Site Sensitivity of Interfacial Charge Transfer and Photocatalytic Efficiency in Photocatalysis: Methanol Oxidation on Anatase TiO2 Nanocrystals

Article abstract of DOI:10.1002/anie.202014037

Photocatalytic oxidation of methanol on various anatase TiO₂ nanocrystals was studied by in situ and time-resolved characterizations and DFT calculations. Surface site and resulting surface adsorbates affect the surface band bending/bulk-to-surface charge migration processes and interfacial electronic structure/interfacial charge transfer processes. TiO₂ nanocrystals predominantly enclosed by the {001} facets expose a high density of reactive fourfold-coordinated Ti sites (Ti_4c) at which CH₃OH molecules dissociate to form the CH₃O adsorbate (CH₃O(a)_Ti4c). CH₃O(a)_Ti4c localized density of states are almost at the valence band maximum of TiO₂ surface, facilitating the interfacial hole transfer process; CH₃O(a)_Ti4c with a high coverage promotes upward surface band bending, facilitating bulk-to-surface hole migration. CH₃O(a)_Ti4c exhibits the highest photocatalytic oxidation rate constant. TiO₂ nanocrystals enclosed by the {001} facets are most active in photocatalytic methanol oxidation.

Full text of DOI:10.1002/anie.202014037

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Accepted Article
Title: Site Sensitivity of Interfacial Charge Transfer and Photocatalytic
Efficiency in Photocatalysis
Authors: Cong Fu, Fei Li, Jiachen Zhang, Dan Li, Kun Qian, Yong Liu,
Junwang Tang, Fengtao Fan, Qun Zhang, Xue-Qing Gong,
and Weixin Huang
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Angewandte Chemie International Edition
10.1002/anie.202014037
RESEARCH ARTICLE
Site Sensitivity of Interfacial Charge Transfer and Photocatalytic
Efficiency in Photocatalysis: Methanol Oxidation on Anatase TiO
Nanocrystals
2
+
+
+
Cong Fu, Fei Li, Jiachen Zhang, Dan Li, Kun Qian, Yong Liu, Junwang Tang, Fengtao Fan, Qun
Zhang,* Xue-Qing Gong,* Weixin Huang*
Abstract: Understanding structural effects and reaction mechanisms
of photocatalysts is of great interest and importance. Herein we report
a comprehensive study of photocatalytic oxidation of methanol on
unambiguously exemplify the surface structure effect in
photocatalysis and elucidate the underlying mechanism.
2
various anatase TiO nanocrystals by means of combined in situ and
time-resolved characterizations and density functional theory
calculations. Surface site and resulting surface adsorbates were
observed to strongly affect the surface band bending/bulk-to-surface
charge migration processes and the interfacial electronic
Introduction
Since the pioneering work on photocatalysis by Honda and
Fujishima in 1972,[1] semiconductor-based photocatalysis has
attracted extensive attention due to the utilization of sustainable
solar energy for chemical fuel production, environmental remedy
and chemical synthesis.[2] A photocatalytic reaction consists of
light absorption and charge generation within photocatalysts,
charge separation and migration to photocatalyst surfaces, and
charge-participated reactions on photocatalyst surfaces (Scheme
structure/interfacial charge transfer processes. TiO
predominantly enclosed by the {001} facets expose a high density of
reactive fourfold-coordinated Ti sites (Ti4c) at which CH OH molecules
dissociate to form the CH O adsorbate (CH O(a)Ti4c). CH
exhibits the localized density of states almost at the valence band
maximum of TiO surface, which facilitates the interfacial hole transfer
process; meanwhile, CH high coverage greatly
2
nanocrystals
3
3
3
3
O(a)Ti4c
2
3
O(a)Ti4c with a
1A). The first two steps mainly occur in the bulk of photocatalysts,
promotes the upward surface band bending, which facilitates the bulk-
to-surface hole migration process. Consequently, among all the
and significant progresses have been achieved in maximizing
their efficiencies via structural optimizations.[3] The last step
involves an interfacial charge transfer process from photocatalyst
surfaces to surface adsorbates and subsequent surface reactions.
Thus it is the rate-limiting step of photocatalytic reactions,
however, it has been much less studied due to the complexity.
observed surface adsorbates, CH
3
O(a)Ti4c exhibits the highest
photocatalytic oxidation rate constant. Consequently, TiO
2
nanocrystals predominantly enclosed by the {001} facets are most
active in photocatalytic methanol oxidation reaction. These results
[*]
C. Fu,[+] D. Li, K. Qian, Prof. W. Huang
Hefei National Laboratory for Physical Sciences at the Microscale,
Key Laboratory of Surface and Interface Chemistry and Energy
Catalysis of Anhui Higher Education Institutes, CAS Key Laboratory
of Materials for Energy Conversion and Department of Chemical
Physics, University of Science and Technology of China
Heifei 230026 (P. R. China)
E-mail: huangwx@ustc.edu.cn
[
+]
F. Li, Prof. X. Gong
Key Laboratory for Advanced Materials, Centre for Computational
Chemistry and Research Institute of Industrial Catalysis, East China
University of Science and Technology,
Shanghai 200237 (P. R. China)
E-mail: xgong@ecust.edu.cn
[
+]
J. Zhang, Prof. Q. Zhang
Hefei National Laboratory for Physical Sciences at the Microscale,
Department of Chemical Physics, Synergetic Innovation Center of
Quantum Information and Quantum Physics, University of Science
and Technology of China
Scheme 1. Schematic Illustration of (A) fundamental processes of surface
photocatalysis and (B) adsorbate-dependent methanol/TiO
transfer processes. “FHT” and “RHT” mean the TiO -to-methanol forward hole
transfer and methanol-to-TiO reverse hole transfer, respectively.
2
interfacial hole
2
2
Heifei 230026 (P. R. China)
E-mail: qunzh@ustc.edu.cn
Y. Liu, Prof. F. Fan
3 2
Photocatalytic conversions of CH OH on TiO are prominent in
environmental photocatalytic reactions, photocatalytic selective
oxidation and photocatalytic reforming reactions, and are used as
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences
Dalian 116023 (P. R. China)
Prof. J. Tang
a
hole scavenging reaction to enhance the efficiency of
Department of Chemical Engineering, University College London
London, WC1E 7JE (UK)
Prof. F. Fan, Prof. W. Huang
Dalian National Laboratory for Clean Energy
Dalian 116023 (P. R. China)
These authors contributed equally to this work.
[
4]
photocatalytic water splitting to hydrogen. Meanwhile, they are
extensively adopted as the probe photocatalytic reaction for
fundamental studies of complex photocatalytic reactions on oxide
photocatalysts. Therefore, adsorption and photochemistry of
[+
]
3 2
CH OH has been much explored using well-defined TiO single
Supporting information for this article is given via a link at the end of
the document.
