Light induced proton pumping with a semiconductor: vision for PhotoProton lateral separation and robust manipulation

: Energy transfer reactions are the key for living open systems, biological chemical networking and development of life inspired nanoscale machineries. It is a challenge to find simple reliable synthetic chemical networks providing a localization of the time dependent flux of matter. In this paper we introduce a reliable, minimal reagent consuming, stable inorganic light promoted proton pump. Localized illumination was applied to a TiO 2 surface in solution for reversible spatially controlled “inorganic photoproton” isometric cycling, the lateral separation of water splitting reactions. The proton flux is pumped during irradiation of the surface of TiO 2 and dynamically maintained at the irradiated surface area in the absence of any membrane or predetermined material structure. Moreover we spatially predetermine a transient acidic pH on the TiO 2 surface in the irradiated area with feedback-driven generation of a base as deactivator. Importantly we describe, how to effectively monitor the spatial localization of the process by in situ scanning ion selective electrode technique (SIET) measurements for pH and scanning vibrating electrode technique (SVET) for local photoelectrochemical studies. This work shows

SIGNIFICANCE: Understanding how protons are spatiotemporally pumped in the system has a broad application in a wide range of pH-responsive adaptive materials. We introduce a reliable, minimal reagent consuming, stable inorganic light promoted proton pump with spatial and temporal separation of oxidation and reduction of water. Localized illumination was applied to a TiO 2 surface in solution for reversible spatially controlled "inorganic photoproton" cycling. The proton flux is pumped during irradiation of the surface of TiO 2 and dynamically maintained at the irradiated surface area, in the absence of any membrane or predetermined material structure.
Mechanisms for this have been elaborated in the paper.
ABSTRACT: Energy transfer reactions are the key for living open systems, biological chemical networking and development of life inspired nanoscale machineries. It is a challenge to find simple reliable synthetic chemical networks providing a localization of the time dependent flux of matter. In this paper we introduce a reliable, minimal reagent consuming, stable inorganic light promoted proton pump. Localized illumination was applied to a TiO 2 surface in solution for reversible spatially controlled "inorganic photoproton" isometric cycling, the lateral separation of water splitting reactions. The proton flux is pumped during irradiation of the surface of TiO 2 and dynamically maintained at the irradiated surface area in the absence of any membrane or predetermined material structure. Moreover we spatially predetermine a transient acidic pH on the TiO 2 surface in the irradiated area with feedback-driven generation of a base as deactivator.
Importantly we describe, how to effectively monitor the spatial localization of the process by in situ scanning ion selective electrode technique (SIET) measurements for pH and scanning vibrating electrode technique (SVET) for local photoelectrochemical studies. This work shows the great potential for time-and space-resolved water splitting reactions for following investigation of pH stimulated processes in open systems with their flexible localization on a surface.
Today increased interest is focused on dynamic, non-equilibrium material properties varying with time: a life inspired nanoscale machinery (1)(2)(3)(4). It involves needs for effective energy conversion with the focus on oscillation reactions (5), chemical networking (6), autocatalytic (7) and autoamplification (8) reactions, mimicking living systems (9), using cell metabolic biomolecules (10) and ions, e.g. proton gradients (11). Biological systems solve such an energymanagement by developing unique sensory and adaptive capabilities, transport mechanisms guided with ions, proton gradients and chemical networks (12). It is very attractive to use light energy (13) conversion for modulation of simple, reliable chemical networking, easy to control, based on existing knowledge on reliable light sensitive materials. We question if a semiconductor, e.g. TiO 2 , surface has a potential as effective photoactive surface to design light controllable networks of chemical reactions with the lateral separation of the two reactions of water splitting. Robust lateral separation and understanding are the keys to derive the system further broad prospects. A simple possible network can be light assisted generation of protons from water molecules and neutralization of H 3 O + with OH - (Figure 1a).
Moreover, recently we have discussed prospects of complex pH-active material systems (14) and introduced light-pH coupled oscillations of polymer assembly (15), cells dynamic switching (16), and protein recognition (17) on TiO 2 modified with polyelectrolytes. A vision of manipulating with spatiotemporal processes on titania remains untouched, which motivates the present work-robust lateral separation of the two reactions of water splitting with prospects of lateral separation of dynamic properties of pH sensitive assemblies. The hypothesis here is, that photoholes and photoelectrons may exhibit anisometric mobilities on surfaces. This may lead to a spatiotemporal separation of reduction and oxidation processes, via the control of the isometry of the carrier mobility. Mechanisms for this have to be elaborated.
In general semiconductor materials have been used for artificial photosynthetic system development (18) and are also known to enable efficient solar water splitting (19). Surprisingly a network of chemical reactions on semiconductor surfaces for open systems is not discussed much, simultaneously having different surface/interface engineering strategies, such as band structure engineering (20), co-catalyst engineering (21). Much is known how to improve heterogeneous semiconductors in terms of charge separation and transfer, enhanced optical absorption, optimized band gap position, lowered cost and toxicity, and improved stability (22).
Thus when the background of formation of spatially separated reactions on TiO 2 surfaces is associated with chemical reaction networking synergy, it is easy to further up-scale and improve the sustainability of the system.
A successful example of an application of such a spatial separation of photoreactions might be a photocatalytic lithography based on a TiO 2 layer (23). The feasibility of the inversion of metal images obtained by the chemical deposition of metals onto nanostructured TiO 2 exposed to high radiation doses has been demonstrated (24), as well as the generation of both negative and positive metallic patterns by varying the dose was reported (25).
Here we focus, at the beginning, on macro-scale, cm dimension surfaces, to control localization of different pH zones on the surface vs. irradiation. Localization of chemical species may lead to a life inspired proton pump machinery for localization of chemical networks on the surface of TiO 2 in an open system, providing further prospects for designing far-from-equilibrium, (26) dynamic (27), oscillation gel materials (28), stimuli-responsive drug delivery systems (29), metastable nanoparticle assemblies (30), reactors to proliferate acidic and basic molecules (7).
Moreover, a thorough understanding of electron and hole transfer thermodynamics and kinetics will lead to elucidating the key efficiency-limiting step and designing highly efficient solar-tofuel conversion systems (31). In this paper, we provide not only evidence of the possibility of spatial and temporal localization of proton pumping on semiconducting TiO 2 with light induced water splitting, but also some potential opportunities for designing an open system with localization of both H + and OH -.

RESULTS AND DISCUSSION
Here we concentrate on the light promoted reaction of water protonation (Figure 1a). Under supra bandgap irradiation of the surface of many semiconductors, e.g. TiO 2 , photohole and photoelectron are generated (Figure 1b). In the scope of photocatalytic reactions one can assume the possible formation of H + and OHdue to oxidation (ox) and reduction (red) reactions involving photogenerated charge carriers on the semiconductor surface. Surprisingly, the dynamics of the simplest reactions of the formation of proton and hydroxyl radical has not been highlighted before. Thus, our key idea is to use water splitting on a semiconductor, not focusing on the products H 2 and O 2 , as usual, but on reactions that provide the formation of H + and OH -.
We focus on the possibility of transforming the energy of electromagnetic irradiation into a pH gradient in time and space near the TiO 2 surface. A photocatalytically active nanostructured titanium dioxide film is the light sensitive part of the model system. An anodized titania nanotubular layer (32) on titanium was used as highly photoactive (15)  After turn-on of illumination, the photocurrent first presents a rapid response with an initial spiking of the photocurrent, indicating a rapid filling and discharging of defect states (35), and then a plateau for relatively constant collection from the active region is reached. In the case of two electrodes (36), TiO 2 working electrode and counter electrode, the photocurrent is caused by the separation of photo-generated electron-hole pairs within the photo-electrode: holes move to the TiO 2 surface, where they are trapped or captured by reduced species in the electrolyte, while the electrons are transported to the back contact via TiO 2 . Here, in our case, we have only one electrode, and photoelectrons are transported to adjacent non-irradiated zones.
A fast and uniform photocurrent response is clearly observed for switch-on and -off events on SVET. A dark current is quickly achieved after irradiation switching off. During the stationary mode the current density line scans and current density maps associated with the photocurrent can be measured to determine the degree of localization. A dash red line of the moving probe is indicated in Figure 2c, and the corresponding current density is shown in Figure 2e. The ionic current can be monitored vs. location of the irradiated spot. The precise value in the centre under the stationary mode is 80 µA/cm 2 .
After irradiation is switched off, the current relaxation in solution is shown in lines acquired every 3 min. Within 3 min after the irradiation is switched off the current density drops to 10 µA/cm 2 . 6 min after the irradiation is switched off no photocurrent is detected.
Imaging and localization (Figure 2h) of the positions of hole-or electron-induced reactions across the surface relative to its irradiation spot are also measured with SVET. No currents are detected before and after irradiation (Figures 2g and 2i), but during irradiation ionic currents appear in solution in response to localized surface irradiation, positive in the irradiated points and slightly negative in the rest of the surface. Moreover, the value of the onset potential (Figure 2f inset) for reverse cycle is shifted to the positive direction, which can be explained by a change of the surface pH (this effect was studied in detail in situ with the scanning ion selective electrode technique (Figure 3)).
For pH modulation with light it is important to understand, how photoinitiated processes on TiO 2 result in the transformation of light into a pH change, including localization of the effect. We apply SIET for mapping of the activity and migration of H + ions over the TiO 2 surface. Maps of the pH are collected for pristine TiO 2 : before illumination (Figure 3a); during illumination ( Figure 3b); after switching off irradiation, and during 40 min relaxation (Figure 3c).
It is seen from the presented proton distribution maps, that under irradiation, indeed, as suggested in Figure 1, the reaction of the generation of protons is observed. The protons are pumped from irradiated TiO 2 and have a gradient over the surface. It is important to note here, that an existing approach for static and localized pH gradients relies on pre-defined configurations of the microelectrodes (37). However, in our case the big advantage is, that we can operate in an open system without any membranes or patterning, just changing the localization of the irradiation spot to provide a proton pump at different places and varying the intensity of illumination. In order to change the gradient patterns, the re-design and re-fabrication of photo-masks, microelectrodes and sometimes chip structures themselves are required (36).
This flexibility of pH gradients is promising for suggestion of life inspired nanoscale machineries.
The action of different photoelectrochemical reactions on the TiO 2 surface under UV illumination seems to be the only plausible explanation for the observed local acidification and is consistent with the mechanism proposed in Figure 1. It is seen from the SIET pH maps (Figure   3a-c), that the system effectively produces microscale pH gradients. Such an engineering of pH gradients, proton pumping on TiO 2 , can be important for various chemical and biological networks. The simplest discussed here is the water protonation and following system deactivation with hydroxyl ions (Figure 1). Instead of immobilized pH gradients in gels, The relaxation time depends on morphology and characteristics of the used semiconductor surface. All above mentioned SIET measurements were performed at 100 µm above the surface.
To measure variation of the pH as a function of the distance from irradiated TiO 2 surface, the probe microelectrode was moved from 2000 µm to 0 µm distance in Z direction. Interestingly, a linear gradient of the pH was observed in the 0-350 µm range. In the distance from 350 to appr. We localized the illumination spot and defocused (decreased the intensity of irradiation from 5 mW/cm 2 to 1 mW/cm 2 ), optical images of the surface with an illumination spot are shown in Figure 3g and Figure 3h. The SIET map of pH activity is shown in Figure 3i, when we decrease the intensity of illumination and locate it at the edge of the measured area. We are able to see on the same surface zone separated H + and OHactivity, which was also assumed looking at Figure 3b. Several other conclusions can be suggested based on SIET measurement shown in Figure 3i: the pH gradient is located in a designed way via the irradiation spot (big advantage in comparison with non-flexible pre-designed electrode pH localization). It is possible to further modify the system to have a pH gradient, (Figure 1) and it is worth to discuss with respect to flexible surface process localization. The question is, if we can also locate the electrodes at a certain distance to have further flexibility.
We placed two TiO 2 electrodes electrically connected from the back and shined light on one of them (Figure 4). The experiment clearly indicates the distinct advantage of the combined use of photo-anodes and -cathodes for the microscale pH gradient profiles of both H + and OH -. On the irradiated electrode the positive current is detected vs. the negative one on the non-irradiated one (Figure 4c), however electrodes are located in the same epoxy holder and the same solution. pH maps follow the tendency of the photocurrent: there is acidification on the irradiated surface and alkalinization on the non-irradiated one (Figure 4d).

CONCLUSION
Light illumination of a photoconductive surface generates a conducting point that serves as a photo-anode and cathode, where protons and hydroxide ions are produced, leading to an increase and decrease in the pH gradient in an open system. The flexible spatial addressability is a great advantage for open system modulation. The spatial-temporal localization of light over photoelectrodes allows characterization of gradient profiles without any re-fabrication of electrode patterns. A bimodal electrolysis enables the fine-tuning of gradient profiles and enriches the variety of available gradient patterns. Thus, the superposition of bimodal pH gradients offers a practical solution to spatially modulate pH gradient patterns. As a corollary, this procedure also enriches the variety of available gradient patterns, which can be applied to many different experiments, such as, to artificially reconstruct and apply complicated biological pH gradient patterns to cells in vitro.
In the study, we introduced a novel life inspired light-addressing method to generate microscale pH gradients, proton pumping, at desired locations on a semiconductor substrate. It is always advantageous, if well studied, reliable materials can be applied to obtain new functions. Here TiO 2 is efficient for "inorganic photoproton" cycling. Light irradiation is spatially patterned by Local illumination of the TiO 2 film surface was performed using a setup equipped with a highintensity UV LED (365 nm; 3 W) supplied by a current stabilizer and a UV light beam focusing system involving several quartz lenses.