High-Performance Photocatalytic Hydrogen Production and

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High performance photocatalytic hydrogen production and degradation of levofloxacin by wide spectrum responsive Ag/Fe3O4 bridged SrTiO3/ g-C3N4 plasmonic nano-junctions: Joint effect of Ag and Fe3O4 Amit Kumar, Anamika Rana, Gaurav Sharma, Mu Naushad, Ala'a H. AlMuhtaseb, Changsheng Guo, Ana Iglesias-Juez, and Florian J. Stadler ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12753 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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ACS Applied Materials & Interfaces

High performance photocatalytic hydrogen production and degradation of levofloxacin by wide spectrum 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

responsive Ag/Fe3O4 bridged SrTiO3/g-C3N4 plasmonic nano-junctions: Joint effect of Ag and Fe3O4 Amit Kumara, b*, Anamika Ranac, Gaurav Sharmaa,b, Mu. Naushadd, Ala'a H. Al-Muhtasebe, Changsheng Guof, Ana Iglesias-Juezg, Florian J. Stadlera* a

College of Materials Science and Engineering, Shenzhen Key Laboratory of Polymer Science and Technology, Guangdong Research Center for Interfacial Engineering of Functional Materials, Nanshan District Key Laboratory for Biopolymers and Safety Evaluation, Shenzhen University, Shenzhen, 518060, PR China b

Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, PR China. c d

School of Chemistry, Shoolini University, Solan, Himachal Pradesh, India-173229

Advanced material Research Chair, Department of Chemistry, College of Science, Building#5, King Saud University, Riyadh, SaudiArabia-11451

e

Department of Petroleum and Chemical Engineering, Faculty of Engineering, Sultan Qaboos University, Muscat, Oman f

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China g

Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, 28049 Madrid, Spain * [email protected] *[email protected]

Abstract Highly photo-responsive semiconductor photocatalysis for energy and environmental applications require judicious choice and optimization of semiconductor interfaces for wide spectral capabilities. This work aims at rational designing of highly active SrTiO3/g-C3N4 junctions bridged with Ag/Fe3O4 nanoparticles for utilizing zscheme transfer and surface plasmon resonance effect of Ag augmented by iron oxide. The SrTiO3/(Ag/Fe3O4)/gC3N4 (SFC) catalyst was employed for photocatalytic hydrogen production and photodegradation of levofloxacin (20 mg/L) under UV, Visible, NIR and natural solar light exhibiting high performance. Under visible light (< 780 nm) SFC-3 sample (30wt% g-C3N4 and 3%Ag/Fe3O4) shows a H2 evolution of 2008 µmol g-1 h-1 which is ~14 times that of bare g-C3N4. In addition 99.3% removal of LFC was degraded in 90 min under visible light with retention of activity under sun. The inherent topological properties, complete, higher charge separation and reduced recombination allowed this catalyst for a high photocatalytic response which was proved by UV-DRS, PL, EIS and PCR measurements. Scavenging experiments and electron spin resonance analysis reveal that the mechanism shifts from a dual charge transfer in case of binary junction to essential z-scheme with incorporation of Ag/Fe3O4. Both •O2- and •OH are main active radicals in visible light while •O2- majorly participate under UV. The synergistic effect of SrTiO3, g-C3N4 and plasmon resonance of Ag/Fe3O4 not only improves light response 1 ACS Paragon Plus Environment


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and reduce recombination but also enhances the redox-ability of charge carriers. A H2 production mechanism and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

LFC degradation pathway (degradataion, defluorination and hydrolysis) has been predicted. This work paves a way for development of photocatalysts working in practical conditions for pollution and energy issues. Key words: Strontium Titanate; graphitic carbon nitride; hetero-junction; hydrogen production; plasmonic Ag; pharmaceutical effluents 1.

Introduction

Rapid industrialization and exploitation of non-renewable energy sources has not only led to their depletion but also accumulation of toxic pollutants in environment. To meet the challenges of environmental pollution and the increasing energy requirements, utilization of renewable sources as well as green technologies have received considerable attention in recent years1-2. Hydrogen energy has been widely tested as alternative to fossil fuels because it is clean with high gravimetric energy density and is to be explored for making it cost effective and efficient3. Energy production and environmental remediation are complimentary issues and both require attention. Tremendous use of personal care and pharmaceutical products for uplifting life standards has led to their end up as pollutants of emerging concern and listed as contaminants of priority control4-5. With increase in use of pharmaceuticals and personal care products, antibiotics abuse has risen as an issue 6-7 as they are detected in drinking water supplies8. Contamination of water bodies by antibiotics has grown as an international issue, due to their low biodegradability and persistence9. They are used for a wide spectrum of ailments in humans and livestock

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and thus their presence in water bodies is spreading which needs to be treated because they cause

adverse effects to aquatic species and increase bacteria resistance11-12. Levofloxacin is a popular antibiotic belonging to fluoroquinolone family used in treating severe bacterial infections13. It enters in the aquatic system because of its resistance to microbial and biological oxidation and escapes conventional waste water treatment 14.

Visible and solar assisted photocatalysis has proven to be one of most effective and cost effective technology for hydrogen evolution and pollutant degradation15. However it is challenging to design and fabricate stable and highly efficient photocatalysts with suitable energy levels and capability to work in different sources of light including solar light16-17. The performance and capability of a photocatalyst is limited by the recombination of photogenerated electron holes and restricted spectral response18-20. Thus design and development of novel photocatalytic systems with high stability, lower recombination and ability to perform in UV, visible, near infrared (NIR) and natural solar light is indispensable. Numerous methodologies have been developed and adopted for enhancing light adsorption range and charge separation, among which formation of heterojunctions especially Z-scheme21 with suitable optical structuring and high interfacial contact for wide spectrum response . Metal-organic hybrid materials and junctions have gained importance22 as they are more stable, have high surface area and dual capabilities of adsorption and photocatalysis. During recent years perovskites and based materials have been utilized as promising photocatalysts owing to their unique structural, optical and electronic 2 ACS Paragon Plus Environment

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properties in addition to low cost23. Strontium titanate (SrTiO3) an n-type semiconductor with perovskite 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

structure is a promising candidate for the photocatalytic technique because of its strong photo-corrosion resistibility, high photocatalytic activity and tremendous structural stability24. Because of its wide band gap and low quantum yield

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it is paramount for SrTiO3 to increase its spectral response which can be done by either

with the formation of the heterojunction structure by coupling two or more different catalysts 26-27 or by adjusting band gap via doping or co-doping with cations28-29. Various heterojunctions of SrTiO3 like SrTiO3/TiO230, SrTiO3/SrCO331, SrTiO3/BiOBr 32, SrTiO3/Bi2O333 have been synthesized for harnessing visible light for various photocatalytic applications. Graphitic carbon nitride (g-C3N4) a robust organic semiconductor has recently gained much interest due to its uninque thermal, electrical and optical properties34. Carbon nitride and based materials have been used in electro catalytic and photocatalytic hydrogen production35. It is an indirect semiconductor with a band gap of ~2.7 eV and is highly stable under thermal and chemical attack because of its tri-s-triazine (C6N7)based building blocks36. However it faces issues as rapid recombination and limited visible-light absorption below 460 nm which renders it less effective. PHE activity of g-C3N4 is limited even it has suitable potential for water splitting. For improving catalytic performance of g-C3N4 various efficient techniques have been investigated such as doping with metal/non-metal37-38 and dye sensitization39. Some researchers have also utilized it as a reinforcing material to provide stability to catalyst and enhancing the photo-activity40. The band structures and conduction band edges of g-C3N4 and SrTiO3 are well matched with capability of production of both the hydroxyl radicals and superoxide ions, which indicates that an environmental benign junction can be formed between the two and should be fabricated. Different published works have reported similar junctions but have not achieve the expected response The introduction of metals like Ag and oxides as Fe3O4 can convert a normal charge transfer mechanism to a z-scheme to ensure the high reactivity of conduction and valence bands. If surface of photocatalyst is modified with Ag, Au etc. in traces the surface plasmon resonance (SPR) effect not only enhances solar absorption but also facilitates surface electron excitation41. In light of photocatalytic ability of SrTiO3 & g-C3N4, magnetic and optical properties of Fe3O4 and SPR effect of Ag, we synthesized ternary SrTiO3/(Ag/Fe3O4)/g-C3N4 nano-heterostructures with ability to work under UV, visible, NIR and natural solar light. Various ratios of SrTiO3/g-C3N4 have been explored and also Ag/Fe3O4 loading was also varied on the heterojunction formulation. On basis of LSV response, scavenging studies, spectral response, ESR analysis and LC-MS results a mechanism has also been proposed for movement of charge carriers under each source of light as well as for the drug degradation and hydrogen production. 2. Materials and Methods 2.1 Materials Titanium tetraisopropoxide (Ti(OCH(CH3)4), strontium chloride (SrCl2.6H2O), absolute ethanol (C2H5OH), sodium hydroxide (NaOH), ferric chloride (FeCl3), ferrous chloride (FeCl2), urea (CH4N2O), silver nitrate 3 ACS Paragon Plus Environment


