Photocatalytic performance of Ag2S under irradiation with visible and near-infrared light and its mechanism of degradation

Article abstract of DOI:10.1039/c4ra15774e

Ag₂S has only rarely been investigated as a photocatalyst on its own, although it has been widely used as an important component of composite photocatalysts. We synthesized Ag₂S by a facile ion-exchange method at room temperature and used it directly as an effective photocatalyst. Our results confirmed the excellent performance of Ag₂S in completely photodegrading methyl orange within 30 min under irradiation with visible light and within 70 min under irradiation with near-infrared light. This good performance is ascribed to the narrow band gap (1.078 eV) of Ag₂S and the lower recombination efficiency of the photogenerated electron-hole pairs of Ag₂S in the photocatalytic process. The active species in the photo-oxidation process was identified as anioic ozone radicals. This simple preparation method and high photocatalytic performance increases the possible future applications of Ag₂S.

Full text of DOI:10.1039/c4ra15774e

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Photocatalytic performance of Ag2S under
irradiation with visible and near-infrared light and
Cite this: RSC Adv., 2015, 5, 24064
its mechanism of degradation†
*
Wei Jiang, Zhaomei Wu, Xiaoning Yue, Shaojun Yuan, Houfang Lu and Bin Liang
Ag2S has only rarely been investigated as a photocatalyst on its own, although it has been widely used as an
important component of composite photocatalysts. We synthesized Ag2S by a facile ion-exchange method
at room temperature and used it directly as an effective photocatalyst. Our results confirmed the excellent
performance of Ag2S in completely photodegrading methyl orange within 30 min under irradiation with
visible light and within 70 min under irradiation with near-infrared light. This good performance is
ascribed to the narrow band gap (1.078 eV) of Ag2S and the lower recombination efficiency of the
photogenerated electron–hole pairs of Ag2S in the photocatalytic process. The active species in the
photo-oxidation process was identified as anioic ozone radicals. This simple preparation method and
high photocatalytic performance increases the possible future applications of Ag2S.
Received 4th December 2014
Accepted 9th February 2015
DOI: 10.1039/c4ra15774e
www.rsc.org/advances
superionic conductors as a result of its good chemical stability
and excellent optical limiting properties.15 Ag2S has oen been
1. Introduction
combined with other semiconductor catalysts to form
a
TiO2 is oen used as a stable and inexpensive photocatalyst
active under ultraviolet (UV) radiation;1–3 however, its applica-
tion is restricted as it cannot be used with visible light. Photo-
catalysts that work with visible light, which makes up about
48% of the solar spectrum, or even infrared light (44% of solar
spectrum)4 have therefore been the focus of much research. To
achieve a high efficiency over a wide range of the solar spec-
trum, various photocatalysts, such as non-metals5 or metals6
doped with TiO2, composite semiconductor photocatalysts7 and
new photocatalysts with a small band gap,8 have been investi-
gated. Photocatalysts that work under visible and near-infrared
(NIR) irradiation include Cu2(OH)PO4,9 YF3:Yb3+Tm3+/TiO2,10
composite photocatalyst, improving the performance of the bulk
photocatalyst. Xie et al.16 reported that TiO2 nanocomposites
coupled with Ag2S absorbed visible light. Subash et al.17 proved
that the photocatalytic efficiency of ZnO was enhanced by loading
with Ag2S and Shen et al.18 conrmed that CdS nanostructures
with highly dispersed Ag2S had the highest photocatalytic activity
for hydrogen evolution when the concentration of Ag2S was 5%
by weight. Other composite photocatalysts containing Ag2S,
such as Ag2S/ZnIn2S4,19 ZnS–In2S3–Ag2S,20 Ag2S/MCM-41 21 and
Ag2S-coupled ZnO/ZnS,22 all exhibit good photocatalytic activity.
These successful examples of increased photocatalytic
performance with the addition of Ag2S suggested that Ag2S may
be an effective photocatalyst. However, there have only been a
few reports of the performance of pure Ag2S as a photocatalyst.
Yang et al.23 reported that only 7.8% of methyl orange (MO) was
degraded by pure Ag2S aer 30 min of irradiation with visible
light, whereas Pourahmad21 reported 41% degradation of
methylene blue by pure Ag2S within 60 min under UV light.
Reddy et al.24 reported the photodegradation of rhodamine B
with Ag2S with a 21.78% rate of conversion aer 120 min of
irradiation with sunlight.
