Effect of electron transfer on the photocatalytic hydrogen evolution efficiency of faceted TiO2/CdSe QDs under visible light

Article abstract of DOI:10.1039/c8nj00180d

Quantum dot (QD)/TiO₂ composites are widely used materials in the field of photocatalysis. However, the influence of different exposed facets of TiO₂ in composites on photocatalytic hydrogen evolution is rarely reported. In this work, CdSe QD-TiO₂ composites with dominant {001} or {101} faceted anatase were synthesized and specifically characterized using X-ray powder diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Photocatalytic hydrogen evolution tests reveal that {001}-TiO₂/CdSe QDs exhibit a 213.1- and 9.0-fold increase in photocatalytic activity compared to {001}-TiO₂ and CdSe QDs, respectively. Notably, the photocatalytic activity of {001}-TiO₂/CdSe QDs is 2.2 times higher than that of {101}-TiO₂/CdSe QDs. Based on the results from UV-Vis diffuse reflectance spectroscopy, Brunauer-Emmett-Teller surface area testing, Mott-Schottky testing and steady-state and time-resolved emission spectroscopy, the main reason for enhanced photocatalytic activity is the faster electron transfer from CdSe QDs to {001}-TiO₂ compared with to {101}-TiO₂. This research on tailored facets in QD/TiO₂ composites provides primary insights for the design of effective photocatalysts.

Full text of DOI:10.1039/c8nj00180d

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Effect of electron transfer on the photocatalytic hydrogen
evolution efficiency of faceted TiO2/CdSe QDs under visible light
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Weiwei Chen,a Shan Yu,a, * Yunqian Zhong,a Xiang-Bing Fan,c Li-Zhu Wuc and Ying Zhoua, b, *
www.rsc.org/
Abstract: Quantum dots (QDs)/TiO2 composite is widely used materials in the field of photocatalysis. However, the
influence of different exposed facets TiO2 in the composites on photocatalytic hydrogen evolution is rarely reported. In
this work, CdSe QDs-TiO2 composites with dominated {001} or {101} faceted anatase were sythesized and specifically
characterized by X-ray powder diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy.
Photacatalytic hydongen evolution test reveals that {001}-TiO2/CdSe QDs exhibit 213.1 and 9.0 times increase in
photocatalytic activity for {001}-TiO2 and CdSe QDs, respectively. Notably, the photocatalytic activity of {001}-TiO2/CdSe
QDs is 2.2 times higher than that of {101}-TiO2/CdSe QDs. Based on the results from UV-Vis diffuse reflectance
spectroscopy, Brunauer-Emmett-Teller surface area test, Mott-Schottky test and steady-state and time-resolved emission
spectroscopy, it indicates that the main reason for enhanced photocatalytic activity is the faster electron transfer from
CdSe QDs to {001}-TiO2 in comparison to {101}-TiO2. This research on tailored facets in QDs/TiO2 composite provides
primary insights for design of effective photocatalysts.
