Effect of Cupric Salts (Cu (NO₃)₂, CuSO₄, Cu(CH₃COO)₂) on Cu₂(OH)PO₄ Morphology for Photocatalytic Degradation of 2,4-dichlorophenol under Near-infrared light irradiation

Abstract

Cu₂(OH)PO₄ microstructures were synthesized by the hydrothermal method using three different types cupric salts (Cu (NO₃)₂, CuSO₄, Cu(CH₃COO)₂) as raw materials. The X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-visible-NIR absorption spectra were used to characterize the as-obtained products. The different anions (SO₄^2-, CH₃COO^-, NO_3-) have different shapes and polarities, which can generate different interactions in reaction bath, induced the difference of structure and morphology of the prepared Cu₂(OH)PO₄. The Cu₂(OH)PO₄ microstructures prepared form Cu(NO₃)₂•3H₂O showed the best photocatalytic activity induced by near-infrared light to degrade 2,4-dichlorophenol (2,4-DCP) solution. Our work suggests that the active morphological surfaces as well as different coordination environments for the metal ions has an important influence on the photocatalytic performance of Cu₂(OH)PO₄ microstructure.

Keywords:
Cu₂(OH)PO₄; Morphology; Near-infrared light; Photocatalytic performance

1. Introduction

In the past decades, semiconductor has attracted significant attention owing to their applicability in solar energy conversion and environmental protection^1-3. However, due to the wide band gaps of the semiconductor photocatalysts, a large part of solar energy cannot be utilized, especially the near-infrared (NIR) region, which accounts for about 44% of the incoming solar energy⁴. Over the past 30 years, the search for near-infrared light photocatalysts has focused mainly on upconversion luminescence of rare earth materials^5-7. The rare earth materials can transfer near-infrared light to UV or visible light active photocatalysts to generate the strongly oxidative holes and reductive electrons⁸. Although this work demonstrated the possibility of using NIR light for photocatalysis, a 980-nm laser had to be used for an overall low conversion, thus exemplifying the limited applicability of this strategy for solar light photocatalysis⁹. Besides up-conversion materials, only a few of near-infrared photocatalysts have been found, such as Bi₂WO₆⁹, carbon quantum dots¹⁰, WS₂¹, Cu₂(OH)PO₄¹¹. It is still a challenging task to further expand the light absorption range of photocatalysts and develop IR-driven photocatalysts to utilize more solar energy.

Among this near-infrared photocatalyst, copper hydroxyphosphate (Cu₂(OH)PO₄) has attracted special attention due to its unique near-infrared photocatalytic properties¹². It has an orthorhombic crystal structure with cell parameters of a=8.062, b=8.384, and c=5.881Ả and consists of a PO₄ tetrahedron, a Cu(1)O₆ octahedron, a Cu(2)O₅ trigonal bipyramid, and an OH group between the two Cu species, in which oxygen atoms are shared with each other (shown in Figure S1)¹³. Cu₂(OH)PO₄ can be synthesized by several methods either the precipitation or hydrothermal methods because it is a simple method and a low-cost technique. Copper nitrate (Cu(NO₃)₂•3H₂O) is the most popular agent to form Cu₂(OH)PO₄ powders^14-16, however, some different copper sources such as Cu(CH₃COO)₂ or CuSO₄ have been reported for use^17-19. However, the effect of different types of copper salts on the morphology of Cu₂(OH)PO₄ has not been reported. It appears that the type of copper salts can control the morphology of Cu₂(OH)PO₄ and then has an effect on its properties, including its photocatalytic activity.

Figure S1
Views showing the corner-sharing between CuO₄(OH)₂ octahedra and CuO₄(OH) trigonal bipyramids in Cu₂(OH)PO₄. For CuO₄(OH)₂ octahedra, O red, H white, and Cu blue; for CuO₄(OH) trigonal bipyramids, Cu cyan. (a) Corner-sharing through OH groups; (b) Corner-sharing through O atoms. All O atoms, except for those in the OH groups, are corner-shared with PO₄ tetrahedra¹.

