Sustainable Design of Hierarchically Porous Ag3PO

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Sustainable Design of Hierarchically Porous AgPO Microspheres through a Novel Natural Template and Their Superior Photooxidative Capacity Subrata Mandal, and Rajakumar Ananthakrishnan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03391 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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ACS Sustainable Chemistry & Engineering

Sustainable Design of Hierarchically Porous Ag3PO4 Microspheres through a Novel Natural Template and Their Superior Photooxidative Capacity Subrata Mandal and Rajakumar Ananthakrishnan* Department of Chemistry, Environmental Materials & Analytical Chemistry Laboratory, Indian Institute of Technology, Kharagpur 721302, India. E-mail: [email protected]; Fax: +91 3222-282252; Tel: +91 3222-282322

Abstract In this context, uniform hierarchical Ag3PO4 porous microspheres were synthesized first time, by a sustainable route based on

novel natural bone glue (BG) assisted one-step precipitation

reaction at room temperature. By varying experimental conditions like bone glue contents, template constituents (amino acid, alginic acid), precursor concentration, temperature, pH and reaction time, we could tune the morphology, porosity, size and properties of Ag3PO4. All the phases, microstructures with different architectures and textural properties of the Ag3PO4 were characterized by Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer– Emmett–Teller (BET) and UV-Vis diffuse reflectance spectroscopy (DRS). Identifications of the oxidation state, local structure and purity of the prepared Ag3PO4 were further characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and thermo gravimetric analysis (TGA). Influence of template on morphology has been characterized by syntheses of Ag3PO4 using different templates like glycine, alanine and alginic acid and products such as pyramidal shape Ag3PO4, Ag3PO4 dodecahedron and coiled-rod like porous Ag3PO4 were obtained, respectively. It revealed that self-assembly of the collagen protein present in the bone glue plays a significant role as a structure directing agent, crystal growth modifier and aggregationorienting agent in the formation of unique Ag3PO4 porous microsphere. Detailed photocatalytic studies on aqueous Rhodamine B (RhB) and 2, 4 dichlorophenol (2, 4 DCP) by Ag3PO4 porous microsphere exhibited enhanced photocatalytic degradation under visible light, which is

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significantly higher than that of other architectures. Findings of this study would offer new insights into the development of porous hierarchical materials as high-performance visible light photocatalysts, and their potential utilization in environmental sustainability.

Keywords: Bone glue, Amino acids, Controlled synthesis, Self assembly, Rhodamine B, 2,4 Dichlorophenol, Photocatalysis

Introduction In recent decades, designing of hierarchical porous semiconductor having well defined morphology in micro/nano scale is proven to be a considerable strategy to enhance the surface activity of the material. Since, these designs bring-up exceptional properties from functional adaptation of the structure at all levels. An enormous progress has been made to design synthesis and use of hierarchically porous semiconductor in the field of gas storage, adsorption, photocatalysis/catalysis, charge storage; etc.1-9 As photocatalysis is surface phenomenon, researchers interested in designing porous semiconductors for enhanced photocatalytic activity. Ag3PO4 being one of the most promising visible light active semiconductor, achieved great attention for many environmental applications. However, Ag3PO4 possesses uncontrolled photocorrosion and rapid charge separation during photocatalysis, which restricts its reusability.10,

11

Recently, various methods have been developed to enhance visible light photocatalytic activity and stability of Ag3PO4. They were based on either fabrication of heterostructure of Ag3PO4 with another semiconductor/ noble metal or Ag3PO4 with well-defined morphology with exposed surface sites to promote the charge separation by synergistic effect.12-20 Very recently, Bi et al. reported fabrication of single-crystalline Ag3PO4 rhombic dodecahedrons with {110} facets exposed and cubes bounded entirely by {100} facets, exhibited higher photocatalytic activity than the micro sized Ag3PO4 particles.21 Dinh et al. also reported a synthesis of homogeneous colloidal Ag3PO4 nanocrystals in large scale with surface area 14.5 m2/g and controlled size ranging from 8–16 nm.22 Though numerous synthesis strategies have been made, construction of Ag3PO4 with very large surface area is still not an easy task. In addition, all those strategies require organic ligands or polymer, which acts as stabilizer and structure directing


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agent with high synthetic costs, thermal pyrolysis at high temperature and chemical etching. These are big stumbling blocks for massive applications, i.e. template-free hydrothermal synthesis, which suffers elevated temperature, prolonged reaction time and low yield. Hence, it remains an urgent challenge to prepare porous hierarchical Ag3PO4 with effective surface area by a rapid, facile, low-cost and environmentally compatible route. Biomolecules are often used, in controlling the nucleation-growth and promoting the selfassembly in syntheses of nanostructures. Meanwhile, they are environment friendly as well as very cheaper in nature. Amino acid template assisted synthesis of nanomaterials is relatively well explored,

23-25

whereas protein directed synthesis is still not fully

understood and required to be developed at the molecular level. Essentially, existence of flexibility along with cohesive forces due to larger size and significant chemical interactions (covalent bonding, ionic bonding and hydrogen bonding) of protein, it could be used as a structure directing and aggregation orienting template. Protein involves the usage of multi-domain cages (template) for specific interactions with inorganic ions, which dictate the size and crystallinity of the nanomaterials.26, 27 In our investigation, we took advantage of very cheap bio-molecule, bone glue (BG), as a novel and natural template for the first time to design porous hierarchical Ag3PO4. Generally BG is a non-edible animal waste product, highly used as a binder in adhesive industry. Chemical responsible for the adhesive property of the glue is collagen, the primary structural protein of animals. It contains many polar and ionizable functional groups, which often make interconnected networks during the binding to a substrate. A facile precipitation technique and green route were adopted for mass production of porous hierarchical Ag3PO4 microspheres by taking advantage of complex structure of bone glue. Oxidation state, local structure and purity of the prepared Ag3PO4 were identified by Xray photoelectron spectroscopy (XPS), Raman spectroscopy and thermo gravimetric analysis (TGA) techniques. Evaluation of phases, morphologies and physiochemical properties were investigated in detail by XRD, FESEM, HRETM, TEM, FTIR, DRS and BET techniques. We are proposing a reasonable evaluation process of Ag3PO4, based on the influence of parameters (temperature, bone glue content, variation of templates) on morphology, size and porosity of Ag3PO4 and interpretation of TEM, FESEM and XRD



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data. Additionally, the prepared uniform Ag3PO4 microspheres not only posses hierarchical porous functional structure but also exhibit enhanced photocatalytic activity towards removal of Rhodamine B (RhB) and 2, 4 DCP by effective mass transfer in comparison with Ag3PO4 having different architectures.

