Water-splitting Reactor Comprising a Combination of TiO 2 -based Photoelectrochemical Cell and Serially Connected TiO 2 /Se Heterojunction Photovoltaic Devices

In this paper, we report on a photocatalytic water-splitting device comprising a TiO 2 photoanode combined with Se/TiO 2 heterojunction photovoltaic (PV) devices. A Se/TiO 2 PV device can supply its photo-induced open circuit voltage to a TiO 2 -Pt photoelectrochemical cell (PEC) in order to assist photoelectrolysis for H 2 and O 2 gas generation without external electrical sources. The open circuit voltage was increased from 0.65 V to approximately 3.2 V by the serial connection of five Se/TiO 2 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 TiO 2 films was used to increase the surface area to attain a stronger photocatalytic reaction. Microdot patterns of Pt were formed as the co-catalyst on a TiO 2 photoanode to suppress the back reaction of the photogenerated carrier. A Pt anode was formed on


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 have developed solar-power-driven water-splitting devices as a carbon-neutral hydrogen generation technology.(5) Powder-based and thin-filmbased photocatalysts have been reported.For example, the use of the dual photocatalyst, which is made by combining different photocatalysts for hydrogen and oxygen generation, is one of the promising 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 an electrical power supply from the photocatalytic water-splitting system is also important to achieve 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., SnO 2 and TiO 2 ) integrated on a Si photoanode have been reported. (8)Khaselev and Turner also reported a monolithic-structure water-splitting device with p/n-GaAs and p-GaInP 2 . (9)ur 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/TiO 2 PV device can supply its light-induced open circuit voltage (V oc ) at 0.65 V as a DC bias voltage to a TiO 2 -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 TiO 2 /ITO layers can be used as common materials for both TiO 2 -Pt PEC and Se/TiO 2 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 TiO 2 working electrode were varied during the characterization of water-splitting devices with constant UV illumination to increase the hydrogen generation rate.

Device design
We propose a water-splitting device composed of TiO 2 -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 TiO 2 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 TiO 2 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 TiO 2 thin films generally exhibit a stronger photocatalytic activity than rutile TiO 2 thin films.In addition, it has been reported that sol-gel-derived porous TiO 2 thin films exhibit a much stronger photocatalytic performance than sputter-deposited TiO 2 thin films. (11)Moreover, it has also been reported that TiO 2 with an anatase/rutile mixture structure often exhibits a higher photocatalytic activity than that with a pure anatase or pure rutile structure. (12)For example, commercially available Degussa P25® (Sigma-Aldrich Co. LLC, USA) is widely known to have highly photoresponsive TiO 2 particles with anatase/rutile mixture crystallographic structures. (13)(16)(17) Such photoanodes showed an anatase/rutile mixture and a mesoporous structure.Accordingly, TiO 2 films with anatase/rutile mixture crystallographic structures deposited by the sol-gel method were employed in this work.The porosity of TiO 2 can be changed by adjusting the amount of polyethylene glycol (PEG) added to the sol-gel precursor solution. (12)Circular Pt dot patterns of 0.2 mm diameter were formed on a TiO 2 photoanode as the co-catalyst to promote the water-splitting reaction.The electrical potential gradient at the Pt/TiO 2 interface promotes the transfer of photogenerated carriers from TiO 2 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 the electrode for hydrogen generation.The TiO 2 photoanode was illuminated by UV light through an indium-tin-oxide (ITO) transparent electrode layer from the substrate side.The exposed area of the TiO 2 photoanode was designed to be 66.5 mm 2 .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 TiO 2 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)oO 3 (20 nm) was employed as a hole transport layer between Se and Au layers.The active area of a single TiO 2 /Se PV device was designed to be 4 mm 2 .Electrical lead patterns were insulated by nonconductive epoxy resin.A water-splitting reactor comprising TiO 2 -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.

