ZnO formation under hydrothermal conditions from zinc ... - Springer Link

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 12, pp. 1811–1816. © Pleiades Publishing, Inc., 2007. Original Russian Text © A.S. Shaporev, V.K. Ivanov, A.E. Baranchikov, O.S. Polezhaeva, Yu.D. Tret’yakov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 12, pp. 1925–1931.

SYNTHESIS AND PROPERTIES OF INORGANIC COMPOUNDS

ZnO Formation under Hydrothermal Conditions from Zinc Hydroxide Compounds with Various Chemical Histories A. S. Shaporeva, b, V. K. Ivanova, b, A. E. Baranchikova, b, O. S. Polezhaevaa, and Yu. D. Tret’yakova, b a

Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia b Moscow State University, Vorob’evy gory, Moscow, 119992 Russia Received May 3, 2007

Abstract—The sequence of phase transformations of zinc hydroxide compounds with various chemical histories during hydrothermal (HT) treatment is studied by X-ray powder diffraction and thermal analysis (TG/DTA). ε-Zn(OH)2 (wulfingite) is the major intermediate in ZnO production by the HT process. The micromorphology and photocatalytic activity of ZnO powders are studied. DOI: 10.1134/S0036023607120017

Zinc oxide is a polyfunctional oxide material with unique characteristics, which is used for manufacturing components of various devices, such as light diodes, gas sensors, and solar cells, and for design of transparent conductors and photocatalysts [1]. The photocatalytic activity of semiconductors (in particular, ZnO) is determined by several parameters, the major being dispersion and imperfection, which in turn are considerably influenced by the preparation technique. Hydrothermal (HT) synthesis is one of the most promising methods for manufacturing materials with various morphology and various structure-sensitive properties; this method is also environmentally friendly, economic, and easy to be implemented [2]. Zinc oxide formation from hydroxide suspensions under HT conditions is a complex, multistage process, whose course is, in particular, directed by the history and composition of precursor suspensions. Sharikov et al. [3] showed that zinc oxide formation during the HT treatment of zinc hydroxide suspensions occurs in different temperature and time intervals depending on whether the precursor had an ammonia or alkali history; the authors assigned this to the considerably differing structures of the precipitated gels. Music et al. [4] attempted to study the processes involved in Zn(NO3)2 hydrolysis in the presence of NaOH at various pHs and during subsequent HT treatment of suspensions. The product obtained at low pHs (~6) was zinc hydroxonitrate Zn5(OH)8(NO3)2(H2O)2, which is stable at ambient temperature for half a year and longer but decomposes to ZnO during HT treatment at 160°ë. However, hydrolysis of zinc nitrate at pH ≈ 13 directly produces zinc oxide. Belomestnykh et al. [5] established that the composition of the

solid products of the reaction between aqueous solutions of zinc nitrate and ammonia (ZnO, Zn5(OH)8(NO3)2(H2O)2, and Zn5(OH)9(NO3)(H2O)n) is determined by the precipitation pH, the rate at which aqueous ammonia is added to the reaction mixture, and the washing parameters. Zinc hydroxide formation was not observed in the cited works, whereas some other reported the formation of crystalline Zn(OH)2 under similar conditions [6]. Thus, the processes involved in ZnO synthesis under hydrothermal and prehydrothermal conditions have not been properly studied; however, they should be taken into account for synthesizing ZnO with controlled micromorphology and functional characteristics. Our goal in this work was to study the effect of the precipitation parameters on the composition of the resulting zinc hydroxide compounds, analyze the scenario of ZnO formation during subsequent HT treatment, and study the effect of the precursor history on the photocatalytic activity of product ZnO powders. EXPERIMENTAL The precursor zinc hydroxide suspensions were prepared by dropping a 0.45 M zinc nitrate solution to an aqueous solution of a base (NaOH or NH4OH). A precipitator solution was taken in the tenfold molar excess; the NaOH concentration was 0.11, 0.22, or 0.43 mol/L (pH was from 13.0 to 13.2), and the NH4OH concentration was 0.075, 0.15, 0.30, or 0.60 mol/L (pH was from 11.2 to 11.7). While adding zinc nitrate, suspensions were vigorously stirred on a magnetic stirrer; then, they were in contact with the mother solution for 30 min, after which precipitates were decanted and six times

