Enhanced photocatalytic activity of CO2reduction to methanol through the use of a novel-structured CuCaAg2Se-graphene-TiO2ternary nanocomposite

Article abstract of DOI:10.1039/d0nj03150j

Effective charge transfer is a key factor in enhancing the catalytic activity of photocatalytic nanocomposite materials. In order to enhance the charge transfer of graphene and TiO2, we combined them with the quaternary chalcogenide (CuCaAg2Se) nanocomposite. The ternary nanocomposite was synthesized by a muffle-assisted hydrothermal method, which was used to obtain decent crystallinity and the polymer precursor solution was prepared by the Pechini method based on the metallic citrate polymerization using ethylene glycol C2H6O2 (Merck) and citric acid C6H8O7 (Merck) as the chelating cation. The morphology, structure, and charge-carrier separation analyses confirmed that the combination of CuCaAg2Se, graphene and TiO2 enhanced the photocatalytic activity of the ternary nanocomposite. All the analysis results pointed out that the ternary nanocomposite was successfully synthesized, which enhanced the performance of the nanocomposite. This ternary nanocomposite showed six times higher photoactivity performance towards CO2 reduction to generate methanol (CH3OH) under UV and visible light irradiation with different amounts of a scavenger, compared to TiO2, pure CuCaAg2Se, and the binary nanocomposite. More importantly, the as-synthesized nanocomposite displayed excellent stability during the recycling test with no obvious loss in the generation of methanol from CO2 photoreduction. The approach presented herein offers a promising route for the rational design of a new class of ternary-nanocomposites for various applications in environmental protection and solar energy conversion.

Full text of DOI:10.1039/d0nj03150j

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ISSN 1144-2546
PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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Enhanced Photocatalytic Activity of CO2 Reduction to MethanolDbOIy: 10t.1h03e9/UD0NsJe03150J
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of a Novel-structured CuCaAg2Se-Graphene-TiO2 Ternary Nanocomposite
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
Zambaga Otgonbayar 1, Kwang Youn Cho 3, Won-Chun Oh 1. 2
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1 Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chungnam,
Korea, 356-706
2 College of Materials Science and Engineering, Anhui University of Science & Technology, Huainan
232001, PR China
3 Korea Institutes of Ceramic Engineering and Technology, Soho-ro, Jinju-Si, Gyeongsangnam-do,
South Korea
Abstract. The effective charge transfer is a key factor to enhance catalytic activity for photocatalytic
nanocomposite materials. In order to enhance the charge transfer of Graphene and TiO2, we combined
with Quaternary chalcogenide (CuCaAg2Se) nanocomposite. The ternary nanocomposite was
synthesized by muffle-assisted hydrothermal method which is afforded to obtaining a decent
crystallinity and the polymer precursor solution was prepared by Pechini method based on metallic
citrate polymerization using ethylene glycol C2H6O2 (Merck) and citric acid C6H8O7 (Merck) as
chelate cation. The morphology structure, charge-carrier separation analysis confirmed that the
combination of CuCaAg2Se, Graphene and TiO2 enhanced the photocatalytic activity of the ternary
nanocomposite. Each results pointed-out that the ternary nanocomposite was successfully synthesized
and enhanced the performance of nanocomposite. This ternary-nanocomposite showed 6-times higher
photo-activity performance towards CO2 reduction to generate methanol (CH3OH) under the UV and
Visible-light irradiation with different amount of scavenger, compared to the TiO2, pure CuCaAg2Se,
binary nanocomposite. More importantly, the as-synthesized nanocomposite displayed excellent
stability during recycling test with no obvious loss in the generation of methanol from CO2 photo-
reduction. This approach presented herein offers a promising route for the rational design of a new
class of ternary-nanocomposite for various applications in environmental protection and solar energy
conversion.
Corresponding author
E-mail: wc_oh@hanseo.ac.kr
Tel: +82-41-660-1337, Fax: +82-41-688-3352

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Keywords: Graphene, semiconductor, chalcogenide nanocomposite, photocatalyDtiOcI:C10O.1023V9ier/weDdA0rNuticJcl0et3Oi1on5li0nnJe,
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methanol
1. Introduction
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Photocatalytic reduction of CO2 with water into renewable hydrocarbon solar fuels is considered as
a promising strategy to simultaneously address the global energy and environmental issues.
Furthermore, the photocatalyst has many advantages over the methods in efficiency of biodegradable
pollutants, simple catalyst regeneration, low energy utilization [1]. TiO2 has a function under UV-light
irradiation with wavelength shorter than 387 nm and recombination and recombination rate of electron
and holes are quickly [2]. A new challenge that scientists need to overcome is the unification of a well-
structured photocatalyst material for transforming CO2 into a product, and the graphene-based
chalcogenide photocatalyst has shown a great-deal of promising attributers in this area. Two-
dimensional (2D) layered materials have gained immense attention in the area of photocatalysis in
recent years [3]. Compared to 0D and 1D materials, these 2D materials possess impressive properties,
including large specific surface area with abundant active sites, superior electron conductivity (e.g., in
the case of graphene) to facilitate charge transfer and separation, and the ability to function as
excellent catalyst supports [4-6]. Graphene united with chalcogenide photocatalyst had a superb
catalytic activity due to variable bandgap energy, such as WSe2-Graphene (2.68 eV) [7], PbSe-
Graphene (2.60 eV) [8], Cu2Se-Graphene (2.80 eV) [9] which showed a high reduction activity of CO2.
Last decade year, manufacturing a ternary structure has turned into an effective tactic to enhance the
absorbing of solar light and photocatalytic capability. According from published papers, the enhanced
photo-activity of Cu2ZnSnS4/ZnO composite catalyst for photocatalytic reduction of CO2 to CH4 [10].
Recently, ZnV2O6/g-C3N4 [11] and Zn2GeO4/Mg-MOF-74 composites with enhanced photocatalytic
activity for CO2 reduction has been reported [12].
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As well, ternary, and quaternary chalcogenide nanocomposite showing a semi-insolating feature
was observed with band-gaps from small (< 1 eV) to large (> 3 eV) and noted a suppleness large
amount of electron hole pairs. The semiconductor of Ag2Se is also used nowadays for their excellent
optical and chemical properties as well as for future potential applications. Also, two stable solid
phases have been noted, one is an orthorhombic phase β-Ag2Se (low - temperature, 0K), the second
one is the cubic phase α-Ag2Se (high - temperature, 409K) [13]. The β-Ag2Se has a role same as a (n-
type) semiconductor and the band-gap energy is 0.07 eV, α-Ag2Se is same as a metal and the band-gap
energy is 0.15 eV [14]. Zheo et al [14] prepared a TiO2/Ag2Se nanocomposite by interface reaction
and their result suggested that the nanocomposite is an excellent photovoltaic with photocurrent
properties towards visible light. Combining with copper (Cu) and into the surface of TiO2 it is noted to

