Surface-Plasmon-Resonance-Induced Photocatalysis by Core–Shell

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Surface-Plasmon-Resonance-Induced Photocatalysis by Core−Shell SiO2@Ag NCs@Ag3PO4 toward Water-Splitting and Phenol Oxidation Reactions Satyaranjan Mohanty, Pradeepta Babu, Kulamani Parida,* and Brundabana Naik* Centre for Nanoscience and Nanotechnology, Siksha ‘O’ Anusandhan, Bhubaneswar 751030, India

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ABSTRACT: A series of core−shell-structured SiO2@Ag NCs@Ag3PO4 photocatalysts with varying percentages of silver nanoclusters (Ag NCs) have been synthesized by using SiO2 as a core material. The crystal structure, morphology, chemical composition, and photophysical properties of assynthesized materials have been thoroughly analyzed through powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, UV−vis diffuse-reflectance spectroscopy, and photoluminescence (PL) spectroscopy techniques. The introduction of Ag NCs has effectively reduced the photogenerated electron−hole recombination rate, as evidenced from PL and Nyquist plots. The electrochemical properties of the photocatalysts were studied through photocurrent measurement, and 23-fold current enhancements in the cathodic direction are observed. The excellent current enhancement by the photocatalyst is attributed to the presence of Ag NCs. The effectiveness of the photocatalysts toward photocatalytic water splitting was studied and produced 2460 μmol h−1 g−1 hydrogen and 1236 μmol h−1 g−1 oxygen by 2 wt % loaded Ag NCs. Again the photocatalytic phenol oxidation has been explored, and the best catalyst is able to oxidize 91% phenol upon visible-light irradiation. The photocatalyst having 2 wt % Ag NCs shows better activity toward both water splitting and phenol oxidation compared to others, which is attributed to better visible-light absorption efficiency, lower electron−hole recombination rate, and low interfacial charge-transfer resistance.

1. INTRODUCTION Green generation of clean and renewable energy and pollution abatement for environmental remediation is the need of the hour. Therefore, photocatalytic water splitting or artificial photosynthesis for the production of clean hydrogen (H2) fuel is emerging as one of the hottest topics in current research.1−3 The overall water-splitting reaction involves a two-electronreduction process for H2 evolution and a four-electron-transfer process for oxygen (O2) evolution through water oxidation.4 In this regard, a semiconductor photocatalyst having a low bandgap energy enables enriched solar-light absorption, and a photon-to-charge-carrier conversion efficiency as well as a low charge-carrier recombination rate is the best.5,6 Among semiconductors, TiO2 has been a promising photocatalyst because of its high efficiency, good stability, high oxidizing power, and low cost. However, it suffers from insufficient utilization of solar light because its wide band gap (3.2 eV) is limited to the UV range.7,8 Hence, alternative visible-light active photocatalysts such as BiVO4, WO3, g-C3N4, and CdS have been highlighted in past decades.9−13 In this regard, silver phosphate (Ag3PO4) is gaining considerable attention because of its high photocatalytic ability, positive conduction band (CB) position for water oxidation to generate O2, and low toxicity.14 The valence (VB) and CB bands of Ag3PO4 are at © XXXX American Chemical Society

+2.9 and +0.45 V. The deeper VB position is responsible for reactions like the oxidation of organic substrate and water oxidation to produce O2. However, the position of the CB is not suitable for the reduction of protons to generate H2 and thereby unfit for an overall water-splitting reaction.15 Hence, various modifications on Ag3PO4 have been demonstrated from time to time to enhance the photoactivity and overall water-splitting reaction to generate H2 and O2. Yi et al. have synthesized Ag3PO4, which shows better photocatalytic activity toward water oxidation reaction compared to BiVO4 and WO3 under similar reaction conditions and generates 636 μmol h−1 O2.16 Yang et al. have decorated Ag3PO4 on 2D g-C3N4, which produces 25 μmol L−1 O2 upon visible-light irradiation.17 Taking the advantages of a multifaceted crystal structure, Indra et al. have reported a faceted Ag3PO4 for the photooxidation of water and compared its activity with irregular-shaped particles. The sizes of polyhedral particles range from 600 to 700 nm, consisting of smooth facets and sharp edges and producing 280 μmol L−1 O2.18 Hou et al. have enhanced the photocatalytic O2 evolution activity of Ag3PO4 by employing the synergistic effect of Ag/AgBr and graphene sheets. The increase in the Received: January 23, 2019

A

DOI: 10.1021/acs.inorgchem.9b00218 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

ance of a Ag NP upon methylene blue degradation.37 The photocatalytic activity of the system is able to surpass the activity compared to nitrogen-doped P-25 TiO 2 and individually dispersed Ag NPs. Zhou et al.’s SiO2/Ag/SiO2/ TiO2 with a SiO2 interlayer of 5 nm shows better photocatalytic activity than its other counterparts upon UV- and visible-light irradiation.38 The enhanced photocatalytic activity for RhB degradation is ascribed to the LSPR and scattering effect provoked by Ag NPs. A Schottky barrier formed at the plasmonic metal NPs and semiconductor interface is responsible for the enhancement of charge separation through transfer of hot charge carriers from metal to semiconductor in a thermodynamically irreversible way.32,34 Lin et al. have designed a plasmonic Ag@ Ag3(PO4)1−x NP photosensitized ZnO nanorod−array photoanodes for photoelectrochemical water oxidation.39 The photosensitized sample shows good photoactivity in visible light, where the photoconversion efficiency is 2% and is attributed to the LSPR of the Ag core and the absorption edge of Ag3(PO4)1−x. Wang et al. have prepared a heterostructured Ag3PO4/AgBr/Ag plasmonic photocatalyst by in situ ionexchange reaction followed by photoreduction.40 The observed photodegradation enhancement can be attributed to the synergistic effect of the LSPR from Ag NPs and the appropriate energy band alignment of the photocatalyst. A novel p−n heterojunction for LaFeO3 and Ag3PO4 has been demonstrated by Yang et al. via an in situ precipitation method that exhibits excellent photoactivity toward phenol oxidation.41 In order to study the plasmonic effect of silver nanoclusters (Ag NCs) on the Ag3PO4 semiconductor, herein we have synthesized a core−shell-nanostructured photocatalyst for the first time. Spherical silica NPs having around 80 nm diameter were synthesized via a modified Stobber method and used as cores, and Ag NCs were deposited using a chemical reduction method along with a Ag3PO4 microcrystal used as the shell. The catalyst shows excellent photocatalytic activity toward a water-splitting reaction and phenol oxidation. The effect of small-sized plasmonic Ag NCs and their weight percent variation on the physicochemical properties of Ag3PO4 have been thoroughly discussed.

