Enhancing the photocatalytic activity of GaN by electrochemical etching

Article abstract of DOI:10.1016/j.materresbull.2015.06.025

Nanoporous (NP) GaN thin films prepared with electrochemical etching method were investigated as photocatalysts in dye photodegradation systematically. The comparison of NP GaN thin films with GaN thin films showed that NP GaN thin films with high surface

Full text of DOI:10.1016/j.materresbull.2015.06.025

Accepted Manuscript
Title: Enhancing the photocatalytic activity of GaN by
electrochemical etching
Author: Dezhong Cao Hongdi Xiao Hangzhou Xu Jishi Cui
Qingxue Gao Haiyan Pei
PII:
S0025-5408(15)00395-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2015.06.025
MRB 8282
To appear in:
MRB
Received date:
Revised date:
Accepted date:
8-10-2014
27-4-2015
11-6-2015
Please cite this article as: Dezhong Cao, Hongdi Xiao, Hangzhou Xu,
Jishi Cui, Qingxue Gao, Haiyan Pei, Enhancing the photocatalytic
activity of GaN by electrochemical etching, Materials Research Bulletin
http://dx.doi.org/10.1016/j.materresbull.2015.06.025
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Enhancing the photocatalytic activity of GaN by electrochemical
etching
Dezhong Caoa, Hongdi Xiaoa∗hdxiao@sdu.edu.cn, Hangzhou Xub, Jishi Cuia,Qingxue Gaoa,
Haiyan Peib
aSchool of Physics, Shandong University, Jinan 250100, People’s Republic of China.
bSchool of Environmental Science and Engineering, Shandong University, Jinan 250100, People’s
Republic of China
*Corresponding author. Tel./fax:+86 531 88377032.
1

Graphical abstract
Dye solution
Degraded
IV
dye solution
⋅OH
H2O
GaN pore
GaN pore
¯
O2
⋅O2 在
e‐ e‐ e‐
CB
II
III
IV
VB
h+ h+ h+
H2O/OH‐
⋅OH
Nanoporous (NP) GaN thin films prepared via electrochemical etching method were first
investigated as photocatalysts. The nanostructures exhibited excellent stability and good
photocatalytic activity in dye photodegradation.
2

Highligts
Nanoporous (NP) GaN thin films showed better photocatalytic activity than GaN thin films.
NP GaN exhibited good ability to photodegrade organic dye at various pHs.
NP GaN had much better photocatalytic activity and more excellent stability than porous Si.
3

Abstract
Nanoporous (NP) GaN thin films prepared with electrochemical etching method were investigated
as photocatalysts in dye photodegradation systematically. The comparison of NP GaN thin films
with GaN thin films showed that NP GaN thin films with high surface-to-volume ratio exhibited
much better photocatalytic activity. In comparison with porous Si wafers, NP GaN thin films with
lower surface area exhibited much better photocatalytic activity, because GaN is efficient not only
for dye reduction like Si, but also for dye oxidation. Due to its ceramic-like chemical inertness,
moreover, NP GaN showed more excellent stability to photodegrade organic dye than porous Si
under basic conditions. The band gap of GaN can be modulated in visible-light region, which will
be beneficial to a photodegradation system with concentrated solar light.
Keywords
Nanoporous GaN; Photocatalytic activity; Porous Si; Electrochemical etching
4

