Silatrane Anchors for Metal Oxide Surfaces - ACS Publications

Forum Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Silatrane Anchors for Metal Oxide Surfaces: Optimization for Potential Photocatalytic and Electrocatalytic Applications Kelly L. Materna,†,‡ Jianbing Jiang,†,‡ Robert H. Crabtree,†,‡ and Gary W. Brudvig*,†,‡ †

Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States Yale Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States

‡

S Supporting Information *

ABSTRACT: Silatrane surface anchors are protected siloxanes that are known to bond firmly (from pH 2−11) to metal oxide electrodes under heating. However, these conditions are not always compatible with the other functionality present. A silatranecontaining porphyrin molecule and a silatrane-containing ruthenium complex have now been designed, synthesized and optimized conditions have been identified for surface binding. Two mild, room-temperature surface binding methods were explored: binding with or without an acidic pretreatment; these methods were compared to the traditional, harsher binding conditions involving strong heating. We find that a preacidified electrode gave comparable surface loadings at room temperature compared to sensitization by using the previous strong heating method. This was also true on TiO2, SnO2, and nanoITO electrodes and thus may be generalizable. The new, milder binding methods also resulted in excellent aqueous and electrochemical stability from pH 2−11. Using a water-insoluble porphyrin with a silatrane anchor further increased the aqueous stability of the deposit, aided by the insolubility of the porphyrin. Finally, X-ray photoelectron spectroscopy (XPS) data confirmed for the first time that the triethanolamine released from the silatrane on deprotection/binding in turn binds to TiO2, SnO2, and nanoITO electrodes. This undesired triethanolamine deposit was easily removed from the surface by electrochemical voltage cycling or with an aqueous acidic wash for 1 h. KEYWORDS: water splitting, surface anchors, metal oxides, silatranes, titanium dioxide, tin oxide, solar energy

1. INTRODUCTION Strong, stable surface anchoring groups are required to heterogenize molecular dyes and catalysts on metal oxide surfaces for use in photocatalytic or electrocatalytic applications.1−5 Specifically, hydrolytically stable anchors are needed when molecules are heterogenized in water-splitting dyesensitized photoelectrochemical cells (WS-DSPECs) to avoid molecule desorption from the electrode surface.1−10 Common surface anchors employed include: carboxylic acids, phosphonic acids, hydroxamic acids, and silatranes.1,2,6−9,11−14 Of these, hydroxamic acids and silatranes provide the greatest stability over a wide pH range, having excellent stability under acidic, neutral, and basic conditions when bound to metal oxide electrodes.2,6,12,13 We recently found silatranes to be electrochemically stable, remaining surface bound from pH 2−11 in water when RuSil (Figure 1) was bound to nanoITO conductive electrodes.6 We have also had success implementing the silatrane anchor for a pentamethylcyclopentadienyl (Cp*) Iridium complex, IrSil (Figure 1), which performed electrochemically driven water-oxidation catalysis when heterogenized on nanoITO conductive electrodes and photocatalytic water oxidation on SnO2 semiconductor electrodes using high potential CF3-substituted porphyrin photosensitizers.7,15 © XXXX American Chemical Society

Because silatranes are among the least explored surface anchors, it is important to explore ways to optimize them for use in catalytic applications. Silatrane surface anchors contain a pentacoordinate silicon atom in a hypervalent caged structure; upon binding to a metal oxide surface, the triethanolamine coproduct is lost and the resulting surface-bound siloxane provides good resistance to hydrolysis, unlike other alkoxy- or chloro-silane derivatives.2,6,7,15−22 In addition, silatranes do not react with metal centers, such as the Ir or Ru ions in wateroxidation catalysts, during synthesis of organometallic complexes, allowing for easy purification of the anchor precursor even if it contains sensitive functional groups.6,7 Traditionally, silatranes are bound to metal oxide surfaces by using high temperatures (60−70 °C), which are needed to deprotect the silatrane cage and form siloxane surface bonds.6,7,15,20,21 Because not all molecules are stable at these high temperatures, Special Issue: Artificial Photosynthesis: Harnessing Materials and Interfaces for Sustainable Fuels Received: March 13, 2018 Accepted: May 31, 2018

A

DOI: 10.1021/acsami.8b04138 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces

Figure 1. Examples of silatrane-based molecules. temperature was held at 400 °C for 1 h, and finally cooled to room temperature at a down rate of 4 °C/min. On average, nanoITO film thicknesses were ∼11.9 μm. 2.4. Electrode Sensitization. TiO2, SnO2, and nanoITO electrodes sensitized with tolSil, RuSil, and PSil were prepared using three sensitization procedures (Figure S1). Prior to sensitization, the sensitization solution was prepared; ∼ 0.045 mM PSil in toluene, ∼0.12 mM RuSil in ethanol, and ∼0.9 mM tolSil in acetonitrile were used as the sensitization solutions. Concentrations of the sensitization solutions varied due to the wide range in solubility of the molecules; the concentration of PSil and RuSil sensitization solutions were chosen to maximize solubility of the molecules in each solvent and to result in a high surface loading after electrode sensitization, which is needed to characterize the electrodes by UV−vis spectroscopy. 2.4.1. Room-Temperature Sensitization. Samples were sensitized at room temperature by placing the electrode (TiO2, SnO2, or nanoITO), having an approximate geometric surface area of 0.5 cm2, in 2.0 mL of the sensitization solution in a capped vial. This was kept in the dark for ∼20 h. Post sensitization, the electrodes were rinsed three times with the solvent it was sensitized in and air-dried. 2.4.2. Acidic Pretreatment & Room-Temperature Sensitization. To perform the acidic pretreatment, the electrode (TiO2, SnO2, or nanoITO), having an approximate geometric surface area of 0.5 cm2, was placed in 2−3 mL of 0.1 M phosphate buffer at pH 2 for 3.5 h in a capped vial at room temperature. The electrode was then removed from the solution and air-dried. Once dry, the electrode was placed in 2.0 mL of the sensitization solution in a capped vial. This was kept in the dark for ∼20 h. Post sensitization, the electrodes were rinsed three times with the solvent it was sensitized in and air-dried. 2.4.3. 70 °C Sensitization. To perform the 70 °C sensitization, the electrode (TiO2, SnO2, or nanoITO), having an approximate geometric surface area of 0.5 cm2, was placed in a vial with 2.0 mL of the sensitization solution. The vial was heated to 70 °C, put under a N2 atmosphere, and kept in the dark for ∼20 h. Post sensitization, the electrodes were rinsed three times with the solvent it was sensitized in and air-dried. 2.5. Electrochemistry. Chronoamperometry (CA) and cyclic voltammetry (CV) were performed using a nanoITO-RuSil working electrode, Ag/AgCl(sat’d NaCl) reference electrode, and Pt wire auxiliary electrode. Prior to measurements, the Pt wire was torched in a flame to clean the surface. Approximately 6 mL of 0.1 M phosphate buffer at pH 2, 7, or 11 was used as the electrolyte. During the CA measurements, electrodes were exposed to a bias of 1.512 V vs NHE at pH 2, 1.217 V vs NHE at pH 7, and 0.981 V vs NHE at pH 11 for 30 min. The bias was chosen to mimic a 400 mV overpotential for wateroxidation catalysis at the respective pH. During CV measurements, a scan rate of 50 mV/s was used.

milder binding methods are desirable if these anchors are to be widely implemented. Here, we report two alternative binding methods for the silatrane anchor (Figure S1, Supporting Information) and examine the stability of the deposit under aqueous and electrochemical conditions from pH 2−11 on TiO2, SnO2, and nanoITO metal oxide surfaces. In addition, we have characterized the binding of triethanolamine that is released upon deprotection of the silatrane to metal oxide surfaces and have explored methods to remove the deposited triethanolamine.

2. EXPERIMENTAL SECTION 2.1. Instrumentation. UV−vis absorption spectra were collected using a Shimadzu UV-2600 spectrophotometer. 1H NMR data were collected using an Agilent 400 MHz NMR instrument. X-ray photoelectron spectroscopic measurements were performed using a PHI VersaProbe II Scanning XPS Microprobe. Electrochemical measurements were performed using a Wavenow potentiostat from Pine Instruments. An aqueous Ag/AgCl (sat’d NaCl) reference electrode was purchased from BASi and used for all electrochemical measurements in water; 0.206 V (calculated by BASi) was added to reference the values to NHE. An Alphastep 200 profilometer was used to measure film thicknesses of the electrodes. 2.2. Synthesis. RuSil,6 tolSil,7 and PSil23 were synthesized by previously reported procedures. 2.3. Electrode (TIO2, SnO2, nanoITO) Preparation. SnO2, TiO2, and nanoITO electrodes were prepared by doctor blading their respective paste onto FTO-coated glass subtracts (TEC-15) supplied by Hartford Glass. SnO224 and nanoITO6,25,26 pastes were prepared using previously reported procedures. Ti-Nanoxide T/SP paste was purchased from Solaronix and was used for the TiO2 paste. A total of two layers of paste were doctor-bladed onto the FTO-coated glass substrates with annealing steps at 80 °C for 9 min after each layer. To complete the preparation of TiO2 and SnO2 electrodes, they were thermally annealed in a box oven by heating the samples from 25 to 370 °C at a rate of 180 °C/h, holding at 370 °C for 10 min, followed by heating to 400 °C at a rate of 180 °C/h, holding at 400 °C for 30 min, and finally cooling to room temperature. On average, TiO2 film thicknesses were ∼9.6 μm and SnO2 film thicknesses were ∼8.5 μm. To complete the preparation of nanoITO electrodes, two annealing steps were performed in a tube furnace. First, the nanoITO electrodes were annealed in air; the samples were heated from 25 to 500 °C at a rate of 4 °C/min, the temperature was held at 500 °C for 1 h, and finally cooled to room temperature at a down rate of 4 °C/min. Lastly, the electrodes were annealed under a 5% H2/N2 atmosphere; the samples were heated from 25 to 400 °C at a rate of 4 °C/min, the B


