Indium Phosphide Quantum Dots Integrated with Cadmium Sulfide Nanorods for Photocatalytic Carbon Dioxide Reduction

Article abstract of DOI:10.1002/cctc.202000616

Photocatalytic conversion of CO₂ into storable fuels is an attractive way to simultaneously address worldwide energy demands and environmental problems. Semiconductor quantum dots (QDs) have gained prominence as candidates for photocatalytic applications due to their many advantages, which include tunability for advanced electronic, optical, and surface properties. Indium phosphide (InP) quantum dots are semiconducting QDs with enormous potential for solar-driven CO₂ reduction. Their advantages include a tunable bandgap, diverse surface chemistry, and nontoxicity. InP QDs and CdS nanorods were integrated using a simple and inexpensive method. CO₂ photoreduction by the CdS-InP composites was evaluated in aqueous solution using triethanolamine as a sacrificial donor. The crystal structures, surface compositions, and morphologies were investigated via X-ray diffraction analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy, respectively. The UV-visible diffuse reflectance spectra of the CdS-InP composites indicated efficient visible light utilization in the 500–550 nm range. The results of photoelectrochemical and photoluminescence analyses illustrated effective charge separation in the composites. The photocatalytic activity of the as-synthesized composites was superior to that of pure CdS. The CO evolution rate of the optimized composite was 216 μmol h^?1 g^?1 during the first three hours of irradiation and increased steadily over the next 62 hours. We also studied the influences of different solvents, the hole scavenger concentration, and catalyst loading on the performance of the optimized composite.

Full text of DOI:10.1002/cctc.202000616

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Indium Phosphide Quantum Dots Integrated with Cadmium
Sulfide Nanorods for Photocatalytic Carbon Dioxide Reduction
Authors: Kahi H. Do, D. Praveen Kumar, A. Putta Rangappa, Yul
Hong, D. Amaranatha Reddy, and Tae Kyu Kim
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.202000616
Link to VoR: https://doi.org/10.1002/cctc.202000616
01/2020

10.1002/cctc.202000616
ChemCatChem
FULL PAPER
Indium Phosphide Quantum Dots Integrated with Cadmium
Sulfide Nanorods for Photocatalytic Carbon Dioxide Reduction
Khai H. Do,†[a] Dr. D. Praveen Kumar,†[a] A. Putta Rangappa,[a] Yul Hong,[a] Dr. D. Amaranatha Reddy[a]
and Prof. Tae Kyu Kim*[a]
[a]
†
Department of Chemistry, Yonsei University, Seoul 03722, Republic of Korea
These authors contributed equally.
E-mail: tkkim@yonsei.ac.kr
Supporting information for this article is given via a link at the end of the document
Abstract: Photocatalytic conversion of CO2 into storable fuels is an
attractive way to simultaneously address worldwide energy demands
and environmental problems. Semiconductor quantum dots (QDs)
have gained prominence as candidates for photocatalytic applications
due to their many advantages, which include turnability for advanced
electronic, optical, and surface properties. Indium phosphide (InP)
quantum dots are semiconducting QDs with enormous potential for
solar-driven CO2 reduction. Their advantages include a tunable
bandgap, diverse surface chemistry, and nontoxicity. InP QDs and
CdS nanorods were integrated using a simple and inexpensive
method. CO2 photoreduction by the CdS-InP composites was
evaluated in aqueous solution using triethanolamine as a sacrificial
donor. The crystal structures, surface compositions, and
morphologies were investigated via X-ray diffraction analysis, X-ray
photoelectron spectroscopy, and transmission electron microscopy,
respectively. The UV-visible diffuse reflectance spectra of the CdS-
InP composites indicated efficient visible light utilization in the 500–
550 nm range. The results of photoelectrochemical and
photoluminescence analyses illustrated effective charge separation in
the composites. The photocatalytic activity of the as-synthesized
composites was superior to that of pure CdS. The CO evolution rate
of the optimized composite was 216 µmol h-1 g-1 during the first three
hours of irradiation and increased steadily over the next 62 hours. We
also studied the influences of different solvents, the hole scavenger
concentration, and catalyst loading on the optimized composite.
