ARTICLE OPEN
Me al oxide single-componen ligh -powe ed mic omo o s o
pho oca aly ic deg ada ion o ni oa oma ic pollu an s
Xia Peng
1
, Ma io U so
1
and Ma in Pume a
1,2,3,4
✉
Mass ans e is a key pa ame e in he e ogeneous eac ions. Mic o/nanomachines, a p omising echnology o en i onmen al
applica ions, significan ly enhance he pe o mance o con en ional pu ifica ion ea men s because o he ac i e mo ion abili y
and hus enhanced di usion (supe di usion) o hese pho oca alys s, which in u n leads o d ama ically imp o ed mass ans e
and highe deg ada ion capabili y compa ed o s a iona y mic opa icles. Howe e , he design o mic omo o s gene ally in ol es
noble me als, o ins ance, Au and P , o achie e an e ec i e au onomous mo ion. Conside ing he expensi e ab ica ion cos and
complica ed s eps, we p esen P - ee single-componen ligh -powe ed WO
3
mic omo o s capable o enhanced di usion and
e ec i e deg ada ion o ni oa oma ic compounds in wa e . These mic oswimme s, syn hesized by a hyd o he mal me hod, which
is highly scalable a low cos , ollowed by calcina ion, exhibi uel- ee ligh -d i en mo ion due o asymme ic ligh i adia ion. Pic ic
acid (PA) and 4-ni ophenol (4-NP) we e selec ed as ep esen a i e ni oa oma ic con aminan s and pho oca aly ically decomposed
by WO
3
mic omo o s hanks o he close con ac wi h he mic omo o s p omo ed by hei sel -p opulsion. This wo k p o ides a low-
cos , sus ainable, scalable me hod o enhancing mass ans e by c ea ing mo ing ca alys s wi h b oad applica ion po en ial o
wa e cleanup.
npj Clean Wa e (2023) 6:21 ; h ps://doi.o g/10.1038/s41545-023-00235-z
INTRODUCTION
Ni oa oma ic compounds a e o ganic composi es ha comp ise
one o mo e ni o g oups (-NO
2
) connec ed o he a oma ic ing
1
.
Taking ad an age o hese g oups, ni oa oma ic compounds ha e
been b oadly applied o ab ica e di e se indus ial p oduc s, such
as explosi es, pes icides, and dyes
2
. Un o una ely, he ni o g oup
also hinde s he biodeg ada ion o hese compounds, esul ing in
con inuous en i onmen al accumula ion. Indeed, hei ex ensi e
employmen has led o se e e con amina ion o g oundwa e and
soil, which is usually associa ed wi h d inking wa e quali y,
se e ely posing a h ea o human heal h
3,4
. T adi ional was e-
wa e pu ifica ion app oaches mainly depend on biological
oxida ion and physical p ocedu es (i.e., ac i a ed ca bon adso p-
ion, nano-fil a ion)
5–7
. The biological p ocess is inadequa e as
hese compounds a e ba ely deg aded due o hei high
cons ancy in he wa e sys em. Physical me hods only ans e
he pollu an s ins ead o des uc i e emo al, which indica es ha
u he ea men s a e necessa y. Ad anced oxida ion p ocesses
(AOPs) ha e been ex ensi ely explo ed in deg ading o ganic
con amina ions in was ewa e
8–10
. AOPs gene ally in ol e highly
eac i e oxygen species (ROS), o ins ance, hyd oxyl adical (•OH)
and supe oxide adical (•O
2-
), which can oxidize haza dous
chemical species in wa e
11
. Pho oca alysis is a p omising ligh -
d i en AOP echnology, necessi a ing a pho oca alys and p ope
ligh i adia ion o gene a e ROS ha b eak down o ganic
pollu an s. Recen ly, nanos uc u ed TiO
2
, ZnO, and Fe
2
O
3
pho o-
ca alys s ha e ecei ed g ea in e es
12–17
. Fo ins ance, Reddy
e al. p oposed Cu-doped ZnO nanopa icles o RhB dye
deg ada ion unde ligh i adia ion
18
. Simila ly, Z-scheme bina y
he e os uc u ed nanocomposi es (i.e., MoS
2
/g-C
3
N
4
and ZnWO
4
/
NiFe
2
O
4
) we e employed o he e ficien pho oca aly ic
deg ada ion o oxic o ganic pollu an s
19,20
. Howe e , hese
app oaches a e es ic ed because o he passi e di usion o
pho oca aly ic ma e ials, equi ing cons an agi a ion. Fu he -
mo e, mos o hem in ol ed mo e han one componen .
