Flower-like CdS/CdV2 O6 composite for visible-light photoconversion of CO 2 into CH4 Sana Ijaz, Muhammad Fahad Ehsan, Muhammad Naeem Ashiq, Nazia Karamt, Muhammad Najam-ul-Haq, Tao He PII: DOI: Reference:
S0264-1275(16)30782-1 doi: 10.1016/j.matdes.2016.06.031 JMADE 1906
To appear in: Received date: Revised date: Accepted date:
28 April 2016 4 June 2016 9 June 2016
Please cite this article as: Sana Ijaz, Muhammad Fahad Ehsan, Muhammad Naeem Ashiq, Nazia Karamt, Muhammad Najam-ul-Haq, Tao He, Flower-like CdS/CdV2 O6 composite for visible-light photoconversion of CO2 into CH4 , (2016), doi: 10.1016/j.matdes.2016.06.031
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Flower-like CdS/CdV2O6 composite for visible-light photoconversion of CO2 into CH4
National Center for Nanoscience and Technology, Beijing 100190, China
IP
1
T
Sana Ijaz1,2, Muhammad Fahad Ehsan1,3 Muhammad Naeem Ashiq2*, Nazia Karamt2, Muhammad Najam-ul-Haq2, Tao He1 2
Institute of Chemical Sciences, Bahauddin Zakariya University Multan (60800), Pakistan School of Natural Sciences, National University of Science and Technology, Islamabad, Pakistan *
SC R
3
= Corresponding author:
[email protected],
[email protected]
NU
Abstract
MA
Flower-like CdS/CdV2O6 composite has been used in the present work for the visiblelight (λ ≥ 420nm) photoconversion of CO2 into CH4. CdV2O6 has been synthesised by
D
simple hydrothermal method. Limited chemical conversion route (LCCR) has been
TE
used for the synthesis of novel efficient visible light driven CdS/CdV2O6 composite. Several characterization techniques such as X-ray diffraction (XRD), high resolution
CE P
electron microscopy (HRTEM), scanning electron microscopy (SEM), UV/visible spectroscopy, X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller
AC
(BET) have been used to characterize the as synthesised photocatalysts. XRD confirms the formation of monoclinic structure of CdV2O6 and orthorhombic phase of CdS in the nanocomposite. HRTEM and SEM results reveal rod-like morphology of CdV2O6 while CdS/CdV2O6 is in the form of well-shaped bunches of flower-like structures. XPS is used to determine the alignment of energy levels against the standard hydrogen electrode (SHE). The as-synthesized flower-like composites have then been employed as efficient photocatalysts for visible-light photoconversion of CO2 into CH4. The CdS/CdV2O6 composite results in visibly higher yield of methane than that of single semiconductors that might be attributed to the suppressed recombination of the
1
ACCEPTED MANUSCRIPT photogenerated charge carriers. Flower-like CdS/CdV2O6 composite also exhibits tremendous photostability than the individual counterparts.
T
Keywords: Composite; Photocatalysis; CdS/CdV2O6; Photoreduction of CO2;
SC R
IP
Methane
1. Introduction
Rapid increase in atmospheric carbon dioxide concentration owing to the elevated
NU
consumption of fossil fuels (coal, natural gas and petroleum) is one of the major environmental issues that modern society is facing today [1]. Other than
MA
environmental issues such as pollution and global warming, several energy-related problems are also associated with the rapid utilization of fossil fuels owing to increase
TE
D
in world population and rapid industrialization [2-3]. It is expected that these energy resources would be replenished due to their high consumption or naturally recurring
CE P
processes by the end of 2050 [4]. As the economic and industrial development mainly relies on energy resources, therefore, it becomes essential for the sustainable human
AC
society to explore alternative energy resources that not only meet rapid growth in energy demand but also prove to be a solution to address the environmental issues. Research in the field of carbon-free alternative energy sources, recycling of carbon dioxide, and towards the higher efficiencies of carbon-based energy systems has been growing since the past few years to address the issues associated with environment and energy [5]. For its practical realization, semiconductor-based photocatalysis is the most significant and rapidly emerging strategy. It has been proved to be a promising solution for these problems due to its tremendous applications in the degradation of organic pollutants, water splitting and reduction of carbon dioxide (CO2) into rapidly
2
ACCEPTED MANUSCRIPT transportable and stable fuels i.e., solar fuels (carbon monoxide (CO), methane (CH 4), methanol (CH3OH) and like) [6].
T
Recently, utilization of solar energy in case of photocatalytic reduction of CO 2 over
IP
semiconductors has attracted much attention as a remedy for environmental problems.
