Guillaume Thomas

Geoffrey Gerer

Laurent Schlur

Fabien Schnell

Thomas Cottineau

Valérie Keller

Denis Spitzer

Double side nanostructuring of microcantilever sensor with TiO 2 -NTs as a route to enhance their sensitivity

We reported a new strategy to enhance the sensing performances of a commercial microcantilever with optical readout in dynamic mode for the vapor detection of organophosphorous coumpounds (OPs). In order to increase significantly the surface area accessible to the molecules in vapor phase, we nanostructured both sides of the microcantilever with ordered, open and vertically oriented amorphous titanium dioxide nanotubes (TiO 2 -NTs) in one step by an anodization method. However, due to the agressive conditions of anodization synthesis it remains a real challenge to nanostructure both sides of the microcantilever. Consequently, we developped and optimized a protocol of synthesis to overcome these harsh conditions which can lead to the total destruction of the silicon microcantilever. Moreover, this protocol was also elaborated in order to maintain a good reflection of the laser beam on one side of the cantilever towards the position sensitive photodiode and limit the light diffusion by the NTs film. The first results related to the detection of dimethyl methylphosphonate (DMMP) showed that TiO 2 and the nanostructuring on both sides of the microcantilever with NTs improved indeed the response of the sensor to vapors compared to a microcantilever nanostructured on only one side. The dimensions and morphology of NTs guaranteed the access of molecule to the surface of NTs. This approach showed promising perspectives to enhance the sensing performances of microcantilevers.

Introduction

Potential exposure to chemical warfare agents (CWAs) from terrorist organizations or military groups is real. [1][2][START_REF] Gupta | Handbook of toxicology of chemical warfare agents[END_REF] They have been used during the 2013 Syrian civil war and more recently, in 2017-2018, against civilians in Syria, Malaysia and United Kingdom. [START_REF] Lane | [END_REF] Among these agents, organophosphorous compounds (OPs) such as tabun (GA), sarin (GB) or VX can be used as weapons of mass destruction and are toxic molecules. 1,[START_REF] Gupta | Handbook of toxicology of chemical warfare agents[END_REF] This potential threat leads to develop sensors with high selectivity and sensitivity. The next generation of sensors must be fast, sensitive, selective, portable and inexpensive. However, it remains a challenge to satisfy all the requirements in a single sensor. Different sensors based on mass spectrometry, 5 colorimetry, 6 photonic crystals, 7 surface acoustic wave 8,9 have been proposed but they often require bulky instrumentations and high costs. Microgravimetric sensors and more particularly microcantilevers are a good candidate and showed promising results due to their low cost, their ease of handle, their reversibility and their good sensitivity. [10][11][12][START_REF] Zhao | Transducers-2015 18th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS)[END_REF][START_REF] Guo | [END_REF][15][16] To date, different strategies have been explored to improve their effectiveness about the detection of OPs. The first strategy consists mostly in functionalizing the microcantilever surface. For example, a selfassembled composite layer of Cu 2+ /11-mercaptoundecanoic acid, 10 a hyper-branched polymer, 12,[START_REF] Guo | [END_REF] or 78-mer 5'-biotinylated aptamer have been fixed on the microcantilever surface. [START_REF] Zhao | Transducers-2015 18th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS)[END_REF] However, the organic molecules are not stable at high temperatures and over long periods of time. The second strategy to improve molecule capture ability of the microcantilevers consists in enhancing their surface of capture which is originally low. 17 It usually achieved by the formation of a nanostructure on its surface. It has begun since a decade with the synthesis of titanium dioxide nanotubes (TiO 2 -NTs) on one side of a microcantilever for the detection of explosives. [17][18][19][20] Thundat et al. also nanostructured the surface of the microcantilever with a nanoporous TiO 2 . 19 Their nanostructuring required a two-step anodization and photolithography process for the detection of trace explosives. However, the use of photolithography process is not costeffective. More recently, Cai et al. developed a metal-organic framework (MOF) of UiO-66 film on a Parylene-C patterned piezoresistive microcantilever in order to increase the active surface area and the probability of DMMP capture. 11 However, the use of MOFs is limited due to drawbacks such as high fabrication costs, low reversibility, pore modulation or stability. 21 Despite the fact that these strategies improve significantly the sensitivity of the microcantilever, the nanostructures were only limited to one side of the microcantilever. They did not nanostructure the opposite side of the microcantilever may be due to harsh conditions of synthesis 17,18 for microcantilevers, important costs 19 or problems of laser beam reflection as reported previously for microcantilevers nanostructured with zinc oxide nanorods. 22 Microcantilever with optical readout consists in reflecting a laser beam on a side of the microcantilever towards a positionsensitive detector in order to follow the resonance frequency during sensing experiments in dynamic mode. This technique is expected to result in a better sensitivity than the piezoresistive detection, especially in this mode. 17,[START_REF] Debéda | Advanced Nanomaterials for Inexpensive Gas Microsensors[END_REF] Indeed, local heating can appear due to flowing current during measurement and piezoresistive layer needed for integration affects the mechanical performance. [START_REF] Debéda | Advanced Nanomaterials for Inexpensive Gas Microsensors[END_REF] Here, to overcome the cited problems, we report for the first time, a cost-effective and optimized synthesis of ordered and vertically aligned TiO 2 nanotubes on both (upper and lower) sides of a commercial microcantilever with optical readout by anodization. This process has the advantage to (i) not require photolithography process (high costs), 19 (ii) to use TiO 2 which is a material widely employed in photochemical and photoelectrochemical applications [START_REF] Cottineau | [END_REF][25][26] , to be easily synthesize 25 and showing a good interaction with simulant of OPs, 20,27,28 (iii) to generate NTs which develop a high surface to volume ratio, (iv) to reflect the laser beam on the microcantilever and (v) to offer a higher enlargement of the surface area due to the double side nanostructuring compared to the single one. The aim of the double nanostructuring is to increase the surface area of the microcantilever and consequently its sensitivity by enhancing the number of captured molecules. Among the different syntheses of TiO 2 -NTs, we chose anodization synthesis. It consists of the anodization of titanium layer for NTs formation. This synthesis has the advantage to be relatively easy on centimeter-sized area, simple, inexpensive and different kind of electrolytes can be used to allow the growth of a wide variety of TiO 2 -NTs morphologies and dimensions. 17,29 However, it remains a big challenge to nanostructure both sides of the commercial microcantilever in a one-step anodization without damaging the microcantilever. Indeed, the presence of fluoride in the electrolyte as well as high applied voltage can easily dissolve the microcantilever which is characterized by a small size (few square micrometer area). The microcantilevers must resist to the synthesis conditions in order to preserve their mechanical properties and consequently their high sensitivity. Moreover, the NTs on the microcantilever surface have to reflect the laser beam light used to measure the resonance frequency. This is also a challenge to conserve this property due to the decrease of the reflective index of TiO 2 compared to Si and also the formation of the porous surface coming from NTs which can prevent the laser reflection.