photocatalyst. It has a narrow bandgap (∼2.4 eV), and its
conduction band edge position is more negative than the
reduction potential of CO2.[4–6] However, the photoconversion
efficiency of bare CdS is low due to its high charge carrier
recombination rate and susceptibility to photocorrosion.[7–10]
Loading the surface of CdS with another semiconductor or a co-
catalyst to enhance photocatalytic activity has been a successful
strategy. The combination of two or more materials can facilitate
photogenerated charges carrier separation, expand the light
absorption window, and improve visible light utilization.[11–13] The
high photocatalytic efficiency of CdS mixed with noble metal (Pt,
Ru, Pd, Ag) nanostructures has made these materials a top
priority in the field of photocatalytic CO2 reduction.[14–16] However,
the high cost of manufacturing them on an industrial scale and the
low abundance of noble metals are obstacles to the practical
application and commercialization of these photocatalysts. A
variety of CdS co-catalysts based on metal oxides,[17–19]
carbon,[20,21] metal sulfide,[5,22] and ZIF[23,24] have been designed
in an effort to overcome these limitations. Nevertheless, the
instability of these co-catalysts during photocatalytic reactions is
a major drawback.
Metal phosphides (MPs) have recently received significant
interest in the field of photocatalysis on account of their
abundance, good stability, low cost, and high electrical
conductivity. These MPs include CoxP,[25–28] Ni2P,[29] Fe2P,[30]
Cu3P,[31] and WP.[32] Due to the large atomic radius and the strong
charge (−3) of phosphorus, the M–P bonds in most metal
phosphides have a highly covalent character.[33–35] The presence
of P moderates the bonding strength of metal phosphides towards
photocatalytic conversion products, which facilitates the
desorption of conversion products from active sites.[36,37] Despite
having suitable physical and chemical properties, including high
electron mobility,[38] indium phosphide (InP) semiconductors have
not been thoroughly studied for photocatalytic CO2 reduction.[39]
The main obstacle to the widespread use of InP for photocatalysis
is the large bulk bandgap (1.35 eV). However, this can be
overcome by reducing the size of InP particles to the quantum dot
(QD) scale. QDs, which are semiconductor nanocrystals, have
unique size-dependent electronic and optical properties as well
as versatile surface features. They have become prominent
candidates for numerous applications, including light-emitting
diodes (LEDs),[40,41] solar energy conversion,[42] and
photocatalysis.[43–45] QDs based on cadmium chalcogenides have
similar emission characteristics, low toxicity, and their fabrication
is cost-effective.[46] InP should be considered as an alternative to
cadmium chalcogenide-based materials, because the large bulk
bandgap (1.35 eV) and Bohr exciton radius (9.6 eV) can be great
Introduction
The uncontrolled consumption of natural resources is depleting
fossil fuel reserves, and combustion emissions are associated
with serious environmental degradation. The development of eco-
friendly and renewable wind and solar energy sources is essential
to solve these problems. To this end, the development of solar
fuels by using sunlight has become a foremost research goal. The
photocatalytic conversion of CO2 into value-added fuels is an
environmentally-friendly, economical, and sustainable strategy for
addressing the global energy demands and environmental issues.
This is because the sun is the ultimate energy source, and water
and CO2 in industrial emissions are abundant.[1–3] There are many
reports of using individual photocatalysts to convert CO2 into
value-added chemicals. However, developing economical high-
performance photocatalysts for CO2 conversion is an ongoing
challenge.
