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Fabricating Appropriate Band-Edge-Staggered HeteroSemiconductors with Optically Activated Au Nanoparticles via Click Chemistry for Photoelectrochemical Water Splitting Arun Prakash Upadhyay, Dilip Kumar Behara, Gyan Prakash Sharma, Maurya Gyanprakash, Raj Ganesh S Pala, and Sri Sivakumar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.6b00335 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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ACS Sustainable Chemistry & Engineering

Fabricating Appropriate Band-Edge-Staggered HeteroSemiconductors with Optically Activated Au Nanoparticles via Click Chemistry for Photoelectrochemical Water Splitting Arun Prakash Upadhyay1, Dilip Kumar Behara1, Gyan Prakash Sharma1, Maurya Gyanprakash1, Raj Ganesh S Pala1,2*, Sri Sivakumar1, 2, 3* 1

Department of Chemical Engineering, Indian Institute of Technology Kanpur

2

Material Science Programme, Indian Institute of Technology Kanpur

3

Centre for Environmental Science & Engineering, Thematic Unit of Excellenceon Soft

Nanofabrication, Indian Institute of Technology Kanpur

Corresponding Author Email Id: [email protected], [email protected]

ACS Paragon Plus Environment


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Abstract Semiconductor-mediated photoelectrochemical (PEC) water splitting to generate hydrogen and oxygen has gained tremendous attention as it has the capability to overcome the world energy crisis. However, limited solar light absorption and high charge carrier’s recombination rate are major bottlenecks in achieving the desired efficiency of PEC devices. Fabrication of semiconductor-metal/metal oxide heterostructure holds a great promise to overcome these bottlenecks as it facilitates solar light absorption, separation and transport of charge-carriers. To this end, we demonstrate a fabrication methodology to design stable tailored heterostructures via click chemistry. To test the proposed methodology, we choose gold (Au), rutile-TiO2 (R-TiO2) and anatase TiO2 (A-TiO2) as model system and designed tailored triad heterostructure i.e. Au@R-TiO2@A-TiO2 over stainless steel and demonstrated its application in photoelectrochemical (PEC) water splitting. We hypothesize that the presence of Au and RTiO2/A-TiO2 interface in triad heterostructures plays an important role in improving the solar light absorption and charge carrier separation respectively. This is supported by the PEC water splitting measurements which shows that the fabricated Au@R-TiO2@A-TiO2@SS triad heterostructure possesses highest applied bias photo-conversion efficiency (ABPE) of ~0.4% at 0.95V applied bias and electrical and solar power-to-hydrogen (ESPH) efficiency of ~4.0% at 1.1V applied bias as compared to other fabricated heterostructures over stainless steel (RTiO2@A-TiO2@SS, R-TiO2@SS, A-TiO2@SS and Au@SS). Further, we have also studied the interfacial kinetics at electrode/electrolyte interface of fabricated heterostructures via electrochemical impedance spectra (EIS) which demonstrate the improved charge carrier transport in Au@R-TiO2@A-TiO2@SS triad heterostructure. Keywords: Titanium di-oxide (TiO2), gold nanoparticles, photoelectrochemical, solar energy, water splitting, surface plasmon resonance and click chemistry.


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Introduction Photoelectrochemical (PEC) splitting of water into hydrogen and oxygen is an important approach to convert solar energy into chemical energy.1-3 Performance of PEC water splitting is largely depends on the efficiency and stability of photoanode where metal oxide semiconductors are mainly used for promoting oxidation reaction. In order to achieve better efficiency in oxidation reaction, photoanode materials consist of oxide semiconductor should possess following characteristics: (1) efficient absorption of solar photons, (2) suitable band alignment with respect to oxygen evolution reaction, (3) efficient electron-hole separation to facilitate redox reaction, and (4) high chemical and photostability.4-7 However, most of the semiconductors do not posses either desirable band gap or reduced charge carrier recombination rate and combination of both which impediment their performance in PEC water splitting reaction.4-7 To circumvent these challenges, many strategies have been developed among which fabrication of semiconductor-metal/metal oxide heterostructures exhibit promising performance because of the improved solar light absorption and effective separation of generated charge carriers at the heterojuntion and therefore increases the reduction/oxidation reaction at heterostructure surface.8 To fabricate such heterostructures, approaches such as physical mixing,9 layer-by-layer (LbL) assembly via electrostatic interaction,10, 11 electro-deposition, dissolutionreprecipitation,12 wet-impregnation,13 dip-coating,14,

15

core-shell approach,16-19sputtering,20,

21

sol-gel methods,17and thermal treatment method have been widely explored so far.21 Though these approaches exhibits improved efficiency, they may suffer from one or combination of following challenges: 1) Lack of precise control in the formation of uniform and desired interface, 2) formation of defects due to lattice mismatch at the interface, 3) stability in different conditions (i.e. pH, temperature), 4) limitation in the choice of materials due to lattice mismatch,



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reaction conditions, etc. and 5) change of composition, porosity, surface area during the fabrication process (e.g. thermal treatment). Thus, it is desirable to develop a generic fabrication methodology to design heterostructure with tailored interface which may overcome the above said challenges. Based on our previous work,22 we believe that utilization of click chemistry to fabricate the tailored heterostructure can circumvent a subset of the above mentioned challenges. It is well known that click chemistry possess several advantages such as orthogonality to different functional group which facilitate the fabrication of different semiconductor/metal oxides heterostructure, simple reaction conditions, high chemical yield, easy and fast reaction. 2326

