Hole dynamic acceleration over CdSO nanoparticles for high-efficiency solar hydrogen production with urea photolysis

Article abstract of DOI:10.1039/c9ta08357j

Solar water splitting to hydrogen (H₂) fuel is considered a sustainable approach to meet green energy demands in the future, but is mainly limited owing to the sluggish hole dynamics involved in the water oxidation reaction and bulk charge separation. Herein, we implemented hole dynamics engineering over cadmium sulfide (CdS) nanoparticles (NPs) via the oxidation reaction substitution with a facile urea oxidation reaction (UOR) and surface conjugation with sulfurous groups (-S/SO_x^2-) as hole-extraction chains, which thermodynamically reduced the reaction barrier and kinetically accelerated the charge separation concomitantly. We then demonstrated a highly efficient photocatalytic H₂ evolution reaction (HER) from a urea solution with valuable urea degradation. An impressive and stable H₂ generation rate of 1.49 mmol h^-1 g^-1 was achieved under 1 sun irradiation with an apparent quantum yield (AQY) of up to 2.4% at 420 nm. In this study, we designed CdS_xO NP photocatalysts to enable hole thermodynamic and kinetic acceleration, which promise a cost-effective and course-smooth solar H₂ production along with urea-rich wastewater purification.

Full text of DOI:10.1039/c9ta08357j

Journal of
Materials Chemistry A
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PAPER
Hole dynamic acceleration over CdSO
nanoparticles for high-efficiency solar hydrogen
production with urea photolysis†
Cite this: J. Mater. Chem. A, 2019, 7,
5650
2
ab
a
c
Xiaoyong Xu, *ab Jingguo Hu,*b
Fengting Luo, Wen Qiao, Huichao He,
d
ad
Yong Zhou * and Dunhui Wang*
2
Solar water splitting to hydrogen (H ) fuel is considered a sustainable approach to meet green energy
demands in the future, but is mainly limited owing to the sluggish hole dynamics involved in the water
oxidation reaction and bulk charge separation. Herein, we implemented hole dynamics engineering over
cadmium sulfide (CdS) nanoparticles (NPs) via the oxidation reaction substitution with a facile urea
2ꢀ
oxidation reaction (UOR) and surface conjugation with sulfurous groups (–S/SO
chains, which thermodynamically reduced the reaction barrier and kinetically accelerated the charge
separation concomitantly. We then demonstrated a highly efficient photocatalytic H evolution reaction
HER) from a urea solution with valuable urea degradation. An impressive and stable H generation rate
was achieved under 1 sun irradiation with an apparent quantum yield (AQY) of up
O NP photocatalysts to enable hole thermodynamic
x
) as hole-extraction
2
(
2
Received 1st August 2019
Accepted 18th October 2019
ꢀ1 ꢀ1
of 1.49 mmol h
g
to 2.4% at 420 nm. In this study, we designed CdS
x
DOI: 10.1039/c9ta08357j
and kinetic acceleration, which promise a cost-effective and course-smooth solar H
2
production along
rsc.li/materials-a
with urea-rich wastewater purification.
An efficient strategy of replacing the formidable WOR with
thermodynamically more favorable reactions has been reported
Introduction
With increasing energy demands and climate concerns, intense to give rise to energy-saving H production in electrochemical
2
1
3–15
16
efforts in developing clean and renewable energy technologies (EC)
have been made to reduce fossil fuel utilization.
and photo-electrochemical (PEC) devices. Several
1–5
16
17
18
Photo- molecules, such as oxysulde, urea, methanol and hydra-
19
catalytic water splitting over semiconductors to produce green zine, have been observed to enable increased energy conver-
hydrogen (H ) using renewable solar input has been considered sion due to their facile oxidization thermodynamics. However,
2
6–8
a promising approach for future energy needs. Unfortunately, the oxidation substitution has been underexplored in photo-
despite huge efforts for more than 40 years in the past, the catalytic H evolution decoupled from water splitting so far.
direct photocatalysis for the overall water splitting process is Notably, the urea emitted from modern agriculture and chem-
2
20
still facing the dilemma of a much lower conversion efficiency. ical industries is one of the most common water pollutants.
