Visible-light-driven selective alcohol dehydrogenation and hydrogenolysis: Via the Mott Schottky effect

Article abstract of DOI:10.1039/c9ta13811k

The Schottky barrier created at the interface between electron sinks and semiconductors can effectively optimize electron transfer for active photocatalysis. However, heterogeneous photocatalytic organic transformations via the Mott Schottky effect are st

Full text of DOI:10.1039/c9ta13811k

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DOI: 10.1039/C9TA13811K
Visible-Light-Driven Selective Alcohol
Dehydrogenation and Hydrogenolysis Via the
Mott Schottky Effect
a, b
Xue Yang+ a, b, Suyuan Zhang+ a, b, e, Huilin Tao c, d, Xuyan Yan a, c, Yanxi Tan
,
Yongfan Zhang c, Shuiying Gao a, b, *, Rong Cao a, b,
*
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
b University of Chinese Academy of Science, Beijing 100049, China
c College of Chemistry, Fuzhou University, Fuzhou 350108, China
d School of Chemistry and Environmental Science, Shangrao Normal University,
Shangrao 334000, China
e School of Physical Science and Technology, ShanghaiTech University, Shanghai
201210, China
+ These authors contributed equally to this work.
Corresponding Author
E-mail: rcao@fjirsm.ac.cn; Fax: (+86)-591-83796710, Tel: (+86)-591-63173153
(Rong Cao)
E-mail: gaosy@fjirsm.ac.cn; (+86)-591-63173153 (Shuiying Gao)

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Abstract
The Schottky barrier created at the interface between electron sinks and semiconductors
can effectively optimize the electrons transfer for active photocatalysis. However,
heterogeneous photocatalytic organic transformations via the Mott Schottky effect are
still rare and their role in the specific photocatalytic system remains ambiguous. Here,
a versatile photocatalytic system integrating the Schottky junctions into the TiO2
particles (Degussa, P25) and Pd NPs based films has been developed. They were readily
available through the facile encapsulation of P25 by Pd-H4BINDI films using an
alternating layer-by-layer (LBL) approach (H4BINDI: N, N'-bis (5-isophthalate acid)
naphthalene-diimide). After H2 reduction process under 200 ℃, the Pd NPs were
obtained, denoted as P25@Pd-H4BINDI-200. Interestingly, the resulting P25@Pd-
H4BINDI-200 photocatalyst presented an exceptionally high activity for
dehydrogenation and hydrogenolysis of benzyl alcohol under visible light irradiation,
which was approximately 17 times higher than that of the parent P25@Pd-H4BINDI
and was superior to any other analogous systems reported to date. Specifically, the
negative charges accumulated on the Pd NPs surface reduce the benzyl alcohol, while
the holes left on the valence band oxidize the benzyl alcohol, effectively promoting the
photocatalytic cycles with enhanced carrier separation. The deeper understanding of the
reaction mechanisms unveiled that Pd NPs play a vital mediating role in storing and
shuttling photogenerated electrons via the Mott Schottky effect, and revealed that the
highly enhanced carrier separation by direct use of photoexcited electrons and holes for
photoredox reactions. This work holds great promises for designing Mott-Schottky

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catalysts in enabling solar-driven photoredox reactions.
Keywords: Mott-Schottky, P25, photocatalysis, Pd, visible light, layer-by-layer, films