1
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Angewandte Chemie International Edition
10.1002/anie.202014037
RESEARCH ARTICLE
crystals and nanocrystals (NCs) as model photocatalysts.[5]
The surface site and corresponding adsorbed methanol species
CH
3
OH mainly adsorbs molecularly at the fivefold-coordinated
surfaces (CH OH(a)Ti5c), and dissociates at
2 3
are successfully demonstrated to determine the TiO -to-CH OH
Ti5c sites on the TiO
the surface oxygen vacancy sites, subsurface interstitial Ti -
related sites and fourfold-coordinated Ti4c sites to form the
2
3
interfacial charge transfer process and subsequently the
photocatalytic efficiency, providing compelling evidence for the
surface structural effect of photocatalysts on the photocatalysis.
3+
methoxy species (CH
argued whether the CH
species at the Ti5c site (CH
3
O(a)).[6] On rutile TiO
OH(a)Ti5c dissociation to the CH
2
(110) surface, it is
O(a)
3
3
3
O(a)Ti5c) on TiO
2
surfaces is induced
Results and Discussion
only thermally [7] or can be photocatalytically. The CH
[8]
O(a)Ti5c
3
species is photocatalytic active to undergo the C-H bond breaking
to form the HCHO(a) species;[7-9] moreover, the photo-induced
Three types of anatase TiO
100} and {101} facets, respectively denoted as TiO
2
NCs primarily enclosed with {001},
-{001}, TiO
-{101}, were synthesized following previous
and characterized microscopically and
{
{
2
2
-
oxidative coupling reaction between co-adsorbed CH
HCHO(a) at the Ti5c sites were observed to produce methyl
formate (HCOOCH O(a)Ti5C is more photoreactive
).[10] The CH
(110) surface than on rutile TiO
3
O(a) and
100} and TiO
recipes
2
[18]
3
3
spectroscopically (Fig. 1 a-c, Supplementary Figs. 1-3). All
samples exhibit a pure anatase phase, uniform morphologies,
similar average sizes of around 12 nm, and similar specific
on rutile TiO
surface.[
2
2
(011)-(2×1)
2
TiO (001)-(14) surface,
10d,10e]
On
anatase
photocatalytic oxidation was observed to occur specifically for the
CH O(a) species at the Ti4c site (CH O(a)Ti4c) without the further
formation of HCOOCH
.[11] Photo-oxidation of methanol on
nanocrystalline anatase TiO showed that bipyramidal
2
surface areas of 100 m /g. The percentages of {001} facets in
3
3
TiO
-{001}, {100} facets in TiO
2
-{100}, and {101} facets in TiO -
2
2
[18]
3
{
101} NCs were estimated to be around 80%.
Commonly
and H
adsorption, such as carbonates, hydroxyl and water, are present
2
observed adsorbates resulting from ambient CO
2
O
2
nanoparticles exposing primarily {101} facets were more active
than platelets exposing primarily {001} surfaces, whereas the
platelets were more active for thermally-driven reactions.[12]
Efficient photocatalytic conversions of methanol molecules over
-
2-
on the surfaces of TiO
and Cl ) used during the synthesis were not detected.
2
NCs, but the capping ligands (F , SO
4
-
2 2
nanoparticles were found to require the presence of O
,[13,14]
TiO
which acts to promote methanol photodecomposition to form
CH O(a), scavenge free electrons and open acceptor sites for the
3
3
injection of new electrons during CH O(a) oxidation, and interact
with surface intermediates.
The TiO
CH OH/TiO
oxidation. The hole transfer dynamics from photoexcited
nanocrystalline TiO film to adsorbed alcohols was measured by
2
-to-methanol interfacial hole transfer process at the
3
2
interfaces plays a key role in photocatalytic methanol
2
highly sensitive femtosecond and nanosecond spectroscopy
under low intensity excitation conditions to avoid fast electron-
hole recombination, from which the rates and yields of
2
photocatalytic oxidation of the studied alcohols on TiO were
evaluated.[15] By measuring the kinetic competition of holes
between recombination with electrons and reaction with methanol,
it was concluded that the lower photocatalytic activity of rutile TiO
than anatase TiO was not due to differences in recombination but
2
2
rather to the deficiency of holes to drive efficient and irreversible
alcohol oxidation, likely related to surface structure and reactive
[
16]
surface species. Theoretical calculation results of photoexcited
hole dynamics at the CH OH/rutile TiO (110) interface suggested
that only the CH O(a)Ti5c species can scavenge the photoexcited
3
2
3
Figure 1. Representative TEM images with inserted HRTEM images and
holes due to its favorable energy level alignment at the
interface.[
Thus, the interfacial hole transfer and photochemistry of
schematic morphology illustrations of (a) TiO
{101} NCs. (d) DRIFTS spectra following CH OH adsorption on various TiO
2
-{001}, (b) TiO
2
-{100} and (c) TiO
2
-
17]
3
2
NCs at 313 K, with DRIFTS spectra of corresponding bare TiO
background spectra. TPD spectra following CH OH adsorption at 353 K with a
heating rate of 10 K/min on (e) TiO -{001}, (f) TiO -{100} and (g) TiO -{101} NCs.
2
NCs as the
3
3 2 2
CH OH on TiO depend on the TiO surface structure and the
2
2
2
adsorbed methanol species, but the underlying mechanism and
the correlation with the photocatalytic efficiency have not been
established. Herein, we comprehensively studied photochemistry
Adsorption and surface reaction of CH
the surface structures of as-synthesized TiO
Figure 1d, CH OH adsorption at 313 K gives rise to the vibrational
3
OH were used to probe
3 2
of CH OH oxidation on various TiO NCs and influences of water
2
NCs. As shown in
coadsorption by means of in situ and time-resolved diffuse-
reflectance infrared spectroscopy (DRIFTS), in situ electron
paramagnetic resonance spectroscopy (ESR), in situ surface
3
bands of CH
3
OH(a) and CH
3
O(a) at 2850/2950 and 2820/2920
OH(a) on TiO
-{101}, while the
O(a) differ greatly and follow an
-{100} > TiO -{101}. Thus, TiO -{001}
O(a):CH OH(a) ratio among the studied
-1
[19]
cm , respectively. The vibrational peak of CH
3
2
-
photovoltage
(SPV)
measurements,
time-resolved
{
100} is stronger than on TiO
vibrational peak intensities of CH
order of TiO -{001} >> TiO
exhibits the largest CH
2 2
-{001} and TiO
photoluminescence spectroscopy (PL) and in situ femtosecond
time-resolved transient absorption (fs-TA) spectroscopy, in
combination with density functional theory (DFT) calculations.
3
2
2
2
2
3
3
2
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Angewandte Chemie International Edition
10.1002/anie.202014037
RESEARCH ARTICLE
TiO
vibrational bands of OH groups on TiO
greatly upon CH OH adsorption (Supplementary Fig. 4),
demonstrating that CH
groups originally on TiO
Since the formation of CH
is accompanied by the OH formation, the loss of OH vibrational
bands follows an order of TiO -{001} < TiO -{100} < TiO -{101},
inverse to that of CH O(a) formation.
