Sol–gel-deposited ZnO thin films A review

Materials Science and Engineering B 174 (2010) 18–30Contents lists available at ScienceDirectMaterials Science and EngineeringBj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m s ebReviewSol–gel-deposited ZnO thin films:A reviewLamia Znaidi ∗Laboratoire d’Ingénierie des Matériaux et des Hautes Pressions (LIMHP),CNRS-UPR 1311,UniversitéParis 13,99Avenue J.B.Clément,93430Villetaneuse,Francea r t i c l e i n f o Article history:Received 31August 2009Received in revised form 6June 2010Accepted 3July 2010Keywords:Zinc oxide Sol–gel Colloids Thin filmsPreferential orientation c -Axisa b s t r a c tDuring the last years,ZnO thin films have been studied extensively due to their potential applications in e.g.piezoelectric and optoelectronic devices or photovoltaic cells.Ordered c -axis orientation of ZnO crystallites is desirable for applications where crystallographic anisotropy is a prerequisite such as for short-wavelength semiconductor diode lasers (SDLs),and piezoelectric surface acoustic wave or acousto-optic devices.Many works were dedicated to c -axis oriented ZnO thin films elaboration and the study of their properties,including physical and chemical methods.For instance,sol–gel processes are partic-ularly well adapted to produce ZnO films in a simple,low-cost and highly controlled way.This review summarizes the main chemical routes used in the sol–gel synthesis of undoped ZnO thin films and high-lights the chemical and physical parameters influencing their structural properties.In this process,the ZnO films synthesis includes three principal steps:(i)solution preparation,(ii)coating and (iii)heat treatment.For the first step,the particle formation is discussed including nucleation and growth,particle size,morphology and colloids stability.These three steps involve several parameters such as:(i)nature and concentration of precursor,solvent and additive,and solution aging time,for the chemical system,(ii)coating method,thickness and substrate for the coating step,and (iii)pre-and post-heat treatment for the last step.The influence of these steps and synthesis parameters on ZnO thin films orientation is discussed.© 2010 Elsevier B.V. All rights reserved.Contents 1.Introduction..........................................................................................................................................192.Sol–gel method toward zinc oxide synthesis........................................................................................................202.1.Chemical systems:precursors,solvents,and additives......................................................................................202.1.1.Precursors...........................................................................................................................202.1.2.Solvents.............................................................................................................................222.1.3.Additives............................................................................................................................232.2.Particle formation ............................................................................................................................232.2.1.Growth mechanisms................................................................................................................232.2.2.Nucleation and growth –particle size .............................................................................................242.2.3.Morphology.........................................................................................................................242.2.4.Colloids stability and thermal behavior............................................................................................243.Formation of oriented film...........................................................................................................................253.1.Film