Photoelectrochemical Solar Cells Prepared From Nanoscale

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Photoelectrochemical Solar Cells Prepared From Nanoscale Zerovalent Iron Used for Aqueous Cd2+ Removal Keyla T Soto, Edwin O. Ortiz-Quiles, Luis Betancourt, Eduardo Larios, Miguel José Yacamán, and Carlos R Cabrera ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00601 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 8, 2016

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ACS Sustainable Chemistry & Engineering

Photoelectrochemical Solar Cells Prepared From Nanoscale Zerovalent Iron Used for Aqueous Cd2+ Removal Keyla T. Soto Hidalgo a, b, ‡, Edwin O. Ortiz-Quiles c, ‡, Luis E. Betancourt c, Eduardo Lariosd, e, Miguel José-Yacamand, and Carlos R. Cabrerac,*

a. University High School, Department of Education, University of Puerto Rico at Río Piedras, San Juan, Puerto Rico 00931. b. Department of Environmental Sciences, University of Puerto Rico at Rio Piedras, San Juan, Puerto Rico 00931. c. Department of Chemistry, University of Puerto Rico at Río Piedras, San Juan, Puerto Rico 00931. d. Physics and Astronomy Department, University of Texas at San Antonio, San Antonio, Texas 78249, USA. e. Departamento de Ingeniería Química, Universidad de Sonora, 83000 Hermosillo, Sonora, México.

KEYWORDS: zerovalent iron, photovoltaic, remediation, electrochemistry, iron oxide nanoparticles, cadmium ferrite

1 ACS Paragon Plus Environment


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Abstract: Nanoscale zerovalent iron (nZVI) particles have been widely studied in the environmental sciences for wastewater treatment. These types of nanoparticles react in aqueous media producing metal oxides, which can be photoactive in the ultraviolet energy region. This prompted us to examine alternatives for the preparation of nanomaterials using nZVI in the presence of 6 ppm and 30 ppm of Cd2+ in aqueous solutions. These Cd2+ concentrations are representative of contaminated regions of Puerto Rico such as the Las Cucharillas Marsh in Cataño. Comprehensive chemical and physical characterization of the resulting nZVI products after their exposure to Cd2+ was done. Further studies of the resulting nanostructures were completed using a photoelectrochemical solar cell (PSC) as the photoanode material. Incident photon-to-current efficiency (IPCE) and electrochemical impedance spectroscopy (EIS) analysis of these PSCs showed active photochemical properties in the ultraviolet range for the sample exposed to 30 ppm of Cd2+. Changes in the structure and chemical oxidation states of the species were observed in transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy analysis was attributed to these photochemical properties. These results show an alternative synthetic method for producing iron oxides for photocatalytic applications, and a possible strategy for reuse of nZVI after water remediation treatments.

Introduction Iron nanoparticles are a new generation of materials for environmental remediation. Various metallic ions including Pb2+, Cr6+, Ni2+, As3+, As5+, Cd2+, Cu2+, Zn2+, and Ba2+ have been fixated from water using this new technology.1 Nanoscale zerovalent iron (nZVI) has been studied as an alternative method for water decontamination processes. In the nZVI reaction, metallic iron is oxidized in the presence of water, which can remove other metal ions from the 2 ACS Paragon Plus Environment

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aqueous media by chemical absorption.2-4 In this process, iron oxides such as hematite (α-Fe2O3) are produced.5 This α-Fe2O3 has been broadly studied for catalytic applications due to its highly active photochemical properties and resistance to corrosion; however, the excited electrons have a short lifetime and the charge mobility is low.6 Therefore, it has been a challenge to use hematite for PSC applications, especially because this material does not absorb in the visible range, yet is highly active in the ultraviolet range. Accordingly, various strategies to improve the photoactivity of this material have been tested, the goal is implementing hematite in commercial solar-based devices, including high temperature synthesis methods for doping structures.6-8 Using hematite instead of TiO2 has been evaluated as an alternative method to produce dyesensitized solar cells.9 Approaches such as doping with other metals and changing the structural arrangement of the system have been employed to overcome challenges regarding electron transfer processes.6 Previous studies from our group have demonstrated that the incorporation of Cd2+ ions in the oxidized nZVI produces changes in the structure of the system associated with both iron oxide species (FexOy) and the formation of cadmium ferrite (CdFe2O4).10 This can be done without using high temperatures to complete the preparation of the material, a common practice that is described in the literature.7-8 In addition to these changes, cadmium species have also been studied for photochemical applications. Despite their reported photochemical properties, there have been few reports that used these particles as photocatalysts.8, 11 nZVI particles were used in Cd2+ removal process as a proof of concept study for further use at the Las Cucharillas Marsh.12 Our previous studies included experiments with Cd2+ concentrations between 1 and 6 ppm.10 The next step of this work entailed studying the effects of



