WS2/g-C3N4 composite as an efficient heterojunction photocatalyst for biocatalyzed artificial photosynthesis

Article abstract of DOI:10.1039/c8ra02807a

A heterogeneous WS₂/g-C₃N₄ composite photocatalyst was prepared by a facile ultrasound-assisted hydrothermal method. The WS₂/g-C₃N₄ composite was used for photocatalytic regeneration of NAD⁺ to NADH, which were coupled with dehydrogenases for sustainable bioconversion of CO₂ to methanol under visible light irradiation. Compared with pristine g-C₃N₄ and the physical mixture of WS₂ and g-C₃N₄, the fabricated WS₂/g-C₃N₄ composite catalyst with 5 wt% of WS₂ showed the highest activity for methanol synthesis. The methanol productivity reached 372.1 μmol h^-1 g_cat^-1, which is approximately 7.5 times higher than that obtained using pure g-C₃N₄. For further application demonstration, the activity of the WS₂/g-C₃N₄ composite catalyst toward photodegradation of Rhodamine B (RhB) was evaluated. RhB removal ratio approaching 100% was achieved in 1 hour by using the WS₂/g-C₃N₄ composite catalyst with 5 wt% of WS₂, at an apparent degradation rate approximately 2.6 times higher than that of pure g-C₃N₄. Based on detailed investigations on physiochemical properties of the photocatalysts, the significantly enhanced reaction efficiency of the WS₂/g-C₃N₄ composite was considered to be mainly benefiting from the formation of a heterojunction interface between WS₂ and g-C₃N₄. Upon visible-light irradiation, the photo-induced electrons can transfer from the conduction band of g-C₃N₄ to WS₂, thus recombination of electrons and holes was decreased and the photo-harvesting efficiency was enhanced.

Full text of DOI:10.1039/c8ra02807a

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WS /g-C N composite as an efficient
2
3 4
heterojunction photocatalyst for biocatalyzed
Cite this: RSC Adv., 2018, 8, 20557
artificial photosynthesis†
Peng Zeng, Xiaoyuan Ji, Zhiguo Su and Songping Zhang *a
ab
ac
a
A heterogeneous WS
hydrothermal method. The WS
NADH, which were coupled with dehydrogenases for sustainable bioconversion of CO
under visible light irradiation. Compared with pristine g-C and the physical mixture of WS
, the fabricated WS /g-C composite catalyst with 5 wt% of WS
methanol synthesis. The methanol productivity reached 372.1 mmol h
times higher than that obtained using pure g-C
the WS /g-C composite catalyst toward photodegradation of Rhodamine B (RhB) was evaluated. RhB
removal ratio approaching 100% was achieved in 1 hour by using the WS /g-C composite catalyst
with 5 wt% of WS , at an apparent degradation rate approximately 2.6 times higher than that of pure g-
. Based on detailed investigations on physiochemical properties of the photocatalysts, the
significantly enhanced reaction efficiency of the WS /g-C composite was considered to be mainly
benefiting from the formation of a heterojunction interface between WS and g-C . Upon visible-light
irradiation, the photo-induced electrons can transfer from the conduction band of g-C to WS , thus
2
/g-C
3
N
4
composite photocatalyst was prepared by a facile ultrasound-assisted
+
2
3
/g-C N
4
composite was used for photocatalytic regeneration of NAD to
2
to methanol
and g-
N
3 4
2
C
3
N
4
2
3
N
4
2
showed the highest activity for
ꢀ1
catꢀ1
g
, which is approximately 7.5
3 4
N . For further application demonstration, the activity of
2
3 4
N
2
3 4
N
2
3 4
C N
Received 1st April 2018
Accepted 27th May 2018
2
3 4
N
2
3 4
N
DOI: 10.1039/c8ra02807a
N
3 4
2
rsc.li/rsc-advances
recombination of electrons and holes was decreased and the photo-harvesting efficiency was enhanced.
is the development of efficient visible-light active
photocatalysts.
Graphitic carbon nitride (g-C N ), a polymeric semi-
3 4
1
. Introduction
10–12
With the increasing concerns regarding the energy crisis and
greenhouse gas emissions, it has become urgent to develop conductor with a band gap of about 2.7 eV, has attracted
a new channel to achieve the conversion of CO to low-carbon signicant attention in recent years because of its excellent
2
1–5
1,13,14
fuel.
conversion from CO
drogenase (FateDH), formaldehyde dehydrogenase (FaldDH) been widely applied for photocatalytic degradation of pollut-
Among the energy conversion systems, methanol visible light response,
non-toxicity, low cost, and abundant
has
by a cascade catalyzed by formate dehy- raw materials. As a visible-light active photocatalyst, g-C
N
3 4
2
15,16
and alcohol dehydrogenase (ADH) is considered to be one of the ants, water splitting, and regeneration of NADH since 2009.
