A porous hybrid material based on calixarene dye and TiO2 demonstrating high and stable photocatalytic performance

Article abstract of DOI:10.1039/c9ta06038c

A highly robust hybrid material based on calixarene dye (HO-TPA) and titanium dioxide with a micro/mesoporous structure and a large surface area (denoted as HO-TPA-TiO₂) has been prepared by a facile sol-gel method. When Pt nanoparticles (Pt NP

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DOI: 10.1039/C9TA06038C
Graphical abstract
A facile strategy to design and prepare a robust porous hybrid material (HO-TPA-TiO2) is
presented by combining TiO2 with calixarene dye HO-TPA through a simple sol-gel method,
offering efficient and persistent dual-function photocatalytic activities for hydrogen production or
CO2 reduction to CO when loaded with Pt nanoparticles or ReP as co-catalysts.

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ARTICLE
A porous hybrid material based on calixarene dye and TiO2
demonstrating high and stable photocatalytic performance
Yi-Fan Chen,a,b Jian-Feng Huang,a Min-Hui Shen,a Jun-Min Liu,a, * Li-Bo Huang,a Yu-Hui Zhong,a Su
Qin,a Jing Guo,a Cheng-Yong Sua, *
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A highly robust hybrid material based on calixarene dye (HO-TPA) and titanium dioxide with micro/mesoporous structure
and large surface area (denoted as HO-TPA-TiO2) has been prepared by a facile so-gel method. When Pt nanoparticles (Pt
NPs) or fac-[Re(4,4’-disphosphonic acid)-2,2’-bipyridine)(CO)3Cl (ReP) as co-catalysts are introduced onto the hybrids
respectively, the two hybrid systems can display dual-functional photocatalytic properties. Under visible-light irradiation,
the optimized Pt/HO-TPA-TiO2 system exhibits high hydrogen production activity (618.3 mmol g−1 h−1 based on Pt mass) and
performs with excellent stability even after used for 15 times (a turnover number of 6417 after 75 h). On the other hand,
the optimum ReP/HO-TPA-TiO2 material displays persistent photocatalysis of CO2 reduction to CO and gives a turnover
number of 534 for 26 h (854.4 mmol g−1 based on ReP mass), which is one of the highest among all the reported
heterogeneous catalytic systems under visible light. The high activity and stability of the HO-TPA-TiO2 based hybrid materials
can be attributed to the stable coordination linkage and efficient electron transfer between HO-TPA and micro/mesoporous
TiO2. The current study offers an inspiration for the design and application of new multi-functional organic-inorganic hybrid
systems in efficient photocatalysis.
Keywords:
Organic-inorganic hybrid materials
Calixarene dyes
Titanium dioxides
Photocatalysis
Stability
poor stability and become inactivated easily because the dye would
detach from semiconductor and degrade under light irradiation.22,23
In a previous paper,24 we utilized cone-calix[4]arene dye Calix-3 to
1. Introduction
Photocatalytic techniques have long been studied to alleviate the
exhaustion of energy resources and deterioration of natural
environments by utilizing solar energy.1,2 To date, enormous
attentions have been focused on photocatalytic hydrogen
production from water-splitting and solar-light-driven reduction of
CO2 to fuels or other useful chemicals, which provide a direct route
to the green chemical industry and a guide for design and
preparation of the multi-functional artificial photosynthesis.3-7
Traditionally, some semiconductor materials (TiO2, Ge3N4, GaN,
K2Ti2O5, and C3N4)8-12 assisted with noble metals (Pt, Pd, and Au)13-15
as the co-catalysts have been exploited and applied in photocatalytic
or photoelectric reactions under UV or limited visible light region. To
take full advantage of solar energy, some strategies, such as
nonmetallic element doping (N, S, and P)16-18 semiconductors and
dye-sensitized semiconductors,19-21 have been employed to utilize
solar energy in the extend visible light region. Nevertheless, their low
photocatalytic activity and stability greatly impede their practical
implementation. Especially the common dye-sensitized systems
loaded with Pt nanoparticles for photocatalytic water splitting have
sensitize Zr-containing metal-organic framework (MOF) embedded
with Pt nanoparticles (Pt@UiO-66-NH2) for photocatalytic H2
production under visible light irradiation. Calix-3 has four -COOH
groups in one molecule and thus multiple hydrogen bonds exist
between UiO-66-NH2 and dyes, which would increase the stability of
the dyes adsorbed on the MOF surface and enhance hydrogen
evolution efficiency and stability. To further enhance dye adsorption
stability on the semiconductor and promote surface interfacial
charge transfer from dye to semiconductor simultaneously, herein
we design a novel organic-inorganic hybrid materials in which
organic dyes and inorganic semiconductors can be combined
through strong coordination bonds.
Hybrid materials containing TiO2 inorganic component has been
applied in many fields due to its high activity and chemical
stability.25,26 Several synthetic methods for hybrid materials have
been developed, such as dispersion polymerization,27 hydrothermal
method,28 template-assisted growth29 and sol-gel method.30-33
Particularly the sol-gel method could be adopted to obtain some
well-compatible organic-inorganic nanomaterials with high surface
areas.34 In addition, in view of the fact that calix[4]arene derivatives
possess large cavity and are easy to form supramolecular complexes
with inorganic molecules,35-38 we set out to design and synthesize a
calix[4]arene organic dye HO-TPA (Scheme 1, Fig S1) consisting of 2-
triphenylamine donor, oligothiophene spacer, and hydroxyl-
substituted calix[4]arene acceptor, which can be combined with TiO2
by simple sol-gel method to obtain a binary organic-inorganic hybrid
system, denoted as HO-TPA-TiO2. This kind of hybrid system loaded
a. School of Chemistry and School of Materials Science and Engineering, Sun Yat-sen
University, Guangzhou, 510275, China.
b. Hainan Provincial Key Lab of Fine Chem, School of Chemical Engineering and
Technology, Hainan University, Haikou, 570228, China.