SC R
Various metal oxides (such as titanium oxide [7-10], magnesium oxide [11], zirconium oxide [12]), and other than oxides (such as phosphides [13] and carbides [14]) have been used as photocatalysts for efficient conversion of carbon dioxide into
NU
value added chemical fuels. But these systems only respond to the UV-light that
MA
corresponds to only ~ 4 % of the total solar spectrum. Therefore, to bring this activity in visible-light that corresponds to 43 % of the solar spectrum is a challenging task
D
[15]. Apart from various metal oxide semiconductors, vanadates are also widely used
TE
as photocatalysts and electrode materials due to their rich structural chemistry and suitable band alignment [16-19]. Among oxides vanadates are visible-light-driven
CE P
photocatalysts. According to theoretical studies, the position of “V” 3d orbital is low in energy spectrum and hence bottom of conduction band that is formed by the “d”
AC
orbital of transition metals in metallic oxides is lower than that of other transition metal oxide [20-21]. Therefore, band gap of vanadate-based semiconductors is narrow which makes them effective visible-light responsive semiconductors. Most of the vanadates such as BiVO4 [22], InVO4 [21], Cd2V2O7 and CdV2O6 [23] have been extensively used as visible light active photocatalysts. Photocatalytic efficiency is closely related to the consumption of photogenerated electron hole pairs and their recombination. Vanadates have low adsorption ability and rapid charge carrier recombination which limit their utility as single photocatalysts [21, 24]. Hence combination of another suitable semiconductor material with vanadate is of great interest and importance. Coupled semiconductors in the form of
3
ACCEPTED MANUSCRIPT heterostructure also provide better charge separation capability due to two different energy level systems. For this purpose, metal sulphides are potential visible-light
T
driven photocatalyst due to their narrow band gap and suitable position of conduction
IP
band [25-26]. Among them, CdS is the most extensively used semiconductor with
SC R
direct band gap and suitable conduction band potential for the photocatalytic reduction of CO2 into solar fuels [27-30]. Combination of CdV2O6 with CdS in the form of superstructure would not only enhance the stability of CdS against undesired
NU
photocorrosion, but will also reduce the chance of electron-hole recombination and
MA
enhance response in visible region of solar spectrum. In the present work, CdV2O6 is chemically converted into CdV2O6/CdS composite by
D
anion exchange under the influence of solubility product difference. By controlling
TE
anion exchange at limited level, small amount of CdV2O6 is converted into CdS that results in the formation of CdV2O6/CdS composite. The structural properties,
CE P
photoreduction efficiency and the stability of the composite system have also been investigated in detail.
AC
2. Experimental
2.1.Materials
Cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O, Sinopharm Chemicals, 99 %), ammonium vanadium oxide (NH4VO3, Alpha Aesar, 99 %) and nitric acid (HNO3, Sinopharm Chemicals, 95 %) were used as starting materials for the synthesis of CdV2O6 and thiourea (CS(NH2)2, Sinopharm Chemicals, 99 %), was used for the chemical conversion of CdV2O6 into CdS/CdV2O6 composite. Sodium sulphide (Na2S Sinopharm Chemicals, 99 %) and Sodium Sodium Sulfite (Na2SO3 Sinopharm
4
ACCEPTED MANUSCRIPT Chemicals, 99 %) were used as scavengers agents for photocatalytic process. All these chemicals were of analytical grade and were used as such without further purification.
T
2.2. Synthesis of CdV2O6
IP
Simple solvothermal method was used for the synthesis of cadmium vanadate
SC R
(CdV2O6). Typically, the amount of 0.608g (5.2 mmol) of NH4VO3 was added in 37 mL of Milli-Q water. 2 to 3 mL of HNO3 was also added drop wise and stirred until ammonium vanadate was completely dissolved.
After 10 minutes, the amount of
NU
0.863g (2.8 mmol) of Cd(NO3)2.4H2O was also added in ammonium vanadate solution
MA
with continuous magnetic stirring at room temperature. 1M NaOH was used to maintain pH of the whole mixture. The whole mixture was then transferred into a
D
Teflon tube of 50 mL capacity which was then filled by the Milli-Q water up to 80 %
TE
of its capacity followed by being sealed in a stainless steel autoclave. The autoclave was kept at 175°C for 18 hours in an oven. After that it was cooled to room
CE P
temperature. The orange coloured CdV2O6 precipitates were obtained after centrifugation. These precipitates were washed several times with Milli-Q water and
AC
finally with absolute ethanol. The precipitates were then dried in oven at 70°C for overnight. The dried CdV2O6 was stored for further characterizations and application. 2.3. Synthesis of CdS/CdV2O6 composite Chemical conversion rout was adopted for the fabrication of CdS/CdV2O6 composite. In a typical procedure, 0.03 mmol of already synthesised CdV 2O6 was dispersed in 40 mL of Milli-Q water with continuous and vigorous stirring at room temperature. After 15 minutes, certain amount of thiourea was also added in it. The whole mixture was then transferred into Teflon tube of 50 mL capacity. The Teflon tube was then sealed in a stainless steel autoclave which was then kept at 175°C for 18 hours in an oven. The autoclave was then cooled down to room temperature and the radish brown 5
ACCEPTED MANUSCRIPT colored CdS/CdV2O6 precipitates were then washed for several times via centrifugation with Milli-Q water and absolute ethanol. The composite was then dried
T
in oven at 60°C for overnight. The dried CdS/CdV2O6 composite was stored for
IP
further characterizations and application.
SC R
2.4. Characterization
The samples purity and the crystal structure of the products was investigated by D8 Advance (Bruker) X-ray diffractometer equipped with Ni monochromatized Cu-Kα
NU
radiation (λ = 0.15406 nm). The XRD patterns were recorded in the range of 10° to
MA
80° at a scan rate of 0.1°/min. Lambda 750 UV‒Visible‒NIR spectrophotometer was used to investigate the optical properties with BaSO4 as a reference. The
D
morphological and compositional analysis of the synthesized samples was done by
TE
using Hitachi S4800 FESEM with energy dispersive X-ray (EDX) spectrometer and Tecnai G2 F20 U-TWIN transmission electron microscopy (TEM). Brunauer–
CE P
Emmett–Teller (BET) analysis (Micromeritics, tristar II 3020) was used to determine the specific surface area of the synthesized samples. X-ray Photoelectron
AC
Spectroscopy (XPS) was also carried out for the oxidation states of element and measurement of valance band positions by using ESCALAB 250Xi X-ray photoelectron spectrometer. The products from the photocatalytic reduction of carbon dioxide were identified by using 7890A GC/LC system from Agilent Technologies. The radiation source used was 300 W Xe lamp. 2.5. Evaluation of Photocatalytic activity 0.02g of catalyst was dispersed in 100 mL of Milli-Q water containing 0.1M Na2S and 0.5M Na2SO3 and transferred to the quartz photoreactor of the photoreaction system. Catalyst solution was saturated with CO2 before irradiation by bubbling the gas through solution for at least 45 min. The ultrapure carbon dioxide gas (99.99 %) was 6
ACCEPTED MANUSCRIPT regulated inside the photoreaction system with definite pressure (25 KPa). Moreover, oxygen was also completely removed from the photoreaction system and was
T
confirmed from GC. Milli-Q water (15°C) was continuously recycled within the
IP
system to maintain the reaction temperature. After that reactor was irradiated by using
SC R
the xenon lamp (300 W) furnished with 420 nm cut-off filters. The reaction was allowed to continue for a total period of 10 hours and the yield of products as a result of CO2 reduction was analysed by gas chromatograph (GC), equipped with flame
NU
ionization detectors (FID) and one thermal conductivity detector (TCD) through an
MA
automated gas valve using helium as the carrier gas. The dark controlled reactions were also performed by repeating same procedure in the absence of any light
D
irradiation, photocatalyst and carbon dioxide in order to confirm that photocatalysts,
CE P
3. Results and discussion
TE
CO2 and light irradiation are necessary for the synthesis of products.