The challenge lies mainly in the four-electron water oxidation Direct urea fuel cells offer great promise for energy-sustainable
9
–11
reaction (WOR) with sluggish kinetics,
the thermodynamic uphill demands a large overpotential for water. Urea oxidative hydrolysis requires a thermodynamic
in which overcoming developments and concurrently remedying urea-rich waste-
21
22
the whole redox course. Therefore, most semiconductors potential as low as 0.37 V, which is much milder than WOR.
appear incapable in hole dynamics toward WOR under strong Thus, it is attractive to replace WOR with the urea oxidation
competition with spontaneous charge recombination.
12
reaction (UOR) for enhanced photocatalytic H
production and
2
simultaneous urea-rich wastewater purication. Moreover, the
ꢀ
+
six-hole UOR (i.e. CO(NH ) + 6OH + 6h / N + CO + 5H O)
2
2
2
2
2
a
College of Electronics and Information, Hangzhou Dianzi University, Hangzhou
can furnish abundant electrons for the H
(HER) and avoids the explosive H /O gas mixing without low-
value O generation, but makes a special demand on catalysts
2
evolution reaction
3
b
10018, China. E-mail: xxy@yzu.edu.cn; wangdh@nju.edu.cn
College of Physics Science and Technology, Yangzhou University, Yangzhou 225002,
2
2
China. E-mail: jghu@yzu.edu.cn
c
2
State Key Laboratory of Environmental Friendly Energy Materials, Southwest in steering hole transfer.
University of Science and Technology, Mianyang 621010, China. E-mail:
Cadmium sulde (CdS) has been considered one of the most
promising photocatalysts for solar H generation because of its
desirable band alignment for substantial sunlight absorption
and suitable redox potentials for water splitting.
hehuichao@swust.edu.cn
d
2
School of Physics, Nanjing University, Nanjing 210093, China. E-mail:
zhouyong1999@nju.edu.cn
23–26
However,
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ta08357j
CdS still shows the low activity and poor stability for
1
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photocatalytic H
2
evolution due to its sluggish hole dynamics (EDX) spectrum (Fig. S3, ESI†) and corresponding elemental
against charge recombination and anodic corrosion. Herein, we mappings (Fig. 1d) illustrate the presence of the Cd, S, and O
report a strategy of modulating hole dynamics over CdS nano- elements in a homogeneous distribution for the CdS O sample.
x
particles (NPs) through conjugation with sulfurous groups (–S/ The Fourier transform infrared (FT-IR) spectra conrm that the
2
ꢀ
SO
impressive solar H
dation. The conjugated sulfurous groups serve as hole extrac- peaks at around 1007, 1107 and 1633 cm can be ascribed to
x
) and integration with accessible UOR, enabling an CdS
x
O NPs are conjugated with abundant sulfurous groups on
2
generation with simultaneous urea degra- the surface, as shown in Fig. 1e, where the observed absorption
ꢀ
1
2
ꢀ
2ꢀ
tion channels toward UOR, evidenced by various kinetics the –SO
3
, –SO
4
and –S groups, respectively, while absent in
analyses on charge separation, signicantly facilitating HER the commercial pure CdS crystals. The FT-IR peak identication
28,29
with a record apparent quantum yield (AQY) of 2.4% at 420 nm is consistent with the previous studies,
and moreover is
for solar-to-H2 conversion without an additional sacricial further veried by controlled experiments herein (Fig. S4, ESI†).
agent.
The Raman spectra (Fig. S5, ESI†) show two typical peaks at 299
ꢀ1
30
and 600 cm of cubic-phase CdS, but the peak intensities of
the CdS O sample are much lower than that of the pure CdS
x
Results and discussion
Preparation and characterization
sample probably due to the shielding effect of the massive
surface groups.