photoactivity is attributed to depletion of the CB of Ag3PO4 and a shifting of the VB due to the pinning of the CB to the Fermi level of silver (Ag).19 Zhai et al. have engineered multiwall carbon nanotubes with Ag3PO4 to form pickering emulsion, which increases the photooxidation rate of water. The multiwall nanotube is responsible for the transfer of photogenerated electrons through the π−π network and increases the photostability of Ag3PO4.20 Xiang et al. have incorporated Ag3PO4 into graphene oxide nanosheets, which enhances the charge separation and increases the photocatalytic activity toward methylene blue degradation.21 A hierarchical porous Ag3PO4 microcube that exhibits 2-fold enhancement in the photocatalytic activity for rhodamine B (RhB) degradation has been prepared by Liang et al. The enhancement in the photocatalytic activity is attributed to an increase in the surface area due to mesoporosity.22 Dong et al. have demonstrated morphology-dependent Ag3PO4, including branch, tetrapod, nanorod, and triangular prism with porous structure, and explored their photocatalytic activity toward dye degradation. The branched Ag3PO4 shows the highest photodegradation activity because of its large specific surface area, which facilitates the adsorption of dye molecules.23 Chen et al. synthesized a visible-light-driven Ag3PO4/AgI photocatalyst through the ion-exchange method. It showed excellent photocatalytic activity toward methyl orange and phenol degradation, which is ascribed to the effective separation of electron−hole pairs through a Z-scheme hybrid system.24 Cai et al. constructed a Ag3PO4-based Z-scheme photocatalytic system, and its photocatalytic activity was studied toward RhB and 2,4-dinitrophenylhydrazine degradation under natural sunlight. The high activity is attributed to the effective transport of electrons away from Ag3PO4, thereby reducing its photocorrosion.25 A Ag3PO4/Ti3C2 Schottky catalyst has also been reported by the same group, where Ti3C2 can enhance the photocatalytic activity and stability of Ag3PO4.26 The addition of plasmonic metal nanoparticles (NPs) with a semiconductor has been pursued to increase their photocatalytic activity.27,28 A core−shell structure has been effective in the field of photocatalysis because of its unique property of inhibiting the agglomeration of catalysts. Metal NPs are more effective under a core−shell structure compared to their monometallic counterparts.29 The increase in efficacy is attributed to their stability against a corrosive catalytic environment. Excitation of the localized surface plasmon resonance (LSPR) in metal NPs through proper incident light produces a collective oscillation of electrons and creates a strong electromagnetic field that can excite the surrounding semiconductor.30 Various mechanisms have been predicted to explain the transfer of hot charge carriers (i.e., electrons and holes) either through radiative energy transfer at metal− semiconductor Schottky junctions31,32 or directly via the nonradiative plasmon-induced energy-transfer process.33,34 Hence, a substantial increase in solar-light absorption from UV to visible light is obtained. However, by tuning the shape and size of the plasmonic NPs, the photocatalytic efficiency can be changed because this affects the light absorption and charge-separation process.34,35 Tang et al. have synthesized the silica−silver core−shell structure with a uniform silver shell layer of 12 nm thickness by a facile and one-step ultrasonic electrode deposition method.36 Chen et al. have demonstrated a silver nanoparticle (Ag NP)-coated silica nanosphere having a diameter of 400 nm and studied the effect of a surface plasmon resonance (SPR)-mediated photocatalytic perform-

2. EXPERIMENTAL PROCEDURES 2.1. Materials. Tetraethyl orthosilicate (TEOS), (3aminopropyl)triethoxysilane (APTES), silver nitrate (AgNO3), disodium hydrogen phosphate (Na2HPO4), sodium borohydride (NaBH4), trisodium citrate (Na3C6H5O7), absolute ethanol (EtOH), and ammonia solution (NH4OH, 25%) of analytical grade were purchased from Merck India and used without purification. Deionized water obtained from a Millipore system was used throughout the reactions. 2.2. Synthesis of SiO2 NPs and Amine Functionalization. Silica NPs were prepared by a modified Stober’s method,42 which involves hydrolytic condensation of TEOS in the presence of NH4OH. In a typical procedure, a solution containing 480 mL of absolute EtOH, 7.75 mL of 25% NH4OH, and 20 mL of deionized water was stirred for 15 min to ensure complete mixing. A total of 25 mL of TEOS was injected into the above solution, and the reaction proceeded at room temperature for 24 h. Thereafter, the colloidal solution was separated by centrifugation at 10000 rpm and subsequently washed with absolute EtOH three times to remove undesirable particles, followed by drying in an oven at 100 °C for 12 h. The as-prepared SiO2 NPs were treated with APTES for amine functionalization. A total of 3 g of silica NPs was dispersed properly in 150 mL of EtOH in a round-bottom flask through sonication, followed by the addition of 3 mL of APTES.38 The solution was then B

DOI: 10.1021/acs.inorgchem.9b00218 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Detailed Scheme Showing the Synthetic Procedure of SiO2@Ag@Ag3PO4, Where the First Step Involves the Preparation of SiO2 Nanoparticles followed by Amine Functionalizationa

In the second step, Ag NCs is deposited by the chemical reduction method and Ag3PO4 by the ion-exchange method and calcined at 500 °C.