† Corresponding authors.
E-mail addresses: liujunm@mail.sysu.edu.cn (J.-M. Liu), cesscy@mail.sysu.edu.cn
(C.-Y. Su).
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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with co-catalyst is expected to work as a persistent photocatalyst reagents were used as received from commercial soVuierwceAsrtiwcleitOhnoliunet
DOI: 10.1039/C9TA06038C
owing to the following advantages: (1) high photo- and thermo- further purification. The P25-TiO2 particle is a commercial product of
stable HO-TPA has four -OH groups in one molecule which can Degussa AG without further treatment. Calix-142,43 and ReP39 were
interact with Ti(OC4H9)4 to form very strong TiO2-O-TPA bonds, prepared according to the methods reported in previous literatures,
leading to great improvement in stability of the doped dyes in hybrid respectively. The detailed synthesis of HO-TPA was described in
system; (2) HO-TPA has four light-harvesting units per molecule, supporting information. The preparation of calixarene-based dyes
which is not only favourable to achieve high molar extinction HO-TPA was performed according to Fig. S1 and detailed synthesis
coefficients but also ensures more effective electron transfer and characterization were included in support information.
between the doped dye and surrounding TiO2; (3) the hybrid material
2.1.1. Synthesis of HO-TPA-TiO2 materials
of HO-TPA and TiO2 has micro/mesoporous structure, large surface
area and abundant active sites for the reactions, thus enhancing light
HO-TPA-TiO2 materials were prepared through a sol-gel process
harvesting from multi-reflections and catalytic-center utilization in
shown in Fig. S2. To prepare 5.0 wt% HO-TPA-TiO2, THF (1 mL), acetic
the pores; (4) in the hybrid materials, TiO2 protects HO-TPA and
acid (168 μL), distilled water (106 μL) and butyltitanate (1 mL) were
subsequently added into 20 mL glass sample bottle, and then HO-
TPA (12.5 mg) in THF (1 mL) was added. The color of mixture changed
from light yellow to dark red, and stirred to become transparent
works as electron transporting mediator and reservoir.
In this paper, we demonstrate the good activity and excellent
stability of this kind of hybrid material with co-catalyst Pt NPs
(denoted as Pt/HO-TPA-TiO2) in photocatalytic H2 production from
gelatum. The gelatum was heated at 50 °C for 10 h to obtain red solid.
water under visible-light, giving a turnover number of 6417 for 75
The residue was soxhleted with distilled water at 100 °C for 30 h and
after used for 15 times and recycled simultaneously. Moreover, we
evaporated to dryness. The hybrid materials were calcined in Muffle
extend the hybrid material HO-TPA-TiO2 toward CO2 photoreduction
furnace at 300 °C for 2 h to become brown powder. To elaborate the
formation process of HO-TPA-TiO2 materials, the intermediate
products at different reaction time before calcination was collected
via immobilization of the Re complex fac-Re(2,2’-bipyridine-4,4’-
bisphosphonic acid)(CO)3(Cl)] (denoted as ReP, Scheme 1), which is
known to be able to photoreduce CO2 to CO in homogeneous
to characterize their crystalline states by XRD patterns (Fig. S3).
systems,39 onto the hybrid material to prepare the ternary hybrid
Similar Gel-TiO2 materials without doping dye and HO-TPA-TiO2
(denoted as ReP/HO-TPA-TiO2) as heterogeneous photosynthesis
(2.5 and 7.5 wt%) materials with different dye amounts were
system. While some interesting results on photocatalytic CO2
prepared with the above method.
reduction using ReP/TiO2 and ReP/dye-sensitized TiO2 (denoted as
ReP/TiO2/dye) hybrids have been reported,39-41 there have been no 2.1.2. Synthesis of Pt/HO-TPA-TiO2 materials
investigations to bind ReP onto the hybrid material in photocatalytic
Pt nanoparticles (0.6 wt%) were loaded onto HO-TPA-TiO2 (0, 2.5,
reaction. Our results disclose that the photocatalyst ReP/HO-TPA-
5.0 and 7.5 wt%) materials with different dye amounts, respectively.
TiO2 could selectively photoreduce CO2 to CO with a high TONCO of
In general, 100 mg of hybrid materials were dispersed in 60 mL H2O
and methanol (5:1, v/v) solution containing H2PtCl6·6H2O (10 mM,
340 μL) in N2, and then stirred and illuminated for 30 min with a 300
W Xe lamp. The solid was centrifuged, washed, and dried in vacuum
534 with 1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole (BIH,
Scheme 1) as the sacrificial agent and triethanolamine (TEOA) as
additive under visible-light irradiation for 26 h. Furthermore, the
mechanisms of photocatalytic hydrogen production and CO2
oven at room temperature. Pt loading (0.6 wt%) was testified by the
photoreduction in the hybrid system have been explored.
inductively coupled plasma atomic emission spectrometry (ICP-AES).
O3PH2
N
2.1.3. Synthesis of surface dye-sensitized Pt/Gel-TiO2/HO-TPA
materials
N
N
N
Cl
N
N
CO
CO
N
N
H
Re
CO
S
S
S
S
S
Ph
The Pt/Gel-TiO2 powder (prepared similarly as above without
addition of HO-TPA) was dispersed in 0.3 mM CHCl3 solution of HO-
TPA at room temperature for 6 h, and then centrifuged and washed
with CHCl3 until no color was observed from the eluent. The solid was
dried in vacuum oven at room temperature for overnight. The
concentrations of dye solutions before and after use were
determined to be 2.4 wt% by UV-vis spectra at 500 nm wavelength
to calculate the adsorbing capacities of the catalysts.