formation................................................................................................................................253.2.Main orientations observed..................................................................................................................253.2.1.General consideration ..............................................................................................................253.2.2.Role of the chemical system:precursor nature and its concentration,solvent,additive,aging time.............................253.2.3.Role of coating:method,speed,thickness,substrate..............................................................................283.2.4.Role of the heat treatments:pre-heat treatment and post-heat treatment.......................................................284.Conclusion............................................................................................................................................29Acknowledgement...................................................................................................................................29References ...........................................................................................................................................29∗Tel.:+33149403445;fax:+33149403414.E-mail address:lamia@limhp.univ-paris13.fr .0921-5107/$–see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.mseb.2010.07.001L.Znaidi/Materials Science and Engineering B174 (2010) 18–30191.IntroductionDuring the last years,ZnO thinfilms have been studied extensively due to their potential applications,as piezoelectric transducers,optical waveguides,acousto-optic media,surface acoustic wave devices,conductive gas sensors,transparent con-ductive electrodes,solar cell windows,and varistors[1–4].ZnO,a II–VI semiconductor,is now recognized as a promising candidate for blue and ultraviolet light-emitting diodes or laser diodes because of its wide-band gap of3.37eV and large exciton binding energy of60meV[2–7].Its large exciton binding energy allows excitonic absorption and recombination even at room tem-perature,which makes this material appealing[7].Excitonic laser oscillation with a very low threshold(24kW/cm2)at room tem-perature was confirmed in50nm thinfilms on sapphire(0001) substrates[7].Such observations indicate that an exciton-related recombination process can be utilized as an optoelectronic device operable at room temperature[7].Reynolds et al.[8]published in1996thefirst report of optically pumped lasing at low temperature(2K)from a ZnO platelet grown from the vapor phase.One year later,ultraviolet spontaneous and stimulated emissions from ZnO thinfilms at room temperature were observed by Zu et al.[9],Bagnall et al.[10]and Segawa et al.[11].These studies were followed by the works by Bagnall et al.[12],Yu et al.[13],Tang et al.[14],Kawasaki et al.[15]and Ohtomo et al.[16]in1998.In these studies,ZnO thinfilms were principally elaborated by laser molecular-beam epitaxy(L-MBE),showing high crystallinity with c-axis orientation(c-axis perpendicular to the substrate).Thesefilms consist of an epitaxially ordered array of hexagonal microcrystallites;and the facets of all hexagons are par-allel to those of the others,forming natural Fabry–Pérot lasing cavities[13,14].The grain size in thesefilms is about50–55nm in most cases;and thefilm thicknesses lie between50and500nm. Yu et al.[13]reported room temperature measurements of optical gain and gain spectra of ultraviolet emission from ZnO thinfilms. They found that the lasing threshold has a minimum near afilm thickness of55nm;increasing rapidly forfilms with thickness of 40nm;and no lasing was observed infilms thinner than30nm. Kawasaki et al.[15]reported that there is an optimum crystallite size of50nm for observing excitonic stimulated emission.On the other hand,the orientation along the c-axis parallel to the substrate surface was also observed in the case of epitaxial ZnO and group-III-nitride thinfilms[17–19].Lin et al.