Template-Free Synthesis of Porous Cu2O Nanospheres at Room Temperature and Investigation on Their Adsorption Property

Porous cuprous oxide (Cu 2 O) nanospheres composed of small nanoparticles with diameters at 10∼20 nm were successfully synthesized without surfactant at room temperature within 5min. The products were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), N 2 adsorption-desorption, and Fourier transform infrared (FT-IR) spectrum. The adsorption ability of the as-prepared products towards methyl orange (MO) as the pollutant was investigated and FT-IR spectrum was employed to identify the adsorbed species. In addition, the reusability of the as-prepared products was studied as well.


Introduction
As a p-type semiconductor material, cuprous oxide (Cu 2 O) has been paid much attention owing to its fundamental significance and widespread potential applications in solar cells, gas sensors, Li-ion battery, and catalysis [1][2][3][4][5].Cu 2 O usually grows into large microcrystals which possess low specific surface area [6], resulting in the limitation of the application of Cu 2 O. Generally, the intrinsic properties of an inorganic nanomaterial could be tuned by controlling its size and microstructure to some extent [7][8][9][10][11][12][13]. Therefore, it is necessary to explore easy method to synthesize cuprous oxide with relatively large surface area.Recently, porous nanostructures have received more and more attention because they are exceptionally useful in sensor [14], catalysis [15], and dye-sensitized solar cells [16] due to their high surface area, tunable pore size, and adjustable framework [17].So far, several methods have been reported to prepare porous Cu 2 O nanostructures [18][19][20][21]; however, the structure control of the Cu 2 O nanocrystals in these reports often relies on organic capping agent to direct the growth of porous structure, thus leading to an environmental burden.In addition, most of these methods involve either elevated temperature or complex process in the preparation of porous Cu 2 O nanostructures.Therefore, it is highly desirable to develop a facile template-free route at room temperature to synthesize porous Cu 2 O nanostructures.
Herein, we present a green and convenient approach for the synthesis of porous Cu 2 O nanospheres assembled from nanoparticles without surfactant at room temperature.In this protocol, only Cu(CH 3 COO) 2 , NaOH, ascorbic acid, and distilled water were used and the product possesses a relative large specific surface area.So it is a highly ideal synthetic route to produce porous Cu 2 O nanomaterials under environmentally benign conditions.Moreover, the as-prepared products exhibit a good adsorption ability towards methyl orange (MO) as the pollutant.stirred for 5 min at ambient conditions.During the process, the color of the reaction mixture turned from blue into dark yellow, then yellow, and orange, which implied the formation of Cu 2 O.At last, the precipitates were separated from the solution by centrifugation at 4000 rpm for 5 min.Then the precipitates were washed with distilled water and absolute ethanol sufficiently and dried in vacuum for several hours.

Characterization.
The crystal structure and phase composition of the sample were characterized using X-ray powder diffraction (XRD, Rigaku D/max-2600/PC, CuK radiation,  = 1.54056Å, 40 kV, 150 mA).The morphology of the products was examined by a field emission scanning electron microscope (FESEM, HITACHI S-4800 electron microscope) operated at 15.0 kV.High resolution transmission electron microscopy (HRTEM) images were recorded on JEOL JEM-2100 microscope.The N 2 adsorption-desorption isotherms at 77 K were measured using a Quantachrome NOVA 2000E analyzer.Before measuring, the samples were degassed at 393 K for 12 h.Surface areas and pore size distributions were calculated using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models, respectively.UV-Vis spectra were taken on a Lambda 45 spectrophotometer.Fourier transform infrared spectroscopy (FT-IR, Bruker, vertex 80) was employed to characterize the residual MO on the porous Cu 2 O nanospheres after adsorption.

Adsorption Property Measurement.
Different amounts of porous Cu 2 O nanospheres (0.02 g-0.12 g) were dispersed into an aqueous solution of methyl orange (MO, 200 mL, 30 mg/L) and the mixed solution was magnetically stirred in the dark.5 mL of suspension was collected at each regular interval and centrifuged to remove the adsorbent powder in order to analyze the adsorption rate of MO by monitoring dye decolorization at the maximum absorption wavelength, using a UV/Vis spectrometer.The separated porous Cu 2 O nanospheres were washed with distilled water and absolute ethanol, respectively, for three times to remove the residual organic species as thoroughly as possible and reused for the next run.

