Lun et l. Nnoscle Reserch Letters (2017) 12:219 DOI 10.1186/s11671-017-1996-x NANO EXPRESS Controlled Hydrotherml Synthesis nd Photoluminescence of Nnocrystlline ZnG 2 O 4 :Cr 3+ Monospheres Tin Lun 1,2, Jinhn Liu 1,2, Xioxue Yun 1,2 nd Ji-Gung Li 1,2,3* Open Access Astrct The hydrotherml synthesis of nnocrystlline ZnG 2 O 4 :Cr 3+ (ZGC) red phosphor monospheres ws ccomplished in this work, nd the effects of system ph, rectnt content, rection time, nd citrte nions (Cit 3 ) on the phse nd morphology evolution of the product were systemticlly studied. Under the optimized conditions of Cit 3 /M = 1.0 molr rtio (M = totl ctions), ph = 5.0, nd 0.2 mmol of Zn 2+, well-dispersed ZGC monospheres with n verge dimeter of ~454 ± 56 nm (verge crystllite size ~15 nm) were successfully otined vi hydrotherml rection t 180 C for 18 h. Cit 3+ ions were demonstrted to e crucil to the formtion of monospheres nd sustntilly ffect the pthwy of phse formtion. The ZGC monospheres clcined t 800 C (verge dimeter ~353 ± 59 nm; verge crystllite size ~30 nm) hve n intensity ~6 times tht of the originl phosphor for the 700 nm red emission of Cr 3+ (the 2 E 4 A 2 trnsition) under excittion with the O 2 G 3+ chrge trnsfer nd t 250 nm. Fluorescence decy nlysis found tht the 700 nm emission hs lifetime vlues of ~5 ms for the ZGC phosphors. Keywords: ZnG 2 O 4, Cr red phosphor, Monospheres, Hydrotherml synthesis, Photoluminescence Bckground The zinc gllte compound of ZnG 2 O 4 elongs to the group of cuic-structured AB 2 O 4 norml spinels (spce group: Fd-3m), in which the Zn 2+ ions occupy the tetrhedrlly coordinted A sites nd the G 3+ ions reside t the octhedrlly coordinted B sites. The compound hs een drwing incresing ttention for wide pplictions in the fields of lighting, disply, nd opticl imging for iology, owing to its excellent therml nd chemicl stility nd wide ndgp (~4.4 4.7 ev) [1]. ZnG 2 O 4 is lso known s type of self-ctivted phosphors nd my emit lue light under short UV or electron em irrdition, owing to the occurrence of O-G chrge trnsfer [1]. As phosphor host, the Mn 2+,Eu 3+, nd Cr 3+ ctivtor ions doped into the ZnG 2 O 4 lttice nd residing t the G 3+ sites re known to emit right green, red, nd red luminescence under proper excittions, respectively [2]. It is lso worth noting tht the * Correspondence: LI.Jigung@nims.go.jp 1 Key Lortory for Anisotropy nd Texture of Mterils (Ministry of Eduction), Northestern University, Shenyng, Lioning 110819, Chin 2 Institute of Cermics nd Powder Metllurgy, School of Mterils Science nd Engineering, Northestern University, Shenyng, Lioning 110819, Chin Full list of uthor informtion is ville t the end of the rticle trnsition metl ion of Cr 3+ my emit ner-infrred persistent luminescence when the chemicl composition nd lttice defects of ZnG 2 O 4 re properly mnipulted, which llows the mteril to hve potentil pplictions in the opticl imging of vsculriztion, tumor, nd grfted cells [3 5]. It is widely ccepted tht phosphor prticles with sphericl shpe my exhiit superior luminescence nd hve dvntges in prcticl ppliction over other morphologies, owing to the fct tht the sphericl shpe my minimize the light scttering on prticle surfces nd denser luminescence lyer cn e constructed vi close pcking of the spheres [6, 7]. For these, developing technique to synthesize Cr 3+ -doped ZnG 2 O 4 (ZnG 2 O 4 :Cr) phosphor spheres is of prcticl importnce. Vrious synthetic pproches hve een estlished up to dte for ZnG 2 O 4 -sed phosphors, typiclly including solid stte rection, therml evportion of ZnO-G powders, pulverizing single crystls grown y the flux method, sol-gel, electrospin, hydrotherml rection, nd chemicl precipittion [8 13]. Morphology control of the product, however, yet remins n issue needed to ddress. We introduced in this work hydrotherml strtegy to produce well-defined ZnG 2 O 4 :Cr 3+ The Author(s). 2017 Open Access This rticle is distriuted under the terms of the Cretive Commons Attriution 4.0 Interntionl License (http://cretivecommons.org/licenses/y/4.0/), which permits unrestricted use, distriution, nd reproduction in ny medium, provided you give pproprite credit to the originl uthor(s) nd the source, provide link to the Cretive Commons license, nd indicte if chnges were mde.