Experiment section Chemicals and materials All chemical reagents utilized in the present work were analytical grade, purchased from Merck Specialities Private Limited and used without further purification. Silver nitrate was obtained from Sigma Aldrich. Deionised water was used throughout. Bone glue is purchased from the regional commercial vender (Zahida Glues, India).

Scheme 1 Schematic presentation of the preparation procedure of porous Ag3PO4 MS.


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Preparations In the synthesis of uniform porous Ag3PO4 microsphere optimized preparation procedure shown in Scheme 1. Briefly, 1 mmol of AgNO3 was added into an aqueous solution of bone glue (1 g) to form an immediate Ag+–Protein complex at room temperature. The resulting solution was stirred for 1 h. Meanwhile aqueous solution containing 10 mmol NaH2PO4 was introduced slowly into the above solution. The color of reaction mixture was observed as turbid yellow within 10 minutes and then stirred for 4 h. Finally, the yellow precipitate obtained was collected by centrifugation at 6000 rpm, thoroughly rinsed repeatedly (six times) with absolute ethanol and distilled water. The resultant was then dried at room temperature for 8 h to obtain the final Ag3PO4 porous microsphere. In addition, detailed investigation of the effect of experimental condition (time, temperature, bone glue content abbreviated as BG, Template variation, pH and AgNO3 to NaH2PO4 ratio) on morphology, shape and size of Ag3PO4, were done. All the microspheres synthesized here, are abbreviated as Ag3PO4 MS. For comparison irregular Ag3PO4 and was prepared by conventional route without adding any template. Characterization Fourier Transform Infra Red spectra (FT-IR) were carried out with a Perkin-Elmer FT-IR spectrophotometer RXI and Thermogravimetric Analysis (TGA) was analyzed on PerkinElmer instrument, Pyris Diamond TG/DTA with Al2O3 crucible to investigate the impurity of the samples after removal of the template. Phase purity of the synthesized samples are characterized by recording XRD on a BRUKER-AXS-D8-ADVANCE defractometer with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 10°-90° at a scanning rate of 0.5° min-1. The X-ray Photoelectron Spectroscopy (XPS) was performed by Specs (German). Electron Microscopy (FESEM) is used with a (Supra 40, Carl Zeiss Pvt. Ltd.) microscope at an accelerating voltage of 5 kV. Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM) were performed with JEOL JEM2010 electron microscope operating at 200 kV. Prior to TEM analysis, the sample was dispersed in absolute ethanol by ultrasonication and then dropped on carbon–copper grids. Images were acquired digitally on a multiple CCD camera. UV-Vis diffuse reflectance spectra (DRS) were obtained by using a Varian Cary 500 UV-Vis- NIR spectrophotometer using BaSO4 as the background. BET surface area and N2 sorption isotherms (77K) were carried out by Quantachrome Autosorb-iQ



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instrument. For adsorption study the sample was dried in the vacuum overnight and 50 mg amount was loaded in a 6 mm sample holder. In desorption study, the sample was used for degassing at 70°C for 2 h and Brunauer–Emmett–Teller (BET) calculations were performed for the analysis of surface area of the sample. Raman spectra of the samples were obtained with a Renishaw Raman microscope, equipped with a Diode laser excitation source emitting at a wavelength of 532 nm. Malvern Zetasizer Nano ZS dynamic light scattering (DLS) instrument equipped with a 4.0 mW He–Ne laser operating at λ = 633 nm was used to characterize the surface charge properties of BG and Ag3PO4. For the measurement, BG or Ag3PO4 suspension of 2 mg/mL concentration was prepared by dispersing glue or Ag3PO4 in deionized water under continuous magnetic stirring. The effect of pH (in the range of 4-8) on the charge of BG or Ag3PO4 was investigated by measuring the zeta potential under distinct pH values, changed by using 0.1 M HNO3 (in the acidic range) and 0.1 M (alkaline range) solutions. Photoluminescence spectra of all the Ag3PO4 with different architecture were taken and recorded by F-7000 FL Spectrophotometer with 325 nm excitation to investigate the recombination of hole and electron upon light irradiation. Electron paramagnetic resonance (EPR) signals of spin-trapped paramagnetic species with 2, 2, 6, 6-Tetramethyl-1-piperidinyloxy (TEMPO) were detected using a Bruker ELEXSYS 580 X-band EPR spectrometer at room temperature (298 K) under visible light irradiation (λ > 420 nm). Photocatalytic activity measurements Photocatalytic activities of Ag3PO4 porous MS (synthesized at different temperatures) and Ag3PO4 with irregular architecture (from another method) were evaluated by degradation of aqueous solutions of RhB (20 mg L-1) and 2, 4 DCP (30 mg L-1) under visible light irradiation (λ≥420 nm, 250 W lamp). The as-obtained Ag3PO4 photocatalysts (20 mg) were dispersed in 30 mL aqueous solution in a pyrex beaker containing RhB or 2, 4 DCP at room temperature. The reaction components in a pyrex glass vessel with the top opened was placed inside a photoreactor set-up (make: Lelesil Innovative System, India) and was maintained a distance of 25 cm from the light source. The solution was continuously (magnetically) stirred in the dark for 1h before the lamp was turned on to ensure the establishment of an adsorption–desorption equilibrium between the catalysts and the pollutant. During the photocatalytic degradation progress, 3 mL of solution was collected at regular intervals of irradiation by pipette, and subsequently centrifuged to remove the


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catalyst. UV-Vis adsorption spectra were recorded using UV-visible spectrophotometer (Thermo Scientific Evolution 201 UV-visible spectrophotometer) to determine the concentration of RhB and the progress of the degradation of 2, 4 DCP was investigated using HPLC chromatography equipped with a Diode Array Detector (Thermo Fisher Dionex UltiMate 3000 SD). Irregular Ag3PO4 and P25 commercial powder were taken as references to compare the photocatalytic activity under the similar experimental conditions.