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washed with distilled water followed by centrifuging (8000 rpm, 5 min). The precipitates were used to prepare aqueous suspensions using 4.5 g of the precipitate for 100 mL of distilled water. These suspensions were then subjected to HT treatment in a Parr 4793 steel HT reactor. The volume of the treated suspension was 50 mL with the 50% autoclave filling. A resistor furnace was used to heat the reactor; the heating rate to the isothermal-exposure temperature was fixed at 5 K/min. Temperature was monitored with a thermocouple directly in the suspension. The treatment temperature ranged from 60 to 125°C. After the treatment was over, the cell was cooled to ~40°ë. The resulting powders were repeatedly rinsed with distilled water and dried at 30°ë for 4 days. X-ray powder diffraction (XPD) analysis was performed on a Rigaku D/MAX 2500 diffractometer using 2500 (CuKα radiation. Diffraction peaks were identified with reference to the JCPDS file. The average grain size of zinc oxide was determined from X-ray diffraction data as the coherent scattering region size using the Selyakov–Scherrer relationship λ D CSD = ---------------, β cos θ

(1)

where β is the physical broadening of the diffraction peak, and λCu = 1.54178 Å. A sapphire single crystal was the reference. Thermal analysis (DTA, TGA) was carried out on a Perkin-Elmer TG7 thermal analyzer in a polythermal mode with the heating rate equal to 10 K/min. Several experiments were carried out to study in situ the composition of the gaseous products eliminated in the course of the controlled heating of samples using an I

‡ b c d e 10

20

30

40

50

60 2θ, deg

Fig. 1. X-ray diffraction patterns for zinc hydroxide xerogels prepared by the hydrolysis of zinc nitrate (a, b) in the presence of (a) 0.11 and (b) 0.44 M NaOH and (c–e) in the presence of (c) 0.075, (d) 0.15, and (e) 0.3 M NH4OH.

STA Jupiter 449C Netzsch thermal analyzer equipped with a Tensor 27 Bruker FT-IR spectrometer. Micromorphology was studied using a Leo912 AB Omega transmission electron microscope (TEM) at the accelerating voltage 100 kV. Test samples lay on polymer grids 3.05 mm in diameter coated with a polymer film. The luminescent properties of powders were studied on a Perkin-Elmer LS55 spectrometer. The exciting wavelength was 300 nm; photoluminescence spectra were recorded in the range from 360 to 800 nm. The photocatalytic activity of ZnO powders was studied in a model reaction of azo dye (Methyl Orange) photodegradation in aqueous zinc oxide suspensions as follows. To a zinc oxide powder sample (60 mg), an aqueous solution (40 mL) of Methyl Orange (13 mg/L) was added, after which the mixture was vigorously stirred on a magnetic stirrer in the dark for 30 min for sorption/desorption equilibrium to be acquired. For the UV treatment of suspensions, a setup based on a Vilber Lourmat VL-6LC lamp (λmax = 312 nm) was used. Before the process, the UV lamp was heated for 30 min to stabilize its spectral characteristics. A suspension sample was placed to an open reactor and exposed to UV radiation for 90 min with vigorous stirring. 3-mL aliquots were taken in the course of the process 0, 5, 15, 30, 60, and 90 min after the start of the experiment. The photocatalyst was removed by centrifuging (8000 rpm; 5 min), after which the residual Methyl Orange concentration in the centrifugates was determined spectrophotometrically (an SF-2000 spectrophotometer, fivefold scans with averaging) as the absorption peak at λ = 461 nm using the Bouguer–Lambert–Beer law. RESULTS AND DISCUSSION When a zinc nitrate solution was added to an NaOH or NH4OH solution with concentrations from 0.22 to 0.43 mol/L or from 0.15 to 0.30 mol/L, respectively, a compact white precipitate was formed. When a highly dilute precipitator (0.11 M NaOH or 0.075 M NH4OH) was used, a loose, jelly precipitate was formed. When a rather high NH4OH concentration (0.60 mol/L) was used, precipitation did not occur. The compounds precipitated by NaOH and NH4OH solutions of different concentrations are considerably different (Fig. 1). The use of alkali as the precipitator yields well-crystallized ε-Zn(OH)2 (wulfingite) and zinc oxide. The use of aqueous ammonia as the precipitator produces zinc hydroxide compounds. Precipitation with 0.15 M NH4OH generates a single-phase product, which was identified in [7] as δ-Zn(OH)2 with the composition Zn(OH)2 · 0.5H2O (in the card PDF#20-1436, this compound is erroneously referred to as α-Zn(OH)2). At higher precipitator concentrations, other zinc hydroxide compounds are formed in addition

