Indian Academy of Sciences
Bull Mater Sci
(2021) 44:5
https://doi.org/10.1007/s12034-020-02281-6
Sadhana(0123456789().,-volV)FT3](012345
6789().,-volV)
Synergistic effect of pH solution and photocorrosion of ZnO particles
on the photocatalytic degradation of Rhodamine B
ANH THI LE, NURUL SYUHADA BINTI SAMSUDDIN, SIN-LING CHIAM
and SWEE-YONG PUNG*
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus,
14300 Nibong Tebal, Malaysia
*Author for correspondence (sypung@usm.my)
MS received 21 March 2020; accepted 8 July 2020
Abstract. This research aims to study the synergistic effect of pH solution and photocorrosion of ZnO particles on the
photodegradation of Rhodamine B (RhB) dye. The stability of ZnO particles was investigated by zeta potential measurement. The structure, morphology and photocatalytic activity of ZnO were analysed by X-ray diffraction, scanning
electron microscope and ultraviolet–visible spectrophotometer, respectively. The ZnO particles were highly crystalline
with a bandgap energy of 3.23 eV. The surface charges of particles were found to be pH-sensitive, where they were more
negative under alkaline condition, i.e., -11.1, -5.3 and -38.2 mV at pH 3, 7 and 11, respectively. ZnO particles that
dissolved in pH 3 solution produce Zn2? ions, while precipitation of Zn(OH)24 was observed in pH 11 solution, resulting
in morphology change of ZnO from rod shape to lamellar shape. The synergistic effect of photocorrosion and dissolution
of ZnO particles in acidic solution reduced RhB dye removal efficiency from *90% at pH 7 to *70% at pH 3. In alkaline
solution, ZnO particles displayed remarkable reusability as it still recorded a 94.8% degradation efficiency after five
successive cycles.
Keywords.
1.
Photocorrosion; pH; zinc oxide; photodegradation; Rhodamine B.
Introduction
Semiconductor photocatalysts have been widely studied for
the treatment of organic pollutants [1–3]. In this process,
photon energy is used to excite semiconductor photocatalysts in order to generate reactive species such as holes (h?),
electrons (e-), hydroxyl free radicals (•OH) and superoxide
free radicals (•O2-). These reactive species are able to
degrade organic pollutants into smaller and less toxic substances [4]. Although titanium dioxide (TiO2) is recognized
as the most effective photocatalysts to date, zinc oxide
(ZnO) differentiates itself by several interesting properties,
i.e., visible-light-responsive photocatalytic activity [5,6],
higher hydrogen peroxide (H2O2) generation rate in water
under UV light irradiation [7] and ease of doping [8,9].
Furthermore, when compared with TiO2 for stability, ZnO
has demonstrated to be more effective for liquid-phase
degradation [5,10,11]. However, the expansion of its
application as photocatalyst is limited by the amphoteric
behaviour of ZnO in different pH solutions, and the photocorrosion effect during illumination.
The photocatalytic performance of ZnO particles to
degrade organic dyes is strongly dependent on the solution
pH and the molecular complexity of organic compounds
[12–15]. The solution pH will determine the photocatalytic
efficiency of ZnO particles for complete mineralization of
the dyes [16]. For instance, in alkaline solution, ZnO surfaces become negatively charged by means of adsorbed
hydroxide ions. The ZnO surface is favourable for the formation of •OH free radicals if there are more adsorbed
hydroxide ions, and thus speed up the degradation of
organic dyes. Furthermore, the electrostatic attraction
between the negatively charged ZnO surfaces and the
cationic organic compounds also contributes to the removal
of dyes [17]. Sakthivel et al [5] found that the aqueous
solution of pH 10 maximized the decomposition of acid
brown 14 dye. Daneshvar et al [18] reported that the optimum working condition of ZnO particles for photodegradation of acid red 14 dye was pH 7. In contrast, the surfaces
of ZnO became positively charged in acidic aqueous solution which is favourable for the adsorption of anionic
organic compound [17]. Chen et al [19] found that the
degradation rate of methylene orange, congo red and direct
black 38 dyes decreased from 90 to 40% when the pH
solution increased from 2 to 10.