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Introduction
Photochemical reaction has become a research focus for the synthesis of various
chemicals due to its environmental friendliness and sustainability.1-3 The application of
solar energy in challenging organic synthesis is an especially promising strategy for
synthetic chemistry.4-7 However, these reactions are often hampered by the sluggish
electrons transfer and rapid recombination of photogenerated electron (e-)-hole (h+) pair.
Integrating metal (mainly noble metal) to semiconductor materials has been recognized
as a promising strategy for facilitating the electron (e-)-hole (h+) separation for efficient
photocatalysis.8-12 Once a metal is in direct contact with a semiconductor,13, 14
a
Schottky barrier will be formed at the metal-support interface due to their different
Fermi level positions,15-17 which can serve as an efficient hot electrons trap to avoid the
e--h+ recombination during photocatalysis.18 The Mott Schottky effect will greatly
contribute to the improvement of heterogeneous photocatalytic activity in the visible-
light region and concomitant effective hot electrons transfer.19-21 However, the
dispersion and stabilization of metal NPs on semiconductor is still a challenge as the
metal-support interactions are not yet sufficient strong.11, 22-24 It is of great interest to
construct effective Schottky barrier between the metal NPs and semiconductive
support.20, 21, 25-27
Photochemical reactions are often hampered by the sluggish electrons transfer and
rapid recombination of photogenerated e-- h+ pair.28-33 In particular, it has become a
research trend that coupling e- and h+ induced redox reactions synergistically in one
reaction system, as photogenerated e- and h+ exhibit remarkable reducing and oxidizing

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peculiarity,34-36 respectively. Earlier, Jiang’s group successfully coupled H2 proDdOuI:c1t0i.o10n39/C9TA13811K
with benzylamine oxidation over a Pt/PCN-777 catalyst in a single system.37 According
to our recent work,38 the photogenerated e- and h+ can fully be utilized for nitrobenzene
photoreduction coupled with alcohols photooxidation. Aromatic alcohols are
significant and fundamental substrates for synthesizing value-added organic
39, 40
chemicals.5,
Consequently, current interests have been directed toward the
oxidation of aromatic alcohols by photoexcited holes, while reduction of aromatic
41
alcohols over photoexcited electrons synergistically in a single system.27,
Unfortunately, related works still constitute a daunting challenge and its reaction
mechanism remains ambiguous.
Bearing the above considerations in mind, a promising approach was developed
to enhance the e--h+ separation via the Mott Schottky effect in a thin film photocatalyst.
The Pd-H4BINDI films were successfully coated on P25 surface through an alternating
layer-by-layer self-assembly method.42 After H2 reduction process under 200 ℃, the
well dispersed Pd NPs were obtained, denoted as P25@Pd-H4BINDI-200. The
ingeniously integration of H4BINDI ligand resisted the aggregation of Pd NPs and
extended the light absorption region of P25. Interestingly, the negative charges
accumulated on the Pd NPs surface reduce the benzyl alcohol, while the holes left on
the VB (valence band) band oxidize the benzyl alcohol, promoting photocatalytic
cycles. The simultaneous exploitation of photogenerated holes and electrons greatly
enhance the e--h+ separation for efficient photocatalysis. This work realizes the
conversions of biomass-relevant substrates and holds great promises for applying Mott-

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Schottky catalysts in enabling solar-driven organic transformations.
Experimental Section
Synthesis Procedures of ligand
N, N’-bis (5-isophthalic acid) naphthalenediimide (H4BINDI) was synthesized
according to some reported literatures.43 In a typical process, a mixture of 1,4,5,8-
tetracarboxydianhydride (5.0 mmol) and acetic acid (25 mL) was stirred for 10 min.
Then, 5-aminoisophthalic acid (10 mmol) was added into the above mixture. The
reaction mixture was refluxed for 12 h, and 20 mL water was added. The product was
collected by filtration washed with ethanol and dried at 70 °C under vacuum for 12 h.
Finally, the H4BINDI compound was recrystallized from DMF.
HOOC
COOH
O
O
O
N
O
O
NH2
AcOH
+
reflux 12 h
HOOC
COOH
N
O
O
O
O
O
HOOC
COOH
Fabrication of Pd-H4BINDI metal-organic films on different substrates
P25, mixture of anatase and rutile (82/18) particles (size, 24 nm; BET surface area, 57
m2 g-1) was purchased from Sinopharm Chemical Reagent Co., Ltd. In a typical
procedure, the P25 pre-coated with Poly(ethylenimine) (PEI, 50 wt% aqueous solution)
as a metal-ion adhesive layer were immersed in an aqueous solution containing K2PdCl4
(5 mM, 5 mL) and an H4BINDI DMF solution (0.5 mM, 5 mL) for 30 min, respectively.