[17]have prepared epitaxial ZnOfilms by metal-organic chemical vapor deposition (MOCVD)on␥-LiAlO2substrate.The authors reported that the polarized Raman and optical transmission spectra indicated that these epitaxialfilms exhibited optical anisotropy,which have appli-cations in certain optical devices,such as the UV modulator and polarization-dependent beam switch.Also,Wraback et al.[18]used the same method(MOCVD)for preparing epitaxial ZnOfilms on the (01¯12)surface of sapphire substrates.They have demonstrated an optically addressed normal incidence ultraviolet light modulator, which exploits the optical anisotropy inherent in ZnO epitaxially grown on(01¯12)sapphire to achieve high contrast.They reported that suchfilms are interesting for acousto-optic,photochromic, and piezoelectric device applications.Schaadt et al.[19]have used the molecular-beam epitaxy for preparing the group-III-nitride thinfilms(GaN/AlN)grown on␥-LiAlO2by molecular-beam epi-taxy.The authors presented a polarization-dependent beam switch based on group-III-nitride thinfilms and they have fabricated a two-color distributed Bragg reflector consisting of alternating (1¯100)-oriented(M-plane)AlN and GaN layers.They have demon-strated that this birefringent distributed Bragg reflector offers two main functionalities,polarization-dependent beam switching and polarization selection.They reported that group-III nitrides of the wurtzite crystal structure exhibit linear birefringence(the refrac-tive index is different for light polarized parallel and perpendicular to the c-axis)and infilms grown on non-polar surfaces,where the c-axis lies in thefilm plane,this effect can be utilized for beam switching and polarization selection.Besides,Romero et al.[20]reported that in order to be prop-erly suitable for acoustic wave devices,polycrystalline ZnO thin films must meet two requirements.First,the basal plane of ZnO crystallites must be oriented parallel to the plane of the substrate (c-axis orientation).Second,the ZnO thinfilms must have a colum-nar structure with void-free grain boundaries.For these reasons, c-axis oriented thin ZnO thinfilms are of interest for the produc-tion of surface and bulk acoustic wave devices.The tilt of the c-axis can determine the type of ultrasonic wave produced by pulse-echo transducers[20].Martin et al.[21]have deposited piezoelectric ZnO thinfilms by reactive magnetron sputtering for use in ultrasonic transducers.They reported that thefilms with the optimum piezo-electric response were achieved in transducers that possessed the hexagonal(002)crystal orientation.The tilt of the c-axis deter-mined the type of ultrasonic wave produced by the pulse-echo transducers.When the angle of the c-axis to the substrate surface was0◦,the longitudinal mode(L-wave)was dominant.At a16◦tilt, the longitudinal and shear waves(S-wave)were mixed,and at a tilt of41◦,the shear wave was dominant[21].So,the both orientation(c-axis perpendicular or parallel to the substrate)were observed in ZnO thinfilms.Nevertheless,the c-axis perpendicular to the substrate stays the one,which is mostly observed and especially when the growth is not made by epitaxy. This point will be discussed in Section3.2.In the following text, c-axis or(002)orientation means c-axis perpendicular to the sub-strate;and a-axis or(100)orientation means c-axis parallel to the substrate.In brief,ZnO crystallites with preferential orientation are desirables for applications where crystallographic anisotropy is a prerequisite such as for UV diode lasers[9–18],and piezoelectric surface acoustic wave or acousto-optic devices[3,5,19–21].A lot of methods have been extensively used for oriented ZnOfilms synthesis,including L-MBE,pulsed laser deposition, metal-organic chemical vapor deposition,sputtering[2],cathodic magnetron sputtering and