Lun et l. Nnoscle Reserch Letters (2017) 12:219 Pge 2 of 8 monospheres, nd the effects of citrte (Cit 3 )nions, system ph, nd rectnt content on the phse structure nd morphology evolution were demonstrted in detil. In the following sections, we report the synthesis nd photoluminescence properties of the nnostructured ZnG 2 O 4 :Cr 3+ monospheres. Methods The stock solutions of Cr 3+ (0.002 M) nd Zn 2+ (0.1 M) were otined y dissolving the corresponding metl nitrtes in distilled wter, nd the G 3+ solution (0.2 M) ws prepred y dissolving G 2 O 3 in nitric cid (HNO 3 ) vi hydrotherml tretment t 100 C. Proper mounts of the ove solutions were then mixed together ccording to the intended chemicl formul of Zn (G 1.995 Cr 0.005 ) O 4. Whenever needed, certin mount of trisodium citrte (Cit 3 ) ws dded into the solution, followed y dilution with distilled wter to totl volume of 75 ml. Under mgnetic stirring, proper mount of HNO 3 (63 wt%) or mmonium hydroxide solution (NH 4 OH, 28 wt%) ws then dded to djust the mixture to certin ph vlue. After homogenizing for 30 min, the s-otined mixture ws trnsferred to Teflon-lined stinless steel utoclve, which ws then put into n ir oven preheted to 180 C for certin period of hydrotherml rection. After nturl cooling to room temperture, the hydrotherml product ws collected vi centrifugtion nd wshed three times with deionized wter nd once with ethnol, followed y drying in n ir oven t 60 C for 12 h. Clcintion of the hydrotherml product ws performed in the ir t 800 C for 2 h. The hydrotherml product will herefter e referred to s nzgc, where n is the mount of Zn 2+ (in mmol) in the hydrotherml rection system for the synthesis of Zn (G 1.995 Cr 0.005 )O 4 phosphors. Phse identifiction ws mde vi X-ry diffrctometry (XRD, Model PW3040/60, Philips, Eindhoven, The Netherlnds) operted t 40 kv/40 ma, using nickel-filtered Cu-Kα rdition (λ = 0.15406 nm) nd scnning rte of 5 /min in the 2θ rnge of 10 70. The morphology nd microstructure of the products were nlyzed y field emission scnning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo, Jpn) under n ccelertion voltge of 15 kv. Thermogrvimetry of the smple ws mde in the ir on Model Thermo Plus TG8120 equipment (Rigku, Tokyo), using heting rte of 10 C/min. Fourier trnsform infrred spectroscopy (FT-IR, Spectrum RXI, PerkinElmer, Shelton, CT, USA) ws performed y the stndrd KBr method. Photoluminescence properties of the phosphors, including excittion, emission, nd fluorescence decy, were mesured t room temperture using n LS-55 fluorospectrophotometer (PerkinElmer). Results nd Discussion Smples Synthesized Without Citrte Anions Without the ttendnce of ny orgnic molecules, the effects of system ph on the phse structure of the hydrotherml product were exmined for 2 mmol of Zn 2+ t the highest ville hydrotherml temperture of 180 C. Figure 1 shows XRD ptterns of the 24 h rection products, where it is seen tht the ph = 5 smple is solely of well-crystllized α-gooh (JCPDS no. 06-0180) hving n orthorhomic crystl structure, while those of ph = 7 nd 9 cn e indexed to the intended ZGC compounds (JCPDS no. 01-071-0843). This is in ccordnce with the literture tht G 3+ undergoes extensive hydrtion nd hydrolysis in n queous solution to form [G (OH) x (H 2 O) y ] 3 x complex ion even under n cidic condition, owing to its reltively high oxidtion stte (3+) nd rther smll ionic size (0.062 nm for CN = 6) [14]. The oltion rection mong [G (OH) x (H 2 O) y ] 3 x (removl of one wter molecule vi rection of two hydroxyls) would then led to the formtion of GOOH. The lck of ny product contining Zn is primrily ecuse the hydrolysis of Zn 2+ ions to induce precipittion is voided y the low solution ph of 5. This is lso understndle from the view point tht either ZnO or Zn (OH) 2 is mphoteric