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enhance improving the catalytic activity and prevent the recombination of an electron-hole, which is
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also altering the band-gap energy. The state of graphene successfully joined the chalcogenide
(inorganic materials) for using the proper method and due to the noted oxygen-containing functional
group of graphene, take-action as a stabilizer and prevent the agglomeration [15].
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This work reports the synthesis of ternary CuCaAg2Se-Graphene-TiO2 nanocomposite, in which a
quaternary chalcogenide nanocomposite and semiconductor TiO2 are dispersed onto the graphene
exterior by muffle-assisted hydrothermal method and analyzed by some physical characteristic
equipment. The photo-catalytic activity of ternary nanocomposite was studied in the CO2 reduction
with a different amount of scavenger and the main final product was methanol (CH3OH) under the
UV/Vis-light irradiation. Sodium carbonate solution as used in CO2 photocatalytic reduction which
can act as the reducing agent. More specifically, there is a two reason: (i)an increase of dissolved CO2
amount, (ii) and reduction of the recombination of hole–electron pairs leading to a longer decay time
of surface electrons and facilitation of CO2 reduction. The ternary nanocomposite (CuCaAg2Se-
Graphene-TiO2) exhibits superior photocatalytic activity owing to an enhanced charge carrier
separation and high stability. The photostability and photoactivity of ternary nanocomposite developed
the research field of photocatalytic carbon dioxide reduction. In this heterostructure, the purpose of
graphene is shown to be a bridge between the quaternary chalcogenide and TiO2 and may improve the
charge-separation, and thereafter may enhance the capacity of photogenerated charge carriers. Each
results pointed-out that the quaternary chalcogenide nanocomposite and TiO2 were successfully
scattered onto the graphene surface. The purpose of this study is the confirmation of exceptional
photocatalytic activity during a reduction of carbon dioxide (CO2) with a target synthesized material
under the Visible light and UV light.
2. Experimental
2.1. Materials
Natural graphite powder, hydrochloric acid (HCl, 36 wt%), sulphuric acid (H2SO4, 97 %) and
potassium permanganate (KMnO4) were purchased from Sigma Aldrich. Titanium n-butoxide (TNB,
C16H36O4Ti) and Ethylene glycol (99.5 %) were purchased from Samchun Pure Chemical Co., LTD.,
Korea. Copper nitrate trihydrate (Cu(NO3)2·3H2O, 99%), Calcium nitrate tetrahydrate
(Ca(NO3)2·4H2O, 98%), Silver nitrate (AgNO3, 98 %), Selenium powder (Se). Citric acid (C6H8O7,
99.5%), Scavenger (Sodium sulfite anhydrous (Na2SO3, 97 %)) were purchased from Duksan Pure
Chemicals Co. Ltd., Korea. It is noted that all solutions were used with distilled water.

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2.2. Synthesis of Graphene
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Hummer Offeman’s method was used to synthesize the graphene by oxidation of graphite [16]. A
certain quantity of natural graphite (20 g) was then dissolved into a sulfuric acid (450 ml) and DI-
water and agitated for one hour under 0 ⁰C in ice bath. Then, the ice bath has been removed and
potassium permanganate (45 g) (graphite + H2SO4) was progressively added into the mixture and
agitated under 35 °C until the color turned into a dim brownish color, continuously. The container was
sealed and later agitated for 30 min under 100 °C, in the meanwhile hydrogen peroxide (H2O2) was
added dropwise in the timeframe of 5 min. After that, the solution was washed with acetone and
hydrochloric acid (HCl, 10%) thrice, then the product was heat-treated in an oven at 90 °C for 12 h
until it formed to a graphite oxide powder. As-prepared graphite oxide powder was added into 200 ml
de-ionized water and rigorously agitated for 30 min, then it was continuously ultra-sonicated for 2 h
(using Ultrasonic Processor, VCX 750). The mixture was cleansed and washed with hot water for
several times, then oven-dried for 6 h to form into a graphene oxide powder.
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2.3. Synthesis of quaternary (CuCaAg2Se) composite
The CuCaAg2Se was synthesized through the use of a muffle-assisted hydrothermal method. The
molar ration of precursor reagents was noted at 0.02:0.02:0.04:0.02. The precursor solution was
prepared through a Pechini-method based on the metallic citrate polymerization using ethylene glycol
(C2H6O2) and citric acid (C6H8O7) as chelate cation. The configuration of numerous coordination
bonds between organic molecules and transition metals ion control to sequestration of the metal its
meaning of chelation. The citric acid was dissolved in DI-water with a constant stirring at 60 °C to
make a citric acid solution. A certain quantity of precursor reagents was dissolved into an as-prepared
acid solvent and stirred until fully dissolved. After that, ethylene glycol (C2H6O2) was added into that
solution, and the proportion ration of citric acid and ethylene glycol is noted at 60:40, and the solution
can become a viscous. Under the same temperature, the solution was stirred for 5 h and put into 100
ml Teflon-lined stainless-steel autoclave. The autoclave was maintained at 150 °C for 12 h, and then
allowed cool to room-temperature (25 °C). The product was rinsed several times using DI-water and
dried at 100 °C for 5 h, and then obtained sample was renamed as CCAS (before). As-prepared
CuCaAg2Se powder was calcined at 700°C for 2 h to obtain, and then renamed by CCAS (after).
2.4. Synthesis of graphene-TiO2 and CuCaAg2Se-graphene-TiO2
First, the graphene-TiO2 was prepared via the use of a hydrothermal method with calcination. A
molar ratio of ethanol to H2O to TNB of 30:15:4 was mixed and stirred continuously for 1 h. Then,
0.5 g graphene was added into that mixture and agitated for 5 h, then it was transferred into the 100 ml

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Teflon-lined autoclave and maintained at 150 °C for 12 h, and then cooled to room temperature
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(25 °C). The product was rinsed several times with DI-water and dried at 100 °C for 5 h and calcined
under 700°C for 2 h. The ternary composite of CuCaAg2Se-graphene-TiO2 was prepared via a muffle-
assisted hydrothermal method. As-prepared CuCaAg2Se was taking 0.50 g and dissolved into metallic
citrate polymerization with agitated for 30 min and then mixed with (0.30 g) as-prepared G-TiO2
nanocomposite and the solution was mixed continuously for 5 h. The mixture put into 100 ml Teflon-
lined stainless-steel autoclave. The autoclave was maintained at 150 °C for 12 h, and then allowed
cool to room-temperature (25 °C). The product was rinsed several times using DI-water and dried at
100 °C for 5 h, and then obtained sample was renamed as CCAS (before). As-prepared powder was
calcined at 700°C for 2 h to obtain, and then renamed
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2.5. Characterization
The phases of samples were examined using X-ray diffraction measurements (SHIMADZU XRD-
6000) equipped with a Cu Ka X-ray source (1.5406ꢀÅ). The Debye-Scherrer equation (Eq. (1)) was
used to analyze the particle size of CuCaAg2Se and TiO2 based on the data acquired via XRD analysis
[17].
퐾휆
0.9 휆
퐿 =
[푑 =
]
(퐹푊퐻푀) cos 휃
훽 푐표푠휃
Where L is the particle size, λ is the wavelength of X-ray radiation (1.5406 Å), β is the full-width at
half-maximum (FWHM) of the peak (in radians) and 2θ is the Bragg angle. The shape and structure of
the nanomaterial surface were examined at high-resolution using SEM (JSM-5600 JEOL, Japan). The
chemical composition was analyzed using an energy dispersive X-ray (EDX) analysis incorporated
into the SEM. Transmission electron microscopy (TEM, Hitachi HT7700, operated at 100ꢀkV) was
used to evaluate the morphologies of the samples. The high-resolution transmission electron
microscopic (HRTEM) images of the samples were obtained using a (Hitachi H9500) operated at
300ꢀkV. The surface composition was studied based on the X-ray photoelectron spectra (KRATOS
AXIS SUPRA) with a monochromatized Al Ka X-ray source (10ꢀkV, 1500ꢀW, pass energyꢀ=ꢀ40ꢀeV).
UV–vis diffuse reflectance spectra (DRS) analysis was conducted using an ultraviolet–visible (UV–vis)
spectrophotometer (SHIMADZU UV-2600) ranging from 200 to 800ꢀnm. The band-gap energies of
the photocatalysts were calculated through the application of a modified Kubelka–Munk function
obtained from UV–vis DRS data. A Raman spectroscopy was performed using a Confocal-Raman
imaging system with a 633ꢀnm excitation laser (Renishaw in Via Reflex). Additionally, the photo-
electrochemical analysis was executed using a self-made photo-electrochemical system installed a

