DOI: 10.1126/science.1096566
, 711 (2004);
304Science
et al.Yadong Yin,Nanoscale Kirkendall Effect Formation of Hollow Nanocrystals Through the
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w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m
Formation of Hollow
Nanocrystals Through the
Nanoscale Kirkendall Effect
Yadong Yin,Robert M.Rioux,Can K.Erdonmez,Steven Hughes, Gabor A.Somorja,A.PaulAl v satos*
Hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect,in which pores form becau of the difference in diffusion rates between two components in a diffusion couple.Starting with cobalt nano-crystals,we show that their reaction in solution with oxygen and either sulfur or lenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides.This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds.A simple extension of the process yielded platinum–cobalt oxide yolk-shell nanostructures,which may rve as nanoscale reactors in catalytic applications.
Porous solid materials are important in many areas of modern science and technology,in-cluding ion exchange,molecular paration, catalysis,chromatography,microelectronics, and energy storage(1–3).Notable examples are microporous(Ͻ2-nm)zeolites and meso-porous(2-to50-nm)silicate and carbona-ceous materials.The ability to manipulate the structure and morphology of porous solids on a nanometer scale would enable greater con-trol of the local chemical environment(4–6). We demon
strate that nanoscale pores can de-velop inside nanocrystals with a mechanism analogous to void formation in the Kirkendall Effect,in which the mutual diffusion rates of two components in a diffusion couple differ by a considerable amount(7).We choo cobalt nanocrystals as a starting material to show that hollow nanocrystals of cobalt oxide and chalcogenides can be successful-ly synthesized through the reaction of co-balt colloidal solution with oxygen and ei-ther sulfur or lenium.
It has been known for more than half a century that porosity may result from dif-ferential solid-state diffusion rates of the reactants in an alloying or oxidation reac-tion.In1947,Smigelkas and Kirkendall reported the movement of the interface be-tween a diffusion ,copper and zinc in brass,as the result of the different diffusion rates of the two species at an elevated temperature(7).This phenome-non,now called the Kirkendall Effect,was the first experimental proof that atomic diffusion occurs through vacancy exchange and not by the direct interchange of atoms. The net directional flow of matter is bal-anced by an opposite flow of vacancies,
which can conden into pores or annihilate
at dislocations.Directional material flows
also result from coupled reaction-diffusion
phenomena at solid/gas or solid/liquid in-
terfaces,leading to deformation,void for-
mation,or both during the growth of metal
oxide or sulfide films(8,9).The voids
are usually explained by outward transport
of fast-moving cations through the oxide
layer and a balancing inward flow of va-
cancies to the vicinity of the metal-oxide
interface.Interface motion and the forma-
tion of pores have been studied becau of
their impact on the reproducibility and re-
liability of solders,passivation layers,dif-
fusion barriers,etc.,but not generally as a
method of preparing porous materials.The
pores produced at a metal-metal diffusion
couple or near the metal-oxide interface of
a growing oxide do not yield monodisper,
ordered arrays but instead form a very het-
erogeneous enmble.The obrved vol-
ume fraction for pores is also commonly
much smaller than would be expected for
the known material flows.The obrva-
tions are a direct result of the large volume
of material that vacancies can diffu into
and the large number of defects with which
they can react(10).
If the faster-diffusing species is confined
into a nanocrystal core,the net rate of vacancy
injection should increa markedly,becau of
修图培训the high surface-to-volume ratio of the particle
and the abnce of defects in the core.Within
the small volume of a transforming nanocrystal,
the supersaturated vacancy cloud is likely to
coalesce into a single void.Previous studies on
the interdiffusion of30-m powders with lay-
ered composition showed a large volume frac-
tion of pores,but the geometry and distribution
of the pores were not uniform,probably be-
cau of aggregation and the bulk-like dimen-
sion of the particles(11).Considerable progress
has recently been made in synthesizing colloidal
nanocrystals with well-controlled size,shape,and
surface properties(12–14).Employing such high-
quality nanocrystals as the starting materials,it
should be possible to produce a relatively uniform
population of hollow nanostructures.