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higher Cd2+ concentrations on nZVI. Moreover, an alternative use of the nZVI product was evaluated, which suggested its use in Photoelectrochemical Solar Cells (PSCs). We report herein the use of nZVI previously exposed to Cd2+ ions as photoanodes in PSCs Photoelectrochemical devices, such as those produced by Brian O’Regan and Michael Grätzel in the 90s, gained great attention during the last decade due to their very low fabrication costs.13 Photoelectrochemical devices are challenging due to the optimization of their quantum conversion efficiency, which is affected by the electron transfer processes in the system. nZVI was exposed to different Cd2+ concentrations (1-30 ppm), similar to the concentrations found in the Las Cucharillas Marsh, Cataño, Puerto Rico.12 The main innovation of this work was to evaluate the product formed after Cd water removal processes using nZVI as a photoactive material by applying extensive characterization techniques. Cd-residual iron products (Cd-RIPs) showed promising photoactive behavior for PSC applications.

Experimental The chemicals used were iron (III) chloride hexahydrate (97%, ACS reagent), sodium borohydride (≥98.5%, reagent grade), trichloroethylene (99.5 %, ACS reagent), Tween® 80, humic acid (sodium salt) obtained from Sigma Aldrich, FeCl3·6H2O (F2877 Sigma Aldrich), NaBH4 (Alfa Aesar), ethanol 200 proof, 99.5% anhydride (Sigma Aldrich), Cd(CH3COO)2·2H2O (Sigma Aldrich), HCl (Alfa Aesar), Light NF/FCC Mineral Oil (Fisher Chemical), laboratory grade Triton (Sigma Aldrich), LiI (99.9%, Sigma Aldrich), I2 (99.8%, Sigma Aldrich) and Acetonitrile (ACS grade, low water, BDH). All solutions were prepared with Nanopure® water (18.2 MΩ·cm, Nanopure Diamond, Barnstead). In the present work, the nZVI materials were tested for their ability to reduce Cd2+ concentrations in waste water. The nZVI was prepared by the reduction of an aqueous iron salt 4 ACS Paragon Plus Environment

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using an easily-controlled and effective reductant, sodium borohydride (NaBH4). A 0.6 M solution of FeCl3·6H2O (molar mass 270.30 g/mol) was prepared in 30.0 mL of ethanol (83% v/v) and nanopure water (17% v/v). The solution was purged with N2 for 30 minutes to remove oxygen prior to the reaction, in order to avoid the rapid oxidation of iron. Then, the solution was titrated by adding 100.0 mL of 0.8 M NaBH4 aqueous solution under nitrogen. After 30 minutes of stirring with a magnetic bar at 700 rpm, the solution was filtered using 0.22 µm filter paper (Millipore) under vacuum at 25 °C. Then, the sample was rinsed three times with 99% absolute ethanol. The filtered samples were immediately placed in a desiccator. Cadmium solutions were prepared using Cd(CH3COO)2·2H2O due to its high solubility in water and ethanol. To represent different samples of wastewater contaminated with cadmium, Cd2+ solutions of 6 ppm and 30 ppm were prepared. The initial pH of each solution was adjusted to 2 using 1.0 M HCl. All reactions were done in a dry box under argon atmosphere to prevent Fe oxidation. Aliquots (200 mL each) of all the prepared cadmium solutions (6 and 30 ppm) were treated with 3.0 g/L of nZVI particles for 5 hours. Concentration units are presented as “ppm” keeping with toxicity nomenclature (ppm = mg·L-1). The pH of each cadmium solution was determined before and after adding the nZVI particles. After 5 hours of stirring with a magnetic bar at 700 rpm, the solution was filtered using filter paper with 0.22 µm pore size (Millipore) under vacuum at 25 °C. The filtered samples were placed immediately in a desiccator to be used as photoanodes in PSCs. CdFe2O4 / FexOy / Fe particles were obtained by the removal process using nZVI particles in Cd ion solutions. This powder was then mixed with 0.500 mL of mineral oil, 5.00 mL of EtOH, and 5 drops (0.33 mL) of Triton to produce a slurry of the active material. The mixed



