graphene spheres cuo nanoflowers composites for use as a high performance photocatalyst

Nanomaterials and Nanotechnology Graphene Spheres-CuO Nanoflowers Composites for Use as a High Performance Photocatalyst Regular Paper Bin Zeng1* and Hui Long2 1 College of Mechanical En

Nanomaterials and Nanotechnology

Graphene Spheres-CuO Nanoflowers

Composites for Use as a High Performance Photocatalyst

Regular Paper

Bin Zeng1* and Hui Long2

1 College of Mechanical Engineering, Hunan University of Arts and Science, Changde, P.R China

2 Department of Applied Physics and Materials Research Center, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P.R China

*Corresponding author(s) E-mail: 21467855@qq.com

Received 30 November 2015; Accepted 22 February 2016

DOI: 10.5772/62634

© 2016 Author(s) Licensee InTech This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the

original work is properly cited

Abstract

Graphene spheres decorated with CuO nanoflowers were

successfully synthesized via a spray-drying method

Scanning-electron and transmission-electron microscopy

were used to confirm that the graphene formed spherical

structures and that the CuO nanoflowers were well

dispersed on the surface of these graphene spheres These

novel nanocomposites had an enhanced photocatalytic

performance, achieving a 95.2% decomposition of methyl

orange after 15 min in the presence of H2O2 when irradiated

by visible light These nanocomposites performed much

better than CuO powders alone

Keywords Graphene, Spray Drying, Photocatalytic

Performance

1 Introduction

Morphology is known to influence the photocatalytic

performances of semiconductor oxides [1] Structures with

nanoflower morphologies have been shown to have unique

properties that differ from those of mono-morphological

structures, because of the combined benefits of their nanoscale building blocks Materials with nanoflower structures show great potential for applications in photo‐ catalysis [2] However, the high recombination rate of photoinduced electron-hole pairs in semiconductor oxides

is the main limitation of their photocatalytic activities [3] Graphene, which is a flat monolayer of carbon atoms, has attracted widespread attention because of its large surface area and unique electronic properties [4] To date, numer‐ ous studies have demonstrated that excellent photocata‐ lytic performances can be achieved by assembling nanomaterials on graphene sheets, because of graphene’s high electron transfer efficiency [5] However, graphene tends to restack through van der Waals interactions, effectively eliminating its outstanding single-layer electri‐ cal properties [6] Spray-drying techniques are preferred for processing graphene, because these techniques are fast and can be used in continuous processes [7] Because solvent drops evaporate rapidly in the hot air streams used

in this process, individual graphene sheets remain dis‐ persed in the water and can form spherical graphene structures with decreased graphene aggregation [7, 8] In previous work, graphene spheres were decorated with urchin-like CuxO (x = 1 or 2) for use as high-performance

1 Nanomater Nanotechnol, 2016, 6:21 | doi: 10.5772/62634

photocatalysts, in which the graphene spheres acted as

co-catalysts to promote the separation and transfer of

photo-generated electrons [8] For the integration of nanoscale

building blocks, CuO nanoflowers were anchored onto the

graphene spheres to generate photocatalysts However, to

the best of our knowledge, such nanomaterials have not

previously been studied

In this work, graphene spheres decorated with CuO

nanoflowers were successfully synthesized using a

spray-drying method These novel composites combined the

merits of both of their components The graphene spheres

acted as a conductive substrate, while the CuO nanoflowers

provided more catalytically active sites Additionally, the

approach presented in this work was simple and highly

efficient for the generation of nanoparticles dispersed on

graphene, as compared with solution-phase methods

Experimental results revealed that these novel nanomate‐

rials exhibited excellent visible-light photocatalytic activi‐

ties for the degradation of the dye methyl orange (MO)