We cho cobalt nanocrystals as the main
starting material.A number of chemical
methods have been developed to synthesize
uniform cobalt nanocrystals in solution(12,
15).Furthermore,cobalt reacts readily with
other species such as sulfur and oxygen.Be-
cau cobalt is the major component in one
class of superalloys,its high-temperature ox-
idation and sulfidation have been well studied
(16,17).It is known that oxidation and sul-
fidation of bulk cobalt under vapor at high
temperature are mainly controlled by outward
movenpickdiffusion of cobalt cations(18).This mode of
growth operating on nanocrystals is expected
to lead to hollow structures.
Sulfidation of cobalt was the first ca in
which we obrved hollow nanostructures.Co-
balt sulfide hollow nanospheres were synthe-
sized in one pot by immediate injection of a
solution of sulfur in o-dichlorobenzene into a
hot cobalt nanocrystal dispersion(Fig.1A)that
was prepared by literature methods(15,19).At
455K,the reaction between cobalt and sulfur
completes within a few conds,resulting in a
black solution of cobalt sulfide nanocrystals.
We confirmed that hollow particles are pro-
duced at temperatures as low as373K.The
hollow particles are very stable in solution,
suggesting that the chemical transformation of
the surface did not disrupt the coating of the
nanocrystals by surfactant molecules.When
washed with methanol,the surfactant layer was
removed,and it was no longer possible to re-
cf英文dissolve the precipitate in o-dichlorobenzene.
shining friends
Outward flow of cobalt through the sul-
fide shell resulted in supersaturation of va-
cancies,which condend to form a single
hole in each nanoparticle(Fig.1,B and D).
Two stable cobalt sulfide phas were ob-
rved,linnaeite(Co
3
S
4
)and cobalt pent-
landite(Co
9
S
8
),depending on the sulfur-
to-cobalt molar ratio ud in the synthesis.
X-ray powder diffraction(XRD)patterns in
Fig.1E show the evolution of the crystal
structure as the molar ratio of sulfur to
cobalt was incread.Co
9
S
8
was the only
sulfide pha obrved when the molar ra-
tio was lower than9:8,whereas Co
3
S
4
also
appeared in the patterns when the molar
ratio slightly exceeded this value.Only
Co
3
S
4
was obtained when the molar ratio of
sulfur to cobalt was above3:4.The size
distribution of the sulfide hollow particles
was similar to that of the starting cobalt
nanocrystals.Monodisper,hollow nano-
crystals lf-asmbled into ordered hexag-
onal arrangements when evaporated slowly
on the surface of a carbon-coated transmis-
sion electron microscopy(TEM)grid.The
Department of Chemistry,University of California at
Berkeley,and Materials Science Division,Lawrence
Berkeley National Laboratory,Berkeley,CA94720,USA.
*To whom correspondence should be addresd.E-
mail:alivis@uclink4.berkeley.edu
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asmbly process was driven by surface tension and van der Waals forces.Cobalt sulfide nanocrystals did not form superlat-tices as readily as cobalt nanocrystals do,probably becau of a diminished van der Waals force (19).Asmblies of hollow nano-particles prent a distinct topology of ordered porous materials.In terms of the accessibility of pores from the outside,they fall between meso-porous materials with accessible channels and void lattices in which pores are confined in a continuous matrix (20).