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250-W halogen lamp as the irradiation-source. The chemical bonds vibration and functional group of
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the product was confirmed by FTIR spectrometer (FTIR iS5, Thermoscience). The photo-current
measurement was conducted by a computer-controlled Versa-STAT-3 electro-chemical analyzer. The
photoluminescence spectroscopy (PL) was carried out at RT in a conventional PL system. The ND 10%
filter united 514 nm laser was utilized as the excitation source and measurement range was noted at the
530-1000 nm region.
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2.6. Photocatalytic CO2 reduction
In terms of the main research field, the photocatalytic CO2 reduction was performed in a reactor
(composed
of
three
part).
To
take
a
precise
amount
(100 mg) of as-
prepared photocatalysts (CuCaAg2Se-Graphene-TiO2) and different amount of sodium sulfite (as a
sacrificial scavenger) was dissolved in 50 ml DI water (0.04 M, containing sodium bicarbonate) and
agitated continuously for 1 h. Then, a high purity carbon dioxide (CO2) gas was purged to the reactor
for 30 min, after the solution was agitated and the visible light turned on via a metal halide
lamp (500 W, SOLAREDGE700, Perfect Light, China). The distance between the light source and the
photocatalyst was constantly at 10 cm, and a heat sink was stocked on the left-side of chamber to
dismiss the lamp-heat. And the reaction time carried on continuously for up to 48 h whereby the
reactor was cool down congenitally for methanol (CH3OH) desorption from the catalyst after every
12 h. The different amounts (10, 20, and 30 ml) of the samples were withdrawn from the suspension at
intervals of 12 h during the light irradiation using a syringe and a membrane filter. Subsequently, the
reaction product was analyzed through the “Quantitative analysis of alcohol” method. In the
“Quantitative analysis of alcohol,” 0.1 M CrO3 was added to the 10 ml sample and agitated for 15 min.
The sample was then centrifuged (10.000 ppm/15 min) and its concentration peak was found using a
UV spectrophotometer (Optizen POP, Korea). The chemical bonds vibration and functional group of
the product was confirmed by FTIR spectrometer (FTIR iS5, Thermoscience). All samples were
measured by the ATR (Attenuated total-reflection) method and a diamond crystal was used. The data
of transmittance was obtained at the 3800 cm-1 to 400 cm-1 wavenumber range.
3. Result and discussion
3.1. X-ray diffraction studies
Figure 1 shows the XRD result of as-synthesized CCAS-G-TiO2, G-TiO2 and pure CCAS (after).

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9.70x102
8.73x102
7.76x102
6.79x102
5.82x102
4.85x102
3.88x102
2.91x102
1.94x102
9.70x101
0.00
CCAS-G-TiO2
G-TiO2
l=CuCaAgSe-G-TiO
2
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CCAS (after)
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=TiO JCPDS
2
(21-1272)
b=CuCaAgSe
a=orthorhombic Ag Se
2
(96-900-8071)
(JCPDF 24-1041)
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2 theta (degree)
Figure 1. The XRD pattern CuCaAg2Se (after), Graphene-TiO2, CuCaAg2Se-G-TiO2
nanocomposite.
The XRD result basically shows the characteristic of CCAS-G-TiO2, G-TiO2 and pure CCAS.
After calcination, the XRD peaks intensity of CCAS nanocomposites was increased and the diffraction
peaks had become sharp, and it was shown that the temperature intensified atomic-mobility and grain
growth after that which was observed as a decent crystallinity. The full-width half maximum amount
of the X-ray diffraction pattern decreased, while the average crystallite-size became bigger, that is the
reason that the CCAS nanocomposite exhibited a decent crystallinity and large crystallite size (as
shown in Figure S1a). The entire diffraction peak confirmed the orthorhombic crystal phase with
lattice parameters a=4.1050 Å, b=4.0700 Å, c=6.3100 Å of CCAS nanocomposite (Card № 96-
9008070). In addition, the peaks were located at 2θ of 23.04⁰, 26.84⁰, 31.06⁰, 36.18⁰, 40.16⁰, 45.20⁰
and 57.98⁰, which were assigned to the (002), (110), (102), (112), (013), (031) and (230) crystal plane
and corresponded to the crystal phase with lattice parameter of a= 4.3359 Å, b= 7.0700 Å,
c= 7.7740 Å of Ag2Se (JCPDS №.24-1041). With the conclusion of G-TiO2 nanocomposite, it is noted
that the XRD pattern showed the similar peaks of anatase TiO2 (JCPDS №.21-1272), and the
characteristic peak of graphene (002) was not shown due to less quantity and peak intensity was low
(as shown in Figure S1b). And the index facet of (101) was exhibited in XRD pattern of G-TiO2. In the
case of anatase TiO2, the index facet of (101), (010) and (001) exhibit a different catalytic activity [18].
After this, it was noted that the CCAS combined with G-TiO2, whereby the most characteristic peak of

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two nanocomposites were overlapped. Herein, this might be due to the consolidation between CCAS
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and G-TiO2. The crystallite sizes of CCAS (before and after calcination) and TiO2 were calculated
using the Debye-Scherrer equation and were found to be 46.16 nm, 54.38 nm, and 43.33 nm,
individually. The crystallites size of pure Ag2Se was noted as 35 nm [19]. After being unified with
copper and calcium and treated with a heat treatment process, the crystallites size of the CCAS
nanocomposite was increased.
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3.2. Morphological characterization
The surface morphology structure was shown to have appeared via SEM and TEM, in Figure 2
(a)-(c) and (d)-(f) shows the SEM, TEM analysis result of all synthesized nanocomposite which
included schematic diagram.
Figure 2. (a-c) SEM images, (d-f) TEM images of CuCaAg2Se-G-TiO2 ternary nanocomposite
The before and after muffle-calcination of pure (CuCaAg2Se) quaternary nanocomposite result
shown in Figure S2 (a)-(d). Before calcination, the pure quaternary nanoparticles showed various
structures, such as resembling the shapes of a sphere and a wrinkle. After calcination, the particle
structure showed it was most resembled to a plate structure, which meant that, in high temperature
nanoparticles grow to slightly larger size and were characteristic in formulating the aggregation.
Figure 2 (a)-(c) presents the CuCaAg2Se quaternary nanocomposite, G-TiO2, and CuCaAg2Se-G-TiO2
nanocomposite morphology analysis. The TiO2 particle regularly dispersed on the graphene-surface,
whereby it is plainly exhibited with the polygonal plates and small spherical shape structure. After, the
G-TiO2 combining with quaternary nanocomposite, both of nanocomposites deposited on the graphene
exterior, in addition to that, the graphene is shown to act as a bridge and acceptor, which is helpful an

