Synthesis of three-dimensional hierarchical CuOflower-like architecture and its photocatalytic activity for rhodamine b degradation

Sambandam Anandan, Muthupandian Ashokkumar, Photocatalytic properties of hierarchical CuO nanosheets synthesized by a solution phase method, J. 69 (2018) (2017) 115e124[r]

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Original Article

Synthesis of three-dimensional hierarchical CuO ﬂower-like

architecture and its photocatalytic activity for rhodamine b degradation

N Phutanona, P Pisitsaka, H Manuspiyab, S Ummartyotina,*

aMaterials and Textile Technology, Faculty of Science and Technology, Thammasat University, Patumtani, Thailand

bThe Petroleum and Petrochemical College, Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Bangkok, 10330,

Thailand

a r t i c l e i n f o

Article history: Received 12 April 2018 Received in revised form 12 May 2018

Accepted 16 May 2018 Available online 23 May 2018 Keywords:

CuO Self-assembly Photocatalyst Rhodamine b

a b s t r a c t

The ﬂower-like CuO materials with a good uniformity were successfully synthesized by the self-assembly method By using pH ranging from to 9, CuO provided different morphologies X-ray diffraction

̣

(XRD) and Scanning electron microscopy (SEM) revealed the purity and uniformity of the CuO particles, respectively The particles showﬂower-like structures composed of CuO nano-sheets Trans-mission electron microscopy (TEM) conﬁrmed the uniformity of nanosheet-like CuO particles with lattice dimensions of 0.2e0.4 nm In a preliminary experiment, the rhodamine b degradation was observed by using CuO as a photocatalyst These semiconducting particles were found to enhance the degradation of the azo dye within 240 It was remarkable to note that as-synthesized CuO particles from the self-assembly method provided a good uniformity in morphology It also exhibited good and suitable properties to serve as a photocatalyst for rhodamine b degradation

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

In thefield of inorganic synthesis, the control of morphological properties with well-defined shapes was considered as an impor-tant target The performance of well-defined architecture of inor-ganic materials is of extraordinary importance because the electronic structure, bonding, surface energy as well as chemical reactivity are directly related to the surface morphology Therefore, numerous approaches have been developed on the methodology of inorganic synthesis, such as solegel technology, hydrothermal, one pot synthesis and precipitation technique [1e3] Although these techniques provided excellent features, such as high purity, high specific surface area and fast processing time, the uniformity on morphological properties was still a troublesome issue if the target inorganic material will be applied in any engineering application The non-uniformity of the inorganic material may include defects and it subsequently reduces performance In order to overcome this issue, novel inorganic synthesis was focused on the colloidal crystal

self-assembly approach From the fundamental point of view, this technique was considered as a common term of the modern sci-entific community to describe the spontaneous aggregation of particles such as atoms, molecules and micelles without the in-fluence of an external force The formation of particles preceeded based on the thermodynamically stable concept and structurally well-defined arrays[4] Up to the present time, this technique gains much interests for the development of nano-scale materials such as nanowires, nanosheets, nanocubes, nanoflowers, nanopods and nanospheres[5]

Among various types of inorganic materials, CuO, as an impor-tant p-type semiconductor with a narrow bandgap (1.4 eV), has been widely employed in many engineering sectors such as superconducting materials and devices, solar energy conversion, magnetic storage media as well as gas sensors[6e9] Due to the importance of CuO, many research groups have been devoting to develop CuO with controllable surface architectures In 2010, Xia et al.[10]investigated the role of the ionic liquid for the hydrothermal synthesis of three-dimensionally hierarchical CuO particles After that, shape-controlled synthesis of CuO by the microwave tech-nique was succeeded by Guo et al.[11] In order to extend on the utilization of CuO, it was electrodeposited on Si substrate and combined with graphene e TiO2 composite as reported by

* Corresponding author Fax: ỵ6625644458

E-mail address:sarute.ummartyotin@gmail.com(S Ummartyotin) Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.05.001

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Chahrour et al.[12]and Nguyen et al.[13], respectively Recently, in 2017, Qi et al.[14]investigated the numerical method for the pre-diction of the growth mechanism of CuO It is known that the density functional theory calculation can be employed to predict the facets of CuO crystals

In this paper, we present the synthesis of CuO crystal with the controlled morphology via the self-assembly method by adding ammonia into the reaction solution With the different pH grades of solution, CuO crystals exhibited various types of morphology The related physicoechemical properties of CuO were presented Pre-liminary investigations on the photocatalytic activity of the as-prepared materials were then reported

2 Experimental 2.1 Chemical reagents

Ammonia (30% purity) was purchased from the Application GmbH-An ITW Company, Germany Copper chloride was purchased from KEMAUS They were employed as chemical reagents Rhoda-mine b and hydrogen peroxide were purchased from Sigma Aldrich, Co, Ltd Analytical grade of ethanol was purchased from Italmar Co LTD., Thailand They were employed as organic ligands and solvent, respectively All of chemical reagents were used as received without further puriﬁcation