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(AgNO3) and cetyltrimethylammonium bromide (CTAB) were acquired from Sigma Aldrich. All reagents were 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of analytical grade and used without purification. 2.2 Synthesis of bare g-C3N4 and SrTiO3 The g-C3N4 powder was produced by the thermal treatment of urea in a muffle furnace as per previously reported method36. SrTiO3 nanoparticles were prepared by sol-gel technique. 12 mL of Ti(OCH(CH3))4 and 4 M citric acid were added to 15ml of distilled water. The solution was then sonicated for 1h (Ultrasonic Frequency: 42,000 Hz, timer: digital timer with 5 cycles, stainless steel SUS304 tank with Capacity: 2500 mL). 20 mL of aqueous strontium chloride (10.56 g) dissolved in 20 ml of water. The solution was then stirred overnight at 60 ºC. The gel formed was then heated at 180 ºC and the obtained brown dried mass was then pyrolysed in a muffle furnace at 450 ºC for 6 h. After cooling to room temperature, the precipitates were collected by filtration, washed with distilled water + ethanol mixture repeatedly and dried at 80 °C for 8 h (Thermo Fisher Scientific, Gravity convection, chamber: Al, temperature range: ambient +5°C to +210 °C). 2.3 Preparation of SrTiO3/g-C3N4 heterojunction (SC) SrTiO3/g-C3N4 binary junction was prepared by wet-impregnation followed by calcination. Different amounts of g-C3N4 were dispersed in the same quantity of ethanol (30 mL) and sonicated for 1h. As synthesized SrTiO3 powder was added and again sonicated for 2 h. The resulting mixture was placed in a fume-hood under stirring to evaporate all the solvent. The mass was then dried in a vacuum oven at 60 ºC for 6 h and grounded. The powder was then pyrolysed at 500 ºC for 3 h in a muffle furnace and cooled to room temperature to obtain SC junction. Binary junctions with different weight percentages of g-C3N4 were prepared [SC-5(5wt %), SC-10(10wt %), SC20 (20wt %), SC-30(30wt %) and SC-50 (50wt %)] 2.4 Preparation of SrTiO3/(Ag/Fe3O4)/g-C3N4 heterojunction (SFC) SFC heterojunctions with different ratios were prepared by a facile co-precipitation cum deposition method. In a typical experiment 300 mg of g-C3N4 were suspended in 30mL methanol and sonicated. In a second beaker FeCl3.6H2O and FeCl2.4H2O (2:1) were dispersed in ethanol with gradual addition of NaOH and stirring. Next, solution silver nitrate (solution 20 mg/L prepared in methanol) was added to this solution. Then, the g-C3N4 suspension was added. The mixture solution was then stirred for 1 h in dark followed by another hour under Xe lamp irradiation (300W) for photo-deposition. The solvent was then evaporated and sample (Ag/Fe3O4/g-C3N4) was dried at 60ºC. 700 mg of SrTiO3 was again suspended in 50 mL of ethanol and synthesized Ag/Fe3O4/gC3N4 was added to it and sonicated for 1 h. The suspension was stirred in a fume-hood for complete methanol evaporation. The final product was collected by filtration and washed with water + ethanol several times and dried at 60 °C for 12 h. The methodology is represented schematically in Fig.1. With this method, samples (with 30wt% g-C3N4/SrTiO3) with different loading of Ag/Fe3O4, were prepared and labelled as SFC-1(1wt %Ag/Fe3O4), SFC-3 (3wt %) and SFC-5(5wt %). 4 ACS Paragon Plus Environment

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2.5 Characterization 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The phase purity and crystal structure of the synthesized hetero-junctions were studied using a Rigaku X ray diffractometer using Cu-Kα1 radiation at a scanning rate (2θ) of 5 min-1 from 10-80º. The morphology was measured by a LEO 435 VP scanning electron microscope and a Hitachi H-7650 transmission electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were carried out using ESCALAB-250 Xray photoelectron spectrometer with monochromatized Al Kα radiation. Fourier transform infrared spectra (FTIR) were obtained by KBr pellet method using a Nicolet 5700 FTIR spectrophotometer. Photoluminescence (PL) of samples was monitored on FLS-920 luminescence instrument. The UV-Visible diffused reflectance (UVVis-DRS) spectra of photocatalysts were obtained using a Cary 300, USA spectrophotometer (BaSO4 as reference). Specific surface area was measured using NOVA 1200 analyser by the Brunauer-Emmett-Teller (BET) method. The total organic carbon (TOC) was measured during degradation tests under UV, visible and solar light using a Shimadzu TOC-VCPH analyser. The electron spin resonance (ESR) signals of photogenerated free radicals (•O2- and •OH) were obtained on a Bruker ER200-SRC spectrometer using 5,5-dimethylLpyrroline N-oxide (DMPO). 2.6 Optical, Electrochemical and photoelectrochemical studies The optical band gap of samples were calculated from the plots of Kubelka-Munk function versus the gap energy. The photoelectrochemical properties of SC, SFC-3, g-C3N4 and SrTiO3 were investigated using a CHI 660D workstation. The composite electrode was prepared using indium-tin oxide (ITO) glass electrode as substrate to form SFC/ITO (for example). For preparing the electrode, 50 mg of photo-catalyst was dispersed in 2 ml ethanol+ ethanol glycol. This mixture was then was then dip-coated onto a 1×1 cm2 indium-tin oxide electrode followed by calcination in air at 80 ºC for 1 h. Electrochemical impedance spectroscopy (EIS) was then carried for the photocatalysts with 0.05 M Na2SO4 over an over-potential of 87 mV (vs Normal Hydrogen Electrode) for each catalyst. The spectra were fitted into an equivalent circuit: Charge transfer resistance (RCT) of 70Ω (determined from the semicircle) in parallel with space charge capacitor (constant phase element) (RSCC) and electrolyte resistance (RS) of 15Ω in series. The AC voltage amplitude was 5.0 mV with frequency varying from 0.01 Hz to 100 kHZ. Mott-Schottky plots were also obtained from capacitance data. The flat band potential of as prepared g-C3N4 and SrTiO3 was calculated by plotting Mott–Schottky plots in 0.1 M Na2SO4 under dark condition. The flat band potential can be determined by extrapolating to C−2= 0. The photo-current response was also studied. A 300 W Xe lamp was used as a light source with cut-off filters. 2.7 Photocatalytic Hydrogen evolution (PHE) The photocatalytic hydrogen evolution ability was tested by water splitting reaction performed in a pyrex reactor with a closed gas circulation system. In a typical test, 50 mg of photocatalyst were dispersed in distilled water (50 mL) and triethanolamine (TEOA) (5 mL) followed by sonication for 1 h. Before exposing the system to light 5 ACS Paragon Plus Environment