The application of ZnO as photocatalyst in aqueous
solution is also limited by photocorrosion under UV irradiation. Generally, photocorrosion occurs due to the residual photo-generated holes on the ZnO surface that are able
to attack the Zn–O bonds and dissociate Zn2? ions; as
5
Page 2 of 10
Bull Mater Sci
shown in equations (1 and 2) [20,21]. With stronger light
intensities, more holes are generated, thus accelerates the
photocorrosion rate of ZnO [22]. Han et al [7] studied the
photocorrosion of magnetron-sputtered ZnO thin films.
They found that the photocorrosion of ZnO thin films was
strongly dependent on pH, with minimum dissolution
around pH 10 and maxima at both very high and very low
pH values. This was due to its amphoteric properties as ZnO
could be dissolved in both strongly acidic and strongly
alkaline solutions [22]. Cao et al [23] investigated the
photocorrosion of ZnO ultrathin film in aqueous solution of
methylene blue dye at a constant pH value. The presence
of methylene blue significantly accelerated the dissolution of
ZnO ultrathin films. However, the effect of photocorrosion
on the degradation of methylene blue dye was not studied in
their work. To the best of our knowledge, there are limited
research works on the influence of both solution pH and
photocorrosion of ZnO to the degradation of organic dyes.
ZnO þ ht ! ZnO þ hþ þ e
þ
ZnO þ h ! Zn
2þ
þ 1=2O2
ð1Þ
ð2Þ
Rhodamine B (RhB) dye is one of the commonly used
water-soluble, persistent synthetic cationic dye. It provides
a vivid pink colour to the textile. It is difficult to remove
RhB dye from wastewater completely by physical methods
due to the conjugations from two or more aromatic rings
(benzene, naphthalene) [24]. The persistent nature of RhB
dye has a detrimental effect on environment, as it imparts
colour to water and grounds [25]. The dye effluents could
also be toxic to humans as well as animals. Although considerable efforts have been devoted to remove RhB dyes by
ZnO photocatalyst [26–31], it is worth to study the performance of ZnO photocatalyst at different solution pHs under
UV light irradiation.
This work aims to study the synergistic effect of solution
pH and photocorrosion on the photodegradation of RhB dye
by ZnO particles. The effect of pH on the ZnO surface
charges was investigated by zeta potential measurement.
The pH solutions used in these experiments were pH 3, pH
7 and pH 11, adjusting using hydrochloric acid (HCl) or
sodium hydroxide (NaOH). The photocorrosion and photocatalytic activity of ZnO particles were examined under
dark condition and UV light irradiation. Various scavenger
agents were used to determine the dominant reactive species
in the degradation of RhB dye and to propose the degradation mechanism.
2.
2.1
Experimental
Chemicals and reagents
Zinc oxide (ZnO, 6623-00) was purchased from Systerm.
Rhodamine B (RhB, 44255) was supplied by Sigma
Aldrich. Hydrochloric acid (HCl, 30%, 100317), sodium
(2021) 44:5
hydroxide (NaOH, 1 M, 106498), potassium iodide (KI,
105043), methanol (ME, Merck, 106018) and benzoquinone
(BQ, Merck, 802410) were purchased from Merck. All
aqueous solutions were prepared with distilled water.
2.2
Characterization
X-ray diffraction (XRD) pattern of ZnO particles was
recorded using X-ray diffractometer (Bruker D8-Discovery,
Cu-Ka, k = 1.54056 Å). The optical property of ZnO particles was studied by UV–VIS reflectance spectroscopy
(Lambda 35, PerkinElmer). The optical bandgap of ZnO
particles was determined by Kubelka–Munk function. The
linear part of the Tauc plot was extrapolated to
½FðRÞht2 ¼ 0, in order to estimate the bandgap of ZnO
particles. The morphology of ZnO particles was analysed by
scanning electron microscope (SEM, 35VP Supra CarlZeiss). The size of ZnO particles was measured by Image J
software with a sample size of 50. The surface charge of
ZnO particles was measured by Zetasizer Nano (Malvern,
633 nm, He–Ne laser). ZnO particles (0.1 mg) was dispersed in 100 ml deionized water by sonication for 30 min.
The operation temperature was 25C.