3.1 Phase analysis
AC
Fig.1 shows the X-ray diffraction (XRD) pattern for CdV2O6 and CdS/CdV2O6 composite synthesized with different amount of thiourea. All the characteristic peaks of CdV2O6 are assigned to the crystal planes of monoclinic CdV 2O6 and this pattern is well matched with standard pattern (JCPDS cards # 22-0133) [31]. XRD Patterns of composites shows that the major peaks (represented by symbol ♦) belong to CdV2O6 in all the patterns while the peaks at 2θ = 26.493⁰, 28.252⁰ and 43.849⁰ (represented by symbol *) with Miller Indices (033), (240) and (107) crystal planes, respectively, are assigned to the orthorhombic CdS (JCPDS cards # 47-1179). The intensity of the peaks related to CdS enhanced gradually with increasing the amount of thiourea from 0.005 mmol to 0.06 mmol. All the peaks in the sample with thiourea (0.005 mmol) are
7
ACCEPTED MANUSCRIPT related to the CdV2O6 and no peak was observed for CdS. However, characteristic diffraction peak of CdS at 43.8491⁰ starts appearing in case of 0.01 mmol thiourea and
T
then became prominent when concentration of thiourea was further increased to 0.06
At very high pressure, some of the
SC R
products at 50⁰C in aqueous solutions [32].
IP
mmol. It is previously reported that thiourea is decomposed to S 2‾ ion and other by-
CdV2O6 sheets of rods dissociate into Cd +2 ions, and it leads to the formation of CdS in the presence of S2‾ ions. It might be attributed to low solubility product of sulfides
NU
as compared to vanadate [32]. Another possibility is that in the presence of water and
MA
high pressure thiourea give H2S gas which then further react with Cd 2+ ions to form CdS crystals [33]. As the amount of thiourea is increased, more CdS was formed may
TE
3.2. SEM and TEM analysis
D
be due to pilling up of CdS crystals.
Scanning electron micrographs of the as prepared samples are shown in Fig. 2(a-e).
CE P
Fig. 2(a) reveals that CdV2O6 appears in the form of bundles of rod-like structures. Length of individual rod ranges from 3μm to 5μm with average diameter of 500 nm.
AC
Morphology of the CdS/CdV2O6 composite synthesized with different amount of thiourea is analysed through SEM images as shown in Fig. 2(b-e). No clear difference in the morphology was observed when 0.005mmol of thiourea is used except the bundle like structure of thickness was converted into separate rod-like structure of width about 100nm. With the increase in amount of thiourea, CdV2O6 rods become thinner and start bending and folding into bunch of flower-like hybrid superstructures. It is clear in the inset of Fig. 2(d) that rod eventually converted into ribbon which is then folded to form flower-like hybrid heterostructure which is uniform in morphology at higher concentration of thiourea (Fig. 2(e)). The flowers are made of nanosheets of thickness 〜20nm and average width of flower is about 1μm. Formation of similar
8
ACCEPTED MANUSCRIPT flower like morphology by using thiourea has also been reported in literature [34, 35]. Fig. 3(a) shows the TEM image of CdS/CdV2O6 composite prepared by 0.06 mmol
T
thiourea. TEM image shows flakes- or petals-like structure that seems to be growing
IP
from the rods. HRTEM image is recorded from the selected area of rod having flakes
SC R
(Fig.3(b)). The d-spacing of 0.206 nm at flakes is indexed to the (444) plane of orthorhombic CdS, while lattice fringe with d-spacing 0.293 nm of rod corresponds to (111) plane of monoclinic CdV2O6. It is confirmed from result of TEM and HRTEM
NU
that these bunches of flower-like structures consist of CdS/CdV2O6 composite. It is
MA
confirmed from result of TEM and HRTEM that these bunches of flower-like structures consist of CdS/CdV2O6 composite.
D
3.3. EDX mapping, XPS and BET analysis
TE
EDX elemental mapping analysis was performed over a single bunch of flower (Fig. 4(a)). The results show that “Cd”, “S”, “V” and “O” elements are homogeneously
CE P
distributed on whole sample which confirms the formation of CdS/CdV 2O6 composite (Fig. 4b-e). XPS was used for surface analysis of prepared composite. The C1s at
AC
284.60 eV has been used to calibrate the peak positions in the XPS spectra. The XPS survey scan of CdS/CdV2O6 composite (Fig. 5) clearly indicates that composite consist of “Cd”, “S”, “V” and “O” without any other impurity. Nitrogen adsorption and desorption isotherm was used to investigate the texture of the composite and surface area was calculated by Barret-Joyner-Halenda (BJH) method. Nitrogen adsorption and desorption isotherm (Fig. 6) for the CdS/CdV 2O6 composite matches corresponds to type II with a hysteresis loop. The sharp increment of adsorption of N2 may be due to the capillary, and according to literature, such behaviour indicates the presence of macropores [36].