Due to the extreme high toxicity of the nerve agents, simulants as DMMP, 2 are preferentially used as model for sensing experiments. These molecules are less toxic and are characterized by a behavior close to the nerve agents making them good candidates for laboratory experiments. 2

Results and discussion

The protocol developed to nanostructure both sides of the microcantilever is presented in Scheme 1: (1) A bare silicon microcantilever (TL-NCL-50 from Nanosensors TM ) with a thickness of 7 ± 1 µm, a length of 225 ± 10 µm, a width of 38 ± 8 µm and a force constant of 21-98 N.m -1 was used as received. ( 2) The front side of the microcantilever was coated with a thick layer (layer No.1) of titanium by physical vapor deposition (PVD). The process used here is sputtering. Deposition was done at a pressure of 2 x 10 -3 mbar under argon atmosphere at 60 °C and the magnetron power density was 9.66 W.cm 2 . (3) The previous step was also applied to the back side of the microcantilever (layer No. 2). The front side (layer No. 1) of the microcantilever was deposited on a gel layer composed of polydimethylsiloxane (PDMS) in order to protect it during titanium deposition. However, the deposition temperature was here fixed at room temperature (23 °C). The deposition parameters were optimized in order to guarantee the growth of TiO 2 -NTs. ( 4) Synthesis of vertically aligned TiO 2 -NTs on the microcantilevers was done by electrochemical anodization of the metallic Ti layers. This method has been reported in a previous work 30 and modified here in order to obtain dense, open and oriented TiO 2 -NTs on both sides without any damage on microcantilever (e.g. dissolution of silicon). Briefly, the anodization was conducted in a two electrodes cell with the titanium covered microcantilever as working electrode and a platinum foil as counter electrode. The electrolyte was composed of 0.5% (w/w) NH 4 F and 10.0% H 2 O (v/v) in ethylene glycol at 45 °C. The same protocol (except step 2) was applied to the microcantilever nanostructured on one side. The anodization process is a competition between the electrochemical oxidation of Ti into TiO 2 and the dissolution of the oxide by the fluoride of the electrolyte according to the Eqs. ( 1) and (2): 17,31 Scheme 1 Schematic synthesis of the double side nanostructuring of microcantilevers with optical readout by TiO 2 -NTs. (2)

Nanoscale Accepted Manuscript

TiO 2 +6F -+4H + →[TiF 6 ] 2 -+2H 2 O
In the case of thin silicon substrate (7 ± 1 µm), it is crucial to pay attention not to dissolve the silicon with the fluoride of the electrolyte as presented in Fig 1. 17 Silicon and silicon dioxide suffer indeed from dissolution in the fluoride solutions. 32,33 Consequently, we used an ethylene glycol based electrolyte having a high viscosity which slows down the diffusion of fluorides and consequently limits the silicon dissolution. Moreover, parameters were specifically adapted to generate the formation and opening of TiO 2 -NTs while limiting the dissolution of silicon by fluorides. Electrolyte was composed of 0.5% (w/w) NH 4 F and 10.0% H 2 O (v/v) in ethylene glycol at 45 °C. The same protocol (except step 2) was applied to the microcantilever nanostructured on one side.

Scanning electron microscopy (SEM) images of a one and double side nanostructured microcantilever with a potential of anodization of 60 V are presented on Fig. 2. We observe the presence of NTs on one side (Fig. 2 a) and on both sides (Fig. 2.b) of the single and double side nanostructured microcantilever, respectively. This protocol of synthesis allows the nanostructuring on both sides. With these conditions, NTs of the single nanostructured microcantilever have a length of 1240 ± 38 nm, an external diameter of 137 ± 13 nm and an internal diameter of 105 ± 13 nm. Concerning the double side nanostructured microcantilever, NTs have a length of 1854 ± 137 nm, an external diameter of 100 ± 6 nm and an internal diameter of 72 ± 7 nm. Then, we optimized the conditions of synthesis by applying a potential of anodization of 20 V and a time of synthesis of 1800 s in order to increase the surface area of the microcantilevers by reducing the diameter and increasing the length of NTs.

