(397ao) Solar Hydrogen Production From Metal Sulfide (ZnS-CuS-CdS) Photocatalysts

SOLAR HYDRPGEN PRODUCTION FROM METAL SULFIDE
(ZnS-CuS-CdS) PHOTOCATALYSTS

Eunpyo Hong, and Jung Hyeun Kim, University
of Seoul, Seoul (Korea)

1.
Introduction

      Hydrogen has long
been considered as a clean and eco-friendly energy source. However,
conventional methods such as steam reforming and electrolysis of the water
require high-energy consumption for hydrogen production. In that sense,
hydrogen production from water splitting reaction using photocatalysts is one
of the ideal ways because it uses only water and solar light, as shown below
for the complete consumption and regeneration cycle.

Generation: H₂O + energy(solar) °æ H₂ + 1/2O₂        (1)

Consumption: H₂ + 1/2O₂ °æ H₂O + energy                (2)

Such a photocatalytic
process was first developed by Fujishima and Honda [1], and later many
researchers have devoted to improve photocatalytic performance in various ways.
In this study, the ZnS photocatalyst is used as a base material. However, the ZnS
photocatalyst solely is not so effective to use because of its relatively large
band gap energy. Therefore, we applied two primary concepts for improving the
electron usage efficiency on hydrogen production.

First,
CuS nanoparticles were introduced on the ZnS photocatalyst surface to improve
interfacial electron transfer. The main role of the CuS nanoparticles is
providing a driving force for electron movement. The CuS materials accept
electrons from ZnS surface, and these electrons reduce the protons (H⁺).
The similar phenomenon is observed from the CuS/ZnO (nanowire) and CuS/ZnS
(nanosheet) [2, 3]. The
major electron transfer mechanisms on photocatalysts under light illumination
are shown below.

CuS/ZnS +
hv °æ CuS(e^-)/ZnS(h⁺)
                           (3)

2CuS + 3e^-
°æ Cu₂S + S^2-                                                                (4)

Cu₂S
+ 2H⁺ + S^2- °æ 2CuS + H₂                                 (5)

Second,
the third material is used for composite semiconductor fabrication for
expanding the light harvesting region. Various composite semiconductor
materials such as CdS, CdSe, CdTe, and CuO were reported. Amongst them, the CdS
material is a promising light sensitizer considering the band gap energy and
band position (valence and conduction bands).

Therefore,
the ZnS-CuS-CdS semiconductor compounds are utilized for
hydrogen production under visible light illumination condition in order to
achieve high electron usage efficiency.

2.
Results and Discussion

          The
ZnS nanoparticles were formed in ionic solutions (Zn²⁺ + S^2-),
and a cation exchange reaction was used for the ZnS-CuS compound fabrication.
Finally, the CdS quantum dots were deposited on the as-prepared particle
surface by a chemical bath deposition method. Schematic view of the ZnS-CuS-CdS
nanoparticle compounds and the ideal electron paths are demonstrated in Fig. 1.

Fig. 1. Schematic energy diagram of ZnS-CuS-CdS
composite photocatalysts.

Three major experimental variables were used
for fabrication of semiconductor nanoparticle materials: 1) thermal sintering
of the ZnS nanoparticles, 2) varying Cu contents for the ZnS-CuS compound, and
3) varying Cd content for the ZnS-CdS. Fig. 2 shows hydrogen production results from only ZnS
case and ZnS-CuS composites. The ZnS-CuS composite evolved much more hydrogen
than from the only ZnS nanoparticle case. Thermal treatment of the ZnS
nanoparticles improves hydrogen production activity for both the only ZnS case and
ZnS-CuS composites.

Fig. 2. Amount
of evolved hydrogen of the ZnS-CuS particles as a function of Cu precursor
content.

Fig. 3 shows amounts of evolved hydrogen
from the photocatalyst samples during the experimental time period. As a
result, photocatalytic performance is greatly increased by introducing two
concepts (CuS and CdS junction), in our experimental system. This result
demonstrates that the electron transfer through the semiconductor composite
material (CdS °æ ZnS °æ CuS) is very
efficient and its usage for hydrogen production is remarkably improved.

Fig. 3.
Amounts of evolved hydrogen from four different types of photocatalysts.

          Other
material properties characterized by HR-TEM, XRD, UV-vis, and XPS analyses are
also helpful in understanding this improved hydrogen production phenomenon with
semiconductor composite photocatalysts under the solar light illumination.

References

1.
A. Fujishima, K. Honda (1972), Nature, 238, 37.

2.
J. Zhang, J. Yu, Y. Zhang, Q. Li, and J.R. Gong (2011), Nano Letters, 11, 4774.

3.
M. Lee and K. Yong (2012), Nanotechnology, 23, 194014.