Methods for light-driven detection and removal of contaminants in water based on hybrid plasmonic core-shell nanostructures

Abstract

Core-shell nanoparticles having a relatively thin shell (< 50 nm thick) are provided for water treatment applications. These nanoparticles have metal cores, semiconductor shells, and are plasmon-resonant. The absorption spectra of such nanoparticles can be engineered to have high overlap with spectra of interest, such as the solar spectrum. The resulting nanoparticles can be used for purification and / or detection of contaminants in water, both biological and chemical.

Description

[0001] Methods for light-driven detection and removal of contaminants in water based on hybrid plasmonic core-shell nanostructures by

[0002] Jenni fer A. Dionne

[0003] Alexander Al-Zubeidi

[0004] FIELD OF THE INVENTION

[0005] This invention relates to photocatalysis for water treatment systems .

[0006] BACKGROUND

[0007] Access to safe drinking water is a maj or challenge faced by humanity . This challenge can be solved using solar photocatalysts , which absorb sunlight - the most abundant from of energy on earth - to puri fy contaminated water .

[0008] Traditionally, photocatalysts made from semiconductors have been proposed for this application because they can produce long-lived excited state carriers to drive chemical reactions . While success ful lab-scale examples exist , ef ficiencies are low ( typically about 5% ) since catalytically active semiconductors like titanium dioxide only absorb ultraviolet light , which forms less than 5% of the sunlight that reaches Earth .

[0009] To ef ficiently use sunlight for the puri fication of water, catalysts need to absorb across the entire solar spectrum . A promising approach is the use of hybrid plasmonic metal nanoparticle-semiconductor nanostructures . Plasmonic metal nanoparticles are the strongest light absorbers per volume and their highly tunable absorption cross-section spans the entire visible and near-IR spectrum.

[0010] Proof-of-concept examples of plasmonic-semiconductor hybrids however relied on noble metals like gold and silver, which prevent this technology from being scalable for real- world applications.

[0011] SUMMARY

[0012] In this work, we developed core-shell hybrid nanoparticles from earth-abundant copper and titanium precursors. The nanostructures are prepared using water and isopropanol as solvents at moderate (100 degrees Celsius) temperatures and ambient pressure, in a two-step approach that does not require specialized equipment or inert gasses. We also consider applications of plasmonic-semiconductor hybrids having noble metal cores (e.g., Au, Ag) to water treatment applications, which appears to be a new use of such nanoparticles.

[0013] Several applications are considered.

[0014] Water purification: Titanium dioxide produces reactive oxygen species which have been used to remove pathogens (bacteria, protozoa, viruses) , chemicals (pharmaceuticals, pesticides, dyes, fertilizers) , and microplastics. This approach completely mineralizes contaminants, leaving behind no toxic by-products.

[0015] A device for contaminant detection and water purification : Nanoparticles are embedded into a support material or directly coated onto a flow channel cut into a support material such as polytetrafluoroethylene (PTFE) . The channel is covered with transparent plastic and contaminated water is flowed through the channel. The nanoparticle- covered channel is illuminated. Reflected and transmitted light is collected and sent to a spectrograph or interferometer. Raman or infrared spectra are taken and analyzed for signatures of contaminants, including pathogens and chemicals. A deep-learning model, trained with spectra of known contaminants, is used to identify contaminants in water. Contaminants are removed by illuminating the nanoparticle-decorated channel with a wavelength of light that triggers the generation of ROS.

[0016] Significant Advantages are provided

[0017] The main alternative technology that can remove both chemicals and pathogens is filtration, but it requires regular, often daily, filter maintenance and filter replacement and can be expensive. Ultraviolet light, boiling, and disinfection (e.g., chlorine tablets) provide an alternative for disinfection alone but are not effective against chemicals. While the sun contains ultraviolet light, in practice UV lamps are usually needed to provide high enough irradiation for effective disinfection, adding the need for an electricity source or solar panels. While some water purification systems measure alkalinity or total dissolved solids, no purification devices provide comprehensive water testing of influent and effluent water. As a result, current water purification devices need to be combined with separate contaminant detection to determine whether purification is needed and whether water is safe after purification. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1A shows a core-shell nanoparticle suitable for use in embodiments of the invention.

[0019] FIG. IB is a schematic energy level diagram corresponding to FIG. 1A.

[0020] FIG. 2 shows overlap of the solar spectrum and nanoparticle absorption spectrum.

[0021] FIG. 3 shows several images of a fabricated core-shell photocatalyst nanoparticle.

[0022] FIG. 4 schematically shows dual function (detection and purification) .

[0023] FIG. 5 shows an example of decomposition of methylene blue via solar photocatalysis.

[0024] FIG. 6A shows an exemplary fabrication sequence for a water treatment device.