We believe that the proposed approach can have the following advantages: (1) tailored

interface formation which can increase the charge-carrier generation and separation upon solar light exposure, (2) increased stability through triazole linkage between preformed nanoparticlenanoparticle and nanoparticle-substrate, and (3) easily extendable to various materials having different morphology such as nanotube, nanoarrays, and nanowires. To test the proposed approach, we use gold (Au), rutile-TiO2 (R-TiO2) and anatase TiO2 (A-TiO2) as model system to fabricate the different heterostructures and showed their application towards PEC water splitting reaction. We have selected polymorphs of TiO2 semiconductor i.e. anatase and rutile as model photocatalyst because of their suitable band edges positions, low cost and high chemical stability.27-29 Further, heterostructure of anatase and rutile can easily facilitate charge migration without the formation of any additional charge-transfer barrier due to structural defects because both possess the similar chemical composition and crystal lattice. Additionally, gold nanoparticle is used to extend the solar light absorption of heterostructure in visible region due to its surface Plasmon resonance effect (SPR) and also it can act as a photo-induced charge carrier sink which further contribute in charge carrier separation.4, 6, 30-32


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The current report comprises of two parts: first part describes the fabrication of different heterostructures via combination of anatase-TiO2, rutile-TiO2, and gold over stainless steel substrates using click chemistry. The reasons behind the selection of stainless steel (SS) substrate are: 1) SS substrate is economically viable compared to FTO and ITO substrate, 2) it is corrosion resistant, 3) its good conducting property makes it appropriate substrate for use in alkaline medium for OER studies and 4) unlike ITO and FTO, stainless steel has high mechanical strength making it suitable for rugged applications.33 In the second part, we demonstrate the application of these fabricated heterostructure towards the photoelectrochemical water splitting as emphasized in the recent reports by Buriak, Gary Hodes and Domen.1, 34, 35 The PEC water splitting performance have been carried out in two electrode configuration instead of three electrode configuration in order to quantify the PEC cell performance taking into account the overpotential losses in both counter and working electrodes.We observed the applied bias photo-conversion efficiency (ABPE) of ~0.40% at ~0.95V applied bias with the Au@R-TiO2@A-TiO2@SS triad heterostructure which is higher as compared to other fabricated heterostructures. Additionally, we have calculated the intrinsic solar to chemical i.e. ISTC efficiency of fabricated heterostructures in three-electrode configuration to evaluate the maximum power which can be generated from heterostructures. The maximum ISTC of ~ 0.175 at 1.55 V (vs. RHE) potential under light is observed with Au@R-TiO2@A-TiO2@SS as compared R-TiO2@A-TiO2@SS (~ 0.085 at 1.456 V vs. RHE) heterostructures. However, none of the above efficiencies i.e. ABPE and ISTC involves the total power output based on the total current density obtained from the contribution from applied bias and solar light. In order to evaluate the performance of PEC cell based on the total observed current density, we have proposed a new efficiency called “electrical and solar power-to-



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hydrogen” (ESPH) and it is based on the definition of efficiency i.e. total power output divided by total power input. In detail, the proposed “electrical and solar power-to-hydrogen” (ESPH) efficiency formula include the contribution of externally supplied electrical power in addition to the power input along with the solar power (detailed explanation is given in section 5). The ESPH efficiency of ~ 4.0% and 11.5% at ~ 1.1V and 1.7 V applied bias respectively was observed with Au@R-TiO2@A-TiO2@SS triad heterostructures which is higher as compared with other combinations (e.g. R-TiO2@A-TiO2@SS, R-TiO2@SS, A-TiO2@SS, Au@RTiO2@SS, Au@A-TiO2@SS, and Au@SS). Further, kinetics study at the electrode/electrolyte interface via electrochemical impedance spectra (EIS) confirms the efficient charge carrier transport in Au@R-TiO2@A-TiO2@SS triad heterostructure. Result and Discussion 1. Rationale for material design Material design for the present work is based on the following principles. Upon light illumination, Au nanoparticles absorbs

visible light and can transfer the energetically hot

electrons to the conduction band of R-TiO2 nanoparticles (pathway (i)) and causes the negative shift in the R-TiO2 Fermi level to achieve Fermi level equilibration.6, 32 Further, the accumulated electrons in the conduction band of R-TiO2 can transfer from R-TiO2 to A-TiO2 (pathway (ii)) because of higher conduction band energy level of R-TiO2 w.r.t A-TiO2 followed by transfer of electrons from A-TiO2 to stainless steel (pathway (iii)).32,

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Due to the anticipated above

mentioned processes, we were motivated to fabricate Au@R-TiO2@A-TiO2@SS triad heterostructures (Figure 1) via click chemistry for photoelectrochemical water splitting.