upshifted significantly, rendering it a larger driving force for
photocatalytic reactions thermodynamically. In specific, CdSe
QDs could function as the photocatalyst independently or in
combination with other semiconductors. There are several
works concerning TiO2/CdSe QDs composite as an effective
1. Introduction
Since 1970s, photocatalytic hydrogen evolution has been
widely studied, as it provides the possibility to collect directly
hydrogen energy from solar energy.1-3 Many semiconductors
have been developed for photocatalytic hydrogen evolution,
such as oxides TiO2, ZnO, sulfide CdS.4-6 Recently, quantum
dots (QDs) have attracted lots of interests due to their
adjustable bandgap, rich surface properties and large surface
area with comparison to bulk semiconductors. Especially,
chalcogenides QDs such as CdS, CdSe and CdTe QDs have been
reported in a series of work for photocatalytic hydrogen
evolution with remarkable efficiency.7-10 Among them, CdSe
QDs are important QDs which have a bandgap of 1.75 eV for
corresponding bulk material and a Bohr exciton radius of 4.6
nm. In the form of QDs, the conduction band of CdSe could be
photocatalyst for dye degradation, reduction of nitro
aromatics or hydrogen evolution.11-15 We also built commercial
P25-TiO2/CdSe QDs based hydrogen evolution system and
found that TiO2 could function as an electron relay between
CdSe QDs and catalytic center to improve the charge
separation efficiency of the system.16
Besides of CdSe QDs, TiO2 with tailored facet for
photocatlysis also draws a lot of attention these years. In 2008,
Yang and Lu successfully obtained anatase TiO2 single crystals
with the reactive facet {001} as the mainly exposed surface.17
Afterwards, research on tailored TiO2 triggered immense
interests. For instance, Xie and Zheng reported that TiO2
nanosheets with a high percentage of exposed {001} facets
exhibited much higher photocatalytic activity towards dye
degradation compared with commercial P25.18 However, Liu
and Cheng et al. discovered that the photoreactivity followed
the order of {010} > {101} > {001} for both oxidative and
reductive reactions.19 More interestingly, Schmuttenmaer and
Kim et al. found that the activity order was {001}-TiO2 > {101}-
TiO2 > {010}-TiO2 in dye sensitized solar cells, but it changed to
{010}-TiO2
> {101}-TiO2 > {001}-TiO2 in photocatalytic
degradation of methyl orange.20 We applied TiO2/Pt composite
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for CO oxidation as well and found that the activity of {101}- chromatograph (Ar carrier gas, molecular sieve 5Å, TCD
TiO2/Pt was higher than that of {001}-TiO2/Pt.21 These reported detector).
activity orders differ a lot for these exposed facets of TiO2 in
different reactions. Therefore, it is necessary to study the 2.4 Characterization methods
influence of different exposed facets in TiO2/CdSe QDs
The X-ray diffraction (XRD) patterns of our samples were
composite on the efficiency of photocatalytic hydrogen
performed by a PANalytical X’pert diffractometer with Cu K
α
evolution in particular.
radiation worked at 40 kV and 40 mA. Transmission electron
microscopy (TEM) and high resolution TEM (HRTEM) were
recorded on a FEI Tecnai G220 electron microscope operated
at 200 kV. BET surface areas and N2 adsorption–desorption
isotherm lines were measured by a nitrogen adsorption
Quanta Chrome exhausted at 150 °C for 4 h. UV-Vis absorption
spectra and diffuse reflection spectra (DRS) were analysed by
the Shimadzu 2600 UV-Vis spectrophotometer with BaSO4
powder as the reflectance standard. X-ray photoelectron
spectra (XPS) was recorded by a Thermo ESCALAB250Xi X-ray
photoelectron spectrometer with Al Kα (E = 1486.6 eV)
emission. C 1s peak at 284.8 eV was used to calibrate the
binding energy of other elements. Electrochemical
measurement was investigated in 0.5 M Na2SO4 aqueous
solution by a CHI660E electrochemical analyser (CHI, China).
The films of working electrode were prepared by the samples
coated on a 1 × 1 cm FTO glass electrode. Pt wire and
saturated calomel electrode were used as the counter-
electrode and the reference electrode, respectively. The
emission spectra of samples were recorded by a HITACHI F-
7000 fluorescence spectrometer. Time-resolved luminescence
decay curves of QDs were analysed by FL-9200 Fluorescence
spectrometer (Edinburgh Instruments).
In this work, we have synthesized {001}-TiO2/CdSe QDs and
{101}-TiO2/CdSe QDs composites and compared their
photocatalytic activity for hydrogen evolution under visible
light irradiation. The result of hydrogen evolution test shows
that {001}-TiO2/CdSe QDs is a better photocatalyst. The
structure, chemical state, light absorption ability, band
structure and charge separation ability of the two composites
were systematically investigated, and it turns out that the
higher activity of {001}-TiO2/CdSe QDs is resulted from the
faster electron transfer from CdSe QDs to {001}-TiO2 and
enhanced electron-hole separation efficiency in {001}-
TiO2/CdSe QDs.
