Facile synthesis of porous Ag₃PO₄ photocatalysts with high self-stability and activity

Abstract

One-dimensional (1D) Ag₃PO₄ hollow porous microrods have been fabricated for the first time by using Ag₂WO₄ microrods as the Ag⁺ ion source and growth template, which exhibit substantively enhanced photocatalytic activity and stability for the degradation of organic contaminations compared with Ag₃PO₄ cubes and particles under visible light irradiation.

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Recently, silver phosphate (Ag₃PO₄), with a moderate band gap (2.45 eV) and high photooxidative capability, has became a promising photocatalytic material for water oxidation as well as organic dyes decomposition under visible light irradiation.¹ However, its practical applications are still hindered mainly due to the relatively poor photocatalytic stability as a result of the self-reduction process. To address this issue, more attention has been paid to the fabrication of various Ag₃PO₄-based composites for improving the stability. Ag/Ag₃PO₄ heterocatalyst had been verified to be an effective strategy for the improvement of the stability of Ag₃PO₄ photocatalyst.^2–4 The stability improvement was attributed to the localized surface plasmon resonance (LSPR) effects of Ag nanoparticles and plenty of negative charge of PO₄³⁻ ions,^5,6 which could effectively inhibit the reducibility of Ag⁺ ions in the Ag₃PO₄ lattice. Yao and Rawal et al. synthesized Ag₃PO₄/TiO₂ visible light photocatalyst that showed much more stable than unsupported Ag₃PO₄.^7,8 The enhanced stability was attributed to the chemical adsorption of Ag⁺ cations in Ag₃PO₄ and O⁻ anions in TiO₂. Besides Ag/Ag₃PO₄ and Ag₃PO₄/TiO₂ composite heterostructures, AgX/Ag₃PO₄ had also attracted much attention due to the excellent photocatalytic activity and stability under visible light irradiation.⁹ The high stability was attributed to the formation of Ag@AgX/Ag₃PO₄@Ag plasmonic system, which effectively retained its activity due to the efficient transfer of photoinduced electrons.¹⁰ In addition, Ag₃PO₄/palygorskite composite photocatalyst also had eminent stability for the degradation of organic contaminant due to the confinement effect as well the porous structure of the palygorskite.¹¹ However, the controllable fabrication of pure-phase Ag₃PO₄ photocatalysts with enhanced stabilities has been rarely reported. In the last few years, it was demonstrated that porous and hollow structures could greatly improve the light harvesting and charge separation efficiency compared with the compact solid counterparts.^12–17 Moreover, the porous structures could facilitate reactants to immerse into photocatalysts contributing to the enhanced surface reaction kinetics and charge transfer. Thereby, the controlled fabrication of hollow and porous Ag₃PO₄ photocatalysts may serve as an effective strategy for improving both photocatalytic stability and activity.

Herein, we demonstrated an anion exchange method for rational construction of one-dimensional Ag₃PO₄ hollow porous structure by using Ag₂WO₄ microrods as Ag⁺ ion sources and growth templates. Benefiting from unique structural features, the as-prepared porous Ag₃PO₄ could show greatly enhanced photocatalytic properties. More importantly, the stability of these hollow porous Ag₃PO₄ photocatalysts had been substantively improved as a result of more efficient electron–hole separation and charge transfer during the photocatalytic process. These demonstrations may provide a new sight for improving the photocatalytic activity as well as stability of Ag₃PO₄, which may also be adapted for the preparation of other silver-based photocatalysts.

Ag₂WO₄ microrods were firstly fabricated through a precipitation process between AgNO₃ and Na₂WO₄ aqueous solution, which served as the starting materials for the subsequent chemical transformation process into Ag₃PO₄ hollow porous microrods. As can be seen from Fig. 1A and S1,† Ag₂WO₄ have been fabricated in large quantities with average diameter of 2 μm. Furthermore, as shown in the inset of Fig. 1A, the TEM image of the product indicates the solid nature of the Ag₂WO₄ precursor. For a typical transformation into Ag₃PO₄ products, the Ag₂WO₄ microrods were treated with 0.05 M Na₂HPO₄ aqueous solution at room temperature until yellow precipitation was formed, the whole process could complete within 10 minutes. As shown in Fig. 1B and S2,† it can be clearly seen that the as-converted Ag₃PO₄ products possess perfect and regular 1D structures about 2 μm in diameter and 20 μm in length, the same size as that of the Ag₂WO₄ microrods precursors. The magnified SEM images (Fig. 1B inset) provide the detailed surface structures of the Ag₃PO₄ products, which are porous and the broken microrods reveals its hollow nature. Besides, the transmission electron microscopy (TEM) image (Fig. 1C) further demonstrates the hollow and porous nature with a shell thickness of about 250 nm and a pore size of 100 nm. The phase purity and crystallographic structure of the Ag₃PO₄ products was characterized by X-ray power diffraction (XRD), as can be seen from Fig. 1D, all the diffraction peaks of the Ag₃PO₄ products can be perfectly indexed to the body-centered cubic structure of Ag₃PO₄ (JCPDS no. 06-0505), and no Ag₂WO₄ diffraction peaks or other additional peaks are detected, indicating complete conversion of Ag₂WO₄ starting materials and high purity of the Ag₃PO₄ products.