Kinematical diffraction simulations (fig.S1)indicated that the XRD peaks are too broad to be consistent with a single crystal shell of the dimensions obrved by TEM (21).We obtained satisfactory fits to the data in Fig.1E,panels (d)and (g),by assuming ϳ4-nm,cubic crystalline do-mains.The fits also provided a confirma-tion of our pha assignments.TEM micro-graphs (Fig.1D)of the same sample show that the average outer diameter of the hol-low Co 9S 8nanocrystals is ϳ15nm.A rea-sonable explanation is that the shell of each hollow sphere is multicrystalline.This was confirmed by high-resolution TEM (HRTEM),which shows that both Co 9S 8and Co 3S 4hollow nanocrystals are com-pod of multiple crystallographic domains (Fig.1C).The arrangement of the domains is analogous to the columnar morphology of grains often obrved in thin film growth.The multicrystalline structure im-plies possible applications of the hollow nanocrystals as nanoscale re
actors,becau small molecules may be able to penetrate the shell through the grain boundaries.In all instances of sulfidation,we found that the diameter of the hole in the center of the nanocrystals was 40to 70%of the initial particle size (starting with cobalt particles with a size distribution of 7%,a single synthesis yielded a hole-size distri-bution of 13%).If sulfur transport through the growing shell were negligible,as shown for bulk sulfidation by marker experiments (18),then the two diameters would be ex-pected to be identical.Significant inward sulfur transport could occur through grain boundaries or during the formation of the first few monolayers of sulfide.It is also possible that inward relaxation of the hole occurs,due to annihilation of vacancies at a micoherent or incoherent cobalt-sulfide interface.Finally,the estimation of the hole size by visual inspection of TEM images may produce systematic errors.We at-tempted to examine the possibility of in-ward sulfur transport by performing the Co 3S 4synthesis at different sulfur concen-trations.Incread sulfur concentration in-cread hole size and enhanced outward growth of the shell,indicating that cobalt mobility rather than sulfur mobility was
affected.This finding is consistent with bulk sulfidation studies (18),in which it is ob-rved that an incread sulfur vapor pressure leads to injection of more cation vacancies into the growing sulfide and enhances the parabolic rate constant for sulfide growth.For bulk cobalt,the rates of oxidation are
3to 4orders of magnitude lower than tho of sulfidation above 750K (18).This is also true under the conditions we ud to
produce hollow nanocrystals,and oxidation of nanocrystals took ϳ3hours at 455K.Figure 2,A to D,shows the evolution of the morphology of the nanocrystals with time as an O 2/Ar mixture is flowed through the cobalt colloidal solution.The XRD shows the prence of metallic cobalt up to 30min after the start of the O 2/Ar flow (Fig.2E).The solution of particles still displayed weak ferrofluidic respon to a 1-T magnet
Fig.1.(A )TEM image of cobalt nanocrystals synthesized by the injection of 0.54g of Co 2(CO)8in 3ml of o -dichlorobenzene into 0.1ml of oleic acid and 0.1g of trioctylphosphine oxide in 15ml of o -dichlorobenzene at 455K.(B )TEM image of the cobalt sulfide pha synthesized by the injection of sulfur in o -dichlorobenzene (5ml)into cobalt nanocrystal solution with
a Co/S molar ratio of 9:12.Co 3S 4particles were synthesized from the cobalt sample shown in (A).(C )HRTEM images of Co 3S 4(left)and Co 9S 8(right).(D )TEM image of the cobalt sulfide pha synthesized as in (B),but with a Co:S molar ratio of 9:8.Co 9S 8particles started from another cobalt sample that had an average diameter of ϳ11nm.(E )XRD patterns of (a)cobalt nanocrystals and (
b to h)cobalt sulfide synthesized with different Co/S molar ratios:(b)9:5,(c)9:7,(d)9:8,(e)9:10,(f)9:11,(g)9:12,and (h)9:18.The dots,triangles,and squares reprent peaks from cobalt,CO 9S 8,and CO 3S 4phas,respectively.
Fig.2.Evolution of CoO hollow nanocrystals over time in respon to a stream of O 2/Ar mixture (1:4in volume ratio,120ml/min)being blown through a cobalt colloidal solution at 455K.(A to D )TEM images of the solutions after flow of
O 2/Ar for (A)0min,(B)30min,(C)80min,and (D)210min.Int:HRTEM of a CoO hollow nanocrystal.(E )XRD patterns of the sample obtained from the solution after flow of O 2/Ar for (a)0min,(b)2.5min,(c)5.5min,(d)10min,(e)30min,(f)80min,and (g)210min.The dots and diamonds reprent peaks from cobalt and CoO phas,respectively.
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30APRIL 2004VOL 304SCIENCE www.sciencemag
712 o n D e c e m b e r 31, 2007
w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m
at that point,suggesting that small cobalt cores remained.It tookϳ3hours for the cobalt cores to be completely consumed; central pores were clearly distinguishable for all nanocrystals under TEM,and the solution showed no magnetic respon. XRD simulations(fig.S1)suggest a multi-crystalline structure with a crystal domain size ofϳ3nm,in agreement with the HRTEM obrvations(Fig.2,int).
