Sensors and Actuators B 178 (2013) 53– 62 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Hollow, porous, and yttrium functionalized ZnO nanospheres with enhanced gas-sensing performances Weiwei Guo a , Tianmo Liu a,∗ , Rong Sun b , Yong Chen a,c , Wen Zeng a , Zhongchang Wang c,∗ a College of Materials Science and Engineering, Chongqing University, Chongqing, China b Institute of Engineering Innovation, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan c WPI Research Center, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan a r t i c l e i n f o Article history: Received 20 June 2012 Received in revised form 18 December 2012 Accepted 20 December 2012 Available online 28 December 2012 Keywords: ZnO Nanospheres Gas sensor Yttrium doping a b s t r a c t We report the synthesis of a hierarchical nanostructure of hollow and porous ZnO nanospheres with a high specific surface area as a novel sensing material to toxic formaldehyde by a simple template-free hydrothermal technique in organic solution. We demonstrate that the liquid mixture ratio and hydro- thermal time play a pivotal role in forming such unique morphology and propose a growth mechanism of Ostwald ripening coupled with grain rotation induced grain coalescence. Comparison investigations reveal that yttrium allows resistance reduction of sensors and enhances significantly gas-sensing per- formances of ZnO nanospheres toward the formaldehyde over the commonly used undecorated ZnO nanoparticles. Such hollow, porous, and yttrium functionalized ZnO nanospheres could therefore serve as hybrid functional materials for chemical gas sensors. The results represent an advance of hierarchical nanostructures in enhancing further the functionality of gas sensors, and the facile method presented could be applicable to many other sensing materials. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Inorganic nanomaterials with hollow and porous superstruc- tures find numerous technological applications where morpholo- gies are known to influence functionality. Gas sensors [1–3], catalysts [4,5], drug delivery carriers [6,7], and photoelectronic building blocks [8–10] are just a few significant examples. In gen- eral, the morphology with a large specific surface area and efficient porosity is often beneficial for the catalytic, gas-sensing and pho- tovoltaic applications due to the likelihood to enhance surface reactions. In this respect, the active search of unusual morphology is currently the subject of intensive research in the nanomaterials world [11,12]. One of the most well-characterized nanomaterials in terms of morphology is ZnO, which is an n-type semiconductor with a direct wide band gap (3.37 eV) and a large excitation binding energy (60 meV) [13,14]. To date, a substantial amount of exper- iments have already provided definitive evidence that size and morphology of ZnO nanomaterials can affect greatly their perform- ances, especially gas-sensing functionality [15–17]. On the other hand, doping ZnO with various elements, e.g., noble metals [18–20], rare-earth metals [21], transition metals [22], and metal oxides [23] ∗ Corresponding authors. Tel.: +81 22 217 5933; fax: +81 22 217 5930. E-mail addresses: tmliu@cqu.edu.cn (T. Liu), zcwang@wpi-aimr.tohoku.ac.jp, wang@cello.t.u-tokyo.ac.jp (Z. Wang). has been suspected to enable modulation of surface charge states of ZnO, modifying significantly its functionality. A general approach to date to fabricate nanomaterials with the hollow and porous morphologies accompanies the use of remov- able or sacrificial templates, including either the hard ones such as monodisperse silica [24], polymer latex spheres, [25,26] and reduc- ing metal nanoparticles [27], or the soft ones such as emulsion micelles [28] and gas bubbles [29]. The disadvantages for the use of templates though rest with the high cost and tedious synthe- sis process, posing a significant hurdle to the large-scale industrial applications. Ideally, one would prefer a one-step template-free method to synthesize the nanomaterials with hollow and porous superstructures in a size tunable manner. Recent breakthroughs in the fabrication of nanomaterials by taking full advantage of known physical phenomena, e.g., oriented attachment [30,31], Ost- wald ripening [32–34], Kirkendall effect [35,36], and etching-based inside-out evacuation [37,38], has brought such “ideal” concept closer to reality. Among all the fabrication techniques, the etching process has been demonstrated as a facile choice for preparing hol- low and porous nanomaterials because it is easy to dissolve inner nano-crystallites via adjusting processing time and temperature [39–41]. Here, we report a technically simple and flexible route: the use of a template-free hydrothermal process to prepare the hollow and porous ZnO nanospheres with a large specific area in a con- trollable manner. We investigate in detail the effect of the liquid mixture ratio and hydrothermal time on the morphology evolution 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.12.073 54 W. Guo et al. / Sensors and Actuators B 178 (2013) 53– 62 and propose a new mechanism that is responsible for the unusual nucleation and self-assembly of ZnO building blocks, i.e., coupling of Ostwald ripening with grain-rotation-induced grain coalescence (GRIGC). Such a unique morphology is maintained after doping yttrium (Y) to produce hybrid functionality of ZnO as a gas-sensing material, which, to the best of our knowledge, has rarely been reported. Our results demonstrate that Y-doped ZnO nanospheres lower remarkably resistance and enhance gas-sensing perform- ances, which may open up a new avenue to develop advanced gas sensors. 2. Experimental All ZnO nanospheres were synthesized by the hydrothermal method. Zinc acetate dehydrate (Zn(CH 3 COOH) 2 ·2H 2 O) (4 mM) was first dissolved into a mixed solution of ethanol (40 mL) and monoethanolamine (MEA) (30 mL) under mechanical stirring for 1 h. The solution was then transferred in autoclaves, which were heated to 160 ◦ C for 24 h to produce precipitate. The pure ZnO powder was prepared by centrifuging the precipitate, washing it with distilled water and ethanol to remove unwanted ions, and drying at 60 ◦ C in air. The obtained powder (0.03 g) was dis- persed in deionized water (20 mL), and 1.5 mL mixed solution of ethanol and yttrium nitrate hexahydrate (N 3 O 9 Y·6H 2 O) (0.01 M) was then added. The solution was stirred thoroughly for 1 h and dried at 80 ◦ C in air before annealing at 400 ◦ C for 2 h to elimi- nate NO 3 – ions. The Y-doped ZnO powder with a mass ratio of Y to Zn of 4% was harvested. To make a straightforward compari- son, the ZnO nanoparticles were also prepared by dissolving 4 mM Zn(CH 3 COOH) 2 ·2H 2 O and 20 mM NaOH in 70 mL distilled water, which was then transferred in autoclaves and heated at 160 ◦ C for 20 h. Microstructure analysis was conducted by the X-ray diffrac- tion (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). For the XRD, a Rigaku D/Max- 1200X diffractometry with Cu K˛ radiation operated at 40 kV and 200 mA was applied. Surface morphologies of the sam- ples were observed using a Hitachi S-4300 SEM. Microstructures and chemical composition were analyzed using the JEOL JEM- 2010F electron microscope operated at an accelerating voltage of 200 kV. Specific surface area was measured upon the multipoint Brunauer–Emmett–Teller (BET) analysis of nitrogen adsorption isotherms, which were recorded on a surface area analyzer (Micromeritics, ASAP 2020M). The powders upon harvest were mixed with diethanolamine and ethanol to form pastes, which were subsequently coated onto an alumina ceramic tube pre-loaded with a pair of gold electrodes at each end. Next, the tube was dried at 400 ◦ C for 2 h in order to eliminate organic binder as well as strengthen the bonding between the pastes and tube. A Ni–Cr wire was placed inside the tube as a heater. The heating wire and tube were soldered on the pedestals to fabricate gas sensors. The sensors were finally aged at 200 ◦ C for 240 h in order to improve stability and repeatabil- ity. Gas-sensing measurements were conducted using a computer controlled measurement system (HW-30A, Hanwei Electronics Co., Ltd.) at room temperature at a humidity of 40%. The sensor was first connected to the circuit board of measurement system, and then the tested gas was introduced into the glass chamber through injecting a given amount of gas. The operating temperature of sen- sors can be adjusted precisely via altering the current flow across the Ni–Cr heater. Resistance (R s ) of the sensors was estimated by R s = R L (V c − V out )/V out , where the R L was resistance of a load resistor (R L = 47 k), and the V c and V out were circuit and output volt- age (V c = 6 V), respectively. The sensor response (S) was defined as S = R a /R g at reductive atmosphere, while as S = R g /R a at oxidative Fig. 1. XRD spectra of Y-free and Y-doped ZnO nanospheres with a series of Y/Zn ratios. Textural orientations of detected matters are given as well for easy reference. atmosphere, where R a and R g were resistance in air and target gas, respectively. The response and recovery time was defined as the interval between when response reached 90% of its maximum and dropped to 10% of its maximum. 3. Results and discussion 3.1. Chemical composition and morphology To identify chemically the prepared samples, we first conducted XRD analyses, as shown in Fig. 1, where textural orientation of the detected matters is shown as well for easy reference. For the ZnO, 2% and 4% Y-doped ZnO samples, all of the peaks are identified as belonging to the wurtzite (hexagonal) structure of ZnO (JCPDS (36- 1451)). No secondary phase is detected although the lattice of the Y-doped sample is found to be somewhat expanded as compared to the Y-free sample. In addition to ZnO, Y 2 O 3 is also detected in the 6% and 8% Y-doped ZnO samples. This suggests that the Y atoms fill the lattice sites of ZnO at the low doping concentration, but tend to form a new Y 2 O 3 phase at high doping concentration (over 6%). However, there are no characteristic secondary-phase XRD peaks in the 2% and 4% Y-doped ZnO