Synthesis of Cubic-relievo Ag3PO4 with High Visible-light Photocatalytic Activity

WEI Ju-Meng, Lü Qiang, WANG Ben-Chi, PAN Jia-Le, YE Xiang-Ju, SONG Chang-Chun

Synthesis of Cubic-relievo Ag3PO4with High Visible-light Photocatalytic Activity

WEI Ju-Meng, Lü Qiang, WANG Ben-Chi, PAN Jia-Le, YE Xiang-Ju, SONG Chang-Chun

(College of Chemistry and Materials Engineering, Anhui Science and Technology University, Fengyang 233100, China)

High visible-light photocatalytic activity silver phosphate (Ag3PO4) was successfully prepareda facile water bath method. Scanning electron microscopy (SEM) images show that the products undergo three morphological changes during the reaction process, and the final product has uniform cubic-relievo morphology with an average particle size of about 1.6 μm. X-ray diffraction (XRD) patterns indicate that the samples are body-center cubic structure. In addition, the high resolution transmission electron microscopy (HRTEM) images show that several crystal facets are located in the surfaces of cubic-relievo Ag3PO4. UV-Vis absorption and photoluminescence (PL) spectra demonstrate that the products possess high visible-light responsive performance and weak PL emission intensity. The cubic-relievo Ag3PO4exhibits significantly higher catalytic performance when applied on the photodegradation of methyl orange (MO) under visible-light irradiation in comparison to the cubic Ag3PO4, intermediate products or commercial nitrided TiO2photocatalyst. This work indicates that the photocatalytic performance of the catalyst can be effectively improved by changing its surface structure.

Ag3PO4; cubic-relievo; crystal faces; visible-light photocatalysis

Photocatalysis is an efficient technology for the degradation of organic contaminants[1]. In the past few decades, the search for photocatalysts is prevalent. To date, many wide band gap semiconductors, such as TiO2, have been demonstrated to be excellent photocatalysts in the fields of photodegradation[2-3]. However, for those semiconductors with high band gap energy, only ultraviolet (UV) light (<400 nm), which is less than 4% of the solar spectrum can be effectively used in the process of photocatalytic reactions. Accordingly, the development of visible-light responsive photocatalysts is becoming increasingly important. Forming semiconductor heterojunction is an effective way to enhance the visible-light absorption[4]. Besides, exploring new type visible-light responsive photocatalysts has aroused growing research interest recently. Since 2010, Ag3PO4with a band gap of 2.36 eV, as an emerging family of promising photocatalysts, has attracted much more attention[5-7].To date, Ag3PO4was confirmed as an excellent visible-light driven photocatalyst for water splitting and degradation of organic contaminants[8-11].For instance, Ye's group[12]first demonstrated that Ag3PO4had high visible-driven (>420 nm) water photooxidation, which could achieve a quantum efficiency as high as 90%.Yang,[13]reportedthat Ag3PO4exhibited high photocatalytic activity for rhodamine B (RhB) degradation. Furthermore, in order to improve and optimize their photoelectric and photocatalytic properties, many methods have been developed. Among these methods, the morphology control has been considered to be one of the most promising avenues[14-15]. The morphology of materials is related to the exposed facets of the crystals, which directly affect the properties of the catalysts[16]. Therefore, adjusting the surface structures is a promising way to improve the photocatalytic activity of Ag3PO4.

In this work, a novel cubic-relievo shape Ag3PO4with rugged surfaces was synthesized by a facile water bath method. The as-prepared final products showed high photocatalytic activity for the degradation of methyl orange (MO) under visible light irradiation.

1 Experimental

1.1 Reagent

Silver nitrate (AgNO3, 99.9%), ammonium hydroxide (NH3·H2O, 25%-28%), disodium hydrogen phosphate (Na2HPO4, 99%), alcohol (99.7%). All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd, and were used without further experimental purification.

