Synthesis and Characterization of a Ternary Nanocomposite Based on CdSe Decorated Graphene-TiO₂ and its Application in the Quantitative Analysis of Alcohol with Reduction of CO₂

Article information

J. Korean Ceram. Soc.. 2018;55(4):381-391

Publication date (electronic) : 2018 June 18

doi : https://doi.org/10.4191/kcers.2018.55.4.03

Asghar Ali ^*, Md Rokon Dowla Biswas ^**, Yonrapach Areerob ^**, Dinh Cung Tien Nguyen ^**, Won-Chun Oh ^**^,

^*University of Chitral, Chitral, Khyber Pakhtunkhwa, Pakistan

^**Department of Advanced Materials Science & Engineering, Hanseo University, Seosan 31962, Korea

^†Corresponding author: Won-Chun Oh, E-mail: wc_oh@hanseo.ac.kr, Tel: +82-41-660-1337, Fax: +82-41-688-3352

Received 2018 May 1; Revised 2018 May 20; Accepted 2018 May 21.

Abstract

In this work, photocatalytic CO₂ reduction over a CdSe-graphene-TiO₂ nanocomposite has been studied. The obtained material was successfully fabricated via ultrasonic technique. The physical properties of the as-synthesized materials were characterized by some physical techniques. The TiO₂ and CdSe dispersed graphene nanocomposite showed excellent results of strong reduction rates of CO₂ compared to the results of bare TiO₂ and binary CdSe-graphene. An outstanding point of the combination of CdSe-TiO₂ and graphene appeared in the form of great photocatalytic reduction capability of CO₂. The photocatalytic activity of the asfabricated composite was tested by surveying for the photoreduction of CO₂ to alcohol under UV and visible light irradiation, and the obtained results imply that the as-prepared CdSe-graphene-TiO₂ nanocomposite is promising to become a potential candidate for the photocatalytic CO₂ reduction.

Keywords: CO₂ reduction; CdSe-graphene-TiO₂; Photocatalytic activity

1. Introduction

The decline in fossil resources and growing concerns about CO₂ emissions are now a major global challenge.1,2) Carbon dioxide (CO₂), the main and most abundant green-house gas, and CO₂ levels in atmospheric are higher than in the past decade.3,4) The increase in the level of atmospheric carbon dioxide (CO₂) caused serious concerns about global warming.5–8) Therefore, it is an urgent need to convert CO₂ into useful energy products. A converting CO₂ to useful products is not only potential way to control the greenhouse effect, but also a very convenient way of decreasing fuel consumption to solve the energy problem.9–13) Artificial photosynthesis (photocatalysis and photo electrocatalysis) is an innovative technology to convert CO₂ into valuable chemicals. 14,15) Solar fuel production by CO₂ photocatalytic conversion provides a best way for carbon cycling and fulfill the deficiency of energy demand and controlling increasing CO₂ levels.16,17) A researcher focuses on a semiconductor-based photocatalytic reduction of CO₂ with H₂O to renewable hydrocarbon fuels by solar energy.18) Among many kinds of semiconductor, titanium dioxide TiO₂ is the outstanding materials due to unique properties (e.g. cheap, nontoxic, and abundant n-type semiconductor), and most used in photocatalysis, photodegradation of organic dyes in a variety of applications.19,20) CH₃OH is a primary product of semiconductor-based material for CO₂ reduction,21,22) but the main drawbacks of TiO₂ are its large band gap (3.2 eV) and fast recombination of photogenerated charge carriers which effected the catalytic capacity.23) To decrease these barriers and enhance the photocatalytic efficiency of photocatalysts, the TiO₂ doped with either anions or captions,24–27) surface coupling with metals or semiconductors, such as CdSe,28–30) CdS31,32) and CdTe33,34) and enhancing the structure of photocatalysts to open their surface area, porosity or reactive facets. 35,36) In recent decades, the application of an ultrasound-based sonochemical process as an advanced oxidation process has gained more attention in terms of the purification of polluted effluents because of its high efficiency and an easy operation. During this process, ultrasound waves lead to a quick growth and collapse of bubbles within the solution, which results in an extremely high temperature and pressure in the bubbles. The use of ultrasound to degrade dye pollutants consumes large amounts of energy, and a complete mineralization of the organic pollutants rarely occurs from the application of sonolysis alone.37) Graphene has unique electrochemical properties such as large theoretical specific area, excellent electron mobility, and high transparency. Graphene oxide (GO) has been used to couple with various photocatalysts to enhance the efficiency of photocatalytic activity.38–48) Metal oxides coupled with graphene enhance visible light absorption and improve the efficiency of charge splitting, which is important for the CO₂ reduction of visible photocatalyst absorption.47–51) In this paper, a new CdSe-graphene-TiO₂ materials were fabricated by ultrasonication techniques. The photocatalytic CO₂ reduction was then conducted UV and visible light irradiation.

