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Sensors and Materials, Vol. 36, No. 8 (2024) 3367–3380
MYU Tokyo
S & M 3737

Water-splitting Reactor Comprising a Combinationof TiO2-based Photoelectrochemical Celland Serially Connected TiO2/Se Heterojunction Photovoltaic Devices
Kenichi Takatsuki,1 Riku Takahashi,1 and Taizo Kobayashi2*
1Graduate School of Science and Engineering, Ritsumeikan University,

1-1-1 Noji Higashi, Kusatsu, Shiga 525-8577, Japan

2College of Science and Engineering, Ritsumeikan University,

1-1-1 Noji Higashi, Kusatsu, Shiga 525-8577, Japan(Received April 15, 2024; accepted June 11, 2024)

Keywords: TiO2, Sephotovoltaic cell, hydrogen production, bias voltage, on-chip device

In this paper, we report on a photocatalytic water-splitting device comprising a TiO2
photoanode combined with Se/TiO2 heterojunction photovoltaic (PV) devices. A Se/TiO2 PV
device can supply its photo-induced open circuit voltage to a TiO2-Pt photoelectrochemical cell
(PEC) in order to assist photoelectrolysis for H2 and O2 gas generation without external electricalsources. The open circuit voltage was increased from 0.65 V to approximately 3.2 V by the serial
connection of five Se/TiO2 PV devices. Hydrogen and oxygen bubbles were successfully
generated by only light illumination without using any wired external voltage source. A
photoanode with sol–gel derived porous TiO2 films was used to increase the surface area toattain a stronger photocatalytic reaction. Microdot patterns of Pt were formed as the co-catalyst
on a TiO2 photoanode to suppress the back reaction of the photogenerated carrier. A Pt anode
was formed on a glass substrate. Parameters related to the photocatalytic reaction, such asatmospheric pressure, the type of chemical additive, and the area ratio of Pt co-catalyst patterns
to the surface area of the TiO2 photoanode, were varied to generate a larger gas volume with thewater-splitting device.

1.

Introduction

Green hydrogen, which is harvested using only renewable energy without the need for fossil
fuels and with little carbon emissions, has been attracting considerable attention as a storable
clean fuel. Various large-scale industrial processes have been developed for hydrogen generation.
However, most of those conventional methods were fossil-derived processes such as the Haber–
Bosch process or a combination of natural-gas- and fossil-derived processes. Current industries
desiring hydrogen generation technology include petroleum refining, ammonia production,
vegetable oils, metal extraction, methanol, steel, and synthetic fuel.(1,2) Many researchers havedeveloped solar-power-driven water-splitting devices as a carbon-neutral hydrogen generation
technology. As such technologies, both electrochemical cells combined with solar cells and/or*Corresponding author: e-mail: tkt07004@fc.ritsumei.ac.jp