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increasing the path of photogenerated electrons, and is another way towards improving the
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photocatalytic efficiency. The graphene was shown as a sheet-like structure that was divided in various
directions. This structure indicates the being on a graphene surface has oxygen functionalities. In this
result, we can realize the disparity of the binary and ternary-composite material. The pure quaternary
nanocomposite showed various sphere-structure, as shown in Figure 2 (d). The round and pentagonal-
shaped TiO2 was occurred as dark-colored and it was successfully dispersed onto the graphene exterior,
which was confirmed by a dark-grey colored graphene surrounded the TiO2 as shown in Figure 2 (e).
The TiO2 was slightly agglomerated due to an oxygen-containing functional group of graphene that
can prevent the metal-oxide agglomeration. The quaternary CuCaAg2Se and TiO2 were scattered onto
the graphene, as shown in Figure 2 (f). The round-shaped quaternary nanocomposite was obtained at
the black-dot, and the CCAS was slightly agglomerated compared with TiO2 and spread onto the
surface of graphene which obtained as a dark-grey colored. Furthermore, the graphene obtained like
double-layer-structure and all nanoparticle were shown to be uniformly spread and strongly attached
onto the exterior during the muffle-assisted hydrothermal method.
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The atomic structure and information of crystallographic structure of nanocomposite was analyzed
via HRTEM. Figure 3 (a) and (c) indicated the lattice space of TiO2. The spacing values can be 0.350
and 0.357 nm which corresponds to the (101) crystal planes in unary and ternary nanocomposites. The
lattice fringes with di-spacing of CuCaAg2Se was 0.235 nm which corresponding to the (102) crystal
planes, as shown in Figure 3 (b). Figure 3 (d-f) shows the inter-active 3D surface-plot of TiO2,
Graphene and CCAS. From this result, the roughness state of each nanoparticle can be analyzed. The
roughness states of the quaternary nanoparticle were normal, and another two nanoparticles states
were shown to be uneven, which was the same as the sunken and convex result. The particle size
histograms of TiO2 and CCAS nanoparticles at different filtered sizes are shown in Figure 3 (g-h).

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500
1000
800
600
400
200
0
CCAS
(h)
TiO2
(g)
450
400
350
300
250
200
150
100
50
Average nanoparticle diameter= 2.93 nm
Average nanoparticle diameter = 5.28 nm
Average NPs diameter error= 2.93 ± 0.8 nm
Average NPs diameter error = 5.28 ± 1.3 nm
Polydispersity ≈ 27.3 %
Polydispersity ≈ 25.75 %
0
0
5
10 15 20 25 30 35 40 45 50
4
8
12 16 20 24 28 32 36
Diameter (nm)
Diameter (nm)

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Figure 3. The HRTEM images of (a) G-
TiO2, (b, c) CuCaAg2Se-G-TiO2
The Cu, Ca, Ag, Se, Ti and O VoiewbtAartiicnleeOdnline
DOI: 10.1039/D0NJ03150J
from the main precursor reagents and C
nanocomposite, (d-f) Interactive 3D surface
plot and (g-h) nanoparticle size histogram of
element was obtained from graphene, which
was one of the absorbent materials in the
catalyst. The strong Kα and Kβ of C and O
element peak observed at 0.28 keV and
0.53 keV [20]. Other main elements peaks
were appeared in characteristic regions. Figure
S4 shows the EDX result and diagram of pure
quaternary nanocomposite (before and after
calcination) which exhibited each element
percent.
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TiO2,
CCAS
in Graphene-TiO2
an
CuCaAg2Se-G-TiO2.These histograms show
the noted calculated average nanoparticle
diameter distributions with 5.28 ± 1.3 nm
(TiO2 in G-TiO2), 2.93 ± 0.8 nm (CCAS in
CCAS-G-TiO2). After CuCaAg2Se united with
G-TiO2 binary nanocomposite, the average
core diameter of TiO2 was reduced during a
muffle-assisted
hydrothermal
synthesis
process. The average particle diameter of TiO2
was 3.39 ± 1.02 nm (TiO2 in CCAS-G-TiO2),
as shown in Figure S3.
In conjunction with UV-vis DRS results,
the slight changes in the feature of
nanoparticles support the light response of
main material, considering their average core
diameter after the coupling reaction as shown
in HRTEM studies. The considerable average
diameter of nanoparticle was produced with
an increased polydispersity.
The quantitative as well as the qualitative
main elements of the catalyst was detected via
EDX analysis and every element has their
characteristic peak (sharp Kα and Kβ). Figure
4 displays the micro-analysis result of as-
synthesized all samples, which reveals the
presence of the main elements.

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Figure 4. The quantitative analysis results.
To explore the detailed surface chemical
state of the elements and interactions between
CuCaAg2Se, graphene, and TiO2, XPS an
analysis was conducted over CCAS-G-TiO2.
The binding energies depend on the chemical
structure and elemental formation of sample.
Figure 5 (a) presents the XPS survey
spectrum, which expresses the existence of
C1s, O1s, Ca2p, Ti2p, Cu2p, Se3d and Ag3d,

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and it is confirmed that there is the successful
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formation of CCAS-G-TiO2.
7x104
6x104
5x104
C1s
(b)
[C-C]
284.56 eV
5x105
CCAS-G-TiO2
C1s
(a)
9
4x104
3x104
2x104
1x104
0
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4x105
3x105
2x105
1x105
0
[C-C] aliphatic
O1s
285.37 eV
[C-OX]
Ag3d
Ti2p
288.72 eV
Cu2p
280 282 284 286 288 290 292 294 296
Binding energy (eV)
Se3d
0
200
400
600
800 1000 1200
Binding energy (eV)
1.3x104 (d)
1.2x104
1.2x104
1.2x104
1.2x104
1.2x104
1.1x104
1.1x104
O1s
Ca2p
Ca2p1/2
(c)
2.5x104
2.0x104
1.5x104
1.0x104
5.0x103
0.0
[X-CO]
Ca2p3/2
532.19 eV
[-C=O]
530.26 eV
346.59 eV
351.30 eV
1.1x104
340
525
528
531
534
537
540
344
348
352
356
360
Binding energy (eV)
Binding energy (eV)
1.0x104
8.0x103
6.0x103
4.0x103
2.0x103
0.0
5.4x103
(f)
Cu2p
Ti2p3/2
Ti2p
(e)
+
Cu 2p
3/2
458.81 eV
4.5x103
3.6x103
2.7x103
1.8x103
9.0x102
0.0
932.26 eV
952.23 eV
+
933.92 eV
Cu 2p
1/2
2+
Cu 2p
3/2
Ti2p1/2
464.57 eV
943.71 eV
453
456
459
462
465
468
471
925 930 935 940 945 950 955 960
Binding energy (eV)
Binding energy (eV)