2 Experimental Details

Graphene oxide (GO) was prepared using the Hummers’

method [9] Graphene/CuO (GR-CuO) was prepared by

adding 50 mL of 20 g L−1 Cu(OAC)2 (~98.5%, Aladdin) to a

GO solution, which was then ultrasonicated for 30 min

Then, 25 mL of 0.2 g mL−1 polyvinylpyrrolidone (PVP) was

added to this solution After 30 min of ultrasonication, this

blended suspension was spray dried at 200 °C, producing

a blue powder This powder was then calcinated at 220 °C

for 30 min to thermally decompose the Cu(OAC)2 and

sintered at 800 °C for 1 h to reduce the GO to graphene and

decompose the PVP under an Ar atmosphere in a tube

furnace The resulting powder was then allowed to cool to

300 °C before being held in air for 1 h to oxidize the

decomposed Cu(OAC)2 Products were produced with

different GR to CuO weight ratios, which are referred to

here as CuO, 1wt.%GR-CuO, 2wt.%GR-CuO and 5wt

%GR-CuO

The as-prepared products were characterized using

scanning electron microscopy (SEM, S4800), transmission

electron microscopy (TEM, JEM-2100F), powder X-ray

diffraction (XRD, D5000), Raman spectroscopy (J-Y, T6400)

and X-ray photoelectron spectroscopy (XPS, K-Alpha

1063), and total organic carbon (TOC) was monitored with

a TOC analyser (Apollo 9000, Terkmar-Dohrmann, USA)

Electrochemical impedance spectroscopy (EIS) was

performed with a CHI660B electrochemical workstation

EIS measurements were carried out in 1 M Na2SO4 using a

three-electrode system, which consisted of a platinum foil

electrode as the counter electrode and a saturated calomel

electrode (SCE) as the reference electrode EIS measure‐

ments were recorded at 0.5 V with an alternating current

(AC) voltage amplitude of 5 mV over the frequency range

of 1 MHz to 5 mHz

The Brunauer-Emmett-Teller (BET) specific surface areas

and porosities of the samples were evaluated on the basis

of nitrogen adsorption isotherms, which were measured at -196 °C using a gas adsorption apparatus (ASAP 2020, Micromeritics, USA)

The photocatalytic degradation of MO was measured at an ambient temperature in the presence of H2O2 The as-prepared products (20 mg) were dispersed in 300 mL of a

20 mg L-1 MO solution to which 2 mL of H2O2 had been added It was placed beside the beaker at a distance of 20

cm After stirring for 30 min in the dark, a 500 W Xe arc lamp was used to irradiate the sample Samples of this mixture (5 mL) were withdrawn every 3 min and immedi‐ ately centrifuged to separate the suspended solids The absorbance of the supernatant was measured at 464 nm using ultraviolet-visible (UV-vis) spectrophotometry

3 Results and Discussion

The XRD patterns of graphene, CuO and 2wt.%GR-CuO are shown in Fig 1a The XRD pattern of graphene had a major peak at 2θ = 25° The as-prepared CuO and 2wt.%GR-CuO nanocomposites produced similar XRD patterns, and their diffraction peaks at 32.5°, 35.5°, 38.7°, 48.8°, 53.3°, 58.4°, 61.5°, 66.3°, 68.0° and 75.1° corresponded to the (110), (002), (111), (-202), (020), (202), (-113), (-311), (113) and (-222) crystal planes of CuO, respectively (JCPDS No 41-0254) However, 2wt.%GR-CuO had an additional peak at 25°, which corresponded to graphene, and a peak at 42.5°, which corresponded to Cu2O (JCPDS No 78-2076) These results suggested that a small amount of CuO nanoparticles were reduced to Cu2O by graphene during the thermal treatment [8]

Raman spectroscopy is an important tool for the character‐ ization of carbon-based materials As shown in Fig 1b, GO displayed a Raman shift in the D (1323 cm−1) and G (1592

cm−1) bands, arising from the disruption of GO’s symmet‐ rical hexagonal graphitic lattice and the in-plane stretching motion of symmetrical sp2 C–C bonds, respectively The D/

G intensity ratio for GO was 1.08 The GR-CuO nanocom‐ posites had Raman peaks in similar positions However, the D/G intensity ratios of 1wt%.GR-CuO, 2wt.%GR-CuO and 5wt.%GR-CuO increased to 1.18, 1.39 and 1.31, respectively These increased D/G ratios indicated a partial reduction of GO to graphene [10], which was also visually observed by the colour change of the product from brown‐ ish for GO to black for graphene

