Metallic tungsten carbide nanoparticles as a near-infrared-driven photocatalyst

Article abstract of DOI:10.1039/c9ta03151k

Employing near infrared (NIR) light for photocatalytic reactions is preferable, considering effective solar utilization. Herein, the metalloid of tungsten carbide (WC) was used as an NIR-driven photocatalyst for the photodegradation of organic pollutants for the first time. The noble metal-like electronic properties of WC were proven via the analysis of its electronic occupied state using ultraviolet photoelectron spectroscopy and valence band XPS. In addition, both the experimental evidence and 3D finite element simulation revealed the NIR-responsive localized surface plasmon resonance (LSPR) behavior of the WC nanoparticles. Accordingly, the WC nanoparticles exhibited excellent UV-visible-NIR full-spectrum absorption, high NIR-triggered photocurrent response and resultant NIR-driven photocatalytic degradation performance. The NIR-mediated photocatalytic mechanism of WC was proposed based on a radical scavenging test, fluorescence observation of radical generation and spin-trapping electron paramagnetic resonance measurements. Hence, metallic WC with NIR absorption and photocatalytic activity may pave the way for the design of full-solar-spectrum-responsive photocatalysts.

Full text of DOI:10.1039/c9ta03151k

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In this work, we present the LSPR effect and full spectrum photoabsorption of metallic
WC for direct NIR-driven photoelectric conversion and photocatalytic degradation.

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Metallic tungsten carbide nanoparticles as near-infrared driven
photocatalyst
Weicheng Huang#a, Hongxue Meng#c, Yan Gaob, Jinxin Wanga, Chunyu Yangb, Danqing Liu*d, Jian
Liua, Chongshen Guo*b, Bin Yang*a, Wenwu Caoa,e
Received 00th January 20xx,
Accepted 00th January 20xx
Employing near infrared (NIR) light for the photocatalytic reactions is preferable way from the consideration of effective
solar utilization. Herein, the metalloid of tungsten carbide (WC) has been used as NIR-driven photocatalyst for the
photodegradation of organic pollutant for the first time. The noble metal-like electronic properties of WC are proved by
electronic occupied state analysis of ultraviolet photoelectron spectroscopy and valance band XPS. In addition, both of
experimental evidence and 3D finite element simulation revealed the NIR-responsive localized surface plasmon resonance
(LSPR) behavior of the WC nanoparticle. Accordingly, WC nanoparticles exhibited excellent UV-Visible-NIR full-spectrum
absorption, high NIR-triggered photocurrent response and resultant NIR-driven photocatalytic degradation performance.
The NIR-mediated photocatalytic mechanism of WC was proposed on the basis of radicals scavenging test, fluorescence
observation of radicals generation and spin trapped electron paramagnetic resonance measurements. Hence, the metallic
WC with NIR absorption and photocatalytic activity may pave the way to design full-solar-spectrum-responsive
photocatalyst.
DOI: 10.1039/x0xx00000x
www.rsc.org/
responsive photocatalyst with up-conversion materials, which
could turn two or more NIR photons into one UV and/or visible
photon to excite the host photocatalyst.5, 6 However, the low
Introduction
Solar energy as the clean and sustainable power source
motivated researchers to devote into the conversion and
utilization of solar energy. Of them, one of the most active
research topics is photodegradation of environmental
pollutants. To achieve this purpose, the photocatalysts should
possess suitable band structure for maximally matching the
solar spectrum and facilitate the charge-carriers generation as
well.1 In this regard, photocatalysts with ultraviolet (UV) and
visible light response have been widely reported, such as TiO2,
WO3, CdS, et.al.2-4 However, the UV light and visible light (Vis)
only occupy 5% and 45% of the total solar spectrum
respectively, and the remnant half belongs to the long-
wavelength near infrared light (NIR). Hence, most of the solar
energy cannot be efficiently utilized for the photocatalytic
reactions. The early attempts for realizing NIR-driven
photocatalyst mainly relied on compositing UV or Vis
conversion and photoluminescence efficiency make it to be a
bad quantum efficiency and photocatalytic activity. Another
strategy is constructing a photosensitized heterojunction,
employing dyes instead of up-conversion materials. However,
this is undesirable either as the dye sensitizers are photoactive
in a wavelength short than 900 nm, as well as toxic and
photobleaching. Hence, photocatalysts with direct NIR
response are still great challenge because of limited
alternative materials,7 and the related study has become
cutting-edge in the solar-to-chemical energy harvesting and
conversion filed.8 The pioneering work in this regard is
Cu2(OH)PO4 (Eg = 0.6 eV), which is the first example using
single matter for realizing NIR-driven photocatalysis.9 Then,
several species also have been highlighted including metal
11
12
WS2,10 BiO2-x
,
Co2.67S4 and black phosphorous,13 et.al.
However, their relative wide band gap (ca. 1.0 eV) make their
light absorption to be cut-off at relative short wavelength NIR
region (up to ca. 1200 nm). In our opinion, the LSPR effect
induced NIR absorption is one of the best strategy for the NIR-
driven photocatalyst design since the optical properties can be
flexibly tuned by changing the morphology.14 LSPR species
benefit from the abundant free carriers and unique dielectric
property for the photocatalysis. When subjected to the
external radiation, the photo-induced oscillator causes strong
light absorption in a specific spectral range. Actually, a variety
of photocatalytic heterojunctions have been decorated with
LSPR materials.15-17 However, the photostability issue,
expensive raw material costs and complicate material
a. School of Instrumentation Science and Engineering, Harbin Institute of
Technology, Harbin 150080, China. E-mail: binyang@hit.edu.cn
b. School of Chemistry and Chemical Engineering, Harbin Institute of Technology,
Harbin 150080, China. E-mail: chongshenguo@hit.edu.cn
c. Department of Pathology, Harbin Medical University Cancer Hospital, Harbin,
China.
d. School of Chemical and Environmental Engineering, Harbin University of Science
and Technology, Harbin 150040, China. Email: danqingliu76@163.com
e. Department of Mathematics and Materials Research Institute, The Pennsylvania
State University, Pennsylvania 16802, USA
# Weicheng Huang and Hongxue Meng contribute equally to this work.
Electronic Supplementary Information (ESI) available: BET, Ar+ etching XPS,
photothermal profiles, degradation of phenol, photostability evaluation, DFT
calculated DOS .
See DOI: 10.1039/x0xx00000x
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synthesis process of noble metal based LSPR materials greatly
hindered their further application in this area. In addition,
most of plasmonic metal as photosensitizer should marriage
with host matrix of semiconductors.18, 19 The mismatched band
structure and charge loss at the interface potentially reduce
their photoelectric conversion efficiency. Therefore, seeking
single component of non-noble metal plasmonic materials that
meet fundamental photocatalytic principle for efficient NIR
utilization are highly demanded.
View Article Online
DOI: 10.1039/C9TA03151K
Recently, tungsten carbide (WC) as one kind of transition
metal carbides catches our attention in virtue of metallic
conductivity and corrosion resistance nature.20 The previous
experimental and theoretical studies have proved that WC
possesses a platinum-like electronic property with d band
electrons dominantly occupying the Fermi level.21, 22 However,
the optical property and the photocatalytic application of the
WC are never studied. The metallic WC with high density of
free electrons may contribute to a similar LSPR-induced NIR
absorption and a direct NIR-driven photocatalytic potency is
expected.23
In this work, we present the full solar spectrum photo
absorption, as well as the metallic nature and LSPR effect of
WC for the direct NIR-driven photoelectric conversion and
photocatalytic degradation performance. 3D Finite element
method (FEM) has also been applied to stimulate the optical
property of WC nanoparticle, which unveils the NIR-excited
LSPR behaviour of the WC nanoparticle. Ultraviolet
photoelectron spectroscopy indicates that the high work
function and proper electronic band structure of WC meet the
NIR light harvesting and the redox potential requirement to
generate reactive radical species. Finally, the NIR-driven
photocatalytic mechanism of WC nanoparticle was proposed.
Figure 1 X-ray diffraction pattern of WC nanoparticle.