Fig. 1 (A) SEM images of Ag₂WO₄ microrods; (B) SEM images of Ag₃PO₄ microrods (inset: showing their hollow and porous nature); (C) TEM image showing the hollow and porous nature; (D) XRD patterns of the Ag₃PO₄ microrods.

X-ray photoelectron spectroscopy (XPS) characterization was undertaken to identify the chemical environment of the element presented in the photocatalyst surface. As shown in Fig. 2A, for characteristic peaks of Ag, P, O and C elements are observed. The carbon peak is attributed to the hydrocarbon from the XPS instrument itself. For Ag 3d (Fig. S3A†), two peaks are observed at binding energies of about 366.94 eV and 372.98 eV, corresponding to Ag 3d^5/2 and Ag 3d^3/2, respectively. The peaks at 132.34 eV and 530.70 eV can be assigned to the binding energies of P 2p and O 1s (Fig. S3B and C†), respectively. These observations can further confirm the high purity of the Ag₃PO₄ products, and the EDS result (Fig. S4†) is also good in agreement with the above XPS results. The UV-Vis diffuse reflectance spectra of the Ag₃PO₄ products have been shown in Fig. 2B, indicating that the hollow porous Ag₃PO₄ microrods can absorb visible light with a wavelength shorter than 520 nm. Furthermore, the bandgap of the Ag₃PO₄ products can be estimated from the equation (αhν)^1/2 = A(hν − E_g), in which α, ν, A are absorption coefficient, light frequency and proportionality constant, respectively. For estimation of the bandgap, (αhν)^1/2 was plotted against hν and then extrapolated the straight line to the hν axis intercept. As shown in the inset of Fig. 1D, the bandgap of the Ag₃PO₄ microrods is 2.42 eV.

Fig. 2 (A) XPS survey spectrum, (B) ultraviolet-visible diffusive absorption spectra of the Ag₃PO₄ microrods, the inset in (B) shows plots of (αhν)^1/2 vs. hν of the Ag₃PO₄ products.

Up to now, the Ag₂WO₄-engaged replacement reaction for the synthesis of 1D hollow porous Ag₃PO₄ microrods should be firstly studied. Owing to the lower solubility of Ag₃PO₄ relative to Ag₂WO₄, the Ag₂WO₄ could adopt a thermodynamically favored direction to transform into Ag₃PO₄ by reacting with HPO₄²⁻ ions. At the beginning of the anion exchange reaction, the released Ag⁺ ions are located very vicinity of the Ag₂WO₄ microrods as Ag₂WO₄ is a solid precursor suspended in the aqueous solution. So far, we have seen many cases where cation diffusion is much faster than anion diffusion,^18–20 and the present case is also of this type where Ag⁺ cations diffuse faster than incoming HPO₄²⁻. As the reaction progresses, the outward Ag⁺ is faster than that of the inward HPO₄²⁻, a continuous outward diffusion of Ag⁺ give rise to a hollow interior from the anion exchange reaction, and the schematic illustration is shown in Scheme 1.

Scheme 1 Schematic illustration of the ion exchange strategy: from Ag₂WO₄ solid microrods to Ag₃PO₄ porous hollow microrods.

To further investigate the ion exchange process for synthesizing Ag₃PO₄ microrods, the effect of Na₂HPO₄ solution on the surface morphology have been investigated. As shown in Fig. 3A, when the Na₂HPO₄ concentration decreased from 0.05 M to 0.01 M, the central section of every rod collapsed seriously. This can be ascribed to the fact that decreasing Na₂HPO₄ concentration made the HPO₄²⁻ scare in the reaction solution, most Ag⁺ ions dissolved and spreaded outside of the Ag₂WO₄ microrods also as the much higher mobility of Ag⁺ than HPO₄²⁻ ions, resulting in the partial destroys of the hollow structured microrods. As the Na₂HPO₄ concentration increasing to 0.025 M (Fig. 3B), the core was filled and 1D structure grew up gradually. With increasing Na₂HPO₄ concentration to 0.10 M (Fig. 3C), the pore of the microrods decrease in quantity and the diameter of the cavity also decrease, as can be seen from the inset of Fig. 3C. A higher Na₂HPO₄ of 0.20 M tends to obtain microrods with solid and imporous (Fig. 3D). We speculate that the continuous supply of HPO₄²⁻ at a higher Na₂HPO₄ concentration made the exchange reaction proceed in the interior of the Ag₂WO₄ microrods, resulting solid and imporous structures. Accordingly, the above facts clearly reveal that the formation of the hollow and porous structures should be due to the synergistic effect of the Ag⁺ dissolution and HPO₄²⁻ supply.