Results and Discussion
The phase and purity of the products were firstly investigated by XRD. Figure 1 shows a typical XRD pattern of the sample.All of the reflection peaks in this pattern can be readily indexed to a pure cubic phase of Cu 2 O (JCPDS no.05-0667).Relatively, wide diffraction peaks imply that the size of the particles is small.
The morphology of the products was observed by SEM. Figure 2(a) is a typical low magnified SEM image of the sample, showing that a large quantity of spherical particles with a narrow size distribution was achieved.The high magnified SEM image (Figure 2(b)) demonstrates that the spherical shaped particles possess an average diameter about 400 nm, and the surfaces of the spheres are rough and porous since they are constructed by aggregation of a lot of small particles with diameters 10∼20 nm, as confirmed by the TEM image (Figure 2(c)).Based on the EM images with different magnifications, it is clear that pores were distributed on both the surface and the interior of the Cu 2 O nanospheres that permits the rapid penetration of the adsorbates from the outside to the inside for deep adsorptions.The aggregating feature is further confirmed by HRTEM image (Figure 2(d)), in which randomly oriented lattice fringes can be found, indicating that the porous spheres are composed of nanocrystals with different growth directions.Circles in the SAED pattern (inset in Figure 2(d)) further show the polycrystalline nature of the porous Cu 2 O nanospheres; that is, the crystal planes of the small Cu 2 O nanoparticles are randomly oriented.
The N 2 adsorption and desorption isotherms of the porous Cu 2 O nanospheres are provided in Figure 3.The specific surface area is calculated to be 26.67 m 2 g −1 .Moreover, the corresponding pore size distribution (the inset of Figure 3) shows a peak pore diameter of 10 nm with pores up to 60 nm in size.Such porous structure provides efficient transport pathways to the interior cavities and is in favor of adsorption.
To demonstrate the potential application of the asprepared porous Cu 2 O nanospheres, we investigated its adsorption ability with methyl orange (MO) as the pollutant.The adsorption was carried out by dispersing different amount of porous Cu 2 O nanospheres in the solution of MO in the dark for various durations under constant stirring.After centrifugation, the UV-Vis absorption of the supernatant was measured and the characteristic band of MO at about 464 nm was selected to monitor the adsorption behavior.As shown in Figure 4, the same trend can be seen that the concentration of the MO progressively decreases following the adsorption time within 120 min and is kept constant beyond 120 min when the amount of Cu 2 O nanospheres was from 0.02 to 0.12 g.Obviously, the adsorption capacity of the porous nanospheres is saturated at about 120 min, which is much shorter than the saturation time of reported polyhedral Cu 2 O on adsorption of MO under identical conditions [22].The more the amount of the porous nanospheres is used, the   larger is the adsorption rate.However, the adsorption behaviors are very similar when the used amount of the porous Cu 2 O nanospheres is 0.10 g and 0.12 g, the concentration of MO lowered by about 60% after about 20 min of adsorption, and the final adsorption rate could reach about 95% when the adsorption time is 120 min.Obviously, the optimum using amount of the as-prepared adsorbent is around 0.1 g.
Since the adsorption experiments were carried out in the dark, the decolorization of the MO solution must result from the adsorption of Cu 2 O particles, which is further corroborated by the FTIR analysis.As shown by curve (a) in Figure 5, the FTIR spectrum of the porous Cu 2 O nanospheres before adsorption exhibits two strong vibration bands.The band at 630 cm −1 corresponds to the Cu-O bond [23] (optically active lattice vibration in the oxide), and the peak at 1628 cm −1 is attributed to the -OH bending vibration, which originates from the surface-adsorbed H 2 O.In comparison with the FTIR spectrum of the pure Cu 2 O nanospheres, some new peaks appear after adsorption (curve (b) in Figure 5).Combined with the FTIR spectrum of pure MO (curve (c) in Figure 5), the new peaks can be assigned to the characteristic vibrations of MO.The peaks at 1446 and 1372 cm −1 are the signals from the methyl group.The peaks at 817 and 843 cm −1 result from the absorption peaks of C-H bonds of di-substituted benzene.The peaks at 697 cm −1 , 1029 cm −1 , and 1120 cm −1 come from the vibrations of the sulfonic group [24].Thus, the FTIR characterization results provide solid evidence for the adsorption of MO on the porous Cu 2 O nanospheres.
Cycling use as well as maintaining high adsorption ability of adsorbent is a critical issue toward long-term adsorption applications.Therefore, the stability of the as-synthesized Cu 2 O adsorbent was evaluated by performing the recycle experiments for adsorbing MO under identical conditions and the results are shown in Figure 6.After five cycles, the adsorption rate does not decrease apparently, indicating that the adsorbent possesses high adsorption stability.

Conclusion
In summary, porous Cu 2 O nanospheres composed of small nanoparticles with diameters 10∼20 nm have been successfully prepared through an environmental benign method in the absence of template within 5 min at room temperature.The BET surface area of the sample is 26.67 m 2 g −1 and the average pore diameter is ca. 10 nm.The porous nanospheres exhibit good adsorption property and stability towards MO as the pollutant, and the initial adsorption rate could reach about 95% within 120 min under optimum conditions.

Figure 1 :
Figure 1: XRD pattern of the as-prepared product.

Figure 2 :
Figure 2: EM images with different magnifications: (a) low magnification SEM image of the sample, (b) high magnification SEM image of the sample, (c) TEM image of the sample, and (d) HRTEM image taken at the edge of porous nanospheres and the corresponding SAED pattern (inset).

Figure 3 :
Figure 3: The N 2 adsorption and desorption isotherms and the pore size distribution curve (inset) of the porous Cu 2 O nanospheres.

Figure 4 :Figure 5 :Figure 6 :
Figure 4: Adsorption curves of the aqueous solution of methyl orange (30 mg L −1 , 200 mL) in the presence of Cu 2 O (0.05 g-0.12 g) at different intervals, where C  denotes the concentrations of MO changing following adsorption time and C 0 denotes the initial concentration of MO.