samples, indicating that the sec- ondary phase is very scarce or highly dispersed. This is because there appear well defined XRD peaks if size of the crystallites is above 1–3 nm [42]. This case is also recognized in two-phase sys- tems, in which the secondary phase with a small concentration is highly dispersed on surfaces of the basic oxide’s grains. These indi- cate that the secondary oxide phase, if have, should have a smaller grain size than the basic oxide in the samples with Y doping con- centrations of 2% and 4%. When the doping concentration is over 6%, a new Y 2 O 3 phase is formed with a grain size larger than 3 nm. Table 1 lists the lattice constants of both the undoped and Y-doped samples obtained from XRD data and the crystallite size calculated using the Scherrer formula. The lattice constants (a and c) and grain sizes increase with the rise of the amount of Y, suggesting that the introduction of Y distorts the crystal structure of the host oxide. Table 1 The lattice constants of the Y-doped ZnO sphere and ZnO nanoparticle, and grain sizes calculated using the Scherrer formula. ZnO Lattice constant Grain size (nm) a (Å) c (Å) 0% Y doped 3.24926 5.20505 18.5 2% Y doped 3.25261 5.20983 19.3 4% Y doped 3.25689 5.21532 21.6 6% Y doped 3.25795 5.21823 22.4 8% Y doped 3.25948 5.21993 23.5 Nanoparticle 3.26889 5.22845 28.9 W. Guo et al. / Sensors and Actuators B 178 (2013) 53– 62 55 This is because the radius of Y 3+ ion (0.92 ˚ A) is larger than that of Zn 2+ (0.74 ˚ A), which should increase the lattice constants of ZnO by Y doping and hence result in a shift in diffraction peak toward lower 2␪ angle. To unveil morphologies of the prepared samples, we show in Fig. 2 SEM images of representative regions in the pristine and doped nanospheres and the nanoparticles. As seen in Fig. 2(a), the Y-free sample is indeed characterized as nanospheres, which are uniformly distributed. These nanospheres are coarse on sur- face (Fig. 2(b) and (c)) and hollow inside, as clearly verified in a broken nanosphere (Fig. 2(d)). Interestingly, the nanospheres are self-assembled to radially aligned nanorods of ∼150 nm in length from their cores yet to self-wrapped irregular nanoparticles at the cores. Pores turn up on the nanosphere surfaces, indicating that the as-synthesized pristine samples are not only hollow but porous. Such a hierarchical morphology are further corroborated from the TEM images showing a difference in the image contrast between the margin and center of nanospheres, i.e., the center seems brighter, which indicates the formation of the well-defined hollow nano- structures (Fig. 3(a)). Fig. 3(b) and (c) gives TEM images of edge regions of the nanospheres, which show unambiguously the pores (Fig. 3(b)), nanorods, and nanoparticles (Fig. 3(c)). Fig. 3(d) presents a high-resolution TEM (HRTEM) image of an edge of a nanosphere (only the edge is likely for imaging due to the large thickness away from surface), from which lattice fringes are clearly visible. The spacing between neighboring lattice planes is estimated to be ∼0.26 nm, in line with that between the (0 0 0 1) planes of a hexag- onal ZnO (inset of Fig. 3(d)), suggesting that the ZnO nanorods grow in the [0 0 0 1] direction. Such interesting hollow and porous nanospheres are not dis- turbed significantly by Y doping (Fig. 2(e)–(g)), although their size becomes larger due to the growth during post-annealing. Fig. 2(h) shows the morphology of nanoparticles for a comparison, which are accumulated with a mean size of ∼50 nm. Fig. 3(e)–(g) presents TEM images of the Y-doped samples, where they retain the porous and hollow nature. Like what was seen in the pristine sample, the nanorods also grow in the [0 0 0 1] direction even when the Y is doped. To identify chemically the samples, we perform an energy- dispersive X-ray spectroscopy (EDS) analysis of a representative nanosphere in the pristine and 4% Y-doped sample, as shown in Fig. 3(h). The nanospheres in the pristine sample are composed of 40.4 at% O and 59.6 at% Zn, while those in the 4% Y-doped sample 34.21 at% O, 63.78 at% Zn and 2.01 at% Y, demonstrating that the embedded additive of Y is really present in the ZnO matrix. Further EDS mapping of both the entire sphere and the edge reveals an even distribution of O (Fig. 3(j)) and Zn (Fig. 3(k)), providing direct evi- dence to the uniform distribution of Y in the doped sample (Fig. 3(l)) and further testifying the second oxide phase is present in the 4% Y-doped ZnO matrix, in consistence with the XRD results. 