Consequen ly, i is highly desi able o explo e mo e acile and
p ac ical s a egies o acili a e he emo al o pollu an s.
Ligh -powe ed mic omo o s, mic o/nano-sized ma e ials ha
exploi ene gy om a powe ul, enewable, and abundan sou ce,
such as ligh , and con e i in o mo ion, ha e ecei ed
conside able in e es
21–26
. P e ious esea ch has shown ha he
ac i e locomo ion o ligh -powe ed mic omo o s based on
pho oca aly ic ma e ials can o e come he limi a ion o passi e
di usions o pho oca alys s by enhancing he in e ac ions wi h
a ge ed pollu an s
27–31
. Fo ins ance, Ma e al. de eloped ubula
mic omo o s based on TiO
2
, magne ic Fe
3
O
4
nanopa icles, and P
o he e ec i e deg ada ion o hodamine 6G
30
. Mo eo e , ligh -
powe ed ZnO/P mic omo o s wi h H
2
O
2
- ee ligh -d i en p opul-
sion abili y and hema i e/P Janus mic o obo s we e explo ed o
ni oa oma ic explosi es decomposi ion
31,32
. Howe e , a noble
me al coa ing was equi ed in bo h cases o unlock he sel -
p opulsion abili y, inc easing ab ica ion cos s and complexi y.
The e o e, low-cos and simple mic omo o s emain o be
explo ed o u u e p ac ical applica ions.
He e, we demons a e he pho oca aly ic deg ada ion o
ni oa oma ic pollu an s by noble me al- ee single-componen
ligh -powe ed WO
3
mic omo o s. Compa ed o Au-WO
3
@C Janus
mic omo o s p oposed by Ren e al. o he pho odeg ada ion o
dye pollu an s
25
, single-componen WO
3
mic omo o s wi hou
he need o addi ional noble-me al coa ing we e p epa ed by a
acile hyd o he mal eac ion ollowed by a calcina ion p ocess.
Once exposed o asymme ic ligh illumina ion, he mic omo o s
1
Fu u e Ene gy and Inno a ion Labo a o y, Cen al Eu opean Ins i u e o Technology, B no Uni e si y o Technology, Pu kyno a 123, 61200 B no, Czech Republic.
2
Depa men o
Medical Resea ch, China Medical Uni e si y Hospi al, China Medical Uni e si y, No. 91 Hsueh-Shih Road, TW-40402 Taichung, Taiwan.
3
Facul y o Elec ical Enginee ing and
Compu e Science, VSB - Technical Uni e si y o Os a a, 17. lis opadu 2172/15, 70800 Os a a, Czech Republic.
4
Depa men o Chemical and Biomolecula Enginee ing, Yonsei
Uni e si y, 50 Yonsei- o, Seodaemun-gu, Seoul 03722, Ko ea. ✉email: ma in.pume a@cei ec. u b .cz
www.na u e.com/npjcleanwa e
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can exhibi uel- ee sel -ac ua ion in pu e wa e , wi h emo e
con ol and apid esponse o e on/o swi ching o UV-ligh .
Mo ion beha io in di e en condi ions demons a ed ha H
2
O
2
enhanced he mo ion and di usion o mic omo o s. Finally, hese
WO
3
mic omo o s we e applied o he e ec i e pho odeg ada-
ion o ni oa oma ic compounds, among which pic ic acid (PA)
and 4-ni ophenol (4-NP) we e selec ed as models because o hei
haza dous na u e and high s abili y in wa e . Such simple and low-
cos mic omo o s capable o deg ading oxic subs ances hold
g ea po en ial in he mic o/nanomo o s sys ems o en i on-
men al emedia ion.
RESULTS
Cha ac e iza ion o WO
3
mic omo o s
WO
3
mic omo o s we e success ully p epa ed ia he combina ion
o a simple hyd o he mal me hod and a calcina ion p ocess, as
desc ibed in Fig. 1a. Specifically, a homogenous solu ion consis -
ing o he ungs en p ecu so and glucose was sealed unde he
hyd o he mal condi ion, ollowed by an annealing p ocess in ai o
p omo e he o ma ion o single-componen WO
3
mic omo o s.