S
S
S
O3PH2
HO
OH OH
OH
BIH
ReP
HO-TPA
Scheme 1. Sensitizer (HO-TPA), reduction catalyst (ReP), and
electron donor (BIH) used for the hybrid system.
2.1.4. Synthesis of ReP/HO-TPA-TiO2 materials
5.0 wt% HO-TPA-TiO2 materials (10 mg) were dispersed in 0.1
μmol ReP aqueous solution (2 mL) and stirred for 4 h, and then
centrifuged and washed with distilled water for 3 times. The solid
was dried in vacuum oven at room temperature for overnight. The
loading amounts of ReP for 10 mg samples were determined to be
0.041 μmol by ICP-AES. For comparison, ReP/Gel-TiO2 and ReP/Gel-
2. Experiments
2.1. Preparation of photocatalysts
All solvents were purified by an MBRAUN MB SPS-800 system.
Anhydrous solvents used in the Suzuki coupling reaction were de-
gassed by bubbling with N2 for 20 min. All other chemicals and
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TiO2/HO-TPA materials were prepared according to the above closed gas circulation, which was swept by high puVriietwy AArtriclbeeOfnolirnee
DOI: 10.1039/C9TA06038C
method for control experiments. The loading amounts of ReP were illumination. 50 mg photocatalysts were dispersed in 100 mL of 10
determined to be 0.054 μmol for 10 mg ReP/Gel-TiO2 materials and vol% TEOA aqueous solution (pH = 8~9). Then the suspension was
0.050 μmol for 10 mg ReP/Gel-TiO2/HO-TPA materials by ICP-AES.
exposed to a 300 W Xe lamp equipped with an optical filter (λ > 420
nm) to cut off the light in the ultraviolet region. The reaction solution
was stirred continuously and cooled to room temperature by a
circulation of cooling water. The amount of hydrogen evolved was
determined at an interval of 1 h with online gas chromatography.
2.2. Characterizations
The 1H and 13C NMR spectra were recorded on a Bruker 400 MHz
spectrometer. Mass spectra data were obtained on an
ultrafleXtreme MALDI-TOF/TOF mass spectrometer (Bruker
Daltonics). The absorption spectra were observed with a Shimadzu
UV-3600 spectrometer and fluorescence spectra were measured
2.4. Photocatalyzed CO2 reduction
Suspensions of photocatalyst particles (10 mg) in 5 mL of DMF
with an Edinburgh Instruments Ltd FLS980 spectrometer. Powder X- containing BIH (134 mg) and TEOA (0 M/0.23 M/0.46 M) were placed
ray diffraction (PXRD) was recorded on a Rigaku Smart Lab in a quartz cell (2 cm pass length; 40 mL total volume), bubbled with
diffractometer (Bragg-Brentano geometry, Cu-Kα1 radiation, λ = CO2 for 30 min, sealed with a septum, and then irradiated using a
1.54056 Å). Transmission electron microscopy (TEM) images, high- xenon lamp while stirring (300 W). The incident light (λ > 420 nm)
resolution transmission electron microscopy (HRTEM), high-angle was obtained by passing the light from the xenon lamp through a
annular dark field scanning transmission electron microscopy water layer of a 10 cm path length and a glass light filter. The
(HAADF-STEM) micrographs, selected area electron diffraction amounts of CO evolved in the overhead space of the cell were
(SAED), energy dispersive X-ray spectroscopy (EDX), and elemental determined by gas chromatography (Fuli Model GC9790 II equipped
mapping were obtained by a JEM-2100F transmission electron with TCD and FID detectors) using a TDX-01 packed column.
microscope at 300 kV. Scanning electron microscopy (SEM)
micrographs were recorded on Hitachi Ultra-high Resolution FE-SEM
SU8010 microscope. Gas adsorption measurements were performed
3. Results and discussion
using ultra-high purity N2 and CO2 gas on Quantachrome Auto-sorb-
iQ2-MP analyzer. The Mott-Schottky curves were measured using a
3.1. Sample characterizations
CHI760E electro-chemical analyzer (China) in a three-electrode cell.
The XRD patterns of HO-TPA-TiO2 with different dye content
Pt plate was used as counter electrode and Ag/AgCl electrode (3 M were recorded (Fig. 1A and Fig. 1B). The obtained peaks at 25.53°,
KCl) was used as reference electrode. The electrolyte was a 0.2 M 38.12°, 48.22°, 54.18°, and 62.94° were found for all the samples,
Na2SO4 solution. The working electrode was prepared on fluorine- corresponding to the (101), (103), (200), (105), and (204) planes,
doped tin oxide (FTO) glass by dipping the mixed slurry containing respectively, denoting formation of anatase TiO2 with similar degree
°
the sample and H2O, and the exposed area of the electrode was 0.25 of crystallization. Calcination at 300 C can remove impurities from
cm2. Cyclic voltammograms (CVs) were recorded using a CHI760E the hybrid system and facilitate the generation and transmission of
electrochemical analyzer with platinum plate as working electrode, photogenerated carriers, but show little impact on the crystallinity.
