Structure, morphology and photocatalytic activity of
Attapulgite / Ag3PO4 nanocomposites synthesized by a facile
chemical precipitation route
Xiuquan Gu * ,
Yongqin Gu, Yulong Zhao,
Yinghuai Qiang, Shuang Zhang
School of Materials Science and Engineering, China University of Mining and Technology,
Xuzhou, 221116, China
* To whom correspondence should be addressed:
Prof. Xiuquan Gu
School of Materials Science and Engineering
China University of Mining and Technology
E- mail [email protected]
Abstract: Attapulgite (ATP) / Ag3 PO4 hybrids have been synthesized via a facile
chemical precipitation route, and we also examined the effect of 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 Ag3 PO 4 nanoparticles
(NPs) with sizes of 5~20 nm, while the as-synthesized hybrids exhibited the higher
photocatalytic activities for MO degradation than the single Ag3 PO4 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
Na3 PO4 , while the ones synthesized using other two phosphates exhibited the higher
photocatalytic activities and stabilities due to the higher loading amount, smaller size
and more uniform coating of Ag3 PO4 NPs. This work presents a new approach for
expanding the possibilities for developing low cost and visible-light responsive
photocatalysts.
Key words: Ag3 PO 4 ; attapulgite; visible- light; photocatalytic
1
1. Introduction
Two serious difficulties of the 21st century are the issues of energy source and
environment. Early in 1972, Fujishima and Honda firstly discovered the
photocatalytic behavior of TiO 2 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 TiO 2 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 TiO 2 [6-8].
Of all the materials, Ag3 PO 4 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 Ag3 PO 4 in several minutes [12, 13]. However, Ag3 PO4 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 (TiO 2 , SnO 2 , AgBr or AgI) on the surface of Ag3 PO4
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 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 (Mg5 Si8 O21 · xH2 O). Such a material was demostrated to be
efficient for using as the supporter of the catalysts such as Pt, TiO 2 , BiOBr and so on
[20, 21]. But up to now, there were still few studies on the ATP/Ag3 PO 4 hybrids [22].
2
In this work, it was demonstrated that the Ag3 PO4 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
Ag3 PO4 microcrystals (MCs). Moreover, the effect of p hosphate salts on the
morphology and photocatalytic performance was examined for both the single
Ag3 PO4 particles and their hybrids with ATP NRs, similar to the previous reports [23].
2. Experimental details
2.1 Synthesis of Ag3 PO4 and ATP/Ag3 PO4 hybrids
Ag3 PO 4 NPs were synthesized by a typical, simple ion-exchange process
described by Ye’s group [10]. Initially, 3 g AgNO 3 and 2.24 g Na3 PO4 ·12H2 O were
dissolved into 50 ml of deionized (DI) water, respectively. Then, the Na3 PO4 aqueous
solution was added to the AgNO 3 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
Ag3 PO4 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 Na2 HPO 4 and
NaH2 PO4 as the phosphate salts. All the above samples were remarked as shown in
Table 1.
The preparation procedure of ATP/Ag3 PO4 hybrids were almost the same with the
single Ag3 PO4 MCs. Herein, the ATP powders with a purity level of 95 % were
provided by Jiangsu ATP Co. Ltd. in Xuyi, China. 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 AgNO 3 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.
3
Table 1 Different types of PO4 3- salts for preparing Ag3 PO4 and their composites.
No.
Composition
1
1-Ag3 PO4
2
2-Ag3 PO4
3
3-Ag3 PO4
4
1-ATP/Ag3 PO4
5
2-ATP/Ag3 PO4
6
3-ATP/Ag3 PO4
Na3 PO4 ·12H2 O
Na2 HPO4
NaH2 PO4 ·2H2 O
√
√
√
√
√
√
2.2 Characterization
The X-ray diffraction spectra (XRD) patterns were obtained using an instrument
(Bruker D8 Advance) with a Cu Ka radiation souce (λ = 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, JEOL2010) at an operation voltage of 200 kV.
2.3 Photocatalytic measure ments
During all the photocatalytic measurements, 0.1 g of as-prepared Ag3 PO4 and
ATP/Ag3 PO4 hybrid samples were dispersed in 100 mL of 10 mg methyl orange (MO)
aqueous solution, 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 (λ > 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.
3. Results and discussion
3.1 Ag3 PO4 MCs
Fig.1 showed the XRD patterns of Ag3 PO4 MCs synthesized using various types
of phosphate salts. It was found that all the samples were clearly crystalized in cubic
4
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 AgPO 3 and Ag2 O,
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 Na3 PO 4 as following, which was consistent with Ref. 23. However, it remained
unclear why the AgPO 3 was formed.
PO 43- + H2O → HPO 42- + OH-
(1)
Ag+ + OH- → AgOH
(2)
2AgOH → Ag2 O + H2 O
(3)
Fig. 1 XRD patterns of single-phase Ag3 PO 4 microparticles:
(a) 1-Ag3 PO 4 ; (b) 2-Ag3 PO4 ; (c) 3-Ag3 PO 4
Fig.2 showed the TEM images of the as-synthesized Ag3 PO4 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,
5
which might be related to the formation of Ag2 O 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 Na2 HPO 4 and NaH2 PO4 were the good phosphate salts for
providing suitable the pH value during the whole ion-exchange process.
