Gel method is a popular approach to prepare crystals and a large number of gel-grown crystals have been obtained during the past decades ^{[1, 2, 3, 4, 5, 6]}. Among these crystals,a few of them have abnormal internal structure in the sense that the crystals incorporate the gel network and become composites. Interestingly,these gel-grown crystals still maintain the long-range order and diffract X-rays and/ or electron beams as single-crystals even though the gel networks are incorporated inside. Initially,in 1969,Nickl and Henisch reported that calcite crystals grown from silica gels incorporated the silica matrix ^[7]. Afterwards,Garcia-Ruiz et al. found that gelgrown protein (lysozyme,ferritin and thaumatin) crystals also incorporate gel (silica and/or agarose) matrix ^{[5, 8, 9]}. More recently,calcite,again,were shown to incorporate gelatine ^[10], poly-acrylamide ^[11],agarose ^{[12, 13, 14, 15]} and xyloglucan gels ^[16]. Although the total amount is still very limited as compared with the large amount of gel-grown crystals,the examples of gelincorporated single-crystals are increasing ^{[17, 18, 19, 20]}.

The unique property of the gel-incorporated single-crystals is the combination of long-range order (single-crystallinity) and composite structures,which are desired for semiconducting and conducting materials because the long-range order provides fast electron transport and the composite structures enable the design for hetero-structure such as p-n junction. For example,mesoporous single-crystal TiO₂ provides higher electron mobility than conventional nanocrystalline TiO₂ ,resulting in solar cells efficiency as high as 7.3% ^[21]. Also,metallic porous composites,such as nanoporous gold that combines high surface area and high conductivity,can be applied in many important technologies including energy conversion and high catalytic activity [22- 24]. Nevertheless,all the gel-incorporated single-crystals are insulators,while no crystals of semiconductors or conductors have been reported yet.

In this work,we prepare crystals of two typical semiconductors,lead sulfide (PbS) and lead iodide (PbI₂ ),in both silica gels and agarose gels to examine if gel networks can be incorporated into these crystals. PbS is a IV-VI semiconductor with a narrow bandgap (0.41 eV) and large excitation Bohr radius (18 nm) ^[25], which has been widely used in many fields such as Pb²⁺ ionselective sensors ^[26],IR detector ^[27],photography ^[28] and hybrid solar cell ^{[29, 30]}. On account of its unique semiconducting and optical properties,extensive effort has been devoted to prepare various PbS crystals ^{[31, 32, 33, 34]}. Different from PbS,PbI₂ is a p-type semiconductor with broad band gap (2.3 eV). It is a very promising room temperature radiation detector material for X-ray andg-ray detection due to its high atomic number and wide energy band gap ^{[35, 36]}. In this work,we demonstrate that both of these two crystals can form gel-incorporated singlecrystals. Chinese Chemical Letters 26 (2015) 504-508 ARTICLE INFO Article history: Received 22 October 2014 Received in revised form 4 January 2015 Accepted 9 January 2015 Available online 24 January 2015 Keywords: Single crystals Gel-incorporation Semiconductor PbS PbI₂ ABSTRACT Gel-incorporated single-crystals provide unique combinational properties of long-range order and composite structures,which is desired for semiconducting and conducting materials. However,the reported gel-incorporated single-crystals are limited to insulating crystals. Here,we examine crystals of two typical semiconductors,lead sulfide (PbS) and lead iodide (PbI₂ ),grown from both silica gels and agarose gels. In all the four crystal-gel pairs,single-crystals of the cubic phase of PbS and the hexagonal phase of PbI₂ were obtained according to the X-ray diffraction analysis. Dissolution of the gel-grown crystals exposed insoluble materials with the shape similar to the original crystals,indicative of gelincorporation inside the crystals. As such,this work creates a facile strategy to construct 3D heterostructures inside semiconducting single-crystals without destroying their long-range order. ·2015 Chinese Chemical Society and Institute of Materia Medica,Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved. 2. Experimental 2.1. PbS crystals 2.1.1. Gel preparation and crystallization

Silica gels with a sodium metasilicate pentahydrate (Na₂SiO₃· 5H₂O) concentration of 7.5 w/v% were prepared by adding a solution of 10 mL 15 w/v% Na₂SiO₃·5H₂O (Aldich,>95.0%) drop by drop into 10 mL 1 mol/L HCl (hydrochloric acid) with constant stirring. The final pH was between 4.5 and 5. The mixture was filtered (1mm,glass fiber,iLab) into a U-tube (20 mm diameter, 120 mm long,40 mm width). 11.25 w/v% and 15 w/v% gels were made in a similar way with the increased concentration (1.5 times and twice) of Na₂SiO₃·5H₂O and HCl.