reactive electron beam evaporation [21–25],spray pyrolysis[20,26–28],and electrodeposition[29,30]. However,sol–gel processes are particularly adapted to produce ZnO colloids[31–42]andfilms[4,43–49]in a simple,low-cost and highly controlled way.The sol–gel process,called also soft chemistry(‘chimie douce’), allows elaborating a solid material from a solution by using a sol or a gel as an intermediate step(Scheme1),and at much lower temperatures than is possible by traditional methods of prepara-tion.It enables the powderless processing of glasses and ceramics, and thinfilms orfibers directly from solution.The synthesis of solid materials via‘chimie douce’often involves wet chemistry reactions and sol–gel chemistry based on the transformation of molecular precursors into an oxide network by hydrolysis and condensation reactions[50,51].Scheme1shows the main steps of preparation of thinfilms and powder by the sol–gel process.We can summarize,for example, thefilm preparation in three parts:(i)preparation of the precur-sor solution;(ii)deposit of the prepared sol on the substrate by the chosen technique;and(iii)heat treatment of the xerogelfilm. The xerogel is the dried gel at ambient pressure(the dried gel in supercritical conditions is called aerogel).The detail of these steps and the process parameters will be discussed in the following sections.The objective of this review is to summarize the main chemi-cal routes used in the sol–gel synthesis of undoped ZnO thinfilms and to highlight the chemical and physical parameters influencing their structural properties.ZnO thinfilm synthesis involves several20L.Znaidi /Materials Science and Engineering B174 (2010) 18–30Scheme 1.Overview showing two synthesis examples by the sol–gel method;(a)films from a colloidal sol;(b)powder from a colloidal sol transformed into a gel.parameters:(1)the nature of the precursor and its concentration,(2)the type of solvent and the acidity of the medium,(3)the type of additive species and their concentrations,(4)the aging time of the early mixture,(5)the method of coating of substrates and its speed,(6)the nature of the substrate,and (7)the pre-and post-heat treatment of the materials.A survey of the literature shows that all these parameters play a key role on the evolution of texture in zinc oxide films.First of all,it should be noted that several works led to signifi-cantly different results particularly concerning the crystallographic orientation even if the experimental conditions were very close to each other.This makes difficult any prediction or any clear correla-tion.Thus the aim of this paper is to underline the main principles that emerge for each parameter and have a great influence on the elaboration of crystallized and textured zinc oxide films.2.Sol–gel method toward zinc oxide synthesis 2.1.Chemical systems:precursors,solvents,and additivesIn the sol–gel process,a molecular precursor in a homogeneous solution undergoes a succession of transformations:(a)hydroly-sis of the molecular precursor;(b)polymerization via successive bimolecular additions of ions,forming oxo-,hydroxyl,or aqua-bridges;(c)condensation by dehydration;(d)nucleation;and (e)growth [52,53].Depending on the nature of the molecular pre-cursors,two sol–gel routes are currently used:metal alkoxides in organic solvents or metal salts in aqueous solutions [50].The main methods of ZnO film elaboration,as reported in the litera-ture,involve several steps and are in fact intermediate between the two sol–gel methods since they use metal salts in alcoholic solu-tions.Indeed,ZnO films are obtained starting from inorganic salts –nitrates,chlorides,perchlorates –or organic salts like acetates and acetylacetonates,dissolved in alcoholic media.It is believed that insuch media the process involves two steps.The first one consists of in situ formation of alkoxide or alkoxy-complexes.In the second step,these complexes undergo transformation