2. Experimental
2.1 Materials and methods
CdCl2∙2.5H2O, Selenium powder and hydrofluoric acid were
purchased from Aladdin, 3-Mercaptopropionic acid (MPA) and
titanium tetrachloride were purchased from Sigma, and other
reagents were purchased from Chengdu Kelong Chemical Co.,
Ltd. All reagents were of analytical grade and utilized without
any further purification.
2.2 Preparation of TiO2/CdSe QDs composites
3. Results and discussion
{001}-TiO2 and {101}-TiO2 were prepared by
a
hydrothermal method,22-23 and water soluble CdSe QDs were
synthesized by our previous method16,24 (as seen from
supporting information). Then {001}-TiO2 and {101}-TiO2 were
combined with CdSe QDs under the assistance of 3-
Mercaptopropionic acid. Typically, 4 mL of NaOH aqueous
solution (1 M) and 0.3 mL of 3-Mercaptopropionic acid were
added into 5 mL of aqueous solution of CdSe QDs. Then the pH
value of the system was adjusted to 7-8. Next {001}-TiO2 or
{101}-TiO2 (50 mg) were dispersed in the mixed solution and
stirred for 30 min. The resulting precipitates were washed and
dried after centrifugation, and 1.5wt% CdSe QDs/TiO2
composites were obtained.
3.1 Crystal structure and composition of TiO2/CdSe QDs
The synthesis of {001}-TiO2/CdSe QDs and {101}-TiO2/CdSe
QDs was based on our previous report,16 in which CdSe QDs
were attached on TiO2 surface with mercaptopropionic acid
(MPA) as the linker. Based on our previous study,16 a loading
amount of 1.5wt% CdSe QDs in the composite can achieve the
highest photocatalytic hydrogen evolution efficiency.
Therefore, the content of CdSe QDs in the {001}-TiO2/CdSe
QDs and {101}-TiO2/CdSe QDs was set at 1.5wt%. The
complete combination of CdSe QDs with {001}-TiO2 or {101}-
TiO2 could be confirmed from the absorption spectra of the
colorless supernatant after the interaction of the two (Fig. S1).
XRD data of CdSe QDs showed three notable diffraction
peaks at 25.45°, 42.4° and 49.8°, and they were assigned to the
(111), (220) and (311) facets of CdSe with zinc blende structure
(Fig. 1). Due to the influence of MPA, there was a slight shift of
these peaks in contrast to the standard data (JCPDS#19-
0191).25-26 The notable widening of these diffraction peaks was
typical for QDs due to their small size. For {001}-TiO2 and
{101}-TiO2 (Fig. 1), all the diffraction peaks were well indexed
to the anatase TiO2 (JCPDS#21-1272).27 In specific, peaks at
25.3°, 37.8°, 48.0°, 53.9°, 55.0°, 62.7° and 68.8° were ascribed to
the (101), (004), (200), (105), (211) and (204) crystal facets of
anatase, respectively. However, the diffraction intensity of
2.3 Photocatalytic hydrogen evolution tests
5 mg of TiO2/CdSe QDs composites were dispersed in 10
mL of isopropanol aqueous solution (isopropanol/H2O = 1/1,
v/v). Then 0.4 mg of NiCl2∙6H2O was added and the pH value of
the system was adjusted to 11. After deoxygenated with argon
gas for 30 min, 1 mL of methane gas as the internal standard
was injected into the reaction systems. The enclosed system
was then irradiated with a 500 W high-pressure mercury lamp
equipped with an optical filter to cutoff light with wavelength
smaller than 420 nm. The amount of evolved hydrogen from
the system was analyzed through a Shimadzu GC-2010 Plus gas
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peaks related to (004) and (200) facets varied for {001}-TiO2 particles was calculated to be 77.9% and 86.2% for {001}-TiO2
and {101}-TiO2, since the mainly exposed facet of the two TiO2 and {101}-TiO2, respectively.23,29-30 Furthermore, small particles
samples is different. According to the full width at half with the size of 2-3 nm and crystal lattice spacing of 0.350 nm
maximum (FWHM) of peaks relative to (004) and (101) facets on the surface of TiO2 were indicative of the existence of zinc
and the Debye–Scherrer equation, the sample thickness along blende structured CdSe QDs with corresponding (111) facet.
