Electronic Supplementary Material (ESI) for ChemComm.
This journal is © The Royal Society of Chemistry 2014

Electronic Supplementary Information†

A photofuel cell comprising titanium oxide and silver(I/0)
photocatalysts for use of acidic water as a fuelYuta Ogura,a Seiji Okamoto,a Takaomi Itoi,b Yukiko Fujishima,c,a Yusuke Yoshida,a andYasuo Izumi*aa Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan., Fax: +81-43-290-2783; Tel: +81-43-290-3696; E-mail:yizumi@faculty.chiba-u.jpb Department of Mechanical Engineering, Graduate School of Engineering, Chiba University,

Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan.
c Department of Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka 1-3,Shinjuku-ku, Tokyo 163-8601, Japan.

Derivation of Equation 2. The kinetic model of photocurrents in the PFCs wasformulated based on the principle as shown in Scheme 1. The equilibrium constants Ka and Kc
were defined in the main text, for the separation of electrons and holes in anodic TiO2 andcathodic Ag-TiO2, respectively, which are irradiated by light. [O2], [H+], [H2O], [Ti4+], [Ti3+],[O2−], [O−], [Ag+], and [Ag0] were the concentrations of O2, H+, and H2O in the electrolyte
solution and the concentrations of Ti4+, Ti3+, O2−, O−, Ag+, and Ag0 in photocatalysts with asubscript “a” (anode) or “c” (cathode). [O−] denotes hole concentrations at VB of TiO2 and[Ti3+]a and [Ag0]c denote electron concentrations at CB of TiO2 and trapping sites of Ag
nanoparticles, respectively.

Ka Ti3  O aaTi 4  O 2 aa1(S1)Ag 0  O ccKc Ag  O 2 c(S2)c

Nitrogen gas was bubbled through the acidic solution for TiO2 to purge O2. In a separate
experiment, O2 was recycled to irreversibly transfer to the gas phase of the anodecompartment through an organic layer and then to the cathode compartment through smallvent holes in the PCP film (Graphical abstract). The reaction rate at anode is formally
proportional to the fourth power of [O−]a as it is four-electron oxidation (Scheme 1).

2rox  kox H 2 Oa  O- a



4(S3)

For the equilibrium of O2 photoreduction, forward and reverse reaction rates were formally
proportional to the fourth powers of [Ag0]c and [O−]c, respectively, because they were fourelectron reactions (Scheme 1).

H O  O  
2K red 2c

O2 c H 
4+c4

c

Ag 0 c(S4)4The net electron flow rate (photocurrent) is the difference in the forward electron flowfrom TiO2 to Ag-TiO2 via the external circuit, and the reverse electron flow from TiO2 to AgTiO2 via the external circuit.

Photocurrent i  k Ti3  O   k 'Ag 0  O a1k1

1K a ' kox '4 K c '2 K red '81
4rox cc

1a1r 4 K ' 2
1
O2 c  H + c   k '  ox 1  c  1
1
1kox '4 K red '8 O2 c 8 H +  2
12

18(2)

cHere, the equilibrium constants with the prime symbol are as follows: K a '  K a Ti 4 a O 2 a ,K c '  K c Ag  O 2  , K red '  K red / [H 2 O]c  , and kox '  kox H 2 Oa  .
2c2c2The solubility of O2 in water is insensitive to the pH value: 4.063 and 4.060 mg per 100
mL of water (1.27 mmol L–1) at pHs of 2.0 and 4.0, respectively. The H+ concentrations (10mmol L–1) were greater than that of dissolved O2 at pH 2.0 for the reaction of O2 + 4H+ + 4e−,whereas the H+ concentrations (0.1–1.0 mmol L–1) were lower than that of dissolved O2 at pH
3.0–4.0 for the reaction. Experimental dependence of photocurrents on pH (Fig. 3B3) wasconsistent with kinetic model (equation 2) in the pH range between 2 and 4.
Experimental Methods. TiO2 powder (1.00 g; P25, Degussa; anatase/rutile phases = 7/3;
specific surface area 60 m2 g−1) was suspended in 3.0 mL of deionized water (< 0.06 S cm−1)and then stirred well. The suspension obtained was dried at 373 K for 24 h and heated in air at673 K for 2 h. The resultant powder was suspended in 3.0 mL of deionized water and placedon ITO (thickness 1.2–1.6 m)-coated Pyrex glass plate. The TiO2/ITO/Pyrex was dried at373 K for 18 h and heated in air at 573 K for 30 min. The amount of TiO2 placed on ITOcoated glass was 5.0 mg, and the area covered was 1.3 cm2.
Silver nitrate (17, 52, or 159 mg; 99.8%, Wako Pure Chemicals) was dissolved in 10 mLof deionized water. The solution was mixed with 3.33 g of untreated TiO2 (P25). The mixture
was magnetically stirred at a rate of 850 rpm and the water was distilled at 353 K. The
obtained powder was dried at 373 K for 24 h and heated in air at 673 K for 2 h. The obtained
Ag-TiO2 powders contained 0.33, 1.0, and 3.0 wt% of Ag. They were suspended in minimum
amount of water and placed on ITO-coated glass in a manner similar to that for theTiO2/ITO/Pyrex.
TiO2/ITO/Pyrex and Ag-TiO2/ITO/Pyrex electrodes were immersed in HCl solutions (40mL on each compartment; initial pH values between 2.0 and 4.0). The two compartments
were separated by a 50 m-thick PCP film (Nafion, Dupont; acid capacity > 9.2 × 10−4
equivalent g−1). N2 and O2 gas were bubbled at 30 mm apart from each photoelectrode at aflow rate of 100 mL min−1 (Graphical abstract). The PFC was equipped with quartz windows( = 80 cm) on both sides. Both TiO2 and Ag-TiO2 photocatalysts were irradiated by UVvisible light through quartz windows via two-way branched quartz fiber light guide (5–2B–1000L, San-ei Electric Co.) from 500-W xenon arc lamp (Ushio, Model SX-UID502XAM).
The distance between the light exit ( = 5 mm) and TiO2 or Ag-TiO2 film was 46 mm. The3light intensity was 8 mW cm−2 at the center of the photocatalyst film on electrodes. Thequantum efficiency of PFC was evaluated based on absorbed light (6.9%) by photocatalyst
among incident light (1.7 × 1017 photons s−1) to photocatalyst layers and short circuit current(74 A ~ 4.6 × 1014 electrons s−1; Fig. 3C). Overall efficiency of cell current per absorbed
light was 0.04, and this can be considered as the product of efficiencies of anode (TiO2) andcathode (Ag-TiO2). If similar efficiency is assumed for electron generation at CB of anodeand hole generation at VB of cathode, the product of 0.2 × 0.2 was 0.04.
Static photocurrent generation tests (Figs. 3A, B) were performed by connecting a
external parallel circuit of a voltmeter, an ammeter, and a resistance of 0.5 Ω. A 30 min cycleof UV-visible light irradiation and 30 min of darkness was repeated on both photocatalysts,
typically five times. The voltage between two electrodes and the current were monitored. The
i–V dependence was also measured using a similar parallel circuit and the resistance wasgradually decreased from 500 kΩ to 0.3 Ω over 20 min (Fig. 3C).
SEM, TEM, HR-TEM, and HAADF-STEM images were observed using TEM apparatus(Hitachi, Model HD2700 with an aberration-corrected STEM and an accelerating voltage of
200 kV. Cyclic voltammetry measurements were performed for Ag-TiO2 (3.0 wt% of Ag) orTiO2 as a working electrode (WE), glassy carbon as counter electrode, and Ag/AgCl as areference electrode immersed in HCl solution of pH 4.0. The voltage of WE was swept
between −1.0 and 1.0 V versus Ag/AgCl at the rate of 50 mV s−1 using a potentio/galvanostat(Model VersaSTAT 3-100, Princeton Applied Research) in O2 or N2 atmosphere at a flow rateof 100 mL min−1 under the irradiation by UV-visible light (8 mW cm−2) or in dark.
Silver K-edge EXAFS spectra were measured at 290 K in transmission mode in thePhoton Factory Advanced Ring at the High Energy Accelerator Research Organization(Tsukuba) on beamline NW10A. The storage ring energy was 6.5 GeV and the ring current
was 537–355 mA. A Si (311) double-crystal monochromator and platinum-coated focusing
cylindrical mirror were inserted into the X-ray beam path. The X-ray intensity was maintained
at 65% of the maximum flux using a piezo translator set to the crystal. The slit opening size
was 1 mm (vertical) × 2 mm (horizontal) in front of the ionization chamber. The silver Kedge absorption energy was calibrated to 25516.5 eV for the spectrum of Ag metal.28 The4

EXAFS data were analyzed using an XDAP package.29 Multiple-shell curve-fit analyses wereperformed for the Fourier-filtered k3-weighted EXAFS data in k- and R-space using empirical
amplitude and phase-shift parameters extracted from the EXAFS data for Ag metal foil, Ag2O,and AgCl powders. The interatomic distance (R) and its associated coordination number (N)for the Ag-Ag, Ag-O, and Ag-Cl pairs were set to 0.2889 nm with the N value of 12, 0.2044
nm with the N value of 2, and 0.2775 nm with the N value of 6 (Table S1).

References(28) J. A.Bearden, Rev. Mod. Phys., 1967, 29, 78–124.
(29) M. Vaarkamp, H. Linders, and D. Koningsberger, XDAP version 2.2.7, XAFS Services International,Woudenberg, The Netherlands, 2006.

5

Table S1. The curve-fit analysis results of Ag K-edge EXAFS for Ag-TiO2 (3.0 wt%-Ag) a

Sample
(a)

673 K-calcined & under air(purple)(b)

Ag-O Ag-Cl Ag-Ag
R (nm)
N2
 (10−5 nm2)0.23050.287(±0.0003)(±0.002)

1.8

2.0––

(±0.1)(±0.1)

7.6

12(±0.3)(±0.2)

Goodnessof fit

1.3 × 1040.2038 0.262 0.287(±0.0003) (±0.001) (±0.007)

In aq. HCl (pH 2.0) & under N2(yellow)(c)
Under UV-visible(ocher)(d)
Light off & under O2(purple)

0.9

1.5

1.5

(±0.2)(±0.3)(±0.1)

−5.9

−15

3.7

(±1.9)(±2.3) (±2.8)0.20460.287(±0.0005)(±0.001)

0.3

6.0––

(±0.1)(±0.1)−7.3

3.8(±3.5)(±0.4)

0.212

0.284

(±0.004)(±0.004)

1.5––

4.0

(±0.1)(±0.02)108.5(±5.6)(±0.4)(e)
0.2044 0.2775 0.2889
Models2612(Ag2O, AgCl, or Ag metal)––––––
a The values in parentheses are evaluated fit errors.

65.4 × 104

1.6 × 105

2.3 × 105

Table S2. Electrochemical data for Ag-TiO2 (3.0 wt% of Ag)

Redox reaction
Reduction
Conditionswave––(A) AgCl + e = Ag + Cl
O2, Light
0.16
O2, Dark
0.22
N2, Light
0.16
N2, Dark
0.22+–(B) Ag2O + 2H + 2e = 2Ag + H2O
O2, Light
0.35
O2, Dark
0.35N2, Light
0.35
N2, Dark––+–(C) 2H + 2e = 2H2O2, Light––
O2, Dark––
N2, Light−0.25
N2, Dark−0.26(D) 4H+ + O2 + 4e– = 2H2O

CV peak (V)Oxidation
wave
CenterEquil. voltage
298 K, pH 4.0

0.75
0.87
0.75
0.87

0.46
0.55
0.46
0.55

0.46

1.16
1.30
1.16
––

0.76
0.83
0.76
––

0.93

––
––––––––––––––

−0.24

0.99

7

Fig. S1. SEM (A), TEM (B), HR-TEM (D, F, H), and HAADF-STEM images (C, E, G) measured for asprepared Ag-TiO2 photocatalyst stored in ambient air.

8