Results and Discussion Uniform hierarchical microspheres synthesized at optimized condition are first characterized. FT-IR spectra of BG assisted synthesized Ag3PO4 MS and irregular Ag3PO4, as shown in Fig. 1a, revealed that both samples had absorption bands at 552 cm1

, 862 cm-1, 1012 cm-1 and 1402 cm-1 which correspond to characteristic peak of Ag-O

bond, asymmetric stretching mode of P-O-P linkage, P-O stretching mode and terminal double bonded oxygen (ν

P=O)

of Ag3PO4, respectively.28,

29

Shifting of P-O- stretching

mode to lower frequency (1012 cm-1) from 1100 cm-1 clearly indicates the existence of covalent bond formed by metal and oxygen.30 The characteristic absorption bands of physiosorbed water molecules appeared at 1660 cm-1 and 3460 cm-1 correspond in-plane bending vibrational mode and symmetric stretching mode of O-H bond.31 The absence of amide I, amide II and amide III stretching modes expected from peptide bond of the template strongly support the easy removal of template after synthesis and purity of the prepared Ag3PO4 MS.32 Fig. S1† shows the Raman spectra of Ag3PO4 MS 30°C (before and after washing) and irregular Ag3PO4. For irregular Ag3PO4 and Ag3PO4 MS 30°C (after washing), the strong absorption peaks at 910 and 1006 cm-1 aroused due to the motion of terminal oxygen bond vibration of phosphate group with some other weak peaks in the spectral range of 200-800 cm-1. Two weak peaks centered at 212 and 342 cm-1 are attributed to the external mode and bending vibration of the tetrahedral phosphate anion, and a broad peak ranged from 510 to 680 cm-1 is ascribed to the symmetric stretching mode of P–O–P bonds. In comparison, main peaks of Ag3PO4 becomes weaker when the Ag3PO4 MS 30°C was not washed, it strongly suggests that during synthesis the strong interaction took place between surface of Ag3PO4 and the template.20,33



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In supporting to the successful removal of template, TG of the prepared Ag3PO4 MS 30°C (before and after washing) was done as shown in Fig. S2† and compare with the BG template. The result shows before washing, Ag3PO4 MS 30°C shows weight loss (8%) at 300°C which is very similar to the weight loss of bone glue. The weight loss corresponds to the organic moieties present in the template but after washing no significant weight loss has been observed for Ag3PO4 MS 30°C and show very similar curves reported in earlier litertaure.34 Thus the results indicate successful removal of the template.

Figure 1 (a) FTIR spectra of irregular Ag3PO4, BG and as-prepared porous Ag3PO4 MS, (b) XRD pattern of irregular Ag3PO4 and as-prepared porous Ag3PO4 MS. Inset shows the polyhedron configuration of cubic Ag3PO4, (c) UV–visible diffuse reflectance spectrum of the porous Ag3PO4MS and a reference solid of irregular Ag3PO4, and the insets show the plots of (F(R)hν)1/2~ hν and (d) N2 sorption isotherm and pore diameter distribution (inset) of the porous Ag3PO4 MS.


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X-ray photoelectron spectroscopy (XPS) was carried out on prepared Ag3PO4 MS 30°C sample to confirm the oxidation state and surface chemical composition in the material (as shown in Fig. S3†). Deconvoluted Ag 3d5/2 and Ag 3d3/2 peaks at the binding energies of 367.2 eV and 373.1 eV represent the Ag+ oxidation state as shown in Fig. S3b†, which is quite reliable with the reported literature value.35, 36 Fig. S3c † represents the XPS spectra of phosphorus present in the phosphate group of Ag3PO4 MS and the peak is situated at 133.2 eV.37 The O1s spectrum shown in Fig. S3d† fitted into two (O2- and -OH) major peaks. The peak at 529.8 eV corresponds to lattice oxygen exists in the crystal of Ag3PO4 MS due to Ag-O bonding, whereas the peak at 531.7 eV is attributed to the surface hydroxyl group on the surface.38 The crystalinity, phase purity and crystallite size of as prepared product was examined by X-ray diffraction and found that microsphere is well crystallized cubic Ag3PO4 with entire diffraction peaks are in accordance with the standard data (JCPDS Card No. 060505) as in Fig. 1b. Generally cubic structure of Ag3PO4, consists of six Ag atoms occupy only the 6d Wyckoff position, and have 4-fold coordination by four O atoms. Phosphorous atoms are each in PO4 tetrahedra, while the O atoms possess 4-fold coordination surrounded by three Ag atoms and one P atom.39 The isolated and regular PO4 tetrahedra construct a body-centred cubic lattice structure. The strong and sharp diffraction peak of irregular Ag3PO4 was clearly observed, which signify high crystalinity and large crystallite size range from 54 nm to 150 nm. BG assisted room temperature synthesized uniform porous Ag3PO4 MS posses broad peak compared to irregular silver phosphate with average crystallite size 19 nm calculated from Debye Schere equation: D = 0.89λ/ (βcosθ), Where λ is the X-ray wavelength (0.15405 nm), β and θ are the full width at half maximum and diffraction angle of an observed peak, respectively. The UV-Vis diffuse reflectance spectra of as-prepared porous Ag3PO4 and micro-sized irregular Ag3PO4 are compared in Fig. 1c. It is notable that Ag3PO4 porous MS exhibits a much broader optical absorption band around 500–700 nm than the solid Ag3PO4 sample. There are no additional absorption traces and splitting of bands due to localized levels in the band gap observed, which indicates high quality of Ag3PO4 and from the viewpoint of oxygen vacancies and metal impurities.40 Prepared Ag3PO4 MS shows bright yellow color than that of solid Ag3PO4, which could be explained due to multiple scattering of light.41 Therefore, Ag3PO4 porous sample exhibiting improved harvesting properties under