3.2. Formation mechanism of hollow and porous nanospheres To gain insight into formation mechanism of the hierarchi- cal nanostructures and how morphology evolves with processing conditions, we first investigate systemically the role of solvent com- position on the structures of nanomaterials. As seen in Fig. 4(a) and (b), the ZnO nanorods are clustered when the MEA is not introduced. Once the MEA is added (5 mL), the nanorods are bun- dled irregularly and loosely with a mean length of 500 nm (Fig. 4(c) and (d)). Further increase in the MEA concentration (15 mL) renders these bundles self-assembled to fan-shaped hemispheres (Fig. 4(e) and (f)). The hollow and porous nanospheres are formed when the concentration of MEA is increased further to 30 mL (Fig. 4(g) and (h)). The nanospheres become denser with fewer holes on surfaces as the MEA concentration is increased to 40 mL (Fig. 4(i) and (j)). However, the nanospheres are nonporous and solid when the con- centration of MEA is increased to 50 mL (Fig. 4(k) and (l)), implying that the precise control of the MEA concentration is essential to producing a hierarchical superstructure. The formation of nanorods in the [0 0 0 1] direction without MEA is understood upon the structural anisotropy and surface polarity of ZnO. The (0 0 0 1) polar plane is the most energetically unfavorable and bears the highest growth rate, followed by (1 0 1 1), (1 0 1 0), ( 1 0 1 1), and (0 0 0 1) planes (inset of Fig. 3(d)) [43,44]. Once the MEA is in the ethanol solution, the coordinated [Zn(MEA) m ] 2+ ions (where m is an integer) are generated, restraining the formation of free Zn 2+ ions and the Zn(OH) 2 , the nuclei of ZnO nanomaterial. The chemical reactions in presence of MEA during hydrothermal process can be expressed as: Zn 2+ + mMEA ↔ [Zn(MEA) m ] 2+ , (1) Zn(OOCCH 3 ) 2 ·2H 2 O + 2C 2 H 5 OH → Zn(OH) 2 + 2H 2 O + 2CH 3 COOC 2 H 5 , (2) Zn(OH) 2 ↔ ZnO ↓ + H 2 O. (3) As the temperature is increased in the autoclaves, the [Zn(MEA) m ] 2+ ions are ready to be decomposed to Zn 2+ ions and ethanolamine molecules (Eq. (1)). Simultaneously, there occurs the esterifica- tion of zinc acetate with ethanol to produce Zn(OH) 2 (Eq. (2)), which is ultimately decomposed to ZnO nanomaterials (Eq. (3)). The ethanolamine molecules, which are adsorbed on the surfaces of ZnO nuclei, can serve as assembling agents, refraining crystals from forming nanorods along the [0 0 0 1] direction [45]. The metastable nanoparticles are produced instead at the initial nucleation stage, which is important for the next-stage Ostwald ripening procedure. Two factors are responsible for the evolution of morphology at solvothermal condition: the initial nucleation status and the solu- bility of precursors in solvent under saturation vapor pressure [46]. Note that the solvent MEA is lower than ethanol in the saturation vapor pressure due to its higher boiling point (78.29 ◦ C for ethanol while 170 ◦ C for MEA). This gives rise to extensive nucleation of metastable nanoparticles, which are aggregated into nanospheres to lower their surface areas and energies [47]. In addition to form- ing the spherical configuration, the MEA also plays a pivotal role in making the nanospheres hollow and porous. In contrast to the fast migration and high nucleation rate of the reactive species in ethanol, it is kinetically slower to form metastable nanocrystals in MEA solution due to the higher boiling point and viscosity of MEA. This allows the mixture of nanocrystals with varying growth orien- tations to assemble into spherical nanoparticles. On the other hand, the MEA is able to facilitate the formation of metastable nanoparti- cles, the interiors of which are susceptible to be dissolved, thereby producing the hollow nanospheres. To shed more light on the formation mechanism of hollow, porous nanospheres, we conduct a series of time-dependent inves- tigations, as shown in Fig. 5. At the early stage (4 h), solid spherical nanoparticles (Fig. 5(a) and (b)) are formed (Fig. 5(c)). As the reaction time is increased to 8 h, hollowing process starts at the nanosphere cores (Fig. 5(d) and (e)), and the surfaces of nanospheres turn rough (Fig. 5(f)), indicating that a portion of particles on surfaces are dissolved. Further extension of reaction time (16 h) enhances the hollowing effect (Fig. 5(g) and (h)), and the numerous nanorods with pores on surfaces are assembled to nanospheres (Fig. 5(h)) due to the dissolution and recrystalliza- tion (Fig. 5(i)). The hollow and porous nanospheres are formed as the reaction time is 24 h. However, there emerge urchin-like struc- tures comprising a large amount of nanorods with a small number of nanoparticles (Fig. 5(j)) as the reaction time is 30 h (Fig. 5(k) and (l)). Interestingly, most of the nanoparticles are dissolved when the 56 W. Guo et al. / Sensors and Actuators B 178 (2013) 53– 62 Fig. 2. SEM images of the pristine ZnO nanospheres taken at (a) low and (b) high magnification. (c) and (d) Magnified SEM images of an open hollow and porous ZnO nanosphere. SEM images of the Y-doped nanospheres taken at (e) low and (f) slightly higher magnification. (g) Magnified SEM image of an open nanosphere in the Y-doped sample. (h) SEM image of the nanoparticles. reaction time is increased to 35 h, leaving behind slim nanorods (Fig. 5(m) and (o)). The disappearance of the nanospheres sug- gests the important role of the nanoparticles in the stabilization of nanospheres (Fig. 5(n)). These imply such a mechanism: the Ostwald ripening [48] cou- pled with the grain rotation induced grain coalescence (GRIGC) [49]. The Ostwald ripening involves the aggregation of nano- crystallites, followed