-ected area electronic diffraction (SEAD) images were collected
on JEOL 2100 Electron Microscope at 200 kV (JEOL Ltd.). The
crystal phase was tested by an X-ray diffractometer
(Empyrean, Malvern Panalytical instruments) using Cu Kα (λ =
0.154184 nm) radiation. X-ray photoelectron spectroscopy
(XPS), valence band XPS (VB-XPS) and ultra-violet
photoelectron spectroscopy (UPS) were recorded on ESCALAB
250Xi, Thermo Fisher Scientific. Visible-NIR absorption spectra
were performed at U-4100 spectrophotometer, HITACHI.
Photoluminescence (PL) spectra were recorded on
Fluoromax-4 fluorescence spectrophotometer (HORIBA Jobin
Yvon, Japan).
a
Photocurrent measurement
Platinum wire and Ag/AgCl electrode were used as counter
electrode and reference electrode respectively, while 0.1 M
Na2SO4 aqueous solution was selected as the electrolyte. The
working electrode was prepared by the following steps: 1) 2
cm x 2 cm ITO glasses were cleaned by ethyl alcohol and
deionized water for three times; 2) the clean ITO glass
underwent hydrophilic treatment by plasma etching (Femto,
Diener electronic); 3) 10 mg WC nanoparticle was added into 1
mL deionized water and then dispersed with ultrasonic for 10
min to form a homogeneous suspension; 4) the WC suspension
was coated on the hydrophilic ITO by spin coating method; 5)
Finally, the obtained working electrode was dried 60 ℃ for 4
h.
Experimental details
Materials
Methylene blue (MB), ethylenediaminetetraacetic acid
disodium salt (EDTA-2Na), sodium oxalate, isopropanol (IPA)
were purchased from Aladdin. Terephthalic acid (TPA) was
purchased from Alfa Aesar with 98% purity. 5, 5-Dimethyl-1-
pyrroline N-oxide (DMPO), 2',7'-dichlorofluorescin diacetate
(DCHF-DA), 1, 3-Diphenylisobenzofuran (DPBF) and 2,2,6,6-
Tetra methyl- piperidine (TEMP) were purchased from Sigma
Aldrich. All the chemical reagents were used without further
purification. The WC nanoparticles (got from Shanghai
Chaowei Nano Technology Co. Ltd.) were synthesized by the
use of a typical high temperature thermal reduction method
assisted by laser-ion beam. Tungsten oxalate is used as raw
material and the operation atmosphere is Argon. The as-
synthesized powder was finally ball milling and sieving to
obtain ultrafine and homogeneous WC nanoparticles.
Photocatalytic degradation experiments
The photocatalytic activity of WC was investigated by the
degradation of methylene blue (MB) under NIR light
irradiation. 50 mg WC nanoparticle was dispersed into 50 ml of
100 mg L-1 MB solution for 1.5
h to arrive at an
adsorption/desorption equilibrium. Then, the WC powder was
centrifugated and transferred to 50 mL of 10 mg L-1 MB
solution for the photocatalytic degradation. An infrared lamp
of 200 W was used as the NIR source, and the wavelengths
smaller than 760 nm were cut off by a high pass filter
(HWB760). During the reaction, cooling water was applied.
After the reaction, 1 mL of the mixed solution was collected
Characterization
The morphological characteristics were observed by scanning
electronic microscope (Carl Zeiss EVO, Merlin Compact.) at 10
kV. Moreover, transmission electron microscopy (TEM), high-
resolution transmission electron microscopy (HRTEM) and sel-
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every hour. The concentration of MB was measured by
referring to the typical absorption peak of MB at 665 nm.