6,7
most promising ways. However, the regeneration of coenzyme Nevertheless, fast recombination of photogenerated electron–
NADH in this multi-enzyme system is the key to the reaction hole pairs and low quantum efficiency limits its further appli-
process, since 3 moles of NADH are consumed for every mole of cations. In general, there are three routes to improve the pho-
8,9
methanol produced. In the past decade, inspired by natural tocatalytic efficiency of g-C
3 4
N , which include construction of
photosynthesis, photocatalytic regeneration of coenzyme with nanostructure, elemental doping, and hybridization. By fabri-
1
7
18
the process of reducing CO
new hope for solving this problem. For the success of such nanocapsules, and some other novel nanostructures, more
articial photosynthesis systems, however, the major challenge active sites were exposed on g-C for enhanced light har-
by multiple enzymes has brought cating nanosheets, diatom frustule structure, multishell
2
19 20
N
3 4
vesting due to signicantly increased specic surface area, and
the migration rate of photo-generated electrons was promoted
State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, and the recombination of electrons and holes reduced.
18–20
a
Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: spzhang@ipe.ac. Elemental doping was another widely adopted method for
cn; Fax: +86 10 82544958; Tel: +86 10 82544958
reducing the recombination of electron–hole pairs by replacing
b
University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
21,22
the lattice defects of g-C N .
Nevertheless, fabrication of new
3
4
c
School of Pharmaceutical Sciences (Shenzhen), Sun Yat-sen University, Guangzhou
nanostructures and elemental doping oen involve compli-
cated preparation process and uncontrollable results.
Compared with these above two methods, hybridization of g-
5
†
1
10275, P. R. China
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra02807a
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3 4
C N with some other semiconductors was is relative simple but (pentamethylcyclopentadienyl) rhodium(III) dimer was added to
efficient. The hybrid materials, which usually have an unsatu- 10 mL of methylene chloride, where the solid was insoluble.
rated outer layer electron, can act as a new electron carrier to Then, 60.07 mg of 1,10-phenanthroline was added to the
trap the electrons in the conduction band (CB) of g-C N
mixture. Aer stirring at room temperature for 3 h, the color of
3
4
through formation heterojunction or “Z” scheme electron the solution changed from dark orange to orange. Aer
4,23,24
transfer chain.
Nevertheless, either construction of heter- removing the solvent by evaporation under reduced pressure, M
ojunction or “Z” scheme transfer chain, there are certain was obtained.
requirements for the band structure of hybrid materials, which
need a good coordinated relationship with that of g-C
3
N
4
.
3 4 2 3 4
2.2. Preparation of g-C N and WS /g-C N composites
In recent years, transition metal dichalcogenides (TMDCs) catalysts
have emerged and received tremendous attentions. TMDCs
3 4
To prepare graphite phase carbon nitride (g-C N ), 10 g of
such as WS , MoS , WSe , and MoSe , have various unique
2
2
2
2
dicyandiamide was ground into a powder and placed in a cap-
ped alumina crucible covered with aluminum foil and then
electronic, optical, mechanical and chemical properties,
promising their potential applications in electronic devices,
ꢁ
calcined at 550 C for 4 hours in a muffle furnace at a rate of
2
5–27
transistors, energy storage devices and catalysis.
TMDCs, the 2H-WS has a band gap of about 2.64 V, with
conduction band (CB) and valence band (VB) positions at
Among
ꢁ
ꢀ1
3
C min . The g-C N was obtained as yellow solid.
3 4
2
WS
2
/g-C
3
N
4
composites were synthesized via a two-step self-
was added into
00 mL deionized water and sonicated for one hour. Into the
assembly procedure. Herein, 100 mg of g-C
3 4
N
26,28
zꢀ1.03 V and 1.61 V versus normal hydrogen electrode,
1
respectively; which means the WS possesses energy level of CB
2
2
above suspension, denite amount of WS powder was added to
obtain a WS mass fraction of 1, 5, 10%. The suspension
2
and VB both lower than that of g-C N , but the CB offset
3
4
between g-C
g-C . Therefore, there would be a perfect band structure
coordination between WS and g-C to construct a hetero-
3 4 2
N and WS is much shorter than the band gap of
mixture was sonicated for 30 min followed by 36 h mechanical
stirring. To consolidate the heterojunction interface in the WS2/
g-C N composites, the mixture was sealed into a 100 mL
3 4
N
2
3 4
N
3
4
junction structure. The good match for the energy level would
possibly enable fast transfer of the photo-induced electrons in
the CB of g-C N to the CB of WS before recombination with
ꢁ
Teon-lined stainless steel autoclave and heated to 140 C for
h. Aer cooling down to room temperature and evaporating
the solvent, WS /g-C composites with different composition
proportions were obtained.
6
3
4
2
2
3 4
N
holes in VB of itself, so that the light harvesting efficiency and
utilization of photo-induced electrons would be improved
29–32
largely.