by an outward mass transfer to form hollow structures. The GRIGC process occurs when particles collide, and the grain rotation takes place thereafter. Such grain rotation low- ers the energy of system and eliminates the grain boundaries, producing single-phase nanocrystals (i.e., coalescence process). Fig. 3. (a) TEM image of the Y-free ZnO sample, highlighting that the nanospheres are hollow. (b) and (c) Enlarged TEM images of the Y-free ZnO nanospheres on edge. (d) HRTEM image of a Y-doped ZnO nanosphere. (e) TEM image of the Y-doped ZnO. (f) and (g) Enlarged TEM images of the doped sample on edge. (h) EDS for the pristine (upper) and doped (lower) ZnO nanomaterials. The horizontal axis denotes the energy and the vertical one the counts (i.e., intensity). (i) Original area and EDS mapping of (j) O, (k) Zn, and (l) Y elements in a Y-doped nanosphere. The insets show mapping of the edge region. W. Guo et al. / Sensors and Actuators B 178 (2013) 53– 62 57 Fig. 4. SEM image of the ZnO nanospheres prepared under different concentrations of monoethanolamine (MEA): (a) and (b) 0 mL, (c) and (d) 5 mL, (e) and (f) 15 mL, (g) and (h) 30 mL, (i) and (j) 40 mL, (k) and (l) 50 mL. The amount of added ethanol is fixed to be 40 mL. Fig. 6 shows schematically formation evolution of the hollow, porous nanospheres. At the initial stage, the ZnO nanocrystals are generated randomly. As the reaction time is increased, the ZnO colloids are aggregated to form solid nanospheres through the Ostwald ripening effect, which is driven by the minimization of sur- face energy. Since crystallites have a higher surface energy in the interiors than on the surfaces, they are more readily to be dissolved. Once being heated, the nano-crystallites are easier to be collided Fig. 5. SEM image illustrating evolution of morphology of the nanospheres with the reaction time: (a–c) 4 h, (d–f) 8 h, (g–i) 16 h, (j–l) 30 h, and (m–o) 35 h. Three images with different magnification are provided in each case. 58 W. Guo et al. / Sensors and Actuators B 178 (2013) 53– 62 Fig. 6. Schematic illustration of morphology evolution of the ZnO nanospheres. and rotated, giving rise to coalescence of neighboring grains to form large single-phase grains. Such a process lowers the inter- facial energy associated with large interfacial area. For the polar ZnO, the (0 0 0 1) plane is most likely to be coalesced due to its highest energy of all planes, which explains the observation that ZnO crystallites are grown in the [0 0 0 1] direction to produce the rod-like ZnO in the shell of hollow nanospheres. Meanwhile, the rotation and migration of particles induce pores, which results in the hollow and porous morphology. 3.3. Resistance as a function of temperature To gain more insights into surface properties of the nanospheres, we show in Fig. 7 nitrogen adsorption–desorption isotherm and size distribution of the pores calculated using the Barret–Joyner–Halenda (BJH) method. Careful analysis of the plot identifies the isotherm as a type IV one, indicating the formation of typical porous structure (Fig. 7(a)). Although the pore spans a large range in size, the majority of pores have a diameter of 5–15 nm, in accord with the above TEM observations (Fig. 7(b)). The specific surface area of the nanospheres reaches 89.5 m 2 g −1 measured using the BET, confirming the porous nature. It should be noted that the specific surface area of the nanoparticles is only 24.2 m 2 g −1 , which indicates that the morphology affects greatly the specific surface area. Such a 3D hierarchical porous nanostructure may hold sub- stantial promise for a wide range of applications, especially as a chemical sensor owing to the large specific surface area that can greatly enhance gas diffusion and mass transport. Extensive effort has been devoted to date to improving gas-sensing prop- erties of ZnO, including the fast response and recovery and high gas response. Among them, the doping of rare-earth elements, e.g., Y, has been demonstrated as one of effective ways to activate host materials, and may enable fictionalization of the hierarchical nanospheres as well for advanced functional gas sensors. To test this scenario, we first present in Fig. 8(a) the resistance (R) as a function of temperature (T) for the sensors fabricated with the Y-free and Y-doped nanospheres in air together with the sen- sor made of pristine ZnO nanoparticles (Fig. 2(g)). Overall feature is different between samples and the sample doped with 4% Y has the lowest resistance. The resistance decreases with increasing amount of Y, but such a decrease comes to a halt when the doping concen- tration of Y is beyond 4%. Carre ˜ no et al. reported the formation of a second phase, Sn 2 Y 2 O 7 , in the SnO 2 doped with a small amount of Y [50]. Likewise, a similar second phase of ZnY m O n may be pro- duced in our samples when the doping concentration is lower than 4%. The second phase has a low resistance, providing conducting channels in the sample and hence lowering the contact resistance of the ZnO grains. This may reduce the resistance of ZnO