Fig. 2 TEM images of single-phase Ag3 PO4 MCs synthesized via
different precursors: (a) 1-Ag3 PO 4 ; (b) 2-Ag3 PO 4 ; (c) 3-Ag3 PO4
Fig.3 showed the visible- light photocatalytic activities of different Ag3 PO4
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 d uring 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 Ag2 O and AgPO 3 , which could lower the light
harvesting efficiency and the active surface area, resulting in the decreased
photocatalytic activity.
6
Fig. 3 Visible- light photocatalytic activities of single-phase Ag3 PO4 MCs:
(a) 1-Ag3 PO4 ; (b) 2-Ag3 PO4 ; (c) 3-Ag3 PO 4
3.2 ATP/Ag3 PO4 hybrids
Fig. 4 showed the XRD patterns of ATP/Ag3 PO 4 hybrids prepared using various
types of phosphate salts with pure Ag3 PO4 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/Ag3 PO 4 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 Ag3 PO4 peaks were observed in the hybrids, with reference to
Ag3 PO4 MCs.
Fig.4 XRD patterns of ATP/Ag3 PO 4 hybrids (a) 1-ATP/Ag3 PO4 ;
(b) 2- ATP/Ag3 PO 4 ; (c) 3- ATP/Ag3 PO4
Fig. 5 compared TEM images of the ATP/Ag3 PO 4 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 Ag3 PO 4 particles, varying in the range of 5~20 nm, were
observed to on the surfaces of ATP rods. The sizes of those Ag3 PO 4 NPs were
dependent on the type of phosphates which were used in the preparation process. Of
7
them, the sample (No. 4) exhibited the largest Ag3 PO 4 particle size of all, which might
be related to the rapid hydrolysis reaction due to higher pH values produced by
Na3 PO4 . 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).
Fig.5 TEM images of ATP/Ag3 PO 4 hybrids (a) 1-ATP/Ag3 PO 4 ;
(b) 2- ATP/Ag3 PO 4 ; (c) 3- ATP/Ag3 PO 4
Fig. 6 displayed the visible- light photocatalytic activities of various ATP/Ag3 PO4
hybrids. The samples (No. 4, 5 and 6) exhibited the higher photocatalytic activities
than the corresponding Ag3 PO 4 MCs. 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 Ag3 PO4
MCs 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 Ag2 O and the
smaller particle sizes of Ag3 PO 4 NPs coated on the ATP surfaces.
Fig. 6 Visible- light photogradation curves over MO of ATP/Ag3 PO4
hybrids (a) 1-ATP/Ag3 PO 4 ; (b) 2- ATP/Ag3 PO4 ; (c) 3- ATP/Ag3 PO 4
8
Fig.7 compared the UV-vis absorbance spectra of ATP, Ag3 PO4 and their hybrids.
It was observed that the Ag3 PO 4 absorbed the sunlight with wavelengths (λ) shorter
than 530 nm while the ATP was just sensitive to the photons with λ < 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
(Ag3 PO 4 ) and 3.72 eV (ATP), respectively. The Ag3 PO 4 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 TiO 2 electrodes for solar energy conversion. Meanwhile, it also provided a
facile and efficient method to obtain ultrathin Ag3 PO4 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 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 Ag2 O
as well as low loading amount of Ag3 PO 4 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 Ag3 PO4 NPs.
Fig. 7 UV-Vis absorbance spectra of (a) pure Ag3 PO4 (b) pure ATP
(c) 1-ATP/Ag3 PO 4 ; (d) 2- ATP/Ag3 PO 4 ; (e) 3- ATP/Ag3 PO 4
9
3.3 Possible mechanis m 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 :
EVB    E e  0.5 E g
(4)
E CB  EVB  E g
(5)
Table 2 Calculation of the CB and VB potentials of Ag 3 PO4 and ATP by terms of Eq. 4 and 5.
χ
Eg (eV)
ECB (eV)
EVB (eV)
Ag3 PO 4
5.96
2.36
0.28
2.64
ATP
6.11
3.72
– 0.25
3.47
where ECB and EVB represented the CB and VB edge potentials, respectively; χ was
the electro- negativity of the semiconductor, which is the geometric mean of the
electro-negativities 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-II band alignment was formed at the
ATP/Ag3 PO4 interfaces, similar with Ag3 PO4 /CoPi reported in our previous studies
[26]. When the hybrids were irradiated, the Ag3 PO4 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 Ag3 PO4 crystals. In a word, we
developed an efficient method to obtain ultrathin Ag3 PO 4 NPs by employing ATP
NRs as the supporter, which exhibited a higher catalytic activity than single Ag3 PO4
microparticles.
10
Fig. 8 Schematic diagram to illustrate the enhanced
photocatalytic activity of ATP/Ag3 PO4 hybrids.
4 Conclusion
In summary, ATP/Ag3 PO 4 hybrids were successfully synthesized by a facile
chemical precipitation route, with Ag3 PO 4 NPs (smaller than 20 nm) being dispersed
onto the surface of ATP NRs. The hybrids exhibited comparable photocatalytic ability
with single Ag3 PO4 MCs, which could degrade 90 % of MO solution in 10 min,
although the actual Ag3 PO4 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 Ag3 PO4 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.
Acknowledge ments
This work was financially supported by the Fundamental Research Funds for the
Central University 2013QNA04 and Natural Science Foundation of Jiangsu Province
BK20130198.
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