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OPEN

received: 04 October 2016
accepted: 04 November 2016
Published: 28 November 2016

Boron-doped diamondsemiconductor electrodes:
Efficient photoelectrochemical
CO2 reduction through surface
modification
Nitish Roy1, Yuiri Hirano1,2, Haruo Kuriyama3, Pitchaimuthu Sudhagar4, Norihiro Suzuki1,
Ken-ichi Katsumata1, Kazuya Nakata1,2, Takeshi Kondo1,2, Makoto Yuasa1,2, Izumi Serizawa3,
Tomoaki Takayama5, Akihiko Kudo1,5, Akira Fujishima1 & Chiaki Terashima1
Competitive hydrogen evolution and multiple proton-coupled electron transfer reactions limit
photoelectrochemical CO2 reduction in aqueous electrolyte. Here, oxygen-terminated lightly borondoped diamond (BDDL) thin films were synthesized as a semiconductor electron source to accelerate
CO2 reduction. However, BDDL alone could not stabilize the intermediates of CO2 reduction, yielding anegligible amount of reduction products. Silver nanoparticles were then deposited on BDDL because of
their selective electrochemical CO2 reduction ability. Excellent selectivity (estimated CO:H2 mass ratio of
318:1) and recyclability (stable for five cycles of 3 h each) for photoelectrochemical CO2 reduction wereobtained for the optimum silver nanoparticle-modified BDDL electrode at −1.1 V vs. RHE under 222-nm
irradiation. The high efficiency and stability of this catalyst are ascribed to the in situ photoactivation of
the BDDL surface during the photoelectrochemical reaction. The present work reveals the potential of
BDDL as a high-energy electron source for use with co-catalysts in photochemical conversion.
Artificial photosynthesis is an emerging process for sustainable conversion of solar energy to chemical energy,
and is important because of the increasing atmospheric CO2 concentration and global energy demand1–3.
Artificial photosynthesis generally uses a semiconductor that absorbs solar radiation and generates electron/
hole pairs1,4,5. These electron/hole pairs produce active radical species on the semiconductor surface prior to theadsorption of water or reacting molecules, or electron/hole pairs directly transfer to guest molecules throughspace6,7. The high energy of these active radicals and electron/hole pairs means they readily take part in certain
chemical conversions under suitable conditions. For example, TiO2 is a well-known photocatalyst for water splitting and pollutant degradation6,7. TiO2 splits water into hydrogen and/or oxygen in the presence of a Pt counterelectrode or hole scavenger using a photon energy greater than its bandgap. Other chemical conversions have also
been achieved through semiconductor/molecular photocatalysis8–11. However, in multistep chemical conversion,
which involve many intermediates to obtain a desired product, each intermediate has the possibility to divert thereaction course, resulting in poor product selectivity12–14. Although product selectivity in multistep reactions
can be increased by choosing a suitable photocatalyst or controlling the reaction conditions, the reliability andefficiency of such reactions are still far from those required for commercial use13. Therefore, efficient conversion
through multistep or multi-intermediate photochemical reaction remains challenging. Chemists typically attempt
to divert such conversions into new mechanistic pathways with the fewest possible intermediates.

1

Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 2788510, Japan. 2Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 2788510, Japan. 3ORC Manufacturing Co., Ltd, 4896 Tamagawa, Chino, Nagano 391-0011, Japan. 4Environmental andSustainability Institute, University of Exeter, Penryn, Cornwall TR10 9EZ, UK. 5Faculty of Science, Tokyo Universityof Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan. Correspondence and requests for materials should be
addressed to A.F. (email: fujishima_akira@admin.tus.ac.jp) or C.T. (email: terashima@rs.tus.ac.jp)
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Photocatalytic or photoelectrochemical CO2 reduction in aqueous solution is one example where productselectivity is limited by both thermodynamics and kinetics. Commercial conversion of CO2 into high-value
chemicals is generally inefficient, laborious, and expensive12–14. Efficient photocatalytic or photoelectrochemical
CO2 reduction with excellent product selectivity is a research goal in the field of sustainable energy conversion,and represents an alternative to natural photosynthesis.
The atmospheric concentration of CO2 has increased tremendously over the last 50 years. Selective, efficientconversion of CO2 into a valuable chemical fuel is not only important for sustainability but also economicallyfavourable15–17. Natural photosynthesis is an indispensable tool to capture CO2, but gross destruction of green
plants and rapid civilization have forced researchers to search for alternatives to natural photosynthesis to efficiently and selectively convert CO2 into high-value chemicals. The selectivity of chlorophyll in natural photosynthesis is unprecedented. In artificial photocatalytic or photoelectrochemical CO2 reduction, conversion of
CO2 into useful chemicals requires suitable catalysts2,4. Photocatalytic or photoelectrochemical CO2 reduction
generally involves multiple proton-coupled electron transfer reactions in aqueous media. These reactions provide
numerous intermediates, and thereby produce multiple CO2 reduction products4. Electrochemical or photoelectrochemical CO2 reduction has been explored using various catalysts with the aim of identifying mechanistic
pathways that finally could lead to efficient conversion of CO2 into a particular product12,18–22.
Halmann et al.22. first reported photoelectrochemical CO2 reduction to produce formic acid, methanol andformaldehyde using a p-type GaP semiconductor in 1978. Since then, many semiconductor photocatalysts
have been used for CO2 reduction23–26. Metal nanoparticles deposition on the semiconductors and fabricationof metal-semiconductor junction have successfully studied for improvement of photocatalytic CO2 reduction
activity27–29. Recently, Hamers and co-workers found that hydrogen-terminated boron-doped diamond (H-BDD)
can produce solvated electrons through injection of photoexcited electrons into aqueous electrolytes30. The solvated electrons produced by H-BDD can reduce CO2 into carbon monoxide (CO) alone via a one-electron transfer reduction mechanism under sufficiently high pressure. However, the extremely high pressure used in thisapproach is undesirable. In addition, H-BDD is easily oxidized under illumination30,31.
In this work, we study an alternative solution to overcome the challenges of one-electron transfer under normal pressure and temperature by using oxygen-terminated lightly boron-doped (1,000 ppm) diamond (BDDL)
incorporated with co-catalysts. The surface of oxygen-terminated BDDL is reduced during photoelectrochemical
reaction so it acts as a semiconducting high-energy electron source30,31. Silver (Ag) co-catalyst deposited on BDDL
stabilizes the intermediates of CO2 reduction reactions under normal pressure to realize selective photoelectrochemical reduction. The semiconducting nature of BDDL is investigated by changing the light sources used inphotoelectrochemical studies.

Results

Surface morphology and photoelectrochemical study. Field-emission scanning electron microscopy(FESEM) and elemental analysis were used to investigate the deposition of Ag nanoparticles on BDDL. First, Ag
nanoparticles were deposited on BDDL by a chronoamperometric method in 0.1 M AgNO3 at −​0.5 V for 60 s to
produce a sample denoted 0.1 Ag-BDDL. Low-magnification FESEM analysis of this sample revealed that Ag
nanoparticles with a diameter of ~300 nm were deposited on BDDL (Fig. 1a). However, a high-magnification
FESEM image indicated that primarily smaller Ag nanoparticles (average size ~20 nm) were deposited on BDDL
(Fig. 1b). The size distributions of smaller Ag nanoparticles are shown in Supplementary Fig. S1 by analysing several high magnified FESEM images. Average size of the smaller Ag nanoparticles was found to be around ~20 nm(Supplementary Fig. S1) with the 0.1 Ag-BDDL. The 0.1 Ag-BDDL sample was then used for electrochemical/
photoelectrochemical CO2 reduction. Figure 1c displays cyclic voltammograms (CVs) of a 0.1 Ag-BDDL electrode in 25 mM Na2SO4. In N2, no cathodic peak was obtained up to −​1.0 V vs. RHE, suggesting no reductionof Ag-BDDL or the electrolyte. The low cathodic current density (onset potential −​1.0 V vs. RHE) may indicate
a poor hydrogen evolution reaction32. Indeed, under CO2-saturated conditions, 0.1 Ag-BDDL displayed a strong
cathodic peak at −​1.1 V vs. RHE in 25 mM Na2SO4, clearly indicating the cathodic electrochemical reduction of
CO2. The cathodic current density gradually decreased with increasing cycle number (Supplementary Fig. S2a).
Careful observation revealed that the difference of cathodic current density between two consecutive cycles
decreased as cycling progressed. After several cycles, the cathodic current density remained almost constant at−​1.1 V (Supplementary Fig. S2a). The opposite behaviour was observed under irradiation at 222 nm with an excimer lamp (7 W). Under irradiation, the cathodic current density increased slowly and reached its maximum value
after several cycles (Supplementary Fig. S2b). The increase in cathodic current density is attributed to the photoexcitation and photoactivation of 0.1 Ag-BDDL by high-energy photons at −​1.1 V vs. RHE. Therefore, underirradiation and applied bias potential, charge transport and cathodic CO2 reduction are facilitated, increasing
cathodic current density. The final cycles in both the dark and light-irradiation conditions show quite different
cathodic current densities (Fig. 1c). This large difference clearly reveals the excellent photoelectrochemical activity of the 0.1 Ag-BDDL electrode for CO2 reduction.
To further confirm the photoelectrochemical behaviour of the 0.1 Ag-BDDL electrode, partial current densities
were measured in both the dark and light in 25 mM Na2SO4 at −​1.1 V vs. RHE. In the first few minutes in the dark,
the partial current density changed sharply from −​0.3 to −​0.2 V and then remained constant (Fig. 1d). This clearlyagrees with the CV characteristics of Ag-BDDL under CO2-saturated conditions. However, under photoexcitation
at 222 nm, the partial current density at −​1.1 V began to increase over time (Fig. 1d). This increase in partial current density is caused by the photoexcitation and photoactivation of 0.1 Ag-BDDL and the subsequent participationof photoexcited electrons in the cathodic CO2 reduction reaction on the Ag surface, as revealed by CV analysis.
Product analysis. CV and chronoamperometric analysis clearly suggest the electrochemical and photoelectrochemical reduction of CO2 by the 0.1 Ag-BDDL electrode at −​1.1 V vs. RHE in 25 mM Na2SO4. Therefore,
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0.1 Ag-BDDL: Photoelectrochemical

activity

L

-0.1

2

-0.1

Current (mA/cm )2

Current (mA/cm )(c) 0.0

N2

-0.2

CO2

CO2 + 222 nm

-0.3
-0.4
-1.5

-1.0

-0.5

Potential (V) vs RHE

0.0

reduction at -1.1 V

-0.2

2

Absence of 222 nm

-0.3
-0.4
-0.5

Presence of 222 nm

-0.6
0

60
120
Time (min)180

Figure 1. Surface morphology and photoelectrochemical CO2 reduction activity of Ag-BDD. (a) Lowmagnification FESEM image of 0.1 Ag-BDDL (0.1 Ag indicates that the AgNO3 concentration during deposition
was 0.1 M; other parameters including potential and time were fixed at −​0.5 V and 60 s, respectively), clearly
indicating its smooth surface. Brighter Ag nanoparticles with a diameter of ~300 nm are indicated by arrows.
(b) High-magnification FESEM image showing smaller Ag nanoparticles (~20 nm) as confirmed by elemental
mapping (inset). (c) CVs of 0.1 Ag-BDDL in 25 mM Na2SO4 aqueous electrolyte. The cathodic peak current at−​1.1 V vs. RHE under CO2-saturated conditions indicates the cathodic reduction of CO2 on the 0.1 Ag-BDDL
electrode. (d) Chronoamperometric current–time curves of the photocurrent generated by the 0.1 Ag-BDDL
electrode in the dark and under irradiation (222 nm). Photocurrent increased in the first hour and then becamealmost constant.

the CO2 reduction products obtained under similar conditions at −​1.1 V vs. RHE with different amounts of Ag
loaded on the BDDL electrodes were detected. Gas chromatography (GC) and ion chromatography (IC) wereused to investigate the reduction products. Figure 2 shows the amount of CO and H2 obtained using Ag-BDDL
electrodes with different Ag loading during irradiation (222 nm). The concentration of AgNO3 used during deposition was slowly increased from 0 to 0.1 M (0 indicates a bare BDDL electrode, while 0.025 Ag-BDDL, 0.05
Ag-BDDL and 0.1 Ag-BDDL denote deposition of Ag from 0.025, 0.05 and 0.1 M AgNO3 aqueous solutions on
BDDL at −​0.5 V for 60 s, respectively). The detailed of surface morphology with 0.025, and 0.05 Ag-BDDL isshown in Supplement Fig. S1. Average sizes of the smaller Ag nanoparticles were found to be ~10 and ~15 nmwith the samples 0.025, and 0.05 Ag-BDDL electrodes, respectively. The photoelectrochemical CO2 reduction
products obtained over these electrodes were analysed (Fig. 2a). The bare electrode (0 Ag-BDDL) produced anegligible amount of CO. The amount of CO produced increased markedly with Ag loading on BDDL. Althoughsimilar amounts of CO were obtained over 0.1 Ag-BDDL (590 μ​g in 3 h) and 0.05 Ag-BDDL (577 μ​g in 3 h), lessH2 was formed by 0.1 Ag-BDDL (1.85 μ​g in 3 h) than 0.05 Ag-BDDL (2.59 μ​g in 3 h). Minor amount of H2 could
be produced from competitive proton reduction in aqueous electrolyte. The weight ratios of CO to H2 were 29:1,
222:1 and 318:1 over 0.025 Ag-BDDL, 0.05 Ag-BDDL and 0.1 Ag-BDDL, respectively. Therefore, the 0.1 Ag-BDDL
electrode produces CO with minimal H2 at −​1.1 V vs. RHE under 222 nm irradiation.
The role of photoelectrochemical CO2 reduction at 0.1 Ag-BDDL is emphasized by comparing the products
obtained with and without light irradiation and under N2-saturated conditions at −​1.1 V. GC analysis reveals
that in the dark, the amount of CO produced (88 μ​g in 3 h) decreased considerably compared with that underirradiation, while the amount of H2 (4.35 μ​g in 3 h) remained similar (Fig. 2c,d). Similarly, under N2-saturated
conditions, a negligible amount of CO (15 μ​g in 3 h) was detected, while the amount of H2 remained similar.
Therefore, the greater amount of CO produced under CO2-saturated conditions and 222 nm irradiation resultsfrom the photoelectrochemical effect of 0.1 Ag-BDDL.

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In presence of 222 nm
0.0 Ag-BDDL

2

CO (µg/cm )600

0.05 Ag-BDDL

400

0.1 Ag-BDDL

300
200

0.1 Ag-BDDL

6
4
0

1

2
Time (h)3(d) 8

0.1 Ag-BDDL

CO2 + 222 nm
CO2

400

N2 + 222 nm

300
200
100
0

0.05 Ag-BDDL

8

1

2Time (h)132Time (h)30.1 Ag-BDDL

6

CO2 + 222 nm

4

N2 + 222 nm

Only CO222

CO (µg/cm )500

0.025 Ag-BDDL

2

H2 (µg/cm )(c) 600

0.0 Ag-BDDL

10

100
0

In presence of 222 nm

120.025 Ag-BDDL

500(b) 142700

H2 (µg/cm )

(a)

2
0

1

2
Time (h)3

Figure 2. CO2 reduction product analysis. (a) Amount of CO produced during irradiation (222 nm) overdifferent Ag-BDDL electrodes in 25 mM Na2SO4 aqueous electrolyte at −​1.1 V vs. RHE. At a particular duration,
the amount of CO increases with the Ag concentration used during deposition on the BDDL substrate.
(b) Production of H2 over the electrodes. At a particular irradiation time, the amount of H2 decreased withincreasing AgNO3 concentration during electrode synthesis. (c) Photoelectrochemical effect of 0.1 Ag-BDDL
under different conditions in 25 mM Na2SO4 at −​1.1 V vs. RHE. The amount of CO produced under irradiation(222 nm) is higher than that produced in the dark, revealing that the very high amount of CO produced
originates from the photoelectrochemical effect of 0.1 Ag-BDDL. Negligible CO was produced over 0.1 AgBDDL under N2-saturated conditions. (d) Amount of H2 produced over the 0.1 Ag-BDDL electrode under
different conditions at −​1.1 V, indicating that H2 is mostly produced through electrochemical reactions.

Ag-BDDL could also produce liquid products because CO2 reduction often follows a multiple proton-coupledelectron transfer reaction mechanism in aqueous electrolyte, providing numerous products. Therefore, IC wasused to detect possible liquid products including formate, acetate and oxalate. No liquid products were detectedfrom the Ag-BDDL electrodes under CO2-saturated conditions in the light (222 nm) or dark at −​1.1 V.

Selectivity of different Ag-modified carbon electrodes. We found that 0.1 Ag-BDDL displayed more
efficient and selective photoelectrochemical CO2 reduction than 0.05 and 0.025 Ag-BDDL electrodes. To confirm
the superior photoelectrochemical activity and selectivity of 0.1 Ag-BDDL, Ag nanoparticles were deposited onheavily boron-doped (10,000 ppm) diamond (denoted BDDH) and glassy carbon electrodes (GCEs) at −​0.5 V for
60 s in 0.1 M AgNO3. The photoelectrochemical performance of the resulting electrodes was studied. Figure 3
shows the total Faradaic efficiency of these electrodes in 25 mM Na2SO4 at −​1.1 V vs. RHE after 3 h of photoelectrolysis under 222 nm irradiation. The total Faradaic efficiency of 0.1 Ag-BDDL was 81.5%. The Faradaic efficiency
for CO with this electrode was 72.5%, and only 9.08% for H2, thereby providing very high selectivity for CO.
Conversely, the total Faradaic efficiency of 0.1 Ag-BDDH was 75.1%. Although this value is close to that of 0.1
Ag-BDDL, the Faradaic efficiency for CO of the 0.1 Ag-BDDH electrode is only 36.8% and the CO:H ratio is ~1:1
indicating facile proton reduction at 0.1 Ag-BDDH. Meanwhile, 0.1 Ag-GCE exhibited a very poor total Faradaic
efficiency (39.8%) with a Faradaic efficiency for CO of just 8.7%. Therefore, Ag-BDDL displays increased conversion and selectivity in the photoelectrochemical reduction of CO2 in 25 mM Na2SO4 at −​1.1 V vs. RHE comparedwith those of 0.1 Ag-BDDH and 0.1 Ag-GCE.
Stability and recyclability of the optimal photocathode.

Electrode stability and recyclability areimportant factors that must be considered for their reliable use in photoelectrochemical solar energy conversion devices. Therefore, we studied the stability and recyclability of the optimized 0.1 Ag-BDDL photocathodeby performing an isotopic experiment because very high energy photons (222 nm, ~5 eV) could degrade the
BDDL surface or the resin used in the electrode contacts. The isotopic experiment was carried out by purging

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Total Faradaic Efficiency

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80

H2

9.08%

H
CO 2

60
40

38.3%
72.5%
36.8%

20
0

31.1%
8.7%

Ag-BDDL

Ag-BDDH

Ag-GCE12(c) 600

3

Time (h)

4

(d)

300
200

3d5/2

367.8 eV

After

380
375
370
365
Binding Energy (eV)360

1st run
2nd run
3rd run
4th run
5th run

10
8
6
4
2

100
0

3d3/2

Before

2

2

400

400

Ag 3d

350
385

5

1st run
2nd run
3rd run
4th run
5th run500

450

H2 (µg/cm )093.13%

100

91.08%

150

50

CO (µg/cm )

500

200

86.42%2

CO (µg/cm )

250

(b)

CO
CO

Counts/sec (a.u.)

13

94.17%

12

93.96%(a) 300

373.8 eVFigure 3. Selectivity of different carbon electrodes. Ag was deposited on BDDL, BDDH and GCE in 0.1 M
AgNO3 at −​0.5 V for 60 s. Total Faradaic efficiency was measured after 3 h of photoelectrolysis at −​1.1 V in
25 mM Na2SO4 under an excimer lamp (222 nm, 7 W).

1

2Time (h)

3

0

1

2
Time (h)3

Figure 4. Stability and recyclability of the optimal photoelectrode. (a) Amount of isotopic 13CO andnormal 12CO produced over time by the 0.1 Ag-BDDL electrode in 25 mM Na2SO4 at −​1.1 V vs. RHE. The
amount of isotopic 13CO increased with irradiation time and reached 94.17% after 5 h. (b) XPS analysis of the
0.1 Ag-BDDL electrode before and after photoelectrolysis at −​1.1 V vs. RHE for 5 h in 25 mM Na2SO4 under
222 nm irradiation. The Ag 3d photoelectron peaks suggest the metallic state of Ag is not changed during thephotoelectrochemical reaction. (c) Recyclability of the 0.1 Ag-BDDL electrode in CO2-purged 25 mM Na2SO4
at −​1.1 V. In each run, the electrolyte was purged with N2 and then CO2 for 1 h. The amount of CO produced
decreased as the number of runs increased. (d) The amount of hydrogen produced in the consecutive runs
indicates that hydrogen evolution increased with run number.

the 25 mM Na2SO4 electrolyte for 10 min with 13CO2 at a flow rate of 3 mL/min. Photoelectrochemical reduction
was then carried out at −​1.1 V vs. RHE under 222 nm irradiation and GC–mass spectrometry (GC-MS) wasused to analyse the isotopic yield. Figure 4a shows the amounts of isotopic 13CO and normal 12CO after different
periods. The amount of 13CO produced (267.56 μ​g after 5 h of illumination at −​1.1 V vs. RHE) is lower thanthat of 12CO obtained previously (590 μ​g after 3 h; see Fig. 2c). This is because CO2 purging was carried out for
1 h in the previous experiment, so more CO2 was dissolved in the electrolyte compared with that in the case of
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isotopic 13CO2 (Fig. 1d, Supplementary Fig. S3). After illumination for 1 and 5 h, the amounts of isotopic 13CO
were 86.42% and 94.17%, respectively. The increase in 13CO content is because only a little non-isotopic 12CO
(16.8 μ​g after 5 h) could enter the system from the resin used to fabricate the electrode contact, which might bedegraded by high-energy photons. The amount of 12CO produced by resin degradation is very small comparedwith that of 13CO produced through photoelectrochemical reduction. Therefore, the relative amount of isotopic
13
CO increased over time. This isotopic experiment clearly reveals that CO originates from the reduction of CO2,
not from degradation of resin, other organics or the BDD surface by the high-energy irradiation. Even after 5 h of
222 nm irradiation, 0.1 Ag-BDDL was stable and produced 13CO with an excellent yield.
X-ray photoelectron spectroscopy (XPS) was performed before and after photoelectrocatalysis to investigate
the elemental states of the 0.1 Ag-BDDL electrode. Figure 4b displays the metallic Ag 3d photoelectron peaks33,34.
The relative amount of Ag and nature of Ag 3d photoelectron peaks remained similar before and after photoelectrolysis at −​1.1 V under 222 nm irradiation, indicating the oxidation states of the Ag nanoparticles did not change
during photoelectrochemical operation; i.e., the metallic state of Ag is stable.
The recyclability of the 0.1 Ag-BDDL electrode was examined by performing five runs for 3 h at −​1.1 V in
25 mM Na2SO4. After each run, the cathodic peak position shifted to lower overpotential with slightly increasedhydrogen evolution current compared with that at higher overpotential. After five runs, the cathodic peak for CO2
reduction moved to lower overpotential by 0.15 V with a current density of 0.2 mA/cm2 (Supplementary Fig. S4).
The amounts of CO and H2 produced were measured (Fig. 4c,d). After five runs, the amount of CO producedin 3 h was 260 μ​g (44%), comparatively lower than that obtained for the first cycle (590 μ​g in 3 h). Conversely,
the amount of H2 produced in the fifth cycle (10.51 μ​g) was greater than that obtained in the first (1.85 μ​g). The
decrease in CO and increase in H2 produced with cycle number are attributed to the shift of the onset potentialof CO2 reduction to lower overpotential as photoelectrolysis was conducted at −​1.1 V (Supplementary Fig. S4).

Discussion

Efficient photoelectrochemical CO2 reduction is very important for use of CO2 as a chemical feed stock and froman environmental viewpoint. Diamond is a highly stable material with a very wide bandgap (5 eV), possessing awide electrochemical potential window, mechanical stability and biocompatibility19,30,35. Another advantage of
diamond is that its conduction band (CB) position is very high, and hydrogen termination can shift its CB above
the vacuum level, providing negative electron affinity towards photoexcited electrons31. Despite the very highenergy of the photoexcited electrons of diamond, it is difficult to use under normal conditions. This is because
pure diamond has very poor active sites for adsorption of foreign molecules. At sufficiently high pressure, thephotoexcited electrons of hydrogen-terminated diamond or H-BDD can be used for photoelectrochemical conversion30,31. The photoexcited electrons in hydrogen-terminated diamond or H-BDD are capable of reducing CO2
via a one-electron transfer mechanism in 0.1 M Na2SO4 at sufficiently high pressure (2.5 MPa)30. Both the requirements of high pressure and hydrogen termination of the catalyst limit the application of diamond as a catalyst.
Therefore, here we modified oxygen-terminated BDDL with Ag nanoparticles to realize photoelectrochemical
CO2 reduction under normal pressure and temperature. Ag nanoparticles were deposited on BDDL because Ag
is known for its selective electrochemical CO2 reduction36,37. The selectivity of Ag originates from its ability toform strong chemical bonds with CO2 under a certain applied potential, which is very important for productselectivity36–38. Strong chemical bonding of a catalyst surface with CO2 reduction intermediates can terminate
the reduction reaction at a particular point without further propagation, unlike Cu surfaces, which are known tofurther propagate reduction reactions to produce multiple CO2 reduction products12,36.
Photoelectrochemical studies and product analysis revealed that the optimized 0.1 Ag-BDDL electrodeexhibited high efficiency and selectivity in the formation of CO in aqueous electrolyte at −​1.1 V vs. RHE with a
Faradaic efficiency of 72.5% after 3 h under 222 nm irradiation (Fig. 3). Photoelectrochemical studies using thisoptimized 0.1 Ag-BDDL electrode were also carried out under similar conditions with 172- and 308-nm irradiation. Irradiation at 308 nm induced a small increase of CO production (235 μ​g in 3 h). Conversely, 172-nm irradiation yielded less CO (97 μ​g in 3 h), similar to -electrochemical CO production alone over the same electrode(Fig. 2c). The marked photoelectrochemical effect of 222 nm irradiation on the performance of the optimized
0.1 Ag-BDDL electrode reveals the role of excitation of valence band (VB) electrons to the CB (Figs 1c,d and 2,
Supplementary Fig. S5) in CO production. This is because the band gap of BDDL (~5 eV) matches well with a
222 nm light source.
Bare BDDL irradiated with 222 nm light did not efficiently produce CO2 reduction products (~19 μ​g CO
that originated from the degradation of impurities; see Fig. 4a) under similar experimental conditions, as
shown in Fig. 2a and Supplementary Fig. S6. This is consistent with the finding of Hamers et al.30. that photoexcited electrons produced in a BDD semiconductor could not reduce CO2 into CO under normal pressure(Supplementary Fig. S6). Thus, the electrochemical CO2 reduction activity of Ag-BDDL is related to the presenceof Ag nanoparticles36. However, electrochemical reduction of CO2 by Ag-BDDL in aqueous electrolyte at −​1.1 V
vs. RHE decreased markedly after a few minutes and then became poor (Fig. 1d). This behaviour is also evident
from the consecutive CV runs conducted under CO2-saturated conditions in the dark (Supplementary Fig. S2a).
In the dark, the cathodic peak current obtained at −​1.1 V vs. RHE in 25 mM Na2SO4 decreased until it reacheda constant value (Supplementary Fig. S2a). Conversely, the cathodic peak current increased until it reached its
maximum value after a certain irradiation time under 222 nm irradiation at −​1.1 V vs. RHE. This increase inphotocurrent (Fig. 1d, Supplementary Fig. S3) is thought to be caused by two factors: first, photoreduction of the
BDDL surface produces photoactive sites at high negative applied potential, and second, simultaneous enhanced
photoelectrochemical reduction of CO2 on the Ag surface. This situation was confirmed by XPS analysis. The C
1 s peaks of 0.1 Ag-BDDL before and after photoelectrolysis at −​1.1 V vs. RHE in 25 mM Na2SO4 for 5 h are presented in Supplementary Figs S7 and S8. A broader C 1 s peak [full width at half maximum (FWHM) of 2.73 eV] isobtained before photoelectrochemical reaction, and becomes narrower (FWHM=​1.51 eV) after photoelectrolysis
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Figure 5. Schematic diagram of half-cell charge transfer with an Ag-BDDL photocathode in 25 mM Na2SO4
for CO2 reduction. Photoexcited electrons are produced in BDDL under 222 nm irradiation. The photoexcitedelectrons are easily transferred to the Ag nanoparticles deposited on BDDL. These photoexcited electrons could
be transferred to the CO2 molecules adsorbed on the Ag surface to form CO2•− anion radicals. The highly
energetic CO2•− anion radicals are readily stabilized by the proton-coupled electron transfer mechanism to
produce CO at −​1.1 V vs. RHE. For clarity, donor levels near the VB of BDDL originating from the boron
impurities are not shown.

for 5 h at −​1.1 V in the presence of 222 nm irradiation (Supplementary Fig. S8). This is because BDDL is oxygen
terminated, so a small amount of C-O-C and C=​O/C-OH bonds are present in addition to C-C bonds at its
surface. The C=​O/C-OH bonds might be reduced at −​1.1 V under 222 nm irradiation. Therefore, the C 1 s peak
displays greater C-C sp3 character after photoelectrolysis because of the decreased amount of C=​O/C-OH bonds,
causing it to narrow (Supplementary Fig. S8). Note that electrochemical bias potential alone cannot reduce thediamond surface; 222 nm irradiation is needed for BDD surface reduction, as evident from XPS, CV, and chronoamperometric analyses (Fig. 1c,d; Supplementary Figs S7 and S8). Our results reveal that 0.1 Ag-BDDL is activatedunder 222 nm irradiation at −​1.1 V vs. RHE to provide very high photoelectrochemical CO2 reduction activity.
There are three plausible formation pathways of CO from CO2 electrochemical or photoelectrochemical
reduction:30,38,39
CO2 + 2H + + 2e− = CO + H 2O

(1)

CO2 + 2e− = CO + CO3 2 −(2)CO2 + e−(h v) = CO + O−

(3)

In aqueous electrolyte, CO is mostly formed from CO2 via reaction (1), where 2 H and 2e participate. In
organic solvent/aprotic electrolyte, reaction (2) is favoured at sufficiently high overpotential or high pressure39,40.
Generally, reaction (3) is rare for photoelectrochemical CO2 reduction because reduction is usually carried out atnegative bias potential, so such photodissociation on the electrode surface is difficult30. Note that in aqueous
media, hydrogen is often produced through proton reduction41, e.g., 2H+ +​  2H+ +​  2e− =​  H22e− in addition to
CO2 reduction at the electrode surface. A negligible amount of hydrogen found with the Ag-BDDL electrodecould produce via proton reduction. However, the experimental results reveal that this smaller amount of hydrogen is produced through electrochemical reduction of proton at the BDD surface (Figs 2 and 3). Hori et al.36.
reported that Ag, Au and Zn can electrochemically reduce CO2 to CO with high selectivity. This is possibly
because of the formation of strong chemical bonds between CO2•− and the catalyst surface. Formation of theCO2•− anion radical involves one-electron transfer (CO2 + e− = CO2•−), and this anion species is very unstable.
The high photoelectrochemical activity of 0.1 Ag-BDDL in aqueous electrolyte at −​1.1 V vs. RHE is attributed tothe transfer of very high energy photoexcited electrons in the CB of BDDL to the Ag nanoparticle surfaces tofacilitate CO2 reduction. The electrochemical cathodic peak of the 0.1 Ag-BDDL electrode under CO2-saturated
conditions suggests that CO2 reduction occurs on the Ag surface. Therefore, the intermediates of CO2 reduction
must be adsorbed on the Ag surface37. It has been reported that Ag surfaces could stabilize CO2•− anion radicals
at certain negative potential depending on the experimental conditions, electrolyte and pH (standard redox
potential for CO2 /CO2•− is −​1.9 V)12,36,37,42. BDD can produce the CO2•−anion radical in 0.1 M Na2SO4 and thisradical can be stabilized as an intermediate product under suitable experimental conditions (high pressure)30.
Therefore, the enhanced photoelectrochemical CO2 reduction of 0.1 Ag-BDDL is thought to be related to theformation of CO2•− anion radicals by the transfer of photoexcited electrons to the Ag nanoparticles. The produced
CO2•− anion radicals are then immediately stabilized by a proton-coupled electron transfer mechanism at the Ag
surface (Fig. 5) to produce the more stable products CO and H2O37.
+

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Conclusion

In summary, the optimized Ag-BDDL electrode exhibits enhanced photoelectrochemical CO2 reduction activity for
CO formation with minimal H2 production under 222 nm irradiation. This Ag-BDDL electrode exhibits superior
activity to that of highly doped BDDH and GCE modified with Ag nanoparticles. The optimized Ag-BDDL electrode is stable after five cycles, retaining 44% of its initial activity for CO production. A high isotopic yield (94.17%
after 5 h) revealed that the electrode is not corroded under 222 nm irradiation at −​1.1 V. The content of surface
oxygenated species decreases slightly during the photoelectrochemical reaction, which might increase the numberof active sites for CO2 reduction. Therefore, the photocurrent increases during the first hour of photoelectrochemical reaction with the 0.1 Ag-BDDL electrode. Such surface photoreduction was not observed after 1 h of photoelectrochemical reaction. The enhanced activity of Ag-BDDL is ascribed to the synergistic interaction of the Ag
nanoparticles with semiconducting BDDL. Ag is known for its selective CO2 reduction, while the very high energyof the CB of BDDL enhanced the formation of CO2•− anion radicals on the Ag surface in aqueous electrolyte. The
CO2•− anion radicals were stabilized on the Ag surface at −​1.1 V via a proton-coupled electron transfer mechanism
to produce CO. The very high energy of the CB electrons of BDDL could be coupled with other co-catalysts topotentially improve the yield of several kinetically and thermodynamically limited photochemical conversions.

Methods

Deposition of Ag nanoparticles. BDD thin films were fabricated on silicon substrates using a reported
microwave plasma chemical vapour deposition method35. Ag nanoparticles were electrodeposited on BDDL
(effective area 1 ×​ 1 cm) in a two-electrode system at −​0.5 V with respect to a Pt counter electrode in aqueous
AgNO3 (0.025–0.1 M) for 60 s. After Ag nanoparticle deposition, the electrodes were cleaned by sonication indeionized water (18.2 MΩ) for 15 min. After washing, electrodes were dried under a N2 flow at room temperature.
Photoelectrochemical studies. Photoelectrochemical CO2 reduction activity was investigated in an H-type
electrochemical cell. Working and counter electrodes were separated by a proton-exchange Nafion membrane, andan aqueous solution of Na2SO4 was used as the supporting electrolyte. Ag/AgCl saturated with KCl was used as areference electrode and Pt mesh served as a counter electrode. The scan rate was 100 mV/s. Excimer lamps (7 W)with wavelengths of 172, 222 and 308 nm were used in the photoelectrochemical studies. Light sources were positioned 2 cm away from the electrode and the reaction temperature was held at 25 °C throughout the experiments.
Gas chromatography analysis. Gaseous products were analysed by a gas chromatograph (GC-2014,
Shimadzu, Japan) equipped with a thermal conductivity detector and flame ionization detector. Isotopic 13CO
was measured by GC-MS (GC-2010 Plus, Shimadzu) and the relative amount was estimated by comparison withthe amount of 12CO produced.
Ion chromatography analysis.

An IC system (Dionex ICS-1000, Thermo Scientific) equipped with
DionexIonPac AS12A and DionexIonPac CS12A columns was used to identify any detectable CO2 reduction
liquid products obtained with the Ag-modified BDDL electrodes.

Sample characterization. An FESEM (JEOL, JEM-3100F) was used to analyse the surface morphologyof the Ag-BDDL electrodes. Energy-dispersive X-ray spectroscopy was conducted for elemental mapping of theAg nanoparticle-modified BDDL substrates. Elemental states of Ag-BDDL electrodes before and after photoelectrochemical reaction at −​1.1 V for 5 h were determined with an X-ray photoelectron spectrometer (ESCA-3400,
Shimadzu) using a monochromatic Mg Kα​ source.

References

1. Fujishima, A. & Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38 (1972).
2. Lewis, N. S. & Nocera, D. G. Powering the planet: Chemical challenges in solar energy utilization. PNAS 103, 15729–15735 (2006).
3. Liu, C., Colón, B. C., Ziesack, M., Silver, P. A. & Nocera, D. G. Water splitting–biosynthetic system with CO2 reduction efficiencies
exceeding photosynthesis. Science 352, 1210–1213 (2016).
4. White, J. L. et al. Light-driven heterogeneous reduction of carbon dioxide: Photocatalysts and photoelectrodes. Chem. Rev. 115,
12888–12935 (2015).
5. Kudo, A. & Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38, 253–278 (2009).
6. Linsebigler, A. L., Lu, G. & Yates, J. T. Jr. Photocatalysis on TiO2 surfaces: Principles, mechanisms, and selected results. Chem. Rev.
95, 735−​758 (1995).
7. Fujishima, A., Zhang, X. & Tryk, D. A. TiO2 photocatalysis and related surface phenomena. Surf. Sci. Rep. 63, 515–582 (2008).
8. Meyer, T. J. Chemical approaches to artificial photosynthesis. Acc. Chem. Res. 22, 163–170 (1989).
9. Hisatomi, T., Kubota, J. & Domen, K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water
splitting. Chem. Soc. Rev. 43, 7520–7535 (2014).
10. Yoon, T. P., Ischay, M. A. & Du, J. Visible light photocatalysis as a greener approach to photochemical synthesis. Nat. Chem. 2,
527–532 (2010).
11. Palmisano, G., Augugliaro, V., Pagliaro, M. & Palmisano, L. Photocatalysis: A promising route for 21st century organic chemistry.
Chem. Commun. 3425, doi: 10.1039/B700395C (2007).
12. Hori, Y., Murata, A. & Takahashi, R. Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper
electrode in aqueous solution. J. Chem. Soc., Farad. Trans. 85, 2309–2326 (1989).
13. Kortlever, R., Shen, J., Schouten, K. J. P., Calle-Vallejo, F. & Koper, M. T. M. Catalysts and reaction pathways for the electrochemical
reduction of carbon dioxide. J. Phys. Chem. Lett. 6, 4073–4082 (2015).
14. Inoue, T., Fujishima, A., Konishi, S. & Honda, K. Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions ofsemiconductor powders. Nature 277, 637–638 (1979).
15. Seneviratne, S. I., Donat, M. G., Pitman, A. J., Knutti, R. & Wilby, R. L. Allowable CO2 emissions based on regional and impactrelated climate targets. Nature 529, 477–483 (2016).
16. Torelli, D. A. et al. Nickel−​gallium-catalyzed electrochemical reduction of CO2 to highly reduced products at low overpotentials.
ACS Catal. 6, 2100–2104 (2016).

Scientific Reports | 6:38010 | DOI: 10.1038/srep38010

8

www.nature.com/scientificreports/
17. Kuriki, R. et al. Nature-inspired, highly durable CO2 reduction system consisting of a binuclear ruthenium(II) complex and anorganic semiconductor using visible light. J. Am. Chem. Soc. 138, 5159–5170 (2016).
18. Hori, Y. Electrochemical CO2 reduction on metal electrodes. Mod. Aspects Electrochem. 42, 89–189 (2008).
19. Nakata, K., Ozaki, T., Terashima, C., Fujishima, A. & Einaga, Y. High-yield electrochemical production of formaldehyde from CO2
and seawater. Angew. Chem. Int. Ed. 53, 890–893 (2014).
20. Iizuka, K., Wato, T., Miseki, Y., Saito, K. & Kudo, A. Photocatalytic reduction of carbon dioxide over Ag cocatalyst-loaded ALa4Ti4O15(A =​Ca, Sr, and Ba) using water as a reducing reagent. J. Am. Chem. Soc. 133, 20863–20868 (2011).
21. Parkinson, B. A. & Weaver, P. F. Photoelectrochemical pumping of enzymatic CO2 reduction. Nature 309, 148–149 (1984).
22. Halmann, M. Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells.
Nature 275, 115–116 (1978).
23. Schreier, M. et al. Covalent immobilization of a molecular catalyst on Cu2O photocathodes for CO2 reduction. J. Am. Chem. Soc.
138, 1938–1946 (2016).
24. Torralba-Pen ́​alver, E., Luo, Y., Compain, J.-D., Chardon-Noblat, S. & Fabre, B. Selective catalytic electroreduction of CO2 at silicon
nanowires (SiNWs) photocathodes using non-noble metal-based manganese carbonyl bipyridyl molecular catalysts in solution and
Ggafted onto SiNWs. ACS Catal. 5, 6138–6147 (2015).
25. Schreier, M. et al. Efficient and selective carbon dioxide reduction on low cost protected Cu2O photocathodes using a molecularcatalyst. Energy Environ. Sci. 8, 855–861 (2015).
26. Bachmeier, A., Hall, S., Ragsdale, S. W. & Armstrong, F. A. Selective visible-light-driven CO2 reduction on a p-type dye sensitizedNiO photocathode. J. Am. Chem. Soc. 136, 13518–13521 (2014).
27. Wang, X. et al. Engineering interfacial photo-induced charge transfer based on nanobamboo array architecture for efficient solar-tochemical energy conversion. Adv. Mater. 27, 2207–2214 (2015).
28. Zhang, Z. et al. Multichannel-improved charge-carrier dynamics in well-designed hetero-nanostructural plasmonic photocatalysts
toward highly efficient solar-to-fuels conversion. Adv. Mater. 27, 5906–5914 (2015).
29. Jang, Y. J. et al. Selective CO production by Au coupled ZnTe/ZnO in the photoelectrochemical CO2 reduction system. Energy
Environ. Sci. 8, 3597–3604 (2015).
30. Zhang, L., Zhu, D., Nathanson, G. M. & Hamers, R. J. Selective photoelectrochemical reduction of aqueous CO2 to CO by solvatedelectrons. Angew. Chem. Int. Ed. 126, 9904–9908 (2014).
31. Zhu, D., Zhang, L., Ruther, R. E. & Hamers, R. J. Photo-illuminated diamond as a solid-state source of solvated electrons in water for
nitrogen reduction. Nat. Mater. 12, 836–841 (2013).
32. Roy, N. et al. Ionic-liquid-assisted selective and controlled electrochemical CO2 reduction at Cu-modified boron-doped diamondelectrode. ChemElectroChem 3, 1044–1047 (2016).
33. Sutter, E. et al. In situ liquid-cell electron microscopy of silver–palladium galvanic replacement reactions on silver nanoparticles.
Nat. Commun. 5, 1–9 (2014).
34. Kumar, P. et al. Plasmonic resonance of Ag nanoclusters diffused in soda-lime glasses. Phys. Chem. Chem. Phys. 17, 8596–8603 (2015).
35. Terashima, C. et al. Charge separation in TiO2/BDD heterojunction thin film for enhanced photoelectrochemical performance. ACS
Appl. Mater. Interfaces 8, 1583–1588 (2016).
36. Hori, Y., Wakebe, H. H., Tsukamoto, T. & Koga, O. Electrocatalytic process of CO selectivity in electrochemical reduction of CO2 atmetal electrodes in aqueous media. Electro. Acta 39, 1833–1839 (1994).
37. Lu, Q. et al. A selective and efficient electrocatalyst for carbon dioxide reduction. Nat. Commun. 5, 1–6 (2014).
38. Goeppert, A., Czaun, M., Jones, J.–P., Prakash, G. K. S. & Olah, G. A. Recycling of carbon dioxide to methanol and derived products–
losing the loop. Chem. Soc. Rev. 43, 7995–8048 (2014).
39. Oh, Y. & Hu, X. Ionic liquids enhance the electrochemical CO2 reduction catalyzed by MoO2. Chem. Commun. 51, 13698–13701 (2015).
40. Abbott, A. P. & Eardley, C. A. Electrochemical reduction of CO2 in a mixed supercritical fluid. J. Phys. Chem. B 104, 775–779 (2000).
41. Roy, N., Leung, K. T. & Pradhan, D. Nitrogen doped reduced graphene oxide based Pt–TiO2 nanocomposites for enhanced hydrogen
evolution. J. Phys. Chem. C 119, 19117–19125 (2015).
42. Lau, G. P. S. et al. New insights into the role of imidazolium-based promoters for the electroreduction of CO2 on a silver electrode.
J. Am. Chem. Soc. 138, 7820–7823 (2016).

Acknowledgements

We thank Japan Science and Technology for financial support through the ACT-C program. C.T. acknowledges
the partial financial support from Hosokawa Powder Technology Foundation and Tokyo Ohka Foundation for
the Promotion of Science and Technology.

Author Contributions

N.R. designed the reaction schemes and performed all the experiments. Y.H. assisted in BDD synthesis, and
FESEM and IC measurements. H.K. supplied excimer light sources and assisted in GC-MS measurement.
T.T. supported the isotopic experiments. N.R. interpreted the data and wrote the manuscript. N.R., P.S., N.S.,
K.K., K.N., T.K., M.Y., I.S., A.K., A.F. and C.T. revised the manuscript and made comments on it. C.T. and A.F.
supervised the work. All authors approved the manuscript before submission.

Additional Information

Supplementary information accompanies this paper at http://www.nature.com/srep
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Roy, N. et al. Boron-doped diamond semiconductor electrodes: Efficientphotoelectrochemical CO2 reduction through surface modification. Sci. Rep. 6, 38010; doi: 10.1038/srep38010 (2016).
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