Unique photoelectrochemical behavior of TiO₂ nanorods wrapped with novel titanium Oxy-Nitride (TiO_xN_y) nanoparticles

Arunachalam, Maheswari,Yun, Gun,Ahn, Kwang-Soon,Seo, Won-Seon,Jung, Dae Soo,Kang, Soon Hyung Elsevier 2018 International journal of hydrogen energy Vol.43 No.34

<P><B>Abstract</B></P> <P>In this work, we developed novel titanium oxynitride (TiO<SUB>x</SUB>N<SUB>y</SUB>) nanoparticles with diameter of 25 ± 2 nm and crystalline size of ∼15 nm on hydrothermally grown one-dimensional (1D) TiO<SUB>2</SUB> nanorod (TNR) arrays. Herein, the TiO<SUB>x</SUB>N<SUB>y</SUB> nanoparticles were synthesized by facile nitridation using TiO<SUB>2</SUB> powder at 100% NH<SUB>3</SUB> gas atmosphere. Titanium oxynitride composed of potentially energetic metal-nitrogen bonds (TiN), compared to the weaker TiO bond, becomes chemically stable in the alkaline environment, and is considered as a suitable material for photoelectrochemical (PEC) system. The PEC performance of TiO<SUB>x</SUB>N<SUB>y</SUB> decorated TNR (abbreviated as TiO<SUB>x</SUB>N<SUB>y</SUB> @TNR) films was evaluated in 0.1 M KOH solution under solar illumination condition, and achieved the potentially high photocurrent density (<I>J</I>) of 2.1 mA/cm<SUP>2</SUP> at 1.23 V versus reversible hydrogen electrode (RHE) (abbreviated as V<SUB>RHE</SUB>) in the TiO<SUB>x</SUB>N<SUB>y</SUB>@TNR arrays, in comparison with the poor photoresponse (0.7 mA/cm<SUP>2</SUP> at 1.23 V<SUB>RHE</SUB>) of the pristine TNR arrays. A nearly three-fold enhancement was attained in the TiO<SUB>x</SUB>N<SUB>y</SUB> decorated TNR arrays, attributed to the high visible light absorption and fast carrier separation, due to the hybridization with the visible active TiO<SUB>x</SUB>N<SUB>y</SUB> nanoparticles in the cascading band alignment between the TiO<SUB>x</SUB>N<SUB>y</SUB> and TNR materials. Furthermore, the introduction of TiO<SUB>x</SUB>N<SUB>y</SUB> layer on the TNR surface quite reduces the interfacial resistance in the solid-liquid interface region, and further, the TiO<SUB>x</SUB>N<SUB>y</SUB> layer contributes to the passivation of the surface states (<I>e.g.,</I> defect, trap sites etc.) where the charge recombination reaction frequently happens, leading to the improvement of PEC performance.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Titanium oxynitride (TiO<SUB>x</SUB>N<SUB>y</SUB>) nanoparticles on 1D TiO<SUB>2</SUB> nanorod was well developed. </LI> <LI> TiO<SUB>x</SUB>N<SUB>y</SUB> nanoparticles were synthesized by nitridation using TiO<SUB>2</SUB> powder at 100% NH<SUB>3</SUB> gas. </LI> <LI> Photocurrent density of 2.1 mA/cm<SUP>2</SUP> at 1.23 V<SUB>RHE</SUB> in the TiO<SUB>x</SUB>N<SUB>y</SUB>@TNR arrays was achieved. </LI> <LI> A three-fold enhancement was attained in the TiO<SUB>x</SUB>N<SUB>y</SUB> decorated TNR film versus TNR film. </LI> </UL> </P>

Oxygen evolution NiOOH catalyst assisted V₂O₅@BiVO₄ inverse opal hetero-structure for solar water oxidation

Arunachalam, Maheswari,Ahn, Kwang-Soon,Kang, Soon Hyung Elsevier 2019 International journal of hydrogen energy Vol.44 No.10

<P><B>Abstract</B></P> <P>Using vanadium oxide (V<SUB>2</SUB>O<SUB>5</SUB>) inverse opal (IO) as a three-dimensional (3D) electron transporting tunnel, bismuth vanadate (BiVO<SUB>4</SUB>) as a light harvester, and Amorphous Nickel Hydroxide (NiOOH) as an oxygen evolution co-catalyst, a V<SUB>2</SUB>O<SUB>5</SUB>@BiVO<SUB>4</SUB>@NiOOH IO architecture was fabricated as an efficient photoanode on a conductive fluorine doped tin oxide (FTO) substrate for photoelectrochemical (PEC) water oxidation. V<SUB>2</SUB>O<SUB>5</SUB> is the visible light absorbing photoanodes for water oxidation; however, the efficiency of this compound remains low (∼0.08 mA/cm<SUP>2</SUP> at 1.23 V vs. reversible hydrogen electrode (V<SUB>RHE</SUB>)) and the unfavorable surface trap states limits the activity of V<SUB>2</SUB>O<SUB>5</SUB> photoelectrodes in a PEC system. We found that the photoactive thin conductive BiVO<SUB>4</SUB> (∼12 nm) in the V<SUB>2</SUB>O<SUB>5</SUB> IO greatly enhanced the charge separation efficiency to achieve better PEC water oxidation through modification of the surface states. The subsequent addition of NiOOH as an effective Oxygen evolution catalyst subsequently reduces the large overpotential and generates the photocurrent density of 1.14 mA/cm<SUP>2</SUP> at 1.23 V<SUB>RHE</SUB>. Electrochemical impedance spectroscopy (EIS) evidenced that NiOOH deposition can substantially lower the charge transfer resistance (R<SUB>ct</SUB>) at the semiconductor interface. Specifically, the consecutive and ordered morphology renders direct conduction pathways for the extraction of photogenerated electron/hole pairs and the convenient structure to penetrate the photogenerated carriers toward the semiconductor surface over the electrolyte. It is expected that the uninterrupted pathways will improve the electron transportation and thus the charge collection properties.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Simple synthesis method adapted to prepare V<SUB>2</SUB>O<SUB>5</SUB> IO photoanode. </LI> <LI> V<SUB>2</SUB>O<SUB>5</SUB>@BiVO<SUB>4</SUB>@NiOOH IO architecture was fabricated as photoanode for water oxidation. </LI> <LI> Thin conductive BiVO<SUB>4</SUB> (∼12 nm) in the V<SUB>2</SUB>O<SUB>5</SUB> IO enhanced the charge separation efficiency. </LI> <LI> Inverse opal morphology prompts uninterrupted pathways to improve the electron transportation. </LI> </UL> </P>

염료감응형 태양전지로의 응용을 위한 얇은 TiO2가 코팅 된 WO3 역오팔 광전극의 개발

Maheswari Arunachalam,곽서의,이인호,김청수,강순형,이상권 한국재료학회 2019 한국재료학회지 Vol.29 No.8

In this study, we prepare pure WO3 inverse opal(IO) film with a thickness of approximately 3 μm by electrodeposition, and an ultra-thin TiO2 layer having a thickness of 2 nm is deposited on WO3 IO film by atomic layer deposition. Both sets of photoelectrochemical properties are evaluated after developing dye-sensitized solar cells(DSSCs). In addition, morphological, crystalline and optical properties of the developed films are evaluated through field-emission scanning electron microscopy(FE-SEM), High-resolution transmission electron microscopy(HR-TEM), X-ray diffraction(XRD) and UV/ visible/infrared spectrophotometry. In particular, pure WO3 IO based DSSCs show low VOC, JSC and fill factor of 0.25 V, 0.89 mA/cm2 and 18.9 %, achieving an efficiency of 0.04 %, whereas the TiO2/WO3 IO based DSSCs exhibit VOC, JSC and fill factor of 0.57 V, 1.18 mA/cm2 and 50.1 %, revealing an overall conversion efficiency of 0.34 %, probably attributable to the high dye adsorption and suppressed charge recombination reaction.

Effects of Al₂O₃ Coating on BiVO₄ and Mo-doped BiVO₄ Film for Solar Water Oxidation

Arunachalam, Maheswari,Yun, Gun,Lee, Hyo Seok,Ahn, Kwang-Soon,Heo, Jaeyeong,Kang, Soon Hyung The Korean Electrochemical Society 2019 Journal of electrochemical science and technology Vol.10 No.4