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irritation of light. According to the Kubelka–Munk Function42, the relation between absorption coefficient and indirect band gap energy can be described by the equation (F(R )hν)1/2 = A (hν – Eg) (where F(R), ν, A and Eg are diffuse reflection absorption coefficient, light frequency, proportionality constant, and band gap, respectively). The plot of (F(R)hν)1/2 ~ Eg based on the indirect transition is shown in the inset Fig. 1c. Estimated from the onset of the absorption edge, the bandgap determined were 2.40 eV and 2.48 eV, respectively, indicating the effect of porous structure on narrowing the bandgap. In general the reduction of size results wider bandgap of the materials due to quantum size confinement according to the relationship of the value of blue-shifting being inversely proportional to the square of the size.43 In contradiction, the narrowing of band gap of the Ag3PO4 porous MS might arises from the larger red-shif due to the formation of surface states in Ag3PO4 porous MS. So the former phenomenon counteracts the blueshifting caused by the reduction of sizes. The narrowed in bandgap and elongated optical path length would offer increase in the production of photo generated electrons and holes, which would favour the photocatalytic reactions. Nitrogen adsorption–desorption isotherms were measured to characterize BET surface areas and the porosity of the obtained Ag3PO4 porous MS. The typical adsorption– desorption isotherms and pore size distribution of the sample prepared at room temperature is presented in Fig. 1d. The obtained type IV isotherm with a typical hysteresis loop shifted to a high relative pressure range of 0.85–1.0 due to the capillary condensation in the inter-particle porosity indicated the presence of inhomogeneous large slit-like pores (6 nm).44 The corresponding pore size distribution curve calculated by BJH (Barrett–Joyner–Halenda) methods revealed a broad distribution around 6 nm (inset of Fig. 1d). These mesopores are presumably aroused from the interstitial spaces formed from the packing of the nanoparticles. The total specific surface area of the Ag3PO4 porous MS as calculated to be 28.13 m²/g. Note that the surface area of Ag3PO4 porous MS is much higher than that of colloidal Ag3PO4 nanocrystals (14.5 m2/g).22 A literature survey of Ag3PO4 synthesized by different template and experimental techniques reveal that in this study porous Ag3PO4 MS exhibits highest surface area as shown in Table S2†. A relatively large surface area and porous architecture of Ag3PO4 provided more active


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sites and diffusion pathways for adsorption and transportation of reactants, which would be beneficial to the photocatalytic process.45, 46

Figure 2 FESEM (a, b) images of the as prepared irregular Ag3PO4 in (1:1), (1:10) ratio of Ag +:H2PO4- respectively and FESEM (c), TEM (d), HRTEM (e) and SAED (f) images of as-prepared Ag3PO4 porous MS.



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FESEM observation in Fig. 2a-c has revealed that the prepared Ag3PO4 from conventional route adopted irregular shape, and size in micrometer range where Ag3PO4 particles from BG assisted synthesis at room temperature are spherical in shape and nano scaled surface roughness. It is observed that the sample consists dominantly of regular separated and uniform Ag3PO4 MS with a diameter of 520 nm. It infers that the peripheral surface of the hierarchical sphere looks coarse and consists of many small Ag3PO4 nanoparticles with an average size of 15 nm. Fig. 2d displays a typical TEM image of the Ag3PO4 porous MS. It seems that the MS are porous in structure and composed of fine nanoparticles with an average size of 20 nm. The surface of the MS is identified as rough. In addition, HRTEM images (Fig. 2e) exhibited that all ultra-fine nanoparticles in the sphere show clear lattice fringes of inter planner distances separated by 0.218 nm and 0.338 nm, which are in good agreement with the plane spacing of {220} and {200} of cubic silver orthophosphate, respectively. The corresponding selected-area electron diffraction (SAED) (Fig. 2f) shows a spot pattern, representing that Ag3PO4 material is single crystalline in nature. Further, the spot patterns can be indexed as (210), (220), (211), and (420) planes of the cubic Ag3PO4 phase.

Figure 3 Morphological evolution of the porous Ag3PO4 MS by TEM with the reaction time at 30°C (a): 10 minute, (b):30 minute, (c):1 hour and (d): 4hour.


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Factors influences the formation of porous Ag3PO4 MS The factors influence the formation of morphology and size of the Ag3PO4 MS are revealed by systematic and controlled studies of the experiments. Reaction temperature, composition (BG content, the molar ratio of Ag+: H2PO4-), Templates and pH value in the precursor solution are crucial to the formation of the MS. Reaction time

Fig. 3 shows the morphological evolution of the Ag3PO4 MS synthesized at room temperature by HRTEM study at different times. At initial stage of the reaction, very small nano sized particles are formed, which then served as seeds for the further growth of Ag3PO4 MS. During the growth BG had aided the aggregation of Ag3PO4 particles via self-assembly process and formed stable uniform microspheres of diameter 520 nm.

Figure 4 FESEM images of Ag3PO4 MS obtained at different temperature (a) 5°C, (b) 30°C, (c) 60°C and (d) 120°C when BG and the precursor Ag+: H2PO4- are 1 gm and 1:10 respectively.



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Reaction temperature

The temperature determines the chemical rate of a reaction, stability of Ag3PO4 and controls the assembly behaviour of the nanoparticles generates as primary seeds. Hence it plays an important role in the formation of porous Ag3PO4 MS.

Figure 5. TEM images of Ag3PO4 MS obtained at different temperature (a) 5°C, (b) 30°C, (c) 60°C and (d)120°C when BG and the precursor Ag+: H2PO4- are 1 gm and 1:10 respectively.

When temperature varies from 5°C to 120°C a continuous increase of average crystallite size of Ag3PO4 nanoparticle from 14.48 nm to 28.7 nm is observed as calculated from the


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PXRD pattern using Debye Schere equation as shown in Fig. S4a† and Table S1† and above all particle size of Ag3PO4 MS at different reaction temperature from FESEM images (Fig. 4) and TEM images (Fig. 5) are in good agreements with above XRD results. FESEM image given in Fig. 4a clearly display that at low reaction temperature (5°C) diameter of the MS significantly increases to 2 µm with close packed structure. At room temperature (30°C) homogeneous and controlled aggregation of Ag3PO4 particles were taking place, which leads to form of stable uniform MS of diameter of 520 nm (Fig. 4b). Significantly non-uniform MS of Ag3PO4 are obtained at 60°C and 120°C (Fig. 4c, d). An interesting observation has been noticed from HRTEM images (Fig. 5) that increase in temperature increased hollowing of MS, which further effectively improved the porosity and the surface area of Ag3PO4 MS (Fig. 6a and Table 1). Ag3PO4 synthesized at 120°C abbreviated as Ag3PO4 MS 120 exhibits highest surface area (40.85m2/g), which is almost double to the surface area (28.13m2/g) of Ag3PO4 synthesized at 30°C.