2. Experimental Procedure

2.1. Materials

Ethanol (C₂H₅OH, 95%), selenium powder (Se, 99%), sodium sulfite (Na₂SO₃·7H₂O, 95%) were purchased from Duskan Pure Chemicals Co. Ltd., Korea. Cadmium acetate dihydrate ((CH₃COO)₂Cd, 98%) was purchased from Daejung Chemicals Co. Ltd., Korea. Titanium (IV) oxide (TiO₂, anatase, and nano power, 99.7%) used as a titanium source was purchased from Sigma-Aldrich Co. (USA).

2.2. Preparation of CdSe composite

In a typical procedure, 1.5 g Na₂SO₃ and 0.3 g Se powder were added to 40 mL distilled water. After vigorous stirring for 1 h at 90°C to form the transparent dispersion, 0.05 g Cd(CH₃COO)₂ and 5 mL NH₄OH were dropped into the above dispersion and stirring continued for 1 h. At the end of that second hour of stirring, the prepared solution was sonicated for 2 h. The mixture was filtered through Whatman filter paper (Φ = 110 mm). The product was washed with distilled water 3 times and with 95% ethanol twice. After drying under a vacuum at 100°C for 8 h, the material was synthesized

2.3. Preparation of CdSe-graphene-TiO₂ composite

Separately, 300 mg graphene oxide, which was dispersed in 20 mL EG using an ultrasonication (250W) for 30 min and the mixture of ethanol : H₂O : titanium (IV) n-butoxide = 35 : 15 : 4 (by mass ratio) were added drop by drop to the above CdSe dispersion to obtain the final dispersion. The obtained dispersion was subjected to hydrothermal treatment at 100°C for 10 h and then naturally reduced to ambient temperature. The mixture was filtered through Whatman filter paper (Φ = 110 mm). The product was washed with distilled water 3 times and with 95% ethanol twice. After drying under a vacuum at 105°C for 24 h before heat treatment at 500°C for 2 h, the materials were prepared.

2.4. Characterization of sonocatalysts

An X-ray diffraction (XRD, Shimadzu XD-D1) was made using monochromatic high-intensity CuKα radiation (λ = 1.5406 Å). Nitrogen adsorption/desorption isotherm studies were investigated using a Micromeritics ASAP 2020 M+C operating at 77 K. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated according to the Barrett-Joyner-Halenda (BJH) method. The morphology, shape, structure, size, and distribution of the nanoparticles of the products were investigated using SEM (JSM-5600 JEOL, Japan) and TEM analysis. The EDS analysis was used to test the element mapping of the as-fabricated materials (attached to the SEM analysis). The XPS analysis was observed using a VG Scientific ESCALAB250 XPS system equipped with a monochromated AlKα X-ray source (hv = 1,486.6 eV) with charge compensation. A UV-vis diffuse reflectance spectra (DRS) analysis was obtained by UV-vis spectrophotometry (Neosys-2000) by using BaSO₄ as a reference at room temperature. The analysis was converted from reflection to absorbance by the Kubelka-Munk method. The Raman spectra were achieved by spectrometry (Jasco Model NRS-3100) with an excitation laser wavelength of 532.06 nm.

2.5. Photocatalytic reduction of CO₂

The photocatalytic capacity of CO₂ reduction was investigated using CdSe-graphene, CdSe-graphene-TiO₂ as a photocatalysts. In this experiment, 150 mg of the photocatalyst was dissolved in a 100 ml NaHCO₃ 0.04 M and continuously stirred, extremely high purity CO₂ was filtered through the reactor for 30 minutes to remove O₂, then the suspension solution was magnetic stirred and then irradiated to visible light by a metal halide (500W, SOLAREDGE700, Perfect Light, China). The distance between the light source and reaction vessel was maintained as 10 cm and the temperature within the reactor was maintained at 10°C by placing a continuous water loop in a jacket around the reactor. The photocatalytic efficiency was tested within 2 days and in each 12 h interval the reactor could cool down naturally for CH₃OH desorption from the catalyst. The obtained solution was centrifuged and analyzed by GC. Further the chromic acid test for alcohols was also introduced to the samples. For example, into the 10 ml sample 0.1 M CrO₃ was put and stirred for 15 min. after that centrifuge the sample and find their concentration peak with the help of UV spectrophotometer.

