢀ0.34 eV, viewpoint, the photoelectrochemical properties of the SrTiO3
respectively. As mentioned above, the production potential of samples are shown in Fig. 8. Fig. 8a shows the photocurrent
O2cꢀ, generated by the reaction of photoinduced electrons and response of the samples under the irradiation of a Xe lamp. It is
surface adsorbed O2 molecules, should be lower than ca. clear that the sample SrTiO3-110-2 with the highest HCHO
ꢀ0.33 eV; moreover, the more negative the position of the CB is, degradation ability presented the highest photocurrent value
the easier the production of O2cꢀ species is. So it can be learned followed by the samples SrTiO3-110-1 and SrTiO3-001, indi-
that the {110} facet exposed sample with a relatively higher CB cating that higher charge separation was produced in the
position of ꢀ0.38 eV is in favor of activating molecular oxygen sample SrTiO3-110-2 as compared to the other two samples. In
into O2cꢀ species, as compared to that of the {001} exposed one addition, electrochemical impedance spectroscopy (as shown in
with a relative CB position of ꢀ0.34 eV. To further verify the Fig. 8b) was also another effective way to check the charge
relative position of the VB and the CB of the samples, DFT separation ability of the samples and generally, the smaller
calculation was carried out as presented in Fig. 5d. It can be slope of the Nyquist plot means higher separation ability for
apparently seen that the VB of SrTiO3 is mainly contributed by O charge carriers.43–45 Obviously, the sample SrTiO3-110-2 still
and Ti atoms and the CB is also dominantly resulting from O exhibited the best separation ability with the lowest slope
and Ti atoms. Furthermore, the intensity of the VB maximum among the three samples, which is in good agreement with the
edge of SrTiO3 with the {110} exposed facet is much higher than photocurrent results in Fig. 8a. Therefore, as compared to the
that of SrTiO3 with the {001} exposed facet, indicating much {001} exposed one, the more {110} facet exposed in the SrTiO3
more charge carriers exist in the VB of the {110} facet exposed crystal, the higher the charge separation ability generated.
crystal, which is good for improving photocatalytic activity. Firstly, the surface of sample SrTiO3-110-2 is constituted by
What's more essential is that the band gap of SrTiO3 with the
{110} exposed facet is larger than that of the one with the {001}
exposed facet; meanwhile, the relative CB position of the former
is more negative than that of the latter. These theoretical results
Table 1 The adsorption energies between the SrTiO3 surface and O2
molecules
are perfectly consistent with the above experimental results as
O2 adsorption energy (eV)
presented in Fig. 5a–c.
SrTiO3
Ti
Sr
In addition to the effect of relative position of the CB of
SrTiO3 on the activation of molecular oxygen into O2cꢀ species,
another key factor, the adsorption ability of the SrTiO3 surface
for O2 molecules, was also considered by calculating the
{001}
{110}
2.83
ꢀ0.59
0.4
ꢀ1.68
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Fig. 6 Adsorption geometry of O2 molecules on the (001) (a) and (110)
(b) surfaces of SrTiO3.
Fig. 9 HCHO photocatalytic oxidation mechanism of SrTiO3 with
various exposed facets.
two ways to be distributed. One way is to transfer to the {001}
facet and the other way is to be captured by O2 molecules
adsorbed on the {110} facet. In this work, the photogenerated
electrons are probably preferred to be captured by O2 molecules
adsorbed on the {110} facet to directly produce O2cꢀ species as
the other substance was in contact with semiconductor, the
distribution of photogenerated electron and hole pairs could be
changed,24 and different photocatalytic reaction types could
lead to various photocatalytic results on some typical exposed
facets.26 Meanwhile, DFT simulation in Fig. 7b also conrmed
the above hypothesis to some extent.
Fig. 7 Charge difference distribution of SrTiO3 adsorbed O2 in the
exposed {001} facet (a) and {110} facet (b).