Average pore size of
CdS/CdV2O6 composite (465.15 Ǻ) is higher than the single CdV2O6 (256.89 Ǻ). 9
ACCEPTED MANUSCRIPT Average pore volume of CdS/CdV2O6 composite at P/P⁰ = 0.975 is found to be 0.13 cm3/g. 14.4837 m²/g BET surface area of the composite is higher than that of prepared
T
CdV2O6 (8.474m²/g). Enhanced surface area and higher pore size distribution in the
IP
composites provides more active sites for the easy transport of charge carrier, and
SC R
efficient adsorption of the reactant molecules, respectively [37]. 3.4. Alignment of energy levels
NU
Alignment of energy level plays an important role in the photocatlytic activity of a catalyst. It is analysed with the help of UV/Visible absorption and XPS valance band
MA
spectra. The UV/Visible diffuse reflectance spectra for CdS, CdV2O6 and CdS/CdV2O6 were recorded in the range of 300-800 nm. These diffuse reflectance
D
spectra are converted in to absorption spectra on the basis of Kubelka-Munk function
TE
[38] and are also shown in Fig. 7(a-c), respectively. A remarkable increase in the absorbance in visible region of spectrum was observed in the case of CdS/CdV2O6
CE P
composite as compare to single CdV2O6. Davis and Mott equation (eq. 1) was used via Tauc plot (inset of Fig. 7(b) & (c)) to
AC
calculate the band gap values for CdS and CdV2O6 –
(1)
Where, α, h and ν are adsorption coefficient, Plank constant and wave number, respectively [39]. The vertical segment of each Tauc plot ((αhν) 2 vs. hν) has been extra-plotted to intersect at x-axis (hν axis) to confirm the band gap for the synthesized materials. The observed band gap values (table 1) are in good agreement with that of previously reported [23, 40]. Valance band position of CdS and CdV2O6 was evaluated by XPS valance band spectra (Fig. 8 (a&b)). The values of valance band (VB) and conduction bands (CB) of CdS and CdV2O6are mentioned in table 1. These positions are used to draw the energy 10
ACCEPTED MANUSCRIPT diagram of CdS/CdV2O6 composite (Fig. 9) that represents the schematic illustration for the photocatalytic reduction of carbon dioxide into methane. The photogenetrated
T
charge carriers can be separated by transfer of electron from the conduction band of
IP
CdS to the conduction band of CdV2O6, and holes might also be transferred from
SC R
valance band of CdV2O6 to the valence band of CdS as shown in Fig. 9. Such an alignment of the energy levels seems to facilitate these complexed photoreactions. 3.5. Photocatalytic reduction of CO2
NU
The as synthesized products (CdV2O6 and CdS/ CdV2O6 prepared by 0.06 mmol of
MA
thiourea) have been applied for the photocatalytic reduction of CO2 with water using visible light ((λ ≥ 420 nm) irradiation for 10 hours in order to check their catalytic
D
activity. Methane (CH4) was found as the only major product. Some controlled
TE
experiments (dark experiments with, without catalyst and carbon dioxide, blank experiment with and without light) were also performed and no CH 4 was detected
CE P
which confirms that methane is being produced only as a result of the photoconversion of carbon dioxide under visible-light and not from any other impurities or surrounding
AC
factors.
The production of methane as a function of time is shown in fig. 10 and the maximum amount of CH4 obtained in case CdV2O6 was 11.53 μmol/g (table 2) which is lower than that obtained in case of CdS/CdV2O6 (29.80 μmol/g). Photocatalytic efficiency mainly depends on the following two factors: (a) rate of electron transfer and (b) rate of
electron-hole
recombination.
The
enhanced
photocatalytic
efficiency
of
CdS/CdV2O6 can be explained on the basis of alignment of energy levels. As the visible-light is illuminated it results in the generation of electron-hole pairs. Since the conduction band minimum of CdV2O6 is less negative and valance band maximum is more positive than that of CdS, photogenerated electron transfer from conduction band
11
ACCEPTED MANUSCRIPT (CB) of CdS to CB of CdV2O6 while holes transfer from valance band of CdV2O6 to that of CdS. This process of electron transfer is faster than the electron-hole
T
recombination because of feasible band positions of composite [41]. According to
IP
previously reported mechanism [42] electrons in the CB of CdV2O6 play major role in
SC R
the photocatalytic reduction of carbon dioxide. Based on the aforementioned discussion, the CdS/CdV2O6 composite provides efficient charge transfer and hence suppresses the recombination of the photogenerated charge carriers which leads to the
NU
higher activity.
MA
Apart from band alignment, existence of common cation (Cd 2+) might be the reason of enhanced photocatalytic activity of CdS/CdV2O6 composite [43]. The presence of
D
common cation might also minimize the electron-hole recombination. The interface in
TE
the present case may act as an additional barrier that would minimize the recombination of photogenerated charge carriers and hence results in enhanced
CE P
photocatalytic activity. That’s why such a low quantity (20 mg) of composite gives high yield of methane than that of single semiconductor.