SEM images (top views) after each optimized synthesis step are displayed on Fig. 3. The side views of the microcantilever after the synthesis of TiO 2 -NTs are exposed on Fig. 4. As observed on the Fig. 3, bare microcantilever shows a smooth surface. After sputtering deposition, Ti layers are characterized by a slight roughness (Fig. 3 b)). The Ti layers are formed of randomly oriented platelet-shaped particles. The thickness of Ti films measured by scanning electron microscopy (SEM) was 2.2 ± 0.1 µm. These Ti layers were then anodized in the optimized conditions (1800 s, 20 V). After anodization, SEM images show ordered and vertically aligned NTs on both surfaces of the microcantilever. Moreover, the top of NTs are completely open. Transmission electron microscopy (TEM) images confirm also the formation of NTs (Fig. 5). This opening ensures the access of molecules into the NTs allowing a possible increase of the molecules capture. The layer No. 1 exhibits NTs with a length of 2736 ± 66 nm, an external diameter of 76 ± 8 nm and an internal diameter of 57 ± 7 nm (Table 1 and Fig. 3 b). The layer No. 2 exhibits NTs with a length of 3193 ± 56 nm, an external and internal diameter of 69 ± 8 nm and 47 ± 7 nm, respectively (Table 1 and Fig. 3 b)). We reach a density of 131 ± 6 and 165 ± 6 NTs.µm -² (determined by counting the NTs on SEM images) on the layer No. 1 and No. 2, respectively. NTs are characterized by a high aspect ratio (length to diameter ratio) of 46 and 36 for layer No. 1 and 2, respectively. The orientation and morphology of NTs lead to increase significantly the surface area until a factor of 343 (theoretical value extracted from dimensions of the NTs) for double side nanostructured microcantilever (Table 1). This value is much higher than the result obtained in a previous study with one side nanostructured microcantilever (80). 18 We also observe that the length of TiO 2 -NTs between the two layers is slightly different. This difference could be explained by the temperature applied during the sputtering No. 2, respectively). We suppose here that a high temperature modifies Ti layer by increasing the grain size as reported in previous works. [34][35][36] Moreover, this effect lead to reduce the defect/gap between the grains resulting in denser film as previously reported. 35 As observed on Fig. 2, the layer No. 2 is characterized by a more pronounced stacking of plateletshaped titanium and less dense film than on layer No. 1. This arrangement leads to more grain boundaries and interfaces on Layer No 2. These interfaces may act like passages for the interstitial rapid diffusions of O 2-according to the diffusion theory that may facilitate chemical reaction between Ti 4+ and O 2-improving the growth of TiO 2 -NTs as observed in a previous report. 37 Moreover, as we used only one counter electrode during anodization process, the local electric field is consequently different between the two sides of the microcantilever which could lead to modify the dimensions of NTs on both sides. Further experiments are necessary to explain completely the mechanism of reaction during this step.

To determine crystallinity of nanostructures, X-ray diffraction (XRD) was employed (Fig. 6). After sputter deposition on a silicon wafer, we observed the (100), (002), ( 101) and (102) planes corresponding to the Ti crystal structure (JCPDS: 00-44-1294). 38,39 After anodization, we observe that the peaks corresponding to the Ti crystal structure remain present on the XRD pattern. However, we noticed that their intensity is reduced indicating the titanium consumption during the anodization. NTs are amorphous because no peaks characteristic of any titanium oxide phase is observed. Additionally, after a thermal treatment of the anodized sample at 450 °C during 4 hours (with a rate of 5 °C/min), we observe three diffraction peaks at 25.4°, 44.5° and 48.2°. The peaks at 25.4° and 48.2° are attributed to (101) and (200) planes of anatase crystalline phase respectively (JCPDS: 01-078-2486). 40 The peak at 44.5° is attributed to the diffusion of Si in Ti grain boundaries. 41 The remaining peaks correspond to the Si wafer. To resume, these results confirm that (i) a layer of Ti remains

Nanoscale Accepted Manuscript

View Article Online DOI: 10.1039/D0NR01596B

Please do not adjust margins

Please do not adjust margins between the substrate (Si) and the layer of TiO 2 -NTs and (ii) TiO 2 -NTs are amorphous after anodization.

We have successfully nanostructured both surfaces of the microcantilever with ordered, completely open and vertically oriented TiO 2 -NTs. The anodization synthesis took place in a single step. This strategy of patterning on both sides increases highly the surface area. This high surface area coming from intratubular, interstitial channels, external groove and external rounded surfaces of NTs could highly enhance the sensing properties of the next generation of microcantilever.