through hydrolysis and polymerization to lead to the oxide.Table 1summarizes the sol–gel undoped ZnO thin film elab-oration methods including chemical systems used and results concerning crystallographic orientation.2.1.1.PrecursorsSeveral zinc precursors have been used:nitrate,chloride,per-chlorate,acetylacetonate and alkoxides such as ethoxide and propoxide,but the most often used is the acetate dehydrate.Metal alkoxides,although they offer several chemical advantages,are not suitable because they are very sensitive to moisture,highly reactive and remain still rather expensive.Because of their low cost,facility of use,and commercial availability,metal salts are interesting as precursors and could be more appropriate for large-scale applica-tions.Since metal salts include inorganic and organics ones,we can underline the comparisons made between them and reported by some authors.Inorganic salts like nitrates are often used,as pre-cursors for sol–gel ZnO-based materials,even though their main drawback is related to the inclusion or difficult removal of anionic species in the final product [51,54].Using zinc acetate as a pre-cursor,the acetate groups,as contaminants of the gel,decompose under annealing producing combustion volatile by-products [54].Bahnemann et al.[33]synthesized transparent colloidal suspen-sions of zinc oxide in water,2-propanol,acetonitrile,and using different zinc salts.They reported that the anion,in zinc salt,is critical for the preparation of transparent and stable ZnO colloids.They mentioned that the use of zinc perchlorate instead of zinc acetate yields a turbid suspension;i.e.,coagulation of the particles takes place,the acetate acts so as stabilizer of the colloidal sol.Also,the experiments with ZnCl 2or Zn(NO 3)2reveal a faster coagulation than in the case of Zn(ClO 4)2following the initial formation of aL.Znaidi/Materials Science and Engineering B174 (2010) 18–3021Table1Main chemical systems used for undoped ZnO thinfilms elaboration by sol–gel process in alcoholic medium and resultingfilm crystallographic orientation.References Precursor(mol L−1)Alcohol Additive(r)H2O(h)AgingtimeSubstrate Pre-heattreatment(◦C)Post-heattreatment(◦C)Thickness(nm)CrystallographicorientationNatsume and Sakata[61]ZAD(0.02)MeOH–––Pyrex80500–575160–230(002)González et al.[62]ZAD(0.59)MeOH––24h Corning glass50300,45035–204(100)(002)(101)150(100)(002)(101) Santos et al.[63,64]ZAD a MeOH–––Glass120350(18L)a(002)Sagar et al.[65,66]ZAD(0.6)MeOH MEA(0–1)–48h Corning glass300400–600180–190(002)Liu et al.[67,68]ZAD(0.6)EtOH,PEG(14g/L)DEA(1)(2)–Glass100500220(6L)(100)(002)(101)Wang et al.[69]ZAD(0.5)EtOH DEA(1)––Quartz400400–800300(6L)(100)(002)(101) Kumar et al.[70]ZAD(0.2)EtOH DEA(1)–48h p-Si(100)250350–450∼250(100)(002)(101) Shaoqiang et al.[4]ZAD(0.46)EtOH–––n-and p-typeSilicon40500(directly)200(002)500(Gradually)(100)(002)(101) Znaidi et al.[47,48]ZAD(0.05)EtOH MEA(2)–72h Glass100–135450–(100)Wang et al.[71]ZAD(1.8)EtOH MEA(1.5)–72h Glass300350–600(5L)a(100)(002)(101) Wang et al.[72]ZAD(0.19)EtOH Ac.Ac.(0.35)(11.1)24h Glass100450–(100)2h Amorphousfilm Bao et al.[46]ZAD(0.4)EtOH Lactic Ac.––Quartz300500,550300(6L)(002)450,600(100)(002)(101) Bole and Patil[73]ZAD a EtOH Lactic Ac.––Glass300300–425375–275(100)(002)(101) Kavanagh andCameron[74]ZAD a EtOH Lactic Ac.(2)–Silicon240700–(100)(002)(101)Bahadur and Rao[75]ZAD a EtOH LiOH·H2O––F:SnO280400800(5L)(100)(002)(101)Brenier and Ortéga [76]ZAD(0.15)1-PrOH TMAH–4weeksSilicon80250(O2)20–60(100)(002)(101)Peterson et al.[77]ZAD(0.3)1-PrOH Glycerol––Si(100),quartz300700180(4L)(100)(002)(101) Raoufiet al.[78]ZAD(0.3)1-PrOH MEA(1)––Glass250300–500500(8L)(100)(002)(101)O’Brien et al.[79]and Rao et al.[80]ZAD(0.3–0.7)2-PrOH MEA(1)–24h UV fused silica60450–65084–437(100)(002)(101) (1.3)(100)(002)(101)Kim et al.[3]ZAD(0.3–0.5)2-PrOH MEA(1)–24h Corning glass250650–Amorphousfilm(0.7)(100)(002)(101)(1–1.3)(100)(002)(101)bLin and Kim[81]ZAD(0.5)2-PrOH DEA(1)–24h Si(100),Glass300450–550280(7L)(100)(002)(101)bSi(100)700,800(100)(002)(101) Aslan et al.