To further conrm the surface bonding of the sulfurous
A facile co-solvent method with a tunable ratio of Cd and S
sources was used to regulate the CdS growth, followed by
extraction washing and air drying, resulting in the formation of
groups on the CdS O NPs, we set the atomic ratios of the Cd to S
x
sources at 1 : 910 and 1 : 1 in the reaction mixture to obtain
different samples (labeled as CdS O-1 and CdS O-2, respec-
x
x
sulfurous group-conjugated CdS (CdS O) NPs (see Method for
x
tively), and probed their surface components before and aer
the annealing treatment. In the absence of a sulfur-rich envi-
synthesis details, Fig. 1a). The transmission electron micros-
copy (TEM) image in Fig. 1b shows that the CdS O product
x
ronment, the resulting CdS
decline in the surface sulfurous groups compared with the
CdS O-1 sample, as evidenced by their FT-IR spectra (Fig. S6,
x
O-2 sample displays an obvious
appears as a occulent colloid containing plenty of NPs with
wide sizes of less than 10 nm. According to the X-ray diffraction
x
(XRD) pattern (Fig. S1, ESI†) and the high-resolution TEM (HR-
ESI†), in which the annealed CdS sample almost loses all FT-IR
TEM, Fig. 1c), these NPs were identied as cubic-phase CdS
27
signals of the surface groups due to the removal of the sulfurous
crystals with resolved (111) lattice planes. The selected area
electron diffraction (SAED) and the fast Fourier transform (FFT)
patterns (Fig. S2, ESI†) further conrm the well-dened cubic
CdS crystallinity. Interestingly, the energy-dispersive X-ray
ꢁ
groups by annealing above 500 C. In addition, their EDX
elemental proportions were analyzed and are shown in Fig. S7
ESI†), in which the maximum ones of the S and O elements and
(
the minimum one of the Cd element appear in the CdS O-1
x
sample. The X-ray photoelectron spectra (XPS, Fig. S8, ESI†)
exhibit the characteristic peaks of S 2p and Cd 3d in the Cd–S
coordination. In particular, the CdS O sample presents an
x
2
ꢀ
emerging asymmetric peak near 169 eV, referring to the SO
/
3
2
ꢀ
31,32
SO4 molecules,
which provides evidence for the coupling
groups. These results unambiguously demonstrate
evidence of surface conjugation of the sulfurous groups on
CdS O NPs grown from the sulfur-rich solution.
Notably, Na S/Na SO is usually employed as a sacricing
reagent to supply negatively charged S /SO
2ꢀ
SO
x
x
2
2
3
2
ꢀ
2ꢀ
3
ions dispersed
in aqueous solution for the rapid capture of photogenerated
holes, and thus make hot electrons more accessible into HER.
2
ꢀ
2ꢀ
However, the direct graing of the S /SO3 groups on the
catalysts has yet been underexplored for photocatalytic H
2
evolution. Therefore, it is worth examining whether the conju-
gated sulfurous groups can serve as hole-extraction chains,
instead of expensive sacricial agents, to reinforce the solar H
2
x
production over the present CdS O NP photocatalyst.
Photo-induced charge dynamics
x
The mean size of the CdS grains in the CdS O sample is esti-
mated at ca. 6.5 nm using the Scherrer equation from the XRD
data (Fig. S1, ESI†) and is consistent with the HR-TEM obser-
vation. In particular, this grain size is close to the depletion
layer width (W, ꢂ6.2 nm) estimated for n-type CdS (Note S1,
Fig. 1 Preparation and characterization. (a) Solvothermal synthetic
route of CdS O NPs. (b) TEM image, (c) HR-TEM image, and (d) EDX
x
x x
elemental mappings of CdS O NPs. (e) FT-IR spectra of CdS O NPs and
pure CdS crystals.
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ESI†), and beyond the quantum-conned space dened by the that can quickly capture the out-diffusing holes from the CdS
3
3
Bohr exciton radius (R , ꢂ3 nm). Thus, the free diffusion in crystal cores (Fig. 2g). Accordingly, the ON/OFF switched SPV
B
the absence of the interior electric force can be the main spectra (Fig. 2b) display the repeatable SPV responses that are
34
mechanism for the charge separation and outow. In fact, the vectorially opposite at two different samples and numerically
high-proportion atoms near the surface in the small CdS grains different by a factor of 20. The SPV nding presents clear
play the main role in the photoelectronic excitation. Therefore, evidence for an important role of the surface conjugation with
the separation and utilization of photogenerated electrons and sulfurous groups in regulating the transfer of photogenerated
holes can be governed by the kinetics of the surface charge charge, concomitant with the optimum charge separation and
35
transfer alone. Here, the surface sulfurous groups on the electron output.