2.3
Photocorrosion study
The photocorrosion study was carried out by measuring the
weight loss of ZnO particles using gravimetric measurement [32]. A sensitive balance with a precision of ±0.1 mg
was used to measure the weight of ZnO particles before and
after the photocorrosion experiment. ZnO (0.5 g) particles
were dispersed in the aqueous solution of RhB dye at pH 3,
pH 7 and pH 11. After 5 days of continuous stirring under
dark condition or UV light irradiation, ZnO particles were
washed with distilled water, dried at room temperature and
then re-weighed. The weight loss percentage (%) was calculated using equation (3):
W0 Wt
Weight loss percentage ð%Þ ¼
100;
ð3Þ
W0
where W0 and Wt are the weight of ZnO particles at initial
and after 5 days, respectively.
2.4
Photocatalytic study
The photocatalytic study of ZnO particles was carried out
by measuring the degradation of RhB dye using a UV–Vis
spectrophotometer (Varian, Carry 50, Victoria, Australia)
[33–36]. The photocatalytic degradation of RhB dye at
various pH levels was carried out in both dark and UV light
irradiated conditions with the presence of ZnO particles. In
brief, 5 ppm of RhB dye solution was first prepared using
distilled water. Then, ZnO particles (0.2 g) were dispersed
Bull Mater Sci
(2021) 44:5
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in 200 ml of RhB dye solution. Either HCl or NaOH was
added dropwise into the mixture to adjust the solution pH to
3, 7 and 11. The mixture was then stirred for 30 min in dark
condition followed by immediate exposure to UV light
(360 W, 254 nm, Philip, TUV F17T8) for 50 min. During
this process, 2.5 ml of solution was taken from the mixture
at 10 min interval. The absorbance of the degraded RhB
solution was measured by a UV–Vis spectrophotometer.
The removal efficiency (RE) was determined based on the
change of RhB dye characteristic absorbance peak at
554 nm, as described in equation (4) [34]:
REð%Þ ¼
A 0 At
100;
A0
ð4Þ
where A0 and At are the absorbance at 554 nm of dye at
initial and time t, respectively.
Scavengers such as potassium iodide (KI) [31,37],
methanol (ME) [38,39] and benzoquinone (BQ) [21,38,40]
were used to investigate the role of reactive species, h?,
•
OH and •O2-; in the photocatalytic process, respectively.
The scavenger test was performed by adding 0.03 g of KI,
8.1 ll of ME or 0.02 g of BQ into the mixture of RhB and
ZnO particles. Subsequently, the RE of RhB dye by ZnO
particles was calculated as described in the above procedure. No scavenger test was performed on e-, as it is known
that it does not actively involve in the degradation of
organic dyes [40,41].
particles was determined by Kubelka–Munk function,
F(R) = (1 - R)2/2R; where R is the absolute value of
reflectance and F(R) is the absorption coefficient [44,45].
The direct bandgap of ZnO particles was approximately
3.23 eV (see inset of figure 2), consistent with the bandgap
energy of bulk ZnO [29,46]. Therefore, UV light was used
to study the photocatalytic performance of ZnO particles.
3.2
Effect of pH on the stability of ZnO particles
The stability and surface charge of ZnO particles in solution
with various pH levels were investigated by zeta potential
measurement. Figure 3 shows the zeta potential results of
ZnO particles with negative surface charges of -11.1, -5.3
and -38.2 mV for pH 3, 7 and 11, respectively. The dissolution of ZnO particles at pH 3 produced Zn2? ions and
H2O, which reduced the concentration of protons in the
solution. Hence, the surface charge of ZnO particles, which
initially could be positively charged, was changed to
slightly negatively charged. This result differs slightly from
the observation reported by Fatehah et al [47], where their
results suggest that the surface of ZnO particles was slightly
positively charged (?1.3 mV) at pH 7. The negatively
charged ZnO particles at pH 7 could enhance its adsorption
capacity for cationic RhB dye.
3.3
3.
5
Photocorrosion behaviour of ZnO particles
Results and discussion
3.1 Structural, morphology and optical properties of ZnO
particles
Figure 1a shows the XRD pattern of the purchased ZnO
particles. Its diffraction pattern is well-indexed to the
characteristic pattern of hexagonal wurtzite ZnO crystal
structure (JSPDS card no.: 006-5190). The hexagonal
wurtzite is a common structure of ZnO [21,42]. The lattice
parameters of ZnO particles are a = b = 3.2470 Å and c =
5.2030 Å. The sharp and narrow diffraction peaks indicate
that the ZnO particles were highly crystalline. The crystallite size (D) of ZnO was estimated by Scherrer’s equation
[43] (D = Kk/b cos h), where k is X-ray wavelength
(0.15418 nm); h the diffraction angle; K a constant (K &
0.9); b presents the full-width of the diffraction line at halfmaximum of the strongest diffraction peak (011) of wurtzite
phase. The calculated crystallite size of the ZnO particles
was 46.2 nm. Figure 1b presents the SEM image of rod-like
ZnO particles. Figure 1c shows the histogram of the length
and the diameter of ZnO particles. The average diameter
and length of ZnO particles were 84.87 ± 24.61 nm and
183.50 ± 64.62 nm, respectively (sample size: 50).
