Magnetic nanoparticles of zinc/calcium ferrite decorated with silver for photodegradation of dyes

ma e ials A icle Magne ic Nanopa icles o Zinc/Calcium Fe i e Deco a ed wi h Sil e o Pho odeg ada ion o Dyes Rica do J. C. Fe nandes 1, Ca los A. B. Magalhães 1, Ca los O. Amo im 2, Ví o S. Ama al 2, Be na do G. Almeida 1, Elisabe e M. S. Cas anhei a 1and Paulo J. G. Cou inho 1,* 1Cen e o Physics (CFUM), Uni e si y o Minho, Campus de Gual a , 4710-057 B aga, Po ugal 2Physics Depa men and CICECO, Uni e si y o A ei o, Campus de San iago, 3810-193 A ei o, Po ugal *Co espondence: [email p o ec ed] Recei ed: 2 Oc obe 2019; Accep ed: 29 Oc obe 2019; Published: 31 Oc obe 2019   Abs ac : Magne ic nanopa icles o zinc/calcium e i e and deco a ed wi h sil e we e p epa ed by cop ecipi a ion me hod. The ob ained nanopa icles we e cha ac e ized by UV/Visible abso p ion, XRD, TEM and SQUID. The mixed zinc/calcium e i es exhibi an op ical band gap o 1.78 eV. HR-TEM imaging showed ec angula nanopla e shapes wi h sizes o 10 ± 3 nm and aspec a io mainly be ween 1 and 1.5. Magne ic measu emen s indica ed a supe pa amagne ic beha io . XRD di ac og ams allowed a size es ima ion o 4 nm, which was associa ed wi h he nanopla e hickness. The sil e -deco a ed zinc/calcium e i e nanopa icles we e success ully employed in he pho odeg ada ion o a model dye (Rhodamine B) and indus ial ex ile dyes (CI Reac i e Red 195, CI Reac i e Blue 250 and CI Reac i e Yellow 145). The nanosys ems de eloped exhibi ed p omising esul s o indus ial applica ion in e luen pho o emedia ion using isible ligh , wi h he possibili y o magne ic eco e y. Keywo ds: magne ic nanopa icles; zinc/calcium e i e; sil e deco a ed nanopa icles; pho odeg ada ion; ex ile dyes 1. In oduc ion Nowadays, one o he majo p oblems wo ldwide is wa e pollu ion. Conside ing popula ion g ow h and he consequen inc ease in indus ializa ion, pollu ion le els in wa e esou ces ha e g own d ama ically. The ex ile indus y appea s as one o he mos pollu ing sec o s wo ldwide. I deals daily wi h millions o li e s o wa e , lea ing an associa ed ace o colo in i s e luen s, which ep esen s one o he main p oblems o his indus ial sec o [1,2]. Recen ly, se e al wo ks ha e d awn a en ion o nano echnology o en i onmen al applica ions and speci ically o magne ic nanopa icles, some o hem being capable o deg ade ex ile dyes by pho odeg ada ion [3]. Fo many yea s, i anium dioxide has been used as he pho oca alys o excellence. Howe e , i s la ge band gap o 3.2 eV dec eases i s applicabili y, by only deg ading in he p esence o UV ligh [ 4 ]. Lowe ene gy adia ion (e.g. in he isible spec um) can be used i he bandgap o he semiconduc o is educed. In his con ex , zinc e i es appea as a p omising semiconduc o (band gap o 1.9 eV), p omo ing pho odeg ada ion o dyes unde isible ligh , as demons a ed in ecen s udies [ 4 ]. Howe e , ocusing on pho o emedia ion o indus ial e luen s, he magne ic p ope ies o he nanopa icles mus be imp o ed, o allow magne ic eco e y and euse o he pho oca alys s. Biocompa ibili y is also a ea u e o pu sue, conside ing applica ions in he pho o emedia ion o wa e na u al esou ces. The inco po a ion o calcium in he nanopa icles composi ion, gi ing mixed zinc/calcium e i e nanopa icles, allows ob aining s able e i es wi h enhanced biocompa ibili y and magne ic p ope ies [5,6]. Ma e ials 2019,12, 3582; doi:10.3390/ma12213582 www.mdpi.com/jou nal/ma e ials Ma e ials 2019,12, 3582 2 o 16 One o he main limi a ions o he applica ion o e i e nanopa icles is hei low sepa a ion e iciency o elec ons and holes, which leads o a much lowe pho oca aly ic ac i i y compa ing o i anium dioxide [ 4 ]. I has been shown ha he deposi ion o a noble me al a nanopa icle su ace inc eases he sepa a ion a e o elec ons and holes, p omo ing he ans e o he in e acial load [ 7 , 8 ]. In he p esen wo k, sil e was used o co e he nanopa icles su ace [ 4 , 7 , 8 ]. Since he a e o ecombina ion o e i es is high, educing hei pho oca aly ic ac i i y, he inco po a ion o sil e educes apid ecombina ion o he gene a ed elec on/hole pai s, inc easing he o ma ion o eac i e species and allowing an enhanced pho oca aly ic ac i i y. In his wo k, mixed zinc/calcium e i e nanopa icles deco a ed wi h sil e clus e s we e es ed as pho odeg ada ion agen s o ex ile eac i e azo dyes, namely Reac i e Red 195 (“Red”), Reac i e Blue 250 (“Blue”) and Reac i e Yellow 145 (“Yellow”). These dyes ha e a gene al s uc u e R − N= N−R ’ (Table 1) and a e he mos used class in indus ial dyeing p ocesses, being gene ally pe sis en in inal indus ial e luen s [9,10]. Rhodamine B (s uc u e in Table 1) was also used as model dye o compa ison, due o i s well-known pho ophysical p ope ies [ 11 , 12 ] and wide use in pho odeg ada ion assays [13–15]. The p oposed nanopa icles a e ad an ageous o was e wa e ea men , as he inco po a ion o Zn ca ions may p omo e an imic obial ac i i y and he p esence o calcium in he e i e s uc u e a o s biocompa ibili y o he nanopa icles. S udies in cell lines ha e shown ha , o a 100 µ g/mL concen a ion o nanopa icles, zinc e i e allows 70.5% o cell iabili y a 24 h, while calcium e i e allows 90.6% o cell iabili y o he same concen a ion and ime o exposu e. Fo compa ison, using cobal e i e nanopa icles, 80% o cell iabili y was obse ed bu wi h only 20 µ g/mL o nanopa icles [ 16 ]. The e o e, enhanced biocompa ibili y is expec ed by inclusion o calcium in zinc e i es. The nanosys ems he e de eloped show p omising esul s o indus ial applica ion in e luen pho o emedia ion. Ma e ials 2019,12, 3582 3 o 16 Table 1. S uc u e o he ex ile dyes and model dye used o pho odeg ada ion assays. Comme cial Name Molecula Fo mula Molecula Weigh (g/moL) Molecula S uc u e C.I. Reac i e Blue 250 (Reac i e Blue RGB) C27H23N5Na4O20S61021.84 Ma e ials 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/jou nal/ma e ials Table 1. S uc u e o he ex ile dyes and model dye used o pho odeg ada ion assays. Comme cial Name Molecula Fo mula Molecula weigh (g/mol) Molecula s uc u e C.I. Reac i e Blue 250 (Reac i e Blue RGB) C 27 H 23 N 5 Na 4 O 20 S 6 1021.84 C.I. Reac i e Yellow 145 (Reac i e Yellow 3RS) C 28 H 20 ClN 9 Na 4 O 16 S 5 1026.25 C.I. Reac i e Red 195 (Reac i e Red 3BS) C 31 H 19 ClN 7 Na 5 O 19 S 6 1136.32 C.I. Reac i e Yellow 145 (Reac i e Yellow 3RS) C28H20ClN9Na4O16S51026.25 Ma e ials 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/jou nal/ma e ials Table 1. S uc u e o he ex ile dyes and model dye used o pho odeg ada ion assays. Comme cial Name Molecula Fo mula Molecula weigh (g/mol) Molecula s uc u e C.I. Reac i e Blue 250 (Reac i e Blue RGB) C 27 H 23 N 5 Na 4 O 20 S 6 1021.84 C.I. Reac i e Yellow 145 (Reac i e Yellow 3RS) C 28 H 20 ClN 9 Na 4 O 16 S 5 1026.25 C.I. Reac i e Red 195 (Reac i e Red 3BS) C 31 H 19 ClN 7 Na 5 O 19 S 6 1136.32 C.I. Reac i e Red 195 (Reac i e Red 3BS) C31H19ClN7Na5O19S61136.32 Ma e ials 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/jou nal/ma e ials Table 1. S uc u e o he ex ile dyes and model dye used o pho odeg ada ion assays. Comme cial Name Molecula Fo mula Molecula weigh (g/mol) Molecula s uc u e C.I. Reac i e Blue 250 (Reac i e Blue RGB) C 27 H 23 N 5 Na 4 O 20 S 6 1021.84 C.I. Reac i e Yellow 145 (Reac i e Yellow 3RS) C 28 H 20 ClN 9 Na 4 O 16 S 5 1026.25 C.I. Reac i e Red 195 (Reac i e Red 3BS) C 31 H 19 ClN 7 Na 5 O 19 S 6 1136.32 Rhodamine B C28H31ClN2O3479.02 Ma e ials 2019, 12, x FOR PEER REVIEW 2 o 18 Rhodamine B C 28 H 31 ClN 2 O 3 479.02 Ma e ials 2019,12, 3582 4 o 16 2. Ma e ials and Me hods 2.1. Nanopa icles P epa a ion 2.1.1. Zinc/Calcium Fe i e Nanopa icles Zinc/calcium e i e nanopa icles we e p epa ed h ough a cop ecipi a ion me hod in e lux condi ions, adap ing a p e iously desc ibed p ocedu e by Cao e al. [ 4 ]. Fi s , 1.082 g o i on (III) chlo ide hexahyd a e, 0.219 g o zinc ace a e and 0.158 g o calcium ace a e we e dissol ed in 200 mL o ul apu e wa e Milli-Q g ade (Millipo eSigma, S . Louis, MO, USA). A e dissolu ion, 1 mL o oleic acid and 1.198 g o u ea we e added o he solu ion. A e comple e dispe sion, he solu ion was e luxed igo ously o a leas 3 h. Fo pu i ica ion, he ob ained sample was washed se e al imes wi h absolu e e hanol and ul apu e wa e , by magne ic decan a ion and cen i uga ion (14,000 g). The mixed e i e nanopa icles we e d ied o 12 h a 90 ◦ C. To imp o e c ys allini y, he zinc/calcium e i e nanopa icles we e calcined a 400 ◦C o 30 min. 2.1.2. Zinc/Calcium Fe i es Deco a ed wi h Sil e Clus e s The as-p epa ed mixed e i e nanopa icles (ei he calcined o non-calcined) we e dispe sed in 100 mL o e hylene glycol. Nex , 0.160 g o sil e ni a e we e dissol ed in 20 mL o ul apu e wa e and added o he p e ious dispe sion. This solu ion was e luxed o 30 min. The p oduc s we e sepa a ed by magne ic decan a ion and cen i uga ion (14,000 g) and washed epea edly wi h absolu e e hanol. The nanopa icles we e d ied o 12 h a 90 ◦C. 2.2. S uc u al Cha ac e iza ion 2.2.1. T ansmission Elec on Mic oscopy (TEM) TEM images o nanopa icles we e acqui ed using a T ansmission Elec on Mic oscope JEOL 2100 (JEOL USA Inc., Peabody, MA, USA) ope a ing a 200 kV coupled o an Elec on Dispe si e X-Ray Spec oscopic analyze (EDS). The solu ions we e sonica ed in e hanol and d opped on o a TEM g id (coppe 400 mesh wi h a ca bon ilm). TEM images we e p ocessed using ImageJ 1.52p so wa e (Na ional Ins i u es o Heal h (NIH), Be hesda, MD, USA). The size o each pa icle was de e mined by equalizing i s a ea wi h he a ea o a ci cle. Howe e , his is a c ude app oxima ion o he ype o pa icles obse ed in TEM images. Thus, an addi ional es ima ion was made by insc ibing ec angula shapes on each pa icle. The a io o esul ing side leng hs o he ob ained ec angles was used as an es ima ion o he aspec a io. 2.2.2. X-Ray Di ac ion (XRD) X-Ray Di ac ion (XRD) analyses we e pe o med using a con en ional Philips PW 1710 (Royal Philips, Ams e dam, The Ne he lands) di ac ome e , ope a ing wi h CuK α adia ion, in a B agg-B en ano con igu a ion. 2.2.3. Magne ic Measu emen s Magne iza ion measu emen s we e done in a MPMS3 SQUID magne ome e (Quan um Design Inc., San Diego, CA, USA). The hys e esis cycles (magne iza ion e sus magne ic ield) o he samples we e measu ed in he con enien ield ange o each sample, wi h a possible maximum +/ − 70 kOe (+/ − 7 Tesla). The measu emen me hod was by DC ex ac ion o VSM oscilla ion a a equency o 14 Hz. A speci ic magne ic ield co ec ion o he apped lux in he supe conduc ing coil was made achie ing an accu acy o esidual less han 2 Oe. Ma e ials 2019,12, 3582 5 o 16 2.3. Pho odeg ada ion Assays To e alua e he pho oca aly ic ac i i y o he as-p epa ed nanopa icles, a home-buil i adia ion appa a us was used. The se up inco po a es a 200 W Xenon A c Lamp (L.O.T.-O iel GmbH & Co. KG, Da ms ad , Ge many), a 400 nm long pass il e (Tho labs Inc., New on, NJ, USA) o isola e he isible spec um adia ion, and a sample cu e e holde . Aqueous solu ions o Rhodamine B (40 mg/L) and o ex ile dyes, C. I. Reac i e Red 195 (“Red”), C. I. Reac i e Blue 250 (“Blue”) and C. I. Reac i e Yellow 145 (“Yellow”) (80 mg/L) we e assayed o 2.5 h. In he i s 30 min, he nanopa icles we e added o he solu ion, in cons an s i ing, unde da k. A e his ini ial ime, he sample solu ion was exposed o ligh unde magne ic s i ing, and aliquo s we e aken a 0, 5, 10, 15, 30, 60, 90 and 120 min. The pho oca alys con en o each aliquo was emo ed by cen i uga ion and he abso p ion spec a we e eco ded in a Shimadzu UV-3600 Plus UV-Vis-NIR (Shimadzu Co po a ion, Kyo o, Japan) spec opho ome e . 3. Resul s and Discussion 3.1. Nanopa icles Cha ac e iza ion 3.1.1. Abso p ion Spec a Figu e 1displays he UV-Visible abso p ion spec a o aqueous dispe sions o mixed zinc/calcium e i e nanopa icles and Ag-deco a ed zinc/calcium e i e nanopa icles. Ma e ials 2019, 12, x FOR PEER REVIEW 2 o 17 2.2.3. Magne ic Measu emen s Magne iza ion measu emen s we e done in a MPMS3 SQUID magne ome e (Quan um Design Inc., San Diego, CA, USA). The hys e esis cycles (magne iza ion e sus magne ic ield) o he samples we e measu ed in he con enien ield ange o each sample, wi h a possible maximum +/−70 kOe (+/−7 Tesla). The measu emen me hod was by DC ex ac ion o VSM oscilla ion a a equency o 14 Hz. A speci ic magne ic ield co ec ion o he apped lux in he supe conduc ing coil was made achie ing an accu acy o esidual less han 2 Oe. 2.3. Pho odeg ada ion Assays To e alua e he pho oca aly ic ac i i y o he as-p epa ed nanopa icles, a home-buil i adia ion appa a us was used. The se up inco po a es a 200 W Xenon A c Lamp (L.O.T.-O iel GmbH & Co. KG, Da ms ad , Ge many), a 400 nm long pass il e (Tho labs Inc., New on, NJ, USA) o isola e he isible spec um adia ion, and a sample cu e e holde . Aqueous solu ions o Rhodamine B (40 mg/L) and o ex ile dyes, C. I. Reac i e Red 195 (“Red”), C. I. Reac i e Blue 250 (“Blue”) and C. I. Reac i e Yellow 145 (“Yellow”) (80 mg/L) we e assayed o 2.5 hou s. In he i s 30 minu es, he nanopa icles we e added o he solu ion, in cons an s i ing, unde da k. A e his ini ial ime, he sample solu ion was exposed o ligh unde magne ic s i ing, and aliquo s we e aken a 0, 5, 10, 15, 30, 60, 90 and 120 minu es. The pho oca alys con en o each aliquo was emo ed by cen i uga ion and he abso p ion spec a we e eco ded in a Shimadzu UV-3600 Plus UV-Vis-NIR (Shimadzu Co po a ion, Kyo o, Japan) spec opho ome e . 3. Resul s and Discussion 3.1. Nanopa icles Cha ac e iza ion 3.1.1. Abso p ion Spec a Figu e 1 displays he UV-Visible abso p ion spec a o aqueous dispe sions o mixed zinc/calcium e i e nanopa icles and Ag-deco a ed zinc/calcium e i e nanopa icles. Figu e 1. UV-Visible abso p ion spec a o aqueous dispe sions o (a) zinc/calcium e i e nanopa icles and (b) sil e -deco a ed nanopa icles. The spec um o he mixed zinc/calcium e i e nanopa icles in Figu e 1a allows he de e mina ion o he op ical band gap, using a Tauc plo (Equa ion (1)), () ( ) g nEhναhν−∝ (1) whe e α is he abso p ion coe icien (p opo ional o he abso bance), n is an exponen ha depends on he na u e o he ansi ion (being n = 2 o a di ec semiconduc o and n = 1/2 o an indi ec one) and Eg is he op ical band gap [17]. A band gap o 1.78 eV (n = 2) was es ima ed om he in e cep o inse o Figu e 1a, in ag eemen wi h he alue o 1.90 eV epo ed by Kim e al. o calcium e i e nanopa icles [18], as well as o zinc e i e [4]. Figu e 1. UV-Visible abso p ion spec a o aqueous dispe sions o ( a ) zinc/calcium e i e nanopa icles and (b) sil e -deco a ed nanopa icles. The spec um o he mixed zinc/calcium e i e nanopa icles in Figu e 1a allows he de e mina ion o he op ical band gap, using a Tauc plo (Equa ion (1)), (αhν)n∝hν−Eg(1) whe e α is he abso p ion coe icien (p opo ional o he abso bance), n is an exponen ha depends on he na u e o he ansi ion (being n =2 o a di ec semiconduc o and n =1/2 o an indi ec one) and E g is he op ical band gap [ 17 ]. A band gap o 1.78 eV (n =2) was es ima ed om he in e cep o inse o Figu e 1a, in ag eemen wi h he alue o 1.90 eV epo ed by Kim e al. o calcium e i e nanopa icles [18], as well as o zinc e i e [4]. Compa ing henanopa icleswi hou andwi hsil e (Figu e1), i canbeobse ed hecha ac e is ic local su ace plasmon esonance (LSPR) band o sil e nanopa icles a ound 435 nm, wi hin he ange o alues p e iously epo ed [19]. 3.1.2. X-Ray Di ac ion (XRD) Measu emen s The calcina ion p ocess allows an imp o emen in c ys allini y and magne ic p ope ies o he nanopa icles, which is essen ial o hei eco e y a he end o he i adia ion p ocedu e, enabling he Ma e ials 2019,12, 3582 6 o 16 possibili y o ecycle and euse he nanopa icles [ 20 , 21 ]. XRD analysis e ealed a s ongly amo phous backg ound o he non-calcined nanopa icles (Figu e 2a). Upon calcina ion, se e al well de ined di ac ion peaks a e obse ed (Figu e 2b). Using FullP o so wa e ( e sion 5.8, J. Rod í guez-Ca ajal, Lab. L é on B illouin, Gi su Y e e, F ance) [ 22 ], Rie eld analysis o calcined zinc/calcium e i e di ac og am was pe o med, by adap ing CIF ile numbe 2300615 (pa ially in e ed cubic spinel phase, space g oup Fd 3 m), co esponding o zinc e i e, o ha e 50% occupa ion wi h Zn and 50% wi h Ca a he zinc la ice si es. Bulk zinc e i e has a di ec spinel s uc u e. Howe e , i was ound ha in nanopa icles he deg ee o in e sion, i, inc eases wi h he dec ease o nanopa icle size [ 23 ], wi h a co esponding enhancemen o magne ic p ope ies. Recen ly, i was epo ed ha mixed zinc/calcium e i es adop an in e ed spinel s uc u e [ 24 ]. Thus, an in e ed spinel s uc u e is conside ed, in which he A 2+ ions in oc ahed al si es a e 50% dis ibu ed be ween zinc and calcium: (Fe) Td (FeZn 0.5 Ca 0.5 ) Oh O 4 . A easonable alue o R F =4.35 (Table 2) was ob ained, indica ing ha he assumed c ys al s uc u e is compa ible wi h he XRD esul s, since all he co esponding di ac ion peaks a e obse ed and ha e nea ly he calcula ed in ensi ies (Figu e 2b). Ma e ials 2019, 12, x FOR PEER REVIEW 4 o 17 Figu e 2. XRD di ac og ams o zinc/calcium e i e nanopa icles: (a) Non-calcined zinc/calcium e i e; (b) calcined zinc/calcium e i e; (c) non-calcined zinc/calcium e i e deco a ed wi h sil e ; (d) calcined zinc/calcium e i e deco a ed wi h sil e . G ay lines: Expe imen al pa e ns; black lines: i ed pa e ns. Mille indices: Black: zinc/calcium e i e; Red: Sil e . Table 2. Selec ed Rie eld analysis pa ame e s. Sample Ox,y,z (*) i (*) Phase size (nm) La ice cons an (nm) Zn/Ca e i e|Ag R Zn/Ca e i e|Ag X2 Zn/Ca e i e non-calcined 0.2405 1 (+) 1.12 | ---- 0.8425 (+) | ---- 3.18 | ---- 1.11 Zn/Ca e i e calcined 0.2464 1 (+) 3.97 | ---- 0.8425 | ---- 4.35 | ---- 1.26 Zn/Ca e i e non-calcined wi h sil e 0.2405 (+) 1 (+) 1.12 (+) | 3.41 0.8425 (+) | 0.4069 3.42 | 0.95 1.32 Zn/Ca e i e calcined wi h sil e 0.2464 (+) 1 (+) 3.97 (+) | 9.90 0.8425 (+) | 0.4078 9.81 | 2.91 1.13 (*) Values in CIF ile 2300615 a e Ox,y,z = 0.2535 and i = 0.62; (+) ixed alue. I can be obse ed (Table 3) ha he Ag coupled calcined nanopa icles exhibi a lowe sil e con en , indica ing ha sil e exhibi s mo e a ini y o he non-calcined amo phous nanopa icles. Howe e , he signi ican enhancemen o he c ys alline s uc u e o he nanopa icles wi h calcina ion is de e minan in ob aining sui able magne ic p ope ies o en i onmen al applica ions. Table 3. Es ima ed pe cen age o sil e in he nanopa icles ob ained by XRD. Nanopa icles Zn0.5Ca0.5Fe2O4 (%) Ag (%) Zn0.5Ca0.5Fe2O4 non-calcined 100 - Zn0.5Ca0.5Fe2O4 calcined 100 - [email protected] non-calcined 57.8 42.2 [email protected] calcined 66.4 33.6 3.1.3. T ansmission Elec on Mic oscopy (TEM) Figu e 2. XRD di ac og ams o zinc/calcium e i e nanopa icles: ( a ) Non-calcined zinc/calcium e i e; ( b ) calcined zinc/calcium e i e; ( c ) non-calcined zinc/calcium e i e deco a ed wi h sil e ; ( d ) calcined zinc/calcium e i e deco a ed wi h sil e . G ay lines: Expe imen al pa e ns; black lines: i ed pa e ns. Mille indices: Black: zinc/calcium e i e; Red: Sil e . The a e age size ha esul s om Debye-Sche e equa ion, as implemen ed by FullP o sui e [ 22 ], is 3.97 nm and he la ice cons an is 8.425 Å. The la ice cons an o bulk ZnFe 2 O 4 is 8.443 Å [ 25 ], bu alues down o 8.411 Å using he mal decomposi ion me hod [ 25 ], and 8.391 Å using mic owa e syn hesis [ 26 ], and up o 8.47 Å when using cop ecipi a ion me hod [ 23 ], we e epo ed. Calcium e i e nanopa icles in he spinel c ys allog aphic o m and using co-p ecipi a ion me hods ha e la ice cons an s be ween 8.34 Å [ 27 ] and 8.37 Å [ 28 ]. Thus, he la ice cons an o he he e ob ained zinc/calcium mixed e i e lies be ween he co esponding single e i e phases. Rie eld analysis on he non-calcined sample (Figu e 2a) is compa ible wi h 1.1 nm size. Bo h calcined and non-calcined samples we e coupled wi h me allic sil e . I s p esence is con i med in he co esponding XRD di ac og ams p esen ed in Figu e 2c,d, espec i ely. The Rie eld analysis using an addi ional phase co esponding o sil e (CIF 9008459) allows an es ima ion o size o he coupled sil e nanopa icles, Ma e ials 2019,12, 3582 7 o 16 as well as o hei amoun in each sample. A summa y o he Rie eld analysis o all samples is shown in Table 2and he esul ing weigh pe cen ages o e i e and sil e a e indica ed in Table 3. Table 2. Selec ed Rie eld analysis pa ame e s. Sample Ox,y,z (*) i (*) Phase Size (nm) La ice Cons an (nm) Zn/Ca Fe i e|Ag R Zn/Ca Fe i e|Ag X2 Zn/Ca e i e non-calcined 0.2405 1 (+)1.12 |---- 0.8425 (+)|---- 3.18 |---- 1.11 Zn/Ca e i e calcined 0.2464 1 (+)3.97 |---- 0.8425 |---- 4.35 |---- 1.26 Zn/Ca e i e non-calcined wi h sil e 0.2405 (+) 1 (+)1.12 (+)|3.41 0.8425 (+)|0.4069 3.42 |0.95 1.32 Zn/Ca e i e calcined wi h sil e 0.2464 (+) 1 (+)3.97 (+)|9.90 0.8425 (+)|0.4078 9.81 |2.91 1.13 (*) Values in CIF ile 2300615 a e Ox,y,z =0.2535 and i =0.62; (+) ixed alue. Table 3. Es ima ed pe cen age o sil e in he nanopa icles ob ained by XRD. Nanopa icles Zn0.5Ca0.5Fe2O4(%) Ag (%) Zn0.5Ca0.5Fe2O4non-calcined 100 - Zn0.5Ca0.5Fe2O4calcined 100 - Ag@Zn 0.5 Ca 0.5 Fe 2 O 4 non-calcined 57.8 42.2 [email protected] 66.4 33.6 I can be obse ed (Table 3) ha he Ag coupled calcined nanopa icles exhibi a lowe sil e con en , indica ing ha sil e exhibi s mo e a ini y o he non-calcined amo phous nanopa icles. Howe e , he signi ican enhancemen o he c ys alline s uc u e o he nanopa icles wi h calcina ion is de e minan in ob aining sui able magne ic p ope ies o en i onmen al applica ions. 3.1.3. T ansmission Elec on Mic oscopy (TEM) TEM images o he calcined zinc/calcium e i e nanopa icles (Figu e 3A,B) e ealed gene ally od-like o p isma ic shapes, wi h a size dis ibu ion o 10 ± 3 nm (Figu e 3C), ob ained conside ing ci cles wi h he same a ea o each o he 206 pa icles ha we e manually delimi ed. Assuming ins ead a ec angula shape (ImageJ bonding ec angle), sizes o longe and sho e sides a e, espec i ely, 12 ±3 nm and 9.8 ±3 nm, wi h a b oad aspec a io dis ibu ion be ween 1.04 and 2 (Figu e 3D). The di e ence in size om XRD es ima ion migh be ela ed o a nanopla e-like s uc u e, al eady epo ed o zinc e i e [ 4 ], whe e i s hickness co esponds o he size de e mined by XRD ( he nanopla es a e lying down, so ha only hei hickness con ibu es o he amoun o la ice planes ha de ine he X- ay di ac ion signal). EDX analysis (a e age o 5 measu emen s) allowed ob aining a a io o Zn/Fe a omic pe cen ages o 26.7%, in acco dance o wha was expec ed o Zn 0.5 Ca 0.5 Fe 2 O 4 (Zn/Fe a io o 25%). TEM images o zinc/calcium e i e nanopa icles deco a ed wi h sil e clus e s (Figu e 3E,F) show he addi ional appea ance o mo e sphe ical shapes (ma ked on Figu e 3E,F) and also o agglome a es o hese sphe ical pa icles. These ha e sizes o 9.4 ± 1 nm and co espond o he sil e con en o he p epa ed sample, being compa ible wi h he size es ima ion ob ained om XRD. EDX analysis es ima ed an a omic sil e pe cen age o 21%, sligh ly smalle han he de e mined by XRD. Ma e ials 2019,12, 3582 8 o 16 Ma e ials 2019, 12, x FOR PEER REVIEW 5 o 17 TEM images o he calcined zinc/calcium e i e nanopa icles (Figu e 3A,B) e ealed gene ally od-like o p isma ic shapes, wi h a size dis ibu ion o 10 ± 3 nm (Figu e 3C), ob ained conside ing ci cles wi h he same a ea o each o he 206 pa icles ha we e manually delimi ed. Assuming ins ead a ec angula shape (ImageJ bonding ec angle), sizes o longe and sho e sides a e, espec i ely, 12 ± 3 nm and 9.8 ± 3 nm, wi h a b oad aspec a io dis ibu ion be ween 1.04 and 2 (Figu e 3D). The di e ence in size om XRD es ima ion migh be ela ed o a nanopla e-like s uc u e, al eady epo ed o zinc e i e [4], whe e i s hickness co esponds o he size de e mined by XRD ( he nanopla es a e lying down, so ha only hei hickness con ibu es o he amoun o la ice planes ha de ine he X- ay di ac ion signal). EDX analysis (a e age o 5 measu emen s) allowed ob aining a a io o Zn/Fe a omic pe cen ages o 26.7%, in acco dance o wha was expec ed o Zn0.5Ca0.5Fe2O4 (Zn/Fe a io o 25%). TEM images o zinc/calcium e i e nanopa icles deco a ed wi h sil e clus e s (Figu e 3E,F) show he addi ional appea ance o mo e sphe ical shapes (ma ked on Figu e 3E,F) and also o agglome a es o hese sphe ical pa icles. These ha e sizes o 9.4 ± 1 nm and co espond o he sil e con en o he p epa ed sample, being compa ible wi h he size es ima ion ob ained om XRD. EDX analysis es ima ed an a omic sil e pe cen age o 21%, sligh ly smalle han he de e mined by XRD. The posi ion o he LSPR band (Figu e 1B) depends on he size, shape and e ac i e index o he medium su ounding he sil e nanopa icle. Conside ing sil e nanosphe es in wa e , o which ci a e was used as s abilizing agen , he plasmon band o 10 nm size should appea a 398 nm [29]. Howe e , in his case, he deposi ed sil e pa icles a e expec ed o be ei he nanodisks o hal -sphe es, wi h one side su ounded by he Zn/Ca mixed e i e and he o he side acing an aqueous en i onmen . An inc ease in e ac i e index is expec ed o induce a ed shi in he plasmon band. The e ac i e index o ZnFe2O4 in 400–500 nm egion is abo e 2 [30], so ha a signi ican ed shi om 398 nm is expec ed. Also, he shape and i s aspec a io ha e a p onounced e ec on he LSPR band posi ion, wi h nanodisks o 10 nm diame e and 2 nm heigh showing wo plasmon bands, one a ∼420 nm and he o he , mo e in ense, a ∼560 nm [19]. Thus, he obse ed plasmon band a 435 nm is no incompa ible wi h he ∼10 nm size de e mined by TEM and XRD. E F C D B A Figu e 3. TEM images o he syn hesized nanopa icles. ( A , B ): Zinc/calcium e i e nanopa icles; ( C ): Pa icle size his og am o image ( B ) and i ing o a Gaussian dis ibu ion; ( D ): Aspec a io his og am o pa icles in image (B); (E,F): Zinc/calcium e i e nanopa icles con aining sil e . The posi ion o he LSPR band (Figu e 1B) depends on he size, shape and e ac i e index o he medium su ounding he sil e nanopa icle. Conside ing sil e nanosphe es in wa e , o which ci a e was used as s abilizing agen , he plasmon band o 10 nm size should appea a 398 nm [ 29 ]. Howe e , in his case, he deposi ed sil e pa icles a e expec ed o be ei he nanodisks o hal -sphe es, wi h one side su ounded by he Zn/Ca mixed e i e and he o he side acing an aqueous en i onmen . An inc ease in e ac i e index is expec ed o induce a ed shi in he plasmon band. The e ac i e index o ZnFe 2 O 4 in 400–500 nm egion is abo e 2 [ 30 ], so ha a signi ican ed shi om 398 nm is expec ed. Also, he shape and i s aspec a io ha e a p onounced e ec on he LSPR band posi ion, wi h nanodisks o 10 nm diame e and 2 nm heigh showing wo plasmon bands, one a ~420 nm and he o he , mo e in ense, a ~560 nm [ 19 ]. Thus, he obse ed plasmon band a 435 nm is no incompa ible wi h he ~10 nm size de e mined by TEM and XRD. Small a ea elec on di ac ion (SAED) images o zinc/calcium e i e samples wi hou (Figu e 4A) and wi h sil e (Figu e 4B) show di ac ion spo s ha can be associa ed wi h e i e phase (cyan ings) and sil e (o ange ings), as ollows. The ci cula p o ile o he images was ob ained using he adial p o ile ImageJ plugin and i ed o a sum o Gaussian unc ions, wi h a iable in ensi ies and hal wid hs, bu wi h cen al posi ions de ined by d-spacing alues calcula ed om he di ac ion c ys al planes, co esponding o ei he spinel o cc c ys al s uc u es by op imizing only he la ice cons an s o each phase. This p ocedu e allowed localiza ion o he ings indica ed in Figu e 4, wi h la ice cons an s o 8.286 Å o zinc/calcium e i e and 4.055 Å o sil e . The di ac ion planes co esponding o he peaks a e (1 1 1); (2 2 0); (3 1 1); (4 0 0); (4 2 2); (3 3 3) +(5 1 1); (4 4 0); (6 2 0) and (5 3 3) o zinc/calcium e i e, and (1 1 1) and (2 2 0) o sil e . Addi ional di ac ion spo s, no used in he ci cula p o ile, can be iden i ied o he (4 4 4) +(7 1 1); (5 5 1); (6 4 2) and (7 3 1) +(5 5 3) di ac ion planes o zinc/calcium e i e and (3 1 1) and (2 2 2) o sil e , being ma ked wi h *in Figu e 4. Ma e ials 2019,12, 3582 9 o 16 Ma e ials 2019, 12, x FOR PEER REVIEW 6 o 17 Figu e 3. TEM images o he syn hesized nanopa icles. (A,B): Zinc/calcium e i e nanopa icles; (C): Pa icle size his og am o image (B) and i ing o a Gaussian dis ibu ion; (D): Aspec a io his og am o pa icles in image (B); (E,F): Zinc/calcium e i e nanopa icles con aining sil e . Small a ea elec on di ac ion (SAED) images o zinc/calcium e i e samples wi hou (Figu e 4A) and wi h sil e (Figu e 4B) show di ac ion spo s ha can be associa ed wi h e i e phase (cyan ings) and sil e (o ange ings), as ollows. The ci cula p o ile o he images was ob ained using he adial p o ile ImageJ plugin and i ed o a sum o Gaussian unc ions, wi h a iable in ensi ies and hal wid hs, bu wi h cen al posi ions de ined by d-spacing alues calcula ed om he di ac ion c ys al planes, co esponding o ei he spinel o cc c ys al s uc u es by op imizing only he la ice cons an s o each phase. This p ocedu e allowed localiza ion o he ings indica ed in Figu e 4, wi h la ice cons an s o 8.286 Å o zinc/calcium e i e and 4.055 Å o sil e . The di ac ion planes co esponding o he peaks a e (1 1 1); (2 2 0); (3 1 1); (4 0 0); (4 2 2); (3 3 3) + (5 1 1); (4 4 0); (6 2 0) and (5 3 3) o zinc/calcium e i e, and (1 1 1) and (2 2 0) o sil e . Addi ional di ac ion spo s, no used in he ci cula p o ile, can be iden i ied o he (4 4 4) + (7 1 1); (5 5 1); (6 4 2) and (7 3 1) + (5 5 3) di ac ion planes o zinc/calcium e i e and (3 1 1) and (2 2 2) o sil e , being ma ked wi h * in Figu e 4. Figu e 4. TEM SAED images o he syn hesized zinc/calcium e i e nanopa icles, wi hou (A) and wi h sil e (B). Below each image, a adial p o ile oge he wi h a i is ep esen ed, conside ing he di ac ion lines o zinc/calcium e i e (ma ked by cyan iangles) and sil e (ma ked by o ange ci cles). Rings ma ked by as e isks pass by addi ional di ac ion spo s, bu did no de ine a comple e ci cle, no being possible o use in he adial p o ile. 3.2. Magne ic P ope ies Figu e 4. TEM SAED images o he syn hesized zinc/calcium e i e nanopa icles, wi hou ( A ) and wi h sil e ( B ). Below each image, a adial p o ile oge he wi h a i is ep esen ed, conside ing he di ac ion lines o zinc/calcium e i e (ma ked by cyan iangles) and sil e (ma ked by o ange ci cles). Rings ma ked by as e isks pass by addi ional di ac ion spo s, bu did no de ine a comple e ci cle, no being possible o use in he adial p o ile. 3.2. Magne ic P ope ies The magne ic p ope ies o he p epa ed zinc/calcium e i e nanopa icles, wi h and wi hou sil e coa ing (Figu e 5) we e cha ac e ized by measu ing hei magne ic hys e esis loop, which shows he ela ionship be ween he induced magne ic momen and he applied magne ic ield (H). The calcined nanopa icles p esen a supe pa amagne ic beha io , as he a io be ween emnan magne iza ion (M ) and maximum magne iza ion (M s ) is below 0.1 (Table 4). I below 0.1, his a io indica es ha mo e han 90% o he magne iza ion is los upon he emo al o he applied magne ic ield [ 31 , 32 ]. The e y low maximum magne iza ion o he non-calcined nanopa icles is jus i ied by hei highly amo phous na u e, as e i ied by XRD. Wi h calcina ion, he maximum magne iza ion inc eases en imes (Figu e 5), main aining a low coe ci i y. Table 4. Coe ci e ield (H c ), sa u a ion magne iza ion (M s ), emnan magne iza ion (M ) and a io M /Ms o zinc/calcium e i es a oom empe a u e. - Hc(Oe) Ms(emu/g) M (emu/g) M /Ms Zn0.5Ca0.5Fe2O4non-calcined 1.8 2.41 8×10−53×10−5 Zn0.5Ca0.5Fe2O4calcined 7.5 20.45 0.022 1×10−3 Sil e coa ed Zn0.5Ca0.5Fe2O4calcined 5.3 18.14 0.016 9×10−4 Ma e ials 2019,12, 3582 16 o 16 26. Solano, E.; F on e a, C.; Puig, T.; Ob ado s, X.; Rica , S.; Ros, J. Neu on and X- ay di ac ion s udy o e i e nanoc ys als ob ained by mic owa e-assis ed g ow h. A s uc u al compa ison wi h he he mal syn he ic ou e. J. Appl. C ys allog . 2014,47, 414–420. [C ossRe ] 27. Pe ei a, D.S.M.; Ca doso, B.D.; Rod igues, A.R.O.; Amo im, C.O.; Ama al, V.S.; Almeida, B.G.; Quei oz, M.-J.R.P.; Ma inho, O.; Bal aza , F.; Calhelha, R.C.; e al. Magne oliposomes con aining calcium e i e nanopa icles o applica ions in b eas cance he apy. Pha maceu ics 2019,11, 477. [C ossRe ] 28. 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