Finite element analysis
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DOI: 10.1039/C9TA03151K
3D finite element analysis was performed in the COMSOL
Multiphysics software package to simulate the local electric
field and light absorption. The tetramer particles model was
built for the calculation. The incident electric field was set to
propagate along the + z direction with + x axis polarization. The
media layer thickness was 10 times of particle size. Perfect
match layer (PML) with radium 2 times of particle size was
used to eliminate the influence of boundary on calculation
results. The refractive index of WC nanoparticle was set as 3.4
by referring to the density functional theory calculation
results. The typical tetrahedral meshing was performed with
the maximal element size no larger than 200 nm, 100 nm, and
5 nm for PML, media, and particle, respectively. Finally, the
Figure 3 X-ray photoelectron spectroscopy (XPS) studies of WC nanoparticle. (a)
Full-range spectrum survey, displaying the presence of C, W, and O elements;
fine resolution scanning of (b) C 1s, (c) O 1s and (d) W 4f orbits.
electric filed distribution and absorption cross-section (σabs
)
were calculated, where the σabs was calculated by integrating
the absorbed power density over the volume of the WC
nanoparticle.24
-edral angle is measured about 63.2°, which is consistent with
the theoretical value of 68.0°. The selected area diffraction
pattern in Figure 2d presents symmetry diffraction spots,
indicating the good crystallinity of the sample. The diffraction
spots can be indexed as (0 1 0), (1 0 1) and (1 1 1) lattice facets,
which is consistent well with the XRD and HRTEM results.
To investigate the chemical state of the WC nanoparticles, the
X-ray photoelectronic spectrum (XPS) measurement was
carried out. Figure 3a shows the full-range XPS survey of WC,
which verified the existent of W, C and O elements. The
presence of O 1s and O Auger peaks (OKLL) imply the existence
of tungsten oxide layer or surface absorbed water. High
resolution XPS spectra are shown in Figure 3b-3d. The core-
level XPS spectra of C1s can be fitted into three peaks (Figure
3b). The strongest peak at 284.8 eV is the standard binding
energy of C1s, and the remnant XPS data were all calibrated by
referring to it. The binding energy peak at 282.9 eV
corresponds to the C-W chemical bond, while the peak at
286.0 eV is related to the C-O band. Figure 3c presents the
core-level XPS (raw and fitted data) of W 4f with the multiple
peaks in the range of 30 to 39 eV, implying the mixed chemical
oxidation states of W atom. As summarized in the Table 1, the
fitted doublet peaks at low binding energy region (33.9 eV and
Results and discussion
Characterizations of WC
The crystal nature of WC nanoparticles was analysed by XRD as
shown in the Figure 1. The diffractive peaks in XRD pattern all
match well with the standard PDF card (JCPDS#59-0939),
indicating the pure hexagonal phase of WC with a high
crystallinity. Three typical diffraction peaks at diffraction angle
of 31.5°, 35.6° and 48.3° are the (0 0 1), (1 0 0) and (1 0 1)
crystal facets of WC, respectively. The morphological features
were observed by SEM and TEM observations in the Figure 2.
The irregular shaped WC nanoparticles have a particle size
from ca. 50 to 200 nm. BET plots and pore size distribution
curves (Figure S1) of WC nanoparticles revealed that the
specific BET surface area is about 4.8 m2/g and the average
pore size is about 10 nm. The high resolution TEM image in
Figure 2c showed clear crystal fringes, presenting two sets of
streaks intersected with each other. The interplanar distance
of 0.181 nm belongs to the (1 0 1) facet spacing, and the other
one of 0.255 nm is assigned to (0 1 0) facet spacing.25 The dih-
31.8 eV) with interval gap about 2.1 eV is corresponding to the
27
W-C chemical bond (W4+ 4f5/2 and W4+ 4f7/2).26,
The rest
peaks at high binding energy region can be fitted into four
symmetric Gauss peaks. The peaks at 36.6 eV and 34.9 eV are
assigned to the W-O bond of W5+ 4f5/2 and W5+ 4f7/2, moreover,
Table 1 W 4f XPS analysis data
Peak
(eV)
Interval
gap (eV)
Atom
ratio
Chemical state Orbital
FWHM
4f7/2
W4+(W-C)
31.8
33.9
34.9
36.6
35.6
37.8
0.85
0.80
0.85
0.85
0.88
0.88
2.1
2.3
2.2
0.65
0.09
0.26
4f5/2
4f7/2
W5+(W-O)
4f5/2
Figure 2 (a) SEM image, (b) TEM image, (c) HR-TEM image, (d) selected area
4f7/2
W6+(W-O)
electron diffraction pattern of WC.