Figure 2 shows the reflectance of ZnO particles characterized by UV–VIS spectroscopy. A steep reflectance edge
from 370 to 410 nm is observed. The bandgap of ZnO
The photocorrosion behaviour of ZnO was investigated by
the weight loss of ZnO particles in dark condition and under
UV light irradiation, as shown in figure 4. Under dark
condition, the weight losses of ZnO were 10.3 and 2.1% at
pH 3 and 11, respectively; while it was only 0.2% at pH 7.
This result indicates that fewer ZnO particles were dissolved in pH 7 solution than in pH 3 and 11 under dark
conditions. Similar findings were reported by other
researchers, where the dissolution of ZnO particles was
found in acidic and strong alkaline solutions [7,22]. Under
UV irradiation, the weight loss of ZnO particles was
accelerated in all pH levels, i.e., 22.3% at pH 3, 4.2% at pH
7 and 2.5% at pH 11. This observation proves the occurrence of photocorrosion on ZnO particles under UV light
irradiation. Furthermore, more ZnO particles were dissolved
in pH 7 solution than the pH 11 solution under ultraviolet
irradiation, indicating that there was a synergistic effect of
photocorrosion and solution pH on the dissolution of ZnO
particles. This result suggests that holes play a major role in
the photocorrosion of ZnO. The importance of holes as
reaction species to decompose RhB dye is verified by the
scavenger test.
To investigate the amphoteric and photocorrosion behaviour of ZnO particles, the morphology of ZnO particles
after photocorrosion experiment, in acidic and alkaline
solutions; were characterized by SEM. Figure 5a shows the
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Bull Mater Sci
20000
(a)
(011)
5
(2021) 44:5
(b)
200 nm
(020)
(013)
(020)
(112)
(021)
(004)
5000
(110)
(012)
10000
(010)
(002)
Intensity (cps)
15000
0
30
40
50
60
70
80
2 theta(degree)
(c)
Figure 1.
particles.
(a) XRD pattern, (b) SEM image and (c) histogram of the diameter and the length of rod-like ZnO
80
pH 3
pH 7
pH 11
0
-5.3
-10
Zeta potential (mV)
200
150
2
40
[F(R)hv]
Reflectance (a.u)
60
100
50
20
0
2.4
2.6
2.8
3.0
3.2
3.4
hv(eV)
0
-11.1
-20
-30
-40
300
400
500
600
-38.2
700
Wavelength (nm)
Figure 2. UV–VIS reflectance spectrum of ZnO particles. The
inset shows the estimated optical bandgap of ZnO particles using
the Kubelka–Munk method.
morphology of the ZnO particles after immersing in acidic
solution. The size of ZnO particles was measured using
Image J software with sample size of 50. Most of the particles were rod-shaped with average diameter and length of
105.6 ± 19.26 and 277.1 ± 104.3 nm, respectively. These
particles are larger than those of ZnO particles before the
Figure 3.
Zeta potential of ZnO particles at various pH values.
photocorrosion experiment, with average diameter and
length of 84.87 ± 24.61 and 183.50 ± 64.62 nm, respectively. This phenomenon implies the dissolution of ZnO
particles in acidic solution. After immersing in alkaline
solution, the morphology of ZnO particles changed to
lamellar shape, as depicted in figure 5b. They could be
passive Zn(OH)2 from the precipitation of saturated zincate
[Zn(OH)24 ] in alkaline solution [48]. Liu et al [49] reported
Bull Mater Sci
(2021) 44:5
Page 5 of 10
analysis shown in figure 5c verified that the lamellar particles are wurtzite ZnO.
Dark
UV
3.4 Effect of pH and photocorrosion on the photocatalytic
activity of ZnO particles
15
Figure 6 shows the photocatalytic performance of ZnO
particles on the removal of RhB dye at various pH levels.