1. Introduction
Research on porous semiconductors has drawn much attention, since the discovery of intense
luminescence from porous Si [1, 2]. The high surface area, band gap shift, and efficient
luminescence suggest uses of porous semiconductors over a wide range extending from
optoelectronics to chemical and biochemical sensors applications. Another attractive application
of porous semiconductor materials is in environmental remediation and hydrogen production
because their high surface-to-volume (S/V) ratio results in enhanced photocatalytic effect [3].
Porous nanostructure fabricated via an electrochemical etching process is generally adhered to a
rigid substrate so that this nanostructure can be easily recycled and reused in environmental
remediation and hydrogen production. Nowadays, porous Si has attracted much interest because of
its potential in utilizing solar energy to degrade organic pollutants [4, 5]. However, chemical
properties of porous Si are unstable under harsh conditions such as low and high pH, which limits
its application.
GaN is a well-known wide band-gap semiconductor that has a bandgap energy of 3.4 eV at
room temperature. The conduction band minimum (ECBM) of GaN is higher than that of
Eo(OH /OH-) (-0.28 versus NHE), whereas the valence band minimums (EVBM) of GaN is lower
than that of Eo(O2/ ) (2.27 V versus NHE) [6, 7]. Based on such a realization, GaN has been
used to split water for the generation of hydrogen gas and decompose organic pollutants into
harmless compounds such as carbon dioxide and water [8, 9]. Due to the ceramic-like chemical
inertness, moreover, it has exhibited much better stability than semiconductors based on metal
oxides, such as ZnO and TiO2, under acidic conditions [9]. Most important of all, the band gap
5

tunability by alloying with InN allows efficient absorption of solar spectrum. Therefore, NP GaN
should be intriguing as a photocatalytic material because of its simple fabrication step as well as
expected advantages of GaN nanostructures in photodegradation for organic pollutants.
In this study, we reported the fabrication of NP GaN thin films by electrochemical etching
in HF solution, and its photocatalytic activity for direct fast scarlet 4BS dye was characterized. In
comparison with porous Si wafers, NP GaN thin films exhibited more excellent chemical stability,
and much better photocatalytic efficiency. It implied that NP GaN and its alloys are promising
photocatalytic materials under concentrated solar light intensity.
2. Experimental
2.1 Fabrication of NP GaN thin films
Electrochemical (EC) porosification experiments were conducted in a two-electrode cell at
room temperature with n-type GaN (0002) thin films or n-type Si (100) wafers (the resistivity of
0.5-1 Ω cm) as the anode and a platinum wire as the counter electrode (cathode) [10, 11]. The
structure of n-type GaN films is that Ga-polar Si doped GaN of 2
thickness was grown on
c-plane sapphire with undoped GaN layer of 500 nm used as an etch stop by metal-organic
chemical vapor deposition (MOCVD) with a doping density of 8×1018 cm-3 [12]. The electrolytes
were prepared by adding ethanol to HF (49%) with a volume ratio of 1:1. The anodization process
was carried out in a potentiostatic (constant voltage) mode controlled by a Gwinstek GPD-3303S
source meter. After anodization, samples were rinsed with deionized water and dried in N2.
Morphology of the samples was characterized with atomic force microscope (AFM) (NanoScope
ⅢA) and scanning electron microscope (SEM) (Hitachi SU-70). Photoluminescence (PL)
spectroscopy measurements with a He-Cd laser emitting at 325 nm were performed to identify the
6

optical characteristics of NP GaN thin films.
2.2 Evaluation of the photocatalytic activity
The molecule structure of direct fast scarlet 4BS (>90% purity) is shown in the following
[13].
There are N=N double bonds which are chromophoric group, phenyl rings and naphthyl rings in
the structure of 4BS dye. The corresponding UV-Vis absorption peaks are centered at ~498, ~334
and ~208 nm, respectively.
The photocatalytic experiments were carried out with the NP GaN films and the porous Si
wafers with the area of 1 cm2 in a 4 ml 4BS dye solution in a sealed quartz tube, under stirring.
The initial concentration of 4BS dye solution was about 1.5×10-5 mol L-1. The reaction system was
irradiated under ultraviolet light using a 30 W Hg lamp (
). The concentration of 4BS
dye was determined by UV-Vis spectrophotometer (UV-2450, Shimadzu, Japan). All of the
photocatalytic reactions were carried out under air ambient and at room temperature.
3. Results and discussion
3.1 Characterization of NP GaN thin films
To evaluate the morphology and structure of NP GaN thin films prepared via the EC etching
technique at 5, 10, 30 and 50 V for 1 min, SEM and AFM measurements were performed. Figs.
1(a)-1(d) show the top view SEM images of the NP GaN films. The SEM image shown in Fig. 1(a)
indicates that the applied bias of 5 V is not sufficient to induce local breakdown which leads to the
pore formation [14]. The root-mean-square (rms) roughness of the film surface measured on a 3 3
7

