Photoinduced Metallic Particle Growth on Single-Crystal Relaxor

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Photoinduced Metallic Particle Growth on Single Crystal Relaxor Ferroelectric Strontium Barium Niobate Eftihia Barnes, Erik M. Alberts, L Christopher Mimun, Jonathon A Brame, Christopher M Warner, Ashley R. Harmon, and Aimee R. Poda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00316 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018

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The Journal of Physical Chemistry

Photoinduced Metallic Particle Growth on Single Crystal Relaxor Ferroelectric Strontium Barium Niobate Eftihia Barnes*,1, Erik M. Alberts2, L. Christopher Mimun3, Jonathon A. Brame3, Christopher M. Warner3, Ashley R. Harmon3, and Aimee R. Poda3 1

Geotechnical and Structures Laboratory, U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180, United States 2

3

HX5 LLC, Vicksburg, Mississippi 39180, United States

Environmental Laboratory, U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180, United States

ABSTRACT The photochemical growth of metallic particles on relaxor ferroelectric tetragonal tungsten bronze (TTB) strontium barium niobate (SBN) is reported in this work. Silver and gold particles were deposited on the unpoled (001) and the (100) surfaces of single crystal SBN:60 and SBN:61 via reduction of silver nitrate and gold chloride solutions under UV illumination. Wavelength dependent experiments reveal that differences in particle deposition on the unpoled (001) and (100) surfaces are primarily due to the optical absorption of the UV light, and not due to the surface termination or local ferroelectric domain structure. Particle deposition on electric field poled (001) surfaces show enhanced particle deposition on positive domains, and suppressed deposition on negative domains. Unusual particle deposition was observed on the perimeter of domains obtained from incomplete switching of the unpoled (001) surface. Based on our experimental observations we propose a band diagram for the SBN interface and we discuss underlying mechanisms influencing the particle deposition process.

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INTRODUCTION Materials with internal electric fields have emerged as promising candidates for advanced oxidation processes (AOPs), which can be used for the degradation of industrial and biological contaminants. The internal electric fields cause band bending at the surface of the semiconductor/solution interface enabling separation of photo-generated electrons and holes, effectively reducing their recombination rate, and allowing them to participate in chemical reactions. The photochemical reactivity of surfaces can be assessed by marker reactions which leave insoluble products on irradiated surfaces. Such reactions have been carried out extensively on non-centrosymmetric materials such as BaTiO3,1-4 LiNbO3,5-8 Pb(Zr0.3Ti0.7)O3 (PZT),9,10 BiFeO3,11 ZnO12 as well as on centrosymmetric materials such as TiO2,13 SrTiO314 and BiVO4.1518

Particle deposition depends on various factors such as the crystallographic orientation of the

sample, the concentration and pH of the solution being reduced or oxidized,8 the wavelength and intensity of the incident light,7,8 the chemical structure and modification of the surface,19 etc. Heterogeneous photocatalysts consisting of oxides decorated with metallic particles have shown promising results in the degradation of organic molecules. For example, ferroelectric BaTiO3 and BiFeO3 powders decorated with nanostructured Ag show enhanced photocatalytic degradation of Rhodamine B,20 and methyl orange respectively.21 Marker reactions may be used to impart substrates enhanced functional properties such as tunable surface wettability22 or make them suitable for plasmonically enhanced Raman scattering.23,24 Materials with internal electric fields include ferroelectrics. Ferroelectrics exhibit spontaneous electrical polarization which can be reversed (switched) by the application of an external electric field. An important subset of ferroelectrics are relaxor ferroelectrics, often referred to as relaxors, which exhibit unique physical properties attributed to their compositional 2 ACS Paragon Plus Environment

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disorder.25

Their large electromechanical response makes them important in ultrasonics,

actuators and motors.25-27 Unlike traditional ferroelectrics, relaxors exhibit slim polarization hysteresis loops and nano-sized polar domains which persist above the dielectric maximum temperature (Tmax), temperature-dependent permittivity with broad peaks and strong frequency dependence, and no sudden change in physical properties (birefringence, refractive index, volume) indicative of macroscopic symmetry breaking at Tmax.28 Complex oxides crystalizing in the tetragonal tungsten bronze (TTB) structure can exhibit composition dependent relaxor ferroelectricity. Strontium Barium Niobate SrxBa1-xNb2O6 (SBN:100x) is a stoichiometrically tunable solid solution which generally crystalizes in the TTB structure for 0.25≤x≤0.75 with the exact compositional limits depending on processing conditions. SBN is a technologically promising material with exceptional pyroelectric,29 electrooptic,30,31 acousto-optic,32 photorefractive,33 nonlinear optical,34 dielectric, and thermoelectric properties35-37. Recently, SBN nanocrystals have been used for photocatalytic water splitting.38 Barium-rich SBN compositions show conventional ferroelectric behavior, whereas strontiumrich compositions exhibit relaxor type ferroelectricity. The TTB unit cell is described as (A1)2(A2)4(C)4(B1)2(B2)8O30 and consists of B1O6 and B2O6 corner sharing octahedra forming three different types of interstitial sites: the square or perovskite (A1) site, the pentagonal (A2) site, and the trigonal (C) site, which can be vacant or occupied by cations.39 In SBN, the A1 sites are occupied only by Sr whereas the A2 sites can be occupied either by Sr or by Ba; however one-sixth of the A1 and A2 sites are vacant.

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Likewise, the C sites are too small to

accommodate any of the cations and remain vacant. The Nb cations occupy the B1 and B2 sites. Ferroelectric properties arise due to small displacements of the Sr, Ba and Nb cations along the c axis leading to the creation of spontaneous polarization.41 When compared to LiNbO3, strontium-



rich SBN has a lower transition temperature to the paraelectric state, lower coercive field, and higher pyroelectric and piezoelectric coefficients.29,42 However, SBN can exhibit ferroelectric fatigue and significant domain backswitching.43 In this work, we investigate the photochemical growth of silver and gold particles on the surface of single crystal relaxor ferroelectric SBN:61 (congruent composition) and SBN:60. We examined the influence of wavelength and intensity of the UV light on the particle size and deposition rate on the (001) and (100) surfaces of congruent SBN (SBN:61). Due to the equivalency of the tetragonal (100) and (010) surfaces, we will interchangeably refer to them as the “(100)” surface. Further, we have used external electric fields to create oriented macroscopic domains in SBN:60, which exhibit selective photochemical reactivity. Our preliminary results can be further refined using more sophisticated domain patterning techniques, such as AFM-tip based nanolithography, that might allow the creation of future devices, sensors and waveguides with precisely patterned metallic nanostructures. Metallic particle deposition can be also exploited to create heterogeneous catalysts with enhanced (photo)catalytic performance. To our knowledge this is the first work reporting photo-induced growth of metallic particles on SBN. EXPERIMENTAL METHODS Single crystal SBN:61 and SBN:60 samples were purchased from Laserand Inc., Canada and MTI Corp., USA, respectively. The SBN:61 crystals were 5 mm cubes whereas the SBN:60 samples were 0.5 mm thick (001) oriented 10 mm × 10 mm plates. All samples were received unpoled. The samples were heated to 473 K for 2 hours, in order to ensure thermal depolarization. Only, the SBN:60 samples were electrically poled at room temperature. The thickness of the SBN:60 samples allowed us to safely apply relatively low voltages ( ≤ 250 V) during poling, but their as-received surface polishing was not as high quality as that of the 4 ACS Paragon Plus Environment

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SBN:61 samples. Electric field poling was carried out within an optically transparent cell containing 10 wt % LiCl aqueous solution (203637, Sigma-Aldrich), while the surface of the sample was monitored with a polarizing optical microscope under transmission mode (Imager.Z1m, Zeiss). The poling voltage, supplied by a DC power supply (PS350, Stanford Research Systems), was ramped until domain nucleation was observed. Transient visualization of the domain nucleation was enabled by the electro-optic effect which causes modulation of the crystal’s refractive index in the presence of an external electric field, producing refractive index contrast across domain walls. 44 Starting from a thermally depolarized sample, electric field of a single polarity was slowly ramped and held at a constant voltage (~250 V) for approximately 10 minutes in order to enable the formation of a nominally single domain state in the area covered by the water electrode. The single domain state was locally probed with PFM. Surfaces connected to the negative poling voltage are c+ orientated with respect to the ground electrode, and likewise surfaces connected to the positive poling voltage are c- orientated. For partial domain poling, electric field was applied either on a thermally depolarized sample or in a single domain area. The amplitude of the electric field was reduced well before completion of the switching process, creating a low density network of reversed domains within the unpolarized or single domain background. The domain structure of the samples was investigated with Piezoresponse Force Microscopy (PFM) (Dimension Icon, Bruker) using Pt coated tips with resonant frequency at ∼300 kHz and a spring constant of ∼16 N/m (HQ:NSC35 cantilever B, MikroMasch). Prior to PFM imaging, the samples were attached to aluminum pucks with conductive paste. PFM scans were carried out with a 5-10 V amplitude ac voltage applied to the sample at a frequency of 50 kHz. The measurement frequency was chosen to be far from the resonant frequency of the cantilever-sample system in order to minimize topographical crosstalk



in the measured data. The piezoresponse signal reported here is the in-phase signal which is the product of the piezoresponse amplitude and the cosine of the phase, and is therefore sensitive to the direction of ferroelectric polarization. Calibration of the vertical PFM lock-in electronics was carried out with a periodically poled lithium niobate (PPLN) standard consisting of alternating c+ and c- domains, with the location of c+ domains revealed by preferential silver particle photodeposition under 300 nm illumination.7 Subsequent PFM measurements were carried out under the same lock-in conditions. Particle photodeposition experiments were carried out as follows: first, the samples were sonicated for 10 min in acetone, 10 min in methanol, and then dried with compressed nitrogen gas. The SBN:61 samples were fully immersed in the solutions with the surface of the sample at ~0.3 cm below the surface of the liquid. For the SBN:60 plate samples, a Viton O-ring (inner diameter ~ 6 mm, thickness ~ 1.8 mm) was placed on the surface, and filled with the appropriate solution. The solutions used were either 10-3 M aqueous AgNO3 (diluted with DI water from LC227002, LabChem) or 10-4 M HAuCl4 aqueous (diluted with deionized (DI) water from LC148957, LabChem). The pH of the silver nitrate and the gold chloride solutions was measured to be ~ 5.5 and ~ 4 respectively. The samples were irradiated with UV light emitting diodes (LED) with peak emission at 265 nm (M265L3, Thorlabs), 300 nm (M300L4, Thorlabs) and 340 nm (M340L4, Thorlabs). The optical power of the UV light reaching the surface of the solutions was measured with a calibrated power meter (843-R, Newport) equipped with a wand-style detector (918D-ST-UV, Newport). A summary of the photodeposition experiments is listed in Table S1. After photodeposition, the samples were immersed in DI water for 1 min, gently rinsed with DI water in order to remove unattached particles, and finally dried with compressed


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nitrogen. Samples were reused by gently cleaning their surface with methanol soaked cotton tipped applicators, and by repeating the cleaning procedure described above. Photodeposition products were imaged with an FEI Nova Nano-SEM 630 Field Emission scanning electron microscope equipped with a secondary electron (SE) detector with accelerating voltages between 5 kV and 10 kV and spot sizes between 2.0 and 5.0. In order to reduce surface charging as well as to prevent electron beam depolarization of the underlying domain structure, the samples were coated with a thin layer of gold prior to SEM imaging. SEM image particle analysis was carried out with ImageJ.45 RESULTS AND DISCUSSION PFM images of the unpoled (001) and (100) surfaces of the SBN:61 sample are shown in Figure 1. Due to crystal symmetry selection rules, only 180° (antiparallel) domains are expected in uniaxial SBN.29 Consistent with previous observations of strontium-rich SBN, the (001) surface consists of fractal-like nanodomains.46-48 The dark and light areas correspond to domains with opposite directions of polarization, orientated in and out of the plane of the figure, whereas areas with intermediate contrast correspond to regions with negligible or unresolved piezoresponse. PFM of the (100) surface reveals stripe or needle like domains with their long direction aligned along the polar c axis. These domains are highly anisotropic, extending several microns along the c axis. Here, light and dark areas correspond to domains with opposite directions of polarization, where the polarizations lies entirely in the plane of the figure and parallel to the c axis. The (001) domain structure of both crystals (SBN:61 and SBN:60) is similar with a representative PFM image from the SBN:60 sample shown in Figure S1.



The (001) and (100) SBN:61 surfaces, after photodeposition under 265 nm illumination, are shown in Figure 2 for high and low intensity illumination. For high intensity illumination (incident power ~ 3 mW) both the (001) and (100) surfaces are densely decorated with silver particles (Figures 2(a), 2(b)). The particle deposition is non-uniform and the shape of the particles is roughly spherical. For low intensity light (Figures 2(c), 2(d)), the deposition pattern is noticeably sparser with smaller particles deposited on either surface. The average size of the deposited particles is larger for the (001) surface than the (100) surface (0.016 ± 0.015 µm2 versus 0.008 ± 0.006 µm2). The energy of the photons reaching the surface of the sample is ~4.68 eV, which is above the direct bandgap of SBN:61; the latter was estimated to be ~3.27 eV and ~3.26 eV for light propagating along the and directions (see Figures S4(a) and S4(c)). Above bandgap illumination creates photogenerated electrons and holes which accumulate on the surface of the crystal, and can participate in photochemical reactions. In our case, the photogenerated electrons reduce the silver cations provided by the silver nitrate solution, leading to the growth of silver particles, Ag(s). The particle morphology depends on a number of factors such as illumination time, surface polarity and termination, as well as the penetration depth of the incident light.7 For SBN, the (001) surface is polar whereas the (100) surface is non-polar; also the optical absorption coefficient of the (001) surface is higher than that of the (100) surface at 265 nm.49 Increased optical absorption leads to a smaller photon penetration depth, which causes an increase of photogenerated carriers near the surface of the material. These shallow carriers have a higher probability of reaching the surface and participating in chemical reactions. Therefore, for the 265 nm photodeposition, the larger particle size observed on the (001) surface could be attributed


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either to a higher intrinsic photo-reactivity of the (001) surface or to its increased optical absorption or due to a combination of these two factors. In order to investigate the intrinsic photoreactivity of the (001) and (100) surfaces, additional deposition experiments were carried out at wavelengths where the optical absorption of the two surfaces is comparable. Based on our experimental capabilities and the optical studies of Dorywalski et al.49 we chose 300 nm (~4.13 eV) and 340 nm (~3.65 eV) as the two wavelengths of interest. The illumination time and light intensity were chosen such that the total number of incident photons was approximately equal to that of the 265 nm illumination (~6×1018 photons⋅cm-2). The particles deposited under 300 nm illumination are shown in Figures 3(a)-(b) and are, on average, smaller than the ones deposited under 265 nm illumination. Also, the average size of the deposited particles on the (001) and the (100) surfaces is similar (average sizes ~ 0.006 ± 0.005 µm2 for both), with a few larger platelet particles nucleating on the (001) surface. The deposited particles under 340 nm illumination are shown in Figures 3(c)-(d). Under the given experimental conditions, sparse nanoparticle deposition was observed on both surfaces. In general, with increasing wavelength, we observe a reduction in the average size of the deposited particles, as well as a significant reduction in the surface area covered by particles when the illumination wavelength increases beyond 300 nm. These results can be explained as follows: as the wavelength of light increases, the penetration depth of the absorbed photons increases. This leads to the creation of photogenerated carriers deeper in the material which have a higher possibility of recombination, thereby reducing the efficiency of the photodeposition process. Growth of gold particles on the SBN surfaces was also observed under UV light illumination. Figure 4 shows gold particle deposition on the (001) and (100) surfaces after illumination of the 9 ACS Paragon Plus Environment


gold chloride solution with 300 nm light. The gold deposition pattern is similar for the (001) and (100) surfaces; however when compared to the silver particle deposition it is much sparser despite a greater number of photons reaching the SBN surface (see Figure S2). These differences are likely due to the complex reduction pathways taking place in the gold chloride solution. Dissolving gold chloride in water forms AuClx- compounds where x strongly depends on the pH of the solution.50-52 The pH of our gold chloride solution was measured to be ~ 4, which according to Wang et al. leads to the formation of AuClx- compounds with x = 2.46, indicating that the Au dissolved species is a mixture of several species such as AuCl4-, AuCl3-, and AuCl2-.52 A one step reduction of AuCl4- into solid gold requires the availability of three photogenerated electrons, whereas multi-step reactions require fewer electrons but as a whole are energetically more demanding.50,52 For LiNbO3, inhibition of gold particle nucleation was observed for high concentrations of the gold chloride solution (>10-5 M), and it was attributed to the screening of the crystal surface by excess chlorine ions, whereas for lower solution concentrations smaller particles were formed, but with higher surface area coverage.51 In order to maintain overall charge neutrality, an oxidation reaction must also take place during the silver and gold reduction reactions; the most likely reaction is water oxidation by photogenerated holes. Silver and gold metallic particle photodeposition on unpoled SBN indicate that the photoreactivity of the (001) and (100) surfaces is comparable. The differences observed for the 265 nm particle deposition can be predominantly attributed to the differences in the optical absorption properties of the two surfaces. In addition, the particle deposition pattern cannot be conclusively correlated to the local ferroelectric domain configuration. This can be explained because of the weak net polarization of the unpoled (001) surface, as is the case of unpoled ferroelectrics. On the other hand, the ferroelectric polarization on the (100) surface lies entirely


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in the plane, and it is not expected to influence surface photoreactivity. When a semiconductor is brought in contact with an electrolyte, the band structure at the interface alters depending on the type of semiconductor (n or p type) with the amount of band bending depending on the pH of the solution.

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Therefore, for the weakly polar and non-polar SBN surfaces we assume a small

upward bending of the bands at the interface. Depending on the height of the barrier and the width of the space charge layer, photogenerated electrons can reach the surface and participate in photochemical reactions. Mechanical polishing of crystal surfaces also leads to the creation of shallow defects such as vacancies and interstitials, which act as preferential nucleation sites for particle deposition.54 The crystals were heated to 473 K, which is sufficient for thermal depolarization, but likely not sufficient for removal of such defects. Therefore, we cannot rule out the possibility that metallic particles preferentially nucleate at such defected sites. In order to evaluate the effect of ferroelectric polarization on the particle photodeposition, electric field poling was carried out on the SBN:60 platelets. By applying a sufficiently high external electric field, the local ferroelectric polarization is aligned, creating a single domain state. Here, we created a nominally single domain surface by slowly ramping the electric field, and optically observing complete switching, before slowly reducing the magnitude of the applied electric field. Silver photodeposition products on the unpoled, positive (c+) and negative (c-) surfaces (domains) under 265 nm illumination are shown in Figure 5. Similar to our previous observations, the unpoled (001) surface shows moderate particle deposition despite the lack of macroscopic polarization. On the other hand, the positive surface exhibits enhanced deposition with dense particle nucleation and high surface area coverage. In addition to the roughly spherical shaped particles, larger platelet particles have also nucleated on the c+ surface. In contrast, metallic particle photodeposition is suppressed on the negative surface: when compared



to the unpoled surface we observe lower surface coverage and sparser particle deposition. The influence of ferroelectric polarization on the surface photoreactivity has been studied in many complex oxides such as BaTiO31-4, 9, LiNbO35-8,19-24, BiFeO311, and PZT9,10. In general, the band structure at the interface between a ferroelectric semiconductor and an electrolyte solution can be altered depending on the direction of ferroelectric polarization.9 It has been suggested that in positive domains downward band bending occurs favoring the participation of photogenerated electrons in reduction reactions, whereas in negative domains the bands bend upward favoring oxidation reactions involving photogenerated holes. Charge neutrality of redox reactions dictates that both of the half reactions must take place at the semiconductor/solution interface because photogenerated carriers in the bulk of the material cannot participate in photochemical reactions. Since the overall reaction rate is limited by the slowest of the two half reactions, the highest rates are likely to occur either on unpoled or multi-domain surfaces, followed by single domain surfaces. In unpoled surfaces, with effectively zero polarization, photogenerated carriers can overcome potential barriers thermally or by tunneling, whereas in multi-domain surfaces, carriers migrating away from the surface can transport to a domain of favorable band structure and reach the interface.55,56 In single domain surfaces, photogenerated carriers that encounter potential barriers are repelled from the surface thereby reducing the overall reaction rate. However, in real world systems, the photoreactivity of ferroelectric surfaces depends not only on polarization but also on a complex balance between polarization charges, adsorbed species and defects. Particle photodeposition on single domain surfaces has been reported for LiNbO3 crystals and thin film PZT. 22,57-59 In LiNbO3, discrete silver nanoparticles were observed on both the c+ and c- surfaces under 276 nm illumination.57 The nucleation of particles on the c+ surface was attributed to band bending and surface or subsurface defects disturbing the space charge layer,


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whereas particle nucleation on the c- surface were attributed to the photoelectric effect; the particle size on either surface was comparable.57 In Al-Shammari et al., silver particles with approximately twice the height and width, than those of the c+ surface, were observed on the cLiNbO3surface.22 Since the potential of the SBN conduction band is significantly lower than that of LiNbO3,57,60 it is unlikely the photoelectric effect plays a significant role on the particle deposition observed on c- SBN. Photodeposition experiments on single domain, electrically poled and self-polarized, PZT films resulted in higher particle deposition (density and size) on the c+ surface.58,59 In particular, particle deposition was reported to be the highest on the c+ surface, followed by the as-grown (unpoled) and the c- surface.58 These results agree with our observations but contradict the reaction rate scenarios described above. In absence of photoanodic dissolution,53 we consider a number of possible explanations. Preferential particle deposition on c+ LiNbO3 domains was recently attributed to the presence of defect dipoles but it is currently unknown if a similar defect dipole state exists in SBN.61 Further, the single domain state in SBN can be unstable, due to domain instabilities originating from residual depolarization and internal fields.62,63 Assuming that the majority of the c+ poled surface remains in the c+ configuration but there exist nano-regions that have reverted to the unpoled state, then these sites can act as hole receptors and promote oxidation. This is a likely scenario because SBN is an n type semiconductor, and in the unpoled state favors oxidation. Since the oxidation reaction does not leave insoluble products, such sites can continuously transport holes to the surface, whereas the c+ surface promotes nucleation of silver particles. The situation is different for the c- surface; namely the photogenerated electrons are repelled from both the c- and the backswitched regions slowing down the overall reaction rate. This appears to be the most plausible explanation for our observations.



A partially poled (001) surface was obtained as a way to mimic ferroelectric lithography. Partial poling was obtained on a thermally depolarized sample or a single domain sample by slowly ramping (~ 2V/s - 5V/s) the electric field up to ~200 V and gradually reducing the electric field before completion of the switching process. PFM images of nucleated domains are shown in Figure 6. In Figure 6(a) the domains are shaped like rounded rectangles: their shape reflects the symmetry of the SBN crystal and has been previously reported by Tian et al. and others.43,62 The PFM signal in the interior of these domains is nearly constant, indicating close to uniform switching. More complex domain structures are also encountered on the same sample, such as the one shown in Figure 6(b). Here the domain ensemble has irregular boundaries, and its interior consists of smaller, partially merged domains. The emergence of these complex domains depends on a number of parameters such as their location within the liquid electrode, electric field magnitude and ramping speed etc.48,62,63 SEM images of the partially poled SBN:60 surface, after 300 nm illumination of the silver nitrate solution are shown in Figure 7. Figures 7(a)-(b) show SEM images of positive domains exhibiting preferential growth of silver particles in their interior surrounded by areas of low or no particle deposition; this is especially pronounced for the domain shown in Figure 7(b). Figures 7(c)-(d) show SEM images of negative domains with suppressed particle deposition surrounded by areas of preferential deposition. The lack of significant particle growth inside the negative domains indicates that for the given intensity (~7.5 mW) and wavelength the photoelectric effect does not play an important role on the metallic particle nucleation process. The anomalous particle depletion observed on the outer boundaries of the positive domains has not been previously reported. The depleted area is too large to be solely attributed to the influence of a single domain wall. Preferential growth of nanoparticles on the domain walls of periodically


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poled LiNbO3 (PPLN) has been observed, but the enhanced deposition was limited to the vicinity of the domain walls (domain wall width

Photoinduced Metallic Particle Growth on Single-Crystal Relaxor

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