Cadmium sulfide (CdS) is responsive to visible light, and it
has attracted a great deal of attention as a semiconductor
1
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Scheme 1. Schematic illustration of material preparation of CdS NRs, InP QDs and CdS-InP composites
advantages for solar energy utilization by tuning the size of InP
QDs. Additionally, InP QDs have higher redox potential than the
bulk InP, which is favorable to photocatalytic reactions. Since
photocatalytic reactions often occur at the interface between a
semiconductor and co-catalyst, zero-dimensional InP QDs could
provide more active sites for photocatalytic reactions than bulk
InP. This would enhance photocatalytic activity at the
semiconductor/InP heterojunction under visible light. A recent
study reported on InP QDs and InP QD/semiconductors for solar-
driven hydrogen production,[47] although the exploration of InP
QDs for photocatalytic CO2 reduction to produce value-added
chemicals has been limited. InP/CdS-based core/shell QDs have
been shown to be quasi- or type II heterostructures,[48,49] which
are suitable for solar energy conversion. CdS integrated with InP
QDs is thus expected to be a promising photocatalyst candidate.
Herein, we report the simple preparation of InP QDs and
their integration with CdS nanorods (NRs) (CdS-InP
nanocomposites). The synthesized materials were evaluated for
the photocatalytic reduction of CO2 in water under simulated solar
light using triethanolamine as a sacrificial donor. The optimized
CdS-InP composite demonstrated high CO2 conversion efficiency,
and its CO evolution rate (216 µmol h-1 g-1) was 2.4 times higher
than that of bare CdS during the first three hours of irradiation.
The enhanced photocatalytic performance of these catalysts
under simulated sunlight illumination was attributed to efficient
charge carrier separation and the efficient utilization of visible light
in the 500–550 nm range.
water moisture, which could react with ZnCl2 to form ZnO. Adding
toluene and slowly increasing the temperature during evacuation
helped prevent this (Figure S1a, Supporting Information). The
dissolution-precipitation technique was utilized to remove the
impurities, such as excess amounts of oleylamine and unreacted
precursors. In addition, ZnCl2 and InCl3 were eliminated due to
high solubility in ethanol. A Schematic illustration of material
preparation showed in Scheme 1.
Results and Discussion
InP
QDs
were
prepared
by reacting
InCl3
and
tris(diethylamino)phosphine in oleylamine. Oleylamine served as
a coordinating solvent and played an integral role throughout the
chemical process.[46,50,51] The presence of ZnCl2 facilitated shell
growth and reduced variability in the size of the InP QDs. This
approach has been shown to prevent the formation of Zn-doped
InP core nanocrystals.[46,50,52] We also added toluene in the
procedure for azeotropic distillation. ZnCl2 and InCl3 absorbed
Figure 1. (a) XRD spectra of the InP QDs, CdS NRs and CdS-InP synthesized
following the reference protocols described in the experimental section. (b) XPS
survey spectrum of the as-prepared composite and high resolution XPS spectra
of (c) Cd 3d, (d) S 2p, (e) In 2p and (f) P 2p regions.
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The crystal structures of the CdS NRs, InP QDs, and CdS-
CdS semiconductor was 2.34 eV, which was consistent with the
reported value [59]
.
InP composites were analyzed via X-ray diffraction (XRD)
analysis. The XRD patterns of the materials are shown in Figure
1a. The XRD pattern of CdS contained a series of diffraction
peaks at 24.81°, 26.51°, 28.18°, 43.68°, 47.84°, and 51.82°,
which were attributed to the (100), (002), (101), (110), (103), and
(112) planes, respectively, of CdS in the pure hexagonal phase
(JCPDS card no. 41-1049). The diffraction pattern of the bare InP
QDs exhibited broad diffraction peaks at 26.3o, 43.6o, and 51.6o,
which were in good agreement with the literature value.[46] Only
CdS peaks were observed in the XRD patterns of the CdS-InP
composites. No InP diffraction peaks appeared, which was due to
the relatively low amount of InP relative to CdS and/or the low
intensities of the InP diffraction peaks and overlap of the InP and
CdS peaks.
The elemental surface chemistry of the CdS-InP was
examined via XPS spectroscopy. The XPS survey spectrum
(Figure 1b) of the CdS-InP composite contained peaks that were
attributed to Cd, S, In, and P elements. The peaks of O 1s and C
1s, associated with the oxidized surface layer and oleylamine,
were detected. The peaks located at 404.8 eV (3d5/2) and 411.5
eV (3d3/2) in the high-resolution XPS spectrum of Cd 3d (Figure
1c) were characteristic of Cd in the +2 oxidation state.