Forum Article


3. RESULTS AND DISCUSSION 3.1. Alternative Binding Methods. To increase the applicability of silatranes as surface anchors, we initially examined alternative surface binding methods. First, and most simply, we examined binding at room temperature. We chose to bind PSil (Figure 1), a CF3-substituted porphyrin, to three metal oxide (TiO2, SnO2, and nanoITO) surfaces because porphyrins have high molar extinction coefficients allowing for surface loadings to be easily quantified. In addition, the excited state potentials of free-base CF3-substituted porphyrins permit them to inject electrons into semiconductor electrodes, such as SnO2, and when oxidized the CF3-substituted porphyrins have potentials high enough to drive a water-oxidation catalyst, making them applicable in photoanodes for photocatalytic water oxidation used in WS-DSPECs.23 A 0.045 mM solution of PSil in toluene was used to sensitize the electrodes at room temperature overnight. TiO2 and SnO2 electrodes were chosen since they are common semiconductor electrodes used as photoanodes in WS-DSPECs,1,8,9,27−29 and conductive nanoITO electrodes were chosen since these are commonly employed as electrodes for electrochemically driven catalysis.7,25,26 Thus, the silatrane-bound electrodes could be applied in photocatalytic (TiO2, SnO2) or electrocatalytic (nanoITO) systems. After sensitization, the electrodes were rinsed three times with toluene, air-dried, and their UV−vis spectra collected. The UV−vis spectrum of surface-bound PSil compared well with the solution-based PSil, showing that the structure of PSil was likely retained on the electrode surface (Figure S4). Surface loadings were calculated from the formula, Γ(mol cm−2) = A(λ)/ (1000ε), where A is the absorbance at wavelength λ, and ε is the molar extinction coefficient at wavelength λ.30 The loadings for all samples were determined using the absorbance at 514 nm, except for the nanoITO electrodes prepared at room temperature, where the absorbance at 420 nm was monitored since the features at 514 nm were too low in intensity. Surprisingly, PSil was detected on TiO2, SnO2, and nanoITO electrodes under room temperature conditions, confirming that more mild conditions than previously described can indeed be used to bind silatranes to metal oxide surfaces. However, the loadings under the room temperature conditions were significantly lower than those at 70 °C (Table 1), being ∼2 times lower on SnO2, ∼ 2.5 times

increase the surface loadings. To test this, we soaked SnO2 electrodes in 0.1 M phosphate buffer at pH 2 to preacidify the electrodes and at pH 11 to prebasify the electrodes. After 3.5 h, the electrodes were air-dried and then exposed to the PSil sensitization solution at room temperature. After sensitization overnight, the electrodes were rinsed three times with toluene, air-dried, and their UV−vis spectra collected; the spectra were then compared to those from other binding methods (Figure S6). The electrodes prepared with an acidic pretreatment at room temperature had similar loadings to the electrodes prepared at 70 °C; in contrast, the samples prepared with a basic pretreatment resembled the loadings of those sensitized at room temperature with no pretreatment; base catalysis was, therefore, not considered further in our study. Silatranes are known to undergo faster hydrolysis in the presence of acid, which may explain the higher loadings with acid.18,31 Since the acidic pretreatment provided higher loadings under mild conditions, we explored this surface binding method on TiO2 and nanoITO electrodes as well. Again, the loadings increased significantly, compared to the room temperature binding without pretreatment; the loadings increased by ∼1.6 times on SnO2, ∼1.4 times on TiO2, and ∼9.5 times on nanoITO (Figure 2, Table 1). On SnO2, loadings under the preacidified conditions compared within error with those obtained by sensitizing PSil at 70 °C. Loadings on TiO2 were the highest using the 70 °C conditions, yet the loadings of the samples prepared from the preacidified surface remained high. The preacidified conditions increased the loadings most on nanoITO surfaces when compared to the 70 °C and room temperature binding conditions, likely due to the higher isoelectric point (IEP) of ITO compared to TiO2 and SnO2.32,33 To test if our alternative, milder, binding conditions were also applicable to other silatrane derivatives, we performed an identical loading analysis using RuSil. This was chosen because it is a known water-oxidation catalyst when phosphonic acid surface anchors are incorporated into the structure and the resulting material has been shown to perform electrochemically driven water-oxidation catalysis on nanoITO electrodes.9,25,34 A 0.12 mM solution of RuSil in ethanol was used to sensitize the electrodes overnight, they were rinsed with ethanol, and their UV−vis spectra collected. The UV−vis spectra of the surfacebound RuSil compares well with the solution based RuSil, indicating that the surface-bound species has again likely retained its structure upon binding (Figure S5). We found that RuSil could bind under room-temperature conditions, but the loadings were again lower than binding at 70 °C (Table 2). The acidic pretreatment of the metal oxide surface significantly improved RuSil loadings on SnO2 by ∼1.5 times, on TiO2 by ∼4 times, and on nanoITO by ∼1.6 times compared to room temperature binding. In addition, the preacidified surface binding method produced loadings within error of those from sensitization at 70 °C (Figure S11). Thus, we have confirmed silatranes can bind to SnO2, TiO2, and nanoITO under mild, room temperature conditions using a preacidified surface, resulting in high loadings comparable to binding at 70 °C. Importantly, the methodology of modifying the metal oxides surface pH using a pH pretreatment could also be applied to other surface anchors (phosphonic acid, carboxylic acid, hydroxamic acid, etc.) to increase and optimize the loading of those molecules on the metal oxide surface of interest. This will depend on the isoelectric point of the metal oxide surface and the nature (binding mode, pKa, presence of