TiO NCs exclusively with CH O(a) species were prepared by
OH adsorption at 353 K and the reactivity of CH O(a) was
studied with TPD spectra (Fig. 1 e–g). A water desorption peak at
50 K appears for all TiO NCs and can be attributed to the
recombinative desorption of OH groups. An additional water
desorption peak at 425 K arises only for TiO -{001} and can be
attributed to the desorption of H O(a) at the Ti sites. With the
temperature increasing, the CH O(a) species undergoes bi-
molecular dehydration coupling reaction to produce CH OCH , bi-
molecular disproportionation reaction to simultaneously produce
CH OH and HCHO, unimolecular dehydrogenation reaction to
produce CO and H , and unimolecular deoxy-hydrogenation
reaction to produce CH . The amounts of total detected products
from CH O(a)/TiO NCs decrease from CH O(a)/TiO -{001} to
CH O(a)/TiO -{100} and further to CH O(a)/TiO -{101}, consistent
with the order of the CH O(a) coverage on TiO NCs. Meanwhile,
the bi-molecular reaction probability of CH O(a) species also
decreases with the coverage of CH O(a) species on TiO NCs.
The CH O(a) species on TiO -{100} undergo all bi-molecular and
unimolecular reactions, but the probability of bi-molecular
reactions is much smaller than that on TiO -{001}. The CH O(a)
species on TiO -{101} only undergo the unimolecular reactions
but not the bi-molecular reactions. Similar results were previously
reported on surface reactions of methanol on the TiO -{001} and
TiO
-{101} nanocrystals.[12b,21]
Adsorption and surface reaction of CH
single crystal surfaces have been well established.[5] The
observed reactivity of CH O(a) species at TiO -{001} are very
similar to those on the restructured anatase TiO (001)-(14)
2
nanocrystals while TiO
2
-{101} exhibits the smallest. The
site of perfect TiO
energy (1.10 eV) of CH
preferential molecular adsorption at the Ti5c sites, while CH
2
(101) is larger than the dissociative adsorption
2
NCs were found to reduce
3
OH (CH
3
O(a)Ti5c), suggesting
a
3
3
OH
3
OH adsorption can displace the OH
preferentially dissociates at the minor surface oxygen vacancy
site (CH O(a)Ov) and molecularly adsorbs at the minor Ti5c site
next to the surface oxygen vacancy sites (CH
NCs, consistent with previous reports.[
20]
2
3
3
O(a) species upon CH
3
OH dissociation
3
OH(a)Ti5c(Ov)),
respectively with adsorption energies of 2.48 and 1.05 eV (Figure
2 and Supplementary Fig. 6). The adsorption energies of
2
2
2
3
CH
(12) are almost identical, respectively being 1.30 and 1.26 eV,
while the dissociative (CH and molecular
(CH OH(a)Ti(int)) adsorption of CH OH at the minor Ti5c site
3 3 2
O(a)Ti5c and CH OH(a)Ti5c at the Ti5c site of perfect TiO (100)-
2
3
CH
3
3
3
O(a)Ti(int))
3
3
3
+
5
2
neighbouring to the subsurface interstitial Ti site give adsorption
energies respectively of 1.64 and 1.33 eV (Figure 2 and
Supplementary Fig. 7), suggesting a preferential dissociation.
Similar subsurface interstitial Ti3 site-induced dissociation of
2
+
2
3
CH
3
2
OH was previously reported on the rutile TiO (110) single
crystal surface.[
various TiO
6b]
Considering the densities of the sites on
3
favoring the CH OH dissociative adsorption and the
3
3
2
3
associated differences between molecular and dissociative
adsorption energies, the DFT calculation results suggest that the
2
4
density of CH
and least on TiO
results. They also indicate that the CH
TiO -{100} than on TiO -{101} but is few on TiO
2
3
O(a) species is largest on TiO
(101) surface, consistent with the above DRIFTS
O(a)Ti5c species is more on
-{001}.
2
(001)-(14) surface
3
2
3
2
2
3
2
3
2
3
3
2
2
2
3
3
2
3
2
2
3
2
2
2
3
OH on various TiO
2
3
2
2
Figure 2. Calculated adsorption energies and PDOS levels relative to the
valence band maximum of various types of methanol adsorbates on anatase
TiO surfaces.
2
single crystal surface exposing the reactive Ti4c sites at the (14)
added row and the Ti5c sites at the (11) basal surface
(
Supplementary Fig. 5).[6c] The (14) surface restructuring of {001}
1
7
2
facets at TiO -{001} was previously confirmed by O solid-state
[
22]
NMR spectroscopy
and environmental transmission electron
O(a) species on
(101) single
crystal surface with minor surface oxygen vacancies
The evolutions of CH
upon UV light illumination were studied using in situ DRIFTS
spectra. In the absence of O , a large increase in the absorbance
3 3 2
OH(a) and CH O(a) at various TiO NCs
microscopy.[ The observed reactivity of CH
23]
3
2 2
TiO -{101} are similar to those on the anatase TiO
2
across the entire spectrum arising from the absorption of
[
24]
(
Supplementary Fig. 6). No studies of CH
TiO (100) single crystal surface was reported, but STM and XPS
characterization results show that the anatase TiO (100) single
3
OH on the anatase
[26]
photoexcited electrons
not for bare TiO
indicating much more abundant photoexcited electrons created in
TiO -{001} than in TiO -{100} and TiO -{101}. Upon CH OH
adsorption, the density of photoexcited electrons greatly
increased for all TiO NCs, due to the efficient trapping of
2
was observed for bare TiO -{001} but
2
2
-{100} and TiO -{101} (Supplementary Fig. 8),
2
2
crystal surface exhibits a typical (12) restructured surface with
the presence of interstitial Ti3+ defects and the absence of surface
oxygen vacancy defects (Supplementary Fig. 7).[25] DFT
2
2
2
3
2
calculations were performed for CH
anatase TiO (001)-(14) surface, TiO
without surface oxygen vacancies, and TiO
without subsurface interstitial Ti3+ defects. CH
dissociatively at the Ti4c site of anatase TiO
CH with an adsorption energy of 2.82 eV and
molecularly at the Ti5c site (CH OH(a)Ti5c) with an adsorption
energy of 0.66 eV (Figure 2 and Supplementary Fig. 5). The
molecular adsorption energy (1.27 eV) of CH OH(a)Ti5c at the Ti5c
3
OH adsorption on the
(101) surfaces with and
(100)-(12) with and
OH adsorbs
(001)-(14)
photoexcited holes by the adsorbed methanol species and the
subsequent enhanced accumulation of photoexcited electrons.