μm area, using AFM, is 6 nm. By increasing the applied biases from 10 to 50 V, the density and
diameter of GaN nanopores vary from 1.44
/cm2 to /cm2, and from 17.5 nm to
35 nm, respectively (Table 1). The variations are ascribed to a stronger electric field that initiates
more avalanche breakdown events per unit time [15].
Figs. 1(e)-1(h) show the cross-sectional images of the corresponding left samples. In Fig.
1(f), the etching depth along the [000 ] direction is about 100 nm at 10 V. At 30 and 50 V, the
nanopores with approximate 2 μm in length are uniform, straight and vertical to the substrate
surface, indicating that the etching rate depends on the applied bias. The trajectory of the pore can
be considered as a mapping of the electric field/carrier flow contours, so a higher applied bias
reflects a higher carrier rate which leads to a higher etching rate [16].
Fig. 2 displays the room-temperature PL spectra of NP GaN samples fabricated under the
different applied biases. The NP GaN samples obtained at high applied bias are observed to be
red-shifted relative to the samples fabricated at low applied bias. The red-shifted can be attributed
to the relaxation of the compressive stress in the porous samples, which can be further confirmed
by Raman scattering [17] and X-ray diffraction [18]. On the other hand, the PL intensity for the
etched samples are found to be increased, which could mainly be attributed to the enhancement of
the photon extraction efficiency due to more photons scattering from the sidewalls of the NP GaN
[19].
3.2 Photocatalytic activity of NP GaN thin films
The photocatalytic activity of GaN thin films etched at the different applied biases was
compared by measuring the amount of dye solutions photodegraded by the etched GaN films
under UV light illumination. Figs. 3(a) and 3(b) show the removal amount of 4BS under different
8

irradiation times and the absorption spectra of 4BS solutions degraded for 8 h, respectively. After
8 h of UV illumination, the amount of organic dye degraded by GaN samples increases from 26%
to 57% with pore density rising from 0 to
/cm2, whereas the linear increases of
removal amount indicate that the prolonged photodegraded times can lead to further increase of
removal amount (see Fig. 3(a)). The high photocatalytic activity of NP GaN thin films obtained at
50 V presumably results from the high S/V ratio of GaN nanopores.
The process of photodegradation can be roughly divided into four different stages (Fig. 4)
[7]. Stage I is photoexcitation in the nanopores, at which electron-hole pairs are created by UV
light illumination (with photo energy greater than the bandgap energy, Eg), transforming the
catalyst into the photoexcited state (eqn. 1). Stage II is that the electron-hole pairs are separated
and subsequently transferred to their sites on the internal surface of GaN nanopores. In parallel to
stage II, stage III is charge recombination, where a fraction of photogenerated electrons and holes
radiatively or nonradiatively recombine, or are trapped by the defects on the internal surface of
GaN nanopores. Stage IV is the redox reaction, at which further react with adsorbed water and
dissolved oxygen to produce
radicals (eqns. 2-7) which are strong and nonselective
oxidizing agents for organic pollutants. Finally, 4BS molecules are oxidized and decomposed by
the
radicals (eqn. 8).
(1)
(2)
(3)
(4)
,
9

(5)
(6)
(7)
(8)
Fig. 5 shows the dependence of photocatalytic activity of NP GaN thin films obtained at 50
V in acidic and basic solutions. The amount of photodegraded dye in acidic solutions with pH of
2.0, 3.0, and 5.0 is 78, 82, and 70 %, respectively, whereas the amount in pH 9 solution is 41%.
This indicates that the photocatalytic activity of NP GaN is more effective under acidic than basic
conditions. Under the acidic conditions, it is easy to form N-H groups on the internal surface of
GaN nanopores, so this pH dependence of the photocatalytic reaction may result from a sequence
of proton-transfer events from the N-H groups to the nearby OH groups and bulk water molecules
[20].
We investigated the changes in surface morphology and crystal structure of NP GaN thin
films obtained at 50 V using SEM (Fig. 6) and XRD (Fig. 7) before and after photodegradation at
pH 2. As shown in Figs. 6 and 7, there are no apparent changes in surface morphology and
diffraction peaks, which can be also observed under basic conditions. Therefore, these results
indicate that NP GaN has stability to photodegrade organic dye under strong acidic or basic
conditions, which is in agreement with the photocatalytic stability of GaN nanowires [9].
3.3 Comparison of the photocatalytic activity of NP GaN with that of porous Si
The photocatalytic activity of NP GaN thin films was compared with that of porous Si wafers
because porous Si wafers have exhibited a very strong photocatalytic activity for some organic
pollutants such as methyl red (MR), phenol, and methyl orange (MO) [4, 5, 21]. Porous Si (100)
10

wafers were fabricated via the EC etching technique at 15 V for 10 min. Fig. 8 shows the typical
top-view and cross-sectional SEM images of porous Si with a diameter of averagely 300 nm, a
pore density of 7.9
9/cm2, and a pore depth of 60 μm. For the porous Si with the area of 1 cm2,
its internal surface area is larger than that of NP GaN obtained at 50 V. We measured the
absorption spectra of 4BS dye solutions photodegraded by the NP GaN and the porous Si. Our
observations clearly show that (i) NP GaN exhibits 57% potodegradation of chromophoric
structure (λ=494 nm) of organic dye, whereas porous Si photodegrades only 33% of the dye (Fig.
9); (ii) the naphthalene and benzene rings can be photodegraded by NP GaN thin films, but
absorbance of aromatic rings in 4BS solutions degraded by porous Si exhibits significant increase
(Fig. 9), which may be explained by the formation of intermediate products such as
NO
NO
2
NO
NO
2
2
2
, andO
. These intermediate products have much bigger
O
N
N
2
2
,
,
absorbency than those in the original azo dye molecules [22].
To evaluate presumably photodegradation mechanisms of NP GaN and porous Si, band
levels of GaN and Si are shown in Fig. 10. The ECBM values of GaN and Si are higher than that of
Eo(OH /OH-), whereas the EVBM value of GaN is lower than that of Eo(O2/ ) [6, 7]. Porous Si
has higher band-gap energy (1.7-1.9 eV) than bulk Si (1.1 eV) [23], but its EVBM value is still
higher than that of Eo(O2/ ). Therefore, GaN is efficient not only for dye reduction like Si (eqns.
4-8), but also for dye oxidation (eqns. 2, 3 and 8). Furthermore, after the photodegradation
experiment under the basic condition, there is no significant change in the surface morphology of
NP GaN, whereas the porous Si layers are dissolved, indicating that NP GaN has more excellent
stability than porous Si.
4. Conclusions
11

In summary, the photocatalytic activity of NP GaN thin films was investigated for the use of
NP GaN thin films as photocatalysts in harsh environments. NP GaN thin flims exhibited higher
photocatalytic activity than GaN thin films because of their larger surface area, which enhanced
hole transport from photocatalyst to dye solution. Furthermore, NP GaN showed higher efficiency
and stability than porous Si due to its large band gap energy and ceramic-like chemical inertness.
This behavior would be beneficial to a photodegradation system with concentrated solar light.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (61376069)
and the Scientific and Technological Development Program of Shandong Province, China
(2012GGX10228).
12

References
[1] L. T. Canham, Silicon quantum wire array fabrication by electrochemical and chemical
dissolution of wafers, Appl. Phys. Lett. 57 (1990) 1046-1048
[2] V. Lehmann, U. Gosele, Porous silicon formation: A quantum wire effect, Appl. Phys. Lett. 58
(1991) 856-858
[3] D. Beytdoun, R. Amal, G. Low, S. McEvoy, Role of Nanoparticles in Photocatalysis, J.
nanoparticles Res. 1 (1999) 439-458
[4] F. Y. Wang, Q. D. Yang, G. Xu, N. Y. Lei Y. K. Tsang, N. B. Wong, J. C. Ho, Highly active
and enhanced photocatalytic silicon nanowire arrays, Nanoscale, 3 (2011) 3269-3276
[5] J. Y. Su, H. T. Yu, X. Quan, S. Chen, H. Wang, Hierarchically porous silicon with
significantly improved photocatalytic oxidation capability for phenol degradation, Appl. Catal.
B-Environ. 138 (2013) 427-433
[6] M. D. Hernández-Alonso, F. Fresno, S. Suárez, M. M. Coronado, Development of alternative
photocatalysts to TiO2: Challenges and opportunities, Energy Environ. Sci. 2 (2009)1231-1257
[7] H. L. Zhou, Y. Q. Qu, T. Zeid, X. F. Duan, Towards highly efficient photocatalysts using
semiconductor nanoarchitectures, Energy Environ. Sci. 5 (2012) 6732-6743
[8] S. W. Ryu, Y. Zhang, B. J. Leung, C. Yerino, J. Han, Improved photoelectrochemical water
splitting efficiency of nanoporous GaN photoanode, Semicond. Sci. Tech. 27 (2012) 015014
[9] H. S. Jung, Y. J. Hong, Y. R. Li, J. H. Cho, Y. J. Kim, G. C. Yi, Photocatalysis Using GaN
Nanowires, ACS Nano, 2 (2008) 637-642
[10] H. Hartono, C. B. Soh, S. Y. Chow, S.J. Chua, E. A. Fitzgerald, Reduction of threading
13

dislocation density in GaN grown on strain relaxed nanoporous GaN template, Appl. Phys. Lett.
90 (2007) 171917
[11] V. P. Parkhutik, V. I. Shershulsky, Theoretical modelling of porous oxide growth on
aluminium, J. Phys. D: Appl. Phys. 25 (1992) 1258-1263
[12] J. Han, T. B. Ng, R. M. Biefield . M. H. Crawford, D. M. Follstaedt, The effect of H2 on
morphology evolution during GaN metalorganic chemical vapor deposition, Appl. Phys. Lett. 71
(1997) 3114-3116
[13] H. D. Xiao, H. Y. Pei, J. Q. Liu, J. S. Cui, B. Jiang, Q. J. Hou, W. R. Hu, Fabrication,
characterization, and photocatalysis of GaN-Ga2O3 core-shell nanoparticles, Mater. Lett. 71 (2012)
145-147
[14] S. Nakamura, S. J. Pearton, G. Fasol, The Blue Laser Diode: The Complete Story (Springer,
New York, 2000)
[15] M. J. Schwab, D. Chen, J. Han, L. D. Pfefferle, Aligned Mesopore Arrays in GaN by Anodic
Etching and Photoelectrochemical Surface Etching, J. Phys. Chem. C 117 (2013) 16890-16895
[16] D. Chen, H. Xiao, J. Han, Nanopores in GaN by electrochemical anodization in hydrofluoric
acid: Formation and mechanism, J. Appl. Phys. 112 (2012) 064303
[17] H. D. Xiao, J. S. Cui, D. Z. Cao, Q. X. Gao, J. Q. Liu, J. Ma, Self-standing nanoporous GaN
membranes fabricated by UV-asisted electrochemical anodizaiton, Mater. Lett. 145 (2015) 304
[18] J. S. Cui, H. D. Xiao, D. Z. Cao, Z. W. Ji, I. Ma, Porosity-induced relaxation of strains at
different depth of nanoporous GaN studied using the Z-scan of Raman spectroscopy, J. Alloy
Comp. 626 (2015) 154
[19] A. P. Vajpeyi, S. J. Chua, S. Tripathy, E. A. Fitzgerald, W. Liu, P. Chen, L. S. Wang, High
14

optical quality nanoporous GaN prepared by photoelectrochemical etching, Electrochem.
Solid-State Lett. 8, (2005) G85
[20] A. V. Akimov, J. T. Muckerman, O. V. Prezhdo, Nonadiabatic Dynamics of Positive Charge
during Photocatalytic Water Splitting on GaN(10-10) Surface: Charge Localization Governs
Splitting Efficiency, J. Am. Chem. Soc. 135 (2013) 8682
[21] H. Z. Xu, H. D. Xiao, H. Y. Pei, J. S. Cui, W. R. Hu, Photodegradation activity and stability
of porous silicon wafers with (100) and (111) oriented crystal planes, Micropor. Mesopor. Mat.
204 (2015) 251
[22] J. Wang, R. Li, Z. Zhang, W. Sun, R. Xu, Y. Xie, Efficient photocatalytic degradation of
organic dyes over titanium dioxide coating up conversion luminescence agent under visible and
sunlight irradiation, Appl. Catal. A 334 (2008) 227-233
[23] M. A. Tischler, R. T. Collins, J. H. Stathis, and J. C. Tsang, Luminescence degradation in
porous silicon, Appl. Phys. Lett. 60,639 (1992).
15

Figure captions
Fig. 1. Plan-view SEM images of GaN samples obtained by the EC etching process at (a) 5 V, (b)
10 V, (c) 30 V and (d) 50 V. (e)-(f) show cross-sectional view of the corresponding left images.
Fig. 2. PL spectra of nanoporous GaN fabricated at the different applied biases.
Fig. 3. (a) Removal amount of 4BS by the etched GaN samples under the different irradiation
times. (b) Absorption spectra of 4BS solution photodegraded by the etched GaN samples for 8 h.
Fig. 4. Schematic illustrating the mechanism for photocatalytic process in GaN nanopores. (I)
photoexcitation in the nanopores, (II) charge separation and transportation, (III) charge
recombination in parallel to stage II, and (IV) photoassisted redox reaction.
Fig. 5. Amount of 4BS photodegraded by NP GaN at 50 V under various pHs.
Fig. 6. SEM images of NP GaN obtained at 50 V (a) before the photocatalytic reaction and (b)
after the reaction at pH 2 for 8 h.
Fig. 7. XRD patterns of NP GaN obtained at 50 V before and after the photocatalytic reaction at
pH 2 for 8 h.
Fig. 8. (a) Plan-view and (b) cross-sectional SEM images of porous Si fabricated by the EC
etching process at 15 V.
Fig. 9. Absorption spectra of 4BS solution photodegraded by porous Si and NP GaN.
Fig. 10. Band gaps (eV) and redox potentials of GaN and Si using the normal hydrogen electrode
(NHE) as a reference [ 6, 7]. This figure can’t indicate that the ECBM of NP GaN is higher or lower
than that of porous Si.
16

Tables
Table 1 Comparison of pore density, pore diameter, and photocatalytic activity of GaN samples
obtained at the different conditions
samplesa
applied bias
rms surface
pore density
pore diameter
(V)
-
roughnessb (nm)
(cm-2)
(nm)
Control
-
6
-
-
-
0
a
5
0
b
10
17
1.4
4.7
4.9
c
30
50
-
-
24
35
d
a Corresponds to samples in Fig.1
b The root-mean-square (rms) surface roughness was determined by atomic force microscopy
measured on a 3 3μm area.
17

Figure 1
(a)
(e)
(f)
(b)
(c)
(g)
(d)
(h)
18

Figure 2
5 V
10 V
20 V
30 V
360
375
390
405
420
Wavelength (nm)
19

Figure 3
105
90
75
60
45
30
(a)
Under UV light
5 V
10 V
30 V
50 V
0
2
4
6
8
Time (h)
Dye solution
5 V
(b)
0.3
10 V
30 V
50 V
0.2
0.1
0.0
400
500
600
700
800
900
Wavelength (nm)
20

Figure 4
UV
I
Dye solution
Degraded
IV
dye solution
⋅OH
H2O
GaN pore
GaN pore
¯
O2
⋅O2 在
e‐ e‐ e‐
CB
II
III
IV
VB
h+ h+ h+
H2O/OH‐
⋅OH
21

Figure 5
100
80
60
40
20
0
2
4
6
8
10
12
pH
22

Figure 6
(a)
(b)
23

Figure 7
After experiment
Before experiment
(0002)
`
31.5
33.0
34.5
36.0
37.5
2θ (degrees)
24

Figure8
(a)
(b)
25

Figure 9
0.5
0.4
0.3
0.2
0.1
0.0
Dye solution
(control)
porous Si
NP GaN
300
400
500
600
700
800
900
wavelength(nm)
26

Figure 10
27