sample. However, when the doping concentration is above 4%, the doped Y in ZnO reaches saturation, forming Y 2 O 3 precipitates that grow along the ZnO grain boundary. The Y 2 O 3 has a higher resistance than the matrix, thereby increasing the contact resistance. This consequently suppresses the dropping of overall resistance signif- icantly, that is, the resistance is increased. Another key feature in Fig. 8(a) is that resistance drops in a less abrupt manner for the Y-doped (Y/Zn = 4%) than Y-free ZnO at the temperature ranging from 300 to 450 ◦ C, which could be attributed to the chemisorbed O on surfaces. However, the reversible reactions take place among O gas (gas), chemisorbed O (ads), and lattice O (lat) with the rise of temperature [51]: O 2 (gas) + e − ⇔ O − 2 (ads), (4) 1 2 O 2 + e − ⇔ O − ads , (5) 1 2 O 2 + 2e − ⇔ O 2− ads , (6) O 2− ads ⇔ O 2− lat , (7) These conclude intuitively that electron transfer from semiconduc- tor to absorbed O is responsible for the increase of resistance. It has been reported that pure ZnO materials exhibit n-type semiconduc- tor characteristics due to the existence of oxygen vacancies [52]. From the EDS analysis, we find that the O/Zn ratio decreased from 67.7% (ZnO) to 53.6% (4% Y-doped ZnO). The fewer amounts of oxy- gen and zinc in the Y-doped ZnO reveal the increase of defects with the introduction of Y in the ZnO nanospheres. Meanwhile the asso- ciated increase in lattice constant gives rise to increased intrinsic defects, e.g., V • O , V •• O , and O // i [53]. During the hydrothermal process, defects can be produced and further enhanced by the doping of Y in the ZnO nanospheres. It is worth noting that ZnO has a hexago- nal close-packed lattice with a relatively open structure in which Zn atoms occupy half of the tetrahedral sites and all the octahe- dral sites are empty. In general, the oxygen vacancy (V •• O ) has lower formation energy than the zinc interstitials (Zn •• i ), resulting in Zn- rich compositions in the real wurtzite ZnO [52]. In this sense, the intrinsic defects and extrinsic dopants can be introduced during the fabrication. Xu also pointed out that the O 2 molecules interact strongly with oxygen vacancies on the surface of ZnO [54]. These imply that the Y doping can increase the concentration of O vacancy and hence absorb more oxygen on the ZnO surface, which as a result increases the concentration of O − . 3.4. Gas-sensing performance To gain insight into gas-sensing properties of the ZnO nano- structures, we present in Fig. 8(b) gas response to formaldehyde (HCHO) gas as a function of temperature at 50 ppm. The Y-free nanospheres show a higher gas response (maximum value of 47.4 at 350 ◦ C) than the undoped nanoparticles, indicating that mor- phology is critical to the enhancement of gas-sensing functionality. Evidently, response of ZnO nanospheres is improved with the addi- tion of Y. However, gas response is saturated to a maximum value of 65.7 when the Y concentration reaches 4%. Further increase in the Y-doping concentration results in an adverse effect, i.e., lowers the gas response. The Y-doped nanospheres have a lower optimal tem- perature (300 ◦ C) than the Y-free ones (350 ◦ C). The enhancement W. Guo et al. / Sensors and Actuators B 178 (2013) 53– 62 59 Fig. 7. (a) Nitrogen adsorption–desorption isotherm and (b) corresponding pore size distribution of the ZnO hollow and porous nanospheres. of gas-sensing properties by Y doping can be understood as fol- lows. In the pristine case, the O molecules are adsorbed on surfaces and capture e from the ZnO semiconductor, forming chemisorbed O species (Eqs. (7)–(10)). Such process gives rise to surface deple- tion layers, which eventually increases resistance of the samples. When being exposed to the HCHO, the HCHO molecules react with the adsorbed O on surfaces in the following manner: HCHO (gas) + 2O − ads ⇒ CO 2 + H 2 O + 2e − . (11) This process releases the trapped electrons back to conduction band of ZnO, increasing thereby concentration of carriers in the semi- conductor [55,56]. The introduction of Y induces oxygen defects in ZnO, increases concentration of O − ads and hence improves gas response. On the other hand, the low optimal operating tem- perature after the Y doping can be ascribed to the formation of weakly bonded complexes ZnY m O n . The chemisorption of oxygen species depends strongly on the temperature and nature of mate- rial. The O 2 is chemisorbed at low temperature while O − and O 2− are chemisorbed at high temperature. Since ZnO is a semiconduc- tor, oxygen absorption and electron transfer are rather difficult to occur at room temperature. The thermal activation of the semicon- ductor is required to observe gas adsorption on surface. This is why change in resistance is not observed when the ZnO nanospheres are exposed in the reduced gases. However, the low-temperature gas adsorption becomes possible by the Y doping due to the pres- ence of the weakly bonded complexes ZnY m O n on the ZnO grain surface. The absorption of oxygen ions can occur on the ZnO sur- face at room temperature due to the