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solution was ultrasonicated for 10 minutes. Fluor tin oxide (FTO) glasses (2.0 cm x 2.0 cm) were used as substrates for the deposition. A 1 cm x 1 cm mask space was prepared with Scotch tape. Afterwards, 3 drops (0.20 mL) of the previously prepared slurry were deposited over the conductive surface of the FTO glasses using a glass rod, until a uniform thickness was achieved. The tape was removed after one hour. Then, the glasses with the deposited material were heated to 80, 200, 300 or 500°C for 5, 15, 10 and 60 minutes respectively, over a hot plate. FTO glasses (10.2 cm x 10.2 cm) from Hartford Glass Co. Inc. were cut into 2.0 cm x 2.0 cm squares. Graphite was deposited over the conductive surface of the FTO glasses. Two drops of a previously prepared electrolyte containing 0.5 M I2 and 0.050 M LiI in acetonitrile was added to the PSC before it was used. A working electrode was prepared using FTO glasses covered with the active material for the electrochemical measurements. A small contact with a copper cable surrounded by a glass cylinder and the glass was made using silver paint. Nonconductive EPOXY was applied to the contact area to assure that the copper and the silver paint were electrochemically insulated from the electrode and from the solution. The phase composition and structures of the CdFe2O4 / FexOy particles were determined by powder X-ray diffraction patterns (PANalytical X’Pert Material Research Diffractometer) using a Cu Kα radiation (λ = 1.54 Å) source. The morphology was observed by high resolution transmission electron microscopy (HRTEM) images with a Cs-corrected JEOL JEMARM200F electron microscope running at 200 kV and equipped with a CEOS Cs corrector on the illumination system. Additional characterization was done using Fourier transform infrared spectroscopy (Nexus 870 FT-IR) and a UV–Vis spectrophotometer (Shimadzu UV-2550). X-ray absorption spectroscopy (XAS) was done at the Cornell High Energy Synchrotron Source (CHESS, Ithaca, NY) using line F3. The Cadmium K edge (26,711 eV) was measured for


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the 6 ppm and 30 ppm samples in transmission mode. Data acquisition consisted of three ionization chamber detectors: incidence (Io); transmittance (IT); and reference (Iref). For the Cd K edge, 100% Ar was used in all chambers. The powder sample data were analyzed using the Demeter Suite Programs. 14 Linear combination fittings were done using Athena software with a solid Cd reference foil and the Cd(OH)2 spectrum calculated from the Hephaestus program 14. All electrochemical measurements were done using a PARSTAT 2273 potentiostat. A trielectrode array using an Ag / AgCl reference electrode, a platinum counter electrode, and previously prepared working electrodes in a 0.5 M H2SO4 solution was employed. Cyclic voltammetry (CV) was done using a xenon/mercury lamp emitting 0.200 W/cm2. Electrochemical impedance spectroscopy (EIS) was carried out using a frequency range of 200 kHz to 50 mHz at an excitation voltage of 5 mV. The data were fitted using ZSimpWin 3.30 assuming a series R(QR)(QR)(CR) equivalent circuit. [[An Oriel quantum efficiency / Incident photon-to-current conversion efficiency measurement system was employed to obtain the photovoltage spectrum. Incident photon-to-current conversion efficiency (IPCE) spectra of PSCs were calculated and normalized to the maximum value on each graph using the following equation: IPCE =

‫ܫ‬ௌ஼ 1240 × ܲ ߣ

where Isc, P, λ and 1240 are the short circuit current of the DSC in amperes (A), power of the light source in watts (W), wavelength of the incident light in nanometers (nm) and a value related to the product between the Planck constant and the speed of light, respectively.