The evolution of hollow morphology is best illustrated by following the reaction of cobalt nanocrystals with lenium.In bulk systems,annihilation of excess vacancies at dislocations and boundaries can produce stress that lead to the formation of cracks near the interface;the cracks then act as nuclei for the further condensation of su-persaturated vacancies(11).Although the exact mechanism is likely to be different,in nanocrystals voids also begin to develop and merge at the boundary(Fig.3).The high defect content and surface energy associated with the boundary favor the nu-cleation of voids there.In addition,as va-cancies diffu inward,they will be more concentrated at the boundary rather than in the interior of the core.As the reaction proceeds in time,more cobalt atoms diffu out to the shell,and the accompanying transport of vacancies leads to growth and merging of the initial voids.This results in the formation of bridges of material be-tween the core and the shell that persist until the core is completely consumed. The bridges provide a fast transport path for outward diffusion of cobalt atoms that can then spread on the inner shell surface.
A similar phenomenon was obrved for bulk powders(11).The growth rate of pores dropped markedly when the cobalt cores became relatively small.Most of the pore volume emed to form during the first few minutes,whereas it tookϳ30min for the cobalt cores to completely disap-pear.This may be becau,as the bridges are also consumed during the reaction,a smaller cross-ctional area is available for solid-state transport of materials.
As an illustration of the generality of the ideas,we synthesized veral other hollow
nanostructures.Sulfidation of disk-shaped
cobalt nanocrystals(21)was obrved to re-
sult in the formation of hollow nanodisks
with cylindrical pores,indicating that spher-
ical symmetry is not required for obtaining
shells of regular thickness.Preliminary stud-
ies on oxidation of iron nanospheres and
sulfidation of cadmium nanospheres also re-
sulted in hollow structures,thus validating
our approach for metallic cores in general.
Theoretically,the mobilities of the reacting
species do not have to be markedly different
to result in vacancy transport.Placing solid
polyethylenenanocrystals containing one reactant in a
comparatively dilute solution creates an ad-
ditional asymmetry that may favor the cre-
ation of hollow structures:The relatively
large change in the concentration of the core
material between the core and the solution
provides a greater driving force for the out-
ward diffusion of the core material.Thus,
numerous combinations of reactants may be
expected to produce various hollow nano-
structures of insulators,miconductors,and
even metals.A recent report on the formation
of gold nanoboxes may involve the same
mechanism at some stage,although the di-
mension of the structures produced is an or-
der of magnitude larger(22).
Hollow nanocrystals offer possibilities in
material design for applications in catalysis,
nanoelectronics,nano-optics,drug delivery sys-
tems,and as building blocks for lightweight
structural materials(23–25).For example,ac-
curate fixation of the catalyst within the pores,
combined with other emerging techniques of
chemical control(26),could result in better
reaction control and new products.To demon-
strate the u of hollow nanocrystals in cataly-
sis,we studied their function as nanoreactors,
each of which contains one noble metal nano-
crystal.A Pt@CoO yolk-shell nanostructure
was synthesized,in which a platinum nanocrys-
tal of a few nanometers was encapsulated in a
CoO shell.Three steps were involved in the
preparation of the nanoreactors:the synthesis
of platinum eds by a modified“polyol”pro-
cess(27),the deposition of cobalt on platinum
to form Pt@Co core-shell nanocrystals,and the
transformation of cobalt into CoO hollow struc-
tures(28).Figure4A shows a typical sample of
platinum particles with an average diameter of
ϳ3nm.The deposition of cobalt onto platinum
at the reaction temperature yielded no alloy,
only Pt core/Co shell particles,as confirmed by
睫毛膏怎么卸XRD analys.The oxidation reaction removed
cobalt atoms away from the platinum particle
surface,leading to the formation of a platinum
yolk/CoO shell structure(Fig.4B).No free
platinum particles were found by TEM inspec-
tion of the Pt@CoO sample.We could control
the size of Pt@CoO particles by changing the
diameter and number of the platinum eds and
the amount of cobalt carbonyl precursor.