Table 1. PSil Loadings on SnO2, TiO2, and nanoITO Electrodes under Different Surface Binding Conditions

SnO2 TiO2 nanoITO

Room Temperature (nmol cm−2)

acidic pretreatment and room temperature (nmol cm−2)

70 °C (nmol cm−2)

16.5 ± 0.3 29.0 ± 0.7 7.5 ± 0.2a

27.7 ± 8.1 41.7 ± 3.6 67.6 ± 18.8

31.4 ± 4.6 77.2 ± 3.7 43.8 ± 9.8

a

Loadings for the room temperature binding conditions were found using the absorbance at 420 nm, whereas the loadings for the other binding methods were found using the absorbance at 514 nm.

lower on TiO2, and ∼6 times lower on nanoITO. Because high loadings are desirable for catalytic applications, other strategies were explored. Because the loadings from sensitization at room temperature were low, we thought that pretreating the electrode under acidic or basic conditions could promote facile silatrane cage deprotection by acid or base-catalyzed deprotection and C


Forum Article


Figure 2. Representative UV−vis spectra of (A) SnO2-PSil, (B) TiO2-PSil, and (C) nanoITO-PSil. Electrodes were sensitized by three binding conditions: at room temperature (blue), under heating at 70 °C (purple), and with an acidic pretreatment and room temperature binding (pink).

Table 2. RuSil Loadings on SnO2, TiO2, and nanoITO

SnO2 TiO2 nanoITO

room temperature (nmol cm−2)

acidic pretreatment and room temperature (nmol cm−2)

70 °C (nmol cm−2)

149 ± 9 130 ± 10 121 ± 36a

236 ± 67a 530 ± 77 202 ± 53

228 ± 29 403 ± 90 242 ± 38

were from surface-bound TEOA or its protonated form. Because the samples were sensitized in acetonitrile, which also contains a N atom, we performed a control with a SnO2 electrode soaked in acetonitrile; the resulting XPS did not show any N 1s peaks indicating that acetonitrile does not bind stably to the electrodes (Figure S19). Taken together, the 1H NMR and XPS results confirm for the first time that TEOA does bind to the metal oxide surface during silatrane deprotection. Because triethanolamine is a known reducing agent, which may induce anodic side reactions or cause undesired oxidative current flow, we also explored routes to remove it from the metal oxide surface. We thought that protonation of TEOA under aqueous acidic conditions could favor TEOA surface desorption. To test this, we soaked tolSil-SnO2, tolSil-TiO2, and tolSil-nanoITO electrodes in 0.1 M phosphate buffer at pH 2 for 1 h, rinsed them with water, and reperformed XPS on the electrodes. The N 1s peaks were gone after this treatment confirming that an acidic wash can indeed remove TEOA from the electrode surface (Figure 3 and Figure S20). 3.3. Triethanolamine Removal. Previously, we saw evidence that cyclic voltammogram (CV) cycling might be a promising route for removing TEOA because it is easily oxidized to the very soluble tricarboxylic acid.7 A CV of tolSil and TEOA on a nanoITO electrode exhibits one irreversible peak at ∼1 V vs NHE. Upon repeated cycling, the feature

a

Used depletion method (see the Supporting Information) to determine loadings.

protecting group, number of surface bonds to form, etc.) of the anchoring group. Thus, a variety of pH pretreatments must be initially screened. 3.2. Triethanolamine Surface Binding. The triethanolamine (TEOA) released on surface-mediated deprotection of the silatrane cage has also been proposed to bind to the metal oxide surface.7 Because our alternative binding methods are milder, we wondered if the silatrane cage would fully deprotect or if triethanolamine would be retained by the metal oxide surfaces. To explore this, p-tolylsilatrane (tolSil) (Figure 1), which contains a single type of N atom, was bound to SnO2, TiO2, and nanoITO electrodes in acetonitrile. Three binding conditions were explored: (1) binding at room temperature, (2) binding at 70 °C, and (3) using an acidic pretreatment with binding at room temperature. 1H NMR spectra of the tolSil sensitization solution before and after surface binding were identical, showing no TEOA peaks, suggesting that TEOA remains on the electrode after sensitization and does not dissolve in the sensitization solution (Figures S17 and S18). X-ray photoelectron spectroscopy (XPS) was performed to examine the nitrogen content on the electrodes. All binding conditions generated N 1s signals on TiO2, SnO2, and nanoITO electrodes, indicating triethanolamine was likely bound to the electrode surfaces (Figure S19). The electrodes prepared via an acidic pretreatment had a N 1s peak shifted to a slightly higher binding energy likely due to protonation of triethanolamine. To test this point, electrodes sensitized with pure TEOA were prepared under identical conditions. Indeed, the N 1s peaks from the TEOA-sensitized samples were identical to those of the tolSil sensitized samples, suggesting that the N 1s signals

Figure 3. X-ray photoelectron spectra in the N 1s region of SnO2 sensitized with tolSil (green), after an acidic wash (blue), after CV cycling (gray), and blank SnO2 (pink). D