[0025] FIG. 6B schematically shows operation of the device of

[0026] FIG. 6A.

[0027] DETAILED DESCRIPTION

[0028] An exemplary embodiment of the invention is a method of water treatment, the method comprising: exposing water to photocatalytic nanoparticles, where each photocatalytic nanoparticle includes a plasmonic and metallic core surrounded by a semiconductor shell; and exposing the photocatalytic nanoparticles to light, where absorption of light in the core causes charge and / or energy transfer to the semiconductor shell.

[0029] FIG. 1A shows a core-shell nanoparticle suitable for use in embodiments of the invention. Here 102 is a plasmonic and metallic core , 104 is a semiconductor shell , and illumination 106 is absorbed by the core-shell nanoparticle .

[0030] A composition of the plasmonic and metallic core can be selected from the group consisting of : Cu, Ni , Mg, Fe , Al and alloys or mixtures thereof .

[0031] Alternatively, a composition of the plasmonic and metallic core is selected from the group consisting of : Au, Ag, Pt , Rh and alloys or mixtures thereof .

[0032] A composition of the semiconductor shell can be selected from the group consisting of : TiCt , CuO, CU2O, ZnO, CdS , BiVO4, and WO3.

[0033] Preferably, the thickness of the semiconductor shell is in a range from 5 nm to 50 nm, and preferably the diameter of the plasmonic and metallic core is in a range from 2 nm to 500 nm .

[0034] FIG . IB is a schematic energy level diagram corresponding to FIG . 1A.

[0035] Due to the core 102 , the particles absorb strongly across the entire visible spectrum . When illuminated, a collective oscillation of electrons is excited, called a plasmon resonance . This collective oscillation allows metallic nanoparticles to absorb light . Therefore , hybrid structures harvest more light than titanium dioxide alone or other comparable semiconductor catalysts . Light incident on the plasmonic core leads to the excitation of electrons into higher energetic states , leaving behind holes . The energy alignment of these electrons and holes allows them to ef ficiently trans fer into the semiconductor . Thus , even though sunlight is inef ficient at exciting electrons in titanium dioxide , the metal core allows visible-light generation of excited electrons and holes . The electrons and holes in titanium dioxide react with water and oxygen dissolved in water to form reactive oxygen species (ROS ) . ROS break down chemical and biological contaminants , including antibiotic and disinfectant-resistant pathogens , allowing our nanoparticles to be used to treat contaminated water . FIG . IB schematically shows generation of ROS by photogenerated electrons and holes .

[0036] Our nanoparticles support a plasmon resonance in the visible region due to the unique combination of nanoparticle material ( Cu, Au, Ag, etc . ) , si ze ( 2-500 nm) and shell thickness (< 50 nm) . Note that a thick shell around the nanoparticles will hinder light penetration and thus plasmon excitation . Additionally, i f a plasmon is excited, a thick shell will dampen the plasmon . Preferably the shell thickness is < 50 nm, thus enabling a plasmon resonance . Preferably, the plasmonic and metallic core has a plasmon resonance wavelength in a range from 300 nm to 1000 nm .

[0037] The photocatalyst nanoparticles can be either directly mixed with potentially contaminated water or deposited onto a support material and then mixed with water . Nanoparticles can be separated from the puri fied water using filtration, precipitation, or centri fugation . Alternatively, the nanoparticles can be assembled on a support , and the water poured over / through this nanoparticle-based filter .

[0038] FIG . 2 shows overlap of the solar spectrum and nanoparticle absorption spectrum . As indicated above , pure TiCt nanoparticles would have far lower overlap with the solar spectrum, since they only absorb UV light .

[0039] FIG . 3 shows several images of a fabricated core-shell photocatalyst nanoparticle . Here the panel on the left is an SEM ( scanning electron microscope ) image , and the panels on the right include a STEM ( scanning transmission electron microscope ) image and energy-dispersive X-ray spectroscopy images for Cu, Ti and 0.

[0040] In this example , we developed copper titanium dioxide core-shell nanoparticles ( CuSTiCt ) for the light driven removal of contaminants in water . The particles have a 50 nm to 200 nm copper core , surrounded by a 5 nm to 100 nm titanium dioxide shell . The hybrid nanoparticles have an extinction peak at 620 nm, and a spectrum spanning from 300 to 800 nm, overlapping with the visible part of the solar spectrum . Attempts at extending the action of titanium dioxide and other semiconductor catalysts into the visible spectrum to overlap with the solar spectrum relied on the engineering of catalysts with many synthesis steps and based on expensive and rare precursors ( e . g . gold) , producing catalysts that cannot be scaled to the device level . The nanoparticles developed here have an earth-abundant plasmonic core , such as copper or nickel , surrounded by a shell of a semiconductor made from earth-abundant precursors .