In this present work, a heterogeneous WS
2.3. Catalyst characterization
2
3 4
/g-C N composite
Scanning electron microscope (SEM) analysis was performed on
a JSM 6700F cold eld emission scanning electron microscope
at beam energy of 10.0 kV. Transmission electron microscope
photocatalyst was prepared via ultrasound assisted hydro-
thermal method. The photocatalytic performance was evaluated
through the regeneration of NADH and degradation of RhB
under visible light. Detailed investigations of the physi-
ochemical and photochemical properties of the photocatalysts
were performed to elucidate the photoinduced electron-transfer
(TEM) images were taken on JEM 2100F instrument at an
acceleration voltage of 200 kV. Atomic force microscopy was
performed using Bruker FASTSCANIO microscope. Thermo
gravimetric analysis (TGA), was carried out in a ow of nitrogen
mechanism involved in the heterogeneous WS
composite photocatalyst. Therefore, our work highlights the
promise of constructing heterojunction based on g-C for
2 3 4
/g-C N
ꢀ
1
ꢁ
ꢀ1
gas (10 mL min ) at a heating rate of 10 C min using
Synchronous DSC-TGA Thermal Analysis (SDT Q600). The
specic surface areas were measured on ASAP 2020HD88 by the
3 4
N
photocatalyzed NADH regeneration and pollutant degradation,
and encourages further in-depth investigations of this novel
type of heterojunction for articial photocatalysis.
2 3 4
BET method. The crystal structures of WS /g-C N composites
were identied by X-ray diffraction (XRD) with Empyrean X-ray
diffractometer from PANalytical B.V. of the Netherlands. X-ray
photoelectron spectroscopy (XPS) analysis was performed on
ESCALAB 250Xi instrument. The Fourier transform infrared
2. Experimental section
ꢀ
1
2.1. Material
(FT-IR) spectra were collected from 400 to 4000 cm at the
ꢀ1
spectral resolution of 0.482 cm on NICOLET iS 50 infrared
spectrometer. UV-vis diffuse reectance spectroscopy (UV-vis
DRS) were recorded in the range of 200–900 nm by a TU-1901
spectrophotometer with the reference of BaSO4. Photo-
luminescence (PL) spectra were determined on a NanoLOG-
TCSPC steady state/transient uorescence spectrometer. The
wavelength of the excitation light was 340 nm.
Formate dehydrogenase (FateDH), formaldehyde dehydroge-
nase (FaldDH), yeast alcohol dehydrogenase (ADH), b-nicotin-
+
amide adenine dinucleotide (b-NAD ), reduced nicotinamide
adenine dinucleotide (NADH), dicyandiamide, triethanolamine
(
TEOA), dichloro (pentamethylcyclopentadienyl) rhodium(III)
dimer, 1,10-phenanthroline were purchased from Sigma-
Aldrich. Tungsten disulde (WS was purchased from
2
)
Shanghai Macklin Biochemical Co., Ltd (Shanghai, China).
2
.4. Photocatalytic regeneration of NADH
The organometallic electron mediator (M), [Cp*Rh(phen)
2
+
H
2
O] , (Cp* ¼ 5-C5Me5, phen ¼ 1,10-phenanthroline) was The photocatalytic regeneration of NADH reaction was carried
synthesized as follows. Briey, 103.01 mg of dichloro out in a side-irradiation vial under an irradiation of 300 W Xe
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lamp with a 400 nm cut-off lter. In the regeneration procedure, temperature. In a typical run, 5 mg of catalysts was added into
+
ꢀ1
the reaction medium (3 mL) was composed of NAD (1 mM), 5 mL of 10 mg L RhB solution. Prior to irradiation, the
TEOA (15 w/v%), M (0.25 mM), phosphate buffer (100 mM, pH suspension was magnetically stirred in the dark for 1 h to
7.0) and catalyst (3 mg). The distance between reactor and Xe ensure the adsorption–desorption equilibrium. The concentra-
lamp was xed at 20 cm. Before irradiation, the reaction solu- tion of RhB was monitored by measuring the absorbance at
tion was placed in a dark environment for 1 h to achieve 554 nm using a UV-vis spectrophotometer (UV-2800, Unico).
adsorption–desorption equilibrium. The concentration of The procedures of the scavenging experiments of reactive
+
NADH regenerated from NAD was monitored by measuring the oxygen species were similar to that of the photo degradation
NADH
ꢀ1
ꢀ1
absorbance at 340 nm (3
¼ 6.22 mM cm ) using a UV-vis experiment. Various scavengers, including isopropyl alcohol
spectrophotometer (UV-2800, Unico).
(IPA, 1 mM), TEOA (1 mM), KI (1 mM), and 0.5 h nitrogen
purging (N ) were subjected into the RhB solution prior to
2
2
.5. Conversion CO
2
to methanol
to methanol was performed in quartz
irradiation.
