Structure, morphology and photocatalytic activity of

J Mater Sci: Mater Electron DOI 10.1007/s10854-015-3058-4

Structure, morphology and photocatalytic activity of attapulgite/ Ag3PO4 hybrids synthesized by a facile chemical precipitation route Yongqin Gu1 • Xiuquan Gu1 • Yulong Zhao1 • Yinghuai Qiang1

Received: 17 January 2015 / Accepted: 7 April 2015 Ó Springer Science+Business Media New York 2015

Abstract Attapulgite (ATP)/Ag3PO4 hybrids have been synthesized via a facile chemical precipitation route, and we also examined the effect of precursor phosphate types on the structure, morphology and photocatalytic property of the obtained products. It was found that the ATP surfaces were uniformly coated by ultrathin Ag3PO4 nanoparticles (NPs) with sizes of 5–20 nm, while the assynthesized hybrids exhibited the higher photocatalytic activities for methyl orange degradation than the single Ag3PO4 crystals although the amount of active species were decreased a lot. The behavior was thus attributed to both the smaller particle sizes and the larger surface. In addition, it was also observed that the photocatalytic activities of those hybrids were dependent on the types of phosphate salts. In details, a few impurities were found in the samples synthesized via Na3PO4, while the ones synthesized using two other phosphates (Na2HPO4, NaH2PO4) exhibited the higher photocatalytic activities and stabilities due to the higher loading amount, smaller size and more uniform coating of Ag3PO4 NPs. This work presents a new approach for expanding the possibilities for developing low cost and visible-light responsive photocatalysts.

& Xiuquan Gu [email protected] 1

School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China

1 Introduction Two serious difficulties of the twentyfirst century are the issues of energy source and environment. Early in 1972, Fujishima and Honda firstly discovered the photocatalytic behavior of TiO2 single crystals, which not only split water into hydrogen but also degraded original pollutants [1–3]. As an important photocatalyst, TiO2 related nanostructures have received a worldwide attention due to high chemical stability and suitable band structure [4]. For instance, Lou et al. recently developed a facile solvothermal method to synthesize anatase TiO2 NWs with a high yield, larger specific surface and excellent ultraviolet (UV) photocatalytic activity [5]. Nevertheless, only less than 5 % of solar energy could be utilized by TiO2 related catalysts due to the relative wide bandgap (3.2 eV). Doping or forming a composite with other semiconductors might be a good way to broaden the light absorption range or enhance the photocatalytic activity/stability of TiO2 [6–8]. Of all the materials, Ag3PO4 could be an ideal choice due to both a visible-light response and a high quantum yield [9–11]. A few organic dyes (e.g. MO, MB or RhB, etc.) were decomposed by Ag3PO4 in several minutes [12, 13]. However, Ag3PO4 also encounters several technical bottlenecks including the high cost, poor stability, large particle size and low surface area [14, 15]. In details, as for the stability, the activity of such a photocatalyst might be attenuated a lot after several cycles of the usage [15]. In order to solve this issue, a few attempts have been tried, including the deposition of a stable and insoluble thin layer (TiO2, SnO2, AgBr or AgI) on the surface of Ag3PO4 crystallites [8, 16–18], or the formation of necklace-like composites based on Ag nanowires (NWs) [19]. By forming a heterostructure, both the stability and activity got

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improved significantly owing to the more efficient charge separation [19]. As well known, the ATP was a kind of natural, low-cost and non-metallic mineral, which exhibited a fibrous structure with large surface area and made up of hydrated magnesium silicate (Mg5Si8O21 xH2O). Such a material was demonstrated to be efficient for using as the supporter of the catalysts such as Pt, TiO2, BiOBr and so on [20, 21]. But up to now, there were still few studies on the ATP/ Ag3PO4 hybrids [22]. In that report [22], the Ag3PO4 NPs (5 nm) were loaded onto the ATP surface with a uniformity, which exhibited the enhancement in photocatalytic activity and stability. In this work, it was demonstrated that the Ag3PO4 NPs, with average size less than 10 nm, were successfully deposited on the surfaces of ATP nanorods (NRs) by a facile chemical precipitation route, which exhibited a comparable catalytic activity with Ag3PO4 microparticles (MPs). Moreover, the effect of phosphate salts on the morphology and photocatalytic performance was examined for both the single Ag3PO4 particles and their hybrids with ATP NRs, similar to the previous report [23].