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2. Fabrication of heterostructures Fabrication of plasmonic metal-semiconductor nanoparticles triad heterostructures on stainless steel (SS) substrate via click chemistry is shown in scheme 1. Surfaces of substrates and nanoparticles were modified with click moieties i.e. alkyne and azide-functionalization by adapting the procedure from our previous work.22 The functional group on substrate and nanoparticles were characterized by FTIR spectroscopy. Peaks at ~ 2100 cm-1 and 2270 cm-1 confirms the presence of alkyne and azide groups on both substrates and nanoparticles. Further, after surface modification these nanoparticles were assembled in different configuration on a stainless steel foil via click chemistry for the fabricate heterostructures.22 Triad heterostructures i.e. gold@rutile-TiO2@anatase-TiO2@stainless steel (Au@RTiO2@A-TiO2@SS) is fabricated via three steps as shown in scheme 1; Firstly, monolayer of azide-functionalized A-TiO2 nanoparticles were formed through click reaction between the azide group of A-TiO2 and alkyne group of SS in the presence of copper (I) solution. Secondly, next layer of R-TiO2 nanoparticle is clicked by reacting alkyne group of R-TiO2 with free azide group of A-TiO2. Finally, azide-Au nanoparticles are clicked over the R-TiO2 nanoparticles with free alkyne group present on R-TiO2 (details are given in electronic supporting information). Additionally, different heterostructures such as A-TiO2@SS, R-TiO2@SS, Au@R-TiO2@SS, Au@A-TiO2@SS, R-TiO2@A-TiO2@SS, A-TiO2@R-TiO2@SS and Au@A-TiO2@R-TiO2@SS are also fabricated using the above described procedure in order to show the applicability and flexibility of approach in designing the optimal and stable heterostructures with tailored interface for efficient PEC water splitting reaction. We believe that though the above photocatalyst heterostructures were anchored with each other through click bond either by any of one or combination of ways as shown in scheme S1. Further, they form Schottky contact (as work



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function of Au is greater than TiO2) with each other and generate enough driving force to facilitate the transfer of charge carriers to carry out the surface redox reactions for efficient water splitting reaction.

3. Characterization Scanning electron microscopy (SEM) images (Figure 2) confirms the formation of different heterostructures. Figure 2a and b shows clearly the monolayer assembly of azidefunctionalized A-TiO2 and R-TiO2 over the large area of alkyne-functionalized SS substrate (~ 2 cm2). Further, Figure 2c and S1c shows the second layer of alkyne-functionalized R-TiO2 and ATiO2 over the monolayer formed in Figure2a and b on SS respectively. Furthermore, Figure 2d and S1d shows the SEM images of triad heterostructures with third (i.e. final) layer of azidefunctionalized Au nanoparticles over the second layer of alkyne-functionalized R-TiO2 and ATiO2 as shown in Figure 2c and S1c respectively, via click chemistry. However, above SEM images show that the nanoparticles were not closely packed which we attributed to the irregular shape of TiO2 nanoparticles. This irregular shape of TiO2 nanoparticles facilitates the assembly of some gold nanoparticles at the interface of R-TiO2 and A-TiO2nanoparticles (shown by arrow in Figure 2d) because of which transfer of electrons from Au nanoparticles to conduction band of A-TiO2 along with transfer to R-TiO2 (pathway (iii) in Figure 1) can also be possible. Based on the above material design and fabrication procedure, we anticipate that the performance of the triad heterostructures (i.e. Au@R-TiO2@A-TiO2@SS) will be enhanced because of the convolution of following; a) increased absorption of light by clicked Au, R-TiO2 and A-TiO2 nanoparticles in triad heterostructures, b) increased charge carrier concentration at the interfaces due to strong SPR effect of Au nanoparticles, and c) enhanced charge carrier’s separation due to the formation of tailored interface and increased redox reaction rate.32 Further, physical presence


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of gold on SS substrate cannot be ruled out completely in the case of Au@R-TiO2@A-TiO2@SS heterostructure (Figure 2d) as both have complementary click moieties; however, this has been minimized by controlling the gold nanoparticles concentration and repetitive washing with ethanol (Figure S1e). Additionally, Figure S1a and S1b show the formation of gold nanoparticles bilayer on the monolayer of R-TiO2 and A-TiO2 over large area of SS substrate. Further, XRD results (Figure 3 and S2a-b) clearly confirm the presence of R-TiO2, A-TiO2 and Au nanoparticles in Au@R-TiO2@A-TiO2@SS fabricated heterostructures without forming the mixed phases of R-TiO2 and A-TiO2. Further, Raman spectra (Figure S2c-d) show the peaks at 143(Eg), 395 (B1g), 513 (A1g) and 639 cm-1(Eg) which are attributed to A-TiO2 and peaks at 210, 400 and 610 cm-1 to R-TiO2 phase. Additionally, a weak peak at 343 cm-1 (A1g, symmetric stretch vibrational mode) confirms the presence of Au nanoparticles in Au@R-TiO2@A-TiO2@SS fabricated heterostructures.

4. Photoelectrochemical measurements The performance of fabricated heterostructures towards photoelectrochemical (PEC) water splitting have been evaluated using two probe method as emphasized by Buriak, Gary Hodes and Domen.1,

34, 35

Briefly, for accurate measurement of PEC water splitting two

conditions have to be enforced. Firstly, photocurrent should be measured at constant potential i.e. chronoamperometry measurement where capacitative current contribution is absent. Secondly, two electrode measurements are required to quantify the bias between the working and the counter electrode so that the overall PEC cell performance can be evaluated. In contrast to the two electrode configuration, the three electrode configuration provides a measure of only the half-cell efficiency of the working electrode.