CdS O NPs were designed as the capture buffer for hole
The UV-vis absorption spectra in Fig. 2c show that both
x
extraction, resulting in the magnied charge separation and samples have a similar optical absorption dominated by the
electron outow. A series of photoelectron spectroscopies, intrinsic band-gap charge transition in CdS, in which the slight
including surface photovoltage (SPV), open-circuit potential blue shi of the absorption edge for CdS
x
O is due to its
(V
oc) decay and electron spin resonance (ESR), were performed composition of smaller CdS crystal grains (Fig. S10, ESI†).
to determine the properties of the photogenerated charge However, the two samples show the distinctly different inten-
transfer. For comparison, the pure CdS sample composed of sities of photoluminescence (PL) associated with the band-gap
large particles (D ꢂ 125 nm, Fig. S9, ESI†) was obtained by the charge recombination. By contrast, the obvious PL quenching
removal of the surface groups with annealing, in which the effect for CdS O corresponds to a more favorable hole extraction
x
space charge region (SCR) exists near the surface of n-type CdS upon surface sulfurous groups that efficiently restrains the
in depletion (D > 2 W). In Fig. 2a, the CdS sample exhibits the recombination of the photogenerated electron–hole pairs. The
st
weakly positive SPV signal with phase angles at the I quadrant; transient Voc decay was measured using an ON/OFF switched
that is, the photogenerated holes are separated toward the light illumination to examine the photo-induced charge
36
38,39
surface. This corresponds to the charge separation model by dynamics.
The temporal Voc prole shows an evidently
the in-built electric eld in the surface SCR of the n-type slower Voc decay rate upon illumination termination for CdS
x
O
37
depletion (Fig. 2g), but the low SPV intensity reects the very (Fig. S11, ESI†), which reects the delaying kinetics of the
nite extent of the upward band bending that appears inade- charge recombination in CdS O relative to CdS. The calculated

x
quate against the charge recombination kinetics. In contrast, lifetime (s) of the photogenerated electrons in CdS O is longer
x
the SPV signal of the CdS O sample is obviously negative with than that in CdS (Fig. 2d), and the prolonged electron survival
x
nd
x
phase angles at the II quadrant; moreover, its absolute value indicates a more effective charge separation in CdS O as well.
is much larger than that of CdS. Thus, different from pure CdS, Spin-trapping experiments based on the ESR technique were
the photogenerated electrons in CdS O are efficiently concen- employed to assess the relative amount of surface-reaching hot
trated on the surface upon the existence of sulfurous groups electrons.
x
40,41
x
As shown in Fig. 2e, CdS O presents a set of strong
ESR signals related to the spin adducts, conrming its high
yield of separated electrons reaching the surface. In contrast, no
ESR signal for the bare CdS sample identies the difficulty of
separated hot electrons reaching the surface in the absence of
sulfurous groups. In addition, the Nyquist plots of electro-
chemical impedance spectroscopy (EIS, Fig. 2f) also show
a smaller semicircle and better interfacial electron transfer in
CdS O compared with bare CdS.
x
These abovementioned analyses on the photo-induced
charge dynamics coherently demonstrate the kinetics accel-
eration of electron output from charge separation in CdS O
x
NPs upon surface conjugation with sulfurous groups. The
surface sulfurous groups consisting of negatively charged –S/
2
ꢀ
SO
x
ions can reasonably work as hole traps to release elec-
tron overows, which is favorable to instigate the photo-
catalytic reduction reaction (such as HER) and is similar to the
function of hole sacricial agents. However, it is conceivable
that the limited sulfurous groups bonding on the surface
cannot maintain the durable capture of photogenerated holes
for photocatalytic HER application. The time-dependent
photocurrent (Fig. S12a, ESI†) and bias voltage-dependent
SPV (Fig. S13, ESI†) present the unstable photo-response
signals associated with the disassembly of the sulfurous
groups. To maintain the surface coordination with sulfurous
groups, we introduced the UOR with low thermodynamic
Fig. 2 Photo-induced charge dynamics. (a) SPV spectra with corre-
sponding phase spectra, (b) ON/OFF switched SPV spectra, (c)
absorption and PL spectra with corresponding samples' photographs
(
inset), (d) electron lifetime spectra, (e) electron-trapping ESR, (f) EIS
spectra with an equivalent circuit model, and (g) schematic diagram for
the photo-induced charge dynamic distribution in CdS and CdS O.
x
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42–46
potential as the ultimate destination for the photogenerated those reported in recent literature (Table S2, ESI†).