After the silica gel formed 24 h later,6 mL 0.5 mol/L HCl was added upon each side of the gel to acidize it. Acidification was done to promote the hydrolyzation of thioacetamide (TAA) in the following procedure. Lower pH is helpful to obtain better crystals ^[37]. After 24 h,the HCl upon the gel was removed away,and the gel was washed twice with DI water to dilute away the remaining acid. Then 10 mL 0.1 mol/L Pb(NO₃)₂ (SCR,>99.0%) and 10 mL 0.4 mol/L TAA ^{[4, 32]} (Sigma,>99.0%) was added into each side of the U-tube. The tube was placed in 158C thermostatic bath after sealed. About 5 days later,PbS crystals appeared in the middle of the U-tube. Crystals in silica gels were separated from the gel with deionized (DI) water and NaOH (SCR,>96.0%). Firstly,the silica gel was smashed with DI water and then 10 mL 5 mol/L NaOH was added. After about 1 h,the gel was dissolved and the crystals were in the bottom of the solution. The crystals were then washed with DI water and ethanol.

Agarose gels were prepared from hot solution of agarose (Sigma,type: 1-B). The 0.5 w/v% agarose solution was prepared by dissolving 0.05 g agarose powder in a hot solution of 0.1 mol/L Pb(NO₃)₂. Then the solution (5 mL) was filtered (0.45mm,nylon, iLab) into a 10 mL plastic centrifuge tube. Gel was formed in 1 h. 5 mL 0.5 mol/L HCl was added upon the gel to acidize it and was removed 24 h later. Subsequently,the gel was washed twice with DI water. 5 mL 0.4 mol/L TAA was added on the top of the gel. About 10 days later,PbS crystals appeared in the gels below the solution/gel interface. Crystal in agarose was separated from gel by dissolving the gels in hot DI water. The crystals were also washed with DI water and ethanol. 2.1.2. Dissolution of the crystals

After separated from gel,crystals of PbS were placed on a slide in a petri dish,then a drop of 12 mol/L HCl was added upon the crystals in a fume hood. After 2 h of dissolution,the petri dish was open for 5 min to allow the H₂S and HCl gas to leave. The crystals were observed with the optical microscope (OM). As a control experiment,the bare agarose gels were immersed in 12 mol/L HCl and partial decomposition of the gels was observed. 2.2. PbI₂ crystals 2.2.1. Gel preparation and crystallization

Silica gels were prepared in a similar way but acetic acid (CH₃COOH) was used instead of hydrochloric acid. 5 mL 10.9 w/v% Na₂SiO₃·5H₂O was added dropwise into 5 mL 2 mol/L CH₃COOH to make silica gels with a Na₂SiO₃·5H₂O concentration of 5.45 w/v%. Denser gels (10.9 w/v%) were prepared by doubling concentration of Na₂SiO₃·5H₂O and CH₃COOH. The final pH was about 4.3. After that PbAc₂ (C₄H₆O₄Pb·3H₂O,SCR,>99.5%) was added into the mixtures with the Pd²⁺ concentration of 0.1 mol/L,10 mL of the mixed solution was filtered (1mm,glass fiber,iLab) into a test tube (20 mL). Gels formed after about 48 h and 10 mL 0.2 mol/L potassium iodide (KI,SCR,>99.0%) was placed on the top of the gels. After about 7 days,hexagon golden crystals appeared in the silica gels. PbI₂ crystals were separated from the silica gels and were rinsed withDI water and ethanol. As PbI₂ is soluble in NaOH solutions,the gels surrounding gel-grown crystals were not completely cleaned using NaOH solutions. Instead,they were rinsed away using DI water and ethanol.Asmallamountof the gel residues might still remain on the surfaces of the crystals.