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degassing was performed to evacuate air. The reaction system was then exposed to a 300 Xe lamp (with 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

controlled filters: Visble-450 mW cm-2 and NIR-200 mW cm-2) with continuous stirring. The temperature of reaction system was maintained (30 ± 0.5 ºC) by cooling water circulation arrangement. The evolved gases were detected and quantified by gas chromatography (GC-14C, Shimadzu, TCD, carrier Ar). In addition various other sacrificial agents other than ethanolamine were used as ascorbic acid (AA), lactic acid (LA), trimethylamine (TEA), ethylene glycol (EG) and N,N-Dimethylformamide (DMF). PHE was also studied in presence of 2 wt% Pt co-catalyst which was photo-deposited onto surface under visible light. The experiments were also performed under natural solar light. 2.8 Photocatalytic degradation of levofloxacin LFC (C18H20N3O4F, > 98% purity) was supplied by Sigma-Aldrich. The degradation of LFC in water was performed under UV, visible (Xe lamp 400-780 nm), NIR (>780nm) light and natural sunlight in a glass photoreactor. A 500 W xenon lamp was used for illumination as artificial visible light source with a 400 and 800 nm cut-off filters for visible and NIR light with approximate intensity of 750 and 325 mW cm-2 respectively. For UV light a 500 W Hg lamp with light intensity of 215 mW cm-2 was used. The temperature was kept constant (30 ± 0.5 ºC) through a water circulation system. In a typical experimental procedure photocatalyst (0.2 g/L) was added to the LFC suspension (10 mg/L). Before the analysis, 2 mL of the slurry was taken out and filtered through a Teflon syringe filter (0.22 μ). The concentration of LFC was determined after regular intervals of time by HPLC (high performance liquid chromatography) (Agilent 1100 model) using a C-18 column (150 x 4.6 mm, 5 μm) and UV detector at 290 nm. Formic acid (0.1%) and acetonitrile were used as the mobile phases at a flow rate of 1 mL/min. A linear gradient elution was followed (Initial 93% formic acid was reduced to 50% over 25 min and returned to 93%. For acetonitrile percentage was 10%, increased to 50% and 90% in 8 min. 33% v/v methanol was applied as an isocratic run with a constant flow rate of 0.55 mL/min. The mass spectrometer with ESI (Waters) was run in positive ion mode with the ion spray needle at 5000 kV. The LC-MS analysis of the generated LFC degradation products was performed with a mass spectrometer with Z-spray electrospray ionization (Waters). Chromatographic separation was done by C18 column (150 mm x 2.1 mm). Water was used as mobile phase with 0.1% formic acid and injection volume of 2 µL. The photocatalytic experiment was performed along consecutive 5 cycles. The SFC catalyst is magnetically separated from reaction system. 3. Results and discussions 3.1 Characterization of as-prepared photocatalysts Phase identification of the as-prepared materials was determined by X-ray diffraction. Fig. 2 displays the diffraction patterns obtained for bare SrTiO3, g-C3N4 materials and SFC-1, SFC-3 & SFC-5 heterojunctions. The low-angle reflection (12.8º) is characteristic of the (100) in-plane structural repeating unit of graphitic carbon nitride (JCPDS 50-1250). The peak at higher angle (27.4º) is originated from the periodic stacking of layers 6 ACS Paragon Plus Environment

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along the (002) axe when exfoliating in nanosheets with inter-planar spacing of d=0.326 nm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

42

. The diffraction

part of bare SrTiO3 exhibits diffraction peaks at 2θ = 22.3º, 32.1º, 39.8º, 46.1º, 52.2º, 68.1º, 77.2º corresponding to (100), (110), (111), (200), (210), (220) and (310) planes respectively which can indexed as cubic structure (JCPDS No. 35-0734) with space group Pm-3m43. The cubic crystal system of SrTiO3 is obtained with lattice parameters a=b=c=3.912 Å. A sharp peak at 25.1º corresponds to some anatase phase of TiO2 formed during calcination and lowers crystallinity of SrTiO3. The diffraction pattern for binary junction presents peaks ascribable to both SrTiO3 and g-C3N4 phases previously described. Therefore, during its synthesis, the nature of both components is preserved, although the observed slight shift in the peaks position respect to the bare components reflect some structural modification by interaccin between both materials. The inetnsity of g-C3N4 is decreased because of its low content. The peak for (002) plane of g-C3N4 shifts to higher angle of 27.9º with d=0.319nm implying a narrowing interlayer distance. This narrowing leads to shorter diffusion length and reduced recombination of charge carriers. In the ternary catalaysts, SFC, along with the peaks asociated to gC3N4 and SrTiO3 components, other additional peaks are observed; the peaks at 38.2º, 43.4º and 63.2º are characteristic of (111), (200) and (220) Ag(0) planes respectively (JCPDS 003-0921)44 and the peaks at 31.6°, 54.2° and 62.6° correspond to (2 2 0), (4 2 2) and (4 4 0) diffraction planes of Fe 3O4 (JCPDS 65-3107

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. In

addition small peaks at 33.6º and 38.4º correspond to Ag2O (JCPDS 72-2108) which may be due to oxidation of some Ag during photo-deposition process conducted in air atmosphere and is consistent with previous findings 46

. In the sample SFC-3 the peaks for Ag have low intensity due to the Ag/Fe3O4 low loading, indicative of very

high dispersion of those entities, but the intensity of those peaks increases when increasing the amount as in SFC-5 where peak at 78.2º is visible for (311) planes of Ago. Slight schanges in the 2θ values suggest that the crystal cell is slightly effected by interaction with the subyacent SC support. The XRD findings confirm the formation of SC and SFC heterojunctions. d110 = 0.235 nm Charcaterization by FTIR was also performed to confirm the presence of above discussed phaes in the ternary heterojunction (Fig S1). The FTIR spectrum of the SFC-3 sample (Fig S1) presents bands at 3354 , 856 and 601 cm-1 that correspond to O-H, Sr-O and Ti-O stretching vibrations of the strontium titanate, respectively

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. The

absorption bands at 1200–1650 cm−1 correspond to the stretching mode of heterocyclic C=N and C-N bonds in gC3N4, while the peak at 812 cm-1 can be assigned to the stretching mode of triazine unit 44 confirms the graphitic structure. The weak feature at 3203 cm-1 is ascribed to the bending mode of heptazine and N-H stretching vibration, probably from residual amino groups due to an incomplete polymerization

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. The band at 550 cm-1

arises from the Fe-O stretching mode of the Fe3O4 phase. XPS was used to study the surface oxidation state and electronic properties of the different components of the synthesized ternary photocatalyst. Fig. 3(a) shows the XPS survey spectrum of the SFC-3 sample. The appeared binding energy peaks conﬁrm the presence of major elements C, N, O, Sr, Ti, Fe and Ag in the composition of 7 ACS Paragon Plus Environment


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the heterojunction catalyst. The C 1s high resolution XPS spectrum (Fig. 3b) displays two peaks at 283.7 eV and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

286.9 eV that are ascribed to C-C bonding in aromatic rings, and to sp2-bonded carbon to N (C=N) respectively 49

. The C 1s peak signals of g-C3N4 (for =C-C-) shift towards lower energy as per reported literature values