nd cnnot exist under sufficiently low ph vlues. It cn lso e inferred from Fig. 1 tht higher system ph produces etter crystllinity for the 2ZGC product, s seen from the shrper XRD peks of the ph = 9 smple. Brodening nlysis of the (311) diffrction with the Scherrer formul yielded verge crystllite sizes of ~7 nd 11 nm for the ph = 7 nd ph = 9 products, respectively. Figure 2 shows FE-SEM morphologies of the three products exhiited in Fig. 1. The α-gooh prticles (Fig. 2) re short rods with rectngulr cross sections, whose lengths nd dimeters re up to ~3 μm nd ~600 nm, respectively. Such crystl morphology seems rising from the crystlliztion hit of α- GOOH nd ws lso oserved for the products synthesized vi homogeneous hydrolysis of G (NO) 3 t ~90 C [15] nd vi hydrotherml rection of GCl 3 -H 2 O-NOH solutions t 180 C nd ph = 6 8 [16]. On the contrry, oth the ph = 7 nd ph = 9 products (2ZGC) re cottonor sponge-like fluffy gglomertes, with the tiny primry crystllites unresolvle with the FE-SEM instrument. Optimiztion of the Synthesis Prmeters to Yield ZGC Monospheres Citrte nions (Cit 3 ) re known to e highly complexing for most of the metl ctions nd hve een frequently used in solution-sed mteril synthesis for rection kinetics nd morphology control. Under the sme hydrotherml conditions (ph = 9 nd rection t 180 C for 24 h), the effects of Cit 3 ddition on prticle
Lun et l. Nnoscle Reserch Letters (2017) 12:219 Pge 3 of 8 c Fig. 1 The 2ZGC products otined y 24 h of hydrotherml rection t 180 C nd under ph vlues of 5, 7, nd c 9 morphology of the 2ZGC phosphors re shown in Fig. 3. It is clerly seen tht sphericl prticles were resulted t the Cit 3 /M (M = totl ctions) molr rtio R of 1.0, though the prticles re yet not uniform in size nd tend to dhere to ech other (Fig. 3). Such sphericl prticles were elieved to hve een formed vi rpid simultneous nucletion/growth in short time durtion [17, 18] nd lso imply the gluing effects of Cit 3 nions. At the insufficient R vlue of 0.5 (Fig. 3), the Cit 3 ions were not le to well glue up the primry prticles/crystllites of 2ZGC into spheres, ut the oserved irregulrly shped gglomertes pper denser thn those shown in Fig. 1d. At the even higher R vlues of 1.5 nd 2.0, the products re simultneously composed of ggregted spheres nd much smller prticles. Such product morphology my hve een resulted from sustntilly heterogeneous nucletion/growth, since the chelting ility of Cit 3 improves t higher Cit 3 content, which mkes the metl ctions needed for 2ZGC precipittion e relesed in rther slow wy, nd s result, multi-step (heterogeneous) nucletion/growth would tke plce since no homogeniztion of the rection system y stirring ws performed during the hydrotherml rection in this work [17]. To further improve the dispersion nd size uniformity of the spheres shown in Fig. 3, we lowered the Zn 2+ content to 0.2 mmol nd the effects of solution ph on prticle morphology of the products (0.2ZGC) were studied t the optiml Cit 3 /M molr rtio R of 1.0. Figure 4 shows FE-SEM morphologies of the products otined vi hydrotherml rection t 180 C for 24 h. It is clerly seen tht lowering the Zn 2+ content is indeed Fig. 2 FE-SEM microgrphs showing morphologies of the 2ZGC products otined y 24 h of hydrotherml rection t 180 C nd under ph vlues of 5, 7, nd c 9
Lun et l. Nnoscle Reserch Letters (2017) 12:219 Pge 4 of 8 Fig. 3 FE-SEM microgrphs showing morphologies of the 2ZGC products otined y 24 h of hydrotherml rection t 180 C nd ph = 9. The Cit 3 /M (M totl ction) molr rtios re 0.5, 1, c 1.5, nd d 2.0 effective to produce etter dispersed prticles of nrrower size distriution (verge size ~840 ± 160 nm) ut only t the low system ph of 5 (Fig. 4). At the higher ph vlues of 7 nd 9, the products turned into reltively dispersed smll prticultes insted of spheres (Fig. 4, c). Compring Fig. 4 with Fig. 3 thus reveled the significnt effects of ction concentrtion (in terms of Zn 2+ content) on the optiml ph needed to produce sphericl prticles, nd this cn e understood s follows. Lowering the Zn 2+ content simultneously decreses the totl mount of Cit 3 in solution since the R rtio is fixed, nd this would in turn lower the gluing effects of Cit 3 towrd the primry prticles/crystllites. Under n cidic condition, for exmple ph = 5, the surfces of the primry prticles/crystllites re protonted, nd the positive chrge llows the surfces to preferentilly dsor the negtively chrged Cit 3 nions. As result, the primry prticles/crystllites were glued together y the dsored Cit 3 to form the spheres shown in Fig. 4. Under the higher ph vlues of 7 nd 9, the Cit 3 nions cnnot e effectively dsored on prticle/crystllite surfces, nd thus, smller dispersed prticultes were formed in the sence of sufficient Cit 3 gluing. Figure 5 shows XRD ptterns of the 0.2ZGC products exhiited in Fig. 4. It is evident tht they cn ll e well indexed to cuic-structured ZnG 2 O 4, whose stndrd diffrctions were included in the figure for comprison. It is interesting to point out tht the hydrotherml product synthesized in the sence of Cit 3 is phse-pure α- GOOH (Fig. 1) rther thn the 0.2ZGC compound shown in Fig. 5. This indictes tht the Cit 3 dditives hve significntly modified the hydrolysis ehviors of Zn 2+ nd G 3+ nd ltered the pthwy of hydrotherml rection, though the exct mechnism yet needs clrifiction. Another oservtion is tht the smple synthesized under lower system ph exhiited more rodened diffrction peks, indicting tht it is less well crystllized nd hs smller crystllite sizes. This is understndle in view tht more Cit 3 nions would e dsored on crystllite surfces under lower ph, which would in turn inhiit crystllite growth. Brodening nlysis of the (311) diffrction with the Scherrer eqution found verge crystllite sizes of ~6.4, 9.7, nd 10.8 nm for the products synthesized under the ph vlues of 5, 7, nd 9, respectively. Time-course phse nd morphology evolution ws studied for the 0.2ZGC smple under the optimized Fig. 4 FE-SEM microgrphs showing morphologies of the 0.2ZGC products otined y 24 h of hydrotherml rection t 180 C. The Cit 3 /M molr rtio R is 1.0 in ech cse, nd the ph vlues re 5, 7, nd c 9
Lun et l. Nnoscle Reserch Letters (2017) 12:219 Pge 5 of 8 c Fig. 5 XRD ptterns of the 0.2ZGC products otined y 24 h of hydrotherml rection t 180 C. The Cit 3 /M molr rtio R is 1.0 in ech cse, nd the ph vlues re 5, 7, nd c 9 conditions of 180 C, ph = 5, nd Cit 3 /M molr rtio R of 1.0. Figure 6 shows XRD ptterns of the products otined for different durtions of hydrotherml rection. It is seen tht the 6 24-h smples re ll well indexle to the ZnG 2 O 4 phse, with the loctions nd reltive intensities of the diffrction peks coincide well with the stndrd diffrction file (JCPDS no. 01-071-0843). It should e noted tht no solid cn e recovered for the shorter rection time of 3 h. The diffrction peks gin intensity with incresing rection time, owing to improved crystllinity. Brodening nlysis of the (311) diffrction yielded verge crystllite sizes of ~8, 13, 15, nd 15 nm for the ZGC phosphors otined vi 6, 12, 18, nd 24 h of rection, respectively. Figure 7 shows the prticle morphology of 0.2ZGC s function of rection time. It is seen tht sphericl prticles hve een resulted fter 6 h of rection. In view tht the spheres re quite uniform in shpe nd size (~295 ± 34 nm) while 3 h of rection did not yield ny solid, it cn thus e inferred tht the spheres were formed in rther short durtion of time vi rpid simultneous nucletion/growth s forementioned. The verge size of the spheres increses with incresing rection time, which reched ~422 ± 47 nm d c Fig. 6 XRD ptterns of the products otined fter 6, 12, c 18, nd d 24 h of hydrotherml rection t 180 C. The Cit 3 /M molr rtio R is 1.0, nd the system ph is 5 in ech cse
Lun et l. Nnoscle Reserch Letters (2017) 12:219 Pge 6 of 8 c d e Fig. 7 FE-SEM microgrphs ( d) showing morphologies of the 0.2ZGC products otined fter 6, 12, c 18, nd d 24 h of rection t 180 C. The Cit 3 /M molr rtio R is 1.0, nd the system ph is 5 in ech cse. e is the size distriution of smple c otined vi dynmic lser scttering. d is the sme smple of Fig. 4 ut viewed under lower mgnifiction t 12 h, ~454 ± 56 nm t 18 h, nd ~840 ± 158 nm t 24 h. The size increment is lrgely cused y Ostwld ripening, which is enhnced y the cidic rection condition (ph = 5). It is lso seen tht the 18 h product hs smoother prticle surfces nd more sphericl shpe thn the 6 nd 12 h products nd is etter dispersed nd more uniform in prticle size thn the 24 h product. Indeed, prticle sizing vi lser diffrction found tht the 18 h product exhiits n lmost single modl size distriution (Fig. 7e) nd hs n verge dimeter of ~454 ± 56 nm. This smple ws therefore chosen for further chrcteriztions. TG nlysis of the 18 h product found three mjor stges of weight losses nd totl weight loss of ~9 wt% up to 1000 C (Fig. 8), the origin of which will lter e clrified with the results of FT-IR. It is cler tht the weight loss of the smple hs lmost terminted t ~800 C. Fig. 8 TG trce for the 18 h product shown in Fig. 7c Fig. 9 FT-IR spectr for the 18 h product (lck line) nd tht clcined t 800 C for 2 h (red line)
Lun et l. Nnoscle Reserch Letters (2017) 12:219 Pge 7 of 8 Fig. 10 A comprison of the 18 h smple efore (line ) nd fter (line ) clcintion t 800 C (left pnel) nd FE-SEM prticle morphology of the clcintion product (right pnel) FT-IR spectroscopy of the s-synthesized 18 h product found O H stretching virtion of wter molecules t ~3425 cm 1,COO sorptions of Cit 3 t ~1588 nd 1395 cm 1, nd CH 2 virtions t ~2923 nd 2850 cm 1 [19 21]. It is noteworthy tht the O H ending mode of wter, usully occurring t ~1640 cm 1,overlps with the ~1588 cm 1 virtion of COO nd contriutes to the rodening of the nd in the ~1440 1750 cm 1 region (the lck line). The two nds locted t ~610 nd 482 cm 1 cn e scried to Zn O ndg O virtions, respectively [22]. After 800 C clcintion, the sorptions corresponding to H 2 OndCOO groups re rely oservle while metl-oxygen virtions were enhnced due to incresed crystllinity of the smple (the red line) (Fig. 9). In ddition, the twin nds t ~2300 cm 1 oserved for oth the originl nd clcined powders re rising from tmospheric CO 2. The FT-IR results thus suggest tht the weight loss oserved for the originl 18 h smple in Fig. 8 is lrgely due to dehydrtion nd the removl of dsored Cit 3 nions. The left-hnd pnel of Fig. 10 compres XRD ptterns of the 18 h powder efore nd fter clcintion t 800 C. It is seen tht the clcintion did not lter the phse purity ut sustntilly improved the crystllinity of the phosphor. Anlysis with the (311) diffrction found verge crystllite sizes of ~15.0 nd 30.4 nm nd lttice constnts of ~0.83402 nd 0.83375 nm for the ssynthesized nd clcined powders, respectively. The lttice prmeters ssyed in this work re close to the vlue of = 0.83349 nm for ZnG 2 O 4 in the stndrd dt file. FE-SEM oservtion indicted tht the clcintion product is solely composed of dispersed monospheres, ut the verge prticle size contrcted from ~454 ± 56 to 353 ± 59 nm due to the mss loss nd densifiction during clcintion. Figure 11 shows the excittion nd emission spectr of the s-synthesized nd 800 C clcined 0.2ZGC phosphors (the 16 h product). It cn e seen tht the excittion spectrum otined y monitoring the ~700 nm red emission of Cr 3+ is composed of four min nds covering wide spectrl region from ultrviolet to red, with those centered t ~250, 275, 440, nd 550 nm rising from O 2 G 3+ chrge trnsfer, O 2 Cr 3+ chrge trnsfer, the 4 A 2 4 T 1 d-d trnsition of the Cr 3+ ctivtor, nd the 4 A 2 4 T 2 d-d trnsition of Cr 3+, respectively Fig. 11 Photoluminescence excittion (lines E1 nd E2) nd emission (lines L1 nd L2) spectr() nd fluorescence decy kinetics () for the s-synthesized 18 h smple (lines E1, L1, ndd1) nd tht clcintion t 800 Cfor2h(lines E2, L2, ndd2)
Lun et l. Nnoscle Reserch Letters (2017) 12:219 Pge 8 of 8 [23]. The ppernce of O 2 G 3+ chrge trnsfer nd y monitoring Cr 3+ emission implies the occurrence of efficient G 3+ Cr 3+ energy trnsfer. Clcintion t 800 C gretly improves the excittion intensity, owing to the removl of wter molecules, orgnic residues, nd prticulrly the improved crystllinity of the phosphor powder. Exciting the phosphor with the O 2 G 3+ chrge trnsfer nd t 250 nm produced the 2 E 4 A 2 emission of the Cr 3+ ctivtors t ~700 nm [1], which further confirms the occurrence of G 3+ Cr 3+ energy trnsfer. It is seen from the PL spectr tht the phosphor clcined t 800 C hs n emission intensity ~6 times tht of the ssynthesized one. Fluorescence decy kinetics of the 700 nm emission under 250-nm excittion is shown in Fig. 11. Both of the decy curves cn e well fitted to the single exponentil function of I = I 0 exp ( t/τ), from which the lifetime of the 0.2ZGC phosphor ws clculted to e 4.75 ± 0.07 ms for the s-synthesized smple nd 4.98 ± 0.06 ms for the clcined smple. The lifetime determined herein is little longer thn the reported vlues of ~1.4 2.5 ms ut is onthesmeorderofmgnitude[23]. Conclusions Nnocrystlline ZnG 2 O 4 :Cr 3+ (ZGC) monospheres were synthesized in this work vi hydrotherml rection t 180 C nd in the presence of Cit 3 ions, which emit red emission t 700 nm (the 2 E 4 A 2 trnsition of Cr 3+ ) upon short UV excittion with the O 2 G 3+ chrge trnsfer nd t 250 nm. The optiml processing prmeters were determined to e Cit 3 /M = 1.0 molr rtio (M = totl ctions), ph = 5.0, 0.2 mmol of Zn 2+,ndrection time of 18 h. Clcining the s-synthesized ZGC monospheres t 800 C for 2 h rought out n ~6-fold intensity increment for the 700 nm emission, owing to dehydrtion, removl of orgnic residues, nd crystllinity improvement. The phosphor monospheres were nlyzed to hve lifetime vlues of ~5 ms for the 700 nm red emission. Acknowledgements This work ws finncilly supported y the Ntionl Trining Progrm of Innovtion nd Entrepreneurship for undergrdutes (201610145430). Authors Contriutions TL, JHL, nd XXY crried out the experiments; JGL nd TL were involved in the results discussion nd drfted the mnuscript. All the uthors hve red nd pproved the finl mnuscript. Competing Interests The uthors declre tht they hve no competing interests. Pulisher s Note Springer Nture remins neutrl with regrd to jurisdictionl clims in pulished mps nd institutionl ffilitions. Author detils 1 Key Lortory for Anisotropy nd Texture of Mterils (Ministry of Eduction), Northestern University, Shenyng, Lioning 110819, Chin. 2 Institute of Cermics nd Powder Metllurgy, School of Mterils Science nd Engineering, Northestern University, Shenyng, Lioning 110819, Chin. 3 Reserch Center for Functionl Mterils, Ntionl Institute for Mterils Science, 1-1 Nmiki, Tsuku, Irki 305-0044, Jpn. Received: 7 Ferury 2017 Accepted: 13 Mrch 2017 References 1. Gu Z, Liu F, Li X, Howe J, Xu J, Zho Y, Pn Z (2009) Red, green, nd lue luminescence from ZnG 2 O 4 nnowire rrys. J Phys Chem Lett 1:354 357 2. Zhng Y, Wu ZJ, Geng DL, Kng XJ, Shng MM, Li XJ, Lin HZ, Cheng ZY, Lin J (2014) Full color emission in ZnG 2 O 4 : simultneous control of the sphericl morphology, luminescent, nd electric properties vi hydrotherml pproch. Adv Funct Mter 24:6581 6593 3. 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