Agarose gels were prepared from hot solution of agarose. The 0.2 w/v% agarose solution was prepared by dissolving 0.02 g agarose powder in a hot solution of 10 mL 0.2 mol/L PbAc₂. Then 5 mL solution was filtered (0.45mm,nylon,iLab) into a 10 mL plastic centrifuge tube. After gelation,acidification wasalsocarried out byadding5mLof2 mol/LCH₃COOHthatwasremovedafter 24 h.Subsequently,the gelswerewashedtwice withDIwater. 5mL0.4 mol/LKIwasaddedonthe top. PbI₂ crystalsgrownin agarosewereseparatedbydissolvingthe gelsin hotDIwatera weeklater. 2.2.2. Dissolution of the crystals

The PbI₂ crystal was dissolved in 4 mol/L Na₂S₂O₃ (Na₂S₂O₃·5H₂O SCR,>99.0%) for 0.5 h,and the residues were observed with optical microscopy. 2.3. Crystal characterization

The morphology of the crystals before and after dissolution was observed with an OM equipped with a digital camera (Nikon LV100 POL) and field emission scanning electron microscope (FESEM S-4800). Powder X-ray diffraction (PXRD) (RIGAKU D/MAX 2550/PC) was used to identify the chemical composition of the obtained crystals. Single Crystal X-ray diffraction (RIGAKU R-AXIS RAPID) was conducted to determine the crystallinity. Si concentration of the crystals grown in silica gels was measured with an inductively coupled plasma atomic emission spectrometer (ICP-AES,Thermo, iCAP6300). 3. Results and discussion

PbS and PbI₂ crystals were grown to investigate whether semiconductor single crystal could incorporated gel matrix. Firstly, the crystals were characterized with Powder X-ray diffraction (PXRD) to identify the crystalline structure of the as-grown solid. As illustrated in Fig. S1 in Supporting information,the strong and sharp peaks indicate the high crystallinity of the as grown solids and the diffraction pattern match well with the JCPDS cards of the cubic phase of PbS and the hexagonal phase of PbI₂ ,respectively. Furthermore,the single crystal XRD results (Table S1 in Supporting information) proved the single-crystallinity of the two crystals ^{[38, 39]}.

After obtaining the crystals from the gels,we proceed to examine the internal structure of the crystals by monitoring the dissolution of the crystals. We first examined the crystals grown from silica gels. The as-prepared PbS crystals were expressed as six branched stars,while the PbI₂ crystals exhibited a hexagonal plate-like morphology (Figs. 1a and 2d). PbS crystals grown in silica gels before and after dissolution were imaged using optical microscopy (OM) (Fig. 1a and b). As the PbS crystals were completely dissolved in HCl,translucent residual materials were left with the similar six branched shape of the origin crystals (Fig. 1b). These insoluble materials were assigned as the silica networks that were incorporated inside the PbS single-crystals when they grew in the silica gels ^[7]. As the gel-grown crystals were gently etched,the incorporated fiber-like and film-like gel materials were exposed in the etch pits and imaged by SEM (Fig. 1d,arrows) ^[8].

Fig. 1

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Fig. 1. (a and b) OM images of PbS crystals grown in a silica gel (7.5 w/v%): (a) as prepared; (b) after dissolving the crystal with 12 mol/L HCl. (c and d) SEM images of PbS crystals grown in a silica gel (11.25 w/v%): (c) as prepared; (d) after slightly etched by 8 mol/L HCl. The arrows highlight the exposed gel materials. (e) Mass ratio of Si in PbS crystals grown in silica gels with varied concentrations.

In addition,ICP-AES was used to quantify the amounts of the incorporation of silica gel. For comparison,these amounts were also calculated. Assuming the growing crystals incorporate the silica gel network completely without water,the mass ratio (W_si) of the incorporated Si element inside the crystals can be calculated from the equation in Ref. ^[19]:

Similarly,when growing in silica gel,PbI₂ crystals incorporated silica networks as well. After the dissolution of the gel-grown crystals,translucent residual materials were left with almost the same hexagonal shape of the origin PbI₂ crystals,indicative of gelincorporation (Fig. 2b and c). Also,ICP-AES showed the existence of substantial amount of Si element in the crystals. In contrast to the case of PbS,the measured weight ratio of Si element in gel-grown PbI₂ crystal is much lower than the calculated values. One possible explanation for the lower amount of incorporated gels is associated with the morphology of the crystals. PbI₂ crystal grown in silica gel exhibits a hexagonal plate-like morphology. During crystallization, the crystals exert anisotropic pressures on the surrounding gel networks,with larger pressures on the six side faces than the top and bottom faces. As a result,the gel network is expanded and ‘‘diluted’’ (Fig. 2a). As a supporting evidence for the gel expansion during crystallization,the gel networks shrank as they were released from the gel-grown PbI₂ crystals. This gel shrinkage was obviously observed for a weaker gel (Fig. 2d and e).