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‡

–12 b –16 100

200

300

400

500 t, °C

Fig. 2. Weight-loss curves for samples prepared by the HT treatment of zinc hydroxide compounds with (a) ammonia and (b) alkali histories at 60°C for 1 h.

to δ-Zn(OH)2, in particular zinc hydroxocarbonate, probably because of the presence of minor ammonium carbonate in the precipitator (NH4OH). From the X-ray diffraction pattern of a sample precipitated by a 0.3 M NH4OH solution, this powder is a mixture of Zn5(OH)6(CO3)2 (PDF#19-1458), δ-Zn(OH)2, and ε-Zn(OH)2. We should mention that zinc hydroxonitrates or single-phase zinc oxides were not formed in this work, unlike in [4, 5], because of the different precipitation and heat-treatment schedules. Repeated washing of the precipitates favored virtually complete elimination of nitrate ions and hindered formation of zinc hydroxonitrate phases [5]. In turn, be think that zinc oxide was formed in [4, 5] due to the sufficiently high drying temperature (up to 100°ë) for Zn(OH)2 dehydration to occur [6].

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Thermal analysis shows that powders with different chemical histories behave differently during heating. The Zn(OH)2 samples prepared by precipitation with NaOH decompose to ZnO; the process mainly occurs in the temperature range from 100 to 150°ë (Fig. 2). For all powders having the ammonia history, the decomposition occurs in two stages: the first stage occurs from 100 to 150°ë and the second from 200 to 250°ë. In all probability, the first stage is water elimination and the second is carbon dioxide elimination. To verify this suggestion, we studied the samples with the ammonia history by TGA combined with the IR spectroscopy of thermolysis gases. In the IR spectra of gaseous decomposition products recorded in the course of heating, rather intense bands due to gaseous CO2 (~2380 cm–1) appear at ~200°ë and disappear at ~250°ë (Fig. 3). The use of different precipitators also affects the micromorphology of xerogels. According to TEM data (Fig. 4), the zinc hydroxide xerogels with the alkali history consist of rather coarse particles (100–1000 nm), whose sizes are far greater than the sizes of the ZnO particles that are formed during subsequent thermal (including HT) treatment. Xerogel particles contain inclusions with relatively small sizes (10–30 nm; Fig. 4a). An analogous situation is observed for zinc hydroxide gels precipitated with NH4OH: these xerogels also contain contrast inclusions with characteristic sizes of 10–100 nm (Fig. 4b). The existence of these inclusions matches our previous heat-flow calorimetry data [3], which indicated that zinc hydroxide degradation starts at 30–40°C. Thus, the inclusions in question can be nuclei of nascent Zn(OH)2 or ZnO. To elucidate the sequence of phase transformations during zinc oxide formation from zinc hydroxide compounds with different chemical history, we heated the suspensions precipitated with 0.5 M Zn(NO3)2 in 0.43 M NaOH and 0.15 M NH4OH to various temperatures under hydrothermal and prehydrothermal conditions. The treatment schedule was chosen proceeding from

Absorbance 80 60

Tim

e, m

0.15 40

0.10 20

0.05 0

in

0.20

3000

2000 ν, cm–1

1000

Fig. 3. IR spectra of the gaseous products eliminated during the decomposition of zinc hydroxide compounds that were prepared by the precipitation of 0.5 M Zn(NO3)2 in 0.3 M NH4OH. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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‡

b

c

10

(‡)

(b)