[82]ZAD(0.4)2-PrOH DEA(1)(0.5)–Glass250450–5501000–1600(10–15L)(100)(002)(101)Chakrabarti et al.[83]ZAD(0.24)2-PrOH DEA(0.006)––Glass100550–(002)p-Si(100),Soda-lime,quartz,alumina700(100)(002)(101)Jiwei et al.[84]ZAD(0.4)2-PrOH DEA(1)(2)–SiO2/Si(111),fused-quartz200(O2)300–650(O2)230–350(100)(002)(101)Wang et al.[85]ZAD(0.32)2-PrOH DEA(1)––Si/SiO2/Ti/Pt300–450550–800500(6L)(100)(002)(101)Bae and Choi[86]ZAD(0.25)2-PrOH DEA(1.5)––Alumina300400–900125–240(3–6L)(100)(002) (101)bOhya et al.[44,87] and Takahashi et al.[88]ZAD(0.25/0.5)2-PrOH DEA(1)(2)–Glass110350–60013–33/L(100)(002)(101)Mridha and Basak[89]ZAD(0.1)2-PrOH DEA––Glass120550260(100)(002)(101)Dutta et al.[90]ZAD(0.03–0.1)2-PrOH DEA––Glass35055036–247(100)(002)(101) Basak et al.[91]ZAD(0.6)2-PrOH DMA––Sapphire120550300(10L)(100)(002)(101) Ghosh et al.[6,92,93]ZAD(0.6)2-PrOH DMA––Quartz120550400(10L)(100)(002)(101)Glass,Si/SiO2(100)(002)(101)GaN(002)Zhang et al.[94]ZAD(0.3)PVA–––Si(100)120600434(100)(002)(101) Pal and Sharon[95]ZA(<0.03)2-PrOH NaOH(1)––Glass–400(5–6L)a(100)(002)(101) Abdel Aal et al.[96]ZAD a2-PrOH NaOH––Glass–550(6L)a(002)Caglar et al.[97]ZAD(1)2-ME MEA(1)––p-type singlecrystal Si300,450550–750(10L)a(100)(002)(101)Ohyama et al.[98]ZAD(0.75)2-ME MEA(1)––Silica300600100–260(002)Li et al.[99,100]ZAD(0.75)2-ME MEA(1)––Silica300600300(7L)(002)Zhu et al.[101]ZAD(0.75)2-ME MEA(1)––Glass60400–550(4L)a(100)(002)(101) Fujihara et al.[102]ZAD(0.75)2-ME MEA(1.1)––Glass400–500400–500200(5L)(002)Znaidi et al.[47,48]ZAD(0.75)2-ME MEA(2)––Glass300500–55076(3L)(002)Ohyama et al.[45]ZAD(0.6)2-ME MEA(1)––Silica500500–(002)DEA(1)(100)(002)(101)bNagase et al.[103]ZAD(0.6)2-ME MEA(1)––Quartz200Laserirradiation35–190(002)Hsieh et al.[104,105]ZAD(0.6)2-ME MEA(1)––SiO2/Si270600–900–(100)(002)(101)Yoon et al.[106]ZAD(0.5)2-ME MEA(1)–72h SiN x/Si,Pt(111)/Si300700140(5L)(002)22L.Znaidi/Materials Science and Engineering B174 (2010) 18–30 Table1(Continued)References Precursor(mol L−1)Alcohol Additive(r)H2O(h)AgingtimeSubstrate Pre-heattreatment(◦C)Post-heattreatment(◦C)Thickness(nm)CrystallographicorientationDEA(1)SiN x/Si,(100)(002)(101)b DEA(1)Pt(111)/Si(002)Choi et al.[107]ZAD(0.5)2-ME MEA(1)––Pt/TiO2/SiO2/Si300400–700180(002)Srinivasan et al. [108–110]ZAD(0.5)2-ME MEA(1)––Glass,quartz350500(8L)a(100)(002)(101)(001)sapphire100,400450–600530(10L)(100)(002)(101)Kokubun et al.[111]ZAD(0.45)2-ME MEA(1)––Silica90/300500/600150(100)(002)(101) Lee et al.[112]ZAD(0.35)2-ME MEA(1)–48h Corning glass350600200(002)Xue et al.[113]ZAD(0.35)2-ME MEA(1)–24h Fused silica300500800(12L)(100)(002)(101)Castanedo-Pérez et al.[114]ZAD(1.14)EG,1-PrOH Glycerol,TEA(0.31)30h Soda-limeglass,silicon100450160(100)(002)(101)Delgado et al.[115]ZAD a EG,1-PrOH Glycerol,TEA–24h Glass100200–600450(5L)(100)(002)(101)Kamalasanan and Chandra[116]ZAD(<1.98)EG,1-PrOH Glycerol,TEA––Soda glass,silicon–450200/L(100)(002)(101)Chatterjee et al.[117]ZNH a PVA––Silicon–700–850∼1000(100)(002)(101)Toyoda et al.[118]ZNH a2-ME–––Platinized Si25250–450a(002)600(100)(002)(101)Okamura et al. [43,119]Zn(OEt)2a1-BuOH Acac––p-Si(111)–700(O2)20/L(100)(002)(101)bOhya et al.[44,87]Zn(OPr n)2(0.2/0.5)2-PrOH DEA(1)(2)–Glass11060011–33/L(100)(002)(101) Abbreviations(Abbrev.):Abbrev.Product name Chemical formula Abbrev.Product name Chemical formula ZAD Zinc acetate dihydrate[Zn(CH3COO)2·2H2O]MeOH Methanol CH3OHZA Zinc acetate anhydrous[Zn(CH3COO)2]EtOH Ethanol C2H5OHZNH Zinc nitrate hexahydrate[Zn(NO3)2·6H2O]1-PrOH1-Propanol C3H7OHMEA Monoethanolamine(HOCH2CH2)NH22-PrOH2-Propanol(CH3)2CHOH DEA Diethanolamine(HOCH2CH2)2NH1-BuOH1-Buthanol C4H9OHTEA Triethanolamine(HOCH2CH2)3N2-ME2-Methoxyethanol CH3O(CH2)2OH DMA Dimethylamine(CH3)2NH EG Ethylene glycol HO(CH2)2OH PVA Polyvinyl alcohol[-CH2CH(OH)-]n PEG Polyethylene glycol H(OCH2CH2)n OH TMAH Tetramethylammonium hydroxide(CH3)4N(OH)Ac.Ac.Acetic