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Results and Discussion Cd-RIPs, simulating their use in the marsh, were done by placing nZVI particles in 30 ppm of Cd2+ solution for 5 hours. This material exhibited two intense XPS peaks, for Cd 3d3/2 and 3d5/2, which were observed in the high resolution XPS spectrum produced from spin-orbit coupling effects. Figure 1a shows the deconvolution of the Cd 3d peaks, with binding energies of 403.3 eV and 405.3 eV for the 3d5/2 signal.15 These two binding energies can be assigned, respectively, to Cd2+ (probably due to CdO or Cd(OH)2 molecules) and to metallic Cd. An absorption process during the removal of Cd2+ from wastewater by nZVI particles has been suggested in a previous work.5 During the development of the reaction, metallic cadmium (Cd0) could be produced by electron transfer processes from metallic iron (Fe0). Studies suggest that this electron transfer reaction is not expected on the surface of the nZVI due to the standard potentials of Cd (-0.40 V vs NHE) and Fe (-0.41 V vs NHE) being so close.4-5 However, Keller’s group recently confirmed the presence of metallic cadmium using XRD after a Cd-removal process.16 We propose that this process could occur in the inner core of metallic iron particles. High energy resolution photoelectron spectrum deconvolution for the O 1s binding energy region shows three peaks – at 528.2, 530.0 and 531.6 eV (see Figure 1b). The binding energies for these peaks can be associated with CdFe2O4, Fe2O3 and hydroxyl groups (e.g. OH-), respectively. Incorporation of OH-1 and O22- shows a broad signal in binding energy ranges between 530.6 and 531.1 eV, which can be observed in the O 1s peak.17 In Figure 1c, the Fe 2p regions have two peaks near 711 and 725 eV, corresponding to the binding energies of the 2p3/2 and 2p½ for Fe3+.18 There is no peak corresponding to Fe0 near 706 and 719 eV. This suggests that the presence of zerovalent iron is relatively small in comparison with the large fraction of


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oxidized iron, most likely due to an extensive oxidation of iron at the particle surface after removal process. Unreacted Fe0 could still be present within these particles, beyond the detection limit of the instrument. In addition, XPS is a surface sensitive technique, which may suggest the presence of oxidized iron species in the surface but does not rule out Fe0 in the bulk of the structure.

(b)

(a) Counts / a.u.

Cd0

Counts / a.u.

Cd2+

414

412

410

408

406

404

402

400

Fe2O3

534

530

528

526

725

(c)

Fe3+

Fe0

Fe0

730

532

Binding Energy / eV Fe3+

735

CdFe2O4

-OH-

Binding Energy / eV

Counts / a.u.

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720

715

710

705

700

Binding Energy / eV Figure 1. X-ray photoelectron spectra of (a) cadmium 3d, (b) oxygen 1s and (c) iron 2p binding energy regions in nZVI particles exposed to 30 ppm of Cd2+ ions in aqueous media.



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To further test our hypothesis regarding the presence of Cd0 and Cd2+ in our proposed core-shell structure model, X-Ray Absorption Near Edge Spectroscopy (XANES) was done at CHESS. XANES is a structure sensitive technique that can give qualitative information on the electronic environment and coordination chemistry in our material. A linear combination fit was applied from the 26,660 eV to the 26,800 eV range with reference spectra of metallic Cd and Cd(OH)2. The percent contribution from each reference was then taken into account to determine the predominant oxidation state of Cd in the bulk of the structure. Figure 2 shows the position of our generated spectra compared to the Cd theoretical spectra. It can be observed that Fe0/FexOy/CdFe2O4 does not exactly match either of the spectra, but shares contributions from both peaks. The linear combination fit suggests the presence of 83% Cd0 and 17% Cd2+, which was attributed to the byproduct containing 30 ppm Cd for the Fe0/FexOy/CdFe2O4 sample. However, these results do not exclude the presence of other oxidized Cd species such as CdO. It implies that in our procedure there is some Cd2+ that reacts directly with the Fe0 at the core shell. Our findings concur with the proposed reduction of Cd2+ to Cd0 due to their differences in standard electrode potentials, which for Cd is (E0= -0.403 V) and for Fe is (E0= -0.447 V).16 The XANES spectra supports the XPS fitting found in Figure 1. However, X-Ray absorption is a bulk sensitive technique while XPS is a surface sensitive technique. Even though (XAS) is sensitive to traces of elements in a sample, the 6 ppm Cd XANES spectra was not useful for further analysis since features were not clear. However, it can be seen in the Supplementary Information that the 30 ppm Cd sample had a much bigger absorption coefficient, which was due to its higher Cd loading in its coordination environment.


