Controllable synthesis of metallic Bi from commercial Bi2O3 via one-pot solvothermal reduction method☆

Lei Yang,Yangyang Zhang*,Hua Li,Hezhou Liu*

State Key Laboratory of Metal Matrix Composites,School of Materials Science and Engineering,Shanghai Jiao Tong University,Shanghai 200240,China

1.Introduction

Elemental bismuth(Bi)crystal is one important and interesting semimetal that has attracted great attention owing to its unique thermal and electrical properties.The relatively small conductionband effective mass(～0.001m0),low charge carrier density(105times smaller than conventional metals at 4.2 K),large mean-free path(～0.4 mm at 4 K)and highly anisotropic Fermi surface make bismuth one ideal material to study quantum con finement effect, finite-size effect,magneto-resistance,transition from semimetalto semiconductor and thermoelectric effects[1–5].Hence,nanoscaled bismuth particles with various morphologies are highly desirable in catalysis,biolabeling,photonics and information storage[6–11].As a nontoxic and lowmelting point metal,Bi also shows great applications in brass plumbing fixtures,ceramic glazes,pigments,lubricants and electronic solder formulations[12,13].

Many efforts have been paid to develop a green and economical method capable of producing Bi particles on a large scale.Fanget al.[14]obtained nanosized Bi particle using an inverse microemulsion method in the presence of NaBH4,an extremely strong reductant.Wanget al.[15]prepared Bi nanospheres by thermolysis of bismuth acetate,which is very expensive,under 315°C.Starket al.[16]initiated flame spray synthesis of Bi particles in an inert atmosphere using bismuth-carboxylate as the starting material while the purity of the products is only～98%and equipment and raw materials investment should be huge.Kanget al.[17]prepared Biparticles by electrochemical reduction of Bi2O3under?1.0 bias in 0.1 mol·L-1KOH solution,but difficulties in the large-scale production of Bi2O3electrodes bound to restrict its application.Although many routes to achieve Bi particles have been developed,most of them either are operated under high temperatures or need expensive raw chemicals,which makes them inappropriate and impractical for the mass production of Bi particles.

In this work,we present a one-pot,low temperature solvothermal method to realize controllable synthesis of Bi particles.A serous system involving only commercialBi2O3,one surfactantand any kind of alcohol is proposed and exemplified.Besides,mechanisms behind the formation of metallic Bi are also analyzed carefully.

2.Experimental

2.1.Materials

Commercial Bi2O3powder(＞99.0%)and polyvinylpyrrolidone(PVP K13–18,Mw=10000)were purchased from Aladdin,other reagents were obtained from Sinopharm Chemical Reagent Co.,Ltd.All the chemical reagents are of analytical grade and used as received without further purification.

2.2.Perpetration of metallic Bi particles

In this work, five kinds ofcommon alcohols including ethanol,glycol,isopropanol and phenylcarbinol were used as reductant separately.Taking ethanol as an example,in a typical procedure,0.2 g commercial Bi2O3(0.4292 mmol)and a certain amount of PVP K13–18 were weighed and dispersed in 40 ml absolute ethanol under sonification and vigorous stirring.PVP K13–18 was employed to control morphologies of final products and three controlled experiments containing 0 g,0.1432 g and 0.2864 g PVP were carried out.Afterwards,three mixtures were transferred into 50 ml Te flon-lined stainless steel autoclaves to 80%of their total volume.These autoclaves were sealed and maintained at 180°C for 24 h before cooling down naturally.After centrifuging the resultant mixture at 14000 r·min-1,supernatants were collected for further characterization while sediments were washed for several times to remove additives and other soluble impurities.Eventually,dark green powders denoted as ET-0,ET-1 and ET-2 were obtained.When ethanol was replaced by glycol,isopropanol and phenylcarbinol,products designated as GC-0,GC-1,GC-2,IS-0,IS-1,IS-2,PH-0,PH-1 and PH-2 could be prepared under similar conditions.

2.3.Characterization

Morphologies were observed with a JEM 2100F field emission transition electron microscope(TEM).Phases were determined by powder X-ray diffraction(XRD)analysis(Rigaku D/MAX255ovl/84,CuKαradiation)at a scan rate of 4(°)·min?1.The chemical states of elements were analyzed by X-ray photoelectron spectroscopy(XPS)on an AXIS ULTRA DLD spectrometer(Kratos).The phase transformation process was investigated by TGA using a NETZSCH DSC 204 F1 instrument with a heating rate of 10 °C·min?1between 40 °C to 400°C in air.Fourier transform infrared(FT-IR)spectra were obtained on a Nicolet6700(ThermoFisher)Fourier transform infrared spectrometer using the KBr pellet technique in the range of 400–4000 cm?1.