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1.4x104
Ag3d
Se3d
2.6x103
2.4x103
2.2x103
2.0x103
1.8x103
1.6x103
1.4x103
Ag3d
(h)
(g)
5/2
1.2x104
1.0x104
8.0x103
6.0x103
368.12 eV
374.06 eV
[Se3d5/2
]
[Se3d3/2
]
[Se-O]
[Se-Se]
Ag3d3/2
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59.21 eV
4.0x103
2.0x103
0.0
54.37 eV
58.53 eV
53.83 eV
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62
366
368
370
372
374
376
378
Binding energy (eV)
Binding energy (eV)
Figure 5. (a) XPS survey spectra and the high-resolution XPS spectra of (b) C1s, (c) O1s, (d)
Ca2p, (e) Ti2p, (f) Cu2p, (g) Se3d and (h) Ag3d.
The electron configuration of C1s is shown to consist of three peaks and those peaks are
corresponding to the [C-C], [C-C] aliphatic and C-OX groups, as shown in Figure 5 (b). In addition,
the existence of C-OX group confirms from the synthesized ternary composite contains the oxygen-
containing functional group [21] which confirms the existence of the chemical-bond between oxygen
and metal, and the X-is the state of metal. The O1s spectrum is shown to compose of two peaks
which are located at the 530.26 and 532.19 eV regions. The first peak indicated the carbonyl group [-
C=O], whereby the second peak are corresponding to the one of the metal carbonates forms, Ti-O-C
as shown in Figure 5 (c) [22]. The Ca2p XPS spectrum consisted of two characteristic peaks which
are located at a 346.59 and 351.30 eV, and those peaks corresponding to the formation of Ca-O, Ca-
TiO2 bonding [23], as shown in Figure 5 (d). The formation of the TiO2 expressed two sharp peaks,
which were centered at the 458.81 and 464.57 eV binding energy regions, and it was assigned to
spin-orbital of Ti2p3/2, Ti2p1/2. In addition, 2p3/2 clearly defined the Ti (IV) (TiO2) and the intensity
ratios of two peaks are also constrained to 2:1, as shown in Figure 5 (e). The Cu2p3/2 and Cu2p1/2
spin orbital peak of ternary composite divided into two major and two satellites peaks, as shown in
Figure 5 (f). The first major peak was defined the Cu(I) oxide and Cu(I) selenide [24]. Se3d is
consist of two characteristic peaks of Se3d5/2 and Se3d3/2 spin orbital which are located at a binding
energy of 53.83 and 54.37 eV regions. In addition, two more peaks were observed in the 58.53 and
58.21 eV regions, which assigned to the Se-Se, Se-O form [25], as shown in Figure 5 (g). Finally,
the Ag3d XPS spectrum of composite consisted of two characteristic peaks which corresponds to the
3d5/2 and 3d3/2 spin orbital, as shown in Figure 5 (h). These two sharp peaks located at 368.12 and
374.06 eV binding-energy regions and those two peaks correspond to the state of Ag (I) in Ag2O and

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Ag2Se, respectively [26]. The binding energy shifts indicate the strong bond-interaction and charge
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transfer among the CuCaAg2Se quaternary nanocomposite, bridge element of graphene, and TiO2 in
the synthesized ternary CCAS-G-TiO2 nanocomposite.
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The charge-separation and transfer process and photo-electro-chemical capacity of sample was
investigated via DRS, PL, and photocurrent analysis. UV-vis DRS analysis was conducted to explore
the optical absorption-properties of prepared sample, as shown in Figure 6 (a).
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5
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5.0
4.8
4.6
4.4
4.2
4.0
3.8
12.9
12.6
12.3
12.0
11.7
11.4
11.1
10.8
10.5
10.2
(a)
2.76 eV
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CCAS
G-TiO2
300
400
500
600
Wavelength (nm)
CCAS-G-TiO2
2.79 eV
2.97 eV
2.4
2.7
3.0
3.3
3.6
3.9
hu(eV)
6.50x10-6
CCAS
G-TiO2
(b)
light on
CCAS-G-TiO2
5.20x10-6
3.90x10-6
2.60x10-6
1.30x10-6
light off
0
30
60
90 120 150 180 210 240 270
Time (s)

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90
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60
50
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30
20
10
CCAS
G-TiO2
596.86 nm
(c)
CCAS-G-TiO2
9
552.53 nm
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500
400
300
200
100
0
560.68 nm
601.73 nm
603.18 nm
601.73 nm
553.09 nm
560.85 nm
596.86 nm
553.09 nm
553.09 nm
550
600
650
700
750
800
Wavelength (nm)
600
660
720
780
840
900
Wavelength (nm)
Figure 6. (a) The DRS result and (inset) UV-vis reflectance spectrum, (b) Photocurrent result
and (c) Photoluminescence (PL) spectra of G-TiO2 and CuCaAg2Se-G-TiO2 nanocomposite.
To get the relation of the energy (E ) and optical absorption, according to the, the absorption
g
spectra can be analyzed by using (Eq. (2)) equation [27]:
[hvF (R∞)]1/n =A(hv Eg ) [2]
where hv= hc/λ is the energy of the incident photon, F (R) (1 R)2/2R is the Kubelka-Munk
function, Eg is the energy of the band gap and A is the constant of proportionality, F(R) is the
absorption coefficient. The composites are exhibit the obvious absorption in the solar-light region of
300-600 nm. The pure quaternary nanocomposite has light-absorption under near-Visible light
region and Kubelka-Munk function was exhibited at 449.27 nm. The synthesized G-TiO2
nanocomposite has photo-absorption from UV-light to Visible light range and Kubelka–Munk
function was exhibited at 417.50 nm. The TiO2 is n-type semiconductor which usually used for
photocatalytic CO2 reduction due can possess a light absorption property under the UV-light region