https://doi.org/10.18494/SAM5075

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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photocatalytic water-splitting devices have been developed.(3–5) Powder-based and thin-filmbased photocatalysts have been reported. For example, the use of the dual photocatalyst, which ismade by combining different photocatalysts for hydrogen and oxygen generation, is one of thepromising technical approaches of the powder-based photocatalyst for highly efficient water
splitting.(6) Although the hydrogen generation cost is reduced to approximately 1.6 $/kg when
the powder-based photocatalyst is used, the cost reduction of hydrogen generation is still an
important issue.(2) Thin-film photocatalysts have advantages such as the availability of
established manufacturing processes for on-chip implementation.(7) Eliminating the need for anelectrical power supply from the photocatalytic water-splitting system is also important toachieve low-cost hydrogen generation. The miniaturization of water-splitting systems into onchip devices will provide great opportunities for a safer on-site hydrogen harvesting technology
using various types of accessible environmental energy. As a tandem configuration of a PV
device and a photocatalyst, mesoporous metal oxide photocatalysts (e.g., SnO2 and TiO2)
integrated on a Si photoanode have been reported.(8) Khaselev and Turner also reported amonolithic-structure water-splitting device with p/n-GaAs and p-GaInP2.(9)Our group previously proposed an optically driven gas generator on a chip with a thinfilm photoelectrochemical cell (PEC) integrated with a thin-film heterojunction PV
cell.(10) A Se/TiO2 PV device can supply its light-induced open circuit voltage (Voc) at 0.65
V as a DC bias voltage to a TiO2-Pt PEC to accelerate photoelectrolysis for gas generation
without the need for external electrical sources. The serial connection of thin-film PV
devices makes it possible to increase the bias voltage with a simple device structure. The
integration of thin-film-based PEC and PV devices on a chip also enables a compact gas
generation system. Since TiO2 /ITO layers can be used as common materials for both
TiO2-Pt PEC and Se/TiO2 PV devices, the proposed gas generator is applicable to on-chip
integration. The ambient pressure, bias voltage, and filling area ratio of the Pt co-catalyst
in the TiO2 working electrode were varied during the characterization of water-splitting
devices with constant UV illumination to increase the hydrogen generation rate.

2.

Materials and Methods

2.1

Device design

We propose a water-splitting device composed of TiO2-based PEC and Se thin-film PV
devices, as illustrated in Fig. 1(a). An open circuit voltage of approximately 3.2 V can be supplied
to the TiO2 photocatalyst from five serially connected PV cells to support the photocatalytic
water-splitting reaction. Figure 1(b) shows a schematic of the device structure of a single cell of
a Se thin-film PV device. Sol-gel-derived mesoporous TiO2 thin films (500 nm) with a large
surface area were employed for the electrode to increase the hydrogen generation rate of PEC. It
is widely known that anatase TiO2 thin films generally exhibit a stronger photocatalytic activity
than rutile TiO2 thin films. In addition, it has been reported that sol–gel-derived porous TiO2
thin films exhibit a much stronger photocatalytic performance than sputter-deposited TiO2 thinfilms.(11) Moreover, it has also been reported that TiO2 with an anatase/rutile mixture structureoften exhibits a higher photocatalytic activity than that with a pure anatase or pure rutileSensors and Materials, Vol. 36, No. 8 (2024)

(a)

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(b)

Fig. 1. (Color online) (a) Device structure of the proposed water-splitting device composed of TiO2-based PEC andserially connected Se thin-film PV devices. (b) Device structure of single cell of TiO2/Se PV device.

structure.(12) For example, commercially available Degussa P25® (Sigma-Aldrich Co. LLC,
USA) is widely known to have highly photoresponsive TiO2 particles with anatase/rutile mixture
crystallographic structures.(13) We previously investigated the photocatalytic activities of
sputter-deposited and sol–gel-derived TiO2 thin films with different anatase/rutile ratios andsurface roughnesses.(14–17) Sol–gel-derived TiO2 thin films exhibited the strongest photocatalytic
activity among samples in our previous work.(14–17) Such photoanodes showed an anatase/rutile
mixture and a mesoporous structure. Accordingly, TiO2 films with anatase/rutile mixture
crystallographic structures deposited by the sol–gel method were employed in this work. The
porosity of TiO2 can be changed by adjusting the amount of polyethylene glycol (PEG) added tothe sol–gel precursor solution.(12) Circular Pt dot patterns of 0.2 mm diameter were formed on a
TiO2 photoanode as the co-catalyst to promote the water-splitting reaction. The electricalpotential gradient at the Pt/TiO2 interface promotes the transfer of photogenerated carriers from
TiO2 to Pt, resulting in the suppression of the hole-electron recombination, and the photocatalytic
reaction can be enhanced.(18,19) A sputter-deposited Pt cathode (200 nm) was used as theelectrode for hydrogen generation. The TiO2 photoanode was illuminated by UV light throughan indium-tin-oxide (ITO) transparent electrode layer from the substrate side. The exposed areaof the TiO2 photoanode was designed to be 66.5 mm2. A Se thin-film PV device was developed
as a superstrate structured device that can be illuminated from the substrate side. The PV device
was composed of a p-type polycrystalline Se photoabsorber layer and an n-type TiO2 window
layer. Se is known as a promising photoabsorber material for visible light because of its very
high absorption coefficient and bandgap energy of approximately 1.9 eV. In indoor PV
applications under LED or cold fluorescent light illumination, the theoretical maximum limit of
the conversion efficiency of the Se PV device is expected to be very high (approximately 55%).(20)
MoO3 (20 nm) was employed as a hole transport layer between Se and Au layers. The active areaof a single TiO2/Se PV device was designed to be 4 mm2. Electrical lead patterns were insulated
by nonconductive epoxy resin. A water-splitting reactor comprising TiO2-based PEC and Sebased PV devices on a chip is characterized by varying the pressure, pH, and supplied bias
voltage, as described in the experimental section.

