Kirigami photo-piezo catalysts for self-sustainable environment remediation

Abstract

A product includes a substrate having a plurality of kirigami cuts in a surface of the substrate, where longitudinal axes of the kirigami cuts are oriented about orthogonal to a longitudinal axis of the substrate, the plurality of kirigami cuts being arranged to allow flexure of the substrate, and a plurality of catalyst nanostructures on the surface of the substrate. The catalyst nanostructures include a photo-piezoelectric material, where the catalyst nanostructures are aligned substantially along the longitudinal axes of the kirigami cuts.

Description

KIRIGAMI PHOTO-PIEZO CATALYSTS FOR SELF-SUSTAINABLE ENVIRONMENT REMEDIATION

[0001] This invention was made with Government support under Contract No. DE- AC52-07NA27344 awarded by the United States Department of Energy. The Government has certain rights in the invention.FIELD OF THE INVENTION

[0002] The present invention relates to photo-piezo catalysts, and more particularly, this invention relates to Kirigami photo-piezo catalysts for self-sustainable environment remediation.BACKGROUND

[0003] Photocatalysts, which convert solar energy directly into chemical energy, were introduced in 1972 and have played a pivotal role in various solar energy applications such as hydrogen production (water splitting), water or air purification, carbon dioxide removal, nitrogen fixation, and sterilization. However, the target efficiency of photocatalysts is yet to be achieved (solar-to-hydrogen conversion efficiency value of 10%), as required for practical applications. The inefficiency of photocatalysts is mainly due to limited charge separation and transfer kinetics, and many attempts have been made to overcome this problem.

[0004] For example, one of the solutions introduced has involved of piezoelectricity- promoted photocatalysts, or photo-piezo catalysts, that have shown improved efficiency realized through the synergistic effects of the coupling between piezoelectric polarization and photoexcitation. Photo-piezo catalysts are particularly suitable outdoors, where there issunlight and mechanical motions such as waves, winds, and life vibrations. For this purpose, various (nano)materials, such as zinc oxide (ZnO)-based nanostructures, metal niobates (KNbCh, NaNbO s, AgNbO;). cadmium sulfide (CdS), bismuth ferrites, and two-dimensional (2D) nanomaterials, which can be used as photo-piezo catalysts, have drawn considerable attention, and their piezo potential and photoinduced charges have been investigated.

[0005] However, most studies have been limited by focusing on characterizing the intrinsic properties of materials as photo-piezo catalysts in a limited lab-setting, using a tool such as ultrasonic vibration that is not a motion found in a natural environment. Moreover, the bending characteristics of ultrasonic vibration correlates to bending the substrate as a single domain that comprises the piezoelectric catalysts on the surface of the substrate. More practical studies on how to integrate emerging nanomaterials to take advantage of natural motions and / or how to increase piezoelectricity by optimized design are necessary. While various (nano)materials have been investigated for enhancing the intrinsic properties related to interfacial band structure, increasing the efficiency by integration of materials with rational design for stress-strain applications has not yet been considered.SUMMARY

[0006] In one aspect, a product includes a substrate having a plurality of kirigami cuts in a surface of the substrate, where longitudinal axes of the kirigami cuts are oriented about orthogonal to a longitudinal axis of the substrate, the plurality of kirigami cuts being arranged to allow flexure of the substrate, and a plurality of catalyst nanostructures on the surface of the substrate. The catalyst nanostructures include a photo-piezoelectric material, where the catalyst nanostructures are aligned substantially along the longitudinal axes of the kirigami cuts.

[0007] In another aspect, a method of forming a photo-piezocatalyst kirigami product includes adding a plurality of catalyst nanostructures onto a surface of a first substrate. The catalyst nanostructures include a photo-piezoelectric material, where longitudinal axes of the catalyst nanostructures are substantially aligned in the same direction. The method includes forming a plurality of kirigami cuts on a surface of a second substrate, where each kirigami cut has a longitudinal axis about orthogonal to a longitudinal axis of the second substrate. Aligned catalyst nanostructures arc transferred to the surface of the second substrate having the kirigami cuts, where longitudinal axes of the catalyst nanostructures are substantially aligned with the longitudinal axes of the kirigami cuts. The first substrate is removed, where the catalyst nanostructures are adhered to the second substrate having the kirigami cuts after removing the first substrate.

[0008] Other aspects and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a flow chart of a method, according to one aspect.

[0010] FIG. 2 is a schematic drawing of a method of forming a kirigami photo-piezo catalyst, according to one aspect.

[0011] FIG. 3 is a series of images of ZnO nanorod alignment using a kirigami substrate, according to one aspect. Part (a) is an image of ZnO nanorods on PDMS before transfer, part (b) is an image of a ZnO nanorod-coated kirigami substrate before stretching, and part (c) is an image of a ZnO nanorod-coated kirigami substrate after stretching.

[0012] FIG. 4 depicts physical properties of aligned ZnO nanorods, according to one aspect. Part (a) is a diagram of FEM analysis of the ZnO nanorods on a kirigami substrate, part (b) is a polarized optical microscopy image of aligned ZnO nanorods and an image of alignment of ZnO nanorods cross-poled with polarized light, and pail (c) is an image of a large kirigami photo-piezo catalyst, before and after stretching.

[0013] FIG. 5 depicts results of bending radius, according to one aspect. Part (a) is a calculated maximum piezoelectric potential as a function of bending radius of a kirigami cell, and part (b) is a calculation result of the piezoelectric potential and polarization induced in a ZnO nanorod under a deformation with a bending radius of 5 mm.

[0014] FIG. 6 depicts reactive oxygen species of photo-piezo effect, according to one aspect. Part (a) is a schematic diagram of radical generation processes in photopiezoelectric effect, and part (b) is a heuristic energy band diagram model with piezoelectric polarization for photo-piezo catalytic effect of kirigami photo-piezo catalyst.

[0015] FIG. 7 is a series of images of a typical setup for photo-piezo kirigami actuator, according to one aspect. Part (a) is an image of a photo-piezo kirigami actuator with light sources, part (b) is an image of an actuator arm and reaction chamber, and pail (c) is an image of a kirigami substrate in the reaction vessel.

[0016] FIG. 8 depicts degradation analyses with respect to the kirigami pattern, according to one aspect. Pails (a)-(d) are images of visual degradation according to different conditions: kirigami pattern and light exposure, and part (e) is a plot of the degradation curves for each condition.

[0017] FIG. 9A is a schematic drawing of parameters considered for kirigami design, according to one aspect. Part (a) alignment angle, part (b) Kirigami A, 13.2 pm length, part (c) Kirigami B, 8.4 pm length, and part (d) Kirigami C, 6 pm length.

[0018] FIG. 9B illustrates an FEM simulation showing von Mises stress distributions for kirigami designs, according to one aspect.

[0019] FIG. 10 depicts plots of degradation according to different parameters, according to one aspect. Pail (a) kirigami pattern, part (b) stretching amplitude, part (c) film thickness, and part (d) alignment angle.

[0020] FIG. 11 depicts a radical scavenger test to understand the mechanism of kirigami photo-piezo catalyst, according to one aspect. Part (a) is a result of photocatalytic effect of reference sample without piezoelectricity, part (b) is result of photo-piezo catalytic effect with mechanical stretching, part (c) is degradation relative to plots depicted in parts (a) and (b).

[0021] FIG. 12 depicts images of ZnO nanorods, according to one aspect. Part (a) is an image of a CuS nanosheet-decorated with ZnO nanorods, part (b) is a magnified view of the sheet depicting atomic distances of z-axis (0001) of ZnO nanorods, and part (c) is a series of EDX analyses indicating ZnO nanorods are fully covered by the CuS nano sheets.

[0022] FIG. 13 depicts images “snapshots” taken at 0.1 second intervals of a wave profile generated at 2.5 Hz, according to one aspect.

[0023] FIG. 14 is a plot of degradation in small water reservoirs with respect to different conditions, according to one aspect.DETAILED DESCRIPTION

[0024] The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.