50

after combining with SrTiO3 but it should be noticed that the binding energy of standard carbon peak i.e 284.7 eV has not changed after formation of the junction. Several works have reported that the shifts or variation in binding energies is related to surface charge density changes, which is due to electron transfer between semiconductor materials with different Fermi levels51. Fig. 3(f) shows Sr 3d spectrum with two peaks at 132.5 eV and 134.2 eV corresponding to Sr 3d5/2 and Sr 3d3/2 respectively32. Ti 2p spectrum (Fig. 3g) shows deconvoluted peaks at 457.1 eV and 468.2 eV for Ti 2p3/2 and Ti 2p½ respectively. The positive moving of binding energies represents movement from higher located fermi level to lower. The Sr 3d peak (3d 5/2) shifts to higher values as per literature52 which hints at increased electron density via transfer of electrons from g-C3N4. Similarly g-C3N4 shows both negative and positive shift which is due to dual effect of electron transfer to SrTiO 3 and by Ag to CB of g-C3N4. The O1s spectrum is shown in Fig. 3(h) with two peaks at 528.5 & 530.5 eV referring to two types of oxygen attributed to oxides and the OH groups respectively. The adsorbed hydroxyl radicals favour the generation of hydroxyl and superoxide radicals during photocatalytic process. Fig. 3(c) shows the XPS spectrum for N 1s which can be deconvoluted into three contributions at 397.5, 398.0 and 399.1 eV assigned to the sp2-bonded N involved in the triazine rings (C=N-C), tertiary N (N–(C)3) and Hbonded N (C-N-H) respectively. C and N 1s XPS results are in concordance with the XRD data and confirms the formation of g-C3N4 53. The Fe 2p spectrum (Fig. 3d) can be deconvoluted into the Fe 2p1/2 and Fe 2p3/2 doublet with peaks located at 709.5 eV and 723.3 eV respectively20. These peaks can be further split in two contributions corresponding to the presence of Fe3+ and Fe2+. The presence of a satellite peak at 715.8 eV could suggest surface co-existence of Fe2+ and Fe3+

54

. This confirms the formation of Fe3O4 also supported by XRD results.

The Ag 3d peak (Fig. 3e) can be fitted into two peaks at 367.7 eV and 373.11 eV attributed to the Ag 3d 3/2 and Ag 3d5/2 double peaks respectively 55. At the same time, these peaks split into the contributions corresponding to Ag+ (367.6 & 373.8 eV) and metallic Ago (368.7 & 374.5eV)56. Observing peaks for Ag+ and Ago shows a partial oxidation of Ag as also observed in XRD findings. The binding energies of Ag and Fe3O4 do not show any noticeable change. The electrons in the valence bands of both the semiconductors SrTiO3 and g-C3N4 can also be excited by X-rays which means there can be excitation to conduction bands and hence binding energy values get shifted. However no change in values for Ag and Fe3O4 shows that they are are bridged in between as we aim by synthetic route. All the changes and no-changes observed in the binding energies are attributed to intense interactions and positioning of the materials in composition; hence existence of SFC junction can be implied from XPS measurements.

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Figure S2 shows the SEM and TEM images of g-C3N4 and SrTiO3. The graphitic carbon nitride shows rough 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sheet like morphology in SEM micrographs which is more evident in the TEM images. SrTiO3 acquires nearly cubic morphology. Fig. 4 (a-c) shows the SEM images for morphology analysis of of SFC-3 at different magnifications. It can be seen that SrTiO3 presents a typical nanocube structure while g-C3N4 shows a porous sheet like morphology. SrTiO3 nanocubes can be clearly seen in contact with g-C3N4 and Ag/g-C3N4 in Fig. 4(c). Small spherical particles represent Ag/Fe3O4. The corresponding elemental mapping for SFC-3 is represented in in Fig. 4(d-k) and shows the uniformity of the junction formed. TEM analysis was further performed conducted to elucidate micromorphology and nanostructure anslysis. Figure 5 (a) shows the cubical SrTiO3 and spherical Ag/Fe3O4 dispersed over thin carbon nitride sheets. It can be observed There is a uniform distribution of nanoparticles on to the g-C3N4 for effective formation of the junction via high interfacial contact (Fig. 5b). The cubical morphology of SrTiO3 and spherical Ag/Fe3O4 is more clear in The corresponding crystal lattices and interfacical contact was further confirmed by HR-TEM analysis. In Fig. 5(c) the lattice spacings of 0.196 nm, 0.282 nm and 0.235 nm are in accordance with (200), (100) and (111) planes of SrTiO3, Fe3O4 and Ag. In addition the higher resolution in Fig 5(d) which confirms the high interfacial contact of all the moieties with lattice spacings of g-C3N4, SrTiO3 and Ag/Fe3O4. This type of architecture favours the electron-mobility promoting photocatalytic performance. The BET surface area of SFC-3 was analysed by BET N2 adsorption-desorption isotherms represented by Fig. 6(a). The ternary catalyst exhibits a typical type-IV isotherm with a high surface area of 56.4 m2 g-1 with a pore size of 10.23 nm and pore volume of 0.643 cm3/g. A higher surface area ensures better adsorption and interaction of the pollutant on to catalyst surface and faster degradation by reactive oxygen species. The magnetizationhysteresis (M-H) curve for ternary SNC junction is represented in Fig. 6(b). The material is soft magnetic due to presence of Fe3O4 with a magnetization of 48 emu/g and low coercivity. However there is a slight unsaturation due to presence of paramagnetic nitrogen. The sample is highly magnetic which allows the catalyst separation and recuperation from the aqueous medium.The point of zero charge (pzc) of photocatalysts was determined for binary and ternary systems SC-30 and SFC-3 (Fig. S3). The obtained values were 4.7 and 5.5 for SC-30 and SFC-3 respectivel.y 3.2 Optical studies and electrochemical response UV-vis diffuse reflectance spectra (DRS) were recorded to assess the light absorption potential of pure SrTiO3, g-C3N4 and binary SrTiO3/30wt%g-C3N4 (SC-30), SrTiO3/(Ag/Fe3O4)(1wt%)/(30wt%g-C3N4 SFC-1), SFC-3 and SFC-5 and test resuts were compared in Fig. 6(c). It is obvious that SrTiO3 shows high UV absorption with almost negligible absorption in the visible region. On the other hand g-C3N4 is able to absorb in visible region too with absorption edge near 450 nm. However, unlike the bare systems, the binary junction with both the components (SC-30) shows absorption in both regions. The enhanced spectral reponse can be attributed to formation of the hetero-junctions between the two semiconductors57. When Ag/Fe3O4 nanospheres are introduced 9 ACS Paragon Plus Environment


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into junction the absorption is broadened upto 1400 nm which refers to near infra-red (NIR) region. A high NIR 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

response is observed with increasing Ag/Fe3O4 content. This is attributed to enhanced visible absorption by Fe3O4 and surface plasmon resonance effect of Ag nanoparticles. In addition it has been proven in previous experiments that magnetitite particles may enhance surface plasmon resonance to a certain extent. Thus ternary photocatalysts SFC-1 (to a lesser extent), SFC-3 and SFC-5 show absorption in UV, visible and NIR region. The corresponding Kubelka–Munk plots are given in Fig. 6(d) for calculaing the band gaps. The bandgaps of the SrTiO3 and g-C3N4 are estimated to be 3.14 eV and 2.74 eV respectively. Table S1 shows the band gap parameters. The Mott-schottky plots (Fig. 7a) were measured for SrTiO3 and g-C3N4 under 750 Hz for assesing the flat band potential. Both the semiconductors have a positive slope which is characteristic of n-type material. In addition it is a fact that top of valence band potential for majority of semiconductors is between 0.10 to 0.30 eV. So it has been taken as + 0.20 eV in this work58. The flat band potentials for SrTiO3 and g-C3N4 were found to be -0.82 eV and -1.12 eV (vs NHE at pH=7) respectively and corresponding conduction bands (ECB) lie at 1.02 eV and -1.32 eV. The corresponding valence bands are calculated using relation: 𝐸𝑉𝐵 = 𝐸𝐶𝐵 + 𝐸𝑔

(1)