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After each immersion, the powders were centrifuged and washed with ethaDnOoIl:.10B.10y39/C9TA13811K
repeating the above steps for several times and finally dried in a vacuum, the catalyst
film P25@Pd-H4BINDI was prepared. The SiO2-Pd-H4BINDI-200 and quartz@Pd-
H4BINDI were prepared in the same manner, but the Pd-H4BINDI composite films
were adsorbed on the surface of the quartz slides (size 25 mm × 12 mm × 1 mm) or
SiO2.
Preparation of P25@Pd-H4BINDI-T
The prepared P25@Pd-H4BINDI sample (100 mg) in a ceramic boat was reduced in a
5 % H2/N2 atmosphere by raising the temperature in a heating rate of 3 °C min-1 to
appropriate pyrolysis temperature to afford P25@Pd-H4BINDI-T, in which T denotes
the pyrolysis temperature (150 °C, 200 °C and 300 °C) and maintaining for 5 h to yield
Pd nanocomposites. The as-obtained black powders were applied for further
characterizations and catalytic reactions. the P25@Pd was prepared according the
former reported process with some modification. In detail, the K2PdCl4 was dissolved
into acetone to disperse P25 and the mixture was ultrasonic agitation for intensive
mixing. The P25@Pd was obtained by the same reduction process as that for P25@Pd-
H4BINDI-200. The resulting P25@Pd was mixed with an DMF solution of H4BINDI
and stirred for 6 h. The P25@Pd/H4BINDI-200 was collected by centrifugation and
dried for the following photocatalysis.
Electrochemical characterization
The electrochemical analysis was carried out on a Zahner Instruments electrochemical
station in a traditional three electrode quartz cell. Briefly, Pt foils plate was used as the

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counter electrode, and Ag/AgCl as the reference electrode. The fluoride-tin oxide (FTO)
glass had been thoroughly cleaned by ethanol and acetone and acted as working
electrode. The catalyst (10 mg) was ultrasonicated in 0.5 mL of DMF to obtain slurry.
Then, 40 mL of the slurry was spread onto the prepared FTO glass, and the side of the
glass was previously protected using scotch tape to ensure the exposed area of the
working electrode was controlled at 1 cm2. Then, the coated working electrode was
dried at room temperature in air without any heating process. The electrolyte was a 0.2
M aqueous Na2SO4 solution without additives. The Nyquist plots were measured
covering the frequency of 1 × 105 to 0.1 Hz intervals. The Mott-Schottky plots were
measured at 800 and 1000 Hz.
Typical experimental procedure for photocatalytic test
In a general procedure for photocatalytic selective oxidation and reduction of alcohol,
10 mg of film photocatalyst was added into 1.5 mL trifluorotoluene (BTF) containing
0.1 mmol alcohol in a quartz tube equipped with a magnetic stirrer. The reaction system
was evacuated and backfilled with oxygen at 1 atm pressure for five times before the
reaction. The reaction system was stirred vigorously (500 rpm) and irradiated with a
300 W xenon lamp equipped with an ultraviolet cut-off filter (> 420 nm). After the
reaction was finished, a certain amount of reaction solutions (60 µL) was filtered by a
one-time plastic filter and extracted with ethanol for GC analysis (Agilent 7890A).
Controlled photoactivity experiments using various radical scavengers were performed
in a similar manner to the photocatalytic experiment with radical scavengers (0.3 mmol)
added to the reaction system.