4f5/2
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Figure 4 (a) UV–vis–NIR diffuse reflectance spectra of WC powder (insert image shows the photograph of the black WC powder); (b) Photocurrent of WC nanoparticle
irradiated by NIR light.
the peaks at 37.8 eV and 35.6 eV are attributable to the W-O showed in Figure 5b, the optical absorption measured in
bond of W6+ 4f5/2 and W6+ 4f7/2. O 1s XPS spectra in Figure 3d deionized water indicates a visible-NIR light absorption up to
show a doublet peaks at binding energy of 530.7 eV (lattice 1400 nm, and the typical absorption peak at ca. 1250 nm may
oxygen) and 532.4 eV (surface water absorption or W-OH be originated from LSPR effect of the metallic WC. What’s
bond), indicating the presence of WO3-x layer as well.17 We more, the LSPR wavelength and absorbance increase with the
used the Ar+ beam to etch the WC with a thickness of 20 nm, refractive index of the surrounding media (nmed) (nmed = 1.333,
and in-situ scanning of the W 4f XPS was performed. As the 1.361, 1.384 and 1.399 for deionized water, ethanol, n-
results shown in Figure S2 and Table S1, the binding energy propanol and butyl alcohol, respectively), verifying an exact
peaks are shift to lower energy region and the doublet peaks LSPR effect (Figure 5b).29 Then, the electric distribution and
at ca. 35 eV~39 eV both decreased after etched 20 nm, which optical absorption property were simulated by 3D finite
indicates that the average chemical state of WC and the element calculations (Figure 5b). To simplify the interaction
content of tungsten oxide are both decreased, verifying the behaviour of particle-light, the tetramer particles with 2
existence of surface tungsten oxide. The high-valent tungsten spheres and 2 ellipsoids were built as the calculation model
atoms in WC may exist in the form of surface (Figure 5a) by referring to the TEM image (Figure 2b). The
nonstoichiometric WO3-x, thus in suit forming WC/WO3-x interval gap between sphere and ellipsoid was set as 5 nm. The
heterojunction with a molar ratio of 0.65/0.35 by the natural wavelength of incident light was set as 1000-1500 nm with
oxidation. Fortunately, it is positive to enhance the separation equal internal of 5 nm step. The calculation results with water
rate of photoexcited charge carriers.26
medium are illustrated in the Figure 5c-5d, which reveals that
the LSPR excitation of WC nanoparticles can significantly
enhance the localized electric field and form plasmonic ‘hot
spots’ between the particles’ interval region, especially for the
narrower region. Moreover, the localized electric field of the
hot spots shows strong dependence on the excitation
wavelength with a maximum enhancement factor peak at ca.
1205 nm, which matches well with the experimental LSPR
absorption (ca. 1250 nm) measured in water (Figure 5b).
What’s more, the simulation results (Figure 5e-5g) in variety of
surrounding media of the WC nanoparticles also show the
enhancement in the localized electric field and optical
absorption cross-section, as well as red shift of the LSPR
optical absorption (Figure 5f-5g), which is identical to our
experimental observations. It is noteworthy that the LSPR-
induced electric fields enhancement can effectively focalize
the incident light (including NIR) field within a particular region
and greatly enlarge the local electromagnetic field intensity,
then forming ‘hot spots’. Finally, the ‘hot spots’ favour the
generation of the energetic hot electrons (or holes).30
Therefore, these plasmonic-excited hot electrons (holes) may
further transfer to the material interface and participate in
photochemical reaction.
Optical and photoelectric properties
Then, we move our attention to the optical and photoelectrical
property of the WC nanoparticles. The UV-visible-NIR diffuse
reflectance spectra of WC powder were measured (Figure 4a).
The black WC powder (insert of Figure 4a) shows a significant
light absorption in the full spectrum of 300-2500 nm.