[0041] In a demonstration, we used a copper core surrounded by a titanium dioxide shell . The core-shell nanoparticles are synthesi zed in two steps , both of which are one-pot syntheses . The nanoparticle cores are synthesi zed by mixing the precursors copper chloride , glucose , and hexadecyl amine in water at 100 degrees Celsius and heating them in a closed container for two hours . The particles are then coated with a titanium dioxide shell by heating the particles in the presence of titanium isopropoxide in isopropyl alcohol . Further synthesis details follow .

[0042] Synthesis example : A procedure where 42 mg CuC12 . 2 H2O is mixed with 260 mg hexadecyl amine (HDA) , 100 mg glucose , and 20 mL deioni zed water in a 22 mL vial . A stir bar is added, the vial is capped, and the solution is stirred for at least 2 hours. The vial is immersed for 2 h in an oil bath set to 104 °C while stirring magnetically. The vial is taken out of the oil bath and cooled under running water. The solution is then poured into a 50 mL centrifuge tube, 20 mL deionized water, and 5 mL hexane is added. The solution is vortexed briefly and centrifuged for 10 minutes at 6, 000 relative centrifugal force (ref) . After centrifugation, the particles settle to the bottom and the HDA molecules collect at the interface between the aqueous and organic phases. The organic phase, the surfactant interlayer, and the aqueous phase are removed. The particles are redispersed 40 mL in deionized water through sonication and hexane is added. The purification process is repeated twice more. Finally, the particles are redispersed in 1.5 mL water and centrifuged at 2,000 ref.

[0043] 0.5 mL of the purified particles are mixed with 5 mL 0.5 mM titanium(IV) ( triethanolaminato ) isopropoxide solution in isopropyl alcohol. The solution is gently boiled under stirring for 15 minutes. Particles are collected by centrifugation for 10 minutes at 2,000 ref and redispersed in water.

[0044] Core-shell nanoparticles can be synthesized in water, alcohols, or a mixture thereof. The particle sizes and shapes can remain intact during shell growth. Preferably, the particles remain stable in solution and in air after shell growth.

[0045] Solution photocatalysis example: Nanoparticle catalysts can be mixed with water that is suspected to contain contaminants and illuminated with light from the sun, an LED, a laser, a halogen lamp, or other light source. For the removal of dyes, an ultraviolet-visible spectrum or photograph of the water-catalyst solution may be taken before and after exposure to light to determine the ability of the catalyst to remove contaminants . For the removal of pathogens , plate cultures of the puri fied water may be grown to test whether pathogens were removed . Alternatively, the presence of contaminants can be determined using vibrational spectroscopy .

[0046] FIG . 4 schematically shows dual function ( detection and puri fication) . As indicated above , a single device can provide both detection and puri fication using the abovedescribed plasmonic photocatalytic nanoparticles .

[0047] Exposing the photocatalytic nanoparticles to light can perform various functions such as : generation of reactive oxygen species , generation of superoxide , generation of peroxides , generation of hydroxyl radicals , removal of contaminants from water, removal of pathogens from water, and solar puri fication of contaminated water .

[0048] We can further perform surface enhanced Raman spectroscopy for detection of contaminants in water, where the photocatalytic nanoparticles enhance a Raman signal of the contaminants . Such surface enhanced Raman spectroscopy can perform various functions such as : detection of contaminants ( organic or inorganic ) in water and detection of pathogens in water .

[0049] Metal-semiconductor hybrid nanoparticles can be used for enhancing vibrational signals of contaminants in water and other liquids by collecting Raman or infrared spectra of the solution to detect contaminants .

[0050] Vibrational spectroscopy can be used to detect chemicals , pathogens , or plastics in water or another liquid .

[0051] Detection of contaminants example : Nanoparticle catalysts are mixed with water that is suspected to contain contaminants . The solution is trans ferred onto a well plate or coverslip with a silicone isolator . Raman or infrared spectra of the water-catalyst solution are taken . The spectra are analyzed for signatures of contaminants , including pathogens and chemicals . A deep-learning model , trained with spectra of known contaminants , can be used to identi fy contaminants in water .

[0052] An apparatus for detecting and removing contaminants can include a nanoparticle-decorated channel that is illuminated and vibrational spectroscopy signals (Raman or infrared spectroscopy) can be detected using a spectrograph or interferometer . The device has a flow channel or membrane that is functionali zed with nanoparticles . Molecular fingerprints for the contaminants are measured and contaminants are identi fied by analyzing the spectra . Contaminants are then removed by illuminating the nanoparticles with a wavelength of light that will trigger the generation of ROS . Such water contaminants can include organic chemicals , plastics , bacteria, protozoa, viruses , and parasites .