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OH
O
P25-(Pd-H4BINDI)n-200
O2
+
General procedures for recyclability test
After photocatalysis, the reacted photocatalyst in the aqueous phase was separated by
filter membrane and washed with ethyl alcohol. The recycled photocatalyst was dried
at 70 °C under vacuum conditions. Then the collected photocatalyst was reused for
another catalytic cycle with the addition of fresh benzyl alcohol and BTF.
Computational methods
The main calculations based on density functional theory (DFT) were carried out using
the Vienna Ab initio Simulation Package (VASP). The kinetic cut-off energy of the
plane-wave expansion was set to 500 eV. The effects of van der Waals interactions
were considered by using the dispersion-corrected vdW-DF2 functional, and the dipole
correction in the surface normal direction was applied. A six-layer slab with a (4 × 2)
supercell was adopted to simulate TiO2 (101) and (001) surface. To avoid the
interactions between the neighboring slabs, the vacuum between adjacent slabs was set
to 20 Å. A part calculation of H4BINDI was carried out by G03 with 6-31G** basis set.
Results and discussion
The synthetic route of Mott-Schottky-type P25@Pd-H4BINDI-T (T denotes the
pyrolysis temperature) film photocatalyst was shown in Scheme 1. In a typical process,
various number layers of Pd-H4BINDI were readily encapsulated on TiO2 substrate
(Degussa, P25) through a facile alternating layer-by-layer (LbL) deposition method.

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The H4BINDI (N, N'-bis (5-isophthalate acid) naphthalene-diimide) ligand was
coordinated with Pd2+ using an LBL approach, which dually provides driving force for
the efficient assembly of the thin films. The well-dispersed Pd NPs were obtained
followed by mild calcination under an 5% H2/N2 atmosphere, affording P25@Pd-
H4BINDI-T as black powders. Unless otherwise specified, the P25 based film catalyst
used in this work was coated with eight layers of Pd-H4BINDI. The Pd content was
determined to be 3.69 wt% by inductively coupled plasma atomic emission
spectroscopy (ICP). FT-IR spectra were compared in P25@Pd-H4BINDI and P25@Pd-
H4BINDI-200 (Fig. S9a) which showed almost no significant change. In more detail
(Fig. S9b), the band at 1580 cm-1, 1670 cm-1 and 1708 cm-1 can be ascribed to the
carboxylate carbonyl functionalities, amide functionalities and imide carbonyl of free
H4BINDI,43 respectively. Such characteristic peaks were also appeared in P25@Pd-
H4BINDI and P25@Pd-H4BINDI-200, indicting the stability of the H4BINDI during
the reduction process (Scheme 1)
Scheme 1. Schematic illustration for the synthesis of P25@Pd-H4BINDI-T.

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In the present work, the ligand N, N'-bis (5-isophthalate acid) naphthalene-DdOiiI:m10i.1d0e39/C9TA13811K
(H4BINDI) (Fig. S1) 44 was chosen to act as a useful building unit for the fabrication of
LBL films. The clear evidence of loading of H4BINDI in multilayer films was obtained
by UV/vis spectra. As expected, the UV/vis absorption of quartz@Pd-H4BINDI films
was aroused from H4BINDI, and its intensity increased systematically with the number
of growth cycles (Fig. 1a). The result indicated the successful adsorption of the
H4BINDI into every layer of film through consecutive, alternating adsorption steps. The
microstructure of films was further investigated by Powder X-ray diffraction (XRD)
analyses. As Fig. 1b showed, the major peaks for films can be perfectly indexed as its
substrate P25. It was noteworthy that the feature diffraction peaks of Pd at 40.1º (JCPDS
no. 46-1043) also displayed on the XRD pattern of P25@Pd-H4BINDI-200, revealing
the successful reduction of Pd NPs on the films. In addition, the Pd valence state in
P25@Pd-H4BINDI film before and after reduction process were investigated by the X-
ray photoelectron spectroscopy (XPS), respectively. As Fig. 1c showed, P25@Pd-
H4BINDI dominating Pd (II) due to two obvious binding energies of the Pd 3d5/2 and
3d3/2 electrons at 338.02 and 343.18 eV, corresponding to Pd2+ ions. After H2 reduction
process under 200 ℃, the Pd 3d signal of P25@Pd-H4BINDI-200 can be fitted to two
pairs of doubles: 340.95/335.65 eV and 343.13/337.89 eV, which can be assigned to Pd
(0) and Pd (II),39 respectively. These results indicated that the Pd (0) NPs were
successfully inlayed into the P25@Pd-H4BINDI-200 multilayer films. The distinct
color change of samples also confirmed the successful adsorption of Pd-H4BINDI films
onto the P25 (Fig. S2). Furthermore, the optical properties of films were investigated

