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W. Wu et al. / Journal of Photochemistry and Photobiology B: Biology xxx (2014) xxx–xxx
and X-ray photoelectron spectroscopy (XPS) were used to confirm
that CQDs as electron reservoir play an important role to enhance
electron transfer from the organic photocatalyst (QuPhÅÀNA) to Pt
salts.
2.4. Photocatalytic hydrogen evolution
A mixed solution (2.0 mL) of an aqueous buffer (pH 4.5, 7.0, 8.0,
9.0 or 10) and MeCN [1:1 (v/v)] containing QuPh+–NA (0.22 mM),
NADH (1.0 mM), CQDs, K2PtCl4 or K2PtCl6 was flushed with N2
gas. The solution was then irradiated with a Xe lamp (Ushio
Optical, Model X SX-UID 500X AMQ) through a color filter glass
(Toshiba Glass UV-35) transmitting k > 340 nm at room tempera-
ture. The gas in the headspace was analyzed using a Shimadzu
GC-14B gas chromatograph (detector, TCD; column temperature,
50 °C; column, active carbon with 60–80 mesh particle size; carrier
gas, N2) to quantify the evolved hydrogen.
2. Materials and methods
2.1. Materials
EDTA-2NaÁ2H2O, K2PtCl4 and K2PtCl6 were purchased from
Sigma–Aldrich. 2-Phenyl-4-(1-naphtyl)quinolinium perchlorate
(QuPh+ÀNA) was synthesized by the reported method [48]. Each
buffer solution was prepared by addition of NaOH to an aqueous
solution containing 50 mM of electrolyte (phthalate for pH 4.5,
phosphate for pH 7.0 or 8.0, or boric acid /potassium chloride for
pH 9.0) or 25 mM of carbonate for pH 10. All chemicals were used
without further purification.
2.5. Kinetic measurements
A mixed solution (2.0 mL) of a deaerated aqueous buffer (pH
7.0) and MeCN [1:1 (v/v)] containing QuPh+–NA (0.44 mM) and
NADH (1.0 mM) was photoirradiated for 15 min with a Xe lamp
through a color filter glass transmitting k > 340 nm. Next, a deaer-
ated aqueous solution containing CQDs, K2PtCl4, K2PtCl6, CQD/K2-
PtCl4 or CQDs/K2PtCl6 was added to the photoirradiated solution
using a microsyringe with stirring. Rate constants of electron
transfer from QuPhÅ–NA (obtained by one-electron reduction of
QuPh+–NA) to the catalyst were determined from the decay of
absorption at 510 nm due to QuPhÅ–NA, which was monitored
using a Hewlett–Packard 8453 diode array spectrophotometer
with a quartz cuvette (path length 10 mm) at 298 K.
2.2. Preparation of CQDs from EDTA-2Na
EDTA-2NaÁ2H2O containing relatively stable carboxylate
anions (COOÀ) was chosen as the precursor. After pyrolysis under
low temperature, COOÀ can remain and thus render the CQDs sol-
uble in water, and easily capture Pt cation. EDTA-2NaÁ2H2O
(1.6 g) was calcined in a tube furnace at 350 °C for 2 h at a heat-
ing rate of 5 °C minÀ1 in a N2 atmosphere. The product was
ground and dispersed in water (100 mL), and then centrifuged
at a high speed (10,000 rpm) for 20 min. The upper brown disper-
sion was filtered with slow-speed quantitative microporous filter
3. Results and discussion
paper (0.25 lm) to remove the non-fluorescent deposit. The
filtrate was dialyzed with MD34 (3500 Da) dialysis tube for 48 h
to remove the remaining salts and small fragments. Pure lumines-
cent CQDs powder was obtained by drying the concentrated solu-
tion at 60 °C for 24 h.
3.1. Characterization of CQDs
The uniformity of the as-synthesized CQDs is clearly shown in
Fig. 1. The diameters of CQDs are centered at 2–3 nm from trans-
mission electron microscopy (TEM) images. High-resolution TEM
(HRTEM) images of CQDs show well-resolved lattice fringes with
interplanar spacings of 0.20 nm, which are close to the (100) dif-
fraction facets of graphite carbon. The Raman peaks centered at
ꢀ1334 and 1561 cmÀ1 for the CQDs are attributed to the D and G
bands of carbon materials representing the sp3 and sp2 carbon,
respectively (see Supporting Information Fig. S1). The intensity
ratio of the D and G bands (ID/IG) is a measure of the disorder
extent, as well as the ratio of sp3/sp2 carbons. The ID/IG ratio of
CQDs is 0.7775, indicating that sp2 hybridization C was much more
than sp3 hybridization C [49]. Therefore, the above results suggest
that CQDs are composed of nanocrystalline cores of graphitic sp2
carbon atoms, and it will benefit for the electron reservoir and
efficient electron transfer [44].
2.3. Characterization of CQDs
High-resolution transmission electron microscopy (HRTEM)
images were taken on a JEOL JEM-2100UHR microscope with an
accelerating voltage of 200 kV. Further evidence for the composi-
tion of the product was inferred from X-ray photoelectron
spectroscopy (XPS), using a Thermo Scientific ESCALAB 250Xi
spectrometer equipped with a pre-reduction chamber. Fourier
transform infrared (FT-IR) spectra were recorded on a Nicolet
6700 spectrometer. Raman spectroscopy was recorded using an
Ar+ ion laser at 514.5 nm (Renishaw in via 2000 Raman micro-
scope, Renishaw plc, UK) to assess the graphitic structure of raw
materials and products.
The chemical compositions and structures of the CQDs were
further investigated. X-ray photoelectron spectroscopy (XPS) mea-
surements show that carbon (C1s, 283 eV), nitrogen (N1s, 398 eV),
and oxygen (O1s, 530 eV) elements were contained in CQDs. The
XPS spectrum of C1s from CQDs can be deconvoluted into three
smaller peaks, which are ascribed to the following functional
groups: sp3 bonded carbon (CAC, 283.0 eV), sp2 bonded carbon
(C@C, 283.8 eV) and epoxy/hydroxyls (CAO, 286.4 eV) (Fig. 2)
[50,51]. The O1s spectrum could be deconvoluted into three peaks
of C@O (529.4 eV), CAO (530.3 eV) and chemisorbed oxygen
(COOH) and/or bound water (533.6 eV) [52,53]. Elemental analysis
also confirmed the composition of CQDs (C: 42.01%, H: 5.83%, N:
9.95%). In the fourier transform infrared spectroscopy (FT-IR) anal-
ysis of CQDs (see Supporting Information Fig. S2), the absorption
bands from 1113 to 1263 cmÀ1 are assigned to a CAOAC stretching
and deformation vibrations, and those at 716, 851 and 1600 cmÀ1
are characteristic of benzene ring. The residue peaks at 1329 cmÀ1
2 QuPh NA+
hv
+
NADH
2 QuPh+ NA
H2
N
NAD++ H+
Me
CQDs/Pt
QuPh+ NA
2 H+
2 QuPh NA
K2PtCl4
or
ET
ET
Pt0
CQDs
CQDs/Pt
K2PtCl6
Scheme 1. Structure of QuPh+ÀNA and the overall catalytic cycle for photocatalytic
hydrogen evolution.
Please cite this article in press as: W. Wu et al., Photocatalytic H2 evolution from NADH with carbon quantum dots/Pt and 2-phenyl-4-(1-naphthyl)quin-