Although
holes, thereby making the surface sulfurous groups into the UOR involvement can promote the activity of CdS O, this
x
durable mediators for hole transfer (Fig. S12b, ESI†). Of effect does not work well on pure CdS, which indicates that the
course, the downstream holes along the downhill thermody- role of the surface sulfurous groups in manipulating the charge
namic potential will be incapable of triggering WOR, but the dynamics is of crucial importance for charge distribution into
accelerated hole kinetics allow the optimum survival and catalytic redox reactions. For example, when using Na
output of hot electrons against charge recombination. More- or lactic acid (LA) as the hole sacricial agents, pure CdS only
over, the consumption of holes into the UOR is also valuable displays a little bit of activity, whereas CdS O renders an
outstanding activity growth (Fig. S16, ESI†). This further proves
2 2 3
S/Na SO
x
for urea-rich wastewater purication.
the desirable regulation of the surface sulfurous groups on
charge transfer and separation, which cannot be imitated by
dissociative sulfurous ions in a Na S/Na SO solution.
Photocatalytic performance
2
2
3
Photocatalytic H
2
evolution experiments were performed with
Moreover, in addition to the enhanced H
a simultaneous urea degradation is demonstrated with nitrogen
) generation detected by gas chromatography. As expected,
CdS presents a negligible N evolution activity, while CdS
enables a N -evolution rate of up to 0.5 mmol h g , corre-
2
generation,
CdS and CdS O as catalysts in water without/with urea under
x
ꢀ
2
simulated 1 sun irradiation (AM 1.5 G, 100 mW cm ) without
any additional sacricial agents (Fig. 3a). The used dosages of
(N
2
2
x
O
ꢀ1
ꢀ1
ꢀ1
the catalyst and urea were pre-optimized at 0.1 g L and 20 g
2
ꢀ
1
L
, respectively, so as to maximize the photocatalytic perfor-
mance (Fig. S14, ESI†). As expected, the pure CdS does not yield
any detectable H generation whether with or without urea due
to the huge charge recombination. In the hole-capturing buffer
of sulfurous groups, CdS O exhibits a denite proton reduction
sponding to urea degradation. The relevant reactions during H2
generation with simultaneous urea degradation are as follows:
2
ꢀ
ꢀ
2H
2
O + 2e / H
2
+ 2OH , 0 V vs. RHE
(1)
x
ꢀ
CO(NH
2
)
2
+ 6OH
/
activity in pure water, which is especially rare in the absence of
sacricial agents, reecting the well-steered charge transfer
ꢀ
N + CO + 5H O + 6e , 0.37 V vs. RHE (2)
2
2
2
toward HER. When integrating with UOR, CdS O shows
x
2 2
Notably, the measured evolution rates of H and N at
a signicantly enhanced HER activity with a H
2
-evolution rate of
(Fig. 3b) and an AQY as high as 2.4% at
20 nm (Note S2 and Fig. S15, ESI†), which are competitive with
ꢀ
1
ꢀ1
a stoichiometric ratio of 3 : 1 match well with an integrated
redox reaction with HER and UOR by electron exchange. The
mild UOR alternative to the harsh WOR can shi the thermo-
dynamic potential from 1.23 V to a mere 0.37 V, lowering the
up to 1.49 mmol h
g
4
photovoltaic standard of solar-to-H conversion.
2
The photocatalytic stability of CdS O for H evolution with
x
2
simultaneous urea degradation was examined by cycling
experiments. Continuous H and N production was observed
with no signicant decay in the sequential runs (Fig. 3d and e).