Finally, based on the above detailed investigation, the mech-
anism for higher HCHO degradation ability of the {110} facet
exposed SrTiO3 over the {001} facet exposed one could be
concluded as follows (as shown in Fig. 9): under the irradiation of
a light source, SrTiO3 with {001} and {110} exposed facets would
be excited to produce photogenerated electrons on the CB and
photogenerated holes on the VB. In the {001} facet exposed
sample, the photogenerated holes would directly decompose the
adsorbed HCHO molecule into CO2 and H2O; while for the {110}
facet exposed sample, not only photogenerated holes and the
corresponding cOH species could be used to degrade HCHO, but
also the photogenerated electrons on the CB could also react with
surface adsorbed O2 molecules to produce active O2cꢀand then to
decompose HCHO owing to boosting of the activation of
molecular oxygen induced by the relatively higher CB position
and lower adsorption energy between the {110} surface and O2
molecules as well as the preferred charge distribution. On the
other hand, due to the various charge densities and band
potentials in both {001} and {110} facets as well as the
consumption of photogenerated electrons by surface adsorbed
O2 molecules, the photogenerated hole–electron pairs in the
87.5% {110} facet exposed sample would be effectively separated.
In this case, more photoinduced electrons and holes would be
used to decompose HCHO. Therefore, {110} facet exposed SrTiO3
is much more active for HCHO photocatalytic oxidation as
compared the {001} facet exposed one.
Fig. 8 The photocurrent responses (a) and electrochemical imped-
ance plots (b) of SrTiO3 samples.
87.5% of the {110} exposed facet and 12.5% of the {001} exposed
facet. This unique facet exposed crystal is benecial for charge
separation owing to the various charge densities and band
potentials in different exposed facets,22,24,27 compared with that
of a 100% {001} facet exposed sample. Secondly, the {110} facet
exposed sample was advantageous for capturing O2 molecules
over the {001} facet exposed one. In this case, the photo-
generated electron on the CB would react with captured O2
molecules to produce O2cꢀ species, nally leading to higher
separation ability of photoinduced electron and hole pairs in
the {110} facet exposed sample. Besides, according to the
studies reported by Mu et al.,22 the photogenerated electron
should be accumulated on the {001} facet of sample SrTiO3-110-
2, owing to the lower CB position of the {001} facet. While in this
work, the trapping experiments, EPR and DFT simulation
conrmed that under the irradiation of light, electrons still
existed in the {110} facet to directly reduce O2 molecules rather
than totally transferring to the {001} facet. This abnormal
phenomenon is probably due to the following reason: to begin
Conclusion
with, the atmospheric O2 molecules are preferably adsorbed on SrTiO3 nanocrystals with adjusted {110} and {001} facet
the {110} facet. In this case, the photogenerated electrons have exposing have been successfully prepared by facile
a
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solvothermal method using various alcohols as solvents. SrTiO3 14 F. Raziq, L. Q. Sun, Y. Y. Wang, X. L. Zhang, M. Humayun,
with the dominant {110} exposed facet presented a 6.8 fold
faster HCHO photocatalytic degradation rate than that of the
S. Ali, L. L. Bai, Y. Qu, H. T. Yu and L. Q. Jing, Adv. Energy
Mater., 2017, DOI: 10.1002/aenm.201701580.
pure {001} facet exposed one. It was found that the much 15 S. Y. Chen, R. Yan, X. L. Zhang, K. Hu, Z. J. Li, M. Humayun,
enhanced photocatalytic activity of the {110} facet exposed Y. Qu and L. Q. Jing, Appl. Catal., B, 2017, 209, 320–328.
sample has mainly resulted from the boosting of the activation 16 F. Raziq, Y. Qu, M. Humayun, A. Zada, H. T. Yu and
of molecular oxygen into O2cꢀactive species, in addition to the
L. Q. Jing, Appl. Catal., B, 2017, 201, 486–494.
much improved separation ability of photogenerated hole– 17 G. G. Liu, G. X. Zhao, W. Zhou, Y. Y. Liu, H. Pang,
electron pairs. Combining with various characterizations, the
H. B. Zhang, D. Hao, X. G. Meng, P. Li, T. Kako and
DFT calculation results further conrmed that the new induc-
J. H. Ye, Adv. Funct. Mater., 2016, 26, 6822–6829.
tion of O2cꢀactive species in the {110} facet exposed SrTiO3 18 Y. P. Xie, Y. Q. Yang, G. S. Wang and G. Liu, J. Colloid
sample was owing to the relatively higher CB position and lower
Interface Sci., 2017, 503, 198–204.