Acid CH3COOHAcac Acetylacetone CH3COCH2COCH3Lactic ctic Acid CH3CHOHCOOH In the thickness column:L:layer.The numbers between brackets for additive and water columns correspond to r and h values,respectively where:r=[additive]/[Zn2+],h=[H2O]/[Zn2+].In the crystallographic orientation column:diffraction peak indicated in bold corresponds to highly orientedfilms according to one of three main orientations.a The precursor concentration in the chemical system and thickness are indicated only when they are mentioned in the corresponding reference.b The peaks intensities are very close.clear colloidal suspension.Matijevi´c[31]reported that the prepara-tion of“monodispersed”sols of metal(hydrous)oxides from metal salts is very sensitive to such factors as salt concentration,nature of the anion,pH,and temperature.He demonstrated that entirely dif-ferent products result when the anions are changed in the studied systems.In a previous work[47],we showed the importance of the counter anion in zinc precursors.Hydrated zinc salts (acetate,nitrate,perchlorate)were dissolved in ethanol or2-methoxyethanol in the presence of monoethanolamine(MEA), which acts at the same time as a base and a complexing agent. Further aging at60–100◦C,during variable periods,leads to translucent colored colloidal sols or precipitates,according to the counter anion and concentration.For nitrate,no sols could be repro-ducibly obtained.In perchlorate solutions,excess MEA leads to the formation of sols,after a slow dissolution of the initially formed precipitates.On the contrary,systems stemming from zinc acetate and MEA lead to reproducible systems,under a great variety of experimental conditions.For example,aging at60◦C for72h results in the formation of stable translucent sols by forced hydrolysis of Zn(II)complexes;the necessary water is supplied by the hydrated salt.Acetate plays an important role in sol formation,by complex-ing Zn(II)cations,in competition with the MEA.The alternative hypotheses for this mechanism will be described in a following section.After this short comparison of the various precursors used in the sol–gel processes,the use of organic salts as precursors in alcoholic media remains advantageous,on the one hand because of the low cost and the facility of the metal salt use in general,and on the other hand by the fact of avoiding the problems presented by certain inorganic anions and the washing steps.2.1.2.SolventsThe solvent must present a relatively high dielectric constant in order to dissolve the inorganic salts[52,55,56].Most alcohols are dipolar,amphiprotic solvents with a dielectric constant that is dependent on the chain length[42].Table2shows the dielectric constants and boiling points of most alcohols used in this work [52,57].We recall that alcohols with low carbon number,up to4,are the most used solvents:methanol,ethanol,1-propanol,2-propanol,1-butanol and2-methoxyethanol(Table1).In addition,a few works use ethylene glycol(HOCH2CH2OH)as a solvent that has a dielectric constant of40.61(at25◦C)and a boiling point of197.4◦C.Among all the monoalcohols,the most used ones are ethanol and2-propanol. It should be noted that2-methoxyethanol,despite its good physical properties,is toxic to reproduction,because it is labelled with Risk Phrase R60(category2:‘May impair fertility’)by the International Programme on Chemical Safety,among others.Hosono et al.[55],made a comparative study of chemical reac-tions from zinc acetate dehydrate to ZnO using different types of alcoholic solvents,i.e.methanol,ethanol,and2-methoxyethanol. Zinc acetate dehydrate(ZAD)was more soluble in methanol than in ethanol or2-methoxyethanol according to dielectric constants of these alcohols(Table2).The reflux time necessary for the formation of ZnO increases with the solutions in order,MeOH(12h) EtOH (48h)<2-ME(72h).Besides,the same authors demonstrated that the XRD analysis of particles,obtained from the three alcoholic。