Ag/AgCl (3 M KCl) as reference electrode, Pt wire as counter Nitrogen adsorption-desorption isotherms measured at 77 K gave
electrode. CVs were measured using 0.1 M tetrabutylammonium Brunauer-Emmett-Teller (BET) surface areas and pore size
hexafluorophosphate (TBAPF6) as a supporting electrolyte in THF distributions for HO-TPA-TiO2 systems, as shown in Fig. S4-S6 and
with a scan rate of 100 mV s-1. The photocatalytic hydrogen Table S1. With the increase of dye content from 0 to 2.5, 5.0, and 7.5
production system was Perfect Light Lab Solar-IIIAG and H2 wt%, the specific surface area (SBET) of Gel-TiO2 and three HO-TPA-
measurement was performed on Agilent Technologies 78. The TiO2 materials increased gradually from 163 to 176, 185, and 188 m2
amounts of Pt NPs and ReP loaded on samples were detected by ICP- g−1, respectively, in comparison with the low SBET value of P25-TiO2
AES (spectra range: 120-800 nm, holographic grating, 2924 line/mm, (55 m2 g−1). A slight decrease in SBET of hybrid materials was observed
SPECTRO CIROS VISION). Attenuated total reflection-infrared after calcination at 300 °C. Nevertheless, a dramatic decrease in SBET
intensity (ATR-IR) spectra were monitored by Frontier FT-IR after calcination at 500 °C was found, which was reasonable because
spectrometer (spectra range 4000-450 cm−1, an average of 64 scans). the degradation temperature of HO-TPA dye was determined to be
X-ray photoelectron spectroscopy (XPS) measurements were carried 400 °C, and thus high temperature treatment resulted in
out using an XR6 monochromated AlKα X-ray source (hν = 1486.6 eV) decomposition of dyes in hybrid materials. More interestingly, two
with a 900 mm spot size in an ultrahigh vacuum chamber (ESCALAB types of pore size distributions, comprising both micropore (1.4~1.6
250Xi). The pass energy was 20 eV. Apparent quantum yields (AQYs) nm) and mesopore (3.4~4.4 nm), were observed in HO-TPA-TiO2
were determined using 420 nm, 450 nm and 500 nm band pass filter materials, in contrast to that (29 nm) of mesoporous P25-TiO2. This
and an irradiation meter, and were calculated by the following suggests that the samples prepared through a sol-gel process
equation: AQY(%) = (2 × The number of evolved CO molecules/The features micro/mesoporous hierarchical structure which contributes
number of incident photons) × 100.
to higher porosity.
2.3. Photocatalytic hydrogen production
The photocatalytic hydrogen evolution by water splitting was
performed in a glass reaction cell with quartz cover connected to a
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(101)
A)
DOI: 10.1039/C9TA06038C
HO-TPA-TiO2(7.5 wt%)
(103)
(200) (105)
(204)
HO-TPA-TiO2(5.0 wt%)
HO-TPA-TiO2(2.5 wt%)
Gel-TiO2
10
20
30
40
50
60
70
80
2 thetas
B)
HO-TPA-TiO (7.5 wt%)
(101)
2
(103)
(200) (105) (204)
Fig. 2 A) TEM, B) HTEM, and C) SAED of HO-TPA-TiO2, and D) and E)
TEM, F) HRTEM, and G) the elemental mappings image of Pt/HO-TPA-
TiO2 (5.0 wt%).
HO-TPA-TiO (5.0 wt%)
2
HO-TPA-TiO (2.5 wt%)
2
To meet the requirement of CO2 reduction, ReP complex was fixed
onto HO-TPA-TiO2 (5.0 wt%) to obtain the photocatalyst ReP/HO-
TPA-TiO2. To monitor ReP adsorbed on the HO-TPA-TiO2, XPS
measurements for ReP and ReP/HO-TPA-TiO2 after photocatalytic
reaction were carried out. A composition of 23% Ti, 51% O, 0.29% Re,
0.56% P, 2.8% N and 0.43% S for ReP/HO-TPA-TiO2 was obtained,
corresponding with the composition of 2.9% Re, 5.8% P, 6.6% N and
2.1% Cl of ReP with the expected 1:2 ratio for Re:P. The excess N
percentage for ReP/HO-TPA-TiO2 was probably derived from HO-
TPA, TEOA and air, and the low Cl percentage suggested that the
active sites resulted from Cl leaving may play an important role in
CO2 immobilization and photoreduction (Fig. S11- S12). To further
confirm the interaction between ReP and HO-TPA-TiO2, the ATR-IR
spectra of ReP and ReP/HO-TPA-TiO2 have been measured. For the
ReP powders, the -C=O stretching bands were observed at 2027 and
1884 cm−1. When the complexes were adsorbed on HO-TPA-TiO2
surface, the -C=O stretching bands became weakened, and red
shifted to 2034 and 1927 cm−1, indicating the robust interaction
between the -PO3H2 moieties of ReP complex and TiO2 (Fig. 3).
Gel-TiO
2
10
20
30
40
50
60
70
80
2 thetas
Fig. 1 The XRD patterns of HO-TPA-TiO2 with different dye content
were recorded without A) and with B) 300 °C calcination.
The SEM and TEM images of HO-TPA-TiO2 (5.0 wt%) materials
were recorded (Fig. S7 and Fig. 2), showing the nanoparticle size
about 10 nm (Fig. 2A and Fig. 2B). The SAED results confirmed the
crystallinity of TiO2 nanoparticles in the hybrid materials (Fig. 2C).
Moreover, the elemental composition of Gel-TiO2 and HO-TPA-TiO2
(5.0 wt%) was measured by XPS measurements (Fig. S8-S9), revealing
that HO-TPA was successfully doped within TiO2 through the sol-gel
process.
For the purpose of H2 production, Pt NPs was introduced into the
porous hybrid materials by reduction of H2PtCl6·6H2O with methanol
under full light. The typical TEM images of the Pt/HO-TPA-TiO2 (5.0
wt%) materials with 0.6 wt% Pt loading showed the size distribution
of Pt particles inside the hybrid materials was about 3-5 nm (Fig. 2D
and Fig. 2E). Analysis of the HRTEM images for the Pt NPs indicated
that the d-spacing between two adjacent lattice planes was about
0.196 nm (Fig. 2F), in agreement with the spacing of Pt (200) plane.
Moreover, the HAADF-STEM elemental mapping of Pt/HO-TPA-TiO2
(5.0 wt%) indicated the elements C, N, O, S and Pt were
homogenously distributed throughout the whole hybrid materials
(Fig. 2G). The EDX results of a point analysis further demonstrated
the existence of Pt NPs encapsulated in hybrids (Fig. S10).