100 nm

500 nm

Fig. 4. Micrographs of the xerogels prepared by the hydrolysis of zinc nitrate in the presence of (a) 0.43 M NaOH and (b) 0.15 M NH4OH.

previous calorimetric data [6] that characterize the temperature ranges of zinc oxide formation. Suspensions having the ammonia history were treated for 1 h at 60, 70, and 85°ë those having the alkali history were treated at 60, 80, and 100°ë. This study was intended to identify the major products and intermediates in the HT synthesis of ZnO. Figure 5 displays the X-ray powder diffraction patterns for the powders prepared by the HT treatment of suspensions having the ammonia history at various

20

30

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50

60 2θ, deg

Fig. 5. X-ray diffraction patterns of samples prepared by the HT treatment of suspensions with the ammonia history for 1 h at (a) 60, (b) 70, and (c) 85°C.

temperatures. After HT treatment for 1 h at 60°ë, peaks of ε-Zn(OH)2 (wulfingite) and ZnO appear in the X-ray diffraction pattern in addition to the peaks of δ-Zn(OH)2. 1-h treatment at 70°ë makes the δ-Zn(OH)2 peaks disappear, and the product is ZnO with minor wulfingite. The product of synthesis at 85°ë is singlephase zinc oxide with the characteristic size of individual grains equal to 55 nm along (100) and 95 nm along (002). TEM indicates a considerable alteration of powder micromorphology with rising treatment temperature: even for the powder synthesized at 60°ë (Fig. 6a), the volume fraction of the high-contrast phase (presumably, ZnO) increases considerably compared to the untreated xerogel (Fig. 4b). Hydrothermal treatment at 85°ë produces a powder of coarse, homogeneous, anisotropic zinc oxide particles (Fig. 6b) extended in the (002) direction, which is typical of ZnO [10]. Thermogravimetric analysis confirms these data: the overall weight loss decreases with increasing synthesis temperature, from 20% for 60°ë to 4.1% for 70°ë and to 1.7% for 85°ë. The hydrothermal treatment of the suspension prepared by zinc nitrate precipitation in NaOH also produces single-phase ZnO. The powders synthesized at 60 and 80°ë are virtually single-phase crystalline Zn(OH)2 (wulfingite) as identified by XPD (Fig. 7). TEM shows inclusions of a contrast phase, which is likely ZnO, in the sample synthesized at 80°ë, and weak maxima due to zinc oxide can be identified in the X-ray diffraction pattern of this sample. The power prepared at 100°ë is single-phase zinc oxide (wurtzite) with a slightly greater average grain size (63 nm along (100) and 110 nm along (002)) compared to the analogously synthesized ZnO samples with the ammonia history. According to TGA, an increase in the HT treatment temperature from 80 to 100°ë decreases the


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‡

b

c 10 (‡)

200 nm

20

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Fig. 7. X-ray diffraction patterns of powders prepared by the HT treatment of suspensions having the alkali history for 1 h at (a) 60, (b) 80, and (c) 100°C.

ment of zinc hydroxide compounds can be represented as follows: NaOH

Zn(NO 3 ) 2 (sln)

Zn(OH) 2 (amorph.)

ε-Zn(OH) 2 Zn(NO 3 ) 2 (sln)

60–80°C

NH4OH ((NH4)2CO3)

α-Zn(OH) 2 + ε-Zn(OH) 2

(b)

200 nm

Fig. 6. Micrographs of powders prepared by the HT treatment of suspensions having the ammonia history for 1 h at (a) 60 and (b) 85°C.

weight loss of the resulting powders from 19 to 0.9%, indicating virtually complete dehydration of Zn(OH)2 at these temperatures in agreement with XPD data and our previous heat-flow calorimetry data [6]. Thus, zinc oxide formation during the HT treatment of suspensions of zinc hydroxide compounds produces crystalline zinc hydroxide (wulfingite); when NaOH is the precipitator, wulfingite is formed as early as during precipitation, while with aqueous ammonia, wulfingite is formed as an intermediate during subsequent HT treatment. The general scheme of the processes involved in the precipitation and subsequent HT treatRUSSIAN JOURNAL OF INORGANIC CHEMISTRY

30–60°C

ZnO. Zn 5 (OH) 6 (CO 3 ) 2 85°C

ZnO.