Bi2WO6/TiO2,11 CQDs/Cu2O,12 BiVO4/CaF2:Er3+, Tm3+, Yb3+
13
and NaYF4:Yb, Tm/CdS/TiO2.14 The preparation of most of these
photocatalysts is too complex for mass production and the time
taken to photodegrade organic compounds under NIR irradia-
tion can be as long as 2, 4 or even 50 hours. It is therefore
desirable to develop an efficient photocatalytic material with a
simple method of preparation and higher activity in both the
visible and NIR regions of the solar spectrum.
Ag2S is an important chalcogenide compound and has been
investigated for use in photoconductors, photovoltaic cells and
These results suggest that the photocatalytic performance of
pure Ag2S is unsatisfactory. However, the low band gap energy
(0.9–1.05 eV)25 of Ag2S with a conduction band (CB) at about
0 eV vs. NHE26 means that it absorbs in the UV, visible and NIR
regions, making it a possible semiconductor material for pho-
tocatalytic applications under visible and NIR irradiation. Thus
we thought that it was worth systematically investigating the
Multi-phases Mass Transfer and Reaction Engineering Laboratory, College of Chemical
Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu,
Sichuan, China, 610065. E-mail: weijiang@scu.edu.cn; Tel: +86-28-85990133
† Electronic supplementary information (ESI) available: EDS and the particle size
distribution of Ag2S; spectrum of methyl orange solution at different
concentrations; extraction of adsorbed MO on Ag2S with DI water; repeatability
experiments of MO degradation with Ag2S under light irradiation. See DOI:
10.1039/c4ra15774e
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photocatalytic ability of pure Ag2S for potential application to FTIR spectrometer (Spectrum II L1600300, PerkinElmer) at
the degradation of persistent organic pollutants. room temperature. The active species were investigated with an
We prepared a pure Ag2S photocatalyst by a facile ion- ER200-SRC ESR analyser.
exchange method and used it to decompose MO under visible
and NIR irradiation. The morphology and structure of the as-
2.4 Measurement of photocatalytic activity
prepared Ag2S were characterized and its photocatalytic
performance was investigated in detail. The working mecha-
nism of Ag2S was determined by simulation, electron spin
resonance (ESR) analysis and photodegradation experiments.
The photocatalytic activities of the samples were evaluated by
the degradation of MO under visible and NIR light irradiation in
an OCRS-IV photoreactor system, which included a 500 W Xe
lamp and a 500 W Hg lamp, a cut-off lter with a specied light
wavelength and a water lter. About 0.1 g of the Ag2S photo-
catalyst was placed in a quartz reactor tube with 10 mL of
aqueous MO solution (16 mg Lꢀ1). The suspension was stirred
for 120 min in the dark to reach adsorption–desorption equi-
librium. Visible light (l > 380 nm) or NIR light (l > 850 nm)
irradiation was switched on to start the photodegradation
reaction. The photoreaction was terminated aer the scheduled
time and the suspension was sampled and centrifuged to
remove the solid particles. The concentration of the MO solu-
tion was determined by UV-visible spectrophotometry. Fig. S3†
shows the corresponding standard absorption curve of the MO
solution.
2. Experimental
2.1 Chemicals
All the reagents used were of analytical-reagent grade and were
used without further purication. All the reagents were
obtained from Chengdu Ke Long Chemical of China.
2.2 Preparation of Ag2S photocatalyst
The Ag2S photocatalyst was prepared by a facile ion-exchange
method at room temperature. In a typical process, 30 mL of
Na2S$9H2O (0.1 M) was dropped into 40 mL of AgNO3 (0.2 M)
solution using an automatic titrator at a speed of 3.2 mL minꢀ1
.
Aer 30 min of magnetic stirring, the black products were
ltered and washed with deionized water at least ve times to
remove the unreacted components and the NaNO3 produced.
Finally, the black powders were vacuum-dried overnight at 60 ꢁC
in the dark. All these operations were carried out as far away
from light as possible.
