J. Qiu et al.
MaterialsResearchBulletin112(2019)297–306
reports were focused on the metal NPs surface loading. Horiuchi and co-
workers deposited Pt NPs as co-catalysts onto MIL-125-NH2 by a pho-
todeposition process for visible-light-promoted photocatalytic hy-
drogen production [32]. Ag/MIL-125-NH2 has been synthesized in situ
by the reduction of Ag+ with the acetylacetonate pendant groups and
the nanocomposites showed efficient methylene blue photocatalytic
degradation [33]. Shen et al. synthesized M/MIL-125 (M = Au, Pd, Pt)
composites via redox reaction between the reductive MIL-125 with
Ti3+ and oxidative metal salt precursors for benzyl alcohol photo-
catalytic oxidation [34]. However, the comprehensive studies and
comparison focused on different noble metal particles (Pt or Au)
loading on MIL-125 with and without amino-functionalization for the
benzyl alcohol oxidation have not been studied. In addition, MIL-125-
NH2 decorated by metal nanoparticles for Cr(VI) photocatalytic re-
duction has not been investigated either.
Here, a series of M/Ti-MOFs (M = Pt and Au, Ti-MOFs = MIL-125-
NH2 and MIL-125) photocatalysts has been synthesized by a facile post-
synthetic method for the selective oxidation of benzyl alcohol. Results
demonstrate that (1) the photocatalytic activity of metal loading on
MIL-125-NH2 is higher than that of MIL-125, (2) Pt/MIL-125-NH2
performs much higher photocatalytic activity than that of Au/MIL-125-
NH2 and MIL-125-NH2 for benzyl alcohol oxidation. In addition, Pt/
MIL-125-NH2 also performed high activity for Cr(VI) photocatalytic
reduction. In order to verify the decreased recombination of photo-in-
duced electrons and holes, photocurrent and Electrochemical
Impedance Spectroscopy measurements have been carried out and a
possible mechanism for the improved photocatalytic activity has been
given.
the above solution and stirred for another 15 min. Finally, 10 mL of
NaBH4 aqueous solution (0.35 mol·L−1) was added into the mixture
drop by drop with a time interval of 3 s and a constant vigorous stirring.
Au NPs solution was synthesized by the same method with
HAuCl4·3H2O instead of H2PtCl6·6H2O.
2.4. Synthesis of M/Ti-MOFs (Pt/MIL-125-NH2, Au/MIL-125-NH2, Pt/
MIL-125 and Au/MIL-125)
M/Ti-MOFs were prepared by a facile method. Typically, 250 mg of
MIL-125 or MIL-125-NH2 was added into 6.2 mL as-prepared Pt or Au
solution (1 wt% to Ti-MOFs). Then the mixture was stirred for 1 h fol-
lowed by 1 h ultrasonic treatment and the same processes were re-
peated for three times. The resulting mixture was washed by deionized
water and ethanol for several times and then dried at 80 °C overnight.
The obtained Pt/MIL-125-NH2, Au/MIL-125-NH2, Pt/MIL-125 and Au/
MIL-125 samples (the estimated loading amounts of Pt or Au on MIL-
125-NH2 are 0.68% and 0.76% respectively, which were evaluated by
XPS measurement) were marked as PtMN, AuMN, PtM and AuM, re-
spectively.
2.5. Characterization
The crystalline structures of these samples were measured by X-ray
diffraction (XRD) using Rigaku Ultima IV with Cu Ka radiation
(k = 0.1542 nm) at 40 kV. The Energy Dispersive X-Ray (EDX)
Fluoresence Spectrometer of the samples was observed by utilizing a
JSM-7600 F (JEOL Ltd., Japan) with an operating voltage of 30 kV.
Transmission electron microscopy (TEM) image and high-resolution
transmission electron microscope (HRTEM) were examined by a JEOL
JEM-2100 instrument at the accelerating voltage of 200 kV. Nitrogen
adsorption-desorption analysis was conducted using Micromeritics
ASAP 2020 at 77 K. All samples were degassed at 120 °C for 120 min
prior to analysis. The specific surface areas were calculated by the
Brunauer-Emmett-Teller (BET) method. X-ray photoelectron spectro-
scopy (XPS, AXIS UltraDLD, Japan) was employed to determine surface
electronic states. All the binding energies were referenced to the C1 s
peak at 284.8 eV of the surface adventitious carbon. The UV–vis diffuse
reflectance spectra (DRS) of the samples over a range of 200–800 nm
were recorded by UV-2600 (Shimadzu, Japan) spectrophotometer with
a BaSO4 reference.