[0025] Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and / or as defined in dictionaries, treatises, etc.

[0026] It must also be noted that, as used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless otherwise specified.

[0027] For the purposes of this application, room temperature is defined as in a range of about 20°C to about 25 °C.

[0028] As also used herein, the term “about” denotes an interval of accuracy that ensures the technical effect of the feature in question. In various approaches, the term “about” when combined with a value, refers to plus and minus 10% of the reference value. For example, a thickness of about 10 nm refers to a thickness of 10 nm ± 1 nm, a temperature of about 50 °C refers to a temperature of 50 °C ± 5 °C, etc.

[0029] A nanoscale, nanoporous, etc. is defined as having a diameter or length (e.g., a pore having an average diameter) less than 1000 nanometers (nm). A microscale, microporous, micron-sized, etc. is defined as having a diameter or length (e.g., a pore having an average diameter) less than about 1000 microns (pm).

[0030] It is also noted that, as used in the specification and the appended claims, wt.% is defined as the percentage of weight of a particular component relative to the total weight / mass of the mixture. Vol.% is defined as the percentage of volume of a particular compound relative to the total volume of the mixture or compound. Mol.% is defined as the percentage of moles of a particular component relative to the total moles of the mixture or compound. Atomic % (at.%) is defined as a percentage of one type of atom relative to the total number of atoms of a compound.

[0031] Unless expressly defined otherwise herein, each component listed in a particular approach may be present in an effective amount. An effective amount of a component means that enough of the component is present to result in a discernable change in a target characteristic of the resin, printed structure, and / or final product in which the component is present, and preferably results in a change of the characteristic to within a desired range. One skilled in the art, now armed with the teachings herein, would be able to readily determine an effective amount of a particular component without having to resort to undue experimentation.

[0032] The following description discloses several preferred aspects for Kirigami photo-piezo catalysts for self-sustainable environment remediation and / or related systems and methods.

[0033] In one general aspect, a product includes a substrate having a plurality of kirigami cuts in a surface of the substrate, where longitudinal axes of the kirigami cuts are oriented about orthogonal to a longitudinal axis of the substrate, the plurality of kirigami cuts being arranged to allow flexure of the substrate, and a plurality of catalyst nanostructures on the surface of the substrate. The catalyst nanostructures include a photo-piezoelectric material, where the catalyst nanostructures are aligned substantially along the longitudinal axes of the kirigami cuts.

[0034] In another general aspect, a method of forming a photo-piezocatalyst kirigami product includes adding a plurality of catalyst nanostructures onto a surface of a first substrate. The catalyst nanostructures include a photo-piezoelectric material, where longitudinal axes of the catalyst nanostructures are substantially aligned in the same direction. The method includes forming a plurality of kirigami cuts on a surface of a second substrate, where each kirigami cut has a longitudinal axis about orthogonal to a longitudinal axis of the second substrate. Aligned catalyst nanostructures arc transferred to the surface of the second substrate having the kirigami cuts, where longitudinal axes of the catalyst nanostructures arc substantially aligned with the longitudinal axes of the kirigami cuts. The first substrate is removed, where the catalyst nanostructures are adhered to the second substrate having the kirigami cuts after removing the first substrate.

[0035] A list of acronyms used in the description is provided below.2D two-dimensional3D three-dimensionalC CelsiusCuS copper sulfateEDX energy dispersive X-rayFEM Finite element method mm millimeterMO methyl orange nm nanometerNR nanorodsPDMS polydimethylsiloxanePET polyethylene terephthalatePMMA polymethyl methyacrylateROS reactive oxygen speciesTC tetracycline m micronUV ultravioletZnO zinc oxide

[0036] Photo(electro)-piezo catalysis has emerged as one of the most effective strategies for sustainable environmental remediation. Herein kirigami strain engineering is introduced to photo-piezo catalysts for enhancing efficiency by increasing the magnitude of applied strain and density of bending. Kirigami is a Japanese art of cutting paper (kiri meaning “cut” and garni meaning “paper”), is a technique where a single piece of paper (i.e., a two- dimensional material) is transformed to a metamaterial or metastructure with properties that did not exist in the initial material, such as enhanced elasticity. Cut-only kirigami involves only cutting — and not folding as in origami — and results in fractal-cut kirigami or ribbon kirigami. Fractal cut creates rotating units, and ribbon creates strip-shaped cutting to form acurved unit. Both types of cutting achieve large strains that cannot be attained by the initial material through external tension or compression.

[0037] As described herein, macroscale stretching motion is converted into localized bending by pliable kirigami structure using similar and lower input energy which can be easily modulated by natural waves. Kirigami structure leads to a significant enhancement (-250%) in the degradation of dyes, resulting in a significant contribution of the oxygen reduction pathway in the charge-transfer mechanism, which corresponds to the observed enhancement. The photo-piezo catalytic effects of kirigami is observed by a small water reservoir test, that shows its feasibility in nature for self-sustainable environmental remediation that can be modulated using motions of winds, waves, and life vibrations.

[0038] Piezocatalysis is a technique that relies on the conversion of mechanical energy to trigger chemical reactions that can be utilized to catalyze the degradation of pollutants in water. By combining piezoelectric material with photocatalytic materials, the built-in electric field generated by the piezoelectric effect promote the separation and migration of photogenerated carriers, thereby enhancing the efficiency, selectivity, and stability of photocatalytic reactions. The efficiency of photo-piezo catalysts inherently depends on both properties: semiconducting (such as photoexcitation and charge transport) and piezoelectricity. While the semiconducting property is more closely related to the intrinsic properties of materials, piezoelectricity strongly relies on mechanical deformations, such as bending and compressing, in addition to the intrinsic piezoelectric tensors of materials.Numerous studies have been performed using ultrasonic experiments to generate piezoelectricity; however, in nature, it is unnatural to expect that photopiezo materials will be under vibrations of kHz or MHz frequency with such high power. To exploit piezoelectricity properly using natural motion, such as winds and waves, pliable structures are highly preferred, provided deformations arc effective for piezoelectricity. Here, as an effective mechanical transducer that can transform natural movement back and forth into localized bending, an approach describes including kirigami and the effect of a kirigami design on performance for photo-piezo catalysts with aligned arrays of nanostructures. A nanostructure may be defined as a structure having at least one dimension in the nanoscale range, such as 500 nanometers (nm) or less). In summary, by rational design, the piezoelectric energydensity may be enhanced simply by increasing the number of local bends per unit area and simultaneously lowering the input energy required for mechanical deformation, e.g., a more pliable structure.

[0039] According to the following aspects, the use of periodic kirigami cuts allows stretching of the substrate and thus a mechanical transducer is introduced, which can convert macroscale stretching motion into localized bending using lower modulation energy. A flat rigid two-dimensional substate transforms into flexible substrate having a plurality of domains, each domain having intricate curvatures. This approach is not only practical but also yields significant enhancements (-250%) in the degradation of dyes, with the oxygen reduction pathway playing a crucial role in the charge-transfer mechanism. Furthermore, the feasibility of using our small water reservoir is demonstrated to showcase the performance of a self- sustainable environmental remediation system that can be modulated using natural motions. The findings described herein have important implications for the development of efficient and sustainable photo-catalysts that may be modulated using natural motions. These aspects are of interest to a wide range of readers in the fields of materials science, environmental science, and nanotechnology.

[0040] FIG. 1 shows methods 100 for forming a photo-piezo catalyst kirigami product, in accordance with one aspect. As an option, the present method 100 may be implemented to construct structures such as those shown in the other FIGS, described herein. Of course, however, method 100 and others presented herein may be used to form structures for a wide variety of devices and / or purposes which may or may not be related to the illustrative approaches listed herein. Further, the methods presented herein may be carried out in any desired environment. Moreover, more or less operations than those shown in FIG. 1 may be included in method 100, according to various approaches. It should also be noted that any of the aforementioned features may be used in any of the approaches described in accordance with the various methods.