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by UV-vis diffuse reflection spectra, and the reflectance date of all samples were
converted into absorbance by the Kubelka-Munk function (Fig. 1d). It was found that
H4BINDI not only provided driving force for the efficient assembly of the thin films,
but also played a key role in extending the photo response of the film catalyst system
to the abundant visible region. Notably, the resulting P25@Pd-H4BINDI-200 film
retained the intrinsic absorption of substrate P25 in the UV region, with remarkably
broaden light adsorption cross section in the whole visible light region.
Fig. 1. (a) Absorbance of P25@Pd-H4BINDI films with various numbers of layer. (b)
XRD patterns and (c) XPS spectra of Pd in P25@Pd-H4BINDI and (d) P25@Pd-
H4BINDI-200 catalysts. (d) UV-vis diffuse reflection spectra of P25@Pd-H4BINDI-
200, P25@Pd-H4BINDI, P25, and H4BINDI.
The scanning electron microscopy (SEM) revealed the microsphere morphology
of bare P25 (Fig. S4). The morphology and particle size of the P25@Pd-H4BINDI-200
films were investigated by transmission electron microscopy (TEM) images. As shown
in Fig. 2a, the well-dispersed Pd NPs within the multilayer films were detected. The

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high-resolution TEM (HRTEM) images showed that the lattice fringe of 0.235 and
0.350 nm were matched (101) and (001) facets of P25, respectively (Fig. 2b). In
addition, Pd NPs dispersed within P25 and H4BINDI layers, and the intervals between
two lattice fringes in Pd were 0.224 nm, corresponding to (111) plane of face-centered
cubic Pd (Fig. 2c, d). HRTEM results revealed the close contact and lattice correlations
between P25 and (Pd-H4BINDI) films. Moreover, the interfacial part of the Pd NPs had
been partially buried in a thin layer of P25 and H4BINDI. Obviously, the P25 acted as
a support and H4BINDI as organic stabilizers for well-dispersed Pd NPs. These results
certainly indicated that strong interactions have been established in P25@Pd-H4BINDI-
200 film, which ensured the formation of compact Schottky contact.

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Fig. 2. (a) TEM and (b-d) HRTEM images for P25@Pd-H4BINDI-200.
The application of solar energy in challenging organic synthesis is an especially
promising strategy for synthetic chemistry. Encouraged by the remarkable absorption
property of P25@Pd-H4BINDI-200 in visible light, photocatalytic dehydrogenation
and hydrogenolysis of benzyl alcohol was further explored. As Table 1 showed, the
P25@Pd-H4BINDI-200 demonstrated remarkable conversion of benzyl alcohol under
mild conditions, affording a benzaldehyde yield of 66% and a toluene yield of 33%
(entry 1), respectively. The activity was more than 17 times higher than those P25@Pd-
H4BINDI (entry 4) and was superior to the reported result on photocatalytic
dehydrogenation and hydrogenolysis of aromatic alcohols in one reaction system.27, 41

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The activity of our work also could compete with other reported analogous systDeOmI:s10f.1o0r39/C9TA13811K
the photocatalytic oxidation of benzyl alcohol (Fig. S7). It's worth mentioning that, the
ratio of benzaldehyde and toluene remained unchanged even extended the reaction time.
This suggested that dehydrogenation and hydrogenolysis of benzyl alcohol occurred
simultaneously with high selectivity. The recent advances on reactive oxygen species
of photocatalysts has emphasized that molecular oxygen is significant for alcohols
photooxidation to aldehydes.45 In addition, the photocatalytic activity was decreased
under air atmosphere (entry 2). There were no byproducts without visible light
irradiation (entry 3), indicating that the photoredox reaction over the P25@Pd-
H4BINDI-200 catalyst was due entirely to a photocatalytic process. P25@Pd-
H4BINDI-150 showed only 10% conversion under the same conditions (entry 5) due to
the inadequate reduction of Pd. However, P25@Pd-H4BINDI-300 showed much
inferior catalytic activity (entry 6). The quartz@Pd-H4BINDI-200 showed no
conversion of benzyl alcohol (entry 7), indicting indispensable role of P25.
The quartz@Pd-H4BINDI-200 showed no conversion of benzyl alcohol (entry 7),
indicting indispensable role of P25. To further demonstrate the key role of P25 for
photocatalytic activity, the contrast photocatalyst of SiO2@Pd-H4BINDI-200 was
prepared. Due to the poor photoactivity of SiO2, SiO2@Pd-H4BINDI-200 photocatalyst
shows only 15 % conversion of benzyl alcohol under the identical conditions (entry
10), indicting the key role of P25 during the photocatalytic process. Nevertheless, bare
P25 exhibit negligible activity, due to its wide band gap (entry 8). Although the
H4BINDI ligand can respond to the visible light, it gives only a minor mount of products

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under the same condition (entry 9). P25@Pd/H4BINDI-200 with low photocatalytic
activity indicted that the ingeniously integration of H4BINDI ligand through layer-by-
layer method also contributed to the efficient photocatalysis (entry 11). The relationship
between catalytic activities and the number of films were further explored. The results
showed that the conversion of benzyl alcohol correlated with the number of bilayers
(Fig. S8). In other words, catalytic activity was related to loading amount of Pd-
H4BINDI. It can be concluded that the catalyst with more Pd-H4BINDI bilayers has
higher efficiency, and the conversion of benzyl alcohol realizes the maximum content
of 100 % with eight layers of Pd-H4BINDI.
Table 1. Photocatalytic dehydrogenation and hydrogenolysis of benzyl alcohol for 2 h
under various conditions a
Entry
Catalyst
Con. A/%b
Yield B/%b
Yield C/%b
1
P25@Pd-H4BINDI-200
P25@Pd-H4BINDI-200
100
73
66
49
33
23
2c
3d
4
P25@Pd-H4BINDI-200
P25@Pd-H4BINDI
0
6
0
6
0
0
0
5
P25@Pd-H4BINDI-150
10
10

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6
7
8
9
P25@Pd-H4BINDI-300
20
0
20
0
0
0
0
0
quartz@Pd-H4BINDI-200
P25
trace
5
trace
5
H4BINDI
10
11
SiO2@Pd-H4BINDI-200
P25@Pd/H4BINDI-200
15
8
15
8
0
0
a
Reaction conditions: 0.1 mmol benzyl alcohol, O2 (1 atm), 1.5 mL BTF, 10 mg
photocatalyst, and visible light illumination (λ > 420 nm).
b Calculated by GC analysis.
c Under air atmosphere.
d In the dark.
The recyclability of a photocatalyst is very meaningful for its practical application.
The stability of P25@Pd-H4BINDI-200 was examined in the dehydrogenation and
hydrogenolysis of benzyl alcohol under direct visible light exposure for 2 h. Strikingly,
the photocatalytic activity of P25@Pd-H4BINDI-200 could be maintained at least 5
runs, indicting the excellent recyclability and photostability of P25@Pd-H4BINDI-200
in this coupled system (Fig. 3a). In addition, the time-resolved photoluminescence
(TRPL) spectra in Fig. 3b showed that the emission induced by the P25@Pd-H4BINDI-
200 exhibited a much longer lifetime than that of P25 and P25@Pd-H4BINDI-300,
indicting the most efficient e--h+ separation efficiency. It was very promising for the
target reactions under visible light irradiation.