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In the accordance to the standardized approach,1,

34, 35

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we measured the applied bias

potential between working and counter electrode using voltmeter during the chronoamperometry measurement and obtained current density vs. applied bias (I-V) curves for all fabricated heterostructures (Figure 4, S3 and S4). Figure 4 shows the I-V curves of fabricated heterostructures up to ~1.5 V applied bias whereas Figure S3 shows the I-V curves of same heterostructures up to large applied bias values i.e. 2.2 V. The current density vs. applied bias curves shown in Figure 4, S3 and S4 clearly suggests that the noble metal and interfaces between each photocatalyst (i.e. Au, R-TiO2, and A-TiO2) plays an important role in enhancing the PEC cell performance. We attribute this enhancement to the convolution of these effects: (1) photon absorption, (2) charge carrier generation, (3) charge carrier separation, and (4) migration to catalyst surface to perform the surface chemical reactions. For example, Figure S4 shows that Au@A-TiO2@SS and Au@R-TiO2@SS heterostructures exhibits higher current density as compared with A-TiO2@SS and R-TiO2@SS heterostructures (shown in Figure 4) respectively. This increment in the current density is mainly because of increased photon absorption due to the gold (Au) nanoparticles present in the heterostructures. Further, we observed that azidefunctionalization of gold nanoparticles (size ~ 20 nm) does not affect the absorption properties of gold nanoparticles (Figure S5a). Furthermore, R-TiO2@A-TiO2@SS (Figure 4 and S3c) shows higher current density as compared to A-TiO2@SS (Figure 4 and S3a) and R-TiO2@SS (Figure 4 and S3b) heterostructures respectively. This is attributed mainly to (a) increased charge carrier separation and migration to catalyst surface and (b) increased surface roughness as a result of R-TiO2/ATiO2 interface which can be further related to higher electrochemical surface area in R-TiO2@ATiO2@SS heterostructures. These two possibilities are further supported by the UV absorption


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(Figure S5b) which clearly suggests that R-TiO2@A-TiO2@SS, R-TiO2 and A-TiO2 heterostructures have nearly same photon absorption and hence the increment in the current density observed is mainly because of the presence of R-TiO2/A-TiO2 interface. Additionally, we have observed that incorporation of Au nanoparticle in R-TiO2@A-TiO2@SS heterostructures further enhances the performance because of the strong SPR effect of Au nanoparticle which shifts the absorption in visible region. For example, Au@R-TiO2@A-TiO2@SS triad heterostructures (Figure 4 and S3d) exhibit the current density of ~ 1.5 mA/cm2 and 45.2 mA/cm2 at 0.9 V and 2.2 V applied bias respectively and under illumination which is higher than the current density observed with all other fabricated heterostructures. Additionally, we believe that comparison of dark and light current density of heterostructures with and without gold nanoparticles can provide supporting evidence for the role of gold nanoparticles SPR effect towards increasing the performance of fabricated heterostructures. For example, under dark condition (at ~1.1 V applied bias), current density of Au@R-TiO2@A-TiO2@SS heterostructure is slightly higher (~ 1.3 times) than the current density observed with R-TiO2@A-TiO2@SS heterostructure which we attributed to the presence of additional catalytic sites provided by the gold nanoparticles. Whereas, under the light illumination the current density of Au@R-TiO2@ATiO2@SS heterostructure is ~ 3 times the current density observed with R-TiO2@A-TiO2@SS heterostructure at ~1.1 V applied bias (Figure 4). We believe that this increment is mainly because of the SPR effect of the gold nanoparticles.

5. Efficiency calculation Performance of fabricated heterostructures is evaluated using applied bias photon conversion (ABPE), intrinsic solar to chemical conversion (ISTC) and the newly proposed ESPH



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efficiency.First we have calculated the ABPE efficiency which evaluates the PEC cell performance under external potential using eq. 1:

ηABPE=

×(. )×

……………….(1)

where 1.23 V is the minimum thermodynamic potential required for water splitting, IPh is the photocurrent density in mA/cm2 (i.e. difference between the total PEC current density and dark current density), Vbias is the applied biased potential between working electrode (WE) and counter electrode (CE) in volts, ηF is the faradaic efficiency (~ 98%) which is calculated as the ratio of rate of hydrogen generated during PEC water splitting and rate of hydrogen based on the observed current density during the collection of hydrogen and Psolar is the intensity of incident solar light (mW/cm2). However, many research groups have used working electrode potential in place of Vb, but such measurements can only give half-cell efficiency. In order to compare the two and three electrode configuration, we calculated the ABPE efficiency in both configurations and observed that wrong selection of Vb values resulted in overestimated PEC cell efficiency (Figure S6 and Table S1). Further, Figure 5 and S4b show the plot of ABPE efficiency vs. applied bias potential (Vb) of fabricated heterostructures. The Au@R-TiO2@A-TiO2@SS exhibits the optimal ABPE efficiency of ~0.4% while R-TiO2@A-TiO2@SS shows optimal conversion efficiency of ~0.30% at 0.82 V applied bias respectively. The ABPE results further supports our anticipation that presence of R-TiO2/A-TiO2interface and Au nanoparticle are playing an important role in enhancing the performance of triad heterostructures in comparison with other heterostructures. However, ABPE efficiency calculated using eq.1 is only applicable when applied bias value is below 1.23V as ABPE value becomes negative for applied bias more than 1.23 V. In addition, most of the efficient photoanodes exhibits much improved performance at the applied