(2) 30–60°C

(3)

To elucidate the effect of the chemical history on the photocatalytic activity of the resulting samples, the suspensions prepared by zinc nitrate precipitation from 0.15 M NH4OH or 0.43 M NaOH were subjected to HT treatment at 125°C for 1 h. The thus obtained powders were single-phase zinc oxide as identified by X-ray powder diffraction. The ZnO powder having the ammonia history had a slightly smaller grain size than the ZnO powder with the alkali history (45 nm along (100) and 88 nm along (002) against 67 nm along (100) and 85 nm along (002)). However, in spite of the higher dispersion, the sample having the ammonia history had a considerably lower photocatalytic activity than the samples having the alkali history: the first-order rate constants of Methyl Orange photodegradation were 0.0027 and 0.0057 min–1, respectively. This result can serve as an indication of the greater imperfection of the sample having the ammonia history. More information on the imperfection of these ZnO samples was gained from luminescent studies. The luminescence spectra (Fig. 8) were typical of polycrystalline ZnO [11]: UV-luminescence peaks were observed at 385 nm, corresponding to electron transition from the conduction band to the valence band (exciton recombination), as well as green luminescence

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zinc compounds. We have shown that the ZnO powders obtained during the HT treatment of Zn(OH)2 suspensions having different chemical history have different defect structures and considerably differ in their photocatalytic activities.

Iabs

‡

ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (project no. 05-03-33036), the Presidium of the Russian Academy of Sciences (program no. 8), and the National Science Foundation.

b

REFERENCES 400

450

500

550

600

650

700 λ, nm

Fig. 8. Photoluminescence spectra of powders prepared by the HT treatment of zinc hydroxide suspensions having (a) alkali and (b) ammonia histories at 125°C for 1 h.

peaks at ~600 nm, whose appearance is usually assigned to the existence of oxygen vacancies in the material [12]. The intensity ratio for the peaks detected at 385 and 600 nm is 0.35 for the sample with the alkali history and 1.0 for the sample with the ammonia history. However, the photoluminescence intensity for the powder with the alkali history is in general higher than the luminescence intensity from the powder with the ammonia history, confirming the high imperfection of the latter and interpreting the different photocatalytic activities of these samples. In summary, our results indicate that the parameters of precipitation (the type and concentration of the precipitator) and subsequent HT treatment influence the chemical and phase compositions of the resulting

1. J. G. Lu, P. Chang, and Z. Fan, Mat. Sci. Eng.: R: Rep. 52, 49 (2006). 2. K. Byrappa and K. Yoshimura, Handbook of Hydrothermal Technology. A Technology for Crystal Growth and Materials Processing (William Andrew, New York, 2000). 3. F. Yu. Sharikov, V. K. Ivanov, and Yu. D. Tret’yakov, Dokl. Akad. Nauk, Ser.: Khimiya 410, 771 (2006). 4. S. Music, D. Dragcevic, and S. Popovic, J. Alloys Comp. 429, 242 (2007). 5. I. P. Belomestnykh, N. V. Voikina, T. A. Markova, et al., Kinet. Katal. 28, 691 (1987). 6. F. Yu. Sharikov, A. S. Shaporev, V. K. Ivanov, et al., Zh. Neorg. Khim. 50, 1947 (2005). 7. R. Giovanoli, H. R. Oswald, and W. Feitknecht, Helv. Chim. Acta 49, 1971 (1966). 8. A. S. Shaporev, V. K. Ivanov, A. E. Baranchikov, and Yu. D. Tret’yakov, Zh. Neorg. Khim. 51, 1621 (2006). 9. A. S. Shaporev, V. K. Ivanov, A. E. Baranchikov, and Yu. D. Tret’yakov, Neorg. Mater. 43, 38 (2007). 10. Y. W. Heo, D. P. Norton, L. C. Tien, et al., Mat. Sci. Eng.: R: Rep. 47, 1 (2004). 11. K. K. Bajaj, Mat. Sci. Eng.: R: Rep. 34, 59 (2001). 12. G. D. Gilliland, Mat. Sci. Eng.: R: Rep. 18, 99 (1997).


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