AC
The mechanism for the photo catalytic reduction of carbon dioxide is described as when the visible light falls on the photocatalyst, it results in the formation of electronhole pairs in CdS as well as in CdV2O6. This process starts with the generation of electron-hole pair on absorption of visible light [44]. The oxidation potential for water is 0.82 (at neutral pH) versus standard hydrogen electrode (SHE) as reported earlier [45]. The valence band of CdS (1.51 V versus SHE) indicates that water can be easily oxidized by the holes present in the valence band. At first •OH is produced by the absorption of photogenerated holes by water which subsequently give H+ and O2 by absorption of further holes. Then •H radical is generated with the reaction of electrons with H+. After the oxidation of water the activation and reduction of CO 2 takes place
12
ACCEPTED MANUSCRIPT which is initiated with the production of negative and metastable superoxide ( •CO2-) radical from the reaction of CO2 with the photogenerated electrons in the CB [46-47]
T
of CdV2O6 and to some extant CdS. The •CO2- radical can easily formed in case of
IP
water and the other solvents having high dielectric constants, [48]. It is reported
SC R
previously that electron hole (M+─O-) surface centres are created upon irradiation on metal oxide semiconductor [45]. The M+ surface centres might be involved in altering the unoccupied molecular orbital of CO2 with the C-O-C bond angle which then
NU
facilitate the conversion of CO2 into •CO2- radical. CO is produced by the reaction of •
CO2- with •H radical which is then converted into •C radicals through successive
MA
reactions followed by the formation of a series of •CH, •CH2 and •CH3 radicals.
D
Finally, generation of CH4 may take place by the reaction of •CH3 radical with H+
TE
through carbide mechanism [3]. 3.6 Stability of CdS/CdV2O6
CE P
Stability of photocatalyst is also very important along with the photocatalytic efficiency for practical applications. The stability was investigated by recording SEM
AC
after photoreduction experiment (Fig.11 (a&b)) and using the same photocatalyst for several runs (Fig.11c). SEM image of CdS/CdV2O6 composite show no remarkable change in morphology even after 10 hours photoreduction experiment. It was observed that there is 5% reduction in the amount of methane after second run (fig. 11c). Thus synthesised CdS/ CdV2O6 composite show not only better photocatalytic efficiency but stability as well.
4. Conclusions Flower-like CdS/CdV2O6 composite has been fabricated by chemical conversion of already synthesised rod shaped CdV2O6 and applied for the photocatalytic reduction of
13
ACCEPTED MANUSCRIPT CO2 with water as a solvent under visible-light illumination. Methane is detected as major products for CdS and CdS/ CdV2O6 composite under visible light. The results
T
show that flower-like CdS/CdV2O6 composite not only shows higher photocatalytic
IP
efficiency but also very good stability due to the suitable band alignment and common
SC R
ion which supresses the electron-hole recombination. Moreover, large surface area of composite also provides large number of active sites that leads to higher photocatalytic activity.
NU
References
MA
1. M.A.A. Mohammed, A. Salmiaton, W.A.K.G.W. Azlina, M.S.M. Amran, A. F. Razi, Hydrogen rich gas from oil palm biomass as a potential source of
D
renewable energy in Malaysia, Renew. Sustain. Energy Review, 15 (2011)
TE
1258-1270.
2. Y. Zhou, Z. Tian, Z. Zhao, Q. Liu, J. Kou, X. Chen, J. Gao, S. Yan, Z. Zou, High-
CE P
Yield Synthesis of Ultrathin and Uniform Bi2WO6 Square Nanoplates Benefitting from Photocatalytic Reduction of CO2 into Renewable Hydrocarbon Fuel under
AC
Visible Light, ACS Appl. Mater. Interfaces, 3 (2011) 3594–3601. 3. M. Tahir and N.S. Amin, Advances in visible light responsive titanium oxidebased photocatalysts for CO2 conversion to hydrocarbon fuels,Energy convers. Manage. 76 (2013) 194-214. 4. Y. Izumi, Recent advances in the photocatalytic conversion of carbon dioxide to fuels with water and/or hydrogen using solar energy and beyond, Coord. Chem. Review, 257 (2013) 171– 186. 5. P. Usubharatana, D. Mc Martin, A. Veawab, and P. TontiwachwuthikulInd, Photocatalytic Process for CO2 Emission Reduction from Industrial Flue Gas Streams, Ind. Eng. Chem. Res. 45 (2006) 2558-2568.
14
ACCEPTED MANUSCRIPT 6. J. Chen, S. Qin, G. Song, T. Xiang, F. Xin, X. Yin, Shape-controlled solvothermal synthesis of Bi2S3 for photocatalytic reduction of CO2 to methyl formate in
T
methanol, Dalton Trans 42 (2013) 15133-15138.
IP
7. T. Mizuno, K. Adachi, K. Ohta, A. Saji, Effect of CO2 pressure on photocatalytic
SC R
reduction of CO2 using TiO2 in aqueous solutions, J. Photochem. Photobiol., A: Chem. 98 (1996) 87-90.
8. S.S. Tan, L. Zou, E. Hu, Kinetic modelling for photosynthesis of hydrogen and
MA
Today, 131 (2008) 125-129.
NU
methane through catalytic reduction of carbon dioxide with water vapour, Catal.
9. S.S. Tan, L. Zou, E. Hu, Photocatalytic reduction of carbon dioxide into gaseous
D
hydrocarbon using TiO2 pellets, Catal. Today 115 (2006) 269-273.
TE
10. G.R. Dey, A.D. Belapurkar, K. Kishore, Photo-catalytic reduction of carbon dioxide to methane using TiO2 as suspension in water, J. Photochem. Photobiol.,
CE P
A: Chem. 163 (2004) 503-508. 11. K. Teramura, T. Tanaka, H. Ishikawa, Y. Kohno, T. Funabiki, Photocatalytic
AC
Reduction of CO2 to CO in the Presence of H2 or CH4 as a Reductant over MgO J. Phys. Chem. B, 108 (2004) 346-354.
12. C.C. Lo, C.H. Hung, C.S. Yuan , J.F. Wu, Photoreduction of carbon dioxide with H2 and H2O over TiO2 and ZrO2 in a circulated photocatalytic reactor, Sol. Energy Mater. Sol. Cells, 91 (2007) 1765-1774. 13. E.E. Barton, D. M. Rampulla, A.B. Bocarsly, Selective Solar-Driven Reduction of CO2 to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell, J. Am. Chem. Soc. 13 (2008) 6342-6344.
