Films of titanate nanosheets (approx. 1.8-nm layer thickness and 200-nm size) having a lamellar structure can form electrolyte- filled semi-permeable channels containing tetrabutylammonium cations. By evaporation of a colloidal solution, persistent de- posits are readily formed with approx. 10- μm thickness on a 6- μm-thick poly(ethylene-terephthalate) (PET) substrate with a 20-μm diameter microhole. When immersed in aqueous solution, the titanate nanosheets exhibit a p.z.c. of −37 mV , consistent with the formation of a cation conducting (semi-permeable) deposit. With a sufficiently low ionic strength in the aqueouselectrolyte, ionic current rectification is observed (cationic diode behaviour). Currents can be dissected into (i) electrolyte cation transport, (ii) electrolyte anion transport and (iii) water heterolysis causing additional proton transport. For all types of electrolyte cations, a water heterolysis mechanism is observed. For Ca 2+and Mg2+ions, water heterolysis causes ion current blocking, presumably due to localised hydroxide-induced precipitation processes. Aqueous NBu 4+is shown to ‘invert ’the diode effect (from cationic to anionic diode). Potential for applications in desalination and/or ion sensing are discussed. Keywords Vo lt am me tr y .Ion valve .Ionic logic .Sensing .Nanostructure .Iontronics Introduction Ion binding and transport through microporous and mesopo- rous materials is of considerable importance in water purifica-tion and in membrane science [ 1,2]. Recent interest has arisenin particular in the porosity of new types of lamellar nano-structures (in particular graphene oxide [ 3,4]), which are linked to ion transport [ 5]a n dw a t e rt r a n s p o r t[ 6] phenomena, and the desire to exploit new types of lamellar 2D-materialsmore widely [ 7]. There are now ranges of novel materials available based, for example on 2D-nanosheets of modifiedcarbons [ 8], phosphorous [ 9], oxides [ 10], chalcogenides [ 11] and mixed oxides such as BiVO 4[12] and MXenes [ 13]. 2D- titanates have been of considerable interest and lamellar tita-nate deposits have been shown to be electrochemically andphoto-electrochemically active depending on the pH and in- tercalated guest species [ 14]. In this report, we focus on tita- nate nanosheet materials that have been developed in pioneering work by Sasaki and coworkers [ 15,16]. Titanate nanosheet materials are produced in two steps [17]; TiO 2reacts with caesium salts to give an intermediate solid lepidocrocite-type product, which is exfoliated intoaqueous solution containing alkylammonium cations (heretetrabutylammonium, [ 18]) to give a stable colloidal solution*Frank Marken f.marken@bath.ac.uk 1Department of Chemistry, University of Bath, Claverton Down,Bath BA2 7AY , UK 2Department of Chemistry, Faculty of Mathematics and Natural Sciences, Bogor Agricultural University, Bogor, West Java, Indonesia 3Energy Technology Research Group, Faculty of Engineering and theEnvironment, University of Southampton, Southampton SO17 1BJ, UK 4Materials and Chemical Characterisation Facility (MCC), University of Bath, Claverton Down, Bath BA2 7AY , UKJournal of Solid State Electrochemistry (2019) 23:1237– 1248 https://doi.org/10.1007/s10008-019-04199-4 of negatively charged titanate nanosheets. The surface charge of the colloid is associated with a point of zero charge (p.z.c.) at approximately pH 4 [ 19]. The colloidal solutions contain single unit cell thick titanate layers, which are typically 200 nm in diameter [ 20]. The deposits of this colloid when dried up (without heating) give a lamellar gel with typically 1.7 to 1.8 nm unit cell spacing (based on the X-ray diffraction pattern, [ 21]). This is consistent with an approximately 1.2- to 1.3-nm-thick titanate unit cell layer sandwiched between elec- trolyte layers of typically 0.5 nm. At elevated temperature, these types of gels can collapse back to the anatase crystal form but, at room temperature, these lamellar nano-structures persist. Applications of titanate nanosheets have been proposed as component in batteries [ 22], in heterogeneous catalysis [ 23], in photocatalysis [ 24] and as functional filler in polymer blends [ 25]. Recently, we have demonstrated that a film de- posit of titanate nanosheet material on a glassy carbon elec-trode allows ferroceneboronic acid to