the direction vertical to (001) and (101) was calculated. Based Other lattice spacing of 0.189, 0.354 and 0.238 nm was from
on the ideal crystal model built by Wulff Construction,28 the the (200) (or (020)), (101) and (002) facets of TiO2,31
percentage of corresponding exposed facets for {001}-TiO2 and respectively (Fig. 2b and 2d). The above data hence verified
{101}-TiO2 was respectively estimated to be 60.5% and 86.6% that CdSe QDs have been successfully loaded on the surface of
to confirm their dominance in all the exposed facets. The {001}-TiO2 or {101}-TiO2.
diffraction patterns of {001}-TiO2/CdSe QDs and {101}-
TiO2/CdSe QDs were quite close to that of individual TiO2. In
specific, after composited with CdSe QDs, the grain sizes of
{001}-TiO2 (14.0 nm) and {101}-TiO2 (16.4 nm) were slightly
increased to 20.5 nm and 17.0 nm, respectively. This is
consistent with a small increase in the crystallinity of {001}-
TiO2 and {101}-TiO2 in the composite. No signals of CdSe QDs
in the composites could be detected in XRD patterns because
of the low content of CdSe QDs in the system.
Fig. 3 XPS fine spectra of Ti 2p (a), Cd 3d (b), Se 3d (c) and S 2p
(d) from {001}-TiO2/CdSe QDs and {101}-TiO2/CdSe QDs.
The chemical valence states of the composites were
studied with X-Ray photoemission spectroscopy (XPS). As
Fig. 1 XRD patterns of different photocatalysts: CdSe QDs,
{001}-TiO2 and {101}-TiO2, {001}-TiO2/CdSe QDs and {101}-
TiO2/CdSe QDs.
shown in Fig. 3, the fine spectra of the two composites
confirmed the existence of Ti, Cd, Se elements. Fig. 3a showed
two peaks at 464.2 and 458.5 eV ascribed to Ti 2p1/2 and 2p3/2
with +4 valence from TiO2, which was in good agreement with
the literatures.32 Due to the interference of the incident
oxygen species during tests, fine spectrum of O was not shown
here. Besides, peaks of Fig. 3b at 411.8 and 405.0 eV separated
by 6.8 eV were assigned to Cd 3d3/2 and 3d5/2 with +2
valence.33 Similarly, peaks at 54.6 and 53.7 eV separated by 0.9
eV were corresponding to Se 3d3/2 and 3d5/2 with -2 valence
(Fig. 3c).12 In addition, signals related to S were detected at
162.8 and 161.5 eV in the spectra (Fig. 3d), which was assigned
to S 2p1/2 and 2p3/2 from the thiol group (–SH) of 3-
mercaptopropionic acid (MPA). Here, MPA was introduced in
the system as the surface ligands of CdSe QDs as well as the
linker between CdSe QDs and TiO2.34-36 No signals related to
the oxidation state of S species could be detected, which
indicated that the stability of the surface ligand of QDs. The
above mentioned results further demonstrated that CdSe QDs
are loaded on surface of different faceted TiO2. Additionally,
we found no difference between {101}-TiO2/CdSe QDs and
{001}-TiO2/CdSe QDs for all fine spectra of the available
elements.
Fig. 2 TEM and HRTEM images of {001}-TiO2/CdSe QDs (a, b)
and {101}-TiO2/CdSe QDs (c, d).