[0053] Device for detection and removal of contaminants example : Nanoparticles are embedded into a support material or directly coated onto a flow channel cut into a support material such as Teflon . The channel is covered with transparent plastic and contaminated water is flowed through the channel . The nanoparticle-covered channel is illuminated . Reflected and transmitted light is collected and sent to a spectrograph or interferometer . Spectra are analyzed for contaminants as described above . Contaminants are removed by illuminating the nanoparticle-decorated channel with a wavelength of light that triggers the generation of ROS .

[0054] FIG . 5 shows an example of decomposition of methylene blue via solar photocatalysis . In the above-described proof-of-concept study, the catalyst was able to photo- catalytically break down methylene blue (MB) when exposed to sunlight. The solid line is before exposure to sunlight, and the dashed line is after 3 hours of sunlight exposure.

[0055] FIG. 6A shows an exemplary fabrication sequence for a water treatment device. Here step 602 shows cutting a channel 614 into a substrate 612 (e.g., a polymer or other suitable material) . Step 604 shows depositing nanoparticles 616 in channel 614. Step 606 shows covering substrate 612 and channel 614 with a lid 618 (e.g., made of transparent plastic to allow illumination to reach nanoparticles 616) .

[0056] Thus the photocatalyst can be embedded into a device. The nanoparticles can be embedded into a support material such as metal oxides (including TiCt, AI2O3, SiCt, MgO) , polymers (including polytetrafluoroethylene) , or other catalytically or non-catalytically active materials. In other words, practice of the invention does not depend critically on the composition of substrate 612. The catalytic nanoparticles can be embedded into a water filter, or a porous material.

[0057] FIG. 6B schematically shows operation of the device of FIG. 6A. Here illumination from light source 620 -- the sun or an artificial light source (light-emitting diode or the like) -- causes nanoparticles 614 to be catalytically active for the treatment of dirty water 622 to provide clean water 624. Thus sunlight and / or artificial illumination can be used to generate ROS, purify water, or initiate chemical reactions .

[0058] As indicated above, there are various device configurations for including nanoparticles in a flow device, and practice of the invention does not depend critically on such details. Suitable configurations include, but are not limited to : a flow channel decorated with the photocatalytic nanoparticles , a water filter in which the photocatalytic nanoparticles are embedded, a support material in which the photocatalytic nanoparticles are embedded, and a porous material in which the photocatalytic nanoparticles are embedded .

Claims

CLAIMS1. A method of water treatment, the method comprising: exposing water to photocatalytic nanoparticles, wherein each photocatalytic nanoparticle includes a plasmonic and metallic core surrounded by a semiconductor shell; and exposing the photocatalytic nanoparticles to light, wherein absorption of light in the core causes charge and / or energy transfer to the semiconductor shell.

2. The method of claim 1, wherein a composition of the plasmonic and metallic core is selected from the group consisting of: Cu, Ni, Mg, Fe, Al and alloys or mixtures thereof .

3. The method of claim 1, wherein a composition of the plasmonic and metallic core is selected from the group consisting of: Au, Ag, Pt, Rh and alloys or mixtures thereof .

4. The method of claim 1, wherein a composition of the semiconductor shell is selected from the group consisting of: TiO2, CuO, Cu2O, ZnO, CdS, BiVO4, and W03.

5. The method of claim 1, wherein the light is sunlight or light from an artificial light source.

6. The method of claim 1, wherein a thickness of the semiconductor shell is in a range from 5 nm to 50 nm, and wherein a diameter of the plasmonic and metallic core is in a range from 2 nm to 500 nm.7 . The method of claim 1 , wherein the plasmonic and metallic core has a plasmon resonance wavelength in a range from 300 nm to 1000 nm .8 . The method of claim 1 , wherein the exposing the photocatalytic nanoparticles to light performs one or more functions selected from the group consisting of : generation of reactive oxygen species , generation of superoxide , generation of peroxides , generation of hydroxyl radicals , removal of contaminants from water, removal of pathogens from water, and solar puri fication of contaminated water .9 . The method of claim 1 , further comprising performing surface enhanced Raman spectroscopy for detection of contaminants in water, wherein the photocatalytic nanoparticles enhance a Raman signal of the contaminants .10 . The method of claim 9 , wherein the surface enhanced Raman spectroscopy performs one or more functions selected from the group consisting of : detection of contaminants in water, and detection of pathogens in water .11 . The method of claim 1 , wherein water flows through a flow device including one or more features selected from the group consisting of : a flow channel decorated with the photocatalytic nanoparticles , a water filter in which the photocatalytic nanoparticles are embedded, a support material in which the photocatalytic nanoparticles are embedded, and a porous material in which the photocatalytic nanoparticles are embedded .

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