0.20
1884
0.16
2027
0.12
ReP
0.08
0.04
1927
2034
ReP/HO-TPA-TiO
1800
2
0.00
1700
1900
Wavenumbers / cm-1
2000
2100
Fig. 3 ATR-IR spectra of ReP and ReP/HO-TPA-TiO2 (5.0 wt%).
The UV/vis solid-state diffraction spectra of HO-TPA, ReP, Gel-
TiO2, HO-TPA-TiO2 and ReP/HO-TPA-TiO2 were showed in Fig. 4A. The
Gel-TiO2 had a strong absorption peak at 325 nm with a band edge
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of 450 nm, corresponding to a 3.2 eV band gap (Fig. 4B). The HO-TPA TiO2, depend on the sample types, solvents and pH for aqueous
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44,45
DOI: 10.1039/C9TA06038C
had an intense absorption band in the region of 400-500 nm, which systems with the Mott-Schottky measurement methods.
The
could be attributed to a π-π* transition with charge-transfer positive slope was in accordance with an n-type semiconductor and
character. After hybridization of HO-TPA sensitizers with TiO2, a the flat band potential was -0.72 V and -1.43 V (vs. Ag/AgCl) for Gel-
typical absorption band of HO-TPA-TiO2 (5.0 wt%) was significantly TiO2 in aqueous solution (Fig. S14A) and DMF solution (Fig. S 14B),
broadened from 320 to 750 nm, which could be attributed to the respectively. Since it is generally accepted that the conduction band
robust linkage of isoreticular organic-inorganic hybrid materials on (CB) potential in n-type semiconductors was approximately equal to
molecular and atomic level. Different from Gel-TiO2 with only a band the Vfb,46,47 the CB potential of Gel-TiO2 was estimated to be -0.72 V
gap in Tauc plots, HO-TPA-TiO2 (5.0 wt%) showed two clear band vs. Ag/AgCl in aqueous solution, which was more negative than the
gaps: a narrow band gap of 2.5 eV and wide band gap of 3.1 eV (Fig. redox potential of H+/H2 and thus favored the proton reduction to
4B). We deduced that the former could be ascribed to HO-TPA and realize photocatalytic H2 generation. The valence band (VB) potential
the latter was attributed to Gel-TiO2. Interestingly, compared with can be calculated to be 2.04 V vs. Ag/AgCl based on the band gap. On
the E0-0 value (2.74 eV) of pure HO-TPA (Table S2), a narrow band gap the other hand, the first oxidation potential of the dyes was
(2.5 eV) of Calix-3 doped in the Calix-3-TiO2 was observed, indicating measured by CVs measurements to obtain HOMO value (Fig. S15 and
the formation of Ti-O bonds in hybrid Calix-3-TiO2 materials (Fig. Table S2). The zero-zero excitation energy was estimated from
S13). Moreover, ReP exhibited an intense absorption band in 350- absorption/emission data (Fig. S16). The redox potential of excited
550 nm regions. Therefore, incorporation of ReP contributed to the HO-TPA* (-1.50 V vs. Ag/AgCl) was calculated from the HOMO value
increase of light absorption intensity of the hybrid material in the of HO-TPA and the zero-zero excitation energy, which was higher
visible-light region, which was consistent with the enhancement of than the CB edge of the Gel-TiO2, the potential of LUMO in Gel-TiO2.
absorption coefficients of ReP/HO-TPA-TiO2.
Therefore, it was thermodynamically beneficial for direct electron
transfer from HO-TPA* to Gel-TiO2. In addition, the Vfb of Gel-TiO2 (-
1.43 V vs. Ag/AgCl) in DMF solution was more negative than the
reduction potential of ReP (E1/2red = -0.83 V vs. Ag/AgCl) shown in Fig.
S17 and Table S2, and thus the electron transfer should be favorable
from the conduction-band of Gel-TiO2 to the catalytic center ReP
complex for the photoreduction CO2 to CO.
1.2
HO-TPA
A)
ReP
Gel-TiO
2
HO-TPA-TiO
2
0.8
0.4
0.0
ReP/HO-TPA-TiO
2
3.2. Effect of dye-doped amount on photocatalytic H2 production
The photocatalytic activities of the HO-TPA-TiO2 with 0.6 wt% Pt
loading were evaluated in visible light region. As shown in Fig. 5, HO-
TPA-TiO2 (2.5 wt%) exhibited an H2 production activity of 3.41 mmol
g−1 h−1. With the mass percentage increase of HO-TPA to 5.0 wt%, the
best photocatalytic activity of 3.71 mmol g−1 h−1 was obtained.
However, further increase in the dye percentage to 7.5 wt% resulted
in the activity decrease to 3.22 mmol g−1 h−1, suggesting the activities
of the photocatalysts were not simply correlated to the dye-doped
amounts. A speculation was that the excessive dye molecules might
cause a loss of light harvesting and hinder effective electron
transport simultaneously. To investigate the effects of different
doping amounts of HO-TPA on the absorption properties and carrier
separation, the UV/vis solid-state diffraction spectra and steady-
state photo-luminescence (PL) spectra of HO-TPA-TiO2 with different
dye-doped amount (2.5/5.0/7.5 wt%) have been measured (Fig.