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1.4 1.2

Normalized µ(x)

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1.0 0.8 0.6 0.4 0.2

Fe0/FexOy/CdFe2O4 (30ppm) Cd0 Cd(OH)2

0.0 26670

26700

26730

26760

26790

Energy / eV Figure 2. XANES spectra of 30 ppm CdFe2O4 probing the Cd K edge with metallic and Cd(OH)2 reference spectra. The changes in phases of nZVI particles before and after the removal process were monitored by XRD. In Figure 3a, the XRD pattern of nZVI particles shows peaks near 35° and 45° corresponding to the phases (104) for hematite (α-Fe2O3) and (110) for Fe0, respectively. The nZVI particles are highly reactive with atmospheric water, readily producing iron oxides. After mixing these particles with a cadmium solution of 6 ppm or 30 ppm, several changes occurred, which are detailed in Figure 3b and 3c.



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Figure 3. XRD patterns of nZVI particles (a) starting material, exposed to (b) 6 ppm and (c) 30 ppm of Cd2+ in aqueous solution. Identification of phases was done for (I) α-Fe2O3, (II) Fe0, (III) FeOOH, and (IV) CdFe2O4. (Right) Photographic images of a magnetic bar in contact with the vials of the three solutions described above. The arrows are pointing towards the product nanomaterials, which maintain their magnetic properties after Cd2+ removal treatment. In addition to α-Fe2O3, the phase (120) of FeOOH appears in both samples.19 Other peaks at 30.1°, 35.6°, and 46.9° of the phases (220), (311), and (331) for cadmium ferrite (CdFe2O4) can also be observed.20-21 The peaks are displaced to the left when compared to the peaks for the iron oxides due to the larger atomic size of cadmium. This information agrees with the CdFe2O4 signal obtained from the XPS analysis, where a reduction in the intensity of the peak of Fe0 was observed after oxidation. This peak is still present in the cadmium solution after 5 hours. Accordingly, the magnetic properties of the 3 samples stayed active as shown in Figure 3 (right). Crystallite sizes were estimated using the Scherrer equation with results of 15.3 nm for 6 ppm, and 18.4 nm for 30 ppm. A larger crystallite was observed in the sample containing 30 ppm, indicating a higher organization in the system.


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In Figure 4a, high resolution transmission electron microscopy (HRTEM) images of nZVI exhibit spherical shapes and well-aligned aggregates with diameters ranging from 25 to 70 nm. These clusters of nanoparticles are caused by magnetic dipole-dipole interactions of the individual particles.22 It shows a contrast between the center and its surroundings, akin to a coreshell structure. Iron oxides form a layer around Fe0, which consequently prevents further oxidation of the particle core. After exposure to Cd2+ in Figure 4b, new particles are organized as nanofibers (ca. 1 µm in length). The formation of nanofibers may have been produced by the diffusion of absorbed Cd2+ ions through the core shell structure, perhaps due to the lattice energy generated in the oxidation of iron.5 Since nZVI particles are mainly covered with an iron oxide shell, any electron transfer reactions between Cd2+ ions and Fe0 should be expected to occur in the core. This hypothesis is akin to the phenomena observed using surface sensitive XPS results with the bulk sensitive XAS analysis where Cd0 provides the predominant contribution. This suggests that after exposure to Cd2+, electron transfers occur between the Fe0 atoms at the core, and the diffused Fe0 provokes a change in both physical and electronic structures of the metal structure. A conceptual model in Figure 4c summarizes the interactions of nZVI with water and Cd2+ in solution.



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Figure 4. HRTEM images of (a) nZVI particles synthesized and (b) iron nanofibers formed during cadmium ion removal. (c) A conceptual model representing Cd²⁺ interactions with the Fe0 core. FTIR spectra were recorded in the range of 500 cm-1 to 4000 cm-1. Figure 5a shows a synthesized nZVI spectrum with a small peak in the 545 cm-1 region associated with α-Fe2O3 nZVI particles as starting materials. Figures 5b and 5c show spectra after exposure of starting materials to 6 ppm or 30 ppm Cd2+ solutions.23 In both samples, characteristic peaks near 1000 cm-1 and 550 cm-1 are present, associated with the Cd-O-Fe bond in tetrahedral building units.24 A more defined signal of the second band is observed when rescaled in Figure 5 (right). There is a small difference in characteristic vibrational frequencies between 30 ppm and 6 ppm samples, mainly because sizes of these particles are at the nanoscale level. Other bands at 2350 cm-1 and 628 cm-1 are related to atmospheric CO2.25 The hydroxide (-OH) band, centered at 3450 14 ACS Paragon Plus Environment

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cm-1 is characteristic of the vibration of water molecules.26 This band is more intense in the nZVI sample because of its reaction with atmospheric water to produce iron oxides.