Fig. 2 showed typical TEM and HRTEM images of {001}-
TiO2/CdSe QDs and {101}-TiO2/CdSe QDs. Notably, the sheet
structure of {001}-TiO2 and pyramid structure of {101}-TiO2
were clearly observed (Fig. 2a and 2c). The average fraction of
dominant facets of TiO2 based on the estimation of a series of
3.2 Photocatalytic activity of TiO2/CdSe QDs composites
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The photocatalytic hydrogen production activity of {001}- shown in the insert of Fig. 5, the band gap of CdSe QDs, {001}-
TiO2 and {101}-TiO2 loaded with 1.5wt% CdSe QDs were TiO2, {101}-TiO2, {001}-TiO2/CdSe QDs and {101}-TiO2/CdSe
investigated in aqueous solution under visible-light irradiation QDs was 2.70 eV, 3.23 eV, 3.19 eV, 2.52 eV and 2.41 eV,
using isopropanol as the electron donor and nickel as the respectively. Notably, the bandgap of {001}-TiO2 was slightly
cocatalyst. Fig. 4 showed that the efficiency of pure CdSe QDs larger than that of {101}-TiO2 (3.23 vs 3.19 eV), which was
was much less than that of TiO2/CdSe QDs composites for comparable to the previous study.38 Meanwhile, the
photocatalytic hydrogen evolution under visible light absorbance of {101}-TiO2/CdSe QDs was larger than that of
illumination. Control experiment further showed that {001}- {001}-TiO2/CdSe QDs in the visible light region. The above data
TiO2 and {101}-TiO2 were very poor catalysts for hydrogen implied that the light capture ability of {001}-TiO2/CdSe QDs is
evolution under visible light irradiation. These results not only weaker than that of {101}-TiO2/CdSe QDs. Therefore, the
demonstrated the vital role of CdSe QDs as photosensitizer in higher photocatalytic efficiency for {001}-TiO2/CdSe QDs was
the system but also confirmed the synergistic effect between irrelevant with their light absorption ability.
CdSe QDs and TiO2.14,16 Further comparison showed that the
amount of hydrogen evolution with {001}-TiO2/CdSe QDs (14.7
μmol) was 2.2 times to that of {101}-TiO2/CdSe QDs (6.6 μmol)
after illumination of 4 h. Moreover, the activity of {001}-
TiO2/CdSe QDs was much better than that of previous
reported commercial P25-TiO2/CdSe QDs (9.0 μmol). Control
experiment further showed that the activity of {001}-
TiO2/CdSe QDs was also better than that of {100}-TiO2/CdSe
QDs (Figure S2). Therefore, {001}-TiO2/CdSe QDs was the best
photocatalyst for hydrogen evolution in our system. Recycling
tests further showed the stability of {001}-TiO2/CdSe QDs
under strong visible-light irradiation, which almost maintained
a high hydrogen generation rate of 6.80 mmol h-1 g-1 of within
12 h (Fig. S3).
Fig. 5 UV–Vis diffuse reflectance absorption spectra and the
plots of [αhν]1/2 vs. photon energy (insert) of samples: CdSe
QDs, {001}-TiO2, {101}-TiO2, {001}-TiO2/CdSe QDs and {101}-
TiO2/CdSe QDs composites.
Fig.
4
Amount of hydrogen evolved from different
photocatalysts under visible-light irradiation (λ ≥ 420 nm, I =
15.56 mW cm-2, 500 W high-pressure mercury lamp, at pH 11).
Fig. 6 N2 adsorption–desorption isotherm lines and pore size
distribution curves (insert) of {001}-TiO2/CdSe QDs and {101}-
TiO2/CdSe QDs.
3.3 Mechanism of different photoactivity of TiO2/CdSe QDs
To investigate the reason behind the different
photocatalytic behavior of {101}-TiO2/CdSe QDs and {001}-
TiO2/CdSe QDs composite, we considered three aspects in the
present system: light absorption, charge separation and
surface catalytic sites. Firstly, UV-Vis diffuse reflection
absorption spectra was measured for the visible region
response and band gap of the as-prepared samples. Fig. 5
showed that {101}-TiO2 and {001}-TiO2 were barely responsive
to visible light (λ > 400 nm), and the edge of absorption
spectra of the composites red shifted from 400 to 500 nm.