Moreover, the CdS O catalyst still maintained a similarly high
2
2
x
catalytic activity aer storage for one week. In addition, the TEM
image, XRD pattern, Raman and EDX spectra of the CdS O
x
catalyst aer stability testing are recorded in Fig. S17 (ESI†).
Altogether, the results show no noticeable change compared
with those before use. These results support the promising
application of CdS
catalyst for solar H
x
O NPs as an efficient and robust photo-
2
production from urea-rich wastewater. In
particular, the activity decay is observed for cycling H
2
evolution
over CdS O in pure water without urea (Fig. S18, ESI†), implying
x
the disassembly of sulfurous groups under hole accumulation.
Therefore, the mild UOR can change the role of the surface
sulfurous groups from a hole collector to mediator with transfer
acceleration; on the other hand, the surface sulfurous groups
can operate hole capture and transfer toward UOR against
strong charge recombination kinetics. The sulfur atoms with
multiple-charged permissible states in the sulfurous groups
enable the charge transfer mediation, and the formed inter-
band energy levels (by surface states) built fast channels of hole
transfer toward UOR. The proposed charge transfer mechanism
is schematically depicted in the energy band structure estab-
lished by estimating the bandgap and atband potentials
Fig. 3 Photocatalytic performance. (a) Time courses of H
b) H -evolution rates, (c) N -evolution rates with urea degradation, (d)
x
-evolution, and (e) N -evolution stabilities in cycling runs for CdS O
and CdS as catalysts in water without/with urea under simulated 1 sun
irradiation. (f) Schematic diagram for photo-induced charge dynamics
2
evolution,
(
H
2
2
2
2
x
with HER and UOR on CdS O.
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(Fig. S19, ESI†). As shown in Fig. 3f, both types of photo- the crystal structure and the phase composition. FT-IR spec-
generated charge carriers are separately steered toward the troscopy was conducted using a Varian Cary 670 spectrometer.
specied redox reactions by hole dynamics modulation, XPS measurements were carried out on an ESCALAB 250Xi
enabling an improved solar H production with simultaneous spectrometer with a monochromatic Al Ka X-ray source oper-
2
urea photolysis.
ating at 150 W, and the binding energies were calibrated
according to the C 1s peak at 284.78 eV. Raman spectra were
measured using a research laser Raman microscope (Renishaw
inVia) with an incident 532 nm laser of 300 mW, and the steady-
state PL spectra were recorded at room temperature using
a research laser Raman microscope (Renishaw inVia) with an
incident 325 nm laser of 300 mW. SPV measurements were
performed with a monochromatic light source (CHF-XM-500 W,
Global Xenon Lamp Power) through grating monochromator
Conclusions
In summary, we report a strategy of modulating hole dynamics to
improve solar H production by engineering surface conjugation
2
and alternative oxidation reaction. Thereby, CdS O NPs conju-
gated with sulfurous groups have been developed as an efficient
x
and robust photocatalyst for solar H production from urea-rich
2
(Omni-l3007-MC300) and lock-in amplier (SR830-DSP) with
wastewater with synchronous urea degradation. The surface
sulfurous groups are demonstrated to function as a hole-
capturing buffer that transports holes downstream into the
thermodynamically mild UOR, enabling the hole-transfer accel-
eration for the optimum separation and utilization of the pho-
togenerated charge into specied redox reactions. The
light chopper (SR540) at a chopping frequency of 23 Hz. UV-vis
absorption spectra were recorded with a Cary 5000 spectro-
photometer (Varian). DMPO spin-trapping ESR spectra were
measured on an A300-10/12 instrument for different catalysts (5
mg) dispersed in 1.5 mL CH OH with 50 mL of DMPO to
3
ꢀ
examine the superoxide radical adducts (DMPO-O2 ).
photocatalytic H2 generation over the CdS O NPs reaches an
impressive rate of 1.49 mmol h
x
ꢀ1
ꢀ1
g
under 1 sun irradiation with
an AQY up to 2.4% at 420 nm. This study not only provides an PEC measurement
interesting waste-to-energy solution, but also gains deep insight
into the charge dynamics for photocatalytic H generation.
2
PEC measurements were performed on an electrochemical work
station (CHI660C Instruments, Shanghai, China) in a standard
three-electrode system with the catalyst-coated FTO (2 ꢃ 1.5
2
cm ) as the working electrode, an Ag/AgCl and a graphite rod
Methods
Sample synthesis
were used as the reference and counter electrodes, respectively.
The working electrodes were prepared by dropping the
CdS O NPs were prepared through a facile co-solvent method suspensions (30 mL) made of various catalysts (1 mg catalysts
x
with a tunable ratio of Cd(OAc) and Na S as the Cd and S added into 50 mL Naon and 450 mL ethanol mixed solution,
2
2
sources, followed by extraction washing and air drying. In respectively) onto the FTO plates. The electrodes were dried
a typical procedure, 4 g of Na S rst was added into 40 mL of with N atmosphere at room temperature. The aqueous solu-
distilled water. The dispersion was sonicated by a sonicator tions without/with 0.33 M urea were used as the electrolytes.
Misonix XL-2000) for 10 min with an output power of 150 W. The potentials were calibrated with respect to the reversible
2
2
(
ꢀ
1
Second, a 2 mL aqueous solution of 7.5 g L Cd(OAc)
2
$2H
2
O
hydrogen electrode (RHE) according to E(RHE) ¼ E(Ag/AgCl) +
was added into a sulfur-rich Na S solution, and then sonicated 0.599 V at pH ¼ 6.8. The “1 Sun” illumination was simulated by
2
for 10 min. The products were collected using centrifugation at a 300 W Xe short arc lamp solar simulator (PerfectLight, Beijing,
1
2 000 rpm for 3 min and washed several times with deionized PLS-SXE 300C) with an AM 1.5 G lter. The light intensity was
ꢀ2
(DI) water and absolute ethanol to remove the residues of other calibrated to 100 mW cm by an irradiatometer (Beijing, FZ-A).
ꢁ
ions. The nal products were dried at 60 C overnight. For The photocurrent responses of the electrodes were measured
comparison, the pure CdS crystals were synthesized by calcining under chopped irradiation at 0 V vs. RHE.
ꢁ
the CdS
x
O sample at 550 C for 4 h in a tubular furnace under
argon blanketing to remove the surface groups. In addition, the
Photocatalytic measurement
Photocatalytic H evolution experiments were performed in
CdS O samples with different contents of surface sulfurous
x
2
groups were synthesized by the same procedure with the
a closed gas circulation and evacuation system tted with a top
window of optical at quartz glass (Labsolar-IV (AG), Perfect-
light, Beijing). The suspension was then stirred in a sealed glass
vessel and purged with Ar for 30 min prior to photocatalytic
experiments to remove dissolved oxygen. The vials were placed
exception of adding different dosages of Na S (from 4.7 mg to 5
2
g). Based on the photocatalytic activity maximum, the ratio of
Cd and S ions was optimized to 1 : 910, and the resultant CdS O
x
NPs were bright yellow in color (Fig. S20, ESI†).
ꢁ
in a thermoregulated rack at 26 C with magnetic stirring, and
Characterization
then irradiated by the simulated solar light from the Xe lamp
i
TEM and HR-TEM characterizations were performed using with an AM 1.5 G lter. The focused light power intensity (I ) on
ꢀ2
a Tecnai F30 electron microscope equipped with EDX spec- the reactor was uniformly calibrated to 100 mW cm . The
troscopy at an acceleration voltage of 300 kV. XRD measure- amounts of evolved H2 and N2 gases were measured by an
ments were conducted on the Shimadzu XRD-7000 online gas chromatography (9790 II, Fuli, Zhejiang) with
diffractometer with Cu Ka radiation (l ¼ 0.15406 nm) to study a thermal conductivity detector (TCD) and Ar as carrier gas.
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Different samples (CdS
x
O and CdS) with the same mass were 10 P. Li, X. Chen, H. He, X. Zhou, Y. Zhou and Z. Zou, Adv.
respectively used as catalysts dissolved into 100 mL aqueous
Mater., 2018, 30, 1703119.
solutions without/with urea. The dosages of catalyst and urea 11 Y. Kuang, T. Yamada and K. Domen, Joule, 2017, 1, 290.