Figure 5. Fourier transformed infrared spectra of nZVI particles: (a) starting material, (b) starting material exposed to 6 ppm Cd2+ in aqueous solution and (c) starting material exposed to 30 ppm Cd2+ in aqueous solution. Three ultraviolet and visible absorption spectra of the studied samples are presented in Figure 6. The behavior of the nZVI particles exposed to water during the experimental procedure show a characteristic band in the ultraviolet region for α-Fe2O3. The shift of this band to the left is associated with smaller particles and aggregate clusters in agreement with the HRTEM results.27 There were no other apparent differences between the nZVI species spectra in Figure 6. CdFe2O4 bands are not observed in the visible range in all probability due to the low concentration of the material when compared with α-Fe2O3.



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Figure 6. Ultraviolet and visible absorption spectra of nZVI particles (a) starting material, exposed to (b) 6 ppm and (c) 30 ppm Cd2+ in aqueous solution. Cyclic voltammetry analyses were done on the photoanode in Figure 7. Bare FTO and TiO2 electrodes do not exhibit any electrochemical activity in a potential window of 0.0 to 1.5 V vs Ag/AgCl.28 In the same windows, nZVI particles show higher capacitances and a small peak near 1.0 V, which is associated with the reduction of Fe3+ to Fe2+.29-30 This peak was better defined in the sample exposed for 5 hours to the 6 ppm Cd2+ solution. It also generated a higher current when compared to the nZVI peak. The electrochemical behavior of the system changed in the sample exposed for 5 hours to the 30 ppm Cd2+ solution. The anodic peak associated with Fe3+ reduction decreased, but a new cathodic peak appeared, indicating the possibility of a reversible electrochemical process. An additional oxidation process occurs at ≈1.1 V and close to 0.3 V, possibly by reactions of cadmium with oxygen and sulfate molecules, respectively.29, 31 A higher double layer capacitance was also observed in this sample. The sample prepared in 30 ppm of Cd2+ proved to be the most electroactive material.


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Figure 7. Cyclic voltammetry results for powders deposited over a conductive fluorine thin oxide glass in a 0.5 M H2SO4 solution at 100 mV/s with 0.0 V as the initial potential. The solution was previously bubbled with nitrogen. Figure 8a illustrates a schematic of the PSC array used for the incident-to-photocurrent efficiency (IPCE) experiments. A thin film was prepared over an FTO glass to produce a photoanode similar to that in Figure 8b. SEM was employed to determine the thin layer thickness deposited over the FTO glass. The resulting image obtained (Figure 8c) has layers of ~ 1 µm thickness.



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Figure 8. (a) Schematic of the assembled parts of the PSC, (b) photoactive material deposited on the anode of the PSC after exposure to 30 ppm Cd2+, and (c) a SEM image of the thin layer deposited on the FTO glass. In Figure 9, IPCE Normalized signals of four PSCs are observed, each one with a different material on the photoanode. The samples, which were prepared using nZVI treated with 6 ppm of Cd2+, do not display significant signals. Particles treated with 30 ppm Cd2+, however, exhibit a relatively broad band from approximately 300 to 450 nm. They have a high absorption and a red shift displacement when compared with the PSC signals of commercial TiO2 in the anatase structure. The results for this region are similar to the absorption results obtained in the UV/Vis analysis. Such a high photovoltage can be explained by improvements in the electron transfer dynamics of the material in the PSC at higher cadmium concentrations due to structural


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changes as previously suggested.8 In the present electrochemical studies, this sample also exhibited the highest activity.

Figure 9. Incident to photocurrent efficiency normalized spectra of the nZVI starting material and nZVI used in wastewater Cd2+ removal process, and TiO2 (anatase). Electrochemical impedance spectroscopy (EIS) provided information about the electron transfer processes in PSCs. The real resistances (Zr) in Nyquist plots provide direct information about the electronic behavior of the system.32 Lower resistance magnitudes imply better electron mobility across the surface of the material. In Figure 10a, the experimental results of EIS analysis show a theoretical simulation of this circuit to complete a semicircle characteristic of the resistances and constant phase elements (CPE) typically found in solution. This CPE is commonly compared with a capacitor; however, some mathematical treatments are needed to complete its transformation. The results of this analysis can be observed in Figure 10b. A comparison of the three nanomaterials showed a reduction in the resistance to the charge transfer that was proportional to the increase in Cd2+ concentrations.