15
ACCEPTED MANUSCRIPT 14. T. Inoue, A. Fujishima, S. Konishi, K. Honda , Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders, Nature, 277
T
(1979) 637–638
IP
15. Y. Zhou, K. Vuille, A. Heel, B. Probst, R. Kontic, G.R. Patzke, An inorganic
SC R
hydrothermal route to photocatalytically active bismuth vanadate, Appl. Catal., A: General, 375 ( 2010) 140–148.
16. D. Ke, T. Peng, L. Ma, P. Cai, K. Dai, Effects of Hydrothermal Temperature on
NU
the Microstructures of BiVO4 and Its Photocatalytic O2 Evolution Activity under
MA
Visible Light, Inorg. Chem. 48 (2009) 4685–4691. 17. J.S. Li, D.Q. Zhang, J.C. Yu, Ordered Mesoporous BiVO4 through Nanocasting:
D
A Superior Visible Light-Driven Photocatalyst,J. Chem. Mater. 20 (2008) 3983–
TE
3992.
18. G. Xi, J. Ye, Synthesis of bismuth vanadate nanoplates with exposed {001} facets
CE P
and enhanced visible-light photocatalytic properties, Chem. Commun. 46 (2010) 1893–1895.
AC
19. H.N. Luitel, R. Chand, T. Torikai, M. Yada and T. Watari , Rare Earth Free Zn3V2O8 Phosphor with Controlled Microstructure and Its Photocatalytic Activity, Int. J. Photoenergy, 2013 (2013) 1.
20. M. Oshikiri, M. Boero, J. Ye, Z. Zou, and G. Kido, Electronic structures of promising photocatalysts
InMO4 (M=V, Nb, Ta) and BiVO4 for water
decomposition in the visible wavelength region, J. Chem. Phys. 117 (2002) 7313– 7318. 21. Y.L.M.K. Zhang, Y. Chen and Y.G. Zhang, Synthesis of novel visible light responding vanadate/TiO2 heterostructure photocatalysts for application of organic pollutants, Chemical Eng. J. 175 (2011) 76– 83.
16
ACCEPTED MANUSCRIPT 22. H. Jiang, H. Dai, X. Meng, L. Zhang, J. Deng, Y, Liu, C, Tong, Au2 Hydrothermal fabrication and visible-light-driven photocatalytic properties of bismuth vanadate
T
with multiple morphologies and/or porous structures for Methyl Orange
IP
degradation,J. Environ. Sci. 24 (2012) 449–457.
SC R
23. D. Li, X. Bai, C. Pan and y. Zhu, Investigations on the Phase Transition between CdV2O6 and Cd2V2O7 and Their Photocatalytic Performances, Eur. J. Inorg. Chem. 2013 (2013) 3070–3075.
NU
24. R. Saravanan, S. Joicy, V. K. Gupta, V. Narayanan, A. Stephen, Visible light
MA
induced degradation of methylene blue using CeO2/V2O5 and CeO2/CuO catalysts, Mater. Sci. Eng. C, 33 (2013) 4725–4731.
D
25. J.G. Yu, B. Yang and B. Cheng, Noble-metal-free carbon nanotube-Cd0.1Zn0.9S
TE
composite for high visible-light photocatalytic H2- production performance, Nanoscale, 4 (2012) 2670-2677.
CE P
26. Q.J. Xiang, B. Cheng and J.G. Yu, Hierarchical porous CdS nanosheet-assembled flowers with enhanced visible-light photocatalytic H2-production performance
AC
Appl. Catal., B, 138–139 (2013) 299-303. 27. C. Wang, R. L. Thompson, J. Baltrus and C. Matranga, Visible light photoreduction of CO2 using CdSe/Pt/TiO2 heterostructured catalyst, J. Phys. Chem. Lett., 1 (2010) 48-53. 28. Q. Li, B. Guo, J. Yu, J. Ran, B. Zhang, H. Yan and J. R. Gong, Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production of CdS-ClusterDecorated Graphene Nanosheets, J. Am. Chem. Soc., 133 ( 2011) 10878-10884. 29. J. Ran, J. Yu and M. Jaroniec, Ni(OH)2 modified CdS nanorods for highly efficient visible-light-driven photocatalytic H2 generation, Green Chem., 13 (2011) 2708-2713.
17
ACCEPTED MANUSCRIPT 30. P. Praus, O. Kozak, K. Kocı, A. Panacek and R. Dvorsky, CdS nanoparticles deposited on montmorillonite: Preparation, characterization and application for
T
photoreduction of carbon dioxide, J. Colloid Interface Sci., 360 ( 2011) 574-579.
IP
31. Y.A. Gonzalez-Rivera, E. Cervantes-Juarez, L. Aquino-Meneses, R. Lozada-
SC R
Morales, S. Jimenez-Sandoval, E. Rubio-Rosas, R. Agustin-Serrano, C. de la Cerna, M. Palomino-Ovando, Photoluminescence in Er-doped V2O5 and Er-doped
NU
CdV2O6, J. Lumins. 155 (2014) 119–124.
32. D.K. Ma, M.L. Guan, S.S. Liu, Y.Q. Zhang, C.W. Zhang, Y.X. He and S.M.
MA
Huang, Controlled synthesis of olive-shaped Bi2S3/BiVO4 microspheres through a limited chemical conversion route and enhanced visible-light-responding
D
photocatalytic activity, Dalton Trans., 41 (2012) 5581.