In order to determine if the Pt@CoO ma-
terials were active as heterogeneous catalysts,
we cho the hydrogenation of ethylene as a
model reaction,becau it readily occurs at
ambient conditions on many transition metal
catalysts.Platinum is one of the most active
metals for this reaction,whereas the activity
of metallic cobalt isϳ2orders of
magnitude
Fig. 3.Evolution of
CoSe hollow nano-
crystals with time by
injection of a suspen-
sion of lenium in o-
dichlorobenzene into
a cobalt nanocrystal
solution at455K,from
top-left to bottom-
right:0s,10s,20s,1
min,2min,and30
min.The Co/Se molar
ratio was
1:1.
Fig.4.(A)Platinum nanocrystals prepared by
the injection of a solution of0.15g of platinum
acetylacetonate in5ml of o-dichlorobenzene
into a refluxing bath of10ml of o-dichlorobenzene
that contained0.3g of1,2-hexadecanediol,0.1ml
of oleic acid,0.1ml of oleylamine,and0.06
ml of trioctylphosphine.The solution was
then heated for another120min.(B)We
formed Pt@CoO yolk-shell nanostructures by
injecting 1.08g Co2(CO)8in6ml of o-
dichlorobenzene into the platinum nano-
crystals solution,and followed by the oxida-
tion of the product particles by blowing a
stream of O2/Ar mixture(1:4in volume ra-
tio,120ml/min)into the colloidal solution at
455K.The system was kept at the same
日语googletemperature under stirring for3hours.
R E P O R T S
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lower (29).We found that pure CoO hollow nanocrystals were inactive for ethylene hy-drogenation (30),even after a 1-hour H 2prereduction at 373K.Only on reduction at 473K for 1hour was ethane detected at temperatures Ͼ300K.Samples containing platinum without pretreatment were active for C 2H 4hydrogenation at temperatures as low as 208K.The steady-state turnover fre-quency for ethane formation at 227K was 8.3ϫ10Ϫ3s Ϫ1(31),which is comparable to the rates of 3.5ϫ10Ϫ2s Ϫ1measured on a 0.04%Pt/SiO 2catalyst (32)and 1.7ϫ10Ϫ2s Ϫ1measured on pure platinum powders (0.2to 1.6m in diameter).The obrvations indicate that the reaction is catalyzed by plat-inum particles,not the CoO shell.This also confirms that a route exists for ethylene and hydrogen entry into the CoO shell interior.The grain boundaries on the shell are the most probable entry points for ethylene and hydrogen diffusion into as well as ethane diffusion out of the shell.
In comparison to catalysts supported on oth-er mesoporous materials,the isolation of cata-lyst nanoparticles within solid shells should minimize condary reaction of the products that degrade lectivity and product distribu-tion.Furthermore,any synergistic interactions between catalyst and support can be more effi-ciently ud when each catalyst particle is in contact with a shell of the support material.
References and Notes
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7.A.D.Smigelskas,E.O.Kirkendall,Trans.AIME 171,130(1947).
8.C.E.Birchenall,J.Electrochem.Soc.103,619(1956).9.J.C.Colson,M.Lambertin,P.Barret,in Proc.7th Int.Symp.Reactivity ofSolids ,J.S.Anderson,F.S.Stone,M.W.Roberts,Eds.(Chapman &Hall,London,1972),pp.283–293.
10.G.B.Gibbs,Oxid.Met.16,147(1981).11.F.Aldinger,Acta Met.22,923(1974).
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291,2115(2001).
16.S.Mrowec,K.Przybylski,Oxid.Met.11,365(1977).17.A.Devin,Cobalt 30,19(1966).
18.S.Mrowec,M.Danlelewski,A.Wojtowicz,J.Mater.
Sci.33,2617(1998).
han
19.V.F.Puntes,K.M.Krishnan,P.Alivisatos,Appl.Phys.
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Aided Mater.Des.8,1(2001).
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Chem.107,7351(2003).