Figure 6 N2 sorption isotherms of (a) Ag3PO4 MS synthesized at different temperature and (b) Ag3PO4 with different morphology synthesized in presence of alginic acid, glycine and alanine.

Table 1 N2 sorption isotherm parameter of Ag3PO4 microsphere synthesized at different temperature. Sl No.

Temperature (°C)

Surface area (m2/g)

Pore volume (cm3/g)

Pore Size (A0)

1 2 3

120 60 30

40.85 28.55 28.13

0.051 0.035 0.039

68.42 61.86 61.24

4

5

27.77

0.019

41.57



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From the results, it is noticed that temperature playing an effective role in the nucleation, self-assembling (s) and dissolution step. Temperature influences Ostwald ripening through its effect on interfacial energy, growth rate coefficients, and equilibrium solubility.47 Hence, it is expected that at higher temperature evacuation of smaller Ag3PO4 particles by Ostwald ripening will be prominent; which leads to form bigger sized particles by creating more space in the interior of the material. It is well known that tendency of nanoparticles to agglomerate through attachment and grow into larger size by self assembly is controlled by effective collision of the particle and with increase in temperature from 30°C to 120°C effective collision overcome the repulsive force forming bigger size MS around 1µm as shown in Fig. 4d. However the MS size increases when reaction temperature was kept at 5°C the reason may be attributed to the structural variation of the protein present in BG. At lower temperatures, structural rigidity and elasticity are developed on protein through cohesive force, and a continuous aggregation leads to the formation of bigger size MS with highly dense structure.48,

49

However,

detailed studies would be required to understand the process in the molecular level. pH value

Porosity and diameter size of Ag3PO4 are greatly influenced by the pH value of the precursor solution. Fig. S5a-5b† Shows the morphology of the samples when reaction takes place with different pH. We observe that at low pH (pH=4) formation of agglomerated Ag3PO4 of indefinite shape consisting of very small size nanoparticles. At low pH the structure of protein in BG became denatured by peptide bond breaking forming small molecule, which may discontinue the self-assembly process. Apart from that formation of Ag3PO4 is highly pH dependent. At low pH, the formations of PO43- are restricted and consumption of PO43- during the nucleation process could not take place and as a result self-assembly of particles into roughness nanotextured MS will be stopped. A well-defined porous MS was obtained when pH was maintained at 6-7, When the precursor was kept in basic medium pH=9 formation of PO43- increases as a result rate of nucleation and release of Ag+ during the process control the self assembly of nanoparticle and restrict the size of the MS under 200 nm with large porosity as confirmed by BET surface area measurement given in Table. S1†


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BG in the precursor solution

In the absence of BG, the unique morphology of Ag3PO4 MS could not be obtained as we already have discussed in result and discussion section. Its amount during the reaction played a crucial role for forming different size nanoseeds and self assembly behaviour. Morphological evolution of the Ag3PO4 synthesized with varying BG content in the precursor solution from 0 g to 2 g clearly tells that without BG in the precursor solution, only irregular Ag3PO4 is formed (Fig. S5c-5d†). Addition of a small amount of BG (0.2 g) leads to start the formation of nanoseeds and irregular MS as shown in Fig. S5c†. Very high quality uniform MS are formed when a proper content of BG (1 g) is added in the precursor solution as shown in Fig. 2c and Fig. 4b. When BG is added with high content (2 g), uniformity of the Ag3PO4 MS is changed, and MS became bigger in size of diameter of 700-800 nm from previous (Fig. S5d†). Interestingly, we observed a continuous broadening of the intense peak in the PXRD pattern of Ag3PO4 as shown in Fig. S4c† It reveals that the crystallite size effectively decreases with increasing BG content. The results suggest that the nucleation step is highly affected by the BG content. When there is an increase in BG contents (from 0.2 g to 2 g), the crystallite size of Ag3PO4 MS decreased from 47.8 nm to 12.8 nm. Template Variation

Collagen is a primary component of BG, and a nearly half of amino acids of collagen are glycine and alanine. Thus it is important to investigate the role of functionality, charge and polymeric structure of protein. Essentially BG templates were substituted by amino acids (glycine and alanine) during the synthesis keeping all the conditions same. It was found that the morphology of porous Ag3PO4 changes to pyramidal and dodecahedron shape with exposed facet as shown in Fig.7a-b. It means the structure directing ability of the amino acid shows different behaviour compared to the BG. The result clearly reveals the self assembly of amino acid bound Ag3PO4 nanoparticle could not be possible, rather they particularly adsorbed on the surface plane of Ag3PO4 crystal, and ultimate growth took place by exposing facets. Interesting when BG was substituted by alginic acid, Ag3PO4 formed with coiled rod like architecture. FESEM images shown in Fig. 7c, d clearly tells assembly of very small Ag3PO4 nanoparticle developed into two dimensional coiled rods like architecture. Noticeably BET surface area of self assembled architectures



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like Ag3PO4 MS and coiled rod like Ag3PO4 posses larger surface area in comparison to pyramidal or dodecahedron shape Ag3PO4 shown in Table S1†.

Figure 7 FESEM images of Ag3PO4 obtained when (a) alanine and (b) glycine and (c) alginic acid used as template keeping all the experimental condition same and (d) High resolution FESEM image of the Figure 7(c).

Effect of Ag+: H2PO4-

In further experiments, we observed that a continuous increase of crystallite and MS size when the molar ratio of Ag+: H2PO4- is changed from 10: 1 to 1:10. (Fig. S6†). When the ratio changes to 1:10, extra H2PO4- influences the assembly process and forms bigger size MS (Fig. S6f†). It indicates that H2PO4- in the solution not only plays a dominant role in the formation of the phase, but also exhibits an important effect on size of Ag3PO4 MS. BG directed self assembly and formation mechanism of Ag3PO4 architectures The detailed investigation reveals formation of Ag3PO4 proceeds through two major steps; nucleation and growth shown in scheme 2. The major change in morphology of Ag3PO4 architecture was found to depend on variation of temperature and templates. Templates are having functional group adopt the positively charged Ag and adsorbed on the surface of newly formed nucleus, once the nucleation stage had finished newly


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formed Ag3PO4 nuclei started to grow. Where growth of a crystal is a phenomenon either proceeds into bigger architectures by gradual consuming of the precursor or via aggregation by assembly process. Here all the templates bound Ag3PO4 nuclei are not essentially found to go self-assembly process. Self-assembly of nanoparticles is driven by strong aggregation behavior promoted by the templates. Shemetov et al. suggested aggregation of nanoparticle in protein mediated synthesis proceeds via either protein– protein interaction through structural perturbation or via completely electrostatic interaction50. Formation of uniform hierarchically Ag3PO4 MS from the subsequent aggregation of Ag3PO4 nanoparticles reveals fibrous protein collagen present in BG template is playing important role as a structure directing, aggregation and stabilizing agent. BG template and Ag3PO4 both posses same charge on the surface at experimental pH 7, as the isoelectric point calculated both are 5.37 and 5.90, respectively (Fig. S7†). It indicates that possibly the electrostatic interaction is not a crucial factor in the assembly process, rather protein-protein interaction through cohesive forces drives the self assembly process leading to hierarchical porous microsphere. Templates like BG, alginic acid also have long chain polymeric structure, structural rigidity and strong intermolecular interaction. Formation of coiled rod like morphology of assembled Ag3PO4 nano particle thus gives a strong evidence of self assembly process. Amino acid like glycine and alanine suffer extra functional groups and have less intermolecular interaction so they would prefer adsorption on crystal facets rather than aggregation and ultimate architecture will be decided by the growth rate of facets. Temperature was also found to tune the porosity and architecture of Ag3PO4 MS. We observed fresh formed Ag3PO4 MS changed to porous or hollow MS with an increase in the reaction temperature. As we know it is thermodynamically controlled phenomena the relatively smaller nanoparticles would dissolve and migrate from the interior to the exterior of the aggregates by evacuation process which is typically called Ostwald ripening process and it becomes prominent at higher temperatures. Once the small crystallites in the interior of the MS were exfoliated completely, it brings Ag3PO4 MS with an enlarged porous interior space by the hollowing effect51 therefore, by simply controlling the reaction time and temperature the MS could be effectively tuned from solid to porous structure.



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Scheme 2. Schematic illustration for the formation of porous (a) Ag3PO4 MS and Hollow microsphere (b) coiled rod like Ag3PO4. (c) Pyramidal shape Ag3PO4 (d) Ag3PO4 dodecahedron.

Photocatalytic activity of Ag3PO4 porous MS Detail investigation of controlled synthesis reveals that Ag3PO4 MS has improved band gap structure, high surface area, good crystalinity and porous structures. PL emission spectra were measured as shown in Fig. S8† and PL intensity of the microsphere was found very small in comparison to the dodecahedron, pyramidal shaped Ag3PO4 which suggests that the lower recombination of active species during photo induced electronic transition would give extraordinary performance in photocatalysis. We have seen porous Ag3PO4 MS synthesized at different temperatures varied in surface area and Size. So to use the surface area as an effective parameter the photocatalytic activity was evaluated by testing their ability to oxidize Rhodamine B (RhB) and 2, 4 DCP as model compound under visible light at room temperature. Fig. 8a illustrates the time dependent absorption


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spectra of the RhB solution after degradation by the porous Ag3PO4 MS synthesized at 30°C and 553.5 nm was taken as a characteristic absorption peak of RhB to monitor the process of degradation. The intensity of the peak decreased very quickly under visiblelight illumination, with a slight shift of the maximum absorption peak to shorter wavelengths, which suggests that the photo degradation of RhB occurs via destruction of the structure52 and after 15 minutes the solution becomes colourless as shown in an inset in Fig. 8a. For comparison, the degradation curves of RhB by porous Ag3PO4 prepared at different temperatures, irregular Ag3PO4 sample, and commercial Degussa P25 powder and in the absence of catalyst under the same conditions are also illustrated in Fig. 8b. The efficiencies of degradation are defined as Ct/C0, where Ct and C0 represent the remnant and initial (after adsorption) concentration of RhB, respectively. It can be revealed that the porous Ag3PO4 MS prepared at 5°C, 30°C and 120°C showed a higher rate in degrading RhB compared with the samples P25 and irregular Ag3PO4 obtained at 15 minute this is because of the relatively large surface area, better crystallinity and narrower bandgap (Fig. 6 and Table 1). Interestingly with increasing surface area the adsorption capacity of the Ag3PO4 MS before visible light irradiation increases as shown in Fig. 8a and Fig. 8b. The degradation of RhB in different catalysts follow the first order kinetics and it was observed the rate of degradation of RhB in presence of Ag3PO4 MS prepared at 120°C becomes 4 time faster than , irregular Ag3PO4 as shown in Fig. 8c. From kinetic data, the apparent rate constant value of the catalysts for the RhB degradation followed the order: k

Ag3PO4

120

(41.87 × 10-2 min-1) ˃ k

Ag3PO4

30

(21.16 × 10-2 min-1) ˃ k Ag3PO4 5 (16.02× 10-2 min-1) ˃ k Ag3PO4irregular (10.28×10-2 min-1) ˃ k P25

(2.24× 10-2 min-1) ˃ k

without

(0.014× 10-2 min-1 min). So the Ag3PO4 MS prepared at

120°C has highest photocatalytic activity and the percentage of degradation of RhB is near to 100 within 15 minute. It has been also observed that our prepared Ag3PO4 MS exhibits a higher rate of photocatalytic activity in compare to synthesized Ag3PO4 dodecahedron, Ag3PO4 pyramidal and coiled Ag3PO4 rod as shown in Fig. 9. The details of the photocatalytic activity and rate constant are given in the Table 2.



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Figure 8 a) Absorption spectrum of a solution of 30 ppm RhB after visible-light irradiation for different times in the presence of porous Ag3PO4 MS. b) Discoloration curves in dark and light and c) first-order kinetic plots for the discoloration of a solution of RhB with (A) no catalyst , (B) P25, (C) irregular Ag3PO4 and different Ag3PO4 MS synthesized at (D)5°C, (E) 30°C and (F)120°C, under visible irradiation. (d) % of dye degradation by catalysts within 15 minute.