be immobilised and binding of fructose within the nano-lamellar space to be de- tected [ 20]. Further study has shown that the titanate deposits behave more similar to organic media (and not to hydrophilic oxides) with the ability to bind hydrophobic organic mole- cules (e.g. anthraquinone) from cyclopentanone and to allow redox conversion of these immobilised inter-lamellar redox systems [ 26]. The ability to host hydrophobic guests has been attributed to the presence of tetrabutylammonium cations within the electrolyte layer. In order to investigate ion transport through the lamellar titanate nanosheets, deposits of the titanate are studied here ona poly(ethylene-terephthalate) substrate (PET with 6- μm thickness) with a microhole (20- μm diameter). The experi- mental arrangement is very similar to that introduced byGirault et al. [ 27] for the study of liquid|liquid micro-inter- faces. Recently, other types of ionomer systems (such as re- constituted cellulose [28], Nafion™ [29], the commercial ionomer Fumasep ™[30], graphene oxide [ 31]a n dap o l y m e r of intrinsic microporosity [ 32]) have been investigated on this type of microhole substrate in order to reveal the mechanisms/ characteristics for ionic current rectification (or ‘ionic diode ’ phenomena). The ionic diode or ionic current rectification effect requires a semi-permeable material. This type of effect allows cation transport (cationic diodes for cation conductors) and anion transport (anionic diodes for anion conductors) tobe distinguished. New ideas for ionic current rectifier applica- tions have been proposed, for example for water purification [33] or for ionic diode sensing [ 34]. The ionic diode effect caused in asymmetrically coated microholes can be explained (at least to a first approximation)based on a combination of ion conductivity and concentrationpolarisation phenomena [ 35]. Figure 1provides a schematic summary of the key processes. With a symmetric ionomerdeposit on both sides of the microhole, transport of ions fromthe left or from the right compartment remains the same and a simple resistance is observed rather than rectifier behaviour. However, with semi-permeable ionomer deposited only onone side (here, always on the side of the working electrode in a classic four-electrode measurement cell [ 36]), a distinct change in current occurs with an ‘op en ’potential domain (due to accumulation of electrolyte in the microhole region; currentflows) and a ‘closed ’potential domain (due to depletion of electrolyte in the microhole region; current is blocked). This is illustrated in Fig. 1for the case of a cationic diode. Ionic diode phenomena are associated with processes that occur at different length scales due to potential dependent com-positional changes. There are d ifferent types of mechanisms, for example based on double-layer changes within nanopores,based on diffusion-migration lay er compositional changes and due to interfacial precipitation or blocking. The diffusion- migration layer –based processes in microhole diodes (as de- scribed in Fig. 1) can be contrasted with processes reported in work on nanochannel diodes [ 37], nano-cones [ 38]a n do n electrolytic microfluidic/nanofluidic channel diodes [ 39,40]. Other types of ionic diodes have been proposed based on gel- gel interfaces [ 41]. Ionic circuits have been proposed based on polymer gels [ 42,43]. Ionic diode switching effects have been demonstrated also due to interfacial precipitation [ 44]a n dd u e to pH gradients [ 45]. The term ‘iontronics ’has been coined by Chun and Chung [ 46] to emphasise the excitement associated with developments functionality, such as that in ionic amplifiers [47], transistors [ 48]o rf l i p - f l o p s[ 36]. More fundamentally, ionic rectifiers under AC-ex citation can be considered as ‘ion pumps ’. A device based on coupling cationic diodes and anion- ic diodes has been proposed for desalination applications [ 33]. Selectivity in ionic diode behaviour is desirable in order to broaden the possible range of applications. In particular, selec- tivity for higher valent cations such as Mg 2+and Ca2+could be very useful for water treatment applications. In this report, ion transport in the inter-lamellar space of tita- nate nanosheets is investigated. The semi-permeable nature of titanate nanosheet