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Fig. 3. (a) Recyclability of P25@Pd-H4BINDI-200 in the dehydrogenation and
hydrogenolysis of benzyl alcohol upon direct visible light exposure for 2 h. (b)
Photoluminescence decay curve of the P25@Pd-H4BINDI-200 and P25@Pd-
H4BINDI-200-T.
The efficient photogenerated charge separation contributes to the prominent
photocatalytic activity. In this regard, the photocurrent and electrochemical impedance
spectroscopy (EIS) measurements were explored to prove the rapid charge separation
efficiency. As Fig. 4a showed, the photocurrent measurements featured good
reproducibility over several on/off cycles of intermittent irradiation (λ > 420 nm).
Moreover, the P25@Pd-H4BINDI-200 produced the highest photocurrent response,
indicating the most efficient transfer and longest lifetime of photoexcited charges.
Inversely, P25 showed no visible light responsiveness due to its wide band gap.
Furthermore, the rapid charge separation efficiency was also proved by open circuit
photovoltaic measurements (Fig. 4b). The P25@Pd-H4BINDI-200 indicated the
smallest radius at high frequencies, suggesting a lowest resistance in charge
transportation and an efficient mechanism for e--h+ separation. The more favorable
electron contacts between Pd and P25 was attributed to the Mott-Schottky effect. These
results clearly revealed that the synergistic constitution of films accelerated the trapping
and transferring of the photoinduced electrons, which should be beneficial for the

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efficient photocatalysis. These results obviously matched well with the measured UV-
visible absorption spectra and photocatalytic activity order. The remarkable
photocatalytic dehydrogenation and hydrogenolysis of benzyl alcohol over
P25@H4BINDI-Pd-200 was initiated by the Schottky junctions of Pd NPs, which
significantly promoted the electrons transfer and reduced the recombination of
photogenerated electron-hole pair. It has been reported that the Schottky junction will
be formed between Pd NPs and TiO2 due to their different Fermi level positions (Fig.
3c-d).46, 47 In general, the work functions of noble metals are usually larger than those
of n-type semiconductor.48, 49 That is, the Fermi level (EF) of P25 is higher than the EF
of Pd NPs. When Pd NPs is in direct contact with a P25 (Fig. 4c), the electrons of the
CB P25 will transfer to the Pd NPs (entitled “metal island”) at the lower energy level
to build up an equilibrium state between their Fermi level. It was mainly attributed the
formation of Mott-Schottky heterojunction at the interface of P25 and metallic Pd that
increased the electron density of Pd NPs, promoting the injection of hot electrons from
photoactivated P25 to Pd NPs (Fig. 4d).
A detailed investigation of energy levels of photocatalysts were explored by
electrochemical Mott-Schottky measurements in conjunction with Tauc plot (Fig. S5-
S6). Evidently, the positive slope of the obtained C-2-V value demonstrated the n-type
character of P25 and H4BINDI ligand. The conduction band (CB) position for P25 and
H4BINDI, which was determined from the intersection with value of -0.58 V, -0.77 V
versus Ag/AgCl (-0.38 V, -0.57 V vs. normalized hydrogen electrode (NHE)),
respectively. With assistant of Tauc plot, the valance band value of P25 and H4BINDI