1.2 Synthesis of Ag3PO4

The typical procedureincludes three steps: (1) pre-paration of Tollens'reagents; (2) addition of Na2HPO4solution; (3) reaction in water bath. Detailedly, 100 mg AgNO3was dissolved in 10mL distilled water. Fresh Tollens'reagents was obtained when 30 mL, 0.1 mol/L ammonia solution was dropwise added to above AgNO3solution. Then, 20 mL, 0.15 mol/L Na2HPO4aqueous solution was dropped into theabove solution. Subsequently, the mixture was placed in a water bath at 30 ℃ for 24 h under magnetic stirring.It is noteworthy thatTollens'reagents and the reaction mixture should put into brown beaker flask in case photocorrosion occurs. Samples were extracted atschedule time (4, 12 and 24 h), corresponding to the initial, intermediate and final products, respectively. The obtained samples were washed with water and alcohol several times and dried invacuum.The synthesis process and morphology evolution of samples are shown in Scheme 1.

1.3 Characterization

The structure of the samples were characterized by XRD (Rigaku D/Max-2400) using Cu-Kα radiation (40 kV, 60 mA,=0.1546 nm). The morphologies of the as-preparedAg3PO4were examined by field emission SEM, (TESCAN, MIRA3) and TEM (JEOL, JEM-2100F). UV-Visible absorption spectroscopy (ABs) was carried out by using a Shimadzu UV-3600 spectrophotometer. Photoluminescence (PL) spectrum was carried out on OmniPL-LF325 spectrofluorometer with 500 nm laser radiation source.

1.4 Photoreactivity measurements

In all of the photocatalytic activity experiments, the samples (10 mg) were made into an 100 mL aqueous MO solution to insure the equilibrium of the MO adsorption on the Ag3PO4. Then the solution was irradiated with a solution (5 mg/L) and stirred in the dark for 30 min to 500 W Xenon lamp with an ultraviolet cut-off filter (> 420 nm). During the irradiation, at given time intervals (10 min), 4 mL solution was sampled and centrifuged (10000 r/min) to remove the catalyst. The concentration of MB was calculated by measuring the absorbance of supernatants with a UV-3600 (Shimadzu) spectrophotometer.

Scheme 1 Synthesis process and morphology evolution of Ag3PO4with the reaction time extending

2 Results and discussion

As shown in Fig. 1(a)-(b), SEM images reveal that the initial products consist of uniform cubic microcrystals with average size of 1.6 μm. Figure 1(c) indicates that the average size of final products is still 1.6 μm, and the enlarged SEM image (Fig. 1(d)) reveals that the as-prepared final products are cubic-relievo shape with rugged surfaces. Based on these results, we propose that the cubic-relievo samples are generated from the cubic products through corrosion process. Furthermore, the morphology of intermediate products (Scheme 1) can also support this viewpoint. The cubic products were obtained within 4 h reaction. With the extension of reaction time, increasing amount of free NH3·H2O was released to the system (as reaction below).

?(2)

When the NH3·H2O concentration was high enough, the edges and the surfaces of the as-formed cubic productswould be corroded by the free NH3·H2O. As the corrosion process continues, the Ag3PO4cubes would be corroded to form the final cubic-relievo shape with rugged surfaces. To the best of our knowledge, the cubic Ag3PO4has been reported, yet this kind of novel morphology has been barely reported previously.

Fig. 1 Different magnification SEM images of initial (a-b), and final products(c-d) with insets showing the correponding histogram of size distribution

In order to investigate the structure of the as-obtained samples, the typical powder X-ray diffraction (XRD) was performed and the XRD patterns were shown in Fig. 2. The results indicate that the XRD patterns of both cubic and cubic-relievosamples can be well indexed to the body-centered cubic structure of Ag3PO4(JCPDS 06-0505)[17].The structure did not change with the different morphologies. The strong and sharp peaks suggest the highly crystalline nature of Ag3PO4microcrystals. Figure 3 shows the high-resolution TEM (HRTEM) images of Ag3PO4samples. As shown in Fig. 3(a), the lattice fringes of cubic-relievo Ag3PO4have spacing of 0.30, 0.27, 0.24 and 0.19 nm, which is in agreement with the spacing of the (200), (210), (211) and (310) planes, respectively. It's worth noting that the (200), (211) and (310) planes locate in surfaces of cubic-relievo, and the Fig. 3(a) shows HRTEM images of (210) planes locating in the interior of the cubic-relievo Ag3PO4crystal. For cubic Ag3PO4, by contrast, the lattice fringe spacing is 0.27 nm corresponding to (210) plane, which located in either surface or interior of the particles. The Fast Fourier Transform (FFT) patterns of HRTEM images are shown in the insets of Fig. 3(a)-(b). Two apparent rings (inset of Fig. 3(a)) indicate that several kinds of crystal facets exist in the surface of cubic-relievo Ag3PO4, and the distinct electron diffraction spots (inset of Fig.3(b)) show mono-crystalline nature of the cubic Ag3PO4crystals. The FFT results are well corresponding to HRTEMimages. However, the XRD pattern does not reveal any information about these crystal facets with different indexes, because these facets just exist in the subgrains below 2-5 nm surface (see the dashed circle in Fig. 3(a)) and account for tiny percentage of the Ag3PO4particle. From this results, it can be inferred that corrosive effect of ammonia makes different crystal facets exposed to air.