deposits (immersed in electrolyte with not toohigh ionic strength) allows cation transport and therefore causes cationic diode effects. This is studied here for a wider range of electrolytes and conditions. Inversion to anionic diode behaviouris observed in the presence of tetrabutylammonium cations. Experimental Chemical reagents Titanate nanosheet material was synthesised as described pre-viously by Sasaki et al. [ 21] and by Harito et al. [ 49,50]. Hydrochloric acid, sodium chloride, potassium chloride, lith- ium chloride, ammonium chloride, tetrabutylammonium chlo- ride, magnesium chloride, calcium chloride, sodium1238 J Solid State Electrochem (2019) 23:1237 –1248 hydroxide, sodium nitrate, sodium perchlorate and sodium sulphate were obtained in analytical grade from Sigma- Aldrich or Fischer Scientific and used without further purifi-cation. Phosphate buffer saline (PBS) was prepared with NaH 2PO4,N a 2HPO 4and NaCl and adjusted to pH 7. Solutions were prepared under ambient condition in volumet- ric flasks with ultrapure water with resistivity of 18.2 M Ωcm (at 22 °C) from an ELGA Purelab Classic System. Instrumentation Electrochemical data (voltammetry and chronoamperometry)were recorded at T= 20 ± 2 °C on a potentiostat system (Ivium Compactstat, The Netherlands). A classic four-electrode membrane electrochemical cell was used. The membrane sep- arates two tubular half-cells (15-mm diameter, see Fig. 2a), one with Pt wire working and saturated calomel (SCE) sense electrode and the other with SCE reference electrode and Pt wire counter electrode. In electrochemical measurements, theworking electrode was always located on the side of titanate nanosheet films. Fluorescence imaging was performed on a Carl Zeiss Confocal Scanning Microscope. For fluorescenceanalysis, rhodamine B was mixed with titanate nanosheetcolloidal solution, which was then applied to the PET films (see below). Procedures The zeta potential for the colloidal titanate solution was mea- sured on a Zetasizer Nano ZS (Malvern Instruments Ltd.,Malvern, UK) confirmed an average of −32.5 mV and an average size of 216 nm [ 26]. In order to form films of titanate nanosheets on PET substrates (20- μmd i a m e t e rh o l ei n6 - μm- thick PET from Laser Micromaching Ltd., Birmingham, UK), 10μL aqueous titanate nanosheet colloidal solution (2.56 g L −1) was applied to a PET film on a glass substrate (the glass was pre-coated with a thin layer of 1% agarose gelby solution casting to stop titanate nanosheet material pene- trating through the PET microhole). A volume of 10 μLt i t a - nate nanosheet colloidal solution was applied to the surfaceover PET to give a 1-cm 2film coating, which after drying produced a thin uniform coating. For the formation of sym- metric deposits on both sides of the PET film, the deposition was repeated on the back side of the PET. In order to image theTiO 2films, stacked fluorescence images were obtained for a symmetric (double-sided) and for the asymmetric (one-sided)deposits. Fig. 1 Schematic depiction of the case of a semi-permeable cation conductor causing a ‘cationic diode ’effect. In the closed state, the flow of cations is limited by electrolyte depletion within the cylindrical sectionof the PET film. In the open state, electrolyte accumulation within the cylindrical section occurs due to oversupply of cations through the semi-permeable cation conductorJ Solid State Electrochem (2019) 23:1237 –1248 1239 Titanate nanosheet film characterisation Fluorescence microscopy with a rhodamine B stain reveals the presence of a uniform ~ 5- to 10- μm-thick film of titanate nanosheets coated over a 20- μm diameter microhole in aPET substrate. Figure 3shows a multi-stack fluorescence mi- croscopy image as top view (with the focus on the PET layer) and as side view. For the symmetric diode (Fig. 3a), it can be seen that the 20- μm microhole was partially filled with tita- nate nanosheet material. It can be clearly observed that there Fig. 3 Fluorescence microscopy image stacks based on rhodamine B embedded into TiO 2nanosheet deposits asymmetric (applied from both sides) and basymmetric (applied form only one side) to the PET substrate with 20- μm diameter microhole Fig. 2 aSchematic description of the experiment with four-electrode control (WE, working electrode; SE, sense electrode; CE, counter