In order to evaluate the benefits of this nanostructuring strategy, bare microcantilever, microcantilever covered by metallic titanium film and microcantilever nanostructured on its both sides with TiO 2 -NTs were exposed to DMMP vapor. The potential of anodization and the synthesis time used to nanostructure the microcantilever were fixed at 20 V and 1800 s. The microcantilever was exposed to 594 ppm of DMMP for 10 minutes at a flow rate of 50 mL.min -1 . The DMMP vapors were generated from an air flow going through a stainless steel bubbler containing a pure solution of DMMP maintained at a temperature of 16 °C. The DMMP concentration was determined by gas chromatography-mass spectrometry with a measured concentration of 594 ppm which was close to the estimated saturated concentration (562 ppm) by using the Antoine equation. 42 The adsorption measurements are illustrated in Fig 7 . The three microcantilevers respond to DMMP vapor with a resonance frequency drop just after the injection of DMMP. The frequency shifts are -13, -50, -632 Hz for bare microcantilever, microcantilever with Ti layer and double side nanostructured microcantilever respectively. Despite the presence of the TiO 2 -NTs, the reflection of the laser beam on the TiO 2 -NTs surface towards the position sensitive photodiode is conserved. The measurement of the resonance frequency shift is thus ensured. Uncovered and Ti covered microcantilevers respond very slightly to DMMP. However, the cantilever covered by a Ti layer shows a higher response (-50 Hz) compared to the uncovered cantilever (-13 Hz). This effect could be attributed to the increase of the surface area due to presence of the platelet-shaped titanium structures on the surface of the microcantilever as observed in Fig. 3. The observed difference can also be due the fact that a native titanium oxide layer formed at air exposure has been probably generated on the surface of titanium layer and which could be more reactive to DMMP than silicon. Previous studies showed that TiO 2 has indeed a good reactivity to DMMP. 20,27,43,44 The initial uptake of DMMP on TiO 2 surface involves hydrogen bonds between the P=O group and hydroxyl groups of the oxide surface. 27,44,45 When both sides of the microcantilever are nanostructured with two dense layers of TiO 2 -NTs, a high response is observed with a shift of -632 Hz (f 0 = 166.105 kHz). The response is multiplied by a factor of 48 compared to the uncovered microcantilever. This result confirms that the nanostructuring offers better response than bare microcantilever. Moreover, it confirms also that TiO 2 is a material having a positive response to DMMP while the dimensions of NTs ensure an access of DMMP molecules on their surface. After 10 min of exposure to DMMP, microcantilevers are again exposed to air. As observed, for all tested microcantilevers, the resonance frequency shift decreases, indicating that DMMP molecules are progressively desorbed from the surface (Fig. 4). For the double side nanostructured microcantilever, we can underline that the response of the cantilever to DMMP vapor is very fast with a response time inferior to 2 s and an initial rate of 527 Hz.s -1 . 90% of the threshold is reached in only 14 s. After 100 s, the frequency of the microcantilever reached an equilibrium value (-632 Hz). However, the desorption process takes more time Please do not adjust margins Please do not adjust margins than adsorption with a response time inferior to 2 s. 90% of the return to the baseline is achieved in 125 s. A time superior to 600 s is necessary to observe a return of the baseline. The comparison of adsorption and desorption times shows that desorption is much slower process than adsorption. It can be explained by the fact that DMMP could be trapped in pores which results in a slow release process. Further studies are necessary to determine in details the mechanisms of DMMP adsorption and desorption on TiO 2 -NTs. According to the mechanism of vapor permeation, the response of the nanostructured microcantilever to DMMP can be considered by three steps: i.e. adsorption, diffusion, and desorption process. First, the frequency changes sharply (initial rate of 527 Hz.s -1 ) when the double side nanostructured mirocantilever is introduced with vapors (DMMP), which results from surface adsorption effect. Then, the frequency shifts very slowly and the frequency nearly not change because of bulk diffusion effect between sensitive sensor material (TiO2) and vapor. This

Conclusion

In summary, we have developed a fast route for enhancing commercial microcantilever response to DMMP vapor. First, we have covered both sides (upper and lower) of a silicon microcantilever with a titanium layer by physical vapor deposition and then, we nanostructured the microcantilever with TiO 2 -NTs by anodization. Anodization of both surfaces has been realized in a one-step process. Through an optimized protocol of titanium deposition and anodization synthesis, we have successfully kept sufficient reflectivity by maintaining surface properties and morphology of the microcantilever as well as the initial structure and morphology of the microcantilever despite the harsh conditions of titanium anodization for silicon. Additionally, we have homogenously nanostructured the sensor with open, dense and vertically oriented amorphous TiO 2 -NTs. The morphology of NTs ensures a good accessibility of the DMMP molecules on the nanostructures surface. Moreover, the NTs morphology and the double side nanostructuring offers a high enhancement of the surface area which should increase the sensor sensitivity. Please do not adjust margins Please do not adjust margins Indeed, NTs allowed to capture more molecules on the surface of the microcantilever and double side nanostructured microcantilever showed an enhancement of the sensor response by a factor of two compared to the single nanostructured microcantilever. In addition, TiO 2 is a good candidate to detect OPs. Double side nanostructured microcantilever showed a fast response and a good recovery. By taking advantage of the high surface area of the double side nanostructuring, further development will be conducted in order to develop a more sensitive and selective sensor to OPs.

Nanoscale Accepted Manuscript

View Article Online DOI: 10.1039/D0NR01596B

DOI: 10 .

 10 1039/D0NR01596B Journal Name COMMUNICATION This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins (1) Ti + 2H 2 O→TiO 2 +4e -+4H +

Fig. 1

 1 Fig.1Optical images of a microcantilever covered by a Ti layer (thickness of 2.2 ± 0.1 µm) a) before anodization and b) after anodization without synthesis optimization (one side of the cantilever was not protected during deposition of Ti layer, potential of anodization and time of synthesis were fixed at 30 V and 1800 s, respectively).

Fig. 2

 2 Fig.2SEM images of microcantilever covered on a) one side (back side) and b) on both sides by TiO 2 -NTs. The potential of anodization is fixed at 60 V for single nanostructured microcantilever and double side nanostructured microcantilever. The potential of anodization was fixed at 60 V.

Fig. 3

 3 Fig. 3 SEM images of bare microcantilever, microcantilever covered by a Ti layer and microcantilever covered by TiO 2 -NTs on both upper (layer No. 2) and lower surfaces (layer No. 1) with a a) large and b) close view. The particles visible on bare microcantilever (b)) are dust particles present on the microcantilever and they were used to adjust the focus on the microcantilever surface.