adsorption energy between the crystal surface and O2 molecules 19 Z. Wan, G. K. Zhang, X. Y. Wu and S. Yin, Appl. Catal., B,
as well as favorable charge accumulation on O2 molecules as
compared to the {001} facet exposed one.
2017, 207, 17–26.
20 X. Y. Wu, J. T. Wang, G. K. Zhang, K. Katsumata,
K. Yanagisawa, T. Sato and S. Yin, Appl. Catal., B, 2017,
201, 128–136.
21 A. Helal, F. A. Harraz, A. A. Ismail, T. M. Sami and
I. A. Ibrahim, Appl. Catal., B, 2017, 213, 18–27.
22 L. C. Mu, Y. Zhao, A. L. Li, S. Y. Wang, Z. L. Wang, J. X. Yang,
Y. Wang, T. F. Liu, R. T. Chen, J. Zhu, F. T. Fan, R. G. Li and
C. Li, Energy Environ. Sci., 2016, 9, 2463–2469.
23 B. Wang, S. H. Shen and L. J. Guo, Appl. Catal., B, 2017, 166–
167, 320–326.
24 L. L. Wang, J. Ge, A. L. Wang, M. S. Deng, X. J. Wang, S. Bai,
R. Li, J. Jiang, Q. Zhang, Y. Luo and Y. J. Xiong, Angew. Chem.,
Int. Ed., 2014, 53, 5107–5111.
25 Y. Zhao, R. G. Li, L. C. Mu and C. Li, Cryst. Growth Des., 2017,
17, 2923–2928.
Conflicts of interest
There are no conicts to declare
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC No. 51472194 and No. 51602237),
the National Basic Research Program of China (973 Program
No. 2013CB632402) and the Fundamental Research Funds for
the Central Universities (WUT:2017 IVA 039).
26 M. Chen, J. Z. Ma, B. Zhang, F. Wang, Y. B. Li, C. B. Zhang
and H. He, Appl. Catal., B, 2017, DOI: 10.1016/
j.apcatb.2017.04.012.
27 R. G. Li, F. X. Zhang, D. G. Wang, J. X. Yang, M. R. Li, J. Zhu,
X. Zhou, H. X. Han and C. Li, Nat. Commun., 2013, 4, 1432.
Notes and references
1 Z. X. Yan, Z. H. Xu, J. G. Yu and M. Jaroniec, Environ. Sci.
Technol., 2015, 49, 6637–6644.
2 G. N. Li and L. Li, RSC Adv., 2015, 5, 36428–36433.
3 J. L. Wang, G. K. Zhang and P. Y. Zhang, J. Mater. Chem. A, 28 S. Bai, L. L. Wang, Z. Q. Li and Y. J. Xiong, Adv. Sci., 2017, 4,
2017, 5, 5719–5725.
1600216.
4 Y. W. Lu, D. H. Wang, C. F. Ma and H. C. Yang, Build. 29 H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou and G. Liu, Nature,
Environ., 2010, 45, 615–621. 2008, 453, 638–641.
5 A. Suligoj, U. L. Stangar, A. Ristic, M. Mazaj, D. Verhovsek 30 A. Selloni, Nat. Mater., 2008, 7, 613–615.
ˇ
ˇ
´
ˇ
ˇ
and N. N. Tusar, Appl. Catal., B, 2016, 184, 119–131.
31 H. Kato, M. Kobayashi, M. Hara and M. Kakihana, Catal. Sci.
Technol., 2013, 3, 1733–1738.
32 B. Wang, S. H. Shen and L. J. Guo, ChemCatChem, 2016, 8,
798–804.