The conversion of CO
2
reactor. The reaction solution was composed of 3 mg catalyst,
NADH (1 mM), KHCO3 (0.1 M), 15 w/v% triethanolamine
3
. Results and discussions
(
TEOA), M (0.125 mM), FateDH (0.2 mg), FaldDH (0.3 mg), and 3.1. Fabrication and characterization of WS /g-C N
2 3 4
ADH (1.5 mg) in 2 mL 0.1 M PBS buffer (pH 7.0). Prior to the composites
addition of enzyme, the PBS buffer was rstly saturated with
The WS
assembly procedure, the morphologies of g-C
wt% WS /g-C N composites were observed by TEM. As shown
2
/g-C
3
N
4
composites were fabricated by a two-step self-
gaseous CO
2
. The amount of methanol was detected by gas
3
N
4
, WS and the
2
11
chromatograph as described in our previous reports.
5
2
3 4
in Fig. 1a, a stacked layer structure is clearly observed in the g-
2.6. Degradation of RhB
33
C N sample, which is consistent with existing reports. The
3
4
The degradation of RhB was performed in quartz reactor. The WS
photocatalytic reaction system consisted of 300 W Xe lamp with (Fig. 1b). For WS
a 400 nm cut-off lter. All experiments were conducted at room covered by WS . The darker part with hexagonal shape in Fig. 1c
2
sample displays a structure of hexagonal lamellar stacking
34
2
3 4 3 4
/g-C N composites, g-C N is found to be
2
Fig. 1 TEM images of (a) g-C
N
3 4
, (b) WS
2
, (c) 5 wt% WS
2
3 4
/g-C N composite and its (d) HRTEM image. (e–i) SEM and the corresponding elemental
mapping of C, N, S, and W elements in 5 wt% WS
2
/g-C composite.
3 4
N
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should be WS
2
and the lighter part is g-C
3
N
4
, which demon- each semiconductor over the other during composite forma-
strates the well distribution of WS2 on g-C N . The high- tion, the XRD patterns of the following materials were presented
3
4
resolution TEM image shows the microstructure of the WS /g- in Fig. 3a, which include the obtained WS , hydrothermally
2
2
C N composite, in which the interplanar spacing is very close treated WS2 (referred as HT WS ) and ultrasonicated WS2
3
4
2
to the 001 plane of WS
wt% WS /g-C
dispersive X-ray spectroscopy (EDS) elemental mapping of hydrothermally treated (HT 5% WS
C, N, S and W, which are characteristic elements of g-C and WS /g-C composites fabricated through combined
WS , respectively, clearly conrmed the uniform coupling of g- ultrasonication-hydrothermal treatments. As shown in Fig. 3a,
C N with WS (Fig. 1f–i). This coupling would be favorable for the XRD pattern of obtained g-C N showed the typical
2
(Fig. 1d). SEM images (Fig. 1e) of the (referred as US WS
composite and the corresponding energy- ultrasonicated 5% WS
2
) in absence of g-C
3
N
4
; as formed g-C
/g-C
3 4
N ), as well as the 5%
3 4
N ;
N ),
3 4
5
2
3
N
4
2
/g-C
3
N
4
composite (US 5% WS
/g-C
2
2
3
N
4
2
3 4
N
2
3
4
2
3 4
ꢁ
ꢁ
the charge transfer between WS and g-C N and then promotes diffraction peaks at 13.1 and 27.3 , perfectly indexed as the
2
3 4
the separation of photo-generated electron–hole pairs. More- (100) and (002) planes, respectively, which were ascribed to in-
over, the TGA measurement shows that the decomposition planar tri-s-triazine unit and the interplanar stacking of
17,40–44
temperature of WS
2
/g-C
3
N
4
composites is lower than the pris- aromatic systems.
The XRD patterns of the obtained WS
2
tine g-C
other g-C
3
N
4
. Similar phenomena were also observed in some exhibit many distinct diffraction peaks, indicating its hexagonal
28,34
3
N
4
-based heterojunction, such as g-C
TiO /g-C DyVO /g-C
3
N
N
4
/AgBr/Fe
g-C
3
O
4
,
/
structure,
the standard card library. The HT WS
which is consistent with the 2H conguration in
and US WS presents the
SnO2ꢀx/g-C
3
N
4
,
2
3
N
4
,
4
3
4
,
3
N
4
2
2
2
4,35–37
SmVO4.
The shi of weight loss towards lower tempera- same XRD as that of the obtained WS , indicating the crystal-
2
ture was considered due to some changes in surface chemistry lization of WS was not affected by ultrasonication and hydro-
2
of the g-C N during forming of heterojunction structure with thermal treatments. For the 5% WS /g-C N composites, the
3
4
2
3 4
3
8,39
other materials (Fig. 2).
XRD were adopted to analyze the composition and structure tion of the two sets of diffraction data for both g-C
of as-prepared WS /g-C composites. To understand the and no other phase is detected, implying that WS
crystallization of the composite and also mutual interface of incorporated into the lattice of g-C
The FT-IR was also adopted to test the obtained g-C
and WS /g-C N composite. As shown in Fig. 3b, pure g-C N
XRD patterns of all the three composites display the combina-
3
N
4
2
and WS
was not
2
2
3 4
N
3 4
N .