2.5 Theoretical calculations
Theoretical calculations were carried out using the plane wave
pseudopotential density functional theory (DFT) method. The
ion–electron interaction was modelled by the ultraso pseu-
dopotential in the Vanderbilt form. The energy cut-off for a
plane wave basis set was 400 eV and a Monkhorst–Pack k-mesh
of 6 ꢂ 6 ꢂ 6 was used. Geometry optimizations were performed
before the single-point energy calculation and self-consistent
convergence accuracy was set at 1 ꢂ 10ꢀ6 eV per atom. The
convergence criterion for the maximum force between atoms
2.3 Characterization
X-ray diffraction (XRD) patterns for Ag2S powder samples were
obtained by glancing-angle X-ray diffraction (X'Pert proMPD,
the Netherlands) with a Cu Ka 40 kV/40 mA X-ray source (l ¼
0.15406 nm). Scanning electron microscopy (SEM) images and
energy-dispersive X-ray spectrometry (EDS) analysis were
carried out using a JSM-7500F eld-emission scanning electron
microscope (JEOL). The transmission electron microscopy
(TEM) and high-resolution TEM (HRTEM) images were
obtained using a Tecnai G2 F20 S-TWIN microscope.
X-ray photoelectron spectrometry (XPS) measurements were
performed on a VG ESCALAB MKII XPS system with an Al Ka
source and a charge neutralizer. UV-visible diffusion reection
spectrometry (UV-visible DRS) was performed using a UV-visible
spectrophotometer (TU-1901, Ge Beijing Spectrometer, China)
and a ne-grained BaSO4 powder was used as the standard.
Photoluminescence (PL) spectra were recorded with a uores-
cence spectrometer (F-7000, Hitachi, Japan). The absorption
was 0.01 eV. The maximum displacement was 5 ꢂ 10ꢀ4 A and
˚
the stress was 60 GPa. The Up parameter for S was 7 eV and the
Ud parameter for Ag was 13.2 eV. As a result of the complete
symmetry between the spin-up and spin-down states, we only
report the spin-up results here.
In general, Ud + Up increases the splitting between the
occupied and empty energy levels because the correction
depends on the occupancy of the orbitals. The energy band
structure using GGA + U, where US,p ¼ 7 eV and UAg,d ¼ 13.2 eV,
was used.
3. Results and discussion
3.1 Characterization of photocatalyst
spectrum of the MO solution was obtained using a UV-visible The as-prepared Ag2S samples were black or dark brown
spectrophotometer (TU-1901, Ge Beijing Spectrometer, China). powders. The surface morphology, structure and band structure
The particle size distribution of the prepared Ag2S samples was were characterized using XRD, XPS, SEM, EDS and UV-visible
determined with a JL-1198 laser particle size analyser (Chengdu DRS.
Jingxin Powder Analyser Instruments Co. Ltd, China). The
3.1.1 XRD analysis. Fig. 1a shows the crystallization and
surface areas of the photocatalysts were determined by the phase identication of Ag2S using XRD. The diffraction peaks
nitrogen adsorption method at ꢀ196 ꢁC with a BET analyser were assigned to monoclinic Ag2S (JCPDS no. 65-2356) with the
˚
˚
˚
(SSA-3500, China). The FTIR spectra of the samples were lattice constants a ¼ 4.230 A, b ¼ 6.910 A and c ¼ 7.870 A. No
recorded with KBr pellets in the range 4000–400 cmꢀ1 on an obvious impurity peak was observed. Based on the XRD data,
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Fig. 1 Structure and component analysis. (a) XRD pattern of Ag2S; (b) XPS survey spectra of Ag2S; (c) Ag3d peaks of Ag2S; and (d) S2p peaks of Ag2S.
the crystallite size of Ag2S was determined to be 32.6 nm using TEM image in Fig. 2b conrms that the size of the Ag2S aggre-
the Debye–Scherrer equation:
gates was about 0.2–0.7 mm, consistent with the results obtained
with the laser particle size analyser (Fig. S2†) and BET analysis;
the average particle size was 0.7226 mm and the specic surface
area was 6.062 m2 gꢀ1. Fig. 2c shows the HRTEM image of an
Ag2S sample in which the lattice fringes can be clearly seen. The
clear fringes with intervals of 0.34 and 0.38 nm can be indexed
to the (012) and (002) lattice planes of Ag2S, respectively.
3.1.4 UV-visible DRS analysis. Fig. 3a shows the UV-visible
absorption spectra of the Ag2S sample. The as-prepared Ag2S
sample had a large amount of absorption in the UV and visible
regions, suggesting that Ag2S should have good photocatalytic
activity under irradiation with UV and visible light. The
absorption edge of the as-prepared Ag2S samples was >800 nm,
suggesting possible photocatalytic activity in the NIR region.