2.2 Methods

2.2.1 Synthesis of theﬂower-like by self-assembly method

An amount of 0.1 M copper chloride was dissolved in the analytical grade ethanol The reaction was stirredly promoted for at room temperature After that, ammonia solution was gently (gradually) dropped into the copper chloride ethanol solu-tion The pH grade of the solution was controlled by the amount of ammonia solution for pH¼ 7, 8, and 10, respectively The solution was heated at 90C for 14 h and then it was centrifuged It was washed several times in order to ensure the homogeneity and purity After that, it wasﬁltered and kept in vacuum in order to avoid the moisture absorbent The schematic diagram of the copper oxide formation is presented inFig

2.2.2 Photocatalytic activity of theﬂower-like CuO material The photocatalytic activity of the CuO as synthesized from the self-assembly technique was evaluated by measuring the photo-catalytic degradation of rhodamine b in the de-ionized water under UV light illumination The concentration of rhodamine b was set to be ppm In the degradation experiment, rhodamine b in an aqueous solution was continuously stirred in the dark for 30 to ensure adsorption and desorption equilibrium An amount of 0.1 ml of hydrogen peroxide was added Rhodamine b was used as the initial concentration for the photo decomposition process UV-Vis spectroscopy was performed to monitor the adsorption change of rhodamine b For this, 0.025 g of as-synthesized CuO from the self-assembly method was added to 100 ml of rhodamine b solution

2.3 Instrumental characterization 2.3.1 Scanning electron microscopy

The sample was investigated by SEM using the JOEL JSM-6301F scanning microscope The SEM was operated at an acceleration voltage of 15.0 kV at a working distance of 15 mm to identify the morphological properties of the powders Before investigation, the samples were sputter-coated with Au to enhance the electrical conductivity A magniﬁcation of 50KX was throughout used for SEM experiments

2.3.2 X-ray diffraction

The crystal structure of the powder was investigated by X-ray diffraction (XRD), using the Phillips P.W 1830 diffractometer sys-tem, with nickel-ﬁltered CuKaradiation The diffraction patterns were recorded over a range of 2qfrom 20 to 80 The step was set to be 2/min

2.3.3 Fourier transform infrared spectroscopy

FTIR spectra were recorded using a Fourier transform infrared spectrometer of PerkinElmer (USA) Spectra were measured at room temperature in the spectral range from 4000 to 400 cm1 with a resolution of±4 cm1and a scan frequency of 32 times.

2.3.4 Transmission electron microscopy

Morphological properties of the as-synthesized powders were investigated by TEM Samples were prepared by the ultrasonic dispersion in ethanol for 180 The product was then dropped onto a molybdenum grid and dried in air prior to the TEM investi-gation The voltage was set to be 200 kV The magniﬁcationwas set to be 60000 and 500000, respectively

2.3.5 UV-vis spectroscopy

A Varian Cary 5000 UV-Vis NIR spectrophotometer (Agilent Technologies, CA, USA), equipped with a transmittance accessory, was employed to record the electronic spectrum of the samples in the wavelength range of 400e700 nm This technique allows to study the absorbance spectra of the samples The transmittance accessory consists of an 110 nm diameter integrating sphere and an

Fig Schematic diagram of CuOﬂower-like material formation

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in-built high-performance photomultiplier Each sample was placed in a sample cell, which was speciﬁcally designed for the instrument A baseline was recorded and calibrated using a polytetra ﬂuoro-ethylene (PTFE) reference cell

3 Results and discussion

3.1 Synthesis and characterization of CuO from the self-assembly method

The nano scale CuOﬂower-like was successfully synthesized by the self-assembly method following the procedure described above The mechanism for the morphological formation of CuO

can be described based on a solution phase decomposition of the complex structure of [Cu (NH3)4]2ỵ It was important to note that

the square-planar amino complex [Cu (NH3)4]2ỵwas formed when

ammonia solution was gradually dropped to the precursor solu-tion as suggested by Ni et al [15] The solution of the copper complex [Cu (NH3)4]2ỵ ion was formed based on the following

reaction

Cu2ỵỵ 4NH3/ [Cu (NH3)4]2ỵ

It is remarkable to note that the square planar structure of the [Cu (NH3)4]2ỵis preferable for the 2D plate-like morphology With

water as solvent, Cu(OH)2 is precipitated based on the following

reactions:

NH3ỵ H2O/ NH4ỵỵ OH

[Cu (NH3)4]2ỵỵ OH/ Cu(OH)2ỵ 4NH3

From the structural point of view, Cu(OH)2is considered as a

layered material and it can be formed in 2D nanostructures based on the thermodynamic equilibrium With the coordinative self-assembly of [Cu (NH3)4]2ỵ, Cu(OH)2 can be formed by a

nucle-ation process It is notable that [Cu (NH3)4]2ỵcan transport the Cu2ỵ

ion to the ligand where the OHion is attached With the incre-ment on the reaction time, the release of NH3 is in form of gas

bubbles This is strongly associated with the previous work re-ported by Wen et al.[16] Formation of CuO occurs by the decom-position of Cu(OH)2based on the following equation

Cu(OH)2/ CuO ỵ H2O

Fig SEM micrographs of CuOﬂower-like at various pH range (a) pH ¼ (b) pH ¼ (c) pH ¼ (d) pH ¼ 10 Fig XRD patterns of the as-synthesizedﬂower-like CuO powerders at various pH

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Fig 2exhibits the FTIR spectra of the CuO structure All samples show the similar characteristic peaks The main adsorption peaks at 3400 cm1and 1400 cm1are assigned to the O-H stretching and bending modes of water [17] The sharp adsorption peaks at 600 cm1 and 500 cm1 are due to the formation of the metal oxygen (Cu-O) bonds It is important to note that no impurity peaks and those of unreacted starting materials were observed Base on the FTIR investigation, it could be concluded that the pure CuO phase with monoclinic structure was successfully synthesized This is in good consistency with a previous article reported by Rao et al [18]

Fig 3presents the XRD patterns of the as-synthesized ﬂower-like CuO structures As can be seen, all investigated powders show a predominant crystalline structure of the monoclinic phase CuO (space group C2/c) The predominant peaks at 2qof 33, 40, 49, 54, 58 and 68, which represent the crystal plane indices of (110), (111), (202), (020), (202) and (113), respectively, corresponded to the monoclinic phase of CuO There are no other impurities found

within the detection limit of the instrument All diffraction peaks are indexed in good agreement with the standard pattern of JCPDF No 48-1548 However, the structure corresponding peaks are slightly shifted due to the possible adsorption of water on the surface of the powder particles, as suggested by Li et al in Ref.[19] Furthermore, it should be noted that the crystallinity of the as-synthesized CuO powder was still low compared to the calcined CuO powder as suggested by Gupta et al.[20] Moreover, from the observed diffraction peaks, the preferential orientation was deter-mined using a texture coefﬁcient of (hkl) This result illustrates that the highest intensity from the XRD measurements was in the (111) plane for all the samples, indicating that the crystal orien-tation was uniform in the x and z directions The crystal size was estimated for the (111) peak using the Scherrer formula D¼ Kl/

bcosq, where D is crystallite size, K is 0.9,lis the X-ray wavelength,

bis the full width at half maximum (FWHM) andqis the angle associated to the considered diffraction peak The statistical average was estimated to be 40e50 nm, respectively

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However, it is important to note that at pH¼ 10, there is still an impurity phase existing in the as-synthesized powder So, the nucleation growth of CuO powder in this condition did not ef ﬁ-ciently proceed

Fig 4presents a typical SEM image of the CuO samples with a variation of the pH values ranging from to 10 It clearly shows that the hierarchical CuO nanosheets of different sizes are composed of aggregating nanorods The size of the sheet was estimated to be 100 nm It is important to note that the nanosheet is formed by the accumulation of the CuO nuclei It is clearly seen that the edge of the nanosheets is very sharp and thick with respect to the incre-ment of the pH values The CuO formation processes start with the nucleation followed then by the aggregation and the self-assembly to form larger particles The morphology of particle was called ﬂower-like CuO 3D architecture Morphological properties of CuO can be controlled by pH of solution as suggested by Gencyilmaz et al.[21]

Fig 5presents the typical TEM images of the CuO samples with pH values ranging from to 10 It was found that the morphological properties of CuO are considered as a secondary structure from the self-assembly of many individual nanosheets The mechanism of aggregating the nanosheets into theﬂower-like shapes is success-fully realized by the self-assembly method It can be explained that the self-assembly of the CuO nanosheets into two- and three-dimensional (2D and 3D) superlattices requires a controlled size distribution and an attractive force, such as the van der Waals

attraction and the balance interaction as suggested by Zheng et al [22] It is important to note that during the self-assembly process, the primary nanosheets are drawn toward each other by the attractive force, and then a stable superlattice is formed under the balance of the steric interactions among the CuO nanosheets

Fig 6shows the high resolution TEM (HRTEM) images of as-synthesized ﬂower-like CuO materials The lattice spacing was found ranging from 0.2 to 0.4 nm, which corresponds to the spacing between the [110] crystal planes