Device fabrication
On-chip photocatalytic reactor devices were fabricated by the following steps (illustrated in Fig. 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 formed on 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 were formed 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. TiO 2 photoanode deposition: Next, mesoporous TiO 2 was formed by the calcination of PEG containing a sol-gel precursor.As reported in the literature, the surface area of mesoporous TiO 2 can be varied by changing the molecular weight of the added PEG. (12,21,22) The patterned TiO 2 photoanode was formed by spin-coating on the masked FTO layer.The TiO 2 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 TiO 2 solgel 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.4. n-type TiO 2 window layer deposition: The TiO 2 window layer (50 nm) was deposited by reactive 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 the desired partial pressure (P Ar ) of 0.1 Pa.Oxygen gas was also introduced at a partial pressure (P O 2 ) 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. TiO 2 thin films were patterned by sputtering through a stencil mask. 5. Te/Se deposition: A very thin Te layer was inserted between the Se and TiO 2 layers to improve the adhesion between these layers.Te and Se layers were sequentially deposited by thermal evaporation.)(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. MoO x and Au deposition: The MoO x (20 nm) and Au (150 nm) hole injection layers were deposited by thermal evaporation through stencil masks.

Preliminary characterization of water splitting
In a preliminary experiment, PEC and PV performance characteristics were characterized using the separated devices to simplify the characterization procedure, considering the yield rate of 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 TiO 2 photoanode are shown in Figs.3(a)-3(c), respectively.The surface roughness and morphology of the TiO 2 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 of porous TiO 2 can be observed in Fig. 3(c).The root mean square surface roughness (R RMS ) of the mesoporous TiO 2 photoanode was measured to be 262 nm.The effects of increasing the surface area of TiO 2 at R RMS larger than 262 nm on enhancing photocatalytic activity have been reported. (23,26)For example, Latthe et al. previously reported that the photocurrent density in cyclic voltammetry with light illumination was increased seven times by increasing the surface roughness of the supersonic aerosol-coated TiO 2 thin films from R a = 350 to 710 nm. (27)Thus, further optimization of the surface morphology is considered to be effective for enhancing the photocatalytic reaction.An open circuit voltage of approximately 3.2 V can be supplied to the TiO 2 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 TiO 2 photoanode for hydrogen and oxygen generation, respectively.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 TiO 2 photoanode are known to be effective parameters in the watersplitting reaction.These parameters were varied, and the generated volume of hydrogen was measured.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 TiO 2 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 with hermetic seal under negative pressure conditions.The acrylic chamber was filled with an electrolyte 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 and supplying bias voltage.The UV-LED (peak wavelength at 365 nm, FOLS-09) was used at an intensity of 10 mW/cm 2 in this experiment.Figure 4(b) shows a photograph of the fabricated reactor 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 TiO 2 /Se PV device shown in Fig. 5 was designed to be 4 mm 2 .The 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 are shown in Fig. 6.Light illumination was conducted at 100 mW/cm 2 at AM 1.5 using a solar simulator (OTENTO-SUNIII, Bunkoukeiki Co., Ltd., Japan).It was observed in Figs.6(a  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.This is also caused by the inadequate performance homogeneity of plural photovoltaic cells on a substrate 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 distance between individual photovoltaic cells is also suggested as an effective approach to decreasing 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 because hydrogen bubbles adhered strongly to the Pt electrode.Thus, the hydrogen volume was calculated 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 TiO 2 photoanode are shown in Fig. 8, when a bias voltage of 3.2 V  was supplied by the wire-connected DC electrical power source and the wire-connected serially connected TiO 2 /Se PV devices.The areal percentage of Pt to TiO 2 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 TiO 2 photoanode.It was observed in Fig. 8 that the rate of hydrogen generation increased from 18.3 to 27.9 µL/h upon adding the Pt co-catalyst to the TiO 2 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 TiO 2 photoanode when a bias voltage of 3.2 V was supplied by the serially connected TiO 2 /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/TiO 2 Schottky interface. (1,14)The TiO 2 -Pt PEC with bias voltages supplied by DC and PV power sources exhibited water-splitting performance similar that of the TiO 2 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)t 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 TiO 2 /Se PV devices by the same procedure as described in the explanation for Fig. 8.It was seen that the rate of hydrogen generation increased with decreasing pressure.The volume of hydrogen generated at a pressure of 20 kPa reached almost five times that obtained in air atmosphere.
The saturation tendencies of hydrogen production were observed at lower pressures as shown in Fig. 9.One of the possible reasons for that may be the increased back reaction due to the increased oxygen and hydrogen amounts with decreasing ambient pressure.The reactive surface area of the Pt cathode was considered to be smaller than that of the porous TiO 2 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 TiO 2 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 in photocatalytic water splitting. (13)According to the literature, the excessively large percentages of the Pt co-catalyst to the TiO 2 photocatalyst cause reaction limitation because of the trade-off between the decreased photon absorption due to a Pt shadow and the promotion of the reaction. (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 TiO 2 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 the generated hydrogen volume increased with the area ratio of Pt to TiO 2 .TiO 2 supported with Pt at an area ratio of 3.14 % increased the amount of hydrogen generated to 1.5 times larger than that without a co-catalyst.On the other hand, the rate of increase in the amount of hydrogen generated showed a saturation tendency with increasing Pt area.This may be a result of the reverse reaction from gas to water at the Pt surface with increasing Pt area.In addition, the excessively 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 TiO 2 photoanode with a 17 × 14 mm 2 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 a methanol fuel processor with dimensions of 70 × 40 × 30 mm 3 demonstrated hydrogen production at 0.498 mol/h for the estimated electric power of 15 W. (31) From the viewpoint of application for hydrogen fuel cells, 15 μL per W/min was reported as an example of the hydrogen consumption rate. (32)Therefore, it will be necessary to increase the hydrogen production rate by approximately twofold.