a

refluxed at 80 °C for 2 h, and the obtained product was centrifuged and dried at 100 °C for 12 h. 2.3. Synthesis of SiO2@Ag. In a typical procedure, 1 g of APTES-functionalized silica NPs was thoroughly dispersed in 500 mL of distilled water. A 1 mM AgNO3 solution was added to the above solution and stirred for 30 min to ensure complete mixing. To this solution was added 10 mL of a 4.28 mM trisodium citrate solution. After 15 min, 5 mL of a 2 mM NaBH4 solution was added dropwise until there is an apparent color change from white to grayish yellow. The final solution was left for an incubation period of 12 h and then centrifuged, followed by drying at 80 °C for 12 h. Different weight percentages (i.e., 1, 2, and 3 wt %) of Ag NCs were loaded onto SiO2 NPs following the same procedure and denoted as SPS-1, SPS-2, and SPS-3, respectively. 2.4. Synthesis of SiO2@Ag@Ag3PO4. Ag3PO4 was synthesized by the ion-exchange method. For all 3 wt % of Ag NCs, Ag3PO4 amount was kept constant (i.e., 10 wt %). In detail, the as-synthesized SiO2@Ag samples were dispersed in 200 mL of distilled water by sonicating for 30 min. AgNO3 and Na2HPO4 were added to the solution in a 3:1 ratio (Ag+/PO43−) for the development of Ag3PO4 microcubes.43 After an incubation period of 12 h, the solution was washed with water and EtOH several times and dried at 80 °C overnight. The dried samples were then calcined at 500 °C for 4 h. For reference, SiO2@Ag3PO4 was denoted as the SPS-0 sample and prepared using the same method. The detailed synthesis procedure is demonstrated in Scheme 1. 2.5. Photocatalytic Water Splitting and Phenol Oxidation. A photocatalytic water-splitting reaction for H2 and O2 evolution was performed by taking 50 mg of each catalyst powder dispersed in 30 mL of a 10 vol % triethanolamine (TEOA) solution and AgNO3, where TEOA and AgNO3 act as hole and electron scavengers, respectively, in a 100 mL sealed quartz batch reactor with a 150 W xenon lamp (≥420 nm) as the light source and with a 420 nm cutoff filter. The solution was stirred with a magnetic stirrer to disperse the photocatalyst in the reaction medium, and the solution was purged with nitrogen several times to remove dissolved gas prior to light irradiation. The evolved gas was collected by downward displacement of water, analyzed by a gas chromatograph fitted with a thermal conductivity detector, and found to be H2 and O2. Phenol was taken as a model pollutant to study the photocatalytic performance of the catalysts, which were dispersed thoroughly in the phenol solution (20 mg L−1, 20 mL) and left in the dark for 30 min to achieve adsorption−desorption equilibrium prior to exposure of sunlight

irradiation for 120 min. After that, the solution was centrifuged at regular intervals, and the concentration was determined using a UV spectrophotometer at a wavelength of 270 nm.

3. CHARACTERIZATION The crystal structure of the synthesized materials was analyzed with a Rigaku Miniflex X-ray diffractometer within the range 10−80° at a scan rate (2θ) of 2° min−1 with a Cu Kα irradiation source (λ = 1.54 Å). High-resolution transmission electron microscopy (HRTEM; JEM 2010, JEOL, Japan) was used to examine the structure and morphology of the photocatalyst, which is built with an energy-dispersive X-ray spectrometer (INCA, Oxford Instruments, U.K.). The microstructure of the photocatalyst was analyzed through fieldemission scanning electron microscopy (FESEM). A Kratos Axis 165 instrument attached with a Mg Kα source was used to perform X-ray photoelectron spectroscopy (XPS) of the photocatalysts. The specific surface area and pore-size distribution were analyzed using TriStar 3000 after degassing the photocatalysts at 250 °C. A JASCO-750 UV−vis spectrophotometer was used to measure the UV−vis diffusereflectance spectra of all synthesized samples in the range of 200−800 nm, taking boric acid as a reference. The photoluminescence (PL) emission spectra were measured by exciting all of the samples at 350 nm using a xenon lamp as the excitation source in a JASCO-FP-8300 spectrophotometer. Electrochemical experiments were done on an IVIUMnSTAT electrochemical workstation using platinum and Ag/AgCl electrodes as the counter and reference electrodes, respectively, and a 300 W xenon lamp as the light source. An electrophoretic deposition method was taken into consideration for the preparation of working electrodes, where 20 mg of each photocatalyst and iodine were dispersed in 20 mL of acetone with proper sonication. Linear-sweep voltammetry (LSV) was done in the potential range from −1.0 to −0.7 V with a scan rate of 25 mV s−1 in the dark as well as under light irradiation. Electrochemical impedance spectroscopy (EIS) of the catalysts was performed at 105−10−2 Hz frequency in the dark at zero biased potential. All electrochemical studies have C

DOI: 10.1021/acs.inorgchem.9b00218 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Powder XRD showing peaks of SiO2, Ag3PO4, and Ag NCs in SPS-0, SPS-1,SPS-2, and SPS-3. (b) HRTEM images of SPS-2. (c) Lattice fringe showing the (210) and (211) planes of Ag3PO4. (d) SAED of SPS-2 showing the (210) and (421) planes of Ag3PO4, (e) Particle size distribution curve showing the average particle size and deviation of Ag NCs. (f) Energy-dispersive X-ray spectra showing the presence of silicon, silver, phosphorus, and oxygen in the sample.

Figure 2. XPS spectra of (a) Si 2p, (b) O 1s, (c) P 2p, and (d) Ag 3d present in the as-synthesized SPS-2 sample.

D

DOI: 10.1021/acs.inorgchem.9b00218 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) UV−vis and (b) PL spectra of as-synthesized SPS-0, SPS-1, SPS-2, and SPS-3.