3.2 Preliminary experiment on photocatalytic activity of CuO from self-assembly method

The photocatalytic experiment of the CuO powders as synthe-sized by the self-assembly method was investigated based on the degradation of rhodamine b The photocatalytic experiment was set up for studying the photocatalytic activity of the CuO particles prepared with pH¼ 7, and 9, respectively At pH ¼ 10, the particle was insufﬁcient for the experiment because only a too small amount of as-synthesized material was yielded.Fig 7presents the change in the UV-Vis spectra of the rhodamine b upon the irradi-ation with artiﬁcial UV light Noticeable changes are observed in the absorbance spectra recorded at different time intervals under UV light irradiation (200e700 nm) From the structural point of view, the chemical structure of the rhodamine b has a considerable effect on the photodecolorization yield The azo group N¼N is susceptible

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to the photodecolorization Also, rhodamine b which has sulfonic groups, exhibits a high adsorption yield, and the additional pres-ence of N¼N increases the reactivity of rhodamine b This is in agreement with literature previously reported by Aljamali et al [23] Moreover, it was observed that the degradation occurred upon the addition of CuO into the rhodamine b solution At t¼ min, the curve represents the absorption spectra of the rhodamine b solu-tion without CuO particles, Upon the addisolu-tion of CuO particles, the decrease in the intensity of the adsorption band at 553 nm occurs It indicates that the absorption of rhodamine b molecule by CuO particle and the possible cleavage of the azo bond as the chromo-phoric group, which cause the decolorization of the solution This phenomenon is a common trend for all CuO particles synthesized under different pH conditions

Fig 8shows the degradation efﬁciencies of the rhodamine b solution in the presence of CuO particles as the photocatalysts A decrease in the concentration of the rhodamine b is observed in a time period of 240 The normalized temporal concentration change in (C/C0) of rhodamine b during the photocatalytic process

seems to be proportional to the normalized maximum absorbance

(A/A0), which can be theoretically derived from the change in the

rhodamine b absorption proﬁle at a given time interval It is also obvious that the kinetics of the rhodamine b degradation can be obtained by taking the reduction of the rhodamine b concentration as a function of the reaction time At the low CuO concentration, the degradation rate is lower, whereas as the CuO concentration in-creases, the degradation rate drastically increases The variation in the relative concentration (C/C0) of the rhodamine b solution is so

clearly emphasized However, as shown inFig 8, the observed data has obviously only a little variation among all CuO samples syn-thesized with different pH values Moreover, it is noted that the enhancement of the photocatalytic performance should be ascribed to the increase in the visible light absorption Furthermore, there is a photocatalytic degradation at the oxygen sites The oxygen va-cancies in the CuO can act as new active sites to reduce O2and form

superoxide radicals, which are thought being responsible for the photocatalytic degradation of the organic pollutants as suggested by Sha et al [24] From the fundamental point of view, CuO is considered as a semiconductor for the photomineralization, which is characterized by aﬁlled valence band and an empty conduction band When CuO is irradiated with UV light, one electron (e) will be excited from the valence band to the conduction band leaving a hole in there The concentration of the electronehole pairs in CuO particles, thus, depends on the intensity of the incident UV light and also on the electronic characteristics of the materials In the lack of the appropriate electronehole scavengers, the photo-generated electronehole pairs may recombine and scatter the input energy as heat within a few minutes Therefore, for a

Fig Time-dependent UV-Vis absorption measurements of rhodamine b degradation of CuO samples with various pH values: (a) pH¼ 7; (b) pH ¼ 8; and (C) pH ¼

Fig Kinetic study of the rhodamine b solution with CuO as a photocatalyst

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photocatalytic process to be efﬁcient, the electron must be promptly removed by an electron acceptor, as suggested by Sasikala et al.[25]

The photocatalytic degradation of rhodamine b is a pseudo first-order reaction and its kinetics is found tofit the expression given in Fig 9: The linear shape of the lnC versus t (in min) graph confirms the pseudo first-order reaction for the rhodamine b degradation The rate constants and the linear regression co-efficients for the degradation of rhodamine b are estimated to be 0.002e0.007 min1 The K value from the linear regression can be an indication of the degradation ability of the organic pollutant Conclusion

The ﬂower-like CuO particles have been successfully synthe-sized by the self-assembly method By using different pH values ranging from to for the synthesis, the obtained CuO materials show different morphologies The as-synthesized CuO powders are in good purity and show good uniformity in size as revealed by X-ray diffraction and Scanning electron microscopy analysis Transmission electron microscopic investigation also revealed the lattice dimension of 0.2e0.4 nm for the as-prepared CuO nano-sheets From the UV-Vis spectra, the adsorption of rhodamine b was also found to occur at the exciting wavelength of 553 nm With the increment of time, the decrease of adsorption intensity was observed It noteworthy that the CuO particles synthesized by the self-assembly method can be used as a photocatalyst for the rhodamine b decomposition

Acknowledgments

The authors would like to acknowledge theﬁnancial support provided by The Faculty of Science and Technology, Thammasat University (grant number 2018)

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