high conducting nature of the ZnY m O n . In this respect, the Y activates reactions of HCHO to the adsorbed O due to the spillover effect [57–59], resulting in a lower optimal operating temperature. Fig. 9 shows response–recovery characteristics for the three sen- sors fabricated with the pristine nanoparticles, Y-free and Y-doped nanospheres at different operating temperatures. Six representa- tive species of volatile organic compound (VOC) gases are chosen purposely, including CH 4 , NH 3 , HCHO, CH 3 OH, CO, and (CH 3 ) 2 CO. The gas concentration is fixed to 50 ppm. The voltage is increased sharply when the test gas is in, yet returns to its original state when gas is out. The key difference among the three samples is that the voltage is increased in the most strikingly manner for the Y-doped sample, verifying again the gas-response enhancement by morphology and Y doping. Moreover, the response and recovery transient of these sensors is superior to HCHO than to the rest of the tested VOC gases, especially in the Y-doped case (Fig. 9(a)–(c)). Upon closer inspection, we find that the response and recovery time is ∼14 and 17 s for the pristine nanoparticles, while ∼10 and 12 s for the Y-free nanospheres. They are further shortened to ∼4 and 6 s for the Y-doped nanospheres (Fig. 9(d)). To shed more light on the Y-doped sample, we further measure the gas-sensing properties at the optimal operating temperature of 300 ◦ C, as shown in Fig. 10. The gas response is increased drasti- cally as the gas concentration is increased up to 250 ppm, yet in a more gentle fashion as the concentration is increased further. The response is saturated at ∼800 ppm. Interestingly, the gas response is increased almost linearly when the gas concentration ranges from 10 to 100 ppm (inset of Fig. 10(a)), implying that the Y-doped ZnO nanomaterial works even at low gas concentration. Fig. 10(b) shows gas response of the Y-doped sensor to the six types of VOC gases at 50 ppm. The result made clear is that the response to the HCHO reaches a maximum value of 65.7 but is no larger than 16 to other gases. This implies that the Y-functionalized nanosphere can act as an efficient gas-sensing material for on-site selective detection of formaldehyde. Since the formaldehyde has a single aldehyde and high reducibility in detecting gases, the unsaturated Y ions tend to absorb HCHO molecules, forming complex species of Y–HCHO [59]. Simultaneously, the absorb oxygen on the surface oxidizes the HCHO into H 2 O and CO 2 , resulting in a good selectiv- ity to HCHO for the sensor. Fig. 10(c) shows a single-cycle response for the Y-doped sensor at different HCHO concentrations at 300 ◦ C. The voltage signal (i) is enlarged with the rise of HCHO concentra- tion, (ii) is stabilized in 4 s when the sensor is exposed in the HCHO atmosphere, and (iii) returns to original state in 6 s once the sen- sor is exposed in air. Fig. 10(d) presents representative reversible cycles of the gas response in HCHO (50 ppm), where one can see Fig. 8. (a) Sensor resistance of Y-free nanoparticles, Y-free and Y-doped nanospheres as a function of temperature in air. (b) Response of the sensors fabricated with Y-free nanoparticles, Y-free and Y-doped nanospheres with various concentrations to HCHO of 50 ppm measured at temperatures from 200 ◦ C to 500 ◦ C. Grain sizes of various ZnO samples are listed in Table 1. 60 W. Guo et al. / Sensors and Actuators B 178 (2013) 53– 62 Fig. 9. Response–recovery characteristic for the sensors fabricated with (a) Y-free ZnO nanoparticles, (b) Y-free and (c) Y-doped ZnO nanospheres. Six types of VOC gases, CH 4 , NH 3 , HCHO, CH 3 OH, CO, and (CH 3 ) 2 OH, are chosen purposely. The operation temperature for the sensors fabricated with the Y-free nanomaterials is 350 ◦ C and that for the sensor fabricated with Y-doped nanomaterial 300 ◦ C. The concentration of the tested gas is fixed to be 50 ppm. (d) Single-cycle response and recovery transients of the three sensors to the HCHO gas at 50 ppm. Fig. 10. (a) Gas response of the sensor made of Y-doped ZnO as a function of HCHO concentration operated at 300 ◦ C. The inset highlights sensing characteristic at low gas concentration. (b) Gas response of the sensor made of Y-doped ZnO to the six types of gases of choice. The concentration of each gas is fixed to be 50 ppm and the operating temperature is 300 ◦ C. (c) Single-cycle response of the Y-doped ZnO to the HCHO gas at different concentrations at 300 ◦ C. (d) Typical response and recovery characteristic of the sensor fabricated with the Y-doped ZnO to the HCHO gas of 50 ppm at 300 ◦ C. A few representative cycles are shown only, demonstrating stability of the Y-doped sensor. that the response and recovery characteristics are reproduced well with no remarkable attenuation. These imply that the Y-doped ZnO nanospheres can improve significantly gas-sensing performances. Such improvement cannot be realized for the nanoparticles and hence the Y-doped nanospheres shall hold substantial promise for the development of a practical sensor device for the on-site detec- tion