indicates the presence of constituent elements in the material. The low particle size of Ag NP decorated in 80 nm SiO2 NPs is linked to the excellent photocatalytic activity by the catalyst toward water-splitting and phenol oxidation reactions. 4.3. Chemical State Analysis. XPS was carried out in order to analyze the chemical state, electronic environment, and oxidation state of the elements present in the photocatalyst.46 The XPS survey spectrum shows four distinct peaks of silicon, silver, phosphorus, and oxygen and is linked to the successful formation of the material. Another peak at 284.5 eV is observed for carbon taken as a reference for XPS analysis (Figure S5). Further, the oxidation states of the elements were investigated by deconvolution of the peaks through CASA XPS software. The core level of Si 2p (Figure 2a) was deconvoluted into two peaks at 103.0 and 102.2 eV, which are attributed to the Si−O bonding in silica and Si−O−Si linkage of silicone present in the sample, respectively.47 As revealed from Figure 2b, the core level of O 1s contains two peaks at 532.2 and 533.1 eV. The peak at 532.2 eV represents the characteristic peak of phosphates, and the peak at 533.1 eV is attributed to the O2 present in silica NPs. The core level of P 2p can be deconvoluted into two peaks at 132.7 and 133.5 eV, which correspond to the 2p3/2 and 2p1/2 states of phosphate, respectively (Figure 2c). Figure 2d represents two distinct peaks of Ag 3d at 367.5 and 373.5 eV, ascribed to 3d5/2 and 3d3/2 spin states, respectively. Ag 3d5/2 upon further deconvolution gives two peaks at 367.5 and 368.6 eV. The peak at 367.5 eV is ascribed to 1+ oxidation state of silver in Ag3PO4, and the peak at 368.6 eV represents Ag NCs consisting of Ag NPs in metallic form.48−50 The relative concentration of metallic silver (Ag NCs) was calculated to be 17.7% by analyzing the area under the Ag0 and Ag+ peaks obtained from deconvolution of Ag 3d XPS spectra using the following equation51 and depicted in Table S1:

been performed by using a 0.1 M Na2SO4 solution. Mott− Schottky analysis was performed at three different frequencies of 500, 1000, and 1500 Hz using 0.1 M Na2SO4 of pH = 6.1.

4. RESULTS AND DISCUSSION 4.1. Crystal Structure. Powder X-ray diffraction (XRD) studies were conducted to analyze the crystal structure, crystallite size, and phase purity of the prepared samples. The XRD pattern of SiO2@Ag@Ag3PO4 exhibits cubic structure with the space group P4̅3n (No. 218) with a unit cell length of 6.004 Å. The diffraction peaks in Figure 1a observed at 2θ = 29.71°, 33.36°, 36.46°, 47.76°, 52.68°, 55.05°, 57.42°, 61.79°, and 72.00° are indexed to planes (200), (210), (211), (310), (222), (320), (321), (400), and (421), respectively, of cubic Ag3PO4 (JCPDF 06-0505).44 The most intense peak observed at 33.36° indicates the crystallinity of Ag3PO4. The peak observed at 20.81° is due the SiO2 sphere used as a hard template. The loading of Ag NCs was not observed because of the low content and uniform disperson throughout the surface of SiO2 NPs. The crystallite sizes were calculated using Scherrer’s equation (L = Kλ/β cos θ), where L = crystallite size, K = shape factor, λ = wavelength of X-ray, β = line broadening at half the maximum intensity, and θ = Bragg’s angle,36 and found to be 44.1, 66.32, 73.3, and 55.6 nm, respectively. 4.2. Morphology Study. HRTEM analysis was performed to study the morphology, lattice fringes, and selected-area electron diffraction (SAED) pattern of the as-synthesized samples.45 The HRTEM image reveals the formation of SiO2 nanospheres of uniform size (∼80−90 nm; Figure S1a−f). From Figure 1b, Ag NCs are well decorated over spherical SiO2 NPs and the cubic Ag3PO4 crystals are attached to the silica surface. FESEM images (Figure S2) demonstrate the deposition of Ag3PO4 microcubes on SiO2 spheres. The spatial distribution and strong interaction of Ag NCs with Ag3PO4 are further confirmed from elemental mapping (Figures S3 and S4). Further, the fringe width of cubic Ag3PO4 is in well agreement with the powder XRD data, where the d-spacing value is 0.26 nm (Figure 1c), which matches with the most intense peak of the (210) plane of Ag3PO4.45 Another lattice fringe having a d-spacing value of 0.22 nm corresponds to the (211) plane. As shown in Figure 1d, the SAED pattern possesses concentric rings linked to the (210) and (421) planes. The particle size distribution curve was plotted for Ag NCs and found to be 1.7 nm with a standard deviation of 0.4 nm and is presented in Figure 1e. Figure 1f represents the energy-dispersive X-ray spectra of the photocatalyst and

relative metallic silver concentration (%) =

[area(Ag 0)] [area(Ag 0) + area(Ag +)]

× 100

4.4. Nitrogen Adsorption−Desorption Study. In order to explore the surface area of the photocatalysts, nitrogen adsorption−desorption studies have been performed and are presented in Figure S6.31,32 The Brunauer−Emmett−Teller surface area and Barrett−Joyner−Halenda pore-size distribution of the samples have been calculated and are tabulated in Table S2. The isotherms are of type IV, with a small fraction of type 1 suggesting mostly mesoporosity along with little E

DOI: 10.1021/acs.inorgchem.9b00218 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. LSV curves (a) before and (b) after light illumination showing the current density. (c) EIS plot of SPS-0, SPS-1, SPS-2, and SPS-3. (d) Mott−Schottky plot of SPS-2 at 500, 1000, and 1500 Hz.

dielectric constant of SiO2 NPs, which act as supports and are due to the formation of Ag NCs. The band-gap energies of the photocatalysts were calculated using the equation54

micropores. The hysteresis loop consists of H3 type, indicating the presence of mesopores not equally distributed throughout the system.52 The surface areas are found to be 37.1, 38.3, 41.2, and 38.1 m2 g−1 for SPS-0, SPS-1, SPS-2, and SPS-3, respectively. From the pore-size-distribution curve, it is observed that there is a minimal change in the pore volume, i.e., 0.339, 0.354, 0.368, and 0.364 cm3 g−1 for SPS-0, SPS-1, SPS-2, and SPS-3 in that order. The increase in the surface area of SPS-2 compared to those of other photocatalysts may increase the active sites and hence the enhanced photocatalytic activity. 4.5. Optical Properties. To gain better insight into the light absorbance properties, UV−vis diffuse-reflectance spectra has been produced and are plotted in Figure 3a. The figure reveals that all of the photocatalysts absorb light in the visible range and the absorbance increases upon loading of Ag NCs.53 SPS-0 shows a characteristic absorption edge at 465 nm. Because the surface plasmon peak of Ag NCs lies close to the absorption peak of Ag3PO4, peak broadening occurs. However, the enhanced image in the range 350−480 nm suggests an increase in the absorbance maximum for SPS-1−SPS-3 compared to the plateau observed for SPS-0. The absorbance edges of the photocatalysts SPS-0, SPS-1, SPS-2, and SPS-3 are at 570, 620, 642, and 634 nm, respectively. From the photophysical property of the photocatalysts, it is clear that SPS-2 shows better visible-light absorbance along with the absorption edge among all photocatalysts and links to the higher photocatalytic activity by SPS-2. Upon silver loading, the absorption values of the photocatalysts were shifted to around 520 nm rather than 410 nm observed as a result of the SPR phenomenon by Ag NPs. The shifting in the absorption spectra is attributed to the high

