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Phosphate-assist

aSchool of Materials, Tianjin Polytechnic UnbSchool of Environment and Chemical Eng

Fiber Membrane Materials and Membrane

Tianjin 300387, China. E-mail: guomlin@

+86 22 83955451cKey Laboratory of Hollow Fiber Membrane

Polytechnic University, Tianjin 300387, Chi

† Electronic supplementary informa10.1039/c4ra07021f

Cite this: RSC Adv., 2014, 4, 42971

Received 12th July 2014Accepted 26th August 2014

DOI: 10.1039/c4ra07021f

www.rsc.org/advances

This journal is © The Royal Society of C

ed one-pot synthesis of zirconiumphosphate-containing mesoporous silica withunique photodegradation ability for rhodamine B†

Jiu-Yang Yang,a Chun-Ming Zheng,*b Yan-Qing Wangb and Ming-Lin Guo*bc

A facile one-pot method for zirconium phosphate-containing meso-

porous silica (ZrP–MS) using sodium silicate and zirconium oxy-

chloride was reported under acidic P123 aqueous solution followed by

an aid of phosphate. The obtained ZrP–MS material is a new type of

mesoporous silica composite, which had a structure similar to SBA-15

but with larger mean pore diameter. Zirconium phosphate was

uniformly distributed in the silica, and it was found to be a unique

photocatalyst for the degradation of rhodamine B (RhB) by compar-

ison with ZrP–MS, silver-activated ZrP–MS and silver phosphate–

mesoporous silica composites on degradation of RhB and methyl

orange, in which the reaction kinetics, luminescence properties of the

final products of RhB degradation, fluorescence of ZrP–MS-adsorbed

RhB, and free radical capture experiment were performed. The

formation and photodegradation mechanism of ZrP–MS are

discussed.

1 Introduction

Mesoporous silicas with ordered nanopores and large surfaceareas have been widely employed as catalyst supports,absorbents, and drug delivery materials1–4 but they them-selves lack diverse functionality. Over the past few years, moreattention has been paid to the functionalization of meso-porous silica (MS), including heteroatoms and organic func-tional groups incorporated into mesoporous frameworks, andmetal oxides and organic groups graed onto mesoporoussilica surfaces,2,5–7 among other types of MS. Of these, post-treatment processes are common because of the limitationsof the conditions of mesoporous silica preparation, for

iversity, Tianjin 300387, China

ineering, and Key Laboratory of Hollow

Process, Tianjin Polytechnic University,

yahoo.com; Fax: +86 22 83955451; Tel:

Materials and Membrane Process, Tianjin

na

tion (ESI) available. See DOI:

hemistry 2014

example, the popular MCM-41 is synthesized under basicconditions with quaternary alkyl ammonium surfactants,8,9

while SBA-15 is typically obtained under strongly acidicconditions in the presence of Pluronic P123 triblock copol-ymer.10,11 These make the functionalization of mesoporoussilica multifarious, time-consuming, and for the applicationsof the resulting materials, there are problems such as thedamage of mesostructure, aqueous leaching, and thermaldecomposition of active components. Moreover, mostsynthesis procedures of ordered mesoporous silica utilizedrelatively expensive silicon alkoxides such as tetraethoxysilane (TEOS) as Si source.8–11 This also results in a very highcost to meet the requirements of extensive application.Sodium silicate is signicantly less expensive compared withTEOS. It is also an attractive silica source for the low-costsyntheses of ordered mesoporous silica.12–16

An attempt to prepare mesoporous silica has been carriedout using sodium silicate under quasi-neutral pH. It wasfound that under direct quenching of the silicate structure ofsodium silicate in Pluronic P123-acidic solution followed bythe addition of Na2HPO4 solution, as-obtained silica exhibi-ted ordered mesoporous structure by the salting-out andocculation of the phosphate. The simple, mild procedureand the absence of chemically aggressive conditions enabledthe loading of metal phosphates and the preparation of theirmesoporous silica composites by one-pot method. Somemetal phosphates have potential catalytic properties17–19 andare thermally stable; a few attempts have been made tofunctionalize mesoporous silica and surfaces by metal phos-phates.20–23 Herein, a one-pot method for the preparation ofzirconium phosphate-containing mesoporous silica (denotedas ZrP–MS) is reported using sodium silicate and zirconiumoxychloride under acidic P123 aqueous solution followed byan aid of phosphate to quasi-neutral pH. Such ZrP–MSmaterial has a similar structure to SBA-15 but with largermean pore diameter. It was found to have a unique photo-catalytic activity for the degradation of rhodamine B (RhB). Inaddition, the reaction kinetics, luminescence properties of

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Fig. 1 (a) TEM images of ZrP–MS. (b) Nitrogen adsorption isotherm of ZrP–MS. Inset: BJH pore size distribution derived from desorption branch.