Planar BiVO₄ and 3 wt% Mo-doped BiVO₄ (abbreviated as Mo:BiVO₄) film were prepared by the facile spin-coating method on fluorine doped SnO₂(FTO) substrate in the same precursor solution including the Mo precursor in Mo:BiVO₄ film. After annealing at a high temperature of 450℃ for 30 min to improve crystallinity, the films exhibited the monoclinic crystalline phase and nanoporous architecture. Both films showed no remarkably discrepancy in crystalline or morphological properties. To investigate the effect of surface passivation exploring the Al₂O₃ layer, the ultra-thin Al₂O₃ layer with a thickness of approximately 2 nm was deposited on BiVO₄ film using the atomic layer deposition (ALD) method. No distinct morphological modification was observed for all prepared BiVO₄ and Mo:BiVO₄ films. Only slightly reduced nanopores were observed. Although both samples showed some reduction of light absorption in the visible wavelength after coating of Al₂O₃ layer, the Al₂O₃ coated BiVO₄ (Al₂O₃/BiVO₄) film exhibited enhanced photoelectrochemical performance in 0.5 M Na₂SO₄ solution (pH 6.5), having higher photocurrent density (0.91 mA/㎠ at 1.23 V vs. reversible hydrogen electrode (RHE), briefly abbreviated as V_RHE) than BiVO₄ film (0.12 mA/㎠ at 1.23 V_RHE). Moreover, Al₂O₃ coating on the Mo:BiVO₄ film exhibited more enhanced photocurrent density (1.5 mA/㎠ at 1.23 V_RHE) than the Mo:BiVO₄ film (0.86 mA/㎠ at 1.23 V_RHE). To examine the reasons, capacitance measurement and Mott-Schottky analysis were conducted, revealing that the significant degradation of capacitance value was observed in both BiVO₄ film and Al₂O₃/Mo:BiVO₄ film, probably due to degraded capacitance by surface passivation. Furthermore, the flat-band potential (V_FB) was negatively shifted to about 200 mV while the electronic conductivities were enhanced by Al₂O₃ coating in both samples, contributing to the advancement of PEC performance by ultra-thin Al₂O₃ layer.

Multilayered Fluorine Doped SnO₂ Inverse Opal/WO₃/BiVO₄ Film for Solar Water Oxidation: Systematic Development and Defined Role of Each Layer

Yun, Gun,Arunachalam, Maheswari,Ahn, Kwang-Soon,Nam, Ki Min,Ha, Jun-Seok,Kang, Soon Hyung Electrochemical Society 2019 Journal of the Electrochemical Society Vol.166 No.15

<P>We developed the multilayered fluorine doped SnO<SUB>2</SUB> (abbreviated as FTO) inverse opal (IO)/WO<SUB>3</SUB>/BiVO<SUB>4</SUB> film by sol-gel based spin coating, subsequently followed by facile electrodeposition. Above FTO IO template, both WO<SUB>3</SUB> and BiVO<SUB>4</SUB> layers were prepared by the electrodeposition methods in the variation of charge amount of 200–1,000 mC cm<SUP>−2</SUP> and of 300–1,500 mC cm<SUP>−2</SUP>, respectively. On increasing the loaded charge amount of WO<SUB>3</SUB> under the constant loaded amount of BiVO<SUB>4</SUB>, the light absorption in the near 400 nm is increased to steadily enhance PEC performance. In contrast, the further loaded WO<SUB>3</SUB> layer of 800–1,000 mC cm<SUP>−2</SUP> on FTO IO film degrades PEC activity, due to the increase of the intrinsic material and interfacial resistances. Meanwhile, on increasing the electrodeposited charge amount of BiVO<SUB>4</SUB> under the constant charge amount of WO<SUB>3</SUB>, the light absorption toward the visible wavelength is sharply increased, and gives rise to the enhancement of PEC performance; and conversely, the more loaded BiVO<SUB>4</SUB> of 1,200–1,500 mC cm<SUP>−2</SUP> induced the decay of PEC performance. The optimum condition in FTO IO/WO<SUB>3</SUB> (600 mC cm<SUP>−2</SUP>)/BiVO<SUB>4</SUB> (600 mC cm<SUP>−2</SUP>) film was well established, and the function or role of each layer was confirmed using photoluminescence, charge separation/transfer efficiency, and photovoltage-time spectra.</P>

Electrolyte effects on undoped and Mo-doped BiVO₄ film for photoelectrochemical water splitting

Das, Pran Krisna,Arunachalam, Maheswari,Seo, Young Jun,Ahn, Kwang-Soon,Ha, Jun-Seok,Kang, Soon Hyung Elsevier 2019 Journal of Electroanalytical Chemistry Vol.842 No.-