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bias more than 1.23 V and for such photoanodes, recently Grätzel and co-workers37 proposed a new measure of efficiency called “Intrinsic solar to chemical conversion (ISTC)” which is based on the decoupling of photoanode into photovolatics effect and electrolysis effect. This efficiency is calculated in the three-electrode configuration (Figure S7) which measures the maximum intrinsic power of photoanode to convert the solar light into chemical energy using eq.2: ISTC=

×. ()

×

)× ()

(

………… (2)

where jphoto is photocurrent in mA/cm2 which is the difference of current density obtained with and without light at the same potential whereas Vphoto is the difference of light (Vlight) and dark (Vdark) potential to obtain the same current density observed at the Vlight potential. Further, photocurrent is plotted against photovoltage (Vph) as shown in Figure S7a-b which gives the maximum internal photovoltaic power of photoanode. For example, under light Au@RTiO2@A-TiO2@SS heterostructures exhibit the current density of ~ 3.1 mA/cm2 at 1.55 V (vs. RHE) potential whereas in dark condition same heterostructures needs 1.64 V (vs. RHE) potential to obtain the same current density which clearly suggests that solar powers saves potential of 0.09 V from the external source or equivalent power of ~ Vph x jph i.e. 0.09 V x 3.0 mA/cm2 = 0.27mW/cm2. Further, they reported that external power saved because of the photoanode photovoltaic power is given by the reduction in the conversion efficiency of water electrolysis reaction i.e. 1.23 (VRHE)/Vdark,RHE ~ 75.0 %. Thus, overall intrinsic solar light to chemical power (ISTC) of this photoanode is ~ ηF × 0.75×0.27 = 0.175 (Figure S7c-d). The maximum ISTC of ~ 0.094 is obtained with Au@R-TiO2@A-TiO2@SS heterostructures as compared to other fabricated heterostructures which clearly suggesting that presence of R-TiO2/A-TiO2 interface and Au nanoparticles increases the intrinsic power of photoanode. Significance of ISTC can be understood as follows; maximum current density



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obtained at 1.55 V potential (vs. RHE) with Au@R-TiO2@A-TiO2@SS triad heterostructures is 3.1 mA/cm2 which can produce the maximum chemical power of ~ 3.1×1.23 = 3.82 mW/cm2 and in this total power, 0.175 mW/cm2 i.e. ~ 4.56% is the contribution from the internal solar conversion power of fabricated heterostructure and rest of power could be from external power source, which is generally, a tandem cell. However, ISTC efficiency is largely used to characterize the coupling efficiency of photoanode and power source (generally tandem cell) which provides the necessary potential required to generate the maximum power from the photoanode. Further, ISTC gives only the half cell efficiency and does not include all the components of PEC cell such as counter electrode, electrolyte, wires etc. Hence, both the above mentioned efficiencies i.e. ABPE and ISTC are largely used to evaluate the performance of PEC device/photoanode based on the photocurrent and photovoltage. However, none of the above efficiency formula involves the total power output based on the total current density obtained as a result of the contribution from both applied bias and solar light. Recently, Lewis and co-workers proposed a methodology to compare the performance of various solar energy conversion technologies generating fuels and/or electricity as output using solar and/or electrical power as inputs to the system.38 For comparison, they proposed a general expression to calculate the system efficiency based on the total power output (chemical plus electrical) and total input power (solar plus electrical) which is similar to our recently reported “electrical and solar power-to-hydrogen (ESPH)” efficiency39 however with the difference that the proposed ESPH efficiency from the present work is based on the total current density obtained from the PEC cell under solar light illumination and applied bias. Hence, in order to evaluate the complete performance of PEC water splitting reaction, we are reporting the


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modified version of “solar-to-hydrogen” efficiency and it is based on total power output divided by total power input. ESPH =

×!"#$ %&/(% )×(. )

-

( ∗ + )(%,/(% )

&. ./ 0

……………… (3)

where, IPEC is the total current density observed under solar light illumination and applied bias, Vb is the applied bias under dark and Idark is the dark current at Vb. Figure 6 shows the ESPH efficiency of fabricated heterostructures based on the current density observed in Figure 4 and S3. We have observed that Au@R-TiO2@A-TiO2@SS triad heterostructures shows the highest ESPH efficiency as compared to other fabricated heterostructures. For instance, in non-faradaic region i.e. at an applied bias of 1.0 V, Au@R-TiO2@A-TiO2@SS triad heterostructures shows the ESPH efficiency of ~ 3.1% whereas same triad heterostructures shows the ESPH efficiency of ~ 11.5% in faradaic region at an applied bias ~ 1.70 V. Further, unlike ISTC, the proposed ESPH efficiency suggests that, if the fabricated heterostructure e.g. Au@R-TiO2@A-TiO2@SS is connected with the external source generally photovolatics (which also harvest the solar light) then it exhibits the efficiency of ~ 3.1% at 1.0 V applied bias. Additionally, it is interesting to note that calculated ESPH efficiency value is lower than the calculated ISTC value for same heterostructures. For example, Au@R-TiO2@A-TiO2@SS triad heterostructures exhibits the ISTC value of ~ 0.175 or 17.5% at 1.55 V potential (vs. RHE) whereas the corresponding calculated ESPH value for same heterostructure at ~ 1.55 V potential (vs. RHE or 1.1 V applied bias) is ~ 4.0% which suggests that proposed ESPH formula is not overestimating the efficiency compare to ISTC. This is because of following two reasons; firstly, ISTC measures the efficiency in three-electrode configuration and measures only half-cell efficiency. Secondly, ISTC does not include the parasitic losses which arise from different