Impressively, the absorbance of WC increases with the
wavelength, which is obviously different from others reported
28
carbides and metallic conductor.8,
Next, the NIR-driven
photoelectrical performance of WC was evaluated by a three
electrodes system with a 200 W infrared lamp as the light
source. As shown in Figure 4b, the NIR light driven
photocurrent with a robust responsivity of ca. 15 μA/cm2 is
very impressive, especially for a non-metal based material.5, 10
The extensive consensus about LSPR stimulated light
absorption of noble metal gives us guidance to further study
the origin of the unique NIR light absorption and NIR
photocurrent response of the metallic WC nanoparticles.
Firstly, the optical absorption spectra of the WC dispersion in
variety of solvents (0.4 mg/mL) have been conducted. As
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Figure 5 FEM simulation and experimental evidence to illustrate the noble-metal like LSPR behaviour of WC nanoparticles. (a) Model used for computing the field electric field
distribution. (b) Absorbance of WC nanoparticle dispersed in various media with refractive index (nmed) from ranging 1.333 to 1.389 (n = 1.333, 1.361, 1.384 and 1.399 for deionized
water, ethanol, n-propanol and butyl alcohol), respectively. (c) The local electric field distribution induced by the plasmonic WC nanoparticle under NIR light radiation (wavelength
of the incident light ranged from 1050 nm to 1400 nm). (d) Wavelength-dependent electric field enhancement at O (0, 0, 0) point corresponding to (c). (e) The electric field
distribution of different surrounding refractive index at the LSPR excitation wavelength of 1200 nm. (f) The calculated absorption cross-section spectra of WC in different
surrounding media. (g) The electric field enhancement dependence of surrounding refractive index at O (0,0,0) point corresponding to (e). The scale bar in (c) and (e) is 50 nm.
NIR-driven photocatalytic performance
reaction system was seen under NIR irradiation. The NIR-
driven photocatalytic degradation curve shown in Figure 6a
(red line) confirmed that the degradation rate of MB is about
50 % after 4 h NIR radiation. Notably, the NIR-driven
On the basis of above fundamental understanding of the
structure, morphology, photoelectrical properties and
remarkable LSPR effect of WC nanoparticles, we next
investigated the NIR-driven photocatalytic performance. The
photocatalytic degradation of methylene blue (MB, 10 mg/L)
was used to evaluate the photocatalytic activity of the WC
nanoparticles. Physical adsorption and localized photothermal
effect are two important factors that may influence the NIR
photocatalytic evaluation. Firstly, we conducted pre-
adsorption process with 100 mg L-1 of MB solution (50 mL) for
1.5 h before the photocatalytic test to make sure a maximum
adsorption-desorption equilibrium. Secondly, we introduced
the cooling water for removing the thermal degradation of MB.
As shown in Figure S3-S4, hardly no temperature rise of
photocatalytic property of WC is better than previously
32
reported W18O49 and tungsten bronzes.31,
Moreover,
colourless 1,3- Diphenylisobenzofuran (DPBF) was also used to
probe the NIR-driven photocatalytic activity of the WC
nanoparticle. DPBF is a compound that could be degraded by
ROS. Hence, the relative DPBF content in the solution can be
recorded in UV-visible spectrophotometer by referring to the
typical absorption at 419 nm. Figure 6b indicates that the WC
can effective decompose DPBF with 25 min NIR radiation,
verifying the excellent NIR photocatalytic performance of WC
again.
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Figure 6 NIR-driven photocatalytic performance of WC nanoparticle and detection of reactive radicals. (a) Photodegradation curve of MB solution；(b) Photodegradation curve of
DPBF solution; (c) Photoluminescence spectra of terephthalic acid after different NIR radiation time, indicating the presence of •OH radicals; (d) Photoluminescence spectra of
DCFH-DA after different NIR radiation time, indicating the presence of 1O2 radicals (the insert pictures show the typical yellow fluorescence); ESR spectra of (e) DMPO-•OH adduct
and (f) TEMP-1O2 adduct spin-trap.
Free radical detection
in the process of photocatalytic degradation of organic
pollutants, which have been confirmed not only in UV-visible
In this work, the WC is brand-new material for the NIR-driven
photocatalytic application. Therefore, the LSPR induced NIR-
driven photocatalytic mechanism has not been acknowledged.