The VB potentials for SrTiO3 and g-C3N4 are estimated as 2.12 eV and 1.42 eV respectively Photo-luminescence (PL) spectroscopy was used to reveal the charge separation efficiency of the catalysts. As visible in Fig. 7(b), SrTiO3 shows low photoluminescence due to its wide band gap. On the contrary, g-C3N4 shows higher recombination as PL intensity is higher. The order of PL intensity is g-C3N4 > SC-30 > SFC-5 > SrTiO3 > SFC-1 > SFC-3. The ternary photocatalyst SFC-3 shows high separation capability with lowest PL intensity, which means that heterojunction formation between SrTiO3 and g-C3N4 and bridging by Ag/Fe3O4 nanoparticles diminished the recombination. But at higher content of both silver and iron oxide they may become centres of recombination and thus show an increase in the photoluminescence intensity as compared to SFC-3. The electron transfer capacity of photocatalysts was tested by electrochemical impedance spectroscopic (EIS) analysis (Fig. 7c). In general, the interfacial properties of such junctions and assemblies a change in the change electrochemical impedance is an essential indicator59. In semiconductor heterojunctions materials a high interfacial transfer facilitates the charge separation and reduces recombination and this is a manifestation of the impedance reduction. The arc radius in the EIS Nyquist curves reflects the electron transfer resistance of conductor/semiconductor SFC-3 exhibits lowest arc radius EIS curve which means it has the best electron transfer capability under visible light exposure as compared to binary junctions, SrTiO3 and g-C3N4. SFC-3 also has lowest arc radius among three ternary nano-junctions of SFC-1 and SFC-5. When Ag/Fe3O4 content is very high on the surface they may act as recombination centres and thus poor electron transfer ability. As far as


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conductivity of the photo-generated electrons is concerned the EIS results present the situation differently i.e 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

synergism of Ag/Fe3O4 and g-C3N4/SrTiO3 enhances the quantity of free electrons. In addition transient photocurrent response (PCR) of photocatalysts was studied. Fig. 7(d) shows the current-time plots of SrTiO3, g-C3N4, SFC-1, SFC-3 and SFC-5 for 5 cycles of visible light exposure. SrTiO3 shows lowest electrical signal due to its large band gap. However ternary photocatalysts show strong response. SFC-3 shows highest photocurrent response owing to separation efficiency, optimum dosage of Ag/Fe3O4 and SPR response. A gradual but minor decrease in photocurrent is due to the slow detachment of sample from ITO glass electrode surface with time. Thus on analysing PL, EIS and PCR response it can be claimed that electron-hole recombination is significantly reduced by formation of Z-scheme heterojunction and bridging of Ag/Fe3O4 nanoparticles. 3.3 Photocatalytic experiments 3.3.1 Optimization of SrTiO3 and g-C3N4 ratio The SrTiO3/g-C3N4 heterojunction was fabricated for overcoming shortcomings of both semiconductors and reducing the electron-hole recombination. The as synthesized SrTiO3, g-C3N4, binary SrTiO3/g-C3N4 and ternary SrTiO3/(Ag/Fe3O4)/g-C3N4 photocatalysts with different mass ratios were employed for photocatalytic hydrogen evolution (PHE) under visible light and degradation of LFC under UV light (< 400 nm). Fig. S4 shows PHE for SrTiO3/g-C3N4 with different wt% of g-C3N4 and Fig. 8(a) for other samples. SC-30 (SrTiO3/30wt% g-C3N4) shows highest H2 evolution of 3000 µmol g-1 in 5h under visible irradiation as compared with 2480, 1780, 1280, 100, 740 and 140 µmol g-1 of SC-20, SC-10, SC-50, SC-5, g-C3N4 and SrTiO3 respectively . Fig. S5 shows the results for degradation of LFC under UV light. SC-30 (30 wt% g-C3N4) showed best degradation rate of 55.0% in 90 min. The order of reactivity for binary junctions with different mass ratios for both H2 production and LFC degradation is: SC-5 < SC-10 < SC-20 < SC-50 < SC-30. The increased degradation with increased percentage of g-C3N4 is due to increased absorption in visible region. However at very high wt%, redundant carbon nitride starts acting as a recombination centre. The explanation also holds good for the PHE. SC-30 was used for formation of ternary junctions. In further modification, nanoparticles were deposited on g-C3N4 and junction was formed with SrTiO3 to form SrTiO3/(Ag/Fe3O4)/g-C3N4. Depending on wt% of Ag/Fe3O4 deposited on junction the catalysts were named as SFC-1 (1 wt%), SFC-3 (3 wt%) and SFC-5 (5 wt%). 3.3.2 Photocatalytic hydrogen evolution After optimization PHE over g-C3N4, SrTiO3, SC-30, SFC-1, SFC-3 and SFC-5 without co-catalyst (2 wt% Pt) was tested under visible light (λ > 420 nm) and results are given in Fig. 8(a) and (b). SFC-3 shows the highest H2 evolution activity followed by SFC-5, SFC-1and SC-30 (Fig. 8a). SrTiO3 due to its high band gap shows negligible activity under visible light. The corresponding hydrogen generation rates are compared in Fig. 8(b). SFC-3 shows 2008 µmol g-1 h-1 PHE followed by 1720, 1440, 600 and 148 µmol g-1 h-1 respectively for SFC-5, 11 ACS Paragon Plus Environment


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SFC-1, SC-30 and g-C3N4 respectively. The same trend is obtained in the PL, PCR and EIS results where lowest 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

recombination and highest charge transfer is observed for SFC-3. The evolution rate for SFC-3 is ~14 times that of g-C3N4. The PHE rate increases on inclusion of 1% Ag/Fe3O4 to SC-30 (SFC-1) and increases to 3wt% but decreases at 5% which is due to superfluous accretion of nano Ag. This leads to more charge recombination and shielding of reactive sites. Pure g-C3N4 shows comparative decent H2 evolution rate but not high because of its limited visible absorption, low specific area and poor charge transport. However SrTiO3 shows a rate of 28 µmol g-1 h-1, even though it has a high band gap which may be due to some vacancies in the crystal. The reactivity is further improved by combining the two moieties in SC-30. This is due to optimum band edge positions of two semiconductors facilitating electrons for generation of H2. As SrTiO3 is inactive in visible region but movement of electrons form g-C3N4 leads to concentration of electrons in CB of SrTiO3 which is also suitably placed and has optimum potential. When Ag/Fe3O4 is introduced in system, PHE rate further increases dramatically. The presence of Ag/Fe3O4 in lower amount effectively shifts the absorption to higher wavelength due to surface plasmon resonance of metallic silver and higher visible absorption of Fe3O4. In addition Ag and Fe3O4 act as recombination sites for unused electrons which maintain the strengths at VB and CB of semiconductors. Under visible irradiation they also become source of electrons too. When 2wt% Pt is used as co-catalyst results improve as seen in Fig. 8(c). SFC-3 shows a PHE of 4200 µmol g-1 h-1 in presence of co-catalyst. The increase in PHE in presence of Pt shows that amino groups may act as a coordinating centre which facilitates the transfer photogenerated charge carriers from graphitic core to Pt. In addition the same role is played by plasmonic silver in visible light. This a high increase in the rate of evolution but if we compare SFC-3 photocatalyst it shows a quite high performance without Pt as co-catalyst. This shows the promising use of these catalysts in hydrogen production via other methods too. In this experiment TEOA was used as a sacrificial agent and in order to study the influence on PHE, various other reagents as AA, EG, LA, DMF, TEA have been used (Fig. 8d). AA, LA and EG which are either alcoholic or acidic materials are feebly active with negligible evolution. However when sacrificial agents are replaced by amines as TEA, DMF and TEOA high PHE is observed which hints that the amino groups containing lone pair of electrons are more prone to holes attack. For practical applications we need to check a wide spectral response for H2 evolution Fig. 8(e) shows performance of SFC-3 under solar and NIR light. SFC-3 shows a hydrogen evolution rate of 1500 and 555 µmol g-1 h-1 under natural solar light and NIR (>780 nm). However g-C3N4 and SrTiO3 don’t show high results under natural solar light also. The activity under NIR is doubtless due to surface plasmon resonance (SPR) effect of metallic silver. The stability and reusability of SFC-3 was investigated for three consecutive cycles (15 h). Incipiently it can be observed that for first two cycles a gradual increase is obtained but for the third cycle a slight saturation is observed in last two hours (Fig. 8f). No distinguishable 12 ACS Paragon Plus Environment