2
2
2
However, no obvious conversions of adsorbed CH
CH OH(a) species could be observed (Supplementary Fig. 9),
consistent with previous results
photoexcited electrons could block acceptor sites on TiO
3
O(a) and
3
3
2
[13]
that the accumulated
surface
(
3
O(a)Ti4c)
2
3
for the injection of new electrons required by the photocatalytic
oxidation of adsorbed methanol species.
3
3
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Angewandte Chemie International Edition
10.1002/anie.202014037
RESEARCH ARTICLE
With the introduction of
dissociation to CH
O(a) [27] was observed to occur only slightly for
all TiO NCs under the dark condition (Supplementary Fig. 10).
Upon UV light illumination, the absorption feature of photoexcited
electrons greatly decreased with the O concentration increasing,
and became similar for all TiO NCs at an oxygen concentration
of 10% (Supplementary Fig. 11), demonstrating the efficient
scavenging of photoexcited electrons by O . Consequently,
photocatalytic oxidation of adsorbed CH O(a) species to
HCOO(a), HCHO(a) and carbonate species occur on all TiO NCs, previous results on the anatase TiO
light illumination leads to photocatalytic oxidation of the
O
2
,
O
2
-promoted CH
3
OH(a)
features of HCOO(a) species emerge and grow; the vibrational
3
features of both CH
TiO -{101} (Fig. 2b), and the vibrational features of HCHO(a)
species and HCOO(a) species appear and grow; the vibrational
features of CH OH(a) species increase on TiO -{100} NCs (Fig.
2c), the vibrational features of CH O(a) species decrease, and
those of HCOO(a) species emerge and grow.
Therefore, the CH OH(a)Ti5c species is photoinactive on TiO
{001} while the CH O(a)Ti4c species is photoactive, consistent with
(001)-(14) surface that UV
3 3
OH(a) and CH O(a) species decrease on
2
2
2
3
2
2
3
2
3
2
-
3
3
2
2
as evidenced by the appearance of the vibrational features of
various types of HCOO(a) and carbonate species and HCHO(a)
CH
CH
3
O(a)Ti4c species but not of the CH
O(a) and CH OH(a) species are photoactive on TiO
(101) surface
3
OH(a)Ti5c species.[11] Both
-{101},
[
13,28]
species
Supplementary Fig. 12, Supplementary Table 1). The further
photocatalytic oxidation of HCOO(a) species forms gaseous CO
the final product of photocatalytic CH OH oxidation in O
Although the photocatalytic oxidation of methanol produces H
species in the form of OH groups TiO surfaces, the OH groups
at the expense of those of CH
3
O(a) species
3
3
2
(
agreeing with previous results on the anatase TiO
that UV light illumination leads to photocatalytic oxidation of both
2
2
,
.
CH
3
OH(a) and CH
3
2
O(a) species.[ The CH
9b]
3
O(a)Ti5c species is
OH(a) upon
3
2
+
photoactive on TiO
UV light illumination can be attributed to a thermal transformation
-{100}, and the formation of CH
3
2
on during the photocatalytic oxidation of methanol adsorbates
change little. This suggests that the generated H species should
of CH
3
O(a) species to CH
3
OH(a) species induced by an oxidation
+
3+
2
of the subsurface interstitial Ti sites of TiO -{100} NCs by
facilely undergo photoexcited electrons-participated reduction
photoexcited holes. Similar processes were proposed for photo
stimulated desorption of O (a) species related to the subsurface
interstitial Ti3+ sites of rutile TiO (110) single crystal surface.[29]
Meanwhile, the HCOO(a) species appears as the dominant
surface intermediate during photocatalytic CH OH oxidation on
TiO -{001} and TiO -{100} while both HCHO(a) and HCOO(a)
species appear on TiO -{101}, indicating more facile
photocatalytic oxidation of HCHO(a) species on TiO -{001} and
TiO -{100} than on TiO -{101}.
reactions with adsorbed O
amounts of adsorbed O on various TiO
be similar (Supplementary Fig. 13).
2
to produce gaseous water. The
2
2
2
NCs were measured to
2
3
2
2
2
2
2
2
Figure 3. (Top) In-situ and time-resolved DRIFTS spectra of photocatalytic
oxidation of CH
a flow rate of 30 cm /min and (Bottom) coverages of CH
3
O(a) and CH
3
OH(a) at 313 K in a 10% O
2
/Ar atmosphere with
3
3
O(a) and CH OH(a),
3
respectively represented by the integrated intensity of the vibrational features at
Figure 4. Rate constants of CH
various TiO NCs at 313 K in a 10% O
cm /min as a function of initial CH O(a)/CH
3
3
O(a) and CH
/Ar atmosphere with a flow rate of 30
OH(a) coverage ratios.
3
OH(a) photocatalytic oxidation on
-
1
2
820 and 2950 cm , as a function of photocatalytic oxidation time: (a) TiO
001}, (b) TiO -{101} and (c) TiO -{100} NCs. DRIFTS spectra of corresponding
NCs were used as the background spectra.
2
-
2
2
3
{
2
2
3
bare TiO
2
2 3
On TiO -{001}, the CH OH(a) coverage does not change with
The photocatalytic oxidation kinetics of CH
3
OH(a) and
the photocatalytic reaction time, thus the variation of CH O(a)
3
coverage results only from the photocatalytic oxidation reaction.
The Ln (I2820) was found to decrease linearly as a function of
photocatalytic reaction time (Supplementary Fig. 17),
demonstrating that photocatalytic oxidation of CH O(a) on TiO -
CH
3
O(a) species on various TiO NCs in a 10% O /Ar atmosphere
2
2
were further studied using in-situ and time-resolved DRIFTS
spectroscopy. Under this condition the photoexcited charge
carrier density in TiO
participated surface reactions of H and O
2
NCs and the photoexcited electrons-
3
2
+
2
are similar for various
{001} follows the first-order reaction kinetics with a rate constant
-1
TiO
Figs. 14-16), and the derived integrated peak areas of the
2
NCs. The acquired spectra were peak-fitted (Supplementary
of 0.02140.0004 min . The photocatalytic oxidation kinetics of
CH O(a) on TiO -{100} and CH O(a) and CH OH(a) on TiO -{101}
3
2
3
3
2
-
1
features at 2950 and 2820 cm , respectively representing the
surface coverages of CH OH(a) and CH O(a), were plotted as a
function of photocatalytic oxidation time. With the UV light
illumination prolonging, the vibrational features of CH OH(a)
species barely change on TiO -{001} (Fig. 2a), the vibrational
features of CH O(a) species keep decreasing, and the vibrational
were analyzed similarly (Supplementary Figs. 18 and 19), found
to follow first-order reaction kinetics, yielding a rate constant of
photocatalytic CH O(a) oxidation on TiO -{100} as 0.1560.0004
3
3
3
2
min-1 and rate constants of photocatalytic CH O(a) and CH OH(a)
3
3 3
2
oxidation on TiO -{101} respectively as 0.01390.0002 and
2
3
4
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
-1
0
.00550.0006 min . Thus, the rate constants of photocatalytic
TiO
2
-{101} NCs, which, consistent with the above TA results,
CH
3
O(a) oxidation follow an order of CH
O(a) /TiO -{100} > CH O(a) /TiO -{101}, which, as shown in
O(a):CH OH(a)
NCs. This suggests that the TiO NCs with
O(a) coverage and CH O(a):CH OH(a) ratio exhibit a
higher rate constant of photocatalytic CH O(a) oxidation.
