High Angle Grain Boundary Movement

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Bond Energy Driven Low/High Angle Grain Boundary Movement Mediated Porous Se-Te Synthesis for Water Splitting Reaction Anup Kumar Sasmal, Arpan Kumar Nayak, Prashant Kartikeya, Debabrata Pradhan, and Tarasankar Pal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10466 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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ACS Applied Materials & Interfaces

Bond Energy Driven Low/High Angle Grain Boundary Movement Mediated Porous Se-Te Synthesis for Water Splitting Reaction

Anup Kumar Sasmal,a Arpan Kumar Nayak,b Prashant Kartikeya,a Debabrata Pradhan,b and Tarasankar Pala,* a

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India Materials Science Centre, Indian Institute of Technology, Kharagpur-721302, India E-mail: [email protected] Tel.: +913222283320; Fax: +913222255303

b

Abstract Herein, for the first time we applied metal-metal bond energy factor for the evolution of porous Se-Te alloy. Porous Se-Te material has been prepared from their elemental states only through heating-cooling process in silicone oil without using any reagent, surfactant or capping agent. Surprisingly the reaction occurred at much lower temperature (240 °C) than the m.p. (450 °C) of Te0. The nucleation and growth have been clarified by means of varied bond energy for the first time. Difference in bond energy between hetero metal-metal bond (Se-Te) and homo metal-metal (Se-Se) directs nucleation and growth towards the fabrication of porous structure even from their elemental state where low angle grain boundary (LAGB) and high angle grain boundary (HAGB) movements play their governing roles. Proper band gap alignment of Se and Te makes the alloy composite applicable to water splitting reaction under Xe arc lamp illumination. PEC efficiency of Se-Te was found to be higher than the reported Se and other composite materials.

Keywords Bond energy factor, low angle grain boundary (LAGB) movement, high angle grain boundary (LAGB) movement, porous alloy, H2-photocathode, water splitting reaction.

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Introduction Plastically deformed metals recrystallize into alloy through thermodynamically controlled process. During such recrystallization process of alloy formation, metal-metal bond formation occurs. In this context, metal-metal bond formations between same metals and different metals happen utilizing the energy stored by the deformation or other cradle. Thus the different rates of utilization of energy for the metal-metal bond formation (be in harmony with bond energy) between same metals and different metals are competitive to be useful for the structural development through nucleation and growth. In this work, for the first time we applied metal-metal bond energy aspect for the evolution of porous structure of Se-Te alloy. The as-synthesized porous Se-Te, prepared from the bulk counterparts without using any reagent, surfactant or capping agent, instrumental manipulation, etc., is found to be expedient for the water splitting reaction under Xe arc lamp illumination. Water splitting reaction, a central theme in catalysis, is a cleaner, cheaper and thus promising avenue for hydrogen and oxygen production which are considered as clean, renewable and carbon-free energy.1 Both the half-cell reactions in photoelectrochemical water splitting reaction, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), are strongly dependent on the electron generation and transport, and thereby play important role to display the efficiency of hydrogen production.2 Enormous effort has been directed for the development of new, cost-effective and efficient non-noble metal catalysts comprised of earth abundant elements beating costly and scarce platinum based materials for the effectual water splitting reaction.3,4 Consequently, H2-evolving photocathodes have been constructed.5 A non-noble and nearly ‘forgotten’ element,6 Tellurium, has not been concentrated towards water splitting reaction although it has very low band gap value (0.45 eV)7 and even exhibited very low efficiency in photochemical H2 evolution on water splitting8 which threw us challenge to utilize it in water splitting reaction. For the synthesis of porous Se-Te alloy,


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we have selectively used Te0 (work function value 4.73-4.95 eV9,10) to combine with nonnoble and cost-effective metalloid Se0 (band gap value ̴ 2.0 eV11 and work function value 5.9 eV12) which is relatively more abundant than platinum. The material was found to be deliberately practical (in rapports to the band structure alignment) for generation and movement of electron effective for water splitting reaction under Xe arc lamp illumination. Reported photocatalysts rendered activity towards water splitting reaction under UV light due to their wide band gap.13 Thus the visible light sensitivity and stability of catalyst in water splitting reaction are onerous.14 Most importantly, it is noteworthy to mention that it is easy to synthesize metal-metal composite material or alloy from their oxidized or reduced form through chemical reaction while the synthesis of the same from their elemental (zero) state is too difficult and uncommon. This work describes the synthesis of a Se-Te alloy but with porous structure through capping agent or surfactant free, reagent-free, easy and facile way directly from their elemental state driven by the metal-metal bond energy factors (Figure 1). On the other hand, only bulk Se materials leads distinctly different structure, smooth Se nanoball, under the similar conditions. Significantly, the semiconductor Se-Te alloy as H2-evolving



Figure 1. Schematic presentation of this work photocathodes (HER catalyst) displayed high efficiency and stability in water splitting reaction under xenon light (Figure 1).

Results and Discussion Synthesis and Structure Evolution of Se-Te Se-Te alloy was synthesized from their bulk counterparts. Selenium powder (Se0 grey material) was dispersed in silicone oil and heated to 230-240 °C to dissolve the total Se material15 (See the detail synthetic procedure in Supporting Information (SI)). As a result, transparent deep yellow solution was produced considered as liquid Se. That yellow hot solution was allowed to cool down, and a brick red colored colloidal solution of selenium nanoballs (SNBs) was obtained. Then tellurium (Te0) powder was added to the brick red colored colloidal solution of SNBs and the solution was heated to 230-240 °C again for few minutes. Black colored precipitate (Se-Te alloy) appeared after the solution cooled down. Herein, cooling rate matters recrystallization. The formation of porous Se-Te occurred through a series of thermodynamically controlled processes (Figure 2) where competitive hetero Se-Te and homo Se-Se/Te-Te bond formations play the vital role. Bulk selenium (grey material) is typically the helical polymeric chain of selenium atom which undergoes plastically deformation16 (step-I in Figure 2) as temperature increases followed by broke down into atomic selenium plausibly (step-II). Those selenium atoms are entrapped into the side chain of siloxane (silicone oil) (step-II).17 The color change of the solution from grey to yellow resembles the transformation of helical polymeric chain (grey bulk Se0) into nano regime of selenium. Herein, silicone oil helps melting of bulk Se material because it has good heat transfer property as well as it is a non-reactive medium for alloying and nanomaterial synthesis.15 However, during cooling of the solution to room temperature the entrapped Se atoms (liquid selenium) escaped from side chain of siloxane and form Se-Se bond (using


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electrons of p orbital), and generates selenium zig-zag three dimensional chain where strong cohesive force persists.15 This three dimensional chain of selenium collapsed18 into selenium nanoparticle as red amorphous selenium nanoball (step-III). The red amorphous selenium nanoball comprised of deformed helical polymeric chain of selenium atoms.19 After that tellurium (Te0) material was added into the solution and the solution was heated at 240 °C (step-IV). The deformed polymeric chain (of amorphous red Se) again broke down into atomistic selenium which were entrapped into the side chain of silicone oil as the temperature increases to 240 °C. Simultaneously network of spiral chains of tellurium (bulk Te0 materials is the large spiral chins of tellurium atoms19) were present in the solution. Because bulk Te would have been little deformed possibly since the reaction temperature did not attain its m.p. Thus there is dramatically reduced prospect of transformation of spiral chain of tellurium into the tellurium atom to be entrapped into the organic side chain of silicone oil like selenium due to the higher m.p. (449.5 °C) and strong cohesive force of Te0 spiral chains although silicone oil (b.p. 250 °C) takes care of melting metals. So both the Se and Te atoms could not be entrapped into the side chain of silicone oil such that release of Se and Te atoms from side chain would be available for the Se-Te chain formation through classical fashion followed by Se-Te nucleus formation upon the collapse of chain (Path-A; step-V). So nucleus of Se-Te formation is not plausible through this pathway (path-A). Even we observed that individual Te0 bulk material did not display any structural development when heated to 250 °C in silicone oil followed by cooling down to room temperature (Figure S1 in SI). The reaction at lower temperature (< 220 °C) was not also successful for alloy formation.



Figure 2. Schematic diagram of porous Se-Te balls synthesis.

However, the most interesting phenomenon occurred as the solution cooled down from 240 °C to room temperature. Surprisingly, during cooling crystallization of Se-Te alloy occurred and porous structure was obtained (step-V). This result appeared although the reaction mixture did not attain the m.p. of bulk Te0 i.e. 449.5 °C instead heated to only 240 °C which is much lower than the m.p. of bulk tellurium material. Elemental bulk materials may only undergo significant deformation or structural evolution only near its m.p. On the other hand, grey selenium material (bulk Se0) under the similar manipulation (heating and cooling) furnished ball shaped structure (smooth nanoballs) (See the Figure S2 in SI).15 Presumably, tellurium has the effect on re-crystallization of selenium.20 Herein the re-crystallization afforded the porous alloy material of selenium and tellurium. As the temperature of the


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solution cooled down, selenium escaped out from organic side chain of silicone oil to form Se-Se bond (using electrons of p orbital) and generates small zig-zag three dimensional chain of Se0 atoms. Plausibly, that chain (or few such small chains) collapsed into small particle which is considered as nucleation centre through intramolecular cross-linking due to its zigzag non-uniform structure (Path-B of step-V in Figure 2).18 Simultaneously, very small Se-Te nucleus having low degree of mis-orientation was generated through the release of Se(0) atoms from organic side chain to form small chain, collapse of small chain, and low angle grain boundary(LAGB) movement of small Te chain during the collapse of small Se0 chains (Path-C of step-V in Figure 2).21 LAGB movement may occur during small Se0 chain formation or collapse of the same. However, theoretically number of nucleus of Se0 would much higher than that of Se-Te since optimum 5% tellurium was used for the reaction considering the eutectic composition. On the other hand, there is no tendency of entrapping and escaping of both Se0 and Te0 under the organic side chain of silicone oil followed by SeTe bond formation to collapse into small Se-Te nucleus (Path-A; as discussed earlier). Because metallic Te0 with a strong cohesive force describes m.p. of 449.5 °C much higher than reaction temperature, and thus tellurium spiral chain (of Te0 bulk material) cannot be broken into tellurium atoms to be entrapped into organic side chain of silicone oil. So the nucleus formation occurred through Path-B most plausibly and Path-C of small extent. Then, grain growth ensued on the nucleation centre. New bond formation for growth of alloy reasonably occurred after immediate release of atoms from organic side chain of siloxane (in case of selenium) and simultaneous high angle grain boundary (HAGB22) movement of Te spiral chain (in case of tellurium). Herein, slightly deformed spiral Te chains are considered as crystallites for the (HAGB) movement for growth of nuclei. Because tellurium spiral chain could not break into atoms to be entrapped into the siloxane side chain at the reaction temperature (reaction temperature 240 °C, high m.p. (449.5 °C) & strong cohesive force of



tellurium chain) as earlier discussed. However, it is important to mention that nucleation and grain growth during recrystallization are dependent on Se-Se, Se-Te, and Te-Te bond formation. Chiefly, there would be a competition between Se-Se and Se-Te bond formation for the growth on nucleus. Both the Se-Se and Se-Te bond formation is prone because Se and Te have the similar electronic structure. Te-Te bond formation may be ignored because of the concentration effect as only 5% Te has been used for the reaction. Nevertheless, Se-Te bond formation would be faster than Se-Se (bond energy order Se-Se>Se-Te>Te-Te; Figure 3a). Hence, there occurred non-uniform grain growth due to the different bond energy utilized for bond formation on Se or Se-Te nuclei as described in Figure 3b-d. The hetero metal-metal (Se-Te) bond formation is faster than homo Se-Se bond formation. This would have also created non-uniform crystal defect (or dislocations).23,24 Additionally, there would have also further generation of crystal defects because of larger bond length of Se-Te than Se-Se (Figure 3e). Recovery is the process by which a crystallite can reduce its intrinsic stored energy by removal or rearrangement of crystal defects.16 Generally, dislocations move when the atoms break their bonds from surrounding planes and re-unites with the atoms at the terminating edge of primary crystallite. So the crystal defect generated because of larger bond length of Se-Te leaving aside Se-Se and this would have been relaxed by bond breaking process at the optimum temperature and creates new different crystal sites also (Te and Se bare sites; Figure 3e) for further bond formation with Se/Te atoms. Anyway, such aforementioned bond formations were non-uniform because these are of different bond length (i.e., Te-Se, Se-Se, or Se-Te on Se or Te atom sites). More importantly, repeated bond formations would have generated larger non-uniform chains (with greater degree of misorientation25 as shown in Figure 3f) which collapsed onto the particle. This is the growth of the particle. Thus all these factors (faster bond formation of Se-Te over Se-Se, generation of non-uniform crystal defect because of


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Figure 3. Mechanistic pathways for growth of porous Se-Te.



the larger bond length of Se-Te than Se-Se bond, crystal defect removal through bond breaking to render different crystal sites and different bond formation to increase the chain length of different lengths, and higher degree of mis-orientation between chains) are responsible for the non-uniform growth of particle furnishing porous Se-Te alloy material. In contrast, in case of grey selenium materials (devoid of Te) into red selenium nanoball conversion under similar conditions uniform directional of bond formation happens, and aforementioned crystal defects are also isotropic around the nucleus centre which permit to the minimization of surface free energy during growth of the nucleus leading to the spherical particle (Figure S2 in SI).

Characterization of Se-Te The as-prepared alloy material, Se-Te, was characterized by XRD unambiguously. The XRD pattern (Figure 4) from all the peaks having proper integration ratio correctly matched with the JCPDS card no. 73-0465 (for Se) and 36-1452 (for Te). Rietveld refinement of X-ray diffraction pattern (Figure S3 in SI) designates the alloy formation of Se and Te. The FESEM images (Figure 4b-c) designate the porosity nature of the as-synthesized materials. EDX analysis (Figure 4d) exhibits the presence of tellurium and selenium in the material as ratio used for the reaction. Additionally, diffuse reflectance spectroscopy (DRS) (Figure S4 in SI) analysis has been performed which matched with earlier literatures with shifting of maxima in case of tellurium.15,26 DRS states the interaction between photon and valence (or conduction) electrons revealing the electronic structure around Fermi surface. Again any shift of these energies, due to the result of transformation, ordering, etc., provides the deeper insight on the nature of the material27. Thus our as-synthesized material, with peak shifting maximum from DRS study, indicates different valence electronic structure than the individual components endorsing the alloy formation. We observed that there is shifting and change of


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valence band spectrum of selenium (Se4p) from bulk Se to Se-Te alloy material whereas peak for tellurium is wrapped (Figure S5). The change of valence band spectrum corroborates the alloy formation28. Additionally, it was also observed some shifting of binding energy of tellurium in core level XPS (Figure S6) supporting alloy formation.28,29 Thus, XPS study also supports the alloy formation.

Figure 4. XRD (a), FESEM (b,c), and EDX (d) of the as-prepared Se-Te

Thus we have successfully fabricated porous Se-Te alloy material from elemental states only without using any reagent, capping agent or surfactant. Earlier literature reports stand on the fabrication of film or glassy materials only through hazardous photoelectrochemical



deposition from their oxides, direct fusion of elements at high temperature (500 °C), laser induction, whereas wire structure was achieved through reduction of oxo-acids (not elemental state) using surfactants.30-33

Photoelectrochemical Study The alignment of energy bands at the interface between semiconductors in their composite material critically directs electron flow reflected in the electronic behavior. Viability of such alignment of composite material is dependent on the work function and band gap of individual components.34 Se and Te have band gap values of 2.0 eV and 0.45 eV, respectively while work function of Se and Te are 5.90 eV and 4.95 eV, respectively.7,9-12 It was envisaged that the Se-Te may combine through staggered gap type way where work function of selenium is higher than tellurium. However, in case of Se-Te alloy where the elements are in intimate combination, interfacial band bending should occur because of the shifting of Fermi levels of Se and Te until thermodynamic equilibrium is reached. Then the material attains common work function. In this circumstance, the material attains newly reconstructed energy band diagram. We performed valence band as well core level XPS spectroscopy. From the XPS analysis we determined the valance band offset (∆EV) as well as conduction band offset (∆EC) and thereby depicted energy band diagram. We observed that there is shifting and change of valence band spectrum of selenium (Se4p) from bulk Se to Se-Te alloy material whereas peak for tellurium is wrapped (Figure S5). The change of valence band spectrum corroborates the alloy formation28. Additionally, it was also observed some shifting of binding energy of tellurium in core level XPS (Figure S6) supporting alloy formation28,29. From the valence band and core level XPS studies, the valence band off-set (∆EV) and conduction band off-set (∆EC) have been calculated as per the equations (1-2).35


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∆EV = ∆ECL –[E(nl)- EV]Se + [E(n ̍ l ̍ )- EV ̍ ]Te

(1)

∆EC = ∆Eg - ∆EV

(2)

where the core level difference ∆ECL is determined from the binding energy difference between (nl) core level of Se and (n ̍ l ̍ ) core level of Te in the Se-Te alloy (Figure S6) . E(nl) and EV are the binding energy of core level and valence band maximum of selenium bulk material while E(n ̍ l ̍ ) and EV ̍ are the binding energy of core level and valence band maximum of tellurium bulk material (Figure S5 and Figure S6). ∆Eg is the difference between band gap of selenium and band gap of tellurium.7,11 The ∆EV and ∆EC were found to be 1.66 eV and 0.11 eV. Thus the consequential energy band diagram of Se-Te alloy was observed to be staggered type as depicted in Figure 5a (in inset). Thus, under light illumination, electrons (after electron-hole separation) in CB of Te pass to CB of Se. Additionally, electron-hole separation occurs in Se under light illumination. All these electrons in CB of Se reduce the H+ to generate H2 (Figure 5a). In such combination between Se and Te, the recombination of electron and hole is reduced also in Te portion under xenon light irradiation during PEC water splitting reaction. Photoelectrochemical (PEC) water splitting reaction has been studied (see the SI for detail method) exploiting the as-synthesized Se-Te alloy material comparing with bulk Se0, bulk Te0, physical mixture of bulk Se0 and bulk Te0 (5% Te0). We observed that as-synthesized Se-Te alloy (having 5% Te0) displayed superior electrochemical property than physical mixture of bulk Se0 (95%) and bulk Te0 (5% Te0), bulk Se0 or bulk Te0. However, on contact Se and Te may act as p-type and n-type semiconductor, respectively. But there is only 5% Te. So uplift of the Fermi level of selenium and lowering of Fermi level of tellurium may not be significant. Hence, the band diagram of Se-Te alloy may be of staggered type. XPS studies and valence band off-set (∆EV) and conduction band off-set (∆EC) values corroborated the staggered type structure.



If the uplift of the Fermi level of selenium was lofty, then straddling gap would arises which assists faster recombination of hole and electrons because of low band gap value of Te. But the alloy of Se-Te (5% Te) was found to be more effective than physical mixture (5%) in water splitting reaction. To evaluate the PEC performance of the as-synthesized Se-Te alloy, bulk Se, bulk Te, Se-Te (5%) [with Se (95%) and Te (5%)] and Se-Te (10%) [with Se(90%) and Te (10%)] based photocathodes, the linear sweep voltammetry (LSV) was carried out in a 0.1 M H2SO4 solution under the illumination of 300 W Xenon lamp as shown in Figure 5b. Pure Se0 and Te0 bulk materials generate photocurrent of -0.106 mA/cm2 and -1.86 mA/cm2 at 0 V (vs RHE), respectively. The samples of physical mixtures, Se-Te (5%), and Se-Te (10%) as photocathodes rendered the photocurrent of -0.814 mA/cm2 (0 V vs RHE) and -2.91 mA/cm2 (0 V vs RHE), respectively. On the other hand, the as-synthesized Se-Te alloy exhibited a much enhanced photocurrent (-4.35 mA/cm2) at 0 V (vs RHE) than sample of physical mixture Se-Te (5%) or other samples. This suggests that uniform alloy nanostructure of SeTe play the important role as photocathode to generate hydrogen from water. The material mechanistically performs according to energy band structure (Figure 5a). The obtained photocurrent value in this report is higher than recently reported selenium or other composite materials (Table 1).36-40 Additionally, the onset potential of as synthesized Se-Te alloy is more positively shifted which is considered to be favorable for the water splitting reaction. Onset potentials are observed to be 0.42, 0.40, 0.35, 0.337 and 0.01 V for Se-Te alloy, Se-Te (10%), Te (bulk), Se-Te (5%) and Se (bulk), respectively. This phenomenon implies that the much higher efficiency of Se-Te alloy having 5% Te0 than the physical mixture Se-Te (5%) and other materials investigated.


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Figure 5. (a) Energy band structure of Se-Te alloy for hydrogen evolution reaction under light irradiation. (b) Linear sweep voltammograms in 0.1 M H2SO4 with different electrocatalysts under dark and light at the scan rate of 5 mV/s. (c) The half-cell solar-tohydrogen efficiency (HC-STH) plots of the samples under illumination of 1 sun. (d) Timedependence photocurrent response with illumination switched ON and OFF for different electrocatalysts at a fixed potential of -0.6 V vs SCE (-0.36 V vs. RHE). (e) Chronoamperometry plots. (f) H2 evolution measured by gas chromatograph with different electrocatalysts, at a fixed potential of -0.259 V vs RHE in 0.1 M H2SO4 under light illumination.



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Table 1: Comparison of PEC efficiency of as-prepared Se-Te with reported semiconductor nanocomposites as photoelectrodes. Sr. No. Photocathode

1 2 3 4a 5 6b a

Se-Te alloy Pt/TiO2/Sb2Se3 p-Si/C3N4−CoSe2 MOS 1|Si CuGaSe2 NiO/Pt/PMI-6T-TPA

Electrolyte

0.1 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.05 M H2SO4 0.5 M H2SO4 0.1 M H2SO4

Metal organic surface (cobalt dithiolene polymer) with p-Si,

Current density at 0 V (vs. RHE) −4.35 mA/cm2 −2.0 mA/cm2 −4.89 mA/cm2 −3.8 mA/cm2 −0.2 mA/cm2 −0.03 mA/cm2 b

References

This work 36 37 38 39 40

at 0.059V (vs RHE).

Furthermore, the half-cell solar-to-hydrogen efficiency (HC-STH) of the as-synthesized electrodes was determined as a function of applied bias potential recorded under the 1 sun using the following equation.41 HC-STH (%) = J x (V−VH+/H2) x 100(%) / P …………………………….(1) Where J, V and VH+/H2 represent photocurrent density (mA/cm2), electrode potential (V vs RHE), and equilibrium redox potential of H+/H2 couple (0 V vs. RHE), respectively. The term P denotes the intensity of simulated sunlight (100 mW/cm2). As shown in Figure 2, the as-synthesized SeTe alloy exhibits the highest HC-STH 0.392 % at 0.186 V (vs RHE), while Se and Te record only HC-STH of 0.0035 % and 0.134 % at 0.186 V (vs RHE), respectively. The physical mixture of SeTe (5 %) and SeTe (10 %) exhibit HCSTH of 0.099 % and 0.333 % at 0.186 V (vs RHE), respectively. Higher HC-STH value of Se-Te alloy than the individual components or physical mixture Se-Te (5%) suggests the higher efficiency of Se-Te alloy material for PEC water splitting reaction. The transient photocurrent was measured using chronopotentiometry at a fixed potential (0.36 V vs RHE) for 1000 s of the as-synthesized samples under illumination on and off modes. Figure 5d shows the rise and fall of photocurrent with illumination on and off mode,


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respectively, indicating a sharp photoresponse of the electrodes during the process. This suggests the higher PEC efficiency of Se-Te alloy than the other material investigated which corroborates the alloy formation out of bulk Se and Te. Additionally, the alloy displays very good stability during the PEC water splitting reaction (Figure 5d; 300-800 sec). We observed that dark current was slightly higher than that on LSV plots. It may be because that the transient measurements were performed after LSV test. With time, the current was found to be improved (as shown in the i-t test). Hence we believe surface might be get cleaned/etched and thus improving the current both under dark and light. To

investigate

the

stabilities

of

the

as-synthesized

electrocatalysts

for

HER,

chronoamperometry was performed for 5 h at a fixed applied potential of −0.259 V vs. RHE under Xe arc lamp illumination as shown in Figure 5e. It was found that all the catalysts remain active and stable even after 5 h. As expected, the alloy catalyst shows higher photocurrent density than those of the individual metals. Interestingly, photocurrent response was slightly improved for SeTe alloy as a function of time, which is believed to be due to the formation of more active sites on its surface. This clearly suggests the effectiveness of SeTe alloy for HER. The evolved hydrogen gas in presence of different electrocatalysts was quantitatively measured using gas chromatography and shown in Figure 5f. The evolved hydrogen was measured to be 2.3 mmol/cm2, 1.47 mmol/cm2, and 5.8 mmol/cm2 with Te, SeTe (5%) and SeTe alloy, respectively, in 5 h at −0.259 V vs. RHE in 0.1 M H2SO4 (Figure 5f). The evolved H2 was found to be correlated to the photocurrent for different catalysts. “So the energy band structure (Figure 5a) is favourable towards reduction for water splitting reaction where recombination of electron and hole is also abridged. Furthermore, the Se-Te alloy was found to be porous structure. This also facilitated the performance of Se-Te alloy in water splitting reaction.



Thus the favourable energy band structure (staggered type), porosity, and alloying in Se-Te alloy make the material as an efficient H2 photocatalyst for water splitting reaction. Simultaneously, there occurs generation of oxygen through the oxidation of water at the anode as descried in Figure 1. The oxidation of water generates also hydrogen ions and electrons. Thus the material is found to be efficient for water splitting reaction.”

Conclusion In conclusion, it has been described method of porous Se-Te alloy synthesis from their elemental state (zero state) only through heating-cooling process in Si oil without using any reagent, surfactant or capping agent, instrumental manipulation, etc. The reaction occurred at 240 °C astonishingly although the reaction temperature was much lower than the m.p. (450 °C) of Te0 material. Difference in bond energy between hetero metal-metal bond (Se-Te) and homo metal-metal (Se-Se) directs nucleation and growth towards the fabrication of porous structure. The nucleation and growth for porosity have been elucidated by means of varied bond energy and LAGB/HAGB movement for the first time. The band gap alignment of Se and Te in the material was found to be effective for the water splitting reaction under xenon light illumination, an evolving research topic in the science of energy solution. It is envisaged that this work on the theory of grain boundary movement combined with bond energy will inspire scientists towards the investigation on the catalytic and other properties of composites or alloy.

Supporting Information Supporting Information (SI) available: The details for Synthetic procedure and characterization of materials including, Rietveld refinement, DRS, Photoelectrochemical


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method, XPS, etc. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors are thankful to the DST and CSIR New Delhi for financial support and IIT Kharagpur for research facilities. Thanks to PKM for his support.

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High Angle Grain Boundary Movement

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