3.Results and Discussions

As it is known to all that commercial Bi2O3powder is vivid yellow in hue[18],however,it becomes black or dark green after being simply solvothermal treated with ethanol,glycol,isopropanol and phenylcarbinol in the existence of various amount of PVP indicating chemical reactions probably occur.Phase structures of all resultant products are carefully examined using XRD and compared in Fig.1,in which standard XRD patterns of metallic Bi and Bi2O3are also listed for contrast[18].Interestingly,except for diffraction peaks originally belong to Bi2O3in ET-0,one new peak appears and can be assigned to the strongest peak of Bi.It is concluded that only a small part of Bi2O3converts to metallic Bi in ethanol because the relative intensity of(012)peak for Bi is very low.Although PVP in this work is mainly used to control the morphology of final products,it also shows obvious in fluence on phase evolution process[19].When PVP is added to the above ethanol/Bi2O3system,no Bi is detected implying that PVP is capable of hindering the phase transformation from Bi2O3to Bi.Obviously,redox reactions must take place during the solvothermal treatment of Bi2O3,and ethanol with--OH group is the only possible chemical that can act as the reducing agent.Ithas been demonstrated that ethanol is a poor reducing agent and the reducibility of R--OH is determined by constitutional formula of--R[20,21],three more simple alcohols including glycol,isopropanol and phenylcarbinol are used to reducing Bi2O3.Glycol as one dihydric alcohol surprises us most for all Bi2O3are reduced to metallic Bi by it in regardless of the addition of PVP,and no any crystalline phase is observed in GC-0,GC-1 and GC-2.The reducibility of phenylcarbinol is comparable with glycol for PH-0 is found to be phase pure metallic Bi,but will be greatly depressed in the presence of PVP.It is reported that PVP consisting of abundant--C=O will strongly attract molecules containing benzene ring[22].Hence,--OH in phenylcarbinol is probably protected by PVP from oxidization which leads to a dramatic drop in its reducibility.Isopropanol that can merely reduce a few of Bi2O3is similar to ethanol and the in fluence of PVP on its reducibility is also alike.Additionally,UV–Visible absorption spectra of bare Bi2O3,IS-0,IS-1 and IS-2 are also collected and displayed in Fig.S1 to further con firm the phase evolution process.Consistent with XRD results,absorption intensities in λ＞550 nmbecome stronger for IS-0,IS-1 and IS-2 due to the generation of Bi.With the increasing of the amount of PVP,the absorption intensity decays because fewer Bi is evolved.As a consequence,glycol is the best reducing agent in producing single-phase Bi and leaves much room in morphology control.The reducing ability strong-to-weak sequence is glycol＞ phenylcarbinol＞isopropanol＞ethanol.

Fig.1.XRD patterns of(a)ET-0,ET-1,ET-2,GC-0,GC-1 and GC-2,(b)IS-0,IS-1,IS-2,PH-0,PH-1 and PH-2.Standard XRD patterns of Bi and Bi2O3 are listed as comparison.

As redox reactions take place during the solvothermal treatment of Bi2O3,extra additives including surfactantetc.are very likely to realize morphology controlling of resultant products.In this work,PVP is taken as an example to demonstrate the possibilities in microstructure tuning,and many other common additives are also viable if PVP works[23,24].Fig.2 displays the TEM images of ET-0,ET-1,ET-2,GC-0,GC-1,GC-2,IS-0,IS-1,IS-2,PH-0,PH-1 and PH-2.The original morphology of commercial Bi2O3is also observed using TEM,as shown in Fig.S2.It can be seen from Fig.S2 that commercial Bi2O3is made up of bunches of connected particles with smooth surface.As has been discussed above,ET-0 is partially reduced Bi2O3while ET-1 and ET-2 are nearly pure Bi2O3and their morphologies are exhibited in(a1)–(a3).It is found that ET-1 and ET-2 have identical shape with initial commercial Bi2O3which is consistent with corresponding XRD results,but smooth spherical Bi2O3particles exploded and become very rough in ET-0(Fig.S3).Apparently,R--OHis capable of destroying the solid structure of Bi2O3layer by layer and finally gives birth to new phases with diverse microstructures.Figure panels(b1)–(b3)are images of GC-0,GC-1 and GC-2 which are phase pure Bi.Surprisingly,their morphologies not only greatly differ from that of commercial Bi2O3,but also show big distinctions among themselves.GC-0 obtained without any additives is composed of large irregular bulky Bifragments while ribbon like Biis gained in the presence of0.1432 g PVP.When the amountofPVP is doubled,hexagonal,spherical and cotton like particles together make up GC-2.Figure panels(c1)–(c3)and figure panels(d1)–(d3)show the morphologies of IS-0,IS-1,IS-2 and PH-0,PH-1,PH-2,respectively.Although IS-0 is similar to ET-0 in phases,their appearances look different and IS-0 is made up of angular particles without exploded surfaces.Both IS-1 and IS-2 are similar to commercial Bi2O3because their reduction degree is low,so are PH-1 and PH-2.The same with GC series products,PH-0 is also single-phase Bi and is composed of Bi spheres attached by plentiful Bi nanorods.It is worth noting that,although Bi2O3is sensitive to high energy electron beam,the morphology showed in Fig.2 should be true morphology because the size of these products are large enough to stay stable and only low magnification TEM is performed[25].The morphology evolution in this work proves that microstructure tuning is feasible and practical by changing types and concentrations of additives and this result is of great importance.As a consequence,it is possible to obtain phase pure Bi with various regular shapes.