S18). In Fig. S18A, the absorption intensity was relevant to the
content of dye and the higher intensity at 350-510 nm appeared for
HO-TPA-TiO2 (5.0 wt%), which was coincident with the maximum H2
evolution observed in this sample. The excessive HO-TPA dye
inversely incurred the optical absorption decline and the
corresponding H2 evolution decrease. The positively related variation
trend between absorption intensity and H2 evolution rates in HO-
TPA-TiO2 indicated that the first step of light absorption to generate
electron-hole pairs was crucial for this hybrid system. On the other
hand, the fluorescence intensity of these hybrids in Fig. S18B
followed the order of HO-TPA-TiO2 (2.5 wt%) > HO-TPA-TiO2 (7.5
wt%) > HO-TPA-TiO2 (5.0 wt%), indicating that the improved
excitation dissociation and more efficient charge transport between
300
400
500
600
700
800
Wavelength / nm
B)
HO-TPA-TiO
2
Gel-TiO
2
3.1 eV
3.2 eV
2.5 eV
2.0
2.5
3.0
hv / eV
3.5
4.0
Fig. 4 A) UV-vis diffraction spectra of HO-TPA, ReP, Gel-TiO2, HO-
TPA-TiO2 (5.0 wt%) and ReP/HO-TPA-TiO2 (5.0 wt%), and B) Tauc
plot of Gel-TiO2 and HO-TPA-TiO2 (5.0 wt%).
To elucidate the possibility for subsequent photocatalytic H2
production and photoreduction of CO2, Mott-Schottky
measurements of Gel-TiO2 were monitored under different
conditions (Fig. S14). It is known that the flat-band potentials (Vfb) of
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HO-TPA and TiO2 interface for hybrid material HO-TPA-TiO2 (5.0 3.3. Stability tests
wt%), in accordance with the photocatalytic activity. It is reasonable
View Article Online
DOI: 10.1039/C9TA06038C
It is known that the loss in H2 production rate is a common and
challenging problem, which could be caused by the dye consumption
or instability under long time visible light irradiation. To check the
photo-stability of our hybrid materials, the photocatalytic
performance of HO-TPA-TiO2 photocatalysts was repeatedly
evaluated to confirm the stability in this study. The details operation
is same as above section 2.3. And the catalyst was recycled by
centrifugation, washing with methanol and vacuum drying after
every 5 h visible light irradiation. The hybrid catalyst was recyclable
for 15 times. As shown in Fig. 6, a general stable H2 production is
evidently maintained during recycling reactions under lasting visible
light irradiation, verifying the excellent stability of organic-inorganic
components. The TONH2 value was calculated as number of mole of
H2 divided by the number of mole of Pt or HO-TPA, giving significant
results of 6417 or 8120 after 75 h photoreactions, respectively.
that lower content HO-TPA could be isolated in the hybrids,
corresponding to lower amount of active sites for carrier separation;
on the contrary, excessive HO-TPA could engender aggregation in
the hybrids and the following decline of activity.
In contrast, the H2 generation rate of Pt/Gel-TiO2/HO-TPA surface
sensitization material with 0.6 wt% Pt loading and 2.4 wt% of HO-
TPA dye amount was only 1.83 mmol g-1 h-1, which was much lower
than all the rates of coordination hybrid Pt/HO-TPA-TiO2 catalysts.
During the photocatalytic reaction process, dye HO-TPA seriously
detached from Pt/Gel-TiO2/HO-TPA material under visible light
irradiation (the corresponding experimental details and results of
doping-dye shedding were described in Fig. S19.), but detaching
phenomena did not occur to the Pt/HO-TPA-TiO2 catalysts even after
used for 15 times and recycled, indicating the hybrid materials have
higher photocatalytic activity and stability than common surface dye-
sensitized system. It was expectable that the inorganic component
TiO2 in the hybrid materials could protect the organic dye HO-TPA,
and the stable linkages between HO-TPA and TiO2 were beneficial to
efficient electron transfer from HO-TPA* to TiO2. In other words,
organic-inorganic components in the hybrid system have synergistic
effect on the photocatalytic reaction.
18
1415
10 11 12 13
1
2
3
5
6
7
8
9
4
15
12
9
24
Pt/HO-TPA-TiO (2.5 wt%)
2
6
Pt/HO-TPA-TiO (5.0 wt%)
20
16
12
8
2
Pt/HO-TPA-TiO (7.5 wt%)
2
3
Pt/Gel-TiO /HO-TPA (2.4 wt%)
2
0
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75
Time / h
Fig. 6 Photocatalytic H2 production over the recyclability of HO-TPA-
TiO2 (5.0 wt%) with 0.6 wt% Pt loading under visible light irradiation
(λ > 420 nm) in 100 mL H2O/TEOA (9:1 v/v).
4
0
0
1
2
3
4
5
Time / h
3.4. Mechanism of H2 production photocatalysis
Fig. 5 Effect of dye-doped amount (2.5/5.0/7.5 wt%) of HO-TPA-TiO2
and sensitized method on photocatalytic activity for H2 production in
100 mL H2O/TEOA (9:1 v/v) under visible light irradiation (λ > 420 nm
).
To elaborate the photocatalytic mechanism of hybrid Pt/HO-TPA-
TiO2 catalysts, the steady-state photo-luminescence (PL) spectra
were measured to monitor the electron transfer from the excited
HO-TPA* to TiO2 for a series of comparable systems as control
experiments (Fig. S21). The HO-TPA compound showed an intensive
emission peak centered at 566 nm, which was attributable to its
strong recombination of excited charge pairs by exciting light (λ = 400
nm). For comparison, PL spectra of the other three different systems
under the same conditions show the decrease of relative
fluorescence intensity of HO-TPA, following the order of hybrid HO-
TPA-TiO2 > surface dye-sensitized Gel-TiO2/HO-TPA > physical-mixing
Gel-TiO2/HO-TPA > HO-TPA. This indicated that the improved
excitation dissociation and more efficient charge transport between
HO-TPA and TiO2 interface occurred for HO-TPA-TiO2 hybrid
material, in contrast to the less effective surface dye-sensitized
system and physical-mixing system. Meanwhile, slight blue shifts
(~24 or 27 nm) of the emission peaks for Gel-TiO2/HO-TPA or HO-
TPA-TiO2 materials further confirmed the interfacial electron transfer
from the sensitized HO-TPA* to the TiO2. Therefore, it was
The photocatalytic action spectra of hydrogen production for
Pt/HO-TPA-TiO2 (5.0 wt%) samples have been obtained under
different wavelengths of incident light irradiation (Fig. S20 and Table
S3). The corresponding apparent quantum yield (AQY) of the samples
followed the order of 2.2% (400 nm) > 1.9% (420 nm) > 1.3% (450
nm) > 0.6% (470 nm) > 0.1% (515 nm). The results showed that the
shorter wavelength suitable for H2 evolution corresponded well with
the main absorption wavelengths of HO-TPA-TiO2 (Fig. 4A), indicating
the H2 production reaction was indeed controlled by light absorption
properties of photocatalyst. Compared with other literatures
reported for heterogeneous catalytic systems under visible-light
(Table S4), the AQYs of Pt/HO-TPA-TiO2 (5.0 wt%) obtained at 420
and 450 nm were moderate.