[0041] The method 100 of FIG. 1 may begin with operation 102 of adding a plurality of catalyst nanostructures onto a surface of a substrate where the catalyst nanostructures are comprised of a photo-piezoelectric material. The substrate may comprise a siloxane rubbermaterial that has a sticky surface. The siloxane rubber material may be coated on one side with an adhesive material for adhering the photo-piezo catalyst nanostructures.

[0042] The plurality of photo-piezo catalyst nanostructures may be added to the substrate by applying, brushing, rubbing, etc. the nanostructures in one direction onto the surface of the substrate. The plurality of photo-piezo catalyst nanostructures are preferably substantially aligned in the same direction. The nibbing, brushing, etc. application process of adding the photo-piezoelectric nanostructure material is an alignment technique for aligning the nanostructures in a uniform direction. The substatc functions as a carrier to transfer aligned photo-piezo catalyst nanostructures to the kirigami substate.

[0043] The catalyst nanostructures may be comprised of a material that has more than one property such as semiconducting, piezoelectric, pyroelectric properties, photocatalytic activity, etc. The nanostructures function as a catalyst In various approaches, the catalyst nanostructure material may include zinc oxide (ZnO)-based nanostructures, metal niobates (KNbCh, NaNbOa, AgNbO.a). metal titanites (BaTiO;, SrTiOrf cadmium sulfide (CdS), bismuth ferrites, 2D transition metal dichalcogenides (TMDCs) (such as M0S2, MoSei, WS2, and WSe2), 2D group IV monochalcogenides (such as GeSe and SnS), 2D group III-V binary compounds (such as AlSb, GaP, GaAs, InP, InAs, and InSb), etc. ZnO nanostructures have multiple properties such as semiconducting, piezoelectric, pyroelectric, and photocatalytic activity. Preferably, the photo-piezo catalyst nanostructures have a substantially hexagonal prism shape, e.g., a shape having an outer surface extending between a hexagonal base and a hexagonal top, such as a nanorod.

[0044] FIG. 2 illustrates a schematic diagram of a method 200 in one approach of forming a photo-piezo catalyst kirigami product 216. In one example, and not meant to be limiting in any way, hydrothermally grown anisotropic ZnO nanorods (NRs) (average diameter and length of 0.47 and 5.37 pm, respectively) may be used as photo-piezo catalyst nanostructures.

[0045] As illustrated in an expanded view in FIG. 2, ZnO NRs 204 have a shape of a hexagonal prism. The longitudinal axis 203 of the hexagonal prism is aligned parallel to the z-axis. The crystallographic orientation may be set such that the

[0001] direction of ZnO aligned with the z-axis (longitudinal axis of the hexagonal prism) in both the spatial andmaterial coordinate systems. As shown in the expanded view, a ZnO NR 204 having a hexagonal prism shape may have a diameter of about 500 nm and length of about 5 pm. Piezoelectric properties of ZnO NRs are listed in Table 1.Table 1. Piezoelectric properties

[0046] Preferably, initial and boundary conditions of piezoelectric nanostructures may include the following characteristics. The nanostructures may be comprised of a linear elastic material and all surfaces may be considered to be free from any mechanical restraints. A nanostructure having no initial deformation, may allow a total charge to be conserved. An initial charge and voltage may be preferably zero.

[0047] In one approach, a bending deformation of a piezoelectric nanostructure may include the characteristics of a hexagonal prism (e.g., a nanorod). The bending deformation of a hexagonal prism may be set with a prescribed displacement of the bending radius R. Components of a hexagonal prism may have prescribed displacement values as listed in Table 2. In one example, a bending deformation may be applied for ZnO NR having a shape of a hexagonal prism having a range of bending radius R varying from 5 mm to 20 mm, and the piezoelectric potential and polarization values of the ZnO NR may be calculated.Table 2. Displacement constraints of the components of a hexagonal prism

[0048] As described herein, ZnO NRs may be aligned and transferred to kirigami substrates that arc prepared separately. As illustrated in FIG. 2, in one example, a substrate 202 may be comprised of polydimethylsiloxane (PDMS) with a coating of polymethyl methacrylate (PMMA) to provide an adhesive material (i.e., sticky coating) for positioning the photo-piezo nanostructure material. A plurality of photo-piezo ZnO nanorods 204 (NR) may be rubbed, brushed, etc. in one direction onto the sticky surface 201 of the substrate 202 along the longitudinal axes 203 of the ZnO NRs thereby causing the ZnO NRs 204 to become aligned with each other. The aligned ZnO NRs 204 on the PDMS substrate 202 forms a transfer substrate 206 for transferring the substantially aligned ZnO NRs 204 to a kirigami substrate.

[0049] Referring back to method 100 in FIG. 1, operation 104 includes forming a plurality of kirigami cuts on a surface of a kirigami substrate, where each kirigami cut has a longitudinal axis about orthogonal to a longitudinal axis of the kirigami substrate. The kirigami substrate may be comprised of a plastic material. Preferably, the material of the kirigami substrate is sufficiently stiff to introduce strain for inducing the piezoelectric material when the kirigami substrate is stretched. In various approaches, the shape of the substrate may be defined by the application of the catalyst nanostructures. The substrate may be two-dimensional (2D) substrate. The substrate may have a rectangular shape. In another approach, the substrate may have a circular shape.

[0050] As illustrated in FIG. 2, a kirigami substrate 214 may be fabricated on a substrate 208 formed of a plastic material 209. In various approaches, conventional plastic films may be included as a substrate. Conventional plastic films have a stiffness that may be configured to bend the piezo material and may be cut by a laser and other etching processes. In one example, material of the substrate 208 may include polyethylene terephthalate (PET) film, poly(vinylidene fluoride-trifluoroethylene) copolymer film, polyvinyl chloride films, polypropylene films, polyethylene films, poly dimethyl siloxane films, etc.

[0051] In various approaches the kirigami cuts in the substrate extend completely through the substrate from one surface of the substrate to the opposite surface of the substrate. In one approach, for example, the substrate 208 may be laser cut with a laser cutting device 210 forming kirigami-style cuts 212. A periodic pattern of the kirigami cuts in the substate induces a flexibility and intricate curvature of the substrate. The periodic pattern of the kirigami cuts allows flexure of the substrate. The longitudinal axes 211 of the kirigami-style cuts 212 is preferably aligned about orthogonal to the longitudinal axis 207 of the substrate 208. In another approach, the kirigami cuts may be fabricated using photolithography techniques. These methods are by way of example only and are not meant to be limiting in any way. The kirigami cuts in the substrate may be fabricated using methodology generally understood in the art.

[0052] The design of the substrate and the design of the kirigami cuts in the substrate may be engineered for performance of the piezoelectric catalyst. In one approach, the substrate may have a rectangular 2D shape. In another approach, the substrate may have a circular 2D shape. The periodic array of kirigami cuts is designed to a plurality of domains that bend when the substrate is stretched, and provides flexibility of the substrate. The periodic array of the kirigami cuts reduces the stiffness of the substrate material. The width cut length w of the kirigami cuts may be in a range of about 100 nanometer (nm) up to millimeter range. For example, nanoscale and micron scale kirigami cuts may be made using photolithography techniques as used in conventional semiconducting processes. The length of the kirigami cuts may be shorter or longer depending on the technology available at the time of fabricating the kirigami cuts.

[0053] Preferably, the size of the kirigami cuts is engineered according to the length of the photo-piezoelectric catalyst nanostructure. In one approach, an average longitudinal length of the kirigami cuts may be defined according to an average longitudinal length of the nanostructures. The average longitudinal length of the kirigami cuts may be at least 10 times the average longitudinal length of the nanostructures. For example, if a photo piezoelectric catalyst nanorod has a length of about 10 pm and diameter of 200 nm, then each kirigami cut may be in a range of 2 to 10 mm. As each the periodic array of kirigami cuts become smaller,shorter, and more densely distributed on the substrate, may result in higher catalytic efficiency.

