Photocatalytic hydrogen evolution with Ni nanoparticles by using 2-phenyl-4-(1-naphthyl)quinolinium ion as a photocatalyst

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Photocatalytic hydrogen evolution with Ni nanoparticles by using2-phenyl-4-(1-naphthyl)quinolinium ion as a photocatalyst†

Yusuke Yamada,a Takamitsu Miyahigashi,a Hiroaki Kotani,a Kei Ohkuboa and Shunichi Fukuzumi*ab

Received 7th November 2011, Accepted 15th December 2011

DOI: 10.1039/c2ee03106j

Photocatalytic hydrogen evolution with 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+–NA) as

a photocatalyst and dihydronicotinamide adenine dinucleotide (NADH) as a sacrificial electron donor

has been made possible for the first time by using nickel nanoparticles (NiNPs) as a non-precious metal

catalyst. The hydrogen evolution rate with the most active Ni nanoparticles (hexagonal close-packed

(hcp) structure, 6.6 nm) examined here was 40% of that with commercially available Pt nanoparticles

(2 nm) using the same catalyst weight. The catalytic activity of NiNPs depends not only on their sizes

but also on their crystal phases. The hydrogen-evolution rate normalized by the catalyst weight

increased as the size of NiNPs becomes smaller, with regard to the crystal phase, the hydrogen-

evolution rate of the NiNPs with hcp structure is more than 4 times higher than the rate of the NiNPs

with face-centred cubic (fcc) structure of similar size. NiNPs act as the hydrogen-evolution catalyst

under the pH conditions between 4.5 and 8.0, although the hydrogen-evolution rate at pH > 7.0 was

much lower as compared with the hydrogen-evolution rate at pH 4.5. A kinetic study revealed that the

rate of electron transfer from photogenerated QuPh_–NA to NiNPs was much higher than the rate of

hydrogen evolution, indicating that the rate-determining step may be proton reduction or desorption of

hydrogen.

1. Introduction

Hydrogen is a promising clean fuel for the next generation

without emitting greenhouse gas and other harmful chemicals

aDepartment of Material and Life Science, Division of Advanced Scienceand Biotechnology, Graduate School of Engineering, Osaka University,ALCA, Japan Science and Technology Agency (JST), Suita, Osaka,565-0871, Japan. E-mail: [email protected]; [email protected]; Fax: +81-6-6879-7370; Tel: +81-6-6879-7368bDepartment of Bioinspired Science, EwhaWomans University, Seoul, 120-750, Korea

† Electronic supplementary information (ESI) available: Recyclabilitytest (Fig. S1), hydrogen generation with NiO nanoparticles (Fig. S2),powder XRD pattern of NiNPs (Fig. S3), interaction of fcc- andhcp-NiNPs with a magnet (Fig. S4), catalytic activity test with differentamounts of NiNPs (Fig. S5) and DLS of CuNPs just after preparation(Fig. S6). See DOI: 10.1039/c2ee03106j

Broader context

Photocatalytic hydrogen evolution has been an attractive method

carbon dioxide. A hydrogen-evolution system composed of an org

utilize visible light. However, the metal catalysts usually contain pre

for hydrogen evolution. Replacing platinum with cheap and abunda

that Ni nanoparticles act as efficient catalysts for the photocatalyti

ion as a photocatalyst and NADH as a sacrificial electron donor.

This journal is ª The Royal Society of Chemistry 2012

after burning.1–4 Currently, hydrogen is produced by the steam

reforming of hydrocarbons followed by the high temperature

and low temperature water gas shift reactions and selective CO

oxidation. Thus, CO2, which is regarded as a typical greenhouse

gas, is produced as a byproduct.5 In order to attain hydrogen by

a really clean process, water splitting by using solar energy is the

most desirable method.6–8 Photocatalytic hydrogen evolution

with a photosensitizer, a sacrificial electron donor, an electron

mediator and a hydrogen-evolution catalyst has been studied for

the last few decades to improve the catalytic efficiency.9–28

Although water should be used as an electron source to realize

the water splitting for the solar-energy conversion, it is still very

important to optimize the photocatalytic reactivity of hydrogen

evolution with a sacrificial electron donor. Recently, the effi-

ciency of photocatalytic hydrogen evolution has been improved

by employing electron donor–acceptor linked dyads with

to convert solar energy to chemical energy without emitting

anic photosensitizer and metal catalysts has large potential to

cious metals, typically platinum, because of a low overpotential

nt metals is still a big challenge. Here we report for the first time

c hydrogen evolution with 2-phenyl-4-(1-naphthyl)quinolinium

Energy Environ. Sci., 2012, 5, 6111–6118 | 6111

http://dx.doi.org/10.1039/c2ee03106j





http://dx.doi.org/10.1039/C2EE03106J

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a long-lived charge-separated state,29–34 which can inject elec-