Meanwhile, the rate constant of photocatalytic CH O(a) oxidation
on TiO -{101} with dominant and photoactive CH OH(a) species
is still larger than that of photocatalytic CH
agreeing with the case of TiO -{001}, confirms that the CH
species on TiO is intrinsically more photoactive than the
3
O(a)/TiO
2
-{001} >
demonstrates more effective trapping of photoexcited electrons
and holes in TiO -{001} NCs.
The recombination processes of photoexcited electrons and
holes were studied with both steady-state and time-resolved
photoluminescence (PL) spectra. The steady-state PL spectra
(Supplementary Fig. 23) show that all TiO NCs exhibit a major
2
peak at 520 nm with two shoulders at 423 and 635 nm,
attributed to the radiative recombination at oxygen vacancies,
CH
3
2
3
2
2
Fig. 4, was found to correlate well with the CH
ratio on various TiO
larger CH
3
3
2
2
3
3
3
3
3
2
3
OH(a) oxidation, which, lattice imperfections and Ti3+ sites.[32] TiO
-{001} NCs show a
greatly stronger PL peak at 520 nm than other two TiO NCs,
thus TiO -{001} NCs have the largest density of oxygen vacancies
3
2
2
3
O(a)
2
2
2
CH
3
OH(a) species.
The observed first-order photocatalytic oxidation kinetics of
O(a) and CH OH(a) suggests that the photoexcited hole
interfaces are large enough
that can act as the hole trap sites. The time-resolved PL spectra
with an excitation at 368 nm and an emission at 515 nm (Fig. 5 d-
f) demonstrate that the photoexcited electrons and holes within
CH
3
3
concentrations at the methanol/TiO
2
2 2 2
TiO -{001} NCs recombine faster than within TiO -{100} and TiO -
to be considered as constants. Therefore, the photocatalytic
oxidation rate constant strongly depends on the photoexcited hole
{101} NCs.
concentration at the methanol/TiO
2
interface and the TiO
2
-to-
methanol interfacial hole transfer efficiency. Various TiO
2
NCs
were measured to exhibit almost identical UV-Vis diffuse
reflectance spectra and similar valence band photoelectron
spectra (Supplementary Fig. 20), and thus they should exhibit
similar processes of light absorption and charge generation. The
kinetics of photogenerated charges were then characterized with
femtosecond time-resolved transient absorption (fs-TA)
spectroscopy using a pump−probe configuration of a 320 nm
pump and a white-light (360–700 nm) probe (Supplementary Fig.
2
21). The TA signals of photo-excited electrons in TiO were
identified in the near-infrared and infrared spectral regions, while
those of photoexcited holes in the UV and near-visible regions (up
to 500 nm).[ Although the TA signals of photoexcited electrons
30]
and holes in TiO
2
overlap in the visible region, the observed
broad, negative profile in the range of about 360-550 nm can be
mainly ascribed to photo-excited hole absorptions, while the
observed broad, positive profile above 550 nm to the overlapping
photo-excited electron and hole absorptions in our case. The
kinetic profiles of photoexcited hole absorptions within the region
2 2
Figure 5. Representative TA kinetics of (a) TiO -{001}, (b) TiO -{101} and (c)
TiO
wavelength from 380 to 410 nm. Time-resolved PL kinetics of (d) TiO
TiO -{101} and (f) TiO -{100} NCs in air recorded with excitation at 368 nm and
emission at 515 nm.
2
-{100} NCs in air taken at the pump wavelength of 320 nm and the probing
2
-{001}, (e)
2
2
of 380-410 nm were analyzed for various TiO
c). Using a global fitting procedure, the acquired TA kinetic
spectra of all TiO NCs can be fitted by three components with the
2
NCs (Figure 5 a-
2
2
Thus, TiO -{001} NCs with the largest density of oxygen
characteristic time constants included in Figure 5 a-c. The first two
components are related to the relaxation processes of the
photoexcited holes from the valence-band maximum to two trap
states, probably with different trap depths.[31] Indicated either by
individual kinetic parameters of three components or by the
vacancies as the hole trap sites exhibit the fastest relaxation
processes of photoexcited holes and meanwhile the fastest
recombination process of photoexcited electrons and holes.
These observations suggest that the defects in TiO facilitate not
2
only the charge separation processes but also the charge
recombination processes. Different bulk charge carrier dynamics
in anatase and rutile TiO single crystals were observed to
2
demonstrate bulk defects-resulted fast recombination rates
through trap-state-assisted channel of charge recombination and
2
to explain the substantially higher activity of anatase TiO in
photocatalytic CO oxidation.[34] A recent pulsed-laser-combined
scanning tunneling microscopy and spectroscopy study of rutile
average kinetic parameter, TiO
relaxation processes of photoexcited holes than other two TiO
NCs, which suggest the presence of more hole trap states in TiO
001} NCs.
TiO NCs upon UV light illumination were characterized with
ESR spectroscopy (Supplementary Fig. 22). TiO
2
-{001} NCs exhibit much faster
2
2
-
{
2
2
-{001} mainly
exhibit the signals of F1+ color centers and adsorbed O
while TiO -{100} and TiO
-
species,
2
1+
2
2
-{101} mainly exhibit the signals of F
2
TiO (110) surface also proposed that the lifetime of photoexcited
[
32]
color centers, consistent with previous reports. Upon UV light
illumination, the photoexcited electrons trapped in the bulk and on
free electrons was correlated with the averaged defect density
within a nanometer-sized area.[35]
The separated photoexcited charges need to migrate from the
bulk of a photocatalyst particle to the surface to participate the
surface redox reactions, which is strongly affected by the surface
band bending of the photocatalyst.[36] We measured surface
the surface of TiO
the ability of TiO
2
NCs give rise to the Ti3+ signal and enhance
to chemisorb O in the ambient condition to form
2
2
-
the O
2
species, respectively; meanwhile, the trapped holes
-
[33]
3+
-
-
generate the O species. The resulting Ti , O
2
and O signals
-{100} and
are much stronger for TiO -{001} NCs than for TiO
2
2
2 2
photovoltage (SPV) signals of TiO NCs and methanol/TiO NCs
5
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
generated upon UV light illumination (Fig. 6 and Supplementary
CH
3
OH(a) at both subsurface interstitial Ti3+ and surface Ti5c sites
Fig. 24). TiO
an upward surface band bending that facilitates the hole migration
from the bulk to the surface. The SPV signals of TiO -{001} and
TiO -{100} are similar and larger than that of TiO -{101}, which
can be attributed to the (14) surface reconstruction of TiO -{001}
and the presence of subsurface interstitial Ti3+ sites in TiO
-{100}.