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DOI: 10.1039/C9TA13811K
was then calculated to be 2.74 V and 2.23 V, respectively.
Fig. 4. (a) Photocurrent tests and (b) EIS plots of P25, P25@Pd-H4BINDI-200 and
P25@Pd-H4BINDI-300. (c) The potential diagram of H4BINDI, P25 and Pd. (d)
Schematic illustration of the Mott-Schottky-type contact of Pd NPs and P25. (EF: work
function; ECB: conduction band; EVB: valence band)
Based on the above experimental and theoretical results, the illustration of carrier
dynamics in selective reduction-oxidation of benzyl alcohol was proposed in Scheme
2. Firstly, the H4BINDI ligand was excited by visible light (λ > 420 nm) to produce
energetic electrons and holes. Density functional theory (DFT) calculations were
performed and the bandgap of H4BINDI and P25 were corrected according to the
experiment results (Fig. S5), respectively. The results noted that (Fig. 3c), the lowest
unoccupied molecular orbital LUMO value of H4BINDI (-3.72 V) was more positive
than that of P25 (-4.33 V), which was thermodynamically allowed for electrons transfer.
Evidently, the photogenerated electrons will inject from the H4BINDI into the P25
conduction band. The unique route for wide band-gap P25 to use visible light is similar

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to the traditional dye-sensitized mechanism. Ultimately, amounts of electronsDwOIi:l1l0.1b0e39/C9TA13811K
accumulated in the CB of P25, making them “hot” enough to diffuse to the Pd NPs
surface due to the Mott Schottky effect. Pd NPs play a vital mediating role in storing
and shuttling photogenerated hot electrons. Finally, the electrons accumulated on the
Pd NPs surface will be consumed by hydrogenolysis process of benzyl alcohol, which
inhibited the recombination of e--h+ pairs and induced the holes to participate in
dehydrogenation of benzyl alcohol continuously. In return, the Schottky barrier also
effectively restrained the electron drift from the NPs back to the P25. Synergistically,
the photogenerated holes left on the valance band (VB) of H4BINDI will participate in
oxidation reactions, promoting photocatalytic cycles. As a result, the dehydrogenation
reaction consumed two holes and released two protons to produce benzaldehyde. This
process essentially separated and synchronously utilized the photogenerated electrons
and holes, which further improved the activity for the photocatalytic reduction-
oxidation of benzyl alcohol. Under visible light irradiation, benzyl alcohol consumes
the photo-generated holes and releases two protons to produce benzaldehyde. In
addition, Pd is reported as a famous protons catcher which can facilely form Pd-H
catalytic sites to promote the reduction half reaction.27 As a result, the benzaldehyde
further depletes two protons to generate toluene and molecular oxygen. Finally, the
production of benzaldehyde and toluene were realized in one single system.27 It was
worth mentioning that, the hydroxyl group is a poor leaving group, it is of great
significance to transform hydroxyl group into its corresponding alkyl group under mild
photocatalysis condition.

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DOI: 10.1039/C9TA13811K
Scheme 2. The proposed reaction mechanism for the photocatalytic selective reduction-
oxidation of benzyl alcohol over P25@H4BINDI-Pd-200 under visible light irradiation.
Conclusions
In summary, a promising strategy for coupling the unique properties of Pd NPs
with the functionality of P25 was proposed through utilization of the LBL technique.
The resulting P25@Pd-H4BINDI-200 photocatalyst displayed a drastically enhanced
photocatalytic performance benefitting from accelerated electrons transfer. This work
realized dehydrogenation and hydrogenolysis of benzyl alcohol simultaneously. The
Schottky barrier created at the interface between Pd NPs and P25 greatly prompted the
hot electrons transfer. The concept demonstrated here will provide a novel solar-driven
catalyst platform for utilizing Mott-Schottky effect for promoting solar-to-chemical
conversion.
Acknowledgement
The authors acknowledge the financial support of National Key Research and
Development Program of China (2017YFA0700100 and 2018YFA0704502 ), financial
support of the NSFC of China (21520102001, 21871263 and 21671188), Key Research

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Program of Frontier Sciences, CAS (Grant No. QYZDJ-SSW-SLH045), and SDtrOaI:t1e0g.1i0c39/C9TA13811K
Priority Research Program of the Chinese Academy of Sciences (XDB20000000).
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This work realized dehydrogenation and hydrogenolysis of benzyl alcohol
simultaneously.