In this work, we report the effect of cupric salts (Cu(NO₃)₂, CuSO₄, Cu(CH₃COO)₂) on the structure, morphology and photocatalytic activity of Cu₂(OH)PO₄ prepared through hydrothermal method. The crystal structures and optical properties of the prepared powders were characterized using X-ray diffraction (XRD) and UV-Vis-NIR diffuse reflectance spectroscopy. Lastly, the photocatalytic properties for the degradation of 2,4-dichlorophenol (2,4-DCP) solution under near-infrared light irradiation were also evaluated.

2. Experimental section

2.1. Synthesis

Cu₂(OH)PO₄ microcrystals were prepared by a hydrothermal method. In a typical procedure, 20 mmol of Cu(NO₃)₂·3H₂O and 10 mmol Na₂HPO₄·12H₂O were mixed into deionized water (ca. 40 mL) under constant stirring for 1h, and the pH of the suspension was adjusted to 7 using 1 mol/L NH₃·H₂O or 1 mol/L HNO₃. The bluish slurry mixture was then transferred to a Teflon-lined stainless steel autoclave, which was filled with deionized water up to 80% of its capacity (50 mL). The autoclave was heated to 120 ^oC in 30 min and maintained at this temperature for 6 h under autogenous pressure and static conditions. After reaction, the suspension was cooled to room temperature naturally. The deep green crystals were collected and washed several times with distilled water and absolute ethanol to remove impurities. The final products were dried at 50 ^oC (more than 10 h) for further characterizations. Cu₂(OH)PO₄ prepared from Cu(NO₃)₂·3H₂O, CuSO₄·5H₂O and Cu(CH₃COO)₂·H₂O have been labeled C-0, C-1 and C-2,respectively.

2.2. Characterization

The structural identification and morphology of the Cu₂(OH)PO₄ powders was carried out using an X-ray diffractometer (XRD, D8 Advance, Bruker AXS) with Cu Kα radiation at a wavelength of 0.15406 nm and a field emission scanning electron microscope (FESEM, S-4800, Hitachi), respectively. UV-Vis-NIR diffuse reflectance spectra (DRS) of the samples were recorded on a UV-Vis-NIR spectrophotometer (Cary 5000, Varian) with an integrating sphere attachment within the range of 200-1400 nm and with BaSO₄ as the reference standard.

2.3. Photocatalytic Study

The photocatalytic properties of the as-obtained samples were evaluated by measuring the degradation of 2,4-DCP under NIR light irradiation. A 350-W infrared lamp as the near-infrared light sources where the λ> 760 nm was filtered out during near-infrared light photocatalysis. Typically, 50 mg of sample powder was dispersed in 50 mL of 2,4-DCP aqueous solution (c=20 mg L^-1). The scheme diagram of the experimental photocatalytic apparatus was shown in Figure S2. Prior to photoirradiation, the suspensions were magnetically stirred in the dark for 30 min (widespread adoption of time) to attain the adsorption/desorption equilibrium between the dye and the surface of the catalyst under ambient conditions. At varied irradiation time intervals, 3 mL suspension was collected, centrifuged and analyzed by UV-Vis spectroscopic measures (Hitachi UV-3100). The 285 nm wavelength was used to quantify the degradation of 2.4-dichlorophenol.

Figure S2
The picture of scheme of our experimental photocatalytic apparatus.

3. Results and Discussion

The X-ray diffraction (XRD) patterns of Cu₂(OH)PO₄ prepared with three different types of cupric salts are shown in Figure 1, all peaks can be clearly indexed as the pure orthorhombic phase as all diffraction peaks of these samples are well matched with JCPDS card number 36-0404. The ration difference between diffraction peaks of different samples may be induced by the different exposed crystal faces. The amount of different exposed crystal faces can induce the difference of intensity of diffraction peak. No diffraction peaks for other phases or materials (such as pseudomalachite [Cu₅(OH)₄(PO₄)₂]) or copper phosphate hydrate [Cu₃(PO₄)₂•3H₂O] are observed in XRD patterns, indicating a high purity and crystallinity of the final products. The average particle diameter is 36.1 nm, 37.5 nm and 38.3 nm for C-0, C-1 and C-2, respectively. The diameter was calculated by using the Scherrer equation, D=kλ/βcosθ, where k is a dimensionless shape factor, with a value close to unity (~0.9); λ is the X-ray wavelength (0.15406 nm); β is the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening (obtained by deducting the background applying the Jade 6.0 software in our experiment); θ is the Bragg angle, which is obtained from the stronger peak in our experiment.

