FULL PAPER
structure. 53 For example, the valence (conduction) band edges
of ZrS 2 is:
VB,CB ( Zr S ) /2 ,
E = χ χ 2
±Egap +E0 (2)
where χ Zr and χ S are the electronegativities of Zr and S, E gap the
calculated bandgap, and E 0 = −4.5 V the difference between the
normal hydrogen electrode (NHE) and vacuum level.
The screening criteria can be summarized as: stability in
water: Δ E ≤ 1.0 eV/atom; bandgap: 1.7 ≤ E gap ≤ 3.0 eV; and band
edges position: CB edge < −0.1 V vs NHE and VB edge > 1.6 V vs
NHE.
Figure 4 shows the 25 stable semiconductors fulfi lling the
screening requirements out of the 2400 calculated materials.
The fi gure combines the evaluation of the stability using Pourbaix
diagrams, calculated at pH = 7 and in a potential range
between −0.4 and 2.2 eV, where stable and unstable compounds
are indicated in green and red, and the indirect and direct positions
of the valence and conduction band edges are drawn with
black and red lines, respectively. In particular, oxides tend to be
more stable at the oxidative potential, as the O 2p orbitals, that
usually form the valence band of oxides, are low in energy and
thermodynamically favorable. In general, the problem of stability
in water is important but not crucial to the design a new
light harvester material. Necessarily, the photoharvester can
be protected by transparent protective shields that remove the
problem of corrosion due to water and oxygen and hydrogen
ions in solution. 54 On the other hand, the use of a transparent
shield increases the manufacturing diffi culties and the total
cost of the photodevice.
We performed a literature search for available information of
the candidate materials of Figure 4 . In particular, we are interested
in data regarding stability in water, light absorption, and
industrial applications. Five materials of Figure 4 (green underlined
formula) have a realistic possibility of success as a onephoton
photocatalytic water splitting material. Ca 2 PbO 4 has an
optical bandgap of approximately 1.8 eV 55 and it is used as a
primer for stainless steel due to its lower toxicity compared to
lead oxide. 56 Cu 2 PbO 2 was originally synthesized by Szillat et al.
and they showed the material was insoluble in basic solutions.
57 This compound has an optical bandgap of 1.7 eV and
is naturally p-type semiconductor. 58 α -AgGaO 2 has been shown
to have a bandgap of 2.4 eV whereas a bandgap of 2.1–2.2 eV
has been found for β-AgGaO 2 . 59,60 AgInO 2 has a bandgap of
1.9 eV. 60 AgGaO 2 and AgInO 2 have been successfully tested
for photocatalytic degradation of alcohols. 59,60 NaBiO 3 has a
bandgap of 2.6 eV, and has already been used for photocatalytic
degradation of pollutants. 61 Using computational modeling,
Liu et al. found a bandgap of 2.2 eV and a valence and conduction
band that straddles the water splitting redox reactions. 62
Some materials show an experimental bandgap above 3.0 eV
and thus are unsuited for an effective water splitting catalyst.
For example, BaSnO 3 , which has already been proposed as a
light harvester material in previous work 7,8 in which the cubic
perovskites have been investigated, has a bandgap of 3.1–3.3 eV
and luminesces at 1.4 eV. 63 It has been tested for photochemical
H 2 and O 2 evolution using sacrifi cial donors, however its
water splitting activity is inhibited due to defect-assisted recombination.
64 In 2 O 3 has a bandgap near 3.4 eV (however some
papers report a bandgap of 2.8 eV 65 ) and a conduction band
www.MaterialsViews.com
near 0.00 V vs RHE. 66 It has been used as a photocatalyst 67 or
to enhance the catalytic performances of photocatalysts, such as
LaTiO 2 N. 68 A detailed analysis of all the candidate materials is
reported in the Supporting Information.
4. Conclusions
In this work, we have calculated the bandgaps of approximately
2400 known materials, available in the Materials Project database,
using a recently implemented functional that includes the
evaluation of the derivative discontinuity.
As a fi rst step, we compared the bandgaps calculated with
the GLLB-SC potential with several levels of the GW approximation
and hybrid HSE06 scheme for 20 materials. We showed
that the agreement between GLLB-SC and GW is rather good,
with a MRE of around 15% better than the agreement between
G 0 W 0 (or HSE06) and GW and with a signifi cant savings in the
computational cost.
Secondly, we have applied a screening procedure to the set
of calculated materials with the goal of fi nding new materials
to be used in a one-photon water splitting device. We combined
the calculation of the bandgaps with the evaluation of Pourbaix
diagrams to estimate the materials’ stability in water with the
evaluation of the band edge positions to determine whether the
photogenerated charges carry the energy necessary to initiate a
water splitting reaction. An a posteriori literature search shows
that at least fi ve of them (Ca 2 PbO 4 , Cu 2 PbO 2 , AgGaO 2 , AgInO 2 ,
and NaBiO 3 ) might be suitable to be used in a water splitting
device and require further experimental investigation.
The calculated data may be of relevance for other applications
within sustainable energy materials and all the data are
made available to the public in the Materials Project database
and in the Computational Materials Repository.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgments
The authors acknowledge support from the Catalysis for Sustainable
Energy (CASE) initiative funded by the Danish Ministry of Science,
Technology and Innovation, from the Danish National Research
Foundation for funding The Center for Individual Nanoparticle
Functionality (CINF) (DNRF54) and from the Center on Nanostructuring
for the Effi cient Energy Conversion (CNEEC) at Stanford University,
an Energy Frontier Research Center founded by the US Department
of Energy, Offi ce of Science, Offi ce of Basic Energy Sciences under
award number DE-SC0001060. Work at the Lawrence Berkeley National
Laboratory was supported by the Assistant Secretary for Energy Effi ciency
and Renewable Energy, under Contract No. DE-AC02-05CH11231. The
Materials Project work is supported by Department of Energy’s Basic
Energy Sciences program under Grant No. EDCBEE.
1400915 (6 of 7) wileyonlinelibrary.com © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: June 3, 2014
Revised: July 29, 2014
Published online: August 21, 2014
Adv. Energy Mater. 2015, 5, 1400915
www.advenergymat.de