Supporting Information. Application of Engineered Si Nanoparticles in Light- Induced Advanced Oxidation Remediation of a

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1 Supporting Information Application of Engineered Si Nanoparticles in Light- Induced Advanced Oxidation Remediation of a Water-Borne Model Contaminant Muhammad Iqbal ǂ, Tapas K. Purkait ǂ, Greg G. Goss, James R. Bolton, Mohamed Gamal El-Din *, and Jonathan G.C. Veinot ǂ* ǂ Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada * Address Correspondence to: jveinot@ualberta.ca, mgamalel-din@ualberta.ca. 1

2 EXPERIMENTAL Si Nanoparticle Synthesis and Functionalization 3 (3NM), 9 (9NM) and 75 (75NM) Si NPs were obtained from thermal processing of hydrogen silsesquioxane (HSQ). 1 Briefly, to obtain 3 nm Si NPs, HSQ (3-4 g) was heated from ambient to 1100 C at 18 C/min in a slightly reducing atmosphere and maintained at 1100 C for 1 h in a tube furnace (Lindberg Blue). Upon cooling to room temperature, the resulting solid was ground first with mortar and pestle to break down larger particles and later finely ground in a Burrell Wrist Action Shaker by shaking with high purity borosilicate beads for 6 h. To synthesize 9 and 75 nm Si nanoparticles, as prepared 3 nm Si/SiO 2 composite after mortar and pestle grinding was transferred to a high temperature furnace (Sentro Tech Corp.) for further annealing under inert atmosphere using Ar gas. 9 nm Si nanoparticles were obtained by heating the 3 nm Si/SiO 2 composite (0.5 g) to 1300 C at 10 C/min and maintaining at this temperature for 1 h followed by cooling down to room temperature. 75 nm Si nanoparticles were synthesized by heating the 3 nm Si/SiO 2 composite (0.5 g) to 1500 C under Ar. The sample was maintained at 1500 C for 1 h followed by cooling down to room temperature. The composite powders obtained were ground following the procedures listed above. 100 nm amorphous (100NM) Si particles were obtained from Strem Chemicals and annealed at 1100 C for 1 h in tube furnace at in an Ar atmosphere. Etching and Functionalization All the Si particles so obtained were etched with HF to liberate/or remove the oxide (SiO 2 ) and were functionalized with dodecene via thermal hydrosilylation as reported by Veinot 2

3 et al. 2 Briefly, hydride-terminated Si NPs were liberated from the Si NPs/SiO 2 composites using the HF etching. 0.2 g of the ground Si NPs/SiO 2 composite was transferred to a PET beaker equipped with a Teflon coated stir bar. Ethanol (3 ml and water (3 ml) were added under mechanical stirring to form a brown suspension followed by 3 ml of 49% HF aqueous solution (Caution! HF must be handled with extreme care). After 1 hour of etching in subdued light the suspension appeared orange/yellow. Hydride-terminated Si NPs were subsequently extracted from the aqueous layer into ca. 30 ml of toluene by multiple (i.e., 3 x 10 ml) extractions. The Si NPs toluene suspension was transferred to test tubes and the Si NPs were isolated by centrifugation at 3000 rpm. After decanting the toluene supernatant, the resulting hydrideterminated particles were re-dispersed into ca. 20 ml 1-dodecene to yield a cloudy suspension which was transferred to a dry 100 ml Schlenk flask equipped with a magnetic stir bar and attached to a Schlenk line. The flask was evacuated and backfilled with argon three times. The temperature was increased to 190 C and the reaction mixture was left stirring for a minimum of 15 hours. The particles obtained were purified by adding appropriate amount of methanol/ethanol mixture as antisolvent and by centrifugation (14,000 rpm) three times and dispersed in chloroform for further functionalization [Figure 1b]. These Si particles obtained above are soluble in non-polar solvents. To render them water soluble an amphiphilic polymer was synthesized as reported by Parak et al. 3 The nanoparticles were coated with amphiphilic polymer by combining amphiphilic polymer (0.8 M monomer units) and Si nanoparticles (0.02 g) in 20 ml of anhydrous chloroform in 50 ml round bottom flask. The solution was stirred for 30 minutes at room temperature. The solvent was removed by rotary evaporation to yield polymer intercalated SiNPs. Aqueous sodium borate buffer (0.05M, ph 8.5, 2-3 ml) was added to the flask and stirred for 10 min. Later, appropriate amount of DI 3

4 water was added to the flask to dilute the nanoparticles. The nanoparticle solutions were centrifuged at 4,000 RPM for 10 min to remove excess polymer using an ultracentrifugation filter (Amicon Ultra, regenerated cellulose membrane, 15 kda molecular weight cutoff). The colorless filtrate was discarded and the concentrated nanoparticle retentate was washed twice using the same procedure. The resulting SiNPs were dissolved in appropriate amount of DI water to obtain required concentrations (100 mg/l and 50 mg/l). 4 All the coated Si particles are soluble in water [Figure 1c]. Characterization FT-IR: Fourier Transformed Infrared (FT-IR) Spectroscopy was performed using a Thermo Nicolet 8700 FTIR Spectrometer. Toluene solutions of samples were drop-coated onto KBr plates to obtain a thin film of the sample. XRD: Powder X-ray diffraction (XRD) data were collected using an INEL XRG 3000 X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å). TEM: Transmission electron microscopy (TEM) images of Si NPs were acquired using a JEOL (LaB 6 filament) electron microscope with an accelerating voltage of 200 kv. The samples were prepared by drop coating a thin layer of sample onto carbon coated Cu TEM grids (300 mesh, Electron Microscopy Science). High-resolution TEM (HR-TEM) images were obtained from a Hitachi-9500 electron microscope with an accelerating voltage of 300 kv. HR-TEM samples were prepared by drop-coating solutions of freestanding Si NPs onto holey carbon films supported on Cu grids (300 mesh, Electron Microscopy Science) The Si NP sizes and size distributions were determined using ImageJ software (version 1.45) by counting at least 200 nanoparticles. The HRTEM images were processed using Gatan DigitalMicrograph software (version ). 4

5 XPS: XPS analyses were carried out using a Kratos Axis Ultra instruments opening in energy spectrum mode at 210 W. XPS samples were prepared by drop-casting Si NPs onto Cu foils. The base pressure and operating chamber pressure were maintained at 10-7 Pa. A monochromatic Al Kα source (λ = 8.34 Å) was used to irradiate the samples while the spectra were obtained with an electron take off angle of 90. Survey spectra were collected using an elliptical spot with major and minor axis lengths of 2 and 1 mm respectively, and 160 ev pass energy with a step of 0.33 ev. CasaXPS software (VAMAS) was used to interpret the high-resolution spectra. All spectra were calibrated with respect to C 1s emission at ev. The background was subtracted using a Shirley-type background. Quantification of each Si oxidation state was obtained using a Voigt profile with 30% Gaussian character. The high-resolution Si 2p region of spectra collected were fit to Si 2p 1/2 /Si 2p 3/2 partner lines, with spin-orbit splitting fixed at 0.6 ev, and the Si 2p 1/2 /Si 2p 3/2 intensity ratio was set to 1/2. Experimental Design for Photodegradation studies A quasi-collimated beam UV apparatus (Calgon Carbon Corporation, Pittsburgh, USA) equipped with a 1 kw medium pressure lamp (HNG, Hanau, Germany) was used to generate a quasi-parallel beam of UV light as shown in Scheme S1. The incident irradiance at the water surface for each sample was quantified using a radiometer (International Light, Model IL1400, Massachusetts, USA) and UV detector (International Light, Model SEL240, Massachusetts, USA). The relative sensitivity of the detector was determined between 200 and 400 nm by the supplier. The irradiance was measured by placing the UV detector at the center of the collimated beam and adjusting the calibration marker on the detector to the height of top of the solution. After obtaining the relative spectral emittance of the MP lamp, the fluence rate from 200 to 400 5

6 nm was calculated proportionally. The correction factors, such as sensor factor and reflection factor, were also applied as per the procedures described by Bolton and Linden. 7 Scheme S1: (a) A quasi-collimated beam UV apparatus and (b) a schematic of remediation experiment in a quasi collimated beam. A known volume (i.e., 10 ml) of a solution containing predetermined concentrations of Si NPs (i.e., 50 mg/l and 100 mg/l) and methanol (concentrations: 2.5, 5, 10, 20, 30, 40, 50 mg/l) were placed in a 20 ml beaker with a 3.0 cm internal diameter and thermostated at 25 ± 2 C. The solution was then mechanically stirred using a magnetic stir bar (3 10 mm) to ensure homogenous and complete mixing during the reaction. For all evaluations the distance between the lamp solution surfaces was maintained at 30 cm. Solutions were exposed to the UV for 10 and 30 min. A schematic of the remediation experiment set up used is presented in Figure 3(b). 6