Figure 1
X-ray diffraction patterns of Cu₂(OH)PO₄ prepared from three different types of cupric salts as raw materials (a) Cu(NO₃)₂•3H₂O; (b) CuSO₄•5H₂O; (c) Cu(CH₃COO)₂•H₂O

Figure 2 (a-d) show the FESEM images for Cu₂(OH)PO₄ with three different cupric salts, respectively. It is clear that the different anions have an obvious effect on the surface morphology of the obtained product. A typical SEM image of Cu₂(OH)PO₄ microstructures with rugby-like morphology is obtained from the precursor of Cu(NO₃)₂·3H₂O and Na₂HPO₄·12H₂O as shown in Figure 2a. The results show that rugby-like Cu₂(OH)PO₄ microstructures was assembled with isotropic and highly aggregated nanoparticles. The size of the nanoparticles was about 100 nm. For the study of sulfate anion effect on the morphology of Cu₂(OH)PO₄ microstructures, an equimolar solution was prepared by dissolving the CuSO₄·5H₂O and Na₂HPO₄·12H₂O. In this case, the Cu₂(OH)PO₄ microstructures with balsam pear-like morphology is obtained as shown in Figure 2b. The balsam pear-like Cu₂(OH)PO₄ microstructures was assembled with highly isotropic and aggregated nanoparticles with rod-like nanocrystals of 100 nm length and 20 nm width. In addition, the rod-like nanocrystals has the smooth surface. For the study of acetate anion effect on the morphology of Cu₂(OH)PO₄ microstructures a spherical-like morphology based on interconnected network of nanoplates were obtained as shown in Figure 2(c-d). Similar reactions were participated in the synthesis of various morphologies of Cu₂(OH)PO₄ microstructures irrespective source of Cu²⁺ ion was different. When SO₄^2-, CH₃COO^-, or NO₃^- is added to the reaction bath, different anions can generate the following problem: (1) different anions (SO₄^2-, CH₃COO^-, or NO₃^-, etc.) have different shapes and polarities, which can generate different interactions in reaction bath; (2) different anions of cupric salts can generate different deposition environments and form different solution effects or bonds between reaction and non-reaction molecules in solution, which affect diffusion and adsorption of complex precursor ions significantly.

Figure 2
Scanning electron microscope images of Cu₂(OH)PO₄ prepared from three different types of cupric salts as raw materials (a) Cu(NO₃)₂•3H₂O; (b) CuSO₄•5H₂O; (c) (d) Cu(CH₃COO)₂•H₂O

To evaluate the optical absorption properties of Cu₂(OH)PO₄ with various morphologies, the UV-Vis-NIR absorption spectra were investigated (Figure 3). The results show that the Cu₂(OH)PO₄ microstructures prepared with Cu(NO₃)₂·3H₂O as raw material (H-0) have very broad absorption band and higher intensity (Figure 3a). The absorption peak of the Cu₂(OH)PO₄ microstructures prepared with Cu(CH₃COO)₂ as raw material (H-2) is located at 520 nm (Figure 3c). It is noteworthy that there is no any absorption in the near-infrared region. Compared to the sample H-2, the Cu₂(OH)PO₄ microstructures prepared with CuSO₄·5H₂O as raw material (H-1) have broader absorption band and higher intensity (Figure 3b). The absorption intensity obtained in the presence of the three copper salts is affected by the anion in the following order: NO₃ ^- >SO₄ ^2->CH₃COO^-. Remarkable absorption enhancement in near-infrared light region is beneficial for improving photocatalytic activity in this irradiation region.