33 L. Q. Dong, H. Shi, K. Cheng, Q. Wang, W. J. Weng and
W. Q. Han, Nano Res., 2014, 7, 1311–1318.
34 J. Boltersdorf, L. Sullivan, T. L. Shelton, Z. K. Wu, M. Gray,
B. Zoellner, F. E. Osterloh and P. A. Maggard, J. Mater.
Chem. A, 2016, 28, 8876–8889.
6 Y. C. Liao, C. S. Xie, Y. Liu, H. Chen, H. Y. Li and J. Wu,
Ceram. Int., 2012, 38, 4437–4444.
7 H. L. Sun, J. Li, G. K. Zhang and N. Li, J. Mol. Catal. A: Chem.,
2016, 424, 311–322.
8 L. Wang, Y. Yang, Y. J. Zhang, Q. Rui, B. B. Zhang, Z. Q. Shen
and Y. P. Bi, J. Mater. Chem. A, 2017, 5, 17056–17063.
9 Y. Ma, Y. Jia, L. Wang, M. Yang and Y. P. Bi, Chem.–Eur. J.,
2016, 22, 5844–5848.
10 J. R. Ran, J. Zhang, J. G. Yu, M. Jaroniec and S. Z. Qiao, Chem. 35 Z. B. Chen, T. F. Jaramillo, T. G. Deutsch, A. Kleiman-
Soc. Rev., 2014, 43, 7787–7812.
11 M. Y. Wang, J. Loccozia, L. Sun, C. J. Lin and Z. Q. Lin, Energy
Environ. Sci., 2014, 7, 2182–2202.
Shwarsctein, A. J. Forman, N. Gaillard, R. Garland,
K. Takanabe, C. Heske, M. Sunkara, E. W. McFarland,
K. Domen, E. L. Miller, J. A. Turner and H. N. Dinh,
J. Mater. Res., 2010, 25, 3–16.
12 H. J. Yu, R. Shi, Y. X. Zhao, T. Bian, Y. F. Zhao, C. Zhou,
G. I. N. Waterhouse, L. Z. Wu, C. H. Tung and T. R. Zhang, 36 X. Y. Wu, M. M. Li, J. Li, G. K. Zhang and S. Yin, Appl. Catal.,
Adv. Mater., 2017, 29, 1605148. B, 2017, 219, 132–141.
13 S. C. Wang, P. Chen, J. H. Yun, Y. X. Hu and L. Z. Wang, 37 H. R. Zhang, G. S. Miao, X. P. Ma and B. Wang, Int. J.
Angew. Chem., Int. Ed., 2017, 56, 8500–8504.
Photoenergy, 2013, 2013, 413507.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
Journal of Materials Chemistry A
38 S. Y. Yu, R. D. Webster, Y. Zhou and X. L. Yan, Catal. Sci. 42 X. Y. Wu, K. K. Zhang, G. K. Zhang and S. Yin, Chem. Eng. J.,
Technol., 2017, 7, 2050–2056. 2017, 325, 59–70.
39 Y. Hu, D. Z. Li, F. Q. Sun, H. B. Wang, Y. Q. Weng, W. Xiong 43 J. Li, X. Y. Wu, W. F. Pan, G. K. Zhang and H. Chen, Angew.
and Y. Shao, RSC Adv., 2015, 5, 54882–54889. Chem., Int. Ed., 2017, DOI: 10.1002/anie.201708709.
40 T. Kimijima, K. Kanie, M. Nakaya and A. Muramatsu, Appl. 44 G. Gong, Y. H. Liu, B. D. Mao, L. L. Tan, Y. L. Yang and
Catal., B, 2017, 144, 462–467. W. D. Shi, Appl. Catal., B, 2017, 216, 11–19.
41 X. D. Meng, G. K. Zhang and N. Li, Chem. Eng. J., 2017, 314, 45 T. Torimoto, T. Kameyama and S. Kuwabata, J. Phys. Chem.
249–256.
Lett., 2014, 5, 336–347.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A