Fig. 3 (A) SEM images of the Ag₃PO₄ microrods obtained in Na₂HPO₄ solutions: (A) 0.01 M; (B) 0.025 M; (C) 0.10 M (inset in (C) is a corresponding TEM image); (D) 0.20 M (inset in (D) is a magnified SEM image).

Finally, the photocatalytic behaviors of the Ag₃PO₄ products were evaluated by monitoring the degradation of rhodamine B (RhB) and methyl orange (MO) dyes under visible light illumination at room temperature. In addition, Ag₃PO₄ cubes and spherical particles were also investigated for comparison. From Fig. 4A, it can be clearly seen that the Ag₃PO₄ microrods exhibit the highest photocatalytic activity, which can completely degrade RhB within 60 s. In contrast, the cubes decomposed RhB in 8 min, while the spherical particles needed 14 min. Fig. 4B shows the absorption spectra of the RhB solution after photocatalytic degradation for various durations over the hollow porous structured Ag₃PO₄ microrods. Under visible light irradiation, the absorption peak at 554 nm diminishes gradually as the exposure time increases. Additionally, the photoreactivity order for the degradation of MO (Fig. 4D) is highly consistent with the above results for the RhB degradation. More specifically, the Ag₃PO₄ microrods can completely degrade MO dye in 60 s, compared with the 14 min and 28 min corresponding to the cubes and the spherical particles. In order to evaluate the stability and reusability of the photocatalyst, we carried out the additional experiments to degrade RhB and MO dyes under the same reaction conditions for five cycles. As shown in Fig. 4C and F, it can be clearly seen that no significant falling in the photocatalytic activity was observed during the repeating 5 cycles, indicating that the as-prepared Ag₃PO₄ microrods are stable and possessed favorable recycling characteristics. The result clearly reveals that the 1D hollow porous structure can serve as a stable and efficient visible-light driven photocatalyst. We attributed the advantages of the Ag₃PO₄ microrods to the 1D hollow and porous structure, which possessed high surface-to-volume ratio, enhanced light scattering and absorption, rapid transport of free electron along the long axis and efficient electron–hole utilization.²¹ Importantly, the hollow and porous structure can provide more surface active sites and better permeability, which is favorable for the special charge transfer and therefore superior photocatalytic performance.^22–25 So these demonstrations indicate that the hollow and porous structured Ag₃PO₄ microrods is a very efficient and stable photocatalyst and their photocatalytic activities can be significantly improved by tailoring the shape and surface structure.

Fig. 4 (A and D) Photocatalytic activities of Ag₃PO₄ microrods, Ag₃PO₄ cubes and spherical particles for the degradation of RhB and MO dyes under visible light irradiation (λ > 420 nm) at room temperature; (B and E) UV-Vis absorption spectra of the RhB and MO solution in the presence of 0.2 g Ag₃PO₄ microrods at different irradiation time. (C and F) Cycling runs in the photocatalytic degradation of RhB and MO in the presence of Ag₃PO₄ microrods under visible light irradiation.

Furthermore, the photoconversion efficiencies of the Ag₃PO₄ samples deposited on fluorine-doped tin oxide (FTO) glass have been explored in detail. As can be seen from the time-dependent photocurrent generation under intermittent light irradiation in Fig. 5, the hollow structured Ag₃PO₄ microrods exhibit a higher photoelectric current and conversion efficiency than Ag₃PO₄ cubes and spherical particles, which has the same order as that observed for photodegradation of organic dyes shown in Fig. 4. Therefore, these demonstrations clearly show that the 1D hollow porous structure is also very feasible for enhancing their photoelectric performance.

Fig. 5 Photoelectric conversion performances of Ag₃PO₄ microrods, Ag₃PO₄ cubes and spherical particles in 0.1 M Na₂SO₄ aqueous solutions.

In summary, this work presents a facile and efficient anion exchange process for the synthesis Ag₃PO₄ hollow porous microrods from the 1D Ag₂WO₄ microrods starting materials under room temperature. The Na₂HPO₄ concentration plays an important role in the formation of the 1D hollow porous structures. Moreover, their photocatalytic performance studies indicated that these hollow porous Ag₃PO₄ microrods exhibit much higher catalytic activities and stability than Ag₃PO₄ cubes and spherical particles. These demonstrations clearly reveal that the rational design and fabrication of 1D hollow porous structure may be an effective technique to improve photocatalytic properties.

Acknowledgements

This work was supported by the “Hundred Talents Program” of the Chinese Academy of Science and National Natural Science Foundation of China (21573264, 21273255, 21303232).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, and additional figures. See DOI: 10.1039/c6ra07398k

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