The electronic state of the final 2wt.%GR-CuO product was characterized using XPS According to Fig 1c, peaks associated with Cu 2p, O 1s and C 1s were observed for the 2wt.%GR-CuO nanocomposite Fig 1d shows the Cu 2p spectra of products The Cu2p spectrum exhibited a Cu 2p1/2 peak at 954.08 eV and a Cu 2p3/2 peak at 933.78 eV, which are characteristics of Cu2+ in CuO Strong shake-up satellite peaks were also observed at 942.33 eV with an overlapping series at 962.38 eV, further confirming the presence of Cu(II) on the surface [11] These observations suggested that 2wt.%-CuO nanocomposites were success‐ fully synthesized

EIS is a powerful diagnostic technique for the investigation

of new materials and electrodes As shown in Fig 1e, the

impedance plots of the GR-CuO nanocomposites had

smaller radii than those of the CuO electrode, indicating

that the high conductivity of graphene decreased the

resistance of the GR-CuO nanocomposites These de‐

creased resistances facilitated electron transfer between

CuO and graphene Ultimately, this substantial decrease in

charge-transfer resistance effectively prevented the

recombination of electrons and holes in the nanocompo‐

sites

Fig 1f shows the nitrogen adsorption-desorption iso‐

therms’ curves The BET surface areas of the CuO, 1wt

%GR-CuO, 2wt.%GR-CuO and 5wt.%GR-CuO were 35.6

m2.g-1, 57.4 m2.g-1, 61.2 m2.g-1 and 73.8 m2.g-1, which were

much larger than those of the pure CuO nanoparticles

(23.1m2.g-1) [12] and the graphene/CuO nanocomposites

(53.2 m2.g-1) [13] These high BET surface areas resulted

from the large surface areas of the CuO nanorods that made

up the CuO nanoflowers, and likely provided more

photocatalytic reaction centres

Figure 1 (a) XRD pattern, (b) Raman spectra, (c) Survey spectra, (d) Cu2p

region XPS spectrum, (e) EIS spectra, (f) Nitrogen adsorption-desorption

isotherm of the as-synthesized products

Fig 2 shows the morphologies of the samples and the effect

of graphene on the samples’ microscopic structures The

2wt.%GR-CuO sample consisted of many monodispersed

graphene spheres with rough surfaces (Fig 2a) According

to the image in Fig 2b, uniform and discrete nanoparticles

covered the surfaces of the graphene spheres without

forming aggregates, indicating that a strong link was

formed between the nanoparticles and the graphene

spheres TEM (Fig 2c) indicated that the graphene was densely decorated with many small nanoparticles A higher resolution image of these nanoparticles (Fig 2d) revealed that these nanoparticles had flower-like structures and that each of the flower-like structures was composed

of multiple smaller nanorods The 1wt.%GR-CuO and 5wt

%GR-CuO samples were also composed of similar gra‐ phene spheres However, the 1wt.%GR-CuO contained more nanoparticles, which were spread uniformly on some surfaces of the graphene spheres (Fig 2e, f) while other large areas lacked nanoparticles and resembled crumpled silk veil waves (Fig 2g, h) In general, spray drying successfully produced composites of graphene spheres with nanoparticles on their surfaces

Figure 2 (a, b) SEM (c, d) TEM image of 2wt.%GR-CuO, (e, f) SEM image of

1wt.%GR-CuO, (g, h) SEM image of 5wt.%GR-CuO

To determine the effect of PVP on the formation of these nanocomposites, experiments were performed using the same chemical reagents in the absence and presence of PVP The products produced in the absence of PVP are shown in Fig 3a, b While graphene still formed spherical structures without PVP, the nanoparticles became aggregated and formed large particles With PVP added to the synthesis solution, only CuO nanoflowers were formed and almost

no large CuO aggregates were observed, as shown in Fig 2d Clearly, PVP played an important role in the successful formation of CuO nanoflowers

3 Bin Zeng and Hui Long:

Figure 3 (a) SEM (b) TEM of 2wt.%GR-CuO without PVP

As a demonstration of a potential application, the photo‐

catalytic performance of GR-CuO was evaluated for the

degradation of MO, which was chosen as a representative

organic dye The degradation ratio was defined as Ce/C0

where C0 and Ce were the initial concentration of MO and

the concentration of MO at a given time, respectively Fig

4a shows MO solutions after their adsorption-desorption

equilibria were reached in the dark in the presence of the

different catalysts The images revealed that the abilities of

the catalysts to adsorb MO were limited (less than 20%)