3. Results and Discussion

The information of crystal structure and composition of CdSe-graphene-TiO₂ via facile ultra-sonication techniques were synthesized by power XRD measurement. Fig. 1 depicts the XRD patterns of CdSe-graphene, graphene-TiO₂ and CdSe-graphene-TiO₂. Fig. 1 shows that TiO₂ is the anatase structure. In graphene-TiO₂ composite, the results of XRD diffraction peaks around 2θ of 25.48, 37.8, 48.0 and 53.7, which can be indexed to the characteristic peaks (101), (004), (200) and (105) [JCPDS PDF#00-21-1272], respectively. For characterization of crystallinity of CdSe and CdSe-graphene-TiO₂, the XRD diffraction peaks appeared at 25.3, 42.1 and 50 belongs to the diffractions of (111), (220) and (311) (JCPDS Card #77-2307). There is no graphene peak in the obtained composites due to the low content.52) XRD patterns exhibited a decrease of TiO₂ intensity peaks in CdSe-graphene-TiO₂ (2θ) at 25.1, 37.0, 37.7, 38.5, 48.0, 53.8, 55.0 and 62.6, [JCPDS PDF#00-65-2891] respectively. Moreover Fig. 1 shows that some Cd peaks is also present around 2θ of 32, 35.2 and 38.3, which can be assigned to the characteristic peaks (002), (001), and (101) [JCPDS PDF#00-21-1272].

Fig. 1

XRD pattern of CdSe, graphene-TiO₂ and CdSe-graphene-TiO₂.

The elemental microanalysis and element weight % of the ultrasonication assisted CdSe, CdSe-graphene-TiO₂ and graphene-TiO₂ nanocomposites were characterized by EDX analysis as shown in the Fig. 2(a–c). The spectra show that the C, O, Ti, Cd and Se are major elements for the CdSe-graphene-TiO₂ nanocomposites. From EDX data, C elemental peak shows evidence of graphene sheet. The Ti and O elemental peaks are the precursor material. And high diffraction Ti peaks appear at 4.54 and 4.98 keV, while Cd and Se elemental peak appear at 3.2, 3.9, 11.4 and 12.5 keV, respectively.53)

Fig. 2

EDX spectra of the (a) CdSe, (b) CdSe-graphene-TiO₂ and (c) graphene-TiO₂.

Figure 3 presented the SEM images of the CdSe, CdSe-graphene and CdSe-graphene-TiO₂ with different magnification. From the Fig. 3(a) to (b), CdSe particles and TiO₂ particles were roughly dispersed and these particles highly grow on the surface graphene. From Fig. 3(c–d), it depicted that spherical-shaped CdSe particles are anchored on the graphene surface. The plate-like structure of graphene shows the existence of oxygen functionalities on the surface of graphene. Owing to a Van der Waals interaction, graphene sheets turn to graphitic structure therefore the attachment of nanoparticles on the graphene sheets is helpful to overcome these interactions.54) Fig. 3(e–f) displays the CdSe-graphene-TiO₂ nanocomposites which clearly exhibits the difference point of the binary and ternary materials. After combination of TiO₂, the brighter spot arises in the ternary composite shown that the CdSe-graphene-TiO₂ particles were successful and of proper distribution pattern. TEM images were taken for furthermore analysis and provide the clear morphology and shape of the CdSe-graphene-TiO₂ nanocomposites. Fig. 4(a) and (b) shows the different magnification of the CdSe-graphene-TiO₂ nanocomposites. From the Fig. 4(a–b), it clearly seen that the spherical CdSe were existed with dark in color and highly agglomerated structure is anchored onto the surface of the graphene sheets. On the other hand, graphene nanosheets covered with TiO₂ particles attached unevenly to the surface of graphene sheets and almost in the circular form. The average size of the TiO₂ particles approximately is 6 to 10 nm. During ultrasonic assisted synthesis of CdSe-graphene-TiO₂, graphene sheet is prevented with partial agglomeration and TiO₂ nanoparticles are remain on the CdSe nanosheets and graphene.55)

Fig. 3

SEM images of the (a–b) CdSe, (c–d) CdSe-graphene, and (e–f) CdSe-graphene-TiO₂.