Water-splitting demonstration using device with TiO 2 /Se PV integrated with TiO 2 -Pt PEC devices
The TiO 2 /Se PV device and TiO 2 -based PEC were integrated and connected to each other on a 30 × 30 mm 2 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 to the sol-gel-derived TiO 2 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 serially connected 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.

Conclusions
Compact fuel cells such as PEFC have recently attracted considerable attention in terms of applicable power supply for indoor small appliances such as IoT devices.Thus, it is considered that 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 TiO 2 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.

Fig. 1 .
Fig. 1. (Color online) (a) Device structure of the proposed water-splitting device composed of TiO 2 -based PEC and serially connected Se thin-film PV devices.(b) Device structure of single cell of TiO 2 /Se PV device.(a) (b)

3 .
Pt cathode and co-catalyst deposition: The Pt cathode and co-catalyst patterns were deposited by 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 FeCl 3 aqueous solution.

Fig. 2 .
Fig. 2. (Color online) Fabrication of the proposed water-splitting device composed of TiO 2 -based PEC and serially connected Se thin PV devices on a chip.
) and 6(b) that the open

Fig. 4 .
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 TiO 2 photoelectrode and their combination are varied.(b) Photograph of the fabricated acrylic reaction chamber.

Fig. 5 .
Fig. 5. (Color online) Photograph of the fabricated PV device and magnified photograph of a single PEC device.

Fig. 6 .Fig. 8 .
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.

Fig. 9 .
Fig. 9. Generated hydrogen volume as a function of UV light illumination time, when the pressure in the acrylic chamber was varied.An open circuit voltage of 3.2 V was applied to the PEC from the wire-connected TiO 2 /Se PV devices by the same procedure as described in the explanation of Fig 8.

Fig. 10 .
Fig. 10.(Color online) Generated hydrogen volume as a function of time, when the area ratio of Pt co-catalyst to TiO 2 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 TiO 2 were 0.63, 1.26, 1.89, 2.51, and 3.14% at a constant pressure of 20 kPa.

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

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