Forum Article


Figure 4. Percent RuSil retained on SnO2 (top left), TiO2 (top right) and nanoITO (bottom) after exposure to 0.1 M phosphate buffer for 24 h at pH 2 (pink), pH 7 (blue), and pH 11 (purple). Three different sensitization conditions were studied: acidic pretreatment with room temperature binding (“acidic”), room temperature binding (“RT”), and 70 °C binding (“70 °C”).

disappears suggesting that TEOA can be oxidatively removed from the electrode. To further examine this and test for TEOA removal on SnO2, TiO2, and nanoITO electrodes, we performed 12 CV cycles from 0 to 1.2 V vs NHE in 0.1 M phosphate buffer at pH 5 (Figure S21). Again, the N 1s peaks in the XPS were gone, suggesting that electrochemical oxidizing conditions can also be used to remove TEOA from electrode surfaces (Figure 3 and Figure S20). 3.4. Hydrolytic Stability. Previously, we observed excellent hydrolytic and electrochemical stability from pH 2−11 using RuSil-derivatized nanoITO electrodes prepared by sensitization at 70 °C.6 Because the new surface binding methods allowed for milder conditions to be used, we wanted to examine the range in stability in water of samples prepared by using the new binding methods. Higher temperatures were thought to be required to bind silatranes to electrodes in order to deprotect the silatrane cage, in order to form strong siloxane surface bonds.2,6,7,20,21 We wondered if the new, milder, binding methods would provide similar stability compared to binding at 70 °C. It seemed possible that milder binding conditions might not fully form the bidentate siloxane surface binding motif or bridging siloxane motif (Si−O−Si), which might limit stability.2,20,21 To probe this stability question, PSil was bound to TiO2, SnO2, and nanoITO electrodes using three binding methods (room temperature with and without acid pretreatment and at 70 °C). The electrodes were then soaked in pure toluene, the sensitization solvent, for 10 min to remove any unbound PSil, air-dried, and the UV−vis spectra collected. The electrodes were then exposed to 0.1 M phosphate buffer at pH 2, 7, and 11 for 24 h to examine the stability of surfacebound PSil under aqueous conditions. After 24 h, the UV−vis spectra (Figures S7−S9) of the electrodes were recollected and the percent PSil retained on the surface was calculated based on changes in UV−vis absorbance (Table S1, Figure S22). The %PSil retained on the nanoITO was the highest under all three

binding conditions, ranging from 93 to 100%; SnO2 electrodes ranged from 71 to 100% and TiO2 ranged from 85 to 100%. No pH effect was observed during the desorption experiments, showing that the new binding methods provided excellent stability under acidic, neutral, and basic aqueous conditions. PSil is also sparingly water-soluble, as evident by the lack of PSil features in the UV−vis spectra of the desorption solutions (Figure S10). The lack of water solubility could result in a hydrophobic effect, keeping a majority of PSil bound on the surface. To see if the silatrane would provide the same aqueous stability in the absence of the hydrophobic effect, we also performed desorption experiments under the same conditions using electrodes sensitized with the more water-soluble RuSil. The UV−vis spectra were collected before and after exposure to pH 2, 7, and 11 on sensitized SnO2, TiO2, and nanoITO electrodes (Figure S12−S14) and the %RuSil retained on the electrode surfaces was estimated (Table S2, Figure 4). For RuSil, no significant pH effect was observed. The %RuSil retained on the electrode ranged from 77 to 98% on SnO2, from 58 to 98% on TiO2, and from 86 to 100% on nanoITO after 24 h of desorption. RuSil was detected in the desorption solutions via UV−vis spectroscopy (Figure S15). The %RuSil retained on the electrode was only slightly lower than the %PSil retained, suggesting that the silatrane anchor was providing the majority of the stability on the electrode surfaces; however, we emphasize the value of having a water-insoluble molecule like PSil for increasing the water stability of the heterogenized species even further. To further support this strategy, an increase in water stability has also been observed previously using water-insoluble Zn-metalated porphyrins bound to TiO2.35 The porphyrins studied contained either a carboxylic acid, phosphonic acid, hydroxamic acid, or acetylacetonate surface anchor; all had excellent stability in water (neutral pH), which was attributed to the lack of water solubility.35 Thus, the E


Forum Article


Figure 5. Percent RuSil retained on TiO2 over 120 min at pH 2 (purple), pH 7 (pink), and pH 11 (blue) in 0.1 M phosphate buffer. Three different sensitization conditions were studied: (A) acidic pretreatment with room-temperature binding, (B) room-temperature binding, and (C) 70 °C binding.