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the nal products of RhB degradation, effect of silver-acti-vated ZrP–MS for the degradation of methyl orange (MO), anda possible photodegradation mechanism in a ZrP–MSsuspension are also discussed. For comparison, silver phos-phate–mesoporous silica composite (Ag3PO4–MS) was alsoprepared the same manner.

Fig. 3 FT-IR spectra of (a) ZrP–MS and (b) AgZrP–MS.

2 ExperimentalPreparation of ZrP–MS

All the chemicals were purchased from Tianjin Kermel Chem-ical Reagent Co. Ltd., China, except Pluronic P123, which wasobtained from Sigma. In a typical synthesis, 1.54 g of P123 wasdissolved in 3.0 mL HNO3 (65%) and 31 mL of water, followedby 0.20 g of ZrOCl2$8H2O. While stirring, the mixture of 3.8 g ofsodium silicate and 30mL of water was slowly added at ambienttemperature; then, the solution of 6.64 g Na2HPO4$12H2O and20 mL of water was added dropwise. A colorless precipitate was

Fig. 2 (a) SAXS pattern of ZrP–MS. (b) XRD of ZrP–MS.

42972 | RSC Adv., 2014, 4, 42971–42976 This journal is © The Royal Society of Chemistry 2014

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formed by occulation with the dropwise addition of Na2HPO4

solution. Aer transfer to a Teon ask for hydrothermaltreatment at 100 �C for 24 h, the solid was ltered, washed withwater, and dried in air. The surfactant was removed with alcoholby Soxhlet extractor. The obtained product was designated asZrP–MS.

Preparation of Ag3PO4–MS

Silver phosphate–mesoporous silica composite was synthesizedwith the same procedure, except that 0.25 g of AgNO3 was addedand denoted as Ag3PO4–MS.

Preparation of AgZrP–MS

Typically, 0.1 g AgNO3 was rst dissolved in 10 mL ethanolsolution. Then, 0.4 g obtained ZrP–MS mentioned above wasadded to the solution. Aer stirring for 4 h at room temperature,

Fig. 4 Photodegradation efficiencies of RhB (a) and MO (c) as a functionand MO (d).

This journal is © The Royal Society of Chemistry 2014

the composites were dried at 60 �C for 24 h to remove ethanol.The nal products were referred to as AgZrP–MS.

Photocatalytic evaluation

Photocatalytic activities of ZrP–MS, Ag3PO4–MS and AgZrP–MSwere evaluated by the degradation of RhB and MO under a 175W Hg lamp with 365 nm wavelength. RhB (110 mL, 10 mg L�1)or MO solutions (110 mL, 30 mg L�1) containing 50 mg ofZrP–MS, Ag3PO4–MS, AgZrP–MS and a blank test were put in aglass beaker. Prior to irradiation, the solution was stored in thedark for 20 min under stirring. 3 mL of the sample solution wasremoved at given time intervals and separated through centri-fugation (12 000 rpm, 2 min). The supernatants were analyzedby recording the variations of RhB and MO at the absorptionband maxima of 554 and 464 nm, respectively, in UV-vis spectrausing a U-3010 spectrophotometer.

of irradiation time and first-order plots for photodegradation of RhB (b)

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Fig. 5 In final degradation products, UV-vis spectra and emissionspectra for residue of RhB: (a/a0) Ag3PO4–MS, (b/b0) ZrP–MS, (c/c0)AgZrP–MS.

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3 Results and discussion

The transmission electron micrograph (TEM) image of ZrP–MSin Fig. 1(a) revealed that the morphology of ZrP–MS was of tinysheet particles and showed that it was of 2D-hexagonalordering. Particles were not observed in the nanopores of theZrP–MS. Fig. 1(b) displayed a typical type IV nitrogen sorptionisotherm with an H3 hysteresis loop, which was the character-istic of a mesostructure ordered from ZrP–MS. BET surface areawas 262.2 m2 g�1, and total pore volume at P/P0 ¼ 0.99 was0.529 cm3 g�1. In addition, the TEM image and nitrogen sorp-tion isotherm of Ag3PO4–MS are shown in Fig. S1 (see ESI†).