3
N
4
, WS
2
,
2
3
4
3
4
has a much stronger IR response than WS , and the addition of
2
WS shows no obvious effect on the IR spectrum of g-C N . For
2
3 4
ꢀ
1
3 4
the g-C N , the strong bands in the region of 1200–1700 cm
are assigned to the typical stretching vibration modes of C–N
ꢀ1
heterocycles. The peaks at 3000–3700 cm represent the N–H
ꢀ1
and O–H stretching vibration. The sharp peak at 810 cm
originates from the characteristic breathing mode of tri-s-
45–49
triazine units.
For the WS /g-C N composites, the overall
2 3 4
patterns of the spectra are the same as the g-C N , conrming
3
4
the existence of heptazine heterocyclic rings in WS /g-C N
3 4
2
composites. In generally, there is no obvious difference from
the FTIR spectrogram before and aer doping, but it's not
difficult for us to see that the position of the characteristic peak
from the tri-s-triazine vibration of g-C
3 4
N has a certain degree of
Fig.
3 4 2 2 3 4
2 TGA profiles of g-C N , WS and 5 wt% WS /g-C N
composite.
shi (Fig. 3c), indicating that there might be some interactions
Fig. 3 (a) XRD patterns and (b) FT-IR spectra of obtained g-C
around 810 cm of spectra in (b).
3
N
4
, WS
2
, WS /g-C N
2 3 4
composites; (c) local magnification of the FT-IR spectra at
ꢀ1
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24
between the “nitrogen pots” of g-C
3
N
4
and W species of WS
2
.
2 3 4
Fig. 4a indicate that the prepared WS /g-C N samples are
As a complement to FT-IR, the Raman spectra of the g-C N and composed of C, N, W and S. Specically, the C1s spectrum
3
4
the 5% WS /g-C N were also recorded. As shown in Fig. S1,† the (Fig. 4d1) of WS /g-C N composites show there are two C1s
2
3
4
2
3 4
catalysts show several very weak distinctive peaks at 458, 679 peaks located at 284.7 and 288.2 eV, the former is ascribed to the
ꢀ
1
and 1220 cm , which is basically consistent with the situation adventitious hydrocarbon from the XPS instrument, while the
5
0,51
2
of g-C
WS /g-C
which may be due to the strong photoluminescence effect of g- g-C
and the small faction of WS in the composite. peaks centered at 398.7 and 399.5 eV were attributed to sp -
X-ray photoelectron spectroscopy (XPS) analysis was hybridized nitrogen atom (C]N–C group) and tertiary nitrogen
employed to determine the chemical composition and bonding (N–C group or H–N–C ), respectively; the peaks at 401.0 and
3
N
4
reported in the literatures.
composite did not lead to apparent new peak, in aromatic rings. The high-resolution N1s spectrum of the WS
composites could be tted into four peaks (Fig. 4d2). The
3 4
The formation of 5% latter is assigned to sp -bonded carbon atom (N–C]N) of g-C N
2
3
N
4
2
/
3 4
N
2
C
3
N
4
2
3
2
conguration of the g-C N , WS and the WS /g-C N compos- 404.7 eV derived from amino function groups and charging effect
3
4
2
2
3 4
4,24,52
ites (Fig. 4a), with the high-resolution spectrum of each sample localization in heterocycles, respectively.
As compared with
were illustrated in Fig. 4b–d. The survey XPS spectra shown in the C1s and N1s spectrum (Fig. 4b1 and b2) of pure g-C
3 4
N , all
Fig. 4 The XPS survey spectra of (a) g-C
3
N
4
, WS
2
and 5 wt% WS
2
/g-C
3
N
4
composite. High resolution C1s (b1), N1s (b2) spectra from g-C
/g-C composite.
3
N
4
and
S2p (c1) W4f (c2) spectra from WS , as well as C1s (d1), N1s (d2), S2p (d3), W4f (d4) spectra from 5 wt% WS
2
2
3 4
N
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those peaks of the WS
with a higher binding energy.
2
/g-C
3
N
4
composites shied to a position
Fig. 6 presents the BET surface areas measurement of 5 wt%
WS /g-C N composites. For comparison, the surface areas of g-
2
3 4
Fig. 4d3 and d4 display the S2p spectra and the W4f spectra C N and the physical mixture of g-C N with 5 wt% WS2
3
4
3 4
of the WS /g-C N composites. The peaks at 161.7 and 163 eV (referred as PM-WS /g-C N in the following text) were also
2
3
4
2
3 4
2
ꢀ
can be ascribed to the S species, donated as S2p3/2 and S2p1/2
respectively; the W4f peaks at 32.1, 34.4, and 37.6 eV can be surface area of g-C N was 7.34 m g , which is similar to the
3 4
,
determined, the results were listed in Table S2.† The specic
34
2
ꢀ1
4
+
24
ascribed to the W species, donated as W4f7/2, W4f5/2 and value reported by He et al., but lower than those reported for
2
8,53
54,57
W5p3/2
.