Scanning elec on mic oscopy (SEM) images in Fig. 1b illus a e
he ob ainmen o WO
3
mic osphe es wi h sizes a ying om 1 o
2 µm. A mo e de ailed analysis displays a highly ough su ace,
asc ibed o a hie a chical s uc u e composed o assembled
nanopa icles (Fig. 1c). Ene gy-dispe si e X- ay spec oscopy (EDX)
elemen al mapping images alida e he exis ence and e en
dis ibu ion o he W and O elemen s, as shown in Fig. 1d, e. The
X- ay di ac ion (XRD) pa e n o WO
3
mic omo o s is displayed in
Fig. 1 , which ag ees wi h he s anda d monoclinic phase (JCPDS
83-0951)
33
. In ac , di ac ion peaks a e obse ed a 2θ=22.9°,
23.4°, 24.1°, 26.4°, 28.1°, 32.8°, 33.1°, 41.6°, 50,43° and associa ed
wi h he (002), (020), (200), (120), (112), (022), (202), (222), and
(400) c ys alline planes o WO
3
. The chemical s a es o WO
3
mic omo o s we e addi ionally de e mined by X- ay pho oelec on
spec oscopy (XPS). Figu e 1g shows he main peaks asc ibed o W
4 and O 1s, highligh ed in he XPS wide spec um. The high-
esolu ion spec um o W 4 demons a es wo dis inguished
peaks in Fig. 1g. The binding ene gies o W 4
7/2
and W 4
5/2
,
posi ioned a 35.3 eV and 37.3 eV, espec i ely, ma ch well wi h
epo ed alues o he W
6+
oxida ion s a e o WO
3
mic osphe es.
The high- esolu ion spec um o O 1sdisplays wo peaks a 531.8
534 532 530 528
In ensi y [a.u.]
Binding ene gy [eV]
Measu ed
Backg oud
Fi ed
W-O
WO(OH)
40 38 36 34
In ensi y [a.u.]
Binding ene gy [eV]
Measu ed
Backg oud
Fi ed
W4 7/2
W4 5/2
W 4
20 25 30 35 40 45 50 55 60
(112)
(400)
(222)
(420)
(202)
(120)
(022)
(200)
(002)
In ensi y [a.u.]
2 The a [deg ee]
(020)
1200 1000 800 600 400 200 0
W 4s
W 5s
C 1s
O KLL
W 4pW 4dW 4
In ensi y [a.u.]
Binding ene gy [eV]
O 1s
O 1s
g
hi
a
W p ecu so Mixing
Hyd o he mal
eac ion
Annealing
S i ing
Cen i uge
D ying
Glucose
WO
bc
e
d
Fig. 1 The p epa a ion p ocess and cha ac e iza ion o WO
3
mic omo o s. a Schema ic illus a ion o he p epa a ion o WO
3
mic osphe es
by hyd o he mal and calcina ion p ocesses. b( he scale ba is 2 µm) and c( he scale ba is 1 µm) SEM images o WO
3
mic omo o s. Elemen al
mapping images o dW and eO. XRD pa e n. gXPS wide spec um. hHigh- esolu ion W 4 XPS spec um. iHigh- esolu ion O 1sXPS
spec um.
X. Peng e al.
2
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and 530.3 eV binding ene gy (Fig. 1i)
34
. The peak a 531.8 eV is
asc ibed o OH g oups on he WO
3
mic omo o s’su ace, and he
o he peak ag ees wi h he la ice oxygen in he c ys al s uc u e
o WO
3
mic omo o s.
Mo ion beha io o WO
3
mic omo o s
The single-componen WO
3
mic omo o s display au onomous
mo ion when exposed o UV-ligh illumina ion bo h in uel- ee
and H
2
O
2
solu ions. P e ious esea ch has elucida ed he
locomo ion mechanism o single-cons i uen pho oca aly ic
mic omachines unde ligh i adia ion
35,36
. As illus a ed in
Fig. 2a, he incidence o pho ons wi h highe ene gy han he
op ical bandgap o WO
3
mic omo o s igge s he gene a ion o
elec on-hole pai s. Acco ding o p e ious wo ks
37–39
, he edge o
he conduc ion band (CB) o WO
3
mic omo o s (E
CB
=0.77 V
NHE
,
whe e E
CB
is he conduc ion band edge a he no mal hyd ogen
elec ode (NHE)) lies below he pho oca aly ic hyd ogen e olu ion
h eshold. The pho ogene a ed elec ons in he CB could no eac
wi h H
+
o gene a e H
2
, whe eas hey can decompose H
2
Oo
H
2
O
2
in p o ons (H
+
). Meanwhile, he pho ogene a ed holes in he
alence band (VB) can con ibu e o b eaking down H
2
Oo H
2
O
2
in o •OH, O
2,
and H
+
. The non-uni o m ligh exposu e o WO
3
mic omo o s esul s in an asymme ical gene a ion o hese
chemical species, es ablishing a p oduc g adien leading o hei
mo emen by sel -pho esis
40,41
. An op ical bandgap o 2.72 eV,
ep esen ing he minimum ene gy o exci e he elec ons om he
VB o he CB, was de e mined om he abso p ion spec um o
WO
3
mic omo o s (Fig. 2b) using he Tauc plo (inse in Fig. 2b)
42
.
Figu e 2c exhibi s he ime-lapse images o a WO
3
mic omo o ’s
ajec o y a in e als o ∼5 s in pu e wa e and 1% H
2
O
2
(Supplemen a y Videos 1 and 2). WO
3
mic omo o s exhibi
B ownian mo ion wi hou ligh and uel- ee p opulsion when
illumina ed by UV-ligh . A longe ajec o y can be obse ed when
1% H
2
O
2
is in oduced due o he mo e p onounced p oduc
g adien a ound he mic omo o . The co esponding speeds o
hese mic omo o s a e also depic ed in Fig. 2d and Supplemen a y
Fig. 1a, u he indica ing he ligh -con olled on/o mo emen in
pu e wa e and H
2
O
2
.
The mo ion o WO
3
mic omo o s unde di e en condi ions was
u he s udied by calcula ing he mean squa ed displacemen
(MSD). As epo ed in Fig. 2e, he MSD o WO
3
mic omo o s in 1%
H
2
O
2
wi hou UV-ligh i adia ion ollows a linea inc ease wi hin
1 s, indica ing me ely B ownian mo ion. The same is obse ed o
he mic omo o s in pu e wa e (Supplemen a y Fig. 1b). When UV-
ligh is in ol ed, he MSD o mic omo o s in pu e wa e and H
2
O
2
mani es s a pa abolic inc ease wi h ime owing o he ligh -
induced mic omo o s’sel -p opulsion (Fig. 2e and Supplemen a y
Fig. 1b)
32,43,44
. Acco ding o MSD analyses, he di usion coe fi-
cien s (D) o WO
3
mic omo o s we e plo ed and inse ed in
Fig. 2e. Unde UV-ligh i adia ion in pu e wa e , D is up o
2.0 ± 0.1 µm
2
s
−1
, whe eas a en old inc ease is no ed in 1% H
2
O
2
(20 ± 1 µm
2
s
−1
). To s udy he pa ame e s ha egula e he
pho oca aly ic locomo ion o WO
3
mic omo o s, di e en con-
cen a ions o H
2
O
2
we e in oduced, as depic ed in Fig. 2 . The
speed o mic omo o s was p og essi ely inc eased om
5±1μms
−1
o 26 ± 2 μms
−1
as a highe concen a ion o H
2
O
2
(1% H
2
O
2
) was added.
Ni oa oma ic pollu an s deg ada ion
Wa e con amina ion caused by ni oa oma ic compounds poses
se e e en i onmen al haza ds due o hei wide usage o he
manu ac u e o explosi es, pes icides, and pha maceu icals and
hei non-biodeg adable na u e wi h high pe sis ence
2
. Recen ly,
a ious en i onmen ally iendly ea men s ha employ ac i e
p opelled mic o/nanomo o s ha e been demons a ed o he
deg ada ion o abso p ion o ni oa oma ic compounds
6,45–48
.
Ne e heless, mos mic omo o s in ol e a p ima y hal -coa ing
wi h expensi e P o Au o achie e he asymme ical s uc u e o
ligh -ac i a ed p opulsion o chemically-d i en mo ion based on
H
2
O
2
decomposi ion. The e is a s ong desi e o de elop easily
ab ica ed, low-cos , and ecyclable mic omo o s as candida es o
acili a e hei p ac ical applica ions. The e o e, we employed he
single-componen WO
3
mic omo o s o deg ade ni oa oma ic
compounds in wa e . PA and 4-NP (molecula s uc u es in he
inse s in Figs. 3a and 4, espec i ely) we e bo h selec ed as
ep esen a i e ni oa oma ic con aminan s. PA was also employed
o in es iga e he eusabili y and pho odeg ada ion mechanism o
WO
3
mic omo o s.
PA pho oca aly ic deg ada ion capabili y o WO
3
mic omo o s
was es ima ed unde UV-ligh i adia ion up o 2 h in 1% H
2
O
2
.
Figu e 3a shows he UV-Vis spec a o PA solu ions a di e en
eac ion imes. The in ensi y o he abso bance peak a e he
ea men s wi h mic omo o s dec eases wi h ime, sugges ing he
deg ada ion o PA. The deg ada ion e ficiency o mic omo o s was
es ima ed acco ding o Eq. (1)
Deg ada ion E iciency ¼C0C
C0
´100%(1)
whe e C
0
ep esen s he o iginal PA concen a ion, while C
is he
PA concen a ion a he ime . Specifically, he decomposi ion
e ficiency o PA is 72% a e 2 h ea men , as shown in Fig. 3c ( ed
line). Addi ionally, se e al con ol expe imen s we e accomplished
o assess o he con ibu ions o he PA pho oca aly ic deg ada ion
using WO
3
mic omo o s: 1) PA +UV-ligh ; 2) PA +UV-ligh +
H
2
O
2
;3)PA+H
2
O
2
;4)PA+WO
3
+UV-ligh ; 5) PA +WO
3
+H
2
O
2
.
As shown in Supplemen a y Fig. 2a, negligible dec eases in he
peak in ensi y can be obse ed a e 2 h pho odeg ada ion
wi hou mic omo o s. Fo uel- ee mo ion (PA +WO
3
+UV-ligh )
and no mo ion (PA +WO
3
+H
2
O
2
), he deg ada ion e ficiencies
a e me ely 28% and 5%, espec i ely, which a e lowe han he
e ficien mo ion wi h H
2
O
2
(PA +WO
3
+UV-ligh +H
2
O
2
). These
esul s sugges ha UV-ligh and he addi ion o H
2
O
2
a e no
su ficien o deg ade PA. The eusabili y o ma e ials has been
conside ed one o he mos c i ical ac o s o hei p ac ical
applica ions. Figu e 3b shows ha he deg ada ion e ficiency s ill
exhibi s up o 55% a e se e al consecu i e deg ada ion cycles
(Supplemen a y Fig. 2b), which indica es ha hese mic omo o s
p ese ed hei high pho oca aly ic capabili y.
Pho ogene a ed adicals (i.e., h
+
,•OH, O
2-
) a e c ucial o
deg ading o ganic pollu an s. Owing o he low CB posi ion o
WO
3
mic omo o s (E
CB
=0.77 V
NHE
), he educ ion o O
2
(O
2
+e
-
→•O
2-
(aq), −0.33 V
NHE
) canno happen unde ligh
i adia ion
37
. Consequen ly, pho ogene a ed h
+
and •OH a e
conside ed he main adicals ha can b eak down PA. Upon UV-
ligh i adia ion, •OH adicals can be o med in wo pa hs: he
oxida ion o wa e and educ ion o H
2
O
2
adso bed on he su ace
o WO
3
mic omo o s as ollows.
H2Oþhþ!OH þHþ(2)
H2O2þe!OH þOH(3)
A sui able po en ial alue o he educ ion o H
2
O
2
is E
CB
=
0.87 V
NHE49
.WO
3
has a highe E
CB
(0.77 V
NHE
), so his eac ion can
happen due o being ene ge ically a o able. The e o e, he
enhanced pho odeg ada ion e ficiency achie ed by PA +WO
3
+UV-ligh +H
2
O
2
elies on he pho oca aly ic capabili y o WO
3
mic omo o s and also he enhanced adical p oduc ion on he
su ace o WO
3
mic omo o s because o he p esence o H
2
O
2
.In
o de o u he unde s and he deg ada ion mechanism, adical
apping expe imen s we e pe o med
50
. In his ega d, EDTA
(10 mg L
−1
) and isop opanol (0.25 μLmL
−1
) we e chosen as ypical
sca enge s o cap u e h
+
and •OH, espec i ely
51
. As shown in
Fig. 3c, he in ol emen o isop opanol sha ply dec eased he
pho o-induced deg ada ion e ficiency o he mic omo o s,
X. Peng e al.
3
Published in pa ne ship wi h King Fahd Uni e si y o Pe oleum & Mine als npj Clean Wa e (2023) 21
esul ing in 37% (Supplemen a y Fig. 3b). The e o e, he addi ion
o isop opanol dec eased he amoun o ee •OH species,
esul ing in a ma kable dec ease in he PA pho odeg ada ion
e ficiency. On he con a y, he addi ion o EDTA accele a ed he
pho odeg ada ion p ocess, enhancing he pho oac i i y o he
mic omo o s a e 2 h ligh exposu e wi h 1% H
2
O
2
(Supplemen-
a y Fig. 3a). On hese bases, i can be concluded ha he adicals
•OH a e he key pho ogene a ed chemical species esponsible o
PA oxida ion (Fig. 3d).
4-Ni ophenol (4-NP) is a poisonous and bio- ebellious ni oa o-
ma ic con aminan ha can ha m human heal h conside ably
8
.In
his wo k, WO
3
mic omo o s we e also used o deg ade 4-NP.
Ini ially, con ol expe imen s we e un o exclude he in e e ence
o o he ac o s (UV-ligh , H
2
O
2
, and hei combina ion). As
depic ed in Supplemen a y Fig. 4a, no no iceable decline in he
abso bance in ensi y can be obse ed when mic omo o s a e no
in ol ed in con ol expe imen s. Figu e 4demons a es he 4-NP
deg ada ion a e he ea men wi h WO
3
mic omo o s unde
ligh exposu e in wa e ( ee- uel mo ion) and in he p esence o
H
2
O
2
( uel-d i en mo ion) o di e en du a ions. Ob iously,
mic omo o s powe ed by he uel p esen ed an enhanced
deg ada ion ac i i y, esul ing in an e ficiency o 40%, highe
han uel- ee mo ion (11%). This phenomenon can be explained
by he lowe p opulsion speed o mic omo o s in pu e wa e
wi hou any agi a ion.
DISCUSSION
He e, we epo ed he la ge-scale syn hesis o p ecious me al- ee
single-componen WO
3
mic omo o s ia a acile hyd o he mal
me hod, which is easily scalable, combined wi h calcina ion. We
demons a ed ha such WO
3
mic omachines exhibi sel -p opulsion
upon ligh exposu e, e en wi hou H
2
O
2
uel. Specifically, hey
displayed pe cep i e on/o mo ion capabili y wi h ligh exposu e.
F om MSD analyses, i can be p o ed ha mic omo o s showed pu e
B ownian mo ion wi hou ligh , whe eas sel -p opulsion was
achie ed unde asymme ic UV illumina ion. The ac i e mo ion
and pho oca aly ic ac i i y o WO
3
mic omo o s we e applied o
deg ade oxic ni oa oma ic pollu an s, such as PA and 4-NP. The
mic omo o s deg aded 70% o PA and 40% o 4-NP in wa e wi hou
any ex e nal agi a ion. The pho odeg ada ion mechanism was also
in es iga ed h ough adical apping expe imen s, which confi med
ha •OH is he key ROS esponsible o pollu an deg ada ion. The
ob ained esul s indica e he possibili y o using single-componen
300 400 500 600 700 800
2.0 2.5 3.0 3.5 4.0
0
1
2
3
4
[h
]
2
Enengy [eV]
Abso bance [a.u.]
Wa eleng h [nm]
0.0 0.2 0.4 0.6 0.8 1.0
0
10
20
30
40
50
R
2
=0.979
UV-o
UV-on
Linea i
Pa abola i
MSD
[
µm2
]
[s]
R
2
=0.999
0
5
10
15
20
H
2
O
2
D
[µm
2
S
-1
]
Ligh o
Ligh on
Wa e
0 5 10 15 20
0
2
4
6
8
10
12
14
16
18
20
Speed [µmS
-1]
Time [s]
00.11
0
5
10
15
20
25
30
Speed [μm S
-1
]
H2O2 [%]
ab
cd
e
H
2
OH
2
O
2
0 s6 s11 s16 s
0 s6 s11 s16 s
Asymme ical
illumina ion
WO3mic omo o
Sel -p opulsion
H2O
H2O2
O2+ 2H+
·OH + H+
H2O2 + 2H+
2H2O
Fig. 2 Mo ion beha io o WO
3
mic omo o s. a Schema ic illus a ion o he p opulsion mechanism o ligh -powe ed WO
3
mic omo o s.
bUV-Vis spec um o WO
3
mic omo o s. The inse shows he bandgap es ima ion om he co esponding Tauc plo . cTime-lapse images o a
WO
3
mic omo o in 1% H
2
O
2
a in e als o ∼5 s. Scale ba s a e 5 µm. dIns an aneous speed alues and eMSD plo s o WO
3
mic omo o s in
1% H
2
O
2
wi hou /wi h UV-ligh i adia ion. The inse shows he co esponding di usion coe ficien s acco ding o MSD plo s fi ing.
Compa ison o mic omo o s’speed in di e en concen a ions o H
2
O
2
. E o ba s ep esen he s anda d de ia ion, n=5 independen
eplica es.
X. Peng e al.
4
npj Clean Wa e (2023) 21 Published in pa ne ship wi h King Fahd Uni e si y o Pe oleum & Mine als
pho oca aly ic mic omo o s o elimina e non-biodeg adable and
haza dous pollu an s om indus ial sewages.
METHODS
Syn hesis o WO
3
mic omo o s
Fi s ly, 50 mL o deionized (DI) wa e was placed in an 80 mL
beake . Then, 1 mmol Na
2
WO
4
was dissol ed in he p epa ed DI
wa e wi h cons an magne ic s i ing. A e wa d, 25 mmol o
glucose was added o he p epa ed suspension. The final mix u e
was magne ically mixed un il ob aining a homogeneous solu ion.
Then, he solu ion was ans e ed in o an au ocla e. The
au ocla e was sealed and placed in a p ehea ed o en a 200 °C
and las ed o 20 h. When he eac ion ended, he au ocla e was
aken ou om he o en and na u ally cooled down o oom
empe a u e. The p ecipi a ed p oduc was cleaned using DI wa e
and e hanol and d ied in an o en o e nigh . Then, he d ied
p oduc s we e sealed wi h aluminum oil and calcined a 550 °C in
ai . The ob ained g een-colo ed p oduc was collec ed o u he
expe imen s.
Cha ac e iza ion o mic omo o s
SEM images o WO
3
mic omo o s we e acqui ed by a Tescan MIRA
3 XMU ins umen . EDX mapping analysis was conduc ed by an
EDX de ec o (Ox o d Ins umen s) coupled o he SEM. The
chemical s a es o WO
3
mic omo o s we e cha ac e ized by a
K a os Analy ical Axis Sup a ins umen .
Mo ion expe imen s
The mic omo o s’mo ion was eco ded by an in e ed mic o-
scope (Nikon ECLIPSE Ts2R) and a came a (BASLER acA1920-
155uc). Specifically, a 5 μL aqueous suspension con aining
WO
3
mic omo o s was used o mo ion expe imen s. Th ee
concen a ions o H
2
O
2
(Me ck, 30%) we e mixed wi h he
p e ious suspension, achie ing final concen a ions o 0, 0.1, and
1%, o obse e and eco d he ideos o mic omo o s’mo ion wi h
a ame a e o 25 ps. No su ac an s we e in ol ed in all
expe imen s. The mic omo o s we e exposed o a UV-ligh sou ce
(Cool LED pE-100, 1.6 W cm
−2
) wi h a 365 nm wa eleng h. In o de
o obse e he on/o beha io o mic omo o s, he ligh sou ce
was swi ched on/o a defined ime in e als (~5 s). The WO
3
mic omo o s’ eloci ies and ajec o ies we e calcula ed om he
eco ded ideos and acked by he NIS Elemen s Ad anced
Resea ch so wa e. The di usion coe ficien o mic omo o s was
0 20 40 60 80 100 120
0.0
0.2
0.4
0.6
0.8
1.0
noi a necnoC[C
/C
0
]
Time [min]
Mic omo o s
EDTA
Isop opanol
300 350 400 450 500 550
0.0
0.2
0.4
0.6
0.8
1.0
1.2
].u.a[ecnab osbA
Wa eleng h [nm]
0 min
10 min
30 min
60 min
90 min
120 min
12345
0
10
20
30
40
50
60
70
]%[n
o
i a
d
a ge
D
AP
Recycling Times
ab
cd
OH
NO2
NO2
O2N
PA
h
H
2
O
·OH
PA
Deg ada ion
p oduc s
CB
VB
2.72 eV
H
2
O
2
Fig. 3 Pho oca aly ic deg ada ion o PA by WO
3
mic omo o s. a UV-Vis spec a o PA a e pho oca aly ic deg ada ion by WO
3
mic omo o s
exposed o UV-ligh i adia ion in 1% H
2
O
2
;bReusabili y assessmen o WO
3
mic omo o s unde 5 successi e cycles o PA deg ada ion;
cRadical sca enge expe imen s conduc ed by employing EDTA and isop opanol; dP oposed WO
3
pho odeg ada ion mechanism. E o ba s
ep esen he s anda d de ia ion, n=3 independen eplica es.
OH
NO
2
4-NP
0 20 40 60 80 100 120
0.5
0.6
0.7
0.8
0.9
1.0
Concen a ion [C
/C
0
]
Time [min]
4-NP + WO3 + H2O2
4-NP + WO3 + UV-ligh + H2O2
Fig. 4 Pho oca aly ic deg ada ion o 4-NP using WO
3
mic omo-
o s unde UV-ligh i adia ion in 1% H
2
O
2
.E o ba s ep esen he
s anda d de ia ion, n=3 independen eplica es.
X. Peng e al.
5
Published in pa ne ship wi h King Fahd Uni e si y o Pe oleum & Mine als npj Clean Wa e (2023) 21

calcula ed acco ding o he ollowing equa ions based on he
esul s o MSD plo fi ing:
MSD ¼4DΔ (4)
MSD ¼4DΔ þ 2Δ 2(5)
Pollu an s deg ada ion expe imen s
In all expe imen s, 2 mg mL
−1
o mic omo o s, 50 μMaqueous
solu ion o pic ic acid (PA, Me ck, 99%), and 1% H
2
O
2
we e
consecu i ely placed in UV- anspa en cu e es. Once UV-ligh
(356 nm, 9 W) was u ned on, he cu e es we e pu in a closed box
o in o al o 120 min. Con ol expe imen s wi hou UV-ligh
i adia ion o H
2
O
2
we e pe o med o elucida e he con ibu ion o
mic omo o s. A e a di e en ligh exposu e ime, he mix u es we e
cen i uged a a ce ain speed o 3 min o ob ain he final solu ion
wi hou he mic omo o s. A UV-Vis spec opho ome e (Jasco V-750)
was used o measu e he abso bance spec a o he solu ions. The
abso bance peak a 354 nm was conside ed o calcula e he
deg ada ion e ficiency. The deg ada ion o 150 μM 4-ni ophenol
(4-NP, Sigma Ald ich, 99%) ollowed he same p ocedu es as PA.
DATA AVAILABILITY
The da a ha suppo he findings o his s udy a e a ailable om he co esponding
au ho upon easonable eques .
Recei ed: 17 Augus 2022; Accep ed: 20 Feb ua y 2023;
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ACKNOWLEDGEMENTS
M.P. acknowledges he financial suppo o he G an Agency o he Czech Republic
(EXPRO: 19-26896X). X.P. was suppo ed by he China Schola ship Council (CSC No.
202008320382). CzechNanoLab p ojec LM2018110 unded by MEYS CR is g a e ully
acknowledged o he financial suppo o he measu emen s/sample ab ica ion a
CEITEC Nano Resea ch In as uc u e.
AUTHOR CONTRIBUTIONS
X.P. p epa ed, and cha ac e ized he mic omo o s, e alua ed he pe o mance o he
mo ion, conduc ed he deg ada ion expe imen s, and w o e he manusc ip .
M.U. designed he expe imen s and con ibu ed o da a in e p e a ion. X.P. and
M.P. o igina ed he idea. M.U. and M.P. supe ised he esea ch. All au ho s ha e
gi en app o al o he final e sion o he manusc ip .
COMPETING INTERESTS
The au ho s decla e no compe ing in e es s.
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