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discrepancy can be noticed which infers that as prepared plasmonic SFC-3 heterojunction possesses splendid 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

stability and high activity in a wider spectrum. 3.3.3 Photocatalytic degradation of LFC under different light sources Plain photolysis of LFC was performed without any photocatalyst under UV, visible and solar light and results are presented in Fig. S6. It is observed that except UV irradiation the degradation is negligible which means that drug is highly resistant to degradation under natural conditions. The photocatalytic degradation of LFC was performed under UV, visible, solar and NIR light for confirming the wide spectral activity and results are given in Fig.9. The degradation efficiencies of SrTiO3, g-C3N4, binary SC-30, ternary SFC-1, SFC-3 and SFC-5 and corresponding kinetics curves under UV light are presented in Fig. 9(a) and (b) respectively. All catalysts show excellent results under UV irradiation. The degradation in presence of SrTiO3 was 62.9% which increased to 77.81% with incorporation of 30wt% g-C3N4 (SC-30). This percentage further increased to 99.1% by incorporation of 3wt% Ag/Fe3O4 (SFC-3). This is due to higher charge separation on formation of junction and presence of small amounts of Ag and Fe3O4 which facilitate absorption and formation of electron-hole pairs. Fig. 9(c) and (d) show performance of photocatalysts under visible light (> 420 nm). The degradation decreased considerably and rate also dropped with pure SrTiO3 showing negligible results. Pure g-C3N4 degraded 41.7% of LFC, which increased to 54.3% and 98.2% for SC-30 and SFC-3 respectively in 90 min. The rate is slower than UV performance as degradation time is stretched. Among SFC-1, SFC-3 and SFC-5, SFC-1 performs best under visible too as in UV light. SFC-5 performs weaker than SFC-1 because at very high concentration Ag and Fe3O3 may become centres of recombination. The results were also compared with Ag/Fe3O4/SrTiO3 and Ag/Fe3O4/gC3N4 under UV, visible and NIR light (Figure S7). Both these catalysts show higher rate of degradation than SrTiO3 and g-C3N4 but comparable to SC-30 under visible and UV light. In the NIR region (> 780 nm) binary and single catalysts could hardly degrade LFC (Fig. S8) the, the catalytic efficiency of SFC-3 also decreased significantly with 47.8% degradation in 120 min. The experimental results prove that the ternary catalyst SFC can perform under a wide spectrum. The kinetic curves (pseudo-first order) for degradation show that apparent rate constants are highest for SFC-3 under UV (0.0628 min-1) and visible regions (0.0478 min-1). Fig. 9(e) shoes the effect of dosage of SFC-3 on degradation of LFC under visible light. Highest degradation was achieved at optimum concentration of 0.3 g L-1. The degradation is obviously lower at low dosage because of limited active sites. At higher dosages exposure to light gets hindered and even scattered due to shielding effect of excess photocatalyst. Fig. 9(f) displays the kinetics plots for LFC degradation in presence of Na+, Ca2+, NO3-, Cl- and HCO3- ions. It is observed that LFC degradation rate is increased in presence of NO3- and HCO3- ions with apparent rate constants 0.0712 min-1 and 0.0587 min-1 respectively. NO3- ions tend to produce ●OH radicals under visible or solar light and increase rate of photo-degradation60. In general HCO3- ions react with •OH to form ●CO3- which are less reactive 6. However ●CO3- has affinity for moieties containing sulphur and amino functionality and fortunately LFC has amino group. But the amino group here is tertiary which limits the 13 ACS Paragon Plus Environment


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interaction with ●CO3-. Thus degradation is enhanced but not to a greater extent. Chloride ions have a negligible 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

effect (k1 = 0.0450 min-1) on degradation. The apparent rate constants in presence of Na+ and Ca2+ are 0.0489 min-1 and 0.0443 min-1. The presence of Na+ ions have little effect on rate of degradation, but Ca2+ lowers the degradation rate which may be due to its complex formation with LFC. The photo-degradation largely depends on the initial pH of the solution and the pzc values of the photocatalyst. Levofloxacin has two pKa values (pKa1 = 5.7 and pKa2 = 7.9)61. The effect of pH on the degradation of LFC in presence of SFC-3 and SC-30 under visible light was investigated over a pH range of 2–10 for a 90 min experiment keeping all parameters the same (Fig. 10a). As quoted above the pzc values for SC-30 and SFC-3 are 4.7 and 5.5 respectively. At initial pH=7 the surface of both the catalysts is negative while that of LFC is neutral or slightly positive. Thus there is a possible interaction between the catalyst and drug. However at low pH both the drug and catalysts are positively charged and at high pH both are negatively charged which leads to repulsion. Thus degradation is very low at highly acidic and basic pH. Hence both the catalysts show higher degradation in the pH range 5-7. After this the catalysts were also used for LFC degradation under natural solar light. The kinetics plots for the solar experiments are given in Fig. 10(b). The degradation is reduced considerably as compared to UV and visible light but higher than NIR performance. The apparent rate constants are 0.0221 min-1, 0.03116 min-1, 0.0384 min-1, 0.0411 min-1 for SC-30, SFC-1, SFC-5 and SFC-3 respectively. Under solar experiments also ternary SFC-3 exhibits the best photocatalytic activity. The obvious decrease in the activity is due to lower energy of natural solar light. However, these experiments prove that the ternary catalyst SFC is active in visible, UV, NIR and natural solar light. 3.3.4 LFC Degradation pathway, TOC removal and reusability To elucidate the degradation route of LFC via reactive oxygen species LC-MS was performed after 20 min of degradation under visible light. The MS spectra of some of intermediates detected are given in Fig. S9. Obviously a typical peak at m/z = 361.1 is for molecular ion peak for LFC. The intermediates detected are compiled in table S2 with corresponding m/z values. The first step is usually replacement of F by OH groups due to hydroxyl radicals attack. The major degradation pathway of LFC as reported by other works also is via attack on piperazinyl group62 followed by decarboxylation and defluorination. The second pathway includes attack of radicals on aromatic rings i.e. benzene and quinolone rings. The third degradation route is hydrolysis and decarboxylation. The scheme 1 shows a possible degradation mechanism combining the above mentioned pathways. The degradation intermediates detected have been similar to as reported earlier63. In first path hydroxyl radicals attack N atom in piperzinyl part (demethylation) to form LFC-1 (m/z =334) which leads to formation of LFC-3 and LFC-4 on successive attacks. In the second step the attack on fluorine leads to formation of LFC-5 (m/Z = 392) which leads to successive ring opening of quinolone ring via decarboxylation and hydrolysis to form LFC-6 (m/Z = 336), LFC-7 (m/z = 368), LFC-8 (m/z=315) and LFC-9 (m/z=230). In 14 ACS Paragon Plus Environment

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another pathway hydroxyl radicals attack on piperzinyl ring with the successive ring opening to form LFC-10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(m/z= 290). Further degradation leads to formation of LFC-4 and complete ring destruction to form aliphatic molecules which may successively led to mineralization in due course. The degradation route may vary from literature in some context. However tis mechanism has been proposed on basis of intermediates detected. The degradation of LFC into intermediates has been supported by LC-MS analysis. Fig. S10(a) represents the mineralization rate or TOC removal rate under different sources of light in presence of binary SC-30 and ternary SFC-3 catalyst (operational conditions are the same). SFC-3 shows a higher TOC removal of 71.2%, 59.3% and 56.1% under UV, visible and natural solar irradiation in 2 h experiment. On the other hand SC-30 shows a lesser TOC removal of 54.5%, 48.2% and 45.3% on UV, visible and solar light. The excellent TOC removal ability suggests higher mineralization. The stability, reusability and easy separation are important factors for practical applications of photocataltsts. SFC-3 was used for degradation of LFC under visible light for 5 repeated cycles. As visible in figure S10 (b) there is an obvious loss in the photodegradation ability of catalyst after 5 cycles. However the decrease is not substantial and catalyst can be used for many further cycles. The XPS scan and SEM image of SFC-3 was obtained after reuse (Fig. S11). No abnormal changes are observed in the binding energies of constituent elements showing no compositional and structural changes occur. In addition we don’t observe many changes in the surface morphology of catalyst. The catalyst has a higher magnetization and is separately magnetically from reaction mixture.