CH OH adsorption greatly enhances the SPV signals of both
TiO
NCs. CH
2
NCs exhibit the positive SPV signals arising from
and CH
vacancy sites and CH
exhibit. Meanwhile, the  value of RHT process of CH
{101} was fitted as 327± 65 ps, larger than that of CH
{100} (122 ± 15 ps)，consistent with the more extensive upward
surface band bending of CH OH/TiO -{101} than of CH OH/TiO -
2
{100}. These observations unambiguous demonstrate the site
sensitivity of interfacial hole transfer processes at the
3
OH/TiO
2
-{101} with CH
3
O(a) at the surface oxygen
OH(a) at surface Ti5c sites
3
O(a) and CH
3
2
3
OH/TiO
2
-
2
2
3
OH/TiO -
2
2
2
3
2
3
3
2
-{001} and TiO
2
-{101} NCs but reduces that of TiO
2
-{100}
surfaces can act as an electron donor,
O(a)
surfaces can act as an electron acceptor, strengthening
the upward surface band bending.[36] Thus CH
OH/TiO -{001}
with dominant CH O(a) species and CH OH/TiO -{101} with more
CH O(a) than CH OH(a) exhibit strengthened upward surface
band bending, while CH OH/TiO -{100} with more CH OH(a) than
CH O(a) exhibits weakened upward surface band bending. The
decreased subsurface interstitial Ti3+ concentration of
CH OH/TiO (100) under UV light illumination (Fig. 3c) can also
3
OH(a) at TiO
2
CH
time scale previously showed efficient, irreversible hole
scavenging by alcohols on mesoporous anatase TiO but
3 2
OH/TiO interface. TA results on the microsecond to second
weakening the upward surface band bending, whereas CH
at TiO
3
2
2
3
2
substantially less efficient and more reversible hole scavenging
by alcohols on mesoporous rutile.[16] It was then proposed that the
lower photocatalytic activity of rutile was not due to differences in
recombination but rather to the deficiency of rutile holes to drive
efficient and irreversible alcohol oxidation.
3
3
2
3
3
3
2
3
3
3
2
weaken the upward surface band bending. As indicated by the
SPV signals, the upward surface band bending extent and the
bulk-to-surface hole migration process follows an order of
3 2 3 2 3 2
CH OH/TiO -{001} > CH OH/TiO -{101} > CH OH/TiO -{100}.
Figure 6. Surface photovoltage (SPV) amplitudes of as-synthesize and
methanol-covered TiO NCs generated upon UV light illumination.
2
3 2 2
The dynamics of photoexcited holes of CH OH/TiO (TiO NCs
dispersed in methanol) were probed with fs-TA spectroscopy
using a pump−probe configuration of a 320 nm pump and a 450
nm probe (Fig. 7 and Supplementary Fig. 25). Similar to the bare
TiO sample, CH OH/TiO -{001} exhibits a monotonic recovery of
2 3 2
probe bleach (PB) with time (Fig. 7a), corresponding to the hole
trapping and recombination processes. In addition to the hole
trapping and recombination processes, both CH
and CH OH/TiO -{101} exhibit a turn-around PB growth beyond
30 ps, demonstrating the occurrence of additional hole transfer
process. CH OH adsorbates on TiO are known to scavenge the
photoexcited holes, therefore, the observed PB growth
superimposing on the relaxation trace of photoexcited holes
corresponds to the photo-induced absorption of a reverse hole
3 2
OH/TiO -{100}
3
2
Figure 7. The PB kinetics with a pump wavelength at 320 nm and a probe

wavelength at 450 nm for (a) CH
CH OH/TiO -{100}. The inset shows expanded kinetic traces in the early-time
window (0–80 ps). The RHT means reverse hole transfer from chemisorbed
methanol to TiO
3 2 3 2
OH /TiO -{001}, (b) CH OH/TiO -{101} and (c)
3
2
3
2
2
.
transfer (RHT) process from the acceptor states of CH
adsorbates to the valence band maximum (VBM) of TiO NCs. A
similar RHT process from adsorbed CH O(a) to the VBM of g-
photocatalyst was previously observed by TA
3
OH
The electronic structures of various TiO
systems were analyzed from the calculated density of states
2 3 2
and CH OH/TiO
2
3
(
(
DOS) of corresponding optimized adsorption configurations
Supplementary Figs. 5-7). The anatase TiO (001)-(1×4) surface
3 4
C N
2
[
37]
spectroscopy.
CH O(a)Ti4c and minor CH
RHT process, while CH
Therefore, CH
OH(a)Ti5c species does not exhibit the
OH/TiO -{100} with CH O(a) and
3 2
OH/TiO -{001} with dominant
shows the VBM and the conduction band minimum (CBM)
respectively localized at the surface oxygen atoms on the added
row and the bulk Ti atoms, while the TiO (100)-(1×2) surfaces
2
3
3
3
2
3
6
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
without and with subsurface interstitial Ti3+ sites and TiO
(101)
2
3 2
site-sensitive adsorbed CH OH species on TiO surfaces strongly
surfaces without and with surface oxygen vacancies show the
VBM and CBM respectively localized at the surface oxygen atoms
affect the hole concentration on the surface and the photocatalytic
efficiency by influencing the surface band bending and bulk-to-
and the bulk Ti atoms. Upon CH
3
OH adsorption, the CBM is still
2
surface hole migration processes of TiO . With photoexcited
localized at the bulk Ti atoms, whereas the VBM involves the
charges-participated surface reactions as the rate-limiting step in
photocatalytic reactions, a quasi equilibrium will establish for the
photoecxited charges in the bulk and on the surface of
photocatalysts, and the dynamics and concentration of
photoexcited charges at the adsorbates/photocatalyst interfaces,
instead of their dynamics and concentration in the bulk,
contributes to the photocatalytic reaction rate.
3
contributions from the CH OH adsorbates, depending on the
adsorption structures.