The chemical nature of products after solvothermal reduction is further examined by XPS analysis.ET-0 and GC-0 are representatives of partially reduced Bi2O3and fully reduced Bi2O3,respectively.Fig.3 displays the Bi 4f high-resolution XPS spectra of ET-0 and GC-0.It is clearly found thatthe peaks ofBi4fspectrum for ET-0,the mixture ofBi2O3and Bi,are asymmetric[26],and can be deconvoluted into two sets of peaks using Gauss–Lorentz fitting method,as shown in Fig.3(a).The resolved peaks with binding energies of 158.5 eV and 163.8 eV match well with reported values for Bi2O3[27],while the rest two peaks at 156.8 eV and 162.2 eV can be well assigned to Bi 4f7/2and Bi 4f5/2of metallic Bi[28,29].On the opposite,the Bi 4f peaks for GC-0 are symmetric and possess identical binding energies with Bi indicating no more Bi3+exists in GC-0.The XPS results further verify that GC-0 is single phase Bi and there is no any amorphous phases containing Bi3+either.

Fig.2.TEM images of(a1)ET-0,(a2)ET-1,(a3)ET-2,(b1)GC-0,(b2)GC-1,(b3)GC-2,(c1)IS-0,(c2)IS-1,(c3)IS-2,(d1)PH-0,(d2)PH-1 and(d3)PH-2.

Thermal responsive behaviors of ET-0,GC-0,IS-0 and PH-0 are analyzed in depth using TGA,as shown in Fig.4.All samples experience mass incrementrather than mass loss at～260 °C owing to the oxidation ofBi.Hence,itcan be concluded thatthe oxidation ofBitakes place prior to its melting[29,30].The sizable differences in mass increase of four samples coincide well with their XRD results indicating TGA curves could be used to analyze the mass percentages of Bi.It has been demonstrated that all samples are made up of either Bi2O3or Bi and the theoretical weight increment of pure Bi is calculated to be 11.48%according to the following equation[29].

Fig.3.High resolution XPS spectra of Bi 4f orbital in ET-0 and GC-0.

Fig.4.Mass loss of ET-0,GC-0,IS-0 and PH-0 in air.

Therefore,the weight percentages of Bi in four samples could be estimated to be 8.01%,99.74%,13.59%and 96.08%for ET-0,GC-0,IS-0 and PH-0,respectively.Consequently,high-purity metallic Bi with controllable microstructure is accessible in large scale using our facile one-pot solvothermal method.It is worth noting that no expensive,toxic chemical is involved during the whole solvothermal process and all alcohols could be recycled.

The phase evolution of Bi2O3during solvothermal treatment has been carefully discussed above.Herein,structural variations of four alcohols are as well investigated using FT-IR,as displayed in Fig.5.Theoretically speaking,the oxidative products of R--OH should be featured with--C=O group whose stretching vibration absorption peak locates at～1720 cm?1[31].Interestingly,all supernatant liquids after generating ET-0,GC-0,IS-0 and PH-0 exhibit obvious peaks at～1720 cm?1and this peak should notexistin any pure alcohols.The relative intensities of--C=O peaks are closely related to the reduction degree of Bi2O3.As a consequence,Bi2O3is reduced to Bi accompanied by the oxidation of--OH into--C=O.

Fig.5.FT-IR spectra of ET-0,GC-0,IS-0 and PH-0.

4.Conclusions

In this work,metallic Bi is successfully obtainedviaa facile,green,cost-effective and sustainable solvothermal process using ethanol,glycol,isopropanol,phenylcarbinol as solvent and commercial Bi2O3as starting material.During the transformation from Bi2O3to Bi,alcohols are served as reductive agents and PVP is employed to realize morphology tuning.Itis found thatallalcohols are capable ofreducing Bi2O3to Bi and the reducibility ofglycolis proved to be the strongest.The mass percentages of Bi in ET-0,GC-0,IS-0 and PH-0 are calculated to be 8.01%,99.74%,13.59%and 96.08%,respectively.The addition and concentration ofPVP show obvious in fluence on the microstructure tuning ofresultant Bi particles.Consequently,our work lays the root for the large scale solvothermalproduction of high-purity Biparticles with various regular morphologies.

Acknowledgements

Instrumental Analysis Center of Shanghai Jiao Tong University and NationalEngineering Research Centerfor Nanotechnology are gratefully acknowledged for assisting with relevant analyses.

Supplementary Material

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.cjche.2016.11.002.

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