αhν = A(hν − Eg )n

where α, ν, A, and Eg are the absorption coefficient, frequency of light, proportionality constant, and band-gap energy, respectively. The value of n defines the type of transition observed in the semiconductors; i.e., when n = 1/2, direct transition is observed, whereas n = 2 suggests an indirect transition. In this case, an indirect transition is observed and found to be 2.13 eV for SPS-0, whereas it decreases to 1.87 eV upon silver loading on Ag3PO4 (Figure S7). The shifting in the band-gap energy up to 0.26 eV is due to the synergistic effect by the two parts and a heterojunction-induced charge-transfer interaction phenomenon, which leads to better photocatalytic activity. 4.6. PL Properties. To have a perception of the photogenerated electron−hole pairs, their recombination, and charge-carrier properties, PL analysis was performed.55,56 As shown in Figure 3b, the PL peaks show similar trends in all of the photocatalysts. Liang et al. have demonstrated the origin of the PL properties of Ag3PO4 and described that the recombination of charge-transfer transition between O 2p and empty Ag 5d is responsible for the emission.57 The PL spectra of Ag3PO4 contain a strong blue emission band with a maximum at 438 nm that is attributed to the former recombination phenomenon or tetrahedral [PO4] clusters. The high degree of distortion caused by [AgO4] clusters or Ag NCs on the surface of Ag3PO4 is responsible for the absence of red emission.54 Moreover, a significant decrease in the F

DOI: 10.1021/acs.inorgchem.9b00218 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) UV−vis spectra showing the degradation of phenol by the photocatalysts. (b) Spectral changes of the phenol solution over SPS-2 after different time intervals. (c) Rate of degradation of phenol over the photocatlysts. (d) First-order kinetics plot of phenol degradation over the samples.

no separation of the electron−hole pairs by the applied electric field, but rather the reaction rates of positive and negative charge carriers contribute to the photocurrent.60,61 In this paper, because of the small particle size of Ag NCs along with Ag3PO4, the applied potential gradient over the space-charge layer could not produce photogenerated electron−hole pairs but rather an efficiency of the charge-carrier-transfer process at different interfaces. However, because of the LSPR phenomenon, the hot holes generated below the Fermi level of Ag NCs have higher energy to overcome the Schottky barrier at the Ag/Ag3PO4 metal−semiconductor junction, which contributes to the cathodic photocurrent.35Furthermore, the overpotential is shifted to a less negative potential upon Ag NC loading and found to be 0.98, 0.96, 0.91, and 0.95 V for SPS-0, SPS-1, SPS2, and SPS-3 in that order. The significant increase of the current density upon light illumination and the lowest negative overpotential by SPS-2 are responsible for better photocatalytic activity toward water-splitting and phenol oxidation reactions. EIS was performed to analyze the generation of photoinduced electron−hole pairs and charge-transfer resistance of the photocatalysts.3,51 The radius of the arc in the Nyquist plot (imaginary part Z′ and real part Z″) measured for SPS-0, SPS1, SPS-2, and SPS-3 at an applied potential of 0.0 V using 0.1 M Na2SO4 as the electrolyte reflects the reaction rate on the electrode surface. As shown in Figure 4c, the arcs of the photocatalysts SPS-0, SPS-1, SPS-2, and SPS-3 are 8740, 2275, 1650, and 7930 Ω, respectively. The larger semicircle observed for SPS-0 among all photocatalysts implies high interfacial charge-transfer resistance, leading to poor separation of the photogenerated electron−hole pair.62 The smaller semicircle for SPS-2 observed as a result of the lower interfacial chargetransfer resistance indicates higher electrical conductivity, as evidenced by the LSV curve.3 The decrease in the charge-

intensity is observed upon silver cluster loading, indicating that the presence of silver clusters suppresses the recombination of charge carriers. The lowest emission of SPS-2 compared to those of other counterparts indicates a smaller electron−hole recombination rate and can improve the efficiency of the photocatalyst.58 The introduction of silver clusters can lead to the effective separation of charge carriers and improved photocatalytic activity by increasing the stability of the photocatalyst. 4.7. Photoelectrochemical Study. Photoelectrochemical analysis was performed in a conventional three-electrode system using 0.1 M Na2SO4 as the electrolyte. In dark conditions, SPS-0, SPS-1, SPS-2, and SPS-3 show photocurrents of 0.04, 0.08, 0.22, and 0.15 mA, respectively, in the cathodic direction (Figure 4a). Upon light illumination, the observed photocurrents for SPS-0, SPS-1, SPS-2, and SPS-3 are 0.20, 0.54, 5.06, and 1.26 mA, respectively. The cathodic photocurrent of SPS-2 increases substantially to 5.06 mA in the cathodic direction, which is 23 times higher compared to the current generated in dark conditions (Figure 4b). Upon light illumination, photocurrent generation is about 25-fold by the photocatalyst compared to SPS-0. The excellent current enhancement in the cathodic direction is due to the presence of Ag NCs on the surface of Ag3PO4, as confirmed by XPS. The excellent cathodic photocurrent by Ag NCs corroborates with the previous results obtained by Chen et al. The group fabricated Ag NCs on a TiO2 photoelectrode, where photocurrent generation is switched from the anodic to cathodic direction by simple tuning of the light wavelength from the UV to visible region.59 According to Gratzel, the particle size is responsible for the photoelectrochemical properties of the semiconductor.60 When the particle size is appreciably small, a space-charge layer is not formed, leading to G

