catalysts
Article
The Spinning Voltage Influence on the Growth of
ZnO-rGO Nanorods for Photocatalytic Degradation of
Methyl Orange Dye
Pierre G. Ramos 1 , Clemente Luyo 1, Luis A. Sánchez 1 , Enrique D. Gomez 2 and
Juan M. Rodriguez 1,*
1 Center for the Development of Advanced Materials and Nanotechnology, Universidad Nacional de
Ingeniería, Av. Túpac Amaru 210, Lima 15333, Peru; pierreramos1990@gmail.com (P.G.R.);
cluyo@uni.edu.pe (C.L.); lasr_uni@hotmail.com (L.A.S.)
2 Department of Materials Science and Engineering, and Materials Research Institute, The Pennsylvania State
University, University Park, PA 16802, USA; egomez9@gmail.com
* Correspondence: jrodriguez@uni.edu.pe




Received: 14 April 2020; Accepted: 4 May 2020; Published: 12 June 2020 

Abstract: In this work, well-designed zinc oxide-reduced graphene oxide (ZnO-rGO) nanorods (NRs)
were synthesized by a hydrothermal method using electrospun ZnO-rGO seed layers. The ZnO-rGO
seed layers were fabricated on fluorine-doped tin oxide (FTO) glass substrates through calcined of
electrospun nanofibers at 400 ◦C in the air for 1 h. The nanofibers were prepared by electrospinning
different spinning voltages and a spinning solution containing zinc acetate, polyvinyl pyrrolidone,
and 0.2 wt% rGO. From a detailed characterization using various analytical techniques, for instance,
X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), Raman spectroscopy,
photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS), the dependence of the structure,
morphology, and optical properties of the ZnO-rGO NRs was demonstrated. The photocatalytic
activities of ZnO-rGO nanorods were evaluated through the degradation of dye methyl orange (MO).
The results show that the change of spinning voltages and the coupling of rGO with ZnO improved
photodecomposition efficiency as compared to pure ZnO. The highest photocatalytic efficiency was
obtained for the ZnO-rGO NRs prepared with a spinning voltage of 40 kV.
Keywords: reduced graphene oxide; ZnO nanorods; photocatalysis; spinning voltage
1. Introduction
Wastewater contaminants that include organic dyes, detergents, and pesticides, which originate
in various industrial fields such as textile, plastic, or agriculture, have become one of the most serious
environmental problems in recent years [1,2]. Various processes have been developed for the treatment
of water contaminated with organic dyes, such as chemical oxidation, adsorption, ion-exchange,
and photocatalytic treatments [3]. From these techniques, adsorption remains as the main technology
used for these purposes [4–6]. However, this treatment method is less effective to cleanse organic
pollutants at very low concentrations [7]. In contrast, photocatalysis which is a promising, simple,
and eco-friendly technology, has been attractive to treat low-concentration organic contaminants
in recent years [8]. So far, various semiconductor photocatalysts, such as ZnO, TiO2, CuO, Fe2O3,
and WO3, have demonstrated promising photocatalytic activity [9–11], highlighting particularly zinc
oxide due to its lower cost, non-toxicity, and size-tunable physicochemical properties [11,12].
Currently, the key challenge for improving the photocatalytic dye degradation efficiency of
ZnO is to inhibit the recombination of photogenerated charge carriers (electron and holes). Thus,
strategies like nanostructuring, doping, and formation of nanocomposites have been adopted to achieve
Catalysts 2020, 10, 660; doi:10.3390/catal10060660 www.mdpi.com/journal/catalysts
Catalysts 2020, 10, 660 2 of 15
this improvement [13,14]; likewise, the photocatalytic activity of the photocatalyst can also be improved
by enlarging its specific surface area through morphological modification [15]. Among the different
types of nanostructures [16], 1D nanostructures like nanorods have received much attention due to
their superior photocatalytic property [17,18]. In particular, these nanorods can be fabricated through
hydrothermal-assisted electrospinning, as previously reported [19–21], which involves depositing a
ZnO seed layer by electrospinning on a substrate and then growing ZnO nanorods. This method can
be considered as an effective and easy method for fabrication of nanostructures.
Additionally, the formation of a nanocomposite with graphene-based materials improves the
photocatalytic activity of ZnO due to more efficient charge separation. Specifically, reduced graphene
oxide (rGO), offers new opportunities in photocatalysis as proved by several studies [22–24].
In addition, our previous work [25] demonstrated the influence of rGO amount on the fabrication
and photocatalytic activity of zinc oxide-reduced graphene oxide (ZnO-rGO) nanorods, finding an
optimal rGO content of 0.2 wt%. Thereby, in this work using this optimized amount of rGO,
ZnO-rGO nanorods were synthesized on fluorine-doped tin oxide (FTO) glass substrates by
low-temperature hydrothermal growth and electrospinning deposition applying different spinning
voltages. Furthermore, we determined the effect of spinning voltages applied and rGO content on the
morphology, crystallinity, optical and photocatalytic properties of the obtained ZnO-rGO nanorods.
Detailed morphological, structural, and optical characterization of ZnO and ZnO-rGO nanorods
were investigated by field emission scanning electron microscopy (FE-SEM), X-ray diffraction
(XRD), Raman spectroscopy, and photoluminescence (PL). Surface properties and chemical
composition of ZnO-rGO nanorods were analyzed by X-ray photoelectron spectroscopy (XPS).
Finally, the photocatalytic methyl orange degradation performances of the ZnO-rGO nanorods
(NRs) were studied.
2. Results and Discussion
2.1. SEM Analysis
Figure 1b–d shows the FE-SEM images of ZnO-rGO seed layers prepared by electrospinning using
a spinning solution with 0.2 wt% rGO and applying different spinning voltages. The FE-SEM image
of pristine ZnO seed layer is shown in Figure 1a. The regions enclosed by white lines indicated in
Figure 1b–d confirm that ZnO nanoparticles were attached and grown on the surface of rGO sheets,
due to covalent coupling/physisorption with the functional groups present in the reduced graphene
oxide (-COOH,-OH and =O) [26,27], which play an important role in the formation of ZnO-rGO seeds
by providing active sites for ZnO nanoparticles to nucleate and grow [28–30]. In addition, it can be
seen from Figure 1 that the increase in the spinning voltage during electrospinning causes a decrease
in the average size of nanoparticles that form the ZnO-rGO seeds compared to the size of particles
that make up the pristine ZnO seed layers (20 ± 3 nm), in the following order: 13 ± 2 nm, 11 ± 1 nm,
and 10 ± 1 nm for ZnO-rGO seed layers fabricated with a spinning voltage of 20 kV, 30 kV, and 40 kV,
respectively. This result could be due to two reasons: (i) the presence of rGO in the spinning solution,
which increased its electrical conductivity [31] and (ii) the increase of the spinning voltage [32].
Previous works have shown that these two reasons produce a decrease in the size of nanoparticles
that make up the fabricated nanostructures by electrospinning [33,34].
The FE-SEM images for the pure ZnO and ZnO-rGO NRs obtained by a wet chemical method
are shown in Figure 1e–h. The higher magnification FE-SEM images are shown as an inset in the
lower left corner of Figure 1e–h and indicate that nanorods come together in a flower-like architecture.
Furthermore, the coupling of rGO in the seed layers produces defective hexagonal nanorods grown in
random orientations as demonstrated in previous work [25]. When only ZnO layers were deposited on
FTO, the diameters of the pure ZnO nanorods obtained (Z20) were in the range 42–58 nm. The adhesion
of the rGO and the spinning voltage increase produce a decrease in the dispersion of diameter size
Catalysts 2020, 10, x FOR PEER REVIEW 3 of 16 
Claaytaelyrst sw20e2r0e,  1d0,e6p6o0sited on FTO, the diameters of the pure ZnO nanorods obtained (Z20) were i3no tfh1e5 
range 42–58 nm. The adhesion of the rGO and the spinning voltage increase produce a decrease in 
the dispersion of diameter size and the mean diameter of the ZnO-rGO nanorods, with almost similar 
asinzdest.h Tehme eaavnerdaigaem seizteer oof fththee ZZnnOO-r-GrGOO NnRasn woraosd: s3,4w ±i t6h namlm, 3o2s t± s3i mnmila, rasnidze 2s9. ±T h3 enamv eforar gZeGs2iz0e, ZoGf t3h0e, 
ZanndO Z-rGG4O0,N reRsspwecatisv: e3l4y.± 6 nm, 32 ± 3 nm, and 29 ± 3 nm for ZG20, ZG30, and ZG40, respectively.
 