ꢀ1
ꢀ1
were pre-optimized at 0.1 g L and 20 g L based on the 12 S. Bai, J. Jiang, Q. Zhang and Y. Xiong, Chem. Soc. Rev., 2015,
photocatalytic performance maximum. The photocatalytic 44, 2893.
stability was evaluated by extracting, washing, and reusing the 13 B. You, X. Liu, N. Jiang and Y. Sun, J. Am. Chem. Soc., 2016,
catalyst with centrifugation in cycling experiments. The AQY 138, 13639.
was dened by the following eqn (3), and was measured using 14 Z. Yu, C. Lang, M. Gao, Y. Chen, Q. Fu, Y. Duan and S. H. Yu,
a 300 W Xe lamp with 400 and 420 nm (ꢄ5 nm) band-pass lter Energy Environ. Sci., 2018, 11, 1890.
and an irradiatometer (FZ-A, Perfectlight, Beijing). The detailed 15 B. You, N. Jiang, X. Liu and Y. Sun, Angew. Chem., Int. Ed.,
calculations are shown in the Note S2 and Fig. S15 (ESI†).
2016, 55, 9913.
1
6 J. Han, X. Zheng, L. Zhang, H. Fu and J. Chen, Environ. Sci.:
Nano, 2017, 4, 834.
7 X. Zhu, X. Dou, J. Dai, X. An, Y. Guo, L. Zhang, S. Tao, J. Zhao,
W. Chu, X. C. Zeng, C. Wu and Y. Xie, Angew. Chem., Int. Ed.,
AQY½%ꢅ
number of reacted electrons
1
¼
¼
ꢃ 100%
number of incident photons
number of evolved H
molecules ꢃ 2
number of incident photons
2
016, 55, 12465.
8 T. Take, K. Tsurutani and M. Umeda, J. Power Sources, 2007,
64, 9.
2
ꢃ 100%
(3)
1
1
2
2
2
1
9 C. Tang, R. Zhang, W. Lu, Z. Wang, D. Liu, S. Hao, G. Du,
A. M. Asiri and X. Sun, Angew. Chem., Int. Ed., 2017, 56, 842.
0 M. Song, Z. Zhang, Q. Li, W. Jin, Z. Wu, G. Fu and X. Liu, J.
Mater. Chem. A, 2019, 7, 3697.
1 C. Li, Y. Liu, Z. Zhuo, H. Ju, D. Li, Y. Guo, X. Wu, H. Li and
T. Zhai, Adv. Energy Mater., 2018, 8, 27.
Conflicts of interest
There are no conicts to declare.
2 Y. Liang, Q. Liu, A. M. Asiri and X. Sun, Electrochim. Acta,
2015, 153, 456.
Acknowledgements
2
2
3 K. H. Cho and Y. M. Sung, Nano Energy, 2017, 36, 176.
4 R. Shi, H. F. Ye, F. Liang, Z. Wang, K. Li, Y. Weng, Z. Lin,
W. Fu, C. Che and Y. Chen, Adv. Mater., 2018, 30, 1705941.
5 L. Cheng, Q. Xiang, Y. Liao and H. Zhang, Energy Environ.
Sci., 2018, 11, 1362.
6 H. Yu, W. Zhong, X. Huang, P. Wang and J. G. Yu, ACS
Sustainable Chem. Eng., 2018, 6, 5513.
7 Y. Zhang, L. Han, C. Wang, W. Wang, T. Ling, J. Yang,
C. Dong, F. Lin and X. W. Du, ACS Catal., 2017, 7, 1470.
8 S. K. Verma, M. K. Deb and J. Agric, Food Chem., 2007, 55,
8319.
This study was nancially supported by the National Natural
Science Foundation of China (11974303, 11574263 and
11674276), the National Basic Research Program of China
2
2
2
2
(
2014CB931702), the Qinglan Project of Jiangsu Province, and
the Personnel Plan of Yangzhou University and Hangzhou
Dianzi University.
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25656 | J. Mater. Chem. A, 2019, 7, 25650–25656
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