milder sensitization condition provides just as stable binding as the harsher 70 °C condition. Therefore, to obtain the best results, electrodes should be sensitized at 70 °C or using an acidic pretreatment as described above. If the silatranecontaining molecule is sensitive to heat or acidic conditions, room temperature binding should be used. 3.5. Electrochemical Stability. Previously, we examined RuSil-nanoITO electrodes prepared at 70 °C and found excellent electrochemically stability from pH 2−11.6 Additionally, we were successful using IrSil (Figure 1) as a silatraneanchored complex on nanoITO for electrochemically driven water-oxidation catalysis at pH 5.8 and for photocatalytic wateroxidation on SnO2 electrodes at pH 6 without notable loss of the IrSil catalyst from either electrode.7,15 To test if the new binding conditions maintained this stability, the electrochemical stability using milder binding conditions was studied. Conductive nanoITO electrodes were chosen because they can be used to perform electrochemically driven catalysis, such as water-oxidation catalysis with heterogenized catalysts.7,25,26 Since the absorption features of RuSil on nanoITO are weak, surface loadings of RuSil on nanoITO were obtained using the depletion method described in the SI. To probe the electrochemical stability, RuSil-nanoITO was sensitized under the three binding conditions. The electrodes were exposed to an applied bias equivalent to a 400 mV overpotential for wateroxidation catalysis at the respective pH values: 1.512 V vs NHE at pH 2, 1.217 V vs NHE at pH 7, and 0.981 V vs NHE at pH 11. The bias was held for 30 min in 0.1 M phosphate buffer and the desorption of RuSil into the electrolyte was monitored, allowing the %RuSil retained on the electrode to be calculated (Table S4, Figure S23). Similar to the water stability studies, there was no evident pH effect and the %RuSil retained ranged from 85 to 90% under room temperature binding conditions, from 93 to 96% under acidic pretreatment and room temperature binding conditions, and between 92 and 97% under 70 °C binding conditions. The large values of %RuSil retained on nanoITO are similar to those in the water stability

results acquired using PSil confirm the effectiveness of this strategy. To further understand the water stability of the silatranederived anchor, we tracked the desorption of RuSil bound to TiO2 over time at pH 2, 7, and 11 to monitor when the majority of the RuSil was desorbed. TiO2 was chosen as the electrode since it provided the highest RuSil loadings and was the easiest to monitor over time. Prior to exposure to water, the electrodes were soaked in pure ethanol, the sensitization solvent, for 10 min to remove any excess RuSil adsorbed to the electrode. Figure 5 shows plots of %RuSil retained over 120 min for the three binding methods. Again, there is no trend with pH, the majority of measurements being within experimental error of each other. Upon exposure to aqueous conditions, a small percentage of RuSil desorbs in the first 30− 60 min; after 60 min, the %RuSil retained on the electrode remains relatively unchanged. Fast desorption in the first 30− 60 min suggested that some RuSil molecules are weakly bound to the electrode and are the first to desorb, but that those which have robust surface bonds remain on the surface over time. This type of desorption behavior has also been observed with hydroxamic acid-anchored molecules on TiO2 surfaces exposed to water and K2CO3 solutions. In the same prior study, the hydroxamic acid anchors were compared to a carboxylic acid anchor, which completely desorbed after 2 h of soaking in water and in a K2CO3 solution.13 This higher degree of lability emphasizes the advantage of using silatrane surface anchoring groups for heterogenizing molecules onto metal oxide surfaces. The %RuSil retained on the electrodes was monitored over 7 days to test for long-term stability. The %RuSil retained was found to range between 52 and 68% using room temperature binding conditions, between 69 and 80% with an acidic pretreatment and room temperature binding conditions, and between 73 and 81% using 70 °C binding conditions over pH 2−11 (Table S3, Figure S16). The results from electrodes sensitized at 70 °C were within experimental error of those prepared with an acidic pretreatment, suggesting that the new, F