28.Platinum acetylacetonate was reduced with a long-chain polyol to form uniform platinum nanoparticles in the prence of surfactants such as oleic acid,oleylamine,and trioctylphosphine.The
size of the platinum particles was tuned from 1to 10nm,depending on the concentration of surfactants.Co 2(CO)8was then injected into the hot solution and decompod to form a conformal coating on plati-num nanocrystals.Oxidation of the Pt@Co nanocrys-tals was performed a few minutes after we intro-duced the cobalt carbonyl by blowing a stream of O 2/Ar mixture (1:4in volume ratio,120ml/min)into the colloidal solution at 455K.The system was kept under stirring for 3hours.A black stable colloidal dispersion in o -dichlorobenzene was obtained.Final-ly,the Pt@CoO particles were precipitated by meth-anol,washed with toluene and methanol three times,and dried under vacuum.Typical nitrogen adsorption/desorption measurement on the powder at 77K showed a type IV isotherm with type H2hysteresis,with a Brunauer-Emmet-Teller surface area of 65m 2/g and a total pore volume of 0.0676cm 3/g.29.G.A.Somorjai,Introduction to Surface Chemistry and
Catalysis (Wiley,New York,1994).
30.The hydrogenation of ethylene was studied at atmo-spheric pressure in a differentially operated plug flow reactor.Standard conditions were 11Torr of C 2H 4,150Torr of H 2,and 208to 353K (sample-dependent).31.Rates were measured on a per-gram basis.They were
normalized per mole of surface platinum species (Pt s )to obtain a turnover frequency (molecule Pt s
Ϫ1s Ϫ1).Moles of Pt s was determined by D ϭ1.13/d ,where D is the platinum dispersion [the ratio of Pt s to the total platinum content (Pt t )]and d is the particle size in nm.The platinum particle size was determined from number average TEM measurements.
32.R.D.Cortright,S.A.Goddard,J.E.Rekoske,J.E.
Dumesic,J.Catal.127,342(1991).33.We thank J.Fre ´chet for the valuable discussions.
Supported by the Air Force Office of Scientific Re-arch under award no.F49620-01-1-0033;by the Director,Office of Energy Rearch,Office of Science,Division of Materials Sciences,of the U.S.Depart-ment of Energy under contract no.DE-AC03-76SF00098;and by the Ford Motor Company and the Berkeley Catalysis Center (R.M.R.).Supporting Online Material
www.sciencemag/cgi/content/full/304/5671/711/DC1
Fig.S1
9February 2004;accepted 16March 2004
Population-Level HIV
Declines and Behavioral Risk
Avoidance in Uganda
Rand L.Stoneburner*and Dan elLow-Beer
Uganda provides the clearest example that human immunodeficiency virus (HIV)is preventable if populations are mobilized to avoid risk.Despite limited resources,Uganda has shown a 70%decline in HIV prevalence since the early 1990s,linked to a 60%reduction in casual x.The respon in Uganda appears to be distinctively associated with communication about acquired immunodeficiency syndrome (AIDS)through social networks.Despite substantial condom u and promotion of biomedical approaches,other African countries have shown neither similar be-havioral respons nor HIV prevalence declines of the same scale.The Ugandan success is equivalent to a vaccine of 80%effectiveness.Its replication will require changes in global HIV/AIDS intervention policies and their evaluation.Projections of the HIV pandemic paint a bleak picture for global health (1,2).Never-theless,becau most cas of HIV occur through connsual xual intercour,it is avoidable if populations are warned and mo-bilized to change risk-taking behaviors.De-spite success from this approach,the appar-ently unrelenting expansion of the pandemic has rved to
emphasize a need for the pro-motion of more effective respons (3–5).HIV risk behaviors and infection rates dropped substantially among homoxual males in North America and Europe in the early to mid-1980s (6–8).The next widely acknowledged success was in heteroxuals
in Thailand,a result that has been unequivo-cally accepted since the early 1990s (9).Then,in 1994–1995,came data from resource-poor Uganda of declines in HIV prevalence among younger pregnant women,coupled with indications of preceding behav-ior change and reductions in HIV incidence (10–15).The Ugandan evidence is still viewed with caution,and confusion persists in its evaluation (16–19).
We reviewed population-level HIV and behavioral data in Uganda and in neighbor-ing countries to evaluate the validity and determinants of HIV declines and to ex-plore possible influences of preventive in-terventions (20,21).
Our initial analysis indicated that HIV incidence was declining in Uganda by the late 1980s (22–24).By 1995,Ugandan surveil-lance of HIV prevalence in pregnant women
Population Health Evaluation Unit,Cambridge Uni-versity,Cambridge,UK.
*To whom correspondence should be addresd.E-mail:randstoneburner@netzero
R E P O R T S
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