The small-angle X-ray scattering (SAXS) in Fig. 2(a) showsthree resolved peaks, which indicate ordered pores. Compar-ison of the structural details of ZrP–MS with other P6mm-typemesoporous silica showed ZrP–MS to be a structural analogueof SBA-15 and MSU-H. The d-spacing of ZrP–MS of 10.467 nmcorresponded to a unit cell parameter a ¼ 12.09 nm. Wallthickness was estimated to be ca. 3.99 nm. ZrP–MS had a meanpore diameter 8.1 nm according to BJH (see inset in Fig. 1(b)).The X-ray diffraction patterns (XRD) of ZrP–MS (Fig. 2(b))exhibited one broad peak in 2q ranges of 15–30�, indicatingtheir amorphous nature, which could be attributed to amor-phous silica composed of the composites. Two other weakdiffraction peaks at 2q values of 23.22� and 31.10� wereconsistent with the earlier determined patterns of zirconiumphosphate with hexagonal crystal system.24

XRF analysis showed that the elemental contents of silicon,zirconium, phosphorous and sodium to be 98.82% in total, andthe Zr/P atomic ratio was determined to be 1.44, suggesting thepresence of some zirconium oxide or zirconium silicate specieson the composites (see ESI Table S1†) except for zirconiumphosphate (ZrP). Combined with the results of TEM, SAXS, XRDand nitrogen adsorption, most of ZrP in ZrP–MS could be in tinyamorphous particles and participated in its mesoporous walls.

In the FT-IR spectra of ZrP–MS and AgZrP–MS (Fig. 3), theSi–O–Si and P–O adsorption bands overlap each other. Both hadonly an observed broad peak at �1090 cm�1.25–27 Differences forboth were seen in the appearance of a strong absorbance bandat 1380.8 cm�1 for AgZrP–MS, which was because of the bond-stretching vibrations of Ag–O.27

As can be seen in plot b of Fig. 4(a), 27.1% of the RhB in thedark was absorbed; however, when ZrP–MS was used as aphotocatalyst, it was found that aer 60 min, 98.03% of the RhBwas degraded, which followed the rst-order kinetics equation(see plot b of Fig. 4(b) and ESI†). A similar Ag3PO4–MScomposite was prepared and investigated for comparison (SAXSand XRD pattern of Ag3PO4–MS are shown in Fig. S2 in ESI†).From plot c of Fig. 4(a), aer 60 min, 96.12% of the RhB wasdegraded. Combined with its kinetics plot (plot a of Fig. 4(b)and ESI†), ZrP–MS catalyst exhibited higher photocatalyticactivities and reaction rates than Ag3PO4–MS composite in thedegradation of RhB. In addition, in the comparison andobservation for the nal products of RhB degradation, materialswith 536 nm green uorescence peaks at 346 nm excitingwavelength were found (see Fig. 5).

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For the degradation of MO, results indicated that ZrP–MShad lower photocatalytic degradation activity (plot c in Fig. 4(c)):aer 60 min, only 25.04% of MO was degraded. Meanwhile, theAg3PO4–MS catalyst had higher photocatalytic activity (plot b inFig. 4(c)): 98.69% of MO was degraded aer 60 min. Their cor-responding kinetics plots are shown in plots a and b in Fig. 4(d).

To improve the photocatalytic activity of the ZrP–MS sample,small amounts of AgNO3 were loaded to it to obtain AgZrP–MS.The results showed that the AgZrP–MS sample presented thehighest photocatalytic performance to RhB and MO with anefficiency of nearly 98.67% (plot a in Fig. 4(a)) and 96.13% (plota in Fig. 4(c)) aer 20 min, respectively. Both the correspondingreaction rates were the highest (see c plot in Fig. 4(b) and c plotin Fig. 4(d)). Their UV-vis and emission spectra also showedthat AgZrP–MS had the highest photocatalytic performance(see Fig. 5).

To understand these observation results, we performedcomparison experiments for the uorescence of ZrP–MS-adsorbed RhB, ZrP–MS, RhB and MO. The results showed thatat 365 nm excited wavelength, MO had a very weak uorescenceat 417 nm, ZrP–MS had a weak uorescence at 404 nm, whereasthe ZrP–MS-adsorbed RhB had the strongest uorescence at601 nm, which shied to a longer wavelength positioncompared with that of RhB (at 589 nm; see Fig. 6(a)). It was alsonoted that at 365 nm excited wavelength, the excited peakintensity of 424 nm for ZrP–MS-adsorbed RhB was enhancedmore than that of RhB (Fig. 6(b)), suggesting the existence ofuorescence resonance energy transfer (FRET).28 Furthermore,free radical capture experiment results showed that there werehydroxyl radicals in the case of ZrP–MS as a catalyst (see ESIFig. S3†).