Interesting to observe that photocatalytic activity of catalyst increased with an increase in surface area. Ag3PO4 MS 120 having larger surface area exhibits superior photocatalytic activity and the lower rate of degradation of RhB in presence of another Ag3PO4 architecture could be well explained by the high rate of recombination process as confirmed by PL measurement earlier.


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Figure 9 (a) Ct/C0 vs time and (b) ln Ct/C0 vs time plot for the degrdation of RhB dye in presence of prepared (i) Ag3PO4 dodecahedron, (ii) Ag3PO4 pyramidal (iii) coiled rod Ag3PO4 and (iv) Ag3PO4 MS 120. Table 2 Photocatalytic activities of synthesized Ag3PO4 with different morphologies and porosity for RhB degradation Entry No.

Ag3PO4

1

Irregular

2

Dodecahedron

Photocatalytic activity (%) in 15 min 81 83

Rate constant (min-1)

Surface area (m2/g)

10.28×10-2

-

10.96×10

-2

21.36

-2

3 4 5

Pyramidal Coiled rod Hollow MS 120

85 92 100

13.43×10 18.71×10-2 41.87×10-2

24.00 29.13 40.85

6 7 8

MS 30 MS 5 P 25 (TiO2)

97 93 29

21.16×10-2 16.02×10-2 2.24×10-2

28.13 27.77 -

Furthermore, 2, 4 DCP was taken as a model compound to check the photocatalytic activity of the prepared Ag3PO4 MS (synthesized at different temperature). The reaction kinetics was studied by a HPLC chromatogram connected to a UV-Visible detector with the detecting wavelength 284 nm.53 Fig. 10a depicts the HPLC chromatograms of the photo degradation study of 2, 4 DCP. The chromatograms of the aliquots show a gradual decrease over time. Fig. 10b,c show Ct/C0 plot and first-order kinetic plots for the degradation of 2, 4 DCP in presence of different Ag3PO4 MS photocatalyst (synthesized at 120°C, 30°C, 5°C) and compared with the rate of degradation with irregular Ag3PO4 and P25. The apparent rate constant values of the photocatalysts were found to follow the following order: k

Ag3PO4

120

(7.9× 10-3 min-1) ˃ k

Ag3PO4

30


(5.2 × 10-3 min-1) ˃ k

Ag3PO4

5


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(2.3× 10-3 min-1) ˃ k

Ag3PO4

irregular

(1.3×10-3 min-1) ˃ k

P25

Page 24 of 37

(0.6× 10-3 min-1).The result

showed that the prepared Ag3PO4 MS synthesized at 120°C follows higher rate (6 time) of removal of 2, 4 DCP compared to irregular Ag3PO4 and it is almost 99% within 60 minute of span of visible light irradiation.

Figure 10. (a) HPLC chromatogram of photo degraded 2, 4 DCP (initial 30 ppm) at regular time interval in presence of Ag3PO4 MS 120°C catalyst (20 mg). (b) Ct/C0 plot in dark and light and (c) first-order kinetic plots of the degradation of 2, 4 DCP in presence of different Ag3PO4 MS synthesized at (A) 120°C, (B) 30°C, (C) 5°C, (D) irregular Ag3PO4 and (E) P25 under visible light irradiation. (d) EPR spectra of radical adduct trapped by TEMPO (h+) in Ag3PO4 MS 30°C in the dark and under visible light irradiation (in aqueous dispersion for TEMPO-h+ )

Interestingly, photocatalytic activity of the catalysts for 2, 4 DCP degradation follows the similar order of rate of RhB degradation. The result may be attributed to the increased surface area of the Ag3PO4 MS as discussed earlier.


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Details of comparative degradation study of different pollutants using Ag3PO4 with another morphology reported in literature are given in Table S2† where we found our prepared Ag3PO4 MS exhibits superior photocatalytic activity. Moreover, reactive species trapping experiments were performed in order to investigate the reactive oxygen species in the photocatalytic process of RhB degradation. Three chemicals, p-benzoquinone (a O2−· radical scavenger), disodium ethylenediaminetetraacetate (Na2-EDTA, a hole scavenger) and tert-butanol (a ·OH radical scavenger) were employed. The rate of RhB degradation is significantly decreased in presence of disodium ethylenediaminetetraacetate, where no significant changes in rate, has not been found in presence of p-benzoquinone. It means O2 is not playing any important role. The results may be attributed to the inability of accumulated electrons in the CB of Ag3PO4, which could not reduce O2 to produce O2−·, due to the more positive edge potential of Ag3PO4 (+0.45 eV) than that of O2/O2−· (−0.33 eV vs. NHE). In contrast, tert-butanol has only smaller scavenging effect, and the rate of degradation affect for a minor extent as shown in Fig. S9†, which indicate that hole is the main reactive species for the direct oxidation of RhB with minor .OH mediated oxidation. It concludes degradation of RhB follows a similar mechanism as reported in earlier literatures54, 21. Moreover, to get better insight of main reactive species EPR techniques had been employed using TEMPO as a spin trapper (TEMPO-h+)

55

and an indirect measurement (fluorescence

techniques) has been chosen for ·OH radical using terepthalic acid (TA) as a probe.56 As shown in Fig. 10d, the signal of spin trapped TEMPO-h+ in dark condition was significant while it decreased to a greater extent when exposed to visible light irradiation and very faint signal at the irradiation of 15 min. The result contributed that holes are playing the main role in the enhancement of photocatalytic performance where in Fig. S10† a non-fluorescent compound TA, which gets converted to fluorescent 2-hydroxy terepthalic acid (HTA) upon reaction with ·OH radical and increases in the fluorescent intensity with respect to time (5 min, 10 min and 15 min) on visible light irradiation confirms the formation of ·OH radical in the presence of Ag3PO4 MS. Reusability and Stability of Ag3PO4 MS The photocatalytic experiment is repeated up to five times under the same conditions to check the photo-stability of porous Ag3PO4 MS prepared at 30°C and a set of five cycles is provided in the discussion. After every cycle, the catalysts are washed with ethanol and water multiple times to remove any adsorbed RhB dye from the surface of the catalyst. As shown in Fig. S11†, the Ag3PO4 MS not only exhibits higher photocatalytic activity, but