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Sensors and Materials, Vol. 36, No. 8 (2024)

Device fabrication

On-chip photocatalytic reactor devices were fabricated by the following steps (illustrated inFig. 2):
1.	FTO patterning: First, a fluorine-doped tin oxide (FTO) transparent conducting layer was
patterned by HCl etching with a Mg particle catalyst. Photoresist mask patterns were formedon an FTO-coated glass substrate (Furuuchi Chemical Corporation, Japan). The photoresist(OFPR-800LB, 50CP) was prebaked at 115 ºC for 3 min after spin-coating at 300 rpm for 30
s. After UV exposure, the photoresist was postbaked at 180 ºC for 2 min. Patterns wereformed by dipping samples in a developer solution. The area of FTO exposed to UV
illumination was etched by applying 2.0 g of zinc powder (90%, Fujifilm Wako Pure
Chemical, Japan) uniformly on the substrate and applying 2.5 mL of HCl (35%, Fujifilm
Wako Pure Chemical). The residual photoresist on FTO patterns was removed by dipping in
acetone (Guaranteed Reagent, Fujifilm Wako Pure Chemical).
2.	TiO2 photoanode deposition: Next, mesoporous TiO2 was formed by the calcination of PEG
containing a sol–gel precursor. As reported in the literature, the surface area of mesoporous
TiO2 can be varied by changing the molecular weight of the added PEG.(12,21,22) The patterned
TiO2 photoanode was formed by spin-coating on the masked FTO layer. The TiO2 sol–gel
precursor was prepared by adding 10 mL of deionized water and 1 mol% PEG into a mixture
solution of 1.25 mL of titanium isopropoxide (TTIP) (95%, Fujifilm Wako Pure Chemical),
10 mL of ethanol (99.5%, Fujifilm Wako Pure Chemical), and 0.3 mL of HCl. The TiO2 sol–gel precursor on the substrate was dried at 100 ºC for 10 min, and the substrate temperature
was increased for annealing at 500 ºC for 1 h.
3.	Pt cathode and co-catalyst deposition: The Pt cathode and co-catalyst patterns were depositedby sputtering with a stencil mask after the deposition of the Ti adhesive layer. Stencil masks
were prepared by forming holes on 60-mm-thick Ni foils by wet etching with 1.2 mol/L FeCl3
aqueous solution.

Fig. 2. (Color online) Fabrication of the proposed water-splitting device composed of TiO2-based PEC and seriallyconnected Se thin PV devices on a chip.