Based on the UV-Vis diffuse reflectance absorption spectra,
Brunauer-Emmett-Teller test demonstrated that the
surface area of {001}-TiO2/CdSe QDs and {101}-TiO2/CdSe QDs
was 67.7 and 89.6 m2/g, respectively. The lower surface area
of {001}-TiO2/CdSe QDs compared with {101}-TiO2/CdSe QDs
indicated that the higher hydrogen evolution efficiency of
{001}-TiO2/CdSe QDs should not result from the amount of
surface reaction sites, either. According to the shapes of
hysteresis loops from Fig 6, {001}-TiO2/CdSe QDs and {101}-
TiO2/CdSe QDs exhibited type H3 and H1 adsorption-
desorption isotherms. In addition, the average pore size of
{001}-TiO2/CdSe QDs (18.3 nm) was bigger than that of {101}-
TiO2/CdSe QDs (12.9 nm). Based on TEM images of samples,
the band gap energy of faceted anatase and composites can be
37
calculated by using the equation of αꢀꢁ ꢂ ꢃꢄꢀꢁ ꢅ ꢆꢇꢈꢉ
.
As
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the wedge holes in {001}-TiO2/CdSe QDs were formed by the electron transfer from CdSe QDs to {001}-TiO2 was
agglomerated sheet structure, which may serve as an available probably more efficient than that to {101}-TiO2.
transmission route to reactant and product to enhance
photocatalytic activity.12
We then focused on the study of charge transfer in the
present system thermodynamically and kinetically. Band
energy levels of {001}-TiO2 and {101}-TiO2 were studied with
Mott-Schottky (M-S) test in combination with UV-Vis DRS
results. As presented in Fig. 7, M-S plots indicated the flat band
potential of {001}-TiO2 and {101}-TiO2 were −0.29 V and −0.17
V (vs. NHE), respectively. Accordingly, the conduction band of
{001}-TiO2 and {101}-TiO2 were −0.39 V and −0.27 V (vs. NHE)
considering the 0.1 V gap between conduction band level and
flat potential.12 Therefore, the valence band edge of {001}-TiO2
Fig. 8 Schematic illustration of the photocatalytic hydrogen
evolution process with {001}-TiO2/CdSe QDs and {101}-
TiO2/CdSe QDs.
and {101}-TiO2 were 2.84 V and 2.92 V (vs. NHE). Note that the
conduction band edge of {001}-TiO2 was more negative than
that of {101}-TiO2.39-41 On the other hand, based on the size
and the excitation absorption peak of CdSe QDs in our system,
the conduction band edge was estimated to be −1.60 V (vs.
NHE).16 As a result, the driving force for electron transfer from
QDs to both {001}-TiO2 and {101}-TiO2 was large enough
thermodynamically. Similarly, the subsequent electron transfer
from {001}-TiO2 and {101}-TiO2 to cocatalyst Ni (E(Ni2+/Ni) =
−0.23 V vs NHE)42 was also feasible, but the driving force of the
former was larger than that of the latter (0.16 vs 0.04 V) from
the thermodynamic point of view (Fig. 8).