In contrast to the W4f and S2p spectrum (Fig. 4c1 most g-C
and c2) of WS , all these above peaks of the WS shied to has multi-layered structure. Slight decrease in surface area was
a position with a lower binding energy aer coupling with g- observed for the PM-WS /g-C N (6.87 m g ); while the 5 wt%
3
N
4
nanosheets,
3 4
which indicates that the g-C N
2
2
2
ꢀ1
2
3 4
C N . Thus, the C1s and N1s peaks of g-C N shied toward the WS /g-C N composites exhibited much higher BET values
3
4
3
4
2
3 4
2
ꢀ1
higher binding energy, while the W4f and S2p peaks of WS2 (12.22 m g ), which indicated that some reactions might have
shis towards the lower binding energy. It was suggested that occurred between g-C and WS during recombination. This
there might be some S–C chemical bonds formed between the larger specic surface area of WS /g-C composites also could
heterojunction interfaces of WS /g-C composites, thus supply more reactive sites for light and substrates, improve the
electron transfer could occur from g-C to WS at this inter- whole efficiency of the reaction. It should be noted that the
faces due to their different electron concentration.
The optical properties of WS , g-C N and the WS /g-C N
3
N
4
2
2
3 4
N
2
3 4
N
3
N
4
2
24,54
specic surface area of the 5 wt% WS
2 3 4
/g-C N composites are
still far lower than the g-C N nanosheets, indicating the
2
3
4
2
3
4
3 4
composites were revealed by the UV-vis diffuse reectance ultrasonication and hydrothermal treatments during composite
spectra (DRS). As shown in Fig. 5a, pure g-C N had absorption formation still could not result in effective exfoliation of the g-
3
4
from UV to visible region, with an absorption edge at 460 nm,
3 4
C N . Aer BJH (Barrett–Joyner–Halenda) plotting, the pore size
which corresponding to a band gap about 2.7 eV according to distribution of each samples were analyzed (Fig. 6b). Compared
5
5,56
the Kubelka–Munk conversion (Fig. 5b).
coupling of WS , the absorption range of the composite became pore diameter.
wider than that of pure g-C , a remarkable enhanced light-
harvesting ability is observed for the WS /g-C N , and the 3.2. Photocatalytic performance for NADH regeneration and
3 4 2
However, with the with the pure g-C N , coupling of WS lead to slight decrease in
2
3 4
N
2
3 4
band gap width estimated from the K–M equation became CO reduction
2
narrower accordingly, which implied that the light absorption
Conversion of CO
cascade reaction has been widely explored.
consumption of the expensive NADH cofactor at stoichiometric
ratios imposes great hurdles to the practice of these processes.
Here in the present work, we intended to develop a biocatalyzed
articial photosynthesis system by coupling the reduction of
2
to methanol through redox multienzymatic
ability of the WS
2
/g-C
3
N
4
composites became stronger. The
7,10,12
However,
coupling of WS
2
on g-C
3
N
4
may generate an impurity energy
level in the valence band (VB) and narrow the band gap. The
defect energy levels increased the light absorption efficiency of
the materials and thus may exhibit photocatalytic performance
superior to that of the individual material. In addition, the DRS
spectra of US 5% WS /g-C N and HT 5% WS /g-C N compos-
CO
2
catalyzed by FateDH, FaldDH, and ADH with the newly
/g-C composite as the photocatalyst to
2
3
4
2
3 4
developed WS
2
3 4
N
ites were also measured and compared with that of the 5% WS2/
g-C , it is indicated that the omitting the ultrasonication or
regenerate NADH using solar energy.
3 4
N
Based on the above photochemical investigations of the g-
C N and the WS /g-C N composites, their performance for
hydrothermal treatments during composite formation lead to
negligible changes in the band structure (Fig. S2†).
3
4
2
3 4
visible light-driven photocatalytic NADH regeneration were
Fig. 5 (a) UV-vis diffuse reflection spectra of pure g-C
3 4 2 2 3 4 3 4
N , WS and 5 wt% WS /g-C N composite; (b) corresponding bandgap of pure g-C N ,
WS and 5 wt% WS /g-C composite estimated from Kubelka–Munk equation.
2
2
3 4
N
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Fig. 6 (a) The N
2
adsorption/desorption isotherms, (b) the corresponding pore-size distribution. BET surface area was measured by nitrogen
sorption isotherms at 77.5 K and up to 1.03 bar.
examined. The reactant solution consisted of 15 wt% TEOA, mesoporous graphitic carbon nitride (mpg-CN) loaded with
+
0
.25 mM electron mediator M, and NAD with an initial WS
2
towards H
content of 1 mg was found optimal to reach highest rate of hydrogen evolution,
mL was maintained in all the experiments. The photocatalytic and heavy loading of WS led to decrease in activity of the
activity comparison among pure g-C N , WS , PM-WS /g-C N heterojunctions photocatalyst.