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Scheme 1: Proposed degradation pathway for LFC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.3.5 Photocatalytic mechanism: PHE and LFC degradation On the basis of electrochemical, optical and structural analysis a tentative mechanism for photocatalytic hydrogen evolution can be put forward. PHE on surface active sites of photocatalyst is similar to that in electrolysis

64

. Therefore H2 generation electrochemically was studied using linear sweep voltammetry (LSV)

method in TEOA solution represented by Fig. S12 and has been compared with commercial Pt/C electrode. SFC3 and SFC-5 show highest current density which indicates higher electron migration at the interface. Both g-C3N4 and SrTiO3 show poor onset values as well as poor current density. In addition binary junction SC-30 shows a slight improvement in onset values and SFC-3 and 5 have proper onset values for H2 production. SFC-3 has an over potential of 0.10 V which is very close to that of commercial Pt/C (0.06V). Fig. 11(a) and (b) shows the charge carries flow in UV and visible irradiation. In Fig. 11 (b), it is observed that both the CB edges of g-C3N4 and SrTiO3 are placed above Eo(H+/H2)= 0.0 eV which facilitates the H2 evolution. However SrTiO3 is not active in visible region, but movement of electrons from CB of g-C3N4 to SrTiO3 leads to its participation and a higher electron flow. In case of SFC, presence of Ag/Fe3O4 leads to higher electron mobility and reduce recombination. They may also act as recombination centres for addition electrons. Under visible light they themselves become source of electrons and increase the density of electrons in conduction bands for a higher hydrogen evolution. SPR effect leads to a higher visible absorption and generation of charge carriers. To elucidate the photocatalytic mechanism regarding the generation of reactive oxygen species involved in case of ternary photocatalyst SFC scavenging tests were performed during photodegradation of LFC. In these experiments 1,4-benzoquinone (BQ), isopropanol (IPA) and ethylenediamminetetraacetic acid disodium salt (EDTA-2Na) were used as scavengers for •O2-, •OH and h+ respectively. The effect of scavengers I presence of SFC-3 under UV light is given in Fig. 10(c). The addition of IPA causes a less diminishing effect as compared to BQ, which means •O2- is important active species. Fig. S13 and Fig. 10(d) show effect of scavengers on LFC degradation in presence of SC-30 and SFC-1 under visible irradiation. For ternary SFC-3, IPA, EDTA-2Na and BQ all show detrimental effect under visible light which means •OH, h+ and •O2- are active species. While under UV light BQ shows lowest degradation in presence of BQ as compared to IPA and EDTA-2Na. This means under UV light •O2- are major active species while under visible light all •O2-, •OH and h+ participate in photodegradation. For validating the results observed for generation of radicals ESR spin trapping was used. Fig. S13 shows the ESR signals for DMPO-•O2- and DMPO-•OH in case of SC-30 under visible light. The intensity for •O2- is higher than •OH. In case of ternary SFC-3 ESR signals for both are obtained under visible and UV light (Fig. 10 e and f). Under UV light a strong signal of •O2- is observed along with a weak signal of •OH. Under visible light irradiation a high intensity ESR signal of •O2- is still obtained and the intensity of •OH is increased 16 ACS Paragon Plus Environment

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considerably. Thus with incorporation of Ag/Fe3O4 the generation of •O2- and •OH radicals is enhanced 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

considerably. After analysis of the scavenging and ESR experiments we can predict a logical explanation for the enhanced photo degradation under ternary catalyst and possible charge transfer. The mechanism is schematically represented in Fig. 11. The mechanism has been discussed separately for UV and visible light. According to ESR experiments in case of SC-30 under visible light the signals for both •O2- and •OH are obtained with a lower intensity for the latter. However for ternary SFC-3 the signals are strong for both the radicals which hints that both holes and electrons are involved in VB of SrTiO3 and CB of g-C3N4. The weaker intensity of •OH signals for SC-30 suggests that electrons get shifted form CB of g-C3N4 (-1.32 eV) to that of SrTiO3 (-1.02 eV) and holes form VB of SrTiO3 (2.12 eV) to VB of g-C3N4 (1.42 eV). Thus a double charge transfer mechanism is obeyed for binary SrTiO3/g-C3N4 catalyst which leads to higher charge separation, reduced recombination and higher photocatalytic activity. The mechanism for ternary SFC-3 has been discussed in light of scavenging and ESR measurements under UV and visible light separately. Fig. 11 (a) shows that under UV light both SrTiO3 and g-C3N4 absorb light and there is a double transfer of electrons and holes among the bands. The redox potential of E (O2/•O2-) = -0.33 eV)65 and CB edges of both SrTiO3 and g-C3N4 are suitable for formation of •O2-. However the redox potential of E (•OH/H2O) is 2.27 eV vs NHE and E (•OH/-OH) is 1.99 eV vs NHE66. So the holes which get transferred form VB of SrTiO3 to VB of g-C3N4 lose their potential to oxidize H2O molecules into hydroxyl radicals. Thus weak hydroxyl radicals signal are observed in the ESR spectrum for the binary junction. Thus the OH radicals are formed by alternative route involving –OH ions or via reaction of •O2- and H+. In case of SrTiO3/Ag@Fe3O3/gC3N4 catalyst the electrons from VB of SrTiO3 and holes from CB of g-C3N4 get transferred to bridging silver or Fe3O4 where they recombine. Thus electrons stay at highly negative CB of g-C3N4 and holes stay at highly positive VB of SrTiO3. Hence the high band potentials are protected for formation of reactive oxygen species. This is also supported by ESR measurements. Now consider the mechanism under visible light irradiation (Fig. 11b). Only g-C3N4 absorbs in visible region as SrTiO3 cannot be excited and electron-hole pairs are formed only in the former. The electrons get transferred to oxygen to form superoxide radical anions and some electrons move to CB of SrTiO3 where they again form •O2-. The superoxide radical anions formed can directly attack LFC molecule or can form •OH which may lead to further degradation. There are other alternative routes to •OH as via superoxide radical anion, via production of H2O2 and possibility of fenton reaction due to presence of Fe3O4. However this is not possible in case of binary SC-30 junction. Thus superoxide radical anions are formed to maximum here which is supported by ESR signals. In case of SFC-3 Ag nanoparticles absorb visible light and produce e-h pairs by surface plasmon resonance effect 17 ACS Paragon Plus Environment


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(SPR) which gets shifted to SrTiO3 and g-C3N4 for further formation of radicals. In addition Fe3O4 also produces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

OH radicals. The photoexcited e- and h+ get overloaded at the surface and consequently changes Fe3+ to Fe2+ ion which reacts with adsorbed oxygen and water molecule to produce reactive species. The Ag/Fe3O4 nanoparticles play a role in transfer of charge carriers to form a Z-scheme heterojunction. Also it has been reported in the literature magnetite nanoparticles with a lower particle size may also contribute to SPR or enhance for Ag67. The heterojunction formation between SrTiO3 and g-C3N4 leads to enhanced charge separation and introduction of Ag/Fe3O4 improves the visible response, generate hydroxyl radicals and superoxide anion radicals and separate the charge carriers. In addition the high valence band of SrTiO3 and conduction band of g-C3N4 are protected for further generation of •O2- and •OH due to presence of Ag/Fe3O4.