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4.	n-type TiO2 window layer deposition: The TiO2 window layer (50 nm) was deposited byreactive sputtering with a Ti metal target (4 N, Ti, 50.8 mm in diameter, 5 mm in thickness,
Furuuchi Chemical Co.).(16,17,23) The sputtering chamber was evacuated using a turbo
molecular pump to a base pressure below 1 × 10−3 Pa. Argon gas was introduced at thedesired partial pressure (PAr) of 0.1 Pa. Oxygen gas was also introduced at a partial pressure(PO2) of 0.3 Pa. The total pressure was set as 6 Pa at the substrate temperature of 350 °C. The
film deposition was conducted with an RF power at 300 W. TiO2 thin films were patternedby sputtering through a stencil mask.
5.	Te/Se deposition: A very thin Te layer was inserted between the Se and TiO2 layers to
improve the adhesion between these layers. Te and Se layers were sequentially deposited bythermal evaporation. After Se deposition, Se/Te stacked layers were annealed at 200 °C for 2
min to crystallize the Se/Te layers.(10,23–25) The Te thickness was controlled by evaporating
all the weighed Te debris because it is difficult to form 1-nm-thick films using a thickness
monitoring system. Se/Te layers were patterned by evaporation through a stencil mask.
6.	MoOx and Au deposition: The MoOx (20 nm) and Au (150 nm) hole injection layers were
deposited by thermal evaporation through stencil masks.

3.

Results and Discussion

3.1

Preliminary characterization of water splitting

In a preliminary experiment, PEC and PV performance characteristics were characterizedusing the separated devices to simplify the characterization procedure, considering the yield rateof manufacturing devices. Bias voltages supplied to PEC were varied using both a wired external
electrical power source and the separated PV devices in the first step of characterization.
PEC devices without PV devices were prepared by fabrication steps 1–3 described in Sect. 3.
A photograph of the fabricated PEC and SEM and AFM images of the mesoporous TiO2
photoanode are shown in Figs. 3(a)–3(c), respectively. The surface roughness and morphology of
the TiO2 thin films were evaluated by scanning at a rate of 1.0 Hz using a dynamic atomic force
microscope (DFM, MFP-3D-BIO, Asylum Research Corp., CA, USA). A rough morphology ofporous TiO2 can be observed in Fig. 3(c). The root mean square surface roughness (R RMS) of themesoporous TiO2 photoanode was measured to be 262 nm. The effects of increasing the surface
area of TiO2 at R RMS larger than 262 nm on enhancing photocatalytic activity have beenreported.(23,26) For example, Latthe et al. previously reported that the photocurrent density incyclic voltammetry with light illumination was increased seven times by increasing the surface
roughness of the supersonic aerosol-coated TiO2 thin films from Ra = 350 to 710 nm.(27) Thus,
further optimization of the surface morphology is considered to be effective for enhancing thephotocatalytic reaction. An open circuit voltage of approximately 3.2 V can be supplied to theTiO2 photocatalyst by the five serially connected PV cells to support the water-splitting reaction.
The PEC shown in Fig. 3(a) is composed of a Pt cathode and a TiO2 photoanode for hydrogen andoxygen generation, respectively.

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Fig. 3. (Color online) (a) Photograph of the fabricated PEC and (b) SEM and (c) AFM images of mesoporous TiO2
photoanode.