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From Table 1, it is clear that the SPS-2 photocatalyst shows a maximum degradation rate constant (0.0199 min−1), which is

transfer resistance can be attributed to the transfer of photogenerated electrons from the semiconductor surface to Ag NCs and prevents direct recombination of the charge carriers, which can contribute to the remarkable enhancement in the photocatalytic activity. Mott−Schottky analysis was performed at different frequencies using 0.1 M Na2SO4 at pH = 6.1 to analyze the band-edge potential and nature of the semiconductor of the catalyst.63 From Figure 4d, it is evident that straight lines with positive slopes were drawn to the potential axis and found to be −0.14 eV versus Ag/AgCl for three different frequencies, i.e., 500, 1000, and 1500 Hz. The potential measured against the Ag/AgCl reference electrode can be converted into normal hydrogen electrode (NHE) potentials by using the following equation:64

Table 1. First-Order Fitting Results of Phenol Degradation over the Photocatalysts catalyst

R2

kobs (min−1)

t1/2 (min)

% of degradation

SPS-0 SPS-1 SPS-2 SPS-3

0.997 0.977 0.999 0.965

0.0041 0.0070 0.0199 0.0078

169.02 99.00 34.82 88.84

38.7 58.1 90.9 61.3

5 times higher than that of SPS-0. In order to know the active species involved in the photodegradation process, quenching experiments were performed by introducing various scavengers into the phenol solution. The photodegradation reaction was repeated by adding 1 mmol of solutions of dimethyl sulfoxide, isopropyl alcohol, methanol, and p-benzoquinone as scavengers for electrons (e−), hydroxyl radicals (•OH−), holes (h+), and superoxide radicals (•O2−), respectively.40,51 The rate of photodegradation decreases significantly when isopropyl alcohol, a hydroxyl radical quencher, was added to the solution. Similar behavior was also observed with the addition of methanol, a hole scavenger, suggesting that the degradation of phenol is dominated by an oxidation reaction of the hydroxyl radical and direct hole oxidation. Figure S8a indicates the role of active species involved in the process. The stability of SPS-2 was also studied by reusing the catalyst, and it was observed that the activity does not decrease significantly even after 4 cycles of use (Figure S8b). Furthermore, the catalysts were tested for the photocatalytic water-splitting reaction and are plotted in Figure 6a,b. In a typical procedure, the photocatalysts were dispersed in 30 mL of an aqueous solution taken in a sealed quartz batch reactor by taking TEOA as a hole scavenger and AgNO3 as an electron scavenger. It is observed that the photocatalysts SPS-0, SPS-1, SPS-2, and SPS-3 produced 700, 750, 1236, and 820 μmol h−1 g−1 of O2, respectively. Because the CB potential of Ag3PO4 is more positive than the H+ reduction potential, Ag3PO4 is unable to produce H2 from water. However, it is observed that the photocatalysts SPS-1, SPS-2, and SPS-3 produce 1496, 2460, and 1630 μmol h−1 g−1 H2 in that order. From the results, it can be deduced that SPS-2 can split water to produce H2 and O2 more effectively compared to other counterparts.17 The role of active species involved in the photocatalytic watersplitting reaction was analyzed through quenching experiments using various scavengers. The photocatalytic water-splitting reaction was repeated by adding a 1 mmol solution of AgNO3, isopropyl alcohol, TEOA, and p-benzoquinone as scavengers for e−, •OH−, h+, and superoxide radicals •O2−, respectively. From the results, it is clear that electrons and holes are the major active species toward photocatalytic H 2 and O2 production, respectively (Figure S9). The effect of the catalyst dosage on photocatalytic water splitting has been plotted in Figure S10, which shows minimal changes in the amount of O2 and H2 evolution with the catalyst amount above 50 mg. Hence, 50 mg of catalyst has been used throughout the photocatalytic water-splitting reaction. The apparent quantum yield was calculated by the following equation:28

Efb(vs NHE) = Efb(pH = 0, vs AgCl) + EAgCl + 0.059pH

The measured pH value of the electrolyte Na2SO4 is 6.1, and EAgCl = 0.197 V. Therefore, the calculated flat-band position of SPS-2 is 0.42 eV versus NHE (pH = 0). The corresponding VB edge potential could be determined from the Kubelka−Munk plot of the UV−vis results using the equation E VB = Eg + ECB and calculated to be 2.29 eV. 4.8. Photocatalytic Activity. The photocatalysts were used for phenol degradation as a step toward environmental pollution abatement and water-splitting reactions under visiblelight irradiation (λ > 420 nm). In a detailed procedure, 20 mg of the photocatalysts were thoroughly dispersed in 20 mL of the aqueous phenol solution (20 mg L−1) and stirred for 30 min in the dark to attain adsorption−desorption equilibrium on the catalyst surface. The suspensions were then irradiated with a 150 W xenon lamp with a 420 nm cutoff filter. After that, solutions were taken at regular intervals and centrifuged to get a clear solution. The concentration of the phenol solution was determined by using an UV−vis spectrophotometer based on the Beer−Lambert law. The photocatalytic degradation efficiency (η) was calculated according to the equation η = (C0 − Ct)/C0 × 100%, where C0 is the absorption intensity of the phenol solution after equilibrium and Ct is the absorption intensity of the phenol solution after irradiation.41 From the UV−vis absorbance spectra, it can be seen that the characteristic peak of phenol is observed around 270 nm and the peak intensity decreases significantly, confirming the oxidation of phenol.51 From Figure 5a, it can be observed that the degradation efficiency of the photocatalysts SPS-0, SPS-1, SPS-2, and SPS-3 are 38%, 58%, 91%, and 61%, respectively. From the results, it is clear that the incorporation of Ag NCs enhances the photocatalytic activity of Ag3PO4. From the experiments, it is observed that SPS-2 shows better photocatalytic activity toward phenol oxidation of around 91%. Further, the absorption peak of phenol decreases rapidly with respect to the irradiation time, indicating the effect of the photocatalyst toward degradation (Figure 5b), and the rate of degradation of the pollutant was calculated by plotting the value of C/C0 versus irradiation time represented in Figure 5c. The kinetics of the photodegradation process is shown in Figure 5d. The data obtained from the experiments are well fitted with the first-order kinetics by the equation −ln(C /C0) = kt

where C0 is the initial concentration, C is the final concentration at time t, and k is the degradation rate constant.