Figure 3
UV-Vis-NIR absorption spectra of Cu₂(OH)PO₄ prepared from three different types of cupric salts as raw materials (a) Cu(NO₃)₂•3H₂O; (b) CuSO₄•5H₂O; (c) Cu(CH₃COO)₂•H₂O

Figure 4 shows the variations of the photocatalytic degradation of 2,4-DCP as a function of irradiation times, where C represents concentration of 2,4-DCP at the irradiation time (t) and C₀ is the concentration of the 2,4-DCP before irradiation. It is evident that without using any photocatalyst, 15% of the 2,4-DCP molecules were degraded after the light irradiation for 6h (Figure 4a). The H-0 sample possesses enhanced near-infrared photocatalytic activities compared with H-1 sample. The 2,4-DCP --degradation degree for Cu₂(OH)PO₄ microstructures (H-1) and Cu₂(OH)PO₄ microstructures (H-0) under 6 h near-infrared light irradiation is 35% and 70%, respectively(Figure 4c and Figure 4d ). Figure S3 shows temporal evolution of the spectral changes during photocatalytic degradation of RhB solution over Cu₂(OH)PO₄ (H-1) under NIR-light irradiation for various time periods. From Figure S3, it could be seen that with increase of irradiation time in aqueous suspension, the intensity of the absorption peak decreased, which indicated that the Cu₂(OH)PO₄ (H-1) exhibited good photocatalytic activity. In contrast, the Cu₂(OH)PO₄ microstructures (H-2) have no near-infrared photodegradation ability to 2,4-DCP (Figure 4b). Therefore, it was observed that rugby-like Cu₂(OH)PO₄ exhibited significantly higher photocatalytic activity for the photodegradation of 2,4-DCP than that of balsam pear-like and spherical-like Cu₂(OH)PO₄ microstructures. The mechanism of photocatalytic reaction for Cu₂(OH)PO₄ under NIR light irradiation is not clear. Huang BB et al ¹¹ examined the electronic band structure of Cu₂(OH)PO₄ by performing density functional theory (DFT) calculations, and pointed out that the photogenerated electrons from the CuO₄(OH) trigonal bipyramids that reach the upper three unoccupied bands are easily transferred to the CuO₄(OH)₂ octahedra. Our experimental results indicate the active morphological surfaces of Cu₂(OH)PO₄ microstructures can determine the photocatalytic activity. As is well-known, the surface area has an important influence on the photocatalytic activity. Especially, the photocatalytic activity of nanomaterial is better than the photocatalytic performance of the micromaterial. In this experiment, due to the larger size of particle, thus, the surface area of the sample has the little influence on the observed photocatalytic performance. In addition, the standard deviations of the degradation efficiency were always below 6%, indicating the good reproducibility of the photocatalytic evaluation.

Figure 4
Photocatalytic degradation of 2,4-DCP by Cu₂(OH)PO₄ prepared from three different types of cupric salts as raw materials under near-infrared light irradiation (a) 2,4-DCP self-decomposition; (b) Cu(CH₃COO)₂•H₂O; (c) CuSO₄•5H₂O; (d) Cu(NO₃)₂•3H₂O

Figure S3
UV-vis spectral changes of 2,4-dichlorophenol in aqueous Cu₂(OH)PO₄ (H-1) sample as a function of irradiation time.

4. Conclusions

The type of cupric salts used as different raw materials can affect the morphology, particle size and photocatalytic activity of the obtained Cu₂(OH)PO₄ powder. Using Cu(NO₃)₂·3H₂O as raw material, rugby-like Cu₂(OH)PO₄ microstructures was obtained and exhibited significantly higher photocatalytic activity for the photodegradation of 2,4-DCP. However, when the raw materials are CuSO₄·5H₂O and Cu(CH₃COO)₂·H₂O, the products showed balsam pear-like and spherical-like morphology, respectively. In addition, the products obtained from CuSO₄·5H₂O and Cu(CH₃COO)₂·H₂O as raw materials has the poor near-infrared light photocatalytic performance. Our work suggest that the active morphological surfaces of Cu₂(OH)PO₄ microstructures have an important influence on the photocatalytic activity.

5. Acknowledgements

This research was financially supported by the Natural Science Foundation of AnHui Provincial Education Department (KJ2015A085, KJ2016A102) and Graduate Student Innovation Fund of Anhui University of Technology.

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Publication Dates

• Publication in this collection
  26 Jan 2017
• Date of issue
  Mar-Apr 2017

History

• Received
  27 July 2016
• Reviewed
  22 Dec 2016
• Accepted
  27 Dec 2016