Fig. 4

 4 Fig. 4 SEM images of double side nanostructured cantilever with TiO 2 -NTs.

Fig. 5

 5 Fig. 5 TEM images of TiO 2 -NTs obtained from nanostructured wafer.

Fig. 7

 7 Fig.7 Resonance frequency shift of the bare microcantilever (Si), microcantilever covered by a Ti layer and double side nanostructured microcantilever with TiO 2 -NTs. The microcantilevers were exposed to 594 ppm of DMMP for 10 min. Double side nanostructured cantilever was synthesized with a potential of anodization of 20 V and a synthesis time of 1800 s.

Fig. 6

 6 Fig.6XRD patterns of silicon wafer before (green) and after deposition of a titanium layer (blue), TiO 2 -NTs before (black) and after thermal treatment at 450 °C (red). TiO 2 -NTs are obtained by anodization.

Fig. 8 a

 8 Fig. 8 a) Resonance frequency shift of a microcantilever nanostructured on one and both sides with TiO 2 -NTs. The microcantilevers were exposed to 594 ppm of DMMP for 10 min. Nanostructured microcantilevers were synthesized with a potential of anodization of 15 V and a synthesis time of 1800 s. b) Frequency responses of the double side microcantilever. Double side nanostructured cantilever was synthesized with a potential of anodization of 20 V and a synthesis time of 1800 s.

Table 1 .

 1 Dimensions of TiO 2 -NTs (length, external (Ø ext ) and internal diameter (Ø int )) and numbers of TiO 2 -NTs on the double side nanostructured microcantilevers (layer No. 1 and layer No. 2).

		Ø ext	Ø int	Length	Number of
		(nm)	(nm)	(nm)	NTs.µm -²
	Double side
	nanostructured microcantilever	69 ± 8	47 ± 7	3193 ± 56	165 ± 6
	(layer No. 1 )
	Double side
	nanostructured microcantilever	76 ± 8	57 ± 7	2736 ± 66	131 ± 6
	(layer No. 2 )

Table of content

 of Double side nanostructured microcantilever with ordered, aligned and open TiO 2 nanotubes

Conflicts of interest

There are no conflicts to declare.

Nanoscale Accepted Manuscript

View Article Online DOI: 10.1039/D0NR01596B


Florian Gelb

Yu-Chien Chueh

Neso Sojic

Valérie Keller

Dodzi Zigah

Thomas Cottineau
email: cottineau@unistra.fr

Electrosynthesis of gradient TiO 2 nanotubes and rapid screening using Scanning PhotoElectroChemical Microscopy

Information (ESI) available: Detailed experimental parameters; Optical image and full set of SEM images for the VLTE sample; CV and EQE of reference TiO

Introduction

Since several years there is a strong interest in light energy conversion motivated by the huge potential of renewable and clean energy provided by the sun combined to the urge related to climate change and to the diminution of fossil fuel resources easily accessible at low cost. 1 New concepts are investigated with the objectives to decrease the price and to improve the conversion efficiency such as dye sensitized solar cells (DSSC) or more recently cells based on perovskite absorber are promising PV systems. 2 Alternatively Photo-electrochemical cells (PEC) would pave the way to the direct production of solar fuel from sunligth. 3,4 Several of these new concepts relies on thick porous semiconducting films that are responsible of the charge carrier transport (for DSSC) and also for light absorption (PEC). 5,6 It is necessary to find the optimal thickness of these films in order to manage the light and charge carrier transportation in the cell. [7][8][9] This is particularly true for the case of tandem PEC cell in which two semi conductors absorbing in two different spectral domains would be associated. 10 The large band gap semiconductor would be in that case in front irradiation configuration and a compromise should be found on the film thickness. Indeed, if a thick layer of photoactive material maximizes the light absorption (mainly at the top of the film), it can result in a reduced photogenerated charge carrier collection at the back contact due to their probability of recombination on long distance.

In this context, the development of rapid screening methods of the film performances as a function of their thickness should be developed. Achieving such measurements requires two conditions: The first requirement is to obtain electrodes with a variable and well controlled thickness of photoactive material. The change of the thickness must not have any impact on the morphology or other properties of the material such as its conductivity or its composition. The second requirement is to have a system that can achieve local PEC analysis with the highest resolution possible which is mainly related to the irradiation area.

So far the large majority of rapid screening experiments were used to determine the optimal composition of ternary oxides. 11,12 For instance, an ambitious distributed combinatorial outreach program was conducted a few years ago using inkjet printing of semi-conductor with variable composition on conductive glass substrates. 13 The most relevant example we found concerning rapid PEC screening to study the influence of geometrical parameters on a single electrode, was done by Loget et al. 14 They used a bipolar anodization method to grow titanium dioxide nanotubes (TiO 2 -NTs) with variable length on a single electrode. The bipolar anodization method was also employed to synthesize gradient of TiO 2 -NTs for photo-catalytic and biomedical applications. 15,16 Nevertheless, considering the formation mechanism of anodic TiO 2 -NTs, 17 the potential variation along the Ti foil in bipolar anodization induces a change of the NTs length but it also changes the diameter and the wall thickness of the tubes. Consequently, the apparent density of TiO 2 in the film will change and the mean free path of light will not be homogeneous for the whole electrode. Furthermore, during the bipolar synthesis, it is not possible to determine the charge transferred at each position of the electrode. The knowledge of this value is important since it can be directly related to the quantity of TiO 2 electrochemically formed during the anodization of the titanium foil. Furthermore, in all the studies mentioned above, the light source used had a spot diameter larger than 1 mm, which limits the accuracy on the local photocurrent determination.