Forum Article


(2) Materna, K. L.; Crabtree, R. H.; Brudvig, G. W. Anchoring Groups for Photocatalytic Water Oxidation on Metal Oxide Surfaces. Chem. Soc. Rev. 2017, 46, 6099−6110. (3) Young, K. J.; Martini, L. A.; Milot, R. L.; Snoeberger, R. C., III; Batista, V. S.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W. Light-Driven Water Oxidation for Solar Fuels. Coord. Chem. Rev. 2012, 256, 2503−2520. (4) Yu, Z.; Li, F.; Sun, L. Recent Advances in Dye-Sensitized Photoelectrochemical Cells for Solar Hydrogen Production Based on Molecular Components. Energy Environ. Sci. 2015, 8, 760−775. (5) Bangle, R.; Sampaio, R. N.; Troian-Gautier, L.; Meyer, G. J. Surface Grafting of Ru(II) Diazonium-Based Sensitizers on Metal Oxides Enhances Alkaline Stability for Solar Energy Conversion. ACS Appl. Mater. Interfaces 2018, 10, 3121−3132. (6) Materna, K. L.; Brennan, B. J.; Brudvig, G. W. Silatranes for Binding Inorganic Complexes to Metal Oxide Surfaces. Dalton Trans. 2015, 44, 20312−5. (7) Materna, K. L.; Rudshteyn, B.; Brennan, B. J.; Kane, M. H.; Bloomfield, A. J.; Huang, D. L.; Shopov, D. Y.; Batista, V. S.; Crabtree, R. H.; Brudvig, G. W. Heterogenized Iridium Water-Oxidation Catalyst from a Silatrane Precursor. ACS Catal. 2016, 6, 5371−5377. (8) Swierk, J. R.; Mallouk, T. E. Design and Development of Photoanodes for Water-Splitting Dye-Sensitized Photoelectrochemical Cells. Chem. Soc. Rev. 2013, 42, 2357−2387. (9) Brennaman, M. K.; Dillon, R. J.; Alibabaei, L.; Gish, M. K.; Dares, C. J.; Ashford, D. L.; House, R. L.; Meyer, G. J.; Papanikolas, J. M.; Meyer, T. J. Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells. J. Am. Chem. Soc. 2016, 138, 13085− 13102. (10) Kirner, J. T.; Finke, R. G. Water-Oxidation Photoanodes using Organic Light-Harvesting Materials: A Review. J. Mater. Chem. A 2017, 5, 19560−19592. (11) Nayak, A.; Roy, S.; Sherman, B. D.; Alibabaei, L.; Lapides, A. M.; Brennaman, M. K.; Wee, K.-R.; Meyer, T. J. Phosphonate-Derivatized Porphyrins for Photoelectrochemical Applications. ACS Appl. Mater. Interfaces 2016, 8, 3853−3860. (12) McNamara, W. R.; Snoeberger, R. C., III; Li, G.; Richter, C.; Allen, L. J.; Milot, R. L.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W.; Batista, V. S. Hydroxamate Anchors for Water-Stable Attachment to TiO2 Nanoparticles. Energy Environ. Sci. 2009, 2, 1173− 1175. (13) McNamara, W. R.; Milot, R. L.; Song, H.-e.; Snoeberger, R. C., III; Batista, V. S.; Schmuttenmaer, C. A.; Brudvig, G. W.; Crabtree, R. H. Water-stable, Hydroxamate Anchors for Functionalization of TiO2 Surfaces with Ultrafast Interfacial Electron Transfer. Energy Environ. Sci. 2010, 3, 917−923. (14) Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. B.; Lee, J. S. Effects of Surface Anchoring Groups (Carboxylate vs Phosphonate) in Ruthenium-Complex-Sensitized TiO2 on Visible Light Reactivity in Aqueous Suspensions. J. Phys. Chem. B 2004, 108, 14093−14101. (15) Materna, K. L.; Jiang, J.; Regan, K. P.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W. Optimization of Photoanodes for Photocatalytic Water Oxidation by Combining a Heterogenized Iridium Water-Oxidation Catalyst with a High-Potential Porphyrin Photosensitizer. ChemSusChem 2017, 10, 4526−4534. (16) Voronkov, M. G.; Belyaeva, V. V.; Abzaeva, K. A. Basicity of Silatranes (Review). Chem. Heterocycl. Compd. 2012, 47, 1330−1338. (17) Voronkov, M. G. Silatranes: Intra-Complex Heterocyclic Compounds of Pentacordinated Silicon. Pure Appl. Chem. 1966, 13, 35−60. (18) Puri, J. K.; Singh, R.; Chahal, V. K. Silatranes: A Review on Their Synthesis, Structure, Reactivity and Applications. Chem. Soc. Rev. 2011, 40, 1791−840. (19) Frye, C. L.; Vincent, G. A.; Finzel, W. A. Pentacoordinate Silicon Compounds. J. Am. Chem. Soc. 1971, 93, 6805−6811. (20) Brennan, B. J.; Llansola Portoles, M. J.; Liddell, P. A.; Moore, T. A.; Moore, A. L.; Gust, D. Comparison of Silatrane, Phosphonic acid, and Carboxylic Acid Functional Groups for Attachment of Porphyrin

studies suggesting that the majority of the RuSil desorbed may likely be from weakly bound RuSil in the presence of water and less likely due to the applied bias. Regardless, this suggests that the new binding conditions are also electrochemically robust.

4. CONCLUSIONS We find that silatrane anchors can be bound to metal oxide surfaces under two new, mild binding conditions: (1) at room temperature and (2) using preacidified electrodes with roomtemperature binding on SnO2, TiO2, and nanoITO electrodes. In addition, we have confirmed that the triethanolamine released on deprototection of the silatrane also binds to the metal oxide electrodes under all binding conditions but can easily be removed using an aqueous acidic wash or by CV cycling. The loadings and stability of the new binding methods were explored and compared to the previous method of binding at 70 °C. The samples prepared from preacidified electrodes with room temperature binding had loadings and hydrolytic and electrochemical stabilities from pH 2−11 comparable to those prepared at 70 °C. Even though the loadings were slightly lower using the room-temperature binding method, the aqueous and electrochemical stability remained robust. Because milder binding conditions can now be employed, silatraneanchored molecules used for heterogenization studies do not need to be stable under high temperatures as was previously needed, allowing for a wider selection of molecules to be heterogenized and used for catalytic applications on metal oxide electrodes with silatrane anchors. Silatrane stabilities on other metal oxide electrodes are currently being explored as well as the stability in reducing environments.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04138. Further details on experimental conditions (surface loadings), extra figures (UV−vis spectra, XPS, electrochemistry), and Tables S1−S4 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robert H. Crabtree: 0000-0002-6639-8707 Gary W. Brudvig: 0000-0002-7040-1892 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science (DE-FG0207ER15909). Additional support was provided by a generous gift from the TomKat Foundation. We thank Dr. Bradley Brennan for helpful discussions.