Then the kinetic decay of the band edge emission of QDs
was tracked by the transient emission spectroscopy. Fig. 9b
showed that the band edge emission of CdSe QDs decayed
completely within a timescale of 100 ns. After introduction of
TiO2, the emission decayed more quickly and it confirmed the
electron transfer from QDs to TiO2. Moreover, the emission
decay from {001}-TiO2/CdSe QDs was more rapid than {101}-
TiO2/CdSe QDs. With triexponential fitting, the average band-
edge emission lifetime of CdSe QDs, {101}-TiO2/CdSe QDs and
{001}-TiO2/CdSe QDs decreased from 8.2 to 4.8 and 4.0 ns,
respectively. Based on the method developed by Kamat et al,35
electron-transfer rate constant from CdSe QDs to {001}-TiO2 or
{101}-TiO2 could be estimated according to the equation Ket
=1/τ(CdSe+TiO2) - 1/τ(CdSe), whereas τ(CdSe) is the lifetime of CdSe
QDs alone, and τ(CdSe+TiO2) is the lifetime of CdSe QDs in the
composite, and Ket is the corresponding electron transfer rate
from CdSe QDs to TiO2. Accordingly, the electron transfer rate
constant from CdSe QDs to {001}-TiO2 was 1.5 times higher
than that to {101}-TiO2 (1.3 × 108 vs 8.6 × 107 s-1). The faster
charge transfer in {001}-TiO2/CdSe QDs might result from the
higher surface energy of {001}-TiO2 (0.9 J/m2) with comparison
to {101}-TiO2 (0.44 J/m2). Especially, the surface Ti atoms from
{001}-TiO2 are all five coordinated, but half of them are five
coordinated and the rest are six coordinated for {101}-TiO2,23,44
which probably makes the tighter attachment of CdSe QDs to
{001}-TiO2.
Fig. 7 Mott-Schottky plots of {001}-TiO2 and {101}-TiO2.
Furthermore, we carried out steady-state emission
spectroscopy measurements to analyze the dynamics of
charge transfer for TiO2/CdSe QDs.33 Fig. 9a showed that CdSe
QDs exhibited a strong emission peak at 496 nm, which was
attributed to its characteristic band edge emission. The
emission peak related to the trap-state emission of CdSe QDs
was missing, indicating the good quality of CdSe QDs in the
present system. The characteristic emission peaks of QDs were
quenched in both {101}-TiO2/CdSe QDs and {001}-TiO2/CdSe
QDs composites, which demonstrated that the chance of
radiative recombination of electrons and holes generated from
CdSe QDs is largely decreased.43 This was probably caused by
the electron transfer from CdSe QDs to {001}-TiO2 or {101}-
TiO2, in accordance with the thermodynamic study mentioned
above. The emission peak intensity of {001}-TiO2/CdSe QDs
was weaker than that of {101}-TiO2/CdSe QDs, implying that
According to the above experiments, {001}-TiO2/CdSe QDs
was proved to be a better photocatalyst for hydrogen
evolution as the result of more efficient charge separation in
contrast to that of {101}-TiO2/CdSe QDs, even though the light
absorption ability and surface area of the former is not as good
as the latter. The important role of charge separation in
photocatalysis and selection of TiO2 with different tailored
facets ({001} or {101} or P25) in the composite has a significant
influence on the efficiency of photocatalytic hydrogen
evolution.
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Fig. 9 (a) Emission spectra of CdSe QDs, {001}-TiO2/CdSe QDs
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Time-resolved fluorescence spectra of single CdSe QDs, {001}-
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In summary, {001}-TiO2/CdSe QDs and {101}-TiO2/CdSe
QDs composites have been synthesized with MPA as the linker
and carefully characterized. The efficiency of {001}-TiO2/CdSe
QDs is higher than that of {101}-TiO2/CdSe QDs for
photocatalytic hydrogen evolution, though the BET surface
area and light absorption capability of the former were weaker
than that of the latter. The thermodynamic and kinetic study
of the system demonstrated that the faster electron transfer
from QDs to {001}-TiO2 contributed to the more efficient
electron-hole separation and the higher photocatalytic
efficiency of {001}-TiO2/CdSe QDs. The presented work has
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on the interaction between CdSe QDs and TiO2, which could
offer certain references in constructing similar photocatalytic
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We gratefully acknowledge financial support from the
scientific research starting project of SWPU (No. 2015QHZ018),
Sichuan Provincial Education Department Project (No.
15ZB0055), Young Scholars Development Fund of SWPU
(201499010096) and Sichuan Provincial Colleges’ Sate Key
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Graphical abstract: Higher photocatalytic hydrogen evolution efficiency of
{001}-TiO2/CdSe QDs was obtained because of its faster electron transfer rate than
{101}-TiO2/CdSe QDs.