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400
reasonable to assume that, in the hybrid system, the electron could
transfer from excited HO-TPA* to TiO2 through the coordination Ti-
O-TPA linkage on atomic level, and finally to Pt NPs surface to
produce H2. Meanwhile, the HO-TPA* became oxidized HO-TPA+ and
then it was reduced back to HO-TPA by sacrificial reagent
triethanolamine (TEOA). Moreover, holes in TiO2 could travel in the
opposite direction from the more positive VB of TiO2 to HO-TPA+,
leading to all-round efficient charge separation and enhanced
photocatalytic activity (Fig. 7).
View Article Online
DOI: 10.1039/C9TA06038C
ReP/HO-TPA-TiO2
ReP/Gel-TiO2
ReP
300
200
100
0
ReP/Gel-TiO2/HO-TPA
0
5
10
15
Time / h
20
25
Fig. 8 Plot of CO formation versus time for 10 mg ReP/HO-TPA-TiO2
(5.0 wt%), ReP/Gel-TiO2, ReP, and ReP/Gel-TiO2/HO-TPA (5.0 wt%) in
5 mL CO2-saturated DMF with 134 mg BIH as electron donor under
visible light irradiation ( λ > 420 nm).
An isotope-labeling experiment was carried out for ReP/HO-TPA-
TiO2 (5.0 wt%) dispersions in 13CO2-saturated DMF-d7 with BIH as the
electron donor. As shown in Fig. S22, the 13C NMR spectrum of the
irradiated sample revealed a signal at δ 185.2 ppm corresponding to
the resonance of 13CO. In addition, the 13C isotope abundance in the
CO formed was analyzed by GC-MS to be > 90% (Fig. S23). Moreover,
in contrast to TONCO of 114 in CO2-saturated DMF for 5 h, no obvious
CO production for this catalyst was found in Ar-saturated DMF under
visible light irradiation, evidently indicating that CO2 is the source of
CO product.
Fig. 7 Proposed mechanism of photocatalytic H2 production over
Pt/HO-TPA-TiO2.
3.5. Photocatalytic CO2 Reduction
Dispersions of ReP/HO-TPA-TiO2 (5.0 wt%) particles (10 mg with
0.041 μmol ReP adsorbed) in 10 mL CO2-saturated DMF containing
BIH (134 mg) were irradiated at > 420 nm using a xenon lamp. The
amounts of CO evolved in the overhead space of the reaction cell
were determined by gas chromatography, whereas the liquid phase
after irradiation was analysed by HPLC. It was confirmed that the
exclusive product is CO accompanied by a trace amount of H2, while
formic acid was not detected at all. Fig. 8 shows plots of turnover
numbers TONCO (representing mole numbers of CO formed relative
to unit mole of the Re complex used) for CO formation versus
irradiation time for the ReP/HO-TPA-TiO2 system, in which the CO
formation steadily continued up to ~25 h with no significant leveling-
off tendency, giving a high TONCO of 300. The apparent quantum
yields on photocatalytic CO production were determined to be 0.37%
at 420 nm, 0.13% at 450 nm and 0.047% at 500 nm for ReP/HO-TPA-
TiO2 (Table S5).
The hybrid system exhibited inherent capability for the two-
electron CO2 reduction under steady supply of electrons through TiO2
due to the energetic match between HO-TPA*, ReP- and CB of TiO2.
The lower rate of photocatalytic CO2 reduction with the surface dye-
sensitized ReP/Gel-TiO2/HO-TPA system compared to the
coordinative ReP/HO-TPA-TiO2 (5.0 wt%) system suggested that the
robust coordination linkage between organic and inorganic
components in nanoscale was more conducive to the electron
transfer from HO-TPA to ReP through the TiO2. For comparison,
ReP/Gel-TiO2 without anchoring HO-TPA was much less efficient for
formation of CO, confirming incorporation of HO-TPA into TiO2 was
indispensable for the efficient reduction of CO2.
3.6. Effect of TEOA as additive on photoreduction CO2
It was reported that the photocatalytic activities of the hybrid
systems were considerably affected by TEOA additive, because
related Re(I) complexes could trap CO2 in the presence of this amine
in the dark to give TEOA-participated Re(I) carbonato complexes.40
Therefore, we also explored potential TEOA effect on the
photocatalytic activity of ReP/HO-TPA-TiO2 (Fig. 9), which showed
the plots of TONCO versus time for ReP/HO-TPA-TiO2 (5.0 wt%) in
photocatalyzed CO2 reduction with BIH as the electron donor using
different concentrations of TEOA additives. It was found that the
photocatalytic activity was significantly enhanced by TEOA at 0.23 M,
exhibiting the highest TONCO of 534 for 26 h and TOFCO of 20.5 h-1,
which represents, to the best of our knowledge, the highest value for
so far reported hybrid catalytic systems. However, the higher
concentration of 0.46 M TEOA would deteriorate the activity of the
hybrid materials (TONCO = 489 for 26 h). As reported previously,40,48
the low-concentration TEOA could enhance the photocatalyzed CO2
reduction by trapping CO2 to form TEOA-bound carbonato complex
(LRe-OCOO-(CH2)2NR2; R = (CH2)2OH), and thus accelerate the
coordination of CO2 after the first one-electron reduction of the Re
complex, while the high-concentration TEOA would induce an excess
negative shift of flat-band potentials (Vfb) of TiO2, unfavorable for the
electron injection from dye to TiO2.