2 2
generation. About 0.3 at% WS loading amount
3 4
concentration of 1 mM and an active g-C N
ꢀ1
2
29
3
4
2
2
3 4
and WS /g-C N composite with WS content of 1%, 5% and
To justify the necessity of multiplicative steps involving
ultrasonication, mechanical stirring and hydrothermal treat-
2
3
4
2
1
0% are presented in Fig. 7a. The monolithic g-C N and WS
3 4 2
had rather low catalytic efficiency, NADH regeneration yield at ment for the preparation of WS
2
/g-C
with g- composites were prepared by simpler steps and evaluated for its
, the NADH photo-regeneration yield enhanced to 15%. activity towards NADH photoregeneration. Results presented in
/g-C composite presents the best Fig. S3 and Table S3† clearly indicated that either omitting of
performance, a NADH yield of 37.1% was achieved, which were the ultrasonication or hydrothermal treatments, or even the
about four times of those obtained by using monolithic g-C N mechanical stirring between these two operations during
and WS . With further increasing the WS content in the composite formation, led to signicantly lower activity of the
3 4 2 3 4
N , a series of WS /g-C N
6
h was below 10%. By physically mixing 5 wt% WS
2
C
3 4
N
While the 5 wt% WS
2
3 4
N
3
4
2
2
composite to 10 wt%, however, leading to decrement in activity fabricated WS /g-C N . The hydrothermal time and tempera-
2
3 4
for the photo-regeneration of NADH. Presumably, coupling of ture did not show remarkable inuence on the activity of the
high amount WS
2
may lead to shielding of active site of g-C
3
N
4
composite photocatalyst. Therefore, the two-step sonication
and subsequent hydrothermal treatments are necessary, though
by WS or loss of favorable interfaces between g-C
2
3
N
4
and WS .
2
Similar phenomenon was also observed on the activity of somewhat tedious, to ensure the photocatalyst a high activity of
Fig. 7 (a) Photocatalytic NADH regeneration by ( ) g-C
3 4 2 2 3 4 2 3 2 3 4
N , ( ) WS , ( ) PM-WS /g-C N , ( ) 1 wt% WS /g-C N4, ( ) 5 wt% WS /g-C N , and (-)
ꢀ1
+
1
0 wt% WS /g-C composite. A reaction medium was composed of photocatalyst (1 mg mL ), NAD (1 mM), TEOA (15 w/v%) and M (0.25 mM)
2
3 4
N
in 100 mM, pH 7.0 phosphate buffer. (b) The production of methanol as a function of time.
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2 3 4 2
the 5 wt% WS /g-C N composite, which presents the highest higher efficiency for methanol synthesis from CO , when the
NADH photo regeneration yield of 37.1%. It was believed that NADH photoregeneration process was coupled with the bio-
the rst step of sonication was to ensure a good dispersion of g- catalytic process. The highest methanol concentration up to
C N particles, the second step of sonication of g-C N and WS
5.58 mM were obtained by using 5 wt% WS /g-C N composite
2 3 4
3
4
3
4
2
mixture was for a full contact between these two materials, as photocatalysts, corresponding to a methanol productivity of
ꢀ
1
ꢀ1
which was crucial for the subsequent hydrothermal reaction.
In another set of experiment, we tried to address the more that obtained by using g-C
challenging question of a potential mediator-free regeneration result clearly indicated that the formation of WS
of NADH. To our surprise, the mediator-free with system with erojunction generated signicant photo-synergistic effect and
wt% WS /g-C N composite as photocatalyst was also played key roles in the enhancement in the photocatalytic
372.1 mmol h
gcat , which was about 7.5 times higher than
ꢀ
1
ꢀ1
3
N
4
(50.31 mmol h
g
cat ). This
2
3 4
/g-C N het-
5
2
3 4
possible, though the regeneration yield at 6 h was only about activity.
half of that obtained in presence of M (Fig. S3, Table S3†). This
can be explained from the following two points. Firstly,
3
.3. Photocatalytic performance for RhB degradation
while in presence of M, 1,4-NADH will be unique production; To further demonstrate the effectiveness of formation of het-
secondly, without a strong redox agent, i.e., mediator, g-C
erojunction structures between the components of the WS /g-
also catalyzes the back reaction, meaning an equilibrium C N composites, the photocatalytic activity of the composites
a mixture of 1,4-NADH and 1,6-NADH would possibly formed,
3 4
N
2
3
4
+
18
between NAD and NADH.
for the degradation of RhB was also evaluated, and results were
We then evaluated the WS /g-C N composite as a photo- compared with pure g-C N and WS . As shown in Fig. 8a, the
2
3
4
3
4
2
catalyst of the articial photosynthesis system for bioconver- degradation ratio of RhB reached to about 100% by using the
sion of CO to methanol. As expected, the photocatalysts 5 wt% WS /g-C N composites under visible light irradiation for
showed higher activity for NADH regeneration also exhibited 60 min, which was much higher than the corresponding
2
2
3 4
Fig. 8 (a) Photocatalytic degradation profiles of RhB under visible light irradiation in the presence of ( ) g-C
wt% WS /g-C 4, ( ) 5 wt% WS /g-C , and (-) 10 wt% WS /g-C composite; (b) the comparison chart of RhB degradation rate after 1 h's
irradiation catalyzed by various catalysts; (c) effects of different scavengers on degradation of RhB in the presence of 5 wt% WS /g-C
composite catalyst for five successive runs (each
3 4 2 2 3 4
N , ( ) WS , ( ) PM-WS /g-C N , ( )
1
2
3
N
2
N
3 4
2
3 4
N
2
3 4
N
composite catalyst under visible-light irradiation; and (d) reusability of the 5 wt% WS
run was lasted for 60 min).