Fig. 2 XRD patterns of cubic and cubic-relievo products

Fig. 3 HRTEM images of (a) cubic-relievo Ag3PO4 and (b) cubic Ag3PO4 with insets showing the correspongding FFT patterns of HRTEM images

The ultraviolet-visible diffuse reflectance spectra of cubic-relievo and cubic Ag3PO4are shown in Fig. 4(a). The light absorption edges of the samples were achieved by extrapolating the steep slopes in the spectra. The cubic- relievo and cubic Ag3PO4exhibits absorbance peak edges around 505 and 514 nm, respectively. Furthermore, for cubic-relievo Ag3PO4, the absorption intensity in the wavelength range from 380 nm to 500 nm is higher than that of cubic Ag3PO4. In our opinion, this enhancement is attributed to several crystal faces in the surfaces of cubic- relievo Ag3PO4. It’s revealed that surface morphology is an important factor affecting diffusive reflectance spectra of the samples. The relationship between the coefficient and band gap energy can be described by the equation: (ν)2=(Eg), in which,,, andgare absorption coefficient, light frequency, proportionality constant and band gap, respectively[18]. The plots oflight energy (ν)2energy () for the as-prepared samples are shown inFig. 4(b), the band gap of cubic Ag3PO4(2.47 eV) and cubic-relievo Ag3PO4(2.45 eV) are evaluated by extrapolating the straight line to theaxis intercept. This slightly broadening of band gap is believed to have little effect on the photocatalytic activity.

Fig. 4 (a) Ultraviolet-visible diffusive reflectance spectra, and (b) plots of light energy (αhν)2vs. photon energy (hν) for the determination of the direct optical band gap of cubic-relievo Ag3PO4 and cubic Ag3PO4

The PL spectra of the as-prepared samples are performed to characterize the separation efficiency of the photo-generated electrons and holes[19]. As shown in Fig. 5, the cubic Ag3PO4possess strong emission intensity in the range between 510-600 nm. Accordingly, this PL emission peaks usually generated from the recombination of electron and holes. However, the emission intensity of cubic-relievo Ag3PO4is weaker than that of the cubic Ag3PO4. As it is well known, weaker PL intensity indicating higher separation efficiency, and which would lead to a higher photocatalytic activity[20]. It means that cubic-relievo Ag3PO4should have higher efficiency for separating the photogenerated electron-holes.

As known to all, photocatalytic activity of materials depends not only on the crystal structure, but also on the surface structure[21]. Compared with cubic Ag3PO4, the novel cubic-relievo Ag3PO4would present more crystal faces. In order to investigate the surface effects on the photocatalytic activity, the photocatalytic performance of cubic and cubic-relievo Ag3PO4was evaluated by determining the degradation of MO under visible-light irradiation (>420 nm). For comparison, the performance of commercial nitrided TiO2and the intermediate product was also investigated, and the degradation efficiency is presented in Fig. 6(a). The adsorption abilities of the catalysts to MO are almost negligible in dark. Therefore, the photocatalytic activities are attributed to the degradation ability of the catalysts under visible-light irradiation. It is obvious that the cubic-relievo Ag3PO4photocatalysts exhibited best photocatalytic activities for the MO degradation, and nearly 100% of MO was degraded after about 20 min irradiation under visible light irradiation. This results indicate that the photocatalytic activity of Ag3PO4was significantly improved after the etching process with the extension of time. The cubic-relievo Ag3PO4degraded 98% of MO and showed a photodegraded rate constant of 0.194 min-1, representing a high photocatalytic activity (as shown in Fig. 6(b)).

Fig. 5 PL emission spectra of cubic and cubic-relievo Ag3PO4

Fig. 6 Photocatalytic activities of MO over cubic-relievo Ag3PO4, cubic Ag3PO4 and commercial nitrided TiO2 under visible- light irradiation (λ>420 nm) (a), the first-order rate constant of three types of Ag3PO4 on degradation of MO (b)

It is known that the photodegradation of organic pollutants is a surface oxidation process, which is driven by photogenerated electron-hole pairs correlated with the surface structure. For the photocatalytic behavior of Ag3PO4, the most crucial factor is the chemical adsorption and reaction of target molecules occurring on the surfaces of Ag3PO4. Therefore, The high photocatalytic activity of cubic-relievo Ag3PO4can be attributed to the active sites exposed on the rugged surfaces. The schematic illustration of the catalytic mechanism is shown in Fig. 7. Several crystal facets exist in the rugged surfaces of cubic-relievo Ag3PO4, as we know, the photogenerated electrons on different crystal facets possess different energy and activity[16,21]. Moreover, the synergy between different crystal facets can enhance photocatalytic performance[22].

Fig. 7 Schematic illustration of the mechanism for the photocatalytic performance of cubic-relievo Ag3PO4

3 Conclusion

In conclusion, a novel cubic-relievo Ag3PO4photocatalyst with rugged surfaces was prepareda corrosion method. The evolution of the morphology form cubic to cubic-relievo shape has been investigated with the reaction time extending. The as-prepared cubic-relievo Ag3PO4exhibited outstanding photocatalytic activity under visible-light irradiation. It is found that the different crystal facets exist on the surfaces of cubic-relievo Ag3PO4which can effectively enhance the photocatalytic performance. This research proposed a new design and synthetic method to improve the performance by changing the surfaces of the materials.

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高可見(jiàn)光催化活性立方體浮雕狀A(yù)g3PO4的合成

魏居孟, 呂強(qiáng), 王奔馳, 潘家樂(lè), 葉祥桔, 宋常春

(安徽科技學(xué)院 化學(xué)與材料工程學(xué)院, 鳳陽(yáng) 233100)

采用簡(jiǎn)單的水浴法成功制備了具有高可見(jiàn)光活性的Ag3PO4材料。掃描電鏡測(cè)試結(jié)果表明反應(yīng)過(guò)程中產(chǎn)物經(jīng)歷了三種形貌的演變, 最終產(chǎn)物具有均一的立方體浮雕形貌, 平均顆粒尺寸大小約為1.6 μm。通過(guò)X射線衍射圖譜可知產(chǎn)物具有體心立方結(jié)構(gòu)。此外, 從高倍透射電鏡圖片中可判斷出樣品表面存在多種晶面。紫外–可見(jiàn)吸收光譜和熒光光譜結(jié)果表明該產(chǎn)物具有高的響應(yīng)可見(jiàn)光的性能和較弱的熒光發(fā)射強(qiáng)度。該產(chǎn)物與立方Ag3PO4、反應(yīng)的中間產(chǎn)物和商用氮化的TiO2催化劑相比, 在可見(jiàn)光下對(duì)甲基橙的降解表現(xiàn)出有顯著的催化活性。研究工作表明, 通過(guò)改變催化劑的表面結(jié)構(gòu)可以有效提高材料的光催化性能。

磷酸銀; 立體浮雕狀; 多種晶面; 可見(jiàn)光催化

TQ174

A

2018-09-26;

2018-12-07

National Natural Science Foundation of China(21603002); Talent Introduction Foundation of Anhui Science and Technology University (ZRC2014448); Key Discipline Foundation of Anhui Science and Technology University (AKZDXK2015A01); The Open Foundation of Chongqing Key Laboratory of Environmental & Remediation Technologies (CEK1502); Foundation of College Students Innovation and Entrepreneurship (2017S10879012)

WEI Ju-Meng (1985-), PhD, lecturer. E-mail: weijm@ahstu.edu.cn

SONG Chang-Chun, professor. E-mail: lzu_alice@163.com

1000-324X(2019)07-0786-05

10.15541/jim20180455