However, H2O2 alone degraded MO only a very little when

irradiated with visible light, as shown in Fig 4b Only the

combined use of H2O2 and the catalysts produced a

significant improvement in degradation efficiency (previ‐

ous research [7] has shown that CuO catalysts without

H2O2 under visible light produces little degradation of

MO) As shown in Fig 4c, the CuO, 1wt.%GR-CuO, 2wt

%GR-CuO and 5wt.%GR-CuO samples degraded by

approximately 74.3%, 90.3%, 95.2% and 78.8% of MO after

15 min of visible-light irradiation in the presence of H2O2

Clearly, 2wt.%GR-CuO had made the best use of visible

light Fig 4d shows typical time-dependent UV-vis

absorption spectra of MO solutions during photodegrada‐

tion in the presence of 2wt.%GR-CuO Over time, the

absorption peak at 464 nm, which corresponded to the

presence of MO, decreased and nearly disappeared after 15

min The inset in Fig 4d shows a significant decrease of the

initial TOC content in the solution after 15 min, further

proving the cleavage of the MO dye molecules These

results indicated that MO was successfully degraded in the

presence of 2wt.%GR-CuO under visible-light irradiation

The superior photocatalytic performances of the GR-CuO

nanocomposites may have resulted from the phenomena

described in the following First, the substantially en‐

hanced specific surface areas of the graphene spheres

provided an ideal substrate for the loading of nanoparticles

without forming aggregates [14] The graphene spheres

were also highly electronically conductive and acted as

electron transporters, allowing the photo-excited electrons

in the composites to be quickly transferred from CuO to

GR, and eventually enabling the enhanced photocatalytic

activities of the composites [15] It should be noted that the

graphene content influenced the extent to which the MO

degradation reaction was enhanced The photocatalytic

activities of the GR-CuO systems were enhanced when the

amount of graphene added was increased This behaviour

was ascribed to graphene’s ability to effectively inhibit the

recombination of photo-excited electron-hole pairs in the nanocomposites However, an increase in the nanocompo‐ sites’ graphene contents above 2wt% decreased the transfer

of electrons from the excitation state of the CuO nanoflow‐ ers, lessening the photocatalytic activity of these materials Finally, the CuO nanoflowers consisted of nanorods that possessed large specific surface areas and provided more photocatalytically active reaction sites This increase in the reactive surface area likely enhanced the photocatalytic performances of the GR-CuO nanocomposites

Figure 4 Photocatalytic degradation efficiency of MO (a) with catalyst and

without H 2 O 2 in the dark, (b) with H 2 O 2 and without catalyst under visible light, (c) different catalysts with H 2 O 2 under visible light, (d) UV-vis absorption spectra of MO solution with 2wt.%GR-CuO at different time intervals (inset figure in (d) shows the change of TOC with time)

The proposed photodegradation mechanism of the GR-CuO nanocomposites is shown in Scheme 1 When irradi‐ ated by visible light, valence band (VB) electrons in the CuO nanoflowers were excited in the conduction band (CB) However, these charge carriers easily recombined, leading

to a low photocatalytic performance When the nanoparti‐ cles were tightly anchored to the surfaces of the graphene spheres, the graphene served as an electron transporter, which efficiently transported those photo-generated electrons and improved electron-hole separation Simulta‐ neously, active radicals (such as ∙OH) generated from the reduction of H2O2 by photo-generated electrons directly decomposed MO into CO2 and H2O

Sheme 1 Proposed scheme of photodegradation mechanism

4 Conclusions

In this study, CuO nanoflowers anchored to graphene

spheres were prepared using a spray-drying method The

photocatalytic performances of the GR-CuO nanocompo‐

sites were increased considerably compared to that of CuO

alone, based on the rate of MO degradation under irradia‐

tion by visible light This work not only demonstrated the

potential of graphene as a support for nanoparticle-based

photocatalysts but also highlighted more generally the

potential applications of graphene-based materials in other

fields

5 Acknowledgements

This work was supported by the Construct Program of the

Key Discipline in Hunan Province (XJF[2011] 76), the

General Project of Department of Science & Technology of

Hunan Province (2014GK3094) and the Project of Hunan

Provincial Education Department (15B158)

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5 Bin Zeng and Hui Long: