Cathodic shift in onset potential of hematite photoanodes by ZrO2 underlayer for efficient water splitting

( Subramanian Arunprabaharan ),( Alagappan Annamalai ),( Jum Suk Jang ) 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.0

Herein we report that influence of Zr underlayer to hematite photoanodes for cathodic shift in onset potential as well as photocurrent. Akaganite (β-FeOOH) nanorods were grown on Zr underlayer treated FTO substrates. Sintering at 800°C transforms the akaganite to hematite phase and induces the Sn atoms from the FTO substrates will penetrate in to the hematite lattices along with Zr atoms. Zr underlayer treated sample shows the better performance of water splitting compared to the pristine photoanode. Cathodic shift in onset potential as well as photocurrent was done by penetration of Zr atoms from the underlayer to hematite lattices. 0.96 mA/cm2 at 1.23 VRHE was achieved for Zr based hematite photoanode with low turn on voltage 0.7 V vs. RHE. The crystalline structure and morphology for the Zr modified hematite photoanode almost identical to the pristine photoanode. Presence of Zirconium and Sn diffusion from the FTO were confirmed by XPS analysis. Electrochemical Impedance Spectroscopy reveals that the presence of Zr underlayer increases the charge transfer resistance along the various interfaces. The presence of ZrO2 underlayer decreases the FTO deformation and the sheet resistance of FTO is decreased. Mott Schottky analysis reveals that the donor density and flat band potential of the pristine and Zr underlayer treated photoanodes shows similar values. This is the first report on Zr underlayer based hematite photoanodes for the increment in photocurrent and cathodic shift in onset potential.

Effect of tetravalent dopants on hematite nanostructure for enhanced photoelectrochemical water splitting

Subramanian, Arunprabaharan,Gracia-Espino, Eduardo,Annamalai, Alagappan,Lee, Hyun Hwi,Lee, Su Yong,Choi, Sun Hee,Jang, Jum Suk Elsevier 2018 APPLIED SURFACE SCIENCE - Vol.427 No.2

<P><B>Abstract</B></P> <P>In this paper, the influence of tetravalent dopants such as Si<SUP>4+</SUP>, Sn<SUP>4+</SUP>, Ti<SUP>4+</SUP>, and Zr<SUP>4+</SUP> on the hematite (α-Fe<SUB>2</SUB>O<SUB>3</SUB>) nanostructure for enhanced photoelectrochemical (PEC) water splitting are reported. The tetravalent doping was performed on hydrothermally grown akaganeite (β-FeOOH) nanorods on FTO (fluorine-doped tin-oxide) substrates via a simple dipping method for which the respective metal-precursor solution was used, followed by a high-temperature (800°C) sintering in a box furnace. The photocurrent density for the pristine (hematite) photoanode is ∼0.81mA/cm<SUP>2</SUP> at 1.23V<SUB>RHE</SUB>, with an onset potential of 0.72V<SUB>RHE</SUB>; however, the tetravalent dopants on the hematite nanostructures alter the properties of the pristine photoanode. The Si<SUP>4+</SUP>-doped hematite photoanode showed a slight photocurrent increment without a changing of the onset potential of the pristine photoanode. The Sn<SUP>4+</SUP>- and Ti<SUP>4+</SUP>-doped hematite photoanodes, however, showed an anodic shift of the onset potential with the photocurrent increment at a higher applied potential. Interestingly, the Zr<SUP>4+</SUP>-doped hematite photoanode exhibited an onset potential that is similar to those of the pristine and Si<SUP>4+</SUP>-doped hematite, but a larger photocurrent density that is similar to those of the Sn<SUP>4+</SUP>- and Ti<SUP>4+</SUP>-doped photoanodes was recorded. The photoactivity of the doped photoanodes at 1.23V<SUB>RHE</SUB> follows the order Zr > Sn > Ti > Si. The onset-potential shifts of the doped photoanodes were investigated using the <I>Ab initio</I> calculations that are well correlated with the experimental data. X-ray diffraction (XRD) and scanning-electron microscopy (FESEM) revealed that both the crystalline phase of the hematite and the nanorod morphology were preserved after the doping procedure. X-ray photoelectron spectroscopy (XPS) confirmed the presence of the tetravalent dopants on the hematite nanostructure. The charge-transfer resistance at the various interfaces of the doped photoanodes was studied using impedance spectroscopy. The doping on the hematite photoanodes was confirmed using the Mott-Schottky (MS) analysis.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Tetravalent dopants on hematite photoanodes for photoelectrochemical (PEC) water oxidation were studied. </LI> <LI> High temperature calcination (800°C/10min) transformed akaganeite to hematite phase and caused doping. </LI> <LI> Onset potential shift of the photoanodes were studied by experimental and computational methods. </LI> <LI> Zr doped hematite exhibited photocurrent density of 1.35mA/cm<SUP>2</SUP> at 1.23V<SUB>RHE</SUB> with lower anodic shift. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Photoelectrochemical water oxidation performance of doped hematite photoanodes in 1 M NaOH solution at 1.23V<SUB>RHE</SUB> under 1 sun illumination.</P> <P>[DISPLAY OMISSION]</P>

Trade-off between Zr passivation and Sn doping on hematite nanorod photoanodes for efficient solar water oxidation: Effects of ZrO2 underlayer and FTO deformation

( Subramanian Arunprabaharan ),( Alagappan Annamalai ),이현휘,최선희,류정호,박정희,장점석 한국공업화학회 2016 한국공업화학회 연구논문 초록집 Vol.2016 No.0

Herein we report the influence of a ZrO2 underlayer on the PEC behavior of hematite nanorod photoanodes for efficient solar water splitting. Akaganite (β-FeOOH) nanorods were grown on FTO substrates treated with a ZrO2 underlayer. Sintering at 800°C transformed akaganite to the hematite (α-Fe2O3) phase and induced Sn diffusion into the crystal structure of hematite nanorods from the FTO substrate and surface migration of Zr atoms from the ZrO2 underlayer toward the top layer. A cathodic shift in the onset potential as well as photocurrent enhancement was achieved by surface passivation from the ZrO2 underlayer and Sn doping from the FTO substrate to the crystal lattice of hematite nanorods.

Trade-off between Zr Passivation and Sn Doping on Hematite Nanorod Photoanodes for Efficient Solar Water Oxidation: Effects of a ZrO₂ Underlayer and FTO Deformation

Subramanian, Arunprabaharan,Annamalai, Alagappan,Lee, Hyun Hwi,Choi, Sun Hee,Ryu, Jungho,Park, Jung Hee,Jang, Jum Suk American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.30

<P>Herein we report the influence of a ZrO2 underlayer on the PEC (photoelectrochemical) behavior of hematite nanorod photoanodes for efficient solar water splitting. Particular attention was given to the cathodic shift in onset potential and photocurrent enhancement. Akaganite (beta-FeOOH) nanorods were grown on ZrO2-coated FTO (fluorine-doped tin oxide) substrates. Sintering at 800 degrees C transformed akaganite to the hematite (alpha-Fe2O3) phase and induced Sn diffusion into the crystal structure of hematite nanorods from the FTO substrates and surface migration, shallow doping of Zr atoms from the ZrO2 underlayer. The ZrO2 underlayer-treated photoanode showed better water oxidation performance compared to the pristine (alpha-Fe2O3) photoanode. A cathodic shift in the onset potential and photocurrent enhancement was achieved by surface passivation and shallow doping of Zr from the ZrO2 underlayer, along with Sn doping from the FTO substrate to the crystal lattice of hematite nanorods. The Zr based hematite nanorod photoanode achieved 1 mA/cm(2) at 1.23 V-RHE with a low turn-on voltage of 0.80 V-RHE. Sn doping and Zr passivation, as well as shallow doping, were confirmed by XPS, I-ph, and M-S plot analyses. Electrochemical impedance spectroscopy revealed that the presence of a ZrO2 underlayer decreased the deformation of FTO substrate, improved electron transfer at the hematite/FTO interface and increased charge-transfer resistance at the electrolyte/hematite interface. This is the first systematic investigation of the effects of Zr passivation, shallow doping, and Sn doping on hematite nanorod photoanodes through application of a ZrO2 underlayer on the FTO substrate.</P>

Effect of Tetravalent Dopants on Hematite Nanostructure for Efficient Solar Water Splitting

( Arunprabaharan Subramanian ),박진우,( Eduardo Gracia-espino ),( Alagappan Annamalai ),이현휘,이수영,최선희,장점석 한국공업화학회 2016 한국공업화학회 연구논문 초록집 Vol.2016 No.1

Tetravalent doping was performed on hydrothermally grown akaganeite (β-FeOOH) nanorods on FTO (Fluorine doped Tin Oxide) substrates by simple dipping method using respective metal precursor solution, followed by high temperature (800 °C) sintering. Photocurrent density for pristine (α-Fe₂O₃) photoanode was ~0.81 mA/㎠ at 1.23 VRHE. Si⁴⁺ doped photoanode shows slight increment in photocurrent with no change in the onset potential than pristine photoanode. However Sn⁴⁺ and Ti⁴⁺ doped photoanode shows an anodic shift in onset potential with an increment in photocurrent. Interestingly, Zr⁴⁺ doped photoanode exhibit a similar onset potential like pristine. ^**This work was supported by the BK21 plus program through the National Research Foundation (NRF) funded by the Ministry of Education of Korea.

Activation of Hematite Photoanodes for Solar Water Splitting: Effect of FTO Deformation

Annamalai, Alagappan,Subramanian, Arunprabaharan,Kang, Unseock,Park, Hyunwoong,Choi, Sun Hee,Jang, Jum Suk American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.7

<P>The sintering at 800 °C is found to induce the diffusion of Sn from the F-doped SnO<SUB>2</SUB> (FTO) into the hematite lattice, enhancing the photoelectrochemical cell (PEC) properties of the hematite photoanodes, but this diffusion also has detrimental effects on the conductivity of the FTO substrate. In the present research we examined the role of FTO deformation during the activation of hematite photoanodes synthesized on FTO substrates. The incorporation of Sn dopants from the FTO substrates in the hematite lattice was confirmed by X-ray photoelectron spectroscopy and was found to increase with sintering time. Further from the extended X-ray absorption fine structure analysis, it was found that the diffused Sn atoms affected the metal sites of the hematite lattice. Increased diffusion of Sn into the hematite lattice caused structural disordering of the FTO, but optimum sintering time compensated for the structural disordering and improved the ordering. Under high-temperature annealing at 800 °C, the FTO substrates underwent a stoichiometric change that directly affected their electrical conductivity; their resistivity was doubled after 20 min of sintering. Activation of hematite photoanodes by high-temperature sintering entails a kinetic competition between Sn dopant diffusion from the FTO substrate into the hematite and the resulting thermal deformation and conductivity loss in the FTO substrates.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2015/jpccck.2015.119.issue-7/jp512189c/production/images/medium/jp-2014-12189c_0013.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp512189c'>ACS Electronic Supporting Info</A></P>

CdS nanograin sensitized Zr:Fe₂O₃ nanorod array for highly efficient solar-light-driven photoelectrocatalytic performance

( Mahadik Mahadeo Abasaheb ),( Arunprabaharan Subramanian ),류정호,조민,장점석 한국공업화학회 2016 한국공업화학회 연구논문 초록집 Vol.2016 No.1

Well-defined CdS nanograins sensitized Zr:Fe₂O₃ nanorods with enhanced photoelectrocatalytic performance are synthesized on F-doped SnO₂(FTO) using hydrothermal method. Owing predominantly to the appropriate band positions and charge collection in Zr:Fe₂O₃ nanorods, CdS/Zr:Fe₂O₃ heterostructure exhibit 1.7 times improved photocurrent density compared to Zr:Fe₂O₃. Unfortunately, CdS nanograins sensitized Zr:Fe₂O₃ nanorod arrays suffers from instability problem. Furthermore, Ni(OH)₂ loading can boost the PEC performance of heterojunction and also acts as a protective layer that improves the stability of the Ni(OH)₂/CdS/Zr:Fe₂O₃ electrode. Such enhanced PEC activity may be ascribed to the effective charge separation between CdS and Zr:Fe₂O₃ nanorods as well as increases the lifetime of holes due to Ni(OH)₂. ^**This work was supported by the BK21 plus program through the National Research Foundation (NRF).

Insights into the enhanced photoelectrochemical performance of hydrothermally controlled hematite nanostructures for proficient solar water oxidation

Park, Jin Woo,Subramanian, Arunprabaharan,Mahadik, Mahadeo A.,Lee, Su Yong,Choi, Sun Hee,Jang, Jum Suk The Royal Society of Chemistry 2018 Dalton Transactions Vol.47 No.12

<P>In this paper, we focus on the controlled growth mechanism of α-Fe2O3 nanostructures <I>via</I> the hydrothermal method. The field emission scanning electron microscopy (FESEM) results reveal that at a lower hydrothermal time, the initial nucleation involves the formation of short and thin β-FeOOH nanorods. The subsequent increase in the hydrothermal time leads β-FeOOH to form thicker and longer nanorods. However, high-temperature quenching (HTQ) at 800 °C for 10 min causes the conversion of akaganeite to the hematite phase and activation of hematite by Sn<SUP>4+</SUP> diffusion from a FTO substrate. Sn<SUP>4+</SUP> diffusion from the FTO substrate to the hematite nanostructure was elaborated by X-ray photoelectron spectroscopy (XPS). An α-Fe2O3 nanorod photoanode prepared by a hydrothermal reaction for 3 h and HTQ exhibits the highest photocurrent density of 1.04 mA cm<SUP>−2</SUP>. The excellent photoelectrochemical performance could be ascribed to the synergistic effect of the optimum growth of α-Fe2O3 nanorod arrays and Sn<SUP>4+</SUP> diffusion. Intensity modulated photovoltage spectroscopy (IMVS) studies revealed that the α-Fe2O3 photoanodes prepared at 3 h and HTQ exhibited a long electron lifetime (132.69 ms), and contribute to the enhanced PEC performance. The results confirmed that the controlled growth of the β-FeOOH nanorods, as well as Sn<SUP>4+</SUP> diffusion, played a key role in charge transfer during the photoelectrochemical application. The charge transfer mechanisms in α-Fe2O3 nanostructure photoanodes prepared at different hydrothermal times and high-temperature quenching are also investigated.</P>

A hydrothermally grown CdS nanograin-sensitized 1D Zr:α-Fe₂O₃/FTO photoanode for efficient solar-light-driven photoelectrochemical performance

Mahadik, Mahadeo A.,Subramanian, Arunprabaharan,Ryu, Jungho,Cho, Min,Jang, Jum Suk The Royal Society of Chemistry 2017 Dalton Transactions Vol.46 No.7

<▼1><P>A CdS nanograin sensitized 1D Zr:Fe2O3 nanorod arrays nanostructure was hydrothermally synthesized and showed an excellent photoelectrochemical performance due to the combined effect of light absorption in CdS and effective charge transport in one dimensional Zr:Fe2O3 nanorod arrays.</P></▼1><▼2><P>Well-defined CdS nanograin-sensitized one-dimensional (1D) Zr:α-Fe2O3 nanostructured arrays with enhanced photoelectrochemical performance are synthesized directly on F-doped SnO2 (FTO) using the hydrothermal method. Owing predominantly to the appropriate photogenerated electron–hole separation and charge collection in 1D Zr:α-Fe2O3 nanorods, hydrothermally deposited CdS/1D Zr:α-Fe2O3 samples exhibit improved photocurrent density over CdS/Fe2O3 nanosheets prepared by other methods. In our work, compared with 1D Zr:α-Fe2O3, the CdS-sensitized 1D Zr:α-Fe2O3 nanorod arrays show 1.9 times improved photoelectrochemical performance. Unfortunately, CdS nanograin-sensitized 1D Zr:α-Fe2O3 nanorod arrays suffer from instability problem. Nickel hydroxide loading, however, can boost the photoelectrochemical performance of the heterojunction and also act as a protective layer that improves the stability of the Ni(OH)2/CdS/1D Zr:α-Fe2O3 electrode compared to CdS/1D Zr:α-Fe2O3. This enhanced PEC activity may be ascribed to the strong heterojunctions between CdS nanograins and 1D Zr:α-Fe2O3 nanorod arrays as well as effective charge separation. This work will provide a new insight into the fabrication and protection of many new photosensitive electrode materials to engineer photoelectrochemical and photocatalytic devices in the near future.</P></▼2>