7 A series of experiments were carried out using 100 mg/l SiNCs and various methanol concentrations and times in presence of radical scavenger tert-butanol (1.5 g/l). To quantify the effect of white light induced photocatalytic performance, control experiments with or without UV-filter (cut off 400 nm) were performed during the advanced oxidation process. A known volume (i.e., 10 ml) of a solution containing predetermined 100 mg/l of Si NPs (i.e., 3NM-P, 9NM-P, 75NM-P and 100NM-P) and 30 mg/l of methanol were placed in a 20 ml beaker with a 3.0 cm internal diameter. The solution was then mechanically stirred using a magnetic stir bar (3 10 mm) to ensure homogenous and complete mixing during the reaction and solutions were exposed to the UV for 30 min. Sample Analysis In the advanced oxidation process, methanol oxidizes to formaldehyde, and the amount of formaldehyde can be determined by means of Nash colorimetric test. 5,6 The Nash reagent was prepared by mixing 2M ammonium acetate, 0.05 M acetic acid and 0.02 M acetylacetone in 1 L of deionized water. A standard curve of the formaldehyde concentration vs. absorbance of DDL at 412 nm is shown in Figure S6. Equal volumes of freshly prepared Nash reagent was added in equal amounts (5.00 ml) to remediated samples and the mixture was incubated at 40 C for 60 min. The amount of diacetyl-dihydrolutidine (DDL) produced was determined by evaluating the absorption spectrum of the solution at 412 nm and comparing with the standard curve data. The absorbance of blank SiNP and Nash (1:1) solution was subtracted to obtain actual absorbance of DDL. The concentrations of unreacted methanol were monitored by means of gas chromatography (GC) with Varian Model 3300 GC equipped with a flame ionization detector 7

8 (FID), a J&W Scientific DB-WAX, DB-5 or DB-624 megabore column. He was employed as the carrier gas. 9 A standard curve of the methanol concentration vs. GC peak height was used to calculate methanol concentration of UV exposed samples. 8

9 Determination of Quantum Yield (Φ) and Yield factor (η) (1) (2) (3) (4) where, Φ = quantum yield = fluence-based first-order rate constant (m 2 J 1 ) k 1 = pseudo first-order rate constant and = the initial and final concentrations of methanol under photolysis, F = fluence (J m 2 ) E p,o = true photon irradiance or incident photon fluence rate (einstein s 1 m 2 ) U λ = molar photon energy (J einstein 1 ) t = time (seconds) The sensor factor (SF), petri factor (PF), and reflection factor (RF) were calculated using the procedure previously reported. 7 The sensitivity of the detector is wavelength dependent, and thus, for polychromatic sources, the radiometer reading will not accurately measure the true irradiance. Therefore, the sensor factor (SF) was calculated based on the sensor spectral 9

10 sensitivity of an International Light SED 240 detector at wavelengths between 200 to 400 nm. The reflection factor (RF) represents the fraction of the incident beam that enters the sample. For air and water, the average refractive indices in the nm region are and 1.372, respectively. Thus for these two media R = 0.025, and the RF is (1 R) = The concentration of the OH radicals generated is quantified by evaluating the formation of formaldehyde in the presence of excess methanol. 8 Yield factors (η) are defined as: (5) where, the change in the total molar hydroxyl radical generation was determined from the concentration of formaldehyde produced during photolysis. Si NP size, morphology, and concentration, as well as methanol concentration and exposure time were evaluated methodically for their influence on Φ, k 1 and. 10

11 Scheme S2: Pictorial representation of water soluble Si NPs after intercalation and dissolution in borate buffer. 11

12 Figure S1: FTIR spectra of alkyl functionalized (a) 3NM (b) 9NM (c) 75NM and (d) 100NM Si NPs. 12

13 Figure S2: UV-Vis absorption spectra (a) and PL-emission spectra upon excitation at 350 nm (b) of water soluble Si NPs. Figure S3: Selected area electron diffraction (SAED) patterns of (a) 3NM (b) 9NM (c) 75NM and (d) 100NM silicon nanoparticles.. 13