The preparation procedure of ATP/Ag3PO4 hybrids were almost the same with the single Ag3PO4 MCs. Herein, the ATP powders with a purity level of 95 % were provided by Jiangsu ATP Co. Ltd. in Xuyi, China. The BET surface area of original ATP is *124 m2/g. Prior to the synthesis, 2 g ATP powders were dispersed into 50 ml DI water to form the dilute slurry by magnetic stirring for 2 h. Afterwards, the AgNO3 aqueous solution and phosphate salts were added into the above ATP slurries in turn, followed by vigorous stirring for several hours. Finally, the hybrids were obtained by filtering, washing, drying and grinding. The obtained products were labeled as No. 1–6 by terms of the anions used in the synthesis reaction, as shown in Table 1. 2.2 Characterization

2 Experimental details

The X-ray diffraction spectra (XRD) patterns were obtained using an instrument (Bruker D8 Advance) with a Cu Ka radiation source (k = 0.15416 nm). X-ray tube voltage and current were set at 40 kV and 40 mA, respectively. The surface morphologies were measured by the transmission electron microscopy (TEM, JEOL-2010) at an operation voltage of 200 kV.

2.1 Synthesis of Ag3PO4 and ATP/Ag3PO4 hybrids

2.3 Photocatalytic measurements

Ag3PO4 NPs were synthesized by a typical, simple ionexchange process described by Ye’s group [10]. Initially, 3 g AgNO3 and 2.24 g Na3PO412H2O were dissolved into 50 ml of deionized (DI) water, respectively. Then, the Na3PO4 aqueous solution was added to the AgNO3 solution dropwise until the emergence of light yellow precipitations. Then, the precipitations were washed in turn with and absolute ethanol in order to dissolve any unreacted raw materials. Finally, the as-prepared Ag3PO4 products were blow-dried using a vacuum oven set at 80 °C for 12 h. The similar procedure was employed to synthesize the samples by using Na2HPO4 and NaH2PO4 as the phosphate salts. All the above samples were remarked as shown in Table 1.

During all the photocatalytic measurements, 0.1 g of asprepared Ag3PO4 and ATP/Ag3PO4 hybrid samples were dispersed in 0.1 g/l of MO, which was laid in a photocatalysis reaction system (Shanghai Bilang). The simulated visible light was provided by a 150 W Xe arc lamp equipped with a UV cutoff filter (k [ 400 nm). Prior to irradiation, the solution suspended with photocatalysts were stirred in the dark for 1 h to ensure that the catalyst surface was saturated with MO. The MO degradation was monitored by measuring the changes of UV–Vis absorption spectra as a function of irradiation time. After the photoreaction, the catalyst powders were removed and recovered from MO solution by centrifuging.

Table 1 Different PO43- salts for preparing of Ag3PO4 and its composites No.

Composition

Na3PO4

1

Ag3PO4

H

2

Ag3PO4

3

Ag3PO4

4

ATP/Ag3PO4

5

ATP/Ag3PO4

6

ATP/Ag3PO4

123

Na2HPO4

3 Results and discussion 3.1 Ag3PO4 MCs

NaH2PO4

H H H H H

Figure 1 shows the XRD patterns of Ag3PO4 MPs synthesized using various types of phosphate salts. It was found that all the samples were clearly crystalized in cubic structure as the main diffraction peaks were well matched with JCPDS card 06-0505. In addition, no any other phases were observed in Sample 2 and 3. However, three weak peaks located at 27.9°, 32.2° and 46.3°, corresponding to


Fig. 1 XRD patterns of Ag3PO4 MPs synthesized by using a Na3PO4; b Na2HPO4; c NaH2PO4

AgPO3 and Ag2O, were discovered in Sample 1. It suggested that there might be the impurity phases in No. 1 due to the chemical reaction between Ag? and OH- produced via the hydrolysis of Na3PO4 as following, which was consistent with Ref. 23. However, it remained unclear why the AgPO3 was formed. 2 PO3 4 þ H2 O ! HPO4 þ OH þ