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(UVA, 315–400 nm, UVB, 280–315 nm, and UVC, 180–280 nm) [28]. AfterDOthI:e10.1q0u39a/Dte0rNnJ0a3r1y50J
chalcogenide (CuCaAg2Se) united with Graphene-TiO2, the light absorption range was changed to
441.48 nm. The reason is: the conglomeration can act on the light-absorption performance resulting
to modify the bandgap. Figure 6 (a) shows the bandgap of CCAS, G-TiO2 and CCAS-G-TiO2, which
was expected to be 2.76, 2.97 and 2.79 eV, respectively. The pure CuCaAg2Se quaternary
nanocomposite, G-TiO2 binary and CuCaAg2Se-G-TiO2 ternary nanocomposite showed the lower
bandgap condition existed rather than a pure anatase TiO2, which can show a high catalytic activity
and can be able to practice application. Furthermore, the ternary CuCaAg2Se-G-TiO2 nanocomposite
can be able to generate more electron-hole pairs under the UV/Vis-light which can exhibit high
catalytic efficiency.
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Figure 6 (b) shows the photo-current response of ternary composite, during four on-off cycles of
light illumination. The ternary composite and pure quaternary nanocomposite showed swift and
velocity current response for each light irradiation. The photo-current state decayed immediately in
the no-light condition and recovered once the light was on. When the light was switched on, the
current-state increased with time in 30 s. This process can be explained in reference to the existence
of charge carriers transfer from the VB to the CB under the light irradiation. The current-state of
CuCaAg2Se was slightly decreased after two on-off cycle (Fig.S5a). The current state of G-TiO2 was
stable during a four-on-off cycles which can confirm the binary nanocomposite showed fluent
current capability. In the case of ternary CuCaAg2Se-Graphene-TiO2 composite, the photo-current
state was obviously amplified after four on-off cycle tests (Fig.S5b). The photocurrent state of G-
TiO2 was 3.90*10-6 A. The photocurrent-time (I-t) curves of quaternary nanocomposite expose the
high-separation capacity for charge-carrier under the light illumination and photocurrent state was
5.45*10-6. In the case of ternary nanocomposite, the photo-current state was almost reached to the
quaternary nanocomposite state. It is shown that it can improve the catalytic activity, after being
combined with Graphene-TiO2 to make a ternary composite. This hypothesis approved in
characterization of UV-vis DRS analysis; the band-gap energy of CCAS was 2.76 eV, G-TiO2 was
2.97 eV, and CCAS-G-TiO2 was 2.79 eV.
Photoluminescence (PL) spectroscopy was conducted to investigate the separation efficiency,
and recombination process of the photo-excited electrons. The PL spectra of composites are
displayed in Figure 6 (c). The unary, binary, and ternary nanocomposite showed a light response to
514 nm laser which is shown in Figure 6 (c) (I-III). The emission-spectra of metal oxide are
categorized into two groups: [1] Near-band-edge (NBE) UV-emission, [2] Deep-level (DL) defect-
related Visible-emission [29]. A broad emission peak was observed in the range between 570 nm to

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625 nm with 514 nm laser. The quaternary nanocomposite has wide-emission peak at 603.18 nm
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wavelength region. The unary nanocomposite had low recombination of photo-generated charge
carriers due to low- intensity emission peak. The Graphene-TiO2 binary composite has a strong and
sharp emission peak at 596.86 nm wavelength region which was assigned to the recombination of
photo-excited carrier. The CuCaAg2Se-Graphene-TiO2 ternary composite exhibited a weak and
broad emission peak at 601.73 nm region, which can confirm a low re-combination rate of photo-
excited carrier compare to binary composite. In CuCaAg2Se-Graphene-TiO2, the excitation
wavelength region was shifted which is due to the photo-excited electrons are transmitted from
graphene-TiO2 to CuCaAg2Se quaternary nanocomposite via the interfacial. The broad emission
band at 596.86, 601.73 nm and 603.48 nm can be assigned to the deep-level visible-emission which
was caused by the crystal defects of nanocomposite. The evidence of the charge separation in ternary
CuCaAg2Se-graphene-TiO2 composite was noted, and the transient photocurrent study was carried
out.
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Raman spectroscopy is the chemical-analysis method which can analyze the chemical structure
phase and molecular interaction of matter. The principle of Raman spectroscopy is a light-scattering
technique, where the light interact to the molecule bond and the result provides the information of
carbon material (D and G-band), structural characteristic between CuCaAg2Se, graphene and TiO2
due to the strong scatter.
3.0x104
CCAS-G-TiO2
2.7x104
G-TiO2
2.4x104
2.1x104
1.8x104
(a) Full raman spectra
1.5x104
1.2x104
9.0x103
6.0x103
3.0x103
200 400 600 800 1000 1200 1400 1600 1800
-1
Raman shift (cm )

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2.4x104
2.1x104
1.8x104
1.5x104
1.2x104
9.0x103
6.0x103
3.0x103
G-TiO2
-1
E (149.92 cm )
g
(b)
9
-1
SOE (254.51 cm )
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-1
(408.75 cm )
B
1g
-1
E (607.41 cm )
g
200 400 600 800 1000 1200 1400 1600 1800
-1
Raman shift (cm )
8x103
7x103
6x103
5x103
4x103
3x103
2x103
1x103
CCAS-G-TiO2
-1
(142.99 cm )
(c)
-1
(274.55 cm )
-1
(388.35 cm )
-1
(591.34 cm )
200 400 600 800 1000 1200 1400 1600 1800
-1
Raman shift (cm )
Figure 7. The Raman full spectra (a) of G-TiO2 (b) and CuCaAg2Se-G-TiO2 (c)
nanocomposite.
Figure 7 (a) shows the full Raman spectroscopy of all composites in detail. The G-TiO2 binary
composite included four different peaks which obtained at 149.92 cm-1, 254.51 cm-1, 408.75 cm-1
and 607.41 cm-1 shift region, as shown in Figure 7 (b). All occurred peaks are assigned to the TiO2.

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In the case of binary composite, there is no peak (D and G-peak) of graphene DdOuI:e10.t1o03V9iet/whDA0erNticJll0eo3Ow1n5li0nJe
concentration of graphene. The TiO2 had three different phases: anatase, rutile and brookite. The as-
prepared TiO2 was corresponded to the anatase phase. The anatase TiO2 had four Raman band at
100-700 cm-1 shift region [30, 31], such as Eg mode at 144, 183 cm-1 region and 639, 629 cm-1, B1g
mode at 399, 382 cm-1, the last one is A1g mode at the 520 cm-1 region. The first intense and sharp
peak assigned to the (left side) Eg mode which obtained at the 149.92 cm-1 shift region. The other
(right side) Eg mode was obtained at 607.41 cm-1 and the peak intensity was shown to be low and
broad. The Eg mode is an asymmetric bend of O-Ti-O bond on the (001) crystal plane, and this peak
was commonly obtained in the Raman spectra of TiO2.
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In the center of Figure 7b, other Raman modes were observed. The asymmetric bend of O-Ti-O
bond on the (001), (110) and (101) crystal planes of TiO2 was corresponded to the B1g mode and this
peak was obtained at 408.75 cm-1. Furthermore, the weak and broad peak was observed at 254.51
cm-1 shift region which renamed by SEO mode and the peak is shown to belong to the motion of
atoms. Figure 7 (c) shows the Raman spectra of ternary composite (CCAS-G-TiO2) which included
four peaks. In 142.99 and 591.34 cm-1 shift regions, these two weak and broad peaks were obtained,
and those peaks are belonging to the Eg mode peak of TiO2. The peak position was shifted to the left
side due to the structural characteristic and asymmetric bend of O-Ti-O bond of TiO2 was changed
which graphene-TiO2 joined with quaternary nanocomposite. There were another two peaks which
were assigned to the Raman band of quaternary nanocomposite at 100-700 cm-1 shift region. The
characteristic peak position of quaternary composite was overlapped with the peak of TiO2 at 200-
600 cm-1. The pure silver selenide had four characteristic peaks at 200-840 cm-1 shift region [32].
Copper and copper oxide had three Raman peaks at 295 cm-1, 342 cm-1, 628 cm-1 shift regions [33].
The combination of Ag2Se with Cu and Ca, the structural property of each part was changed, and the
Raman peaks were indicated at 274.55, 388.35 and 591.34 cm-1. All peak position was shifted to the
(left side) red shift, which means that the peak belongs at the low Raman shift side. In Raman
spectra, the shifting of peaks towards lower or higher-wavenumber is related to the chemical bond
and symmetry of molecules were changed in quaternary nanocomposite and Graphene-TiO2.
Metal oxygen and functional group in the composites were confirmed by FTIR spectra. The
FTIR spectra of the synthesized nanocomposites are show in Figure 8.