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300
be transferred to the PReX site. At last, the catalytic cycle can be
completed after the recovery of HO-TPA by the reduction of HO-
TPA·+ with BIH as the electron donors.
View Article Online
DOI: 10.1039/C9TA06038C
0 M TEOA
0.23M TEOA
0.46M TEOA
200
100
0
0
1
2
3
4
5
6
7
8
Time / h
Fig. 9 Plot of CO formation versus time for 10 mg ReP/HO-TPA-TiO2
(5.0 wt%) in 5 mL CO2-saturated DMF with 134 mg BIH as electron
donor and 0/0.23/0.46 M TEOA as the additive under visible light
irradiation (λ > 420 nm ).
Fig. 10 Proposed mechanism of photoreduction CO2 to CO
production over ReP/HO-TPA-TiO2 under visible light irradiation.
3.7. Mechanism of CO2 photoreduction to CO
A possible outline of the major pathways for the photocatalytic
reaction, demonstrates the charge transfer processes of the hybrid
materials ReP/HO-TPA-TiO2 in selective reduction of CO2 to CO with
considerably high turnover numbers (Fig. 10). The excited-state
oxidation potential (E*) of HO-TPA (-1.53 V vs. Ag/AgCl) was much
more negative than the flat-band potential (Vfb) of Gel-TiO2 (-1.43 V
vs. Ag/AgCl) in DMF solution, which allows the ultrafast electron
injection under visible light irradiation. The TiO2 in the hybrid system
worked as electron reservoir, which might be easily to transfer
electrons in dynamics to the ReP sites for the reduction of CO2 to
CO.49 The injected electron from the TiO2 flowed to the ReP complex
to motivate the two-electron catalytic reduction of CO2 on the ReP
4. Conclusions
We present an unprecedented strategy to design and prepare a
robust porous hybrid material (HO-TPA-TiO2) with combination of
TiO2 and calixarene dye HO-TPA containing four D-π-A units through
a sol-gel method, offering efficient and persistent dual-function
photocatalytic activities when loaded with Pt nanoparticles or ReP as
co-catalysts. For comparison, the coordination binding Pt/HO-TPA-
TiO2 with 0.6 wt% of Pt loading exhibits much higher H2 production
activity and more excellent stability than analogous Pt/Gel-TiO2/HO-
TPA system under the similar photocatalytic conditions. The
enhancement in H2 evolution efficiency and stability could be
ascribed to stable Ti-O-TPA linkage and more efficient electron
transfer between calixarene dye and TiO2. On the other hand, the
HO-TPA-TiO2 hybrid system loading 0.26 wt% ReP complex works as
a stable photocatalyst for the selective reduction of CO2 to CO in DMF
in the presence of an electron donor BIH, of which the photocatalytic
activities can be tuned by TEOA additives. A remarkable TONCO of 534
(2.19 mmol g−1 based on the mass of hybrids or 854.4 mmol g−1 based
on ReP mass) has been achieved via a single-run photoreaction of
Re/HO-TPA-TiO2 in the presence of 0.23 M TEOA with no leveling-off
tendency for 26 h, which represents one of the highest among all the
reported heterogeneous catalytic systems under visible-light. The
convenient methodology reported here might provide a potential
approach to develop highly active, robust and multi-functional
organic-inorganic hybrid systems for the efficient photocatalysis.
red
sites, where the Vfb of TiO2 should be more negative than E1/2 of
ReP (-0.83 V vs. Ag/AgCl) (Fig. S16). However, the mechanistic details
of CO2 reduction catalyzed by PReX have not yet been fully
understood (Fig. S24). In principle, a 17-electron species (PRe•)
formed from the one-electron reduced state (PReX•−) has been
generally accepted for the coordination of CO2 in equilibrium with
the coordination of a solvent molecule (DMF),50 which resulted in
complicated intermediates, such as, CO2-bridged dimer complex
(LRe-COOReL),51 CO2-coordinated complex,52 metallocarboxylic acid
or TEOA-bound carbonato complex by the introduction of H2O or
TEOA.49 Among them, the CO2-bridged dimer complex was
unfavorable to the catalytic activity of CO2 photoreduction to CO. In
our system, the phosphonated ReP complex was immobilize onto the
hybrid nanoparticle (HO-TPA-TiO2) surface at extremely low mass
percentage (0.26%), which avoided forming the inactive ReP complex
dimers, therefore improving the TONCO value in comparison to the
homogeneous ReP complex.39 Furthermore, the CO-formation from
CO2 by Re catalyst was a second-reduction step accompanied by a
multi-electron process, involving the generation of a higher-
oxidation state Re complex, which must be reduced by the second
electron transfer to regenerate the starting-state complex, PReX. In
present hybrid system, the second-reduction step could occur with
the electrons “pooled” in TiO2, which might be long-lived enough to
Acknowledgements
This work was supported by NSFC (21572280, 21720102007,
21890382 and 21821003), Local Innovative and Research Teams
Project of Guangdong Pearl River Talents Program
(2017BT01C161), STP Project of Guangzhou (201504010031
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[29] Y. Chen, X. Luo, Y. Luo, P. Xu, J. He, L. Jiang, J. Li, ZV.iYewanA,rtJic.leWOannlinge,
and 201707010114), NSF of Guangdong Province
(S2013030013474), and FRF for the Central Universities
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Conflicts of interest
There are no conflicts to declare.
Appendix A. Supplementary data
The Supporting Information is available free of charge on the
ACS Publications website at DOI: XXXX.
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