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components of PEC cell whereas proposed ESPH formula includes these losses to evaluate the full PEC cell efficiency. Next, we have collected the gases (i.e. H2 and O2) evolved during water splitting from the fabricated (i.e. working) and Pt (i.e. counter) electrode using inverted beaker method (Figure S8). It is clear from the setup that inverted beaker method increases the solution and other associated losses which in turn decrease the current density as compared to current density observed in Figure 4 and S3 and thus decreases the amount of gas generation. However, inverted beaker setup possesses the advantage of safe and pure collection of oxygen and hydrogen gases. The collected gases from working and counter electrode are in perfect stoichiometry and analysed from gas chromatography (Figure S9a) which clearly suggests that no corrosion product is forming other than hydrogen and oxygen and hence can be directly plotted vs. applied bias without further analysis.

ESPH =

(%%12 3 /4)×( 5 67/%12) ( ∗ + )(%,/(% )×&89:((% ) &. ./ 0

× 100………… (4)

Figure 7a and 7b shows the plots between applied bias and amount of oxygen and hydrogen collected experimentally and calculated based on the current density observed during collection of oxygen and hydrogen gases from Au@R-TiO2@A-TiO2@SS and R-TiO2@ATiO2@SS heterostructures. The ESPH efficiency based on moles of hydrogen collected experimentally and current density observed during collection is calculated using eq. 3 and 4 respectively (Figure S10). Further, based on the ESPH efficiency calculations, it is observed that Au@R-TiO2@A-TiO2@SS triad heterostructures shows the highest ESPH efficiency as compared to other fabricated heterostructures. For instance, in non-faradaic region i.e. at an applied bias of 1.0 V, Au@R-TiO2@A-TiO2@SS triad heterostructures shows the ESPH


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efficiency of ~ 1.2% based on H2 collected and also based on the observed current density during collection. Furthermore, in faradaic region at an applied bias ~ 1.4 V, same triad heterostructures shows the ESPH efficiency of ~ 3.8% in both based on H2 collected and observed current density during collection which is higher than the ESPH efficiency calculated for R-TiO2@A-TiO2@SS heterostructure that clearly suggests the role of Au towards enhancing the performance of heterostructures. Additionally, we have studied the long term stability of clicked Au@R-TiO2@A-TiO2 triad heterostructures at constant applied bias (~ 0.90 V) in 0.1M NaOH under light (Figure S9b). It is observed that triad heterostructures exhibit a constant current of ~ 1.3 mA over the long period of time which clearly suggests that important role of strong covalent triazole linkage between each photocatalyst i.e. Au, R-TiO2, A-TiO2 and SS dye and P25-TiO2 for long term stable performance of heterostructure as photoanode. Moreover, chronoamperometry measurement further supports that evolved gases from electrode is purely from water splitting not from the oxidation of any organic ligands which was used during the functionalization of photocatalyst. Additionally, we have also performed PEC water splitting measurement and efficiency calculation with other fabricated triad heterostructures (Figure S11ab) i.e. Au@A-TiO2@R-TiO2@SS which shows lesser current density (~ 2.9 mA/cm2) and ABPE (~ 0.36%) as compared to Au@R-TiO2@A-TiO2@SS. This is mainly attributed to efficient electron transfer in Au@R-TiO2@A-TiO2@SS as proposed in Figure 1 as compared toAu@ATiO2@R-TiO2@SS (Figure S11c). However, they have showed good performance as compared to other heterostructures which further supports the importance of presence of all three nanoparticles i.e. Au, R-TiO2 and A-TiO2 in heterostructure. Additionally, we have also fabricated multilayer’s of photocatalyst e.g. four tailored multilayer’s of (a) A-TiO2, (b) R-TiO2 and (c) R-TiO2@four layers of A-TiO2 nanoparticles over



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stainless steel using same approach to study the role of amount of catalyst in PEC water splitting. PEC performance of above mentioned heterostructures (Figure S12) showed that increasing the multilayer’s of photocatalyst results in decreased performance as compared to photocatalyst monolayers. This can attributed to convoluted effect of increased charge carrier recombination rate and IR resistance. Further, electrochemical impedance spectroscopy (EIS) analysis has been performed using fabricated heterostructures at different potentials from non-faradaic to faradaic potential under light to explore the effects of interfacial conductivity and charge transfer mechanism (Figure 8a). Impedance across the interface is represented by Nyquist plot which is a plot between real and imaginary part of complex impedance. As proposed by Bisquert et al. each semicircle of Nyquist plot represents presence of number of capacitive resistive (RC) element which is directly related to charge transfer event at electrode/electrolyte interface where (1) high frequency semicircle denotes the effect of double layer and space charge capacitance with trapping-detrapping events from conduction band of semiconductor to trap states, (2) intermediate frequency semicircle denotes the effect of charge transfer from the trapped state to electrolyte and (3) lower frequency semicircle denotes the charge transfer event from valence band to the electrolyte.40, 41 Based on above points, there are generalized possibilities, (1) transfer of holes either from trapped state or valance band with electron and hole recombination in bulk (equivalent circuit showed in Figure 8b), (2) recombination of electrons and holes in semiconductor bulk (equivalent circuit showed in Figure 8c) and (3) transfer of holes either from trapped state or valance band without electron and hole recombination in bulk (equivalent circuit showed in Figure 8d).