If we consider that all the Ag/Fe3O4

nanoparticles are not distributed on surface or bridged at correct interface, then the transfer of photogenerated electrons and holes can’t proceed by Z-scheme as predicted earlier. Thus the electrons from CB of g-C3N4 could be transferred to CB of SrTiO3 and holes from VB of SrTiO3 to VB of g-C3N4. As explained earlier Ag and Fe3O4 can capture electrons from surface or interface and still some of electrons can still be transferred to Ag and Fe3O4 thus inhibiting the recombination. 4. Conclusion In summary this work reports fabrication of binary SrTiO3/g-C3N4 and ternary SrTiO3/(Ag/Fe3O4)/g-C3N4 nanohetero-junctions by precipitation-wet impregnation-photodeposition route for H2 evolution and LFC degradation. The ratio of SrTiO3 and g-C3N4 was optimized first and then loading of Ag/Fe3O4 was also optimized to get best performance. The fact that ternary photocatalyst has a wide spectral response and higher charge separation, was supported by UV-DRS, PL, EIS and PCR measurements. SFC-3 without any co-catalyst exhibits a higher PHE rate of 2008 µmol g-1 h-1 under visible light which is ~14 times that of bare carbon nitride. The synergistic effects of SrTiO3/g-C3N4 heterojunction, protection of higher VB & CB by Ag/Fe3O4, electron capture and SPR effect lead to high photocatalytic activity under various light sources. LC-MS was performed to predict a degradation pathway based on intermediates: decarboxylation, defluorination and hydrolysis. Scavenging experiments and ESR measurements for binary and ternary catalysts under UV and visible light helped in predicting a suitable photocatalytic mechanism in addition to identification and thermodynamic feasibility of the major active species. Under UV light •O2- is the major active species while both •O2- and •OH radicals participate in visible irradiation. The formation of hydroxyl radicals is facilitated by electron capture by Ag and Fe3O4, reaction of superoxide radical anions by H+ and via H2O2 route. Ag/Fe3O4 helps in Z-scheme transfer of electrons and holes in the ternary junction SFC-3 meanwhile utilizing the high band potentials of two semiconductors to the maximum. SFC catalyst with wide spectral response is highly efficient, magnetically recoverable and reusable for many photocatalytic cycles. This research study helps in understanding rational designing of ternary photocatalytic


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systems utilizing the Z-scheme transfer and surface plasmon resonance for degradation of noxious pollutants and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hydrogen production. Supplementary information FTIR spectrum of SFC-3 (Figure S1), SEM and TEM image of g-C3N4 and SrTiO3 (Figure S2), pzc curve for SC-30 and SFC-3 (Figure S3), Time course of H2 evolution for different wt % of g-C3N4 with SrTiO3 under visible light (Figure S4), Photodegradation potential of various samples with varying weight percentages of gC3N4 in SrTiO3/g-C3N4 junction under UV light (Figure S5), Photolysis of LFC (Figure S6), Photodegradation potential of Ag/Fe3O4/g-C3N4 and Ag/Fe3O4/SrTiO3 under UV, visible and NIR light (Figure S7), Performance of photocatalysts under Near infra-red irradiation (>780nm) (Figure S8), LC-MS spectrum for various intermediates (Figure S9), (a) TOC removal for LFC degradation by SFC-3 (b) Reusability studies for SFC-3 under visible light irradiation (Figure S10), XPS survey scan of SFC-3 and SEM image of SFC-3 after five cycles of reuse under visible light (Figure S11), LSV curves for various samples coated on ITO in TEOA solution with scan rate 1 mV s-1 (Figure S11), Effect of scavengers on degradation of LFC in presence of SC-30 under visible light and ESR spectrum of DMPO-•O2- and DMPO-•OH under visible light for SC-30 (Figure S12), Band gap parameters (Table S1), Photodegradation products of LFC identified by LC-MS (Table S2) Acknowledgments The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group no. RG-1436-034. References (1) Acerce, M.; Voiry, D.; Chhowalla, M. Metallic 1T Phase MoS 2 Nanosheets as Supercapacitor Electrode Materials. Nat. Nanotech. 2015, 10(4), 313. (2) Zarrin, S.; Heshmatpour, F. Photocatalytic Activity of TiO2/Nb2O5/PANI and TiO2/Nb2O5/RGO as New Nanocomposites for Degradation of Organic Pollutants. J Hazard. Mater. 2018, 351, 147-159. (3) Sumboja, A.; An, T.; Goh, H. Y.; Lübke, M.; Howard, D. P.; Xu, Y.; Handoko, A. D.; Zong, Y.; Liu, Z. OneStep Facile Synthesis of Cobalt Phosphides for Hydrogen Evolution Reaction Catalysts in Acidic and Alkaline Medium. ACS Appl. Mater. Interfaces 2018, 10 (18), 15673-15680. (4) Xu, J.; Hao, Z.; Guo, C.; Zhang, Y.; He, Y.; Meng, W. Photodegradation of Sulfapyridine Under Simulated Sunlight Irradiation: Kinetics, mechanism and Toxicity Evolvement. Chemosphere 2014, 99, 186-191. (5) Zhou, C.; Lai, C.; Xu, P.; Zeng, G.; Huang, D.; Li, Z.; Zhang, C.; Cheng, M.; Hu, L.; Wan, J.; Chen, F.; Xiong, W.; Deng, R. Rational Design of Carbon-Doped Carbon Nitride/Bi12O17Cl2 Composites: A Promising Candidate Photocatalyst for Boosting Visible-Light-Driven Photocatalytic Degradation of Tetracycline. ACS Sustainable Chem. Eng. 2018, 6 (5), 6941-6949. 19 ACS Paragon Plus Environment


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Figure 1: Synthesis scheme for SrTiO3/Ag/Fe3O4/g-C3N4 (SFC)


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2: XRD patterns of as synthesized photocatalysts


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Figure 3: XPS spectra for SFC-3: (a) Survey (b) C 1s (c) N 1s (d) Fe 2p (e) Ag 3d (f) Sr 3d (g) Ti 2p (h) O 1s 27 ACS Paragon Plus Environment


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Figure 4: (a-c) SEM micrographs of SFC-3 at different magnifications; (d-k) Elemental colour mapping of SFC3


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Figure 5: (a-b) Low resolution TEM images of SFC-3: (c-d) High resolution images of SFC-3



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Figure 6: (a) BET isotherm for SFC-3 (b) Magnetization-Hysterisis curve for SFC-3 (c) UV-Diffusion reflectance spectra (d) Kubelka-Munk function versus the gap energy


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Figure 7: (a) Mott-Schottky plots (b) PL spectra (c) EIS Nyquist plots for photoelectrodes (INSET-equivalent circuit) (d) Photo-current response of photocatalysts



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Figure 8 : (a) Time course of H2 evolution under visible light ; The corresponding photocatalytic H2 production rates of photocatalysts under visible light (b) without Pt as co-catalyst (c) With Pt as co-catalyst (d) With different kinds of sacrificial agents (e) Performance of SFC-3 under solar and NIR light (f) Cyclic stability under visible light for SFC-3 32 ACS Paragon Plus Environment

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Figure 9: Photocatalytic activity (a-b) Degradation of LFC and corresponding kinetics plots under UV (c-d) Degradation of LFC and corresponding kinetics plots under visible light; Operational parameters for LFC degradation by SFC-3 (e) Effect of dosage (f) Effect of inorganic ions [photocatalyst] = 0.3 mg/ml (variable for dosage effect) [LFC] = 10 mgL-1, Initial pH= 7, Temperature = 30±0.5 °C 33 ACS Paragon Plus Environment


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Figure 10: (a) Effect of pH under visible light (b) Kinetics plots for degradation of LFC under natural solar light; Scavenging studies for LFC degradation: (c) Under UV, (d) Under visible radiation; ESR spectrum of DMPO•O2- and DMPO- •OH (e) Under visible light (f) Under UV light; [SFC-3] = 0.3 mg/ml) [LFC] = 10 mgL-1, Initial pH= 7, Temperature = 30±0.5 °C


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Figure 11: Schematic diagram for the charge separation and movement under (a) UV light and (b) Visible light



Graphical abstract


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High-Performance Photocatalytic Hydrogen Production and

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