trons directly to a hydrogen-evolution catalyst without an elec-

tron mediator upon photoexcitation of the donor–acceptor

linked dyads.35–38

A bottleneck of the photocatalytic hydrogen-evolution system

with heterogeneous catalysts is the demand for a precious metal

such as Pt,9,16 although Fe and Co complexes act as the catalysts

in homogeneous systems39–50 and natural systems utilize non-

precious metals, i.e., Fe and Ni, in the active centres of

hydrogenases.51–57 In heterogeneous systems, Fe nanoparticles

have been employed as a hydrogen-evolution catalyst in the

reaction system with an electron donor–acceptor dyad with long-

lived charge-separated state and NADH as a photocatalyst and

a sacrificial electron donor, respectively.35 However, the catalytic

efficiency is by far lower than that of Pt nanoparticles. On the

other hand, an Ni based catalyst, which is a component metal of

Fe–Ni hydrogenases,51–55 has yet to be examined as a catalyst for

the photocatalytic hydrogen evolution.

We report herein an efficient photocatalytic hydrogen-evolu-

tion system composed of a donor–acceptor linked dyad,

2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+–NA), as a pho-

tocatalyst34,38 and NADH as a sacrificial electron donor using Ni

nanoparticles (NiNPs) together with the catalytic activity of

nanoparticles of Fe, Co and Cu. This is the first report to employ

NiNPs as a catalyst for efficient photocatalytic hydrogen

evolution. The whole reaction scheme is depicted in Scheme 1.

The size effects of NiNPs as well as effects of crystal phase on

both the catalytic reactivity for the hydrogen evolution and the

electron injection from QuPh_–NA were also examined in detail

to optimize the catalytic reactivity.

2. Experimental section

All chemicals were obtained from a chemical company and

used without further purification. 1-Octadecene (ODE, 90%),

poly(vinylpyrrolidone) (PVP, Mw ¼ 40 000), cobalt carbonyl

[Co2(CO)8], tri-n-octylphosphine oxide and hexadecylamine were

obtained from Tokyo Chemical Industry Co., Ltd. Pt nano-

particles (2 nm) capped with PVP were supplied by Tanaka

Kikinzoku Kogyo. Nickel acetylacetonate [Ni(acac)2], tri-n-

octylphosphine and oleylamine (70%) were obtained from Sigma-

AldrichCo.LLC.Oleic acid (60%), sodiumborohydride (NaBH4)

and sodium hydroxide were obtained fromWako Pure Chemical

Industries. Copper(II) acetate [Cu(OAc)2] was obtained from

Nacalai Tesque. 2-Phenyl-4-(1-naphthyl)quinolinium perchlo-

rate was synthesized by the reported method.34 Each buffer

solution was prepared by addition of NaOH to an aqueous

solution containing an electrolyte (50mMof phthalate for pH4.5,

Scheme 1 Structure of QuPh+–NA and the overall catalytic cycle for

photocatalytic hydrogen evolution.

6112 | Energy Environ. Sci., 2012, 5, 6111–6118

5.3, 6.0; 50 mM of phosphate for pH 7.0, 8.0 and 25 mM of

carbonate for pH 10). All chemicals were used without further

purification unless otherwise noted. Purified water was provided

by aMilli-Qwater purification system (Millipore, Direct-Q 3UV)

where the electronic conductance was 18.2 MU cm.

2.1. Synthesis of particles

Synthesis of 6.6 nm Ni nanoparticles (NiNPs).58 Ni(acac)2(275 mg, 1.1 mmol), tri-n-octylphosphine (0.50 mL, 1.1 mmol)

and oleic acid (0.50 mL, 2.0 mmol) were dissolved in oleylamine

(7 mL) in a 50 mL three-neck round-bottom flask at room

temperature. The solution was degassed under reduced pressure

for 20 min at 403 K, then heated to 518 K (at 4.8 K min�1) and

kept at this temperature for 30 min under an Ar atmosphere.

After the solution was cooled to room temperature, ethanol was

added to cause flocculation and then the suspension was centri-

fuged (15 000 rpm, 10 min) to separate black precipitates. The

black precipitates were washed three times by dispersion/

precipitation (n-hexane/ethanol) cycles. The final product was

dispersed in n-hexane (5 mL).

Synthesis of 11 nm NiNPs.58 Ni(acac)2 (275 mg, 1.1 mmol) and

tri-n-octylphosphine (1.3 mL, 3.0 mmol) were dissolved in

oleylamine (7 mL) in a 50 mL three-neck round-bottom flask at

room temperature. The solution was degassed under reduced

pressure for 20 min at 403 K, then heated to 523 K (at

4.0 Kmin�1) and kept at this temperature for 30 min under an Ar

atmosphere. After the solution was cooled to room temperature,

ethanol was added to flocculate and then the suspension was

centrifuged (15 000 rpm, 10 min) to separate black precipitates.

The black precipitates were washed three times by dispersion/



Synthesis of 36 nm NiNPs.58 Ni(acac)2 (275 mg, 1.1 mmol) was

dissolved in oleylamine (15 mL) in a 50 mL three-neck round-

bottom flask at room temperature. The solution was degassed

under reduced pressure for 20 min at 403 K, then heated to 543 K

(at 5.2 K min�1) and kept at this temperature for 2 h under an Ar


ethanol was added to flocculate and then the suspension was

centrifuged (15 000 rpm, 10 min) to separate black precipitates.

The black precipitates were washed three times by dispersion/



Synthesis of 210 nm hcp-NiNPs.58 Ni(acac)2 (275 mg,

1.1 mmol) was dissolved in a solution of oleylamine (4 mL) and

octadecene (4 mL) in a 50 mL three-neck round-bottom flask at

room temperature. The solution was degassed under reduced

pressure for 20 min at 403 K, then heated to 543 K (at

5.2 Kmin�1) and kept at this temperature for 30 min under an Ar


ethanol was added to cause flocculation and then the suspension

was centrifuged (15 000 rpm, 10 min) to separate black precipi-

tates. The black precipitates were washed three times by disper-

sion/precipitation (n-hexane/ethanol) cycles. The final product

was dispersed in n-hexane (5 mL).



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Synthesis of Co nanoparticles (CoNPs).59 Co2(CO)8 (540 mg,

1.58 mmol) was dissolved in 1,2-dichlorobenzene (3 mL). The

solution was injected into 1,2-dichlorobenzene (15 mL) con-

taining a mixture of oleic acid (0.20 mL, 0.62 mmol) and tri-n-

octylphosphine oxide (100 mg, 0.26 mmol) in a 50 mL three-neck

round-bottom flask at 455 K and then refluxed for 30 min. After

the solution was cooled to room temperature, ethanol was added

to cause flocculation. Then the suspension was centrifuged

(15 000 rpm, 10 min) to separate black precipitates. The black

precipitates were washed three times by dispersion/precipitation

(n-hexane/ethanol) cycles. The final product was dispersed in

n-hexane (5 mL).

Synthesis of Cu nanoparticles (CuNPs).60 Cu(OAc)2 (60 mg,

0.30 mmol) and PVP (400 mg) were dissolved in water (20 mL) in

a 50 mL three-neck round-bottom flask at room temperature.

Then, an aqueous solution of NaBH4 (11 mg, 0.30 mmol) and

NaOH (12 mg, 0.30 mmol) was injected into the Cu(OAc)2/PVP

aqueous solution and stirred for 15 min under a N2 atmosphere

at room temperature. After the reaction, acetone was added to

cause flocculation and then the suspension was centrifuged



(acetone/water) cycles. The final product was dispersed in

ethanol (5 mL).

Synthesis of Fe nanoparticles (FeNPs).61 ODE (20 mL) and

oleylamine (0.3 mL) were mixed and degassed by passing Ar gas

at 393 K for 30 min. The temperature was raised to 453 K (at

3.3 K min�1) and Fe(CO)5 (0.7 mL, 5.2 mmol) was slowly added.

The mixture was kept at the temperature for 20 minutes. After

the solution was cooled to room temperature, ethanol was added

to flocculate and then the suspension was centrifuged




n-hexane (5 mL).

Synthesis of 16 nm fcc-NiNPs.62 Ni(acac)2 (200mg, 0.78mmol)

was dissolved in 1,2-dichlorobenzene (5mL) at 373K, andquickly

injected into 1,2-dichlorobenzene (40mL) containing amixture of

hexadecylamine (4.0 g, 17 mmol) and NaBH4 (400 mg, 11 mmol)

at 393 K during vigorous stirring. The resulting mixture was

heated to 453 K and kept at this temperature for 1 h under an Ar







Synthesis of 22 nm fcc-NiNPs.62 Ni(acac)2 (200 mg,

0.78 mmol) was dissolved in 1,2-dichlorobenzene (5 mL) at

373 K, and quickly injected into 1,2-dichlorobenzene (40 mL)

containing a mixture of hexadecylamine (1.5 g, 6.2 mmol), tri-

n-octylphosphine oxide (500 mg, 1.3 mmol) and NaBH4

(150 mg, 4.0 mmol) at 393 K during vigorous stirring. The

resulting mixture was heated to 453 K and kept at this

temperature for 45 min under an Ar atmosphere. After the


solution was cooled to room temperature, ethanol was added to

cause flocculation and then the suspension was centrifuged




n-hexane (5 mL).

Synthesis of 80 nm fcc-NiNPs.63 Ni(acac)2 (275 mg, 1.1 mmol)

was dissolved in oleylamine (15 mL) in a 50 mL three-neck

round-bottom flask at room temperature. The solution was

degassed under reduced pressure for 20 min at room tempera-

ture, then, heated to 408 K and kept at this temperature for

30 min. Then, the solution was further heated up to 488 K (at

4.7 K min�1) and kept at this temperature for 1 h under an Ar







Capping agent exchange with PVP.35 An n-hexane solution

(2.0 mL) containing nanoparticles was mixed with a solution

containing PVP (200 mg) in CHCl3 (8 mL) at 323 K. The solution

was stirred for 9 h to exchange a surfactant. A certain amount of

acetone was added to the solution to cause flocculation and then

the mixture was centrifuged at 15 000 rpm for 10 min. The

collected water-soluble particles were washed three times by

dispersion/precipitation (ethanol/acetone) cycles.

2.2. Photocatalytic hydrogen evolution

Amixed solution (2.0 mL) of an aqueous buffer (pH 4.5, 5.3, 6.0,

7.0, 8.0 or 10) and MeCN [1 : 1 (v/v)] containing QuPh+–NA

(0.44 mM), NADH (1.0 mM) and NiNPs (25 mg) was flushed

with N2 gas. The solution was then irradiated with a Xe lamp

(Ushio Optical, Model X SX-UID 500X AMQ) through a colour

filter glass (Toshiba Glass UV-35) transmitting l > 340 nm at

room temperature. After the solution was stirred for 1 min in the

dark, the gas in the headspace was analyzed using a Shimadzu

GC-14B gas chromatograph (detector, TCD; column tempera-

ture, 50 �C; column, active carbon with 60–80 mesh particle size;

carrier gas, N2) to quantify the evolved hydrogen.

2.3. Kinetic measurements

A mixed solution (2.0 mL) of a deaerated aqueous buffer (pH

4.5, 6.0, 7.0, 8.0 or 10) andMeCN [1 : 1 (v/v)] containing QuPh+–

NA (0.44 mM) and NADH (1.0 mM) was photoirradiated for 1

min with a Xe lamp through a colour filter glass transmitting l >

340 nm. Then, a deaerated aqueous solution containing NiNPs

(25 mg) was added to the photoirradiated solution using

a microsyringe with stirring. Rate constants of electron transfer

from QuPh_–NA (obtained by one-electron reduction of QuPh+–

NA) to the catalyst were determined from the decay of absorp-

tion at 510 nm due to QuPh_–NA, which was monitored using

a Hewlett-Packard 8453 diode-array spectrophotometer with

a quartz cuvette (path length 10 mm) at 298 K.



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2.4. Catalysts characterization

Transmission electron microscope (TEM) images of nano-

particles, which were mounted on a copper microgrid coated

with elastic carbon, were observed by a JEOL JEM 2100 oper-

ating at 200 keV. Powder X-ray diffraction patterns were

recorded by a Rigaku RINT 2000. Incident X-ray radiation was

produced by a Cu X-ray tube, operating at 40 kV and 200 mA

with Cu Ka radiation of 1.54 �A. The scanning rate was 2� min�1

from 20� to 80� in 2q.

3. Results and discussion

3.1. Photocatalytic hydrogen evolution with catalytic

nanoparticles

Catalytic nanoparticles were prepared by reduction of metal ion

sources followed by reported methods with some modifications.

Ni nanoparticles (NiNPs) were prepared by the reduction of

Ni(acac)2 complex in oleylamine at high temperature in the

presence or absence of tri-n-octylphosphine and oleic acid. Cu

nanoparticles (CuNPs) were obtained by Cu(OAc)2 reduction by

NaBH4 in the presence of PVP at room temperature. Fe nano-

particles (FeNPs) and Co nanoparticles (CoNPs) were prepared

from corresponding carbonyl complexes by thermal decompo-

sition (see the Experimental section). The obtained Co, Cu and

Fe nanoparticles were characterized by TEM and dynamic laser

scattering (DLS) as displayed in Fig. 1. The TEM images and

DLS data for NiNPs are shown in Fig. 4a (vide infra). The

diameters of NiNPs, CuNPs, FeNPs and CoNPs were 6.6 �1.6 nm, 200 � 100 nm, 16 � 5.0 nm and 4.5 � 2.0 nm,

Fig. 1 TEM images and particles size distributions determined by

dynamic laser scattering (DLS) of (a) CoNPs, (b) CuNPs and (c) FeNPs.


respectively.64 The capping agents of the NiNPs, FeNPs and

CoNPs were exchanged to PVP before catalysis measurements in

order to increase the dispersity to an aqueous solution.

Fig. 2 shows the time courses of photocatalytic hydrogen

evolution under photoirradiation (l > 340 nm) of a mixed

solution [MeCN/buffer (pH 4.5) ¼ 1/1 (v/v)] containing NADH,

QuPh+–NA and metal nanoparticles (MNPs, M ¼ Ni, Cu, Fe or

Co) as a sacrificial electron donor, a photocatalyst and

hydrogen-evolution catalysts, respectively. Commercially avail-

able Pt nanoparticles (PtNPs, 2 nm) were also employed as

a hydrogen-evolution catalyst for a reference. No hydrogen

evolution was observed from the mixed solution in the dark. The

photoirradiation (l > 340 nm) of the solution resulted in

hydrogen evolution. Among the nanoparticles used here, only

NiNPs (Fig. 2, red closed square) and PtNPs (blue diamond)

provided the evolution of the stoichiometric amount of hydrogen

(2.0 mmol) in a certain reaction time. The hydrogen-evolution

rates with PtNPs and NiNPs were 28 mmol h�1 and 11 mmol h�1,

respectively. The catalytic reactivity of NiNPs was maintained

for the repetitive use for three times as shown in Fig. S1 in the

ESI†. CuNPs and FeNPs also acted as catalysts for hydrogen

evolution, however, their catalytic reactivity was rather modest

compared with NiNPs. No hydrogen evolution was observed

with CoNPs. Under atmospheric conditions, Ni metal might be

oxidized to form nickel oxide. The catalytic activity of NiO

nanoparticles was investigated in the photocatalytic hydrogen

evolution under the same conditions described above. As indi-

cated in Fig. S2†, only 10% of the stoichiometric amount of

hydrogen was evolved for the first reaction cycle. When the NiO

catalyst was used repeatedly, the catalytic activity was gradually

improved, because NiO was probably reduced to Ni metal under

the reaction conditions. Thus, we focused on examining the

catalytic reactivity of NiNPs depending on the particle sizes and

crystal phases including the kinetic measurements.

3.2. Electron transfer from QuPh_–NA to NiNPs

Rates of electron transfer from QuPh_–NA to NiNPs in a mixed

solution of a phthalate buffer (pH 4.5) and MeCN [1 : 1 (v/v)]

were determined by UV-vis spectral change to compare the

results with the corresponding rates of hydrogen evolution

Fig. 2 Time courses of hydrogen evolution under photoirradiation (l >

340 nm) of a deaerated mixed solution (2 mL) of a phthalate buffer

(pH 4.5) and MeCN [1 : 1 (v/v)] containing QuPh+–NA (0.44 mM),

NADH (1.0 mM) and various catalysts [12.5 mg L�1, closed square,

NiNPs (6.6 nm); open square, CuNPs; closed circle, FeNPs; open circle,

CoNPs and closed diamond, PtNPs (2 nm)] at 298 K.



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determined by gas chromatography. This is the first example to

observe the electron-transfer rate from a radical species to NiNPs

compared with the hydrogen-evolution rate. An aliquot (2.0 mL)

of QuPh+–NA (0.44 mM) was photoirradiated in the presence of

NADH (1.0 mM) for several minutes to generate QuPh_–NA.

Then, a small portion of an aqueous solution containing NiNPs

(12.5 mg L�1) was added to the mixed solution containing QuPh_–

NA to initiate the hydrogen evolution. Fig. 3 depicts the time

profiles of UV-vis absorption change at 510 nm (blue circles) and

those of the amount of evolved hydrogen quantified by gas

chromatography (red circles) for NiNPs. The total amount of

evolved hydrogen was 0.4 mmol, which corresponds to 80% of the

stoichiometric amount of hydrogen predicted from the absorp-

tion change due to QuPh_–NA. As shown in Fig. 3, the decay of

QuPh_–NA due to electron transfer from QuPh_–NA to NiNPs is

completed within 30 s (blue lines), whereas hydrogen evolution

lasts for a longer period (red line, ca. 500 s). These results indicate

that the rate-determining step is not electron transfer from

QuPh_–NA to NiNPs but the hydrogen evolution step, which

includes the proton reduction, hydrogen association and

hydrogen desorption.

Fig. 4 TEM images and particles size distributions determined by

dynamic laser scattering of hcp-NiNPs with sizes of (a) 6.6 nm, (b) 11 nm,

(c) 36 nm and (d) 210 nm.

3.3. Size effects of NiNPs on the photocatalytic hydrogen

evolution

TEM images and dynamic laser scattering (DLS) of NiNPs with

different sizes are displayed in Fig. 4. The size-controlled NiNPs

were prepared by thermal decomposition of Ni(acac)2 in oleyl-

amine with oleic acid, tri-n-octylphosphine or octadecene at the

temperature higher than 523 K (see the Experimental section).

The sizes determined for NiNPs were 6.6 � 1.6 nm (Fig. 4a),

11 � 2 nm (Fig. 4b), 36 � 12 nm (Fig. 4c) and 210 � 80 nm

(Fig. 4d). The TEM images indicated that no shape-controlled

particles were obtained. The powder X-ray diffraction patterns

of smaller NiNPs (6.6 nm and 11 nm) were very broad,

however, those of larger NiNPs (36 nm and 210 nm) indicated

that the crystal phase of these NiNPs is mainly a hexagonal

close-packed (hcp) structure (Fig. S3 in the ESI†). More obvi-

ously, these particles were not attracted to a magnet as shown in

Fig. S4 in the ESI†.

Fig. 3 Decay time profile of absorption at 510 nm due to QuPh_–NA in

electron transfer from QuPh_–NA to NiNPs (12.5 mg L�1) (solid line) and

corresponding time profile of hydrogen evolution (broken line) in mixed

solutions of a phthalate buffer (pH 4.5) and MeCN [1 : 1 (v/v)]. QuPh_–

NA was produced by photoirradiation of QuPh+–NA (0.44 mM) in the

presence of NADH (1.0 mM).


The photocatalytic hydrogen evolution was conducted under

photoirradiation of a mixed solution (2.0 mL) of a deaerated

buffer (pH 4.5) and MeCN [1 : 1 (v/v)] containing QuPh+–NA

(0.44 mM), NADH (1.0 mM) and NiNPs (12.5 mg L�1) with

different sizes (6.6–210 nm).When smaller NiNPs with the size of

6.6 nm or 11 nm were employed as hydrogen-evolution catalysts,

nearly a stoichiometric amount of hydrogen was evolved (closed

and open circles in Fig. 5a), although the amount of evolved

hydrogen was �80% of the stoichiometric amount in the case of

NiNPs with the size of 36 nm (closed square in Fig. 5a) and only

50% with the size of 210 nm. The hydrogen-evolution rates (VH2)

Fig. 5 (a) Time courses of hydrogen evolution under photoirradiation

(l > 340 nm) of mixed solutions of deaerated phthalate buffer (pH 4.5)

and MeCN [1 : 1 (v/v)] containing NADH (1.0 mM), QuPh+–NA

(0.44 mM), and NiNPs with different sizes (12.5 mg L�1) at 298 K. (b)

Plots of hydrogen-evolution rates normalized by weight concentration of

NiNPs vs. the size of NiNPs.



Fig. 7 Dynamic laser scattering (DLS) of fcc-NiNPs with different sizes.

(a) 16 � 4.0 nm, (b) 22 � 6.0 nm and (c) 80 � 20 nm. (d) Powder X-ray

diffraction patterns obtained from NiNPs with different sizes.

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normalized by the weight concentrations of NiNPs were plotted

against the size of NiNPs in Fig. 5b. The smallest NiNPs with the

size of 6.6 nm exhibited the highest VH2 of 1.1 mmol h�1 mg L�1.

This result is quite reasonable, because smaller NiNPs have

a higher specific surface area per weight.

The rate constants (ket) of electron transfer from QuPh_–NA to

NiNPs with different sizes were determined from the slopes of the

linear plots of kobs vs. the weight concentration of NiNPs in

Fig. 6a where kobs is the pseudo-first-order rate constant deter-

mined from the first-order plots. A mixed solution (2.0 mL) of

a phthalate buffer and MeCN [1 : 1 (v/v)] containing QuPh+–NA

(0.44 mM) was photoirradiated in the presence of NADH

(1.0 mM) to generate QuPh_–NA until no significant absorption

change was observed. Then, a small portion of an aqueous

solution containing NiNPs was added to the solution containing

QuPh_–NA to initiate the electron transfer to NiNPs. Larger ketvalue was achieved at smaller NiNPs with the size of 11 nm as

shown in Fig. 6b. Although the electron transfer is much faster

than hydrogen evolution as shown in Fig. 3, the slower electron

transfer from QuPh_–NA to NiNPs with large particles may be

a disadvantage for hydrogen evolution.

3.4. Effects of crystal phase of NiNPs

Two different types of crystal phases have been reported for

NiNPs, which are hexagonal close-packed (hcp) and face-centred

cubic (fcc) structures. The fcc-Ni nanoparticles with three

different sizes were prepared by the reported method. Fig. 7a–c

show the particles size distributions of fcc-NiNPs determined by

DLS. The sizes of fcc-NiNPs were 16 � 4.0 nm, 22 � 6.0 nm and

80 � 20 nm. The crystal phase of these fcc-NiNPs was confirmed

by the X-ray diffraction patterns as indicated in Fig. 7d. The

three characteristic peaks appeared at 44.5�, 51.9� and 76.5� areoriginated from the diffractions from (111), (200) and (220)

planes, respectively.65

Photocatalytic hydrogen evolution with fcc-NiNPs as

a hydrogen-evolution catalyst was investigated by photo-

irradiation of mixed solution of phthalate buffer (pH 4.5) and

MeCN containing NADH (1.0 mM) and QuPh+–NA (0.44 mM).

Fig. 8 shows the time courses of hydrogen evolution with fcc-

NiNPs (12.5 mg L�1) at 298 K. The fastest hydrogen evolution

was observed with fcc-NiNPs (16 nm) and the slowest hydrogen

Fig. 6 (a) Plots of the pseudo first-order rate constants (kobs) of electron

transfer from QuPh_–NA to NiNPs vs. weight concentrations of NiNPs at

298 K. (b) Size dependence of the rate constants (ket) of electron transfer

from QuPh_–NA to NiNPs with various sizes determined from the slopes

of the plots in (a).


evolution was attained with fcc-NiNPs (80 nm). This result is

quite reasonable, because the specific surface area increases at

smaller particles. Even at smallest particles, the hydrogen yield

was lower than 40% of the stoichiometric amount. The

hydrogen-evolution rates with fcc-NiNPs normalized by the

weight concentrations (black circles) are compared with NiNPs,

mainly composed of hcp structure (red circles) in Fig. 8b. The

hydrogen-evolution rate with hcp-NiNPs (11 nm) was 4.8 times

higher than that with fcc-NiNPs (16 nm).

Ni nanoparticles are often used as hydrogen-production

catalysts for steam reforming of hydrocarbons.66 The catalytic

activity of Ni catalysts has been reported as highly dependent on

their surface structures.67 Certain step sites are considerably

more reactive than close-packed facets. For steam reforming of

glycerol, high product selectivity to hydrogen has been reported

on hcp-NiNPs compared with fcc-NiNPs.67 Additionally,

intrinsic activity of hcp-Ni is much higher than fcc-Ni in propene

hydrogenation.68 The fcc-Ni structure is thermally stable and


(l > 340 nm) of mixed solutions of deaerated phthalate buffer (pH 4.5)

and MeCN [1 : 1 (v/v)] containing NADH (1.0 mM), QuPh+–NA

(0.44 mM), and fccNiNPs with different sizes (12.5 mg L�1) at 298 K. (b)

Plots of hydrogen-evolution rates with different structures of NiNPs

(circle, hcp; square, fcc) vs. the sizes of NiNPs.



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hcp-Ni structure is known as a metastable structure.66 The

superior catalysis of hcp-NiNPs can be ascribed to the loose

packing of hcp structure. Thus, the hcp-NiNPs are more active

catalysts for the photocatalytic hydrogen evolution than the fcc-

NiNPs.

Fig. 10 (a) Plots of the pseudo-first-order rate constants (kobs) for

electron transfer from QuPh_–NA vs. weight concentrations of NiNPs at

298 K. Time courses of hydrogen evolution with different weights of

NiNPs are indicated in Fig. S5†. (b) pHDependence of the rate constants

(ket) for electron transfer from QuPh_–NA to NiNPs determined from

slopes of the plots in (a).

3.5. Photocatalytic hydrogen evolution with NiNPs under

various pH conditions

The photocatalytic hydrogen-evolution experiments were per-

formed with a mixture of an aqueous buffer and MeCN [1 : 1

(v/v)] containing QuPh+–NA (0.44 mM), NADH (1.0 mM) and

NiNPs (12.5 mg L�1) under various pH conditions. Fig. 9a shows

the amount of evolved hydrogen as a function of photo-

irradiation time under the conditions of pH 4.5, 5.3, 6.0, 7.0, 8.0

and 10. When the pH of the buffer was as low as 4.5, the stoi-

chiometric amount (2.0 mmol) of hydrogen was evolved,

however, the amount of hydrogen evolved decreased with

increasing the pH. Fig. 9b plots the hydrogen-evolution rates

normalized by the weight concentration of the NiNPs against pH

values of the buffer solution. Although the concentration of

proton in the buffer solution decreases logarithmically by

increasing pH, the hydrogen-evolution rate moderately

decreased. For example, proton concentration decreases to 1/30

when pH changes from 4.5 to 6.0, whilst the hydrogen-evolution

rate at pH 6.0 maintained 1/3 of that at pH 4.5. These results also

indicate that Ni nanoparticles can catalyze H2 evolution with

little overpotential when provided with electrons of energy near

the H+/H2 couple. The standard potential for NAD+ to NADH

at pH 0 is�0.11 V vs.NHE, and this couple exhibits a 29 mV per

pH unit dependence. Thus, at pH 4, the reduction of H+ with

NADH is essentially thermoneutral and it becomes endergonic

as the pH is increased.

Fig. 10a shows the plots of the pseudo-first-order rate

constants (kobs) for electron transfer from QuPh_–NA against

weight concentrations of NiNPs at 298 K under various pH

conditions. Fig. 10b indicates the rate constant of electron

transfer from QuPh_–NA to NiNPs determined from the slope of

Fig. 10a in logarithmic scale. The small pH dependence of ketindicates that the electron-transfer process is not coupled with

proton.


(l > 340 nm) of deaerated mixed solutions (2 mL) of aqueous buffers with

various pH values and MeCN [1 : 1 (v/v)] containing QuPh+–NA

(0.44 mM), NADH (1.0 mM), and NiNPs (12.5 mg L�1) at 298 K. (b)

The pH dependences of hydrogen-evolution rates (VH2) of NiNPs

(12.5 mg L�1).


4. Conclusions

We have revealed for the first time that Ni (non-precious metal)

nanoparticles act as an efficient catalyst for the photocatalytic

hydrogen evolution with 2-phenyl-4-(1-naphthyl)quinolinium

ion as a photocatalyst and NADH as a sacrificial electron donor.

Among metal nanoparticles (NPs) of Ni, Fe, Co and Cu, which

were examined as hydrogen-evolution catalysts in the photo-

catalytic hydrogen-evolution system, NiNPs (hcp, 6.6 nm)

exhibited the highest catalytic activity to provide the stoichio-

metric amount of hydrogen evolution. The catalytic reactivity of

NiNPs depends on both their sizes and also crystal phases. The

smaller size and hexagonal close-packed (hcp) surface are merited

for the high specific surface area and loose-packed structure

compared with the face centred cubic (fcc) structure, respectively,

leading to the high catalytic reactivity of hydrogen evolution.

Acknowledgements

This work was supported by Grants-in-Aid (20108010 and

23750014) and a Global COE Program, ‘‘The Global Education

and Research Centre for Bio-Environmental Chemistry’’, from

the Ministry of Education, Culture, Sports, Science and Tech-

nology, Japan and NRF/MEST of Korea through the WCU

(R31-2008-000-10010-0) and GRL (2010-00353) Programs. We

sincerely acknowledge the Research Centre for Ultra-Precision

Science & Technology, Osaka University, for TEM

measurements.

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Photocatalytic hydrogen evolution with Ni nanoparticles by using 2-phenyl-4-(1-naphthyl)quinolinium ion as a photocatalyst