As shown in Figure 2, the projected density of states (PDOS)
of the CH O(a)Ti4c species on the anatase TiO (001)-(1×4) surface
and the CH O(a)Ti5c species on the anatase TiO (100)-(1×2) and
TiO (101) surfaces are localized almost at the VBM of the
corresponding TiO surfaces, while those of CH OH(a)Ti5c species
and minor CH OH adsorbates (CH O(a) and CH OH(a)) at the
defective sites are localized far below the VBM. From the view of
thermodynamics, this suggests that the CH
CH O(a)Ti5c species have higher probabilities to accept the
photogenerated holes than the CH OH(a)Ti5c species. From the
view of kinetics, the transferred holes can either oxidize the
CH OH(a)Ti5c species or reversely transfer to the valence band of
TiO , whereas the transferred holes can only oxidize the
CH O(a)Ti4c and CH O(a)Ti5c species. These theoretical
calculation results suggest that the TiO -to-
CH O(a)Ti5c interfacial hole transfer is more efficient
than the TiO -to-CH OH(a)Ti5c interfacial hole transfer, consistent
with the observed experimental results. The CH O(a) species on
all TiO NCs are intrinsically more photoactive than the CH OH(a)
species; meanwhile, CH OH/TiO -{100} and CH OH/TiO -{101}
with high coverages of CH OH(a)Ti5c species exhibit the RHT
processes, but on TiO -{001} with dominant CH O(a)Ti4c species,
the minor CH OH(a)Ti5c species barely accepts the photoexcited
holes, subsequently exhibits no RHT process and is photo-
inactive. Therefore, the adsorbed CH OH species on TiO
3
2
3
2
2
2
3
Photocatalytic activity of various TiO
in gas-phase photocatalytic CH OH oxidation in O
UV light illumination (Supplementary Fig. 26). The morphology-
dependent photocatalytic activity were observed, in which TiO
{001} exhibits the highest CO production while TiO -{100} is least.
As shown above, the specific surface areas, the amounts of
adsorbed O and the bulk photoexcited charge carrier density are
similar for various TiO NCs, thus they are not responsible for the
observed different photocatalytic behaviors. The observed
morphology-dependent photocatalytic activity of various TiO
NCs can be reasonably related to the photocatalytic oxidation
rate constants and surface coverages of photoactive CH OH
adsorbates on various TiO NCs. The photoactive CH
species on TiO -{001} NCs exhibits a higher photocatalytic
oxidation rate constant and a larger surface coverage than the
photoactive CH O(a)Ti5c species on TiO -{101} and TiO -{100}
NCs, therefore, TiO -{001} NCs are more photocatalytic active
than TiO -{101} and TiO -{100} NCs. These results demonstrate
2
NCs were evaluated in
3
3
3
3
2
to CO under
2
3
O(a)Ti4c and
2
-
3
2
2
3
2
3
2
2
3
3
2
2
3
O(a)Ti4c/CH
3
3
2
3
2
3
O(a)Ti4c
3
2
2
3
3
2
3
2
3
2
2
3
2
2
3
2
2
3
the key roles of site-dependent surface species and interfacial
charge transfer in photocatalysis.
3
2
surfaces exhibit the site-sensitive electronic structures and
subsequently the interfacial hole transfer processes (Scheme 1B).
These results also suggest that the CH
anatase TiO (100)-(1×2) and TiO (101) surfaces are photoactive
and responsible for the observed photocatalytic oxidation of
CH O(a) species respectively on anatase TiO -{101} and TiO
100}, whereas the minor CH O(a) species at the defective sites
of anatase TiO (101) and TiO (100)-(1×2) surfaces are not.
Similar theoretical calculation results were previously reported for
the CH OH/rutile TiO
(110) system.[17] This is different from the
3
O(a)Ti5c species on
2
2
3
2
2
-
{
3
2
2
3
2
Figure 8. (a) DRIFTS spectra following CH
precovered TiO -{100} NCs at 313 K, with DRIFTS spectra of corresponding
bare TiO NCs as the background spectra. (b) DRIFTS spectra after CH OH
adsorption on H -{100} NCs at 313 K followed by
UV light illumination, with corresponding DRIFTS spectra prior to UV light
illumination as the background spectra. (c) DRIFTS spectra after CH OH
-{100} NCs at 313 K followed by
)/Ar atmosphere with a flow rate of
3
OH adsorption on bare and water-
case in heterogeneous catalysis where minor defective sites
usually contribute much to catalytic activity. Consequently, the
2
2
3
experimental observations of photoactive CH
TiO -{101} and TiO -{100} NCs demonstrate the occurrence of
CH OH dissociation at the surface Ti5c sites of anatase TiO
1×2) and TiO (101) surfaces.
Although the CH O(a)Ti4c species on the anatase TiO
1×4) surface and the CH
(100)-(1×2) and TiO (101) surfaces exhibit the PDOS
localized similarly at the VBM of corresponding TiO surfaces,
CH O(a)Ti4c/TiO -{001} exhibits the largest photocatalytic
oxidation rate constant (Fig. 4). All types of anatase TiO NCs
3
O(a)Ti5c species on
16
18
2
O (H O)-precovered TiO
2 2
2
2
3
3
2
(100)-
(001)-
16 18
2 2 2
O (H O)-precovered TiO
adsorption on H
UV light illumination under a 10%
(
(
2
16
18
O ( O
2 2
3
3
2
30 cm /min, with corresponding DRIFTS spectra prior to UV light illumination as
O(a)Ti5c species on the anatase
the background spectra.
3
TiO
2
2
2
Water was reported to affect the methanol photocatalysis over
3
2
TiO
adsorption of reactants,[4,39] forming OH• radicals and/or
additional surface-trapped holes,[40]
and decelerating
recombinative decay of holes.[41] Influences of water on
photochemistry of CH OH on various TiO NCs were examined.
Exposure of water at 313 K on TiO NCs forms molecularly-
adsorbed H O(a) and various types of surface OH groups, whose
intensities reduce greatly upon subsequent CH OH exposure,
2
photocatalysts in various aspects,[4,14,38], such as influencing
2
exhibit similar charge generation and separation efficiencies upon
UV light illumination, thus this can be attributed, on one hand, to
the most extensive upward surface band bending and subsequent
,
the largest hole concentration on the TiO
2
surface for
-{001}, on the other hand, to the fact that the minor
OH(a) species barely competes with the dominant
O(a)Ti4c species for the holes on TiO -{001}. Therefore, the
3
2
CH
CH
CH
3
3
3
OH/TiO
2
2
2
2
3
7
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Angewandte Chemie International Edition
10.1002/anie.202014037
RESEARCH ARTICLE
resulting from displacement of adsorbed H
2
O(a) and OH groups
emerges as the surface intermediate for photooxidation of
by CH
features of molecularly-adsorbed H
evacuation. The DRIFTS spectra of CH
and H O-precovered TiO NCs (Fig. 8a and Supplementary Fig.
8) show less formations of CH O(a) and CH OH(a) and less
consumptions of original OH groups on H O-precovered TiO NCs, presence of
indicating that pre-adsorption of water on TiO NCs inhibits the photooxidation of CH
can be related to the isotope effect, including decreased coverage
of OH groups on TiO O dissociation, comparing by H
by H 18 16O
dissociation (Supplementary Fig. 34). No HCO O species could
3
OH adsorption (Supplementary Fig. 28). The vibrational
O(a) disappears upon further
OH adsorption on bare
CH
photooxidation of CH
observed for photooxidation of CH
presence of O , implying the involvement of surface OH groups in
photooxidation of adsorbed methanol species even in the
However, this species disappears for
OH/H in the presence of O , which
18O/TiO
3
OH/H
2
O/TiO
2
in the absence of
O
2
but not for for
, was
3 2 2
OH/H O/TiO in the
2
3
OH/TiO in the presence of O
2
2
1
6
3
2
2
2
2
3
3
2
2
2
O .