Fig. 2

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Fig. 2. (a) Schematic representation of plate-like crystal growing in gel media. A growing crystal exerts anisotropic pressures on the surrounding gel networks and expands them. The gel networks incorporated inside the crystal recoil and shrink after the crystal is dissolved. (b and c) OM images of PbI2 crystals grown in a silica gel (10.9 w/v%): (b) as prepared; (c) after dissolving the crystal with 4 mol/L Na2S2O3. (d and e) OM images of PbI2 crystals grown in a silica gel (5.45 w/v%): (d) as prepared; (e) after dissolving the crystal with 4 mol/L Na2S2O3. The outline of the crystal is highlighted by dash lines. (f) Mass ratio of Si in PbI2 crystals grown in silica gels with varied concentrations.

Both PbS and PbI₂ crystals were grown from silica gels previously but the internal structures of the crystals were not examined and gel-incorporation was not found ^{[4, 40, 41, 42, 43]}. Here, we clearly demonstrate that the silica gels were incorporated by the PbS and PbI₂ single-crystals,providing an approach to construct internal composite structures in these semiconducting single-crystals by gel networks. Assuming that the Si element exists as SiO₂ in the crystal,the highest amounts of gel materials inside the crystals we achieved are 0.57 w/w% in PbS and 0.31 w/ w% in PbI₂. As we tried to use higher gel concentrations to achieve higher amounts of gel incorporation,we failed to obtained singlecrystals (Fig. S2 in Supporting information).

Further,we examined the agarose gel,an organic (polysaccharide) gel,to investigate the possibility of the incorporation of organic gel network inside PbS and PbI₂ single-crystals. As grown in agarose gels,hexagonal phase of PbI₂ crystals appeared tribranched (Fig. 3a and c). After the crystals were dissolved completely,the translucent residual gels (Fig. 3b) indicated that agarose gels could also be incorporated into PbI₂ crystals. In addition,gel fibers could be observed clearly in the etch pits after the crystal was slightly etched (Fig. 3d,arrows).

Fig. 3

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Fig. 3. (a and b) OM images of PbI2 crystal grown in 0.2 w/v% agarose gels: (a) as prepared; (b) after the crystal was dissolved in 4 mol/L Na2S2O3,with gel left. (c and d) SEM images of PbI2 crystals grown in 0.2 w/v% agarose gels: (c) as prepared; (d) after the crystal was slightly etched by 4 mol/L Na2S2O3. The arrows highlight the exposed gel fibers.

PbS crystals grown in agarose gels were expressed as truncated octahedron with a few small holes on the surfaces (Fig. 4a and c). Although the crystals were too small for single crystal X-ray diffraction,their faceted shapes implies the single-crystallinity. When these crystals were dissolved in concentrated hydrochloric acid,the incorporated agarose gels were revealed (Fig. 4b). The reason less agarose gel residue was found after the dissolution of the PbS crystals is that agarose gels partially decompose in concentrated hydrochloric acid. Instead of complete dissolution, partially etching revealed the incorporated gel fibers in the etch pits (Fig. 4d,arrows). Therefore,we clearly demonstrate that the gels,both inorganic and organic ones,were incorporated by the PbS and PbI₂ single-crystals.

Fig. 4

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Fig. 4. (a and b) OM images of PbS crystals grown in 0.5 w/v% agarose gels: (a) as prepared,(b) after the crystals were dissolved in 12 mol/L HCl. (c and d) SEM images of PbS crystals grown in 1 w/v% agarose gels: (c) as prepared; (d) after the crystals were slightly etched by 3 mol/L HCl. The arrows highlight the exposed gel fibers.

In summary,we have prepared PbS and PbI₂ single-crystal grown from both silica and agarose gels. Through crystallographic analysis by X-ray diffraction and OM images of the dissolved crystals,we demonstrated that the obtained crystals were gelincorporated single-crystals for all the four crystal-gel pairs.

Together with molecule ^[44] and particle ^[45] incorporation,this work provides a possible way to construct internal hybrid structure inside semiconducting single-crystals. One of the future directions is to prepare semiconducting single-crystals with incorporated semiconducting gels ^{[46, 47, 48]} and these semiconductor hybrids may be useful for high-performance electronic applications.

Acknowledgments

This work was supported by Zhejiang Province Natural Science Foundation (No. LZ13E030002),the 973 Program (No. 2014CB643503),the National Natural Science Foundation of China (Nos. 51222302,51373150,51461165301),and Fundamental Research Funds for the Central Universities.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.01. 020.