TE
33. S. Wang, A. Pan, H. Yin, Y. He, Y. Lei, Z. Xu, B. Zou, Synthesis of PbS
CE P
microcrystals via hydrothermal method. Mater Lett 2006; 60: 1242-1246 34. X. Liang, Q. Cai, W. Xiang, Z. Chen, J. Zhong, Y. Wang, M. Shao, Z. Li, Preparation and Characterization of Flower-like Cu2SnS3 Nanostructures by
AC
Solvothermal Route, J. Mater. Sci. Technol., 29 (2013)231-236
35. R. Poulomi, S.S. Kumar, Solvothermal Growth of Flower-Like Morphology from Nanorods of Copper Sulfides, J. Nanosci. Nanotechnol. 8 (2008) 1523-1527. 36. B. Subramanian and S. Meenakshisundaram, The simple, template free synthesis of a Bi2S3–ZnO heterostructure and its superior photocatalytic activity under UVA light, J. Daltan Trans., 43 (2013) 5338-5347. 37. Z. Shen, G. Chen, Y. Yu, Q. Wang, C. Zhou, L. Hao, Y. Li, L. He and R. Mu, Sonochemistry synthesis of nanocrystals embedded in a MoO3–CdS core–shell
18
ACCEPTED MANUSCRIPT photocatalyst with enhanced hydrogen production and photodegradation, J. Matter. Chem., 22 (2012) 19646-19651.
T
38. Y. Lzumi, T. Itoi, S. Peng, K. Oka, Y. Shibata, Site Structure and Photocatalytic
IP
Role of Sulfur or Nitrogen-Doped Titanium Oxide with Uniform Mesopores under
SC R
Visible Light, J. Phys. Chem. C, 113 (2009) 6706-6718
39. X. Gao, I. E. Wachs, Investigation of Surface Structures of Supported Vanadium
Chem., B, 104 (2000) 1261-1268.
NU
Oxide Catalysts by UV−vis−NIR Diffuse Reflectance Spectroscopy, J. Phys.
MA
40. J. Yu, J. Jin, B. Chenga and M. Jaroniec, A noble metal-free reduced graphene oxide–CdS nanorod composite for the enhanced visible-light photocatalytic
D
reduction of CO2 to solar fuel, J. Mater. Chem., A, 2 (2014) 3407-3416.
Wong,
TE
41. L. Zhang, K.H. Wong, Z. Chen, J.C. Yu, J. Zhao, C. Hu, C.Y. Chan and P.K. AgBr-Ag-Bi2WO6
nanojunction
system:
A
novel
and efficient
CE P
photocatalyst with double visible-light active components, Appl. Catal. A, 363 (2009) 221-229.
AC
42. M.F. Ehsan, M.N. Ashiq, F.Bi, Y. Bi, S. Palanisamy and T. He, Preparation and characterization
of
SrTiO3–ZnTe
nanocomposites
for
the
visible-light
photoconversion of carbon dioxide to methane, RSC Adv. 4 (2014) 48411-48418.
43. M.F. Ehsan and T. He, In situ synthesis of ZnO/ZnTe common cation heterostructure and its visible-light photocatalytic reduction of CO2 into CH4, Appl. Catal., B: Environ., 166-167 (2015) 345-352. 44. I.H. Tseng, W.C. Chang and J.C.S. Wu, Photoreduction of CO2 using sol–gel derived titania and titania-supported copper catalysts, Appl. Catal. B: Environ 37 (2002) 37-48.
19
ACCEPTED MANUSCRIPT 45. V.P. Inrakanti, J.D. Kubicki and H.H. Schobert, Photoinduced activation of CO2 on Ti-based heterogeneous catalysts: Current state, chemical physics-based
T
insights and outlook, J. Energy & Environ. Sci., 2 (2009) 745-758.
IP
46. X.H. Xia, Z.J. Jia, Y. Yu, Y. Liang, Z. Wang and L.L. Ma, Preparation of multi-
SC R
walled carbon nanotube supported TiO2and its photocatalytic activity in the reduction of CO2 with H2O, Carbon, 45 (2007) 717–721.
Photocatalytic
reduction
of
NU
47. M.A. Asi, C. He, M. Su, D. Xia, L. Lin, H. Deng, Y. Xiong, R. Qiu and X.Z. Li, CO2
to
hydrocarbons
using
AgBr/TiO2
MA
nanocomposites under visible light, Catal. Today, 175 (2011) 256–263. 48. B.J. Liu, T. Torimoto, H. Yoneyama, Photocatalytic reduction of CO2 using
D
surface-modified CdS photocatalysts in organic solventsJ. Photochem. Photobiol.,
AC
CE P
TE
A: Chem. 133 (1998) 93-97.
20
ACCEPTED MANUSCRIPT Figure 1: X-ray diffraction patterns for CdV2O6 and CdS/CdV2O6 composite prepared with different amount of thiourea. Figure 2: Scanning electron microscopic images of (a) = CdV2O6 and (b-e) = CdS/CdV2O6
T
composite prepared by different amount of thiourea: (b) = 0.005 mmol, (c) = 0.01 mmol, (d)
IP
= 0.03 mmol, (e) = 0.06 mmol.
SC R
Figure 3: (a) The transmission electron microscopic image for CdS/CdV2O6 composite and (b) = the high transition transmission electron microscopic image for CdS/CdV2O6 composite.
(a) transmission electron microscopic image of CdS/CdV2O6 composite,
NU
Figure 4:
corresponding energy dispersive elemental mapping images of (b) = Cd, (c) = S, (d) = V and
MA
(e) = O
Figure 5: X-ray photoelectron spectroscopic (XPS) survey scan of CdS/CdV2O6 composite
TE
D
Figure 6: Nitrogen adsorption and desorption isotherm for CdS/CdV2O6 composite Figure 7: UV/Visible spectra of (a) = CdS/CdV2O6 composite (b) CdS and (c) CdV2O6
CE P
Figure 8: XPS Valance band spectra of (a) = CdS (b) = CdV2O6 Figure 9: Schematic energy diagram for the Photocatalytic reduction of CO2
AC
Figure 10: Yield of Methane during photocatalytic reduction of carbon dioxide under visible light by using CdV2O6 and CdS/ CdV2O6 composite as a function of time. Figure 11: Scanning electron microscopic image of CdS/CdV2O6 composite (a) = before and (b) = after photoreduction (c) = various runs for the Visible light irradiated catalytic conversion of CO2 to methane over flower like CdS/CdV2O6 composite
21
ACCEPTED MANUSCRIPT
*
0.03mmolTU
*
*
D
CE P
35
*
(003)
0.01mmolTU
0.005mmolTU
(-403)
40
45
2-theta(degree)
AC
30
TE
(201)
25
0.06mmolTU
NU
*
MA
Intensity (a.u)
* *
(107)
(240)
(033)
SC R
IP
T
Figure1
22
CdV2O6 50
55
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
SC R
IP
T
Figure 2
23
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
SC R
IP
T
Figure 3
24
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
SC R
IP
T
Figure 4
25
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
26
ACCEPTED MANUSCRIPT Figure 5
V2p
4
4x10
C1s
4
3x10
4
2x10
NU
S2p
4
1x10
600
500
400
MA
0 300
CE P
TE
D
Binding Energy(e.V)
AC
Counts/sec (C/s)
4
IP
Cd3d
6x10 5x10
T
O1s
4
SC R
4
7x10
27
200
100
ACCEPTED MANUSCRIPT Figure 6
T IP
80
SC R
60
40
NU
20
0.0
0.2
0.4
MA
0 0.6
0.8
o
CE P
TE
D
Relative Pressure (P/P )
AC
Quantity Adsorbed (cm3/g STP)
100
28
1.0
ACCEPTED MANUSCRIPT Figure 7
1.6
(a)
CdS/CdV2O6 Composite
SC R
0.8 0.6
NU
0.4
0.0 300
400
500
MA
0.2
600
CE P
TE
D
(nm)
AC
Absorbance
1.0
IP
CdV2O6
1.2
T
1.4
29
700
800
ACCEPTED MANUSCRIPT
30
(b)
25
h
1.5 1.0
T
15
IP
(eV 2 m -2 )
2.0
Absorbance
Tauc plot
20
10 5
SC R
2.5
0 -5 1.6
0.5
1.8
2.0
2.2
2.4
2.6
2.8
NU
h(eV)
0.0 400
500
600
700
800
MA
300
(C)
(eV 2 m -2 )
15
h
CE P
0.6
10
5 0
AC
Absprbance
0.8
Tauc Plot
20
1.2 1.0
25
TE
1.4
D
(nm)
0.4 0.2
-5 2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
h (e.V)
0.0 -0.2 300
400
500
600
700
(nm)
30
800
900
ACCEPTED MANUSCRIPT Figure 8
600
IP
T
(a)
500
SC R
300 200
NU
100
MA
0 -100 2
1
0
-1
-2
600
(b)
CE P
500
TE
D
Binding Energy (e.V)
400 300
AC
Intensity(a.u)
intensity (a.u)
400
200 100 0
-100 5
4
3
2
1
0
Binding Enerdy (e.V)
31
-1
-2
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
SC R
IP
T
Figure 9
32
ACCEPTED MANUSCRIPT
25
CdV2O6 Control Control Control
15
SC R
20
T
CdS CdS/CdV2O
IP
30
NU
10 5
0
2
MA
0 4
6
CE P
TE
D
time (h)
AC
CH4 (mol/g)
Figure 10
33
8
10
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
SC R
IP
T
Figure 11
34
ACCEPTED MANUSCRIPT
IP
25
T
30
(c)
20
SC R
20 15
15 10
NU
10 5
0
5
10 0
5
10 0
MA
0
CE P
TE
D
Time(h)
AC
CH4 (mol/g)
25
3rd Run
2nd Run
1st Run
30
35
5
5 0 10
ACCEPTED MANUSCRIPT
CdV2O6
Valance band position (V) Vs SHE Conduction band position (V) Vs SHE
1.51
2.38
-0.81
-0.35
2.32
2.73
AC
CE P
TE
D
MA
NU
Band Gap (eV)
IP
CdS
SC R
Compound
T
Table1: Band positions and Band gap values of CdS and CdV2O6
36
ACCEPTED MANUSCRIPT
Maximum yield
(µmol/g-h)
(µmol/g)
1.010
11.53
(visible light)
8.208
NU
0.704
CdS (Visible light)
MA
2.816
CdS/ CdV2O6
IP
CdV2O6
Rate
SC R
Product
AC
CE P
TE
D
(visible light)
37
18.368
T
Table 2: Maximum yield and Rate of production of Methane and Methanol during Photocatalytic reduction of CO2 under visible light using CdS/CeO2 composite
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
SC R
IP
T
Graphical Abstract
38
ACCEPTED MANUSCRIPT Flower-like CdS/CdV2O6 composite for visible-light photoconversion of CO2 into CH4
T
Sana Ijaz, Muhammad Fahad Ehsan, Muhammad Naeem Ashiq*, Nazia Karamat and Tao He
SC R
1.
IP
Highlights:
The fabrication of nanocomposite increases the charge transfer and minimizes the
CdS/CdV2O6 nanocomposite can be utilized as an artificial photosynthetic system for the
MA
2.
NU
recombination of the photogenerated charge carriers.
photoreduction of CO2 into CH4 under the visible light irradiations.
CE P
TE
D
The formation of composite improves the stability of the photocatalyst.
AC
3.
39