Interestingly, the S 2p spectrum exhibited two doublets signals
(Figure 1d). The first doublet, S 2p3/2 (162.7 eV) and S 2p1/2 (163.9
eV), was assigned to sulfur (S2-) on the surface exposed to the
more electronegative environment near InP, whereas the second
component with S 2p spin-orbit splitting of ∼1.2 eV (S 2p3/2
binding energy = 161.3 eV) corresponded to S2− in the CdS lattice.
[53,54] The In 3d5/2 and In 3d3/2 peaks appeared at 444.7 eV and
452.3 eV, respectively (Figure 1e). The high-resolution P 2p
spectrum (Figure 1f) contained two contributions. The 2p1/2 and
2p3/2 peaks were found at respectively 130.7 eV and 129.8 eV,
implying the presence of the phosphide (P3-).[55,56] Relative to
peaks in the XPS spectrum of pure InP (Figure S4), peaks in the
XPS spectra of In and P in the CdS-InP composite were shifted
slightly due to the modified chemical environment.
The morphology of the CdS-InP composites was
investigated via transmission electron microscopy (TEM) (Figure
2) and energy dispersive X-ray spectral (EDS) mapping (Figure
S3). Figure 2a shows a trace amount of InP QDs deposited onto
CdS NRS. The high-resolution TEM (HRTEM) image of the
composite (Figure 2b) clearly demonstrated two lattice spacings
of 0.33 nm and 0.34 nm, which were assigned to CdS,[57] and
InP,[47] respectively. The elemental compositions determined
through EDS mapping (Figure S3) evidenced the distribution of
InP on CdS.
Figure 2. (a) TEM and (b) HR-TEM images of CdS-InP nanocomposites.
The UV-visible spectroscopy was performed to study the
optical properties of the InP QDs, the CdS NRs, and the
composites (Figure 3a, b). The absorption spectrum of InP QDs
in CHCl3 contained a peak at 525 nm. This was close to the band
edge position of bare CdS (~520 nm) and corresponded to
intrinsic bandgap absorption by the CdS NRs.[58] Combining the
CdS NRs and InP QDs did not alter the band structure of CdS.
Rather, the InP QDs enhanced the absorption because of the
similar absorption in the range from 500 to 550 nm. The more
efficient utilization of visible light by the as-synthesized
nanocomposites led to an increase in photocatalytic efficiency.
The bandgap energy of the InP QDs was calculated using Tauc’s
method and found to be 2.20 eV. The estimated bandgap of the
Photoluminescence (PL) emission spectroscopy is a useful
technique for examining the trapping and charge recombination
efficiencies of bare CdS and the related composites. The PL
spectra of CdS and CdS-InP were recorded at an excitation
wavelength of 380 nm at room temperature (Figure 4a). The rapid
recombination of photogenerated charge carriers within the CdS
nanorods generated a strong PL peak with band-edge emission
appearing at ~520 nm.[28] In contrast, the lower peak intensity in
the spectrum of as-prepared composites indicated the fast
transfer of photoinduced electrons along with the suppression of
charge carrier recombination, leading to enhancing photocatalytic
activity.
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Figure 3. (a) UV-Visible absorption spectrum of InP QDs. (b) UV/Vis diffuse
reflectance spectra of the CdS NRs and the related composite (b). Tauc plots
of InP QDs and CdS NRs are inset into two panels (a) and (b), respectively to
determine the band-gap energy
The dynamics of interfacial charge separation of as-
synthesized materials were photoelectrochemically examined
under simulated sunlight using a potentiostat and indium tin
oxide (ITO) working electrodes coated with CdS and the CdS-InP
nanocomposites. The time-dependent photocurrent responses of
the bare CdS nanostructures and the nanocomposite (Figure 4b)
were recorded over 360 seconds of 30 s on/off irradiation cycles.