AQY (%) = H

2 × number of H 2 molecules × 100 number of photons absorbed DOI: 10.1021/acs.inorgchem.9b00218 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Rates of evolution of (a) O2 and (b) H2 over various photocatalysts and reusability plots of the SPS-2 photocatalyst for (c) O2 and (d) H2 evolution.

By using the band-pass filter (>420 nm) and an irradiatometer of 0.3 W, the AQY was found to be 4.6%. The higher photocatalytic activity of SPS-2 toward phenol oxidation and water-splitting reactions is attributed to the following observations: (1) a better visible-light absorption efficiency and (2) a lower recombination rate of photogenerated electron−hole pairs, as explained by the UV−vis and PL spectra, respectively. Further the smaller semicircular Nyquist plot implies a lower interfacial charge-transfer resistance in the catalyst, which enhances the photocatalytic activity significantly. The reusability test has been performed, and no significant decrease of the H2 and O2 evolution rates up to 4 cycles, as shown in Figure 6c,d, implies excellent photostability of the catalyst. For comparative studies, various strategies that have been employed to enhance the photocatalytic properties of the Ag3PO4 photocatalyst have been demonstrated in Table 2. On the basis of the high photocatalytic activity of the assynthesized Ag/Ag3PO4 core−shell plasmonic nanocatalyst, a plausible mechanism has been predicted and depicted in Scheme 2. Under visible-light irradiation, the photoexcited electrons of Ag3PO4 moved to the CB from the VB, whereas the holes are transferred from the VB to the outer surface of Ag3PO4 and involved in an oxidation reaction of water splitting and a phenol molecule. A similar photoexcitation process also takes place on the Ag NC surface to generate hot electron− hole pairs. The Fermi level of the Ag NPs (+0.8 V) is well below the CB of Ag3PO4, and hence the photoinduced electrons travel from the semiconductor to the Fermi level of silver. However, the hot electrons generated on the Ag NC surface have enough potential to reduce H+ to H2 because of the LSPR.48 Meanwhile, the hot holes remaining below the Fermi level of the Ag NCs migrate to the Ag NCs/Ag3PO4

Table 2. Literature Review on Various Ag3PO4-Based Photocatalysts photocatalyst Ag3PO4

fabrication method ion exchange

Ag3PO4/2D g-C3N4 electrostatic assembly polyhedral Ag3PO4

microemulsion

Ag3PO4/Ag/AgBr/ RGO Ag3PO4/graphene

hydrothermal

porous Ag3PO4 microcube morphologydependent Ag3PO4 Ag@Ag3(PO4)1−x/ ZnO nanorod p-LaFeO3/nAg3PO4 core−shell Ag NCs@Ag3PO4

in situ growth in toluene template-assisted synthesis soft chemical method, ultrasonication in situ electrochemical deposition in situ precipitation ion exchange

activity O2 evolution, 636 μmol h−1 O2 evolution, 25 μmol L−1 O2 evolution, 280 μmol h−1 O2 evolution, 76 μmol h−1 MB degradation, 95%, 20 min RhB degradation, 99%, 24 min RhB degradation, 98%, 35 min PEC O2 evolution, 34 μmol h−1 phenol oxidation, 90%, 120 min phenol oxidation, 91%, 120 min O2 evolution, 1236 μmol h−1 H2 evolution, 2460 μmol h−1

ref 16 17 18 19 21 22 23 36 38 this work

interface and contribute to the water oxidation and phenol degradation reactions. As reported by Moon et al., the particle sizes of Au NPs contribute to the charge-carrier-transfer process in a Au/TiO2nanocomposite by tuning the Schottky barrier height at the metal−semiconductor junction and increase the PEC water oxidation efficiency.35 Because the current system possesses Ag/Ag3PO4 with respect to the I

DOI: 10.1021/acs.inorgchem.9b00218 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Mechanistic Insight into the Photocatalytic Reaction

XPS measurement. Furthermore, the photocatalytic activity has been explored toward a water-splitting reaction to generate H2 and O2 and phenol oxidation reactions. The photocatalyst SPS2 is able to generate 2460 μmol h−1 g−1 of H2 and 1236 μmol h−1 g−1 of O2 through water splitting along with phenol oxidation of up to 91% in 120 min. A LSPR-induced mechanism has been predicted for the photocatalytic activity of the synthesized catalyst, which shows effective separation of the photoexcited electron−hole pairs, thereby increasing the activity and stability of the photocatalysts. The work demonstrates a facile and cost-effective pathway for the morphology-directed synthesis of Ag3PO4−Ag NCs as an effective catalyst toward water-splitting and phenol oxidation reactions under visible-light irradiation.

metal−semiconductor junction, the small particle size (∼1.7 nm) may decrease the Schottky barrier height, thereby facilitating the charge-transfer process at the metal−semiconductor junction. The molecular mechanism for the photocatalytic activity is as follows: Ag 3PO4 + hν → Ag 3PO4 (eCB− + hVB+) Agclusters + hν → Agclusters+ + e− Ag 3PO4 (hVB+) + OH− → •OH

Ag 3PO4 (2eCB−) + O2 + 2H+ → H 2O2 Ag 3PO4 (eCB−) + H 2O2 → •OH + OH− •

■

OH, Ag 3PO4 (hVB+) + phenol → CO2 + H 2O

e−, Agclusters + 2H+ → H 2 +

h + 2H 2O → O2 + 4H

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00218.