The water-splitting reaction was characterized using the fabricated PEC with an acrylic
attachment. Ambient pressure, the type of chemical additive in the aqueous solution, and bias
voltage applied to the TiO2 photoanode are known to be effective parameters in the watersplitting reaction. These parameters were varied, and the generated volume of hydrogen wasmeasured. An acrylic attachment with gas reservoirs was designed to observe the water-splitting
reaction and measure gas volume as parameters such as pressure, bias voltages and area
percentage of co-catalyst on TiO2 photoelectrode and their combination are varied. The
generated gas volume was estimated by measuring the reserved gas volume, as shown in Fig.
4(a). Ambient pressure can be adjusted by evacuation to the targeted pressure while monitoring
the pressure gauge. Bias voltages can be applied to the photoanode through the flange withhermetic seal under negative pressure conditions. The acrylic chamber was filled with anelectrolyte solution in such a way that the pressure-measuring port is not completely immersed.
The water-splitting reaction was started by UV illumination from the back of the substrate andsupplying bias voltage. The UV-LED (peak wavelength at 365 nm, FOLS-09) was used at an
intensity of 10 mW/cm2 in this experiment. Figure 4(b) shows a photograph of the fabricatedreactor chamber. Gas volumes that accumulated in reservoirs as a function of time were
measured.
PV devices without PEC were prepared by fabrication steps 4–6 described in Sect. 3. A
photograph of the simplified PV device is shown in Fig. 5.
The active area of a single TiO2/Se PV device shown in Fig. 5 was designed to be 4 mm2. The
measurement results of current density and voltage curves for (a) a single PV device and (b) aserially connected PV device (five cells) with and without light illumination are shown in Fig. 6.
Light illumination was conducted at 100 mW/cm2 at AM 1.5 using a solar simulator (OTENTOSUNIII, Bunkoukeiki Co., Ltd., Japan). It was observed in Figs. 6(a) and 6(b) that the openSensors and Materials, Vol. 36, No. 8 (2024)

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(a)

(b)
Fig. 4. (Color online) (a) Structure of acrylic attachment with gas reservoirs for the observation of water-splitting
reaction and measurement of gas volumes as parameters such as pressure, bias voltages and area percentage of cocatalyst on TiO2 photoelectrode and their combination are varied. (b) Photograph of the fabricated acrylic reaction
chamber.

circuit voltage was increased from 0.69 to 3.32 V by connecting five PV cells serially. The
serially connected device had lower short-circuit current density and fill factor than the single
PV device. This is probably a result of the increased resistance at the connecting parts between
the five cells and/or the inadequate matching of the photocurrent from individual PV cells. Thisis also caused by the inadequate performance homogeneity of plural photovoltaic cells on asubstrate by our laboratory equipment. It will be important to improve the uniformity of PV
performance to decrease the mismatched current of serially connected PV cells. Decreasing the3374

Fig. 5.

Sensors and Materials, Vol. 36, No. 8 (2024)(Color online) Photograph of the fabricated PV device and magnified photograph of a single PEC device.

(a)

(b)

Fig. 6. (Color online) Measurement results of current density and voltage curves for (a) a single PV device and (b)a serially connected PV device (five cells) with and without light illumination at 100 mW/cm 2 at AM 1.5.

distance between individual photovoltaic cells is also suggested as an effective approach todecreasing the electrical resistance at connecting parts.
A photograph of the fabricated PEC with the wire-connected PV devices used for the
preliminary characterization of water splitting and its demonstration are shown in Figs. 7(a) and
7(b), respectively. It can be observed in Fig. 7(b) that the generated oxygen was accumulated in
the reservoir. However, it was difficult to measure the hydrogen volume accurately becausehydrogen bubbles adhered strongly to the Pt electrode. Thus, the hydrogen volume wascalculated to be two times the oxygen volume.
The generated hydrogen volumes as a function of UV illumination time with and without Pt
co-catalyst patterns on the TiO2 photoanode are shown in Fig. 8, when a bias voltage of 3.2 V

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(a)

(b)
Fig. 7. (Color online) Photographs of (a) PEC with the wire-connected PV devices for preliminary water-splitting
characterization and (b) water-splitting demonstration.

Fig. 8. Generated hydrogen volume as a function of UV illumination time, when the experiment was conducted at
the ambient pressure of 101 kPa. A bias voltage of 3.2 V was supplied by the wire-connected DC electrical powersource and the wire-connected serially connected PV devices.