Scanning PhotoElectroChemical Microscopy (SPECM) is a scanning probe technique that uses an optical fiber (or an optoelectrode) as a probe to analyze a surface locally. [18][19][20] Unlike conventional photoelectrochemical measurements, which give a global response of the surface, in SPECM a local response is obtained. In this configuration the spatial resolution is determined by the illuminated area. The use of an optical fiber allows the photoelectrochemical reaction to be triggered only where the surface is illuminated. [21][22][23] The small aperture of the optical fiber and the short distance between the fiber and the sample result in a small spot size of around 200 m. Therefore, SPECM is a technique of choice to study an inhomogeneous surface or to screen different materials present on the same substrate. As an example, it was used to determine the optimal Zn doping of WO 3 on material spots deposited on a single electrode 24 and also to show that addition of Ag improves the behaviour of Fe 2 O 3 by screening micrometer spots with different composition. 25 In our case, we were able, with the SPECM, to make a rapid screening, with a high resolution, of the nanotubes properties.

In the present work, we developed a new method to synthesize anodic TiO 2 -NTs with a changing length on a single electrode (here after named VLTE for Variable Length TiO 2 -NTs Electrode). This simple method allows making an electrode fully covered with these NTs which have an increasing length in one direction of the film. These model electrodes with variable nanotube length were then tested with SPECM in order to determine to optimal TiO 2 -NTs film thickness for their use as photoanode for water splitting.

Variable length TiO 2 -NTs electrode synthesis

To achieve the growth of TiO 2 -NTs with a gradual length on a single electrode, we developed a simple method based on the control of the exposure time of the titanium to the electrolyte. Such approach was attempted earlier but in a configuration in which both the electrolyte exposure time and the applied potential were changed. 26 For this reason and also due to the use of aqueous based electrolyte, mainly the diameter of the NTs was changed and the length only increased from 300 to 500 nm.

In our case for growing NTs with different lengths, a titanium foil and the platinum electrode are placed vertically in a Teflon cell as schematically represented in figure 1. At the beginning of the synthesis, both electrodes are immerged in an ethylene glycol based electrolyte which is then slowly removed from the cell by a peristaltic pump. As the level of electrolyte goes down progressively, an increasing part of the titanium is not anymore in contact with the electrolyte and the nanotube growth is inhibited in that area. After the synthesis, the electrode is then carefully rinsed with deionized water and dry under nitrogen flux and annealed at 500°C under air for 4h. (detailed synthesis conditions are given in Electronic Supplementary Information ESI)

After the experiment, the electrode exhibits horizontal iridescences which are a first indication of the change of the thickness of the TiO 2 -NTs film (ESI figure S1). These lines arise from light interference in the porous film and as the thickness changes, different wavelength are successively reflected. 27 The whole electrode was then introduced in a SEM and observed all along the z axis. In the figure 2 are presented some SEM images of the TiO 2 -NTs films at different height of the electrode (the whole set of image can be seen in ESI figure S2). They clearly show the growth of the nanotubes from the initial state with small nanotubes of a few hundred nm at the top of the electrode to NTs with a length of several micrometers at the bottom. It should be noticed that for the shortest nanotubes, an initiation layer resulting from a preanodization step is still present. This first anodization and the removal of the primary nanotube layer is necessary to ensure a better adhesion of the nanotubes to the film and without this initial step, delaminating of the longer NTs is observed on the VLTE. 28 The length of the NTs at different positions on the electrode was measured and plotted in figure 3.a. It can be seen that this length increase from 780 to 5950 nm ± 200 nm and then is slightly reduced at the bottom of the nanotubes. The initial length is not zero since the electrolyte pumping starts 100 s after the beginning of the anodization. This duration is enougth to form small NTs at the top of the electrode. The decrease of the length observed at the bottom of the electrode can be explained by the formation mechanism of the nanotubes which can be schematically described according to the following equations:

(1)

+ + + 4H e 4 + TiO O H 2 Ti 2 2 (2) [ ] O 2H + TiF 4H + 6F + TiO 2 2 6 + 2
TiO 2 is formed electrochemically at the Ti surface while the nanotubular structure is achieved by its partial dissolution by the fluoride present in the electrolyte. The top of the nanotubes is then formed at the beginning of the anodization. A too long exposure to the electrolyte results in the formation of a so-called "nanograss" layer at the top of the film. 17 The anodization data as a function of time are represented in figure 3.b and c. At the beginning, the applied voltage (U) increases from 0 to 45 V this is accompanied by an increase of the current (I) that reaches a maximum and slowly decrease when the potential stabilized to 45 V. It should be noticed that the current tends to 0 in our case since less surface is exposed, while for classical anodization the current reaches a plateau when an equilibrium between oxide formation and dissolution is reached (eq. 1&2). According to equation 1 the quantity of TiO 2 formed during the anodization should be proportional to the charge transferred. In order to correlate this value with the length of the TiO 2 nanotubes, we used the VLTE anodization data (figure 3.b) to determine the charge density transferred at each position of the electrode: First, the height of electrolyte pumped out as a function of time, h(t), can be calculated from the flow imposed by the pump (f p ) according to: (pump starts at t=100 s; for t<100s, h(t)=0)