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97.5
96.2
94.9
93.6
92.3
91.0
2989.15
(C-H)
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1575.19
(Ti-O-Ti)
1211.63
(C-OH)
3236.12
(O-H)
1074.41
(C-O)
2379.06
(CH2)
CCAS
G-TiO2
CCAS-G-TiO2
872.86
1403.87
C=O
(CH2)rocking
710.07 (Se-O)
800 1200 1600 2000 2400 2800 3200 3600
Wavenumbers (cm-1)
Figure 8. FTIR spectra of CuCaAg2Se, G-TiO2 and CuCaAg2Se-G-TiO2 nanocomposite.
The adsorption band of O-H stretching (R-C(O)-OH) at around 3236.12 cm-1, 2379.06 cm-1
corresponds to CH2, C=O stretching (C(O)-OH) at 1403.87 cm-1, and CH2 rocking at 872.86 cm-1.
The result is attributed to the vibrational modes of citric acid molecule [34]. Moreover, the other
appearance bands confirmed the evidence of successful chemical bonding state of CuCaAg2Se. More
specifically, 710.07 cm-1 corresponding to Se-O, Se-Se vibration [35]. Additionally, the Graphene
exhibited the C-H stretching at 2989.15 cm-1, C-OH stretching at and 1213.63 cm-1, and C-O
stretching at and 1074.41 cm-1, implying the vibration of hydroxyl group and carboxyl group of
Graphene [36]. The strong absorption band at 1575.49 cm-1 is corresponding to the Ti-O-Ti bond in
TiO2. All functional groups confirmed the strong chemical formation of Graphene and TiO2. After
quaternary nanocomposite (CuCaAg2Se) combined with G-TiO2 via muffle-assisted hydrothermal
method. The peak intensity and position were changed, indicating the formation of due to formation
of CuCaAg2Se-G-TiO2 bonding.
Recently, some of research papers with Ag2Se-based nanocomposites for the CO2 reduction have
not been published. There are several silver doped or silver-based photocatalyst for the CO2
reduction such as Ag-Cu2O core-shell nanoparticles [37], Ag3PO4 [38], novel Ag/Ag2WO4/ZnWO4
[39] heterojunctions. Generally, Ag2Se and Ag2Se-graphene nanocomposite were used in solar cells,
sensor, and conductor due to high chemical and optical properties. Some of techniques were applied
to the fabrication of silver selenide nanostructures include combination of the precursors at high

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temperatures [40], hydrothermal method, microwave [41], and molecular precursor methods [42]. In
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our research study, the nanocomposite was synthesized by muffle-assisted hydrothermal method
which is afforded to obtaining a decent crystallinity and the polymer precursor solution was prepared
by Pechini method based on metallic citrate polymerization using ethylene glycol C2H6O2 (Merck)
and citric acid C6H8O7 (Merck) as chelate cation. The surface and morphology structure of pure
Ag2Se is spherical type and diameter of particles were around 5–10 nm. The Ag2Se-rGO possesses
good conductivity and considerably low charge transfer resistance (Rct) due to the high synergistic
effect, which can confirm that silver chalcogenide with graphene nanocomposite exhibit a high
catalytic activity. The Ag2Se-rGO possesses good conductivity and considerably low charge transfer
resistance (Rct) due to the high synergistic effect, which can confirm that silver chalcogenide with
graphene nanocomposite exhibit a high catalytic activity.
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3.3. Photocatalytic performance and reduction of CO2
Figure 9 shows the result of the “Quantitative analysis of alcohol” which used an as-synthesized
ternary composite for using a different amount of scavenger. Besides, to investigate the role of
ternary nanocomposite, the photocatalytic CO2 reduction process was carried out for a unary and
binary nanocomposites, such as TiO2, CuCaAg2Se, G-TiO2.

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TiO2
TiO2
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3
2
1
0
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12
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8
(with 0.1 g scavenger)
(with 0.3 g scavenger)
CCAS
G-TiO2
8.28
(a)
CCAS
G-TiO2
12.28
CCAS-G-TiO2
CCAS-G-TiO2
10.17
6.21
(a) Under Vis- light irradiation
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4.02
6.04
6
4.24
2.34
4
2.17
2.14
1.96
3.1
3.02
2.89
2.59
1.14
1.99
1.02
0.94
0.96
2
1.56
1.57
1.44
0.68
0.52
0.89
0.35
0.71
0.58
0.18
0.12
0.33
0
12h
24h
36h
48h
12h
24h
36h
48h
Time (hour)
Time (hour)
TiO2
(with 0.6 g scavenger)
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8
CCAS
G-TiO2
12.68
CCAS-G-TiO2
10.88
7.28
6
4.78
4.62
4
3.59
3.14
2.81
2.58
1.93
1.73
1.66
2
1.02
1.01
0.71
0.56
0
12h
24h
36h
48h
Time (hour)