It is of great significances to take efforts to study related
mechanism. Herein, we started our study from the detection
of free radicals. As we all known, the hydroxyl radical (•OH),
singlet oxygen (1O2) and hole (h+) are the active radical species
driven photocatalyst, but also in some of NIR-driven
31, 33, 34
photocatalytic process.12,
Firstly, we tested the
photocatalytic performance with reactive specie scavengers.
Sodium oxalate (or EDTA-2Na) and isopropanol (IPA) are
usually used as the scavenge of •OH and h+. As shown in Figure
6a, the degradation rate of MB was greatly suppressed when
the extra 5 mM of sodium oxalate (or EDTA-2Na) or 20 wt% of
IPA was introduced into the photocatalytic reactor. We thus
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Figure 7 (a) Schematic diagram of the NIR-triggered generation of holes and electrons. (b) Temperature-dependent resistivity spectra. (c) UPS valance band spectra of
WC nanoparticle. (d) Valence band spectra of WC nanoparticles before and after Ar+ beam etching. (e) Illustration of NIR-driven photocatalytic process.
can conclude that both of •OH and h+ play an important role in photoluminescence (PL) intensity increased with NIR radiation
the photocatalytic process, especially for the •OH as the WC time, and even could be evidenced by the naked eyes (insert of
1
was almost deactivated in presence of EDTA-2Na or sodium Figure 6d), confirming the generation of •OH and O2 by NIR-
oxalate. Secondly, to confirm the production of radial species triggered WC. Finally, the ESR spectroscopy was used to detect
1
by NIR-excited WC, free radical fluorescent probes were the •OH and O2 during the NIR-driven photocatalytic process
employed. Terephthalic acid can react with •OH and then form as well. As shown in Figure 6e, the typical quadruple peaks of
2,5-dihydroxyterephthalic acid, which can be excited by DMPO-•OH adduct with peak intensity ratio of 1:2:2:1
incident light of 320 nm and give a luminescence at 425 nm.35 appeared after NIR irradiation for 20 min. Moreover, the
Moreover, 2',7'-dichlorofluorescin diacetate (DCFH-DA) was generation of singlet oxygen (1O2) under NIR irradiation was
used as probe for singlet oxygen (1O2) detection, which could confirmed by TEMP probe. In the Figure 6f, the sharper and
be oxidized by 1O2 to a fluorescent molecular with an emission stronger triplet peaks of TEMP-1O2 adduct indicated the
1
enhanced signal of O2 after 20 min NIR radiation, in contrast
at 520 nm.36 The typical photoluminescence spectra of the
terephthalic acid and DCFH-DA that collected at different
reaction time are shown in Figure 6c-6d. The
to the weak signal in the dark condition. Thus, free radicals of
•OH, 1O2 and h+ were identified above, all of which
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Journal of Materials Chemistry A
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Journal Name
participated in the NIR-driven photocatalytic degradation mechanism to interpret the NIR-driven photocatalytic property
View Article Online
process.
of WC. The relevant chemical reactions arDeOliIs: t1e0d.10a3s9/fCo9llToAw03in15g1:K
NIR-driven photocatalytic mechanism
WC + hv (NIR) → WC* + e- + h+
(Eq.1)
(Eq.2)
(Eq.3)
(Eq.4)
(Eq.5)
-
The schematic diagram in Figure 7a shows the LSPR-excited
carries (e- and h+) generation under NIR light radiation, which
is pivotal in the whole photocatalytic procedure. In addition,
the chemical potentials relating to the generation of radial
O2 + e- → •O2
-
•O2 + H+ + → •HO2
WC* + h+ + OH- → WC + •OH
•O2 + •OH → 1O2 + OH-
-
-
species are presented as well. The redox potentials of O2/•O2
and OH-/•OH are -0.146 eV (vs NHE) and 1.99 eV (vs NHE),
respectively.37 For confirming the metallic nature of WC, we
performed the temperature-dependent resistivity test, UPS
analysis and VB XPS measurement, respectively. The
temperature-dependent resistivity curve of WC shows that the
resistivity of WC increases with temperature elevation in the
range of -100 ℃ to 0 ℃ (Figure 7b), disclosing that the WC is a
(1) NIR light illumination triggers LSPR in WC and plenty of ‘hot
electrons’ are excited up to LSPR state, leaving the matched
holes in the OS state (Eq.1).