2 3 4
/g-C N
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the 5 wt% WS
2
/g-C
3
N
4
composites was examined by measuring
the RhB degradation during repeated usages. As shown in
Fig. 8d, aer ve successive runs, there is no remarkable
decrease in its activity. Hence, the photocatalyst has a good
stability during the photocatalytic reactions.
3.4. Proposed mechanism
High efficient charge separation was the crucial fact for the
improvement of photocatalytic activity. Construction of heter-
ojunction structure is one of the most effective means to
improve photocatalytic activity via fast and efficient transfer
photo-excited electron in the CBs in the heterojunction. In this
Fig. 9 Schematic diagram of electron flow and energy level during
coenzyme regeneration.
degradation ratios obtained by using pristine g-C
3
N
4
(ꢂ70% at
/g-
study, WS
heterojunction structure. Based on the energy band structure of
g-C and WS , as shown in Fig. 9, the CB and VB of g-C are
all higher than that of WS Because of the unstable state of
2 3 4
was hybridized with g-C N to generate an efficient
1
C
20 min) and WS
2
(ꢂ10% at 120 min), as well as the PM-WS
2
3
N
4
(ꢂ70% at 120 min). Fig. 8b presents the apparent degra-
N
3 4
N
3
4
2
24,28
dation rate derived by simple linearly plotting of RhB concen-
tration decrease against irradiation time. The results indicate
that the degradation rate using composite photocatalyst con-
2
.
photo-generated electrons and holes, they will tend to recom-
bine quickly to achieve steady state. However, due to the
distance between CB of g-C N and CB of WS is much shorter
taining 5 wt% WS
2
was about 2.6 and 19.7 times higher than
, respectively.
3
4
2
that of g-C and WS
3
N
4
2
than the band gap of g-C N , the CB offset of 0.16 eV will
3
4
In general, some reactive oxygen species including hydroxyl
possibly drive the photo-excited electrons on the CB of g-C
transfer to the CB of WS before recombination with holes in
the VB of g-C . Based on the similar mechanism, the holes in
the VB of WS tend to transfer to the VB of g-C , driven by the
3 4
N
ꢀ
radicals (cOH) and superoxide radicals (cO
2
), as well as holes
2
+
(
h ) are expected to be involved in the photocatalytic process of
3 4
N
55,58
RhB degradation.
To investigate the role of these reactive
2
3 4
N
species, the effects of some radical scavengers and N purging
2
VB offset of 0.10 eV. Therefore, an orientated transfer of the
charge carrier across the heterojunction interface was realized.
To further demonstrate the photoinduced electron-transfer
mechanism, as well as elucidate the effect of amount of WS2
on the photodegradation of RhB were studied to propose the
possible photocatalytic mechanism. Results in Fig. 8c indicate
that the addition of IPA (cOH scavengers) had almost no effect
+
on RhB degradation. While addition of TEOA (h scavengers)
in WS
detailed investigations of the photochemical properties were
performed on WS /g-C heterojunction hybridized with
2 3 4
/g-C N heterojunction on the overall reaction efficiency,
+
and KI (scavenger of both cOH and h ) led the degradation rate
aer 1 h's irradiation decreased to about 70% and 58%,
2
3 4
N
respectively. When N purging was conducted which acts as
2
different amount of WS ranging from 1 wt% to 10 wt%. For
most semiconductors, photogenerated electrons and holes can
bind aer the excitation by incident light, leading to the transfer
of partial energy to uorescence. In general, the binding effi-
ciency and photoelectron lifetime can be measured by moni-
2
ꢀ
cO2 scavenger, the degradation rate decreased dramatically to
ꢀ
about only 22%. These results suggested that the cO2 plays
major roles in the photodegradation of RhB catalyzed by the
WS
2 3 4
/g-C N heterojunction photocatalyst, which is consistent
55,58
1
7,57,59,60
with results in other reports.
toring uorescence intensity and lifetime.
presents the uorescence spectrum of pure g-C
shows strong uorescence intensity at 437 nm. However, the
Fig. 10a
Reusability of photocatalyst is a very important parameter
from an economical viewpoint. The reusability and stability of
N , which
3 4
Fig. 10 (a) PL spectra of g-C
and WS /g-C composite.
3 4 2 2 3 4 3 4 2
N , WS and WS /g-C N composite photocatalysts. (b) Excited state electron radioactive decay of g-C N , WS
2
3 4
N
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2 3 4 2 3 4 2
physical mixing WS and g-C N (PM-WS /g-C N ) and the WS /
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3
4

uorescence spectra. More interesting, the WS /g-C N hetero-
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