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In all equivalent circuits, Rsol represents the solution resistance, Rct is the charge transfer resistance for charge transfer from either from valence band or trapped state, CT is either valance band capacitance or the trapped state capacitance, Rtrapping is the combined effect of resistance due to trapping and de-trapping events of electrons and holes from conduction band and valance band to trap state, Rrecom,bulk is the electron hole recombination in bulk of semiconductor, Cdl is the combined effect of Helmholtz capacitance and space charge capacitance. We observed that, for Nyquist plot obtained from fabricated heterostructure; equivalent circuit shown in Figure8c is best fit circuit and it supports the hypothesis of negligible charge carrier recombination in bulk as a result of transfer of hole either from trapped state or valence band. However, calculated capacitance value shown in Table S2 reveals that accumulated holes in the trap states (generally refer as chemical capacitance) is higher than the capacitance accumulation of the holes in valance band (generally called space charge capacitance). Additionally, chemical capacitance depends on number of trapped state (Nss) and number density of holes (fSS) is calculated using eq. (5): A

=>8:? = B D E F44 (1 − F44 )……………………… (5) C

=B H(>,>8:?

9

CD

J4 K44 F44 (1 − F44 )LM ……………… (6)

Further, resistance for transfer of holes from the trapped state i.e. Rct,trap is given by eq. 6, where, ks, nνare rate constant of electron transfer from trap state to electrolyte and valence band hole density respectively. It is clear from eq. 6 that Rct,Trap varies inversely with the number density of holes in trapped state which signifies that higher the number of holes in trapped state, lower the value of Rct,Trap, and higher the activity of photocatalyst. Furthermore, we have calculated ratio of charge transfer resistance (Rct,Trap) to chemical capacitance (CTrap) which gives charge transfer resistance per site (Table S2) to compare the performance of fabricated



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heterostructures. Based on this analysis, we observed that Au@R-TiO2@A-TiO2@SS triad heterostructures exhibit lower Rct,Trap/CTrap ratio as compared to other heterostructures (Table S2) which clearly supports the best performance of Au@R-TiO2@A-TiO2@SS triad heterostructures observed in Figure 4.

Conclusion We demonstrate an approach for fabricating the Au@R-TiO2@A-TiO2@SS triad heterostructures which showed improved photoelectrochemical performance as compared with other metal-semiconductor heterostructures i.e. R-TiO2@A-TiO2, Au@R-TiO2, Au @A-TiO2, A-TiO2, and R-TiO2 over stainless steel. Further, efficiency calculation clearly suggests that Au@R-TiO2@A-TiO2@SS triad heterostructures show maximum applied bias photo-conversion efficiency (ABPE) and ESPH efficiency of ~ 0.4% and 4.0% respectively at ~ 0.955 V and 1.1 V applied bias as compared to other heterostructures. We attributed this enhancement to the presence of gold nanoparticles and interface formation between each photocatalyst i.e. Au, RTiO2 and A-TiO2 that facilitates 1) increase in the absorption cross section in visible region, 2) increases the charge carrier concentration at the interface, 3) decrease in the charge carrier’s recombination rate through subsequent transfer of photo-excited electrons from goldRTiO2A-TiO2. Furthermore, EIS analysis shows the importance of interfaces in triad heterostructures and improved conductivity. Our results demonstrate that proposed approach provides the flexibility in fabrication and can be extended to photocatalysts of widely different morphology.


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Acknowledgements We gratefully acknowledge the support from the Technology System Development program of the

Department

of

Science

and

Technology,

Government

of

India

via

project

DST/TSG/SH/2011/106. References 1. Hisatomi, T.; Kubota, J.; Domen, K., Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43, 7520-7535. 2. Linic, S.; Christopher, P.; Ingram, D., Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911-921. 3. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S., Solar Water Splitting Cells. Chem. Soc. Rev. 2010, 110, 6446-6473. 4. Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y., Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269-271. 5. Christopher, P.; Xin, H. L.; Marimuthu, A.; Linic, S., Singular Characteristics and Unique Chemical Bond Activation Mechanisms of Photocatalytic Reactions on Plasmonic Nanostructures. Nat. Mater. 2012, 11, 1044-1050. 6. Linic, S.; Christopher, P.; Ingram, D. B., Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911-921. 7. Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G., Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science 2011, 334, 645-648. 8. de Mendonca, V. R.; Dalmaschio, C. J.; Leite, E. R.; Niederberger, M.; Ribeiro, C., Heterostructure Formation from Hydrothermal Annealing of Preformed Nanocrystals. J. Mater. Chem. A 2015, 3, 2216-2225. 9. Ohno, T.; Tokieda, K.; Higashida, S.; Matsumura, M., Synergism between Rutile and Anatase TiO2 Particles in Photocatalytic Oxidation of Naphthalene. Appl. Catal., A 2003, 244, 383-391. 10. Liu, Z.; Zhang, X.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A., Anatase TiO2 Nanoparticles on Rutile Tio2 Nanorods: A Heterogeneous Nanostructure via Layer-by-Layer Assembly. Langmuir 2007, 23, 10916-10919. 11. Zhang, G.; Pan, K.; Zhou, W.; Qu, Y.; Pan, Q.; Jiang, B.; Tian, G.; Wang, G.; Xie, Y.; Dong, Y.; Miao, X.; Tian, C., Anatase TiO2 Pillar-Nanoparticle Composite Fabricated by Layer-by-Layer Assembly for High-Efficiency Dye-Sensitized Solar Cells. Dalton Trans. 2012, 41, 12683-12689. 12. Kawahara, T.; Ozawa, T.; Iwasaki, M.; Tada, H.; Ito, S., Photocatalytic Activity of RutileAnatase Coupled TiO2 Particles Prepared by a Dissolution-Reprecipitation Method. J. Colloid Interface Sci. 2003, 267, 377-381. 13. Loddo, V.; Marci, G.; Martin, C.; Palmisano, L.; Rives, V.; Sclafani, A., Preparation and Characterisation of TiO2 (Anatase) Supported on TiO2 (Rutile) Catalysts Employed for 4-Nitrophenol Photodegradation in Aqueous Medium and Comparison with TiO2 (Anatase) Supported on Al2O3. Appl. Catal., B 1999, 20, 29-45.