(2) The electrons then relax to the lower energy state and
transit to the surface state WO3-x preferentially, thus realizing
the separation of electron and holes (Figure 7e).
metal-like conductor. The occupation of Ef level by continuous (3) The combination of dissolved O2 and the electrons at WC
-
electronic occupied states and the right-shift of the VB-XPS surface generates •O2 and subsequent •HO2 (Eq.2 and Eq.3).
spectra (Region II) after Ar+-etching disclose the metallic Meanwhile, the holes located at OS2 would react with OH- to
-
nature of the WC nanoparticles as well (Figure 7c and Figure produce •OH (Eq.4), which may further react with •O2 to
7d)38. In addition, the work function of WC can be calculated generate 1O2 (Eq.5).
as 6.62 eV by referring to the high energy edge (14.6 eV). Thus,
(4) All the reactive radical species participate in the
Ef level relative to NHE is 2.12 eV, being larger than 1.99 eV of
degradation process to destroy the organic pollutant.
OH-/•OH. Two significant occupied states (noted as OS1 and
OS2, Figure 7c) near the Ef level can be fitted with energy level
of -0.7 eV and 1.1 eV relative to the Ef. In the Figure 7d of VB
Conclusions
XPS spectra, there were obvious changes in both of region I
In summary, we have demonstrated a new application of WC
and region II after Ar+ etching. The first change is the
nanoparticles for the NIR-responsive photocatalytic
disappearance of binding energy falling edge in the region I,
degradation of pollutant. The powder absorbance spectra
while the second one is the shift of region II toward the fermi
demonstrated a broad photoabsorption of WC, especially in
energy level for the etched WC. Both of these two changes
the NIR region. Accordingly, photoelectrical test indicated an
happened as the result of surficial tungsten-oxide layer
obvious NIR-mediated photocurrent response. 3D finite
removal. Hence, it is clearly that the region I is attributable to
element simulations revealed that the LSPR effect of the
the valence band maximum of surface tungsten oxide and the
metallic WC accounted for the NIR-harvesting and
region II is the tailing electronic state of the metallic WC38. The
photoelectrical properties. As a result, WC nanoparticles
binding energy intercept of region II shifted towards right side
showed a desired photocatalytic degradation (ca. 50% removal
and crossed the fermi level well (typical metallic nature)
rate of MB) performance under NIR radiation.
proved above assumption again as we have confirmed the
metallic nature of WC above. In a word, the two significant
axial intercepts (OS1 and OS2) in binding energy falling edge of
the UPS (-2 eV~6 eV) reflect the typical electron occupied state
of the surface tungsten oxide and metallic WC, respectively.
Noteworthy, OS1 relating to the valence band maximum of the
Conflicts of interest
There are no conflicts to declare.
surface tungsten oxide is formed by the hybrid orbit electron
of W 4d and O 2p, while the OS2 of metallic WC tailing
electronic state is constructed by the hybrid orbit electron of
Acknowledgements
W 4d and C 2p (Figure S5)22, 39, 40. Consequently, the electrons
can transit between the occupied states and LSPR state by
absorbing NIR photonic energy (Figure 7e). According to
previous XPS results, we have confirmed the existence of the
nonstoichiometric tungsten oxide on the surface of WC (noted
as surface state WO3-x). The in suit formation of WC/WO3-x
heterojunction not only offers a suitable chemical potential for
This work was financially supported by the National Natural
Science Foundation of China (No. 51572056, 51572059,
51602083
& 81600539), Heilongjiang Provincial Natural
Science Foundation for Distinguished Young Scholar (Grant No.
JC2017001), and Harbin Applied Technology Research and
Development Project (2017RAXXJ003).
-
the generation of •O2 (Figure 7e),41 but also promotes the
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