Figure 6a–c depicts the UV–Vis absorbance spectra of RhB
dye. Generally, the absorbance of RhB solution, regardless
of solution pH, decreased slightly in the first 30 min when
stirred in dark condition. However, it dropped significantly
with time when the UV lamps were switched on, indicating
the degradation of RhB dye. Figure 6d shows RE of RhB
dye by ZnO particles. Under dark condition, the RE of RhB
dye at all pH levels are below 20%. The removal of RhB
dye by ZnO particles was saturated after 20 min. As no UV
light was introduced in this stage, the removal of RhB dye
by ZnO particles was caused by the physical adsorption of
the positively charged dye molecules onto the negatively
charged surface of ZnO particles [51]. Significant
enhancement of RE was obvious under UV light exposure.
Figure 6e presents the RE of RhB after 10 min of UV light
irradiation. The REs increased with increasing pH level;
from 69.6 to 90.5 and 96.4% when pH was raised from pH 3
10
5
0
pH 3
pH 11
Weight losses of ZnO particles at various pH values.
9000
(011)
that, as the concentration of NaOH increased from 0.1 to
0.5 M; ZnO nanorods photo-anode collapsed completely
into ZnO plates. Furthermore, some pits were observed on
the surface of these ZnO plates, an indication of the photocorrosion of ZnO. A similar observation was reported by
Ishioka et al [50] on the photocorrosion of ZnO nanoflowers. They found that the outer surfaces of ZnO nanoflowers
were corroded after UV light irradiation for 72 h. The XRD
(c)
(020)
3000
1500
(020)
(012)
4500
(013)
6000
(010)
(002)
Intensity (cps)
7500
(112)
(021)
(004)
Figure 4.
pH 7
(110)
Weight loss percentage (%)
25
20
5
0
20
30
40
50
60
70
80
2 theta (degree)
Figure 5. (a) SEM images of ZnO particles after photocorrosion study in acidic solution; (b) SEM image and
(c) XRD pattern of ZnO particles after photocorrosion study in alkaline solution.
1.2
Intensity (a.u)
1.0
Bull Mater Sci
(a)
Dark
0.8
0.6
1.2
0 min
10 min
20 min
30 min
40 min
50 min
60 min
70 min
80 min
UV
0.4
0.2
0 min
10 min
20 min
30 min
40 min
50 min
60 min
70 min
80 min
(b)
1.0
0.8
0.6
0.4
0.2
1.2
0.0
0.0
400
450
500
550
600
650
450
0.8
0.6
0.4
0.2
500
550
600
650
700
400
Wavelength (nm)
Wavelength (nm)
450
500
550
600
650
700
Wavelength (nm)
(e) 120
(d)
100
pH 3
pH 7
pH 11
80
UV light
Dark
100
Removal efficiency (%)
Removal efficiency (%)
0 min
10 min
20 min
30 min
40 min
50 min
60 min
70 min
80 min
0.0
400
700
(2021) 44:5
(c)
1.0
Intensity (a.u)
Page 6 of 10
Intensity (a.u)
5
60
40
dark
UV
20
0
90.5
80
96.4
69.6
60
40
20
0
0
20
40
60
pH 3
80
pH 7
pH 11
Time (min)
2.5
(f)
KpH11 = 0.047
2.0
pH 3
pH 7
pH 11
log(Co/C)
1.5
K pH7
1.0
2
.04
=0
0.5
KpH3 = 0.026
0.0
0
10
20
30
40
50
Reaction time (min)
Figure 6. UV–Vis absorbance spectra as a function of time at (a) pH 3, (b) pH 7 and (c) pH 11; RhB dye removal
efficiency of ZnO particles at pH 3, 7 and 11 (d) as a function of time, (e) as a function of pH (with 30 min in dark and
20 min in UV light); (f) first-order kinetic reaction of the degradation of RhB under UV light irradiation.
to pH 7 and pH 11. The degradation of RhB dye across pH
levels under UV light irradiation obeyed the first-orderkinetic as shown in figure 6f. Low RE of RhB dye in acidic
solution could be attributed to the dissolution of ZnO
particles. It could be concluded that the pH solution and
photocorrosion play major roles in the photocatalytic performance of ZnO particles for the removal of RhB dye.
3.5
Scavenger study
In order to understand the photocatalytic mechanism of
ZnO with synergistic effect of pH and photocorrosion, the
main reactive species of the degradation of RhB dye were
determined. Scavengers such as KI, ME and BQ were used
to quench reactive species, h?, •OH and •O2-, respectively.