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was supplied by the wire-connected DC electrical power source and the wire-connected seriallyconnected TiO2/Se PV devices. The areal percentage of Pt to TiO2 was fixed at 3.14%. Five PV
devices were serially connected, and the open circuit voltage of the connected PV was 3.2 V
with one-sun illumination. UV was also applied to the sol–gel-derived TiO2 photoanode. It wasobserved in Fig. 8 that the rate of hydrogen generation increased from 18.3 to 27.9 µL/h uponadding the Pt co-catalyst to the TiO2 photoanode when a bias voltage of 3.2 V was supplied by
the DC power source. It was also observed in Fig. 8 that the rate of hydrogen generation
increased from 17.4 to 26.7 µL/h upon adding the Pt co-catalyst to the TiO2 photoanode when abias voltage of 3.2 V was supplied by the serially connected TiO2/Se PV devices. These results
indicate that the photocatalytic reaction is enhanced by suppressing the hole-electron
recombination due to the creation of an electrical potential gradient at the Pt/TiO2 Schottky
interface.(1,14) The TiO2-Pt PEC with bias voltages supplied by DC and PV power sources
exhibited water-splitting performance similar that of the TiO2 electrode.
The generated hydrogen volume as a function of time is shown in Fig. 9, when the pressure in
the acrylic chamber was varied. Generally, photocatalytic water splitting reactivity decreases
with increasing ambient pressure because of the activation of back reactions such as water
formation by hydrogen and oxygen molecules and/or the photoreduction of those molecules.(28,29)
At high pressures, the photocatalytic activity for overall water splitting generally decreases
because of the slower bubble formation and desorption of product gases.(28) An open circuit
voltage of 3.2 V was applied to the PEC from the wire-connected TiO2/Se PV devices by thesame procedure as described in the explanation for Fig. 8. It was seen that the rate of hydrogengeneration increased with decreasing pressure. The volume of hydrogen generated at a pressureof 20 kPa reached almost five times that obtained in air atmosphere.
The saturation tendencies of hydrogen production were observed at lower pressures as shownin Fig. 9. One of the possible reasons for that may be the increased back reaction due to theincreased oxygen and hydrogen amounts with decreasing ambient pressure. The reactive surface

Fig. 9. Generated hydrogen volume as a function of UV light illumination time, when the pressure in the acrylicchamber was varied. An open circuit voltage of 3.2 V was applied to the PEC from the wire-connected TiO2/Se PV
devices by the same procedure as described in the explanation of Fig 8.

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area of the Pt cathode was considered to be smaller than that of the porous TiO2 photoanode.
Thus, the reaction rate limitation at the Pt cathode is also considered as the other possible reason
for the nonlinear tendency of hydrogen production when ambient pressure was decreased. The
optimization of device dimensions is one of our future works. The effects of adding a Pt cocatalyst with TiO2 on the water-splitting activity were characterized at a constant pressure of 20
kPa. A co-catalyst such as Pt or Au is considered to suppress the back reaction of molecules inphotocatalytic water splitting.(13) According to the literature, the excessively large percentages of
the Pt co-catalyst to the TiO2 photocatalyst cause reaction limitation because of the trade-off
between the decreased photon absorption due to a Pt shadow and the promotion of thereaction.(30) The number of pieces of Pt co-catalyst circle shaped patterns (200 um in diameter)
was varied as 0, 5, 10, 15, 20, and 25. The Pt co-catalyst patterns were formed as circular dots of
200 mm diameter. The area ratios of Pt to TiO2 were 0.63, 1.26, 1.89, 2.51, and 3.14%. The
calculated hydrogen volume as a function of time is shown in Fig. 10. It is suggested that thegenerated hydrogen volume increased with the area ratio of Pt to TiO2. TiO2 supported with Pt at
an area ratio of 3.14 % increased the amount of hydrogen generated to 1.5 times larger than thatwithout a co-catalyst. On the other hand, the rate of increase in the amount of hydrogengenerated showed a saturation tendency with increasing Pt area. This may be a result of thereverse reaction from gas to water at the Pt surface with increasing Pt area. In addition, theexcessively increased hydrogen production may cause the increased back reaction.
The hydrogen production reactor for a mobile power source is one of our future applications.
In this study, the hydrogen production rate can be calculated as 5.36 mmol/h using a TiO2
photoanode with a 17 × 14 mm2 projection area. Further increasing the hydrogen production rate
up to sub-mol/h will become one of our future works. As a reference, it has been reported that amethanol fuel processor with dimensions of 70 × 40 × 30 mm3 demonstrated hydrogenproduction at 0.498 mol/h for the estimated electric power of 15 W.(31) From the viewpoint ofapplication for hydrogen fuel cells, 15 μL per W/min was reported as an example of the hydrogenconsumption rate.(32) Therefore, it will be necessary to increase the hydrogen production rate byapproximately twofold.