The CdS-InP nanocomposite generated a stronger photocurrent
than the bare CdS nanorods, indicating that the separation of
photoexcited electrons and holes was more efficient in the CdS-
InP nanocomposite. The repeatability and stability of the
photocurrent response over several on-off cycles demonstrated
that the presence of InP on CdS NRs can prevent the
photocorrosion. Photoelectrochemical (PEC) analysis confirmed
that the InP QDs enhanced the photocatalytic activity of the CdS
nanostructures. The semicircle in the Nyquist plot of the ITO
electrode coated with CdS-InP was smaller in diameter than the
semicircle in the Nyquist plot of CdS under simulated solar light
(Figure 4c). This suggested that InP facilitated the migration of
charge carriers from the InP QDs to the CdS NRs and other
components of the reaction system.
The photocatalytic activities of the synthesized materials
were examined by using TEOA as a sacrificial agent under
simulated solar light (AM 1.5ꢀG). During the photocatalytic
reduction of CO2, CO was generated as the major product. In
detail, the blank experiments were performed for ensuring that
light, TEOA, CO2, and composites are the integral parts of CO2
photoreduction systems (Figure S6a). Especially, the
performance of the system in the Ar atmosphere was lower than
that in the normal condition owing to the lower pH of the reaction
mixture in the CO2 atmosphere. The effect of the InP weight
percentage in the InP-CdS nanocomposites on CO evolution is
displayed in Figure 4d. The high recombination rate of as-
synthesized CdS nanostructures resulted in very low solar-driven
photocatalytic CO evolution rates of 287 µmol g-1 after 190
minutes of irradiation and approximately 400 µmol g-1 after 390
minutes. The coexistence of InP and CdS had a beneficial impact
on the CO production rate. The CO generation rate of the
composite containing 5.0 wt% InP reached a maximum of 216
µmol h–1 g–1 during the first three hours. The CO evolution rate
under the higher InP loadings was lower. Excessive loading with
InP covered active sites of CdS and hindered photocatalytic
activity. The external quantum efficiency (EQE) of the optimized
CdS-InP nanocomposite under visible light filtered through a
425 nm bandpass filter was estimated to be 0.44% during the first
three hours of irradiation. The EQE then remained constant up to
6.5 hours of irradiation.
Figure 4. (a) PL spectra, (b) Transient photocurrent, and (c) Impedance
measurements of pristine CdS and CdS-InP nanostructures. (d) Effect of InP
QDs loading ratio on CdS NRs, (d) Long-term stability, and (f) Recyclability of
CdS-5% InP with 4 mg of catalyst dispersed in 15 mL of 33.3 vol% aqueous
TEOA solution under simulated solar light irradiation.
A Long-term photocatalysis experiment was performed with
the optimized composite for up to 65 hours, as shown in Figure
4e. The CO yield increased with respect to the irradiation time
while hydrogen evolution was suppressed after 5 hours and its
rate rose slightly with increasing time. We concluded that CO2
intermediates accumulated on the surface over time, which
inhibited the adsorption of water and limited the water reduction
process.[60] Furthermore, The pH of the solution increased during
the photocatalysis experiment. This increased the solubility of
CO2 and affected positively photoreduction of CO2.[61] The cycling
experiments highlighted the stability and reusability of catalysts
(Figure 4f). The optimized composite was reused after entirely
recovering the photocatalyst particles from the reaction via
centrifugation, washing with ethanol, and drying. It retained the
activity after four recycling tests. In the case of evacuating for 3
hours and filling the reactor with CO2 for 1 hour, there was a
decrease in efficiency to nearly 50% in the second run (Figure
S6c). The adsorption of the intermediates on the material surface
in the first cycle hampered the interaction of reactants with the
catalyst particles.
5% CdS-InP was collected after 6.5 hours of photocatalytic
reaction to further examine the change of the photocatalyst. The
used composite sample was characterized by means of XPS and
TEM images. Both XPS spectra (Figure S7) and TEM images
(Figure S8, 9) illustrated the presence of InP QDs on CdS
surfaces, firmly suggesting the photostability of the composites.
There was no change in the high resolution XPS spectrum in In
3d region, while the deconvolution of the P 2p signals was
complicated by the low intensity of P peaks.