2. Experimental
2.1. Materials
Titanium isopropoxide (Ti(O-iPr)4, 99%) was obtained from Sam
Chemical Technology (Shanghai) Co., Ltd. Poly (diallyldimethy-
lammonium chloride) solution (Mw 200000-350000, 20 wt%,
(C8H16ClN)n, PDDA) and Chloroplatinic
acid hexahydrate
(H2PtCl6·6H2O, Pt ≥ 37.5%) were purchased from Aladdin Industrial
Company, China. 1,4-benzendicarboxylic acid, N, N-dimethylforma-
mide (DMF), Sodium borohydride (NaBH4, 98%), NaOH and methanol
were supplied by Sinopharm Chemical Reagent CO., Ltd., China.
Hydrogen Tetrachloroaurate (III) Trihydrate (HAuCl4·3H2O) was ob-
tained from Shanghai Titan Scientific Co., Ltd., China. All chemicals
were used as received without further treatment.
2.6. Electrochemical measurements
Photocurrent measurements and electrochemical impedance spec-
troscopy (EIS) tests were carried out on a CHI-760E electrochemical
workstation (Chenhua Instrument, Shanghai, China) in a standard
three-electrode system. Namely, a saturated calomel electrode (SCE)
was used as the reference electrode, a Pt foil as the counter electrode,
and the samples as the working electrodes. A 0.5 mol·L−1 Na2SO4 (pH
= ˜6) aqueous solution was utilized as the electrolyte. The working
electrodes were prepared as follows: 5 mg of the sample was dispersed
in 0.5 mL DMF and ultrasonicated for 30 min, then 0.1 mL of colloidal
solution was dropped on a 1.5 cm × 1.0 cm indium-tin oxide (ITO)
glass substrate with an active area of about 1 cm2 and allowed to dry at
100 °C overnight. For photocurrent measurements, a 300 W xenon lamp
(CEL-HXF300) served as the light source. Mott-Schottky plot was tested
at the same conditions with Ag/AgCl reference electrode instead of SCE
and without light irradiation.
2.2. Synthesis of Ti-MOFs (MIL-125 and MIL-125-NH2)
MIL-125 and MIL-125-NH2 were synthesized referring to previous
report [35]. In a typical synthesis, 1.6 mL of Ti(O-iPr)4 and 3 g of 1,4-
benzendicarboxylic acid were dissolved in 56 mL of DMF solution
containing 6 mL of methanol, and then continuous stirring for 2 h. After
that, the mixed solution was placed into a 100 mL Teflon-lined stainless
steel autoclave and heated at 150 °C for 48 h. The solid products were
collected by centrifugation and washed with DMF and methanol to
remove the residual reactant and exchange the DMF. MIL-125-NH2 was
synthesized by the same method with 3.3 g of 2-NH2-terephthalic acid
and 72 h instead of 3 g of 1,4-benzendicarboxylic acid and 48 h heating
time, respectively.
2.3. Preparation of Pt and Au NPs solution
2.7. Photocatalytic oxidation of aromatic alcohol
The Pt and Au NPs solutions in a homogeneous dispersion were
prepared based on a previous report with modifications [36]. Typically,
2 mL of KOH solution (0.1 mol·L−1) added into 26 mL of H2PtCl6·6H2O
with a concentration of 3.8 × 10-3 mol·L−1. After vigorous stirring of
15 min, 10 mL of PDDA aqueous solution (0.02 g·mL−1) was added into
The photocatalytic oxidation of benzyl alcohol was carried out
under
a 300 W Xenon lamp (CEL-HXF300, a CEL-VisREF filter,
350–780 nm), 10 cm to the reaction liquid) at ambient temperature
(˜20 °C). In general, 100 mg of the as-prepared photocatalyst was
298