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TiO2
TiO2
(with 0.3 g scavenger)
(with 0.1 g scavenger)
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(b)
CCAS
G-TiO2
CCAS
G-TiO2
8.
16
15.53
CCAS-G-TiO2
CCAS-G-TiO2
14
12
10
8
(b) Under UV light irradiation
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11.08
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3.57
3.54
2.31
1.
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1.
2.36
2.3
1.
89
1.95
1.86
0.
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0.
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1.12
18
0.
0.93
0.76
0.
0.53
0.
0
0
12h
24h
36h
48h
12h
24h
36h
48h
Time (hour)
Time (hour)
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16
14
12
10
8
TiO2
(with 0.6 g scavenger)
CCAS
G-TiO2
16.84
CCAS-G-TiO2
15.35
11.02
7.13
6.58
6
5.26
3.76
4
2.88
2.68
2.52
2.16
2.11
2
1.32
1.25
1
0.72
0
12h
24h
36h
48h
Time (hour)
Figure 9. (a) Quantitative analysis of the Alcohol result of unary, binary, and ternary
composite under the Visible light, (b) UV light for using a different amount of scavenger.
The first comparison made with as-prepared TiO2, under identical experimental conditions. However,
TiO2 exhibited rather poor CO2 reduction activity (Figure 9 (a, b)) owing to its UV-only light
absorption and unmatched CB potential to drive the CO2 reduction effectively. The improved CO2
conversion efficiency of the TiO2 is due to facilitated charge transfer and separation property of
graphene. More importantly, the methanol evolution over CCAS-G-TiO2 is also superior to
graphene-based nanocomposite, TiO2 photocatalytic systems. Under the Vis-light, the ternary
composite exhibited high final product yield (CH3OH), and when increasing the amount of
scavenger (0.1 to 0.6 g) the yield was increased. With (0.1 g) scavenger, the methanol yield was
noted as 2.17, 4.02, 6.21, 8.28 %. With (0.3 g) scavenger, the final product yield was 4.24, 6.04,
10.17 and 12.28 % while with (0.6 g) scavenger, the methanol yield was 4.62, 7.28, 10.88 and 12.68 %
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under the vis-light after 12 to 48 hours as shown in Figure 8 (a). Figure 8 (b) shoDwOsI: 1t0h.1e03V9rie/ewDsA0urNtilcJtl0e3Oo1n5fli0nJe
methanol yield under the UV-light, the target ternary composite exhibited high methanol (CH3OH)
yield with various quota of scavenger. With (0.1 g) scavenger, the methanol yield was 2.35, 5.00,
6.38 and 8.71 %. When using a (0.3 g), the yield was 4.82, 7.31, 11.08 and 15.53 % while using a
(0.6 g) scavenger the methanol yield was 7.13, 11.02, 15.35 and 16.84 % under the UV-light after 12
to 48 hours.
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To analyze the consequence of “Quantitative Analysis of Alcohol,” the ternary composite
showed a good catalytic activity and reduction performance for CO2 into methanol (CH3OH) with
using various amount of scavenger. Sodium sulfite (Na2SO3) was applied as the scavenger since it is
a great efficient electron provider and cycling electron donor, to combine the Carbon dioxide (CO2)
compound to photo-reduction with water to enhance all potency. In addition, the scavenger boosts
the photo-catalytic activity of graphene-based ternary composite in CO2 reduction. Under the UV-
light irradiation, the final product quota was high with using a high amount of scavenger. Moreover,
with using a 0.3g and 0.6g scavenger affects was not high.
In the practical application of a reusable state, the reproducibility and sturdiness of a
photocatalyst never be dismissed. In the cycling test, the photocatalyst (CCAS-G-TiO2) was recycled
for 9 times (432 h) under UV/Vis-light irradiation with scavenger (0.3 g) in CO2 reduction. As can
be seen in Figure 10,
24
Under Vis-light
Under UV-light
22
cycle 9
cycle 8
cycle 7
cycle 6
cycle 5
cycle 4
cycle 3
cycle 2
cycle 1
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8
.36
.15
.13
.06
.99
7
1
.92
.75
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1
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1
.53
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.95
4
1
.22
.23
.17
.14
.02
.86
2
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.75
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.59
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.22
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1
6
4
2
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Recycling time (days)

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Figure 10. Photocatalytic activity of CO2 reduction using CuCaAg2Se-GrapheDnOeI-: T10i.1O0329/Dsa0NmJ0p3l1e50J
after 432 h (9 days) illumination in the cycling experiment. the ternary nanocomposite retains its
notable activity after nine cycle under identical conditions with no significant deactivation towards
to Methanol evolution. According from recycling test, the synthesized ternary nanocomposite is
stable for practical application in generating solar fuel.
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The FTIR spectrum of first intermediate product (after 4 h) and final product (after 12 h of
reaction) was shown in Figure 11.
Table 1. Characteristic vibrational group of the intermediate and final-product from the FTIR
measurement.
№
1
2
3
4
Vibrational state
Hydroxyl group
Carbonyl group
Alcohol group
Methyl group
Mode
(O-H)
C=O
C-O-H
(C-H) in
(CH3) 훿
Experimental (cm-1)
3294.57 cm-1
1648.3 cm-1
Theoretical (cm-1)
3400-3300 cm-1
1715-1700 cm-1
1274.41 cm-1
1402.32 cm-1
1440-1220 cm-1
1450 and 1375 cm-1
100
80
60
40
20
(C-O)
(C-O-H)
(C=O)
(C-H)d
Formic acid
Methanol (ref)
Methanol
(O-H)
500
1000
1500
2000
2500
3000
3500
-1
Wavenumbers (cm )
Figure 11. The FTIR result and result of the final product: reference (red line), final
methanol product (blue line), formic acid (black line).

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The first intermediate product (formic acid) and final product consisted of hydroxyl group (O-H),
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carbonyl group (C=O), alcohol group (C-O-H), methyl group (CH3-deformation) and the peak
appeared at the exact wavenumber region which can confirm the state of each product. Table 1
summarized the results of each obtained peak position with the theoretical key data. The position of
hydroxyl and carbonyl group was noted as the same. The intensity and position of C-H in (CH3)-
deformation was different in methanol as compared with Formic acid. In addition, the FTIR result of
the final product was compared with the result of commercial methanol. Each peak position was
overlapped, and it can prove that photocatalyst material can convert the CO2 into a methanol.
Figure 12 shows the morphology and crystal structure of photocatalyst after UV/Vis-light
irradiation which analyzed via SEM and XRD. After the experiment, it was possible to collect the
photocatalyst, and it was rinsed with DI-water and dried at 100 °C.
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1.30x103
1.17x103
1.04x103
9.10x102
7.80x102
6.50x102
5.20x102
3.90x102
2.60x102
1.30x102
0.00
(e)
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Before light irradiation
After Vis-light irradiation
After UV-light irradiation
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2 theta (degree)
Figure 12. (a-b) The formation mechanism of CuCaAg2Se-G-TiO2 ternary nanocomposite
with the SEM images of after Vis-light irradiation, and (c-d) after UV-light irradiation, (e)
the XRD pattern of ternary photocatalyst after light irradiation.
The morphology structure of the photocatalyst was clear and each peak position is noted as the
same as a fresh photocatalyst in SEM and XRD. The total result showed no apparent retrogression as
compared to the pure-photocatalyst which can prove the high stability and reusability of this material.
For the suggested photocatalytic reduction mechanism, the light (photon) interacts onto the
exterior of TiO2 absorbing the energy. When the light energy (Ev) is equal or greater than the
bandgap energy (Eg) of semiconductor, then the electron can be excited from the VB to CB creating
an electron-hole pair. During a reaction, an excited electron and hole move to the semiconductor-
exterior. Nevertheless, the absence of a co-catalyst, or an insufficient active site determines that the
electron-hole recombines very quickly on the semiconductor-surface and shows that the catalytic-
efficiency was modified. During a photocatalytic reaction, the role of graphene is an electron
acceptor/transporter, whereby this presents the most important part of the partition of electron-hole
transport. The oxygen-function of graphene were bonded by un-paired π-electrons with Ti-atoms on
the exterior of TiO2, which affecting the band-gap of graphene-TiO2 nanocomposites. The
photocatalytic activity can be further enhanced through the contributions of a junction/interaction
between the co-catalyst and the semiconductor to increase the electron-hole partition or charge
transport. In the heterogeneous system, the performance of the co-catalyst acts as an electron-trap to
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