+

5. CONCLUSION A series of core−shell-structured Ag NCs@Ag3PO4 photocatalysts with varying percentages of Ag NCs have been synthesized using SiO2 as a core material. Loading of Ag NCs enhances more visible-light absorption through the SPR phenomenon. The significant decrease in the PL spectra and the smaller semicircular curve in the Nyquist plot imply a lower recombination rate of photogenerated electron−hole pairs and a lower charge-transfer resistance in the photocatalyst. An excellent photocurrent enhancement of up to 25fold in the cathodic direction compared to neat Ag3PO4 leads to the formation of Ag NCs and also has been confirmed from

HRTEM images of SPS-2, particle size distribution curve of silica NPs, FESEM images, elemental mapping of the as-synthesized sample from FESEM and TEM, XPS survey spectra, C 1s reference spectra, relative concentration of metallic silver, surface area and poresize distribution (inset), Tauc’s plot, histogram showing the role of active species in photocatalytic process, stability curve of the photocatalysts, effect of the different radical scavengers on the photocatalytic water-splitting reaction, and effect of the catalyst dosage on the photocatalytic water splitting (PDF) J

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Water Using Visible Light and Structure Controlled Graphitic Carbon Nitride. Angew. Chem., Int. Ed. 2014, 53, 9240−9245. (14) Cai, T.; Liu, Y.; Wang, L.; Zhang, S.; Zeng, Y.; Yuan, J.; Ma, J.; Dong, W.; Liu, C.; Luo, S. Silver Phosphate-Based Z-Scheme Photocatalytic System with Superior Sunlight Photocatalytic Activities and Anti-Photocorrosion Performance. Appl. Catal., B 2017, 208, 1− 13. (15) Martin, D. J.; Liu, G.; Moniz, S. J.; Bi, Y.; Beale, A. M.; Ye, J.; Tang, J. Efficient Visible Driven Photocatalyst, Silver Phosphate: Performance, Understanding and Perspective. Chem. Soc. Rev. 2015, 44, 7808−7828. (16) Yi, Z.; Ye, J.; Kikugawa, N.; Kako, T.; Ouyang, S.; StuartWilliams, H.; Yang, H.; Cao, J.; Luo, W.; Li, Z.; Liu, Y.; Withers, R. L. An Orthophosphate Semiconductor with Photooxidation Properties under Visible-Light Irradiation. Nat. Mater. 2010, 9, 559−564. (17) Yang, X.; Tang, H.; Xu, J.; Antonietti, M.; Shalom, M. Silver Phosphate/Graphitic Carbon Nitride as an Efficient Photocatalytic Tandem System for Oxygen Evolution. ChemSusChem 2015, 8, 1350−1358. (18) Indra, A.; Menezes, P. W.; Schwarze, M.; Driess, M. Visible Light Driven Non-Sacrificial Water Oxidation and Dye Degradation with Silver Phosphates: Multi-Faceted Morphology Matters. New J. Chem. 2014, 38, 1942−1945. (19) Hou, Y.; Zuo, F.; Ma, Q.; Wang, C.; Bartels, L.; Feng, P. Ag3PO4 Oxygen Evolution Photocatalyst Employing Synergistic Action of Ag/AgBr Nanoparticles and Graphene Sheets. J. Phys. Chem. C 2012, 116, 20132−20139. (20) Zhai, W.; Li, G.; Yu, P.; Yang, L.; Mao, L. Silver Phosphate/ Carbon Nanotube-Stabilized Pickering Emulsion for Highly Efficient Photocatalysis. J. Phys. Chem. C 2013, 117, 15183−15191. (21) Xiang, Q.; Lang, D.; Shen, T.; Liu, F. Graphene-Modified Nanosized Ag3PO4 Photocatalysts for Enhanced Visible-Light Photocatalytic Activity and Stability. Appl. Catal., B 2015, 162, 196−203. (22) Liang, Q.; Ma, W.; Shi, Y.; Li, Z.; Yang, X. Hierarchical Ag3PO4 Porous Microcubes with Enhanced Photocatalytic Properties Synthesized with the Assistance of Trisodium Citrate. CrystEngComm 2012, 14, 2966−2973. (23) Dong, P.; Wang, Y.; Li, H.; Li, H.; Ma, X.; Han, L. ShapeControllable Synthesis and Morphology-Dependent Photocatalytic Properties of Ag3PO4 Crystals. J. Mater. Chem. A 2013, 1, 4651−4656. (24) Chen, Z.; Wang, W.; Zhang, Z.; Fang, X. High-Efficiency Visible-Light-Driven Ag3PO4/AgI Photocatalysts: Z-Scheme Photocatalytic Mechanism for Their Enhanced Photocatalytic Activity. J. Phys. Chem. C 2013, 117, 19346−19352. (25) Cai, T.; Liu, Y.; Wang, L.; Zhang, S.; Zeng, Y.; Yuan, J.; Ma, J.; Dong, W.; Liu, C.; Luo, S. Silver Phosphate-based Z-Scheme Photocatalytic system with Superior Sunlight Photocatalytic Activities and Anti-photocorrosion Performance. Appl. Catal., B 2017, 208, 1− 13. (26) Cai, T.; Wang, L.; Liu, Y.; Zhang, S.; Dong, W.; Chen, H.; Yi, X.; Yuan, J.; Xia, X.; Liu, C.; Luo, S. Ag3PO4/Ti3C2MXene Interface Materials as a Schottky Catalyst with Enhanced Photocatalytic Activities and Anti-photocorrosion Performance. Appl. Catal., B 2018, 239, 545−554. (27) Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911. (28) Patra, K. K.; Gopinath, C. S. Bimetallic and Plasmonic Ag−Au on TiO2 for Solar Water Splitting: An Active Nanocomposite for Entire Visible Light Region Absorption. ChemCatChem 2016, 8, 3294−3311. (29) Shown, I.; Ujihara, M.; Imae, T. Synthesis of Β-CyclodextrinModified Water-Dispersible Ag-TiO2 Core−Shell Nanoparticles and Their Photocatalytic Activity. J. Nanosci. Nanotechnol. 2011, 11, 3284−3290. (30) Pany, S.; Naik, B.; Martha, S.; Parida, K. Plasmon Induced Nano Au Particle Decorated over S, N-Modified TiO2 for Exceptional Photocatalytic Hydrogen Evolution Under Visible Light. ACS Appl. Mater. Interfaces 2014, 6, 839−846.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Satyaranjan Mohanty: 0000-0002-1603-6812 Brundabana Naik: 0000-0001-5568-1171 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.N. acknowledges Siksha ‘O’ Anusandhan for the infrastructure and Science and Engineering Research Board (SERB), India, for funding (Grant ECR/2016/000602). S.M. acknowledges SERB, India (Grant ECR/2016/000602), for a fellowship to perform the research work. P.B. acknowledges Siksha ‘O’ Anusandhan for a fellowship.

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