Sustainable Energy & Fuels Accepted Manuscript

(3) ( ) ( ) K 100 t f t h p =
The constant (K) depends on the geometry of the container (its horizontal section) and correspond to the volume needed to change the height of 1 mm. In our case, the cell is a cylinder with a diametrer of 7 cm and K=3.848 mL.mm -1 . The value of h(t), represented in figure 3.b (green curve) can be used to determine the surface of the electrode in contact with the electrolyte at any time. Knowing the anodization current from the synthesis data I(t) the charge used for the nanotube synthesis as a function of time and consequently each position of the electrode, can be calculated by the following integral: This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins

(4) ( ) ( ) ( ) ( ) = t 0 dt t h L W t I t Q
with W and L the width and the length of the electrode. Then, the evolution between the nanotube length and the charge density can be represented (figure 3.c). Excluding the area were the nanograss is formed (i.e. h>9.1 mm or Q>7.7 C.cm -2 ), this evolution is linear with a growing ratio of 865 nm.C -1 .cm 2 . This confirms, on a single electrode, that accordingly with equation 1, the NTs length is proportional to the quantity of TiO 2 formed electrochemically. The length of the nanotubes in the VLTE can thus be estimated by knowing the total charge transferred at each position of the electrode for a given set of synthesis conditions.

SPECM mapping

The VLTE electrode was fixed horizontally on a glass slide and then a cell made in PDMS was built around the sample. The SPECM experiment was run in a three electrodes configuration. A platinum mesh was used as the counter electrode and a Ag/AgCl (NaCl 3 M) as the reference electrode. The VLTE was the working electrode and its potential was set at 0.6 V vs Ag/AgCl. The sample was irradiated trough an optical fiber with a numerical aperture of 0.22 and a core diameter of 200 µm. The light source was a mercury-xenon lamp (LC8 from Hamamatsu) providing a spectrum centered at 365 nm. The optical fiber was moved along xy axes at a scan rate of 400 µm.s -1 , with steps every 40 µm each 0.1 s, z, the distance between the tip and the surface was constant (20 µm) during the measurement. When the UV source was turned on, the optical fiber illuminated locally the TiO 2 -NTs, and the current was simultaneously recorded as a function of the optical fiber position. This current corresponds to the local photoelectrochemical oxidation of water, which happens only below the light spot. With this system, a 2D image of TiO 2 nanotube gradient was obtained on an area of 10*2 mm 2 , located in the middle of the electrode, first in the dark then under illumination with UV light (figure 5.a).

In the dark, the current has a residual value below 6.5 µA. Under UV light irradiation, an increase of the current clearly appears on the sample even for short nanotubes. This current increases until a maximum located approximately in the center of the electrode. Then, for the longer nanotubes, the photocurrent decreases. In figure 5.b are represented the photocurrent evolutions on three positions of the electrode and their average. The photocurrent appears for short nanotubes and reaches a local maximum at 0.5 mm from the top of the electrode. This local maximum, might be attributed to the partial contribution of the dense underlying TiO 2 layer located at the interface between the NTs and Ti substrate. 6 With longer nanotubes, the contribution of this dense layer is reduced since less light can reach it and mainly the NTs contribute to the photocurrent. After this local maximum, the photocurrent increases with the length of the NTs until a maximum of 10.1 µA at 5.5 mm from the top of the electrode; a position for which TiO 2 -NTs have a length of 4.3±0.2 µm. The length corresponding to this maximal photocurrent is coherent with measurements performed for nanotubes of different lengths, synthesized by a classical method. As detailled in supplementary information (ESI figure S6), TiO 2 -NTs films with different lengths, were tested in a normal 3 electrodes PEC setup. The highest photocurrent and yield were measured for NTs with a length of approximately 4 m. This value is also in agreement with the optimal thickness of NTs film reported, by other teams, to be between 4 and 6 m in the case of photo-(electro)catalytic applications. 29,30 This maximum of efficiency

Sustainable Energy & Fuels Accepted Manuscript

View Article Online DOI: 10.1039/C9SE00895K

Please do not adjust margins Please do not adjust margins for this length is explained by a balance between two properties governing the photocurrent generation: (i) a good light absorption, resulting in high number of e -/h + pairs generated which increases with the thickness and (ii) the efficienct transport of the electron to the back contact (the Ti foil). The latter is limited by the diffusion of electrons in TiO 2 and will then be reduced for thick nanotubes films. 6,9 To determine the reliability of our approach combining variable length nanotubes synthesis and SPECM measurements, we compared our results with a reference TiO 2 -NTs electrode. For this, nanotubes were grown in the same electrolyte and the charge transferred was limited to 5 C.cm -2 , resulting in a titanium foil covered with NTs having a length of 4050 ± 110 nm. The external quantum efficiency (EQE) of this electrode was then measured (data are in figure S7). EQE represents the ratio between the quantity of electron collected and the number of incident photon at a given wavelength according to:

(5) ( ) ( ) ( ) = P j e c h EQE ph
With j ph (λ) and P(λ) are the measured photocurrent and the incident light power density at a given wavelength λ, respectively. For the reference electrode an EQE value of 0.15 was obtained at 365 nm which is the central wavelength of the UV light source used in SPECM setting. The same calculation can be done for the SPECM system knowing the irradiation power at the optical fiber exit (115±50 µW). For NTs having a length of 4000 nm the measured photocurrent triggered by this irradiation is 3.5 µA. By applying equation 5, an EQE of 0.15±0.06 is obtained. This value is in good agreement with the EQE obtained at 365 nm for the reference sample, confirming the good response of the VLTE electrode even with the localized spot of the SPECM system (estimated spot diameter 215±50 µm). The high uncertainty is explained by the difficulty to measure the power at the fiber output.