■

REFERENCES

(1) Ashford, D. L.; Gish, M. K.; Vannucci, A. K.; Brennaman, M. K.; Templeton, J. L.; Papanikolas, J. M.; Meyer, T. J. Molecular Chromophore-Catalyst Assemblies for Solar Fuel Applications. Chem. Rev. 2015, 115, 13006−13049. G


Forum Article

ACS Applied Materials & Interfaces Sensitizers to TiO2 in Photoelectrochemical Cells. Phys. Chem. Chem. Phys. 2013, 15, 16605−14. (21) Brennan, B. J.; Keirstead, A. E.; Liddell, P. A.; Vail, S. A.; Moore, T. A.; Moore, A. L.; Gust, D. 1-(3′-Amino)propylsilatrane Derivatives as Covalent Surface Linkers to Nanoparticulate Metal Oxide Films for Use in Photoelectrochemical Cells. Nanotechnology 2009, 20, 505203. (22) Brennan, B. J.; Gust, D.; Brudvig, G. W. Organosilatrane Building Blocks. Tetrahedron Lett. 2014, 55, 1062−1064. (23) Jiang, J.; Spies, J. A.; Swierk, J. R.; Matula, A. J.; Regan, K. P.; Romano, N.; Brennan, B. J.; Crabtree, R. H.; Batista, V. S.; Schmuttenmaer, C. A.; Brudvig, G. W. Direct Interfacial Electron Transfer from High-Potential Porphyrins into Semiconductor Surfaces: A Comparison of Linkers and Anchoring Groups. J. Phys. Chem. C 2018, DOI: 10.1021/acs.jpcc.7b12405. (24) Antoniuk-Pablant, A.; Terazono, Y.; Brennan, B. J.; Sherman, B. D.; Megiatto, J. D.; Brudvig, G. W.; Moore, A. L.; Moore, T. A.; Gust, D. A New Method for the Synthesis of β-cyano Substituted Porphyrins and Their Use as Sensitizers in Photoelectrochemical Devices. J. Mater. Chem. A 2016, 4, 2976−2985. (25) Chen, Z.; Concepcion, J. J.; Hull, J. F.; Hoertz, P. G.; Meyer, T. J. Catalytic Water Oxidation on Derivatized nanoITO. Dalton Trans. 2010, 39, 6950−6952. (26) Sheehan, S. W.; Thomsen, J. M.; Hintermair, U.; Crabtree, R. H.; Brudvig, G. W.; Schmuttenmaer, C. A. A Molecular Catalyst for Water Oxidation that Binds to Metal Oxide Surfaces. Nat. Commun. 2015, 6, 6469. (27) Kamire, R. J.; Materna, K. L.; Hoffeditz, W. L.; Phelan, B. T.; Thomsen, J. M.; Farha, O. K.; Hupp, J. T.; Brudvig, G. W.; Wasielewski, M. R. Photodriven Oxidation of Surface-Bound IridiumBased Molecular Water-Oxidation Catalysts on Perylene-3,4-dicarboximide-Sensitized TiO2 Electrodes Protected by an Al2O3 Layer. J. Phys. Chem. C 2017, 121, 3752−3764. (28) Lapides, A. M.; Sherman, B. D.; Brennaman, K.; Dares, C. J.; Skinner, K. R.; Templeton, J.; Meyer, T. J. Synthesis, Characterization, and Water Oxidation by a Molecular Chromophore-Catalyst Assembly Prepared by Atomic Layer Deposition. The “Mummy” Strategy. Chem. Sci. 2015, 6, 6398−6406. (29) Moore, G. F.; Blakemore, J. D.; Milot, R. L.; Hull, J. F.; Song, H.-e.; Cai, L.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W. A Visible Light Water-Splitting Cell with a Photoanode Formed by Codeposition of a High-Potential Porphyrin and an Iridium WaterOxidation Catalyst. Energy Environ. Sci. 2011, 4, 2389−2392. (30) Trammell, S. A.; Meyer, T. J. Diffusional Mediation of Surface Electron Transfer on TiO2. J. Phys. Chem. B 1999, 103, 104−107. (31) Párkányi, L.; Hencsei, P.; Bihátsi, L.; Kovács, I.; Szöllő sy, Á . The Effect of the C-methyl Substitution on the Molecular Geometry of the Silatrane Skeleton: The Crystal Structures of 1-(4′-tolyl)-silatrane and 1-(4′-tolyl)-3,7,10-trimethylsilatrane. Polyhedron 1985, 4, 243−249. (32) Son, D. Y.; Lee, C. R.; Shin, H. W.; Jang, I. H.; Jung, H. S.; Ahn, T. K.; Park, N. G. Understanding the Role of the Dye/Oxide Interface via SnO2-Based MK-2 Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2015, 17, 15193−200. (33) H. Pulker, H. S., Aegerter, M.A. Coatings on Glass 1998; Elsevier Science: Amsterdam, 1999; p 484. (34) Chen, Z.; Concepcion, J. J.; Jurss, J. W.; Meyer, T. J. Single-site, Catalytic Water Oxidation on Oxide Surfaces. J. Am. Chem. Soc. 2009, 131, 15580−15581. (35) Martini, L. A.; Moore, G. F.; Milot, R. L.; Cai, L. Z.; Sheehan, S. W.; Schmuttenmaer, C. A.; Brudvig, G. W.; Crabtree, R. H. Modular Assembly of High-Potential Zinc Porphyrin Photosensitizers Attached to TiO2 with a Series of Anchoring Groups. J. Phys. Chem. C 2013, 117, 14526−14533.

H


Silatrane Anchors for Metal Oxide Surfaces - ACS Publications

Recommend Documents