This reduction in resistance improves the electron transfer processes of the system, increasing the potential of the material to be employed in the PSCs. 600

R1

400

-Zi / Ohm

(a)

Q1

500

R2

300 Original Fitted

200 100 0 0

100

200

300

400

500

600

Zr / Ohm 600

Feo / FexOy Feo / FexOy / CdFe2O4 (6 ppm)

500

Feo / FexOy / CdFe2O4 (30 ppm)

(b)

400

-Zi / Ohm

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300 200 100 0 0

100

200

300

400

500

600

Zr / Ohm

Figure 10. Electrochemical impedance spectroscopy of (a) a theoretical simulation of the circuit to complete a semicircle characteristic of the resistances and constant phase elements (CPE) and of (b) the PSCs with the following photoanodes: starting material, exposed to 6 ppm and to 30 ppm of Cd2+.


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Conclusions Nanoscale zerovalent iron particles were produced by a chemical reduction technique to complete Cd2+ fixation from water. The concentrations of Cd2+ used were 6 ppm and 30 ppm, which are within the range of values found in Cd-contaminated areas of Puerto Rico. The material was physicochemically characterized, and further tested in a photoelectrochemical device as a photoanode. High photoelectrochemical values were observed in the particles produced from nZVI that had been in contact with the 30 ppm Cd2+ solution. These particles also possessed better structural organization and electrochemical properties when compared to the 6 ppm Cd-RIP sample and the TiO2. We conclude that there are better electron transfer processes from the hematite due to the presence of Cd0 and Cd2+, including morphological changes of the system shown by (i) morphological differences found in TEM and (ii) electronic behavior monitored using XPS and XAS analysis. These results provide a new strategy: to reuse materials used in remediation processes based on nZVI. They also offer an alternative method to prepare modified iron oxide particles for photocatalysis without using high temperature treatments. These results are important for future applications in photoactive materials synthesis. However, the matrix in contaminated sites is complex and might affect the photocurrent due to other physicochemical processes. Preliminary results from our group show the presence of other metals such as arsenic and lead at contaminated sites, which can alter PSC efficiency. Corresponding Author: [email protected] and [email protected]



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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes

The authors declare no competing financial interest.

Acknowledgements The authors acknowledge the use of facilities of the Materials Characterization Center of the University of Puerto Rico. This work had financial support from NASA-URC Grant Nos. NNX08BA48A and NNX10AQ17A. Financial support of the NSF-NSEC Center for Hierarchical Manufacturing, Grant No. CHM-CMMI-0531171, is also gratefully acknowledged. The authors of this work would like to thank the NSF PREM Grant No. DMR-0934218, Title: “Oxide and Metal Nanoparticles. The Interface between life sciences and physical sciences.” The authors would also like to acknowledge The Welch Foundation Agency Project # AX-1615, ‘‘Controlling the Shape and Particles Using Wet Chemistry Methods and Its Application to Synthesis of Hollow Bimetallic Nanostructures” and to NSF for its support through grant DMR1103730, “Alloys at the Nanoscale: The Case of Nanoparticles Second Phase”. We would like to thank Pedro Carrión at Corredor Yaguazo Inc., and Víctor Domínguez of PUMA Energy of the Caribbean for supporting this project. Special thanks to Rubén Mendoza (UTSA) for the TEM images. Part of this work is based upon research conducted at the Cornell High Energy Synchrotron Source (CHESS, Ithaca, NY), which is supported by the National Science Foundation under NSF award DMR-1332208.


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Table of Contents (TOC) Image: Photoelectrochemical Solar Cells Prepared From Nanoscale Zerovalent Iron Used for Aqueous Cd2+ Removal Keyla T. Soto Hidalgo a, b, ‡, Edwin O. Ortiz-Quiles c, ‡, Luis E. Betancourt c, Eduardo Lariosd, e, Miguel José-Yacamand, and Carlos R. Cabrerac,*

Synopsis: nZVI used in Cd2+ water removal process can be recycled as nanomaterials for light harvesting devices.


Photoelectrochemical Solar Cells Prepared From Nanoscale

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