On the basis of the results described above, the suggestedhypothesis of the formation of ZrP–MS and photodegradationwas as follows: (1) when Na2HPO4 was added into surfactantP123-acidic-silicic acid solution, the solution went rapidly tonear-neutral pH conditions. This favored formation of tinyuncharged silicate oligomers,16 which were low negativelycharged or uncharged; (2) phosphates acted as occulating

This journal is © The Royal Society of Chemistry 2014

Fig. 6 (a) Emission spectra of fluorescence of ZrP–MS-adsorbed RhB, ZrP–MS, RhB and MO; (b) excitation spectra of ZrP–MS-adsorbed RhBand RhB.

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agent29 and bridging agent between hydroxyl groups on thesurface of the silicate oligomers and the PEO moiety of P123 byhydrogen bonds (see Fig. 7). An assembly process might bedeparted from spherically uncharged core–shell P123-silicamicelles;16 (3) on the other hand, during the course of theformation of mesoporous silica, interaction existed betweenZrO(II) cation, hydroxyl groups on the surface of the silicateoligomers and the PEO moiety of P123. This resulted in a gooddispersion of zirconium oxychloride. At the same time, asNa2HPO4 was added it also induced ZrP to form tiny particlesand limited their accumulation; (4) the high salt concentrationin the solution promoted hydrophobic interactions withsurfactant micelles, silica assembly and tiny ZrP particles in thePEO shell, and further, mesoporous silica composites with tinyamorphous ZrP were formed; (5) based on the advantage of the

Fig. 7 Schematic representation of the bridging role of phosphates inthe formation of mesoporous silica by hydrogen bonds.

This journal is © The Royal Society of Chemistry 2014

large mesopore size and high specic surface area, diffusionresistance of RhB will be effectively reduced; besides, the activesites and adsorption capacity of the composites were greatlyincreased. The resulting ZrP–MS could produce photogeneratedelectrons and holes at 365 nm wavelength UV irradiationbecause the strong adsorption interaction between RhB andZrP–MS and photosensitive activity, the RhB was easily oxidizedby the holes in the valence band (based on hydroxyl radicalexistence experiment). On the other hand, there might havebeen the FRET effect of ZrP–MS for RhB, which enhanced RhBadsorption efficiency for UV irradiation and resulted in RhBbeing more completely degraded. The MO was hardly adsorbedon the surface of ZrP–MS and was insensitive for UV radiation;there was no FRET effect. When ZrP–MS was used as catalyst, itsphotogenerated electrons and holes were easily combined.Finally, the MO was not observably degraded; (6) in the cases ofsilver-activated ZrP–MS and Ag3PO4–MS (see ESI Fig. S4 andTable S2†), the silver ions that existed on their surfaces couldhave combined with the photogenerated electrons. It wasbenecial for the separation of photogenerated electrons andholes30–33 and promoted their photodegradation efficiency.

4 Conclusions

In summary, when ZrP–MS was synthesized with sodium sili-cate and zirconium oxychloride, the silica source rst needed tobe mixed into dilute acidic-surfactant solution while beingstirred at room temperature. As Na2HPO4 was added, surfactantP123 silicic acid and zirconium oxychloride acidic solutionrapidly reached near-neutral pH conditions. Tiny zirconiumphosphate particles and uncharged silicate oligomers wereformed, and they mixed together to build the ZrP–MS under aidof P123 and phosphate buffer system. This process is rapid,reproducible, and suitable as commercially viable synthesisconditions. Such obtained ZrP–MS has a larger mean porediameter and is a unique photocatalyst for the degradation of

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RhB. The simple one-pot process also favors the preparation ofother salt phosphate–mesoporous silica composites such asAg3PO4–MS. Both ZrP–MS-loaded silver nitrate and Ag3PO4–MShave very strong photocatalytic performance. This method notonly provides efficient and reproducible control for theimprovement of the photocatalytic performance of silverphosphate, it also reduces the high cost of silver phosphate forpractical environmental applications. These are new materialsfor the functionalization of mesoporous silica, and further workis ongoing.

Acknowledgements

We are grateful for the support of the National Natural ScienceFoundation of China (no. 51208357).

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