4
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We used a wide variety of solvents for the optimized
composite photo-experiments, which revealed the vital role of
water in CO2 conversion. The photocatalytic efficiency of the
reaction system was generally better in the presence of water,
which acted as a hydrogen source. The highest activity was
observed in water (Figure S6d). The second position was
MeCN/water, followed by DMF/water. To the best of our
knowledge, lower activity in organic solvents was due to phase
separation.[62,63] The influence of the scavenger concentration on
the photocatalytic performance of the optimized composite was
investigated. These results are shown in Figure S6e. The gas
evolution rate was initially proportional to the TEOA concentration,
which ranged up to 5 mL in a 15 mL reaction solution. The
maximum CO production rate was 216 μmol h−1 g−1. The
intermediate formation was more pronounced at high TEOA
concentrations, which reduced photocatalytic efficiency.[64] The
photocatalytic activity of the optimized CdS-InP composite
catalyst in 15 ml of the reaction solution was profoundly
dependent on the amount added (3 – 5 mg), as illustrated in
Figure S6f. Although the selectivity for CO (~30%) was roughly
the same regardless of the amount of catalyst added, the rates of
CO production differed. The CO evolution rates with 3, 4, and
5 mg of the photocatalyst were 200, 216, and 149 μmol h−1 g−1,
respectively. A larger quantity of the catalyst had a shielding effect
on the suspended nanocomposites, which resulted in a lower CO
evolution rate.
Mott-Schottky (MS) measurements were carried out to
further research the CO2 photoreduction mechanism of as-
synthesized materials (Figure S10). The flat band potentials (VFB)
of CdS and the CdS-InP nanocomposite in the MS measurements
were -0.847 V and −0.919 V vs. EAg/AgCl, respectively, at the
frequencies of 0.5 and 0.1 kHz. Following potential conversion
from the Ag/AgCl reference to NHE, the VFB values were -0.650 V
and -0.722 V vs. NHE.[65] The conduction band potential (ECB) of
the CdS NRs with a bandgap of 2.34 eV was −0.65 V vs. NHE
and the ECB of the InP QDs with a bandgap of 2.20 eV equaled
−0.72 V, in line with the report,[47] because of the closeness
between the conduction band potential of an n-type
semiconductor and the flat band potential.[66] Therefore, the
valence band potential (EVB) values of CdS NRs and InP QDs are
estimated to be 1.69 V and 1.48 V vs. NHE, respectively.
Based on PL, PEC, and MS measurements, a plausible
mechanism of the solar-driven CO2 reduction using the CdS-InP
nanocomposites and TEOA as a hole scavenger is depicted
in Scheme 2. The excitations of the CdS and InP in the CdS-InP
nanocomposites by solar light generated electron-hole pairs. The
electrons were located in the conduction bands (CBs), and the
holes were in the valence bands (VBs). Since CdS-InP
composites were type II heterostructures, the photogenerated
electrons migrated directly from the CB of InP to the CB of CdS.
The photogenerated holes in the VB of CdS traveled in the
opposite direction to the VB of InP. Consequently, the
photoinduced electrons in the CB of CdS reduced CO2 on the
surface into CO. Meanwhile, the holes in the VB of InP were
consumed by the sacrificial electron donor (TEOA). The InP QDs
enabled the effective separation of the photogenerated charge
carriers and enhanced the harvesting of solar radiation, which
was supported by the UV-Vis measurements. Moreover, the
conduction band potentials of CdS and InP were very similar,
which allowed the rapid migration of electrons from InP to CdS.
Scheme 2. Schematic representation of the reaction mechanism for
photocatalytic CO2 reduction of CdS-InP composite.
Conclusion
A simple and inexpensive process for the preparations of pure InP
QDs, CdS NRs, and CdS-InP nanocomposites was successfully
developed. The CdS-InP composites demonstrated enhanced
sunlight-driven photocatalytic CO2 reduction activity in water
relative to bare CdS. This was due to the improved separation of
photogenerated electron-hole pairs and better absorption
capability in the 500–550 nm range. The performance of the
optimized composite was investigated in different solvents,
indicating the important role of water in CO2 photoreduction. In
addition, the suppression of H2 evolution after five hours of
irradiation in the long-term photocatalysis experiment resulted in
enhanced CO selectivity during CO2 reduction. Considering each
of these factors, the InP-CdS nanocomposites are promising
candidates for harvesting solar energy, photochemical conversion,
and storage.