2
3
2
2
2
subsequent adsorption of methanol by blocking the surface sites.
This leads to weaker intensities of photoexcited electrons in the
2
2
2
1
8
in situ DRIFTS spectra of CH
3
OH adsorption on H
NCs upon UV light illumination than on corresponding bare
NCs, due to the decreased coverages of adsorbed methanol
2
O-precovered
1
8
TiO
TiO
2
be observed for photooxidation of CH
presence of 16O
Moreover, for photooxidation of
CH O/TiO , only the COO stretch
OH/H 16 in the presence of 18O
vibration features of HCO O were observed, but those of the
3 2 2
OH/H O/TiO in the
2
2
.
species that can efficiently trap photoexcited holes
Supplementary Fig. 29). In the presence of O , the intensity of
photoexcited electrons in the in situ DRIFTS spectra of CH OH
adsorption on H O-precovered TiO -{001} NCs upon UV light
illumination is stronger than on bare TiO -{001} NCs. Thus, the
pre-adsorption of water also inhibits the subsequent adsorption of
that acts as efficient scavenger of photoexcited electrons.
Meanwhile, the SPV measurements demonstrate less extensive
upward surface band bending for CH OH adsorption on H O-
precovered TiO NCs than on corresponding bare TiO NCs
Supplementary Fig. 30), resulting from the samller
CH O(a):CH OH ratios for CH OH adsorption on H O-precovered
TiO NCs. All these results demonstrate the adverse effects of
pre-adsorbed water on the surface photochemistry of methanol
on TiO NCs.
Despite the above-observed adverse effects, we observed the
photooxidation of methanol species on water-precovered TiO
NCs in the absence of O , evidenced by the appearances of the
vibrational features of HCOO, HCO, OH and H O species at the
expense of those of adsorbed methanol species in the DRIFTS
3
2
2
2
18
(
2
1
6
3
corresponding HCO O species were not. Therefore, in the
presence of O , surface OH groups on TiO can still act as
2
2
2
2
2
scavenger of photoexcited electrons to facilitate photooxidation of
adsorbed methanol species, but the resulting OH• radicals
seldom contribute to the formation of surface formate
intermediates due to the presence of much more reactive O
radicals. It was also proposed that surface OH groups can open
O
2
2
•
3
2
2
2
up a pathway of protonation in O reduction to form •OOH
radicals,[ a species more reactive than the O
• species in
2
2
42]
(
promoting the HCOO formation.[43] Similar to photooxidation of
CH OH/TiO in the presence of O (Supplementary Fig. 12), an
obvious vibration feature of carbonate at 1580 cm was observed
in all cases of photooxidation of CH OH/H O/TiO in the presence
of O and does not exhibit the isotope effect. We tentatively assign
this carbonate species to COCH3OH TiO2 strongly bound to
TiO surface.
3
3
3
2
2
3
2
2
-
1
2
3
2
2
2
2
TiO2
O O
2
2
2
spectra (Fig. 8b and Supplementary Fig. 31), although the Conclusion
coverages of formed surface intermediates are significantly lower
than those formed during photooxidation of methanol species on
bare TiO NCs in the presence of O (Supplementary Fig. 12).
This is different from the case of methanol species on bare TiO
NCs in the absence of O , in which methanol species barely
exhibits photooxidation reactivity (Supplementary Fig. 9). Thus,
surface OH groups formed by water dissociation on TiO NCs can
act as the acceptor sites on TiO surface for the injection of new
In summary, we have successfully unveiled the site sensitive
interfacial charge transfer and photocatalytic efficiency of CH OH
oxidation on anatase TiO NCs. The CH O(a)Ti4c species on TiO
001} and the CH O(a)Ti5c species on TiO -{100} and TiO -{101}
with the PDOS localized almost at the valence band maximum of
corresponding TiO surfaces are intrinsically more photoactive
than the CH OH(a)Ti5c species with the PDOS localized within the
valence band, while the CH O(a) species at the defect-related
sites is hardly photoactive. The CH O(a)Ti4c species of
CH OH/TiO -{001} with the highest CH O(a) coverage, the most
extensive upward surface band bending and the largest hole
concentration on the TiO surface exhibits the largest
2
2
3
2
2
-
2
3
2
{
3
2
2
2
2
2
3
electrons to trigger the photooxidation of adsorbed methanol
species, forming the HCO and HCOO species. Moreover, the
3
3
1
8
HCO O species was observed In the DRIFTS spectra of
methanol species on -{100} NCs
18O-precovered TiO
illuminated by UV light in the absence of O (Fig. 8b and
3
2
3
H
2
2
2
2
Supplementary Fig. 32), proving that the OH• radicals can directly
photocatalytic oxidation rate constant. These results exemplify
surface site- and adsorbate-dependent surface band
bending/bulk-to-surface charge migration processes and
electronic structure/interfacial charge transfer processes during
photocatalytic reactions and demonstrate surface structure
engineering of photocatalysts as an effective approach to
maximize photocatalytic efficiencies.
oxidize the HCO intermediate. However, the major formate
1
6
species is still HCO O, likely formed by oxidation of HCO
intermediate by origially-existing 16OH• radicals and by binding of
HCO intermediate with surface lattice oxygen of TiO
TiO2). These observations demonstrate the existence
of surface OH groups-induced photooxidation pathway of
adsorbed methanol species on TiO surface.
The abilities of surface OH group and O
photooxidation of adsorbed metahnol species on TiO
2
(
HCOCH3OHO
2
2
to participate in
NCs were
2
Acknowledgements
further examined by comparing surface photochemistry of
18
adsorbed CH
{
(
3
OH species on H
2
16O (H
2
O)-precovered TiO
2
-
)
This work was financially supported by the National Natural
Science Foundation of China (21525313, 91745202, 91945301,
16
18
2
100} NCs at 313 K followed by UV light illumination in O ( O
Fig. 8c and Supplementary Fig. 33). The HCO species, which
2
21825301, 21573211 and 21633007), the National Key Research
8
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1
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
Research Article
+
+
+
Surface sites and resulting methanol
adsorbates strongly affect the surface
band bending/bulk-to-surface charge
migration processes and the
interfacial electronic
Cong Fu, Fei Li, Jiachen Zhang, Dan
Li, Kun Qian, Yong Liu, Junwang Tang,
*
Fengtao Fan, Qun Zhang, Xue-Qing
Gong, Weixin Huang*
*
structure/interfacial charge transfer
processes, and consequently play a
key role in the photocatalytic
efficiency.
Page No. – Page No.
Site Sensitivity of Interfacial Charge
Transfer and Photocatalytic
Efficiency in Photocatalysis:
Methanol Oxidation on Anatase TiO
2
Nanocrystals
1
1
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