ð1Þ

Ag þ OH ! AgOH

ð2Þ

2AgOH ! Ag2 O þ H2 O

ð3Þ

Figure 2 shows the TEM images of the as-synthesized Ag3PO4 samples. As seen, the sample (No. 1) exhibited the rhombic dodecahedral morphology, which were made up of a few irregular nanospheres (NSs), owing to the different growth rates of the crystallographic planes in the order of V{110} [ V{100} [ V{111} [23, 24]. In addition, it was also observed that the average size of No. 1 was much lower than other ones, which might be related to the formation of Ag2O and other impurities. They acted like an obstacle that

inhibited the grain growth [25]. It was noted that there were no obvious differences in the surface morphologies or particle sizes of Sample 2 and 3, suggesting that both the Na2HPO4 and NaH2PO4 were the good phosphate salts for providing suitable pH value during the whole ion-exchange process. Figure 3 shows the visible-light photocatalytic activities of different Ag3PO4 samples. As seen, the MO exhibited a good stability under the visible-light irradiation for a long time. Both of No. 2 and 3 exhibited the excellent photocatalytic performance, which could degrade over 80 % of MO solution during initial 10 min. However, the photocatalytic activity of No. 1 was rather low, just degrading less than 40 % of MO during the same duration. The behavior might be ascribed to the coexistence of secondary phases as Ag2O and AgPO3, which could lower the light harvesting efficiency and the active surface area, resulting in the decreased photocatalytic activity. 3.2 ATP/Ag3PO4 hybrids Figure 4 shows the XRD patterns of ATP/Ag3PO4 hybrids prepared using various types of phosphate salts with pure Ag3PO4 MCs as a reference. Evidently, several peaks localized at 8.3°, 19.79°, 20.81°, 26.6° and 35.5° from ATP were clearly identified in the ATP/Ag3PO4 hybrids. A small weak peak appeared at 27.5°, which might be related to the present of the quartz impurity in the raw materials. Besides, no obvious shifts of Ag3PO4 peaks were observed in the hybrids, with reference to Ag3PO4 MPs. Figure 5 shows TEM images of the ATP/Ag3PO4 hybrids synthesized using various phosphate salts. The ATP exhibited a rod-like structure with average diameter of *50 nm, which facilitated carrier transporting and achieving a high specific surface area. Meanwhile, the Ag3PO4 particles, varying in the range of 5–20 nm, were coated on the surfaces of ATP rods. There might be two

Fig. 2 TEM images of Ag3PO4 MPs synthesized via different precursors: a Na3PO4; b Na2HPO4; c NaH2PO4

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factors leading to the reduced size of Ag3PO4 particles. First, the ATP NRs provided a large surface for the growth or nucleation of Ag3PO4. Besides, the space steric

Fig. 3 Visible-light photocatalytic activities of Ag3PO4 MPs synthesized by using a Na3PO4; b Na2HPO4; c NaH2PO4

Fig. 4 XRD patterns of ATP/Ag3PO4 hybrids synthesized by using a Na3PO4; b Na2HPO4; c NaH2PO4

hindrance was formed between the adjacent NRs, limiting the further growth of Ag3PO4 NPs. The sizes of those Ag3PO4 NPs were dependent on the type of phosphates which were used in the preparation process. Of them, the sample (No. 4) exhibited the largest Ag3PO4 particle size of all, which might be related to the rapid hydrolysis reaction due to higher pH values produced by Na3PO4. On the contrary, the lower the hydrolysis rate, the smaller NPs or better loading level were deposited on the ATP supporters like other samples (No. 5 and 6). Figure 6 shows the visible-light photocatalytic activities of various ATP/Ag3PO4 hybrids. The samples (No. 4, 5 and 6) exhibited the higher photocatalytic activities than the corresponding Ag3PO4 MPs. Especially, 90 % of MO was degraded during 10 min by No. 5 and 6, while it might take 30 min or longer time for single Ag3PO4 MPs to finish such a task. It was also worthy noting that the degradation rates of No. 5 and 6 were obviously higher than No. 4 due to the less impurities like Ag2O and the smaller particle sizes of Ag3PO4 NPs coated on the ATP surfaces. Figure 7 shows the UV–Vis absorbance spectra of ATP, Ag3PO4 and their hybrids. It was observed that the Ag3PO4 absorbed the sunlight with wavelengths (k) shorter than 530 nm while the ATP was just sensitive to the photons with k \ 320 nm, suggesting that the ATP didn’t exhibit any visible-light activity during the photochemical process. As a result, their band gaps could be calculated to be 2.36 eV (Ag3PO4) and 3.72 eV (ATP), respectively. The Ag3PO4 owned a good visible-light response characteristic while the ATP provided both the large surface areas and the carrier transport pathways. The behavior of the hybrids was similar with the dye sensitized TiO2 electrodes for solar energy conversion. Meanwhile, it also provided a facile and efficient method to obtain ultrathin Ag3PO4 particle, which was demonstrated to be very difficult via the traditional routes. As also could be seen, those hybrids synthesized via various Na? salts exhibited different

Fig. 5 TEM images of ATP/Ag3PO4 hybrids synthesized by using a Na3PO4; b Na2HPO4; c NaH2PO4

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J Mater Sci: Mater Electron Table 2 Calculation of the CB and VB potentials of Ag3PO4 and ATP by terms of Eqs. 4 and 5

Fig. 6 Visible-light degradation curves over MO of ATP/Ag3PO4 hybrids synthesized by using a Na3PO4; b Na2HPO4; c NaH2PO4

optical absorbance behaviors. In details, for the samples (No. 1), no obvious changes were observed in the absorbance spectra compared to pure ATP except a very weak adsorption in the visible regions, which might be caused by the coexistence of narrow-band-gap Ag2O as well as low loading amount of Ag3PO4 NPs on the surface of ATP NRs. But for the other two hybrids, both the high visible-light absorption and the redshifted band edge were observed due to the efficient loading of Ag3PO4 NPs. 3.3 Possible mechanism for photocatalytic property Further, the conduction band (CB) and valence band (VB) potentials of these semiconductor samples were deduced according to the following empirical equation and the calculated results were indicated in Table 2:

v

Eg (eV)

Ag3PO4

5.96

2.36

0.28

2.64

ATP

6.11

3.72

–0.25

3.47

ECB (eV)

EVB (eV)

EVB ¼ v Ee þ 0:5Eg

ð4Þ

ECB ¼ EVB Eg

ð5Þ

where ECB and EVB represented the CB and VB edge potentials, respectively; v was the electro-negativity of the semiconductor, which is the geometric mean of the electronegativities of the constituent atoms; Ee is the free electron energy on the hydrogen scale (about 4.5 eV), Eg was the band gap energy of the semiconductor. In terms of Table 2, a schematic diagram about the band alignment was drawn, as showed in Fig. 8. It was clearly seen that a type-I band alignment was formed at the ATP/ Ag3PO4 interfaces, similar with Ag3PO4/CoPi reported in our previous studies [26]. Both the CB and VB edges of Ag3PO4 laid in the band gap of ATP. When the hybrids were irradiated, the Ag3PO4 NPs were excited by the visible light to generated e- and h?, both of which might contribute to the MO degradation. As could be seen in Fig. 8, the suppressed recombination of photogenerated carriers was ruled out, since that the ATP exhibited a more postive VB edge and more negative CB edge. Thus, the enhanced photocatalytic activity could only be ascribed to the lowered average size or surface area of active Ag3PO4 crystals. In a word, we developed an efficient method to obtain ultrathin Ag3PO4 NPs by employing ATP NRs as

Fig. 7 a UV–Vis absorbance spectra of ATP, Ag3PO4 and various ATP/Ag3PO4 hybrids; b Plot of (ahm)2 versus energy (hm) of ATP and Ag3PO4 for obtaining the band gap energies

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Fig. 8 Schematic diagram to illustrate the enhanced photocatalytic activity of ATP/Ag3PO4 hybrids.

the supporter, which exhibited a higher catalytic activity than single Ag3PO4 microparticles.

4 Conclusion In summary, ATP/Ag3PO4 hybrids were successfully synthesized by a facile chemical precipitation route, with Ag3PO4 NPs (smaller than 20 nm) being dispersed onto the surface of ATP NRs. The hybrids exhibited comparable photocatalytic ability with single Ag3PO4 MPs, which could degrade 90 % of MO solution in 10 min, although the actual Ag3PO4 content of the former was much lower than the latter. The enhanced photocatalytic performance might be attributed to the smaller size and the larger surface of Ag3PO4 crystals after forming a hybrid structure. The result suggested that it might be a low cost and high efficient method to prepare the visible-light photocatalysts. Acknowledgments This work was financially supported by the Fundamental Research Funds for the Central University 2015QNB03.

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