<P><B>Abstract</B></P> <P>As the electrolyte in a photoelectrochemical system used for solar water splitting is closely associated with the charge transfer phenomenon, the fundamental study of the electrolyte is an increasingly crucial issue. In this paper, the relation between the photoelectrochemical performance and the variation of electrolyte composition is investigated in depth. Here, the potassium phosphate (KPi) containing different anions (CH<SUB>3</SUB>COO<SUP>−</SUP>, Cl<SUP>−</SUP> and NO<SUB>3</SUB> <SUP>−</SUP>) or cations (NH<SUB>4</SUB> <SUP>+</SUP>, K<SUP>+</SUP> and Na<SUP>+</SUP>) was prepared at the electrolyte with the different pH values of 5, 7, 9, and 11 in order to investigate how anions or cations affect the PEC performance. Herein, we compared the differences in the undoped and Mo-doped BiVO<SUB>4</SUB> films due to the different bulk/surface states, and found different morphological and crystalline features as well as remarkably different PEC performance. At first, the pH in the electrolyte significantly influences the solar water oxidation reaction, revealing that the electrolyte with a high pH grants better PEC activity, because the higher pH solution can provide more hydroxide ions (OH<SUP>−</SUP>) to react with holes to form hydroxyl radicals, which are recognized as important intermediates for PEC water oxidation in the presence of O<SUB>2</SUB>. Further, in the electrolyte containing the different cations, the NH<SUB>4</SUB> <SUP>+</SUP> ions exhibit the enhanced PEC performance, due to its cation size which increases the ionic dissociation and viscosity. By contrast, the anion effect can be negligible in the used electrolyte. From this fundamental research, it can be known that the optimization of electrolyte is quite a vital parameter for advancing the PEC performance.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Electrolyte study in BiVO<SUB>4</SUB> and Mo-doped BiVO<SUB>4</SUB> films in the photoelectrochemical water splitting </LI> <LI> Cation and anion, pH and long-term stability should be investigated to measure the PEC activity. </LI> <LI> Best performance was achieved in the NH<SUB>4</SUB> <SUP>+</SUP> cation in the alkaline solution. </LI> <LI> Faradaic efficiency of Mo-doped BiVO<SUB>4</SUB> film exhibit the higher than that of undoped BiVO<SUB>4</SUB> film. </LI> </UL> </P>

Nanolayered CuWO₄ Decoration on Fluorine-Doped SnO₂ Inverse Opals for Solar Water Oxidation

Cho, Ha Eun,Yun, Gun,Arunachalam, Maheswari,Ahn, Kwang-Soon,Kim, Chung Soo,Lim, Dong-Ha,Kang, Soon Hyung The Korean Electrochemical Society 2018 Journal of electrochemical science and technology Vol.9 No.4

The pristine fluorine-doped $SnO_2$ (abbreviated as FTO) inverse opal (IO) was developed using a 410 nm polystyrene bead template. The nanolayered copper tungsten oxide ($CuWO_4$) was decorated on the FTO IO film using a facile electrochemical deposition, subsequently followed by annealing at $500^{\circ}C$ for 90 min. The morphologies, crystalline structure, optical properties and photoelectrochemical characteristics of the FTO and $CuWO_4$-decorated FTO (briefly denoted as $FTO/CuWO_4$) IO film were investigated by field emission scanning electron microscopy, X-ray diffraction, UV-vis spectroscopy and electrochemical impedance spectroscopy, showing FTO IO in the hexagonally closed-pack arrangement with a pore diameter and wall thickness of about 300 nm and 20 nm, respectively. Above this film, the $CuWO_4$ was electrodeposited by controlling the cycling number in cyclic voltammetry, suggesting that the $CuWO_4$ formed during 4 cycles (abbreviated as $CuWO_4$(4 cycles)) on FTO IO film exhibited partial distribution of $CuWO_4$ nanoparticles. Additional distribution of $CuWO_4$ nanoparticles was observed in the case of $FTO/CuWO_4$(8 cycles) IO film. The $CuWO_4$ layer exhibits triclinic structure with an indirect band gap of approximately 2.5 eV and shows the enhanced visible light absorption. The photoelectrochemical (PEC) behavior was evaluated in the 0.5 M $Na_2SO_4$ solution under solar illumination, suggesting that the $FTO/CuWO_4$(4 cycles) IO films exhibit a photocurrent density ($J_{sc}$) of $0.42mA/cm^2$ at 1.23 V vs. reversible hydrogen electrode (RHE, denoted as $V_{RHE}$), while the FTO IO and $FTO/CuWO_4$(8 cycles) IO films exhibited a $J_{sc}$ of 0.14 and $0.24mA/cm^2$ at $1.23V_{RHE}$, respectively. This difference can be explained by the increased visible light absorption by the $CuWO_4$ layer and the favorable charge separation/transfer event in the cascading band alignment between FTO and $CuWO_4$ layer, enhancing the overall PEC performance.