Fig. 3b showed the results of UV-visible DRS, which is used to
estimate the band gap of semiconductors. By plotting the graph
of (ahv)2 versus hv, where a is the absorption coefficient and hv is
the photon energy,29 the band gap of the as-prepared Ag2S
samples was estimated to be 1.078 eV, which was in agreement
with previously reported values.30
kl
D ¼
bcos q
where D is the crystallite size for the catalyst, k is a dimen-
sionless constant, l is the wavelength of the X-rays (0.15406
nm), b is the full-width at half-maximum of the diffraction peak
and q is the diffraction angle.
3.1.2 XPS analysis. XPS was used to determine whether
impurity elements such as N and Na were present and to
determine the valence state of the Ag atoms. Fig. 1b conrms
that no heteroatom was introduced into the crystal, despite the
fact that the catalyst was obtained from the reaction of AgNO3
and Na2S in aqueous solution. This agrees with the results of
the EDS analysis given in Fig. S1.†
The peaks at 368.21 and 374.26 eV in Fig. 1c can be assigned
to the Ag3d5/2 and Ag3d3/2 of Ag+ ions in the Ag2S photocatalyst.27
The peaks at 161.3 and 162.5 eV of the S2p photoelectron
spectrum in Fig. 1d were in perfect agreement with the binding
energy of S2p3/2 and S2p1/2 of S2ꢀ 28 It was concluded that the as-
.
prepared sample consisted of pure Ag2S without any Ag and S
atoms of other valencies.
3.1.5 PL analysis. The PL signals of semiconductor mate-
rials result from the recombination of photoinduced charge
carriers. In general, the lower the PL intensity, the lower the
recombination rate of the photoinduced electron–hole pairs
and the higher the photocatalytic activity of the semiconductor
photocatalysts.31,32 PL was therefore used to identify the pho-
tocatalytic activity of Ag2S.
3.1.3 Microscopic surface structure and morphology of
Ag2S. The crystalline structure of a photocatalyst affects its
photocatalytic activity. We therefore investigated the typical
microscopic surface structure and morphology of the as-
prepared Ag2S by SEM and TEM.
Fig. 2a is a typical SEM image of the Ag2S samples showing
that the Ag2S particles were an aggregation of crystal grains with
diameters of 30–80 nm, consistent with the XRD results. The
The PL emission spectra of pure Ag2S and pure TiO2 were
examined in the wavelength range 300–700 nm (Fig. 4). The
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Fig. 2 (a) SEM, (b) TEM and (c) HRTEM images of Ag2S.
Fig. 3 (a) The UV-vis DRS spectra of Ag2S, (b) plot of (ahn)2–hn.
of the photocatalyst. Fig. 5 shows that there was a sharp
decrease in the MO concentration from the start time to time
zero. However, adsorption–desorption equilibrium was ach-
ieved aer 1 h and no signicant decrease in the MO concen-
tration was observed aer this time.
3.2.2 Photodegradation of MO with Ag2S. Aer the
adsorption equilibrium of MO with Ag2S had been achieved,
we conducted a photocatalytic decomposition experiment of
MO with Ag2S under irradiation with visible and NIR light to
evaluate the photocatalytic performance of Ag2S. The degra-
dation curve for MO (Fig. 5) shows that Ag2S completely
degraded MO within 30 min under irradiation with visible
light and within 70 min under irradiation NIR light. The
repeatability experiments for the degradation of MO over Ag2S
are shown in Fig. S5.†
The fast rate of degradation of MO with Ag2S under irra-
diation with NIR light can be attributed to the huge surface
area resulting from the aggregated structure of the Ag2S
crystal grains and the strong absorption in visible and NIR
regions. However, the relatively slow decomposition rate of
MO under irradiation with NIR light compared with visible
light can be ascribed to the weak illumination of the NIR light
from the same Xe lamp, which supplied a visible light illu-
mination of 18–21 klx and an NIR light illumination of 0.09–
0.12 klx.
Fig. 4 PL spectra of Ag2S and TiO2.
intensity of the PL spectrum of pure TiO2 was much higher than
that of the Ag2S samples. It can be reasonably inferred that pure
Ag2S should have a higher photocatalytic performance than
TiO2.
3.2 Photocatalytic activity
A typical persistent organic dye, MO, was selected as the target
compound in the photodecomposition experiment to evaluate
the photo-oxidation ability of Ag2S. The adsorption and photo-
degradation of MO were determined with Ag2S as a photo-
catalyst under irradiation with visible and NIR light.