Defect identification with SPECM

In figure 6 is presented the SPECM mapping of a part of a second VLTE electrode. For this electrode, the evolution of nanotubes length as a function of the anodization charge was calculated according the same protocol as earlier, and the growing factor of 850 nm.C -1 .cm 2 is consistent with the first sample. The calculation and data for this sample are presented in figure S12. On the photocurrent mapping, lines with lower photocurrent can be observed on the left of the image which corresponds to the bottom of the electrode. Their positions correspond to scratches that were done on one side of the electrode surface. As observed by SEM in figure 6.b, these scratches are 10 m in width. Even if the optical fiber aperture is larger, the good spatial and electrochemical resolution of the SPECM, allows localizing these stripes. Similarly, a small round area can be observed in the center of the mapping image. Its position perfectly matches with a small punch that was done with an needle on the VLTE sample that can be observed optically in image 6.c. The mapping of this VLTE surface was done on the side of the titanium foil and small strip with low photocurrent can be observed at the top and the left of the figure 6.a. This observation can be related to side effect phenomenon occurring during the growth of the nanotubes film. This can be optically related to the observation of figure 6.d: on this dark field image of the surface, at the top edge of the electrode, the TiO 2 -NTs film appears in red and a strip of approximately 100 m non covered with nanotubes appears on the side of the electrode in grey. This can be explained by a peeling of the nanotubes film on the side of the electrode during the synthesis or rincing/annealing step. This side effect was also observed for other samples (example of dark field images in Figure S5).

Besides confirming the reproducibility of the VLTE electrode synthesis, the observation of this second sample allows to highlight the good resolution and the potential application of the SPECM method to detect point defects in electrodes for PEC applications.

Conclusions

In this work we report a simple and reproducible synthesis method of aligned TiO 2 -NTs electrode on which the length of the nanotubes can be tuned along one direction. By changing This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins the contact time between the electrolyte and the titanium substrate, NTs having a length ranging from a few hundred nm up to few µm can be grown on a single electrode. Furthermore, by this approach the diameter of the nanotubes does not change significantly and this type of electrode allows local studies of the influence of the NTs length independently of others parameters.

In order to measure the performance of these original samples with a variable thickness of photoactive material, we used by SPECM in order to study the influence of the nanotubes length on the generated photocurrent for water splitting reaction. The mapping image revealed significant photocurrent change in the direction of the length gradient while a good homogeneity of the photocurrent appears in the lateral direction. The photocurrent profile indicates a maximum for nanotubes having a length of 4300 nm; a value which is consistent with previously reported studies. This approach combining gradient synthesis and SPECM measurement be used to measure quickly the optimal value of a parameter while limiting the synthesis time by doing all the test locally on a single electrode.

Furthermore the SPECM measurements present an excellent spatial and electrochemical resolution that could allows to detect defects in photo-electrochemical films in the perspective of electrodes diagnostic. We believe this resolution can further be improved by playing on the electrochemical parameters and the aperture of the optical fiber.

Figure 1 :

 1 Figure 1 : Schematic principle of VLTE synthesis by electrolyte pumping.

  DOI: 10.1039/C9SE00895K COMMUNICATION Please do not adjust margins Please do not adjust margins

Figure 2 :

 2 Figure 2 : SEM images of TiO 2 -NTs observed at different height of the nanotubes (observed with a tilt angle of 45°).

Figure 3 :

 3 Figure 3: a) Length and diameter of the nanotubes as a function of the position from the top of the electrode determined by SEM. b) Anodization parameters: applied voltage and height of electrolyte pumped out as a function of time. c) Anodization current and calculated charge density evolution during the VLTE synthesis. d) Measured TiO 2 -NTs length as a function of the calculated charge density (Q) (correlation parameter estimated from linear regression: 865 nm.C -1 .cm 2 ).

  This phenomenon results directly from a thinning of the nanotubes wall according to equation 2 and eventually the total dissolution and collapse of the nanotube top, consequently the length does not increase continuously. This thinning of the NTs wall at the top can be observed in ESI figure S3: the SEM images of the top of the NTs film at different heigh (or anodization time) clearly show the transition from the initiation layer to the nanograss. In figure 3.a is also represented the values of NTs external diameter measured at their half height (diameters at the top and bottom of the nanotubes are presented in ESI figure S4). As observed in these synthesis conditions, the diameter has an average value of 102±10 nm and does not change significantly depending on the position of the NTs on the electrode.

Figure 4 :

 4 Figure 4: Scheme of the setup with the optical fiber scanning the gradient of TiO 2 -NTs during the SPECM experiment.

Figure 5 :

 5 Figure 5: a) SPECM results in the dark and under illumination at λ=365 nm, b) Photocurrent profile at 3 positions and c) corresponding NTs length for the same VLTE.

Figure 6 :

 6 Figure 6: a) SPECM mapping of a second VLTE sample. b) SEM images of scratches made at the bottom of the nanotubes. c) and d), dark field optical images of a punching on the TiO 2 -NTs electrode and of the side of the electrode.

Acknowledgements

FG is grateful to CNRS for funding is PhD research through the ANR BAGETE (ANR-16-CE05-0001-01). Thierry Dintzer and the SEM-CRO Platform are acknowledged for SEM images.

Sustainable Energy & Fuels Accepted Manuscript

Conflicts of interest

There are no conflicts to declare.

Sustainable Energy & Fuels Accepted Manuscript