[0054] In various approaches, an average longitudinal length of the nanostructures may be in a range of greater than 10 nm to less than 10 pm, greater than 100 nm to less than 1 pm, etc. In some approaches, the average longitudinal length of the kirigami cuts may be in a range of greater than 100 nm to less than 50 mm, greater than 1 pm to less than 10 mm, greater than 100 pm to less than 10 mm, greater than 1 mm to less than 10 mm, etc.

[0055] In various approaches, the periodicity of the kirigami cuts may be engineered and arranged in rows extending along the longitudinal axes of the cuts in the respective rows. In particular, as illustrated in FIG. 2, in-line spacing spi defines the distance between the longitudinal lengths of the kirigami cuts in the same row along the longitudinal axes and row spacing sp2 defines the distance between alternate rows of kirigami cuts (e.g., alternate rows have the same cut pattern). The in-line spacing spi and row spacing sp may be predefined according to the length of the piezoelectric nanostructures. In preferred approaches, for optimal deposition of piezoelectric nanostructures, the spacings spi and sp2 may be in a range with a minimum of about 100 times the length of the piezoelectric nanostructures. In exemplary approaches, the distance between adjacent kirigami cuts, e.g., the spacings spi and sp2, may have an average length of greater than about 5% to about 50% of an average longitudinal length of the kirigami cuts. For example, a nanorod having a longitudinal length of 5 pm may preferably be applied to a kirigami substrate having in-line spacing spi and row spacing sp in a range of about 500 pm up to 5 mm. The in-line spacing spi may be different than the row spacing spi.

[0056] In one approach, the substrate may include an array of periodic kirigami cuts that results in a three-dimensional (3D) substrate during stretching of the substrate, e.g., during exposure of natural environmental movement, such as wind, wave, light vibration, etc. For example, a plurality of circular shaped kirigami cuts may result in a kirigami substrate that has the characteristic of forming a 3D structure in response to environmental motion (e.g., wave, wind, light vibration, etc.).

[0057] The cutting of kirigami not only imparts elasticity to the substrate but also may tune many other mechanical properties. For example, it is known that the in-plane and out-of-plane deformation modes of each domain are determined by the ratio of the cut width / thickness of the substrate. The Fbppl-von Karman number in thin-film mechanics directly indicates this property, and important parameters for piezoelectricity, such as the bending radius of each segment and the mechanical force required for stretching, should be thoroughly investigated. In general, a wider cut length w may benefit from a lower radius of bending (i.e., higher bending stress), however a wider cut length w decreases the number of domains owing to the limited active area of the sample. In preferred approaches, although a shorter width cut length w may result in a lower bending stress, a larger number of domains (i.e., a higher density of kirigami cuts) may demonstrate higher performance of photocatalytic degradation in response to environmental motion. In one example, a photo- piezocatalyst kirigami product having a high density of kirigami cuts demonstrates significant MO dye degradation at lower stretching amplitudes (e.g., 4.9-mm stretching amplitude).

[0058] Moreover, periodic kirigami cuts on a substrate also provide flexibility and induced curvature of materials that are deposited, applied, added to the surface of the kirigami cut substrate. The kirigami cuts forming a plurality of domains transform the substrate into a pliable, flexible substrate that bends in response to less environmental force (e.g., waves, wind, light vibrations, etc.) than the environmental force needed for bending an uncut substrate characterized as having one domain. The kirigami cuts of the substate may be engineered for bending electric or piezoelectric materials for an environmental purpose. In preferred approaches, the length w of the kirigami cuts and the periodicity of the kirigami cuts spi, sp2) are engineered to provide optimal elasticity of the substrate such that the substrate returns to the original form after stretching induced by environmental motion.

[0059] In various approaches, an environmental motion (e.g., wind, waves, etc.) stretches a kirigami-cut substrate that in turn causes a bending of aligned electric or piezoelectric materials on the kirigami-cut substrate to generate electricity. The alignment of the piezoelectric materials (e.g., crystal nanostructures) is maximized according to the alignment of the kirigami cuts to induce the piezoelectric material to generate electricity. In one approach, piezoelectric material that relies on mechanical information to generate electricity and electric field, the bending direction is critical of piezoelectric material. The bendingdirection of the kirigami cut substrate is preferably aligned with the plane of the crystal of piezoelectric material applied to the surface of the substrate. Strain may be more largely applied to the piezoelectric material because the piezoelectric material has a smaller bending radius. The bending of the substrate induces electricity from the piezoelectric material. In various approaches, increasing the stretching amplitude of the substrate correlates increased piezoelectric catalytic activity.

[0060] The thickness of the kirigami substate is important to kirigami mechanics. In one example, referring to the photo-piezo catalyst kirigami product 216, the thickness (f) of the PET film kirigami substrate 208 imparts a strain that affects the generation of electricity from the aligned ZnO NRs. The thickness of the substrate correlates with strain such that a thicker substrate has more strain and thus results in improved performance of the photo-piezo catalyst nanostructures. A greater amount of applied stress (e.g., environmental motion) is involved in achieving the desired stretching amplitude for inducing piezoelectric activity of the nanorods. Moreover, the thickness of the substrate correlates to the distance of the piezoelectric nanostructures on one surface of the substrate to the strain of the substrate during applied stress (e.g., environmental motion). For instance, piezoelectric nanostructures are positioned on a neutral plane of the cross section of the kirigami substrate, and thus, a thicker substrate may promote a more desirable the bending radius of the nanorods correlating to an increased thickness of the substrate. In various approaches, a thickness t of the substrate may be in a range of greater than 1 nanometer (e.g., an atomically thin layer such as graphene, 2D materials, etc.) to less than 1 mm. In preferred approaches, the thickness t of the substrate may be in a range of 10 pm to about 100 pm. In some approaches, a thickness of about 100 pm is preferred for increased degradation performance of the photo- piezo catalyst kirigami substate.

[0061] Referring back to method 100 in FIG. 1, operation 106 includes transferring the plurality of aligned photo-piezo catalyst nanostructures to the surface of the substrate having the kirigami cuts. The longitudinal axes of the photo-piezo catalyst nanostructures may be substantially aligned with the longitudinal axes of the kirigami cuts. In preferred approaches, when natural environmental movement (e.g., wind, wave, light vibration, etc.) is applied to the kirigami substrate, strain along the longitudinal axes of the kirigami cuts (that areorthogonal to the longitudinal axis of the kirigami substrate) induces the similarly aligned piezoelectric nanostructures to generate electricity. Randomly aligned piezoelectric nanostructures may also generate electricity on a kirigami substate, however, the catalytic efficiency and performance of the piezoelectric nanostructures would be significantly reduced compared to piezoelectric nanostructures aligned along the longitudinal axes of the kirigami cuts. Preferably, the alignment angle of the aligned piezoelectric nanostructures relative to the longitudinal axes of the kirigami cuts is about 0°.

[0062] Referring to the schematic drawing in FIG. 2, in one example, the transfer substate 206 having aligned ZnO NRs is added to the kirigami substrate 214. The side of the transfer substrate 206 having aligned ZnO NRs 204 is positioned onto the kirigami cuts 212 of the kirigami substrate 214, such that the longitudinal axes 203 of the aligned ZnO NRs 204 is aligned in the same direction as the longitudinal axes 211 of the kirigami cuts 212.

[0063] The transfer substrate 206 having the aligned ZnO NRs 204 allows the transfer of the aligned ZnO NRs 204 to the kirigami substrate 214 in the presence of elevated temperature and application of a gentle pressure on the transfer substrate 206 toward the kirigami substrate 214 — the aligned ZnO NRs 204 are positioned in between the two substrates.

[0064] In one example, the aligned ZnO NRs 204 are transferred from the transfer substrate 206 to the kirigami substate 214 using a process of decal printing on a hot plate at around 110 °C transfers the aligned ZnO NRs 204 to the kirigami substrate 214.

[0065] Referring back to method 100 in FIG. 1, operation 108 includes removing the substrate that transferred the catalytic nanostructures to the kirigami substrate. The catalyst nanostructures remain adhered to the substrate having the kirigami cuts after removing the transfer substrate. A photo-piezo catalyst kirigami substrate is formed having the longitudinal axes of the catalyst nanostructures aligned along the longitudinal axes of the kirigami cuts.

[0066] As illustrated in the schematic drawing of FIG. 2, the photo-piezo catalyst kirigami product 216 is formed upon removal of the transfer substrate 206. A photo-piezo catalytic kirigami product 216 depicted in FIG. 2 is an example of a product, in accordance with one inventive aspect.

[0067] According to one aspect, a product includes a substrate having a plurality of kirigami cuts in a surface of the substrate and a plurality of catalyst nanostructures on the surface of the substrate. As described herein the catalyst nanostructures include a photopiezoelectric material. As an option, the present product may be implemented in conjunction with features from any other inventive concept listed herein, such as those described with reference to the other figures. Of course, however, such product and others presented herein may be used in various applications and / or in permutations which may or may not be specifically described in the illustrative embodiments listed herein. Further, the product presented herein may be used in any desired environment.

[0068] The longitudinal axes of the kirigami cuts may be oriented about orthogonal to a longitudinal axis of the substrate. The plurality of cuts are arranged to allow flexure of the substrate. The plurality of kirigami cuts arranged in a uniform pattern. The plurality of kirigami cuts extends completely through the substrate from one surface of the substrate to the opposite surface of the substrate. As described herein, the kirigami cuts have essentially uniform length relative to one another.

[0069] According to one aspect, the degree of alignment of catalyst nanostructures on the kirigami substrate may be an important factor since piezoelectricity is highly associated with the applied stress tensor. The catalyst nanostructures are aligned substantially along the longitudinal axes of the kirigami cuts. As illustrated in FIG. 2, the alignment angle 0of the longitudinal axes 203 of the piezoelectric nanostructures (e.g., ZnO NRs 204) relative to the longitudinal axes 211 of the kirigami cuts 212 is preferably about 0°. The photo-piezo catalyst kirigami product 216 illustrates an alignment angle 0 being about 0°, such that the longitudinal axes 203 of the aligned ZnO NRs 204 is about parallel to the longitudinal axes 211 of the kirigami cuts 212. Increasing the alignment angle of the longitudinal axes of the nanostructures from greater than 0° to 90° may cause a decrease in piezoelectric and catalytic performance of the aligned piezoelectric nanorods on the kirigami substrate (see Experiments, part (d) of FIG. 8).

[0070] As illustrated in FIG. 3, in one example, an almost perfect alignment of ZnO NRs parallel to the longitudinal axis (e.g., z-axis) of the kirigami cut has an alignment angle fi’of about 0° (part (a) and part (b), respectively). This 2D pattern of kirigami transforms uponstretching the kirigami substrate in the orthogonal direction (i.e., the v-dircction) into a sinusoidal 3D surface with alternating convex and concave curvatures (part (c)).

[0071] In various approaches, nanostructures having susceptibility to bending stress is preferable. For example, bending stress can be directly and effectively applied to the c-axis of the ZnO crystal structure, where the c-axis is geometric midline of the ZnO crystal rod, perpendicular to the easy cleavage planes. For example, in the magnified view of the ZnO NR 204 in FIG. 2, the c-axis may be parallel to the longitudinal axis 203 of the ZnO NR 204. It is known that wurtzite ZnO NRs generate built-in potentials depending on the applied bending radius. Mechanical deformation of the nanostructured piezoelectric material induces an electric field. As illustrated in part (a) of FIG. 4, a NR structure generates a potential depending on the bending radius. As the bending radius is increased from 20 mm to 5 mm, the potential increases with increased stretching from 10% stretching to about 50% stretching. Specifically, it is expected from calculation via the finite element method (FEM) that local bending of kirigami structure can induce approximately ±0.6 V with 50% of stretching, as noted with dark shading on the edges of the bending NR.

[0072] In various approaches, the piezoelectric nanostructures are preferably uniformly aligned over a large surface of the substrate. In one approach, to align the ZnO NRs in one direction (as illustrated on the composite substrate 206 of FIG. 2), a brush-aided mechanical rubbing method, similar to that described elsewhere, may be used. Briefly, in one example, fully dried ZnO NRs may be applied to a typical art brush (e.g., a nylon fiber flat brush, 27- mm fiber length, 40- pm fiber width) and swept directionally onto the polydimethylsiloxane (PDMS) substrates. The moderate stickiness of the PDMS (e.g., having a cured base to curing agent ratio of 10: 1) allows for proper rotation of ZnO NRs that are not parallel to the direction of brush motion, thereby maintaining their alignment. After repeated sweeps (e.g., approximately 10 times), most of the ZnO NRs may be aligned (e.g., as illustrated in pail (a) of FIG. 3). Note that this alignment is not just local, but global, as the laser diffraction patterns of different spots show highly anisotropic features and high similarity between the data. In addition, as illustrated using an optical method in part (b) of FIG. 4, polarized optical microscope images show distinctly different light transmissions, producing a high extinction ratio depending on the TE and TM modes of light. The arrows indicate polarized light ineach image. The upper image depicts the substrate rotated 45°, and some light is visible, and the bottom image depicts the light vertical to the aligned nanorods, all the light can be blocked, thereby demonstrating the nanorods are uniformly aligned in the same direction.

[0073] The fabrication of substrates with kirigami cuts is scalable using commercial laser tools and conventional patteming / etching methods. Together with gram scale synthesis of uniform ZnO NRs, alignment and PDMS-mediated transfer methods, a large-scale (bigger than typical A4 size) kirigami photo-piezo catalysts may be fabricated that can be directly adopted for various energy and environmental applications (part (c) of FIG. 4).

[0074] It is intriguing that the piezoelectric effect changes the portion of ■ O2radicals significantly from the least important factor to the most, not to mention the increase in the total amount of decomposition. FIG. 5 illustrates a stretching effect of the kirigami catalyst in further detail, in particular, the formation of piezoelectric potential. Moreover, the piezoelectric effect of the nanorods aligned on the kirigami substrate is formed within the nanorods as a function of the bending radius of a kirigami cell. In one example described herein, through FEM calculations, a maximum piezoelectric potential may be formed within the ZnO NRs as a function of bending radius of a kirigami cell (as illustrated in the plot of part (a)). When a kirigami substrate is stretched, the aligned nanorods (e.g., ZnO NRs) experience bending deformation. As further illustrated in part (b), the piezoelectric polarization within the nanorods (e.g., ZnO NRs) is oriented in the diameter direction of the rods, with an increasing polarization value towards the end.

[0075] Normally, the surface energy band bending of n-type oxide semiconductors creates an energy barrier for electron surface reactions, facilitating hole surface accumulation by forming a potential well. As illustrated in part (a) of FIG. 6, the piezoelectric polarization induced in a ZnO NR under deformation with a bending radius that causes a curvature of 1 / R, generates an energy barrier for electron surface reactions that is reduced on the positively polarized surface, thereby promoting superoxide generation reactions. Conversely, the thickness of the depletion layer increases on the negatively polarized surface, leading to greater hole surface accumulation, which promotes water oxidation and direct organic oxidation reactions.

[0076] According to various aspects, bending of the kirigami substrate results in the generation of reactive oxygen species from the photo-piezoelectric catalyst nanostructures thereby inducing a series of reactions that may result in a degradation of antibiotics and organic pollutants. The bending of the kirigami substrate may be caused by environmental motion, such as wind, waves, light vibrations, etc.

[0077] Furthermore, to understand the influence of piezoelectric polarization on the generation processes of free radicals, part (b) of FIG. 6 illustrates a heuristic band diagram for decomposing organic compounds in the ZnO photocatalyst. It is well known that electron-hole pairs are created when ZnO absorbs photons, and these are separated into free electrons and holes. The electrons readily reduce oxygen in solution, generating ■while the holes oxidize water, producing ■ OH on the surface of ZnO NRs, whose energy band near the surface is bent due to the aqueous solution (indicated by the black dotted line in the diagram of part (b)). This band bending acts as an energy barrier for the electron reduction reaction, increasing its activation energy, but provides an energy space for the surface accumulation of holes, accelerating the water oxidation reaction. Through additional reactions of ■and ■ OH, hydrogen peroxide and singlet oxygen radicals could also be generated. Considering that the decomposition of organic matter occurs either by reaction with these four reactive oxygen species (ROSs) or by direct oxidation with h+, without wishing to be bound by any theory, one may conclude that the photocatalytic decomposition of MO without stretching was mainly governed by ■ OH from water oxidation (as shown, for example, in Experiments, part (c) of FIG. 11).

[0078] Meanwhile, when piezoelectric polarization is induced within the ZnO NR (solid line in the diagram of part (b) of FIG. 6), an electric field forms along with the polarization, and the energy band bending becomes asymmetric. This modification in the band diagram changes the reaction scheme as follows: 1) It promotes the separation of electron-hole pairs, which in turn, lowers the energy barrier for electrons during oxygen reduction. 2) Then, it creates additional energy space to facilitate the surface accumulation of holes. As a result, the increase in the concentration of photo-electrons and photo-holes in ZnO NRs leads to a decrease in the activation energy for ■generation, followed by the promotion of the water oxidation reaction.

[0079] The concentration of ■ O2in the reaction shown in part (a) of FIG. 6 may play a pivotal role in the generation of hydrogen peroxide and singlet oxygen and the energy barrier for electron transfer in the ■ O2generation reaction on the ZnO NR surface, induced by energy band bending, has a significant impact on delaying organic decomposition reactions. Consequently, the reduction of this energy barrier by piezoelectric polarization enhances the production of superoxide, leading to a substantial improvement in overall organic pollutant decomposition. On the other hand, the effect of piezoelectric polarization on the water oxidation is expected to increase the generation of hydroxyl radicals due to a slight increase in hole accumulation resulting from the expansion of the thickness of depletion region. In one example, without wishing to be bound by any theory, the theoretical hypothesis is well- matched with the results in Experiments section, part (c) of FIG. 11, that demonstrates the contribution of ■ O2was greatly promoted by the piezoelectric effect, and those of h+and ■ OH were slightly improved.

[0080] In various approaches, the combination of the kirigami substrate for the photo- catalytic piezoelectric nanostructures enhances the photo-catalytic performance of the piezoelectric nanostructures. As demonstrated herein (Experiments, FIG. 8), in one example, the photo-catalytic effect of ZnO nanorods on a kirigami substrate is significantly enhanced during application of mechanical motion of the kirigami substrate in the presence of UV light in experiments measuring the degradation of methyl orange dye, (e.g., 35 % degradation without mechanical motion versus 90% degradation with mechanical motion).

[0081] The combination of the high catalytic effect and structural pliability of the ZnO NR-kirigami structure enables its utilization in practical applications for self- sustainable environmental remediation. Kirigami photo-piezo catalysts can use the movement of water and air, neither interrupting their flow nor shielding sunlight, which are important for achieving harmony with nature.

[0082] Tetracycline (TC) is a representative groundwater pollutant threatening terrestrial and aquatic biodiversity. In one example, as described in the Experiments section, FIG. 13, TC may be used to demonstrate feasibility of the kirigami photo-piezo catalyst for practical application in a real environmental condition. Systematic testing under varying rocking frequency (frock) may represent motions of waves and how these waves affect theperformance of kirigami stretching. While most frequencies cause similar degradation, including irregular degradation, a frequency of 2.5 Hz presents a distinctly high photocatalytic effect. Without wishing to be bound by any theory, the high performance of the frequency 2.5 Hz may be explained such that the size of the testing reservoir is particularly well matched with frock of 2.5 Hz, producing the most stretched kirigami among the various conditions.

[0083] In natural environments, where the scale is larger and the rocking frequency is lower (ranging from 0.2 to 2.5 Hz), a set of multiple kirigami substrates may be used, similar to the concept of a HEPA (high-efficiency particulate air) filtration system. The mechanical and chemical robustness of kirigami photopiezo catalysts have additional advantages such as recyclability of the catalysts. For example, the same kirigami piezo-photo catalysts exhibited almost identical results as the initial state after six cycles of photo-catalytic experiments, with no significant chemical or morphological degradation observed, indicating exceptional durability.

[0084] This work considered just one of the exemplary cases that can be found in real systems, and it should be emphasized that kirigami is particularly suitable for many practical applications because the cut pattern and design can be further tuned to accommodate the frequency and flow that are required for the environment and system. Various emerging photo-piezo catalytic nanomaterials can be integrated with kirigami structures, and this approach could possibly suggest a solution for self-sustainable environmental remediation that takes advantage of the natural motions of winds, waves, and life vibrations.

[0085] Experiments

[0086] ZnO NR synthesis

[0087] ZnO NRs were synthesized using a wet chemical precipitation method.Hexamethylenetetramine (HMTA) (0.5 g) was dissolved in 1.8 L of deionized (DI) water at 85 °C while stirring at 200 rpm. Zinc acetate dihydrate 0.5 g was dissolved in 0.2 L DI water at 4 °C separately, and then the solution was dropped for 30 min into the heated HMTA aqueous solution with continuous stirring. The mixed solution was aged at 85 °C for 60 min and then cooled to room temperature. Synthesized ZnO NRs were purified using vacuum- assisted filtration with a 0.2-pm pore cellulose acetate membrane filter. The filtered ZnONRs were redispersed in fresh ethanol and dried completely for future use in powder form. The crystallinity of the ZnO NRs was analyzed by high-resolution X-ray diffraction (HR- XRD, Rigaku SmartLab) with a Cu K-alpha X-ray source at the Core Facility for Bionanomatrials at Gachon University.

[0088] Preparation of kirigami photo-piezo catalyst

[0089] For the kirigami substrates, PET films were cut with various patterns using a laser system (Laserbox, Makeblock®). On the cut films, poly (methyl methacrylate) (PMMA) 5 wt.% solution in toluene was spin-coatcd at 2,000 rpm for 30 s. An aligned monolayer of ZnO NRs was prepared using one-direction dry rubbing on a flat polydimethylsiloxane (PDMS) pad and transferred onto the PMMA-coated cut film at 110 °C. The aligned ZnO NRs on the kirigami cut film were observed via scanning electron microscopy (SEM, Hitachi S-4300) with the accelerating voltage of 15.0 kV.

[0090] Photo-piezo catalytic degradation tests

[0091] The catalytic activity of the kirigami photo-piezo catalyst was estimated using an methyl orange (MO) dye degradation test. The images of FIG. 7 illustrate specifically designed actuators for kirigami samples employed for mechanical modulation to activate piezoelectricity. Part (a) illustrates a typical setup for photo-piezo kirigami actuator with light sources (365-nm UV LED or Xe lamp). Part (b) illustrates the actuator arm and reaction chamber. Part (c) shows the kirigami piezoelectric catalyst in the reaction vessel.

[0092] The as-prepared kirigami catalyst substrate was placed into a glass reaction vessel filled with 5 LI M MO 60 mL aqueous solution with one end of the substrate fixed to the bottom. Another end of the substrate was connected to a motorized linear actuator (L12- 12PT-3, Mighty ZAP) for stretching with a steady speed and amplitude, and the glass reactor was irradiated by light sources (18-W power, 365-nm UV LED and 300-W Xe Lamp) for activating photocatalytic degradation. To estimate MO degradation, the change in absorbance of MO in the solution with photocatalytic reaction time was measured using a UV-VIS spectrometer (Jasco V-770).

[0093] Tests were conducted on every combination of mechanical modulation and UV photo-catalytic effects as illustrated in FIG. 8. In part (a), the case labeled as "Adsorption" represents the scenario where kirigami is present without any modulation or light. It is worthnoting that certain dyes, such as Rhodamine 6G, exhibit a strong affinity for surfaces, enabling changes in solution concentration through simple dipping and collection. In part (b), the case labeled as "Kirigami (w / stretching)" corresponds to mechanical modulation alone, while in part (c), "UV + Kirigami (w / o stretching)" represents the scenario involving only the photo-catalytic effect. Dramatic color changes from orange to transparent were observed as the decomposition of dyes progressed, particularly when mechanical modulation and UV irradiation were achieved simultaneously, as shown in part (d). As shown in part (e) of the plot of degradation curves of each sample: part (a) adsorption (•), part (b) Kirigami with stretching (■), part (c) UV + Kirigami without stretching (o) and part (d), UV + Kirigami with stretching (□). Kirigami sample with both mechanical modulation and UV light (□) decomposed ~90 % of dye in 90 min, whereas Kirigami sample with UV light but without mechanical modulation (without stretching) (o) only dissolved 35 % of dye in 90 min. Likewise, sole modulation of kirigami samples without UV irradiation showed low dye degradation (below ~2%). The photo-catalytic effects of the ZnO nanorods was significantly enhanced by the mechanical modulation of the kirigami substrate.

[0094] Parameters of kirigami cuts

[0095] To determine how the parameters of kirigami affect the performance and establish the optimized structure, various design factors of kirigami were tested as illustrated in FIG. 9A. . Part (a) illustrates the alignment angle 0. and the film thickness t that affect the sketching amplitude A. Each example in pails (b), (c), and (d) represents a variation of the periodicity of the kirigami cuts. Kirigami A, Kirigami B, and Kirigami C were fabricated with each having a different width cut lengths iv (13.2, 8.4, and 6 mm, respectively) of kirigami cuts. The kirigami cuts were characterized based on the standard kirigami pattern where the cut is arrayed with a face-centered unit cell. The longitudinal length (£) from end to end of the kirigami substrate was the same for each sample (i.e., 32.4 mm). The spacings spi, sp2 between the kirigami cuts were also standardized between the samples. The spacing in a longitudinal direction, the in-line spacing spi between the kirigami cuts was about 1.2 mm. The spacing in a direction about orthogonal to the longitudinal direction of each kirigami cut, the row spacing sp2, was about 2.4 mm.

[0096] FIG. 9B illustrates the results of the finite element method (FEM) simulation for each kirigami sample, kirigami A, kirigami B, and kirigami C. Commercial software (COMSOL Multiphysics 6.0) was used to investigate the von Mises stress distributions in the kirigami substrate and calculate the piezo-voltage generated by bending. Kirigami structures were directly created by the tools in COMSOL software, and stretching was performed as described. Briefly, boundary conditions were applied to the ends of the kirigami structure. At one end, a fixed boundary condition was placed, while at the opposite end, the described displacements in the stretching direction was enforced. An infinitely small bias load (l x 10-3 times smaller than the axial load) was also applied on the top edge of each cut to break the symmetry, which is inevitable to create a curved kirigami structure.

[0097] Each kirigami sample was stretched according to a fixed boundary condition. An increase in piezo- voltage generated by the bending is illustrated by lighter shading. The number of domains and the magnitude of von Mises stress were larger for shorter cut length (w = 6 mm, kirigami C).

[0098] The results of the catalytic effect with respect to various parameters of the kirigami substrate as demonstrated with photo-piezo catalytic degradation tests is shown in FIG. 10. Part (a) shows the degradation results for each sample, kirigami A, B, and C. Each sample had a stretching amplitude A of 4.9 mm and a substrate thickness t of 100 pm. The experimental results clearly show that a larger number of domains (i.e., a higher density of kirigami cuts) as shown for the kirigami C design is more effective in degradation at the tested amplitude. The higher performance in the catalytic effect of the kirigami C design mirrored the results demonstrated in the FEM analysis.

[0099] The effect of stretching amplitude A on degradation was tested with the kirigami C sample in part (b) of FIG. 10. The greater stretching amplitude of 4.9 mm resulted in significantly higher degradation of MO dye.

[0100] To determine the importance of the thickness of the substrate, three different thicknesses (t) of substrate, polyethylene terephthalate (PET) films, were used for kirigami cut. Results of the test show that thicker films having the kirigami C pattern are more suitable in accordance with the FEM results part (c) of FIG. 11. The effect of film thickness was related to applied strain. The experimental data of the stress-strain curve of the kirigamistructure demonstrates extremely high elasticity and stretchability, which closely matches the results obtained from FEM analysis.

[0101] The results of the amplitude of stretching (part (b)) and the angle dependence (part (d)) of the kirigami C sample show that the induced voltage of piezoelectricity is directly related to the performance of the photo-piezo catalysts, confirming the critical role of the kirigami structure (parts (b) and (d)).

[0102] Piezoelectric Effect on Photocatalytic Activity

[0103] The piezoelectric effect of the ZnO NRs aligned on the cyclically stretched kirigami substrate enhanced the efficiency 2.5-fold of MO photocatalytic degradation. Cyclically stretching includes repeatedly applying periodic stretching to a kirigami substrate while maintaining a constant stretching amplitude. To identify the catalytic degradation mechanism of the kirigami photo-piezo catalyst, radical scavenger tests were conducted. The three active species (superoxide radical ■ O ), hydroxyl radicals ( ■ OH), holes ( / fl)), L- ascorbic acid, isopropanol, and methanol were used as scavengers. Each scavenger (225 pM) was added to 5 pM of the MO solution, and the influence of each scavenger on catalytic degradation was quantified.

[0104] To determine the correlation between such a huge improvement in photocatalytic efficiency and the piezoelectric effect, different scavengers were added to the MO solutions to selectively trap the free radical species participating in decomposition reactions, as illustrated in FIG. 11. Specifically, radical scavengers of L-ascorbic acid, isopropyl alcohol, and methanol were used to capture superoxide anion radicals (■ O£) hydroxyl radicals (■ OH), and holes (h+), respectively. Based on the photocatalytic decomposition efficiency in the absence of scavengers, the influence of each scavenger was investigated. Part (a) is a plot of the influence of each scavenger in the absence of a piezoelectric effect (no stretching “stretching X), and pail (b) is a plot of the influence of each scavenger in the presence of a piezoelectric effect (stretching O). Part (c) is comparison of the estimated contribution of each free radical to the degradation of MO. In the absence of stretching (part (a)), the total degradation of 34% was achieved solely via the photocatalytic effect of ZnO NRs; specifically, ■ OH contributed 17.69%, h+4.16%, and ■ O2 : 2.53%, respectively. In contrast, when the piezoelectric effect was involved by stretching the kirigami substrate (part (b)), asignificant MO decomposition of 87.4% was observed with contributions from ■ 0^ of 59.58%, ■ OH of 21.99%, and h+of 7.23%. Note that MO usually shows a lower degradation ratio due to the presence of C=N and N=N double bonds. However, quite strikingly, kirigami photo-piezo catalysts effectively degrade under relatively mild wave conditions.

[0105] Demonstration of Wave Simulation of Photo-Piezo Catalysts

[0106] A small water reservoir system using tetracycline (TC) was set up that has a solar like irradiation and mechanical motion to generate waves to demonstrate the feasibility of the kirigami photo-piezo catalyst. ZnO NRs were decorated with copper sulfate (CuS) nanosheets for being activated by the visible light. CuS nanosheets were grown by successive ionic layer adsorption and reaction (SILAR) method. As precursors of cation and anion, 10 mM copper sulfate aqueous solution and 10 mM sodium sulfide aqueous solution were prepared.

[0107] Parts (a) and (b) of FIG. 12 illustrate high-resolution transmission electron microscopy (HRTEM) of CuS nano sheets -decorated ZnO NRs. CuS nanosheets were adopted so the photocatalysts can work under natural light. Images. The magnified view of part (b) shows the atomic distances of 0.26 nm and 0.31 nm are z-axis (0001) of ZnO NRs and (102) direction of CuS nanosheets, respectively.

[0108] The images of part (c) illustrate the energy dispersive X-ray (EDX) analysis that the ZnO NRs were fully covered by the CuS 2D nanosheets. The as-prepared ZnO NRs coated kirigami photo-piezo catalyst was soaked in 80 mL of copper sulfate solution for 1 minute, allowing copper ions (Cu2+) to adsorb on the surface of ZnO NRs. Then, it was rinsed with DI water for 10 seconds to remove weakly absorbed cations. As the next step, the cation-absorbed catalyst was soaked in 80 mL of sodium sulfide solution for 1 minute, causing sulfur anions (S2‘) to react with Cu2+and form CuS nanoparticlcs. Finally, it was also rinsed by DI water for 10s to remove unreacted ions. As the result of CuS decoration, the catalyst was shown dark green color (image O K series of part (b)). The existence of CuS on the surface of ZnO NRs was observed by high-resolution transmission electron microscope (HR-TEM, JEOL JEM-F200) (part (a)) and energy dispersive X-ray spectrometer (EDS) (part (b)).

[0109] The degradation experiments of the kirigami catalyst structures in mild wave conditions were conducted using a small water reservoir, as illustrated in the series of snap shot images of FIG. 13. Rocking was performed to generate the waves using a linear shaker, which is programmable, and a small sponge was attached to one end of the kirigami as a buoy. One end of the CuS / ZnO NRs Kirigami photo-piezo catalyst was fixed to the bottom and a Styrofoam buoy was suspended from the other end in the small reservoir filled with 160 mL of 5 pM tetracycline (TC) aqueous solution. The water wave in the small reservoir was generated using a linear shaker in the frequency range of 100 - 200 Hz. A Xc lamp (300W) was used as the visible light source, the intensity was set to 0.1361 W / cm2, which is the same as solar constant, adjusted by a light detector (SED033, International Light). Each snapshot at 0.1 second intervals represented about 2.50 Hz. The dashed line represents the water wave profile.

[0110] FIG. 14 represents a systematic study of degradation in small water reservoirs with respect to conditions, including a Xe light source similar to natural light, to study whether there is frequency dependent behavior. As expected, small degradation (-15%) occurs when there is no aid of piezoelectricity, i.e., when the water is calm (0 Hz). At a frequency of 2.5 Hz, there was a large effect of degradation compared to frequencies lower (1.67 Hz and 2.17 Hz) and higher (2.83 Hz and 3.33 Hz). These results showed the photo- piezo catalyst kirigami substrates demonstrated frequency dependent behavior.

[0111] In other studies, 10 kirigami substrates were tested in similar conditions of for decomposing 5 pM TC in a 1.6 L tank, and the results showed highly promising outcomes (-80% decomposition with 1 Hz rocking) for the application in environmental remediation.

[0112] In Use

[0113] Various aspects of an inventive concept described herein may be used as piezoelectric devices, photopiezo catalysts, photopiezo catalysts for self-sustainable environment remediation.

[0114] The inventive concepts disclosed herein have been presented by way of example to illustrate the myriad features thereof in a plurality of illustrative scenarios, aspects of an inventive concept, and / or implementations. It should be appreciated that the concepts generally disclosed are to be considered as modular, and may be implemented in anycombination, permutation, or synthesis thereof. In addition, any modification, alteration, or equivalent of the presently disclosed features, functions, and concepts that would be appreciated by a person having ordinary skill in the ail upon reading the instant descriptions should also be considered within the scope of this disclosure.

[0115] While various aspects of an inventive concept have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of an aspect of an inventive concept of the present invention should not be limited by any of the above-described exemplary aspects of an inventive concept but should be defined only in accordance with the following claims and their equivalents.

Claims

CLAIMSWhat is claimed is:

1. A product, comprising: a substrate having a plurality of kirigami cuts in a surface of the substrate, wherein longitudinal axes of the kirigami cuts are oriented about orthogonal to a longitudinal axis of the substrate, the plurality of kirigami cuts being arranged to allow flexure of the substrate; and a plurality of catalyst nanostructures on the surface of the substrate, wherein the catalyst nanostructures comprise a photo-piezoelectric material, wherein the catalyst nanostructures are aligned substantially along the longitudinal axes of the kirigami cuts.

2. The product as recited in claim 1, wherein each of the kirigami cuts extends through the substrate from the surface to the opposite side of the substrate.

3. The product as recited in claim 1, where the kirigami cuts have essentially uniform length.

4. The product as recited in claim 1, wherein the catalyst nanostructures are comprised of a material that has at least one property selected from the group consisting of: a semiconducting property, a piezoelectric property, and a photocatalytic property.

5. The product as recited in claim 4, wherein the catalyst nanostructures are comprised of a material selected from the group consisting of: a zinc oxide (ZnO)-based nanostructure, a metal niobate, a metal titanite, cadmium sulfide (CdS), a bismuth ferrite, a two-dimensional metal dichalcogenide, a two-dimensional group IV monochalcogenide, and a two-dimensional group III-V binary compound.

6. The product as recited in claim 1, wherein the catalyst nanostructures have a substantially hexagonal prism shape.

7. The product as recited in claim 1, wherein an average longitudinal length the catalyst nanostructures is in a range of greater than 10 nanometers to less than 10 microns.

8. The product as recited in claim 7, wherein an average longitudinal length of the kirigami cuts is at least 10 times an average longitudinal length of the nanostructures.

9. The product as recited in claim 8, wherein the average longitudinal length of the kirigami cuts is in a range of greater than 100 nanometers to less than 50 millimeters.

10. The product as recited in claim 1, wherein the substrate is comprised of a plastic material.

11. The product as recited in claim 1, wherein an alignment angle of longitudinal axes of the catalyst nanostructures relative to the longitudinal axes of the kirigami cuts is about 0°.

12. The product as recited in claim 1, wherein an average distance between adjacent kirigami cuts has an average length of greater than about 5% to about 50% of an average longitudinal length of the kirigami cuts.

13. The product as recited in claim 1, wherein a thickness of the substrate is in a range of greater than 1 nanometer to less than 1 millimeter.

14. A method of forming a photo-piezocatalyst kirigami product, the method comprising: adding a plurality of catalyst nanostructures onto a surface of a first substrate, wherein the catalyst nanostructures comprise a photo-piezoelectric material, wherein longitudinal axes of the catalyst nanostructures arc substantially aligned in the same direction; forming a plurality of kirigami cuts on a surface of a second substrate, wherein each kirigami cut has a longitudinal axis about orthogonal to a longitudinal axis of the second substrate; transferring aligned catalyst nanostructures to the surface of the second substrate having the kirigami cuts, wherein longitudinal axes of the catalyst nanostructures are substantially aligned with the longitudinal axes of the kirigami cuts; and removing the first substrate, wherein the catalyst nanostructures are adhered to the second substrate having the kirigami cuts after removing the first substrate.

15. The method as recited in claim 14, wherein the first substrate comprises a poly siloxane rubber material.

16. The method as recited in claim 14, wherein the second substrate comprises a plastic material.

17. The method as recited in claim 14, wherein the catalyst nanostructures have a substantially hexagonal prism shape.

18. The method as recited in claim 14, wherein the catalyst nanostructures are comprised of a material selected from the group consisting of: a zinc oxide (ZnO)-based nanostructure, a metal niobate, a metal titanite, cadmium sulfide (CdS), a bismuth ferrite, a two-dimensional metal dichalcogenide, a two- dimensional group IV monochalcogenide, and a two-dimensional group III-V binary compound.

19. The method as recited in claim 14, wherein an alignment angle of the longitudinal axes of the catalyst nanostructures relative to the longitudinal axes of the kirigami cuts is about 0°.

20. The method as recited in claim 14, wherein the catalyst nanostructures are transferred to the second substrate using a decal printing technique.

21. The method as recited in claim 20, wherein the transferring includes heating the second substrate above 100 degrees Celsius.

22. The method as recited in claim 14, wherein an average longitudinal length of the catalyst nanostructures is in a range of greater than 10 nanometers to less than 10 microns.

23. The method as recited in claim 22, wherein an average longitudinal length of the kirigami cuts is at least 10 times the average longitudinal length of the nanostructures.

24. The method as recited in claim 23, wherein the average longitudinal length of the kirigami cuts is in a range of greater than 100 nanometers to less than 50 millimeters.

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