These excellent properties conrmed the proposal that Ag2S
could work under NIR irradiation with a high degradation rate
because of the narrow band gap (1.078 eV) and high quantum
3.2.1 Adsorption of MO on Ag2S. The adsorption of MO on
the surface of the Ag2S nanoparticles in the dark was deter-
mined rst to exclude the extra effect of this on the performance
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pollutants. However, as the band gap of Ag2S is only 1.078 eV
and cOH has a large electrode potential of 2.80 eV, cOH may
not play a part in the working mechanism for Ag2S. It was
therefore necessary to determine the active species and work
out the photocatalytic mechanism for Ag2S under light
irradiation.
3.3.1 Determination of radical species. A spin-trapping
technique is oen used to detect free radicals by trapping a
short-lived radical to generate a long-lived nitroxide radical,
which can then be monitored using conventional ESR meth-
odology.33 The photocatalytic mechanism of Ag2S with and
Fig. 5 Photodegradation of MO in aqueous solution (a) with fresh without light irradiation was investigated by ESR (Fig. 7). No
Ag2S under irradiation with visible light and (b) with fresh Ag2S under
irradiation with NIR light.
signicant ESR signal was detected for Ag2S in the dark. In
contrast, a septet peak group with an intensity ratio of
1 : 2 : 2 : 2 : 2 : 2 : 1 was observed, consistent with the nitroxide
DMPO–cO3ꢀ. The determined free radical should therefore be
efficiency. This outstanding photocatalytic activity and wide
the ozone anion radical.34 As the unique signal of ozone anion
working spectrum of Ag2S is attractive for further development.
radicals was the only signal observed in solution with Ag2S
3.2.3 FTIR analysis. It was necessary to determine the
under light irradiation, we concluded that this free radical was
degradation of the adsorbed MO on the surface of Ag2S under light
photogenerated by Ag2S in aqueous solution.
irradiation as about 40% of the MO had been adsorbed, despite
3.3.2 Radical-trapping experiments. Although the ozone
the fact that the MO in aqueous solution had been completely
anion radical was detected during the light irradiation of
degraded under light irradiation. Two Ag2S samples were analysed
Ag2S, it was also necessary to determine the role that this
with FTIR spectrometry aer reaching MO adsorption equilibrium
radical played in the photo-oxidation process. A radical-
to determine the functional groups adhered to the surface of the
trapping experiment was conducted to identify the major
Ag2S and to conrm the photodegradation of the adsorbed MO.
active species in the degradation of MO on the surface of Ag2S.
One sample was exposed to light for 30 min (sample 1) and the
Two scavengers, methanol and indigo carmine, were used to
other sample was kept in the dark (sample 2).
trap the photogenerated holes and ozone. Possible distur-
The results in Fig. 6 conrm the complete decomposition of
bances caused by the masking effect of the two scavengers in
the MO adsorbed on the surface of the Ag2S. The peaks at 1601
the detection of MO with UV-visible spectrophotometry was
and 814 cmꢀ1 of sample 2 can be assigned to the framework
excluded based on the results shown in Fig. 8a. The degra-
vibrations of the benzene ring and the two substituted phenyl
dation of MO aer the addition of various scavengers is
groups, respectively. These two characteristic peaks of MO were
shown in Fig. 8b. The addition of methanol decreased the rate
not observed in the FTIR spectrum of sample 1.
of photodegradation of MO signicantly because the photo-
Samples 1 and 2 were both soaked in deionized water. This
generated holes were trapped. However, the addition of
phenomenon (Fig. S4†) and the results of FTIR analysis proved
indigo carmine stopped the photo-oxidation process
that MO can be completely photodegraded with Ag2S under
completely. This clear result conrmed that the combined
light irradiation in both the adsorbed and dissolved states.
function of the photogenerated holes and the ozone anion
radicals under light irradiation dominated the oxidative
degradation of MO. The ozone anion radicals may be the
3.3 Photocatalytic mechanism of Ag2S photocatalyst
major active species in the photocatalytic degradation of
Ag2S.
cOH radicals and photogenerated holes are both possible
active species in the photocatalytic degradation of organic
Fig. 7 ESR analysis of Ag2S under irradiation with visible light and in the
dark.
Fig. 6 FTIR spectra of Ag2S with adsorbed MO: after (sample 1) and
before (sample 2) light illumination.
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Fig. 8 Photodegradation of MO with Ag2S. (a) Spectrophotometric spectra of different mixtures composed of pure MO, MO + methanol and
MO + indigo carmine. (b) Degradation curve of Ag2S with MO + indigo carmine, MO + methanol and pure MO.
Fig. 10 Energy band structure and density of states of Ag2S.
Fig. 9 Unit cell structure of Ag2S. Yellow and blue spheres represent S
and Ag atoms, respectively.
3.3.3 DFT simulation of Ag2S. To further understand the
electronic structure and mechanism of chemical bonding of the
prepared Ag2S, ab initio DFT calculations were carried out. Fig. 9
shows the crystal cell model used for the DFT simulation of
Ag2S. The space group was based on P21/c (no. 14), corre-
sponding with the measured XRD results of the as-prepared
samples. The provided bond length of the Ag2S bond was
Fig. 11 TDOS and PDOS of Ag2S.
ꢁ
˚
2.692 A and the bond angle was 105.894 .
The calculated band structure (Fig. 10) conrmed that Ag2S
was a semiconductor with a direct band gap of 1.063 eV at the
G point. This calculated value for the Ag2S band gap was
slightly lower than the determined value of 1.078 eV, but closer
to the previously reported value of 1.05 eV.30 This slight
discrepancy may be ascribed to the underestimation of the
DFT, which considered only the excited states in the
calculation.
respectively. The valence band of Ag2S is clearly split into two
regions. The lower region probably has S3p features, whereas
the upper region is dominated by the Ag4d states. There were
hybridized S2p states near the Fermi level, with little practical
contribution from the Ag5p states that have a dominant weight
at the bottom of the CB. The width of the valence band for Ag2S
was 10 eV. The S3p states had an intensity peak at 0.68 eV. In the
upper region, two Ag4d maxima were found at ꢀ9.2 and
ꢀ7.18 eV.
Fig. 11 plots the calculated total density of states (TDOS) and
the partial density of states (PDOS) for Ag2S. The valence
atomic congurations were 3s23p6 for S and 4d10 for Ag,
3.3.4 Working mechanism of Ag2S. The photocatalytic
activity of a photocatalyst is determined by the band gap and the
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ꢀ
O3 + dye / oxidation products
(3)
(4)
h+ + dye / oxidation products
4. Conclusion
An Ag2S photocatalyst has been successfully synthesized by a
facile ion-exchange method. The as-prepared Ag2S was an
aggregation of crystal grains with diameters of 30–80 nm and
was an excellent photocatalyst for the decomposition of MO
under light irradiation. The complete degradation of MO with
Ag2S was reached within 30 min under visible light illumina-
tion and within 70 min under NIR light illumination. MO was
completely photodegraded by Ag2S under light irradiation
whether it was in the adsorbed or dissolved state. Such an
attractive performance is ascribed to the narrow band gap of
Ag2S (1.078 eV) obtained both by theoretical calculations and
by experiment. The PL spectrum of pure Ag2S conrmed its
high quantum efficiency. The combined function of the pho-
togenerated holes and the ozone anion radicals of Ag2S in
aqueous solution under light irradiation dominated the
reaction process of the oxidative degradation of MO. The
simple preparation method, excellent photocatalytic perfor-
mance and ability to work under NIR light irradiation
broadens the possible applications of Ag2S as a promising
photocatalyst.
Fig. 12 Schematic diagram of the charge-transfer process in Ag2S.
oxidation potential of the photogenerated holes. The CB and
valence band (VB) potentials of Ag2S at the point of zero charge
were calculated by the following equation35 to investigate the
oxidation potential of the photogenerated holes:
EVB ¼ X ꢀ Ee + 0.5Eg
where X is the Pearson absolute electronegativity of the semi-
conductor, dened as the geometric mean of the absolute
electronegativity of the constituent atoms,36 Ee is the energy of
free electrons on the hydrogen scale (about 4.69 eV), EVB is the
VB edge potential and Eg is the band gap of the semiconductor.
The CB position can be determined by:
Acknowledgements
We appreciate the nancial support from the National Natural
Science Foundation of China Project (no. 21176157 and
21476146).
ECB ¼ EVB ꢀ Eg
The value of X for Ag2S is about 4.968 eV.37 Based on these
equations, the top of the VB and the bottom of the CB of Ag2S
were calculated to be 0.992 and ꢀ0.072 eV, respectively. This
result agrees with previously published data.38
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Fig. 12 is a schematic diagram of the charge-transfer process
in Ag2S.
Ag2S + hv / eꢀ + h+
O2 + 2OHꢀ + 2h+ + eꢀ / O3 + H2O
(1)
(2)
ꢀ
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