Experimental Section
Chemicals: Indium(III) chloride (99.999%), Oleylamine (70%),
Tris(diethylamino)phosphine (97%) were purchased from Sigma-Aldrich.
Acetonitrile (ACN), Cadmium acetate dihydrate (Cd(CH3COO)2ꞏ2H2O),
Chloroform, Ethanol, N,N-Dimethylformamide (DMF), Triethanolamine
and Zinc(II) chloride were purchased from Daejung Chemicals & Metals
Co. Ltd., Korea. Thiourea (NH2CSNH2) was obtained from Kanto Chemical
Co. Inc., Japan.
Synthesis of InP QDs: Indium phosphide (InP) was prepared from indium
chloride and tris (diethylamino)phosphine in the presence of ZnCl2,
according to the method reported with some modifications.[46]
A
suspension of 100 mg (0.45 mmol) indium(III) chloride, 300 mg (2.2 mmol)
zinc(II) chloride, and 3.0 ml toluene in technical oleylamine (5.0 mL) was
stirred and degassed at ambient temperature for 30 minutes. The mixture
was then stirred at 80 °C for 30 minutes, at 120 °C for 30 minutes, and at
135 °C for 30 minutes. Once the temperature reached 180 °C, a volume
of 0.45 mL (1.6 mmol) of tris(diethylamino)phosphine (phosphorus:indium
ratio = 3.6:1) was quickly injected into the mixture under an inert
atmosphere. The temperature of the mixture was held at 180 °C for 30
minutes. When the reaction was complete, the mixture was allowed to cool
to room temperature. The raw InP QDs were obtained by adding 20.0 mL
ethanol, centrifuging and discarding the supernatant. The samples were
purified twice using 10.0 mL chloroform (CHCl3) as the solvent and 15.0
mL ethanol as the nonsolvent and washing them again with ethanol. Finally,
the QDs were added to 20.0 mL of CHCl3 and centrifuged to remove the
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insoluble impurities. The QD solution was stored in the absence of free
oxygen and moisture.
reduced pressure at room temperature. Then, the operation was repeated
as noted in CO2 photoreduction experiments.
To determine the mass concentration of InP QDs, the exact volume
of the solution was taken, and then the residue was weighed after
evaporation of CHCl3 under reduced pressure at room temperature. It was
estimated that 1 mL mixture contained about 3 mg InP QDs.
QE was determined under similar experimental conditions, except
the 150 W Xe irradiation source was fitted with a 425 nm bandpass filter
with an optical density of >4 in the rejection band and a slope factor of <1%
at the adjusted intensity. The liquid level was ~16 cm from the lamp window,
and a 21.24 cm2 area was illuminated. The apparent quantum efficiency
(QE) was calculated using the following equation:
Synthesis of cadmium sulfide nanorods: Cadmium sulfide nanorods
(CdS NRs) were prepared using the solvothermal technique. A mixture of
1335.5 mg (5 mmol) cadmium acetate (Cd (CH3COO)2. 2H2O) and 381.4
mg (5 mmol) thiourea (CH4N2S) was suspended in the solvent of
ethylenediamine (60.0 mL) under constant magnetic stirring for 30 minutes.
The subsequent mixture was transferred to a Teflon-lined stainless-steel
autoclave vessel of 100 mL capacity. The vessel was placed in a 160 °C
hot air oven and heated for 48 hours. The mixture after the reaction was
cooled normally off to ambient temperature and obtained yellow solid
precipitate. The precipitate was cleaned with de-ionized water and ethanol
several times to eliminate the impurities, then dried at 80 °C for 12 hours.
Numbers of reacted electrons




QE % 
100


Numbers of incident photons
n
CO  2  nH  2  N


A
2

100
Numbers of incident photons


where nCO and nH represent moles of produced CO and H2, respectively
and NA denotes Av2ogadro constant.