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Figure Legends Figure 1: Band alignment analysis of Au@R-TiO2@A-TiO2 on alkyne-functionalized stainless steel substrates via click chemistry. Scheme

1:

Schematic

representation

of

fabrication

of

Au@R-TiO2@A-TiO2 triad

heterostructures over stainless steel via click chemistry. Figure 2: FE-SEM images of (a) A-TiO2, (b) R-TiO2, (c) R-TiO2@A-TiO2, and (d) Au@RTiO2@A-TiO2 heterostructures over alkyne-functionalized stainless steel (SS) substrates via click chemistry. Figure 3: XRD spectra of Au@R-TiO2@A-TiO2 triad heterostructure over alkynefunctionalized stainless steel substrates via click chemistry. Figure 4: Two electrode analysis for photoelectrochemical water splitting using A-TiO2@SS, RTiO2@SS, R-TiO2@A-TiO2@SS, and Au@R-TiO2@A-TiO2@SS heterostructures fabricated via click chemistry. Figure 5: Applied-bias photo-conversion efficiency (calculated from eq.1) based on photocurrent measured in two electrode configurations for fabricated heterostructures via click chemistry. Figure 6: (a) Electrical and solar power-to-hydrogen (EPSH) efficiency vs. applied bias based on current density observed in Figure 4 from different heterostructures. Figure 7: Amount of hydrogen and oxygen gases collected during PEC water splitting and based on the current density observed during collection of gases from (a) Au@R-TiO2@A-TiO2 and (b) R-TiO2@A-TiO2 heterostructures. Figure 8: (a) Electrochemical impedance spectroscopy analysis of Au@R-TiO2@A-TiO2@SS triad heterostructure over alkyne-functionalized stainless steel substrates via click chemistry at


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different applied bias under light. Equivalent fitted circuit when (b) transfer of hole takes place either from trapped state or valence band with electron and hole recombination in bulk, (c) hole does not transfer from valence band or trapped state (only electron hole recombination takes place in bulk), and (d) transfer of hole takes place either trapped state or valence band without electron and hole recombination in bulk.



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Figure 1


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Page 27 of 36

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Scheme 1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2


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Page 29 of 36

1.0

Normalized Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


*

0.8

R_TiO R-TiO22 * ---A-TiO A-TiO2 # #--- Au 2 ^ ---- Au

*

^ *

^

0.7 * #

0.5 0.3

#

^

#

* #

^ * *

*

#^

#*

0.2 A-TiO R-TiO2 2 A-TiO2

1.0 0.8 0.6 0.4 0.2 0.0 20

R-TiO2

30

40

50

2 Theta, degrees Figure 3


60

70


4

3

j, (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2

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A-TiO2@SS_dark A-TiO2@SS_light R-TiO2@SS_dark R-TiO2@SS_light R-TiO2@A-TiO2@SS_dark R-TiO2@A-TiO2@SS_light Au@R-TiO2@A-TiO2@SS_dark Au@R-TiO2@A-TiO2@SS_light

1

0 0.0

0.2

0.4

0.6

0.8

1.0

Applied Bias, V Figure 4


1.2

1.4

1.6

Page 31 of 36

0.4 0.3

ABPE, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A-TiO2@SS R-TiO2@SS R-TiO2@A-TiO2@SS Au@R-TiO2@A-TiO2 @SS

0.2 0.1 0.0 -1.2

-0.8

-0.4 0.0 0.4 Applied Bias, V

Figure 5


0.8

1.2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6


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Figure 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8


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Table of Contents Graphic

Stable Au@R-TiO2@A-TiO2 triad clicked heterostructure with tailored interfaces shows enhanced PEC performance due to increased solar light absorption and charge carrier separation.



Table of Contents Graphic

e-

Stainless Steel

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(iv)

(iv)

e-

e-

(iii)

e- e-

AuNp CB

e- e-

e-

e-

e-

(ii)

e-

e-

(i)

Ef

e- e- eAuNp

CB

e-

e-

e-

e-

3.03 eV

A-TiO2

R-TiO2

3.20 eV

h+ VB

h

+

h+ h+ h+ VB


Ef

Page 36 of 36

Fabricating Appropriate Band-Edge-Staggered ... - ACS Publications

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