FFiiggurree 11.. FFieieldld ememisissisoionn scsacnaninign geleecletrcotrno nmimcriocrsocoscpoyp (yFE(F-SEE-SME)M im) iamgeasg oesf soefesde leadyelarsy e(ar–sd(a) –adn)da tnodp 
tvoiepwv ioewf noafnnoarondosro (des–h(e)– lha)blealbedel eads aZs2Z0 2(0a,(ea),,e )Z, GZ2G02 0(b(,bf,)f, ),ZZGG303 0(c(c,g,g),) ,aanndd ZG4400 ((d,,h)).. Thee ssiizzee 
diissttrriibuttiionss and corrrresspondiing ssttandarrd deviiattiionss ((SD)) arre sshown aass iinsseettss.. 
 
Catalysts 2020, 10, 660 4 of 15
Catalysts 2020, 10, x FOR PEER REVIEW 4 of 16 
22..22.. XRD Annaallyyssiiss 
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pthaet tderifnfroafctrieodnu pceadttegrrna pohf reendeuocxeidd egrisapshhoenwen oaxsidaen iisn ssheot winnt haes atonp inlesfett cionr tnheer toofpF liegfut rceor2n. eTrh oifs Fimigaugree 
r2e. vTehailss itmwaogme areinvepaelask tswcoo rmreasipno pnedaiknsg ctoorrrGesOpoanscdriinbge dtot orGthOe iars(c1r0i0b)edan tdo (t0h0e2ir) p(1la0n0)e san[3d6 ](,0w02h) ipchlanaeres 
n[3o6t],d wisthinicghu aisrhee ndoitn dthisetidnigffuriaschteiodn ipna tthteer ndsifofrfaZctnioOn- rpGaOtteNrnRss mof aZinnlOy -druGeOt oNthRes dmeastirnulyc tidouneo tfor GthOe 
sdheesetrtuscdtiuornin ogf trhGeOf osrhmeeattsi odnuroifntgh tehne afnoormstrauticotnu roef atnhed ntoantohsetrluimctiutered aanmdo tuon tthoef lirmGiOtedu saemdo[u37n,t3 8o]f. 
OrGthOe rusdeidffr [a3c7t,i3o8n].p Oeathkesro dfiifmfrpacutrioitny ppehaaksse sowf iemrepunroittyd eptheactseeds ,winedreic naotitn dgettheectpehda, sinedpiucaretinfogr mthaet piohnasoef 
ZpunrOe- froGrOmaNtiRosn. of ZnO-rGO NRs. 
 
Fiigurre 22.. XX-r-arayyd idffirfafrcaticotnio(nX R(XDR) Dpa) ttpearnttseornf s( a)ofZ n(aO) nZanOor ondasn(oNroRds)s a(nNdRzsin) caonxdid zei-nrecd uoxceideg-rreadpuhceende 
ogxraidpehe(Znne Oox-riGdeO ()ZNnROs-rgGroOw) nNfRros mgrsoewedn lfaroymer ssefaebdr ilcaayteerds bfayberliecacttreods pbiyn enliencgtrwosipthin0n.2inwgt w%itrhG O0.2i nwtth%e 
srGpiOnn iin gthseo lsuptiinoniunsgi nsgolduitffioerne nutsisnpgin dniifnfgerveonltt asgpeins:n(ibn)g2 v0oklVta,g(ce)s:3 0(bk)V 2,0a nkdV(, d(c))4 030k Vk.VT, haenXdR (Dd) p4a0t tkeVrn. 
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sduobpsetdra ttiena orxeimdea r(FkTedOb) ysu“bs”t.rate are marked by “♦”. 
22..33.. RRaammaann AAnnaallyyssiiss 
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ppeeaakk aatt aarro
−
ouunndd 443388 ccmm−1 iinn ZZnnOO RRaammaann ssppeeccttrruumm ccoorrrreessppoonnddss ttoo EE2 ((hhiigghh)) mmooddee aanndd iiss rreellaated to
2 1 ted to 
vibra − −1
vibrattiioonnss oofft htheeo xoyxgyegnensu sbulbatltaitcteicien iZnn ZOn[O39 []3, w9]h, ewrehaesresaecso snedcoonrdde orrpdeeark ps eaat k3s3 1act m331 acnmd−11 1a4n2dc 1m142, 
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1 −
a3t 4183,4185, 8175,827,7 1247,14a,n adn2d9 20990c9m cm−1w whhicihcha arerea assccrribibeeddt toot thhee DD,, GG,, 22DD,, aanndd DD++GG bbaannddss,, rreessppeeccttiivveellyy.. 
TThhee DD bbaannd 3 2
d rreepprreesseennttss tthhee sspp3 ccaarrbboonn ddeeffeeccttss [[4422]],, wwhhiillee tthhee GG bbaanndd ccoorrrreessppoonnddss ttoo tthhee oorrddeerreedd sspp2 
ccaarrbboonn nneettwwoorrkk [[4433]].. TThhee 22DDb baannddc acannb eber erlealtaetdedto toth tehsee cseocnodnodr doerdr emr omdoedoef tohfe tDheb Dan bda[n4d4 ][4a4n]d atnhde 
Dth+e GD+bGan bdarnedp rreespernetssenants easnti emstaitmioantioofnt hofe tdhies odridsoerrd[4er2 ][.4R2]a. mRaamn aspne scptreactorfa Zonf OZn-rOG-OrGNOR Ns Robs toabintaeidnebdy 
by electrospinning using different voltages show bands related to ZnO around at 438 cm−1 (Figure 
3b) and related to reduced graphene oxide at ~1366 cm−1 and ~1593 cm−1 for D and G band, 
 
Catalysts 2020, 10, 660 5 of 15
eClaetacltyrsotss 2p0i2n0,n 1i0n, gx FuOsRin PgEEdRiff ReErVeInEtWv oltages show bands related to ZnO around at 438 cm−1 (Figur5e o3f b16) 
and related to reduced graphene oxide at ~1366 cm−1 and ~1593 cm−1 for D and G band, respectively,
wrehsipcehcstiuvgeglye,s wtshthicehp seurgmgaenstesn tcheeo pfetrhme acanrebnocnea ocfe tohues cmarabtoerniacleinouths emZanteOri-arGl iOn tnhaen ZonroOd-sr.GFOu rnthaneromroodrse., 
aFurerdth-sehrmiftoirseo, bas reerdve-sdhinft tihse oGbsaenrvdeDd bina ntdhse tGha atncodn Dfir mbanthdes ftohramt actoionnfiromf C t-hOe- Zfonrmlinaktiaogne o[4f 5C] -iOn -tZhne  
ZlinkOa-grGe O[45n]a ino trhoed Zs ndOue-rtGoOth neainoteroradcst idouneb teot wtheee ninthereaoctxiiodne bsetmwieceon dthuec tooxridane dsetmheiccoanrdbuonctaocre aonuds  
mthaet ecrairabl.oTnhaceeionutes nmsiatyterraiatilo. TofhDe ibnatnends(iItDy )rtaotiGo boaf nDd (bIGan) dco (rIrDe)l attoe sGw bitahntdh e(IdGi)s ocordrreerlaetveasl uwaittiho nthine 
cdairsboordnearc eovualsumataiotenr inal csa[r4b6o]n. aTcheeouvsa lmueasteorfiaIlDs /[I4G6]a.r Teh0e.8 v4a,l1u.e0s1 o, 1f .I0D6/I,Ga anrde 01.8040, f1o.r01r,G 1O.0,6Z, aGn2d0 1, .Z0G0 f3o0r, 
arGndOZ, ZGG4020s,a ZmGp3l0e,s ,arneds pZeGct4i0v eslaym. Tphlees,l arergsperecItDiv/IeGlyr.a Ttihoeo lfaZrgneOr -IrDG/IGO rNatRios ocfo ZmnpOa-rreGdOto NrRGsO cosumgpgaersetds  
thoe rcGreOa tisoungogfesmtso rtehed ecfreecatstiionnt hoef ZmnoOr-er GdOefeNcRtss ionb ttahien eZdnbOy-hrGydOr oNthResr moabltatrienaetdm ebnyt hcayudsreodthbeyrmthael 
ptrreastemnceento fcazuinscedo xbiyd eth[e4 6p,4re7s]e. nce of zinc oxide [46,47]. 
 
FFiigguurree 33.. ((aa)) RRaamaann ssppeeccttrraa ooff rrGO,, ZZnnO NRRss,, aanndd tthhee ZZnnO--rrGO ((ZZG2200,, ZZG3300,, ZZG4400)) NRRss;; aanndd ((bb)) 
eennllaarrggeedd RRaamaann ssppeeccttrruumss aat t442200–4–6406 0cmcm−1 −o1f oZfnZOn-rOG-OrG (OZG(Z20G, 2Z0G, 3Z0G, Z30G,4Z0)G N40R)sN gRroswgnr ofrwonm fsreoemd 
slaeyederlsa yfaebrsrifcaabterdic abteyd ebleycetrloecsptrionsnpiinngn iwngithw i0t.h2 0w.2t%w t%rGrOG Oini nthteh essppininnniningg ssoolluuttioionn uussiinngg ddiiffffeerreenntt 
ssppiinnnniinngg vvoollttaaggeess.. 
2.4. PL Analysis
2.4. PL Analysis 
Measuring photoluminescence (PL) is an effective means of investigating the separation process
- Measuring photoluminescence (PL) is an effective means of investigating the separation process 
of e and h+ pairs in semiconductors. Figure 4a shows the PL spectra with a 325 nm excitation
of e- and h+ pairs in semiconductors. Figure 4a shows the PL spectra with a 325 nm excitation 
wavelength of pure ZnO and ZnO-rGO NRs, fabricated by electrospinning using different spinning
wavelength of pure ZnO and ZnO-rGO NRs, fabricated by electrospinning using different spinning 
voltages and a spinning solution with 0.2 wt% rGO. It was found that all samples have the same
voltages and a spinning solution with 0.2 wt% rGO. It was found that all samples have the same 
emission shape with three emission peaks located around 385, 600, and 760 nm. Enlarged PL
emission shape with three emission peaks located around 385, 600, and 760 nm. Enlarged PL 
spectrums of ZnO and ZnO-rGO NRs in the range at 370–400 nm wavelengths are shown in Figure 4b.
spectrums of ZnO and ZnO-rGO NRs in the range at 370–400 nm wavelengths are shown in Figure 
Near-band edge (NBE) emission around 385 nm (shown in Figure 4b) is attributed to the free exciton
4b. Near-band edge (NBE) emission around 385 nm (shown in Figure 4b) is attributed to the free 
recombination process [48]. Furthermore, a blue-shift in this peak was observed for ZnO-rGO
exciton recombination process [48]. Furthermore, a blue-shift in this peak was observed for ZnO-rGO 
NRs compared to ZnO NRs. These shifts as demonstrated by Jayalakshmi et al. [49] are due to
NRs compared to ZnO NRs. These shifts as demonstrated by Jayalakshmi et al. [49] are due to the 
the presence of reduced graphene oxide in the fabricated ZnO-rGO NRs. The emission peaks in
presence of reduced graphene oxide in the fabricated ZnO-rGO NRs. The emission peaks in the 
tvhiesibvlies ibrelegiroeng io(~n6(0~06 0n0mn) ma)ndan tdheth neenaera-irn-ifnrafrraerde drergeigoino n(~(7~6706 0nnmm) )wweerree aassssiiggnneedd rreessppeeccttiivveellyy ttoo 
ssttrruuccttuurraall//iinnttrriinnssiicc ddeeffeeccttssa annddi mimppuurirtiiteisesi nint htehes tsrturcutcutruerse[s5 [05,501,5]1, ]a,n adndth tehsee csoecnodn-odr-doerdr edri ffdriaffcrtaiocntioonf  
tohfe thNeB NEBeEm eimssiisosniobna bnadn[d5 2[5].2]M. Moroeroevoevre,rt,h tehesp sipninninnigngv volotlatgagesesu useseddf oforrt htheef faabbrriiccaattiioonn ooff ZZnnOO--rrGGOO 
NNRRss aanndd tthhee pprreesseennccee ooff rrGGOO iinn tthhee ssaammpplleess pprroodduucceedd aa ssiiggnniifificcaanntt ddeeccrreeaassee iinn iinntteennssiittiieess ooff ZZnnOO 
eemmiissssiioonn ppeeaakkssa assf ofolllolowws,sI, ZInO > > IIZG20 >
ZnO ZG20 > IIZG3 0>> I IZG,4 0w, hwihchic his isasaZG30 ZG40 ssoscoicaitaetded wwitihth ththee ddeeccrreeaassiinngg iinn tthhee 
ddiiaammeetteerrss ooff NNRRss ((sseeee FFiigguurree 11)) dduuee toto ththee inincrcereaases eofo fspsipninninnign gvovlotaltgaegse asnadn dthteh perpesreensecne coef orGf rOG Oin 
itnhet hneannaonstorsutcrutucrteusr e[s53[5,534,5].4 ]T. hTish isdedcreecarseea sien inPLP Linintetnesnistyit yininddiciacatetse saa rreedduuccttiioonn ooff eelleeccttrroonn--hhoollee 
rreeccoommbbiinnaattiioonn iinn tthhee ZZnnOO--rrGGOO ssaammpplleess aanndd eennhhaanncceemmeenntt ooff cchhaarrggee ttrraannssffeerr aatt tthhee iinntteerrffaaccee [[5522,,5544]],, 
wwhhiicchh ccaann iimmpprroovvee tthhee pphhoottooccaattaallyyttiicc aaccttiivviittyy ooff tthhee oobbttaaiinneeddn naannoorrooddss.. 
 
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FigFuigreur4e. 4. (a(a) ) PPhhoototolulumiinescence (PL)) speeccttrraa aanndd (b(b) ) eennlalargrgeded PPLL spsepcetcrutrmums sat at37307–04–040 00nmn m
wawFvieaglvueenrlege nt4hg.s th(osaf )o ZfP nZhOnoOt(oZ (lu2Z0m2)0ia)n naendscdZe ZnncnOeO -r-(rGPGLOO) ( (sZpGec2t0r,a,  ZaGn3d0 , (ZbG) 4e00n)) lNarRgRses dgg rorPowLwn n sfprfoermoctmr suesmeedse  dlaalyta ey3res7 r0fsa–b4far0ib0cr aintcemadt e d
bwya veleelcetnrgotshpsi nonf iZnngO w (iZth2 00).2 and ZnO-rGO (ZG20, ZG30, ZG40) NRs grown from seed layers fabricated 
by electrospinni g with 0.2 w wt%t%r GrGOOi nint hthees sppiinniing solution ussiingg ddififffeerreennt tssppini nniningg vovlotaltgaegse. s.
by electrospinning with 0.2 wt% rGO in the spinning solution using different spinning voltages. 
2.52. .5X. PXSPAS nAanlyalsyissis 
2.5. XPS Analysis 
ThTehceh echmeimcailcaclo mcopmopsoitsiiotnioonn otnh ethseu srufarcfaecaen adndth tehset astteatoef oefl eemleemnetns tps rpesresnetnotn oZn nZOnOan adndZ nZOn-OrG- O
NRrGs hOa TNvheRebs c ehheaenvmeci hcbaaelre acnco tcmehrpairozasecidttieobrniyz eoXdn-  rbtahyye X ps-uhrarofyta ocpeeh laeonctotdre oltenhcets rpsotenact estrp ooefsc cteorloepsmycoe(npXtyPs  S(pX)r.PeFSsie)g.n uFt irogenu5 rZae ns5Oha o sawhnodsw tZhsn etOhXe-P S
surXrvGPeOSy  sNsuprRvesce thyr aasvpefeo cbrterteahn fe ocroh tbahrteaa ciontbeetrdaizineedad nb noyar oXndo-rrsaoyad nsp dhanordteo vreelevaceltsarloosn no lnsyplyet hctthereop sprcreoespseeyn (cXeP oSff) z.z iFninicgc u(Z(rZen )n5, )ao, xsoyhxgoyewgnse ( ntOh)(e,O  ),
aXnPdS  csaurrbvoeny  s(Cpe) cetlream foern tths.e I onb atdaidnietido nn,a nito craond sb ea nsdee rne vtheaalts t ohnel ysp tehcet rpar eosfe tnhcee  Zonf zOi nacn (dZ nZ)n, Oox-ryGgOen (O), 
and  NRs 
caonnd
c acrbon (
taianr bCo1ns (
C) el
 (C28) 4e.lements.. InIn adadidtiiotino, nit, citanc abne bseense tehnatt hthaet stphectsrpae ctra of the ZnO and ZnO-rGO
NRs contain C1s (5238 4e.V53), eOV1)s, (O5219s.3(35 2eV9.)3, 3anedV )Z,na2npd (1Z0n202.p53( 1e0V2)0 p.5e3akosf . tThhe ZnO and ZnO-rGO NRs 
contain C1s (284.53 eV), O1s (529.33 eV), and Zn2p (1020.53 eV) peakesV. T) hp
e
ee
 aCk1s. peTahke aCls1os found in 
 C1s peak also fopeak also
fouZn2d0 ifnullZ 2sp0efcutrlul msp eisc tbruecmauisse btehcea upsueret hZenOpu NreRZs naOre NexRpsosaerde etox ptohsee datmtoosthpheeartem wohsperhee cuanrdb oinn  
cZo2n0t afmuliln saptieocntr ucamn  itsa kbee cpaulascee  t[h5e5 ]p. uFrieg uZrne O5 bN Rprse saerne tse xtphoes ehdig tho- rtehseo luattmionos pZhne2rpe w re w
 XhePrSe  scpaerbohne re
carcboonntacmonintaatmioinn actaionn tcaakne tpaklaecep la[5c5e].[ 5F5i]g.uFrieg u5rbe 5pbrepserenstse ntthset hheighhig-rhe-sroelsuotliuonti oZnnZ2n23/2
p p 3X/2PXS PsSpsepctercat ra
memaseuasreudred 3/2 ctra 
measurefdo r
for
foarl
 
 l
aZll nZOnO
all ZnO-r
-GrGOON NR
-rGO NRRs.s. Th
s.T Thhe
eo obtai
e obbttaaiin
ne
need
d r
d rre
essuullttss are consistent with the oxidation state of Zn2+ 2+
in ZnO [56,57]. A remarkable shift of ZnO-rGOe sNulRtss  a
arree ccoonnssiisstteenntt wwitihth ththee ooxxididataitoino nstsattaet oefof Z2n+
in ZnO [56,57]. can be seen in Zn2p3/2 region (Figure 5b Z) nin  
cino mZpnaOr is[5o6n, 5w7]A. Ar eremmaarrkkaabbllee sshhiifftt ooff ZZnnOO--rrGGOO NNRRss ccaann bbee sseeeenn inin ZZnn22pp3/23 /r2ergeigonion (Figure 5b) in
com ith the pure ZnO NRs, caused by a strong interaction between zinc oxid e(F aingudr ree d5ub)c eidn  
gcopmapriasroisnown iwthithth tehep upruereZ ZnnOON NRRss, ,c caauusseedd bbyy aa ssttrroonngg iinntteerraaccttioionn bbetween zinc oxide and reduced
graprahpehneenoex oidxied[e5 [85,589,5]9. ].T This interaction produces a good separatione twofe tehne z pinhco otox-iidned auncded r ecdhuacregde  
cgarrarpiehrsn. eT houxisd, eth [e5 p8,e5r9f]o. rT
h
mhisains ici
n
en
tterraccttiion prroducceess aa ggoooodd sseeppaararatitoionn ofo fththe epphohtoot-oin-idnudcuecde dchcahrgaer ge
carcraierris. Thus, t  of the photocatalyst was enhanced [58,59]. 
ers. Thus,h tehep eprefrofromrmaanncceeo offt htheep phhoottocatalyst was enhaanncceedd [5[588,5,599].] .
 
 
Figure 5. (a) X-ray photoelectron spectroscopy (XPS) survey spectra and (b) Zn2p3/2 spectra of Z20, 
FiguFrigeu5r.e( 5a.) (Xa)- rXa-yrapyh pohtoteolelcetcrtornonsZ psGpe2ec0ctrt, orZosGscco3op0p,y ya (n(XdP ZSG) s4u0r nvaeenyyo srspopedecsct.rt ara anandd (b()b Z) nZ2np23p/2 3s/p2escptreac torfa Zo2f0Z, 20,
ZG20, ZG30, and ZG40 nanorods. ZG20, ZG30, and ZG40 nanorods. 
The high-resolution scan of C1s (left side) and O1s (right side) for ZnO-rGO NRs is shown in 
FigTuhrTeeh h6ei.  ghFhiigr-hrset-,rs eotshlouel tuCitoi1nosn sX csPacanSn os opffeC cCt1r1sus (m(lle,e fftatc ssqiiudier)e dan fdr oOm1 s2 (8(rr1ii ggthoht t 2ss9idi2de ee) )Vfof,o rw rZaZnsnO dO-re-GcroGOnO vNoNRlusRt eissd i sihnsothowo nfwi vine  in
FigcFuhigraeurar6ec.t e6rF. iisFrtisircts ,pt,te htahekesC ,C l1a1sbse XXlePPdSS a sssp pCeecc,t tCru,m C, , aCcq, uainirreded dC ffr.r oTomhme  2p28e81a1 kto tao t2 a29r92o 2ueVenVd, , w2w8asa sddeceocnovnovloultuedte dintion tfoivfie ve
1 2 3 4 5 2.4 eV (C1) corresponds to 
chatchrhaeac rCtae-crOties-rtZiicsntp icbe opanekdask, [ls5a,8 bl]ae.b lTedhleeda sb aiCsn 1dC,i1nC, gC2 ,2e,C nC3e3,r, gCy44 ,,a atn adr oCu55n.. TdTh h2e8e p4p.e8ea akek Va ta( taCraor) uocunondrrd 2e8s2p28.o24n. 4edVesV  (tCo( 1Ct)h 1ce)o crCroe-rCsrpe osop
2 fn ordGns dOtos,  to
thethCe- OC--OZ-nZbno bnodnd[5 [85]8.].T Thheeb bininddiningg eenneerrggyy aatt aarroouunnd 228844..88 eeVV ((CC2)2 )cocrorrersepsopnodnsd tso ttoheth Ce-C- oCf orGf rOG, O,
 
 
Catalysts 2020, 10, 660 7 of 15
Catalysts 2020, 10, x FOR PEER REVIEW 7 of 16 
whwilhe itlhe ethoeth oetrhethr rteherepee pakeasklos claotceadteadp apprporxoimximatealtyelayt a2t8 268.46,.42,8 288.85.,5a, nadnd2 92292e VeVa raerea sassisgingnededt otoC C-O-O/C/C-O- H
(C3O),HC (=CO
3),( CC4=)O,  a(nCd O-C=
4), and OO-HC=(OCH5) (fCunct
5) fuionnctailognraol ugprosu, rpess,p reescptievcetliyve[l6y0 ,[6610],6. 1T]h. TehXeP XSPhSi ghhig-rhe-sroesluoltuiotnioOn 1s
speOc1trsa sapreecstrhao warne isnhFoiwgun rein6 F(rigiguhrte s6id (er)i,gwhth iscihdew), ewrehaicchq uwireerde fraocmqui5r2e5d tofro53m9 e5V25a ntod 5d3e9c oenVv oalnudt ed
intdoefcoounrvooxluytgeedn insptoe cfioeusr,  doexnyogteend sapseOcie1s, ,O d2e,nOo3te, dan ads OO4., OTh,e Obi,n adnindg Oen. eTrhgey bpie1 2 3 4 nadkins go betnaeinrgeyd paeta5k2s9 .7,
531 2-
o.b0t,a5in33ed.6 ,aat n5d295.73,6 5.731e.V0,, c5o33rr.6e,s panodn d5i3n6g.7t oeVO, coirornesspoofntdhiengZ ntoO Ola2- titoicnes (oOf 1t)h[e6 2Z]n, Oox lyagtteicne d(Oefi1)c i[e6n2c],i es
lattoixcyeginenZ dneOfic(Oien2)c[ie4s8 ]la, totxicyeg ienn ZsnpOec (iOes2)c [h4e8m], iosxoyrbgeedn sopnetchieess cuhrefamcieso(Orb3e)d[ 6o3n] ,tahne dsuorxfyacgee n(Oin3) t[h63e]O, a-nCd= O
bonoxdy(gOen4) i[n5 t7h,6e4 O],-rCe=sOpe bcotinvde l(yO. 4) [57,64], respectively. 
 
FigFuigreur6e. 6H. Higihg-hr-erseosloultuitoinonC C11ss( l(elefftts siiddee)) and O1s (rightt ssiiddee)) ssppeecctrturumm ofo (fa()a Z) 2Z02, 0(,b()b Z)GZ2G0,2 (0c,)( Zc)GZ3G0,3 0,
andan(d )(dZ)G Z4G04n0a nnaonrordosd.s. 
 
Catalysts 2020, 10, 660 8 of 15
2.6. Photocatalytic Decolorization of Methyl Orange
The extent of adsorption depends on the physicochemical characteristics of the catalyst and
pollutant [65]. Hence, methyl orange (MO) dye adsorption experiment was carried out in the presence
of catalyst and the absence of light. This experiment showed no significant adsorption of MO onto the
catalyst. In addition, as a reference we applied light irradiation on methyl orange in the absence of a
photocatalyst; no degradation of dye was observed, corroborating that the degradation was effectively
driven by a photocatalytic process. The photocatalytic dye degradation performances of the ZnO and
ZnO-rGO NRs were evaluated in aqueous solution of methyl orange dye (5 ppm) under UV light
irradiation, and the results are plotted in Figure 7a. Figure 7a shows the change in the methyl orange
concentration in aqueous solution in the presence of ZnO-rGO NRs as a function of irradiation time.
C is the concentration of MO at the irradiation time (t) and C0 represents the initial concentration
of MO. The results indicate that ZnO-rGO NRs synthesized by electrospinning, applying different
spinning voltages and a spinning solution with 0.2 wt% rGO can improve the photocatalytic activity of
single ZnO NRs. The ZG40 photocatalyst shows the highest photocatalytic activity compared with
Z20, ZG20, and ZG30 photocatalysts. The degradation efficiency of the photocatalyst based on ZG40
NRs shows a maximum degradation of ~99% at 6 h, whereas the degradation efficiency of Z20, ZG20,
and ZG30 is ~77%, ~95%, and ~97% at 7 h of irradiation time, respectively. The enhancement of
photocatalytic performance of ZnO-rGO NRs as compared to pristine ZnO NRs is mainly attributed
to the fact that the photogenerated electrons from ZnO excited by a light source are trapped by the
reduced graphene oxide, avoiding recombination of electron-hole pairs [66], and also to the increase
in specific surface area that in turn relies on the decrease in particle size (see Figure 1), caused by
the increase of voltage during electrospinning [67,68]. According to the XPS analysis, it can be seen
that the peak’s intensity at 288.5 eV (C3) of ZnO-rGO NR photocatalysts is much stronger than that
of pure ZnO, indicating the significant increase of hydroxy groups on the surface of ZnO-rGO NRs,
compared with pure ZnO NRs. The hydroxy groups can act as adsorption centers on which the
degradation of pollutants takes place and are helpful to enhance the photocatalytic activity of ZnO-rGO
NRs [69,70]. Moreover, a high amount of oxygen vacancies was found on the surface of the ZG40
sample. The presence of oxygen-deficient centers on the surface can reduce the rate of electron-hole
pair recombination [69–71]. Thereby, the highest photocatalytic activity was obtained for the ZnO-rGO
NRs fCaabtarlyisctas 2te02d0, u10s,i xn FgO4R0 PkEEVR oRfEVsIpEiWn ning voltage as was demonstrated by the photocatalytic ex9p oef 1r6im ent.
 
FigurFeig7u.r(ea 7).P (ah)o Ptohdoteogdreagdradtiaotinonc ucruvrveesso off methyl orraannggee wwithit hdidffieffrenret nZtnZOn-rOG-OrG NOR NphRotpohcaottaolycsattsa. lysts.
(b) K(ibn)e Kticinseptilcost pslcoatsl ccuallcauteladtefdro fmrom(a ()ao) foZf ZnnOO-r-rGGO NR phoottooccaatatalylsytsst. s.
The MThOe idmepgrroavdeamtieonnt pinr othcee spshwotaoscafutarltyhteicr dinevgreasdtiagtaiotned efbfiycitehnecyfi rosft t-hoer dnearnroeraocdtsi ofanbkriicnaetetidc imn othdies l [72].
The rwatoerke xisp rreelsasteiodn toi st:hlen i(nCcre/aCse )in= spkitn, nwinhge rveolCtagiess tahned itnhiet iraGl Oab asdohrebraennccee ,inCto
0 t 0 t i sthteh ZenaOb-sroGrOba snecede after
time lta,yaenrsd. Tkhise pthoessfiibrlset p-ohrodtoecrartaaltyeticco dnesgtraandta.tAionp mloetcbheantwismee nof lMn(OC is/ pCr)opaonsdedr efoarc tZionOn -triGmOe NisRssh, oasw n in
illustrated in Figure 8. When the ZnO nanostructure is irradiated 0from the light source, the excited 
Figureele7cbtr.oTnhs e(eestimated degradation rate constants for the Z20, ZG20, ZG30, and ZG40 nanorods were
−) migrate from the valence band (VB) of ZnO to its conduction band (CB) with generation 
simultaneous to the same number of holes (h+) in the VB (Equation (2)). The rGO sheets receive these 
photo-excited electrons from the conduction band of ZnO nanorods and act as an electron transporter 
phase hindering the electron-hole recombination [55]. This is mainly because the energy levels of rGO 
and ZnO are different from each other. The value of the work function of rGO is −4.42 eV, which is 
lower than the value of −4.05 eV obtained for the ZnO conduction band [67]. The separated e−–h+ pairs 
react with oxygen molecules and with the surface absorbed water molecules as described in 
Equations (1)–(7), producing the strongly oxidizing hydroxyl radical • OH  and oxygen radical 
anion O• , which react with the MO dye molecules in solution and degrade them [73,74]. Based on 
these results we conclude that the variation of spinning voltages and the presence of rGO enhance 
the photocatalytic activity performance of pure ZnO nanorods, suppressing photoinduced charge 
recombination effectively. Finally, the overall chemical and photocatalytic reactions are shown 
below: / →→ /	 	
→ 	 	  (1) 
 (2) 
/ 	 	 		→ 	  (3) 
O• 	 → 	 → 	/O• 	•  (4) 
 	 (5) 
• 	 → 	 	  (6) 
 (7) 
 
Catalysts 2020, 10, 660 9 of 15
found to be 0.192 ± 0.011 h−1, 0.412 ± 0.011 h−1, 0.506 0.020 h−1
± , and 0.688 ± 0.059 h−1, respectively.
Furthermore, a linear dependence between this kinetic parameter and the spinning voltage was
found. The high values obtained for ZnO-rGO nanorods compared to ZnO signify an improvement
of the photocatalytic activity due to the increase of voltage during spinning fabrication and by the
incorporation of rGO into ZnO seed layers. The calculated values of the correlation coefficient (R2) for
Z20, ZG20, ZG30, and ZG40 nanorods were 0.977, 0.995, 0.989, and 0.958, respectively, which are close
to 1. It demonstrates that the degradation process of MO through ZnO and ZnO-rGO NRs follows
first-order reaction kinetic. The results obtained for the photocatalytic efficiency of the nanorods
obtained are in good agreement with the results obtained by photoluminescence (see Figure 4).
The improvement in the photocatalytic degradation efficiency of the nanorods fabricated in this
work is related to the increase in spinning voltages and the rGO adherence into the ZnO-rGO seed
layers. The possible photocatalytic degradation mechanism of MO is proposed for ZnO-rGO NRs,
as illustrated in Figure 8. When the ZnO nanostructure is irradiated from the light source, the excited
electrons (e−) migrate from the valence band (VB) of ZnO to its conduction band (CB) with generation
simultaneous to the same number of holes (h+) in the VB (Equation (2)). The rGO sheets receive these
photo-excited electrons from the conduction band of ZnO nanorods and act as an electron transporter
phase hindering the electron-hole recombination [55]. This is mainly because the energy levels of
rGO and ZnO are different from each other. The value of the work function of rGO is −4.42 eV,
which is lower than the value of −4.05 eV obtained for the ZnO conduction band [67]. The separated
e−–h+ pairs react with oxygen molecules and with the surface absorbed water molecules as described
in Equ(atio)ns (1)–(7), producing the strongly oxidizing hydroxyl radical (OH) and oxygen radical
anion O−2 , which react with the MO dye molecules in solution and degrade them [73,74]. Based on
these results we conclude that the variation of spinning voltages and the presence of rGO enhance
the photocatalytic activity performance of pure ZnO nanorods, suppressing photoinduced charge
recombination effectively. Finally, the overall chemical and (photocataly)tic reactions are shown below:
ZnO/rGO + hν→ ZnO(/rGO e−CB +) h+VB (1)
ZnO + hν→ ZnO e−CB + h+VB (2)
ZnO(e−CB)→ rGO(e−tr) (3)
ZnO/rGO(h+ −
VB() +)OH → ZnO/rGO + OH (4)
rGO e− + O −
tr 2(ads) → O2 (5)
O−2 + pollutent (MO)→ decomposed products (6)
OH + pollutent (MO)→ decomposed products (7)
Catalysts 2020, 10, x FOR PEER REVIEW 10 of 16 
 
FigFuirgeu8re.  8A. Asc shcehmemataicticd diaigagrarmamf oforrt thheec chaarrge--ttransfer processs aanndd pphhoototoccaatatalylytitci cacatcivtiivtyit yofo ZfnZOn-OrG-rOG O
NRNsRfosr fomr emtheythl yolr oanragneg(eM (MOO) d) edgergardadaatitoionn. . 
3. Materials and Methods 
3.1. Materials, Reagents, and Chemicals 
In this work, zinc acetate dihydrate (Zn(CH3COO)2 2H2O, Merck, Darmstadt, Germany), 
polyvinylpyrrolidone (PVP, Sigma-Aldrich, St. Louis, MO, USA), and N,N-dimethylformamide 
(HCON(CH3)2, Merck, Darmstadt, Germany) were used as precursors of ZnO seed layers, while 
commercial reduced graphene oxide (Sigma-Aldrich, St. Louis, USA) was additionally used to obtain 
the ZnO-rGO seed layers. In addition, zinc nitrate hexahydrate (Zn(NO3)2 6H2O, Sigma-Aldrich, 
Steinheim, Germany) and sodium hydroxide (NaOH, Merck, Darmstadt, Germany) were employed 
in the preparation of ZnO and ZnO-rGO nanorods growth solutions. All of the chemicals used to 
obtain the samples were used as received. 
FTO glass conductive plates of 3.5 cm × 2 cm of area and 7 Ω/cm2 of resistance were used as 
substrates for the deposition of ZnO and ZnO-rGO nanorods. 
3.2. Preparation of ZnO and ZnO-rGO Nanorods 
ZnO and ZnO-rGO seed layers were deposited onto FTO glass plates using the electrospinning 
technique, and subsequently, they were used as substrates to grow ZnO and ZnO-rGO NRs, 
respectively, by hydrothermal treatment. To obtain a pure ZnO seed layer, a spinning solution 
containing 1 g of zinc acetate dihydrate and 1 g of polyvinylpyrrolidone dissolved in N-N 
dimethylformamide was used. ZnO-rGO seed layers were obtained by using the same spinning 
solution seen above, adding now 0.2 wt% rGO and varying the spinning voltage in three values: 20 
kV, 30 kV, and 40 kV. The obtained spinning solutions were loaded into a plastic syringe fitted with 
a 0.6 mm diameter needle made of stainless steel. The syringe supplied the feeding solution at a speed 
of 2 mL h−1 with a deposition time of 3 h. Then, the ZnO-rGO seed layers were obtained by calcination 
in a muffle furnace at 400 °C of the coated substrates. The solution medium used for the growth of 
the ZnO and ZnO-rGO NRs was prepared according to Rodriguez et al. [75] by mixing equal volumes 
of zinc nitrate hexahydrate (0.15 M) and sodium hydroxide (2.1 M) in water. Then, the substrates 
seeded with ZnO and ZnO-rGO films were placed in 100 mL screw-capped glass flasks containing 
40 mL of the solution for the growth. These glass flasks containing the substrates and the solutions 
for the growth were placed in an oven at 90 °C for 1 h. After that, the substrates covered with ZnO 
and ZnO-rGO NRs were removed from the solution, cleaned with distilled water and ethanol and 
finally dried at 70 °C. ZnO nanorods grown from ZnO seeds prepared with 20 kV of spinning voltage 
were labeled as Z20, whereas the ZnO-rGO nanorods grown from ZnO-rGO seeds prepared with 20 
kV, 30 kV, and 40 kV of spinning voltages were labeled as ZG20, ZG30, and ZG40, respectively. Figure 
9 illustrates the formation procedure of ZnO-rGO nanorods arrays. 
 
Catalysts 2020, 10, 660 10 of 15
3. Materials and Methods
3.1. Materials, Reagents, and Chemicals
In this work, zinc acetate dihydrate (Zn(CH3COO)2 2H2O, Merck, Darmstadt, Germany),
polyvinylpyrrolidone (PVP, Sigma-Aldrich, St. Louis, MO, USA), and N,N-dimethylformamide
(HCON(CH3)2, Merck, Darmstadt, Germany) were used as precursors of ZnO seed layers,
while commercial reduced graphene oxide (Sigma-Aldrich, St. Louis, USA) was additionally
used to obtain the ZnO-rGO seed layers. In addition, zinc nitrate hexahydrate (Zn(NO3)2 6H2O,
Sigma-Aldrich, Steinheim, Germany) and sodium hydroxide (NaOH, Merck, Darmstadt, Germany)
were employed in the preparation of ZnO and ZnO-rGO nanorods growth solutions. All of the
chemicals used to obtain the samples were used as received.
FTO glass conductive plates of 3.5 cm × 2 cm of area and 7 Ω/cm2 of resistance were used as
substrates for the deposition of ZnO and ZnO-rGO nanorods.
3.2. Preparation of ZnO and ZnO-rGO Nanorods
ZnO and ZnO-rGO seed layers were deposited onto FTO glass plates using the electrospinning
technique, and subsequently, they were used as substrates to grow ZnO and ZnO-rGO NRs, respectively,
by hydrothermal treatment. To obtain a pure ZnO seed layer, a spinning solution containing 1 g of zinc
acetate dihydrate and 1 g of polyvinylpyrrolidone dissolved in N-N dimethylformamide was used.
ZnO-rGO seed layers were obtained by using the same spinning solution seen above, adding now
0.2 wt% rGO and varying the spinning voltage in three values: 20 kV, 30 kV, and 40 kV. The obtained
spinning solutions were loaded into a plastic syringe fitted with a 0.6 mm diameter needle made of
stainless steel. The syringe supplied the feeding solution at a speed of 2 mL h−1 with a deposition
time of 3 h. Then, the ZnO-rGO seed layers were obtained by calcination in a muffle furnace at 400 ◦C
of the coated substrates. The solution medium used for the growth of the ZnO and ZnO-rGO NRs
was prepared according to Rodriguez et al. [75] by mixing equal volumes of zinc nitrate hexahydrate
(0.15 M) and sodium hydroxide (2.1 M) in water. Then, the substrates seeded with ZnO and ZnO-rGO
films were placed in 100 mL screw-capped glass flasks containing 40 mL of the solution for the growth.
These glass flasks containing the substrates and the solutions for the growth were placed in an oven at
90 ◦C for 1 h. After that, the substrates covered with ZnO and ZnO-rGO NRs were removed from the
solution, cleaned with distilled water and ethanol and finally dried at 70 ◦C. ZnO nanorods grown
from ZnO seeds prepared with 20 kV of spinning voltage were labeled as Z20, whereas the ZnO-rGO
nanorods grown from ZnO-rGO seeds prepared with 20 kV, 30 kV, and 40 kV of spinning voltages
were labeled as ZG20, ZG30, and ZG40, respectively. Figure 9 illustrates the formation procedure of
CZatnalOyst-sr G20O20,n 1a0,n xo FrOoRd sPEaErrRa RyEsV. IEW 11 of 16 
 
FFigiguurer e9.9 S. cShcehmemataicti cdidaigargarmam shsohwowinign gthteh epprorcoecsess sapappplileided toto pprorodduucec eththe eZZnnOO-r-GrGOO nnaannoororodd aarrrarayys.s .
3.3. Photocatalytic Characterization 
The photocatalytic activity of ZnO and ZnO-rGO nanorods was measured through degradation 
of an aqueous solution of methyl orange (MO) using a 220 W OSRAM Ultravitalux lamp (OSRAM, 
Wilmington, DE, USA) placed approximately at 20 cm from the system, where 70 Wm−2 in the UV-A 
range of intensity was measured. The initial concentration of MO was 5 ppm. In the typical 
experiment, 3 mg of ZnO and ZnO-rGO nanorods were added in 50 mL MO in a 100 mL beaker. 
Experiments were performed at room temperature of 23 °C +/− 2 °C. During the irradiation, 3 mL of 
the treated solution was collected each hour in order to analyze the methyl orange concentration by 
a Lambda 25 UV–Vis spectrophotometer (PerkinElmer, Waltham, MA, USA) at 462 nm. The 
photocatalytic activity of the ZnO and ZnO-rGO NRs fabricated with different spinning voltages 
during the electrospinning process was compared. 
3.4. Characterization Methods 
The obtained nanostructures were characterized by X-ray diffraction using a Bruker D8 Advance 
(Bruker, Billerica, MA, USA). The 2θ range was from 10 to 70 degrees with CuKα radiation (λ = 1.5418 
Å). The morphologies of ZnO-rGO seeds as well as of ZnO-rGO NRs were visualized by field 
emission scanning electron microscope HITACHI SU8230 (Hitachi, Omuta, Japan). ImageJ 1.52r 
software was used in order to obtain the size distributions of nanoparticles and fabricated nanorods. 
Room temperature Raman and photoluminescence spectroscopy were obtained on a Renishaw inVia 
equipment (Renishaw, Wotton-under-Edge, UK) equipped with a 514 nm wavelength of Ar laser 
light and by He-Cd laser source with a wavelength of 325 nm, respectively. The Raman spectrums 
were recorded over a range of 100 cm−1 to 3200 cm−1, while the photoluminescence measurements 
were recorded over the wavelength range of 350 nm to 950 nm. X-ray photoelectron spectroscopy 
was utilized to identify chemical states of prepared samples by using a Physical Electronics 
VersaProbe II (Physical Electronics, Chanhassen, MN, USA) instrument equipped with a 
monochromatic Al (Kα) X-ray source (hν = 1486.7 eV) and a concentric hemispherical analyzer. 
4. Conclusions 
In summary, a series of ZnO and ZnO-rGO NRs were synthesized on ZnO and ZnO-rGO seed 
layers by a wet chemical method, and the seed layers were obtained by electrospinning using a 
precursor solution with 0.2 wt% rGO and applying different spinning voltages. Furthermore, the 
photocatalytic methyl orange degradation performances were evaluated. The obtained results show 
an effective formation of ZnO-rGO NRs, where the adhered rGO sheets and different spinning 
voltages applied influenced the structure, morphology, photocatalytic performance, and optical 
properties of the nanorod samples. Significant enhancement of the photocatalytic efficiency of 
fabricated ZnO-rGO nanorods as compared with pure ZnO nanorods was determined. The ZnO-rGO 
nanostructure fabricated with 40 kV of spinning voltage exhibited the highest photocatalytic activity 
for dye photodegradation. The enhancement in photocatalytic activity of ZnO-rGO NRs is attributed 
 
Catalysts 2020, 10, 660 11 of 15
3.3. Photocatalytic Characterization
The photocatalytic activity of ZnO and ZnO-rGO nanorods was measured through degradation
of an aqueous solution of methyl orange (MO) using a 220 W OSRAM Ultravitalux lamp (OSRAM,
Wilmington, DE, USA) placed approximately at 20 cm from the system, where 70 Wm−2 in the UV-A
range of intensity was measured. The initial concentration of MO was 5 ppm. In the typical experiment,
3 mg of ZnO and ZnO-rGO nanorods were added in 50 mL MO in a 100 mL beaker. Experiments were
performed at room temperature of 23 ◦C 2 ◦± C. During the irradiation, 3 mL of the treated solution
was collected each hour in order to analyze the methyl orange concentration by a Lambda 25 UV–Vis
spectrophotometer (PerkinElmer, Waltham, MA, USA) at 462 nm. The photocatalytic activity of the
ZnO and ZnO-rGO NRs fabricated with different spinning voltages during the electrospinning process
was compared.
3.4. Characterization Methods
The obtained nanostructures were characterized by X-ray diffraction using a Bruker D8 Advance
(Bruker, Billerica, MA, USA). The 2θ range was from 10 to 70 degrees with CuKα radiation
(λ = 1.5418 Å). The morphologies of ZnO-rGO seeds as well as of ZnO-rGO NRs were visualized by
field emission scanning electron microscope HITACHI SU8230 (Hitachi, Omuta, Japan). ImageJ 1.52r
software was used in order to obtain the size distributions of nanoparticles and fabricated nanorods.
Room temperature Raman and photoluminescence spectroscopy were obtained on a Renishaw inVia
equipment (Renishaw, Wotton-under-Edge, UK) equipped with a 514 nm wavelength of Ar laser light
and by He-Cd laser source with a wavelength of 325 nm, respectively. The Raman spectrums were
recorded over a range of 100 cm−1 to 3200 cm−1, while the photoluminescence measurements were
recorded over the wavelength range of 350 nm to 950 nm. X-ray photoelectron spectroscopy was
utilized to identify chemical states of prepared samples by using a Physical Electronics VersaProbe II
(Physical Electronics, Chanhassen, MN, USA) instrument equipped with a monochromatic Al (Kα)
X-ray source (hν = 1486.7 eV) and a concentric hemispherical analyzer.
4. Conclusions
In summary, a series of ZnO and ZnO-rGO NRs were synthesized on ZnO and ZnO-rGO seed
layers by a wet chemical method, and the seed layers were obtained by electrospinning using a precursor
solution with 0.2 wt% rGO and applying different spinning voltages. Furthermore, the photocatalytic
methyl orange degradation performances were evaluated. The obtained results show an effective
formation of ZnO-rGO NRs, where the adhered rGO sheets and different spinning voltages applied
influenced the structure, morphology, photocatalytic performance, and optical properties of the
nanorod samples. Significant enhancement of the photocatalytic efficiency of fabricated ZnO-rGO
nanorods as compared with pure ZnO nanorods was determined. The ZnO-rGO nanostructure
fabricated with 40 kV of spinning voltage exhibited the highest photocatalytic activity for dye
photodegradation. The enhancement in photocatalytic activity of ZnO-rGO NRs is attributed to
the higher transfer rate of photo-generated electrons from ZnO to rGO, the high efficiency in light
utilization, and inhibited recombination of the photoinduced hole-electron pairs of ZnO. Moreover,
electrospinning is a convenient method, compared with traditional methods, for the large-scale
fabrication of ZnO-rGO nanorods arrays; we speculate that this approach can be extended further for
the growth of other nanomaterials on substrates.
Author Contributions: Conceptualization, J.M.R. and P.G.R.; methodology, P.G.R.; investigation, P.G.R. and
L.A.S.; resources, L.A.S., C.L., and E.D.G.; writing—original draft preparation, P.G.R.; writing—review and editing,
E.D.G. and J.M.R.; visualization, C.L.; supervision, J.M.R.; funding acquisition, J.M.R. All authors have read and
agreed to the published version of the manuscript.
Funding: This research was funded by the Peruvian National Fund for Scientific, Technological Development
and Technological Innovation (FONDECYT), grant numbers 168-2015-FONDECYT and 32-2019-FONDECYT-BM-
INC.INV, P.G.R, E.D.G. and J.M.R. want to thanks to the PSU-UNI-PUCP seed projects program for support.
Catalysts 2020, 10, 660 12 of 15
Acknowledgments: The work described in this paper was financially supported by the projects INNOVATE
(project number 113-INNOVATE PERU-ISASS-2018) and FINCYT (project number 133-FINCYT-IB-2015). P.G.R.
wants to thanks the Ministry of Education of Peru for the PhD scholarship. The authors thanks Jeff
Shallenberger of the Materials Characterization Lab at Penn State for his excellent technical assistance with X-ray
photoelectron spectroscopy.
Conflicts of Interest: The authors declare no conflict of interest.
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