Fig. 4

TEM images of CdSe-graphene-TiO₂ (a) 50 nm and (b) 100 nm.

The chemical states of the components elements of the CdSe-graphene-TiO₂ nanocomposites were analyzed by XPS. Fig. 5(a) presented the XPS survey spectrum indicated peaks corresponding to Se, Cd, Ti, O and C components exhibited the formation of CdSe-graphene-TiO₂ nanocomposites. Fig. 5(b) has clearly shown that C 1s has a strong peak located between 285 to 290 eV. These values of binding energies have the C-C at 284.8 eV, C-O at 286 eV and C=O at ~ 289 eV corresponding to each functional group. This consequence shows that some partial oxygen functional groups contain in studied nanocomposites.56) Fig. 5(c) shows the O 1s peak located at 532.5 eV assigned to C-O and C=O groups. After the heat treatment, the O 1s region may be overlapped by other chemical species peaks.57,58) The present of C–OH group can be proven by a peak at 534.7 eV.57) Fig. 5(d) illustrates the Ti_2p peaks, which located different energy position (460.4 eV and 466.5 eV), which indicates that the Ti⁺⁴ oxidation state.59) Fig. 5(e) and (f) shows the binding energies positions of Cd_3d3/2 and Cd_3d5/2 located at 407 eV and 413 eV. And, Se_3d core level peak was confirm at 55.2 eV. The CdSe sample proves the present of Cd⁺² and Se⁻² ions.60)

Fig. 5

XPS results of the CdSe-graphene-TiO₂ nanocomposite (a) survey scan spectra (b) C_1s (c) O_1s (d) Ti_2p (e) Cd_3d and (f) Se_3d.

Further the Raman spectroscopy used for the study of structural properties of the CdSe-graphene-TiO₂. Fig. 6 provides detail about GO and nanocomposites. It has observed that the peak around the 220 cm⁻¹ waves can be assigned CdSe phase.61,62) The peaks around to 200 cm⁻¹ to 300 cm⁻¹ and 410 cm⁻¹ show the presence of the Se⁰ and anatase TiO₂.63,64) The characteristic D band appears at 1354 cm⁻¹ and G bands appear at 1590 cm⁻¹, which are corresponding to the vibration of carbon atoms in disorder or defect sites and in plane vibration of sp² and sp³ bonded carbon atoms, respectively.65) The nature of the defects can be determined from the intensity ratio of the corresponding D to G band. Moreover, the calculated I_D/I_G was found to be ~ 1.07 eV. The intensity ratio I_D/I_G ratio helps to evaluate the defects of CdSe-graphene-TiO₂ samples where a higher ratio Shown more defects. Figure 7(a–b) shows the UV-visible diffuse reflectance spectra of CdSe, TiO₂ and CdSe-graphene-TiO₂. As shown in Fig. 7, an increase of absorbance toward the visible region 550 nm was observed with the ternary compound. And, it clearly shows the homogenous distribution of the nanoparticles with increase of the absorbance. After the anchoring of CdSe on graphene, the absorption region shifts to the visible light region.66) From Fig. 8(a–b), shown that after the synthesis of a composite (TiO₂ and CdSe-TiO₂) with graphene, a threshold wavelength was extended to 405 and 797 nm and band gap energies were 3.4 and 2.6 eV estimated by using the Kubelka-Munk theory, respectively.67,68) The reason of the variation of band gap energies of as-prepared composites may possibly be introduction of CdSe and TiO₂ particles on the graphene sheet surface or partial agglomeration, which may affect the optical property of nanocomposites.69,70) The decreasing Eg of the CdSe-graphene-TiO₂ nanocomposite can be attributed to both the inherent light absorption capacity of carbon materials and the electron transitions between carbon of graphene and metal oxide (CdSe and TiO₂) phase.71,72)

Fig. 6

Raman spectra of CdSe-graphene-TiO₂ nanocomposites.

Fig. 7

Diffuse reflectance spectra (DRS) of (a) CdSe, CdSe-graphene-TiO₂ (b) TiO₂.

Fig. 8

Diffuse reflectance spectra (DRS) obtained from Kubelka-Munk transformation function versus photon energy (a) TiO₂ and (b) CdSe-graphene-TiO₂.

The photocatalytic capacity of the as-synthesized CdSe-graphene-TiO₂ material was tested by surveying the photocatalytic reduction of CO₂ under UV-visible and visible light irradiation (in the region of 300 ~ 450 nm). Using alcohol oxidation method with CrO₃ four types of sample were synthesized. Fig. 9 exhibited the photocatalytic capacity of the photocatalytic reduction of CO₂ four different samples. Fig. 9(a) shows the reduction effect of CO₂-dissolved water samples, and Fig. 9(b) is the corresponding reduction effect for commercial carbonated water without additional CO₂, showing reaction rate with different reaction conditions with photocatalyst. With increasing time, the photocatalytic efficiency increases. Fig. 9 shows that the amount of alcohol increases with the time of light irradiation.

Fig. 9

(a) Photocatalytic reduction of CO₂ with carbonated water without CO₂ gas under visible light and (b) photocatalytic reduction of CO₂ with water dissolve with CO₂ gas under UV light.

(1) CO2+CdSe-graphene-TiO2→Alcohol (Under visible light irradiation)

(2) CO2+2 e-+2H+→HCOOHAlcohol+CrO3→Oxidation of alcohol

(2′) RCH2OH (1°alcohol)+[O]→RCHO (Aldehyde)+[O]→RCOOH (Carboxylic acid)CrO3+reduction→Cr (III) ion (UV/VIS Spectra response)

To find the methanol concentration in CdSe-graphene-TiO₂ nanocomposites, CrO₃ use as an oxides agent. During experiment methanol was oxides and the result the CrO₃ oxidation form change to reduce form then the color of the sample was change. CH₃OH can follow indirectly on the CrO₃, increasing methanol concentration the color of CrO₃ were changed.

Further for conformation chromic acid test result, CdSe-graphene-TiO₂ were also evaluated by gas-chromatography (GC). Photocatalytic reaction time is up to 48 h and, after 12 h, liquid product is manually taken for GC analysis. A series of test were conducted for the confirmation of CH₃OH, 1^st control experiment was conducted in dark with catalysts, and 2^nd experiment was conducted under light radiation but no catalysts, both results shown no CH₃OH were detected. These control experiments show that both irradiation and light catalysts are important factors for the uptake of CO₂ for CH₃OH formation.

The photocatalytic efficiency of the CdSe-graphene-TiO₂ composites with different time interval was evaluated in term of photoreduction of CO₂ to CH₃OH under visible and UV light irradiation. Fig. 10(a–b) shows the effect of methanol yield. The results show that the photocatalytic CO₂ reduction of CdSe-graphene-TiO₂ under UV light were found to be higher than that visible light. The methanol yield for CdSe-graphene-TiO₂ (12 h), CdSe-graphene-TiO₂ (24 h), CdSe-graphene-TiO₂ (36 h) and CdSe-graphene-TiO₂ (28 h) under visible light are 0.12, 0.44, 0.49 and 0.69 μmolg⁻¹h⁻¹, and methanol yield for CdSe-graphene-TiO₂ (12 h), CdSe-graphene-TiO₂ (24 h), CdSe-graphene-TiO₂ (36 h) and CdSe-graphene-TiO₂ (48 h) under UV light are 0.36, 0.54,0.71 and 1.12 μmolg⁻¹h⁻¹, respectively. Graphene oxide plays the role of an electron acceptor, photosensitizer, and the adsorbent to efficiently enhance the photodegradation of organic dyes. The presence of the high density of oxygen-containing functional groups, such as hydroxyl groups, epoxy, and carboxyl at the edge or on the large surface area of graphene oxide provides the more space for ionic/electro interaction between dye molecules and the aromatic rings of graphene oxide sheets, and from that, the better absorptivity can be achieved.73–76)

Fig. 10

The methanol yield in the photo reduction of CO₂ under (a) visible light irradiation and (b) UV light irradiation using CdSe-graphene-TiO₂ nanocomposites as photocatalysts.

4. Conclusions

In conclusion, we profitably prepared CdSe-graphene-TiO₂ nanocomposites by facial ultrasonic method. SEM and TEM results indicated that the TiO₂ particles were uniformly dispersed on the surface graphene sheets supported by CdSe, TiO₂ and CdSe-TiO₂ are in anatase structure with size approximately 6 to 10 nm. The samples were tested under UV-visible and visible light irradiation to convert CO₂ to CH₃OH, the experimental results shown excellent light absorption and enhanced photocatalytic efficiency for CO₂ reduction under both UV-visible and visible light irradiation. The maximum CH₃OH yield 1.12 μmolg⁻¹h⁻¹ was achieved under UV-visible light irradiation. This work reveals that CdSe-graphene-TiO₂ could be used as a promising candidate for CO₂ reduction.

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