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also higher photo-stability and rate became 88% in 5th cycle, which indicates the photocorrosion of the catalyst after 4th cycle started. The bright yellow colored porous Ag3PO4 turned into black (Fig. S11† inset) and PXRD pattern of the catalyst after 4th cycle gives extra diffraction peaks at (111), (200), (220) plane, which are the characteristic PXRD pattern of metallic Ag deposited on Ag3PO4 MS (Fig. S4d†). For more significant information, XPS has been done of the used samples and Fig. 11a, shows the XPS survey scan of used Ag3PO4 MS 30°C. The Ag 3d5/2 and Ag 3d3/2 peaks could be divided into four characteristic peaks, where 373.1 and 374.3 eV belonged to Ag 3d3/2 and the other two peaks (367.2 and 368.2 eV) were for Ag 3d5/2, respectively as shown in Fig. 11b. The two new peaks situated at 374.3 and 368.2 eV were assigned to elemental Ag.57, 58 The relative atomic concentration of Ag0 was estimated to be 4.6% and 7.4% after 3rd and 4th cycle as shown in Table S3†. The photocatalytic activity was not affected abruptly due to slight increase of Ag0 content on the material. The Ag3PO4 MS does not lose photocatalytic activity significantly even at the 5th cycle, and it was almost similar (88%) compared to 4th cycle (90%). The slight decrease in photocatalytic activity may be attributed the shielding of absorption of incident light by Ag0 content (7.40%) on Ag3PO4 MS. Thus, the result implies that SPR effect of Ag58,

59

is also counteracting with

aforementioned shielding effect in the photocatalytic process.

Figure 11. (a) XPS survey scans of fresh Ag3PO4 MS 30°C and used Ag3PO4 MS 30°C (after 4th cycle). (b) Comparison of the Ag 3d XPS spectra of Ag3PO4 MS 30°C and used Ag3PO4 MS 30°C (3rd cycle and 4th cycle).


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There are few factors playing a vital role in enhancing the photocatalytic ability of the porous Ag3PO4 MS. In the list, narrowing the band gap of the MS facilitates easier production of electron and hole; improved porous structure accelerates the rate of diffusion as well as exchange of reaction intermediates, and overall synergistic effect of hierarchical porous structure improves the light harvesting property of the catalyst resulting enhancement in the photocatalytic behaviour.

Conclusions In summary, we have provided a facile and very efficient synthetic route to design hierarchical Ag3PO4 porous MS assembled by well-defined nanosized particles with the assistance of BG. In addition, we have proposed a possible formation mechanism based on the investigation of the effect of reaction parameters, e.g.; Temperature, time, pH, precursor concentration, BG content and templates in controlling the morphology, porosity and crystallite size of Ag3PO4. Interestingly, the self-assembly of Ag3PO4 nanoparticles as well as their Ostwald ripening (in different temperature) processes are identified as crucial factors in the formation of porous MS, which regulate the ultimate shape and porosity of the prepared Ag3PO4 MS. The obtained Ag3PO4 porous MS has uniform size, porous structure and narrow bandgap, which exhibit superior photocatalytic activity under visible light as compared to that of irregular Ag3PO4 and commercial P25 and other Ag3PO4 architectures. The enhanced photocatalytic activity could be attributed to the large surface to the volume ratio, more surface active sites, narrow bandgap and synergistic effect of porous structure. Mean while obtained photocatalysts can be advantageous for degradation of organic pollutants as our method for synthesis of photocatalysts involves simple synthetic strategy, green and inexpensive biomolecule as a template. Moreover, the BG template could be useful to design other porous materials for various new applications.

Acknowledgements The author, Subrata Mandal, thank the UGC for research fellowship. The authors extend their acknowledgements to DST, New Delhi (for FESEM facility), CRF-IITKGP (for TEM and HRTEM analyses), and Department of Physics, IIT KGP (for XPS analysis).



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Associated Content Supporting Information Raman spectra of Ag3PO4 MS 30°C (before and after wash) and irregular Ag3PO4; TG Curve of Ag3PO4 MS 30°C and Template; XPS spectra of Ag3PO4 MS 30°C; XRD patterns, FESEM images and TEM images of Ag3PO4 MS prepared at different condition; Zeta potential of BG and Ag3PO4 measured at different pH; PL spectra of various Ag3PO4 architecture; Kinetics of reactive species trapping experiment; Fluorescence spectra of ·OH radical adducts by terepthalic acid as a probe; Recyclability and stability check of Ag3PO4 MS; Table of synthesized parameters of Ag3PO4 microsphere; Table synthesized parameters of reported Ag3PO4 with different morphologies and their photocatalytic activities; Table of surface atomic concentration of Ag and Ag+ calculated from XPS.

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Highlights 

Fabrication of uniform porous Ag3PO4 microspheres assisted by noble and cheap biomolecule “Bone glue” as a green template and control of microspheres to dodecahedron and pyramidal shapes Ag3PO4 by Glycine or Alanine (constituents of the template) has been demonstrated in this work.



Characterization of porous Ag3PO4 Microspheres synthesized in different experimental condition by FT-IR, XRD, RAMAN, XPS, DRS, BET, FESEM, HRTEM, and TEM to reveal the formation mechanism.



Investigation of Time-dependent evolutions of morphologies of Ag3PO4 and discussion of the influence of experimental condition.



Prepared porous Ag3PO4 shown significant visible-light photocatalytic activity towards the degradation of Rhodamine B (RhB) and 2, 4 dichlorophenol.



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Table of Contents Synopsis Uniform hierarchical Ag3PO4 porous microspheres were synthesized by natural bone glue assisted one-step reaction. Characterization studies revealed, morphology and textural properties of the material could be controlled by template constituents. The optimized strategy resulted hierarchical structures having large surface area and superior photooxidative capacity towards organic dye and 2, 4 dichlorophenol than simple Ag3PO4 structures.


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TOC: Sustainable Design of Hierarchically Porous Ag3PO4 Microsphere through a Novel Natural Template and Their Superior Photooxidative Capacity 2341x965mm (96 x 96 DPI)


Sustainable Design of Hierarchically Porous Ag3PO

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