electrode; RE, reference electrode) of the applied potential and TiO 2nanosheet deposit on the side of the working electrode. bA typical TEM image of the titanate nanosheet material is shown1240 J Solid State Electrochem (2019) 23:1237 –1248 are two layers of titanate nanosheets on opposite sides of the PET. However, for the asymmetric diode (Fig. 3b), the layer of titanate nanosheet material is seen on one side (ca. 5 –10μm thick). The extent of the deposit is not clearly resolved due tolight reflection effects. Results and discussion Ion transport through lamellar titanate nanosheet deposits: Na+transport Figure 4a shows typical cyclic voltammetry data over a po- tential range of + 4 to −4 Vapplied to the nanosheet coated or uncoated PET membrane immersed in 10 mM NaCl on both sides. Three cases are shown: (i) a PET film with an empty 20-μm diameter hole, (ii) a PET film with titanate nanosheets deposited symmetrically on both sides and (iii) a PET film with titanate nanosheets deposited asymmetrically only on one side. For the empty microhole, the currents are dominated by the specific resistivity (giving a constant resistance) of theaqueous NaCl electrolyte solution filling the microhole [ 51]. With titanate nanosheets applied symmetrically on both sides,the current decreases for both negative and positive appliedpotentials due to the titanate restricting the flow of ions. For the asymmetric case, current rectification behaviour is ob- served with a higher current generated at positive applied po-tentials and a lower current generated at negative applied po- tentials. Figure 1illustrates the corresponding cases of an open cationic diode (positive applied potential) and a closed cation- ic diode (negative applied potential) for the case of negatively charged titanate nanosheet material. Next, rectification phe- nomena are investigated with different concentrations of aque-ous NaCl electrolyte solution. Figure 4b shows cyclic voltammetry data for asymmetri- cally deposited titanate nanosheets. Rectified currents flowwith positive applied potentials and therefore cation conduc- tion must be possible through the titanate deposit. With the increasing concentration of NaCl from 1 to 1000 mM, a well-defined steady-state voltammetric response is observed. For experiments at lower NaCl concentration, the current in the negative potential range is lower compared to the current inthe positive potential range. This is consistent with cationic diode behaviour as seen in cases such as Nafion ™[29]a n d Fumasep™FKS-30 ionomer [ 30]. The effect of NaCl electro- lyte concentration on the currents and on the switching time for the cationic diode is shown more clearly in chronoamperometry data in Fig. 4c. From these data, the rectification ratio can be calculated (obtained by dividing theabsolute currents at + 1 V and at −1 V; see inset in Fig. 4b). An increase in NaCl concentration causes an increase in both the current in the open state and the current in the closed state. An optimum in rectification effect is observed in the range Fig. 4 aCyclic voltammograms (scan rate 50 mV s−1) for (i) an empty microhole, (ii) a symmetric titanate deposit and (iii) an asymmetric titanate deposit (microhole diameter 20 μm) immersed in 10 mM NaCl. bData for asymmetrically deposited titanate nanosheets showing cyclic voltammograms (scan rate 25 mV s−1) in aqueous NaCl and c chronoamperometry responses (stepping from + 1 to −1 V) immersed in aqueous NaCl with a concentration of (i) 1, (ii) 10, (iii) 50, (iv) 100, (v) 500, (vi) 1000 mM NaCl on both sides. Inset: rectification ratios fromchronoamperometry data at ± 1 VJ Solid State Electrochem (2019) 23:1237 –1248 1241 from 50 to 100 mM NaCl solution. The decrease in rectifica- tion ratio when going to higher ionic strength can be explained due to loss of semi-permeability and onset of anion transportthrough the titanate nanosheet film. Ion transport through lamellar titanate nanosheet deposits: electrolyte cations versus anions Next, the effects of electrolyte cations and anions on titanate nanosheet transport are investigated in order to better understand cationic diode processes. Figures 5a–bs h o was u m m a r yo fd a t a from cyclic voltammogram, chr onoamperometry and rectifica- tion ratio bar plots for aqueous 10 mM HCl, NaCl, KCl, LiCl,NH 4Cl, MgCl 2and CaCl 2. All salt systems show some conduc- tivity, but there are distinct changes depending on the type ofcation. Currents for the open diode are considerably higher inthe presence of monovalent cations such as H +,L i+,N a+,K+and NH 4+compared to those for divalent cations such as Mg2+and Ca2+. This could be consistent with a higher mobility for mono- valent cations relative to divalent cations. However, the ability of cations to bind/transfer is affected by water heterolysis and willbe investigated in more detail below. Among the monovalent cations, protons gave the highest currents and rectification ratio (Fig. 5a–b). Next, the effects of electrolyte anions are investigated. Figures 5c–d show a summary of data from cyclic voltam- mogram, chronoamperometry and a rectification ratio bar plotfor aqueous 10 mM NaOH, NaNO 3, NaCl, NaClO 4,N a 2SO4 and PBS pH 7. For all types of anions, similar levels of recti-fication are observed. Currents for the open diode (in the Fig. 5 Data for asymmetric titanate nanosheet deposits. aCyclic voltammograms (scan rate 25 mV s−1),bchronoamperometry data (stepping from + 1 to −1 V), immersed in aqueous (i) HCl, (ii) LiCl, (iii) NaCl, (iv) KCl, (v) NH 4Cl, (vi) MgCl 2and (vii) CaCl 2(10 mM on both sides). cCyclic voltammograms (scan rate 25 mV s−1),dchronoamperometry data (stepping from + 1 to −1 V), for films immersed in aqueous (i) NaOH, (ii) NaNO 3, (iii) NaCl, (iv) NaClO 4, (v) Na 2SO4and (vi) phosphate buffer saline pH 7 (all 10 mM on both sides). Insets show bar graphs for current rectification (see text)1242 J Solid State Electrochem (2019) 23:1237 –1248 positive potential range) become significantly higher for higher valent anions (SO 42−and PO 43−), but effects of the type of anion on the rectification ratio data seem insignificant. Clearly, anions at a sufficiently high concentration can enter into the inter-lamellar space and significantly change the rate of transport. This suggests that there could be always an un-derlying contribution from anion transport, even when the cation transport appears dominant. Sulphate and phosphate are likely to more strongly collapse the internal double layerand thereby to open up the nanochannels to both cation and anion transport. This can give rise to a higher ion current and less semi-permeability. Chronoamperometry data show rela-tively complex transients (rising and falling) with transient features mainly taking place in the first 0.5 s of the data. It is likely that during this period, not only concentration gradientsin the solution in the vicinity of the microhole develop, but also that concentration gradients within the nanosheet material change. The current traces meas ured during chronoamperometry may contain a component of ‘adsorption currents ’that are asso- ciated with compositional changes in the inter-lamellar space.Although electrolyte anion effe cts are clearly significant, it is currently difficult to disect these and to provide a more detailed assessment of the nature of these effects. Ion transport through lamellar titanate nanosheet deposits: competing cation and proton transport The observation of current throug h the titanate nanosheet depos- it can be associated generally with three types of processes based on (i) flow of cations, (ii) flow of anions or (iii) the potential-driven water heterolysis process leading to the formation of mobile protons and mobile hydroxide (with a resulting net pH gradient). The latter process has been reported for biological[52] and for bipolar membrane processes [ 53]. Catalysts for heterolytic water splitting such as graphene oxide [ 54] have been proposed to considerably enhance the potential-driven protonand hydroxide formation. In measurements as those reported in Fig. 5, it is not immediately clear whether water heterolysis c a nc o n t r i b u t et oo re v e nd o m inate the observed currents. One way of directly monitoring the simultaneous formation of protons and hydroxide is by placing a secondary working electrode (WE2) into the two-compartment cell. Figure 6 shows a schematic drawing with WE2 place in the vicinity of the membrane and in the left counter electrode compart- ment. Initially, a cyclic voltammetry experiment was per- formed (with the three electrodes in the left compartment) to observe the onset potential for hydrogen evolution. In Fig. 6a, data for 10 mM HCl are shown and the inset shows the onset at−0.4 V vs. SCE (indicated by an arrow). Next, the five- electrode membrane cell is operated in cyclic voltammetry mode with the collector (WE2) potential fixed at the onset potential. Transport of protons associated with the open diode (at positive applied potentials) causes an increase in protonconcentration at the location of WE2 and thereby leads to a current. Figure 6a shows both the generator current (WE1, mem- brane) and the collector current (WE2). The magnitude of thecollector current approaches that of the generator current (with the opposite sign) in the case of a pure proton current.Therefore, the data for the proton transport for 10 mM HCl in Fig. 6a provide proof for the methodology to work. Data in Fig. 6b are shown for 10 mM LiCl. Perhaps surprisingly, again a significant proton transport is observed with the col-lector current almost mirroring the generator current. This suggests that water heterolysis at titanate nanosheets is impor-tant and that the transport of Li +is likely to be accompanied by proton transport. Results for NaCl (Fig. 6c), KCl (Fig. 6d) and NH 4Cl (Fig. 6e) are very similar, and also suggest proton formation by heterolysis and transport through the titanate film. Data observed for MgCl 2(Fig. 6f) and for CaCl 2(Fig. 6g) are different not only with much lower proton fluxes, but alsomuch lower generator currents. The shape of the voltammetric signal for Ca 2+is more complex and indicative of some blocking of the diode as the potential is scanned into the pos- itive potential range. For both Mg2+and Ca2+, small transient peaks are seen on the negative going potential scan, whichcould be associated with an ‘unblocking ’process. These ob- servations are consistent with a blocking effect for Mg 2+and Ca2+caused by a localised pH changes at the titanate film surface. Water heterolysis occurs at the titanate nanosheets, and this then causes locally the formation of hydroxide whichthen in combination with Mg 2+or Ca2+causes blocking of the titanate nanochannels. However, the importance of this effectand the relative importance of proton versus electrolyte cationtransport through the titanate film are difficult to quantify from these data. Further evidence for these processes comes from direct pH measurements. In Fig. 7, a schematic drawing illustrates how two pH probes (glass membrane-based, battery controlled) are incor-porated into the four-electrode electrochemical cell. The pHprobes give a reading for the local pH (as an average) close to the PET film on the left and on the right. The photograph in Fig.7a shows the system before starting the experiment. With 10 mM NaCl in the left and right compartments, the pH is close to neutral. When operating the cationic diode at + 4 V (open), an average current of approximately 23 μA flows and the pH value start to divert. After 10 min of operation (see Fig.7b) the pH in the left compartment reaches 6.56 (more acidic) and in the right compartment 6.91 (less acidic). Figure 7c–d summarise pH data for experiments in 1 mM NaCl and in 10 mM NaCl. For a concentration of 100 mMNaCl, pH changes were less significant. It is possible to relatethese observations to the water heterolysis process. The effect of protonic current flow through the diode can be estimated in terms of an upper limit for the pH changes inJ Solid State Electrochem (2019) 23:1237 –1248 1243 the right/left compartment when assuming a fixed volume Vof ca. 0.01 dm3(an estimated volume where pH changes occur close to the membrane surface). Eqs. ( 1)a n d( 2) express the shift in pH (from the initial value pH initial) in the right com- partment (where protons are lost) and in the left compartment (where protons are gained). pHleft¼−log1010−pHinitialþIt=FV/C0/C1 ð1ÞpHright¼14þlog1010−14−pHinitialðÞþIt=FV/C16/C17 ð2Þ Parameters in these equations are the time-average absolute current I, the time t= 600 s and the Faraday constant F= 96,487 C mol−1. For the case of a 1 mM NaCl electrolyte (see Fig. 7c), the average current at an applied voltage of 4 V was 10 μA over 600 s, which (with pH initial= 6.39) translates to pHleft=5 . 2a n dp H right= 8.8. Comparison to the experimental results in Fig. 7c shows that the predicted trend is indeed Fig. 6 Schematic drawing showing the five-electrode cell with additional (WE2, 3-mm diameter platinum disk) collector electrode for sensing proton flux. Data sets for a10 mM HCl, b10 mM LiCl, c10 mM NaCl, d10 mM KCl, e10 mM NH 4Cl,f10 mM MgCl 2andg10 mM CaCl 2including the cyclic voltammogram (scan rate 50 mV s−1)a tW E 2showing onset of hydrogen evolution (blue arrow) and the generator-collector voltammogram (three consecutive potential cycles, scan rate50 mV s −1).hBar graph comparing the ratio of collector current and generator current (at + 4 V; in %) for different electrolyte media1244 J Solid State Electrochem (2019) 23:1237 –1248 observed. For a concentration of 10 mM NaCl (Fig. 7d), the observed average current was 23 μA at 4 V applied voltage. This gives pH left=4 . 8a n dp H right= 9.2. Experimental data sug- gest a less strong change in pH and therefore less competition between Na+transport and water heterolysis at the higher NaCl concentration. Additional experiments for 100 mM NaCl (not shown) suggest even lower pH drift even at higher currents. Water heterolysis appears to be strongly electrolyte concentra- tion dependent and probably insignificant at electrolyte concen-trations of 10 mM or higher. Although conclusive in terms of providing direct evidence for the water heterolysis process, the data provide only qualitative insights and further work will berequired to provide a quantitative measure of the water hetero- lysis process under these conditions. Ion transport through lamellar titanate nanosheet deposits: interference of NBu 4+with proton transport T h ep r e s e n c eo fN B u 4+cation in the original titanate host ma- terial is due to the exfoliation process requiring a bulky tetraalkylammonium reagent. When depositing titanate nano-sheets onto the PET film, NBu 4+cations are present and they define the ability of the titanate lamellae to bind to host mole-cules [ 24] and to conduct ions. It is therefore important to also investigate the effect of NBu 4+cations on the ionic current rectification and on the ion conductivity. Figure 8as h o w s membrane voltammetry data for 10 mM HCl electrolyte initial-ly in both sides of the two-compartment cell. The typical cat- ionic diode response is observed (Fig. 8a(i)). When adding NBu 4Cl into the right compartment, this can equilibrate with the titanate nanosheet deposit and change the ability of thelamellar structure to conduct ions. Data in Fig. 8a demonstrate a systematic decrease in ion current (and in rectification ratio) as the concentration of NBu 4Cl increases. Ultimately, when adding 50 mM NBu 4Cl, the ionic diode is inverted. The switch of the open diode state from positive potential to negative po-tential is a strong indication for a change from a cationic diode to an anionic diode. This is also confirmed in chronoamperometry data in Fig. 8b. The current transient shows a rising current when opening the diode. In the presence of 50 mM NBu 4Cl, the behaviour switches and the rising cur- rent transient is observed in the negative potential domain. It is possible to envisage the processes responsible for the ion transport in the lamellar stru cture based on the equilibration of competing cation in the titanate double layer. Both protonsand NBu 4+can be bound (Fig. 8c). In the absence of NBu 4+ vacancies exist for protons (or other cations) to bind and to conduct through the titanate deposit. However, when adding NBu 4+cations these bind and block the transport of other cat- ions (protons). For 10 mM NBu 4+cations both the current in the open state and the rectification ratio half. When addingmore NBu 4Cl, it is possible to ‘invert ’the diode behav- iour. This can be explained by excess of NBu 4Cl binding Fig. 7 Illustration of the experimental system. aPhotograph showing the experimental cell with the left compartment (counter electrode and reference electrode) and the right compartment (working electrode andsense electrode). In both compartments, an additional battery-driven pH sensor is added to probe the pH close to the PET film in the middle. b After 10 min + 4 Vand 23 μA current the pH has become more acidic on the left and more alkaline on the right. Bar graphs for the effect at 10 min +4Vf o r c1m MN a C l( 1 0 μA) and d10 mM NaCl (23 μA)J Solid State Electrochem (2019) 23:1237 –1248 1245 into the inter-lamellar sp ace and a net anion transport mechanism (due to mobile ch loride) being activated.More work will be needed to further exploit this switch from cationic to anionic current rectification. Summary and conclusions It has been shown that asymmetry in the deposition of a tita-nate nanosheet film can be used to induce current rectification phenomena or ionic diode effects, as long as the semi-permeable character due to cation transport is maintained. In this case, a ‘cationic diode ’has been produced with rectifica- tion effects observed for many types of cations. A cation va- cancy model is proposed based on NBu 4+cations occupying part of the inter-lamellar space with vacancies for other typesof cations to bind. The excess of negative charges on thetitanate allow cation binding and transport. The rectification ratio for NaCl electrolyte considerably de- creased when concentration of aqueous NaCl (or the ionicstrength) was increased on the side of titanate nanosheet film (consistent with anion uptake into the lamellar space and anion transport in addition to cation tra nsport at higher ionic strength). Currents for the open diode are shown to be associated with additional water hydrolysis and proton transport rather than pure Li +,N a+,K+,N H 4+,M g2+and Ca2+transport. In the case of Mg2+and Ca2+, this resulted in blocking of transport presumably due to precipitation effects in the presence of hydroxide. Waterheterolysis requires catalytic sites for the potential-driven disso-ciation reaction. Sharp edges at the 2D-titanate surfaces (causing high local field gradients) with a pK Aclose to 4 (resulting in active sites for field-driven wa ter dissociation) could provide well-suited reaction environments, but further study will be need-ed to confirm this. A more quantitative observation of the relative contributions of proton and electrolyte cation transport to thediode current will be required. NBu 4+cations have been shown to suppress cation conduction and ultimately ‘invert ’the diode to anion conduction due to an increase in positive charge in thelamellar space. In order to improve the understanding of these processes, in future, theory could be developed at both the atomic level and the meso-level to take in account localised field effectsas well as interfacial concentration gradient/polarisation effects that are caused by the current flow. Applications of titanate films could be possible (i) under low ionic strength conditions as a catalyst for water heterolysis[55] or (ii) under higher ionic strength conditions in desalina- tion. However, the ability of the lamellar structures to maintaineffective rectification at higher ionic strengths is currently poor. Therefore, the inter-lamellar space needs to be betterdesigned to maintain a higher rectification effect (by maintain- ing semi-permeability). This could be achieved by tuning the concentration and structure of the tetraalkylammonium guestcations. Further work will be directed toward the effects of inter-lamellar guests on the mechanism and the use of nano- sheet materials in desalination and sensing. Fig. 8 aAsymmetric TiO 2nanosheet data for cyclic voltammograms (scan rate 25 mV s−1) in aqueous electrolyte (for 10 mM HCl on the left and (i) 10 mM HCl, (ii) 10 mM HCl + 0.12 mM NBu 4Cl, (iii) 10 mM HCl + 0.37 mM NBu 4Cl, (iv) 10 mM HCl + 0.62 mM NBu 4Cl, (v) 10 mM HCl + 0.87 mM NBu 4Cl, (vi) 10 mM HCl + 1 mM NBu 4Cl, (vii) 10 mM HCl + 1.87 mM NBu 4Cl, (viii) 10 mM HCl + 3.12 mM NBu 4Cl, (ix) 10 mM HCl + 4.37 mM NBu 4Cl, (x) 10 mM HCl + 10 mM NBu 4C l ,( x i )1 0m MH C l + 5 0m MN B u 4Cl, on the right). b Chronoamperometry data (stepping from + 1 to −1 V) immersed in aqueous electrolyte. Inset: rectification ratio calculation from chronoamperometry data at ± 1 V. cSchematic description of the competition of protons and tetrabutylammonium cations1246 J Solid State Electrochem (2019) 23:1237 –1248 Acknowledgements B.R.P. would like to thank the Indonesian Endowment (LPDP RI) for a PhD scholarship. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appro- priate credit to the original author(s) and the source, provide a link to theCreative Commons license, and indicate if changes were made. Publisher ’sn o t e Springer Nature remains neutral with regard to jurisdic- tional claims in published maps and institutional affiliations.