of the harmful HCHO gas. 4. Conclusions We have fabricated successfully novel hollow and porous ZnO nanospheres via the simple template-free hydrothermal technique in organic solution, and investigated the gas-sensing functions. We demonstrate that the ratio of MEA in solution is critical to mor- phology because it facilitates formation of metastable nanoparticle, restrains the growth of nanorods, and serves as fundamen- tal building blocks for nanospheres. Systematic microstructural studies reveal a coupling of the Ostwald ripening with the grain-rotation-induced grain coalescence growth mechanism which is responsible for the formation of the hollow and porous nanospheres. Such hierarchical nanospheres possess a large spe- cific surface area and can be functionalized with Y for advanced chemical gas-sensing application. Gas-sensing performance to the HCHO is found to be enhanced in the doped sample with the Y con- centration of 4%. This work indicates that the Y-doped hierarchical structures represent an important step forward to exploring the novel gas sensors for future on-site detection of harmful gases. Acknowledgements This work was supported in part by the National Natural Science of China (51202302) and China Postdoctoral Science Foundation (No. 2012M511904). Z.W. appreciates financial supports from the Grant-in-Aid for Young Scientists (A) (grant no. 24686069) and the Challenging Exploratory Research (grant no. 24656376). W. Guo et al. / Sensors and Actuators B 178 (2013) 53– 62 61 References [1] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sensors and Actuators B 140 (2009) 319–336. [2] H. Zhang, Q. Zhu, Y. Zhang, Y. Wang, L. Zhao, B. Yu, One-pot synthesis and hier- archical assembly of hollow Cu 2 O microspheres with nanocrystals-composed porous multishell and their gas-sensing properties, Advanced Functional Mate- rials 17 (2007) 2766–2771. [3] W. Guo, T. Liu, H. Zhang, R. Sun, Y. Chen, W. Zeng, Z. Wang, Gas-sensing per- formance enhancement in ZnO nanostructures by hierarchical morphology, Sensors and Actuators B 166–167 (2012) 492–499. 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Xu, Au nanoparticle modi- fied WO 3 nanorods with their enhanced properties for photocatalysis and gas sensing, The Journal of Physical Chemistry C 114 (2010) 2049–2055. [59] S. Morrison, Selectivity in semiconductor gas sensors, Sensors and Actuators B 12 (1987) 425–440. 62 W. Guo et al. / Sensors and Actuators B 178 (2013) 53– 62 Biographies Weiwei Guo is currently a PhD candidate at the College of Materials Science and Engineering, Chongqing University in China. He is now engaged in the synthesis and characterization of the semiconducting materials and in the investigation of their gas sensing properties. Tianmo Liu is a professor of College of Materials Science and Engineering at Chongqing University in China since 2001. He received Dr. Eng. from Department of Solid Mechanics, Chongqing University in 1999. His current research interest involves functional materials for gas sensors and magnesium alloys. He is now also holding a group leader position at the National Engineering Research Center for Magnesium Alloys at Chongqing University. Rong Sun is currently a PhD candidate in the Institute of Engineering Innovation, The University of Tokyo in Japan. Her research interest involves the characterization of materials using advanced transmission electron microscopy. Yong Chen is currently a PhD candidate at the College of Materials Science and Engineering, Chongqing University in China, and also an exchange student at Tohoku University in Japan since 2011. He is now engaged in fabricating nanomaterials and in characterization using advanced transmission electron microscopy. Wen Zeng received his PhD degree in material Science from Chongqing University in China. He is currently a lecture at the College of Materials Science and Engineering, Chongqing University. He is focusing on synthesis of low-dimensional functional materials, on fabrication of semiconducting sensors and on first-principles calcula- tions. Zhongchang Wang is currently an assistant professor at the WPI Research Center, Advanced Institute for Materials Research, Tohoku University in Japan. He received his master degree in 2004 from Chongqing University in China and PhD in 2008 from the University of Tokyo in Japan. He is now mainly focusing on gas-sensing materials, interfaces, grain boundaries, dislocations in oxides, and quantum electron transport by combining the state-of-the-art transmission electron microscopy with the first-principles calculations. . Chemical journa l h o mepage: www.elsevier.com/locate/snb Hollow, porous, and yttrium functionalized ZnO nanospheres with enhanced gas-sensing performances Weiwei Guo a , Tianmo . formaldehyde over the commonly used undecorated ZnO nanoparticles. Such hollow, porous, and yttrium functionalized ZnO nanospheres could therefore serve as hybrid . 2012 Keywords: ZnO Nanospheres Gas sensor Yttrium doping a b s t r a c t We report the synthesis of a hierarchical nanostructure of hollow and porous ZnO nanospheres