Photoelectrochemical
measurements:
Photoelectrochemical
measurements were conducted using
a
CHI 617B electrochemical
workstation in a standard three-electrode configuration. Pt wire was used
as a counter electrode, and an Ag/AgCl electrode served as the reference
electrode. The aqueous electrolyte solution contained 0.5 M Na2SO4. A
150 W Xe lamp equipped with an AM 1.5G filter was used for irradiation at
an intensity of 1 sun. A uniform suspension containing 5.00 mg of an as-
synthesized material, 450 μL ethanol, and 50 μL Nafion was spread onto
glass pretreated with conductive indium tin oxide (ITO) and dried under
reduced pressure. The transient photocurrent responses were measured
at 0.0 V during the on-off cycling of the solar simulator. Electrochemical
impedance spectroscopy (EIS) was performed at open circuit potential
using an AC voltage of 5 mV. A standard potentiostat equipped with an
impedance spectral analyzer was used to record Mott-Schottky plots at
frequencies of 0.1 and 0.5 kHz over a potential range of −1.2 to 1.2 V in
darkness. The measured potentials vs. Ag/AgCl were converted to the
Synthesis of InP QDs/CdS NRs composites: The CdS-InP composites
were prepared using the wet impregnation method. 35.0 mg of CdS NRs
were dispersed in 3.5 mL volumes of ethanol, and the mixtures were
ultrasonicated for 15 minutes at room temperature. The requisite amounts
of InP QDs in CHCl3 were added to the mixtures, which were kept for
ultrasonic treatment again for 15 minutes under an inert atmosphere. The
solvent was evaporated under reduced pressure at room temperature to
obtain CdS-InP composites, which were well kept in the absence of free
oxygen and water moisture.
Characterization methods: To specify the crystal structures of the
powder samples, Ultima IV X-ray diffractometer using Cu Kα radiation as
the X-ray source was used. The crystal structure of InP QDs was
determined using
a
SmartLab high-resolution X-ray diffractometer
normal hydrogen electrode (NHE) scale according to ENHE = EAg/AgCl
0.197.
+
(Rigaku). The chemical states and elemental compositions of the samples
were evaluated via X-ray photoelectron spectroscopy (XPS).
Measurements were performed using a K-alpha system (ThermoScientific,
USA) equipped with a monochromated Al Kα X-ray source (1,486.6 eV)
operated at 36 W (12 kV, 3 mA). The C 1s at 284.8 eV was selected as
the standard peak to calibrate the energy scale. Transmission electron
microscopy (TEM) images and elemental maps were obtained using a
JEM-200 TEM (JEOL, Japan) equipped with an energy dispersive X-ray
spectrometer (EDS) at an accelerating voltage of 200 kV to characterize
the microstructure properties. The UV-VIS absorption measurements were
Acknowledgements
This work was supported by National Research Foundation of
Korea (NRF) grants, funded by the Korean Government
(MSIP) (2014R1A4A1001690 and 2016R1E1A1A01941978).
operated
using
a
V-770
UV-VIS/NIR
spectrophotometer.
Keywords: CdS NRs Photocatalytic • Heterogeneous • InP QDs
Photoluminescence (PL) spectra of the photocatalysts were recorded on a
F-7000 fluorescence spectrometer (Hitachi, Japan) at ambient
temperature. Inductively Coupled Plasma Optical Emission Spectrometer
(OPTIMA 8300 ICP-OES) was employed to analyze the compositions of
the samples. The mass ratios between In and Cd elements in different
samples characterized by ICP-OES are shown in Table S1.
• Photocatalysis CO2 Reduction • Suppression of H2 Evolution
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Entry for the Table of Contents
The simple and cost-effective synthesis of noble metal-free CdS-InP composites for high photocatalytic CO2 conversion into CO
in aqueous solution is demonstrated. These synthesized materials are the potential candidates for pouring efforts into research on solar
energy harvesting applied to photochemical conversion and storage fields.
8
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