Fig. 10. (Color online) Generated hydrogen volume as a function of time, when the area ratio of Pt co-catalyst to
TiO2 photoanode was varied. The number of pieces of Pt co-catalyst circle shaped patterns (200 um in diameter) was
varied as 0, 5, 10, 15, 20, and 25. The area ratios of Pt to TiO2 were 0.63, 1.26, 1.89, 2.51, and 3.14% at a constantpressure of 20 kPa.

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3.2

Water-splitting demonstration using device with TiO2/Se PV integrated with TiO2-Pt
PEC devices

The TiO2/Se PV device and TiO2-based PEC were integrated and connected to each other on
a 30 × 30 mm2 chip, as shown in Fig. 11. Five PV devices were serially connected, and the open
circuit voltage of the connected PV was 3.2 V with one-sun illumination. UV was also applied tothe sol–gel-derived TiO2 photoanode under the same conditions as in the preliminary
experiment. The hydrogen volume as a function of light illumination time is shown in Fig. 12,
when a bias voltage of 3.2 V was supplied from an external voltage source and the seriallyconnected PV devices on a chip. The hydrogen generation rate was calculated from the reserved
oxygen volume to be 26.6 μL/h. The PEC and PV integrated device on a chip showed a smaller
volume of hydrogen generated. The increased series resistance due to the serial connection of
PV devices is considered to be one of the possible reasons for the smaller hydrogen volume.

Fig. 11. (Color online) Photograph of TiO2/Se PV devices and TiO2-based PEC integrated and connected to eachother on a 30 × 30 mm 2 chip.

Fig. 12. Hydrogen volume as a function of light illumination time, when a bias voltage of 3.2 V was supplied froman external voltage source and serially connected PV devices on a chip.

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Conclusions

Compact fuel cells such as PEFC have recently attracted considerable attention in terms ofapplicable power supply for indoor small appliances such as IoT devices. Thus, it is consideredthat a miniaturized hydrogen generator will become more important for such applications.
Microreactor technology enables a reduction of risk in terms of the use of reactive and/or toxic
materials, and its compactness will be suitable for mobile on-site hydrogen generation
applications. In this work, a water-splitting device composed of a combination of a TiO2
photocatalyst and a PV bias voltage source on a chip was developed. The water-splitting reaction
was successfully induced by only light illumination without the need for an external wired
electrical power source.

Acknowledgments
The authors would like to acknowledge the support of the Mazda Foundation.

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About the Authors
Kenichi Takatsuki received his B.S. degree from Ritsumeikan University, Shiga, Japan, in
2023. Since 2023, he has been a master course student at Ritsumeikan University.
Riku Takahashi received his B.S. degree in 2021 and M.S. degree in 2023 in mechanicalengineering from Ritsumeikan University, Shiga, Japan. He is currently working at Mitsubishi
Electric Inc.
Taizo Kobayashi received his B.S. degree in 1999, M.S. degree in 2001, and Ph.D. degree in2009 in mechanical engineering from Ritsumeikan University, Shiga, Japan. His research
focuses on optical functional materials in MEMS applications. He is currently a professor at
Ritsumeikan University, where he joined the faculty in 2017. (tkt07004@fc.ritsumei.ac.jp)