Figure 7 shows the RE of RhB dye with and without the
presence of scavengers. The introduction of ME slightly
reduced the RE of RhB dye in solutions with pH 7 (56.6%)
and pH 11 (69.7%), but not in the case of pH 3 solution
(67.9%). The inhibition of the degradation of ME in pH 7
and pH 11 solutions can be explained by equation (5) [52].
This result indicates that the •OH free radicals were not the
dominant species generated in acidic solution. On the other
hand, both KI and BQ significantly reduced the RE,
regardless of pH. The degradation of RhB dye was
Bull Mater Sci
(2021) 44:5
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Figure 7. Removal of RhB dye by ZnO particles at various pHs
with the presence of scavengers (ME for •OH; BQ for •O2– and KI
for h?).
significantly reduced to 2.8, 4.6 and 5.8% in the presence of
KI at pH 3, pH 7 and pH 11, respectively. This reduction of
RE reveals that iodide quenched most of the h? generated
from ZnO particles according to equation (6) [38]. The
quenching of h? reduced the formation of •OH free radicals
according to equation (7). In addition, RE of RhB dye was
suppressed to 6.8, 32.1 and 11.3%, respectively, in the
presence of BQ at pH 3, pH 7 and pH 11. This reduction of
RE reveals that BQ quenched most of the •O2- generated
from ZnO particles as shown in equation (8) [38]. This
result suggests that both h? and •O2- free radicals were the
dominant species responsible for the degradation of RhB
molecules at all pH levels, while the •OH free radicals only
contribute to the degradation of RhB dye in neutral and
alkaline solutions.
OH ðMEÞ þ OH ! H2 O
ð5Þ
I þ hþ ! I2
ð6Þ
þ
h þ H2 O ! OH
BQ þ
3.6
O2
ð7Þ
! BQ þ O2
ð8Þ
Photocorrosion and photocatalytic mechanism of ZnO
The experimental findings are summarized in table 1, while
the photocorrosion mechanism of ZnO and photocatalytic
Table 1.
pH
3
7
11
5
degradation mechanism of RhB dyes by ZnO particles are
proposed below.
Photocorrosion of ZnO particles depends on the solution
pH and UV light irradiation. The chemical dissolution
corresponds to the direct reaction of ZnO with protons (H?)
and hydroxyl ions (OH-) [49]. In acidic solution and dark
condition, the dissolution of small sized ZnO particles were
preferred until all the protons (H?) were consumed. Thus,
the products were Zn2? ions in aqueous solution as shown
in equation (9) [7]. This dissolution reaction was accelerated with UV light irradiation as more h? were produced on
the surface of ZnO particles, as shown in equation (10). In
alkaline solution, the dissolution of ZnO also occurred. At
high pH, Zn2? ions dissolved and formed Zn(OH)24 precipitates as described in equation (11), resulted in morphology change of ZnO particles [7]. Under UV light
irradiation, the photogenerated h? on the surface of ZnO
were quenched by OH- ion. Thus, the photocorrosion was
not as significant in alkaline solution as compared to that for
acidic condition.
ZnO þ 2Hþ ! Zn2þ þ H2 O
ð9Þ
ZnO þ hþ þ Hþ ! 1=2H2 O2 þ Zn2þ
ð10Þ
ZnðOHÞ2
4
ð11Þ
ZnO þ 2H2 O þ 2OH !
The photocatalytic degradation mechanism of RhB dye
with the effect of solution pH and photocorrosion of ZnO
particles is presented in figure 8. Under dark condition, no
reactive species was produced by ZnO particles. The
removal of RhB molecules by ZnO particles was done by
physical-adsorption. In acidic solution, the dissolution of
ZnO particles reduced most of the active sites on its surfaces, thus relatively poor and saturated RE were expected,
as shown in figure 8a. In alkaline solution, ZnO particles
were more negatively charged (pH 11, -38.2 mV) due to
the adsorbed hydroxide ions. This favoured the attraction of
positively charged RhB molecules (cations) to be adsorbed
on the surface of ZnO particles, as illustrated in figure 8a
[51]. Therefore, the RE of RhB by ZnO particles was higher
under alkaline solution, i.e., 14.8% at pH 11. However, high
pH solution could dissolve ZnO particles, i.e., 2% at pH 11,
which also reduces the photocatalytic activity of ZnO
particles.
Upon UV light illumination, as depicted in figure 8b,
reactive species such as h?, e-, •OH free radicals and •O2free radicals were produced by ZnO particles, presented in
Synergistic effects of photocorrosion and pH to the photocatalytic activity of ZnO particles under UV light irradiation.
Surface charge (mV)
-11.1
-5.3
-38.2
; decrease, : increase.
Photocorrosion (%)
12.2
4.1
0.4
:
;
;
Photodegradation efficiency (%)
69.6
90.5
96.4
;
:
:
Dominant species
•
O2–, h?
OH, •O2–, h?
•
OH, •O2–, h?
•
5
Page 8 of 10
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(2021) 44:5
Figure 8. Removal mechanisms of RhB dye in acidic and alkaline solutions via (a) adsorption
under dark condition and (b) photocatalytic degradation under UV light irradiation.
ZnO þ ht ! ZnO þ hþ þ e
ð12Þ
hþ þ OH ! OH
ð13Þ
e þ O2 ! O2 ð14Þ
þ
h þ RhB ! intermediate substances
ð15Þ
OH or O2 þ RhB ! ROH þ CO2 þ H2 O
ð16Þ
80
60
40
20
0
pH3
pH7
pH11
Figure 9. Stability test of ZnO particles for degradation of RhB
dye in pH 3, 7 and 11.
even at the 5th cycle. While the ability of dye removal of
ZnO particles deteriorated from 69.6 to 60.8% after five
successive cycles at pH 3. This could be explained by the
dissolution and photocorrosion in acidic solution, reducing
the amount of ZnO photocatalyst in the photocatalytic
reaction. Thus, the optimum performance of ZnO particles
under UV light to decompose RhB dye occurs in neutral and
alkaline solutions.
4.
3.7
cycle 1
cycle 2
cycle 3
cycle 4
cycle 5
100
Removal efficiency (%)
equations (12–14) [19,24,29]. In acid solution, ZnO particles suffered from both dissolution and photocorrosion
effects. Thus, the synergistic effect of pH solution and
photocorrosion could deteriorate the photocatalytic performance of ZnO particles. The main reactive species
responsible for the degradation of RhB dye are h? and •O2free radicals. In alkaline solution, the large number of
hydroxide ions would be favourable to react with h? to
produce more •OH free radicals and to inhibit the photocorrosion of ZnO, which logically enhanced the RE of RhB
dye. Besides, the RE was improved because the surface of
ZnO particles had more negative charge, which could
attract more RhB cations moving towards the ZnO particles
for degradation by holes. Finally, h?, •OH free radicals and
•
O2- free radicals generated from ZnO particles, effectively
degraded RhB dye into less harmful minerals in alkaline
solution according to equations (15 and 16) [31,53].
Conclusion
Stability of ZnO particles
The stability of the ZnO particles at various pH levels was
tested for RhB dye degradation for five successive cycles as
shown in figure 9. At pH 7 and 11, ZnO particles displayed
excellent stability of RE of 87.8 and 94.8%, respectively,
The synergistic effect of pH solution and photocorrosion to
the photocatalytic performance of ZnO particles was successfully investigated in pH 3, 7 and 11 solutions under UV
light irradiation and dark condition. The surface of ZnO was
found to be negatively charged regardless of pH level. The
Bull Mater Sci
(2021) 44:5
RhB dye was considerably degraded by *70% at pH 3
within 50 min over the ZnO particles, while neutral and
alkaline conditions accelerated the RE of dye to above 90%.
The dissolution and photocorrosion of ZnO were the main
factors for low RE of RhB dye in acidic solution. The
degradation of dye in all conditions was strongly suppressed
(\20% of RE) in the presence of KI and BQ scavengers.
The h? and •O2- free radicals are crucial for the mineralization of RhB dye; however, h? reduces the photocatalytic
performance of ZnO by photocorrosion. ZnO particles displayed excellent stability of RE in neutral and alkaline
solutions even after five successive cycles. The finding of
this work provides insight and explanation on the interrelationship between photodegradation of RhB dye and
photocorrosion/pH effect of ZnO particles.
Acknowledgements
We would like to acknowledge the research funding (RUI
USM 1001/PBAHAN/8014095) from Universiti Sains
Malaysia and the support from USM Fellowship.
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