14 Figure S4: XRD patterns of (a) 3NM (b) 9NM (c) 75NM and (d) 100NM silicon nanoparticles. 14

15 Figure S5: Size distribution analysis of (a) 3NM (b) 9NM nm (c) 75NM and (d) 100NM Si NPs. Figure S6: Survey XPS, high resolution XPS of O1s and C1s regions for 3NM-P 15

16 Table S1: Ratio of Si to O from survey XPS of SiNPs. Si NPs Si/O ratio 3NM-P 66:34 9NM-P 83:27 75NM-P 90:10 100NM-P 85:15 Figure S7: Partitioning of Rhodamine 6G with 100NM-P Si NPs (a) under regular light and (b) under UV illumination at 365 nm. Samples on the left in (a) and (b) are before centrifugation while samples on the right are after centrifugation. (Note: the color is arising from the dye not from the Si NPs). 16

17 Figure S8: Characterization of 3NM-P after remediation: (a) FTIR spectra, (b) high resolution XPS of Si 2p region, and (c) PL image under handheld UV light (λ= 365 nm). 17

18 Figure S9: (a) Standardization curve used to determine the formaldehyde concentration. (b) A representative UV-Vis spectrum of Si NPs upon incubation with Nash reagent. Figure S10: Photocatalytic activities of Si NPs upon 30 min UV exposure (a) decomposition of methanol studied via GC (b) formaldehyde formation investigated via Nash reagent. Methanol concentration at (0.944 mm; 30 mg/l) was used to determine yield factors shown in Figure. S12 18

19 Figure S11: Formaldehyde (HCHO) formation in the presence of TBA as scavenger. The samples with scavenger are represented with (S) in the parentheses. (a) HCHO formation as function of time in presence of TBA (b) HCHO formation as function of fluence in presence of TBA (c) comparison HCHO formation in the presence and absence of TBA as function of time (d) comparison of HCHO formation in the presence and absence of TBA as a function of fluence. 19

20 Figure S12: Photocatalytic QYs of various SiNPs for formaldehyde formation with and without window glass filter (filter cut off 400 nm). 20

21 Figure S13: Formaldehyde formed and yield Factors for Si NPs upon UV-exposure of 30 min for indicated SiNPs with concentration of 100mg/L. 21

22 References: 1. Hessel, C. M.; Henderson, E. J.; Veinot, J. G. C. Hydrogen silsesquioxane: A Molecular Precursor for Nanocrystalline Si-SiO 2 Composites and Freestanding Hydride-Surface- Terminated Silicon Nanoparticles. Chem. Mater. 2006, 18, Kelly, J. A.; Veinot, J. G. C. An Investigation into Near-UV Hydrosilylation of Freestanding Silicon Nanocrystals. ACS Nano 2010, 4, Lin, C.-A. J.; Sperling, R. A.; Li, J. K.; Yang, T.-Y.; Li, P.-Y.; Zanella, M.; Chang, W. H.; Parak, W. J. Design of an Amphiphilic Polymer for Nanoparticle Coating and Functionalization. Small 2008, 4, Hessel, C. M.; Rasch, M. R.; Hueso, J. L.; Goodfellow, B. W.; Akhavan, V. A.; Puvanakrishnan, P.; Tunnel, J. W.; Korgel, B. A. Alkyl Passivation and Amphiphilic Polymer Coating of Silicon Nanocrystals for Diagnostic Imaging. Small 2010, 6, Nash, T. The Colorimetric Estimation of Formaldehyde by Means of the Hantzsch Reaction. Biochem. J. 1953, 55, Chan, P. Y.; Gamal El-Din, M.; Bolton, J. R. A Solar-Driven UV/Chlorine Advanced Oxidation Process. Water Res. 2012, 46, Bolton, J.; Linden, K. Standardization of Methods for Fluence (UV Dose) Determination in Bench-Scale UV Experiments. J. Environ. Eng. 2003, 129, Sun, L.; Bolton, J. R. Determination of the Quantum Yield for the Photochemical Generation of Hydroxyl Radicals in TiO 2 Suspensions. J. Phys. Chem. 1996, 100, Safarzadeh-Amiri, A.; Bolton, J. R.; Cater, S. R. Ferrioxalate-Mediated Photodegradation of Organic Pollutants in Contaminated Water. Water Res. 1997, 31,