Supporting Information for
Efficient Photocatalytic H2 Production Using Visible-Light Irradiation and
(CuAg)xIn2xZn2(1-2x)S2 Photocatalysts with Tunable Band Gaps
Guangshan Zhang1,*,†, Wen Zhang2,*,†, Daisuke Minakata3, Peng Wang1, Yongsheng
Chen4, and John Crittenden4,5
1
State Key Laboratory of Urban Water Resource and Environment, School of
Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin
150090, China
Department of Civil and Environmental Engineering, New Jersey Institute of
Technology, Newark, New Jersey 07102, United States
2
3
Department of Civil and Environmental Engineering, Michigan Technological
University, Houghton, MI 49931, United States
4
School of Civil and Environmental Engineering, Georgia Institute of Technology,
Atlanta, Georgia 30332, United States
5
Brook Byers Institute for Sustainable Systems, Georgia Institute of Technology,
Atlanta, Georgia 30332, United States
Page 1 of 8
This supporting information contains the following sections:
S1. The bench-scale water-splitting reactor
S2. Calculating process for the conduction band and valence band potentials
S3. Optimization of photocatalytic reaction conditions
Page 2 of 8
S1. The bench-scale water-splitting reactor
300-W Xe lamp
780-mL gas-closed
stainless steel reactor
water bath
N2 inlet
Magnetic stirrer
and heater
Figure S1. A photograph of the bench-scale system with the major components such as
the 780-mL gas-closed stainless steel reactor.
300 W Xe lamp
Cutoff filter
Black cylinder
2
1
H2O
N2
Magnetic stirrer
Teflon tube (sampling)
4
3
water circulation system
Catalyst
N2
H2
Reaction solution
#1–#4 indicate four air valves
This graph is not drawn to scale
Hydrogen storage tank
Figure S2. The overall schematic design of the H2 generation system.
Page 3 of 8
S2. Calculating process for the conduction band and valence band potentials
Table S1. Values of the electron affinity, ionization energy and element
electronegativity.
Constituent
Electron affinity
Ionization energy Element electronegativity
elements
(eV)
(eV)
(eV)
Cu
1.235
7.726
4.48
Ag
1.302
7.576
4.44
In
0.30
5.786
3.04
Zn
-0.87
9.394
4.26
S
2.077
10.36
6.22
Table S2. X, Eg, ECB, and EVB at the point of zero charge for (CuAg)xIn2xZn2(1-2x)S2.
Value of x
X (eV)
Eg (eV)
ECB (eV)
EVB (eV)
3.47
0
5.15
-1.09
2.38
2.72
0.025
5.13
-0.73
1.99
2.36
0.05
5.11
-0.57
1.79
0.1
5.07
2.12
-0.49
1.63
1.93
0.15
5.04
-0.43
1.50
1.78
0.25
4.96
-0.43
1.35
1.62
0.3
4.93
-0.38
1.24
0.5
4.79
1.51
-0.47
1.04
Table S3. PZZP, conduction and valence band edge energies for (CuAg)xIn2xZn2(1-2x)S2
at pH 2.0 with respect to NHE.
Value of x
0
0.025
0.05
0.1
0.15
0.25
0.3
0.5
PZZP
ECB (eV)
EVB (eV)
1.7
-1.11
2.36
3.1
-0.67
2.05
3.4
-0.49
1.87
4.2
-0.36
1.76
4.0
-0.31
1.62
4.2
-0.30
1.48
4.7
-0.22
1.40
6.8
-0.19
1.32
Page 4 of 8
S3. Optimization of photocatalytic reaction conditions
From the above analysis, (CuAg)0.15In0.3Zn1.4S2 was chosen as the model
photocatalyst that presumably has the highest visible light photocatalytic activity for
the following experiments, which systematically evaluated the reaction parameters and
pinpoint the optimal photocatalytic conditions for H2 production. In particular, the
optimal initial pH for the photocatalytic H2 production has been analyzed in our
previous work [1], which indicated that the lower the initial pH is, the faster the H2
production rate could be obtained. Thus, in this study, a constant initial solution pH of
2.0 was employed without the further investigation.
S3.1. Effect of the Ru loading amount on H2 production rate
Figure S3 shows the influence of the Ru loading amount on photocatalytic H2
production. Without any Ru addition, the H2 production was zero, indicative of the
important role of Ru as the cocatalyst in this photocatalytic process [2]. The
photocatalytic activity increased significantly with the increasing Ru loading and
reached the maximum level of 230 µmol m−2 h−1 at a Ru loading dose of 0.7 wt% in the
suspension, which compares well with previous reports [3-5]. In a previous study,
additional increases in the Ru loading dose did not enhance the H2 production rate but
inhibited photocatalytic H2 production [3]. This is probably because the number of
catalytic active sites on the (CuAg)0.15In0.3Zn1.4S2 interface increases with the
increasing Ru loading to some extent and thus increases the photocatalytic activity [4].
However, the excessive deposition of Ru on the photocatalyst surface will negatively
affect light absorption and thus H2 production [3]. In the following tests, 0.7 wt% was
selected as the optimal dose of Ru.
(a)
(b)
Figure S3. (a) H2 production as a function of irradiation time and (b) photocatalytic H2
production rates per Ru loading amount on the (CuAg)0.15In0.3Zn1.4S2 photocatalyst.
S3.2. Effect of the initial KI concentration on H2 production rate
Figure S4 shows the H2 production rate at different initial KI concentrations. The
H2 production rate gradually increased as the KI concentration increased from 0 to 0.2
Page 5 of 8
M. However, further increasing the KI concentration from 0.2 to 0.25 M did not change
the H2 production rate. Similar results were observed when methanol [6,7], HPr [8], and
glucose [9] were used as electron donors. Because the surface adsorption of I− follows
the Langmuir isotherm equation [8,9], at high KI concentrations, the H2 production rate
leveled off, which indicates that the photocatalyst surface may be saturated with I− and
no longer provide available reaction sites for photocatalytic H2 evolution.
The photocatalytic H2 production rate varies as a function of KI concentration,
which is reported to follow the Langmuir-Hinshelwood-Hougen-Watson kinetic model
[10,11]:
r
dnH2
dt

k ' K [I- ]
1  K [I- ]
(1)
where r represents the H2 production rate, µmol m−2 h−1; k' represents the maximum
specific H2 production rate when the I− concentration reaches infinity, µmol m−2 h−1;
and K represents the adsorption constant of KI on the photocatalyst surface, L mol−1.
We fitted the experimental H2 production rate data with this Langmuir isotherm
equation. The best fit was achieved by minimizing the Objective Function (OF), which
is defined as below [12]:
1 N  rexp,i  rcal ,i

N  1 i 1  rexp,i
2

OF 


(2)
where rexp,i and rcal,i are the experimental and calculated H2 production rates for a
particular KI concentration (i) at a certain reaction time, respectively, and N is the
number of data points. As shown in Figure S4, the OF was 0.069, indicating that the
model fit (the dashed line) was within ± 0.069 of the experimental data for 68% of the
fitted values, assuming that the difference follows a normal distribution and the OF is
the sample deviation. The fitting results show that k' and K are 301 µmol m−2 h−1 and
15.8 L mol−1, respectively.
Page 6 of 8
Figure S4. Effect of initial KI concentration on photocatalytic H2 production over the
Ru/(CuAg)0.15In0.3Zn1.4S2 photocatalyst.
S3.3. Effect of the photocatalyst dose on H2 production rate
Figure S5 shows the effect of photocatalyst dose on the H2 production rate. As
expected, the optimal photocatalyst dose with the greatest apparent H2 production rate
was approximately 1.0 g L−1. Further increases in the photocatalyst dose decreased the
photocatalytic H2 production dramatically, because the high concentration of
photocatalyst interfered with light transmission or penetration into the suspension
according to the Beer-Lambert law [13,14], which was verified in our previous paper
[15]. Consequently, the photocatalytic reactor underwent a significant loss of incident
light energy penetration [4].
Figure S5. Effect of the photocatalyst dose on photocatalytic H2 production over the
Ru/(CuAg)0.15In0.3Zn1.4S2 photocatalyst.
S3.4. Effect of the reaction temperature on H2 production rate
Figure S6 shows the photocatalytic H2 production rate at various temperatures
ranging from 25C to 55C. The H2 production rate slightly increased with the increase
in temperature, from 230 µmol m−2 h−1 at 25C to 249 µmol m−2 h−1 at 55C. The
temperature dependence of the H2 production rate is consistent with previous results
[16,17] and obeys the Arrhenius equation over the range of 25−55C.
Page 7 of 8
Figure S6. H2 production rate over the Ru/(CuAg)0.15In0.3Zn1.4S2 photocatalyst at
different reaction temperatures.
When the other reaction conditions were fixed, the H2 production rate for the
Ru/(CuAg)0.15In0.3Zn1.4S2 photocatalyst, considering reaction temperature, can be
expressed as:
E
r = k0  exp(- A )
(3)
R T
where r is the H2 production rate (µmol cm−2 h−1), k0 is the reaction constant (µmol
cm−2 h−1), EA is the activation energy of the reaction (kJ mol−1), R is the universal gas
constant (8.31 × 10−3 kJ mol−1 K−1) and T is the reaction temperature (K). Figure S7
shows an Arrhenius plot of the photocatalytic H2 production rate. The activation energy
was estimated a approximately 2.3 kJ mol−1 from the Arrhenius plot, indicating that an
elevated reaction temperature would be favorable for H2 production. Considering the
energy consumption, because the photocatalytic activity of the photocatalyst increased
only 8.3% from 25C to 55C, the room temperature (25C) is used as reaction
temperature.
Figure S7. Arrhenius plot of the photocatalytic H2 production rate.
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