Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2020 Jul 21;5(30):19050–19060. doi: 10.1021/acsomega.0c02410

Biomaterial-Induced Stable Resistive Switching Mechanism in TiO2 Thin Films: The Role of Active Interstitial Sites/Ions in Minimum Current Leakage and Superior Bioactivity

Misbah Sehar Abbasi †, Muhammad Sultan Irshad ‡, Naila Arshad §, Iftikhar Ahmed ∥,*, Muhammad Idrees ⊥,*, Shafiq Ahmad #, Zhou Wei ‡, Mohamed Sharaf #, Muhammad Dzulqarnain Al Firdausi #
PMCID: PMC7408193  PMID: 32775907

Abstract

graphic file with name ao0c02410_0010.jpg

Leakage of current in oxide layers is the main issue for higher speed and denser resistive random-access memory. Defect engineering played a substantial role in meeting this challenge by doping or producing controlled interstitial defects or active sites. These controlled active sites enabled memory cells to form a stable and reproducible conduction filament following an electrochemical metallization model. In this study, a defect-abundant lime peel extract (LPE)-mediated anatase TiO2 thin film was fabricated using a simple hydrothermal route. The detailed structural and morphological analysis of the bioactive anatase TiO2-LPE thin film reveals the homogeneous growth of TiO2 flowers and distinct features in terms of controlled defects as compared to simple anatase TiO2. These interstitial defects (Ti+3 and Ti+4) behave as active sites for cation migrations along highly conductive K1+ ions because of the mediation of LPE. The defect-free surface reveals slight surface roughness (4.8 nm) that successfully minimizes leakage of current. The strategy enabled a reliable conductive bridge filament, which can replicate with no more electric degradation. The Ag/TiO2-LPE/FTO-based memory cell demonstrates reproducible bipolar resistive switching along with a high ON/OFF ratio (>105), excellent endurance (1.5 × 103 cycles), and long-term retention (105 s) without any electrical degradation. Furthermore, green-synthesized TiO2-LPE nanoparticles have shown superior antibacterial activity as compared to other green syntheses of different plants or fruits against the toxic microorganisms present in inorganic media.

1. Introduction

Expeditious development in the field of electronic technology, higher speed, low cost, and denser memories are required. Resistive random-access memory (RRAM) is one of the possible contenders to meet the need for the next generation of nonvolatile memories. RRAM has a simple metal/insulator/metal sandwich structure and excellent complementary metal-oxide semiconductor compatibility, which are prominent factors for its applied applications and mass creations. The working principle of resistive switching depends on the formation of a conduction filament (CF). The resistance of RRAM cells can be changed by applying electric stimuli; initially, the device exhibits a high resistance state (HRS) and converts into a low resistance state (LRS), and these two resistance states are referenced to logic 0 and logic 1, respectively.1−4 Substantial efforts have been made to explore CF formation, such as the valence change mechanism (VCM) and electrochemical metallization (ECM) phenomena. Recently, Xue et al. reported ECM-based flexible resistive switching in hydrothermally grown TiO2 thin films.5 Both have high competition with each other in terms of efficient and reliable CF formation and switching parameters of the device, for example, operational speed, endurance, Joule heating effect, and retention time.6,7 Noticeably, significant improvements are needed toward the stability of the CF and sneak path issues or leakage of the current.8 The complementary resistive switching also suggests that sneak path current may occur because of many cells in a multilevel selected layer.9 Different in situ measurements were carried out to investigate this challenge and revealed the abnormal behavior in resistive switching because of pre-existing defects in the oxide layer.10−14

Chang et al. reported HfO2-based abnormal resistive switching behavior because of pre-existing defects.15 These pre-existing defects should be addressed to achieve the control of variability and reproducible resistive switching behavior in the oxide layer, especially.16 Recently, Ebenhoch et al. have reported retention relaxation at high temperature and volatile resistive switching behavior because of redistribution of point defects or oxygen vacancies.14

Comparatively, ECM is the most developed and quite mature theory to meet this challenge efficiently in terms of defect chemistry of transition metal oxides (TMOs).8,17 ECM phenomena provide a substantial way for the migration of metal ions through these pre-existing defects (interstitial defects) by easily approaching the lattice of TMOs.4,18 Moreover, green synthesis of TMOs played a significant role in green electronics toward low-cost recycling of waste materials for the development of higher speed and sustainable memory devices.19,20 In this study, green-synthesized titanium dioxide (TiO2) mediated by a lime peel extract (LPE)-based memory cell is developed to address the sneak path current issue with the utilization of active interstitial defects or active sites followed by the ECM mechanism. These active sites or interstitial defect sites have been confirmed in many in situ measurements and other optical investigations. The incorporation of green synthesis facilitates the migration of active metal ions through these interstitial defect states.21,22 In the ECM model, the CF formation leads to oxygen vacancies, but these other interstitial defect sites may interrupt the charge localization.23 The current study exploits these active interstitial sites to form a conduction channel by the combination of active metal Ag+1 ions of the top electrode and highly conductive potassium of LPE through these Ti+3 and Ti+4 defect interstitial sites. Chen et al. also reported the migration ability of highly conductive potassium ions (K1+).24

TMOs are captivating the elementary and technological attention for demonstrating unique properties that are driven by various factors, including the number of d-electrons in transition metal ions, their crystalline structures, and point and interstitial defects.4,8,17,18,25,26 TiO2 is a white color and n-type semiconductor, which is thermally stable, has excellent optical and dielectric characteristics with a high dielectric constant value (86–170), and is biocompatible and nontoxic as compared to other metal oxide elements. TiO2 has its functionality in large applications such as photocatalysts, lithium-ion batteries, dye-sensitized solar cells, H2O2 reduction, optoelectronic devices, solar cells, and so forth because of its outstanding absorption properties.27−30 Furthermore, TiO2 NPs have also been declared as a nontoxic element to the environment by the United Nations.31 Titanium dioxide has zero d-electrons; the number of d-electrons could be controlled by doping it with desired elements. Anatase TiO2 has higher mobility of charge carriers than rutile because of the difference in Fermi energy.32 Several plant part (stems, leaves, flowers, and peel) extracts are used as oxidizing, reducing, and capping agents to control aggregations and agglomerations of plants in the NPs. Green-synthesized nanoparticles have emerged as active antimicrobial and antibacterial agents because of their high surface area to volume ratio.33−37 In this context, green-synthesized TiO2 NPs have shown better antibacterial activity in retarding the growth of both Gram-positive and Gram-negative bacteria as compared to pure TiO2 NPs. Besides, their oxidizing activity could be increased because of the presence of flavonoids or reducing agents, which give rise to additional antibacterial activity to play a vital role in many infectious diseases.34

Herein, a Ag/TiO2-LPE/FTO-based memory cell successfully demonstrates the reproducible bipolar resistive switching behavior in a low voltage regime, excellent endurance, and a high ON/OFF ratio without any electrical degradation. The fabricated memory cell utilizes the pre-existing interstitial defects in the selected layer following the quite mature ECM model. The incorporation of LPE enabled the fabricated memory cells to build a mature, stable, and reproducible CF with the aid of highly conductive K1+ and Ag+1 ions as compared to other simple TiO2-28,30,38 and HfO215-based memory cells. The lime peel-mediated TiO2-based memory cell exhibits abundant interstitial defects, which may facilitate Ag+1 ions to move from the cathode to anode. Furthermore, the mediation of LPE in TiO2 NPs contains carboxylic groups, amino acids, and citric acid, which act as reducing, capping, and antibacterial agents against both Gram-positive and Gram-negative bacterial strains, which give additional applications in the field of healthcare.

2. Results and Discussion

The crystalline growth of TiO2-LPE-based thin films acquires the same tetragonal structure to avoid the large mismatch between the thin films and substrate. The purpose of choosing FTO (tetragonal) glass as a substrate is to achieve crystalline tetragonal epitaxial growth of TiO2-LPE-based thin films instead of bare glasses. The hydrolysis of inorganic species during reaction time was controlled by HCl. The chemistry of desired synthesis acquired pH should approach 3 or less.28 The purely acidic nature of solution promotes the homogeneous growth of crystalline tetragonal TiO2-LPE-based thin films. Moreover, the presence of HCl plays a vital role toward defect chemistry of TiO2-LPE-based thin films to control the oxidation state of Ti4+. The acidic media govern the conversion of Ti4+ into Ti2+ at high pressure and excessive temperature during chemical reactions in an autoclave.28

Primarily, the hydrolysis of a titania precursor (C12H28O4Ti) into titanium hydroxide is performed in an aqueous acidic medium. At that moment, the titanium hydroxide forms a complex when reacting with water, which can be controlled in an acidic medium.28 The complete procedure is illustrated in the following equations.

Step 1. Hydrolysis

2. i

Step 2. Condensation

2. ii

Step 3. Drying/Annealing

2. iii

In Figure 1a, the X-ray diffraction (XRD) pattern of TiO2-LPE deposited on the substrate of FTO glass in the two theta range of 20–80° is displayed. The observed spectrum demonstrates a crystalline tetragonal phase structure that matches with standard JDCPS card (21–1272). The objective behind utilizing the substrate of conductive FTO crystalline (tetragonal) glass is to make sure the maximum possible crystallinity and small lattice mismatch (less than 2%) in the TiO2-LPE thin film structure. The sample validated high crystallinity in structure with diffraction angles of 25.4, 38.3, 48.1, 54.5, 63.1, 69.7, and 75.5° with the corresponding planes (101), (112), (200), (211), (204), (220), and (215). These corresponding planes confirm the crystalline tetragonal structure of anatase TiO2. The diffraction angle at 25.4° is associated with crystallographic plane (101) of the anatase phase only.39 The surface morphology and thickness of green-synthesized anatase TiO2-LPE thin films were analyzed using scanning electron microscope investigations. Figure 1b demonstrates the surface view of the TiO2-LPE thin film, which reveals the growth of anatase 3D TiO2 flowers on an anatase 1D TiO2 nanoparticle bed. These flowers are much beyond from one other in a way that the TiO2 particle layer is subjected to the silver paste coated for the formation of an active electrode. Hence, it is inferred that the observed switching behavior is due to the TiO2-LPE particle layer below these flowers. The surface morphology of thin films can be very decisive for resistive switching phenomena; for example, the surface defects may cause leakage of current, introduce different paths for current flow, and disturb localization of the CF.21,29 Moreover, some isolated cracks appear on the 1D TiO2 nanobelt because of induced material contraction coupled with adhesion to a FTO substrate, which leads to stress in the material. Taleb et al. also extensively discussed the crack formation because of adhesion stress between a film and substrate.40 Fortunately, surface roughness plays an important role in minimizing the leakage of current. If surface defects exist, then the active metal top electrode material also may cause short circuit, which could result in a low dielectric response or no resistive switching behavior of the sandwiched layer.41 From the cross view (Figure 1c), results showed that the thickness of the deposited anatase TiO2-LPE layer is ∼ 4 μm. Figure 1d represents the energy-dispersive spectromtery (EDS) spectrum and indicates that the elemental composition of the green-synthesized TiO2-LPE thin film is purely constituents of C, O, and Ti while carrying no impurity. Here, the absence of H confirms that the solvent has evaporated from the coated surface. It contains 18% carbon, 62% oxygen, and 20% Ti. The EDS results showed that they are contiguous to the XRD results.

Figure 1.

Figure 1

Open in a new tab

(a) Represents the X-ray diffraction spectrum, which reveals the tetragonal crystalline structure of green-synthesized TiO2-LPE thin films. (b) Demonstrates the surface morphology of green-synthesized anatase TiO2 thin films, which revealed the homogenous growth of TiO2 flowers grown on the TiO2 NP bed and these TiO2 flowers apart from each other. (c) Cross-view image of TiO2-LPE thin films, which estimates the thickness of the films on the FTO substrate. (d) Elemental composition of green-synthesized anatase TiO2 mediated by LPE.

Figure 2 shows the surface topography of LPE-mediated TiO2 thin films explained by atomic force microscopy, for example, surface roughness, phase, and surface waviness. TiO2-LPE thin films reveal defect-free surface topography, which plays an important role in sustaining controlled charge localization instead of sneak path current. As shown in Figure 2(a), the selected atomic force microscopy (AFM) 3D view region (11.9 × 13 μm) of TiO2-LPE thin films exposed a large bump, which is inferred from the anatase TiO2 flower (supported scanning electron microscopy (SEM) morphology results). Besides this large bump, a small bump also appears, which corresponds to the TiO2 NP belt on which TiO2 flowers grow. As we know, thin-film surfaces are susceptible, and their surface roughness may cause several effects, especially in electronic devices such as interfacial ohmic conduction, etc.42 The surface roughness spectrum reveals the minimum surface roughness (4.8 nm) of the as-prepared TiO2-LPE thin films. This slight roughness emerged because of TiO2 flower bumps along with root mean square (RMS) roughness (6 nm), as illustrated in Figure 2(b). Statistical analysis measurements bring out the surface area 5.410 μm2 with a median of 0.53 μm. The minimum surface roughness plays a substantial role in minimizing leakage of current in thin film-based electronic devices and supports the local charge transport.43 As shown in Figure 2(c), phase analysis of TiO2-LPE thin films expressed a bright area, which confirmed the existence of anatase TiO2 flowers, which are far from each other and have short circuit or a sneak path issue. The waviness image represents the regularities in a pattern along with a smooth structure, as shown in Figure 2(d). Collectively, the maximum waviness height of the TiO2-LPE thin films, average spacing of waviness, and RMS waviness were 596, 46.3, and 37.1 nm, respectively.

Figure 2.

Figure 2

Open in a new tab

(a) Represents the 3D view of surface topography of TiO2-LPE thin films, which inferred that the abundant small bumps correspond to the anatase TiO2 nanobelt on which large bumps (TiO2 flowers) grow homogeneously. (b) Surface roughness spectra of TiO2-LPE thin films, which depicted the slight surface roughness, which is beneficial for electrical switching to avoid leakage of current, which may occur because of surface defects. (c) Phase diagram replicates the distance of TiO2 flowers from each other. (d) Surface waviness reveals the smooth regularity in the pattern.

Fourier transform infrared (FTIR) spectroscopy was carried out to identify surface constitutional functional groups of green-synthesized TiO2-LPE-based thin films. The FTIR spectrum was recorded from a range of 500–1200 cm–1 for TiO2-LPE-based thin films as shown in Figure 3a. The investigation shows that the characteristics of TiO2-LPE are associated with pectin of lime peel and anatase TiO2 with corresponding peaks to relative functional groups. From the FTIR spectrum, several peaks at 2886, 2345, 1470, 1276, 1125, 947, and 840 cm–1 could be observed. Among all, the most intensive band appeared at 2886 cm–1 and represents the stretching of hydroxyl (−CH) alkenes of the carbohydrates.31,39 The observed peak at 2345 cm–1 represents a vibrational −CH2 band, and 1276 and 1470 cm–1 represent stretching bands of potassium ions of LPE.24,44,45 The vibrational bands from 1300–1200 cm–1 depict the vibration of the methoxy group O–CH3 and the alcohol group C-OH that are responsible constituents, which precede the fundamental structure of lime peel. A stretching band at 1125 cm–1 is associated with Ti–O–Ti. Whereas a band at 947 cm–1 shows bond stretching of O–Ti–O, that at 840 cm–1 corresponds to stretching of Ti–O. Figure 3b shows Raman spectra of green-synthesized anatase TiO2-LPE-based thin films. The observed Raman bands for LPE-TiO2 appeared at 188, 390, and 625 cm–1 respectively. Among these characteristic peaks, the band that appeared at 625 cm–1 belongs to carotenoids (class of flavonoids). Carotenoids impart color to citrus fruits and are permissible to π-π* transition, which appears in the visible region. Sharp bumps at 390 and 188 cm–1 are associated with the different vibrating mode of anatase TiO2. These vibrating modes of anatase TiO2 spread over the wavenumber zones covering some typical vibration peaks, which is the confirmation of the crystalline nature of TiO2. The peak at 1297 cm–1 corresponds to potassium ions, which are present in LPE.46 It is inferred that the Raman spectrum of anatase TiO2 exhibits weak bonds compared to the rutile phase because of two-phonon scattering, whereas in the rutile phase, second-order scattering is more intense.39

Figure 3.

Figure 3

Open in a new tab

(a) Shows the FTIR spectra of green-synthesized anatase TiO2 mediated by LPE, which exposed the functional group of TiO2-LPE thin films. (b) Raman scattering reveals that the green-synthesized anatase TiO2 covering the specific frequencies modes because of two-photon scattering and the class of flavonoids (carotenoids) allows the π- π* transition in the visible region and anatase TiO2-associated characteristics.

For detailed investigation of defect chemistry of thin films in terms of intrinsic and extrinsic defects or surface dangling bonds, photoluminescence (PL) spectroscopy is employed. It offers deep understanding of charge recombination and charge excitation. During the synthesis of anatase TiO2-LPE thin films, lattice or point defects such as oxygen vacancies or interstitial defects are generated, which are responsible for dielectric behavior.47Figure 4a represents the PL spectra of LPE-based thin films on a FTO glass substrate at 10s laser exposure time and using 457 nm excitation wavelength. Generally, PL emission intensity is very low in anatase TiO2 because it has an indirect band gap type of a semiconductor.39,47 Because TiO2 is a semiconductor with an indirect band gap, no band gap photoluminescence appears. However, with exposure of ultraviolet light, broad visible photoluminescence arose as a result of recombination of free carriers and oppositely trapped charges.28,32,48 For TiO2-LPE-based thin films, the rise in emission intensity could be because of interstitial and point defects and also may occur because of potassium shift.24 These additional defects were generated because of green synthesis of lime peel and possessing additional active sites for accepting electrons as compared to simple anatase TiO2. The spectrum has revealed several emission peaks centered at 596, 613, and 666 nm, respectively. Furthermore, the spectrum shifting to the visible region suggests more lattice or interstitial defects, which give birth to additional properties to the host material to build a tunable CF.24,28 The observed PL spectra of TiO2-LPE-based thin films comprise a single sharp intensity peak, which confirms the good quality of the synthesized material. The observed peak has a wavelength of about 613 nm, which lies in the visible region (yellow) corresponding to 2.03 eV optical bandgap of TiO2-LPE thin films. Both radiative recombination and nonradiative recombination of electron–hole pairs in the green-synthesized anatase TiO2 thin film are illustrated in Figure 4b. Mostly, during radiative recombination, the involvement of two types of defects causes the formation of a sharp intensity peak. Radiative transfer of mobile electrons may be caught at low bulk close to the valence band during electron trapping phenomena.28 Consequently, the electron being caught in the trapped state is bound to a structural defect that may be interstitial (Ti+3 and Ti+4). The trapped electron peaks lie in the 590–700 nm range corresponding to the red spectrum.28

Figure 4.

Figure 4

Open in a new tab

(a). Demonstrates the photoluminescence spectra of green-synthesized anatase TiO2-LPE thin films, which show the additional interstitial defects such as Ti+2 and Ti+3, respectively. (b) R and NR denote the radiative and nonradiative recombination of electron traps, hole traps, and electron holes in green-synthesized anatase TiO2-LPE thin films.

Resistive switching characteristics of the Ag/TiO2-LPE/FTO-based memory cell were scrutinized by a voltage drop from 0 to 4 V applied on the top Ag electrode along with a step-up voltage of 0.1 V, while the bottom FTO electrode was kept grounded during the forming process. The compliance current was kept as low as 0.01A to abstain from permanent breakage.22,27,28Figure 5a represents the complete resistive switching device behavior; it shows the immediate switching from the HRS to LRS at 3 V with an increase in current from 0A to 10 mA, which leads to the establishment of the CF. The whole switching cycle could be observed to follow the sweep voltage values from 0 V → −4 V → 4 V → 0 V alienated with a constant sweep of 0.1 V shown by the arrows. In the beginning, a linear switching behavior was observed up to a value of 2 V, and a rapid change in switching behavior was recorded. The device switched into the LRS at 3 V, and current leads up to 10 mA under appliance current, which was termed set state or ON state as anticipated in a sweeping cycle (sweep 1) from 0 V → −4 V. Noticeably, the device retained its LRS state under applied polarity of −4 V → +4 V as illustrated in sweep 2, which revealed the bipolar nature of the device. The abrupt change in resistance from the LRS to HRS was observed on applying reverse polarity from 4 V to 0 V, and the current reduced from 10 mA to 0A. Therefore, the threshold voltage for reset for the fabricated Ag/TiO2-LPE/FTO-based memory cell is −3 V as shown in sweep 3. The switching from the LRS to HRS is associated with the reset state or OFF state of the device. Therefore, 3 V/–3 V is a converging point, where both states could be achieved and procreated. Figure 5b represents the complete resistive switching cycle of the Ag/TiO2-LPE/FTO-based memory cell at the semilogarithmic scale. The resistance variance versus voltage sweep was also recorded to visualize the complete resistance variation profile of the Ag/TiO2-LPE/FTO-based memory cell as anticipated in Figure 5c. The resistance variation graph validates the huge difference of resistance in both states, and this difference controls the variability of the device. Figure 5d shows the voltage distribution of the Ag/TiO2-LPE/FTO-based memory cell. The mean values (μ) for both the LRS and HRS are clearly stated in the voltage distribution diagram, which turn out to be −3 V/3 V, respectively.

Figure 5.

Figure 5

Open in a new tab

(a). Demonstrates the complete resistive switching cycle of the green-synthesized Ag/TiO2-LPE/FTO-based memory cell, which inferred a stable and reproducible bipolar resistive switching response. (b) Typical IV characteristics of the Ag/TiO2-LPE/FTO-based memory cell on the logarithmic scale. (c) Resistance variation profile reveals how resistance response varies under electric stimuli. (d) Statistical voltage distribution of both set and reset states confirmed the control variability of the device.

The Ag/TiO2-LPE /FTO-based memory cell was characterized by some tests such as the cycle to cycle (C/C) test of the LRS and HRS such as the endurance test and retention test to determine its capability. Figure 6a demonstrates the cycle to cycle endurance test of the Ag/TiO2-LPE/FTO-based memory cell. While performing the endurance test, it came out that the device sustained its HRS and LRS with no apparent electrical deterioration over 1.5 × 103 cycles. The retention test of the Ag/TiO2-LPE /FTO-based memory cell was also carried out as illustrated in Figure 6b. The retention test revealed out that the Ag/TiO2-LPE /FTO-based memory cell perfectly preserved its HRS and LRS over >105 s with no apparent electrical degradation attributed to outstanding retention. The detailed comparison of switching parameters of the Ag/TiO2-LPE /FTO-based memory cell against other TiO2-based memory cells is provided in Table 1. Moreover, the Ag/TiO2-LPE/FTO resistive switching device exhibits an excellent ON/OFF ratio (>105) as compared to the recently reported TiO2-based resistive switching device.5

Figure 6.

Figure 6

Open in a new tab

(a). Represents the endurance test of the green-synthesized Ag/TiO2-LPE/FTO-based memory cell, which shows the stability between the LRS and HRS over 1500 cycles along without any electrical degradation; (b) fabricated memory cell exhibits retention over >105 s without any electrical deterioration.

Table 1. Demonstrates the Comparison of Green-Synthesized Defect-Abundant TiO2-LPE-Based Resistive Switching Performance over Other Previous TiO2-Based Investigations.

memory cell TiO2 growth technique I ∼ V curve RHRS/RLRS retention (s) endurance (cycles) ref.
Ag/TiO2/FTO drop casting bipolar ∼10 NA 100 (30)
Pt/Co-TiO2/FTO Dc sputtering bipolar ∼10 ∼105 8 × 102 (38)
Ti/TiOx-LCMO/Pt pulsed layer deposition bipolar ∼103 NA 80 (49)
Pt/TiO2/Pt TiO2 adhesive layer thermal oxidation unipolar ∼10 10 years NA (50)
Ag/TiO2-LPE/FTO hydrothermal method bipolar >105 ∼105 1500 this work
Open in a new tab

Resistive switching properties of the Ag/TiO2-LPE /FTO-based memory cell were measured by plotting current–voltage (iv) characteristic curves in both positive and negative sides on a log–log scale at room temperature. It was observed that the conducting properties of the Ag/TiO2-LPE /FTO-based memory cell appear to be higher than simple TiO2. It can be seen from the figure that in the voltage test range, the device obeys Ohmic conductance as explained in Figure 7a.

2. iv

Figure 7.

Figure 7

Open in a new tab

Provides the suitable slope of the logarithmic scale of the Ag /TiO2-LPE /FTO-dependent memory cell conductive model. (a) Positive area curve and (b) negative area resistant to the TiO2-LPE layer switching curve.

Here, Johm is the current density, μ is termed charge mobility, q is electronic charge, no represents the charge density,

and d is the active layer thickness.

Throughout the HRS, the behavior of conduction could be explained by a classical SCLC-model, which supports that defects in the Ag/TiO2-LPE/FTO memory cell will generate the conduction band just below traps and reject injected carriers. At the same time, there may occur an electrophile site where the current transmission complies with the law of the child square law in the chemical composition of green-synthesized anatase TiO2 mediated by LPE as demonstrated in Figure 7b.

2. v

The whole resistive switching phenomena could be explained by the well-known ECM conduction mechanism. The ECM is a quite mature theory as compared to other CF mechanisms.1,4,6,17,27 When positive voltage is applied on the Ag top layer, the Ag atoms get oxidized electrochemically at the interface of the active top electrode. The oxidation occurs with the generation of Ag → Ag1++ 1e–1 ions at the interface of the Ag/TiO2-LPE layer, and potassium ions of LPE present in anatase TiO2 also oxidize K → K1++ e–1 within the TiO2-LPE as shown in Figure 8a. Positively charged Ag1+ ions drift from the top electrode along highly conductive K1+ ions through Ti interstitial defects (Ti+3 and Ti+4). Both migrated ions combined and led toward the bottom electrode, which is chemically inert and reduced as Ag1+ + 1e– → Ag, and K1++ e–1 → K, respectively. At the value of threshold voltage (3 V), the population of both positive ions increases leading toward a path to the development of a conduction bridge (CB) or a high CF. This CF creates a connection between top and bottom electrodes, which may cause the device to switch into the LRS as illustrated in Figure 8b, whereas there is electrochemical deprivation or suspension at the feeble area of the CB matrix, which causes the breakdown of the filament leading the device to the HRS when reverse polarity is fed. The dissolution occurs because of charge attraction of positively charged ions under negative polarity as depicted in Figure 8c. The Ag/TiO2-LPE /FTO-based memory cell device demonstrates the improvement in stable resistive switching by the incorporation of K1+ ions, which combine to Ag+1 ions and exhibits an efficient switching response as compared to simple anatase TiO2 with high storage capability owing to its excellent combination of active metal ion transportation, which may facilitate the stable and tunable CF of the Ag/TiO2-LPE/FTO-based memory cell device. The described conduction mechanism strengthens the electrochemical metallization theory owing to incorporation of its highly conductive K1+ ions along Ag+1 ions as compared to recently published VCM-based TiO2 thin film equipped resistive switching behavior.

Figure 8.

Figure 8

Open in a new tab

(a–c) Defect chemistry of green-synthesized Ag/TiO2-LPE/FTO followed by ECM phenomena with the incorporation of K+1 ions with active Ag+1 ions through Ti interstitial defects: (a) migration of active metal positive ions leading to the electroforming process and (b) shows CF formation of the TiO2-LPE-based memory cell under positive sweep, and both electrodes are connected to each other via this conduction channel, and the device switched into the LRS. (c) Reveals the electrochemical dissolution under negative sweep because of the bipolar nature of the device, and the rupture or dissolution occurs at the interface of the TiO2-LPE and Ag top electrode; as a result, the device would be switched into the HRS.

The mediation of LPE in anatase TiO2 NPs introduces soluble sugars and insoluble polysaccharides, polyphenols, carboxylic groups, and amino acids, and citric acid acts as a stabilizing agent and promotes antibacterial ability of the green-synthesized NPs.51 Recent studies demonstrate that green synthesis of TiO2 NPs via different plant extracts revealed outstanding antibacterial activity against various pathogens, which may cause many serious infectious diseases.52−55 The nanoparticle mechanism is still hypothetically toward the bactericidal effect, and it has not been fully understood until now. Different mechanisms have been proposed to explain the antibacterial effect of nanoparticles. Except large area / volume ratio, the green-synthesized NPs exhibit better contact with bacterial cells, and the antibacterial effect formed because of oxidative stress caused by the free-state radical formation.56−58 The superior bioactivity of mediation of LPE against bacterial pathogens as compared to other green syntheses of TiO2 NPs is discussed in the Supporting Information (ESI discussion and Table S1). Antimicrobial tests of TiO2-LPE NPs (20, 50, and 80 ug / ml) with different doses toward Gram-positive and Gram-negative bacterial strains have been performed using an agar well diffusion process. In vitro this approach was implemented to promote awareness that TiO2-LPE anatase can cure many infectious disorders caused by bacterial pathogens. Such pathogens have been identified and recognized by scientists at the Lahore Scientific and Industrial Research Complex and Laboratories Council of Pakistan. The shown statistics indicate that the in vital studies of TiO2-LPE NPs have resulted in a strong dose-dependent antimicrobial activity. The control medium used was dimethyl sulfur oxide (DMSO) and the standard antibiotic used in the study of antimicrobial agents was tetracycline. The TiO2-LPE NP amount has been held at 100 μL, while antibiotic concentrations are 0.005 mg / ml. The antibacterial inhibition region of TiO2-LPE NPs increases with four bacterial strains as shown in Figure 9 because of the existence of the antioxidant quality of flavonoids in LPE.

Figure 9.

Figure 9

Open in a new tab

(a–d) TiO2-LPE NP antibacterial activity in various strains at different doses with a positive and negative regulation. (a) E. coli(b) K. pneumonia, (c) B. subtilis, and (d). S. aureus.

3. Conclusions

In summary, LPE-mediated anatase TiO2-based thin films were fabricated using the one-pot hydrothermal route. The defect-abundant thin films were characterized briefly and exhibited unique additional features such as more point defects or active interstitial sites as compared to simple anatase TiO2 thin films. The vibrational or stretching modes of the green-synthesized anatase TiO2 thin films in the presence of active potassium sites and the bioactive flavonoid, quercetin, were induced because of the LPEs are exposed by FTIR and Raman spectroscopy. The PL spectra of TiO2-LPE thin films reveal the presence of additional interstitial defects, such as Ti+3 and Ti+4, which may allow the Ag+1 ion to migrate between the anode and cathode with the aid of active highly conductive K1+ ions. The surface morphology depicted the homogeneous growth of TiO2 flowers over the TiO2 NP bed. The Ag /TiO2-LPE / FTO-based memory cell demonstrates stable and reproducible bipolar resistive switching behavior under low voltage (SET / Reset, 3 V/–3 V) and a high ON/OFF ratio (>105) under 10 mA compliance current. Moreover, the green-synthesized memory cell shows unprecedented endurance over 1.5 × 103 consecutive cycles without inducing any electrical degradation in the retention test (105). Though TiO2 NPs are antibacterial but the incorporation of flavone-enriched LPE mediation enabled TiO2 to be a superior antibacterial agent against different infectious pathogens such as Gram-positive and Gram-negative strains.

4. Materials and Methods

4.1. Materials

From the eminent citrus valley (city of Sargodha) in Pakistan, fresh lime fruits were collected. The essential glass substrate (16 Ω/cm2) of fluorine-doped tin oxide (FTO) was bought from Sigma-Aldrich. Chemicals such as titanium tetra isopropoxide (TTIP) (98.5% Dae-Jung) and hydrochloric acid were used, and absolute ethanol was bought from Aladdin Chemical Reagent Co., Ltd. All chemical reagents were employed without additional purification. The reactions and manipulations were conducted in air and ignoring the nitrogen atmosphere.

4.2. Preparation of Lime Peel Extract

Lime fruits, a dietary supplement, have been effectively used in various fields such as in medicines, optoelectronics, antibacterial agents, and much more. Having peculiar flavonoid compounds such as quercetin, phenolic compounds, etc., they came forth as a potential candidate for green synthesis. The collected lime peel was cut transversely, and the surface was cleaned and washed with double deionized water many times. Afterward, the cleaned lime peel was dehumidified under sunlight at room temperature and further ground with a mortar pestle to obtain a powdered form. An extract of lime peel was obtained by the maceration method (herbal: DI water) at 1:1 (w/w) ratios requiring continuous stirring for 48 h at room temperature. The extracted powder was obtained after the filtration process in which raw materials were evaporated at 40 °C under reduced pressure and frozen at 4 °C for next processes. The obtained LPE powder was dissolved in DI water and further made a dilution of 200 mg/mL. A Metrohm (827-lab pH meter) was utilized to analyze the pH of the final concentration. By evaluating the pH of the lime extract, dilution was 7.98. The final refined powder contains flavonoids that are quercetin, isoquercetin, rutin, and pectin. Furthermore, a high concentration of pectin also plays a significant role toward resistive switching behavior.22

4.3. Green Synthesis of TiO2 NPs and Thin Films

Green synthesis of anatase TiO2 NPs mediated by LPE was performed using a simple solvothermal route. The TiO2 precursor (titanium tetra isopropoxide known as TTIP, 98.5% Dae-Jung). The transparent FTO glass was cleaned thoroughly using an ultrasonic bath for 20 min with a blended solution of 2 propanol and acetone at a volume ratio of 1:1. The following factors affecting the defective chemistry of solvothermally grown nanostructures are (a) precursor concentration of the synthesized solution, (b) treatment time, and (c) treatment temperature. For the synthesis of anatase TiO2, 15 mL of deionized water and HCl were mixed with the ratio (1:1). Afterward, TTIP (1.2 mL) and LPE powder (1 g/mL) was added to the synthesized solution using a herbal to metal ratio (1:1) under continuous stirring for 1 h. Lime peel has a property of being a base material for alkaline medium. Later, the obtained solution was moved into a Teflon vessel, and the cleaned FTO substrate was placed in a way that its conducting side is faced up.28 The vessel is sealed airtight and closed into an autoclave. Then the autoclave was given a slowly raising temperature of 5 °C per minute up to 150 °C in a convection oven for 3 h. Then, it was allowed to cool after achieving room temperature; the substrate was taken out of the autoclave and allowed to dry at 60 °C for an hour in an open air atmosphere. A circular shaped silver (Ag) contact area (8.5 mm2) coated by conductive silver paste with the help of a brush was used for the top electrode. The Ag contact coated area was cured at 80 °C to avoid short circuit. The whole process of green synthesis of TiO2 NPs mediated by LPE was repeated for TiO2-LPE powder to investigate antibacterial activities. The detailed material characterization tool is discussed in the Supporting Information.

4.4. Antibacterial Activity

A well-known diffusion technique was employed to investigate the antibacterial activity of green-synthesized TiO2 NPs mediated by LPE against both Gram-positive and Gram-negative strains. These strains of bacteria were cultured on Muller-Hinton agar plates, i.e., Klebsiella pneumonia, Bacillus subtilis, Escherichia coli, and Staphylococcus aureus, respectively. By employing a germ-free cotton swab, incubated bacterial cultures (107/mL) were washed homogenously. Different concentrations of (10e40 mg/mL) green-synthesized TiO2-LPE NPs were poured into each well of all the plates. The bacterial swab plates at 37 °C temperature were incubated for 48 h. A clear zone emerged after incubation, which was termed the zone of inhibition (Table 2). For the positive control, tetracycline as a standard antibiotic was utilized.

Table 2. All Experiments Were Replicated Three Times with Their Mean Values ± Standard Deviations.

  zone of inhibition (mm)
  gram-positive
 gram-negative
TiO2-LPE NP concentrations E. coli K. pneumonia B. subtilis S. aureus
20 ug/ml 5 ± 3 13 ± 1 12 ± 3 6 ± 1
50 ug/ml 10 ± 1 17 ± 2 14 ± 2 13 ± 2
80 ug/ml 20 ± 0 21 ± 1 19 ± 2 21 ± 1
DMSO 0 10 ± 2 0 9 ± 2
tetracycline 30 ± 1 30 ± 3 33 ± 2 10 ± 1
Open in a new tab

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group No (RG- 1438-089).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02410.

  • Information on material characterization tools such as XRD, SEM, AFM, PL, FTIR, and IV source measurement unit; detailed discussion regarding antibacterial activity; and antibacterial activity comparison of lime peel-mediated TiO2 nanoparticles versus other (plants and flowers) green-synthesized TiO2 nanoparticles (PDF)

Author Contributions

∇ M.S.A. and M.S.I. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ao0c02410_si_001.pdf (438.7KB, pdf)

References

  1. Chang T.-C.; Chang K.-C.; Tsai T.-M.; Chu T.-J.; Sze S. M. Resistance random access memory. Mater. Today 2016, 19, 254–264. 10.1016/j.mattod.2015.11.009. [DOI] [Google Scholar]
  2. Feng P.; Chao C.; Wang Z. S.; Yang Y.-C.; Jing Y.; Fei Z. Nonvolatile resistive switching memories-characteristics, mechanisms and challenges. Prog. Nat. Sci.: Mater. Int. 2010, 20, 1–15. 10.1016/S1002-0071(12)60001-X. [DOI] [Google Scholar]
  3. Pan F.; Gao S.; Chen C.; Song C.; Zeng F. Recent progress in resistive random access memories: materials, switching mechanisms, and performance. Mater. Sci. Eng. R 2014, 83, 1–59. 10.1016/j.mser.2014.06.002. [DOI] [Google Scholar]
  4. Zhu L.; Zhou J.; Guo Z.; Sun Z. An overview of materials issues in resistive random access memory. J. Materiomics 2015, 1, 285–295. 10.1016/j.jmat.2015.07.009. [DOI] [Google Scholar]
  5. Xue D.; Song H.; Zhong X.; Wang J.; Zhao N.; Guo H.; Cong P. Flexible resistive switching device based on the TiO2 nanorod arrays for non-volatile memory application. J ALLOY COMPD 2020, 822, 153552. 10.1016/j.jallcom.2019.153552. [DOI] [Google Scholar]
  6. Kügeler C.; Rosezin R.; Linn E.; Bruchhaus R.; Waser R. Materials, technologies, and circuit concepts for nanocrossbar-based bipolar RRAM. Appl. Phys. A: Mater. Sci. Process. 2011, 102, 791–809. 10.1007/s00339-011-6287-2. [DOI] [Google Scholar]
  7. Waser R.; Aono M. Nanoionics-based resistive switching memories. J. Nanosci. Nanotechnol. 2007, 6, 833–840. 10.1038/nmat2023. [DOI] [PubMed] [Google Scholar]
  8. Waser R.; Dittmann R.; Staikov G.; Szot K. Redox-based resistive switching memories–nanoionic mechanisms, prospects, and challenges. Adv. Mater. 2009, 21, 2632–2663. 10.1002/adma.200900375. [DOI] [PubMed] [Google Scholar]
  9. Karakulak E.; Mutlu R.; Uçar E. Sneak path current equivalent circuits and reading margin analysis of complementary resistive switches based 3D stacking crossbar memories. Inf. MIDEM 2015, 44, 235–241. [Google Scholar]
  10. Li H.; Chen Z.; Ma W.; Gao B.; Huang P.; Liu L.; Liu X.; Kang J. Nonvolatile logic and in situ data transfer demonstrated in crossbar resistive RAM array. IEEE Electron Device Lett. 2015, 36, 1142–1145. 10.1109/LED.2015.2481439. [DOI] [Google Scholar]
  11. Lim S.; Sung C.; Kim H.; Kim T.; Song J.; Kim J.-J.; Hwang H. Improved synapse device with MLC and conductance linearity using quantized conduction for neuromorphic systems. IEEE Electron Device Lett. 2018, 39, 312–315. 10.1109/LED.2018.2789425. [DOI] [Google Scholar]
  12. Wang Z.-R.; Su Y.-T.; Li Y.; Zhou Y.-X.; Chu T.-J.; Chang K.-C.; Chang T.-C.; Tsai T.-M.; Sze S. M.; Miao X.-S. Functionally complete Boolean logic in 1T1R resistive random access memory. IEEE Electron Device Lett. 2016, 38, 179–182. 10.1109/LED.2016.2645946. [DOI] [Google Scholar]
  13. Zhou J.; Kim K.-H.; Lu W. Crossbar RRAM arrays: Selector device requirements during read operation. IEEE Trans. Electron Devices 2014, 61, 1369–1376. 10.1109/TED.2014.2310200. [DOI] [Google Scholar]
  14. Ebenhoch C.; Kalb J.; Lim J.; Seewald T.; Scheu C.; Schmidt-Mende L. Hydrothermally Grown TiO2 Nanorod Array Memristors with Volatile States. ACS Appl. Mater. Interfaces 2020, 12, 23363–23369. 10.1021/acsami.0c05164. [DOI] [PubMed] [Google Scholar]
  15. Chang K.-M.; Tzeng W.-H.; Liu K.-C.; Chan Y.-C.; Kuo C.-C. Investigation on the abnormal resistive switching induced by ultraviolet light exposure based on HfOx film. MICROELECTRON RELIAB 2010, 50, 1931–1934. 10.1016/j.microrel.2010.05.012. [DOI] [Google Scholar]
  16. Robertson J.; Gillen R. Defect densities inside the conductive filament of RRAMs. Microelectron. Eng. 2013, 109, 208–210. 10.1016/j.mee.2013.03.010. [DOI] [Google Scholar]
  17. Wong H.-S. P.; Lee H.-Y.; Yu S.; Chen Y.-S.; Wu Y.; Chen P.-S.; Lee B.; Chen F. T.; Tsai M.-J. Metal–oxide RRAM. P IEEE 2012, 100, 1951–1970. 10.1109/JPROC.2012.2190369. [DOI] [Google Scholar]
  18. Zheng L.; Sun B.; Mao S.; Zhu S.; Zheng P.; Zhang Y.; Lei M.; Zhao Y. Metal ions redox induced repeatable nonvolatile resistive switching memory behavior in biomaterials. ACS Appl. Bio Mater. 2018, 1, 496–501. 10.1021/acsabm.8b00226. [DOI] [PubMed] [Google Scholar]
  19. Lim Z. X.; Sreenivasan S.; Wong Y. H.; Zhao F.; Cheong K. Y. Filamentary conduction in aloe vera film for memory application. Procedia Eng. 2017, 184, 655–662. 10.1016/j.proeng.2017.04.133. [DOI] [Google Scholar]
  20. Wang Z.; Nminibapiel D.; Shrestha P.; Liu J.; Guo W.; Weidler P. G.; Baumgart H.; Wöll C.; Redel E. Resistive switching nanodevices based on metal–organic frameworks. ChemNanoMat 2016, 2, 67–73. 10.1002/cnma.201500143. [DOI] [Google Scholar]
  21. Liu J.; Wöll C. Surface-supported metal–organic framework thin films: fabrication methods, applications, and challenges. Chem. Soc. Rev. 2017, 46, 5730–5770. 10.1039/C7CS00315C. [DOI] [PubMed] [Google Scholar]
  22. Wang X.; Tian S.; Sun B.; Li X.; Guo B.; Zeng Y.; Li B.; Luo W. From natural biomaterials to environment-friendly and sustainable nonvolatile memory device. Chem. Phys. 2018, 513, 7–12. 10.1016/j.chemphys.2018.06.013. [DOI] [Google Scholar]
  23. Clima S.; Sankaran K.; Chen Y. Y.; Fantini A.; Celano U.; Belmonte A.; Zhang L.; Goux L.; Govoreanu B.; Degraeve R. RRAMs based on anionic and cationic switching: a short overview. PHYS STATUS SOLIDI-R 2014, 8, 501–511. 10.1002/pssr.201409054. [DOI] [Google Scholar]
  24. Chen H.; Gal Y.-S.; Kim S.-H.; Choi H.-J.; Oh M.-C.; Lee J.; Koh K. Potassium ion sensing using a self-assembled calix [4] crown monolayer by surface plasmon resonance. SENSOR ACTUAT B-CHEM 2008, 133, 577–581. 10.1016/j.snb.2008.03.038. [DOI] [Google Scholar]
  25. Song F.; Bai L.; Moysiadou A.; Lee S.; Hu C.; Liardet L.; Hu X. Transition metal oxides as electrocatalysts for the oxygen evolution reaction in alkaline solutions: an application-inspired renaissance. J. Am. Chem. Soc. 2018, 140, 7748–7759. 10.1021/jacs.8b04546. [DOI] [PubMed] [Google Scholar]
  26. Woo J.; Lee W.; Park S.; Kim S.; Lee D.; Choi G.; Cha E.; Hyun Lee J.; Young Jung W.; Gyung Park C.. Multi-layer tunnel barrier (Ta2 O5/TaO x/TiO2) engineering for bipolar RRAM selector applications, Symposium on VLSI Technology. IEEE: 2013; pp T168-T169.
  27. Acharyya D.; Hazra A.; Bhattacharyya P. A journey towards reliability improvement of TiO2 based resistive random access memory: a review. MICROELECTRON RELIAB 2014, 54, 541–560. 10.1016/j.microrel.2013.11.013. [DOI] [Google Scholar]
  28. Irshad M. S.; Abbas A.; Qazi H. H.; Aziz M. H.; Shah M.; Ahmed A.; Idrees M. Role of point defects in hybrid phase TiO2 for resistive random-access memory (RRAM). Mater. Res. Express 2019, 6, 076311 10.1088/2053-1591/ab17b5. [DOI] [Google Scholar]
  29. Jensen S. C.; Friend C. M. The dynamic roles of interstitial and surface defects on oxidation and reduction reactions on titania. Top. Catal. 2013, 56, 1377–1388. 10.1007/s11244-013-0135-x. [DOI] [Google Scholar]
  30. Sahu D. P.; Jammalamadaka S. N. Remote control of resistive switching in TiO 2 based resistive random access memory device. Sci. Rep. 2017, 7, 1–8. 10.1038/s41598-017-17607-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Amanulla A. M.; Sundaram R. Green synthesis of TiO2 nanoparticles using orange peel extract for antibacterial, cytotoxicity and humidity sensor applications. Mater. Today 2019, 8, 323–331. 10.1016/j.matpr.2019.02.118. [DOI] [Google Scholar]
  32. Riga J.; Tenret-Noel C.; Pireaux J.-J.; Caudano R.; Verbist J.; Gobillon Y. Electronic structure of rutile oxides TiO2, RuO2 and IrO2 studied by X-ray photoelectron spectroscopy. Phys. Scr. 1977, 16, 351. 10.1088/0031-8949/16/5-6/027. [DOI] [Google Scholar]
  33. Annadhasan M.; Muthukumarasamyvel T.; Sankar Babu V.; Rajendiran N. Green synthesized silver and gold nanoparticles for colorimetric detection of Hg2+, Pb2+, and Mn2+ in aqueous medium. ACS Sustainable Chem. Eng. 2014, 2, 887–896. 10.1021/sc400500z. [DOI] [Google Scholar]
  34. Dhanavade M. J.; Jalkute C. B.; Ghosh J. S.; Sonawane K. D. Study antimicrobial activity of lemon (Citrus lemon L.) peel extract. Br. J. Pharmacol. & Tox 2011, 2, 119–122. [Google Scholar]
  35. Irshad M. S.; Aziz M. H.; Fatima M.; Rehman S. U.; Idrees M.; Rana S.; Shaheen F.; Ahmed A.; Javed M. Q.; Huang Q. Green synthesis, cytotoxicity, antioxidant and photocatalytic activity of CeO2 nanoparticles mediated via orange peel extract (OPE). Mater. Res. Express 2019, 6, 0950a4 10.1088/2053-1591/ab3326. [DOI] [Google Scholar]
  36. Roy A.; Bulut O.; Some S.; Mandal A. K.; Yilmaz M. D. Green synthesis of silver nanoparticles: biomolecule-nanoparticle organizations targeting antimicrobial activity. RSC Adv. 2019, 9, 2673–2702. 10.1039/C8RA08982E. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Yang Z.; Hao X.; Chen S.; Ma Z.; Wang W.; Wang C.; Yue L.; Sun H.; Shao Q.; Murugadoss V. Long-term antibacterial stable reduced graphene oxide nanocomposites loaded with cuprous oxide nanoparticles. J. Colloid Interface Sci. 2019, 533, 13–23. 10.1016/j.jcis.2018.08.053. [DOI] [PubMed] [Google Scholar]
  38. Hirose S.; Nakayama A.; Niimi H.; Kageyama K.; Takagi H. Improvement in Resistance Switching and Retention Properties of Pt/TiO2 Schottky Junction Devices. J. Electrochem. Soc. 2011, 158, H261–H266. 10.1149/1.3529247. [DOI] [Google Scholar]
  39. Reddy K. M.; Reddy C. G.; Manorama S. Preparation, characterization, and spectral studies on nanocrystalline anatase TiO2. J. Solid State Chem. 2001, 158, 180–186. 10.1006/jssc.2001.9090. [DOI] [Google Scholar]
  40. Taleb A.; Mesguich F.; Onfroy T.; Yanpeng X. Design of TiO2/Au nanoparticle films with controlled crack formation and different architectures using a centrifugal strategy. RSC Adv. 2015, 5, 7007–7017. 10.1039/C4RA13829E. [DOI] [Google Scholar]
  41. Chen C. Y.; Goux L.; Fantini A.; Redolfi A.; Clima S.; Degraeve R.; Chen Y. Y.; Groeseneken G.; Jurczak M.. Understanding the Impact of Programming Pulses and Electrode Materials on the Endurance Properties of Scaled Ta2 O5 RRAM Cells. In 2014 IEEE International Electron Devices Meeting; 2014; pp 12–14.
  42. Buckwell M.; Montesi L.; Hudziak S.; Mehonic A.; Kenyon A. J. Conductance tomography of conductive filaments in intrinsic silicon-rich silica RRAM. Nanoscale 2015, 7, 18030–18035. 10.1039/C5NR04982B. [DOI] [PMC free article] [PubMed] [Google Scholar]
  43. Sartore M.; Pace R.; Faraci P.; Nardelli D.; Adami M.; Ram M. K.; Nicolini C. Controlled-Atmosphere Chamber for Atomic Force Microscopy Investigations. Rev. Sci. Instrum. 2000, 71, 2409–2413. 10.1063/1.1150628. [DOI] [Google Scholar]
  44. Giovannitti A.; Nielsen C. B.; Rivnay J.; Kirkus M.; Harkin D. J.; White A. J. P.; Sirringhaus H.; Malliaras G. G.; McCulloch I. Sodium and Potassium Ion Selective Conjugated Polymers for Optical Ion Detection in Solution and Solid State. Adv. Funct. Mater. 2016, 26, 514–523. 10.1002/adfm.201503791. [DOI] [Google Scholar]
  45. Arslanoglu H.; Altundogan H. S.; Tumen F. Preparation of Cation Exchanger from Lemon and Sorption of Divalent Heavy Metals. Bioresour. Technol. 2008, 99, 2699–2705. 10.1016/j.biortech.2007.05.022. [DOI] [PubMed] [Google Scholar]
  46. de Moraes Barros H. R.; de Castro Ferreira T. A. P.; Genovese M. I. Antioxidant Capacity and Mineral Content of Pulp and Peel from Commercial Cultivars of Citrus from Brazil. Food Chem. 2012, 134, 1892–1898. 10.1016/j.foodchem.2012.03.090. [DOI] [PubMed] [Google Scholar]
  47. Choudhury B.; Choudhury A. Tailoring Luminescence Properties of TiO2 Nanoparticles by Mn Doping. J. Lumin. 2013, 136, 339–346. 10.1016/j.jlumin.2012.12.011. [DOI] [Google Scholar]
  48. Liu B.; Zhao X.; Zhao Q.; He X.; Feng J. Effect of Heat Treatment on the UV--Vis--NIR and PL Spectra of TiO2 Films. J. Electron Spectros. Relat. Phenomena 2005, 148, 158–163. 10.1016/j.elspec.2005.05.003. [DOI] [Google Scholar]
  49. Liu X. J.; Li X. M.; Wang Q.; Yu W. D.; Yang R.; Cao X.; Gao X. D.; Chen L. D. Improved Resistive Switching Properties in Ti/TiOx/La0. 7Ca0. 3MnO3/Pt Stacked Structures. Solid State Commun. 2010, 150, 137–141. 10.1016/j.ssc.2009.09.032. [DOI] [Google Scholar]
  50. Cao X.; Li X.; Yu W.; Zhang Y.; Yang R.; Liu X.; Kong J.; Shen W. Structural Characteristics and Resistive Switching Properties of Thermally Prepared TiO2 Thin Films. J. Alloys Compd. 2009, 486, 458–461. 10.1016/j.jallcom.2009.06.175. [DOI] [Google Scholar]
  51. Fitri N.; Fatimah I.; Chabib L.; Fajarwati F. I. Formulation of Antiacne Serum Based on Lime Peel Essential Oil and in Vitro Antibacterial Activity Test against Propionibacterium Acnes. In AIP Conference Proceedings 2017, 1823, 20123. 10.1063/1.4978196. [DOI] [Google Scholar]
  52. Hudlikar M.; Joglekar S.; Dhaygude M.; Kodam K. Green Synthesis of TiO2 Nanoparticles by Using Aqueous Extract of Jatropha Curcas L. Latex. Mater. Lett. 2012, 75, 196–199. 10.1016/j.matlet.2012.02.018. [DOI] [Google Scholar]
  53. Tarafdar A.; Raliya R.; Wang W.-N.; Biswas P.; Tarafdar J. C. Green Synthesis of TiO2 Nanoparticle Using Aspergillus Tubingensis. Adv. Sci. Eng. Med. 2013, 5, 943–949. 10.1166/asem.2013.1376. [DOI] [Google Scholar]
  54. Rao K. G.; Ashok C. H.; Rao K. V.; Chakra C. S.; Rajendar V. Synthesis of TiO2 Nanoparticles from Orange Fruit Waste. Synthesis (Stuttg). 2015, 1, 82–90. [Google Scholar]
  55. Rao K. G.; Ashok C. H.; Rao K. V.; Chakra C. S.; Tambur P. Green Synthesis of TiO2 Nanoparticles Using Aloe Vera Extract. Int. J. Adv. Res. Phys. Sci 2015, 2, 28–34. [Google Scholar]
  56. Parham S.; Wicaksono D. H. B.; Bagherbaigi S.; Lee S. L.; Nur H. Antimicrobial Treatment of Different Metal Oxide Nanoparticles: A Critical Review. J. Chin. Chem. Soc. 2016, 63, 385–393. 10.1002/jccs.201500446. [DOI] [Google Scholar]
  57. Liu S.; Zeng T. H.; Hofmann M.; Burcombe E.; Wei J.; Jiang R.; Kong J.; Chen Y. Antibacterial Activity of Graphite, Graphite Oxide, Graphene Oxide, and Reduced Graphene Oxide: Membrane and Oxidative Stress. ACS Nano 2011, 5, 6971–6980. 10.1021/nn202451x. [DOI] [PubMed] [Google Scholar]
  58. Applerot G.; Lellouche J.; Lipovsky A.; Nitzan Y.; Lubart R.; Gedanken A.; Banin E. Understanding the Antibacterial Mechanism of CuO Nanoparticles: Revealing the Route of Induced Oxidative Stress. Small 2012, 8, 3326–3337. 10.1002/smll.201200772. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao0c02410_si_001.pdf (438.7KB, pdf)

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES